Electricians' operating and testing manual

Front matter

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Preface

The object of this work is to instruct the practical electrician in the management, operation and testing of the more important electrical devices now in use.

Almost every line of industry, great or small, now has much to do with electric motors and lights, to say nothing of the ever increasing number of small isolated plants using gas or gasoline engines in connection with electrical generators.

From observation of many such plants in actual operation it has long been apparent to the authors that some hand book giving in a condensed, simple manner all of the instructions needed for the intelligent installation and operation of electrical devices is greatly needed.

The method adopted has been that of familiarizing the student w T ith the underlying principles governing the design of motors, dynamos, arc lamps, etc., rather than to go overmuch into detail on the construction of particular commercial forms. It is confidently believed that the student who has mastered the general theory of dynamos, motors, etc., will have no difficulty in comprehending such variations in their application as he may meet with.

In order to avoid unnecessary bulkiness and to give the reader as much of the necessary information as

4 Preface

possible within the limits of the space, all catalogue cuts have been omitted; it being assumed that the reader is familiar with the general appearance of motors, arc lamps, storage battery, etc.

It is in the hope that this work may meet with the same success and kindly reception as previous efforts of the authors that this volume is offered to the public.

THE AUTHORS.

Chapter I The Electric Current

By the electric current is meant that agency which comes into action when a circuit containing an electro-motive force is closed. Electro-motive force is, as the name implies, the impelling force, and the circuit is the system of conductors along which alone electrical action takes place. This flow of current is quite analogous to the flow of water in a system of piping or over the surface of the earth. Such a flow of water can take place only when the water is at different levels, or, when from any cause, a difference of pressure exists between different points. When either or both of these conditions exist the flow always takes place in a certain direction, i. e., from the high level or pressure to the lower, and this flow is always more or less diminished by obstacles or resistances. The same observations hold true of electric currents ; they flow only in obedience to electrical pressure ; they flow always in a certain direction determined by that pressure and the quantity of the flow depends, or is governed, other things being equal, by obstacles which are spoken of as resistances.

AYe cannot prove, and it is not necessary, that there is any actual direction, or, much less, a change of direction, or even a flow of current, but the phenomena noticed make the assumption a very convenient one and it is, to say the least, very helpful in the study and application of these phenomena.

Refer now to Figure 1 which shows a common glass jar nearly filled with water and which also contains a small quantity of sulphuric acid and one plate of zinc Z and another of copper C. While the two ends of the wires connected to the plates remain apart there is no flow of current, but an electrical pressure exists, as can readily be shown. As soon, however, as we bring the two ends of wire together a flow of current takes place. It is the high resistance of the air between the two terminals of the wires which prevents the flow of current in this case, just as the resistance of the valve in a water pipe prevents the flow of water when it is closed.

The direction of the flow of current is said to be from the zinc to the copper inside of the cell and from the copper back to the zinc in the exterior circuit or

The Elect pic Current outside of the cell. In all batteries (a battery is a number of cells coupled together) the copper plate or terminal is spoken of as the positive or + pole from which the current flows and the zinc plate as the negative or — pole toward which the current flows. From the cell shown, which is the simplest of all forms, we can obtain but a very insignificant current, and that for only a very short time, for reasons to be given later on. If we couple a number of cells together, as conventionally indicated at B in Figure 2, we shall be able to obtain a considerable current. In this representation of a battery the long thin lines stand for the copper plate from which the current flows to the outside circuit and the short thick lines stand for the zinc element from which the current flows inside of the cell to the copper element.

Figure 2 has been drawn principally to acquaint the reader with the general effects obtainable from the electric current. In the outer system of wires W, W, etc., the same current passes through all of the devices, and this is known as a series circuit. R represents fine wire, which will be heated to redness or even melted if the current is made strong enough. At A is shown the manner in which an arc cr a flash may be produced; first bring the ends of carbon or metal together until the current is started, then separate them a little and the current will continue, thus forming a very hot flame known as the electric arc.

If a wire carrying a current be wound about an iron core M, magnetism will be generated and enable the iron bar to attract other pieces of iron. This magnetism will exist only while the current is flowing.

If the current is forced to pass through water, as indicated at E, it will decompose it and this decomposition will be noticeable by the bubbles of gas given off. If the current pass through a suitably prepared bath in which metals are properly connected to the wires, the metal will be eaten away from the positive pole and deposited at the negative.

