Hawkins Electrical Guide v. 08 (of 10) Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

LXIII. WAVE FORM MEASUREMENT

The great importance of the wave form in alternating current work is never denied, though it has sometimes been overlooked. The application of large gas engines to the driving of alternators operated in parallel requires an accurate knowledge of the wave form, and a close conformation to a sine wave if parallel operation is to be satisfactory. It is also important that the fluctuations in magnetism of the field poles should be known, especially if solid steel pole faces be used.

If an alternator armature winding be connected in delta, the presence of a third harmonic becomes objectionable, as it gives rise to circulating currents in the winding itself, which increase the heating and lowers the efficiency of the machine.

That the importance of having a good wave form is being realized, is proved by the increasing prevalence in alternator specifications of a clause specifying the maximum divergence allowable from a true sine wave. It is however perhaps not always realized that an alternator which gives a good pressure wave on no load may give a very bad one under certain loads, and the ability of the machine to maintain a good wave form under severe conditions of load is a better criterion of its good design than is the shape of its wave at no load.

The question of wave form is of special interest to the power station engineer. Upon it depends the answer to the questions: whether he may ground his neutral wires without getting large circulating currents; whether he may safely run any combination of his alternators in parallel; whether the constants of his distributing circuit are of an order liable to cause dangerous voltage surges due to resonance with the harmonics of his pressure wave; what stresses he is getting in his insulation due to voltage surges when switching on or off, etc. It has been shown by Rossler and Welding that the luminous efficiency of the alternating current arc may be 44 per cent. higher with a flat topped than with a peaked pressure wave, while on the other hand it is well known that transformers are more efficient on a peaked wave. Also the accuracy of many alternating current instruments depends upon the wave shape.

In making insulation breakdown tests on cables, insulators, or machinery, large errors may be introduced unless the wave form at the time of the test be known. It is not sufficient even to know that the testing alternator gives a close approximation to a sine wave at no load; since if the capacity current of the apparatus under test be moderately large compared with the full load current of the testing alternator, the charging current taken may be sufficient to distort the wave form considerably, thus giving wrong results to the disadvantage of either the manufacturer or purchaser.

The desirability of a complete knowledge of the manner in which the pressure and current varies during the cycle, has resulted in various methods and apparatus being devised for obtaining this knowledge. The apparatus in use for such purpose may be divided into two general classes,

1. Wave indicators; 2. Oscillographs.

and the methods employed with these two species of apparatus may be described respectively as,

1. Step by step; 2. Constantly recording.

that is to say, in the first instance, a number of instantaneous values are obtained at various points of the cycle, which are plotted and a curve traced through the several points thus obtained. A constantly recording method is one in which an infinite number of values are determined and recorded by the machine, thus giving a complete record of the cycle, leaving no portion of the wave to be filled in.

The various methods of determining the wave form may be further classified as:

{ Joubert's method; { Four part commutator method; { Modified four part commutator method; 1. Step by step { Ballistic galvanometer method; { Zero method; { By Hospitalier ondograph.

{ cathode ray; { by use of various types { glow light; 2. constantly recording { of =oscillograph=, { moving iron; { such as { moving coil; { hot wire.

=Joubert's Method.=--The apparatus required for determining the wave form by this step by step method, consists of a galvanometer, condenser, two, two way switches, resistance and adjustable contact maker, as shown in fig. =2,589=.

The contact maker is attached to the alternator shaft so that it will rotate synchronously with the latter. By means of the adjustable contact, the instant of "making" that is, of "closing" the testing circuit may be varied, and the angular position of the armature, at which the testing circuit is closed, determined from the scale, which is divided into degrees.

A resistance is placed in series with one of the alternator leads, such that the drop across it, gives sufficient pressure for testing.

=Ques. Describe the method of making the test.=

Ans. For current wave measurement switch No. 1 is placed on contact F, and for pressure wave measurement, on contact G, switch No. 2 is now turned to M and the drop across the resistance (assuming switch No. 1 to be turned to contact F) measured by charging the condenser, and then discharging it through the galvanometer by turning the switch to S. This is repeated for a number of positions of the contact maker, noting each time the galvanometer reading and position of the contact maker. By plotting the positions of contact maker as abscissæ, and the galvanometer readings as ordinates, the curve drawn through them will represent the wave form.

The apparatus is calibrated by passing a known constant current through the resistance.

=Ballistic Galvanometer Method.=--This method, which is due to Kubber, employs a contact breaker instead of a contact maker. The distinction between these two devices should be noted: A contact maker keeps the circuit closed during each revolution for a short interval only, whereas, a contact breaker keeps the circuit open for a short interval only.

Fig. 2,592, shows the necessary apparatus and connections for applying the ballistic galvanometer method. The contact breaker consists of a commutator having an ebonite or insulating segment and two brushes.

In operation the contact breaker keeps the circuit closed during all of each revolution, except the brief interval in which the brushes pass over the ebonite segment.

The contact breaker is adjustable and has a scale enabling its various positions of adjustment to be noted.

=Ques. Describe the test.=

Ans. The contact breaker is placed in successive positions and galvanometer readings taken, the switch being turned to F, fig. 2,592, in measuring the current wave, and to G in measuring the pressure wave. The results thus obtained are plotted giving respectively current and pressure waves.

=Ques. How is the apparatus calibrated?=

Ans. By sending a constant current of known value through the resistance R.

=Zero Method.=--In electrical measurements, a zero method is one in which the arrangement of the testing devices is such that the value of the quantity being measured is shown when the galvanometer needle points to =zero=.

In the zero method either a contact maker or contact breaker may be used in connection with a galvanometer and slide wire bridge, as shown in figs. 2,595 and 2,596.

=Ques. What capacity of battery should be used?=

Ans. Its voltage should be as great as the maximum pressure to be measured.

=Ques. What necessary condition must be maintained in the battery?=

Ans. Its pressure must be kept constant.

=Ques. How are instantaneous values measured?=

Ans. The bridge contact A is adjusted till the galvanometer shows no deflection, then the length AS is a measure of the pressure.

The drop between these points can be directly measured with a voltmeter if desired.

=Ques. How did Mershon modify the test?=

Ans. He used a telephone instead of the galvanometer to determine the correct placement of the bridge contact A.

=Ques. How can the instantaneous values be recorded?=

Ans. By attaching to the contact A, a pencil controlled by an electro-magnet arranged to strike a revolving paper card at the instant of no deflection, the paper being carried on a drum.

=Hospitalier Ondograph.=--The device known by this name is a development of the Joubert step by step method of wave form measurement, that is to say, the principle on which its =action is based=, consists in automatically charging a condenser from each 100th wave, and discharging it through a recording galvanometer, each successive charge of the condenser being automatically taken from a point a little farther along the wave.

As shown in the diagram, fig. 2,597, the ondograph consists of a synchronous motor A, operated from the source of the wave form to be measured, connected by gears B to a commutator D, in such a manner that while the motor makes a certain number of revolutions, the commutator makes a like number diminished by unity; that is to say, if the speed of the motor be 900 revolutions per minute, the commutator will have a speed of 899.

The commutator has three contacts, arranged to automatically charge the condenser cc' from the line, and discharge it through the galvanometer E, the deflection of which will be proportional to the pressure at any particular instant when contact is made.

In fig. 2,597, GG' are the motor terminals, HH' are connected to the condenser cc' through a resistance (to prevent sparking at the commutator) and I, I' are the connections to the service to be measured.

A permanent magnet type of recording galvanometer is employed. Its moving coil E receives the discharges of the condenser in rapid succession and turns slowly from one side to the other.

The movable part operates a long needle (separately mounted) carrying a pen F, which traces the curve on the rotating cylinder C. This cylinder is geared to the synchronous motor to run at such a speed as to register three complete waves upon its circumference.

By substituting an electromagnetic galvanometer for the permanent magnet galvanometer, and by using the magnet coils as current coils and the moving coil as the volt coil, the instrument can be made to draw watt curves. Fig. 2,598 shows the general appearance of the ondograph.

=Cathode Ray Oscillograph.=--This type of apparatus for measuring wave form was devised by Braun, and consists of a cathode ray tube having a fluorescent screen at one end, a small diaphragm with a hole in it at its middle, and two coils of a few turns each, placed outside it at right angles to one another. These coils carry currents proportional to the =pressure= and =current= respectively of the circuit under observation.

The ray then moves so as to produce an energy diagram on the fluorescent screen.

The instrument is much used in wireless telegraphy, as it is capable of showing the characteristics of currents of very high frequency.

=Glow Light Oscillograph.=--This device consists of two aluminum rods in a partially evacuated tube, their ends being about two millimeters apart. When an alternating current of any frequency passes between them a sheath of violet light forms on one of the electrodes, passing over to the other when the current reverses during each cycle. The phenomenon may be observed or photographed by means of a revolving mirror.

=Moving Iron Oscillograph.=--This type is due to Blondel, to whom belongs the credit of working out and describing in considerable detail the principles underlying the construction of oscillographs.

The moving iron type of oscillograph consists of a very thin vane of iron suspended in a powerful magnetic field, thus forming a polarized magnet. Near this strip are placed two small coils which carry the current whose wave form is to be measured.

The moving iron vane has a very short period of vibration and can therefore follow every variation in the current.

Attached to the vane is a small mirror which reflects a beam of light upon some type of receiving device.

The Siemens-Blondel oscillograph shown in fig. 2,604, is of the moving coil type, being a development of the moving iron principle.

=Moving Coil Oscillograph.=--The operation of this form of oscillograph is based on the behaviour of a movable coil in a magnetic field.

It consists essentially of a modified moving coil galvanometer combined with a rotating or vibrating mirror, a moving photographic film, or a falling photographic plate. The galvanometer portion of the outfit is usually referred to as the oscillograph as illustrated in figs. 2,608 to 2,612, representing diagrammatically the moving system.

In the narrow gap between the poles S, S of a powerful magnet are stretched two parallel conductors formed by bending a thin strip of phosphor bronze back on itself over an ivory pulley P. A spiral spring attached to this pulley serves to keep a uniform tension on the strips, and a guide piece L limits the length of the vibrating portion to the part actually in the magnetic field.

A small mirror M bridges across the two strips as shown. The effect of passing a current through such a "vibrator" is to cause one of the strips to advance while the other recedes, and the mirror is thus turned about a vertical axis.

Each strip of the loop passes through a separate gap (not shown in the figure). The whole of the "vibrator," as this part of the instrument is called, is immersed in an oil bath, the object of the oil being to damp the movement of the strips, and make the instrument dead beat. It also has the additional advantage of increasing by refraction the movement of the spot of light reflected from the vibrating mirrors.

The beam of light reflected from the mirror M is received on a screen or photographic plate, the instantaneous value of the current being proportional to the linear displacement of the spot of light so formed.

With alternating currents, the spot of light oscillates to and fro as the current varies and would thus trace a straight line.

To obtain an image of the wave form, it is necessary to traverse the photographic plate or film in a direction at right angles to the direction of the movement of the spot of light.

=Ques. How are the oscillograms obtained in the Duddell moving coil oscillograph?=

Ans. In all cases the oscillograms are obtained by a spot of light tracing out the curve connecting current or voltage with time. The source of light is an arc lamp, the light from which passes first through a lens, and then, excepting when projecting on a screen, through a rectangular slit about 10 mm. long by 1 mm. wide. The position of the lamp from the lens is adjusted till an image of the arc is obtained covering the three (two moving, one fixed) small oscillograph mirrors. The light is reflected back from these mirrors and, being condensed by a lens which is immediately in front of them, it converges till an image of the slit is formed on the surface where the record is desired. All that is necessary now to obtain a bright spot of light instead of this line image is to introduce in the path of the beam of light a cylindrical lens of short focal length.

=Ques. What is the function of the mirrors on the vibrating vane?=

Ans. They simply control the direction of a beam of light in a horizontal plane in such a manner that its deflection from a zero position depends on the current passing through the instrument, and it is therefore evident that the oscillograph is not complete without means of producing a time scale.

=Ques. How is the time scale produced?=

Ans. Either the surface on which the beam of light falls may be caused to move in a vertical plane with a certain velocity, so that the intersection of the beam and the plane surface traces out a curve connecting current with time (a curve which becomes a permanent record if a sensitized surface be used); or, the surface may remain stationary and in the path of the horizontally vibrating beam may be introduced a mirror which rotates or vibrates about a horizontal axis, thus superposing a vertical motion proportional to time on the horizontal vibration which is proportional to current, and causing the beam of light to trace out a curve connecting current and time on the stationary surface.

=Ques. What kind of recording apparatus is used with the Duddell oscillograph?=

Ans. A falling plate camera, or a cinematograph film camera.

=Ques. Explain the operation of the falling plate camera.=

Ans. In this arrangement a photographic plate is allowed to fall freely by the force of gravity down a dark slide. At a certain point in its fall it passes a horizontal slit through which the beams of light from the oscillograph pass, tracing out the curves on the plate as it falls.

The mean speed of the plate at the moment of exposure is about 13 feet per second. This speed is very suitable for use with frequencies of from 40 to 60 periods per second. A cloth bag is used to introduce the plate to the slide.

A catch holds the plate until it is desired to let it fall. Inside the case, is a small motor, 100 or 200 volts direct current, driving four mirrors which are fixed about a common axis with their planes parallel to it.

By looking through a small slot in the end of the camera into these rotating mirrors, the observer sees the wave form which the oscillograph is tracing out and is thus able to make sure that he is obtaining the particular wave form or other curve desired before exposing the plate.

The plate falls into a second red cloth bag which is placed on the bottom of the slide. The plates used are "stereoscopic size", 6¾" × 3¼" (17.1 × 8.3 cm.).

=Ques. For what use is the cinematograph camera adapted?=

Ans. For long records.

For instance, in investigations, such as observation on the paralleling of alternators, the running up to speed of motors, and the surges which may occur in switching on and off cable, etc. The cinematograph camera fits on to the falling plate case and by means of which a roll of cinematograph film can be driven at a uniform speed past the exposure aperture, enabling records up to 50 metres in length to be obtained. An interior view of the cinematograph camera is shown in fig. 2,621.

SOME OSCILLOGRAPH RECORDS

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LXIV. SWITCHBOARDS

=General Principles of Switchboard Connections.=--The interconnection of generators, transformers, lines, bus bars, and switches with their relays, in modern switchboard practice is shown by the diagrams, figs. 2,636 to 2,645. The figures being lettered A to J for simplicity, the generators are indicated by black discs, and the switches by open circles, while each heavy line represents a set of bus bars consisting of two or more bus bars according to the system of distribution. It will be understood, also, in this connection, that the number of pole of the switches and the type of switch will depend upon the particular system of distribution employed.

Diagram A, shows the simplest system, or one in which a single generator feeds directly into the line. There are no transformers or bus bars and only one switch is sufficient.

In B, a single generator supplies two or more feeders through a single set of bus bars, requiring a switch for each feeder, and a single generator switch.

In C, two generators are employed and required and the addition of a bus section switch.

D, represents a number of generators supplying two independent circuits. The additional set of bus bars employed for this purpose necessitates an additional bus section switch, and also additional selector switches for both feeders and generators.

E, shows a standard system of connection for a city street railway system having a large number of feeders.

This arrangement allows any group of feeders to be supplied from any group of generators.

It also permits the addition of a generator switch for each generator.

F, represents the simplest system with transformers.

It requires a single generator transformer bank, switch and line. The arrangement as show at F is used where a number of plants supply the same system.

G, represents a system having more than one line.

In this case a bus bar and transformer switch is used on the high tension side.

H, shows a number of generators connected to a set of low tension bus bars through generator switches, and employing a low tension transformer switch.

I, shows the connections of a system having a large number of feeders supplied by several small generators. In this case, the plant is divided into two parts, each of which may be operated independently.

J, represents the arrangement usually employed in modern plants where the generator capacity is large enough to permit of a generator transformer unit combination with two outgoing lines. By operating in parallel on the high tension side only, any generator can be run with any transformer. The whole plant can be run in parallel, or the two parts can be run separately.

=Switchboard Panels.=--The term "panel" means the slab of marble or slate upon which is mounted the switches, and the indicating and controlling devices. There are usually several panels comprising switchboards of moderate or large size, these panels being classified according to the division of the system that they control, as for instance:

1. Generator panel; 2. Feeder panel; 3. Regulator panel, etc.

In construction, the marble or slate should be free from metallic veins, and for pressures above, say, 600 volts, live connections, terminals, etc., should preferably be insulated from the panels by ebonite, mica, or removed from them altogether, as is generally the case with the alternating gear where the switches are of the oil type.

The bus bars and connections should be supported by the framework at the back of the board, or in separate cells, and the instruments should be operated at low pressure through instrument transformers.

The panels are generally held in position by bolting them to an angle iron, or a strip iron framework behind them.

=Generator Panel.=--This section of a switchboard carries the instruments and apparatus for measuring and electrically controlling the generators. On a well designed switchboard each generator has, as a rule, its own panel.

In the case of a dynamo, a good representative panel would have mounted upon it a reverse current circuit breaker, an ammeter, a double pole main switch (or perhaps a single pole switch, since the circuit breaker could also be used as a switch) a double pole socket into which a plug could be inserted to make connection with a voltmeter mounted on a swinging bracket at the end of the board; a rheostat handle, the spindle of which operates the shunt rheostat of the machine, the rheostat being placed either directly behind the spindle, if of small size, or lower down with chain drive from the hand wheel spindle, if of larger size, a field discharge switch and resistance, a lamp near the top of the panel for illuminating purposes, a fuse for the voltmeter socket, and, if desired, a watthour meter. If the dynamo be compound wound, the equalizing switch will generally be mounted on the frame of the machine, and in some cases the field rheostat will be operated from a pillar mounted in front of the switchboard gallery. If the generator be for traction purposes, the circuit breaker is more often of the maximum current type, and a lightning arrester is often added, without a choke coil, the latter as well as further lightning arresters being mounted on the feeder panels.

In the case of a high pressure alternating current plant of considerable size, the bus bars oil switches, and the current and pressure transformers are generally mounted either in stoneware cells, or built on a framework in a space guarded by expanded metal walls, and no high pressure apparatus of any sort is brought on to the panels themselves.

=Feeder Panel.=--The indicating and control apparatus for a feeder circuit is assembled on a panel called the feeder panel.

The most common equipment in the case of a direct current feeder panel comprises an ammeter, a double pole switch, and double pole fuses or instead of the fuses, a circuit breaker on one or both poles; in the case of a traction feeder a choke coil and a lightning arrester are often added.

The equipment of a typical high pressure three phase feeder panel is an ammeter (sometimes three ammeters, one in each phase) operated by a current transformer, and oil break switch with two overload release coils, or three if the neutral of the circuit be earthed, the releases being operated by current transformers.

The switch when on a large system is often in a cell some distance behind the panel, and is then controlled by a system of levers, or by a small motor which is started and stopped by a throw over switch on the panel, in which case there is generally a lamp or lamps on the panel to show whether the switch is open or closed.

Air brake switches or links are placed between the bus bars and the oil switch to allow of the latter being isolated for inspection purposes, and as a general rule no apparatus carrying high pressure current is allowed on the front of the panel. With both direct and alternating current feeders, a watthour meter is often added to show the total consumption of the circuit.

A typical three phase generator panel is provided with three ammeters, one in each phase, operated from three current transformers, one to each ammeter, a volt meter, a power factor indicator, and an indicating watthour meter, all operated from one or more pressure transformers, and the necessary current transformers, the operating handle of the oil switch, which is connected to the switch itself by means of rods, two maximum releases operated by current transformers, or a reverse relay for automatically tripping the switch, lamps for indicating when the switch is tripped, a socket for taking the plug which makes connection between the secondary of a pressure transformer and the synchronizer on the synchronizing panel, and a lamp for illuminating purposes, while on the base of the panel or on a pillar at the front of the gallery is mounted the gear for the field circuit. This consists of a double pole field switch and a discharge resistance, an ammeter, a handle for the rheostat in the generator field, and (if each alternator have its own direct coupled exciter) possibly also a small rheostat for the exciter field.

NOTE.--In some cases where the capacity of the plant is not very great, the oil switch is mounted on the back of the panel, and the bus bars, current transformers, &c., on the framework, also just at the back of the panel, but under no circumstances, in good modern practice, is high pressure apparatus permitted on the front of the board. Where the capacity of the plant is very large, the oil switches are operated electrically by means of small motors, and in this case the small switch gear for starting and stopping this motor is mounted on the generator panel, also the lamp or lamps to indicate when the switch is open, and when closed.

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Chapter LXV

=ALTERNATING CURRENT WIRING=

In the case of alternating current, because of its peculiar behaviour, there are several effects which must be considered in making wiring calculations, which do not enter into the problem with direct current.

Accordingly, in determining the size of wires, allowance must be made for

1. Self-induction; 2. Mutual-induction; 3. Power factor; 4. Skin effect; 5. Corona effect; 6. Frequency; 7. Resistance.

Most of these items have already been explained at such length, that only a brief summary of facts need be added, to point out their connection and importance with alternating current wiring.

=Induction.=--The effect of induction, whether self-induction or mutual induction, is to set up a back pressure of spurious resistance, which must be considered, as it sometimes materially affects the calculation of circuits even in interior wiring.

Self-induction is the effect produced by the action of the electric current upon itself during variations in strength.

=Ques. What conditions besides variations of current strength governs the amount of self-induction in a circuit?=

Ans. The shape of the circuit, and the character of the surrounding medium.

If the circuit be straight, there will be little self-induction, but if coiled, the effect will become pronounced. If the surrounding medium be air, the self-induction is small, but if it be iron, the self-induction is considerable.

=Ques. With respect to self-induction, what method should be followed in wiring?=

Ans. When iron conduits are used, the wires of each circuit should not be installed in separate conduits, because such arrangement will cause excessive self-induction.

The importance of this may be seen from the experience of one contractor, who installed feeders and mains in separate iron pipes. When the current was turned on, it was found that the self-induction was so great as to reduce the pressure to such an extent that the lamps, instead of giving full candle power, were barely red. This necessitated the removal of the feeders and main and re-installing them, so that those of the same circuit were in the same pipe.

=Ques. What is mutual induction?=

Ans. Mutual induction is the effect of one alternating current circuit upon another.

=Ques. How is it caused?=

Ans. It is due to the magnetic field surrounding a conductor cutting adjacent conductors and inducing back pressures therein.

This effect as a rule in ordinary installations is negligible.

=Transpositions.=--The effect of mutual induction between two circuits is proportional to the inter-linkage of the magnetic fluxes of the two lines. This in turn depends upon the proximity of the lines and upon the general relative arrangement of the conductors.

The inductive effect of one line upon another is equal to the algebraic sum of the fluxes due to the different conductors of the first line, considered separately, which link the secondary line.

The effect of mutual induction is to induce surges in the line where a difference of frequency exists between the two currents, and to induce high electrostatic charges in lines carrying little or no current, such as telephone lines.

INDUCTANCE PER MILE OF THREE PHASE CIRCUIT

---------+-------+----------+------------ | Diam. | Distance | Self Size | in | d in | Inductance B. & S. | inch. | inches. | L henrys. ---------+-------+----------+------------ 0000 | .46 | 12 | .00234 | | 18 | .00256 | | 24 | .00270 | | 48 | .00312 | | | 000 | .41 | 12 | .00241 | | 18 | .00262 | | 24 | .00277 | | 48 | .00318 | | | 00 | .365 | 12 | .00248 | | 18 | .00269 | | 24 | .00285 | | 48 | .00330 | | | 0 | .325 | 12 | .00254 | | 18 | .00276 | | 24 | .00293 | | 48 | .00331 | | | 1 | .289 | 12 | .00260 | | 18 | .00281 | | 24 | .00308 | | 48 | .00338 | | | 2 | .258 | 12 | .00267 | | 18 | .00288 | | 24 | .00304 | | 48 | .00314 | | | 3 | .229 | 12 | .00274 | | 18 | .00294 | | 24 | .00310 | | 48 | .00351 | | | 4 | .204 | 12 | .00280 | | 18 | .00300 | | 24 | .00315 | | 48 | .00358 | | | 5 | .182 | 12 | .00286 | | 18 | .00307 | | 24 | .00323 | | 48 | .00356 | | | 6 | .162 | 12 | .00291 | | 18 | .00313 | | 24 | .00329 | | 48 | .00369 | | | 7 | .144 | 12 | .00298 | | 18 | .00310 | | 24 | .00336 | | 48 | .00377 | | | 8 | .128 | 12 | .00303 | | 18 | .00325 | | 24 | .00341 | | 48 | .00384 | | | 9 | .114 | 12 | .00310 | | 18 | .00332 | | 24 | .00348 | | 48 | .00389 | | | 10 | .102 | 12 | .00318 | | 18 | .00340 | | 24 | .00355 | | 48 | .00396 ---------+-------+----------+------------

This effect may be nullified by separating the lines and by transposing the wires of one of the lines so that the effect produced in one section is opposed by that in another. Of two parallel lines consisting of two wires each, one may be transposed to neutralize the mutual inductance.

Fig. 2,678 shows this method. The length L' should be an even factor of L so that to every section of the line transposed there corresponds an opposing section.

The self inductance of lines is readily calculated from the following formula:

L = .000558 {2.303 log (2A ÷ d) + .25} per mile of circuit

where

L = inductance of a loop of a three phase circuit in henrys. Note.--The inductance of a complete single phase circuit = L × 2 ÷ √3. A = distance between wires; d = diameter of wire.

CAPACITY IN MICRO-FARADS PER MILE OF CIRCUIT FOR THREE PHASE SYSTEM

---------+-------+----------+-------------- | Diam. | Distance | Capacity Size | in | A in | C in B. & S. | inch. | inches. | micro-farads ---------+-------+----------+-------------- 0000 | .46 | 12 | .0226 | | 18 | .0204 | | 24 | .01922 | | 48 | .01474 | | | 000 | .41 | 12 | .0218 | | 18 | .01992 | | 24 | .01876 | | 48 | .01638 | | | 00 | .365 | 12 | .0124 | | 18 | .01946 | | 24 | .01832 | | 48 | .01604 | | | 0 | .325 | 12 | .02078 | | 18 | .01898 | | 24 | .01642 | | 48 | .01570 | | | 1 | .289 | 12 | .02022 | | 18 | .01952 | | 24 | .01748 | | 48 | .0154 | | | 2 | .258 | 12 | .01972 | | 18 | .01818 | | 24 | .01710 | | 48 | .01510 | | | 3 | .229 | 12 | .01938 | | 18 | .01766 | | 24 | .01672 | | 48 | .01480 | | | 4 | .204 | 12 | .01874 | | 18 | .01726 | | 24 | .01636 | | 48 | .01452 | | | 5 | .182 | 12 | .01830 | | 18 | .01690 | | 24 | .01602 | | 48 | .01426 | | | 6 | .162 | 12 | .01788 | | 18 | .01654 | | 24 | .01560 | | 48 | .0140 | | | 7 | .144 | 12 | .01746 | | 18 | .01618 | | 24 | .01538 | | 48 | .01374 | | | 8 | .128 | 12 | .01708 | | 18 | .01586 | | 24 | .01508 | | 48 | .01350 | | | 9 | .114 | 12 | .01660 | | 18 | .01552 | | 24 | .01478 | | 48 | .01326 | | | 10 | .102 | 12 | .01636 | | 18 | .01522 | | 24 | .01452 | | 48 | .01304 ---------+-------+----------+--------------

=Capacity.=--In any given system of electrical conductors, a pressure difference between two of them corresponds to the presence of a quantity of electricity on each. With the same charges, the difference of pressure may be varied by varying the geometrical arrangement and magnitudes and also by introducing various dielectrics. The constant connecting the charge and the resulting pressure is called the capacity of the system.

All circuits have a certain capacity, because each conductor acts like the plate of a condenser, and the insulating medium, acts as the dielectric. The capacity depends upon the insulation.

For a given grade of insulation, the capacity is proportional to the surface of the conductors, and universally to the distance between them.

A three phase three wire transmission line spaced at the corners of an equilateral triangle as regards capacity acts precisely as though the neutral line were situated at the center of the triangle.

The capacity of circuits is readily calculated by applying the following formulae:

C = 38.83 sc 10^{-3} / log (D ÷ d) per mile, insulated cable with lead sheath; C = 38.83 × 10^{-3} / log (4h ÷ d) per mile, single conductor with earth return; C = 19.42 × 10^{-3} / log (2A ÷ d) per mile of parallel conductors forming metallic circuit;

in which

C = Capacity in micro-farads; for a metallic circuit, C = capacity between wires;

sc = Specific inductive capacity of insulating material; = 1 for air, and 2.25 to 3.7 for rubber;

D = Inside diameter of lead sheath; d = Diameter of conductor; h = Distance of conductors above ground; A = Distance between wires.

