Hawkins Electrical Guide v. 07 (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

LV. ALTERNATING CURRENT SYSTEMS

The facility with which alternating current can be transformed from one voltage to another, thus permitting high pressure transmission of electric energy to long distances through small wires, and low pressure distribution for the operation of lighting systems and motors, gives a far greater variety of systems of transmission and distribution than is possible with direct current.

Furthermore, when the fact that two phase current can be readily transformed into three phase current, and these converted into direct current, and vice versa, by means of rotary converters and rectifiers, is added to the advantages derived by the use of high tension systems, it is apparent that the opportunity for elaboration becomes almost unlimited. These conditions have naturally tended toward the development of a great variety of systems, employing more or less complicated circuits and apparatus, and although alternating current practice is still much less definite than direct current work, certain polyphase systems are now being generally accepted as representing the highest standards of power generation, transmission and distribution.

A classification of the various alternating current systems, to be comprehensive, should be made according to several points of view, as follows:

1. With respect to the arrangement of the circuit, as

a. Series; b. Parallel; c. Series parallel; d. Parallel series.

2. With respect to transformation, as

Transformer;

3. With respect to the mode of transmitting the energy, as

a. Constant pressure; b. Constant current.

4. With respect to the kind of current, as

a. Single phase { two wire; { three wire;

b. Monocyclic

{ four wire; c. Two phase { three wire; { five wire;

{ six wire; { three wire; { four wire; d. Three phase { star connection; { delta connection; { star delta connection; { delta star connection;

e. Multi-phase { of more than { three phases;

5. With respect to transmission and distribution, as

a. Frequency changing; b. Phase changing; c. Converter; d. Rectifier.

In order to comprehend the relative advantages of the various alternating current systems, it is first necessary to understand the principle of vector summation.

~Vector Summation.~--This is a simple geometrical process for ascertaining the pressure at the free terminals of alternating current circuits. The following laws should be carefully noted:

1. If two alternating pressures which agree in phase are connected together in series, the voltage at the free terminals of the circuit will be equal to their arithmetical sum, as in the case of direct currents.

When there is phase difference between the two alternating pressures, connected in series, the following relation holds:

2. The value of the terminal voltage will differ from their arithmetical sum, depending on the amount of their phase difference.

When there is phase difference, the value of the resultant is conveniently obtained as explained below.

~Ques. How are vector diagrams constructed for obtaining resultant electric pressure?~

Ans. On the principle of the parallelogram of forces.

~Ques. What is understood by the parallelogram of forces?~

Ans. It is a graphical method of finding the resultant of two forces, according to the following law: If two forces acting on a point be represented in direction and intensity by adjacent sides of a parallelogram, their resultant will be represented by the diagonal of the parallelogram which passes through the point.

Thus in fig. 2,124, let OA and OB represent the intensity and direction of two forces acting at the point O, Draw AC and BC, respectively parallel to OB and OA, completing the parallelogram, then will OC, the diagonal from the point at which the forces act, represent the intensity and direction of the resultant, that is, of a force equivalent to the combined action of the forces OA and OB, these forces being called the components of the force OC.

~Ques. Upon what does the magnitude of the resultant of two forces depend?~

Ans. Upon the difference in directions in which they act, as shown in figs 2,125 to 2,128.

~Ques. Is the parallelogram of forces applied when the difference in direction or "phase difference" of two forces is 90 degrees?~

Ans. It is sometimes more conveniently done by calculation according to the law of the right angle triangle.

According to this principle, if two alternating pressures have a phase difference of 90 degrees they may be represented in magnitude and direction by the two sides of a right angle triangle as OA and OB in fig. 2,129; then will the hypotenuse AB represent the magnitude and direction of the resultant pressure. That is to say, the resultant pressure

AB = √((OA)² + (OB)²) (1)

EXAMPLE.--A two phase alternator is wound for 300 volts on one phase and 200 volts on the other phase, the phase difference being 90°. If one end of each winding were joined so as to form a single winding around the armature, what would be the resultant pressure?

By calculation, substituting the given values in equation (1),

Resultant pressure = √(300² + 200²) = √(130,000) = 360.6 volts.

This is easily done graphically as in fig. 2,129 by taking a scale, say, 1" = 100 volts and laying off OA = 3" = 300 volts, and at right angles OB = 2" = 200 volts, then by measurement AB = 3.606" = 360.6 volts.

~Ques. When the two pressures are equal and the phase difference is 90°, is it necessary to use equation (1) to obtain the resultant?~

Ans. No. The resultant is obtained by simply multiplying one of the pressures by 1.41.

This is evident from fig. 2,130. Here the two pressures OA and OB are equal as indicated by the dotted arc. Since they act at right angles, OB is drawn at 90° to OA. According to the equation of the right angle triangle, the resultant AB = √(1² + 1²) = √2̅ = 1.4142 which ordinarily is taken as 1.41.

This value will always represent the ratio between the magnitude of the resultant and the two component forces, when the latter are equal, and have a phase difference of 90 degrees.

~Forms of Circuit.~--Alternating current systems of distribution may be classed, with respect to the kind of circuit used, in a manner similar to direct current systems, that is, they may be called series, parallel, series parallel, or parallel series systems, as shown in figs. 2,131 to 2,134.

~Series Circuits.~--These are used in arc lighting, and series incandescent lighting, a constant current being maintained; also for constant current motors and generators supplying secondary circuits.

Several forms of constant current alternator, analogous to the Thompson-Houston and Brush series arc dynamos, have been introduced. In the design of such alternators self-induction and armature reaction are purposely exaggerated; so that the current does not increase very much, even when the machine is short circuited. With this provision, no regulating device is required.

An objectionable feature is that the voltage of a constant current alternator will rise very high if the circuit be opened, because it is then relieved of inductance drop and armature reaction.

To guard against a dangerous rise of voltage, a film cut out or equivalent device is connected to the terminal of each machine so that it will short circuit the latter if the voltage rise too high.

~Ques. What advantage have constant current alternators over constant current dynamos?~

Ans. The high pressure current is delivered to the external circuit without a commutator, hence there is no sparking difficulty.

The above relates to the revolving field type of alternator. There are, however, alternators in which the armature revolves, the current being delivered to the external circuit through collector rings and brushes. This type of alternator, it should be noted, is for moderate pressures, and moreover there is no interruption to the flow of the current such as would be occasioned by a tangential brush on a dynamo in passing from one commutator segment to the next.

In the revolving field machine, though the armature current be of very high pressure, the field current which passes through the brushes and slip rings is of low pressure and accordingly presents no transmission difficulties.

~Ques. State a disadvantage.~

Ans. Some source of direct current for field excitation is required.

~Ques. In a constant current series system, upon what does the voltage at the alternator depend?~

Ans. The number of devices connected in the circuit, the volts required for each, and the line drop.

~Parallel Circuits.~--These are used for constant pressure operation. Such arrangement provides a separate circuit for each unit making them independent so that they may vary in size and each one can be started or stopped without interfering with the others. Parallel circuits are largely used for incandescent lighting, and since low pressure current is commonly used on such circuits they are usually connected to step down transformers, instead of direct to the alternators.

~Parallel Series Circuits.~--Fig. 2,137 shows the arrangement of a parallel series circuit and the pressure conditions in same. Such a circuit consists of groups of two or more lamps or other devices connected in parallel and these groups connected in series.

Such a circuit, when used for lighting, obviously has the disadvantage that if a lamp filament breaks, the resistance of the group is increased, thus reducing the current and decreasing the brilliancy of the lamps. This arrangement accordingly does not admit of turning off any of the lights.

~Series Parallel Circuits.~--The arrangement of circuits of this kind is shown in fig. 2,134; they are used to economize in copper since by joining groups of low pressure lamps in series they may be supplied by current at correspondingly higher pressure.

Thus, if in fig. 2,134, 110 volt, ½ ampere lamps be used, the pressure on the mains, that is, between any two points as A and B would be 110 × 3 = 330 volts. Each group would require ½ ampere and the five groups ½ × 5 = 2½ amperes.

~Transformer Systems.~--Nearly all alternating current systems are transformer systems, since the chief feature of alternating current is the ease with which it may be transformed from one pressure to another. Accordingly, considerable economy in copper may be effected by transmitting the current at high pressure, especially if the distance be great, and, by means of step down transformers, reducing the voltage at points where the current is used or distributed.

Ordinarily and for lines of moderate length, current is sent out direct from the alternator to the line and transformed by step down transformers at the points of application.

With respect to the step down transformers, there are two arrangements:

1. Individual transformers; 2. One transformer for several customers.

Individual transformers, that is, a separate transformer for each customer is necessary in rural districts where the intervening distances are great as shown in fig. 2,139.

~Ques. What are the objections to this method of distribution?~

Ans. It requires the use of small transformers which are necessarily less efficient and more expensive per kilowatt than large transformers. The transformer must be built to carry, within its overload capacity, all the lamps installed by the customer since all may be used occasionally.

Usually, however, only a small part of the lamps are in use, and those only for a small part of the day, so that the average load on the transformer is a very small part of its capacity. Since the core loss continues whether the transformer be loaded or not, but is not paid for by the customer, the economy of the arrangement is very low.

In the second case, where one large transformer may be placed at a distribution center, to supply several customers, as in fig. 2,140, the efficiency of the system is improved.

~Ques. Why is this arrangement more efficient than when individual transformers are used?~

Ans. Less transformer capacity is required than with individual transformers.

~Ques. Why is this?~

Ans. With several customers supplied from one transformer it is extremely improbable that all the customers will burn all their lamps at the same time. It is therefore unnecessary to install a transformer capable of operating the full load, as is necessary with individual transformers.

~Ques. Does the difference in transformer capacity represent all the saving?~

Ans. No; one large transformer is more efficient than a number of small transformers.

~Ques. Why?~

Ans. The core loss is less.

For instance, if four customers having 20 lamps each were supplied from a single transformer, the average load would be about 8 lamps, and at most not over 10 or 15 lamps, and a transformer carrying 30 to 35 lamps at over load would probably be sufficient. A 1,500 watt transformer would therefore be larger than necessary. At 3 per cent. core loss, this gives a constant loss of 45 watts, while the average load of 8 lamps for 3 hours per day gives a useful output of 60 watts, or an all year efficiency of nearly 60 per cent., while a 1,000 watt transformer would give an all year efficiency of 67 per cent.

For long distance transmission lines, the voltage at the alternator is increased by passing the current through a step up transformer, thus transmitting it at very high pressure, and reducing the voltage at the points of distribution by step down transformers as in fig. 2,141.

~Ques. In practice, would such a system as shown in fig. 2,141 be used?~

Ans. If the greatest economy in copper were aimed at, a three phase system would be used.

The purpose of fig. 2,141 is to show the importance of the transformer in giving a flexibility of voltage, by which the cost of the line is reduced to a minimum.

~Ques. Does the saving indicated in fig. 2,141 represent a net gain?~

Ans. No. The reduction in cost of the transmission is partly offset by the cost of the transformers as well as by transformer losses and the higher insulation requirements.

Every case of electric transmission presents its own problem, and needs thorough engineering study to intelligently choose the system best adapted for the particular case.

~Single Phase Systems.~--There are various arrangements for transmission and distribution classed as single phase systems. Thus, single phase current may be conveyed to the various receiving units by the well known circuit arrangements known as series, parallel, series parallel, parallel series, connections previously described and illustrated in figs. 2,131 to 2,134.

Again single phase current may be transmitted by two wires and distributed by three wires. This is done in several ways, the simplest being shown in fig. 2,143.

~Ques. Under what conditions is the arrangement shown in fig. 2,143 desirable?~

Ans. This method of treating the neutral wire is only permissible where there is very little unbalancing, that is, where the load is kept practically the same on both sides of the neutral.

~Ques. What advantage is obtained by three wire distribution?~

Ans. The pressure at the alternator can be doubled, which means, for a given number of lamps, that the current is reduced to half, the permissible drop may be doubled, the resistance of the wires quadrupled, and their cost reduced nearly 75 per cent.

~Ques. What modification of circuit A (fig. 2,143), should be made to allow for unbalancing in the three wire circuit?~

Ans. An auto-transformer or "balance coil" as it is sometimes called should be used as at B.

This is a very desirable method of balancing when the ratio of transformation is not too large.

~Ques. For what service would the system shown in fig. 2,143 be suitable?~

Ans. For short distance transmission, as for instance, in the case of an isolated plant because of the low pressure at which the current is generated.

The standard voltages of low pressure alternators are 400, 480, and 600 volts.

~Ques. In practice are single phase alternators used as indicated in fig. 2,143?~

Ans. Alternators are wound for one, two or three phases. Three phase machines are more commonly supplied and in many cases it will pay to install them in preference to single phase, even if they be operated single phase temporarily.

For a given output, three phase machines are smaller than single phase and the single phase load can usually be approximately balanced between the three phases. Moreover, if a three phase machine be installed, polyphase current will be available in case it may be necessary to operate polyphase motors at some future time.

Standard three phase alternators will carry about 70 per cent. of their rated kilowatt output when operated single phase, with the same temperature rise.

~Ques. How are three phase alternators used for single phase circuits?~

Ans. The single phase circuit is connected to any two of the three phase terminal leads.

~Ques. What form of single phase system should be used where the transmission distance is considerable?~

Ans. The current should be transmitted at high pressure, a step down transformer being placed at each distribution center to reduce the pressure to the proper voltage to suit the service requirements as shown in fig. 2,146.

Thus, if 110 volt lamps be used on the three wire circuit, the pressure between the two outer wires would be 220 volts. A transformation ratio of say 10:1 would give 2,220 volts for the primary circuit. The current required for the primary with this ratio being only .1 that used in the secondary, a considerable saving is effected in the cost of the transmission line as must be evident.

With the high pressure alternator only one transformation of the current is needed, as shown at the distribution end.

In place of the high pressure alternator, a low pressure alternator could be used in connection with a step up transformer as shown in fig. 2,147, but there would be an extra loss due to the additional transformer, rendering the system less efficient than the one shown in fig. 2,146. Such an arrangement as shown in the fig. 2,147 might be justified in the case of a station having a low pressure alternator already in use and it should be desired to transmit a portion of the energy a considerable distance.

~Ques. How could the system shown in fig. 2,147 be made more efficient than that of fig. 2,146?~

Ans. By using a high pressure alternator in order to considerably increase the transmission voltage.

Thus, a 2,200 volt alternator and 1:10 step up transformer would give a line pressure of 22,000 volts, which at the distribution end could be reduced, to 220 volts for the three wire circuit, using a 100:1 step down transformation.

~Ques. Would this be the best arrangement?~

Ans. No.

~Ques. What system would be used in practice for maximum economy?~

Ans. Three phase four wire.

~Ques. What are the objections to single phase generation and transmission?~

Ans. It does not permit of the use of synchronous converters, self-starting synchronous motors, or induction motor starting under load. It is poorly adapted to general power distribution, hence it is open to grave objections of a commercial nature where there exists any possibility of selling power or in any way utilizing it for general converter and motor work.

~Ques. For what service is it desirable?~

Ans. For alternating current railway operation.

There are advantages of simplicity in the entire generating, primary, and secondary distribution systems for single phase roads. These advantages are so great that they justify considerable expense, looked at from the railway point of view only, the single phase system throughout may be considered as offering the most advantage.

~Ques. What are the objectionable features of single phase alternators?~

Ans. This type of alternator has an unbalanced armature reaction which is the cause of considerable flux variation in the field pole tips and in fact throughout the field structure.

In order to minimize eddy currents, such alternators must accordingly be built with thinner laminations and frequently poorer mechanical construction, resulting in increased cost of the machine. The large armature reaction results in a much poorer regulation than that obtained with three phase alternators, and an increased amount of field copper is required, also larger exciting units. These items augment the cost so that the single phase machine is considerably more expensive than the three phase, of the same output and heating.

~Ques. What factor increases the difficulties of single phase alternator construction?~

Ans. The difficulties appear to increase with a decrease in frequency.

The adoption of any lower frequency than 25 cycles may result in serious difficulties in construction for a complete line of machine, especially those of the two or four pole turbine driven type where the field flux is very large per pole.

~Monocyclic System.~--In this system, which is due to Steinmetz, the alternator is of a special type. In construction, there is a ~main~ single phase winding an auxiliary or ~teaser~ winding connected to the central point of the main winding in quadrature therewith.

The teaser coil generates a voltage equal to about 25 per cent. of that of the main coil so that the pressure between the terminals of the main coil and the free end of the teaser is the resultant of the pressure of the two coils.

By various transformer connections it is possible to obtain a practically correct three phase relationship so that polyphase motors may be employed.

In this system, two wires leading from the ends of the single phase winding in the alternator supply single phase current to the lighting load, a third wire connected to the end of the teaser being run to points where the polyphase motors are installed as shown in fig. 2,150.

The monocyclic system is described at length in the chapter on alternators, Guide No. 5, pages, 1,156 to 1,159.

~Two Phase Systems.~--A two phase circuit is equivalent to two single phase circuits. Either four or three wire may be employed in transmitting two phase current, and even in the latter instance the conditions are practically the same as for single phase transmission, excepting the unequal current distribution in the three wires. Fig. 2,151 shows a two phase four wire system.

~Ques. For what service is the system shown in fig. 2,151 desirable?~

Ans. It is adapted to supplying current for lighting and power at moderate or short distances.

Either 110 or 220 volts are ordinarily used which is suitable for incandescent lighting and for constant pressure arc lamps, the lamps being connected singly or two in pairs.

~Ques. Where current for both power and light are obtained from the same source how should the circuits be arranged?~

Ans. A separate circuit should be employed for each, in order to avoid the objectionable drop and consequent dimming of the lights due to the sudden rush of current during the starting of a motor.

Disagreeable fluctuation of the lights are always met with when motors are connected to a lighting circuit and the effect is more marked with alternating current than with direct current, because most types of alternating current motor require a heavy current usually lagging considerably when starting. This not only causes a large drop on the line, but also reacts injuriously upon the regulation of transformers and alternators, their voltage falling much more than with an equal non-inductive load.

~Ques. What voltages are ordinarily used on two phase lines of more than moderate length?~

Ans. For transmission distances of more than two or three miles, pressures of from 1,000 to 2,000 volts or more are employed to economize in copper. For long distance transmission of over fifty miles, from 30,000 to 100,000 volts and over are used.

~Ques. For long distance transmission at 30,000 to 40,000 volts, what additional apparatus is necessary?~

Ans. Step up and step down transformers.

~Ques. Explain the method of transmitting two phase current with three wires.~

Ans. The connections at the alternator are very simple as shown in fig. 2,152. One end of each phase winding is connected by the brushes a and b', to one of the circuit wires, that is to A and B respectively. The other end of each phase winding is connected by a lead across brushes a' and b, to which the third wire C is joined.

The current and pressure conditions of this system are represented diagrammatically in fig. 2,153. The letters correspond to those in fig. 2,152, with which it should be compared.

As shown in the figure each coil is carrying 100 amperes at 1,000 volts pressure. Since the phase difference between the two coils is 90°, the voltage between A and B is √2̅ = 1.414 times that between either A or B and the common return wire C.

The current in C is √2̅ = 1.414 times that in either outside wire A or B, as indicated.

~Ques. How should the load on the two phase three wire system be distributed?~

Ans. The load on the two phases must be carefully balanced.