The arrangement of wires shown at I, I 1 is drawn to illustrate the method of inducing currents of electricity in transformers. When a current is passed around the bar at I a current will be induced in I'. Tlii \ current will last only a very short time, but if I is connected to a circuit in which the current is contin-

The Electric Current 11

ually changing in value the induction of currents will follow every change in current strength and it is possible to arrange these variations in such a manner that light may be obtained from these induced currents.

The inner circuit shown connected to the battery by heavy lines is known as a parallel circuit, all of the devices being connected in parallel instead of in series as in the other. In such a circuit each device is independent of the others and the current increases in proportion to the needs of the devices connected. The more there are connected the greater becomes the current, but the voltage need not be increased. In a series circuit the more devices there are connected the more cells of battery must be provided to increase the pressure so that the current may remain the same. Only such devices as use the same amount of current can be run in a series circuit.

In the parallel circuit at C there is shown a condenser. A condenser is an arrangement of plates which is capable of taking a charge of electricity at rest much as a jar is capable of taking a charge of water. The positive and negative plates of a condenser are perfectly insulated from each other and a current of electricity cannot pass from one to the other. When, how T ever, the plates are connected to a source of current a small quantity of current w T ill pass into the condenser. Such a charge may be held in a condenser for some time or will pass out of it when the voltage at the terminals is withdrawn or the circuit around the condenser closed. Enough current can be made to pass in and out of a small condenser to affect 9 telephone receiver T, or a sensitive polarized bell,

CONDUCTORS AND INSULATORS

An appreciable current flow can take place only in a system of electrical conductors. The best conductors are the various metals in the order here given : silver, copper, gold, platinum, tin, lead, etc. The difference in favor of silver as against copper is so small compared to the higher price of silver that the latter is seldom used. A pound of copper will, under the same circumstances, carry about 6 or 7 times as much current as a pound of iron and as this fact makes copper much cheaper than iron, copper is the metal almost universally used for electrical purposes.

It is not, however, sufficient to provide conductors along which the current can flow ; it is also necessary to surround these conductors with some material which will prevent the current from flowing anywhere except along these conductors. A bare copper wire lying on some other conducting material can no more be depended upon to carry the current than a lot of broken or disconnected pipes can be depended upon to carry a stream of water. Even under such conditions the current may flow along the wires and the water may flow along the pipes if these happen to offer the easiest path, but in neither case will it be possible to get any work done. To get the proper service from either we must be able to force the flow where our machinery needs it ; whatever portion of it we can not so confine is a direct loss.

Such materials as resist the flow of current sufficiently to prevent its escape from the conductors in appreciable quantities are known as insulators. Sonic

The Electric Current 13

of them are: air, glass, silk, rubber, dry asbestos, porcelain, slate, marble, wood, mica, shellac, paraffine, etc. All insulators to give the best service require to be dry. If they are wet current will leak through the body of those that are porous and over the surface of those that are not. It should be borne in mind that there is no such thing as, either a perfect conductor or a perfect insulator. Every conductor offers some resistance to the flow of current and no matter what the insulator may consist of, if we but make the pressure great enough we can force some current through it.

Chapter II Electrical Units

In order to make any practical use of electricity we must be able to measure it, and for the purpose of measurement and calculation the following units have been adopted by electricians, and in turn legalized by the U. S. government :

The ohm as the unit of resistance.

The ampere as the unit of current flow or current strength.

The volt as the unit of electro-motive-force or electrical pressure.

The coulomb as the unit of quantity. The farad as the unit of capacity.

The joule as the unit of work.

The watt as the unit of power.

The henry as the unit of induction.

THE OHM

The ohm is the unit of resistance. Resistance is a property possessed by all materials, but in varying degrees. It always varies inversely as the cross-section of the material; that is, the larger the wire the less will be its resistance and the smaller the wire the

Electrical Units 15

greater will be its resistance. The resistance of all materials increases directly as the length, and is, also, to a small extent, affected by a rise in temperature. This resistance acts electrically much as friction does mechanically; it is very useful in the proper place and very objectionable in the wrong place. It is this resistance in the filament of a lamp that gives us light when current is forced through it, and heat in the heater, but it is also this resistance which causes the loss in voltage or pressure which makes it so difficult to transmit currents of magnitude over wide areas. Resistance tends to diminish current flow and, when great enough, prevents it entirely.