=Frequency.=--The number of cycles per second, or the frequency, has a direct effect upon the inductance reactance in an alternating current circuit, as is plainly seen from the formula.

X_{i} = 2π_f_L

In the case of a transmission line alone; the lower frequencies are the more desirable, in that they tend to reduce the inductance drop and charging current. The inductance drop is proportional to the frequency.

The natural period of a line, with distributed inductance and capacity, is approximately given by __ P = 7,900 / √LC

where L is the total inductance in millihenrys, and C, the total capacity in micro-farads. Accordingly some lower odd harmonic of the impressed frequency may be present which corresponds with the natural period of the line. When this obtains, oscillations of dangerous magnitude may occur. Such coincidences are less likely with the lower harmonics than with the higher.

=Skin Effect.=--The tendency of alternating current to confine itself to the outer portions of a conductor, instead of passing uniformly through the cross section, is called skin effect. The effect is proportional to the size of the conductor and the frequency.

=Ques. What effect has "skin effect" on the current?=

Ans. It is equivalent to an increase of ohmic resistance and therefore opposes the current.

If the conductor be large, or the frequency high, the central portion of the conductor carries little if any current, hence the resistance is therefore greater for alternating current than for direct current.

=Ques. For what condition may "skin effect" be neglected?=

Ans. For frequencies of 60 or less, with conductors having a diameter not greater than 0000 B. & S. gauge.

=Ques. How is the "skin effect" calculated for a given wire?=

Ans. Its area in circular mils multiplied by the frequency, gives the ratio of the wire's ohmic resistance to its combined resistance.

That is to say, the factor thus obtained multiplied by the resistance of the wire to direct current will give its combined resistance or resistance to alternating current.

The following table gives these ratio factors for large conductors.

RATIO FACTOR FOR COMBINED RESISTANCE

+---------------+---------+---------------+---------+ | Cir. mils. | Ratio | Cir. mils. | Ratio | | × frequency | factor | × frequency | factor | +---------------+---------+---------------+---------+ | 10,000,000 | 1.00 | 70,000,000 | 1.13 | | 20,000,000 | 1.01 | 80,000,000 | 1.17 | | 30,000,000 | 1.03 | 90,000,000 | 1.20 | | 40,000,000 | 1.05 | 100,000,000 | 1.25 | | 50,000,000 | 1.08 | 125,000,000 | 1.34 | | 60,000,000 | 1.10 | 150,000,000 | 1.43 | +---------------+---------+---------------+---------+

=Corona Effect.=--When two wires, having a great difference of pressure are placed near each other, a certain phenomenon occurs, which is called corona effect. When the spacing or distance between the wires is small and the difference of pressure in the wires very great, a continuous passage of energy takes place through the dielectric or atmosphere, the amount of this energy may be an appreciable percentage of the power transmitted. Therefore in laying out high pressure transmission lines, this effect must be considered in the spacing of the wires.

=Ques. How does the corona effect manifest itself?=

Ans. It is visible at night as a bluish luminous envelope and audible as a hissing sound.

=Ques. What is the critical voltage?=

Ans. The voltage at which the corona effect loss takes place.

=Ques. Upon what does the critical voltage depend?=

Ans. Upon the radius of the wires, the spacing, and the atmospheric conditions.

The critical voltage increases with both the diameter of the wires, and the spacing.

The losses due to corona effect increase very rapidly with increasing pressure beyond the critical voltage.

The magnitude of the losses as well as the critical voltage is affected, by atmospheric conditions, hence they probably vary with the particular locality, and the season of the year. Therefore, for a given locality, a voltage which is normally below the critical point, may at times be above it, depending upon changes in the weather.

Such elements as smoke, fog, moisture, or other particles (dust, snow, etc.) floating in the air, increase the losses; rain, however, apparently has no appreciable effect upon the losses. It follows then that in the design of a transmission line, the atmospheric conditions of the particular locality through which the line passes should be considered.

=Ques. How should live wires be spaced?=

Ans. They should be so spaced as to lessen the tendency to leakage and to prevent the wires swinging together or against towers. The spacing should be only sufficient for safety, since increased spacing increases the self-induction of the line, and while it lessens the capacity, it does so only in a slight degree.

The following spacing is in accordance with average practice.

SPACING FOR VARIOUS VOLTAGES

+---------+----------+ | Volts | Spacing | +---------+----------+ | 5,000 | 28 ins. | | 15,000 | 40 ins. | | 30,000 | 48 ins. | | 45,000 | 60 ins. | | 60,000 | 60 ins. | | 75,000 | 84 ins. | | 90,000 | 96 ins. | | 105,000 | 108 ins. | | 120,000 | 120 ins. | +---------+----------+

=Resistance of Wires.=--For quick calculation the following method of obtaining the resistance (approximately) of wires will be found convenient:

1,000 feet No. 10 B. & S. wire, which is about .1 inch in diameter (.1019), has a resistance of one ohm, at a temperature of 68° F. and weighs 31.4 pounds. A wire three sizes larger, that is No. 7, has twice the cross section and therefore one-half the resistance. A wire three sizes smaller than No. 10, that is No. 13, has one-half the cross section and therefore twice the resistance.

Thus, starting with No. 10, any number three sizes larger will double the cross sectional area and any wire three sizes smaller will halve the cross sectional area of the preceding wire. This is true to the extreme limits of the table, so that the area, weight and resistance of any wire may be at once calculated to a close approximation from this rule, intermediate sizes being obtained by interpolation.

For alternating current, the combined resistance, that is, the total resistance, including skin effect, is obtained by multiplying the resistance, as found above by the "ratio factor" (see table page 1,894).

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Chapter XLVII

=Impedance.=--The total opposition to the flow of electricity in an alternating current circuit, or the impedance may be resolved into two components representing the ohmic resistance and the spurious resistance; these components have a phase difference of 90°, and they may be represented graphically by the two legs of a right angle triangle, of which the hypothenuse represents the impedance.

Similarly, the volts lost or "drop" in an alternating circuit may be resolved into two components representing respectively

1. The loss due to resistance.

2. The loss due to reactance.

These components have a phase difference of 90° and are represented graphically similar to the impedance components. This has been explained at considerable length in Chapter XLVII (Guide V).

=Power Factor.=--When the current falls out of step with the pressure, as on inductive loads, the power factor becomes less than unity, and the effect is to increase the current required for a given load. Accordingly, this must be considered in calculating the size of the wires. As has been explained, the current flowing in an alternating current circuit, as measured by an ammeter, can be resolved into two components, representing respectively the active component and the wattless component or idle current. These are graphically represented by the two legs of a right triangle, of which the hypothenuse represents the current measured by the ammeter.

This apparent current, as is evident from the triangle, exceeds the active current and lags behind the pressure by an amount represented by the angle ϕ between the hypothenuse and leg representing the energy current as shown in fig. 2,694.

=Ques. What determines the heating of the wires on alternating current circuits with inductive loads?=

Ans. The apparent current, as represented by the hypothenuse of the triangle in fig. 2,694.

=Ques. How is the apparent current obtained?=

Ans. Divide the true watts by the product of the power factor multiplied by the voltage.

Example.--A certain circuit supplies 20 kw. to motors at 220 volts and .8 power factor. What is the apparent current?

true watts 20,000 Apparent Current = -------------------- = -------- = 113.6 amperes power factor × volts .8 × 220

=Ques. What else, besides power factor, should be considered in making wire calculations for motor circuits?=

Ans. The efficiency of the motor, and the heavy starting current.

The product of the efficiency of the motor multiplied by the power factor gives the apparent efficiency, which governs the size of the wires, apparatus, etc., necessary to feed the motors.

Allowance should be made for the heavy starting current required for some motors to avoid undue drop.

TABLE OF APPROXIMATE AMPERES PER TERMINAL FOR INDUCTION MOTORS

Column headings: A-110 volts B-220 volts C-440 volts D-550 volts

+-----+----------------+------------------+------------------------+ | | | Two phase | Three phase | |Horse| Single phase | four wire | three wire | |power+----+-----+-----+-----+-----+------+-----+-----+------+-----+ | | A | B | C | A | B | C | A | B | C | D | +-----+----+-----+-----+-----+-----+------+-----+-----+------+-----+ | .5| 6.6| 3.4 | 1.8| 3.3 | 1.7 | .9 | 3.7 | 1.8 | 1 | | | 1 | 14 | 7 | 3.5| 6.4 | 3.2 | 1.6 | 7.4 | 3.7 | 1.9 | | | 2 | 24 | 12 | 6 | 11 | 5.7 | 2.9 | 13 | 6.6 | 3.3 | 2.5 | | 3 | 34 | 17 | 8.5| 16 | 8.1 | 4.1 | 19 | 9.3 | 4.7 | 3.5 | | 4 | 52 | 26 | 13 | 26 | 13 | 6.5 | 30 | 15 | 7.5 | 6 | | 5 | 74 | 37 | 18.5| 38 | 19 | 9.5 | 44 | 22 | 11 | 9 | | 10 | 94 | 47 | 23.5| 44 | 22 | 11 | 50 | 25 | 12.5 | 11 | | 15 | | | | 66 | 33 | 16.5 | 76 | 38 | 19 | 16 | | 20 | | | | 88 | 44 | 22 | 102 | 51 | 25.5 | 22 | | 25 | | | | 111 | 55 | 28 | 129 | 64 | 32 | 25 | | 30 | | | | 134 | 67 | 33.5 | 154 | 77 | 38.5 | 32 | | 40 | | | | 178 | 89 | 44.5 | 204 | 107 | 53.5 | 44 | | 50 | | | | 204 | 102 | 51 | 236 | 118 | 59 | 52 | | 75 | | | | 308 | 154 | 77 | 356 | 178 | 89 | 77 | | 100 | | | | 408 | 204 | 102 | 472 | 236 | 118 | 100 | +-----+----+-----+-----+-----+-----+------+-----+-----+------+-----+

=Ques. What are the usual power factors encountered on commercial circuits?=

Ans. A mixed load of incandescent lamps and induction motors will have a power factor of from .8 to .85; induction motors above .8 to .85; incandescent and Nernst lamps .98; arc lamps, .85.

=Wire Calculations.=--In the calculation of alternating current circuits, the two chief factors which make the computation different from that for direct current circuits, is induction and power factor. The first depends on the frequency, and physical condition of the circuit, and the second upon the character of the load.

=Ques. Under what conditions may inductance be neglected?=

Ans. In cases where the wires of a circuit are not spaced over an inch apart, or in conduit work, where both wires are in the same conduit.

Under these conditions the calculation is the same as for direct current after making proper allowance for power factor.

=Ques. Under what conditions must induction be considered?=

Ans. On exposed circuits with wires separated several inches, particularly in the case of large wires.

=Sizes of Wire.=--The size of wire for any alternating circuit may be determined by slightly modifying the formula used in direct current work, and which, as derived in Guide No. 4, page 748, is

amperes × feet × 21.6 circular mils = --------------------- (1) drop

The quantity 21.6, is twice the resistance (10.8) of a foot of copper wire one mil in diameter (mil foot). This resistance (10.8) is multiplied by 2, giving the quantity 21.6, because the length of a circuit, or feet in the formula, is given as the "run" or distance one way, that is, one-half the total length of wire in the circuit, must be multiplied by 2 to get the total drop, viz.:

amperes × feet × 10.8 × 2 amperes × feet × 21.6 circular mils = ------------------------- = --------------------- drop drop

It is sometimes however convenient to make the calculation in terms of watts. Formula (1) may be modified for such calculation.

In modifying the formula, the "drop" should be expressed in percentage instead of actual volts lost, that is, instead of the difference in pressure between the beginning and the end of the circuit.

In any circuit the loss in percentage, or

drop % loss = ------------------ × 100 impressed pressure

from which

% loss × impressed pressure drop = --------------------------- (2) 100

Substituting equation (2) in equation (1)

amperes × feet × 21.6 circular mils = ----------------------------- % loss × imp. pressure ------------------------ 100

amperes × feet × 2,160 = -------------------------- (3) % loss × imp. pressure

Equation (3) is modified for calculation in terms of watts as follows: The power in watts is equal to the applied voltage multiplied by the current, that is to say, the power is equal to the volts at the consumer's end of the circuit multiplied by the current, or simply

watts = volts × amperes

from which

watts amperes = ----- (4) volts

Substituting this value for the current in equation (3) and remembering that the pressure taken is the volts at the consumer's end of the line

watts ----- × feet × 2,160 volts circular mils = --------------------- % loss × volts

watts × feet × 2,160 = -------------------- (5) % loss × volts^{2}

This formula (5) applies to a direct current two wire circuit, and to adapt it to any alternating current circuit it is only necessary to use the letter M instead of the number 2,160, thus

watts × feet × M circular mils = ------------------ (6) % loss × volts^{2}

in which M is a coefficient which has various values according to the kind of circuit and value of the power factor. These values are given in the following table:

=VALUES OF M=

--------+----------------------------------------------------------- | POWER FACTOR SYSTEM +-----+-----+-----+-----+-----+-----+-----+-----+-----+----- | 1.00| .98| .95| .90| .85| .80| .75| .70| .65| .60 --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- Single |2,160|2,249|2,400|2,660|3,000|3,380|3,840|4,400|5,112|6,000 phase | | | | | | | | | | --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- Two |1,080|1,125|1,200|1,330|1,500|1,690|1,920|2,200|2,556|3,000 phase | | | | | | | | | | (4 wire)| | | | | | | | | | --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- Three |1,080|1,125|1,200|1,330|1,500|1,690|1,920|2,200|2,556|3,000 phase | | | | | | | | | | (3 wire)| | | | | | | | | | --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----

NOTE.--The above table is calculated as follows: For =single phase= M = 2,160 ÷ power factor^{2} × 100; for =two phase= four wire, or three phase three wire, M = ½ (2,160 ÷ power factor^{2})× 100. Thus the value of M for a single phase line with power factor .95 = 2,160 ÷ .95^{2} × 100 = 2,400.

It must be evident that when 2,160 is taken as the value of M, formula (6) applies to a two wire direct current circuit and also to a single phase alternating current circuit when the power factor is unity.

In the table the value of M for any particular power factor is found by dividing 2,160 by the square of that power factor for single phase and twice the square of the power factor for two phase and three phase.

=Ques. For a given load and voltage how do the wires of a single and two phase system compare in size and weight, the power factor being the same in each case?=

Ans. Since the two phase system is virtually two single phase systems, the four wires of the two phase systems are half the size of the two wires of the single phase system, and accordingly, the weight is the same for either system.

=VALUES OF T=

+-------------------+--------------------------------------------+ | | POWER FACTOR | | SYSTEM +--------+--------+--------+--------+--------+ | | 1.00 | .98 | .90 | .80 | .70 | +-------------------+--------+--------+--------+--------+--------+ | Single phase | 1.00 | .98 | .90 | .80 | .70 | +-------------------+--------+--------+--------+--------+--------+ | Two phase, 4 wire | 2.00 | 1.96 | 1.80 | 1.60 | 1.40 | +-------------------+--------+--------+--------+--------+--------+ |Three phase, 3 wire| 1.73 | 1.70 | 1.55 | 1.38 | 1.21 | +-------------------+--------+--------+--------+--------+--------+

NOTE.--This table is for finding the value of the current in line, using the formula I = W ÷ (E × T), in which I = current in line; E = voltage between main conductors at receiving or consumers' end; W = watts. For instance, what is the current in a two phase line transmitting 1,000 watts at 550 volts, power factor .80? I = 1,000 ÷ (550 × 1.60) = 1.13.

=Ques. Since there is no saving in copper in using two phases, what advantage has the two phase system over the one phase system?=

Ans. It is more desirable on power circuits, because two phase motors are self-starting.

That is to say, the rotating magnetic field that can be produced by a two phase current, permits an induction motor to start without being equipped with any special phase splitting devices which are necessary on single phase motors, because the oscillating field produced by a single phase current does not produce any torque on a squirrel cage armature at rest.

=Ques. For equal working conditions, what is the comparison between the single, two and three phase system as to size and weight of wires?=

Ans. Each wire of the three phase system is half the size of one of the wires of the single phase system, hence the weight of copper required for the three phase system is 75% of that required for the single phase system. Since in the two phase system half of the load is carried by each phase, each wire of the three phase system is the same size as one of the wires of the two phase system, hence, the copper required by the three phase system is 75% of that required by the two phase system.

=MISCELLANEOUS FORMULÆ FOR COPPER WIRES=

Diameter squared = circular mils Circular mils × .7854 = square mils .000003027 × circular mils = pounds per foot .003027 × circular mils = pounds per 1,000 feet .0159847 × circular mils = pounds per mile .003879 × square mils = pounds per 1,000 feet .33033 ÷ circular mils = feet per pound .0000002924 × circular mils = pounds per ohm .342 ÷ circular mils = ohms per pound .096585 × circular mils = feet per ohm 10.353568 ÷ circular mils = ohms per foot

Breaking weight of wire ÷ area = breaking weight per square inch.

Breaking weight per square inch × area = breaking weight of wire.

The weight of copper wire is 1-1/7 times the weight of iron wire of same diameter.

EXAMPLE.--What size wires must be used on a single phase circuit 2,000 feet in length to supply 30 kw. at 220 volts with loss of 4%, the power factor being .9?

The formula for circular mils is

watts × feet × M circular mils = ------------------ (1) % loss × volts^{2}

Substituting the given values and the proper value of M from the table, in (1)

30,000 × 2,000 × 2,660 circular mils = ---------------------- = 82,438 4 × 220^{2}

Referring to the accompanying table of the properties of copper wire, the nearest larger size wire is No. 1 B. & S. gauge having an area of 83,690 circular mils.

=TABLE OF THE PROPERTIES OF COPPER WIRE=

Giving weights, length and resistances of wires of Matthiessen's Standard Conductivity for both B. & S. G. (Brown & Sharpe Gauge) and B. W. G. (Birmingham Wire Gauge) from Transactions October 1903, of the American Institute of Electrical Engineers.

__________________________________________________________________ | | | Gauges. To the nearest fourth | | Length.|Resistance. significant digit. | | | _____________________________________| |________|_________ | | | | Weight. | | | | Diameter. | Area. | Lbs. | Feet | Ohms per | | | | per | per lb.|1,000 ft. ______|_______|___________|__________| 1,000 |________|__________ | | | | feet. | | | | | Circular | | | B.& S.| B.W.G.| Inches. | mils. | | | @ 68° F. ______|______ |___________|__________|_________|________|_________ | | | | | | 0000 | | 0.460 | 211,600 | 640.5 | 1.561 | .04893 | 0000 | 0.454 | 206,100 | 623.9 | 1.603 | .05023 | 000 | 0.425 | 180,600 | 546.8 | 1.829 | .05732 | | | | | | 000 | | 0.4096 | 167,800 | 508.0 | 1.969 | .06170 | 00 | 0.380 | 144,400 | 437.1 | 2.288 | .07170 00 | | 0.3648 | 133,100 | 402.8 | 2.482 | .07780 | | | | | | | 0 | 0.340 | 115,600 | 349.9 | 2.858 | .08957 0 | | 0.3249 | 105,500 | 319.5 | 3.130 | .09811 | 1 | 0.3000 | 90,000 | 272.4 | 3.671 | .1150 | | | | | | 1 | | 0.2893 | 83,690 | 253.3 | 3.947 | .1237 | 2 | 0.2840 | 80,660 | 244.1 | 4.096 | .1284 | 3 | 0.2590 | 67,080 | 203.1 | 4.925 | .1543 | | | | | | 2 | | 0.2576 | 66,370 | 200.9 | 4.977 | .1560 | 4 | 0.2380 | 56,640 | 171.5 | 5.832 | .1828 3 | | 0.2294 | 52,630 | 159.3 | 6.276 | .1967 | | | | | | | 5 | 0.2200 | 48,400 | 146.5 | 6.826 | .2139 4 | | 0.2043 | 41,740 | 126.4 | 7.914 | .2480 | 6 | 0.2030 | 41,210 | 124.7 | 8.017 | .2513 | | | | | | 5 | | 0.1819 | 33,100 |100.2 | 9.98 | .3128 | 7 | 0.1800 | 32,400 | 98.08 | 10.20 | .3196 | 8 | 0.1650 | 27,230 | 82.41 | 12.13 | .3803 | | | | | | 6 | | 0.1620 | 26,250 | 79.46 | 12.58 | .3944 | 9 | 0.1480 | 21,900 | 66.30 | 15.08 | .4727 7 | | 0.1443 | 20,820 | 63.02 | 15.87 | .4973 | | | | | | | 10 | 0.1340 | 17,960 | 54.35 | 18.40 | .5766 8 | | 0.1285 | 16,510 | 49.98 | 20.01 | .6271 | 11 | 0.1200 | 14,400 | 43.59 | 22.94 | .7190 | | | | | | 9 | | 0.1144 | 13,090 | 39.63 | 25.23 | .7908 | 12 | 0.1090 | 11,880 | 35.96 | 27.81 | .8715 10 | | 0.1019 | 10,380 | 31.43 | 31.82 | .9972 | | | | | | | 13 | 0.0950 | 9,025 | 27.32 | 36.60 | 1.147 11 | | 0.09074 | 8,234 | 24.93 | 40.12 | 1.257 | 14 | 0.08300 | 6,889 | 20.85 | 47.95 | 1.503 | | | | | | 12 | | 0.08081 | 6,530 | 19.77 | 50.59 | 1.586 | 15 | 0.07200 | 5,184 | 15.69 | 63.73 | 1.997 13 | | 0.07196 | 5,178 | 15.68 | 63.79 | 1.999 | | | | | | | 16 | 0.06500 | 4,225 | 12.79 | 78.19 | 2.451 14 | | 0.06408 | 4,107 | 12.43 | 80.44 | 2.521 | 17 | 0.0580 | 3,364 | 10.18 | 98.23 | 3.078 | | | | | | 15 | | 0.05707 | 3,257 | 9.858 | 101.4 | 3.179 16 | | 0.05082 | 2,583 | 7.818 | 127.9 | 4.009 | 18 | 0.04900 | 2,401 | 7.268 | 137.6 | 4.312 | | | | | | 17 | | 0.045260 | 2,048 | 6.200 | 161.3 | 5.055 | 19 | 0.042000 | 1,764 | 5.340 | 187.3 | 5.870 18 | | 0.040300 | 1,624 | 4.917 | 203.4 | 6.374 | | | | | | 19 | | 0.035890 | 1,288 | 3.899 | 256.5 | 8.038 | 20 | 0.035000 | 1,225 | 3.708 | 269.7 | 8.452 | 21 | 0.032000 | 1,024 | 3.100 | 322.6 | 10.11 | | | | | | 20 | | 0.031960 | 1,022 | 3.092 | 323.4 | 10.14 21 | | 0.028460 | 810.1 | 2.452 | 407.8 | 12.78 | 22 | 0.028000 | 784.0 | 2.373 | 421.4 | 13.21 22 | | 0.025350 | 642.4 | 1.945 | 514.2 | 16.12 | 23 | 0.025000 | 625.0 | 1.892 | 528.6 | 16.57 23 | | 0.022570 | 509.5 | 1.542 | 648.4 | 20.32 | | | | | | | 24 | 0.022000 | 484.0 | 1.465 | 682.6 | 21.39 24 | | 0.020100 | 404.0 | 1.223 | 817.6 | 25.63 | 25 | 0.020000 | 400.0 | 1.211 | 825.9 | 25.88 | | | | | | | 26 | 0.018000 | 324.0 | .9808 | 1,020 | 31.96 25 | | 0.017900 | 320.4 | .9699 | 1,031 | 32.31 | 27 | 0.016000 | 256.0 | .7749 | 1,290 | 40.45 | | | | | | 26 | | 0.015940 | 254.1 | .7692 | 1,300 | 40.75 27 | | 0.014200 | 201.5 | .6100 | 1,639 | 51.38 | 28 | 0.014000 | 196.0 | .5933 | 1,685 | 52.83 | | | | | | | 29 | 0.013000 | 169.0 | .5116 | 1,955 | 61.27 28 | | 0.012640 | 159.8 | .4837 | 2,067 | 64.79 | 30 | 0.012000 | 144.0 | .4359 | 2,294 | 71.90 | | | | | | 29 | | 0.011260 | 126.7 | .3836 | 2,607 | 81.70 30 | | 0.010030 | 100.5 | .3042 | 3,287 | 103.0 | 31 | 0.010000 | 100.0 | .3027 | 3,304 | 103.5 | | | | | | | 32 | 0.009000 | 81.0 | .2452 | 4,078 | 127.8 31 | | 0.008928 | 79.70 | .2413 | 4,145 | 129.9 | 33 | 0.008000 | 64.0 | .1937 | 5,162 | 161.8 | | | | | | 32 | | 0.007950 | 63.21 | .1913 | 5,227 | 163.8 33 | | 0.007080 | 50.13 | .1517 | 6,591 | 206.6 | 34 | 0.007000 | 49.0 | .1483 | 6,742 | 211.3 | | | | | | 34 | | 0.006305 | 39.75 | .1203 | 8,311 | 260.5 35 | | 0.005615 | 31.52 | .09543 |10,480 | 328.4 36 | 35 | 0.005000 | 25.0 | .07568 |13,210 | 414.2 | | | | | | 37 | | 0.004453 | 19.83 | .06001 |16,660 | 522.2 | 36 | 0.004000 | 16. | .04843 |20,650 | 647.1 38 | | 0.003965 | 15.72 | .04759 |21,010 | 658.5 | | | | | | 39 | | 0.003531 | 12.47 | .03774 |26,500 | 830.4 40 | | 0.003145 | 9.888 | .02993 |33,410 |1047. --------------------------------------------------------------------

=Drop.=--In order to determine the drop or volts lost in the line, the following formula may be used

drop = ((% loss × volts) / 100) × S (1)

in which the % loss is a percentage of the applied power, that is, the power delivered to the consumer and not a percentage of the power at the alternator. "Volts" is the pressure at the consumer's end of the circuit.

=VALUE OF "S" FOR 60 CYCLES=

-----------------+------------------------+------------------------+ | .98 power factor | .90 power factor | --------+--------+------------------------+------------------------+ Size of | Area | Spacing of | Spacing of | wire | in | conductors | conductors | B.&S. |circular| | | gauge | mils. | | | | +----+----+----+----+----+----+----+----+----+----+ | | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"| --------+--------+----+----+----+----+----+----+----+----+----+----+ 500,000 |500,000 |1.21|1.45|1.61|1.77|1.92|1.32|1.80|2.11|2.44|2.75| 300,000 |300,000 |1.15|1.29|1.38|1.48|1.57|1.19|1.47|1.66|1.84|2.02| 0,000 |211,600 |1.12|1.22|1.28|1.34|1.41|1.13|1.33|1.45|1.58|1.63| 000 |167,800 |1.09|1.18|1.22|1.28|1.29|1.08|1.23|1.33|1.44|1.53| 00 |133,100 |1.07|1.14|1.18|1.21|1.25|1.03|1.16|1.24|1.32|1.40| 0 |105,500 |1.05|1.10|1.14|1.17|1.20|1.00|1.09|1.16|1.22|1.28| 1 | 83,690 |1.04|1.08|1.10|1.13|1.15|1.00|1.05|1.09|1.14|1.19| 2 | 66,370 |1.02|1.05|1.08|1.10|1.12|1.00|1.00|1.04|1.08|1.12| 3 | 52,630 |1.02|1.04|1.06|1.07|1.09|1.00|1.00|1.00|1.03|1.06| | | | | | | | | | | | | 4 | 41,740}|1.00|1.02|1.03|1.04|1.07|1.00|1.00|1.00|1.00|1.00| 5 | 33,100}| | | | | | | | | | | | | | | | | | | | | | | 6 | 26,250}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 7 | 20,820}| | | | | | | | | | | | | | | | | | | | | | | 8 | 16,510}| | | | | | | | | | | 9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 10 | 10,380}| | | | | | | | | | | --------+--------+----+----+----+----+----+----+----+----+----+----+ --------+--------+----+----+----+----+----+----+----+----+----+----+ Size | | .80 power factor | .70 power factor | of | Area +------------------------+------------------------+ wire | in | Spacing of | Spacing of | B.&S. |circular| conductors | conductors | gauge | mils. +----+----+----+----+----+----+----+----+----+----+ | | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"| --------+--------+----+----+----+----+----+----+----+----+----+----+ 500,000 | 500,000|1.27|1.89|2.25|2.64|3.03|1.14|1.72|2.12|2.53|2.92| 300,000 | 300,000|1.11|1.46|1.68|1.90|2.12|1.00|1.33|1.56|1.78|2.01| 0,000 | 211,600|1.03|1.27|1.43|1.58|1.75|1.00|1.14|1.29|1.45|1.69| 000 | 167,800|1.00|1.16|1.28|1.41|1.53|1.00|1.02|1.15|1.28|1.50| 00 | 133,100|1.00|1.07|1.15|1.22|1.00|1.00|1.00|1.03|1.13|1.21| 0 | 105,500|1.00|1.00|1.07|1.15|1.00|1.00|1.00|1.00|1.01|1.09| 1 | 83,690}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 3 | 52,630}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| | | | | | | | | | | | | 4 | 41,740}| | | | | | | | | | | 5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| | | | | | | | | | | | | 6 | 26,250}| | | | | | | | | | | 7 | 20,820}| | | | | | | | | | | | | | | | | | | | | | | 8 | 16,510}| | | | | | | | | | | 9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 10 | 10,380}| | | | | | | | | | | --------+--------+----+----+----+----+----+----+----+----+----+----+ The coefficient S has various values as given in the accompanying tables. As will be seen from the table, the value of S to be used depends upon the size of wire, spacing, power factor and frequency.