~Ques. Why should the power factor be kept high?~

Ans. A high power factor should be maintained in order to keep the voltage on the phases nearly the same at the receiving ends.

~Ques. How should single phase motors be connected and what precaution should be taken?~

Ans. Single phase motors may be connected to either or both phases, but in such cases, no load should be connected between the outer wires otherwise the voltages on the different phases will be badly unbalanced.

Fig. 2,154 shows a two phase three wire system, with two wire and three wire distribution circuits, illustrating the connection for lighting and for one and two phase motors.

~Ques. Describe another method of transmission and distribution with two phase current.~

Ans. The current may be transmitted on a four wire circuit and distributed on three wire circuit as in fig. 2,155.

The four wire transmission circuit is evidently equivalent to two independent single phase circuits.

In changing from four to three wires, it is just as well to connect the two outside wires A and B together (fig. 2,152), as it is to connect a´ and b. It makes no difference which two secondary wires are joined together, so long as the other wires of each transformer are connected to the outside wires of the secondary system.

~Ques. For what service is the two phase three wire system adapted?~

Ans. It is desirable for supplying current of minimum pressure to apparatus in the vicinity of transformers. It is more frequently used in connection with motors operating from the secondaries of the transformers.

~Ques. How should the third or common return wire be proportioned?~

Ans. Since the current in the common return wire is 41.4 per cent. higher than that in either of the other wires it must be of correspondingly larger cross section, to keep the loss equal.

~Ques. What is the effect of an inductive load on the two phase three wire system and why?~

Ans. It causes an unbalancing of both sides of the system even though the energy load be equally divided. The self-induction pressure in one side of the system is in phase with the virtual pressure in the other side, thus distorting the current distribution in both circuits.

~Ques. Describe the two phase five wire system.~

Ans. A two phase circuit may be changed from four to five wires by arranging the transformer connections as in fig. 2,158.

As shown, the secondaries of the transformers are joined in series and leads brought out from the middle point of each secondary winding and at the connection of the two windings, giving five wires.

With 1,000 volts in the primary windings and a step down ratio of 10:1, the pressure between A and C and C and E will be 100 volts and between the points and the connections B or D at the middle of the secondary coils, 50 volts.

The pressure across the two outer wires A and E is, as in the three wire system, √2̅ or 1.41 times that from either outer wire to the middle wire C, that is 141 volts.

The pressure across the two wires connected to the middle of the coils, that is, across B and D, is 50 × √2̅ = 70.5 volts.

~Three Phase Systems.~--There are various ways of arranging the circuit for three phase current giving numerous three phase systems.

1. With respect to the number of wires used they may be classified as

a. Six wire; b. Four wire; c. Three wire.

2. With respect to the connections, as

a. Star; b. Delta; c. Star delta; d. Delta star.

The six wire system is shown in fig. 2,160. It is equivalent to three independent single phase circuits. Such arrangement would only be used in very rare instances.

~Ques. How can three phase current be transmitted by three conductors?~

Ans. The arrangement shown in fig. 2,160 may be resolved into three single circuits with a common or grounded return.

When the circuits are balanced the sum of the current being zero no current will flow in the return conductor, and it may be dispensed with, thus giving the ordinary star or ~Y~ connected three wire circuit, as shown in fig. 2,163. The transformation from six to three wires being shown in figs. 2,161 to 2,163.

Fig. 2,166 is a view of an elementary three phase three wire star connected alternator.

~Ques. What are the pressure and current relations of the star connected three wire system?~

Ans. These are shown in the diagram, fig. 2,166 and 2,167.

Assuming 100 amperes and 1,000 volts in each phase winding, the pressure between any two conductors is equal to the pressure in one winding multiplied by √3̅, that is 1,000 × 1.732 = 1,732 volts.

The current in each conductor is equal to the current in the winding, or 100 amperes.

~Ques. Describe the delta connection.~

Ans. In the delta connection, the three phase coils are connected together forming an endless winding, leads being brought out from these points.

Fig. 2,168 shows a delta connected three phase alternator, the pressure and current relation being given in fig. 2,169.

~Ques. What are the pressure and current relations of the delta connected three wire system?~

Ans. They are as shown in fig. 2,169.

Assuming 100 amperes and 1,000 volts in each phase winding, the pressure between any two conductors is the same as the pressure in the winding, and the current in any conductor is equal to the current in the winding multiplied by √3̅, that is 100 × 1.732 = 173.2 amperes, that is, disregarding the fraction, 173 amperes.

~Ques. What are the relative merits of the star and delta connections?~

Ans. The power output of each is the same, but the star connection gives a higher line voltage, hence smaller conductors may be used.

When it is remembered that the cost of copper conductors is inversely as the square of the voltage, the advantage of the ~Y~ connected system can be seen at once.

Assuming that three transformers are used for a three phase system of given voltage, each transformer, star connected, would be wound for 1 ÷ √3̅ = 58% of the given voltage, and for full current.

For delta connection, the winding of each transformer is for 58% of the current. Accordingly the turns required for star connection are only 58% of those required for delta connection.

~Ques. What is the objection to the star connection for three phase work?~

Ans. It requires the use of three transformers, and if anything happen to one, the entire set is disabled.

~Ques. Does this defect exist with the delta connection?~

Ans. No.

One transformer may be cut out and the other two operated at full capacity, that is at ⅔ the capacity of the three.

~Ques. Describe the ~T~ connection.~

Ans. In this method two transformers are used for transforming three phase current. It consists in connecting one end of both windings of one transformer to the middle point of like windings of the other transformer as in fig. 2,170.

~Ques. What is the open delta connection?~

Ans. It is a method of arranging the connections of a bank of three delta connected transformers when one becomes disabled as in fig. 2,171.

~Change of Frequency.~--There are numerous instances where it is desirable to change from one frequency to another, as for instance to join two systems of different frequency which may supply the same or adjacent territory, or, in the case of a low frequency installation, in order to operate incandescent lights satisfactorily it would be desirable to increase the frequency for such circuits. This is done by motor generator sets, the motor taking its current from the low frequency circuit.

Synchronous motors are generally used for such service as the frequency is not disturbed by load changes; it also makes it possible to use the set in the reverse order, that is, taking power from the high frequency mains and delivering energy at low frequency.

~Ques. In the parallel operations of frequency changing sets what is necessary to secure equal division of the load?~

Ans. The relative angular position of the rotating elements of motor and generator must be the same respectively in each set.

~Ques. How is this obtained?~

Ans. Because of the mechanical difficulty of accurately locating the parts, the equivalent result is secured by arranging the stationary element in one of the two machines so that it can be given a small angular shift.

~Transformation of Phases.~--In alternating current circuits it is frequently desirable to change from one number of phases to another. For instance, in the case of a converter, it is less expensive and more efficient to use one built for six phases than for either two or three phases.

The numerous conditions met with necessitate various phase transformations, as

1. Three phase to one phase; 2. Three phase to two phase; 3. Two phase to six phase; 4. Three phase to six phase.

These transformations are accomplished by the numerous arrangements and combinations of the transformers.

~Three Phase to One Phase.~--This transformation may be accomplished by the use of two transformers connected as in fig. 2,175 in which one end of one primary winding is connected to the middle of the other primary winding and the second end of the first primary winding at a point giving 86.6 per cent. of that winding as shown. The two secondary windings are joined in series.

~Three Phase to Two Phase.~--The three phase system is universally used for long distance transmission, because it requires less copper than either the single or two phase systems. For distribution, however, the two phase system presents certain advantages, thus, it becomes desirable at the distribution centers to change from three phase to two phase. This may be done in several ways.

~Ques. Describe the Scott connection.~

Ans. Two transformers are used, one having a 10:1 ratio, and the other, a ½√3̅:1, that is, an 8.66:1 ratio. The connections are arranged as in fig. 2,176.

It is customary to employ standard transformers having the ratios 10:1, and 9:1.

~Ques. What names are given to the two transformers?~

Ans. The one having the 10:1 ratio is called the ~main~ transformer, and the other with the 8.66:1 ratio, the ~teaser~ transformer.

In construction, the transformers may be made exactly alike so that either may be used as main or teaser.

In order that the connections may be properly and conveniently made, the primary windings should be provided with 50% and 86.6% taps.

~Ques. Describe another way of transforming from three to two phases.~

Ans. The transformation may be made by three star connected transformers, proportioning the windings as in fig. 2,177, from which it will be seen that two of the secondary windings are tapped at points corresponding to 57.7 per cent. of full voltage.

~Three Phase to Six Phase.~--This transformation is usually made for use with rotary converters and may be accomplished in several ways. As these methods have been illustrated in the chapter on Converters (page 1,462), it is unnecessary to again discuss them here. Fig. 2,178, below shows the diametrical connection for transforming three phase to six phase.

~Alternating Current Systems.~--The saving in the cost of transmission obtained by using alternating instead of direct current is not due to any difference in the characteristics of the currents themselves, but to the fact that in the case of alternating current very high pressures may be employed, thus permitting a given amount of energy to be transmitted with a relatively small current.

In the case of direct current systems, commutator troubles limit the transmission pressure to about 1,000 volts, whereas with alternating current it may be commercially generated at pressures up to about 13,000 and by means of step up transformers, transmitted at 110,000 volts or more.

~Relative Weights of Copper Required by Polyphase Systems.~--A comparison between the weights of copper required by the different alternating current systems is rendered quite difficult by the fact that the voltage ordinarily measured is not the maximum voltage, and as the insulation has to withstand the strain of the maximum voltage, the relative value of copper obtained by calculation depends upon the basis of comparison adopted.

As a general rule, the highest voltage practicable is used for long distance transmission, and a lower voltage for local distribution. Furthermore, some polyphase systems give a multiplicity of voltages, and the question arises as to which of these voltages shall be considered the transmission voltage.

If the transmission voltage be taken to represent that of the distribution circuit, and the polyphase system has as many independent circuits as there are phases, the system would represent a group of several single phase systems, and there would be no saving of copper. Under these conditions, if the voltage at the distant end be taken as the transmission voltage, and the copper required by a single phase two wire system as shown in fig. 2,180, be taken as the basis of comparison, the relative weights of copper required by the various polyphase systems is given in figs. 2,181 to 2,188.

In the case represented in fig. 2,180, if the total drop on the line be 100 volts, the generated voltage must be 1,100 volts, and the resistance of each line must be 50 ÷ 1,000 = .05 ohms. Calculated on this basis, a two phase four wire system is equivalent to two single phase systems and gives no economy of copper in power transmission over the ordinary single phase two wire system. This is the case also with any of the other two phase systems, except the two phase three wire system.

In this system two of the four wires of the four wire two phase system are replaced by one of full cross section.

The amount of copper required, when compared with the single phase system, will differ considerably according as the comparison is based on the highest voltage permissible for any given distribution, or on the minimum voltage for low pressure service.

If E be the greatest voltage that can be used on account of the insulation strain, or for any other reason, the pressure between the other conductors of the two phase three wire system must be reduced to E ÷ √2̅.

The weight of copper required under this condition is 145.7% that of the single phase copper.

On the basis of minimum voltage, the relative amount of copper required is 72.9% that of the single phase system.

Figs. 2,187 and 2,188 are two examples of three phase four wire systems. The relative amount of copper required as compared with the single phase system depends on the cross section of the fourth wire. The arrangement shown in fig. 2,188, where the fourth wire is only half size, is used only for secondary distribution systems.

~Choice of Voltage.~--In order to properly determine the voltage for a transmission system there are a number of conditions which must be considered in order that the economy of the entire installation shall be a maximum.

The nature of the diversely various factors which affect the problem makes a mathematical expression difficult and unsatisfactory.

~Ques. What is the relation between the cross sectional area of the conductors and the voltage?~

Ans. For a given circuit, the cross sectional area of the conductors, or weight varies inversely as the voltage.

~Ques. Would the highest possible voltage then be used for a transmission line?~

Ans. The most economical voltage depends on the length of the line and the cost of apparatus.

For instance, alternators, transformers, insulation and circuit control and lightning protection devices become expensive when manufactured for very high pressures. Hence if a very high pressure were used, it would involve that the transmission distance be great enough so that the extra cost of the high pressure apparatus would be offset by the saving in copper effected by using the high pressure.

In the case of the longest lines, from about 100 miles up, the saving in copper with the highest practicable voltage is so great that the increase in other expenses is rendered comparatively small.

In the shorter lines as those ranging in length from about one mile to 50 or 75 miles, the most suitable voltage must be determined in each individual case by a careful consideration of all the conditions involved. No fixed rule can be established for proper voltage based on the length, but the following table will serve as a guide:

Usual Transmission Voltages

+----------------------------------------+ | Length of line | Voltage | | in miles | | +------------------+---------------------+ | 1 | 500 to 1,000 | | 1 to 2 | 1,000 to 2,300 | | 2 to 3 | 2,300 to 6,600 | | 3 to 10 | 6,600 to 13,200 | | 10 to 15 | 13,200 to 22,000 | | 15 to 20 | 22,000 to 44,000 | | 20 to 40 | 44,000 to 66,000 | | 40 to 60 | 66,000 to 88,000 | | 60 to 100 | 88,000 to 110,000 | +------------------+---------------------+

~Ques. What are the standard voltages for alternating current transmission circuits?~

Ans. 6,600, 11,000, 22,000, 33,000, 44,000, 66,000, 88,000.

The amount of power to be transmitted determines, in a measure, the limit of line voltage. If the most economical voltage considered from the point of view of the line alone, be somewhere in excess of 13,200, step up transformers must be employed, since the highest voltage for which standard alternators are manufactured is 13,200. In a given case, the saving in conductor by using the higher voltage may be more than offset by the increased cost of transformers, and the question must be determined for each case.

~Ques. What are the standard transformer ratios?~

Ans. Multiples of 5 or 10.

~Mixed Current Systems.~--It is often desirable to transmit electrical energy in the form of alternating current, and distribute it as direct current or vice versa.

Such systems may be classed as mixed current systems. The usual conversion is from alternating current to direct current because of the saving in copper secured by the use of alternating current in transmission, especially in the case of long distance lines. Such conversion involves the use of a rotary converter, motor generator set, or rectifier, according to the conditions of service.

The suburban trolley forms a good example of a mixed system, in which alternating current is generated at the central station and transmitted to sub-stations, where it is transformed to low pressure, and converted into direct current for use on the line. Fig. 2,195 shows the interior of a sub-station of this kind.

~Ques. What direct current pressure is usually employed on traction lines?~

Ans. 500 volts.

~Ques. Mention another important service performed by a mixed system.~

Ans. If the generator furnish alternating current it must be converted into direct current in order to charge storage batteries.

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LVI. AUXILIARY APPARATUS

For the proper control of the alternating current in any of the numerous systems described in the previous chapter, various devices, which might be classed as "auxiliary apparatus," are required. These may be grouped into several divisions, according to the nature of the duty which they perform, as

1. Switching devices;

a. Ordinary switches; b. Oil break switches; c. Remote control switches.

2. Current or pressure limiting devices;

a. Fuses; b. Reactances; c. Circuit breakers; d. Relays.

3. Lightning protection devices;

a. Air gap arresters; b. Multi-gap arresters; c. Horn gap arresters; d. Electrolytic arresters; e. Vacuum tube arresters; f. Choke coils; g. "Static" interrupters.

4. Regulating devices;

a. Induction voltage regulators;

b. Variable ratio transformer regulators { drum type; { dial type;

c. Compensation shunts; d. Pole type regulators; e. Small feeder voltage regulators; f. Automatic voltage regulators; g. Line drop compensators; h. Starting compensators; i. Star delta switches.

5. Power factor regulating devices;

a. Condensers; b. Synchronous condensers.

6. Indicating devices;

{ plunger type; a. Moving iron instruments { inclined coil type; { magnetic vane type;

b. Hot wire instruments;

c. Induction instruments { shielded pole type; { repulsion type;

d. Dynamometers; e. Instrument transformers;

{ commutator type; f. Watthour meters { induction type; { Faraday disc type;

{ synchronous motor type; g. Frequency indicators { resonance type; { induction type;

{ lamp type; h. Synchronism indicators { voltmeter type; { resonance type; { rotating field type;

i. Power factor indicators { wattmeter type; { rotating field type;

j. Ground detectors; k. Earth leakage cut outs; l. Oscillographs.

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LVII. SWITCHING DEVICES

A switch is a piece of apparatus for making, breaking, or changing the connections in an electric circuit.

The particular form and construction of any switch is governed by the electrical conditions under which it must operate.

Since the electric current cannot be ~stopped instantly~ when the circuit in which it is flowing, is broken, ~an arc is formed~ as the switch contacts separate; this tends to burn the contacts, and to short circuit, the severity of such action depending on the voltage and the proximity of the switch terminals. Accordingly in switch design, provision must be made to counteract these tendencies. Thus,

1. The contacts should separate along their entire length, rather than at a point; 2. The terminals should be far enough apart and properly protected to prevent short circuiting of the arcs; 3. The break should be quick; 4. The gap should be surrounded by the proper medium (air or oil) to meet the requirements of the electrical conditions.

A great variety of switches have been introduced to suit the different requirements. Knife switches are used for low pressure service, the multiple break form being used where it is desired to reduce the arcing distance.

~Ques. How should single throw switches be installed?~

Ans. They should open downward so gravity will keep them open.

~Ques. How should double throw switches be installed?~

Ans. Horizontally.

~Ques. What is a plug switch?~

Ans. A switch in which the current is ruptured in a tube enclosed at one end, thereby confining the arc and limiting the supply of air.

They are used on high pressure circuits of from 10,000 to 20,000 volts, for transferring live circuits and for voltmeter and synchronizing circuits where there is very little energy. The usual current capacity is from 4 to 7½ amperes.

~Forms of Break.~--On high pressure circuits there are several types of switch: they are classified with respect to the break, that is to say, according as the break takes place,

1. In open air; 2. In an enclosed air space; 3. Aided by a metal fuse; 4. Aided by a horn; 5. In oil.

~Ques. What is the objection to open air break?~

Ans. The relatively long gap required to extinguish the arc, limiting this form of switch to low or moderate pressure circuits.

The open air arc may cause very high voltage oscillations when the circuit contains inductance and capacity unless the break occur at zero value.

~Ques. What are disconnecting switches?~

Ans. Knife switches in series with other switches so that the apparatus controlled by the latter may be repaired in safety by entirely disconnecting it from the bus bars or live circuit.

Such switches are not intended to rupture the load current.

~Ques. What are the features of the enclosed air break?~

Ans. The switch is more compact than the open air break type, but pressure oscillations are caused on opening the circuit the same as with the open air break, and it is not desirable for heavy current.

~Ques. How is the fuse arranged in the metal fuse break type of switch?~

Ans. It is placed in a tube fitted with powdered carbonate of lime or some other insulating powder.

~Ques. Describe its operation.~

Ans. The moving arm of the switch draws the fuse through the tube, thus opening the circuit without much disturbance.

~Ques. What is the objection to the metal fuse switch?~

Ans. The powder is set flying by the explosion of the arc, which, as it settles, gets into the bearings of any machine that may be in the vicinity.

~Ques. What is a horn break switch?~

Ans. One provided with horn shaped extensions to the contacts, as shown in fig. 2,219.

The arc formed on breaking the circuit, as it travels toward the extremities of the horns, becomes attenuated and is finally ruptured.

