Hawkins Electrical Guide Number Two, by Hawkins and Staff, a Project Gutenberg eBook.

The armature of a dynamo consists of coils of insulated wire wound around an iron core, and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field or the field magnets rotated and armature held stationary.

THE ARMATURE	221 to 228
Definition— how continuous current is obtained —type of armature—comparison ring and drum armatures— why drum armature is the prevailing type —disc armatures—why disc armatures were abandoned.
ARMATURE WINDINGS	229 to 256
Preliminary considerations— winding diagrams and winding tables —lap and wave winding—angular pitch or spread of drum coils—parallel or lap winding —series or wave winding —double-windings—Siemens winding—objection to Siemens winding— chord winding —multiplex windings—number of brushes required—number of armature circuits— equalizer rings — drum winding requirements .
THEORY OF THE ARMATURE	257 to 282
Current distribution in ring and drum armatures— connection of brushes —variation of voltage around the commutator— cross magnetization ; field distortion—remedies for field distortion—angle of lead—demagnetizing effect of armature reaction— effect of lead — eddy currents ; lamination—remedy for eddy currents—magnetic drag on the armature—smooth and slotted armatures—comparison of smooth and slotted armatures—magnetic hysteresis in armature cores— core loss or iron loss—dead turns—friction. 4
COMMUTATION AND THE COMMUTATOR	283 to 302
Period of commutation —commutating plane—normal neutral plane—neutral plane—plane of maximum induction—commutation—position of the brushes— sparking —effect of self-induction—construction of commutators— points relating to commutators—types of commutator.
BRUSHES AND THE BRUSH GEAR	303 to 320
Classification—gauze brushes—wire brushes—strip brushes— carbon brushes —adjustment—comparison of copper and carbon brushes—size of brushes—number— contact angle of brush — brush contact—drop in voltage at brushes— brush holders —brush rigging—multipolar brush gear.
ARMATURE CONSTRUCTION	321 to 348
Parts—shaft—core—slotted core— core laminations —core bolts—attachment to shaft—insulation of core discs—teeth—advantages and defects of slotted armatures—slotted cores; built up construction— ventilation — insulation of core —armature windings—construction of inductors—objection to copper bars— various windings : hand winding—evolute or butterfly winding—connectors—barrel winding—bastard winding—former winding—former coils—peculiarity of evolute coil—"straight out" coil— coil retaining devices —driving horns.
MOTORS	349 to 388
Definition— principles —propelling drag—essential requirements of construction— the reverse electromotive force —hydraulic analogy—action of current supplied to motor— armature reaction in motors—method of starting a motor— classes of motor; series, shunt, and compound —power of a motor—brake horse power—mutual relations of motor torque and speed— speed regulation —series parallel controller— interpole motors . 5
SELECTION AND INSTALLATION	389 to 406
General conditions governing selection — construction —efficiency—adaptation of series and shunt motors—location—foundations— erection of dynamos and motors —connecting up dynamos—marine generating set—belt clamp—belt lacing—belt speed— points relating to belts —gear drive—friction drive— electrical connections .
AUXILIARY APPARATUS	407 to 430
Switches —switch classification and construction—difficulty encountered in opening the circuit— various switches : knife, snap, and quick break types— fuses —circuit breakers: maximum, minimum, reverse current, maximum and reverse current, no voltage breaker— discriminating cut out —time limit attachments— rheostats —starting boxes— switchboards . 221

Ques. What are the practical objections to the elementary armature, described in fig. 165?

Ans. It induces a very feeble current, which is not of constant pressure, but pulsating; that is, it consists of two pronounced impulses in each revolution as shown in fig. 168.

Ans. The pulsations are due to the coil moving alternately into, and out of, the positions of best and least action in the magnetic field.