These values are accurate enough for all practical purposes, and may be used for distances of 20 miles or less and for voltages up to 25,000.

The capacity effect on very long high voltage lines, makes this method of determining the drop somewhat inaccurate beyond the limits above mentioned.

=VALUE OF "S" FOR 25 CYCLES=

---------+--------+------------------------+------------------------+ Size | | .98 power factor | .90 power factor | of | Area +------------------------+------------------------+ wire | in | Spacing of | Spacing of | B.&S. |circular| conductors | conductors | gauge | mils. +----+----+----+----+----+----+----+----+----+----+ | | 1" | 2" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"| ---------+--------+----+----+----+----+----+----+----+----+----+----+ 500,000 |500,000 |1.01|1.17|1.23|1.29|1.36|1.02|1.22|1.35|1.43|1.61| 300,000 |300,000 |1.04|1.10|1.13|1.18|1.21|1.00|1.08|1.16|1.25|1.31| 0,000 |211,600 |1.03|1.07|1.09|1.11|1.14|1.00|1.02|1.07|1.13|1.15| 000 |167,800 |1.00|1.05|1.06|1.09|1.10|1.00|1.00|1.02|1.07|1.11| 00 |133,100 |1.00|1.03|1.05|1.06|1.08|1.00|1.00|1.00|1.02|1.05| 0 |105,500 |1.00|1.01|1.02|1.03|1.04|1.00|1.00|1.00|1.00|1.00| | | | | | | | | | | | | 1 | 83,690}| | | | | | | | | | | 2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 3 | 52,630}| | | | | | | | | | | | | | | | | | | | | | | 4 | 41,740}| | | | | | | | | | | 5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 6 | 26,250}| | | | | | | | | | | | | | | | | | | | | | | 7 | 20,820}| | | | | | | | | | | 8 | 16,510}| | | | | | | | | | | 9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 10 | 10,380}| | | | | | | | | | | ---------+--------+----+----+----+----+----+----+----+----+----+----+ Size | | .80 power factor | .70 power factor | of | Area +------------------------+------------------------+ wire | in | Spacing of | Spacing of | B.&S. |circular| conductors | conductors | gauge | mils. +----+----+----+----+----+----+----+----+----+----+ | | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"| ---------+--------+----+----+----+----+----+----+----+----+----+----+ 500,000 | 500,000|1.00|1.15|1.30|1.47|1.62|1.00|1.00|1.16|1.33|1.49| 300,000 | 300,000|1.00|1.00|1.09|1.16|1.25|1.00|1.00|1.00|1.02|1.12| 0,000 | 211,600|1.00|1.00|1.00|1.03|1.10|1.00|1.00|1.00|1.00|1.00| 000 | 167,800|1.00|1.00|1.00|1.00|1.01|1.00|1.00|1.00|1.00|1.00| 00 | 133,100|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 0 | 105,500|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| | | | | | | | | | | | | 1 | 83,690}| | | | | | | | | | | 2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 3 | 52,630}| | | | | | | | | | | | | | | | | | | | | | | 4 | 41,740}| | | | | | | | | | | 5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 6 | 26,250}| | | | | | | | | | | | | | | | | | | | | | | 7 | 20,820}| | | | | | | | | | | 8 | 16,510}| | | | | | | | | | | 9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| 10 | 10,380}| | | | | | | | | | | ---------+--------+----+----+----+----+----+----+----+----+----+----+

EXAMPLE.--A circuit supplying current at 440 volts, 60 frequency, with 5% loss and .8 power factor is composed of No. 2 B. & S. gauge wires spaced one foot apart. What is the drop in the line?

According to the formula

% loss × volts drop = -------------- × S 100

Substituting the given values, and value of S as obtained from the table for frequency 60

5 × 440 drop = ------- × 1 = 22 volts 100

=Current=.--As has been stated, the effect of power factor less than unity, is to increase the current; hence, in inductive circuit calculations, the first step is to determine the current flowing in a circuit. This is done as follows:

apparent load current = ------------- (1) volts

and

watts apparent load = ------------ (2) power factor

Substituting (2) in (1)

watts ------------ power factor watts current = ------------ = --------------------- (3) volts power factor × volts

EXAMPLE.--A 50 horse power 440 volt motor has a full load efficiency of .9 and power factor of .8. How much current is required?

Since the brake horse power of the motor is given, it is necessary to obtain the electrical horse power, thus

brake horse power 50 E.H.P. = ----------------- = -- = 55.5 efficiency .9

which in watts is

55.5 × 746 = 41,403

which is the actual load, and from which

actual load 41,403 apparent load = ------------ = ------ = 51,754 power factor .8

The current therefore at 440 volts is

apparent load 51,754 ------------- = ------ = 117.6 amperes volts 440

EXAMPLE.--A 50 horse power single phase 440 volt motor, having a full load efficiency of .92 and power factor of .8, is to be operated at a distance of 1,000 feet from the alternator. The wires are to be spaced 6 inches apart and the frequency is 60, and % loss 5. Determine: =A=, electrical horse power; =B=, watts; =C=, apparent load; =D=, current; =E=, size of wires; =F=, drop; =G=, voltage at the alternator.

=A=. Electrical horse power

brake horse power 50 E. H. P. = ----------------- × --- = 54.3 efficiency .92

or,

54.3 × 746 = 40,508 watts

=TABLE OF WIRE EQUIVALENTS= (Brown and Sharpe gauge) ------+--------+--------+--------+---------+---------+--------- 0000 | 2 # 0 | 4 # 3 | 8 # 6 | 16 # 9 | 32 # 12 | 64 # 15 000 | 2 " 1 | 4 " 4 | 8 " 7 | 16 " 10 | 32 " 13 | 64 " 16 00 | 2 " 2 | 4 " 5 | 8 " 8 | 16 " 11 | 32 " 14 | 64 " 17 0 | 2 " 3 | 4 " 6 | 8 " 9 | 16 " 12 | 32 " 15 | 64 " 18 1 | 2 " 4 | 4 " 7 | 8 " 10 | 16 " 13 | 32 " 16 | 64 " 19 2 | 2 " 5 | 4 " 8 | 8 " 11 | 16 " 14 | 32 " 17 | 64 " 20 3 | 2 " 6 | 4 " 9 | 8 " 12 | 16 " 15 | 32 " 18 | 64 " 21 4 | 2 " 7 | 4 " 10 | 8 " 13 | 16 " 16 | 32 " 19 | 64 " 22 5 | 2 " 8 | 4 " 11 | 8 " 14 | 16 " 17 | 32 " 20 | 64 " 23 6 | 2 " 9 | 4 " 12 | 8 " 15 | 16 " 18 | 32 " 21 | 64 " 24 7 | 2 " 10 | 4 " 13 | 8 " 16 | 16 " 19 | 32 " 22 | 64 " 25 8 | 2 " 11 | 4 " 14 | 8 " 17 | 16 " 20 | 32 " 23 | 64 " 26 9 | 2 " 12 | 4 " 15 | 8 " 18 | 16 " 21 | 32 " 24 | 64 " 27 10 | 2 " 13 | 4 " 16 | 8 " 19 | 16 " 22 | 32 " 25 | 64 " 28 11 | 2 " 14 | 4 " 17 | 8 " 20 | 16 " 23 | 32 " 26 | 64 " 29 12 | 2 " 15 | 4 " 18 | 8 " 21 | 16 " 24 | 32 " 27 | 64 " 30 13 | 2 " 16 | 4 " 19 | 8 " 22 | 16 " 25 | 32 " 28 | 14 | 2 " 17 | 4 " 20 | 8 " 23 | 16 " 26 | 32 " 29 | 15 | 2 " 18 | 4 " 21 | 8 " 24 | 16 " 27 | 32 " 30 | 16 | 2 " 19 | 4 " 22 | 8 " 25 | 16 " 28 | | 17 | 2 " 20 | 4 " 23 | 8 " 26 | 16 " 29 | | 18 | 2 " 21 | 4 " 24 | 8 " 27 | 16 " 30 | | 19 | 2 " 22 | 4 " 25 | 8 " 28 | | | 20 | 2 " 23 | 4 " 26 | 8 " 29 | | | 21 | 2 " 24 | 4 " 27 | 8 " 30 | | | ------+--------+--------+--------+---------+---------+---------

=B.= Watts

watts = E.H.P. × 746 = 54.3 × 746 = 40,508

=C.= Apparent load

apparent load or kva = (actual load or watts ÷ power factor)

= 40,508 ÷ .8 = 50,635

=D.= Current

current = (apparent load or kva ÷ volts) = 50,635 ÷ 440 = 115 amperes

=E.= Size of wires

cir. mils = (watts × feet × M) ÷ (% loss × volts^{2}) = (40,508 × 1,000 × 3,380) ÷ (5 × 440^{2}) = 141,443

From table page 1,907, nearest size larger wire is No. 00 B.&S. gauge.

=F.= Drop

drop = ((% loss × volts) ÷ 100) × S = ((5 × 440) ÷ 100) × 1.17 = 25.74 volts

NOTE.--Values of S are given on page 1910.

=G.= Voltage at alternator

alternator pressure = (volts at motor + drop) = 440 + 25.74 = 465.7 volts.

Spotted a typo or a problem in this chapter? Tell us

LXVI. POWER STATIONS

The term power station is usually applied to any building containing an installation of machinery for the conversion of energy from one form into another form. There are three general classes of station:

1. Central stations; 2. Sub-stations; 3. Isolated plants.

These may also be classified with respect to their function as

1. Generating stations; 2. Distributing stations; 3. Converting stations.

and with respect to the form of power used in generating the electric current, generating stations may be classed as

1. Steam electric; 2. Hydro-electric; 3. Gas electric, etc.

=Central Stations.=--It must be evident that the general type of central station to be adapted to a given case, that is to say, the general character of the machinery to be installed depends upon the kind of natural energy available for conversion into electrical energy, and the character of the electrical energy required by the consumers.

This gives rise to a further classification, as

1. Alternating current stations; 2. Direct current stations; 3. Alternating and direct current stations.

The alternators or dynamos may be driven by steam or water turbines, reciprocating engines, or gas engines, according to the character of the natural energy available.

=Ques. Why is the reciprocating engine being largely replaced by the steam turbine, especially for large units?=

Ans. Because of its higher rotative speed, and absence of a multiplicity of bearings which in the case of a high speed, reciprocating engine must be maintained in close adjustment for the proper operation of the engine.

The higher speed of rotation results in a more compact unit, desirable for driving high frequency alternators.

=Ques. Is the steam turbine more economical than a high duty reciprocating engine?=

Ans. No.

=Location of Central Stations.=--As a rule, central stations should be so located that the average loss of voltage in overcoming the resistance of the lines is a minimum, and this point is located at the center of gravity of the system. In fig. 2,706 is shown a graphical method of locating this important spot.

Suppose a rough canvass of prospective consumers in a district to be supplied with electric light or power shows the principal loads to be located at A, B, C, D, E, etc., and for simplicity assume that these loads will be approximately equal, so that each may be denoted by 1 for example:

The relative locations of A, B, C, D, E, etc., should be drawn to scale (say 1 inch to the 1,000 feet) after which the problem resolves itself into finding the location of the station with respect to this scale.

The solution consists in first finding the center of gravity of any two of the loads, such as those at A and B. Since each of these is 1, they will together have the same effect on the system as the resultant load of 1 and 1, or 2, located at their center of gravity, this point being so chosen that the product of the loads by their respective distances from this point will in both cases be equal.

The loads being equal in this case the distances must be equal in order that the products be the same, so that the center of gravity of A + B is at G, which point is midway between A and B.

Considering, next, the resultant load of 2 at G and the load of 1 at C, the resultant load at the center or gravity of these will be 3, and this must be situated at a distance of two units from C and one unit from G so that the distance 2 times the load 1 at C equals the distance 1 times the load 2 at G. Having thus located the load 3 at H, the same method is followed in finding the load 4 at I. Then in like manner the resultant load 4 and the load 1 at E gives a load 5 at S.

The point S being the last to be determined represents, therefore, the position of the center of gravity of the entire system, and consequently the proper position of the plant in order to give the minimum loss of voltage on the lines.

=Ques. Is the center of gravity of the system, as obtained in fig. 2,706, the proper location for the central station?=

Ans. It is very rarely the best location.

=Ques. Why?=

Ans. Other conditions, such as the price of land, difficulty of obtaining water, facilities for delivery of coal and removal of ashes, etc., may more than offset the minimum line losses and copper cost due to locating the station at the center of gravity of the system.

=Ques. How then should the station be located?=

Ans. The more practical experience the designer has had, and the more common sense he possesses, the better is he equipped to handle the problem, as the solution is generally such that it cannot be worked out by any rule of thumb method.

=Ques. What are the general considerations with respect to the price of land?=

Ans. The cost for the station site may be so high as to necessitate building or renting room at a considerable distance from the district to be supplied.

If the price of land selected for the station be high, the running expenses will be similarly affected, inasmuch as more interest must then be paid on the capital invested.

The price or rent of real estate might also in certain instances alter the proposed interior arrangement of the station, particularly so in the case of a company with small capital operating in a city where high prices prevail. In general, however, it may be stated that whatever effect the price of real estate would have upon the arrangement, operation and location of a central station it can quite readily and accurately be determined in advance.

=Ques. With respect to the cost of the land what should be especially considered?=

Ans. Room for the future extension of the plant.

Although such additional space need not be purchased at the time of the original installation it is well, if possible, to make provision whereby it can be obtained at a reasonable figure when desired. The preliminary canvass of consumers will aid in deciding the amount of space advisable to allow for future extensions; as a rule, however, it is wise to count on the plant enlarging to not less than twice its original size, as often the dimensions have to be increased four and even six times those found sufficient at the beginning.

=Ques. What trouble is likely to be encountered with an illy located plant after it is in operation?=

Ans. It may be considered a nuisance by those residing in the vicinity, occasioning many complaints.

Thus, if the plant be placed in a residential section of the community the smoke, noise and vibration of the machines may become a nuisance to the surrounding inhabitants, and eventually end in suits for damage against the company responsible for the same. For these and the other reasons just given a company is sometimes forced to disregard entirely the location of a central station near the center of gravity of the system, and build at a considerable distance; such a proceeding would, if the distance be great, necessitate the installation of a high pressure system.

There might, however, be certain local laws in force restricting the use of high pressure currents on account of the danger resulting to life, that would prevent this solution of the problem. In such cases there could undoubtedly be found some site where the objections previously noted would be tolerated; thus, there would naturally be little objection to locating next to a stable, a brewery, or a factory of any description.

=Ques. Why is the matter of water supply important for a central station?=

Ans. Because, in a steam driven plant, water is used in the boilers for the production of steam by boiling, and if the engines be of the condensing type it is also used in them for creating a vacuum into which the exhaust steam passes so as to increase the efficiency of the engine above what it would be if the exhaust steam were obliged to discharge into the comparatively high pressure of the atmosphere.

The force of this will be apparent by considering that the water consumption of the engine ordinarily is from 15 to 25 lbs. of "feed water" per horse power per hour, and the amount of "circulating water" required to maintain the vacuum is about 25 to 30 times the feed water, and in the case of turbines with their 28 or 29 inch vacuum, much more. For instance, a 1,000 horse power plant running on 15 lbs. of feed water and 30 to 1 circulating water would require (1,000 × 15) × (30 + 1) = 465,000 lbs. or 55,822 gals. per hour at full capacity.

=Ques. Besides price what other considerations are important with respect to water?=

Ans. Its quality and the possibility of a scarcity of supply.

It is quite necessary that the water used in the boilers should be as free as possible from impurities, so as to prevent the deposition within them of any scale or sediments. The quality of the water used for condensing purposes, however, is not quite so important, although the purer it is the better.

If the plant is to be located in a city, the matter of water supply need not generally be considered, because, as a rule, it can be obtained from the waterworks; there will then, of course, be a water tax to consider and this, if large, may warrant an effort being made to obtain the water in some other way. In any event, however, the possibility of a scarcity in the supply should be reduced to a minimum.

If the plant be located in the country, some natural source of water would be utilized unless the place be supplied with waterworks, which is not generally the case. It is usual, however, to find a stream, lake or pond in the vicinity, but if none such be conveniently near, an artesian or other form of well must be sunk.

If abundance of water exist in the vicinity of the proposed installation, not only would the location of the plant be governed thereby, but the kind of power to be used for its operation would depend thereon. Thus, if the quantity of the water were sufficient throughout the entire year to supply the necessary power, water wheels might be installed and used in place of boilers and steam engines for driving the generators. The station would then, of course, be situated close to the waterfall, regardless of the center of gravity of the system.

=Ques. What should be noted with respect to the coal supply?=

Ans. The facility for transporting the coal from the supply point to the boiler room.

In this connection, an admirable location, other conditions permitting, is adjacent to a railway line or water front so that coal delivered by car or boat may be unloaded directly into the bins supplying the boilers.

If the coal be brought by train, a side or branch track will usually be found convenient, and this will usually render any carting of the fuel entirely unnecessary.

In whatever way the coal is to be supplied, the liability of a shortage due to traffic or navigation being closed at any time of the year should be well looked into, as should also the facility for the removal of ashes, before deciding upon the final location for the plant.

=Choice of System.=--The chief considerations in the design of a central station are economy and capacity. When the current has to be transmitted long distances for either lighting or power purposes, economy is attainable only by reducing the weight of the copper conductors. This can be accomplished only by the use of the high voltage currents obtainable from alternators.

Again, where the consumers are located within a radius of two miles from the central station, thereby requiring a transmission voltage of 550 volts or less, dynamos may be employed with greater economy.

Alternating current possesses serious disadvantages for certain important applications.

For instance, in operating electric railways and for lighting it is often necessary to transmit direct current at 500 volts a distance of five or ten miles. In such cases, the excessive drop cannot be economically reduced by increasing the sizes of the line wire, while a sufficient increase of the voltage would cause serious variations under changes of load. Hence, it is common practice to employ some form of auxiliary generator or booster, which when connected in series with the feeder, automatically maintains the required pressure in the most remote districts so long as the main generators continue to furnish the normal or working voltage.

The advantage of a direct current installation in such cases over a similar plant supplying alternating current line is the fact that a storage battery may be used in connection with the former for taking up the fluctuations of the current, thereby permitting the dynamo to run with a less variable load, and consequently at higher efficiency.

=Ques. Name some services requiring direct current.=

Ans. Direct current is required for certain kinds of electrolytic work, such as electro-plating, the electrical separation of metals, etc., also the charging of storage batteries for electric automobiles.

=Ques. How is direct current supplied?=

Ans. Sometimes the central station is equipped with suitable apparatus for supplying both direct and alternating current. This may be accomplished in several different ways: By installing both direct and alternating current generators in the central station; by the use of double current generators or dynamotors, from which direct current may be taken from one side and alternating current from the other side; or by installing, in the sub-station of an alternating current central station, in addition to the transformers usually placed therein, a rotary converter for changing or converting alternating current into direct current.

Thus, it is evident that the character of a central station will be governed to a great extent by the class of services to be supplied.

An exception to this is where the entire output has to be transmitted a long distance to the point of utilization.

In such cases a copper economy demands the use of high tension alternating current, and its distribution to consumers may be made directly by means of step down transformers mounted near by or within the consumers' premises, or it may be transformed into low voltage alternating current by a conveniently located sub-station.

Where the current is to be used chiefly for lighting and there are only a few or no motors to be supplied, the choice between direct current and alternating current will depend greatly upon the size of the installation, direct current being preferable for small installations and alternating current for large installations.

If the current is to be used primarily for operating machinery, such as elevators, travelling cranes, machine tools and other devices of a similar character, which have to be operated intermittently and at varying speeds and loads, direct current is the more suitable; but if the motors performing such work can be operated continuously for many hours at a time under practically constant loads, as, for instance in the general work of a pumping station, alternating current may be employed with advantage.

[A] NOTE.--The diversity factor of a customer's group of lamps, namely, the ratio of maximum demand to connected load is usually called the demand factor of the customer.

=Size of Plant.=--Before any definite calculation can be made, or plans drawn, the engineer must determine the probable load. This is usually ascertained in terms of the number and distances of lamps that will be required, by making a thorough canvass of the city or town, or that portion for which electrical energy is to be supplied. The probable load that the station is to carry when it begins operation, the nature of this load, and the probable rate of increase are matters upon which the design and construction chiefly depend.

=Ques. What is the nature of the load carried by a central station?=

Ans. It fluctuates with the time of day and also with the time of year.

=Ques. How is a fluctuating load best represented?=

Ans. Graphically, that is to say by means of a curve plotted on coordinate paper of which ordinates represent load values and the corresponding abscissæ time values, as in the accompanying curves.

=What is the nature of a power load?=

Ans. Where electricity is supplied for power purposes to a number of factories, the load is fairly steady, dropping, of course, during meal hours. In the case of traction, the average value of the load is fairly steady but there are momentarily violent fluctuations due to starting cars or trains.

=Ques. What is the peak load?=

Ans. The maximum load which has to be carried by the station at any time of day or night as shown by the highest point of the load curve.

=Ques. Define the load factor.=

Ans. The machinery of the station evidently must be large enough to carry the peak load, and therefore considerably in excess of that required for the average demand. The ratio of the average to the maximum load is called the load factor.

There are two kinds of load factor: the annual, and the daily.

The annual load factor is obtained as a percentage by multiplying the number of units sold (per year) by 100, and dividing by the product of the maximum load and the number of hours in the year. The daily load factor is obtained by taking the figures for 24 hours instead of a year.

=Ques. What must be provided in addition to the machinery required to supply the peak load?=

Ans. Additional units must be installed for use in case of repairs or break down of some of the other units.

EXAMPLE.--What would be the boiler horse power required to generate 5,000 kw. under the following conditions: Efficiency of generators 85%; efficiency of engines 90%; feed water of engines and auxiliaries 15 lbs. per I. H. P.; boiler pressure 175 lbs.; temperature of feed water 150° Fahr? With a rate of combustion of 15 lbs. of coal per sq. foot of grate per hour and an evaporation (from and at 212°) of 8 lbs. of water per lb. of coal, what area of grate would be required and how much heating surface?

5,000 kw. = 5,000 ÷ .746 = 6,702= electrical horse power

To obtain this electrical horse power with alternators whose efficiency is 85% requires

6,702 ÷ .85 = 7,885 brake horse power at the engine

This, with mechanical efficiency of 90% is equivalent to

7,885 ÷ .9 = 8,761 indicated horse power

Since 15 lbs. of feed water are required for the engines and auxiliaries per indicated horse power per hour, the total feed water or evaporation required to generate 5,000 kw. is

15 × 8,761 = 131,415 lbs. per hour.

that is to say, the boilers must be of sufficient capacity to generate 131,415 lbs. of steam per hour from water at a temperature of 150° Fahr. This must be multiplied by the factor of evaporation for steam at 175 lbs. pressure from feed water at a temperature of 150°, in order to get the equivalent evaporation "from and at 212°."

The formula for the factor of evaporation is

H - h factor of evaporation = ------- (1) 965.7

in which

H = total heat of steam at the observed pressure; h = total heat of feed water of the observed temperature; 965.7 = latent heat, of steam at atmospheric pressure.

Substituting in (1) values for the observed pressure and temperature as obtained from the steam table

1,197 - 118 factor of evaporation = ------------ = 1.117 965.7

for which the equivalent evaporation "from and at 212°" is

131,415 × 1.117 = 146,791 lbs.= per hour

=FACTORS OF EVAPORATION=

-------------+-----------------------------------------------------+ Temp of | STEAM PRESSURE BY GAUGE | feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+ Deg. Fahr. | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | -------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ 32 |1.214|1.216|1.220|1.222|1.225|1.227|1.229|1.231|1.232| 40 |1.206|1.209|1.212|1.214|1.216|1.219|1.220|1.222|1.224| 50 |1.195|1.197|1.201|1.204|1.206|1.208|1.210|1.212|1.214| 60 |1.185|1.188|1.191|1.193|1.196|1.198|1.200|1.202|1.203| 70 |1.175|1.178|1.180|1.183|1.185|1.187|1.189|1.191|1.193| 80 |1.164|1.167|1.170|1.173|1.175|1.177|1.179|1.181|1.183| 90 |1.154|1.157|1.160|1.162|1.165|1.167|1.169|1.170|1.172| 100 |1.144|1.147|1.150|1.152|1.154|1.156|1.158|1.160|1.162| 110 |1.133|1.136|1.139|1.142|1.144|1.146|1.148|1.150|1.152| 120 |1.123|1.126|1.129|1.131|1.133|1.136|1.138|1.140|1.141| 130 |1.113|1.116|1.118|1.121|1.123|1.125|1.127|1.129|1.130| 140 |1.102|1.105|1.108|1.110|1.113|1.115|1.117|1.119|1.120| 150 |1.091|1.095|1.098|1.100|1.102|1.104|1.106|1.108|1.110| 160 |1.081|1.084|1.087|1.090|1.092|1.094|1.096|1.098|1.100| 170 |1.070|1.074|1.077|1.079|1.081|1.083|1.085|1.087|1.089| 180 |1.060|1.063|1.066|1.069|1.071|1.073|1.075|1.077|1.079| 190 |1.050|1.053|1.056|1.058|1.060|1.063|1.065|1.066|1.068| 200 |1.039|1.043|1.045|1.048|1.050|1.052|1.054|1.056|1.058| 210 |1.029|1.032|1.035|1.037|1.040|1.042|1.044|1.046|1.047| -------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ Temp of | STEAM PRESSURE BY GAUGE | feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+ Deg. Fahr. | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 | -------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ 32 |1.234|1.236|1.237|1.239|1.240|1.241|1.243|1.244|1.245| 40 |1.226|1.227|1.229|1.230|1.232|1.233|1.234|1.236|1.237| 50 |1.215|1.217|1.218|1.220|1.221|1.223|1.224|1.225|1.226| 60 |1.205|1.207|1.208|1.210|1.211|1.212|1.214|1.215|1.216| 70 |1.194|1.196|1.197|1.199|1.200|1.202|1.203|1.205|1.206| 80 |1.184|1.186|1.187|1.189|1.190|1.192|1.193|1.194|1.195| 90 |1.174|1.176|1.177|1.179|1.180|1.181|1.183|1.184|1.185| 100 |1.164|1.165|1.167|1.168|1.170|1.171|1.172|1.174|1.175| 110 |1.153|1.155|1.156|1.158|1.159|1.160|1.162|1.163|1.164| 120 |1.143|1.145|1.146|1.147|1.149|1.150|1.151|1.153|1.154| 130 |1.132|1.134|1.136|1.137|1.138|1.140|1.141|1.142|1.144| 140 |1.122|1.124|1.125|1.127|1.128|1.129|1.131|1.132|1.133| 150 |1.111|1.113|1.115|1.116|1.118|1.119|1.120|1.121|1.123| 160 |1.101|1.103|1.104|1.106|1.107|1.108|1.110|1.111|1.112| 170 |1.091|1.092|1.094|1.095|1.097|1.098|1.099|1.101|1.102| 180 |1.080|1.082|1.083|1.085|1.086|1.088|1.089|1.090|1.091| 190 |1.070|1.071|1.073|1.074|1.076|1.077|1.078|1.080|1.081| 200 |1.059|1.061|1.063|1.064|1.065|1.067|1.068|1.069|1.071| 210 |1.049|1.051|1.052|1.053|1.055|1.056|1.057|1.059|1.060| -------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ Temp. of | STEAM PRESSURE BY GAUGE | feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+ Deg. Fahr. | 230 | 240 | 250 | 260 | 270 | 280 | 290 | 300 | | -------------+-----+-----+-----+-----+-----+-----+-----+----+------+ 32 |1.246|1.247|1.248|1.250|1.251|1.252|1.253|1.254| 40 |1.238|1.239|1.240|1.241|1.242|1.243|1.244|1.245| 50 |1.228|1.229|1.230|1.231|1.232|1.233|1.234|1.235| 60 |1.217|1.218|1.219|1.220|1.221|1.222|1.223|1.224| 70 |1.207|1.208|1.209|1.210|1.211|1.212|1.213|1.214| 80 |1.196|1.198|1.199|1.200|1.201|1.202|1.203|1.204| 90 |1.186|1.187|1.188|1.189|1.190|1.191|1.192|1.193| 100 |1.176|1.177|1.178|1.179|1.180|1.181|1.182|1.183| 110 |1.166|1.167|1.168|1.169|1.170|1.171|1.172|1.173| 120 |1.155|1.156|1.157|1.158|1.159|1.160|1.161|1.162| 130 |1.145|1.146|1.147|1.148|1.149|1.150|1.151|1.152| 140 |1.134|1.135|1.136|1.137|1.138|1.139|1.140|1.141| 150 |1.124|1.125|1.126|1.127|1.128|1.129|1.130|1.131| 160 |1.113|1.115|1.116|1.117|1.118|1.119|1.120|1.121| 170 |1.103|1.104|1.105|1.106|1.107|1.108|1.109|1.110| 180 |1.093|1.094|1.095|1.096|1.097|1.098|1.099|1.100| 190 |1.082|1.083|1.084|1.085|1.086|1.087|1.088|1.089| 200 |1.072|1.073|1.074|1.075|1.076|1.077|1.078|1.079| 210 |1.061|1.062|1.063|1.064|1.065|1.066|1.067|1.068| -------------+-----+-----+-----+-----+-----+-----+-----+-----+

One boiler horse power being equal to an evaporation of 34½ lbs. of water from a feed water temperature of 212° Fahr., into steam at the same temperature, the boiler capacity is accordingly

148,105 ÷ 34.5 = 4,293 boiler horse power.