~Ques. What are the objections to this type of switch?~

Ans. The considerable space required for the horns and arcs, and the line surges caused by the arc.

Horn switches were used extensively for high pressure alternating current circuits before the introduction of oil switches.

~Oil Switches.~--The extensive use of high pressure currents and alternating current motors and other devices introducing inductance make it necessary to use switches radically different from the ordinary air break types.

The opening of circuits of considerable current value with inductive loads is not possible with old style switches which were quite adequate for the service for which they were designed. These circuits are controlled with ease and certainty by the oil switch.

~Ques. What is an oil switch?~

Ans. One in which the contact is broken under oil.

This type of switch is the one almost universally used on high pressure alternating current circuits, because of the fact that the oil tends to cause the current to break when at its zero value, thus preventing the heavy arcing which would occur with an air break switch, and the consequent surges in the line which are so often the cause of breakdown of the insulation of the system.

~Ques. What is the nature of an oil break?~

Ans. It is not a quick break.

Oscillograph records show that the effect of the oil is to allow the arc to continue during several cycles and then to break the current, usually at the zero point of the wave.

~Remote Control Oil Switches.~--It is desirable in the case of switches on high pressure circuits to locate the parts which carry the high pressure current at some distance from the switchboard in order that they may be operated with safety.

With respect to the manner in which the switches are operated they may be classed, as

1. Hand operated; 2. Power operated.

~Ques. What kind of power is used?~

Ans. Electricity is used in most cases; in some installations, switches are operated by compressed air.

~Ques. For what pressures should remote control switches be used?~

Ans. For pressures above 1,100 volts.

~Ques. Describe the operating mechanism of a remote control, hand, and electrically operated switch.~

Ans. For hand operation, the mechanism between the operating lever and switch proper, consists simply of a system of links and bell cranks. Various shapes of bell crank are used, to permit change in direction or position of the force applied to operate the switch.

~Ques. Name two classes of electrically operated remote control switch.~

Ans. Those operated by solenoids, and those operated by motors.

The solenoid type are closed by the action of a plunger solenoid, and opened either by another solenoid called a "tripping coil" or by gravity. Some examples of remote control are shown in the accompanying illustrations.

~Ques. What indicating devices are used with electrically operated switches?~

Ans. Red and green lamps; ~red~ for closed and ~green~ for open as shown in fig. 2,231.

~Ques. For what service are motor operated switches used?~

Ans. For exceptionally heavy work where the kilowatt rupturing capacity is greater than that for which the other types are suitable.

~Rupturing Capacity of Oil Switches.~--While an oil switch may be designed for a given pressure and to carry a definite amount of current, it should not be understood that the switch will necessarily rupture the amount of normal energy equivalent to its volt ampere rating.

Oil switches are often used on systems with generator capacity of many thousand kilowatts. It is therefore essential that the switches shall be able to break not only their normal current, but also greatly increased current that would flow if a short circuit or partial short circuit occur.

Under short circuit conditions alternators develop instantaneously many times their normal load current, while the sustained short circuit current is approximately two and a half to three times normal, or even higher with turbine alternators. Hence, circuit breakers of the so called instantaneous type must be capable of rupturing the circuit when the current is at a maximum, whereas, non-automatic switches, or circuit breakers with time limit relays will be required to interrupt only the sustained short current circuit. The reason is evident, since the delay in opening the switch allows the current to approach the sustained short circuit conditions.

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LVIII. CURRENT AND PRESSURE LIMITING DEVICES

In any electric installation there must be provided a number of automatic devices to secure proper control. The great multiplicity of devices designed for this purpose may be divided into two general classes, as

1. Current limiting; 2. Pressure limiting.

Because of the heating effect of the current which increases in proportion to the square of the strength of the current, it is necessary to protect circuits with devices which do not allow the current to exceed a predetermined value.

Accordingly fuses, circuit breakers, reactances, etc., are used, each possessing certain characteristics, which render it suitable for particular conditions of service.

For instance, just as in analogy, steam boilers must be protected against abnormal pressures by safety valves, electric circuits must be guarded against excessive voltages by pressure limiting devices, otherwise much damage would occur, such as the burning out of incandescent lamps, grounding of cables, etc.

The control of steam is simple as compared to the electric current, the latter being the more difficult to manage because of its peculiar behaviour in certain respects, especially in the case of alternating current which necessitates numerous devices of more or less delicate construction for safety both to the apparatus and the operator.

~Fuses.~--A fuse is "an electrical safety valve", or more specifically, the actual wire or strip of metal in a cut out, which may be fused by an excessive current, that is to say, by a current which exceeds a predetermined value. A fuse, thus serves to protect a circuit from any harm resulting from an undue overload.

Fuses have been treated at such length in Guide No. 2, Chapter XXV, that very little can be said here, without repetition.

~Ques. What effect have the terminals on a fuse?~

Ans. The current at which a fuse melts may be greatly changed by the size and shape of the terminals.

If near together and large, they may conduct considerable heat from the fuse thus increasing the current required to blow the fuse.

~Ques. What is the objection to large fuses?~

Ans. The discharge of molten metal when the fuse blows is a source of danger.

~Ques. What should be used in place of large fuses?~

Ans. Circuit breakers.

~Ques. What are the objections to fuses in general?~

Ans. The uncertainty as to the current required to blow them; the constant expansion and contraction is liable to loosen the terminal screws when screws are used.

~Ques. What is the advantage of fuses?~

Ans. They form an inexpensive means of protecting small circuits.

~Ques. Describe a plug fuse.~

Ans. It is constructed as shown in fig. 2,239, the fuse wire being visible and stretching between the two metal portions of the plug.

~Ques. What is a cut out fuse?~

Ans. One similar to a simple fuse, but provided with clip contacts as used for knife switch contacts.

The fuse wire is usually contained in a china or porcelain tube, which also serves the purpose of a handle for withdrawing the fuse.

~Ques. What is an expulsion fuse?~

Ans. One in which the fuse is placed in an enclosed chamber with a vent hole.

In operation, when the fuse blows, the hot air and molten metal are expelled through the vent.

~Ques. What is a no arc fuse?~

Ans. A cartridge type fuse, in which the space surrounding the fuse wire is filled with powdered material.

The object of the powdered material is to assist in extinguishing the arc formed when the fuse blows.

~Ques. What is a magnetic blow out fuse?~

Ans. An enclosed fuse which is subject to the action of a magnetic field produced by the current, the magnetic field tending to blow out the arc when fusing occurs.

~Ques. What is a quick break fuse?~

Ans. One having a weight suspended from its center, or springs attached to its ends so that the arc formed at fusing is quickly attenuated and extinguished.

~Ques. What is the disadvantage of a fuse as compared to an oil switch circuit breaker?~

Ans. When a fuse blows, the arc causes oscillations in the line, which cause excessive rise of pressure under certain capacity conditions, whereas this disturbance is reduced to a minimum with an oil switch.

~Ques. What metal is used for fuse wires?~

Ans. Various metals. Ordinary fuse wire is made of lead or an alloy of lead and tin.

~Ques. What is the objection to aluminum?~

Ans. It becomes coated with oxide or sulphide, which acts as a tube tending to retain the metal inside and prevent rupture.

~Ques. What is the objection to copper?~

Ans. Its high fusing point.

~Current Limiting Inductances.~--The great increase in capacity of power stations, for supplying the demands of densely populated centers and large manufacturing districts, together with the decrease in the reactance of modern alternators and transformers due to improvement in design to obtain better regulation, has presented a problem in apparatus protection not contemplated in the earlier days of alternating current distribution. This problem is entirely separate and distinct from that of eliminating the tendency toward short circuit, incident to the high voltages now common in transmission lines. It accepts that all short circuits must occasionally occur and considers only the protection of the connected apparatus against the mechanical forces due to the magnetic stresses of such enormous currents.

~Ques. What means are employed to limit the value of a short circuit current?~

Ans. A current limiting inductance coil (called a reactance) is placed in series with the alternators or transformers.

~Ques. What are its essential features of construction?~

Ans. It consists of bare stranded cable wound around a concrete core and held in place by wooden supports as shown in fig. 2,243.

In order to avoid the prohibitive expense of high voltage insulation, the reactance coil is designed for the low tension circuit. This requirement prohibits the use of a magnetic core which, if economically designed for normal operation, would become saturated at higher densities, or, if designed large enough to avoid saturation at short circuit conditions, would become prohibitive in cost and dimensions.

The elimination of all magnetic material from the construction of the concrete core reactance permits of no saturation, and assures a straight line voltage characteristic at all current loads.

~Ques. Where is the proper location for a current limiting reactance?~

Ans. As near the alternator as possible.

~Ques. Why?~

Ans. To lessen the possibility of a short circuit occurring between the reactance and the alternator.

~Ques. Beside limiting the current, what other service is performed by the reactance?~

Ans. It protects the alternator from high frequency surges coming in from the outside, and limits the current from other machines on the same bus.

~Circuit Breakers.~--The importance of circuit protective devices, commonly called circuit breakers, is fully recognized. The duty of a circuit breaker is to protect the apparatus in an electrical circuit from undesirable effects arising from abnormal conditions, by automatically breaking the circuit. Accordingly a circuit breaker must comprise a switch in combination with electrical control devices designed to act under abnormal conditions in the circuit.

A circuit breaker is a device which ~automatically~ opens the circuit in event of abnormal conditions, in the circuit.

In the design of circuit breakers, there are several methods used to effect the rupturing of the arc between contacts when opened on heavy overload, such as:

1. Magnetic blow out; 2. Thermal break; 3. Carbon break.

In the magnetic blow out type, the arc is extinguished between auxiliary contacts confined by a chute in which the arc is rapidly blown out due to a powerful magnetic field from one or more electromagnets. This type may be used in air or watertight boxes and is peculiarly adapted for service where the arc must be confined.

In a carbon break type, the arc is finally ruptured between carbon break contacts. The breaking of the circuit is accomplished progressively, that is to say, it is done in three stages, by several sets of contact, known respectively as

1. The main contacts; 2. The intermediate contacts; 3. The carbon contacts.

In operation, as the circuit breaker acts to break the circuit, first the ~main~ contacts, separate, then the ~intermediate~ contacts, and finally the ~carbon~ contacts between which the arc is ruptured.

~Ques. What is the object of the intermediate contacts?~

Ans. To prevent the forming of an arc on the main contacts.

~Ques. What is the object of the carbon contacts?~

Ans. First to protect the intermediate contacts by providing a path for the current after the intermediate contacts separate, and 2, to "slow down" the current by means of the considerable resistance of the carbon, thus reducing to a minimum the arc which is formed when the carbon contacts separate.

~Ques. How is the automatic operation of a circuit breaker usually accomplished?~

Ans. Usually through the medium of a solenoid, or electromagnet energized by current from the circuit controlled by the breaker.

The essential features of construction and operation of a circuit breaker is shown in the elementary diagrams, figs. 2,250 to 2,253. ~In construction~ as shown in fig. 2,250 it consists essentially of three sets of contacts, a swinging contact arm which is set in the closed position by the handle operating through the toggle joint, the movement of which is limited in the closing direction by the stop. The latter is made adjustable by an eccentric pin or equivalent. Connected to the toggle is the plunger of the solenoid whose winding is energized by current from the circuit which the circuit breaker is to control.

~In operation~, the circuit is closed by hand by turning the handle downward to the position shown in fig. 2,250, that is as far as it will go.

Since the toggle has passed the center line the arm will be held normally in this position because of the spring action of the contacts. Now, if the current rise above a predetermined limit, the pull exerted by the solenoid will overbalance the tendency of the toggle to remain in the closed position, and pull the two toggle links downward below the center line, drawing the contact arm back and breaking the circuit.

The progressive action which takes place during this operation is shown in figs. 2,250 to 2,253 in which the main contacts separate first, then the intermediate, and finally the carbon contacts as mentioned before.

~Ques. What name is given to this type of circuit breaker?~

Ans. It is called an overload circuit breaker.

~Automatic Features.~--There are three methods of connecting the winding of the solenoid, or trip coil as it is called:

1. In series with the main circuit; 2. In shunt with the main circuit; 3. In shunt with an auxiliary circuit.

The automatic controls arising from these connections give various kinds of protection to the circuit and are known as

1. Overload trip; 2. Underload trip; 3. Low voltage trip; 4. Auxiliary circuit trip.

~Ques. What is the object of the overload trip?~

Ans. It is intended to open the circuit when the current exceeds a predetermined value.

~Ques. What modifications are made in the mechanism shown in the elementary diagrams?~

Ans. Sometimes a latch is used in place of the toggle and a magnet in place of the solenoid as in figs. 2,265 and 2,266.

~Ques. Why is a magnet used in combination with a latch?~

Ans. Because with this arrangement very little movement is required to trip the breaker, and for such conditions, a magnet is more efficient than a solenoid.

~Ques. How does the latch arrangement work?~

Ans. When the proper current is reached, the magnet pulls open the latch and the contact arm of the breaker moves by the force of gravity or other means and opens the circuit.

~Ques. How does the underload trip operate?~

Ans. The same as the overload type except that they operate on a diminution of current instead of an excess.

~Ques. Describe the no voltage trip.~

Ans. The energy for the trip of this breaker is derived from a high resistance or fine wire coil which is arranged to be placed directly across the line, in operation, when the current flowing through the circuit falls below a predetermined value, the energy of the coil is insufficient to counteract the force of a spring, which then trips the breaker.

~Ques. Describe the auxiliary circuit trip.~

Ans. A pressure coil is used which is energized by current from an auxiliary circuit. The coil is only momentarily energized, by push button, relay or other control, as distinguished from the preceding types, in which the coil is constantly energized.

~Ques. What other name is given to the auxiliary circuit trip?~

Ans. It is sometimes called the shunt trip, though ill advisedly so.

~Relays.~--Oil break switches and carbon break circuit breakers are commonly used to open electrical circuits at some given overload and on short circuit. To secure additional protection under a variety of abnormal conditions or to provide for a certain predetermined operation or sequence of operations, relays may be employed.

A relay is defined as: A device which ~opens~ or ~closes~ an ~auxiliary circuit~ under predetermined electrical conditions in the main circuit.

The object of a relay is generally to act as a sort of electrical multiplier, that is to say, it enables a comparatively weak current to bring into operation a much stronger current.

~Ques. For what service are relays largely used?~

Ans. They are employed in connection with high voltage switches where the small amount of energy derived from an ordinary instrument transformer is insufficient for tripping.

The connections between relays and circuit opening devices are usually electrical. Combinations of this nature are extremely flexible since they permit the use of a number of devices, each having a different function, with a single circuit breaker or oil switch as well as with two or more switches, to secure the desired operation and protection.

~Selection.~--In all electrical installations protection of apparatus is important, but in some large central stations this is secondary to continuity of service.

To combine maximum protection without interruptions of service is not always possible, but these requirements can be approximated very closely by the use of reliable and simple controlling or protecting devices if proper care be taken to select the relays suited to the special conditions of the installation. To do this intelligently, a knowledge of the various types of relay is necessary.

There is a multiplicity of types and a classification to be comprehensive, should, as in numerous other cases, be made from several points of view. Accordingly relays may be classified:

1. With respect to the nature of the service performed, as

a. Protective; b. Regulative; c. Communicative.

2. With respect to the operating current, as

a. Alternating current; b. Direct current.

3. With respect to the manner of performing their function, as

a. Circuit opening; b. Circuit closing.

4. With respect to the operating current circuit, as

a. Primary; b. Secondary.

5. With respect to the abnormal conditions which caused them to operate, as

a. Overload; b. Underload; c. Over voltage; d. Low voltage; e. Reverse energy; f. Reverse phase.

6. With respect to the time consumed in performing their function, as

a. Instantaneous (so called); b. Definite time limit; c. Inverse time limit.

7. With respect to the character of its action, as

a. Selective; b. Differential.

8. With respect to whether it acts directly or indirectly on the circuit breaker, as

a. Main; b. Auxiliary.

~Protective Relays.~--These are used to protect circuits from abnormal conditions of voltage, or current, which would be undesirable or dangerous to the circuit and apparatus contained therein.

~Ques. How do protective relays operate?~

Ans. They act in combination with automatic circuit breakers, operating when their predetermined setting has been reached, energizing the trip coil of the circuit breaker and opening the circuit.

Fig. 2,279 shows the principles of relay operation. When the current or pressure in the main circuit reaches the predetermined value at which the protective system should operate, the relay magnet attracts the pivoted contact arm and closes the auxiliary circuit; this permits current to flow from the current source in that circuit and energize the trip coil thus opening the main circuit.

~Regulative Relays.~--This class of relay is used to control the condition of a main circuit through control devices operated by a secondary circuit.

~Ques. For what service are relays of this class employed?~

Ans. They are used as feeder circuit or generator regulators.

~Ques. How do they differ from protective relays?~

Ans. They have differentially arranged contacts, that is to say, arranged for contact on either side of a central or normal position.

~Communicative Relays.~--These are used for signalling in a great variety of ways for indicating the position of switching apparatus or predetermining the condition of electric circuits.

~A. C. and D. C. Relays.~--As here used, the classification refers to the kind of current used on the auxiliary circuit. In some cases direct current is used to energize the trip gear of the circuit breaker or oil switch, and in others, alternating current.

A. C. and D. C. relays are respectively known as circuit opening and circuit closing relays, being later fully described.

~Circuit Opening Relays.~--The duty of a circuit opening relay is to open the ~auxiliary circuit~, usually alternating current, nd thereby cause the oil switch or circuit breaker to be opened by the use of a trip coil in the secondary of a current transformer, or by low voltage release coil.

The trip coil of the breaker is generally shunted by the relay contacts and when the moving contact of the relay disengages from the stationary contact, the current from the transformer which supplies the relay, flows through the trip coil thus opening the breaker. These features of operation are shown in fig. 2,282.

~Ques. Where are circuit opening relays chiefly employed?~

Ans. In places where direct current is not available for energizing the trip coil.

~Ques. What is the objection to alternating current trip coils?~

Ans. They have relatively high impedance and impose a heavy volt ampere load on the transformers.

~Circuit Closing Relays.~--The duty of a circuit closing relay is to close the auxiliary circuit at the time when the predetermined abnormal condition is reached in the primary circuit. The closing of the auxiliary circuit energizes the trip coil and opens the breaker.

~Ques. What kind of current is generally used for the auxiliary circuit of a circuit closing relay?~

Ans. Direct current.

~Ques. At what pressure?~

Ans. From 125 to 250 volts.

~Ques. Where is this current usually obtained?~

Ans. From a storage battery, or from the exciter.

~Ques. For what current are the contacts ordinarily designed?~

Ans. About 10 amperes.

~Primary and Secondary Relays.~--Primary relays are sometimes called series relays as they have the current coils connected directly in series with the line, both on high and low tension circuits.

Secondary relays receive their current supply from the secondary circuits of current transformers. Alternating current relays connected to secondary of pressure transformers and relays with both current and pressure windings are included in this class.

~Ques. What is the usual winding of the coils?~

Ans. The current coils are usually wound for 5 amperes and the pressure coils for 110 volts.

~Ques. What refinement is made in the design of relays and why?~

Ans. Care is exercised to reduce to a minimum the volt ampere load imposed by the relay on the current transformer to permit the use of un-stranded meters and relays upon the same transformer.