Ans. If an additional coil be added to the elementary armature, at right angles to the existing coil, and its ends suitably connected to a four part commutator, as in fig. 185, so that 222 one coil is in the position of best action, while the other is in the position of least action, the pulsations of the resulting current will be of less magnitude. By increasing the coils and suitably altering the construction of the commutator to accommodate the ends of these coils, the resultant current may be represented by practically a straight line, indicating the so called continuous current , instead of the wavy resultant curve No. 6, as illustrated in fig. 187.

Fig. 247.—Ring armature of four pole dynamo: diagram of winding and connections, showing direction of the induced currents. The currents in the windings under the upper N and S poles are opposed to each other and flow to the external circuit by the positive brush 1, and back to this half of the armature by the negative brushes 3 and 4. At the same instant the opposed currents in the lower windings flow to the external circuit by positive brush 2 and return to the armature through negative brushes 3 and 4. The armature is thus divided into four circuits and four brushes are required which must be placed between the poles so as to short circuit the coils as they pass through the neutral space. In this form of winding there is no difference of potential between the + brushes, so that they are connected in parallel, as are also the negative brushes, and then to the external circuit. In multipolar machines there are as many brushes as pole pieces. Since opposite commutator bars are of the same potential on this four pole dynamo they may be joined by a cross connecting wire and two brushes, as 2 and 4, dispensed with. This can only be done when there is an even number of coils. The armature is said to be "cross connected."

Types of Armature. —Although there are many forms of armature, all may be divided into three classes, according to the arrangement of the coils or winding on the core, as:

Fig. 248.—Early form of Gramme ring armature, the core being shown cut through, and some of the coils displaced to make it clearer. The core, F, consists of a quantity of iron wire wound continuously to form a ring of the shape shown by the section. Over this is wound about thirty coils of insulated copper wire, B C D, etc., the direction of the winding of each being the same, and their adjacent ends connected together. The commutator segments consist of a corresponding number of brass angle pieces, m , n , which are fixed against the wooden boss, o , carried on the driving shaft. The junction of every two adjacent coils is connected to one of the commutator segments, as shown at n .

Ans. The drum armature is electrically and mechanically the more efficient, possessing, as it does, possibilities in the way of better mechanical construction of the core, and in the 224 arrangement and fixing of the inductors thereon not to be found in the ring form. Less wire and magnetizing current are required for the field magnets for a given output than with the ring armature. Drum winding is not so simple as ring winding, and it is more difficult to ventilate a drum than a ring armature, it being necessary to provide special ventilating ducts.

Fig. 249.—Modern form of Gramme ring armature. The core consists of a number of thin flat rings of well annealed charcoal iron, the outer diameter of each ring or disc being 11½ inches, and its inner diameter 9¼ inches. Sheets of thin paper insulate each disc from its neighbors to prevent the flow of eddy currents. The armature is mounted on a steel shaft to which is keyed a four armed metal "spider," the extremities of whose arms fit into notches cut in the inner edges of the soft iron core rings, so that a good mechanical connection is obtained between the core and the shaft. The spider is made of a non-magnetic metal, to reduce the tendency to leakage of lines of force across the interior of the armature. The armature inductors consist of cotton covered copper wire of No. 9 standard wire gauge, wound around the core in one layer, and offering a resistance, from brush to brush, of 0.048 ohm. There are two convolutions in each section, the adjacent ends of neighboring sections being soldered to radial lugs projecting from the commutator bars.

Ans. It consists essentially of an iron ring, around which is wound a number of coils. These various coils are wound on separately, the wire being carried over the outside of the ring, then through the center opening and again around the outside, this operation being repeated until the winding for that individual section is completed. The adjacent coil is then wound in the same way, the ends of each being brought out to the commutator side of the armature, the arrangement of 225 the coils on the ring and connections with the commutator being shown in fig. 247 , examples of actual construction being shown in figs. 248 and 249 .

Ques. For what conditions of operation is the ring armature specially adapted, and why?

Ans. It is well suited to the generation of small currents at high voltage, as for series arc lighting, because the numerous coils can be very well insulated.

Ques. Why does a ring armature require more copper in the winding than a drum armature?