The rate of evaporation is given at 8 lbs. of water (from and at 212° Fahr.), for which the fuel required is

148,105 ÷ 8 = 18,513 lbs. of coal per hour.

For a rate of combustion of 15 lbs. of coal per hour per square foot of grate,

grate area = 18,513 ÷ 15 = 1,234 sq. ft.

For stationary boilers the usual ratio of heating surface to grate area is 35:1, accordingly the heating surface corresponding to this ratio is

1,234 × 35 = 43,190 sq.ft.

The above calculation is based on a rate of evaporation of 8 lbs. of water per lb. of coal and a rate of combustion of 15 lbs. of coal per sq. ft. of grate. For other rates the required grate area may be obtained from the following table:

---------------------------------------------------------------------- GRATE SURFACE PER HORSE POWER (KENT) ------------+------+-----+-------------------------------------------- |Pounds| | | of | Lbs.| Pounds of coal burned per square foot of |water | of | grate per hour | from | coal+----+----+----+----+----+----+----+----+---- |and at| per | | | | | | | | | | 212° | h.p.| 8 | 10 | 12 | 15 | 20 | 25 | 30 | 35 | 40 | per | per | | | | | | | | | |pound | hour+----+----+----+----+----+----+----+----+---- | of | | Square feet grate per horse power | coal | | ------------+------+-----+----+----+----+----+----+----+----+----+---- Good coal | }10 | 3.45| .43|.35 | .28| .23| .17| .14| .11| .10| .09 and boiler | } 9 | 3.83| .48| .38| .32| .25| .19| .15| .13| .11| .10 | | | | | | | | | | | Fair coal |{8.61 | 4. | .50| .40| .33| .26| .20| .16| .13| .12| .10 or boiler |{8 | 4.31| .54| .43| .36| .29| .22| .17| .14| .13| .11 |{7 | 4.93| .62| .49| .41| .33| .24| .20| .17| .14| .12 | | | | | | | | | | | Poor coal |{6.9 | 5. | .63| .50| .42| .34| .25| .20| .17| .15| .13 or boiler |{6 | 5.75| .72| .58| .48| .38| .29| .23| .19| .17| .14 |{5 | 6.9 | .86| .69| .58| .46| .35| .28| .23| .22| .17 | | | | | | | | | | | Lignite and|}3.45 |10. |1.25|1.00| .83| .67| .50| .40| .33| .29| .25 poor boiler|} | | | | | | | | | | ------------+------+-----+----+----+----+----+----+----+----+----+----

=General Arrangement of Station.=--In designing an electrical station, it is preferable that whatever rooms or divisions of the interior space are desired should determine the total outside dimensions of the plant in the original plans of the building than that these latter dimensions be fixed and the rooms, etc., be fitted in afterward.

=SAVING DUE TO HEATING THE FEED WATER=

Table showing the percentage of saving for each degree of increase in temperature of feed water heated by waste steam.

---------------------------------------------------------------------- Init| Pressure of steam in boiler, lbs. per sq. inch above atmosphere temp|----------------------------------------------------------------- of | feed| 0 | 20 | 40 | 60 | 80 | 100 | 120 | 140 | 160 | 180 | 200 ----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- 32°|.0872|.0861|.0855|.0851|.0847|.0844|.0841|.0839|.0837|.0835|.0833 40 |.0878|.0867|.0861|.0856|.0853|.0850|.0847|.0845|.0843|.0841|.0839 50 |.0886|.0875|.0868|.0864|.0860|.0857|.0854|.0852|.0850|.0848|.0846 60 |.0894|.0883|.0876|.0872|.0867|.0864|.0862|.0859|.0856|.0855|.0853 70 |.0902|.0890|.0884|.0879|.0875|.0872|.0869|.0867|.0864|.0862|.0860 80 |.0910|.0898|.0891|.0887|.0883|.0879|.0877|.0874|.0872|.0870|.0868 90 |.0919|.0907|.0900|.0895|.0888|.0887|.0884|.0883|.0879|.0877|.0875 100 |.0927|.0915|.0908|.0903|.0899|.0895|.0892|.0890|.0887|.0885|.0883 110 |.0936|.0923|.0916|.0911|.0907|.0903|.0900|.0898|.0895|.0893|.0891 120 |.0945|.0932|.0925|.0919|.0915|.0911|.0908|.0906|.0903|.0901|.0899 130 |.0954|.0941|.0934|.0928|.0924|.0920|.0917|.0914|.0912|.0909|.0907 140 |.0963|.0950|.0943|.0937|.0932|.0929|.0925|.0923|.0920|.0918|.0916 150 |.0973|.0959|.0951|.0946|.0941|.0937|.0934|.0931|.0929|.0926|.0924 160 |.0982|.0968|.0961|.0955|.0950|.0946|.0943|.0940|.0937|.0935|.0933 170 |.0992|.0978|.0970|.0964|.0959|.0955|.0952|.0949|.0946|.0944|.0941 180 |.1002|.0988|.0981|.0973|.0969|.0965|.0961|.0958|.0955|.0953|.0951 190 |.1012|.0998|.0989|.0983|.0978|.0974|.0971|.0968|.0964|.0062|.0960 200 |.1022|.1008|.0999|.0993|.0988|.0984|.0980|.0977|.0974|.0972|.0969 210 |.1033|.1018|.1010|.1003|.0998|.0994|.0990|.0987|.0984|.0981|.0979 220 | -- |.1029|.1019|.1013|.1008|.1004|.1000|.0997|.0994|.0991|.0989 230 | -- |.1039|.1031|.1024|.1018|.1012|.1010|.1007|.1003|.1001|.0999 240 | -- |.1050|.1041|.1034|.1029|.1024|.1020|.1017|.1014|.1011|.1009 250 | -- |.1062|.1052|.1045|.1040|.1035|.1031|.1027|.1025|.1022|.1019 ----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----

NOTE.--An approximate rule for the conditions of ordinary practice is a saving of 1 per cent. made by each increase of 11° in the temperature of the feed water. This corresponds to .0909 per cent. per degree. The calculation of saving is made as follows: Boiler pressure, 100 lbs. gauge; total heat in steam above 32° = 1,185 B.T.U. feed water, original temperature 60°, final temperature 209°F. Increase in heat units, 150. Heat units above 32° in feed water of original temperature = 28. Heat units in steam above that in cold feed water, 1,185-28 = 1,157. Saving by the feed water heater = 150 ÷ 1,157 = 12.96 per cent. The same result is obtained by the use of the table. Increase in temperature 150° × tabular figure .0864 = 12.96 per cent. Let total heat of 1 lb. of steam at the boiler pressure = H; total heat of 1 lb. of feed water before entering the heater = h', and after passing through the heater = h''; then the saving made by the heater is (h''-h') ÷ (H-h').

Under usual conditions the plans of an electrical station are readily drawn, as they are generally of a simple nature. The engines and generators will occupy the majority of the space, and these are usually placed in one large room; in some stations, however, they are located respectively in two adjacent rooms. The boilers are generally located in a room apart from the engines and dynamos, and in some cases a separate building is provided for them; the pumps, etc., must be installed not far from the boilers, and space must also be allowed near the boilers for coal and ashes.

Fig. 2,720 shows the floor plan of an electrical station, in which a countershaft and belted connections are used between the engines and generators. Referring first to the plan of the building itself, A represents the engine and dynamo room, B denotes the boiler room, C the office, D the store room, and E the chimney connected with the boilers by means of the uptake w. Referring next to the apparatus installed, S, S, S, S represents a battery of four boilers; these are connected by steam piping VV to the two steam engines, M and M, which are belted to the countershaft O. Belted to the countershaft are the generators, T, T, T, T, the circuits from which are controlled on the switchboard, H.

=Ques. What are the objections to the arrangement shown in fig. 2,720.=?

Ans. The large space required by the belt drive especially in locations where land is expensive. Another objection is the frictional loss due to the belt drive with its countershaft, etc.

=Ques. What are the desirable features of the belt drive?=

Ans. High speed generators may be used, thus reducing the first cost, and the multiplicity of speeds and flexibility of the system resulting from the use of a friction clutch.

Thus in fig. 2,720, each pulley may be mounted on the counter shaft O with a friction clutch. A jaw clutch may also be provided at Z, thus permitting the shaft O to be divided into two sections. It is therefore possible by this arrangement to cause either of the engines to drive any one of the generators, or all of them, or both of the engines to drive all of the generators simultaneously.

=Ques. Under what condition is the counter shaft belt drive particularly valuable?=

Ans. In case of a break down of any one of the engines or generators, and also when it becomes necessary to clean them without interrupting the service.

=Ques. How may the design in fig. 2,720 be modified for the installation of a storage battery?=

Ans. If a storage battery be necessary, a partition may be constructed across the room A, as indicated by the dotted lines, and the battery installed in the room thus formed.

=Ques. Mention a few details in the general arrangement of the building fig. 2,720.=

Ans. Two doors to the room A may conveniently be provided at K and L, the former connecting with the boiler room B, and the latter serving as the main entrance to the station. There is little that need be added to what has already been stated regarding the boiler room B. The door at F provides for the entrance of coal and the removal of ashes, while at P, the pump and heaters may conveniently be located. In the office C, visitors may be received, the station reports made out, bulletins issued from time to time, and whatever engineering problems arise may here be solved on paper by the engineer in charge of the plant. The store room D will be found convenient for various supplies, tools and appliances needed in the operation of the station. These may here be kept under lock and key and the daily waste and loss resulting from carelessness avoided.

=Ques. What important point should be noted in locating the engines and boilers?=

Ans. They should be so placed that the piping between them will be as short and direct as possible.

=Ques. Why?=

Ans. The steam pipe should be short to reduce the loss of heat between engine and boiler to a minimum, and both short and direct to avoid undue friction and consequent drop in pressure of the steam in passing through the pipe to the engine.

Entirely too little attention is given to this matter on the part of designers and it cannot be too strongly emphasized that, for economy, the steam pipe between an engine and boiler should be as short and direct as possible, having regard of course, for proper piping methods.

=Ques. What should be provided for the steam pipe?=

Ans. A heavy covering of approved material should be placed around the pipe to reduce the loss of heat by radiation. For this purpose hair felt, mineral wool and asbestos are used.

=Ques. How should the piping be arranged between the engine and condenser, and why?=

Ans. It should be as short and direct as possible; especially should elbows be avoided so that the back pressure on the engine piston will be reduced as near as can be to that of the condenser.

That is to say, in order to get nearly the full effect of the vacuum in the condenser the frictional resistance of the piping should be reduced to a minimum.

Where 90° turns are necessary, easy bends should be used instead of sharp elbows. The force of this argument must be apparent by noting the practice of steam turbine builders of placing the turbine right up against the condenser, and remembering that a high vacuum is necessary to the economical working of a turbine. See fig. 1,445, page 1,182.

=Ques. What are the considerations respecting the number and type of engine to be used?=

Ans. In the illustration fig. 2,720, two engines M and M' are employed, one belted to each end of the countershaft O. These engines should be of similar or identical pattern; for a small output they may be either simple or compound, as the conditions of fuel expenditure may dictate, but if the output be large, triple expansion engines or turbines are advisable.

Corliss or similar slow speed engines may advantageously be used in either case. In all cases the engine should be run condensing unless the cost for circulating water is prohibitive; even in such cases cooling towers may be installed and effect a saving.

In operation, during the greater part of the day, one engine running two or perhaps three of the generators, will carry the load, but when the load is particularly heavy, as in the morning and evening, both engines and all the generators may be required to meet the demands.

By exercising a little ingenuity in shifting the load on different machines at different times, both engines and dynamos, may readily be cleaned and repaired without interrupting the service.

=Ques. For economy what kind of steam should be used?=

Ans. Super-heated steam.

The saving due to the use of superheated steam is about 1% for every ten degrees Fahr. of super-heat. It should be used in all cases.

=Ques. How should the machines be located?=

Ans. Sufficient space should be allowed between them that cleaning and repairing may be done easily, quickly and effectually.

=Ques. How should the switchboard be located?=

Ans. In fig. 2,720, the switchboard H is mounted against the wall dividing the room A from the room B, and is in line with the machines.

The advantages arising from a switchboard thus installed are, that the switchboard attendant working thereon can obtain at any time an unobstructed view of the performance of each individual machine, and he has in consequence a much better control of them; then, too, while he is engaged at the engines or generators he can also see the measuring instruments on the switchboard, and ascertain approximately the readings upon them.

In cases of emergency it is sometimes necessary for the engineer in charge of a plant to be in several places at the same time in order to prevent an accident, and that this seemingly impossibility may be approximated as nearly as possible, it is essential that the controlling devices be located as closely together as is consistent, and that no moving belt or pulley intervene between them.

These conditions are well satisfied in fig. 2,720, and owing to the short distances between the generators and the switchboard the drop of voltage in each of the conducting wires between them will be low.

This latter advantage is worthy of notice in a station generating large currents at a low pressure. To offset the advantages just mentioned, the location of the switchboard in line with the machines introduces an element of danger to the switchboard, its apparatus, and the attendant, on account of the possible bursting of a flywheel or other parts of the machines from centrifugal force.

If the switchboard be placed in the dotted position at H', or, in fact, at the opposite end of the room A, the damage to life and property that might result from the effects of centrifugal force would be eliminated, but in place thereof would be the disadvantages of an obstructed view of the machines from the switchboard, an obstructed view of the switchboard from the machines, inaccessibility between these two, and a greater drop of voltage in the majority of the conducting wires between the generators and the switchboard.

=Ques. Describe a second arrangement of station with belt drive and compare it with the design shown in fig. 2,720.=

Ans. A floor plan somewhat different from that presented in fig. 2,720 is shown in fig. 2,731. Here a belt drive is employed, but no countershaft is used. Each generator, therefore, is dependent upon its respective engine, and in consequence the flexibility obtained by the use of a countershaft is lost. On the other hand, there is less loss of mechanical power between the engines and generators in the driving of the latter, and less floor space is necessary in the room A. If, however, the floor area of this room be made the same as in the previous arrangement and the same number of machines are to be installed, they may be spaced further apart, affording in consequence considerably more room for cleaning and repairing them.

In operation, the normal conditions should be such that any two of the engine and generator sets may readily carry the average load, the third set to be used only as a reserve either to aid the other two when the load is unusually heavy or to replace one of the other sets when it becomes necessary to clean or repair the latter.

The switchboard may perhaps be best located at H, as a similar position on the opposite side of the room A would bring it beneath one or more of the steam pipes and thus endanger it should a possible leakage occur from these pipes. If located at H, however, it will be in line with the machines, and therefore will be subject to the disadvantages previously mentioned for such cases; consequently it might be as well to place it at the further end of the room, either against the partition (shown dotted) of the storage battery room if this be built, or else (if no storage battery is to be installed), against the end wall itself. The nearer end of the room A would not be very desirable for the switchboard installation on account of being so far removed from the machines, and therefore more or less inaccessible from them. Outside of what has now been mentioned, the division of the floor plan and the arrangement therein is practically the same as in fig. 2,720, accordingly what has already been stated regarding the former installation applies, therefore, with equal force to the present installation.

=Ques. Describe a plant with direct drive.=

Ans. This type of drive is shown in fig. 2,732. Each engine is directly connected to a generator, that is, the main shafts of both are joined together in line so that the generator is driven without the aid of a belt.

=Ques. What is the advantage of direct drive?=

Ans. The great saving in floor space, which is plainly shown in fig. 2,732, the portion A' representing the saving which results over the installations previously illustrated in figs. 2,720 and 2,731.

=Ques. How could the floor space be further reduced?=

Ans. By employing vertical instead of horizontal engines.

=Ques. What should be done before drawing the plans for the station?=

Ans. The types of the various machines and apparatus to be installed should, as nearly as possible, be selected in advance so that their approximate dimensions may serve as a guide in drawing up the plans of the building.

Owing to the great difference in these dimensions for the various types, and in fact for the same types as manufactured by different concerns, no definite rules regarding the necessary space required can here be given. In a general way, however, the author has endeavoured to indicate by the drawings the relative amounts of space that ordinarily would be considered sufficient.

=Ques. What is the disadvantage of direct drive?=

Ans. A more expensive generator is required because it must run at the same speed as the engine, which is relatively low as compared with that of a belted generator.

=Station Construction.=--The construction or rearrangement of the building intended for the plant is a problem that under ordinary conditions would be solved by an architect, or at least by an architect with the assistance of an electrical or mechanical engineer, still there are many installations where the electrical engineer has been compelled to design the building.

In such instances he should be equipped with a general knowledge of the construction of buildings.

=Foundations.=--The foundation may be either natural or artificial; that is, it may be composed of rock or soil sufficiently solid to serve the purpose unaided, or it may be such as to require strengthening by means of wood or iron beams, etc. In either case any tendency toward a considerable settling or shifting of the foundation due to the action of water, frost, etc., after the station has been completed must be well guarded against. To this end special attention should be given to the matter of drainage.

=Ques. How should the foundation be constructed for the machines?=

Ans. The foundations constructed for the machines should be entirely separate from that built for the walls of the building, so that the vibrations of the former will not affect the latter.

If there be several engines and dynamos to be installed, it is best to construct two foundations, one for the engines and one for the dynamos. If, however, there be considerable distance between the units, it may be advisable to build a separate foundation for each engine and for each dynamo. The material of which these foundations are composed should if the machines be of 20 horse power or over, possess considerable strength and be impervious to moisture. Brick, stone and concrete are desirable for the purpose, and only the best quality of cement mortar should be employed. Care must be taken that lime mortar is not used in place of cement mortar, as the former is not well adapted to withstand the vibrations of the machines without crumbling.

=Ques. Describe a method of constructing foundations.=

Ans. An excavation is made to the desired depth and a form inserted corresponding to the desired dimensions for the foundation. A template is placed on top locating all the centers, with iron pipes suspended from these centers, two or three sizes larger than the anchor bolts. At the lower end of the pipes are core boxes. Concrete is poured into the mould thus formed, and when hard, the forms are removed thus leaving the solid foundation. The anchor bolts are inserted through the pipes and passed through iron plates at the lower end as shown in fig. 2,734, being secured by nuts. By using pipe of two or three bolt diameters a margin is provided for adjustment so the bolts will pass through the holes in the frame of the machine thus allowing for any slight errors in laying out the centers on the template.

=Ques. What is the object of the openings in the bottom of the foundation?=

Ans. In case of a defective bolt, it may be replaced by a new one without injury to the foundation.

=Walls.=--Regarding the material for the walls of the station iron, stone, brick and wood may be considered. Of these, iron in the form of sheets or plates would be entirely fireproof, but being itself a conductor would introduce difficulties in maintaining a high insulation resistance of the current carrying circuits; it would also make the building difficult to heat in winter and to keep cool in summer. Stone in the form of limestone, granite or sandstone, as a building material is desirable for solidity and attractiveness; it is also fireproof and an insulator, but the high cost of such a structure for an electrical station usually prohibits its use except in private plants or in electrical stations located in large cities.

Brick is a good material and is readily obtained in nearly all parts of the country; it is comparatively cheap, and is also an insulating and fireproof material. The bricks selected for this purpose should possess true sharp edges, and be hard burned.

=Ques. What are the features of wood?=

Ans. Wood forms the cheapest material that can be used for the walls of electrical stations, and it usually affords satisfaction, but has the disadvantage of high fire risk.

=Roofs=.--In fig. 2,736 is shown one form of construction for the roof of an electrical station. The end view here presented shows the upper portion of the walls at B and D; these support the iron trusses C, and the roof proper MN. In many stations there is provided throughout the length of the building, a monitor or raised structure on the peak of the roof for ventilation and light. The end view of the monitor is shown at S in the figure; its sides should be fitted with windows adjustable from the floor.

=Floors.=--The floor of the station should be so designed that it will be capable of supporting a reasonable weight, but as the weights of the machines are borne entirely by their respective foundations the normal weight upon the floor will not be great; for short periods, however, it may be called upon to support one or two machines while they are being placed in position or interchanged, and due allowance must be made for such occurrences.

Station floors for engine and dynamo rooms are, as a rule, constructed of wood. Where very high currents are generated, however, insulated floors of special construction mounted on glass are necessary as a protection from injurious shocks. Brick, concrete, cement, and other substances of a similar nature are objectionable as a floor material for engine and dynamo rooms on account of the grit from them, caused by constant wear, being liable to get into the bearings of the machines.

Where there are no moving parts, however, as in the boiler room, the materials just mentioned possess no disadvantages and are preferable to wood on account of being fireproof.

=THEORETICAL DRAFT PRESSURE IN INCHES OF WATER IN= =A CHIMNEY 100 FEET HIGH=

(For other heights the draft varies directly as the height)

Temp. in TEMP. OF EXTERNAL AIR. (BAROMETER 30 INCHES) Chimney, °F. 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100° 200° .453 .419 .384 .353 .321 .292 .263 .234 .209 .182 .157 220 .488 .453 .419 .388 .355 .326 .298 .269 .244 .217 .192 240 .520 .488 .451 .421 .388 .359 .330 .301 .276 .250 .225 260 .555 .528 .484 .453 .420 .392 .363 .334 .309 .282 .257 280 .584 .549 .515 .482 .451 .422 .394 .365 .340 .313 .288 300 .611 .576 .541 .511 .478 .449 .420 .392 .367 .340 .315 320 .637 .603 .568 .538 .505 .476 .447 .419 .394 .367 .342 340 .662 .638 .593 .563 .530 .501 .472 .443 .419 .392 .367 360 .687 .653 .618 .588 .555 .526 .497 .468 .444 .417 .392 380 .710 .676 .641 .611 .578 .549 .520 .492 .467 .440 .415 400 .732 .697 .662 .632 .598 .570 .541 .513 .488 .461 .436 420 .753 .718 .684 .653 .620 .591 .563 .534 .509 .482 .457 440 .774 .739 .705 .674 .641 .612 .584 .555 .530 .503 .478 460 .793 .758 .724 .694 .660 .632 .603 .574 .549 .522 .497 480 .810 .776 .741 .710 .678 .649 .620 .591 .566 .540 .515 500 .829 .791 .760 .730 .697 .669 .639 .610 .586 .559 .534

=Chimneys.=--These are generally constructed of brick and iron, sometimes of concrete. Iron chimneys cost less than brick chimneys, necessitate less substantial foundations, and are free from the liability of cracking. They must be painted to prevent corrosion, are less substantial, and lose considerably more heat by radiation than do brick chimneys.

Both brick and iron chimneys, require an inner wall or lining of brick, which forms the flue proper, and in order that this wall be not cracked by sudden cooling an air space is left between it and the outer wall. In a brick chimney the inner wall need not extend much beyond half the height of the chimney, but when iron is used it should reach to the top.

=Ques. Upon what does the force of natural draught in a chimney depend?=

Ans. It depends upon the difference between the weight of the column of hot gases inside the chimney and the weight of a like column of the cold external air.

=Ques. How is the intensity of the draught expressed?=

Ans. In terms of the number of inches of a water column sustained by the pressure produced.

=Ques. Are high chimneys necessary?=

Ans. No.

Chimneys above 150 feet in height are very costly, and their increased cost is not justified by increased efficiency.

[Illustrations: FIGS. 2,741 to 2,744.--Installation of forced draft system to old boiler plant. The figures illustrate the simplest method. The fan which is of steel plate with direct connected double cylinder engine, is placed immediately over the end of a brick duct into which the air is discharged. This duct is carried under ground across the front of the boilers, to the ash pits of each of which connection is made through branch ducts. Each branch duct opening is provided with special ash pit damper, operated by notched handle bar, as illustrated in the detail. This method of introduction serves to distribute the air within the ash pit, and to secure even flow through the fuel upon the grate above. Of course, the ash pit doors must remain closed in order to bring about this result. A chimney of sufficient height to merely discharge the gases above objectionable level is all that is absolutely necessary with this arrangement. Although the introduction of a fan in an old plant is usually evidence of the insufficiency of the existing chimney to meet the requirements, such a chimney, will, however, usually serve as a discharge pipe for the gases when the fan is employed. The fan thus becomes more than a mere auxiliary to the chimney; it practically supplants it so far as the method of draught production is concerned.]

The latest chimney practice is to build two or more small chimneys instead of one large one. A notable example is the Spreckels Sugar Refinery in Philadelphia, where three separate chimneys are used for one boiler plant of 7,500 horse power. The three chimneys are said to have cost several thousand dollars less than an equivalent single chimney.

=Very tall chimneys= have been characterized by one writer as "monuments to the folly of their builders."

=Ques. How is mechanical draft secured?=

Ans. In two ways, known respectively as induced draught and forced draught.

=Ques. Describe the method of induced draft.=

Ans. A fan is located in the smoke flue, and which in operation draws the gases through the furnace and discharges them into a short chimney.

=Ques. Describe the method of forced draft.=

Ans. In this method, air is forced into the furnace underneath the grate bars by means of a fan or a steam jet blower.

=Ques. What is the application of the two systems?=

Ans. Induced draft is installed mostly in new plants, while forced draft is better adapted to old plants.

=Steam Turbines=.--It is not the author's intention to discuss at length the steam end of the electric plant, because too much space would be required, and also because the subject belongs properly to the field of mechanical engineering rather than electrical engineering. However, because of the recent introduction of the steam turbine for the direct driving of large generators, and the fact that it is now almost universally used in large central stations, a detailed explanation of its principles and construction may not be out of place.

A turbine is a machine in which a rotary motion is obtained by transference of the momentum of a fluid or gas. In general the fluid is guided by fixed blades, attached to a casing, and, impinging on other blades mounted on a drum or shaft, causing the latter to revolve.

Turbines are classed in various ways as: 1, radial flow, when the steam enters near the center and escapes toward the circumference; and 2, parallel flow, when the steam travels axially or parallel to the length of the turning body.

Turbines are commonly, yet erroneously classed as:

1. Impulse; 2. Reaction.

=Ques. What is the distinction between these two types?=

Ans. In the so called impulse type, steam enters and leaves the passages between the vanes at the same pressure. In the so called reaction type, the pressure is less on the exit side of the vanes than on the entrance side.

Fig. 2,750 is a sectional view of the Parsons-Westinghouse parallel flow turbine. Steam from the boiler enters first a receiver in which are the governor controlled admission valves. These valves are actuated by a centrifugal governor.

Steam does not enter the turbine in a continuous blast, but intermittently, or in puffs. The speed regulation is therefore accomplished by proportioning the duration of these puffs to the load of the engine, this being effected by the governor, fig. 2,752.

The governor of the turbine has only to move a small pilot valve, or slide, E, which admits steam under the piston F, and lifts the throttle valve proper off its seat.