The use of circuit opening relays to cut out the trip coil of an oil switch during normal operation, has been described, and in the short time that the trip coil is in circuit, it does not affect the accuracy of the instrument readings. This practice, however, does not apply in the case of curve drawing meters, voltage compensators or other devices which have in themselves sufficient load for separate current transformers. In this connection it should be noted that to obtain accurate instrument and meter readings; the current transformers should not be loaded beyond certain limits which depend upon the volt ampere load and power factor of each of the connected devices.

So great is the variety of combination used and the variations of these factors in their several combinations at different loads and settings, that special consideration of each arrangement is advisable.

~Overload Relays.~--Series relays are connected directly in series with the line and are chiefly used with high pressure oil break switches for overload protection. If current transformers are to be used on the same circuits for other purposes, and have sufficient capacity to admit of adding a relay coil, secondary relays would be more economical; otherwise, the series relays are less expensive.

By means of a specially treated wooden rod, the relay operates a tripping switch, closing a separate tripping circuit, usually 125 or 250 volts direct current. Series relays are essentially the same as secondary relays except in the coil winding and insulation.

~Underload Relays.~--These are similar in construction to low voltage relays but have current instead of pressure windings.

~Over Voltage Relays.~--These are usually of the circuit closing type and are similar to secondary overload relays, but have pressure instead of current windings.

~Low Voltage Relays.~--Relays of this class are in most cases used

[Illustration: FIG. 2,304.--Condit 600 volt, 1,500 ampere single pole back connected type K circuit breaker, ~motor operated~. The mechanical and electrical features of the circuit breakers are no different than when hand operated, the only difference being that the motor is used for the operating means. This motor is so arranged that even should it over travel, due to an accident to the controlling circuit, it cannot produce more than a predetermined strain on the circuit breaker. In other words, after the motor has closed the circuit breaker, further travel of the motor will not result in putting a strain on the operating parts. Suitable motors are supplied for this service, the type of motor varying in accordance with the character of the operating current supplied. The advantage of this type of electrical operation is that it puts very little strain on the switch mechanism, takes very little operating current, allows the use of standard parts, and makes an extremely substantial and flexible structure. Its disadvantage is that it closes slowly, and it must not, therefore, be used in places where quick closing is essential.] for the protection of motors in the event of a temporary weakening or failure of the pressure. They are also used in connection with a low voltage release or shunt trip coil on an oil switch or a circuit breaker.

~Reverse Energy Relays.~--The chief object of this species of relay is to protect the generator. When so used, the overload adjustment is set at the maximum value to give overload protection only at the maximum carrying capacity of the generator and a sensitive reverse protection to prevent a return of energy from the line.

~Reverse Phase Relays.~--This type of relay is used chiefly to prevent damage in case of reversal of leads in reconnecting wiring to two or three phase motors.

~Time Element.~--It is often inconvenient that a circuit breaker should be opened immediately on the occurrence of what may prove to be merely a momentary overload, so that time lag attachments are frequently provided, particularly with relays. These devices, which may form part of the relay or may be quite distinct from it, retard its action until the overload has lasted for a predetermined time--several seconds or more.

~Ques. What should preferably govern the time lag?~

Ans. It should depend on the extent to which the overload is reduced as the time elapses.

~Instantaneous Relays.~--The so called instantaneous relays operate almost instantly on the occurrence of the abnormal condition that they are to control.

There is of course a slight time element comparable with that of an overload circuit breaker, but for practical purposes, the operation may be considered as instantaneous.

~Time Limit Relays.~--Under this classification there are two sub-divisions.

1. Definite time limit; 2. Inverse time limit.

~Ques. Describe the time mechanism of a definite time limit relay.~

Ans. It consists of an air dash pot, and an air diaphragm or equivalent retarding device connected to the contact mechanism.

~Ques. How does it operate?~

Ans. In some designs, when the contacts are released, they descend by gravity against the action of the retarding device thereby making contact a definite interval after the occurrence of the abnormal condition.

~Ques. How does the inverse time limit type operate?~

Ans. The actuating and contact mechanism is attached directly to an air bellows and in operation tends to compress the bellows against the action of a specially constructed escape valve in the latter.

~Ques. Why is the arrangement called inverse time limit?~

Ans. Because the retardation varies inversely with the pressure on the bellows, and therefore inversely with the magnitude of the abnormal condition.

~Ques. What other device may be used to retard the operation?~

Ans. A damping magnet is sometimes used which acts on a disc or drum and which may be adjustable.

~Ques. How is the inverse time element introduced by this arrangement?~

Ans. The retardation is due to eddy currents induced by moving the disc or drum through the magnetic field. The reaction thus induced varies inversely with the magnitude of the force with which the disc or drum is urged through the field and hence inversely with the abnormal condition.

~Ques. What are the ordinary limits of adjustment for inverse time limit relays?~

Ans. From one-half second to 30 seconds, depending upon the time setting and magnitude of the overload current.

A setting of from two to six seconds is ordinarily used, depending upon the requirements. Where selective operation is desired a minimum setting of two seconds is recommended.

~Differential Relays.~--In this type of relay there are two electromagnets. In normal working these oppose and neutralize each other. Should, however, either winding become stronger or weaker than the other, the balance is upset, the magnet energized, and the relay comes into operation.

A modification of such a relay for alternating current is shown in fig. 2,322, from which it will be seen that when the currents are as indicated, the circuit A has the larger pressure induced in it, whereas, should the main current reverse with reference to the shunt current, the circuit B would have the larger induced pressure. [Illustration: FIG. 2,323.--Diagram of modern power house wiring and busses showing location of relays.]

~[1]How to Select Relays.~--The following general information on relays, together with reference to the one line diagram, fig. 2,323, will be of interest and assistance in making a selection from the various relays previously described to meet the requirements of modern power house and sub-station layouts.

[1] NOTE.--As suggested by the General Electric Co.

~Single pole relays~ are used on single phase and on balanced three phase circuits.

~Double pole relays~ are used on ungrounded three phase and on quarter phase.

~Triple pole relays~ are used on three phase grounded neutral and interconnected quarter-phase.

~Circuit closing relays~ are recommended in all cases where a constant source of direct current is available for operating trip coils.

The conditions for which relays have been designed for power circuits may perhaps be best described, by considering a one line diagram from the generator end to the sub-station auxiliary machines and feeders.

Considering first alternating current circuits, the prevailing practice is to make the circuit breakers by which the alternators are connected to the low tension bus non-automatic, in order to insure minimum interruption of alternator service. The chance of trouble in this part of the circuit is remote, but should it occur, the station attendant could generally open the circuit breaker before the machines would be injured.

~Reverse current relays~ of instantaneous or time limit types are often connected to the secondaries of current and of pressure transformers to indicate by lamp or bell any trouble that may occur in the generator circuit.

These relays operate with a low current reversal at full pressure and conversely with a proportionally greater current at voltages less than normal. At zero pressure, the relay would act as an overload one, set for high overload. At zero current, a voltage considerably in excess of normal would be required to operate it.

Specifications sometimes call for automatic generator circuit breakers: in this case definite time limit overload relays are used. They are connected in the secondaries of current transformers and are designed to give the same time delay for all trouble conditions; they allow the defective circuit to be opened, if possible, at a point more remote from the generator than the generator circuit breaker.

When the total generator capacity exceeds the rated rupturing capacity of the circuit breakers, one or more sectionalizing circuit breakers are placed in each bus.

If operating conditions admit, these devices are made non-automatic and are left disconnected except in case of emergency; but if it be necessary for them to be continually in service, they may be made automatic by means of instantaneous overload relays connected to current transformers in the low voltage bus; the relays being adjusted to trip the circuit breaker under short circuit conditions, confining the trouble to one section and preventing the circuit breakers rupturing more than their rated capacity.

Installations with but one bank of power transformers, and without high voltage bus, are provided with automatic circuit breakers operated by an inverse time limit relay.

The relay is connected to the secondaries of current transformers, which in turn are connected in the low voltage side of the power transformer.

Stations with more than one bank of power transformers, a high voltage bus, and high and low voltage circuit breakers, may have both circuit breakers arranged to trip at the same time or one after the other. As in the former case, they are operated from the inverse time limit relay connected in the low voltage side.

In plants in which two or more banks of transformers are operated in parallel between high and low voltage busses, it is desirable to have for each transformer bank, an automatic circuit breaker equipment which will act selectively and disconnect only the bank in which trouble may occur. With a circuit breaker on each side of transformer bank, selective action may be secured in two ways as follows:

1. By means of an instantaneous differential relay connected in the secondaries of current transformers installed on both the high and low voltage sides of each transformer bank.

The relay operates on a low current, reversal on either side of the bank.

2. By means of one inverse time limit, secondary or series relay installed on that side of the transformer bank which is opposite the source of power, the relay being arranged to trip both the high and low voltage circuit breakers.

The first method has the disadvantage of high first cost due to the high voltage current transformers required, but is more positive than the second method and is independent of the number of transformer banks in parallel.

The second method is the less expensive of the two and protects against overloads as well as short circuits in the transformers, but it is less positive and introduces delay in the disconnection of the transformer when trouble occurs. Furthermore, it is not selective when less than three banks are operating in parallel.

The automatic circuit breakers in the outgoing line may be operated from inverse time limit relays connected in the secondaries of current transformers; or in case transformers are not necessary for use with instruments, series high voltage inverse time limit relays connected directly in the line may be used.

Whether to select current transformers with relays insulated for low voltage, or to choose series relays, is a question of first cost and adaptability to service conditions. Below 33,000 volts, the commercial advantages in favor of the series relay are slight, and since it is somewhat difficult to design this device for the large current capacities met with at the lower voltage, it is generally the practice to use the relay with current transformer, because of its operating advantage. This practice, however, is not entirely followed, since some service conditions (described later) make the use of series relays very desirable and practical.

Inverse time limit relays are satisfactory for one, or more than two outgoing lines in parallel as they act selectively to disconnect the defective line only, but installations with only two outgoing lines in parallel have the same load conditions in both lines and selective tripping of the circuit breakers in the defective line is obtained by means of a selective relay acting instantaneously under short circuit conditions only.

The relay design and action is similar to the reverse current relay previously mentioned, and is connected to the secondaries of current transformers in each high voltage line and pressure transformers in the low voltage bus.

In the sub-station, the conditions are the reverse of those in the main station, the incoming lines becoming the source of power.

If there be only one incoming line and no high voltage bus, the line circuit breaker is generally non-automatic. With one incoming line and high voltage bus, the circuits from the service side of the bus are equipped with automatic circuit breakers and relays. These relays and those used for other arrangements of two or more incoming lines in parallel, as well as high and low voltage circuit breakers, are of the same design and are applied in the same manner as for the generating station.

Regarding the relay equipments for auxiliary machines, the same practice is recommended with the generator end of alternating current motor generator sets as with the main generators, the outgoing feeder circuit breakers being tripped from inverse time limit or instantaneous relays.

With several synchronous machines in parallel, the relays are arranged to operate with the least time delay with which it is possible to get selective action, in order to prevent the machines being thrown out of step in event of trouble conditions causing a decrease of voltage.

The various types of induction motor and various conditions under which they are employed, have brought about the development of several types of relay to protect the motors and the apparatus with which they are used.

It is desirable to disconnect a large motor in case of voltage failure, and with conditions requiring either a motor operated, or a solenoid operated circuit breaker, a low voltage relay is used to close the tripping circuit whenever the voltage decreases to, approximately, 50 per cent. below normal.

Up to 550 volts, these relays may be connected across the line, but for higher voltages they are connected to secondaries of pressure transformers. Smaller motors with which hand operated circuit breakers are used, are generally provided with low voltage release attachments that perform the same function as the relay.

Induction motors are sometimes subjected to high voltage conditions and to protect them from injury, high or excess voltage relays are employed to trip the automatic circuit breaker. These relays are of similar design and wired in the same manner as the low voltage relays.

Reverse phase relays have been developed for operating conditions under which a reversal of phase would cause trouble, as for example, in the case of elevator motors.

These are so designed that any phase reversal that would reverse an induction motor, would operate the relay and disconnect the automatic circuit breaker.

The design is based on the principle of the induction motor, and in the case of low voltage motors of limited capacity, the relay may be connected in series in the motor leads. If the voltage or capacity of the motor make this arrangement inexpedient, the relay may be placed in the secondaries of current or pressure transformers connected in the motor leads.

Underload relays are often used to trip the automatic circuit breaker that is placed in the primaries of arc lighting circuits to prevent an abnormal rise of secondary voltage in case of a break in the secondary circuit.

The underload relay is similar in design to the low voltage relay excepting that it acts on a decrease of current.

The problem of protecting induction motors, from injury, that may result from running on single phase, or from an overload, and at the same time permit the motor to be started with the necessarily high starting current that may be greatly in excess of the overload current, has caused the development of the series relay.

This device may be connected in series with the motor leads for voltages up to 2,500; it is designed with an inverse time limit device which may be adjusted to give the desired protection.

The field for relays is more extensive for alternating current than for direct current power circuits, the latter being generally confined to much smaller and simpler systems and areas of distribution, and generally sufficient selective action can be obtained by the use of fuses or circuit breakers arranged with instantaneous trip.

Operating conditions sometimes make it advisable for the generator circuit breakers to open only after the auxiliary and feeder circuit breakers have failed to isolate the trouble.

This is accomplished by using direct current series inverse time limit relays to trip the generator circuit breakers.

Instantaneous reverse current relays are used to trip the machine circuit breaker of battery charging sets, rotaries and motor generator sets to prevent their running as a motor on the charging or direct current end. These relays can act only in case of current reversal.

To prevent serious unbalancing of voltages in Edison three-wire systems causing trouble, differential balance relays are used to trip the circuit breakers on a small percentage of unbalancing.

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LIX. LIGHTNING PROTECTION DEVICES

Lightning protection devices, or lightning arresters, are devices for providing a path by which lightning disturbances or other static discharges may pass to the earth.

Lightning arresters, designed for the protection of transmission lines, must perform this function with a minimum impairment of the insulation of the lines.

In general the construction of lightning arresters comprise

1. Air gaps; 2. Resistances; 3. Inductances; 4. Arc suppressing devices.

~Ques. What are the causes of static charges?~

Ans. They may be caused by sandstorms in dry climates, or may be due to grounds on the high pressure side of a system.

~Ques. What causes high frequency oscillations?~

Ans. They are usually due to lightning discharges in the vicinity of the line.

~Ques. What are the requirements of lightning protection devices?~

Ans. They must prevent excessive pressure differences between line and ground, line and line, and between conductor turns in the electrical apparatus.

~Air Gap Arresters.~--method of relieving any abnormal pressure condition is to connect a discharge air gap between some point on an electric conductor and the ground. The resistance thus interposed between the ground and the conductor is such that any voltage very much in excess of the maximum normal will cause a discharge to ground, whereas at other times the conductor is ungrounded because of the air gap. This forms the principle of air gap arresters.

The single gap while adequate for telegraph line protection, was found insufficient for electric light and power circuits, because since the current in such circuits is considerable and usually at high pressure it would follow the lightning discharge across the gap. Thus the problem arose to devise means for short circuiting the line current resulting in various modifications of gap arrester.

~Multi-gap Arresters.~--The essential elements of an arrester of this type are a number of cylinders spaced with a small air gap between them and placed between the line to be protected and the ground, or between line and line.

In operation, the multi-gap arrester discharges at a much lower voltage than would a single gap having a length equal to the sum of the small gaps. In explaining the action of multi-gaps, there are three things to consider:

1. The transmission of the static stress along the line of the cylinders; 2. The sparking at the gaps; 3. The action and duration of the current which follows the spark, and the extinguishment of the arc.

~Ques. What is a spark?~

Ans. The conduction of electricity by air.

~Ques. What is an arc.~

Ans. The conduction of electricity by vapor of the electrode.

~Distribution of Static Stress.~--The cylinders of the multi-gap arrester act like plates of condensers in series. This condenser function is the essential feature of its operation.

When a static stress is applied to a series of cylinders between line and ground, the stress is immediately carried from end to end.

If the top cylinder be positive it will attract a negative charge on the face of the adjacent cylinder and repel an equal positive charge to the opposite face and so on down the entire row.

The second cylinder has a definite capacity relative to the third cylinder and also to the ground; consequently the charge induced on the third cylinder will be less than on the second cylinder, due to the fact that only part of the positive charge on the second cylinder induces negative electricity on the third, while the rest of the charge induces negative electricity to the ground. Each successive cylinder, counting from the top of the arrester, will have a slightly smaller charge of electricity than the preceding one.

~Sparking at the Gaps.~--The quantity of electricity induced on the second cylinder is greater than on any lower cylinder and its gap has a greater pressure strain across it as shown in fig. 2,357. When the voltage across the first gap is sufficient to spark, the second cylinder is charged to line voltage and the second gap receives the static strain and breaks down. The successive action is similar to overturning a row of ten-pins by pushing the first pin against the second. This phenomenon explains why a given length of air gap concentrated in one gap requires more voltage to spark across it than the same total length made up of a row of multi-gaps.

As the spark crosses each successive gap, the voltage gradient along the remainder readjusts itself.

~How the Arc is Extinguished.~--When the sparks extend across all the gaps the line current will follow if, at that instant, the line pressure be sufficient. On account of the relatively greater line current, the distribution of pressure along the gaps becomes equal, and has the value necessary to maintain the line current arc on a gap.

The line current continues to flow until the voltage of the generator passes through zero to the next half cycle, when the arc extinguishing quality of the metal cylinders comes into action.

The alloy contains a metal of low boiling point which prevents the reversal of the line current. It is a rectifying effect, and before the pressure again reverses, the arc vapor in the gaps has cooled to a non-conducting state.

~Effect of Frequency.~--The higher the frequency of the lightning oscillation, the more readily will the multi-gap respond to the pressure.

Briefly stated, the problem is to properly limit the line current so that the arc may be extinguished; to arrange a shunt circuit so that the series resistance will be automatically cut out if safety demand it on account of a heavy lightning stroke and, while retaining these properties, to make the arrester sensitive to a wide range of frequency.

It should be noted that series resistance limits the rate of discharge of the lightning as well as of the line current. The greater the value of the line current, the greater the number of gaps required to extinguish the arcs.

~Graded Shunt Resistances.~--Any arc is unstable and can be extinguished by placing a properly proportioned resistance in parallel with it. All the minor discharges then pass over the resistances and the unshunted spark gaps, the resistance assisting in opening the line current after the discharge.

Very heavy discharges pass over all the spark gaps, as a path without resistance, but those spark gaps which are shunted by the resistance, open after the discharge.

The line current, after the first discharge is accordingly deflected over the resistances, and limited thereby, the circuit being finally opened by the unshunted spark gaps. The arrangement of shunted resistances is shown in fig. 2,358.

~The Cumulative or "Breaking Back" Effect.~--The graded shunt resistance gives a valuable effect, where the arrester is considered as four separate arresters. This is the "cumulative" or "breaking back" action.

When a lightning strain between line and ground takes place, the pressure is carried down the high resistance H (figs. 2,365 and 2,366), to the series gaps GS, and the series gaps spark over.

Although it may require several thousand volts to spark across an air gap, it requires relatively only a few volts to maintain the arc which follows the spark. In consequence, when the gaps GS spark over, the lower end of the high resistance is reduced practically to ground pressure.

If the high resistance can carry the discharge current without giving an ohmic drop sufficient to break down the shunted gaps GH, nothing further occurs--the arc goes out.