The legal ohm is equivalent to the resistance of a column of mercury 106.3 centimeters long, 14.4521 grammes in mass and at the temperature of melting ice. As an illustration of more practical value : 2 3/10 feet of No. 36 B. & S. gauge wire has a resistance of one ohm; 380 feet of No. 14 wire a resistance of one ohm and 1,000 feet of No. 10 wire a resistance of one ohm.

THE AMPERE

The ampere is the unit of current strength. It expresses the rate of current flow. It is not correct to speak of it as measuring quantity. To obtain the quantity we must multiply the amperes flowing by the length of time they flow. The heating of a wire, the chemical action and the magnetism produced are all due to the amperes flowing in the circuit. The legal ampere is that current, which, when passed through a solution of nitrate of silver, prepared in accordance with certain specifications, deposits silver at the rate of 0.001118 grammes per second.

It is the current which results from a pressure! ol one volt acting in a closed circuit on a resistance of one ohm. A 16 c. p. 110 volt incandescent lamp requires a current of one-half ampere; an open, series arc lamp a current of about 10 amperes.

THE VOLT

The volt is the unit of electro-motive-force, or electrical pressure. It is this pressure which is the immediate cause of current flow and we speak of it as of so many volts, just as we speak of steam pressure as of so many pounds.

The volt is defined as the electro-motive-force which will force a current of one ampere through a resistance of one ohm. This is equal to about 1000/1434 of the electro-motive-force of a Clarkes cell. The common wet carbon battery gives about 1.2 volts; a storage battery about 2 volts per cell.

THE COULOMB

The coulomb is the unit of quantity. It is the current delivered by one ampere in a second. To find th number of coulombs we multiply the number of amperes by the time in seconds. This unit is seldom used.

THE FARAD

The farad is the unit of capacity. Under certain circumstances electrical conductors and certain appliances chiefly known as condensers can be heavily charged with static electricity (electricity in a state of rest) and can be again discharged; and, in fact, if

Electrical Units 17

subjected to an alternating electro-motive-force this charging and discharging is continually taking place. This charge depends upon the nature of the material out of which the condenser is made upon the number and size of the plates and upon the electro-motive-force of the circuit. The more current there is forced into a given condenser the greater will be its potential difference. This can not, of course, be greater than that maintained at its terminals unless a static charge be given.

Such a conductor or condenser is said to have a capacity of one farad, when a charge of one coulomb produces a difference of potential of one volt. This unit comes into use very seldom in ordinary work.

THE JOULE

The joule is the unit of work. It is equal to the energy expended in forcing one ampere through a resistance of one ohm, in one second. This unit is also seldom used.

THE WATT

The w r att is the unit of power. Just as the ampere expresses the rate of current flow, without telling us anything about the actual quantity delivered, so the watt measures the rate of doing work, or the rate of energy consumption in the circuit. The watts in any circuit are equal to the volts multiplied by the amperes. An incandescent lamp requiring 110 volts and i/o ampere is said to consume energy at the rate of 55 watts. Seven hundred and forty-six watts equal one horsepower. The watt is a much used unit and charges for use of light or power are usually based upon watt-hours. The watts supplied, multiplied by the time, measure the power delivered.

THE HENRY

The henry is the unit of induction. It is seldom used in ordinary practical calculations but an understanding of its meaning is important.

The henry represents the induction in the circuit when the induced electro-motive-force is equal to one volt while the inducing current varies at the rate of one ampere per second.

Chapter III

The simplest form of magnet and also the one with which people are most familiar is the compass needle. This needle is merely a bit of magnetized steel and has the property of pointing toward the north with one of its ends and, of course, south with the other. Such a compass needle is a very convenient instrument to have about and many instructive little experiments can be made with it. If we take a compass and explore any piece of steel that has been lying in a north and south direction, as, for instance some portion of a steel building, we shall quickly see that the north end of the piece of steel has a tendency to repel the north seeking end of the needle, but w r ill attract the south seeking end of the same needle with as much force as it repelled the other end. In making this experiment and in all other similar experiments care must be exercised not to bring the needle, especially if it should not be free to swing, too close to the bar of steel, if this be strongly magnetized, otherwise the powerful magnetism of the bar may overpower that of the needle and reverse its polarity. In such a case the former north seeking end would become the south seeking end.