As soon as the pilot valve closes, the spring shifts the main throttle valve. Thus, at light loads, the main throttle or admission valve is continually opening and shutting at uniform intervals, the length of time during which it remains open depending upon the load.

As the load increases, the duration of the valve opening also increases, until at full load the valve does not reach its seat at all and the steam flows steadily through the turbine. The steam thus admitted flows into the annular passage A, fig. 2,750, by the opening S, and then past the blades, revolving the rotor.

When the load increases above the normal rated amount a secondary pilot valve is moved by the same means, this in turn admitting steam to a piston, similar to F, which lifts another throttle valve. This admits steam into the annular space I, so that it acts upon the larger diameter of the drum or rotor, giving largely increased power for the time being.

The levers or arms of the governor are mounted upon knife edges instead of pins, making it extremely sensitive. The tension spring may be adjusted by hand while the turbine is running.

The governor does not actually move the pilot valve, but shifts the point L in fig. 2,752. A reciprocating motion is given to the rod I by a small eccentric on the governor shaft; this is driven by worm gearing shown near O in fig. 2,750, so that the eccentric makes one revolution to about eight of the turbine. Thus, with a turbine running 1,200 revolutions, the rod I would be moved up and down 150 times per minute. As the points A and H are fixed, the motion is conveyed to the small pilot valve E, thus giving 150 puffs a minute. The governor in shifting the point L brings the edge of the pilot valve nearer the port and so cuts off the steam earlier.

The annular diameter or space between the rotor and the stator is gradually increased from inlet to exhaust, the blades being made longer in each ring. When the mechanical limit is reached, the diameter of the rotor is increased as at I and D so as to keep the length of blade within bound.

Balance pistons as at B, C, F are attached to the rotor, their office being to oppose end thrust upon those blades in corresponding diameter of the rotor. Communication is established through the passage V and pipe M between the eduction pipe and the back of these pistons, thus increasing the efficiency of their balancing and also taking care of any leakage past them.

A small thrust bearing T prevents end play of the rotor, and is adjustable to maintain the proper clearance between the rings of blades; this varies from ⅛ inch at the admission to 1 inch at the exhaust. This bearing also takes up any extra unbalanced thrust. A turbine should operate with a high vacuum, because without this it does not compare favorably with an ordinary reciprocating engine from the point of economy.

Separate air pumps are provided to create the vacuum.

Where the ordinary type of vertical air pump is employed, a booster or vacuum increaser is added, as nothing below 26 inches is advisable, 28 and 29 inches being always striven for. It is also preferable to use a certain amount of super-heat with steam turbines.

To assist in producing the high vacuum, exhaust passages are made large, the eduction passage E in fig. 2,750 being nearly twenty-three times the area of the steam pipe.

Among other details, a noteworthy feature is a small oil pump K, which circulates oil through bearings of the machinery, the oil being drawn from the tank under the governor shaft and gravitating there after use. No pressure of oil is employed. Stuffing rings prevent leakage; these consist of alternate grooves and collars in shaft and bearing, like the grooves in an indicator piston.

=Ques. Why is a high vacuum desirable?=

Ans. Because the turbine is capable of expanding the steam to a very low terminal pressure, and this is necessary for economy.

=Ques. What may be said of the working pressures for turbines?=

Ans. To meet the varied conditions of service, turbines are designed to operate with: 1, high pressure, 2, low pressure, or 3, mixed pressure.

High pressure turbines operate at about the same initial pressure as triple expansion engines.

Low pressure, as here applied, means the exhaust pressure of the reciprocating engine from which the exhaust steam passes through the turbine before entering the condenser.

Mixed pressure implies that the exhaust steam is supplemented, for heavy loads, by the admission of live steam.

=Ques. What determines the working pressure?=

Ans. When all the power is furnished by the turbine, it is designed for high pressure; when operated in combination with a reciprocating engine, low pressure is used for constant load, and mixed pressure for variable load.

NOTE.--There are logical engineering reasons for the existence of the several types of turbine, viz., single flow, double flow, and semi-double flow. The double flow turbine is not inherently superior to the single flow design, but is used under conditions for which the single flow machine is unsuitable. Similarly, the semi-double flow is recommended only for conditions which it can meet more satisfactorily than either of the other types.

NOTE.--Low pressure turbines use exhaust steam from non-condensing engines and are valuable as an adjunct to existing plants for the purpose of increasing economy and capacity with a minimum outlay for new equipment.

NOTE.--Bleeder turbines are for use in plants which are required to furnish, not only power, but also considerable and varying quantities of low pressure steam for heating purposes. In these turbines a part of the steam after it has done work in the high pressure stages may be diverted to the heating system, and the remainder expanded through the low pressure blading and exhausted into the condenser. In this way none of the energy of the heating steam, due to the difference of pressure between the boiler and the heating system is wasted. On the other hand if no steam is required for heating purposes, the turbine operates just as efficiently as though the bleeder feature were absent.

The De Laval steam turbine is termed by its builders a high speed rotary steam engine. It has but a single wheel, fitted with vanes or buckets of such curvature as has been found to be best adapted for receiving the impulse of the steam jet. There are no stationary or guide blades, the angular position of the nozzles giving direction to the jet. The nozzles are placed at an angle of 20 degrees to the plane of motion of the buckets. The best energy in the steam is practically devoted to the production of velocity in the expanding or divergent nozzle, and the velocity thus attained by the issuing jet of steam is about 4,000 feet per second. To attain the maximum efficiency, the buckets attached to the periphery of the wheel against which this jet impinges should have a speed of about 1,900 feet per second, but, owing to the difficulty of producing a material for the wheel strong enough to withstand the strains induced by such a high speed, it has been found necessary to limit the peripheral speed to 1,200 or 1,300 feet per second.

It is well known that in a correctly designed nozzle the adiabatic expansion of the steam from maximum to minimum pressure will convert the entire static energy of the steam into kinetic energy. Theoretically this is what occurs in the De Laval nozzle. The expanding steam acquires great velocity, and the energy of the jet of steam issuing from the nozzle is equal to the amount of energy that would be developed if an equal volume of steam were allowed to adiabatically expand behind the piston of a reciprocating engine, a condition, however, which for obvious reasons has never yet been attained in practice with the reciprocating engine. But with the divergent nozzle the conditions are different.

The Curtis turbine is built by the General Electric Company at their works in Schenectady, N. Y., and Lynn, Mass. They are of the horizontal and vertical types. In the vertical type the revolving parts are set upon a vertical shaft, the diameter of the shaft corresponding to the size of the machine.

The shaft is supported by and runs upon a step bearing at the bottom. This step bearing consists of two cylindrical cast iron plates bearing upon each other and having a central recess between them into which lubricating oil is forced under pressure by a steam or electrically driven pump, the oil passing up from beneath.

A weighted accumulator is sometimes installed in connection with the oil pipe as a convenient device for governing the step bearing pumps, and also as a safety device in case the pumps should fail, but it is seldom required for the latter purpose, as the step bearing pumps have proven after a long service in a number of cases, to be reliable. The vertical shaft is also held in place and kept steady by three sleeve bearings one just above the step, one between the turbine and generator, and the other near the top.

These guide bearings are lubricated by a standard gravity feed system. It is apparent that the amount of friction in the machine is very small, and as there is no end thrust caused by the action of the steam, the relation between the revolving and stationary blades may be maintained accurately. As a consequence, therefore, the clearances are reduced to the minimum.

The Curtis turbine is divided into two or more stages, and each stage has one, two or more sets of revolving blades bolted upon the peripheries of wheels keyed to the shaft. There are also the corresponding sets of stationary blades bolted to the inner walls of the cylinder or casing.

The governing of speed is accomplished in the first set of nozzles and the control of the admission valves here is effected by means of a centrifugal governor attached to the top end of the shaft. This governor, by a very slight movement, imparts motion to levers, which in turn work the valve mechanism.

The admission of steam to the nozzles is controlled by piston valves which are actuated by steam from small pilot valves which are in turn under the control of the governor.

Speed regulation is effected by varying the number of nozzles in flow, that is, for light loads fewer nozzles are open and a smaller volume of steam is admitted to the turbine wheel, but the steam that is admitted impinges against the moving blades with the same velocity always, no matter whether the volume be large or small. With a full load and all the nozzle sections in flow, the steam passes to the wheel in a broad belt and steady flow.

WEIR TABLE giving cubic feet of water per minute that will flow over a weir one inch wide and from ⅛ to 20⅞ inches deep.

--------+------+------+------+------+------+------+------+------ Depth | | | | | | | | inches | | ⅛ | ¼ | ⅜ | ½ | ⅝ | ¾ | ⅞ --------+------+------+------+------+------+------+------+------ =0= | .00 | .01 | .05 | .09 | .14 | .19 | .26 | .32 =1= | .40 | .47 | .55 | .64 | .73 | .82 | .92 | 1.02 =2= | 1.13 | 1.23 | 1.35 | 1.36 | 1.58 | 1.70 | 1.82 | 1.95 =3= | 2.07 | 2.21 | 2.34 | 2.48 | 2.61 | 2.76 | 2.90 | 3.05 =4= | 3.20 | 3.35 | 3.50 | 3.66 | 3.81 | 3.97 | 4.14 | 4.30 =5= | 4.47 | 4.64 | 4.81 | 4.98 | 5.15 | 5.33 | 5.51 | 5.69 =6= | 5.87 | 6.06 | 6.25 | 6.44 | 6.62 | 6.82 | 7.01 | 7.21 =7= | 7.40 | 7.60 | 7.80 | 8.01 | 8.21 | 8.42 | 8.63 | 8.83 =8= | 9.05 | 9.26 | 9.47 | 9.69 | 9.91 |10.13 |10.35 |10.57 =9= |10.80 |11.02 |11.25 |11.48 |11.71 |11.94 |12.17 |12.41 =10= |12.64 |12.88 |13.12 |13.36 |13.60 |13.85 |14.09 |14.34 =11= |14.59 |14.84 |15.09 |15.34 |15.59 |15.85 |16.11 |16.36 =12= |16.62 |16.88 |17.15 |17.41 |17.67 |17.94 |18.21 |18.47 =13= |18.74 |19.01 |19.29 |19.56 |19.84 |20.11 |20.39 |20.67 =14= |20.95 |21.23 |21.51 |21.80 |22.08 |22.37 |22.65 |22.94 =15= |23.23 |23.52 |23.82 |24.11 |24.40 |24.70 |25.00 |25.30 =16= |25.60 |25.90 |26.20 |26.50 |26.80 |27.11 |27.42 |27.72 =17= |28.03 |28.34 |28.65 |28.97 |29.28 |29.59 |29.91 |30.22 =18= |30.54 |30.86 |31.18 |31.50 |31.82 |32.15 |32.47 |32.80 =19= |33.12 |33.45 |33.78 |34.11 |34.44 |34.77 |35.10 |35.44 =20= |35.77 |36.11 |36.45 |36.78 |37.12 |37.46 |37.80 |38.15 --------+------+------+------+------+------+------+------+------

NOTE.--The weir table on this page contains figures 1, 2, 3, etc., in the first vertical column which indicates the inches depth of water running over weir board notches. Frequently the depths measured represent also fractional inches, between 1 and 2, 2 and 3, etc. The horizontal line of fraction at the top represents these fractional parts, and can be applied between any of the numbers of inches depth, from 1 to 25. The body of the table shows the cubic feet, and the fractional parts of a cubic foot, which will pass each minute for each inch in depth, and for each fractional part of an inch by eighths for all depths from 1 to 25 inches. Each of these results is for only one inch width of weir. To estimate for any width of weir the result obtained for one inch width must be multiplied by the number of inches constituting the whole horizontal length of weir.

=Hydro-Electric Plants.=--The economy with which electricity can be transmitted long distances by high tension alternating currents, has led to the development of a large number of water powers in more or less remote regions.

This economy is possible by the facility with which alternating current can be transformed up and down. Thus at the hydro-electro plant, the current generated by the water wheel driven alternator is transformed to very high pressure and transmitted with economy a long distance to the distributing point where it is transformed down to the proper pressure for distribution.

A water wheel or turbine is a machine in which a rotary motion is obtained by transference of the momentum of water; broadly speaking, the fluid is guided by fixed blades, attached with a casing, and impinging on other blades mounted on a drum or shaft, causing the latter to revolve.

There are two general classes of turbine: 1. Impulse turbines; 2. Reaction turbines.

=Ques. What is an impulse turbine?=

Ans. One in which the fluid is directed by means of a series of nozzles against vanes which it drives.

=Ques. What is a reaction turbine?=

Ans. One in which the pressure or head of the water is employed rather than its velocity. The current is deflected upon the wheel by the action of suitably disposed guide blades, the passages being full of water. Rotary motion is obtained by the change in the direction and momentum of the fluid.

=Ques. Name three classes of reaction turbines.=

Ans. Parallel flow, inward flow, and outward flow.

Parallel flow turbines have an efficiency of about 70% and are suited for low falls not over 30 feet. Inward and outward flow turbines have an efficiency of about 85%. Impulse turbines are suitable for high heads.

=Isolated Plants.=--When electric power transmission from central stations first came into commercial use, the distance from the station at which current could be obtained at a reasonable cost was exceedingly limited.

Consequently, persons desiring electrical power were in the majority of cases forced to install their own apparatus for producing it, this being the origin of isolated plants.

From the nature of the case it is evident that an isolated plant is as a rule smaller and more simple in construction than a central station, and in consequence much more readily operated and managed. It is generally owned by a private individual or a corporation and operated in conjunction with other affairs of a similar character. A basement or other portion of a building is usually set aside in which the necessary apparatus is installed.

Although electricity is now transmitted economically to great distances from central stations, there is still a field for the isolated plant.

The average type of isolated plant has enlarged from a small dynamo driven by a little slide valve engine located in an out of the way corner to direct connected generators and engines of hundreds and even thousands of horse power assembled in a large room specially adapted to the purpose.

In the more modern of these, the electrical outputs are each frequently equal to that of a town central station of respectable size, and the auxiliary equipments are similar in every particular. As a matter of fact, in certain modern isolated plants the only feature that distinguishes them from central stations is that in the former case the owner of the plant represents the sole consumer and conducts other business in connection with it, whereas in the latter case there are a large number of consumers uninterested financially in the enterprise, which is itself generally owned and operated by a company conducting no other business.

=Sub-Stations.=--According to the usual meaning of the term, a sub-station is a building provided with apparatus for changing high pressure alternating current received from the central station into direct current of the requisite pressure, which in the case of railways is 550 to 600 volts.

Where traffic is heavy and the railway system of considerable distance, sub-stations are provided at intervals along the line, each receiving high pressure current from one large central station and converting it into moderate pressure direct current for their districts.

=Ques. Upon what does the arrangement of the sub-station depend?=

Ans. Upon the character of the work and the type of apparatus employed for converting the high pressure alternating current into direct current.

In general it should be substantial, convenient to install or replace the heavy machines, and the layout arranged so that the apparatus can be readily operated by those in attendance.

An overhead traveling crane is the most convenient method of handling the heavy machinery, and is frequently used in large sub-stations.

Fig. 2,776 shows a sectional view, and fig. 2,777, a plan for a small sub-station containing two rotary converters and two banks of three single phase static transformers operating on a three phase system at 11,000 or 13,200 volts, together with the auxiliary apparatus.

=Ques. For three phase installations, what are the merits of separate and combined transformers?=

Ans. With separate transformer for each phase, repairs are more readily made in case of accident or burnouts in the coils. The three phase units have the advantage of low first cost.

Sub-station transformers produce considerable heat, due to the hysteresis and eddy currents, and it is necessary to get rid of it.

Small transformers radiate the heat from the shell and the medium sizes have corrugated shells which increase the surface and provide more rapid radiation.

Large transformers are cooled by an air blast supplied by motor driven blowers or by water pumped through a coil of pipe which is immersed in the insulating oil of the transformer. The large size oil insulated, water cooled transformers are used on circuits of 33,000 volts or more. In water turbine plants, the water may be piped to the transformer under pressure and the pump omitted which cuts down the cost of operating. Air blast transformers usually have a damper or shutter for air control.

=Ques. Explain the use of reactance coils in sub-stations.=

Ans. In order that the direct current voltage of the ordinary rotary may be regulated by a field rheostat, which calls for a corresponding change in the alternating current voltage, a reactance coil is provided between the low tension winding and the converter.

Without such a reactance the maintenance of the same voltage at full load as at no load involves excessive leading and lagging currents and consequently excessive heating in the armature inductors, unless the resistance drop from the source of constant pressure is small, or the natural reactance of the circuit high.

=Ques. What is the effect of weakening the converter field?=

Ans. A lagging current is set up which causes a drop in the reactance coil.

=Ques. State the effect of strengthening converter field.=

Ans. A leading current is set up which gives a rise of voltage in the reactance coil.

Hence when a heavy current passes through the series coil of a compound wound converter and tends to produce a leading current, the reactance coil will balance it, and improve the power factor of the whole line.

=Portable Sub-Stations.=--A portable sub-station constitutes a spare equipment for practically any number of permanent sub-stations and renders unnecessary the installation of spare equipment in each.

It can be used to increase the capacity of a permanent sub-station when the load is unusually heavy, or to provide service while a permanent sub-station is being overhauled or rebuilt.

The transformer can be used for emergency lighting, the primary being connected to a high pressure line and the secondary to the load, if special provision be made at the time the transformer is built to adapt it for these applications.

When an electric railway has a portable sub-station, direct current can be provided at any point on the system where there is track at the high pressure line. The direct current can be made available very quickly as its production involves only the transferring of the sub-station, and its connection to the high pressure line.

Portable sub-stations range in capacity from 200 to 500 kw., and for all alternating current voltages up to 66,000, and frequencies of 25 and 60 cycles.

Although portable sub-stations usually must be of more or less special design to adapt them to the conditions under which they must operate, there are certain general features that are common to all. All members are readily accessible and there are no unnecessary parts. The weight and dimensions are a minimum insuring ease of transportation. Live parts are so protected that the danger of accidental contact with them is minimized.

=Ques. What are the advantages of using outdoor transformers on portable sub-stations?=

Ans. All high pressure wiring is kept out of the car. The transformer is more effectively cooled and the heat dissipated by the transformer does not warm the interior of the cab. The transformer is much more accessible. The car can be run under a crane and the transformer coils pulled out with a hoist.

Taps for different high and low pressure voltages can be readily provided at the time the transformer is being built.

Spotted a typo or a problem in this chapter? Tell us

LXVII. MANAGEMENT

The term "management," broadly speaking, includes not only the actual skilled attention necessary for the proper operation of the machines, after the plant is built, but also other duties which must be performed from its inception to completion, and which may be classified as

1. Selection; 2. Location; 3. Erection; 4. Testing; 5. Running; 6. Care; 7. Repair.

That is to say, someone must select the machinery, determine where each machine is to be located, install them, and then attend to the running of the machines and make any necessary repairs due to the ordinary mishaps likely to occur in operation.

These various duties are usually entrusted to more than one individual; thus, the selection and location of the machinery is done by the designer of the plant, and requires for its proper execution the services of an electrical engineer, or one possessing more than simply a practical knowledge of power plants.

The erection of the machines is best accomplished by those making a specialty of this line of work, who by the nature of the undertaking acquire proficiency in methods of precision and an appreciation of the value of accuracy which is so essential in the work of aligning the machines, and which if poorly done will prove a constant source of annoyance afterward.

The attention required for the operation of the machines, embracing the running care and repair, is left to the "man in charge," who in most cases of small and medium size plants is the chief steam engineer. He must therefore, not only understand the steam apparatus, but possess sufficient knowledge of electrical machinery to operate and maintain it in proper working order.

The present chapter deals chiefly with alternating current machinery, the management of direct current machines having been fully explained in Guide No. 3, however, some of the matter here presented is common to both classes of apparatus.

* * * * *

=Selection.=--In order to intelligently select a machine so that it will properly harmonize with the conditions under which it is to operate, there are several things to be considered.

1. Type; 2. Capacity; 3. Efficiency; 4. Construction.

The general type of machine to be used is, of course, dependent on the system employed, that is, whether it be direct or alternating, single or polyphase.

Thus, the voltage in most cases is fixed except on transformer systems where a choice of voltage may be had by selecting a transformer to suit.

In alternating current constant pressure transmission circuits, an average voltage of 2,200 volts with step down transformer ratios of 1/10 and 1/20 is in general use, and is recommended.

For long distance, the following average voltages are recommended 6,000; 11,000; 22,000; 33,000; 44,000; 66,000; 88,000; and higher, depending on the length of the line and degree of economy desired.

In alternating circuits the standard frequencies are 25, and 60 cycles. These frequencies are already in extensive use and it is recommended to adhere to them as closely as possible.

In fixing the capacity of a machine, careful consideration should be given to the conditions of operation both =present= and =future= in order that the resultant efficiency may be maximum.

Most machines show the best efficiency at or near full load. If the load be always constant, as for instance, a pump forcing water to a given head, it would be a simple matter to specify the proper size of machine, but in nearly all cases, and especially in electrical plants, the load varies widely, not only the daily and hourly fluctuations, but the varying demands depending on the season of the year and growth of the plant's business. All of these conditions tend to complicate the matter, so that intelligent selection of capacity of a machine requires not only calculation but mature judgment, which is only obtained by long experience.

In selecting a machine, or in fact any item connected with the plant its construction should be carefully considered.

Standard construction should be insisted upon so that in the event of damage a new part can be obtained with the least possible delay.

The parts of most machines are interchangeable, that is to say, with the refined methods of machinery a duplicate part (usually carried in stock) may be obtained at once to replace a defective or broken part, and made with such precision that little or no fitting will be required.

The importance of standard construction cannot be better illustrated than in the matter of steam piping, that is, the kind of fittings selected for a given installation.

With the exception of the exhaust line from engine to condenser, where other than standard construction may sometimes be used to reduce the frictional resistance to the steam, the author would adhere to standard construction except in very exceptional cases. Those who have had practical experience in pipe fitting will appreciate the wisdom of this.

For installations in places remote from large supply houses, the more usual forms of standard fittings should be employed, such as ordinary T's, 45° and 90° elbows, etc.

In such locations, where designers specify the less usual forms of standard fittings such as union fittings, offset reducers, etc., or special fittings made to sketch, it simply means, in the first instance that they usually cannot be obtained of the local dealer, making it necessary to order from some large supply house and resulting in vexatious delays.

As a rule, those who specify special fittings have found that their making requires an unreasonable length of time, and the cost to be several times that of the equivalent in standard fittings.

An examination of a few installations will usually show numerous special and odd shape fittings, which are entirely unnecessary.

Moreover, a standard design, in general, is better than a special design, because the former has been tried out, and any imperfection or weakness remedied, and where thousands of castings of a kind are turned out, a better article is usually the result as compared with a special casting.

In the matter of construction, in addition to the items just mentioned, it should be considered with respect to

1. Quality; 2. Range; 3. Accessibility; 4. Proportion; 5. Lubrication; 6. Adjustment.

It is poor policy, excepting in very rare instances, to buy a "cheap" article, as, especially in these days of commercial greed, the best is none too good.

Perhaps next in importance to quality, at least in most cases, is range. This may be defined as scope of operation, effectiveness, or adaptability. The importance of range is perhaps most pronounced in the selection of tools, especially for plants remote from repair shops.

For instance, in selecting a pipe cutter, there are two general classes: wheel cutters, and roller cutters. A wheel cutter has three wheels and a roller cutter one wheel and two rollers, the object of the rollers being to keep the wheel perpendicular to the pipe in starting the cut and to reduce burning. It must be evident that in operation, a roller cutter requires sufficient room around the pipe to permit making a complete revolution of the cutter, whereas, with a wheel cutter, the work may be done by moving the cutter back and forth through a small arc, as illustrated in figs. 2,786 and 2,787. Thus a wheel cutter has a greater range than a roll cutter.

Range relates not only to ability to operate in inaccessible places but to the various operations that may be performed by one tool.

PROPERTIES OF STANDARD WROUGHT IRON PIPE

--------------------------+------+-----------------+ | | | | | | | | | | | | Diameter |Thick-| Circumference. | | ness.| | --------+--------+--------+ +--------+--------+ Nominal| Actual | Actual | |External|Internal| internal|external|internal| | | | --------+--------+--------+------+--------+--------+ Inches | Inches | Inches |Inches| Inches | Inches | --------+--------+--------+------+--------+--------+ ⅛ | .405 | .27 | .068 | 1.272 | .848 | ¼ | .54 | .364 | .088 | 1.696 | 1.144 | ⅜ | .675 | .494 | .91 | 2.121 | 1.552 | ½ | .84 | .623 | .109 | 2.639 | 1.957 | ¾ | 1.05 | .824 | .113 | 3.299 | 2.589 | 1 | 1.315 | 1.048 | .134 | 4.131 | 3.292 | 1¼ | 1.66 | 1.38 | .14 | 5.215 | 4.335 | 1½ | 1.9 | 1.611 | .145 | 5.969 | 5.061 | 2 | 2.375 | 2.067 | .154 | 7.461 | 6.494 | 2½ | 2.875 | 2.468 | .204 | 9.032 | 7.753 | 3 | 3.5 | 3.067 | .217 | 10.996 | 9.636 | 3½ | 4. | 3.548 | .226 | 12.566 | 11.146 | 4 | 4.5 | 4.026 | .237 | 14.137 | 12.648 | 4½ | 5. | 4.508 | .246 | 15.708 | 14.162 | 5 | 5.563 | 5.045 | .259 | 17.477 | 15.849 | 6 | 6.625 | 6.065 | .28 | 20.813 | 19.054 | 7 | 7.625 | 7.023 | .301 | 23.955 | 22.063 | 8 | 8.625 | 7.982 | .322 | 27.096 | 25.076 | 9 | 9.625 | 8.937 | .344 | 30.238 | 28.076 | 10 | 10.75 | 10.019 | .366 | 33.772 | 31.477 | 11 | 12. | 11.25 | .375 | 37.699 | 35.343 | 12 | 12.75 | 12. | .375 | 40.055 | 37.7 | --------+--------+--------+------+--------+--------+ ------+-------------------------+---------------+--------+-------+------ | | | | | | | | | | | | | Length | | | |Length of pipe |of pipe |Nominal| Diam.| Transverse areas. | per square |contain-|weight |Number | | foot of |ing one | per | of ------+--------+--------+-------+-------+-------+ cubic | foot. |thread Nom. |External|Internal| Metal |Ext'nal|Int'nal| foot. | | per intern| | | |surface|surface| | | inch ------+--------+--------+-------+-------+-------+--------+-------+ of Inches|Sq. ins.|Sq. ins.|Sq.ins.| Feet | Feet | Feet |Pounds |screw ------+--------+--------+-------+-------+-------+--------+-------+----- ⅛ | .129 | .0573| .0717| 9.44 |14.15 |2513. | .241 | 27 ¼ | .229 | .1041| .1249| 7.075 |10.49 |1383.3 | .42 | 18 ⅜ | .358 | .1917| .1663| 5.657 | 7.73 | 751.2 | .559 | 18 ½ | .554 | .3048| .2492| 4.547 | 6.13 | 472.4 | .837 | 14 ¾ | .866 | .5333| .3327| 3.637 | 4.635 | 270. | 1.115 | 14 1 | 1.358 | .8626| .4954| 2.904 | 3.645 | 166.9 | 1.668 | 11½ 1¼ | 2.164 | 1.496 | .668 | 2.301 | 2.768 | 96.25 | 2.244 | 11½ 1½ | 2.835 | 2.038 | .797 | 2.01 | 2.371 | 70.66 | 2.678 | 11½ 2 | 4.43 | 3.356 | 1.074 | 1.608 | 1.848 | 42.91 | 3.609 | 11½ 2½ | 6.492 | 4.784 | 1.708 | 1.328 | 1.547 | 30.1 | 5.739 | 8 3 | 9.621 | 7.388 | 2.243 | 1.091 | 1.245 | 19.5 | 7.536 | 8 3½ | 12.566 | 9.887 | 2.679 | .955 | 1.077 | 14.57 | 9.001 | 8 4 | 15.904 | 12.73 | 3.174 | .849 | .949 | 11.31 |10.665 | 8 4½ | 19.635 | 15.961 | 3.674 | .764 | .848 | 9.02 |12.34 | 8 5 | 24.306 | 19.99 | 4.316 | .687 | .757 | 7.2 |14.502 | 8 6 | 34.472 | 28.888 | 5.584 | .577 | .63 | 4.98 |18.762 | 8 7 | 45.664 | 38.738 | 6.926 | .501 | .544 | 3.72 |23.271 | 8 8 | 58.426 | 50.04 | 8.386 | .443 | .478 | 2.88 |28.177 | 8 9 | 72.76 | 62.73 |10.03 | .397 | .427 | 2.29 |33.701 | 8 10 | 90.763 | 78.839 |11.924 | .355 | .382 | 1.82 |40.065 | 8 11 |113.098 | 99.402 |13.696 | .318 | .339 | 1.450|45.95 | 8 12 |127.677 |113.098 |14.579 | .299 | .319 | 1.27 |48.985 | 8 ------+--------+--------+-------+-------+-------+--------+-------+----

Open construction should be employed, wherever possible, so that all parts of a machine that require attention, or that may become deranged in operation, may be accessible for adjustment or repair.