If, on the contrary, the lightning stroke be too heavy for this, the pressure strain is thrown across the shunted gaps, GH, equal in number to the previous set. In other words, the same voltage breaks down both of the groups of gaps, GS and GH, in succession. The lightning discharge current is now limited only by the medium resistance M, and the pressure is concentrated across the gaps, GM.

If the medium resistance cannot discharge the lightning, the gap GM spark, and the discharge is limited only by the low resistance.

The low resistance should take care of most cases but with extraordinarily heavy strokes and high frequencies, the discharge can ~break back~ far enough to cut out all resistance.

In the last steps, the resistance is relatively low in proportion to the number of shunt gaps, GL, and is designed to cut out the line current immediately from the gap, GL. This "breaking back" effect is valuable in discharging lightning of low frequency.

After the spark passes, the arcs are extinguished in the reversed order. The low resistance, L, is proportioned so as to draw the arcs immediately from the gaps, GL. The line current continues in the next group of gaps, GM, until the end of the half cycle of the generator wave.

At this instant the medium resistance, M, aids the rectifying quality of the gaps, GM, by shunting out the low frequency current of the alternator.

On account of this shunting effect the current dies out sooner in the gaps, GM, than it otherwise would.

In the same manner, but to a less degree, the high resistance, H, draws the line current from the gaps, GH.

This current now being limited by the high resistance, the arc is easily extinguished at the end of the first one-half cycle of the alternator wave.

~Ques. What is the difference between arrester for grounded Y and non-grounded neutral systems?~

Ans. The connections are shown in figs. 2,365 and 2,366. The difference in design lies in the use of a fourth arrester leg between the multiplex connection and ground or ungrounded system.

~Ques. Why is the fourth leg introduced?~

Ans. The arrester is designed to have two legs between line and line. If one line become accidentally grounded, the full line voltage would be thrown across one leg if the fourth or ground leg were not present.

On a ~Y~ system with a grounded neutral, the accidentally grounded phase causes a short circuit of the phase and the arrester is relieved of the strain by the tripping of the circuit breaker. Briefly stated, the fourth or ground leg of the arrester is used when, for any reason, the system could be operated, even for a short time with one phase grounded.

~Ques. Describe the multiplex connection.~

Ans. It consists of a common connection between the phase legs of the arrester above the earth connection and provides an arrester better adapted to relieve high pressure surges between lines than would otherwise be possible.

Its use also economizes in space and material for delta and partially grounded or non-grounded ~Y~ systems.

~Horn Gap Arresters.~--A horn gap arrester consists essentially of two horn shaped terminals forming an air gap of variable length, one horn being connected to the line to be protected and the other to the ground usually through series resistance as shown in fig. 2,378.

~Ques. How does the horn gap arrester operate?~

Ans. The arc due to the line current which follows a discharge, rises between the diverging horn and becoming more and more attenuated is finally extinguished.

~Ques. What is the objection to the horn gap on alternating current circuits?~

Ans. The arc lasts too long for synchronous apparatus to remain in step.

~Ques. What provision was made to shorten the duration of the arc?~

Ans. A series resistance was inserted in the arrester circuit as shown in fig. 2,377.

~Ques. What difficulty was caused by the series resistance?~

Ans. With sufficient series resistance to prevent loss of synchronism, the arrester failed to protect the system under severe conditions.

~Ques. With these objections what use was found for the horn gap arrester?~

Ans. It is used as an emergency arrester on some overhead lines, to operate only when a shut down is unavoidable, also for series lighting circuits.

The necessity of service requires that series lightning systems be fully equipped against damage by lightning and similar trouble. The most common disturbances occurring on series circuits are the surges set up by the sudden opening of the loaded circuit. These disturbances are especially severe where circuits are accidentally grounded, due to contact of the wires where they pass through other circuits.

~Ques. How are the spark gaps adjusted?~

Ans. They are set to give a low spark pressure relative to the voltage of the line.

~Ques. Why are horn arresters well suited to protect series lighting circuits against surges?~

Ans. Because the surges are damped out before the arc which forms across the horn gaps is interrupted.

These arcs last for several cycles, since the length of the time of action of the arrester depends upon the lengthening of the arc between the horn gaps, limited by the series resistance.

Since practically all disturbances on lighting circuits are of low frequency, the series resistance can be used with good results; it aids the horn in extinguishing the arc, limits the size of the arc and prevents short circuits occurring during the period of discharge.

~Electrolytic Arresters.~--Arresters of this class are sometimes called aluminum arresters because of the property of aluminum on which their action depends; that is, it depends on the phenomenon that a non-conducting film is formed on the surface of aluminum when immersed in certain electrolytes.

If however, the film be exposed to a higher pressure, it may be punctured by many minute holes, thus so reducing its resistance that a large current may pass. When the pressure is again reduced the holes become resealed and the film again effective.

In construction, the aluminum arrester consists essentially of a system of nested aluminum cup shaped trays, supported on porcelain and secured in frames of heated wood, arranged in a steel tank.

The system of trays is connected between the line and ground, and between line and line, a horn gap being inserted in the arrester circuit which prevents the arrester being subjected to the line voltage except when in action.

The electrolyte is poured into the cones and partly fills the space between the adjacent ones. The stack of cones with the electrolyte between them is then immersed in a tank of oil. The electrolyte between adjacent cones forms an insulation. The oil improves this insulation and prevents the evaporation of the solution.

A cylinder of insulating material concentric with the cone stack is placed between the latter and the steel tank, the object being to improve the circulation of the oil and increase the insulation between the tank and the cone stack. The arrester, as just described consists of a number of cells connected in series.

~Ques. Of what does a single cell consist and what are its characteristics?~

Ans. It consists of two of the cone shaped aluminium trays or plates and an electrolyte, which forms a condenser that will stand about 350 volts before breaking down. When this voltage is exceeded the cell becomes a fairly good conductor of electricity, but as soon as the voltage drops its resistance again resumes a very high value.

~Ques. What is the critical voltage?~

Ans. The voltage at which the current begins to flow freely.

Up to a certain voltage the cell allows an exceedingly low current to flow, but at a higher voltage the current flow is limited only by the internal resistance of the cell, which is very low. A close analogy to this action is found in the well known safety valve of the steam boiler, by which the steam is confined until the pressure rises above a given value, when it is released. On the aluminum plates there are myriads of minute safety valves, so that, if the electric pressure rise above the critical voltage, the discharge takes place equally over the entire surface. It is important to distinguish between the valve action of this hydroxide film and the failure of any dielectric substance.

~Ques. When a cell is connected permanently to the circuit what two conditions are involved?~

Ans. The temporary critical voltage and the permanent critical voltage.

For instance, if the cell have 300 volts applied to it constantly, and the pressure be suddenly increased to, say 325 volts, there will be a considerable rush of current until the film thickness has been increased to withstand the extra 25 volts; this usually requires several seconds. In this case 325 volts is the ~temporary~ critical voltage of the cell.

Similar action will occur at any pressure up to about the ~permanent~ critical voltage, or the voltage at which the film cannot further thicken, and therefore allows a free flow of current.

If the voltage be again reduced to 300 the excess thickness of film will be gradually dissolved, and if it vary periodically between two values, each of which is less than the permanent critical value, the temporary critical voltage will be the higher value. This feature is of great importance as it provides a means of discharging abnormal surges, the instant the pressure rises above the impressed value.

~Ques. How is the number of cells required for a given circuit determined?~

Ans. The number required for a given operating voltage is determined by allowing about 250 to 300 volts per cell.

~Ques. In putting cells in commission how is the electrolyte introduced?~

Ans. It is poured into the aluminum trays and the overflow drawn off at the bottom of the tank.

~Ques. Describe the further operations in putting cells in commission.~

Ans. After putting in the electrolyte it is allowed to stand for a few days until part has evaporated, then the oil is poured over the surface to prevent further evaporation.

~Ques. What action takes place when the trays stand in the electrolyte and cell is disconnected from the circuit?~

Ans. Part of the film deteriorates.

~Ques. What is the nature of the film?~

Ans. The film is composed of two parts, one of which is hard and insoluble, and apparently acts as a skeleton to hold the more soluble part. The action of the cell seems to indicate that the soluble part of the film is composed of gases in a liquid form.

~Ques. What action takes place when a cell which has stood for some time disconnected, is reconnected to the circuit?~

Ans. There is a momentary rush of current which reforms the part of the film which has dissolved.

This current rush will have increasing values as the intervals of rest of the cell are made greater.

Many electrolytes have been studied, but none has been found which does not show this dissolution effect to a greater or lesser extent.

If the cell has stood disconnected from the circuit for some time, especially in a warm climate, there is a possibility that the initial current rush will be sufficient to open the circuit breakers or oil switches. This current rush also raises the temperature of the cell, and if the temperature rise be great, it is objectionable.

When the cells do not stand for more than a day, however, the film dissolution and initial current rush are negligible.

~Ques. What is the object of using horn gaps on electrolytic arresters?~

Ans. The use is threefold: 1, it prevents the arrester being subjected continually to the line voltage; 2, acts as a disconnecting switch to disconnect the arrester from the line for repairs, etc., and 3, acts as a connecting switch for charging.

~Charging of Electrolytic Arresters.~--In electrolytic arresters all electrolytes dissolve the film when the arrester is on open circuit, the extent of the dissolution depending upon the length of time the film is in the electrolyte, and upon its temperature. It is therefore necessary to charge the cells from time to time and thus prevent the dissolution and consequent rush of current which would otherwise occur when the arrester discharges.

~Ques. Describe the charging operation for arresters with grounded circuits.~

Ans. It consists in simply closing simultaneously the three horn gaps so that the full pressure across the cells causes a small charging current to flow and form the films to their normal condition.

~Ques. Describe the charging operation for arresters for non-grounded circuits. ~ Ans. First, the horn gaps are closed for five seconds and opened again to normal position, thus charging the cells of the three line stacks. Second, with the horn gaps still in normal position, the position of the transfer device is reversed and the horn gaps are again closed for five seconds and returned to the normal position.

The complete charging operation takes but a few moments and should be performed daily. The operation is valuable, not only in keeping the films in good condition, but also in giving the operator some idea of the condition of the arrester by enabling him to observe the size and color of the charging spark.

~Grounded and Non-grounded Neutral Circuits.~--It is important to avoid the mistake of choosing an arrester for a thoroughly grounded neutral when the neutral is only partially grounded, that is, grounded through an appreciable resistance. Careful consideration of this condition will make the above statement clear.

In an arrester for a grounded neutral circuit, each stack of cones normally receives the neutral pressure when the arrester discharges, but if a phase become accidentally grounded, the line voltage is thrown across each of the other stacks of cones until the circuit breaker opens the circuit. The line voltage is 173 per cent. of the neutral or normal operating voltage of the cells and therefore about 150 per cent. of the permanent critical voltage of each cell. This means that when a grounded phase occurs, this 50 per cent. excess pressure is short circuited through the cells until the circuit breaker opens.

The amount of energy to be dissipated in the arrester depends upon the kilowatt capacity of the generator, the internal resistance of the cells, and the time required to operate the circuit breakers. It is evident that the greater the amount of resistance in the neutral, the longer will be the time required for the circuit breakers to operate. Therefore, in cases where the earthing resistance in the neutral is great enough to prevent the automatic circuit breakers opening practically instantaneously, an arrester for a non-grounded neutral system should be installed.

~Ground Connections.~--In all lightning arrester installations it is of the utmost importance to make proper ground connections, as many lightning arrester troubles can be traced to bad grounds. It has been customary to ground a lightning arrester by means of a large metal plate buried in a bed of charcoal at a depth of six or eight feet in the earth.

A more satisfactory method of making a ground is to drive a number of one inch iron pipes six or eight feet into the earth surrounding the station, connecting all these pipes together by means of a copper wire or, preferably, by a thin copper strip. A quantity of salt should be placed around each pipe at the surface of the ground and the ground should be thoroughly moistened with water. It is advisable to connect these pipes to the iron framework of the station, and also to any water mains, metal flumes, or trolley rails which are available.

The following suggestions are made for the usual size station.

1. Place three pipes equally spaced near each outside wall, making twelve altogether, and place three extra pipes spaced about six feet apart at a point nearest the arrester.

2. Where plates are placed in streams of running water, they should be buried in the mud along the bank in preference to being laid in the stream. Streams with rocky bottoms are to be avoided.

3. Whenever plates are placed at any distance from the arrester, it is necessary also to drive a pipe into the earth directly beneath the arrester, thus making the ground connection as short as possible. Earth plates at a distance cannot be depended upon. Long ground wires in a station cannot be depended upon unless a lead is carried to the parallel grounding pipes installed as described above.

4. As it is advisable occasionally to examine the underground connections to see that they are in proper condition, it is well to keep on file exact plans of the location of ground plates, ground wires and pipes, with a brief description, so that the data can be readily referred to.

5. From time to time the resistance of these ground connections should be measured to determine their condition. The resistance of a single pipe ground in good condition has an average value of about 15 ohms. A simple and satisfactory method of keeping account of the condition of the earth connections is to divide the grounding pipes into two groups and connect each group to the 110 volt lighting circuit with an ammeter in series.

~Choke Coils.~--A lightning discharge is of an oscillatory character and possesses the property of self-induction, accordingly it passes with difficulty through coils of wire. Moreover, the frequency of oscillation of a lightning discharge being much greater than that of commercial alternating currents, a coil can readily be constructed which will offer a relatively high resistance to the passage of lightning and at the same time allow free passage to all ordinary electric currents.

Opinions on the design of choke coils for use with lightning arresters vary considerably. Some engineers recommend the use of very large choke coils, but while large choke coils of high inductance do choke back the high frequency currents better than smaller coils of less inductance, they cost more, and under many conditions they are a menace to the insulation unless the lightning arresters be installed on both sides of them.

Part of the functions of the choke coil are performed by the end turns of a transformer and extra insulation is invariably installed in all power transformers built in recent years.

The choice of choke coils must be influenced by the condition of insulation in the transformers as well as by the cost, pressure regulation, and nature of the lightning protection required.

~Ques. What are the primary objects of a choke coil?~

Ans. To hold back the lighting disturbance from the circuit apparatus during discharge, and to lower the frequency of the oscillation so that whatever charge gets through the choke coil will be of a frequency too low to cause serious pressure drop around the first turns of the end coil in either alternator or transformer.

If there be no arrester, the choke coil cannot perform the first function, accordingly a choke coil is best considered as an auxiliary to an arrester.

~Ques. What is the principal electrical condition to be avoided with a choke coil?~

Ans. Resonance. The coil should be so arranged that if continual surges be set up in the circuit, a resonant voltage due to the presence of the choke coil cannot build up at the transformer or generator terminals. This factor is a menace to the insulation. Another way of stating the condition is as follows: So arrange the choke coil as not to prevent surges, originating in a transformer, passing to the arrester.

~Ques. What is another electrical condition to be avoided and why?~

Ans. Internal static capacity between adjacent turns

[Illustration: FIG. 2,410.--Westinghouse choke coil for high pressure transmission circuits, 2,200 to 25,000 volts. Choke coils of this type are wound without iron cores on circular or elliptical center blocks. They have a large number of layers and few turns per layer (except those made for small currents, they usually have one turn per layer), which give the best condition for insulating and cooling. They are air cooled, heavily insulated and have a line lead at the top, as shown. Choke coils are designed to prevent the short circuits sometimes caused by the local concentration of pressure such as may be produced by a lightning discharge. They limit, to some extent, an abnormal rise of pressure on the apparatus by delaying the advance of a static wave from the line and thus give the arrester more time to act. The disturbance caused by a lightning flash passes along the line in the form of a surge or "tidal wave." If this wave pass a choke coil, it is flattened out, and if the coil be of sufficient power, becomes practically harmless. It is evident, however, that the choke coil receives the full force of the wave, and that, consequently, it must be heavily insulated; moreover, the choke coil must not overheat under load, nor introduce into the circuit excessive inductive resistance.] of the choke coil, because this lowers the effectiveness of the coil.

~Ques. What is the object of making choke coils in the form of an hour glass?~

Ans. To prevent sagging between the supports.

~Ques. How are choke coils cooled?~

Ans. By air, or by oil.

~Ques. For what service are oil cooled choke coils used?~

Ans. On circuits of pressures above 25,000 volts, choke coils immersed in oil, as are transformer coils, have advantages in that the coil is amply insulated not only from the ground but against side flash, and that copper of comparatively small section may be used without undue heating.

~"Static" Interrupters.~--A static interrupter is a combination of a choke coil and a condenser, the two being mounted together and placed in a tank and oil insulated.

It is used on high pressure circuits and its function is to so delay the erroneously called "static" wave in its entry into the transformer coil, that a considerable portion of the latter will become charged before the terminal will have reached full pressure.

A choke coil alone sufficiently powerful to accomplish this would be too large and costly on very high pressure and would interfere with the operation of the system.

~Ques. How is the condenser and choke coil connected?~

Ans. The condenser is connected between the line and ground behind the choke coil near the apparatus to be protected as shown in fig. 2,413.

~Ques. What is the effect of the condenser?~

Ans. The condenser, which has a very small electrostatic capacity, has no appreciable effect upon the normal operation, but a very powerful effect upon the static wave on account of its extremely high frequency.

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LX. REGULATING DEVICES

~Regulation of Alternators.~--Practically all the methods employed for regulating the voltage of direct current dynamos and circuits, are applicable to alternators and alternating current circuits. For example: in order that they shall automatically maintain a constant or rising voltage with increase of load, alternators are provided with composite winding similar to the compound winding of direct current dynamos, but since the alternating current cannot be used directly for exciting the field magnets, an accessory apparatus is required to rectify it or change it into direct current before it is used for that purpose.

It is a fact, however, that composite wound alternators do not regulate properly for inductive as well as non-inductive loads.

In order to overcome this defect compensated field alternators have been designed which automatically adjust the voltage for all variations of load and lag. These machines have already been described.

~Alternating Current Feeder Regulation.~--With slight modification, the various methods of feeder regulation employed with direct current, may be applied to alternating current distribution circuits. For instance, if a non-inductive resistance be introduced in any electric circuit, the consequent drop in voltage will be equal to the current multiplied by the resistance. Therefore, feeder regulation by means of rheostats is practically the same in the case of alternating current as in that of direct current. In the case of the former, however, the effect of self-induction may also be utilized to produce a drop in voltage. In practice, this is accomplished by the use of self-induction coils which are commonly known as reactance coils.

~Application of Induction Type Regulators.~--In supplying lighting systems, where the load and consequently the pressure drop in the line increases or decreases, it becomes necessary to raise or lower the voltage of an alternating current, in order to regulate the voltage delivered at the distant ends of the system. This is usually accomplished by means of alternating current regulators or induction regulators. A device of this kind is essentially a transformer, the primary of which is excited by being connected directly across the circuit, while the secondary is in series with the circuit as shown in fig. 2,414. By this method the circuit receives the voltage generated in the secondary.

~Ques. Name two types of pressure regulator.~

Ans. The induction regulator, and the variable ratio transformer regulator.

~Ques. Of what does an induction regulator consist?~

Ans. It consists of a primary winding or exciting coil, a secondary winding which carries the entire load current.

The primary is wound for the full transmission voltage, and is connected across the line, while the secondary is connected in series with the line.