Ans. For the reason that those inductors which lie on the inner side of the iron ring, being screened from practically all the lines of force, as shown in fig. 250 , do not generate any current.

Ans. It reduces considerably the large amount of dead wire necessary with the ring type.

Ans. By winding the wire entirely on the outer surface of a cylinder or drum , as it is called, as shown in fig. 251 , thus none of the wire is screened by the metal of the core.

Disc Armatures. —The inductors of a disc armature move in a plane, perpendicular to the direction of the lines of force, about an axis parallel to them as shown in fig. 253 . The main difficulty with this type has been in constructing it so that it will be strong and capable of resisting wear and tear. It was introduced in an effort to avoid the losses due to eddy currents and hysteresis present in the other types of armature.

Fig. 253.—Disc armature of Niaudet. It is equivalent to a ring armature, having the coils turned through an angle of 90°, so that all the coils lie in a plane perpendicular to the axis of rotation. The connections of the coils with each other and with the commutator remain the same, the beginning and the end of adjacent coils leading to a common commutator bar as shown. The magnetic field is arranged by the use of two magnets, so arranged as to present the north pole of one to the south pole of the other, and vice versa . In the figure one of these magnets is considered as above the paper, and the other below. If this armature be rotated through the magnetic field as shown, a reversal of current takes place in each coil, when it is in such a position that one of its diameters coincides with the pole line, NS . If the brushes be set so as to short circuit the coils that are in this position, the armature will be divided into two branchings, the current flowing in an opposite direction in each, and a direct current will flow in the exterior circuit.

CHAPTER XVIII
ARMATURE WINDINGS

To connect up rightly the inductors on an armature so as to produce a desired result is a simple matter in the case of ring winding, for bipolar or multipolar machines. It is a less easy matter in the case of drum winding, especially for multipolar machines. Often there are several different ways of arriving at the same result, and the fact that methods which are electrically equivalent may be geometrically and mechanically different makes it desirable to have a systematic method of treating the subject.

The elementary arrangement of drum and disc armatures has already been considered, which is sufficient explanation for small armature coils of only a few turns of wire, but in the case of larger machines which require many coils, further treatment of the subject is necessary.

Winding Diagrams and Winding Tables. —In the construction of armatures, instructions to winders are given in the form of diagrams and tables. In the tables the letters F and B stand for front and back , meaning toward the front end, and from the front end respectively. The letters U and D stand for up and down .

The end view is simply a view showing the arrangement of the armature inductors and connections looking from the front or commutator end, such as shown in fig. 254 .

In the radial diagram the inductors of the armature are represented by short radial lines, while the end connectors are represented by curves or zigzags, those at one end of the armature being drawn within, those at the other end, without the circumference of the armature. With the radial diagram it is easier to follow the circuits and to distinguish the back and front pitch of the winding.

The developed diagram is a mode of representation, originally suggested by Fritsche of Berlin, in which the armature winding is considered as though the entire structure had been developed out of a flat surface. This is best explained by aid of figs. 255 and 256 .

Lap Winding and Wave Winding. —In winding armatures there are two distinct methods employed, known respectively as lap and wave winding . The distinction arises in the following manner: Since the inductors, in passing a north pole generate electromotive forces in one direction, and in passing a south pole generate electromotive forces in the opposite direction, it is evident that an inductor in one of these groups ought to be connected to one in nearly a corresponding position in the other group, so that the current may flow down one and up the other in agreement with the directions of the electromotive forces. The order followed in making these connections gives rise to lap and wave windings.

Ans. One in which the ends of the coils come back to adjacent segments of the commutator; the coils of such a winding lap over each other.

Ans. One in which the coil ends diverge and go to segments widely separated, the winding to a certain extent resembling a wave.