The design should be such that there is ample strength, and the bearings for moving parts should be of liberal proportions to avoid heating with minimum attention.

A comparison of the proportions used by different manufacturers for a machine of given size might profitably be made before a selection is made.

The matter of lubrication is important.

Fast running machines, such as generators and motors, should be provided with ring oilers and oil reservoirs of ample capacity, as shown in figs. 2,788 to 2,794.

All bearings subject to appreciable wear should be made adjustable so that lost motion may be taken up from time to time and thus keep the vibration and noise of operation within proper limits.

=Selection of Generators.=--This is governed by the class of work to be done and by certain local conditions which are liable to vary considerably for different stations.

These variable factors determine whether the generators must be of the direct or alternating current type, whether they must be wound to develop a high or a low voltage, and whether their outputs in amperes must be large or small. Sufficient information has already been given to cover these various cases; there are, however, certain general rules that may advantageously be observed in the selection of generators designed to fill any of the aforementioned conditions, and it is well to possess certain facts regarding their construction.

=Ques. Name an important point to be considered in selecting a generator.=

Ans. Its efficiency.

=Ques. What are the important points with respect to efficiency?=

Ans. A generator possessing a high efficiency at the average load is more desirable than a generator showing a high efficiency at full load.

=Ques. Why?=

Ans. The reason is that in station practice the full load limit is seldom reached, the usual load carried by a generator ordinarily lying between the one-half and three-quarter load points.

=Ques. How do the efficiencies of large and small generators compare?=

Ans. There is little difference.

=Ques. How are the sizes and number of generator determined?=

Ans. The sizes and number of generator to be installed should be such as to permit the engines operating them being worked at nearly full load, because the efficiencies of the latter machines decrease rapidly when carrying less than this amount.

=Ques. What is understood by regulation?=

Ans. The accuracy and reliability with which the pressure or current developed in a machine may be controlled.

It is generally possible if purchasing of a reputable concern, to obtain access to record sheets on which may be found results of tests conducted on the generator in question, and as these are really the only means of ascertaining the values of efficiency and regulation, the purchaser has a right to inspect them. If, for some reason or other, he has not been afforded this privilege, he should order the machine installed in the station on approval, and test its efficiency and regulation before making the purchase.

=Installation.=--The installation of machines and apparatus in an electrical station is a task which increases in difficulty with the size of the plant. When the parts are small and comparatively light they may readily be placed in position, either by hand, by erecting temporary supports which may be moved from place to place as desired, or by rolling the parts along on the floor upon pieces of iron pipe. If, however, the parts be large and heavy, a traveling crane such as shown in fig. 2,797, becomes necessary.

=Ques. What precaution should be taken in moving the parts of machines?=

Ans. Care should be taken not to injure the bearings and shafts, the joints in magnetic circuits such as those between frame and pole pieces, and the windings on the field and armature.

The insulations of the windings are perhaps the most vital parts of a generator, and the most readily injured. The prick of a pin or tack, a bruise, or a bending of the wires by resting their weight upon them or by their coming in contact with some hard substance, will often render a field coil or an armature useless.

Owing to its costly construction, it is advisable when transporting armatures by means of cranes to use a wooden spreader, as shown in fig. 2,798 to prevent the supporting rope bruising the winding.

=Ques. If an armature cannot be placed at once in its final position what should be done?=

Ans. It may be laid temporarily upon the floor, if a sheet of cardboard or cloth be placed underneath the armature as a protection for the windings; in case the armature is not to be used for some time, it is better practice to place it in a horizontal position on two wooden supports near the shaft ends.

=Ques. What kind of base should be used with a belt driven generator or motor?=

Ans. The base should be provided with V ways and adjusting screws for moving the machine horizontally to take up slack in the belt, as shown in fig. 2,799.

Owing to the normal tension on the belt, there is a moment exerted equal in amount to the distance from the center of gravity of the machine to the center of the belt, multiplied by the effective pull on the belt. This force tends to turn the machine about its center of gravity. By placing the screws as shown, any turning moment, as just mentioned, is prevented.

=Ques. How should a machine be assembled?=

Ans. The assembling should progress by the aid of a blue print, or by the information obtained from a photograph of the complete machine as it appears when ready for service. Each part should be perfectly clean when placed in position, especially those parts between which there is friction when the machine is in operation, or across which pass lines of magnetic force; in both cases the surfaces in contact must be true and slightly oiled before placing in position.

Contact surfaces forming part of electrical circuits must also be clean and tightly screwed together. An important point to bear in mind when assembling a machine is, to so place the parts that it will not be necessary to remove any one of them in order to get some other part in its proper position. By remembering this simple rule much time will be saved, and in the majority of instances the parts will finally be better fitted together than if the task has to be repeated a number of times.

When there are two or more parts of the machine similarly shaped, it is often difficult to properly locate them, but in such cases notice should be taken of the factory marks usually stamped upon such pieces and their proper places determined from the instructions sent with the machine.

=Ques. What should be noted with respect to speed of generator?=

Ans. Each generator is designed to be run at a certain speed in order to develop the voltage at which the machine is rated. The speed, in revolutions per minute, the pressure in volts, and the capacity or output in watts (volts × amperes) or in kilowatts (thousands of watts) are generally stamped on a nameplate screwed to the machine.

This requirement frequently requires calculations to be made by the erectors to determine the proper size pulleys to employ to obtain the desired speed.

=Example.=--What diameter of engine pulley is required to run a dynamo at a speed of 1,450 revolutions per minute the dynamo pulley being 10 inches in diameter and the speed of engine, 275 revolutions per minute?

The diameter of pulley required on engine is 10 × (1,450 ÷ 275) = 53 inches, nearly.

=Rule.=--To find the diameter of the driving pulley, multiply the speed of the driven pulley by its diameter, divide the product by the speed of the driver and the answer will be the size of the driver required.

Example.--If the speed of an engine be 325 revolutions per minute, diameter of engine pulley 42 inches, and the speed of the dynamo 1,400 revolutions per minute, how large a pulley is required on dynamo?

The size of the dynamo pulley is

42 × (325 ÷ 1,400) = 9¾ inches.

=Rule.=--To find the size of dynamo pulley, multiply the speed of engine by the diameter of engine wheel and divide the product by the speed of the dynamo.

Example.--If a steam engine, running 300 revolutions per minute, have a belt wheel 48 inches in diameter, and be belted to a dynamo having a pulley 12 inches in diameter, how many revolutions per minute will the dynamo make?

The speed of dynamo will be 300 × (48 ÷ 12) = 1,200 rev. per min.

=Rule.=--When the speed of the driving pulley and its diameter are known, and the diameter of the driven pulley is known, the speed of the driven pulley is found by multiplying the speed of the driver by its diameter in inches and dividing the product by the diameter of the driven pulley.

=Example.=--What will be the required speed of an engine having a belt wheel 46 inches in diameter to run a dynamo 1,500 revolutions per minute, the dynamo pulley being 11 inches in diameter?

The speed of the engine is 1,500 × (11 ÷ 46) = 359 rev. per min. nearly.

=Rule.=--To find the speed of engine when diameter of both pulleys, and speed of dynamo are given, multiply the dynamo speed by the diameter of its pulley and divide by the diameter of engine pulley.

=Ques. How are the diameters and speeds of gear wheels figured?=

Ans. The same as belted wheels, using either the pitch circle diameters or number of teeth in each gear wheel.

=Ques. What should be noted with respect to generator pulleys?=

Ans. A pulley of certain size is usually supplied with each generator by its manufacturer, and it is not generally advisable to depart much from the dimensions of this pulley. Accordingly, the solution of the pulley problem usually consists in finding the necessary diameter of the driving pulley relative to that of the pulley on the generator in order to furnish the required speed.

=Ques. What is the chief objection to belt drive?=

Ans. The large amount of floor space required.

=Ques. How may the amount of space that would ordinarily be required for belt drive, be reduced?=

Ans. By driving machines in tandem as in fig. 2,810, or by the double pulley drive as in fig. 2,811.

=Ques. What is the objection to the tandem method?=

Ans. The most economical distance between centers cannot be employed for all machines.

=Ques. What is the objectionable tendency in resorting to floor economy methods with belt transmission?=

Ans. The tendency to place the machines too closely together. This is poor economy as it makes the cleaning of the machines a difficult and dangerous task; it is therefore advisable to allow sufficient room for this purpose regardless of the method of belting employed.

=Ques. What is the approved location for an alternator exciter?=

Ans. To economize floor space the exciter may be placed between the alternator and engine at S in fig. 2,811.

=Belts.=--In the selection of a belt, the quality of the leather should be first under consideration. The leather must be firm, yet pliable, free from wrinkles on the grain or hair side, and of an even thickness throughout.

If the belt be well selected and properly handled, it should do service for twenty years, and even then if the worn part be cut off, the remaining portion may be remade and used again as a narrower and shorter belt.

Besides leather belts, there are those made of rubber which withstand moisture much better than leather belts, and which also possess an excellent grip on the pulley; they are, however, more costly and much less durable under normal conditions.

In addition to leather and rubber belts, there are belts composed of cotton, of a combination of cotton and leather, and of rope. The leather belt, however, is the standard and is to be recommended.

Equally important with the quality of a belt is its size in order to transmit the necessary power.

The average strain under which leather will break has been found by many experiments to be 3,200 pounds per square inch of cross section. A good quality of leather will sustain a somewhat greater strain. In use on the pulleys, belts should not be subjected to a greater strain than one eleventh their tensile strength, or about 290 pounds to the square inch or cross section. This will be about 55 pounds average strain for every inch in width of single belt three-sixteenths inch thick. The strain allowed for all widths of belting--single, light double, and heavy double--is in direct proportion to the thickness of the belt.

=Ques. How much horse power will a belt transmit?=

Ans. The capacity of a belt depends on, its width, speed, and thickness. A single belt one inch wide and travelling 1,000 feet per minute will transmit one horse power; a double belt under the same conditions, will transmit two horse power.

This corresponds to a working pull of 33 and 66 lbs. per inch of width respectively.

=Example.=--What width double belt will be required to transmit 50 horse power travelling at a speed of 3,000 feet per minute?

The horse power transmitted by each inch width of double belt travelling at the stated speed is

3,000 1 × ----- × 2 = 6, 1,000

hence the width of belt required to transmit 50 horse power is

50 ÷ 6 = 8.33, say 8 inches.

=Ques. At what velocity should a belt be run?=

Ans. At from 3,000 to 5,000 feet per minute.

=Ques. How may the greatest amount of power transmitting capacity be obtained from belts?=

Ans. By covering the pulleys with leather.

=Ques. How should belts be run?=

Ans. With the tight side underneath as in fig. 2,814.

=Ques. What is a good indication of the capacity of a belt in operation?=

Ans. Its appearance after a few days' run.

If the side of the belt coming in contact with the pulley assume a mottled appearance, it is an indication that the capacity of the belt is considerably in excess of the power which it is transmitting, inasmuch as the spotted portions of the belt do not touch the pulley; and in consequence of this there is liable to be more or less slipping.

Small quantities of a mixture of tallow and fish oil which have previously been melted together in the proportion of two of the former to one of the latter, will, if applied to the belt at frequent intervals, do much toward softening it, and thus by permitting its entire surface to come in contact with the pulley, prevent any tendency toward slipping. The best results are obtained when the smooth side of the belt is used next to the pulley, since tests conducted in the past prove that more power is thus transmitted, and that the belt lasts longer when used in this way.

=Ques. What is the comparison between the so called endless belts and laced belts?=

Ans. With an endless belt there is no uneven or noisy action as with laced belts, when the laced joint passes over the pulleys, and the former is free from the liability of breakage at the joint.

=Ques. How should a belt be placed on the pulleys?=

Ans. The belt should first be placed on the pulley at rest, and then run on the other pulley while the latter is in motion.

The best results are obtained, and the strain on the belt is less, when the speed at which the moving pulley revolves is comparatively low. With heavy belts, particular care should be taken to prevent any portion of the clothing being caught either by the moving belt or pulleys, as many serious accidents have resulted in the past from carelessness in regard to this important detail. The person handling the belt should, therefore, be sure of a firm footing, and when it is impossible to secure this, it is advisable to stop the engine and fit the belt around the engine pulley as well as possible by the aid of a rope looped around the belt.

=Ques. Under what conditions does a belt drive give the best results?=

Ans. When the two pulleys are at the same level.

If the belt must occupy an inclined position it should not form a greater angle than 45 degrees with the horizontal.

=Ques. What is a characteristic feature in the operation of belts, and why?=

Ans. Belts in motion will always run to the highest side of a pulley; this is due partially to the greater speed in feet per minute developed at that point owing to the greater circumference of the pulley, and also to the effects of centrifugal force.

If, therefore, the highest sides of both pulleys be in line with each other, and the shafts of the respective pulleys be parallel to each other, there will be no tendency for the belt to leave the pulleys when once in its proper position. In order that these conditions be maintained, the belt should be no more than tight enough to prevent slipping, and the distance between the centers of the pulleys should be approximately 3.5 times the diameter of the larger one.

=Ques. What minor appurtenances should be provided in a station?=

Ans. Apparatus should be installed as a prevention against accidents, such as fire, and protection of attendants from danger.

In every electrical station there should be a pump, pipes and hose; the pump may be either directly connected to a small electric motor or belted to a countershaft, while the pipes and hose should be so placed that no water can accidentally reach the generators and electrical circuits. A number of fire bucket filled with water should be placed on brackets around the station, and with these there should be an equal number of bucket containing dry sand, the water being used for extinguishing fire occurring at a distance from the machines and conductors, and the sand for extinguishing fire in current carrying circuits where water would cause more harm than benefit. To prevent the sand being blown about the station, each sand bucket, when not in use, should be provided with a cover.

Neat cans and boxes should be mounted in convenient places for greasy rags, waste, nuts, screws, etc., which are used continually and which therefore cannot be kept in the storeroom.

While it is important to guard against fire in the station, it is equally necessary to provide for personal safety. All passages and dark pits should therefore be thoroughly lighted both day and night, and obstacles of any nature that are not absolutely necessary in the operation of the station, should be removed. Moving belts, and especially those passing through the floor, should be enclosed in iron railings. If high voltages be generated, it is well to place a railing about the switchboard to prevent accidental contact with current carrying circuits, and in such cases it is also advisable to construct an insulated platform on the floor in front of the switchboard.

=Switchboards.=--The plan of switchboard wiring for alternating current work depends upon the system in use and this latter may be either of the single phase, two phase, three phase, or monocyclic types. The general principles in all these cases, however, are practically identical.

Fig. 2,820 shows the switchboard wiring for a single phase alternator. As an aid in reading the diagram, the conductors carrying alternating current are represented by solid lines, and those carrying direct current, by dotted lines.

The exciter shown at the right is a shunt wound machine. By means of the exciter rheostat, the voltage for exciting the field winding of the alternator is varied; this, in turn, varies the voltage developed in the alternator since the main leads of the exciter are connected through a double pole switch G to the field winding of the alternator.

A rheostat is also introduced in the alternator field winding circuit to adjust the alternator pressure. It may seem unnecessary to employ a rheostat in each of two separate field circuits to regulate the voltage of the alternator, but these rheostats are not both used to produce the same result. When a considerable variation of pressure is required, the exciter rheostat is manipulated, whereas for a fine adjustment of voltage the alternator rheostat is preferably employed.

Sometimes a direct current ammeter is introduced in the alternator's field circuit to aid in the adjustment.

The main circuit of alternator after being protected on both sides by fuses, runs to the double pole switch K. These fuses serve as a protection to the alternator in case of a short circuit at the main switch. It will be noticed the fuses are of the single pole type and are mounted a considerable distance apart; this is to prevent any liability of a short circuit between them in case of action. Enclosed fuses are now used entirely for such work, since in these there is no danger of heated metal being thrown about and causing damage when the fuse wire is melted. Enclosed fuses are also more readily and quickly replaced than open fuses, the containing tube of each being easy to adjust in circuit, and when the fuse wire within is once melted the tube is discarded for a new one.

The main circuit after passing through the main switch is further protected on both sides by circuit breakers. Leaving these protective devices, the left hand side of the circuit includes the alternating current ammeter, and then connects with one of the bus bars. The right hand side of the circuit runs from the circuit breaker to the other bus bar. As many feeder circuits may be connected to the bus bars and supplied with current by the alternator as the capacity of this machine will permit. If, however, there be more than one feeder circuit, each must be wired through a double pole switch.

In alternating current work the pressures dealt with are much greater than those in direct current installations, so that proportionate care must be taken in the wiring to remove all possibility of grounds.

To locate such troubles, however, should they occur, a ground detector is provided. For this class of work the ground detector must be an instrument especially designed for high pressure circuits. Two of its terminals should be connected to the line wires and the third, to ground; in case of a leak on the line, a current will then flow through the detector and by the position of the pointer the location and seriousness of the leak may be judged.

A step down transformer is also rendered necessary for the voltmeter and the pilot lamps, owing to the high voltage in use. The primary winding of the transformer is connected across the main circuit of the alternator. This connection should never be made so that it will be cut out of circuit when the main switch is open, for it is always advisable to consult the voltmeter before throwing on the load by closing this switch.

=Ques. How does the switchboard wiring for a two phase system differ from the single phase arrangement shown in fig. 2,820?=

Ans. It is practically the same, except for the introduction of an extra ammeter and a compensator in each of the outside wires, and in the use of a four pole switch in place of the two pole main switch.

The ammeters, of course, are for measuring the alternating currents in each of the two phases or legs of the system, and the compensators are two transformers with their primary coils in series with the outside wires and their secondary coils in series with each other across the outside wires. The transformers thus connected are known as compensators or pressure regulators, and as such compensate for the drop in pressure on either side of the system.

=Ques. How is the four pole main switch wired?=

Ans. Its two central terminals which connect directly with the line wires, are joined together by a conductor, and from this point one wire is led off. This wire, together with the two outside wires, form the feeders of the system.

=Ques. How many voltmeters are required for the two phase system?=

Ans. One voltmeter is sufficient on the board if a proper switching device be employed to shift its connections across either of the two circuits; otherwise, two voltmeters will be necessary, one bridged across each of these respective circuits.

The same reasoning holds true in regard to ground detectors, so that one or two of these will be required, depending upon the aforementioned conditions.

=Ques. What are the essential points of difference between the single phase switchboard wiring as shown in fig. 2,820, and that required for a three wire three phase system?=

Ans. The three phase system requires the use of a three pole switch in place of the two pole switch; the insertion of an ammeter, a circuit breaker, and a compensator in each of the three wires of the system; the presence of two ground detectors instead of one, and the addition of a voltmeter switch if but one voltmeter be provided, or else the installation of two voltmeters, connected the one between the middle wire and outer right hand wire, and the other between the middle wire and outer left hand wire.

=Ques. Mention a few points relating to lightning arresters.=

Ans. In most cases where direct current is used they are mounted on the walls of the station near the place at which the line wires enter. If they be mounted outside the station at this point, special precautions should be taken to keep them free from moisture by enclosing them in iron cases, but no matter where they are located it is necessary that they be dry in order to work properly.

If possible, one place should be set aside for them and a marble or slate panel provided on which they may be mounted.

Wooden supports are undesirable for lightning arresters on account of the fire risk incurred; this, however, may be reduced to a minimum by employing skeleton boards and using sheets of asbestos between the arresters and the wood.

In parts of the country where lightning is of common occurrence and where overhead circuits are installed which carry high pressures, heavy currents, and extend over considerable territory, it is advisable to have the station well equipped with lightning arresters of the most improved types.

In each side of the main circuit, between the lightning arrester connections and the switchboard apparatus there should be connected a choke coil or else each of the main conductors at this point should be tightly coiled up part of its length to answer the same purpose.

A quick and effective way of coiling up a wire consists in wrapping around a cylindrical piece of iron or wood that part of the conductor in which it is desired to have the coils, the desired number of times, and then withdrawing the cylindrical piece. The coils, each of which may contain 50 or 200 turns, thus inserted in the main circuit introduce a high resistance or reluctance to a lightning current, and thus prevent it passing to the generator; there will, however, be an easy path to earth afforded it through the lightning arrester, and so no damage will be done. Coils of the nature just mentioned may advantageously be introduced between the generator and switchboard to take up the reactive current developed upon the opening of the circuit, and in the case of suspended conductors, the coils may be used to take up the slack by the spring-like effect produced by them.

The safety of the operator should be especially considered in the design of high pressure alternating current switchboards.

Such protection may be secured by screening all the exposed terminals, or preferably by mounting all the switch mechanism on the back of the board with simply the switch handle projecting through to the front; by pushing or pulling the switch handle, the connections can thus be shifted either to one side of the system or to the other.

=Ques. Upon what does the work of assembling a switchboard depend?=

Ans. It depends almost entirely upon the size of the plant, varying from the simple task of mounting a single panel in the case of an isolated plant, to the more difficult problem of supporting a large number of panels in a central station.

=Ques. When the material chosen for a switchboard must be shipped a considerable distance, what form of board should be used?=

Ans. The board units or "slabs" should be of small dimensions, to avoid the liability of breakage and expense of renewal when a unit becomes cracked or machine injured.

Ordinarily, switchboards vary from five to eight feet in height and the widths of the panels vary from five to six feet. In some boards the seams between the slabs run vertically, and in others horizontally. In order to render the assembling of the switchboard as simple as possible, and its appearance when finished the most artistic, these seams should run horizontally rather than vertically. The edges of each of the slabs should also be chamfered so that there will be less danger of their breaking out when being mounted on the framework.

=Ques. In assembling a switchboard, how should the lower slabs be placed, and why?=

Ans. They should be suspended a little distance from the floor to prevent contact with any oil, dirt, water or rubbish that might be on the floor.

=Ques. How are the slabs or panels supported?=

Ans. They are carried on an iron or wooden framework with braces to give stability.

The braces should be securely fastened at one end to the wall of the station, and at the other end to the framework of the board, as shown in fig. 2,836.

To fasten the switchboard end of the brace directly to the slate, marble or other material composing the board is poor practice and should never be attempted.

If the station be constructed of iron, these switchboard braces must be such that they will thoroughly insulate the board and its contents from the adjoining wall.

=Ques. What is the usual equipment of a switchboard?=

Ans. It comprises switching devices, current or pressure limiting devices, indicating devices, and fuses for protecting the apparatus and circuits.

On some switchboards are also mounted small transformers for raising or lowering the voltages, and lightning arresters as a protection from lightning. In addition to the apparatus previously mentioned nearly all switchboards carry at or near their top two or more incandescent lamps provided with shades or reflectors, for lighting the board.

=Ques. What should be done before wiring a switchboard?=

Ans. The electrical connections between the various apparatus mounted on the face or front of the board, are made on the back of the board. It is necessary that these connections be properly made else considerable electrical power will be wasted at this point. The wiring on the back of the board should therefore be planned out on paper before commencing the work.

In laying out the plan of wiring care must be taken to allow sufficient contact surface at each connection; there should be not less than one square inch of contact surface allowed for each 160 amperes of current transmitted.

For the bus bars, which, by the way are always of copper, one square inch per 1,000 amperes is the usual allowance; this is equal to 1,000 circular mils of cross sectional area per ampere.

Every effort should be made to give the bus bars the greatest amount of radiation consistent with other conditions, in order that their resistances may not become excessive owing to the heat developed by the large currents they are forced to carry. Suppose, for instance, the number of amperes to be generated is such as to require bus bars having each a cross sectional area of one square inch. If the end dimensions of these bars were each 1 inch by 1 inch, there would be less radiating surface than if their dimensions were each 2 inches by ½ inch.

=Operation of Alternators.=--The operation of an alternator when run singly differs but little from that for a dynamo.

As to the preliminaries, the exciter must first be started. This is done in the same way as for any shunt dynamo. At first only a small current should be sent through the field winding of the alternator; then, if the exciter operates satisfactorily and the field magnetism of the operator show up well, the load may gradually be thrown on until the normal current is carried, the same method of procedure being followed as in the similar case of a dynamo.

On loading an alternator, a noticeable drop in voltage occurs across its terminals. This drop in voltage is caused in part by the demagnetization of the field magnets due to the armature current, and so depends in a measure upon the position and form of the pole pieces as well as upon those of the teeth in the armature core. The resistance of the armature winding also causes a drop in voltage under an increase of load.

Another cause which may be mentioned is the inductance of the armature winding, which is in turn due to the positions of the armature coils with respect to each other and also with respect to the field magnets.

=Alternators in Parallel.=--When the load on a station increases beyond that which can conveniently be carried by one alternator, it becomes necessary to connect other alternators in parallel with it. To properly switch in a new machine in parallel with one already in operation and carrying load, requires a complete knowledge of the situation on the part of the attendant, and also some experience.

The connections for operating alternators in parallel are shown in fig. 2,843. In the illustration the alternator A is in operation and is supplying current to the bus bars. The alternator B is at rest. The main pole switch B' by means of which this machine can be connected into circuit is therefore open.

Now, if the load increase to such extent as to require the service of the second alternator B, it must be switched in parallel with A. In order that both machines may operate properly in parallel, three conditions must be satisfied before they are connected together, or else the one alternator will be short circuited through the other, and serious results will undoubtedly follow.

Accordingly before closing main switch B, it is necessary that

1. The frequencies of both machines be the same; 2. The machines must be in synchronism; 3. The voltages must be the same.

=Ques. How are the frequencies made the same?=

Ans. By speeding up the alternator to be cut in, or change the speed of both until frequency of both machines is the same.

=Ques. How are the alternators synchronized or brought in phase?=

Ans. The synchronism of the alternators is determined by employing some form of synchronizer, as by the single lamp method of fig. 2,843, or the two lamp method of fig. 2,845.

=Ques. In synchronizing by the one lamp method, when should the incoming machine be thrown in?=

Ans. It is advisable to close the switch when the machines are approaching synchronism rather than when they are receding from it, that is to say, the instant the lamp becomes dark.

=Ques. What are the objections to the one lamp method?=

Ans. The filament of the lamp may break, and cause darkness, or the lamp may be dark with considerable voltage as it takes over 20 volts to cause a 100 volt lamp to glow.

=Ques. What capacity of single lamp must be used?=

Ans. It must be good for twice the voltage of either machine.

=Ques. What modification of the synchronizing methods shown in the accompanying illustrations is necessary when high pressure alternators are used?=

Ans. Step down transformers must be used between the alternators and the lamps to obtain the proper working voltages for the lamps.

=Ques. How is the voltage of an incoming machine adjusted so that it will be the same as the one already in operation?=

Ans. By varying the field excitation with a rheostat in the alternator field circuit.

=Ques. How may two or more alternators be started simultaneously?=

Ans. After bringing each of them up to its proper speed so as to obtain equal frequencies, the main switches may be closed, thereby joining their armature circuits in parallel. As yet, however, their respective field windings have not been supplied with current, so that no harm can result in doing this. The exciters of these machines after being joined in parallel, should then be made to send direct current simultaneously through the field windings of the alternators, and from this stage on the directions previously given may be followed in detail.

=Ques. What are the conditions when two or more alternators are directly connected together?=

Ans. If rigidly connected together, or directly connected to the same engine, they must necessarily run in the same manner at all times.

When machines connected in this way are once properly adjusted so that they are in phase with each other, their operation in parallel is even a simpler task than when they are all started together but are not directly connected.

=Ques. When an alternator is driven by a gas engine, what provision is sometimes made to insure successful operation in parallel?=

Ans. An amortisseur winding is provided to counteract the tendency to "hunting."

=Ques. What is the action of the amortisseur winding?=

Ans. Any sudden change in the speed of the field, generates a current in the amortisseur winding which resists the change of velocity that caused the current.

The appearance of an amortisseur winding is shown in the cut below (fig. 2,850) illustrating the field of a synchronous condenser equipped with amortisseur winding.

=Ques. How are three phase alternators synchronized?=

Ans. In a manner similar to the single phase method.

Thus the synchronizing lamps may be arranged as in fig. 2,581, which is simply an extension of the single phase method.

=Ques. Are three lamps necessary?=

Ans. Only to insure that the connections are properly made, after which one lamp is all that is required.