~Ques. What is its principle of operation?~

Ans. When the primary coil is turned to various positions the magnetic flux sent through the secondary coil varies in value, thereby causing corresponding variation in the secondary voltage, the character of which depends upon the value and direction of the flux.

~Ques. What is the effect of turning the secondary coil to a position at right angles with the primary coil?~

Ans. The primary will not induce any voltage in the secondary, and accordingly it has no effect on the feeder voltage.

~Ques. What is this position called?~

Ans. The neutral position.

~Ques. What are the effects of revolving the primary coil from the neutral position first in one direction then in the other?~

Ans. Turning the primary in one direction increases the voltage induced in the secondary, thus increasing the feeder voltage like the action of a booster on a direct current circuit while turning the primary in the opposite direction from the neutral position, correspondingly decreases the feeder voltage.

~Ques. It was stated that for neutral position the primary had no effect on the secondary; does the secondary have any effect on the feeder voltage?~

Ans. The secondary tends to create a magnetic field of its own self-induction, and has the effect of a choke coil.

~Ques. How is this tendency overcome?~

Ans. The primary is provided with a short circuited winding, placed at right angles to the exciting winding. In the neutral position of the regulator, this short circuited winding acts like the short circuited secondary of a series transformer, thus preventing a choking effect in the secondary of the regulator.

~Ques. What would be the effect if the short circuited winding were not employed?~

Ans. The voltage required to face the full load current through the secondary would increase as the primary is turned away from either the position of maximum or minimum regulation, reaching its highest value at the neutral position.

The short circuited winding so cuts down this voltage of self-induction that the voltage necessary to force the full load current through the secondary when the regulator is in the neutral position is very little more than that necessary to overcome the ohmic resistance of the secondary.

~Ques. What effect is noticeable in the operation of a single phase induction regulator?~

Ans. It has a tendency to vibrate similar to that of a single phase magnet or transformer.

~Ques. Why?~

Ans. It is due to the action of the magnetizing field varying in strength from zero to maximum value with each alteration of the exciting current, thus causing a pulsating force to act across the air gap, which tends to cause vibration when the moving part is not in perfect alignment.

~Ques. Explain the effect produced by bad alignment?~

Ans. If the bearings of the primary be not in perfect alignment with the bore of the secondary, thereby making the air gap on one side smaller than that on the other, the crowding over of the flux to the smaller air gap will cause an intermittent pull in that direction, which will develop vibration unless the primary bearings are tight and the shaft sufficiently stiff to withstand the pull.

~Ques. Upon what does the regulator capacity for any given service depend?~

Ans. It depends upon the range of regulation required and the total load on the feeder.

~Ques. How is the capacity stated?~

Ans. In percentage of the full load of the feeder.

For instance, on a 100 kilowatt circuit, a 10 kw. regulator will give 10 per cent. regulation, and a 5 kw. regulator, 5 per cent. regulation.

~Polyphase Induction Regulators.~--The polyphase induction regulator is similar to the single phase regulator except that both the primary and secondary elements are wound with as many sets of coil as there are phases in the circuit.

In construction these windings are distributed throughout the complete circumference of the stationary and moving elements and closely resemble the windings of an induction motor.

Polyphase regulators have but little tendency to vibrate because the field across the air gap is the resultant of two or more single phase fields and is of a constant value at all times. This field rotates at a rate depending upon the number of poles and the frequency of the circuit. This produces a mechanical pull of constant value which rotates with the magnetic field varying its position from instant to instant.

It is evident that this pull is of an entirely different character from that produced by the single phase field and that there is no tendency to set up the vibration that the mechanical pull of the single phase regulator tends to establish.

There is, however, considerable torque developed, and the device for revolving the moving element must be liberally designed so as to withstand the excess torque caused by temporary overloads or short circuits.

~Ques. In what respects do polyphase induction regulators differ in principle from single phase regulators?~

Ans. The induced voltage in the secondary has a constant value, and the regulation is effected by varying the phase relation between the line voltage and the regulator voltage.

~Ques. How is the primary wound?~

Ans. It is wound with as many separate windings as there are phases in the circuit, and these primary or shunt windings are connected to the corresponding phases of the feeder.

~Ques. What kind of magnetizing flux is produced by the primary windings?~

Ans. A practically constant flux which varies in direction.

~Ques. How is the secondary wound?~

Ans. There is a separate winding for each phase.

~Ques. Why is the voltage induced in the secondary constant?~

Ans. Because of the constant magnetizing flux.

~Ques. How is the line voltage varied by a polyphase regulator?~

Ans. When the regulator is in the position of maximum boost, the line AB, fig. 2,425 represents the normal busbar voltage, BC the regulator voltage, and AC the resultant feeder voltage. When the regulator voltage is displaced 180 degrees from this position, the regulator is in the position to deliver minimum voltage to the feeder, the regulator voltage being then represented by BD, and the resultant feeder voltage by AD. When the regulator voltage is displaced angularly in the direction BF, so that the resultant feeder voltage AF becomes equal to the normal busbar voltage AB, the regulator is in the neutral position. Intermediate resultant voltages for compensating the voltage variations in the feeders may be obtained by rotating the moving element or primary in either direction from the neutral position. For example, by rotating the primary through the angle FBE, the resultant voltage may be made equal to AE or AJ, thereby increasing the feeder voltage by an amount BJ; or by rotating it in the opposite direction through the angle FBG, the feeder voltage may be reduced by an amount BH.

~Ques. How are induction regulators operated?~

Ans. By hand or automatically.

~Ques. How is automatic operation secured?~

Ans. By means of a small motor, controlled by voltage regulating relays.

~Ques. How is the control apparatus arranged?~

Ans. Two relays are employed with each regulator, a primary relay connected to the feeder circuit and operating under changes of voltage therein, and a secondary relay connected between the primary relay and the motor, and operated by the contacts of the former, for starting, stopping and reversing the motor in accordance with changes in the feeder voltage, thereby causing the regulator to maintain that voltage at its predetermined normal value.

~Ques. Why are two relays used?~

Ans. For the reason that a primary relay, of sufficient accuracy and freedom from errors due to temperature and frequency variations, could not be made sufficiently powerful to carry the relatively large current required for operating the motor.

~Ques. What names are given to the relays?~

Ans. Primary and secondary.

~Ques. What difficulties were encountered in the operation of relays? ~

Ans. Vibration or chattering at the contacts of both relays and tendency of the movable contact arm of the primary relay to hug closer to one of the stationary contact points than to the other, thereby operating too often.

~Ques. What causes vibration or chattering at the contacts?~

Ans. This is due to the voltage frequently approximating the value required for closing a contact, thereby causing the contact points to barely touch and make several poor contacts in succession.

~Ques. What objectionable action is produced by vibration at the contacts?~

Ans. Arcing, burning and pitting of the contacts, even when alloys of the rarer metals are used, such as those of the platinum group, having extreme hardness and high melting points.

~Ques. What effect is produced by poor contact of the primary relay?~

Ans. It causes chattering in the secondary relay; which burns out and wears away its contact points, increasing the heating of the motor, creating objectionable noise and entailing wear and tear on the whole outfit.

~Ques. Why does the movable contact arm of the primary relay tend to remain nearer one of the stationary contact points than the other?~

Ans. This is due to the tendency of the relay to open the contact whenever the voltage equals that at which the contact closes.

~Ques. What provision is made in the primary relay to prevent vibration or chattering?~

Ans. Two auxiliary windings are provided: one in series with each of the stationary contact points and so arranged as to assist in making the contact by increasing the pressure on the contact points at the instant of closure.

The best effect of the compounding action of the auxiliary coils is obtainable when arranged for ¾ per cent. of the torque of the main coil.

A non-inductive resistance placed in parallel with each coil of the secondary relay, takes current approximately in phase with the current in the main coil of the primary relay, and of proper strength to make the number of ampere turns in the auxiliary coil three-fourths per cent. of the number in the main coil. The resistances have the additional effect of absorbing the "discharge" from the main coils of the secondary relay when the contacts are broken, thereby obviating sparking at the primary contact points.

~Variable Ratio Transformer Voltage Regulators.~--The principle of operation of this class of regulator is virtually the same as that of the induction type regulator; that is to say, both consist of regulating transformers, but ~in the variable ratio method~ the primary or series coil is divided into a number of sections which may be successively cut in or out of the circuit to be regulated, instead of varying the flux through the entire coil, as in the induction type. There are two distinct mechanical forms of variable ratio regulator:

1. Drum type; 2. Dial type.

~Drum Type Regulators.~--This form of variable ratio transformer consists essentially of a drum and finger type switch, similar to a railway controller.

There are many contacts, giving a large number of points of regulation, obtained by the use of changing switches and floating coils.

The floating coil is a part of the secondary winding of the regulating transformer which is insulated from the main portion of the winding, and is sub-divided by taps into a number of equal sections.

The sub-divisions of the main secondary winding are much larger, each one being equivalent to the whole of the floating coil.

~Ques. Describe the operation of a drum regulator.~

Ans. The floating coil and main windings are first connected in series with each other and with the line to be regulated. The floating coil is then cut out of the circuit step by step. When entirely cut out it is transferred to the next lower tap on the main winding, after which it is again cut out step by step and then transferred again. By continuing this process a large number of steps are provided with but comparatively few actual taps on the transformer.

~Ques. How many floating coils are used and why?~

Ans. Two floating coils are included in each regulator so that one can be transferred while the other is supplying the current to the line.

~Dial Type Regulators.~--This form of variable ratio transformer regulator consists of a regulating transformer and a dial type switch as shown in the accompanying illustrations. The regulating transformer is similar to a standard transformer except that the secondary winding is provided with a number of taps leading to the contact of the dial switch as shown in the diagram fig. 2,437.

~Ques. What modification is made to adapt dial regulators for heavy current?~

Ans. A dial with a series transformer, and a shunt or auto-transformer are employed as shown in fig. 2,436.

~Ques. Why is such modification desirable?~

Ans. Because, the additional cost of a series transformer is small in comparison with the cost of building a dial with a large current carrying capacity, and the cost of bringing out a number of heavy leads from a small transformer.

~Ques. How are dial regulators modified for high voltage?~

Ans. Standard dials may be used with series and shunt transformers similar to the method used for heavy current circuits.

~Ques. Describe the connections.~

Ans. The primary of the shunt transformer is connected across the line and the secondary has a number of taps which are connected to contacts on the dial. The primary of the series transformer is connected in series with the line and two leads from the secondary winding are connected to the dial.

The connections are similar to those shown in fig. 2,437, except that shunt transformers are used instead of auto-transformers.

It will be seen that the circuit comprising the dial, the secondary of the shunt, transformer and the secondary of the series transformer form a circuit which is not electrically connected to the main circuit. It can therefore be grounded without disturbing the main circuit as a safeguard to render it impossible for the pressure of the dial to be higher above the ground than the secondary voltage of the shunt transformer.

~Small Feeder Voltage Regulators.~--In some generating stations the voltage is maintained constant at the busbars and the line drop compensated by automatically operated regulators connected in the main feeders. It is possible in this way to obtain constant voltage at all loads at the various distribution centers, that is, at those points on the feeders where the lines of the majority of consumers are connected as shown in fig. 2,447.

It is evident, however, that, while the voltage at the center of distribution can be maintained constant, no account can be taken of the drop in the lines between this center and the consumers. This drop is generally negligible, except in some particularly long lines, as, for example, consumer B in fig. 2,447.

In order to obtain perfect regulation at B, it would be necessary to install a separate regulator in that line, this regulator to be installed either at the center C or preferably at B.

In a great many cases the power distribution is not as ideal as indicated in fig. 2,447, but rather as shown in fig. 2,448, that is, the consumers are connected all along the feeder. In this case there is no definite center of distribution, and the automatic regulator installed in the station can be adjusted to give only approximately constant voltage at an imaginary center of distribution C; that is, the voltage cannot be held constant at any definite point during changes of load distribution.

The majority of the consumers may, however, obtain sufficiently good voltage while a few may have reason for criticism. To overcome this difficulty it is necessary either to increase the copper in the feeder or else to install small automatic regulators.

There are also cases where a small amount of power is transmitted a long distance through a feeder direct from the station.

The amount of copper required to reduce the line drop is usually too great to be considered and the cost of the ordinary automatic regulator is also comparatively high. In such cases small pole type regulators as shown in fig. 2,449 are desirable.

~Ques. Describe the operation of the regulator mechanism shown in fig. 2,450.~

Ans. Assuming the voltage to be normal, the balance arm of the relay will be held horizontal, the trips F will not engage with the triggers E, and no movement is therefore transmitted to the ratchet wheel C. If the voltage drops below normal, the left hand trip will descend until it finally gets in the way of the left hand trigger just before it reaches the limit of its counterclockwise travel. This trigger will therefore release the left pawl D, which will engage with the ratchet wheel and will consequently turn it clockwise until the rocker arm reaches its right hand limit. Before the rocker arm reaches the left hand limit, the released pawl must be locked by its trigger, so that if the voltage has reached its normal value, further movement of the ratchet wheel will not take place, whereas if the voltage be still too low, the trigger will again release the pawl by striking the trip of the relay.

~Ques. How is this automatic locking of the pawl obtained?~

Ans. By having a lip G on the under side of the pawl strike a finger H fastened to the bearings in front of the ratchet wheel.

The pawl is thus raised just before it reaches the limit of its clockwise travel sufficient to be locked by its trigger.

~Ques. How does the mechanism operate when the voltage rises above normal?~

Ans. As described above, with the exception that the right hand trip causes a rotation of the regulator in the opposite direction.

~Ques. How is adjustment made for various voltages?~

Ans. Taps are provided on the resistance in series with the relay, and finer adjustment can be obtained by means of the helical spring on the right hand end of the balance arm.

In order to adjust the sensitiveness of regulation, the bearing for the balance arm can be raised or lowered by means of a stud J, fig. 2,450, connecting this bearing with the bearing of the operating shaft, and the regulator can be made to maintain the voltage within 1 per cent. above or below normal.

~Ques. What provision is made for convenient inspection?~

Ans. A snap switch is provided by means of which the power to the motor and relay can be disconnected.

~Automatic Voltage Regulators for Alternators.~--The accurate regulation of voltage on any alternating current system is of importance. The desired voltage may be maintained constant at the alternator terminals by rapidly opening and closing a shunt circuit across the exciter field rheostat.

~Ques. Describe in more detail this method of regulation.~

Ans. The rheostat is first turned in until the exciter voltage is greatly reduced and the regulator circuit is then closed. This short circuits the rheostat through contacts in the regulator and the voltage of the exciter and alternator immediately rise. At a predetermined point, the regulator contacts are automatically opened and the field current of the exciter must again pass through the rheostat. The resulting reduction in voltage is arrested at once by the closing of the regulator contacts which continue to vibrate in this manner and keep the generator voltage within the desired limits. The connections are shown in fig. 2,457.

~Line Drop Compensators.~--In order that the actual voltage at a distant point on a distribution system may be read at the station some provision must be made to compensate for the line drop, that is to say, for the difference in voltage between the alternator and the center of distribution.

In order to do this a device which is known as a "line drop compensator" is placed in the voltmeter circuit as shown in the diagram, fig. 2,458.

~Ques. What are the essential parts of a line drop compensator?~

Ans. The elements of a line drop compensator are a variable resistance, and a variable inductance.

~Ques. Describe the connections.~

Ans. The secondary of a pressure transformer is connected in series with the compensator inductance and resistance, and the secondary of a current transformer as shown in the diagram, fig. 2,458.

~Ques. How are the inductance and resistance wound?~

Ans. They are wound so that any proportion of the winding of either can be put in or out of the voltmeter circuit.

~Ques. How can the voltmeter indicate the pressure at the center of distribution?~

Ans. If the amount of inductance and resistance be properly adjusted, there will be produced a local circuit corresponding exactly in all its characteristics to the main circuit. Hence, any change in the main circuit produces a corresponding change in the local circuit, and causes the voltmeter to always indicate the pressure at the end of the line or center of distribution or at any point for which the adjustment is made.

~Ques. How should the adjustment be made?~

Ans. It is advisable to calculate the ohmic drop for full load and set the resistance arm at the point which will give the required compensation and then adjust the inductance arm until the voltmeter reading corresponds to the voltage at the point on the line selected for normal voltage.

[2] NOTE.--It is desirable, in any system of distribution, to read the active voltage at the point of distribution, by means of the voltmeters in the station. A compensator proper consists of a variable resistance and a variable inductance, and sometimes a current transformer. In wiring, the voltmeter, instead of being connected directly across the secondaries of a pressure transformer, has inserted in series with it, portions of the resistance and inductance of the compensator. These are so connected that the drop in pressure across them will be combined with that of the pressure transformer, so that the voltmeter reading indicates the pressure at the center of distribution or end of the line.

~Starting Compensators.~--These are used for starting induction motors and consist of inductive windings (one for each phase) with a number of taps connecting with switch contacts as shown in fig. 2,463. A starting compensator is similar to a rheostat except that inductive windings are used in place of the resistance grids.

~Ques. Describe the inductive windings.~

Ans. The compensator winding consists of an inductive coil in each phase with each coil placed on a separate leg of a laminated iron core. Each coil is provided with several taps so located that a number of sub-voltages may be obtained.

~Ques. Are starting compensators necessary for small motors? Why?~

Ans. No, because the full voltage starting current taken, although equal to several times the load current, is nevertheless so small, compared with the capacity of the station alternators or feeders, that it does not materially affect the regulation of the circuit.

Motors larger than about 7 horse power cause an objectionably heavy rush of current if thrown directly on the line. Starting compensators obviate such sudden variations of line load and are accordingly recommended for motors above 7 horse power except in cases where voltage variations and excessive starting currents are not objectionable.

~Ques. What should be noted with respect to the compensator winding taps?~

Ans. The choice of a tap giving so low a voltage as to require over one minute for starting should be avoided so as to prevent the overheating to which starting compensators, in common with other motor starting devices, are liable if left in circuit unnecessarily long, or if the motor be started several times in rapid succession.

It should also be noted that the starting current diminishes rapidly as full speed is approached. It is, therefore, important that the switch be kept in the starting position until the motor has finished accelerating to prevent any unnecessary rush of current when the switch is thrown to the running position.

~Ques. What is the usual arrangement of starting compensators for large motors?~

Ans. Starting compensators may be wound for any voltage or current for which it is practicable to build motors. For very large motors the switching device is generally separate from, the compensator itself and consists of triple and four pole switches for three phase and two phase motors respectively. One double throw switch or two interlocked single throw switches are required for the motor and a single throw switch for energizing the compensator, the running side of the motor circuit being provided with fuses or automatic circuit breakers, or the switches provided with low voltage and overload release attachments.

~Star Delta Switches.~--These are starting switches, designed for use with small three phase squirrel cage motors having their windings so arranged that they may be connected in star for starting and in delta for running.

~Ques. Describe the operation of a star delta switch.~

Ans. In starting the motor, the drum lever is thrown in the starting direction which connects the field windings of the motor ~in star~. When the motor has accelerated and has come partially up to speed the starting lever is quickly thrown to the running position in which position the field windings are connected ~in delta~. The effect of connecting the field winding in star at starting is to reduce the voltage applied to each phase winding, while in the running position each phase of the field winding has full line voltage impressed upon it.