Angular Pitch or Spread of Drum Coils. —Before taking up the winding as a whole, the form of the individual coil should be considered. Fig. 260 shows an end view of one coil in position on a drum armature of a multipolar machine. The two slots X and Y contain the sides of the coil and the distance between them on the surface of the drum is called the angular pitch or spread of the coil . Theoretically this is equal to the 234 235 pitch of the poles, represented by the angle M, which is the angle between the pole centres.

Parallel or Lap Drum Winding. —In order to avoid much of the difficulty usually experienced by students of drum winding, the beginner should construct for himself a wooden armature core upon which he can wind strings of various colors, or wires with distinctive insulation, to represent the numerous coils that are used on real armatures. A few windings attempted in this way will make clear many points that cannot be so easily grasped from a written description.

The type of drum core best adapted for this work is the slotted variety as shown in fig. 258 , as it will facilitate the winding. The core as shown in the illustration has twelve slots and six commutator segments, the number of each required for the example of lap winding indicated in the winding table fig. 259 .

Ans. As given in the table, it consists of six loops of wire presenting twelve inductors on the cylindrical surface of the core or drum. In the table, six wires are shown, having 237 distinctive and varied insulation so as to readily distinguish the different coils. Opposite these are letters and figures designating the path and connections of each coil.

Ans. According to the table it is:

A — 1 — 6 — B

that is, one end of the wire is connected to commutator segment A ( fig. 262 ) and then wound to the back of the drum through slot 1, across the back of the drum to slot 6, returning through this slot, and then connected with commutator segment B.

Ans. The second coil, having the block insulation, is wound according to the table, in the order: 238

B — 3 — 8 — C

that is, beginning at segment B, thence to back of drum through slot 3, across the back to slot 8, returning through this slot and ending at segment C.

Ans. Each of the succeeding coils are wound as indicated in the table, the last connection being made to segment A, the one from which the winding started.

Ans. It may be considered simply as a wire wound spirally around the drum, with loops brought down to the commutator segments, and ending at the segment from which the start was made.

Ans. The wire must not be wound around the drum to diametrically opposite positions, as for instance 1 to 7 in fig. 265 .

Ans. The reason will be clearly seen by attempting the winding on the wooden core. A winding of this kind on the drum fig. 258 , would proceed as follows:

In order now to continue winding in a regular way, the wire from segment d should pass to the rear of the armature along space 7, but this space is already occupied by the return of the first coil. Continuing the winding from this point, it would be necessary to carry the wire from segment d to 6 or 8, resulting in an unbalanced winding .

Ans. The inductors, in passing from the front to the rear of the armature, fig. 263 , must occupy positions 1, 3, 5, 7, 9, 11, and the even numbered positions will then serve as the returns for these wires.

Ans. No, for practical use each coil would consist of several turns, the diagram then merely indicates the end connections and slots for the several turns of each coil.

Series or Wave Drum Winding. —In this mode of winding, the inductors are arranged around the armature so that they do not turn back, thus describing a zigzag or wave-like path; that 242 is, the coil ends instead of connecting with adjacent segments of the commutator, are attached to segments more or less remote.

Ans. Only two sets of brushes are required for such a winding, but as many brushes as there are poles can be used.

Ans. They are generally used on armatures designed to furnish a current of high voltage and low amperage.

Double Windings. —In the various drum windings thus far considered, each coil had its individual slots, that is, no two occupied the same two slots. This arrangement gave twice the number of slots as commutator segments.

In a double winding there are as many segments as slots, each of the latter containing two inductors, comprising part of two coils.

The Siemens Winding. —In winding drum armatures for bipolar dynamos of two horse power or less, and especially for very small machines as used in fan or sewing machine motors, a form of winding, known as the Siemens winding, which is shown in fig. 271 , is largely used. It consists in dividing the surface of the armature core in one equal number of slots, say 16, and using a 16 part commutator.

Ans. It produces an unsightly head where the wires pass 246 around the shaft and requires considerable skill to make it appear workmanlike.

Ans. By using the chord windings of Froehlich or Breguet, which are improvements over the Siemens in appearance and are more easily carried out.