=Ques. How is it known that the connections of fig. 2,851 are correct?=

Ans. If, in operation, the three lamps become bright or dark simultaneously, the connections are correct; if this action takes place successively, the connections are wrong.

If wrong, transpose the leads of one machine until simultaneous action of the lamps is secured.

=Ques. What is the disadvantage of the lamp method of synchronizing?=

Ans. Lack of sensitiveness.

=Ques. Which is the accepted lamp method, dark or brilliant?=

Ans. In the United States it is usual to make the connections for a dark lamp at synchronism, while in England the opposite practice obtains.

With the dark lamp method, the breaking of a filament might cause the machines to be connected with a great phase difference, whereas, with the brilliant lamp it is difficult to determine the point of maximum brilliancy. This latter method, therefore may be called the safer.

=Ques. What may be used in place of lamps for synchronizing?=

Ans. Some form of synchroscopes, or synchronizers.

=Ques. How does the Lincoln synchronizer work?=

Ans. The construction is such that a hand moves around a dial so that the angle between the hand and the vertical is always the phase angle between the two sources of electric pressure to which the synchronizer is connected.

If the incoming alternator be running too slow, the hand deflects in one direction, if too fast, in the other direction. When the hand shows no deflection, that is, when it stands vertical, the machines are in phase. A complete revolution of the hand indicates a gain or loss of one cycle in the frequency of the incoming machine, as referred to the bus bars.

=Cutting Out Alternator.=--When it is desired to cut out of circuit an alternator running in parallel with others, the method of procedure is as follows:

1. Reduce driving power until the load has been transferred to the other alternators, adjusting field rheostat to obtain minimum current; 2. Open main switch; 3. Open field switch.

=Ques. What precaution should be taken?=

Ans. Never open field switch before main switch.

=Ques. What is the ordinary method of cutting out an alternator?=

Ans. The main switch is usually opened without any preliminaries.

=Ques. What is the objection to this procedure?=

Ans. It suddenly throws all the load on the other alternators, and causes "hunting."

=Ques. What forms of drive are especially desirable for running alternators in parallel, and why?=

Ans. Water turbine or steam turbine because of the uniform torque, thus giving uniform motion of rotation.

With reciprocating engines, the crank effect is very variable during the revolution, resulting in pulsations driving the alternator too fast or too slow, and causing cross current between the alternators.

=Ques. Is a sluggish, or a too sensitive governor preferable on an engine driving alternators in parallel?=

Ans. A sluggish governor.

=Alternators in Series.=--Alternators are seldom if ever connected in series, for the reason that the synchronizing tendency peculiar to these machines causes them to oppose each other and fall out of phase when they are joined together in this way. If, however, they be directly connected to each other, or to an engine, so that they necessarily keep in phase at all times, and thus add their respective voltages instead of counteracting them, series operation is possible.

NOTE.--According to the practice of the General Electric Co., 2½ degrees of phase difference from a mean is the limit allowable in ordinary cases. It will, in certain cases, be possible to operate satisfactorily in parallel, or to run synchronous apparatus from machines whose angular variation exceeds this amount, and in other cases it will be easy and desirable to obtain a better speed control. The 2½ degree limit is intended to imply that the maximum departure from the mean position during any revolution shall not exceed 2½ ÷ 360 of an angle corresponding to two poles of a machine. The angle of circumference which corresponds to the 2½ degree of phase variation can be ascertained by dividing 2½ by ½ the number of pole; thus, in a 20 pole machine, the allowable angular variation from the mean would be 2½ ÷ 10 = ¼ of one degree.

=Transformers.=--These, as a whole, are simple in construction, high in efficiency, and comparatively inexpensive. Their principles of operation are also readily understood.

The efficiency of a transformer, that is, the ratio between full load primary and full load secondary is greatest when the load on it is such that the sum of the constant losses equals the sum of the variable losses.

In general, transformers designed for high frequencies and large capacities are more efficient than those designed for low frequencies and small capacities. As a whole, however, a transformer leaves but little to be desired as regards efficiency, a modern 60 cycle transformer of 50 kilowatts capacity or more possesses an efficiency of approximately 98 per cent. at full load and an efficiency of about 97 per cent. at half load.

=Ques. How should a transformer be selected, with respect to efficiency?=

Ans. One should be chosen, whose parts are so proportioned that the point of maximum efficiency occurs at that load which the transformer usually carries in service.

In many alternating current installations, comparatively light loads are carried the greater part of the time, the rated full load or an overload being occurrences of short durations. For such purposes special attention should be given to the designing or selecting of transformers having low core losses rather than low resistance losses, because the latter are then of relatively small importance.

=Ques. What kind of efficiency is the station manager interested in?=

Ans. The "all day efficiency."

This expression, as commonly met with in practice, denotes the percentage that the amount of energy actually used by the consumer is of the total energy supplied to his transformer during 24 hours. The formula for calculating the all day efficiency of a transformer is based upon the supposition that the amount of energy used by the consumer during 24 hours is equivalent to full load on his transformer during five hours and is as follows:

5w E = ------------- 24c + 5r + 5w where E = the all day efficiency of the transformer, w = the full load in watts on the primary, c = the core loss in watts, r = the resistance loss in watts.

=Ques. What are the usual all day efficiencies?=

Ans. The average is about 85 per cent. for those of 1 kilowatt capacity, 92 per cent. for those of 5 kilowatts capacity, 94 per cent. for those of 10 kilowatts capacity, and about 94.5 per cent. for those of 15 kilowatts capacity.

=Ques. What becomes of the energy lost by a transformer?=

Ans. It reappears as heat in the windings and core.

This heat not only increases the resistances of the windings and core, producing thereby a further increase of their respective losses, but in addition causes in time a peculiar effect on the iron core which is intensified by the reversals of magnetism constantly going on within it.

After about two years' service, the iron apparently becomes fatigued or tired, and this phenomenon is called aging of the iron. Since the life of the transformer depends to a great extent upon this factor, the conditions responsible for its existence should as far as possible be removed. Means must therefore be provided in the construction to radiate the heat as quickly as it is generated.

=Ques. What kind of oil is used in oil cooled transformers?=

Ans. Mineral oil.

=Ques. How is it obtained?=

Ans. By fractional distillations of petroleum unmixed with any other substances and without subsequent chemical treatment.

=Ques. What is the important requirement for transformer oil?=

Ans. It should be free from moisture, acid, alkali or sulphur compounds.

=Ques. How may the presence of moisture be determined?=

Ans. By thrusting a red hot iron rod in the oil; if it "crackle," moisture is present.

=Ques. Describe the Westinghouse method of drying oil.=

Ans. It is circulated through a tank containing lime, and afterwards, through a dry sand filter.

=Ques. What is the objection to heating the oil (raising its temperature slightly above boiling point of water) to remove the moisture?=

Ans. The time consumed (several days) is excessive.

=Ques. What effect has moisture?=

Ans. It reduces the insulation value of the oil. .06 per cent. of moisture has been found to reduce the dielectric strength of oil about 50 per cent. "dry" oil will withstand a pressure of 25,000 volts between two 9½ inch knobs separated .15 inch.

=Ques. What is understood by transformer regulation?=

Ans. It is the difference between the secondary voltage at no load and at full load, and is generally expressed as a percentage of the secondary voltage at no load.

=Ques. What governs its value?=

Ans. The resistance and reactance of the windings.

=Ques. How may the regulation be improved?=

Ans. By decreasing the resistances of the windings by employing conductors of greater cross section, or decreasing their reactance by dividing the coils into sections and closely interspersing those of the primary between those of the secondary.

NOTE.--The term ="regulation"= as here used is synonymous with "drop." The voltage drop in a transformer denotes the drop of voltage occurring across the secondary terminals of a transformer with load. This drop is due to two causes: 1, the resistance of the windings; and 2, the reactance or magnetic leakage of the windings. On non-inductive load, the reactive drop, being in quadrature, produces but a slight effect, but on inductive loads it causes the voltage to drop, and on leading current loads it causes the voltage to rise. As the voltage drop of a good transformer is very small even on inductive load, direct accurate measurement is difficult. It is best to measure the copper loss with short circuited secondary by means of a wattmeter, and at the same time the voltage required to drive full load current through. From the watts, the resistance drop can be found, and from this and the impedance voltage, the reactive drop may be calculated. From these data a simple vector diagram will give, near enough for all practical purposes, the drop for any power factor, or the following formula may be used which has been deduced from the vector diagram. _________________________ D = √(W + X)^{2} + (R + P)^{2} - 100 where R = % resistance drop; X = % reactive drop; P = % power factor of load; W = % wattless factor of ________ load (√1 - P^{2}); D = % resultant secondary drop. For non-inductive loads where P = 100 and W _____________________ = 0, D = √X^{2} + (100 + R)^{2} - 100. In the case of leading currents it should be considered negative.

In transformers where there is a great difference in voltage between the primary and secondary windings, however, this remedy has its limitations on account of the great amount of insulation which must necessarily be used between the windings, and which therefore causes the distances between them to become such as to cause considerable leakage of the lines of force.

=Ques. How does the regulation vary for different transformers, and what should be the limit?=

Ans. Those of large capacity usually have a better regulation than those of small capacity, but in no case should its value exceed 2 per cent.

=Ques. What advantages have shell type transformers over those of the core type?=

Ans. They have a larger proportion of core surface exposed for radiation of heat, and a shorter magnetic circuit which reduces the tendency for a leakage of the lines of force into the air.

Both types have advantages and disadvantages as compared with the other. In the shell type, there is less magnetic leakage, but also less surface exposed for radiation, and greater difficulty in providing efficient insulation between the two circuits; in the core type there is more surface exposed for radiation and less difficulty in insulating the windings, but there is also a great leakage of the lines of magnetic force into the outer air.

=Ques. How are the windings usually arranged?=

Ans. As a rule, there is only one primary winding but the secondary winding is generally divided into two equal sections, the four terminals of which are permanently wired to four connection blocks which may be connected so as to throw the secondary sections either in parallel or in series with each other at will.

=Ques. What is necessary for satisfactory operation of transformers in parallel?=

Ans. They must be designed for the same pressures and capacities, their percentages of regulation should be the same and they must have the same polarity at a given instant.

One may satisfy himself as to the first of these conditions by examining the name plates fastened to the transformers, whereon are stamped the values of the respective pressures and capacities of each.

Although equal values of regulation is given as one of the conditions to be satisfied, transformers may be operated in parallel when their percentages of regulation are not the same. Ideal operation, however, can be attained only under the former state of affairs. Suppose, for instance, a transformer having a regulation of two per cent. be operated in parallel with another of similar size and design but having a regulation of one per cent. The secondary pressures of these transformers at no load will of course be the same, but at full load if the secondary pressure of the one be 98 volts, that of the other will be 99 volts. There will, therefore, be a difference of pressure of one volt between them which will tend to force a current backward through the secondary winding of the transformer delivering 98 volts. This reversed current, although comparatively small in value, lowers the efficiency of the installation by causing a displacement of phase and a decrease in the combined power factor of the transformers.

=Ques. Describe the polarity test.=

Ans. The test for polarity consists in joining together by means of a fuse wire, a terminal of the secondary winding of each transformer, and then with the primary windings supplied with normal voltage, connecting temporarily the remaining terminals of the secondary windings. The melting of the fuse wire thus connected indicates that the secondary terminals joined together are of opposite polarities, and that the connections must therefore be reversed, whereas if the fuse wire do not melt, it shows that the proper terminals have been joined and that the connections may be made permanent.

The object of this test is, obviously, not to determine the exact polarity of each secondary terminal, but merely to indicate which of them are of the same polarity.

=Motor Generators.=--In motor generator sets, either the shunt or series wound type of motor may be employed at the power producing end of the set, but the field of the generator is either shunt or compound wound, depending upon whether or not it is desired to maintain or to raise the secondary voltage near full load. In either case a rheostat introduced in the shunt field winding of the generator will be found very essential. Both generator and motor are so mounted on the base that their respective commutators are at the outer ends of the set. By this means ample space surrounds all of the working parts, and repairs can readily be made.

Motor generators are frequently used as boosters to raise or boost the voltage near the extremities of long distance, direct current transmission lines. Of these, electric railway systems in which it is desired to extend certain of the longer lines, form a typical example.

Owing to the great cost of changing such a system over to one employing alternating current, or storage batteries, or of constructing an additional power station, these solutions of the problem are usually at variance with good judgment and the amount of money at hand. The choice then remains between the purchase of additional wire for feeders, the connection of a booster in the old feeders, or the installation of both larger feeders and a booster. Of these, it is generally found that either the second or the third mentioned alternative meets the conditions most satisfactorily.

A booster installed in a railway system for the purpose just mentioned, would have a series wound motor, and the conditions to which it must conform would be as follows: The motor having a series winding must provide for the full feeder current passing through both armature and field windings.

Owing to the varying loads on a railway system, due to the frequent starting and stopping of cars, the feeder current varies between zero and some such value as 150 amperes. This fluctuation of current through the field winding will, in ordinary cases, vary the magnetization of the pole pieces from zero almost to the point of saturation; that is, the maximum feeder current will so nearly fill the magnet cores with lines of force that it would be quite difficult to cause more lines of magnetic force to pass through them.

So long as the point of saturation is not reached, however, the proportion of current to field strength remains constant, and therefore the ratio of amperes to volts will not vary.

The severe fluctuations of the feeder current would, if the motor were shunt or compound wound, cause most serious sparking and various other troubles, but in a series motor where the back ampere turns on the armature that react on the field vary in precisely the same proportion as the ampere turns in the field, there exists at all times a tendency to balance the active forces and produce satisfactory operation. If, however, the field magnet cores be very large, they cannot so quickly respond, magnetically, to changes in the strength of the current, and there is then greater liability of the armature reaction momentarily weakening the field and thereby producing temporary sparking.

=Ques. Are motor generators always composed of direct current sets?=

Ans. No.

=Ques. Describe conditions requiring a different combination.=

Ans. For purposes where for instance direct currents of widely different voltages are to be obtained from an alternating current circuit, and it is desired to install but one set, a motor generator consisting of an alternating current motor such as an induction motor, and a dynamo must necessarily be employed.

In such sets, it is common to find both motor and dynamo armatures mounted on a common shaft, and the respective field frames resting on a single base, although for connection on a very high pressure alternating current circuit, separate armature shafts insulated from each other but directly connected together, and separate bases resting on a single foundation, are usually employed to afford the highest degree of insulation between the respective circuits of the two machines.

=Ques. What is the objection to a set composed of alternating current motor and alternator?=

Ans. The commercial field that would be naturally covered by such a set is better supplied by a transformer.

=Ques. Why?=

Ans. Because a transformer contains no moving parts, and is therefore simpler in construction, cheaper in price, and less liable to get out of order.

=Dynamotors.=--A dynamotor differs from a motor generator in that the motor armature and the generator armature are combined into one, thereby requiring but one field frame. Since the motor and generator armature windings are mounted on a single core, the armature reaction due to the one winding is neutralized by the reaction caused by the other winding. There is, consequently, little or no tendency for sparking to occur at the brushes, and they therefore need not be shifted on this account for different loads.

=Ques. How is a dynamotor usually constructed?=

Ans. It is usually built with two pole pieces which are shunt wound.

=Ques. Why does the voltage developed fall off slightly under an increase of load?=

Ans. Because a compound winding cannot be provided.

=Ques. Describe the armature construction and operation.=

Ans. It consists of two separate windings; one of which is joined to a commutator mounted on one side of the armature for motor purposes, and the other to the commutator on the other side of the armature for generator purposes.

By means of two studs of brushes pressing on the motor commutator, current from the service wires is fed into the winding connected to this commutator, and since the shunt field winding is also excited by the current from the service wires, there is developed in the generator winding on the rotating armature a direct voltage which is proportional to the speed of rotation of the armature in revolutions per second, the number of conductors in series which constitute the generator winding, and the total strength of the field in which the armature revolves. This pressure causes current to pass through the generator winding and the distributing circuit when the distributing circuit to which this winding is connected by means of its respective commutator, brushes, etc., is closed.

=Ques. How is a dynamotor started?=

Ans. It is connected at its motor end and started in the same manner as any shunt wound motor on a constant pressure circuit.

=Ques. What precautions should be taken in starting a dynamotor?=

Ans. The necessary precautions are, to have the poles strongly magnetized before passing current through the motor winding on the armature; to increase gradually the current through this winding, and not to close the generating circuit until normal conditions regarding speed, etc., are established in the motor circuit.

=Ques. How is the current developed in the machine regulated?=

Ans. It can be regulated by the introduction of resistance in one or the other of the armature circuits, or by a shifting of the brushes around the commutator.

=Ques. Are dynamotors less efficient than motor generators of a similar type?=

Ans. No, they are more efficient.

=Ques. Why?=

Ans. Because they have only one field circuit and at least one bearing less than a motor generator.

A motor generator has at least three bearings, and occasionally, four, where the set consists of two independent machines directly connected together.

=Rotary Converters.=--An important modification of the dynamotor is the rotary converter. This machine forms, as it were, a link between alternating and direct current systems, being in general a combination of an alternating current motor and a dynamo.

It has practically become a fixture in all large electric railway systems and in other installations where heavy direct currents of constant pressure are required at a considerable distance from the generating plant. In such cases a rotary converter is installed in the sub-station, and being simpler in construction, higher in efficiency, more economical of floor space, and lower in price than a motor generator set consisting of an alternating current motor and a dynamo which might be used in its place, it has almost entirely superseded the latter machine for the class of work mentioned.

=Ques. What is the objection to the single phase rotary converter?=

Ans. It is not self-starting.

=Ques. What feature of operation is inherent in a rotary converter?=

Ans. A rotary converter is a "reversible machine."

That is to say, if it be supplied with direct current of the proper voltage at its commutator end, it will run as a direct current motor and deliver alternating current to the collector rings. While this feature is sometimes taken advantage of in starting the converter from rest, the machine is not often used permanently in this way, its commercial application being usually the conversion of alternating currents into direct currents.

=Ques. How does a rotary converter operate when driven by direct current?=

Ans. The same as a direct current motor, its speed of rotation depending upon the relation existing between the strength of the field and the direct current voltage applied.

If the field be weak with respect to the armature magnetism resulting from the applied voltage, the armature will rotate at a high speed, increasing until the conductors on the armature cut the lines of force in the field so as to develop a voltage which will be equal to that applied.

Again, if the field be strong with respect to the armature magnetism, resulting from the applied voltage, the armature will rotate at a low speed. If, therefore, it be desired to operate the converter in this manner and maintain an alternating current of constant frequency, the speed of rotation must be kept constant by supplying a constant voltage not only to the brushes pressing on the commutator, but also to the terminals of the field winding.

=Ques. How does it operate with alternating current drive?=

Ans. The same as a synchronous motor.

=Ques. What is the most troublesome part and why?=

Ans. The commutator, because of the many pieces of which it is composed and the necessary lines along which it is constructed, its peripheral speed must be kept within reasonable limits.

=Ques. What should be the limit of the commutator speed?=

Ans. The commutator speed, or tangential speed at the brushes should not exceed 3,000 feet per minute.

=Ques. Name another limitation necessary for satisfactory operation.=

Ans. The pressure between adjacent commutator bars should not exceed eight or ten volts.

If the commutator bars be made narrow in order to obtain the necessary number for the desired voltage with the minimum circumference and therefore low commutator speed, the brushes employed to collect the current are liable to require excessive width in order to provide the proper cross section and yet not cover more than two bars at once.

=Ques. How can the commutator speed be kept within reasonable limits, other than by reducing the width of the commutator bars?=

Ans. By using alternating current of comparatively low frequency.

For a rotary converter delivering 500 volt direct current, the proper frequency for the alternating current circuit has been found to be 25 cycles per second.

=Ques. When a rotary converter is operated in this usual manner on an alternating current circuit, how can the direct current be varied?=

Ans. It may be varied (from zero to a maximum) by changing the value of the alternating pressure supplied to the machine, or it may be altered within a limited range by moving the brushes around the commutator, or in a compound wound converter by changing the amount of compounding.

Under ordinary conditions, varying the voltage developed by changing the voltage at the motor end is not practical, hence the voltage developed can be varied only over a limited range. In addition to this, the voltage developed at the direct current end bears always a certain constant proportion to the alternating current voltage applied at the motor end; this is due to the same winding being used both for motor and generator purposes. In all cases the proportion is such that the alternating current voltage is the lower, being in the single phase and in the two phase converters about .707 of the direct current voltage, and in the three phase converter about .612 of the direct current voltage. It is thus seen that whatever value of direct current voltage be desired, the value of the applied alternating current voltage must be lower, requiring in consequence the installation of step down transformers at the sub-station for reducing the line wire voltage to conform to the direct current pressure required.

=Ques. What is the efficiency of a rotary converter?=

Ans. It may be said to have approximately the same efficiency as that in the average of the same output, although in reality the converter is a trifle more efficient on account of affording a somewhat shorter average path for the current in the armature, reducing in consequence the resistance loss and the armature reaction.

=Ques. May a converter be overloaded more than a dynamo of the same output, and why?=

Ans. Yes, because there is usually less resistance loss in the armature of the converter than in the armature of the dynamo.

Thus, a two phase converter may be overloaded approximately 60 per cent., and a three phase converter may be overloaded about 30 per cent. above their respective outputs if operated as dynamos.

=Ques. Describe how a converter is started.=

Ans. There are several methods any one of which may be employed, the choice in any given case depending upon which of them may best be followed under the existing conditions.

If it be found advisable to start the converter with direct current, the same connections would be made between the source of the direct current and the armature terminals on the commutator side of the converter as would be the case were a direct current shunt motor of considerable size to be started; this naturally means that a starting rheostat and a circuit breaker will be introduced in the armature circuit.

The shunt field winding alone is used, and this part of the wiring may be made permanent if, as is usually the case, the same source of direct current is used normally for separate field excitation.

The direct current may be derived from a storage battery, from a separate converter, or from a motor generator set installed in the sub-station for the purpose.

An adjustable rheostat will, of course, be connected in the field circuit for regulation. Before starting the converter, however, it is necessary to do certain wiring between the terminals on the collector side of the machine and the alternating current supply wires, in order that the change over from direct current motive power to alternating current motive power may be made when the proper phase relations are established between the alternating current in the supply wires and the alternating current in the armature winding of the converter.

In order that proper phase relations exist, the armature of the converter must rotate at such a speed that each coil thereon passes its proper reversal point at the same time as the alternating current reverses in the supply wires. This speed may be calculated by doubling the frequency of the supply current and then dividing by the number of pole pieces on the converter, but a far more accurate method of judging when the converter is in step or in synchronism with the supply current consists in employing incandescent lamps as shown in fig. 2,872.

=Ques. How is a polyphase converter started with alternating current?=

Ans. This may be done by applying the alternating pressure directly to the collector rings while the armature is at rest. There need be no field excitation; in fact the field windings on the separate pole pieces should be disconnected from each other before the alternating voltage is applied to the armature, else a high voltage will be induced in the field windings which may prove injurious to their insulation. The passage of the alternating current through the armature winding produces a magnetic field that rotates about the armature core, and induces in the pole pieces eddy currents, which, reacting on the armature, exert a sufficient torque to start the converter from rest and cause it to speed up to synchronism.

=Ques. How much alternating current is required to start a polyphase converter?=

Ans. About 100 per cent. more than that required for full load.

=Ques. How may this starting current be reduced?=

Ans. Transformers may be switched into circuit temporarily to reduce the line wire voltage until the speed become normal.

In conjunction with this method, the method of synchronizing shown in fig. 2,872 may be used, thus, in starting, there is an alternating current between the brushes which pulsates very rapidly, but when synchronism is approached, the pulsations become less rapid until finally with the converter in step with the alternator the pulsations entirely disappear.

The light given by the lamps thus connected indicates accurately the condition of affairs at any one time, varying from a rapidly fluctuating light at the beginning to one of constant brilliancy at synchronism.

=Ques. If the armature of the starting motor have a starting resistance, how must this be connected?=

Ans. It should be connected in series with the armature inductors before the alternating voltage is applied.

As the motor increases in speed, the starting resistance is gradually short circuited until it is entirely cut out of circuit.

NOTE.--Some converters are provided with a small induction motor for starting mounted on an iron bracket cast in the converter frame, and whose shaft is keyed to that of the converter. Allowing for a certain amount of slip in the induction motor, the field of this machine must possess a less number of magnet poles than the converter in order to enable the latter machine to be brought to full synchronism. To start the induction motor, it is simply necessary to apply to its field terminals the proper alternating voltage. The bracket, and therefore the motor, is usually mounted outside the armature bearing on the collector side of the converter.

=Ques. Describe the usual wiring for the installation of a rotary converter in a sub-station.=

Ans. Commencing at the entrance of the high pressure cables, first there is the wiring for the lightning arresters, then for the connection in circuit of the high tension switching devices, from which the conductors are led to bus bars, and thence to the step down transformers.

On a three phase system the transformers should be joined in delta connection, as a considerable advantage is thereby gained over the star connection, in that should one of the transformers become defective, the remaining two will carry the load without change except more or less additional heating. Between the transformers and rotary converter the circuits should be as short and simple as possible, switches, fuses, and other instruments being entirely excluded. The direct current from the converter is led to the direct current switchboard, and from there distributed to the feeder circuits.

=WATTMETER ERROR FOR A LOAD OF 1,000 VOLT-AMPERES= (For a lag of 1 degree in the pressure coil)

+------------+----------+-------+-------------------+ | | | |Error of indication| |Power factor|True watts| Error | in per cent | | | | | of true value | +------------+----------+-------+-------------------+ | 1. | 1,000 | .3 | 0.03 | | .9 | 900 | 7.6 | 0.85 | | .8 | 800 | 10.5 | 1.31 | | .7 | 700 | 12.5 | 1.78 | | .6 | 600 | 13.9 | 2.32 | | .5 | 500 | 15.1 | 3.02 | | .4 | 400 | 15.9 | 3.98 | | .3 | 300 | 16.6 | 5.54 | | .2 | 200 | 17.1 | 8.55 | | .1 | 100 | 17.3 | 17.30 | +------------+----------+-------+-------------------+

NOTE.--In the iron vane type instrument when used as a wattmeter, the current of the series coil always remains in perfect phase with the current of the circuit, provided series transformers are not introduced. The error, then, is entirely due to the lag of the current in the pressure coil, and this error in high power factor is exceedingly small, increasing as the power factor decreases. In the above table it should be noted that the value of the error as distinguished from the per cent. of error, instead of indefinitely increasing as the power factor diminishes, rapidly attains a maximum value which is less than 2 per cent. of the power delivered under the same current and without inductance. It should also be noted that the above tabulation is on the assumption of a lag of 1 degree in the pressure coil. The actual lag in Wagner instruments for instance, is approximately .085 of a degree, and the error due to the lag of the pressure coil in Wagner instruments is, therefore, proportionally reduced from the figures shown in the above tabulation.

=Ques. In large sub-stations containing several rotary converters how are they operated?=

Ans. Frequently they are installed to receive their respective currents from the same set of bus bars; that is, they may be operated as alternating current motors in parallel. They are also frequently operated independently from single bus bars, but very seldom in series with each other.

=Ques. How may the direct current circuit be connected?=

Ans. In parallel.

NOTE.--In motor testing, by the methods illustrated in the accompanying cuts, it is assumed that the motor is loaded in the ordinary way by belting or direct connecting the motor to some form of load, and that the object is to determine whether the motor is over or under loaded, and approximately what per cent. of full load it is carrying. All commercial motors have name plates, giving the rating of the motor and the full load current in amperes. Hence the per cent. of load carried can be determined approximately by measuring the current input and the voltage. If an efficiency test of the apparatus be required, it becomes necessary to use some form of absorption by dynamometer, such as a Prony or other form of brake. The output of the motor can then be determined from the brake readings. The scope of the present treatment is, however, too limited to go into the subject of different methods of measuring the output of the apparatus, and is confined rather to methods of measuring current input, voltage, and watts. The accuracy of all tests is obviously dependent upon the accuracy of the instruments employed. Before accepting the result obtained by any test, especially under light or no load, correction should be made for wattmeter error. See table of wattmeter error on page 2,075.

=Ques. What provision should be made against interruption of service in sub-stations?=

Ans. There should be one reserve rotary converter to every three or four converters actually required.

=Ques. Why does a rotary converter operate with greater efficiency, and require less attention than does a dynamo of the same output?=

Ans. There is less friction, and less armature resistance, the latter because the alternating current at certain portions of each revolution passes directly to the commutator bars without traversing the entire armature winding as it does in a dynamo; there is no distortion of the field and consequently no sparking, or shifting of the brushes, since the armature reaction resulting from the current fed into the machine and that due to the current generated in the armature completely neutralizes each other.