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LXI. SYNCHRONOUS CONDENSERS

~Synchronous Condensers.~--A synchronous motor when sufficiently excited will produce a leading current, that is, when over excited it acts like a great condenser, and when thus operated on circuits containing induction motors and similar apparatus for the purpose of improving the power factor it is called a synchronous condenser.

Although the motor performs the duty of a condenser it possesses almost none of the properties of a stationary condenser other than producing a leading current, and is free from many of the inherent defects of a stationary condenser.

The relation of power factor to the size and efficiency of prime movers, generators, conductors, etc., and the value of synchronous condensers for improving the power factor is generally recognized.

Induction motors and other inductive apparatus take a component of current which lags behind the line pressure, and thereby lowers the power factor of the system, while a non-inductive load, such as incandescent lamps, takes only current in phase with the voltage and operates at unity power factor.

Since transformers require the magnetizing current, they may seriously affect the power factor when unloaded or partially loaded, but when operating at full load their effect is practically negligible.

The relative effect of fully loaded and lightly loaded induction motors on the power factor is indicated by the diagram, fig. 2,478. The magnetizing current is nearly constant at all loads and is wattless, lagging 90 degrees behind the impressed pressure, or at right angles to the current which is utilized for power.

In the figure, AB is the magnetizing component, which is always wattless, and CB the power component. The angle ACB gives the phase relation between voltage and current; the cosine of this angle CB ÷ AC is the power factor.

It is evident from the diagram that if the load be reduced, the side CB is shortened, and as AB is practically constant, the angle of lag ACB is increased. It therefore follows that the cosine of this angle, or the power factor is reduced.

The figure clearly shows the reason for the low power factor of induction motors on fractional loads and also shows that since the magnetizing current is practically constant in value, the induction motor can never operate at unity power factor.

With no load, the side CB (real power) is just sufficient to supply the friction and windage. If this be represented by DB, since AB remains constant, the power factor is reduced to 10 or 15 per cent. and the motor takes from the line about 30 per cent. of full load current. It therefore follows that a group of lightly loaded induction motors can take from the system a large current at exceedingly low power factor.

The synchronous motor when used as a condenser, as before stated, has the property of altering the phase relation between pressure and current, the direction and extent of the displacement being dependent on the field excitation of the condenser.

It can be run at unity power factor and minimum current input, or it can be over excited and thereby deliver leading current which compensates for the inductive load on other parts of the system. The synchronous condenser, therefore, can supply magnetizing current to the load on a system while the power component is supplied by the generators.

~Effects of Low Lagging Power Factors.~--Transformers are rated in kva. output; that is, a 100 kva. transformer is supposed to deliver 100 kw. at unity power factor at normal voltage and at normal temperatures; but, if the power factor should be, say .6 lagging, the rated energy output of the transformer would be only 60 kw. and yet the current and, consequently, the heating would be approximately the same as when delivering 100 kw. at unity power factor.

The regulation of transformers is inherently good, being for small lighting transformers about 1½ to 2 per cent. for a load of unity power factor, and about 4 to 5 per cent. at .7 power factor. Larger transformers with a regulation of 1 per cent. or better at a unity power factor load, would have about 3 per cent. regulation at .7 power factor.

Alternators also are rated in kva. output, usually at any value of power factor between unity and .8.

The deleterious effects of low power factor loads on alternators are even more marked than on transformers. These are, decreased kw. capacity, the necessity for increased exciter capacity, decreased efficiency, and impaired regulation.

Assume the case of a 100 kva. .6 power factor, 60 kw. output. It is probable that normal voltage could be obtained only with difficulty, unless the alternator was especially designed for low power factor service. The lagging power factor current in the armature sets up a flux which opposes the flux set up by the fields, and in consequence tends to demagnetize them, resulting in low armature voltage.

It is often impracticable, without the installation of new exciters, to raise the alternator voltage by a further increase of the exciting voltage and current. The field losses, and therefore the field heating of the alternator, when it is delivering rated voltage and current, are greater at lagging power factor than at unity. Increased energy input and decreased energy output both cause a reduction in efficiency.

The regulation at unity power factor of modern alternators capable of carrying 25 per cent. overload, is usually about 8 per cent. Their regulation at .7 power factor lagging is about 25 per cent. The effect of low power factor on the lines can best be shown by the following example:

EXAMPLE.--Assuming a distance of five miles and a load of 1,000 kw. and desiring to deliver this load at a pressure of about 6,000 volts, three phase, with an energy loss of 10 per cent., each conductor at unity power factor would have to be 79,200 c.m., at .9 power factor, 97,533 c.m., and at .6 power factor, 218,000 c.m. In other words, at the lower power factor of .6, the investment in copper alone would be 2.8 times as much.

If the same size of wire were used at both unity and .6 power factor lagging, the energy loss would be about 2.8 times the loss at unity power factor, or about 28 per cent. Low lagging power factor on a system, therefore, will generally mean limited output of prime movers; greatly reduced kilowatt capacity of generator, transformer and line; and increased energy losses. The regulation of the entire system will also be poor.

~Cost of Synchronous Condenser vs. Cost of Copper.~--Referring to the example given in the preceding paragraph, and calculating the necessary extra investment in copper with the .6 power factor load, and copper at 17 cents per pound, the result is that 29,292 pounds more copper is required than with the power factor of .9 which means a total extra investment in copper alone of $5,000 (29,292 × $.17). A synchronous condenser of sufficient capacity to accomplish the same result would cost about the same amount. It would therefore cost less to install the condenser because at the same time a considerably increased capacity would be obtained from the alternators, transformers, etc.

~Synchronous Condenser Calculations.~--In figuring on the installation of a condenser for correcting power factor troubles, a careful survey of the conditions should be made with a view of determining just what these troubles are and to what extent they can be remedied by the presence of a leading current in the system.

It is necessary to possess a thorough knowledge of the system, covering the generating capacity in energy and kva., average and maximum load, and power factor on the alternators, average and maximum load, and power factor on the feeders, system of distribution, etc.

The desirable location of a condenser is, of course, nearest the inductive load in order to avoid the transmission of the wattless current, but it often happens that a system is so interconnected that one large condenser cannot economically meet the conditions, in which case it may be better to install two or more smaller ones.

The question of suitable attendance should also be considered and, for this reason, it may be necessary to compromise on the location. When the location of the condenser has been decided upon and the load and power factor within its zone determined, the proper size of condenser to raise the power factor to a given value can be found as follows:

The method of procedure can best be explained by reference to a concrete case. Assume a load of 450 kw. at .65 power factor. It is desired to raise the power factor to .9. What will be the rating of the condenser?

Referring to the diagram, fig. 2,488, it is necessary to start with 450 kw. At .65 power factor, or 692 kva., this has a wattless lagging component of √(692² - 450²) = 525 kva. With the load unchanged and the power factor raised to .9, there will be 500 apparent kva., which will have a wattless component of √(500² - 450²) = 218 kva.

It is obvious that the condenser must supply the difference between 525 kva. and 218 kva., or 307 kva. A 300 kva. condenser would, therefore, meet the requirements.

If it be desired to drive some energy load with the condenser and still bring the total power factor to .9, proceed as indicated in fig. 2,489. Assume a total load of 150 kw. on the motor. As before, 450 kw. at .65 power factor, or 692 kva., with a wattless component of 525 kva.

The energy load will be increased from 450 to 600 kw. as indicated, and with the power factor raised to .9 there will be a kva. of 667 with a wattless component of √(667² - 600²) = 291.

There must be supplied 525 - 291 = 234 in leading kva.

The synchronous motor then must supply 150 kw. energy and 234 kva. wattless, which would give it a rating of √(150² + 234²) = 278 kva. at .68 power factor.

The standard 300 kva. condenser would evidently raise the power factor slightly above .9 power factor leading.

By reference to the chart, fig. 2,490, the size of the required condenser can be obtained direct without the use of the above calculation. The method of using this curve is as follows: Assume a load of say 3,000 kw. at .7 power factor and that it be desired to raise the power factor to .9. Run up the vertical line at 3,000 kw. to the .7 power factor line, and from there along the horizontal line to the margin and find a wattless component at this power factor of 3,000 kva., approximately. Again run up the 3,000 kw. vertical line to the .9 power factor line and from there along the horizontal line to the margin and find a wattless component of 1,500 kva. The rating of the condenser will then be 3,000 kva. - 1,500 kva. = 1,500 kva. This table of course can be used for hundreds of kilowatts as well.

For determining the rating of a synchronous motor to drive an energy load this curve is not so valuable, although it can be used in determining the wattless component direct in all cases where the energy component and power factor are known. Knowing this energy component and power factor or wattless component, the energy load can obviously be found by referring to the curved lines on the diagrams, the curve that crosses the junction of the vertical energy line and the power factor or wattless component line giving the total apparent kva.

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LXII. INDICATING DEVICES

Alternating current ammeters or voltmeters indicate the virtual values of the current or pressure respectively, that is to say, they indicate, the square root of the mean square of a variable quantity.

The virtual value of an alternating current or pressure is equivalent to that of a direct current or pressure which would produce the same effect.

For instance an alternating current of 10 virtual amperes will produce the same heating effect as 10 amperes direct current.

The relation of the virtual value of an alternating current to the other values is shown in fig. 2,491. When the current follows the sine law, the square root of the mean square, value of the sine functions is obtained by multiplying their maximum value by 1 ÷ √2̅ or .707.

The word ~effective~ is commonly used erroneously for ~virtual~, even among the best writers and the practice cannot be too strongly condemned[3].[4] The difference between the two is illustrated in Guide No. 5, page 1,013, fig. 1,237, the mechanical analogy here given may make the distinction more marked.

[3] NOTE.--I adhere to the term virtual, as it was in use before the term efficace which was recommended in 1889 by the Paris Congress to denote the square root of mean square value. The corresponding English adjective is efficacious, but some engineers mistranslate it with the word effective. I adhere to the term virtual mainly because effective is required in its usual meaning in kinematics to represent the resolved part of a force which acts obliquely to the line of motion, the effective force being the whole force multiplied by the cosine of the angle at which it acts with respect to the direction of motion.--S. P. Thompson.

[4] NOTE.--The author adheres to the term virtual because in mechanics the adjective effective is used to denote the difference of two opposing forces; for instance, at any instant in the operation of a steam engine, effective pressure = forward pressure - back pressure, hence, to be consistent in nomenclature, the term effective cannot be used for the forward or virtual pressure, that is, the pressure impressed on an electric circuit.

In the operation of a steam engine, there are two pressures acting on the piston:

1. The forward pressure; 2. The back pressure.

The forward pressure on one side of the piston is that due to the live steam from the boiler, and the back pressure, on the other side, that due to the resistance or opposition encountered by the steam as it exhausts from the cylinder.

In order that the engine may run and do external work, it is evident that the forward pressure must be greater than the back pressure, and it follows that the pressure available to run the engine is the difference between these two pressures, this pressure difference being known as the ~effective pressure~, that is to say

effective pressure = forward pressure - back pressure

Thus, electrically speaking, the effective voltage is that voltage which is available for driving electricity around the circuit, that is,

effective volts = virtual volts - back volts = virtual pressure - (virtual pressure - drop)

In the case of the steam engine, the forward pressure absolute, that is, measured from a perfect vacuum is the virtual pressure (not considering the source). The back pressure may vary widely for different conditions of operation as illustrated in figs. 2,493 and 2,494.

In the measurement of alternating current, it is not the average, or maximum value of the current wave that defines the current commercially, but the square root of the mean square value, because this gives the equivalent heating effect referred to direct current. There are several types of instrument for measuring alternating current, and they may be classified as

1. Electromagnetic (moving iron); 2. Hot wire; 3. Induction; 4. Dynamometer.

~Electromagnetic or Moving Iron Instruments.~--This type of instrument depends for its action upon the pull of flux in endeavoring to reduce the reluctance of its path. This pull is proportional to the product of the flux and the current, and so long as no part of the magnetic circuit becomes saturated, the flux is proportional to the current, hence the pull is proportional to the square of the current to be measured.

~Ques. What are some objections to moving iron instruments?~

Ans. Instruments of this type are not independent of the frequency, wave form, or temperature and external magnetic fields may affect the readings temporarily.

There are several forms of moving iron ammeters, which may be classified as

1. Plunger; 2. Inclined coil; 3. Magnetic vane.

~Ques. Describe the plunger type.~

Ans. This type of ammeter consists of a series coil and a soft iron plunger forming a solenoid, the plunger is so suspended that the magnetic pull due to the current flowing through the coil is balanced by gravity, as shown in fig. 2,497.

~Ques. How should the plunger be constructed to adapt it to alternating current, and why?~

Ans. It should be laminated to avoid eddy currents.

~Ques. What is the character of the scale and how should it be constructed?~

Ans. The scale is not uniform and should be hand made and calibrated under the conditions which it is to be used.

~Ques. What is the objection to moving iron ammeters?~

Ans. Since the coil carries the entire current they are large and expensive.

~Ques. What precaution should be taken in installing moving iron ammeters?~

Ans. Since gravity is the controlling force, the instrument should be carefully levelled.

~Ques. Describe an inclined coil instrument.~

Ans. It consists of a coil mounted at an angle to a shaft carrying the vane and pointer, as shown in fig. 2,498. A spring forms the controlling force and holds the pointer at zero when no current is flowing.

~Ques. What is the principle of operation of the inclined coil instrument?~

Ans. When a current is passed through the coil, the iron tends to take up a position with its longest sides parallel to the lines of force, which results in the shaft being rotated and the pointer moved on the dial, the amount of movement depending upon the strength of the current in the coil.

~Ques. Describe a magnetic vane instrument.~

Ans. It consists of a small piece of soft iron or vane mounted

[Illustration: FIG. 2,500.--Magnetic vane movement of a Wagner instrument; it is used both for voltmeters and ammeters. This type differs from the dynamometer movement in that a vane of very soft iron replaces the moving coil. The magnetic vane movement makes use of its controlling spring only for the purpose of resisting the pull on the vane and the returning of the needle to zero. The spring does not carry any current.] on a shaft that is pivoted a little off the center of a coil as shown in fig. 2,499, and carrying a pointer which moves over a scale.

~Ques. How does it work?~

Ans. Its principle of operation is that a piece of soft iron placed in a magnetic field and free to move, will move into such position as to conduct the maximum number of lines of force.

The current to be measured is passed around the coil, producing a magnetic field through the center of the coil. The magnetic field inside the coil is strongest near the inner edge, hence, the vane will move against the restraining force of a spring so that the distance between it and the inner edge of the coil will be as small as possible.

The operation of moving iron instruments of the plunger type may be explained by saying that the current flowing in the coil produces a pole at its end and induces an unlike pole at the end of the plunger nearest the coil, thus attracting the plunger, as illustrated in fig, 2,501 above.

~Hot Wire Instruments.~--Instruments of this class depend for their operation on the expansion and contraction of a fine wire carrying either the current to be measured or a definite proportion of that current.

The expansion or contraction of the wire is caused by temperature changes, which in turn are due to the heating effect of the current flowing through the wire.

Since the variations in the length of the wire are extremely small, considerable magnification is necessary. Pulleys or levers are sometimes used to multiply the motion, and sometimes the double sag arrangement shown in fig. 2,504.

As shown here, A is the active wire carrying the current to be measured and stretched between the terminals T and T´. It is pulled taut at its middle point by another wire C, which carries no current, and is, in its turn, kept tight by a thread passing round the pulley D attached to the pointer spindle, the whole system being kept in tension by the spring E.

Hot wire instruments are equally accurate with alternating or direct current, but have cramped scales (since the deflection is proportional to the square of the current), and are liable to creep owing to unequal expansion of the parts. There is also the danger that they may be burnt out with even comparatively small overloads. They are not affected by magnetic fields but consume more current than the other types, these readings are inaccurate near either end of the scale.

~Induction Instruments.~--These were invented by Ferraris, and are sometimes called after him. They are for alternating current only, and there are two forms:

1. Shielded pole type; 2. Rotary field type.

~Ques. Describe the shielded pole type of induction instrument.~

Ans. As shown in figs. 2,505, and 2,506 it consists, essentially of a disc A, or sometimes a drum and a laminated magnet B. Covering some two-thirds of the pole faces are two copper plates or shields C, and a permanent magnet D.

~Ques. How does it work?~

Ans. Eddy currents are induced in the two copper plates or shields C, which attract those in the disc, producing in consequence a torque in the direction shown by the arrow, against the opposing action of a spring. Magnet D damps the oscillations.

~Ques. Describe the rotary field type of induction instrument.~

Ans. The parts are arranged similar to those of wattmeters, the necessary split phase being produced by dividing the current into two circuits, one inductive and the other non-inductive.

~Dynamometers.~--This type of instrument is used to measure volts, amperes, or watts, and its operation depends on the reaction between two coils when the current to be measured is passed through them. One of the coils is fixed and the other movable.

~Ques. Describe the construction of a dynamometer.~

Ans. It consists, as shown in fig. 2,513, of a fixed coil, composed of a number of turns of wire, and fastened to a vertical support. The fixed coil is surrounded by a movable coil composed of a few number of turns or often of only one turn of wire. The movable coil is suspended by a thread and a spiral spring attached to a tortive head which passes through the center of a dial. The ends of the movable coil dip into mercury cups, which act as pivots and electrical contacts, making connection with one end of the fixed coil and one terminal of the instrument as shown. The tortion head can be turned so as to place the planes of the coils at right angles to each other and to apply tortion to the spring to oppose the deflection of the movable coil for this position when a current is passed through the coils. A pointer attached to the movable coil indicates its position on the graduated dial between the two stops. Another pointer attached to the tortion head performs a similar function.

~Ques. How does the dynamometer operate?~

Ans. When current is passed through both coils, the movable coil is deflected against one of the stop pins, then the tortion head is turned to oppose the movement until the deflection has been overcome and the coil brought back to its original position.

[Illustration: FIG. 2,518.--Keystone dynamometer movement. Since the law governing this type of instrument is the law of current squares, it follows that in the case of voltmeters, equally divided scales cannot be obtained. In the case of ~wattmeters~, the scale is approximately equally divided, due to the fact that the movement of the moving coil is proportional to the product of the current in the fixed and moving coils. ~The moving parts~ have been made as light in weight as is consistent with mechanical strength, and the entire moving system is supported on jeweled bearings. The motion of the pointer is rendered aperiodic by the use of an aluminum air vane moving in a partially enclosed air chamber. This method of damping the oscillations of the moving parts renders unnecessary the use of mechanical brakes or other frictional devices, which tend to impair the accuracy of the instrument. The illustration shows a ~voltmeter~, which, however, differs but little from a wattmeter. In the case of a wattmeter the fixed coils are connected in series with the line, either directly or through a current transformer, while the moving coil is connected in shunt to the line.] The angle through which the tortion head was turned, being proportional to the square root of the angle of tortion, the current strength in amperes is equal to the square root of the angle of tortion multiplied by a calculated constant, furnished by the maker of the instrument.

~Ques. How is the dynamometer arranged to measure watts?~

Ans. When measuring watts, the instrument should be so arranged that one coil carries the main current, and the other a small current which is proportional to the pressure.

~Ques. In the construction of a dynamometer what material should not be used and why?~

Ans. No iron or other magnetic material should be employed because of the hysteresis losses occasioned thereby. The frame should be of non-conducting material so as to avoid eddy currents.