20 Opt rating and Testing

We have mentioned the needle and the bar as being made of steel because steel differs from iron in that it has the power to retain whatever magnetism it may be charged with for an indefinite time, while iron, especially if it be well annealed and soft, looses its magnetism the instant the magnetizing force ceases to act. A bar of iron will attract the needle as well as the steel but to a lesser extent ; with the steel bar the attracting or repelling force will be due to the action of both magnets while with the iron bar it will be only the force of the needle that does the attracting.

Magnets consisting of hardened steel are known as permanent magnets and usually made up either in horse-shoe shape or in the form of straight bars as shown in the following figures. It is impossible to make a magnet with one pole only. No matter into how many pieces we may divide a magnetized bar. each piece wiH possess a north and a south pole. This is illustrated in Figure 3 where the different poles are designated by the iron filings which cling to them. If these pieces be all joined again perfectly the intermediate poles will all disappear and there will be only the two poles, one at each end. It is, however, possible to arrange a bar magnet so that it shall have a number of poles throughout its length, even while it remains solid. This is illustrated in Figure 4, where the two ends of the bar are magnetized in opposite direction so that two poles of same sign are formed in the center and oppose each other.

Consider now the horse-shoe magnet shown in Figure 5. If we take a magnet of this kind and sprinkle a lot of iron filings or small tacks about the ends they will be attracted and form around it in the manner shown. If the magnet is weak it will be necessary to assist the formation somew r hat by gently placing the tacks where they will.stick. If we now raise the magnet, a large part of the filings or tacks will follow and we can carefully put on a number more, but shall soon learn that there is a limit and that as we put on

S. s more tacks in one place some of the others will fall off. If we now take the armature A and place it across the pole of the magnet, say at 0, by far the greater part of the tacks will fall off. The reason for this is that the magnetic flow or flux, as it is usually termed, follows along the lines of least resistance like any other flow. The armature offering a path of much lower resistance than the partially disconnected tacks simply shunts the flow around them and they cease to be attracted.

The magnetism is conceived to consist of a flow of lines of force as indicated in Figure 6. These lines are supposed to leave the magnet at the north pole N and passing through the intervening space, to return to the south pole S of the same magnet. If we take a compass and beginning, say at the right hand end, move the needle along the path of the lines of force we shall see it align itself to them and as it is brought to the other end of the bar the other end of the needle will meet it. The flow of these lines of force is greatly facilitated By iron or steel but there is no medium

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W which can be interposed that will prevent the flow entirely ; in other words, there is no insulation for magnetism. It is, however, possible to shield bodies from it by introducing an easier path for it as we have seen in the experiment with the tacks.

Permanent magnets are generally made by "touching, ' ' that is, a magnet is wiped across the end of the bar which is to be magnetized as shown in Figure 7. Tf the inducing magnet is strong it is not even necessary to bring it in contact with the bar to be magnetized. One must proceed in a systematic manner, however, that is, always touch the same end of the bar to be magnetized with the same end of the magnetizing bar and move it over the bar in the same direction. If this is not done one "touch" will neutralize the other and the result will be either no magnetism at all or at best but a very little of it.

Permanent magnets are often made up of a number of bars of equal length and shape fastened together and are then known as compound magnets. Permanent magnets can be demagnetized by heating to a red heat.

ELECTRO-MAGNETS

Electro-magnets differ from permanent magnets; first, in being made of soft iron instead of steel ; second, the magnetizing force is not another magnet, but a current of electricity ; third, the magnetism lasts only while the current is circulating in the wire wound around the core ; fourth, the strength of magnetism is variable and within certain limits in proportion to the current flowing; fifth, the polarity, or the direction of the lines of force changes with the direction of the current and can therefore be instantly reversed.

We may now consider the generation of magnetism by means of the electric current.

If we assume the black circle (Figure 8) to be an electrical conductor in which the current is flowing in the positive direction (i. e., away from us) that conductor will be surrounded by lines of force circling about it in the direction of the arrows shown. The number of lines of force w T ill be directly in proportion

Figure 8 Figure 9 Figure 10

to the current strength (number of amperes) in the circuit. If we reverse the direction of the current the lines of force will circulate in the opposite direction. If we lay two wires, carrying current in the opposite direction, side by side as shown in Figure 9 the lines of force will repel each other and tend to separate the wires ; if, however, these wires be carrying current in the same direction the lines of force will act in harmony and tend to draw the wires together as in Figure 10. These lines of force are, of course, only conceptions, but they are very natural ones. If wires such as those shown be thrust through a piece of paper or cardboard at right angles to it and if iron filings be sprinkled on the paper and near the wires, these filings will have a tendency to arrange themselves in circles around the wires as outlined in the figures. In order to properly get such an ouline it will be necessary to gently jar the paper several times thus temporarily annulling the friction which tends to hold them in their place, so that they may more readily align themselves to the lines of force surrounding the wire.