=What electrical difficulty is experienced with a rotary converter?=

Ans. Regulation of the direct current voltage.

=Ques. How is this done?=

Ans. It can be maintained constant only by preserving uniform conditions of inductance in the alternating current circuit, and uniform conditions in the alternator.

While changes in either of these may be compensated to a certain extent by adjustment of the field strength of the converter, they cannot be entirely neutralized in this manner; it is therefore necessary that both the line circuit and the alternator be given attention if the best results are to be obtained from the converter.

=Ques. What mechanical difficulty is experienced with rotary converters?=

Ans. Hunting.

=Ques. What is the cause of this?=

Ans. It is due to a variation in frequency.

The inertia of the converter armature tends to maintain a constant speed; variations in the frequency of the supply circuit will cause a displacement of phase between the current in the armature and that in the line wires, which displacement, however, the synchronizing current strives to decrease. The synchronizing current, although beneficial in remedying the trouble after it occurs, exerts but little effort in preventing it, and many attempts have been made to devise a plan to eliminate this trouble.

NOTE.--Three phase motor test; polyphase wattmeter method. This is identical with the test of fig. 2,882, except that the wattmeter itself combines the movement of the two wattmeters. Otherwise the method of making the measurements is identical. If the power factor be known to be less than 50 per cent., connect one movement so as to give a positive deflection; then disconnect movement one and connect movement two so as to give a positive deflection. Then reverse either the pressure or current leads of the movement, giving the smaller deflection, leaving the remaining movement with the original connections. The readings now obtained will be the correct total watts delivered to the motor. If the power factor be known to be over 50 per cent., the same methods should be employed, except that both movements should be independently connected to give positive readings. An unloaded induction motor has a power factor of less than 50 per cent., and may, therefore, be used as above for determining the correct connections. For a better understanding of the reasons for the above method of procedure, the explanation of the two wattmeter method, fig. 2,882, should be read. The power factor may be calculated as explained under fig. 2,882. Connect as shown in fig 2,882. The following check on connection may be made. Let the polyphase induction motor run idle, that is, with no load. The motor will then operate with a power factor less than 50 per cent. The polyphase meter should give a positive indication, but if each movement be tried separately one will be found to give a negative reading, the other movement will give a positive reading. This can be done by disconnecting one of the pressure leads from the binding post of one movement. When the power factor is above 50 per cent. then both movements will give positive deflection.

=Ques. What are the methods employed to prevent hunting?=

Ans. 1, the employment of a strongly magnetized field relative to that developed by the armature; 2, a heavy flywheel effect in the converter; 3, the increasing of the inductance of the armature by sinking the windings thereon in deep slots in the core, the slots being provided with extended heads; and 4, the employment of damping devices or amortisseur winding on the pole pieces of the converter.

=Ques. What method is the best?=

Ans. The damping method.

The devices employed for the purpose are usually copper shields placed between or around the pole pieces, although in some converters the copper is embedded in the poles, and in others it is made simply to surround a portion of the pole tips.

In any case its action is as follows: The armature rotating at a variable speed has a field developed therein which is assumed to be also rotating at a variable speed; the magnetism of this rotary field induces currents in the copper which, however, react on the armature and oppose any tendency toward a further shifting of the magnetism in the armature and therefore prevent the development of additional currents in the copper. Since copper is of low resistance, the induced currents are sufficient in strength to thus dampen any tendency toward phase displacement, and so exert a steadying influence upon the installation as a whole.

=Electrical Measuring Instruments.=--In the manufacture of most measuring instruments, the graduations of the scale are made at the factory, by comparing the deflections of the pointer with voltages as measured on standard apparatus. The voltmeters in most common use have capacities of 5, 15, 75, 150, 300, 500 and 750 volts each, although in the measurement of very low resistances such as those of armatures, heavy cables, or bus bars, voltmeters having capacities as low as .02 volt are employed.

The difference between the design of direct current voltmeters of different capacities lies simply in the high resistance joined in series with the fine wire coil. This resistance is usually about 100 ohms per volt capacity of the meter, and is composed of fine silk covered copper wire wound non-inductively on a wooden spool.

In the operation of an instrument, if the pointer when deflected do not readily come to a position of rest owing to friction in the moving parts, it may be aided in this respect by gently tapping the case of the instrument with the hand; this will often enable the obstruction, if not of a serious nature, to be overcome and an accurate reading to be obtained.

=Ques. Describe a two scale voltmeter.=

Ans. In this type of instrument, one scale is for low voltage readings and the other for high voltage readings; on these scales the values of the graduations for low voltages are usually marked with red figures, while those for high voltages are marked with black figures. A voltmeter carrying two scales must also contain two resistances in place of one; a terminal from each of these coils must be connected with a separate binding post, but the remaining terminal of each resistance is joined to a wire which connects through the fine wire coil with the third binding post of the meter. The two first mentioned binding posts are usually mounted at the left hand side of the meter and the last mentioned binding post and key at the right hand side.

The resistance corresponding to the high reading scale is composed of copper wire having the same diameter as that constituting the resistance for the low reading scale, but as the capacity of the former scale is generally a whole number of times greater than that of the latter scale, the resistances for the two must bear the same proportion.

=Ques. How is a two scale voltmeter connected?=

Ans. In the connection of a two scale voltmeter in circuit, the single binding post is always employed regardless of which scale is desired. If, then, the voltage be such that it may be measured on the low reading scale, the other binding post employed is that connected to the lower of the two resistances contained within; if, however, the pressure be higher than those recorded on the low reading scale, the binding post connected to the higher of the two resistances contained within is used.

NOTE.--Three phase alternator load test. By means of the connection shown in fig. 2,888, readings of armature current and field amperes can be obtained with any desired load. The field current can be varied also so as to maintain constant armature voltage irrespective of load; or the field current may be kept constant and the armature voltage allowed to vary as the load increases. The connections may also be used to make a temperature test on the alternator by loading it with an artificial load. In some cases after the alternator is installed the connection may be used to make a temperature test, using the actual commercial load the alternator is furnishing.

Inasmuch as the capacities of the scales are usually marked on or near the corresponding binding posts, there will generally be no difficulty in selecting the proper one of the two left hand binding posts.

=Ques. How is a two scale voltmeter connected when the binding posts are not marked?=

Ans. If only an approximate idea is possessed of the voltage to be measured, it is always advisable to connect to the binding post corresponding to the high reading scale of the meter in order to determine if the measurement may not be made safely and more accurately on the low reading scale. In any case, some knowledge must be had of the voltage at hand, else the high reading portion of the instrument may be endangered.

Too much care cannot be taken to observe these precautions whenever the voltmeter is used, for the burning out or charring of the insulation either in the fine wire coil or in the high resistance of the meter by an excessive current, is one of the most serious accidents that can befall the instrument.

If a voltmeter has been subjected to a voltage higher than that for which it was designed, yet not sufficiently high to injure the insulation, but high enough to cause the pointer to pass rapidly over the entire scale, damage has been done in another way. The pointer being forced against the side of the case in this manner, bends it more or less and so introduces an error in the readings that are afterward taken.

The same damage will be done if the meter be connected in circuit so the current does not pass through it in the proper direction, although in this case the pointer is not liable to be bent so much as when it is forced to the opposite side of the meter by an abnormal current, since then it has gained considerable momentum which causes a severer impact. The extent of the damage may be ascertained by noting how far away from the zero mark the pointer lies when no current is passing through the instrument. If this distance be more than two-tenths of a division, the metal case enclosing the working part should be removed and the pointer straightened by the careful use of a pair of pinchers.

=Ques. What should be noted with respect to location of instruments?=

Ans. If they be placed near conductors carrying large currents, the magnetic field developed thereby will produce a change in the magnetism of the instruments and so introduce an error in the readings.

=Ques. How should portable instruments be wired?=

Ans. The wires must be firmly secured to the supports on which they rest, so as to reduce the possibility of their being pulled by accident, and so causing the instruments to fall.

A fall or a rough handling of the meter at once shows its effect on the readings, for as much harm is done as would result from a similar treatment of a watch.

The hardened steel pivots used in all high grade voltmeters are ground and polished with extreme care so as to secure and maintain a high degree of sensitiveness. The jewels on which the moving parts revolve are of sapphire, and they too must necessarily be made with skill and carefulness; if, therefore, the jewels become cracked and the pivots dulled by careless handling, the meter at once becomes useless as a measuring instrument.

=Ques. How should readings be taken?=

Ans. The deflection of the pointer should be read to tenths of a division; this can be done with considerable accuracy, especially after a little practice.

For very accurate results, a temperature correction should be applied to compensate the effect which the temperature of the atmosphere has upon the resistance of the meter when measurements are being taken. In ordinary station practice the temperature correction is negligible, being for resistance corresponding to the high scale in first class meters, less than one-quarter of 1 per cent. for a range of 35 degrees above or 35 degrees below 70 degrees Fahrenheit.

=Ques. What attachment is sometimes provided on station voltmeters used for constant pressure service?=

Ans. A normal index.

=Ques. What precaution must be taken in connecting station voltmeters?=

Ans. Care must be taken to guard against any short circuiting of the voltmeter, which, would mean a short circuiting of the generator, and as a result the probable burning out of its armature.

The high resistance of the voltmeter prevents any such occurrence when it is connected in the proper way, but should one side of the circuit be grounded to the metal case or frame of the meter, a careless handling of the lead connected with the other side of the circuit would produce the result just mentioned.

=Ques. Why do station voltmeters indicate a voltage slightly lower than actually exists across the leads?=

Ans. Since they are usually connected permanently in circuit; a certain amount of heat is developed in the wiring of the instrument.

The effect of this heat increases the voltmeter resistance and consequently reduces the current below that which otherwise would pass through the meter; since the deflections of the pointer are governed by the strength of the current, station voltmeters invariably indicate a voltage slightly lower than that which actually exists across their leads.

NOTE.--=Checking up of a recording wattmeter.= This may conveniently be done by noting the deflections at short intervals on an ammeter connected in circuit, and also the readings on the dial of the recording wattmeter during this period. If this test be continued for an appreciable time, the product of the pressure in volts, the current in amperes, and the time in hours, should equal the number of watthours recorded on the counters of the dial.

NOTE.--=Transformer testing.= In the early days of transformer building, before the commercial wattmeter had been perfected, leakage or exciting current was the criterion of good design. After the introduction of the wattmeter, core loss became the all important factor, and for a long time the question of leakage current was lost sight of. With the introduction of silicon steel, leakage or exciting current again assumed prominence. Keeping in mind the fact that all characteristics of a transformer are of more or less importance, it is essential that the user of such apparatus have at hand the necessary facilities for making tests of all such variable quantities. The tests which all users of transformers should make, are given in this chapter.

=Ques. Can direct current be measured by an alternating current voltmeter?=

Ans. Yes.

NOTE.--=Transformer copper loss test.= The usual and best method of obtaining copper losses is to separately measure the primary and secondary resistance and calculate from these the primary and secondary copper losses. For general diagram of connections and discussion of the drop method, see fig. 2,875. The current should be kept well within the load current of the transformer to avoid temperature rise during the test. In other words, the resistance of the coil is the voltage across its terminals divided by the current. The resistance of the primary coil can be measured similarly. The copper loss in watts in each coil will then be the product of the resistance and the square of the rated current for that coil. The total copper loss will be the sum.

=Ques. What would be the effect of placing a direct voltmeter across an alternating current circuit, and why?=

Ans. There would be no deflection of the pointer owing to the rapid reversals of the alternating current.

=Ques. What are the usual capacities of alternating current voltmeters?=

Ans. They are 3, 7.5, 10, 12, 15, 20, 60, 75, 120, 150, 300 and 600 volts, but these capacities may each be increased by the use of a multiplier.

=Ques. How are station voltmeters usually attached to the switchboard?=

Ans. They are usually bolted to the switchboard by means of four iron supports mounted on the back of the instrument; two of these are fastened near each side of the case.

Under certain conditions, however, as in paralleling of alternators, it is convenient to have the alternating current voltmeter mounted on a swinging bracket at the side of the switchboard. The voltmeter may then be swung around in any desired direction so as to enable the attendant to keep informed of the voltage while switching in each additional alternator.

=Ques. How should an ammeter be operated to get accurate readings, and why?=

Ans. It should be cut out of circuit except while taking a reading, because of the error introduced by the heating effect of the current.

In an ammeter having a capacity of 50 amperes, the error thus introduced will be less than 1 per cent. if connected continuously in circuit with a current not exceeding three-quarters this capacity.

An ammeter of 100 amperes capacity may be used indefinitely in circuit with less than 1 per cent. error up to one-half its capacity, and for five minutes at three-quarters capacity without exceeding the 1 per cent. limit.

The 150 scale ammeter may be left in circuit for an indefinite length of time at one-third its full capacity, and for three minutes at one-half its full capacity, with a negligible error.

Ammeters of 200 and of 300 ampere capacities must not continuously carry more than one-quarter of these loads respectively if the readings are to have an accuracy within 1 per cent. nor more than one-half these respective number of amperes for three minutes if the same degree of accuracy be desired.

In order to cut or shunt the ammeter out of circuit when not in use, it is customary when wiring the instrument in place, to introduce a switch as a shunt across it; this switch is kept closed except when a measurement is being taken.

When currents larger than 300 amperes have to be measured, ammeter shunts are generally employed, although ammeters up to 500 amperes capacity are manufactured.

=Ques. What is used in place of instrument shunts for high pressure alternating current measurements?=

Ans. Instrument transformers.

=Ques. What important attention should be periodically given to measuring instruments?=

Ans. They should be frequently tested by comparison with standards that are known to be correct.

Electrical measuring instruments, owing to the nature of their construction and the conditions under which they must necessarily be used, are subject to variations in accuracy. This feature is an annoying one on account of the difficulty of detecting it; a meter may, as far as appearances go, be in excellent working order and yet give readings which are not to be relied upon.

Ridiculous as it may appear, the average station attendant may frequently be seen straining his eyes to read to tenths of a division on the scale of a meter which, if subjected to test, would show an inaccuracy of over 2 per cent.

In testing a meter, by comparing it with a standard, in order to obtain the best results there should be one man at each meter so that simultaneous readings may be taken on both instruments, and the man at the standard meter should maintain the voltage constant while a reading is being taken, by means of a rheostat in the field circuit of the generator supplying the current.

Each meter should be checked or calibrated at five or six approximately equidistant points over its scale; the adjustable resistance being varied each time to give a deflection on the standard meter of an even number of divisions and the deflection on the other meter recorded at whatever it may be. Having obtained the necessary readings, the calculation of the constant or multiplying factor of the meter undergoing test is next in order.

This may best be shown by taking an actual case in which a 150 scale voltmeter is being tested to determine its accuracy. The data and calculations are as follows:

Readings on Readings on Constant standard meter meter tested 150 149.2 150 ÷ 149.2 = 1.005 125 125.0 125 ÷ 125.0 = 1.000 100 98.9 100 ÷ 98.9 = 1.011 75 73.6 75 ÷ 73.6 = 1.019 50 50.0 50 ÷ 50.0 = 1.000 25 24.8 25 ÷ 24.8 = 1.008 ------ 6.043

Average constant for six readings, 6.043 ÷ 6 = 1.007.

It may be stated in general that before taking the readings for this test, the zero position of the pointer on the meter tested should be noted, and if it be more than two-tenths of a division off the zero mark, the case of the meter should be removed and the pointer straightened.

Furthermore, it will be noticed from the readings here recorded that the test is started at the high reading end of the scale; this is done in order that the pointer may gradually be brought up to this spot, by slowly cutting out of circuit the adjustable resistance, and thus show whether or not the pointer has a tendency to stick at any part of the scale. If the meter seem to be defective in this respect, it should be remedied either by bending the pointer or scale, or by renewing one or both of the jewels, before the comparison with the standard is commenced.

It is obvious from the readings recorded for the 150 scale voltmeter, that as compared with the corresponding deflections of the standard, the former are a trifle low.

In order to determine for each observation how much too low they are, it is necessary to divide each reading on the standard by the corresponding reading on the meter tested. The result is the amount by which a deflection of this size on the meter tested must be multiplied in order to obtain the exact reading. This multiplier is called a constant, and as shown, a constant is determined for each of the six observations.

The average constant for the six readings is then found, and this is taken as the constant for the meter as a whole; that is, whenever this 150-scale voltmeter is used, each reading taken thereon must be multiplied by 1.007 in order to correct for its inaccuracy.

The most convenient and systematic way of registering the constant of a meter is to write it, together with the number of the meter and the date of its calibration, in ink on a cardboard tag and loop the same by means of a string to the handle or some other convenient part of the meter.

NOTE.--=Transformer polarity test.= A test of importance in the manufacture of transformers, and sometimes necessary for the user, is the so called banking or polarity test. The transformers from any particular manufacturer have the leads brought out in such a manner that a transformer of any size can be connected to primary and secondary lines in a given order without danger of blowing the fuses due to incorrect connections. All manufacturers of transformers, however, do not bank transformers in the same way, so that it is necessary in placing transformers of different makes to test for polarity. This is done as shown in fig. 2,906. One transformer is selected as a standard and the leads of the second transformer connected as indicated in the diagram. If the transformers be 1,100-2,200 volts to 110-220, two 110 volt lamps are connected in the secondaries of the transformers as indicated, while the primary of the transformer is connected across the line. In transformers built for two primary and two secondary voltages, it is necessary to test each primary and each secondary. The diagram shows the method of connecting one 2,200 volt coil and one 110 volt coil to the transformer to be tested. When the primary circuit of the transformer under test is closed, and if the secondary leads of the 110 volt coil under test be brought out of the case properly, the two 110 volt lamps should be brightly illuminated. If, on the other hand, the two 110 volt terminals have been reversed, no current will flow through the lamps. If these two terminals be found to be brought out correctly, transfer the secondary leads of the transformer under test to the second 110 volt coil. Upon closing the primary circuit, the lamp should again be brightly illuminated. Repeat this process with each of the secondary coils and the other primary coil, and if the lamps show up brightly in every case on closing the primary circuit, all leads have been properly brought out. If on any tests the lamps do not light up brightly, the leads on the transformer must be so changed as to produce the proper banking.

=Ques. What are the usual remedies applied to a voltmeter to correct a 3 or 4 per cent. error?=

Ans. They consist of straightening the pointer, varying the tension of the spiral springs, renewing the jewels in the bearings, altering the value of the high resistance, and, in the case of a direct current instrument, strengthening the permanent magnet.

=Ques. How is the permanent magnet strengthened?=

Ans. After detaching it from the instrument, wrap around several turns of insulated wire, and pass through this wire for a short time 3 or 4 amperes of direct current in such a direction as to reinforce the magnet magnetism.

=Ques. How may the value of the high resistance of a voltmeter be altered?=

Ans. Determine the resistance of the voltmeter and add or subtract, according as the reading is high or low, a certain length of wire whose resistance is in per cent. of the voltmeter resistance the same as the per cent. of error.

NOTE.--The complete calibration of a two scale voltmeter does not, as might be supposed, necessitate that the readings on both scales be checked with standards, for since the resistance corresponding to the one scale is always some multiple of the resistance of the other, the constants of the two scales are proportional. For instance, if S = the reading at the end of the high scale of the voltmeter; S^{1} = the reading at the end of the low scale of the voltmeter; R = the resistance in the meter corresponding to the high scale; R^{1} = the resistance in the meter corresponding to the low scale; K = the constant for the high scale, and K^{1} = the constant for the low scale. Then

SK ÷ R = S^{1}K^{1} ÷ R^{1}

from which

K^{1} = SKR ÷ S^{1}R

That is to say, if the respective resistances corresponding to the two scales be known, and the constant of the high scale be determined by comparison with a standard, then by aid of these known values and the maximum readings on the two scales, the constant of the low scale may be calculated. It is also possible to calculate the constant of the high scale if the constant of the low scale be known, together with the values of the resistances corresponding to the two scales; for from the equation previously given.

K = RS^{1}K^{1} ÷ R^{1}S

=Ques. What is a frequent cause of error in an alternating current meter, and why?=

Ans. The deterioration of its insulation, which permits the working parts of the instrument coming in contact with the surrounding metal case.

A convenient method of testing for deterioration of insulation is shown in fig. 2,905.

=How to Test Generators.=--In the operation of electrical stations, many problems dealing with the generators installed therein can be readily solved by the aid of characteristic curves, which bear a relation to the generators similarly as do indicator diagrams to steam engines.

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Part M

In steam engineering, a man who did not fully understand the method of taking an indicator diagram would be considered not in touch with his profession, and in electrical engineering the same would be true of one ignorant of the method of obtaining characteristic curves.

The necessary arrangement or connection of the generator from which it is desired to obtain a characteristic curve, consists in providing a constant motive power so that the machine may be run at a uniform speed, and when the field magnets of the generator are separately excited the field current from the outside source must also be maintained constant, preferably by a rheostat connected in the field of the auxiliary exciting machine. It is also necessary in every case that means be provided for varying the main current of the generator step by step from zero to maximum. This may best be done by employing a water rheostat, as shown in fig. 2,909.

=Ques. What instruments are needed in making a test of dynamo characteristics?=

Ans. A voltmeter, ammeter, speed indicator, the usual switches and rheostats.

=Ques. How is the apparatus connected?=

Ans. It is connected as shown in fig. 2,910.

=Ques. Describe the test.=

Ans. Having completed the preliminaries as in fig. 2,910, the test should be started with the main circuit of the generator open. Then, in the case of the shunt machine, the speed should be made normal and the field rheostat adjusted until the voltmeter reading indicates the rated voltage of the machine at no load and readings taken. The electrodes of the water rheostat should be adjusted for maximum resistance and main circuit closed, and a second set of readings taken. Several sets of readings are taken, with successive reductions of water rheostat resistance. The results are then plotted on coordinate paper giving the characteristic curve shown in fig. 2,908.

=Ques. What does the characteristic curve (fig. 2,911) show?=

Ans. An examination of the curve shows that the highest point of the curve occurs at no load or 0 amperes; that as the current is increased, the voltage drops, first slightly to the point B and then rapidly until the point E is reached, when any further lowering of resistance in the main circuit to increase the current causes not only a rapid decline in the voltage but also of the current until both voltage and current become approximately zero.

In some generators, a very slight current results even when the terminals of the machine are actually short circuited; that is, due to residual magnetism in the pole pieces, the lower portion of the curve often terminates, not exactly at zero, but at a point some distance along the current line.

The working portion of the curve is from A to C, at which time the machine is supplying a fairly constant voltage. From C to E shows a critical condition of affairs, while the straight portion D O represents the unstable part of the curve caused by the field current being below its proper value.

The position of the point C determines the maximum power the machine is capable of developing, being in this case (47.5 × 25) ÷ 746 = 1.59 horse power.

=Ques. How may the commercial efficiency of a generator be determined?=

Ans. To obtain the commercial efficiency, the input and output must be found for different loads.

The input may be found by running the generator as a motor at its rated speed, loading it by means of a Prony brake. The generator must be stripped of all belting or other mechanical connections, supplied with its normal voltage and full load current, and the pressure of the Prony brake upon its armature shaft or pulley adjusted until the rated speed of the armature is obtained. The data thus obtained is substituted in the formula.

2π L W R input in brake horse power = ---------- (1) 33,000

in which

L = length of Prony brake lever; W = pounds pull at end of lever; R = revolutions per minute.

The output or electrical horse power for the same load is easily calculated from the formula

amperes × volts output in electrical horse power = --------------- (2) 746

After obtaining value for (1) and (2) the commercial efficiency for the load taken is obtained from the formula

output commercial efficiency = ------ (3) input

Having obtained the commercial efficiency, the difference between the ideal 100 per cent. and the efficiency found will be due to certain losses in the generator. These losses may be classified as

1. Mechanical. 2. Electrical.

The mechanical losses are the friction of the bearings and brushes, and air friction. The electrical losses consist of the eddy current loss, hysteresis loss, armature resistance loss, and field resistance loss.

In testing for these losses, the generator to be tested should be belted to a calibrated motor which latter machine should preferably be of the constant pressure, shunt wound type.

The friction of the bearings and belt of the generator are determined together by raising the brushes off its commutator and running it at the rated speed by means of the calibrated motor.

The amount of power as ascertained from the calibration curve of the motor for the voltage and current used therein when driving the generator as just explained, is a measure of these two losses. The power thus used is practically constant at all loads and is about 2 per cent. of that necessary to drive the generator at full load.

The friction of the brushes can very conveniently be determined next by lowering them on the commutator and giving them the proper tension.

The increase in power resulting from the greater current that will now be taken by the motor to run the dynamo at its rated speed, will be a measure of this loss. In general, its value will be about .5 per cent. of the total power required to drive the dynamo at full load, and this also will remain constant at all loads.

The friction of the air upon the moving armature of the dynamo cannot be determined experimentally, but theoretically this loss is small and may be estimated as .5 per cent.; it is also constant at all loads.

The core loss may be determined experimentally by exciting the field magnets of the dynamo with the normal full load field current through the magnet coils, and noting the increase of power required by the motor to maintain the rated speed of the dynamo thus excited under no load, over that necessary under the same conditions with no field excitation. This increase of power will be the value of the core loss. The core loss is approximately 3 per cent. of the power required to operate the dynamo at full load, and it is constant at varying loads. If it be desired to divide the core loss into its component parts, it is necessary also to run the dynamo under the same conditions as before with field excitation but at half its rated speed. If, then,

H = the power lost in hysteresis at rated speed, E = the power lost in eddy currents at rated speed, T = the power lost in hysteresis and eddy currents at rated speed, S = the power lost in hysteresis and eddy currents at half speed.

there may be formed the two following equations:

H E T = H + E, and S = --- + ---, 2 2

from which the elimination of H will give E = 2T - 4S.

The value of the eddy current loss thus found will be about 1½ per cent., and constant at all loads.

Having previously ascertained the power lost in both eddy currents and hysteresis, and knowing now the power lost in eddy currents alone, it is easy to find that lost in hysteresis by simply subtracting the latter known value from the former. The value of the hysteresis loss is therefore approximately 1½ per cent., and it is constant at different loads.

There yet remains to be determined the armature resistance loss and the field resistance loss. As for the calibrated motor, this may be disconnected from the dynamo, as it need not be used further in the test.

The armature resistance is the resistance of the armature winding of the dynamo, between the commutator bars upon which press the positive and negative brushes. Assume that the value of the armature resistance be known, call this value R ohms, together with that of the full load armature current, which is also known and which call I amperes, this is sufficient data for calculating the armature resistance loss at full load. It is evident that to force the full load current I through the armature resistance R will require a pressure of R volts, and that the watts lost in doing so will be the voltage multiplied by the current. The armature resistance is consequently

IR × I = I^{2}R watts

or, expressed in horse power it is

I^{2}R ÷ 746

At full load it is usually about 2 per cent. of the total power required to drive the generator fully loaded. The armature resistance loss varies in proportion to the load, in fact, as the last expression shows, it increases as the square of the armature current.

The field resistance loss is calculated in the same manner as just explained for the armature resistance loss, it being equal in horse power to the square of the full load field current multiplied by the resistance of the field winding and divided by 746. In a shunt dynamo it is practically constant at 2 per cent. of the total power at full load, but in a series or in a compound generator it will vary in proportion to the load.

HAWKINS PRACTICAL LIBRARY OF ELECTRICITY

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=ELECTRICAL GUIDE, NO. 1= Containing the principles of Elementary Electricity, Magnetism, Induction, Experiments, Dynamos, Electric Machinery.

=ELECTRICAL GUIDE, NO. 2= The construction of Dynamos, Motors, Armatures, Armature Windings, Installing of Dynamos.

=ELECTRICAL GUIDE, NO. 3= Electrical Instruments, Testing, Practical Management of Dynamos and Motors.

=ELECTRICAL GUIDE, NO. 4= Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage Batteries.

=ELECTRICAL GUIDE, NO. 5= Principles of Alternating Currents and Alternators.

=ELECTRICAL GUIDE, NO. 6= Alternating Current Motors, Transformers, Converters, Rectifiers.

=ELECTRICAL GUIDE, NO. 7= Alternating Current Systems, Circuit Breakers, Measuring Instruments.

=ELECTRICAL GUIDE, NO. 8= Alternating Current Switch Boards, Wiring, Power Stations, Installation and Operation.

=ELECTRICAL GUIDE, NO. 9= Telephone, Telegraph, Wireless, Bells, Lighting, Railways.

=ELECTRICAL GUIDE, NO. 10= Modern Practical Applications of Electricity and Ready Reference Index of the 10 Numbers.

=Theo. Audel & Co., Publishers. 72 FIFTH AVENUE=, =NEW YORK.=

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