~Watt Hour Meters.~--A watt hour meter is a watt meter that will register the watt hours expended during an interval of time. Watt hour meters are often erroneously called ~recording~ or ~integrating~ watt meters.

There are several types of the electromotor form of watt hour meter, which may be classified as

1. Commutator type; 2. Induction type; 3. Faraday disc type.

~Ques. What are the essential parts of a watt hour meter?~

Ans. A motor, generator, and counting mechanism.

~Ques. What is the function of the motor?~

Ans. Since the motor runs at a speed proportional to the energy passing through the circuit, it drives the counting mechanism at the proper speed to indicate the amount of energy consumed.

~Ques. What is the object of the generator?~

Ans. It furnishes a suitable counter torque or load for the motor.

~Ques. Is there any other resistance to be overcome by the motor?~

Ans. It must overcome the friction of all the moving parts.

~Ques. Is the friction constant?~

Ans. No.

~Ques. What provision is made to correct the error due to friction?~

Ans. The meter is compensated by exciting an adjustable auxiliary field from the shunt or pressure circuit.

~Ques. What is the construction of the generator?~

Ans. In nearly all meters it consists of a copper or aluminum disc carried on the same shaft with the motor and rotated in a magnetic field of constant value.

~Ques. How is the counter torque produced?~

Ans. When the disc is rotated in the magnetic field, eddy currents are induced in the disc in a direction to oppose the motion which produces them.

~Ques. For what services is the commutator type meter used?~

Ans. It is used on both direct and alternating current circuits.

~Ques. What is the objection to the commutator meter?~

Ans. The complication of commutator and brushes, and the fact that the friction of the brushes is likely to affect the accuracy of the meter.

~Ques. What are its characteristics?~

Ans. It is independent of power factor, wave form, and frequency when no iron is used in the motor.

~Ques. What meter is chiefly used on A. C. circuits?~

Ans. The induction meter.

~Principles of Induction Watt Hour Meters.~--Every commercial meter of this type is made up of a number of elements, described below. Each of these elements and parts has certain functions, and each is therefore necessary to the successful operation of the meter; moreover, each element, unless correctly designed, may introduce a source of inaccuracy. These elements are:

1. The field producing element; 2. The moving element; 3. The retarding element; 4. The registering element; 5. The mounting frame and bearings; 6. The friction compensator; 7. The power factor adjustment; 8. Frequency adjustment; 9. The case and cover.

~1. The Field Producing Element.~--This consists of the electromagnetic circuit and the measuring coils. One of these coils, connected in series with the circuit to be metered, is wound of few turns and is therefore of low inductance. The current through it is in phase with the current in the metered circuit. The other coil, connected across the circuit, is highly inductive, and therefore the current in it is nearly 90 degrees out of phase with, and proportional to the voltage of the metered circuit across its terminals. Therefore, when the current in the circuit is in phase with the voltage (100 per cent. power factor) the currents in the meter coils are displaced almost 90 degrees with respect to each other.

~Ques. How is this angle made exactly 90 degrees?~

Ans. By means of the power factor adjustment.

~Ques. How are the coils mounted?~

Ans. They are so mounted on the core that the currents in them produce a rotating or shifting field in the air gap, in somewhat the same manner that the currents in the primary windings of an induction motor produce a rotating field.

~Ques. What is the strength of the rotating field with 90 degrees phase difference between the currents?~

Ans. It is proportional to the product of the currents in the two coils and therefore proportional to the product of current and voltage in the metered circuit.

At any other power factor the field is proportional to this product multiplied by the sine of the angle of phase difference between the two meter currents. If the current in the voltage coil be in quadrature with the voltage of the metered circuit, at any power factor the sine of the angle of phase difference between the currents in the meter circuits will be equal to the cosine of the angular displacement between the current and voltage in the metered circuit. Under these conditions therefore the strength of the shifting field is proportional also to the power factor of the circuit. In other words, the strength of the rotating field is proportional to the product of the volts, amperes and power factor and is therefore a measure of the actual power.

~Ques. In what part of the meter is energy consumed?~

Ans. In the field producing element.

It is upon the design of this element that the losses in the meter depend. Current is flowing through the shunt coil continuously, even when no energy is being taken, and the higher the inductance of this coil, the smaller will be the energy component of the constant flow. The series coil causes a loss of energy proportional to the square of the current flowing. It also causes a drop in voltage, both inductive and resistive, hence, the resistance and inductance of the series coil of the meter should be as low as possible.

~Ques. How should the magnetic circuit be designed?~

Ans. The design should be such that the increase of magnetic flux with high voltage or high current will not have a retarding action but will act only to increase the torque.

If the retarding effect be not prevented, the meter will, of course, run slow at overloads. A comparative test of meters at varying load and at varying voltage will reveal the characteristics of the magnetic circuit.

~2. The Moving Element.~--This usually consists of a light metal disc revolving through the air gap in which the rotating field is produced.

~Ques. What is the action of the disc?~

Ans. It acts like the squirrel cage armature of an induction motor, developing the motive torque for the meter.

~Ques. How is this torque counter balanced?~

Ans. By the retarding element so that the speed is proportional to the torque.

~Ques. How should the disc be made and why?~

Ans. As light as possible to reduce wear on the bearings to a minimum.

~3. The Retarding Element.~--This part acts as a load on the induction motor and enables the adjustment of its speed to normal limits. In order that the speed shall be proportional to the driving torque, which varies with the watts in the circuit, it is necessary that the torque of the retarding device be proportional to the speed. For this reason a short circuited constant field generator, consisting of a metal disc rotating between permanent magnet poles, has been generally adopted.

~Ques. How is the retarding torque produced?~

Ans. Eddy currents are induced in the disc in rotating through the magnetic field which, according to Lenz's law, oppose the force that produces them, thus developing a retarding torque.

~Ques. How is the constant field for the retarding disc produced?~

Ans. By permanent magnets.

The retarding disc may be the same disc used for the moving element, in which case the meter field acts on one edge while the permanent magnet field acts on the edge diametrically opposite. This arrangement simplifies the number of parts and saves space and weight of moving element.

~Ques. What error is likely to be introduced by the retarding element?~

Ans. If the strength of the permanent magnets change from any cause, the retarding torque will be changed and the calibration of the meter rendered inaccurate.

~Ques. How may the strength of the permanent magnets be changed?~

Ans. They may become weak with age, or affected by the proximity of other magnetic fields. The series coil of the meter may, under short circuit so affect the strength of the permanent magnets as to render the meter inaccurate.

~Ques. What precautions are taken to keep the strength of the permanent magnets constant?~

Ans. Weakening with age is prevented by the process of "Aging." The effect of neighboring fields is overcome by iron shields; this prevents the electromagnets affecting, through overloads, the strength of the permanent magnets.

~4. The Registering Element.~--This mechanism comprises the dials, pointers, and gear train necessary to secure the required reduction in speed. This gear train is driven directly by the rotor and therefore its friction should be low and constant. The dials should be easily read and should register directly in kilowatt hours. If a constant be used to reduce the reading to kilowatt hours, it should be some multiple of 10, to avoid errors in multiplication. By means of suitable gears in the meters this is easily accomplished.

~5. The Mounting Frame and Bearings.~--These parts have an important influence on the accuracy of the meter, as it is in the bearings that most of the friction in the meter occurs. The frame should be rigid and free from vibration, so that the bearings will be at all times in perfect alignment.

Initial friction is unavoidable in any meter construction and can be easily compensated for. A change in the initial friction, however, due to wear of bearings, makes readjustment necessary.

In selecting a meter the special attention should therefore be given, to the construction of the bearings, particularly the lower, or "step" bearing which supports the weight of the moving element.

~Ques. Describe a good construction for the step bearing.~

Ans. A desirable construction would consist of a very highly polished and hardened ball with jewel seats.

~6. The Friction Compensator.~--The object of this device is to overcome the initial friction of the moving parts. It is evident that if this initial friction were not compensated some of the driving torque of the meter would be used in overcoming it, and the meter would therefore not rotate at very light load, and not fast enough at other loads, thus rendering the registration inaccurate, especially at light loads.

Since meters are usually run at light loads it is important that an efficient light load adjustment or friction compensator should be provided.

~Ques. What important point should be considered in the design of the friction compensator?~

Ans. The compensating torque must not cause the moving element to rotate or "creep" without current in the series coil.

The rotation of a meter is caused by two distinct torques, the varying meter torque, dependent on the power in the circuit, and the constant torque adjusted to compensate the initial friction.

The friction at all speeds is not exactly the same as the initial friction, and therefore the friction compensating torque may be in error a few per cent. at high speeds.

If the compensating torque be small compared with the driving torque, this small error percentage is negligible in its effect on accuracy. The smaller it is, the greater will be the accuracy at all loads, and therefore, as the compensating torque is adjusted to balance the initial friction, the initial friction should be small compared with the driving torque.

A high driving torque and low initial friction are therefore desirable, but any increase in the driving torque which necessitates an increase in friction, is obviously useless.

The desirable feature of a meter is high ratio of torque to friction. As the friction is practically proportional, to the weight of the moving element, in meters having the same form of bearing, the ratio of torque to weight of rotor gives an approximation to the ratio of torque to friction, but the design of bearing should not be overlooked.

A meter having a high torque obtained by using a thick and consequently heavy disc, often has a lower ratio of torque to weight than another with lower torque, and is consequently likely to be less accurate over a given range. Furthermore, the heavy disc is a distinct disadvantage because it produces more wear on the bearings and thus reduces the life.

~7. The Power Factor Adjustment.~ This adjustment is necessary to make the phase angle between the shunt and series field components 90° with unity power factor in the metered circuit. Owing to the resistance and iron loss in the shunt field circuit, that field is not shifted quite 90° with respect to the voltage. Yet exact quadrature is necessary to make the strength of the resultant field, and consequently the rotor speed, proportional to the power factor, as explained in the discussion of the field producing element.

~Ques. What is the usual construction of the power factor adjustment?~

Ans. It usually consists of a short circuited loop enclosing part or all of the shunt field flux.

~Ques. How does this loop act?~

Ans. It acts like the secondary of a transformer.

The flux induces a current in it which, acting with the current in the shunt coil, produces a slightly lagging field. By shifting the position of the resistance of the short circuited loop, the lag may be so adjusted that the shunt field flux is in exact quadrature with the voltage. It should be noted, however, that this adjustment makes the meter correct at or near one frequency only. This feature is not objectionable if reasonable accuracy be maintained within the limits of normal variation of frequency.

~8. Frequency Adjustment.~--This is often desirable, particularly for systems operating at 133 cycles. Most makes of meter are provided with means for changing the adjustment from 133 to 60 cycles in case of change in the system.

~9. The Case and Cover.~--These parts should be dust and bug proof, to avoid damage to the bearings, insulation and moving parts, and should of course be provided with means for sealing.

Terminal chambers so arranged that the cover of the meter element need not be removed in connecting up, are an important feature, particularly in meters that require no adjustment at installation, as they prevent entrance of dust into the main meter chamber.

A window through which the rotation of the disc can be observed in checking, should be provided for the same reason.

~The Faraday Disc, or Mercury Motor Ampere Hour Meter.~--On this type of meter the mercury motor consists essentially of a copper disc floated in mercury between the poles of a magnet and provided with leads to and from the mercury at diametrically opposite points. The theoretical relations of the various parts are shown in fig. 2,558.

~Ques. Explain its operation.~

Ans. The electric current, as shown in fig. 2,558, enters the contact C, passes through the comparatively high resistance mercury H to the edge of the low resistance copper disc D across the disc to the mercury H and out of contact C'. The magnetic flux cuts across the disc on each side from N to S, making a complete circuit through M and M'. The relative directions of the magnetic flux and the current of electricity as well as the resulting motion are shown in fig. 2,559. According to the laws of electromagnetic induction, if a current carrying conductor cut a magnetic field of flux at right angles, a force is exerted upon the conductor, tending to push it at right angles to both the current and the flux. When connected to an eddy current damper or generator which requires a driving force directly proportional to the speed of rotation, the mercury motor generator becomes a meter. The speed of such a meter is a measure of the current or rate of flow of the electricity through the motor element, and each revolution of the motor corresponds to a given quantity of electricity. Then, by connecting a revolution counter to this motor generator, a means is provided for indicating the total quantity of electricity in ampere hours that is passed through the meter.

~Ques. How is the flux produced in the alternating current form of Faraday disc meter?~

Ans. By the secondary current of a series transformer.

~Frequency Indicators.~--A frequency indicator or meter is an instrument used for determining the frequency, or number of cycles per second of an alternating current. There are several forms of frequency indicator, whose principle of operation differs, and according to which, they may be classed as

1. Synchronous motor type; 2. Resonance type; 3. Induction type.

~Ques. How is a synchronous motor employed as a frequency indicator?~

Ans. A small synchronous motor is connected in the circuit of the current whose frequency is to be measured. After determining the revolutions per minute by using a revolution counter, the frequency is easily calculated as follows:

frequency = (revolutions ~per second~ × number of poles) ÷ 2.

~Ques. Describe the resonance method of obtaining the frequency.~

Ans. In construction, the apparatus consists of a pendulum, or reed, of given length, which responds to periodic forces having the same natural period as itself. The instrument comprises a number of reeds of different lengths, mounted in a row, and all simultaneously subjected to the oscillatory attraction of an electromagnet excited by the supply current that is being measured. The reed, which has the same natural time period as the current will vibrate, while the others will remain practically at rest.

The construction and operation of the instrument may be better understood from figs. 2,565 and 2,566, which illustrates the indicating part of the Frahm meter. This consists of one or more rows of tuned reeds rigidly mounted side by side on a common and slightly flexible base.

The reeds are made of spring steel, 3 or 7 mm. wide, with a small portion of their free ends bent over at right angles as shown in fig. 2,566 and enameled white so that when viewed end on they will be easily visible. The reeds are of adjustable length, and are weighted at the end.

A piece of soft iron, rigidly fastened on the base plate which supports the reeds, forms the armature of a magnet.

When the magnet is excited by alternating current, or interrupted direct current, the armature is set in vibration, and that gives a slight movement to the base plate at right angles to its axis, thereby affecting all the reeds, especially those which are almost in tune with its vibrations.

The reed which is in tune will vibrate through an arc of considerable amplitude, and so indicate the frequency of the exciting current.

~Ques. For what use is the resonance type of frequency meter most desirable?~

Ans. For laboratory use.

~Ques. Describe the induction type of frequency meter.~

Ans. It consists of two voltmeter electromagnets acting in opposition on a disc attached to the pointer shaft. One of the magnets is in series with an inductance, and the other with a resistance, so that any change in the frequency will unbalance the forces acting on the shaft and cause the pointer to assume a new position, when the forces are again balanced. The aluminum disc is so arranged that when the shaft turns in one direction the torque of the magnet tending to rotate it decreases, while the torque of the other magnet increases. The pointer therefore comes to rest where the torques of the two magnets are equal, the pointer indicating the frequency on the scale.

~Ques. What is the object of the aluminum disc?~

Ans. Its function is to damp the oscillations of the pointer.

~Synchronism Indicators.~--These devices, sometimes called synchroscopes, or synchronizers indicate the exact difference in phase angle at every instant, and the difference in frequency, between an incoming machine and the system to which it is to be connected, so that the coupling switch can be closed at the proper instant. There are several types of synchronizer, such as

1. Lamp or voltmeter; 2. Resonance or vibrating reed; 3. Rotating field.

The simplest arrangement consists of a lamp or preferably a voltmeter connected across one pole of a two pole switch connecting the incoming machine to the busbars, the other pole of the switch being already closed.

If the machines be out of step, the lamps will fluctuate in brightness, or the voltmeter pointer will oscillate, the pulsation becoming less and less as the incoming machine approaches synchronous speed. Synchronism is shown by the lamp remaining out, or the voltmeter at zero.

~Ques. How does the resonance type of synchronism indicator operate?~

Ans. On the same principle as the resonance type of frequency indicator, already described.

~Ques. What is the principle of the rotating field type of synchronism indicator?~

Ans. Its operation depends on the production of a rotating field by the currents of the metered circuits in angularly placed coils, one for each phase in the case of a polyphase indicator. In this field is provided a movable iron vane or armature, magnetized by a stationary coil whose current is in phase with the voltage of one phase of the circuit. As the iron vane is attracted or repelled by the rotating field, it takes up a position where the zero of the rotating field occurs at the same instant as the zero of its own field. In the single phase meter the positions of voltage and current coils are interchanged and the rotating field is produced by means of a split phase winding connected to the voltage circuit.

~Power Factor Indicators.~--Meters of this class indicate the phase relationship between pressure and current, and are therefore sometimes called phase indicators. There are two types:

1. Wattmeter type; 2. Disc, or rotating field type.

In the wattmeter type, the phase relation between the pressure and the current fluxes is such that on a non-inductive load the torque is zero.

For instance, in a dynamometer wattmeter, the pressure circuit is made highly inductive and the instrument then indicates volts × amperes × sin φ instead of volts × amperes × cos φ, that is to say, it will indicate the wattless component of the power. A dynamometer of this type is sometimes called an idle current wattmeter.

~Ques. Describe a single phase power factor meter of the disc or rotating field type.~

Ans. It consists of two pressure coils, as shown in fig. 2,579, placed at right angles to each other, one being connected through a resistance, and the other through an inductance so as to "split" the phase and get the equivalent of a rotating magnetic field.

The coils are placed about a common axis, along which is pivoted an iron disc or vane. The magnetizing coils FG are in series with the load. If the load be very inductive, the coil M experiences very little torque and the system will set itself as shown in the figure. As the load becomes less inductive, the torque on S decreases and on M increases so that the system takes up a particular position for every angle of lag or lead.

~Ground Detectors.~--Instruments of this name are used for detecting (and sometimes measuring) the leakage to earth or the insulation of a line or network and are sometimes called ground or earth indicators, or leakage detectors.

For systems not permanently earthed anywhere, these instruments are nearly all based on a measurement of the pressure difference between each pole and earth, two measurements being required for two wire systems, and three for three wire, whether direct current single phase, or polyphase alternating current. ~In the case of direct current~ systems, the insulation, both of the network and of the individual lines, can be calculated from the readings, ~but with alternating current~, the disturbance due to capacity effects is usually too great. ~In any case~, however, the main showing the smallest pressure difference to earth must be taken as being the worst insulated.

For low tension systems ~moving coil~ (for alternating current) or ~moving iron~ instruments (for direct current) are the most used, while for high tension systems electrostatic voltmeters are to be preferred. ~On systems having some point permanently earthed~ at the station, as for instance the neutral wire of direct current system, or the neutral point of a three phase system, an ammeter connected in the earth wire will serve as a rough guide. It should indicate no current so long as the insulation is in a satisfactory state, but on the occurrence of an earth it will at once show a deflection. The indications are, however, often misleading, and serve more as a warning than anything else.

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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.

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Alternating Current Systems, Circuit Breakers, Measuring Instruments.

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Alternating Current Switch Boards, Wiring, Power Stations, Installation and Operation.

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Telephone, Telegraph, Wireless, Bells, Lighting, Railways.

~ELECTRICAL GUIDE, NO. 10~

Modern Practical Applications of Electricity and Ready Reference Index of the 10 Numbers.

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TRANSCRIBER'S NOTES

Silently corrected simple spelling, grammar, and typographical errors.

Retained anachronistic and non-standard spellings as printed.

Enclosed bold markup in ~tildes~.

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