If we now wrap a wire carrying a current of electricity around a bar of iron as shown in Figure 11 these lines of force will pass through the iron as depicted and we shall have a similar circulation of lines of force in this case as was observed in a magnetized bar of steel. Magnetism is also produced by a coil of ware wound in this way that contains no iron but the magnetic flux will be much less. This is due to the fact that the iron offers a much lower resistance to the flow of lines of force than does air and hence their number is greatly increased. The law that governs magnetic flux is exactly similar to Ohm's law which governs current flow It is :

Magneto-motive force Magnetic flux =

Magnetic resistance

The magneto-motive-force is produced by the current circulating in the coil of wire and so far as magnetism is concerned it matters not at all whether the 100 amperes circulate once around the bar or certain air space or whether one ampere circulates 100 times. The magnetizing force is always proportional to the product of the number of turns of wire and the current flowing in the wire. This product is generally known as the " ampere turns. "

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The magnetic resistance varies with the material used inside of the coil, or helix, as it is often termed. It is greatest with air (with exception of a few substances that need not be considered in practical work) and least with well annealed wrought iron. The magnetic resistance also decreases as the cross-section of the iron increases and, conversely, increases as the cross-section of the iron decreases, i. e., the core becomes smaller. The above is rigidly true only for air and approximately only within certain limits for iron. The conductivity of iron for lines of force has a limit and if we attempt to force too many lines of force through a given core a point will soon be reached where the iron assists the magnetization only to a very small extent and any further increase in the strength of that magnet will be little more than in the ratio that is possible for air. This is illustrated by Figure 12 which shows graphically, by means of the curves the relative magnetism produced in different kinds of iron and steel. The highest magnetization is possible with annealed sheet iron, the lowest curve shown is for cast iron. The numbers along the bottom line indicate the relative ampere-turns or magnetizing force and those along the vertical line the resulting magnetic flux or magnetism.

We have seen by Figure 11 how the lines of force produced by currents of electricity produce magnetism, either in the surrounding air or in a bar of iron. If we now coil another wire about the bar and send current through it in the opposite direction, or reverse a part of the winding in any coil so that current will circulate in the opposite direction we shall neutralize the influence of the first coil; that is, the lines of force produced by the two windings will oppose each other and there will be no magnetism if they are exactly equal to each other. If they are not equal then the resulting magnetism will be proportional to the difference in strength of the two opposing coils. It must not be understood that the number of turns of wire, size of wire, and current in the wire must be the same, but that the "ampere turns" in me two coils must be equal. To find the difference we must subtract the ampere turns in the weaker coil from those in the stronger. It is not possible to quite neutralize the action of one coil by the action of the other but we can come very close to it and the remaining magnetism can be detected by the most delicate instruments only. Such a winding is known as ' ' differential" and is made use of in many dynamos, motors, arc lamps, measuring instruments, etc.

We may now briefly consider the forms of magnets suitable for different purposes. The attraction of an

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In electromagnet for its armature varies as the square of the number of lines of force passing through both. If then we desire to obtain the greatest traction, we must see that we obtain the greatest number of lines of force that a given current can produce. For this purpose it is necessary to arrange the magnetic circuit so that it shall have the least possible resistance. This we obtain when we make the cross-section of the core large enough so that the magnetization need never be pushed beyond the nearly straight rise of the curves shown in Figure 12. The iron core should also be made as short as possible. We must, however, have space to place a certain number of turns of wire around the core and it must be long enough to allow this. It need not be a bit longer. We must not, however, imagine that very much can be gained by crowding the windings together as shown in Figure 13; for in such a case the outer layers of wire become very long and introduce unnecessary resistance into the electrical circuit and also, this construction makes necessary an unusual length of the yoke. As a general rule it will be found advisable to make the thickness of the coil about equal to the thickness of the core ; to make the yoke just long enough so that the coils