Chord Winding. —In cases where the front and back pitches 2 are so taken that the average pitch differs considerably from the value obtained by dividing the number of inductors by the number of poles, the arrangement is called a chord winding.

In this method each coil is laid on the drum so as to cover an arc of the armature surface nearly equal to the angular pitch of the poles; it is sometimes called short pitch winding .

Ques. What is the difference between the Siemens winding and the chord winding?

Ans. This is illustrated in fig. 271 , which shows one end of an armature. In the Siemens winding, a wire starting, say at A, crosses the head and enters the slot marked B. If it enters slot C it is a chord winding.

Ans. The winding is started in the same manner as described in the Siemens method, only instead of crossing the head and returning in the section diametrically opposite, the section A C , fig. 271 , next to it is used for the return of the wire to the front end. Leads for connecting to the commutator are left at the beginning and end of each section as before stated and the only difference between the two methods will be noticed when the first layer is nearly complete in that two sections lying next to each other have no wire in them. This will cause the winder to think he has made a mistake, but by continuing the winding and filling in these blank spaces in regular order when the two layers are completed, all the sections will be filled with an equal number of turns and there will be the required number of leads from the coils to connect up to the commutator bars.

Multiplex Windings. —An armature may be wound with two or more independent sets of coils. Instead of independent commutators for the several windings, they are combined into one having two or more sets of segments interplaced around the circumference. Thus, in the case of two windings, the brush comes in contact alternately with segments of each set. 248 The brush then must be large enough to overlap at least two segments, so as to collect current from both windings simultaneously. Both windings then are always in the circuit in parallel.

Ans. It reduces the tendency to sparking, because only half of the current is commutated at a time, and also because adjacent commutator bars belong to different windings.

Fig. 272.—A progressive wave winding. If the front and back pitches of a wave winding be such that in tracing the course of the winding through as many coils as there are pairs of poles, a segment is reached in advance of the one from which the start was made, the winding is said to be progressive. The figure shows three coils of a winding having 18 inductors. From the definition, the number of coils to consider to determine if the winding be progressive is equal to the number of poles divided by 2, which in this case is equal to 2. These coils are shown in the figure as follows: A—1—4—F and F—11—14—B. The second coil ends at segment B which is in advance of segment A from which the winding began, indicating that the winding is progressive. Fig. 272 is given simply to illustrate the definition of a progressive winding, and not to represent a practical winding.

Ans. It involves an increased number of inductors and commutator segments, which is undesirable in small machines, but for large ones might be allowable.

Ans. In these windings, the division of what otherwise would be very stout inductors into several smaller ones, has the effect of reducing eddy current loss.

Ques. For what service are machines with multiplex windings specially adapted?

Ans. Multiplex windings are used in machines intended to supply large currents at low voltages, such as is required in electrolytic work.

Number of Brushes Required. —The number of places on the commutator at which it is necessary or advisable to place a set of collecting brushes can be ascertained from the winding diagrams. All that is necessary is to draw arrows marking the directions of the induced electromotive forces. Wherever two arrow heads meet at any segment of the commutator, a positive brush is to be placed, and at every point from which two arrows start in opposed directions along the winding, a negative brush should be placed.

Ques. How many brushes are required for lap windings and ordinary parallel ring windings?

Ans. There will be as many brushes as poles, and they will be situated symmetrically around the commutator in regular order and at angular distances apart equal to the pole pitch.

Ans. If arrows be drawn marking the direction of the induced electromotive forces to determine the number of brushes, it will be found that only two brushes are required for any number of poles.

Figs. 274 and 275.—Right and left hand windings. These consist respectively of turns which pass around the core in a right or left handed fashion. Thus in fig. 274, in passing around the circle clockwise from a to b , the path of the winding is a right handed spiral. In fig. 275, which shows one coil of a drum armature, if a be taken as the starting point, in going to b , a must be connected by a spiral connector across the front end of the drum to one of the descending inductors such as M, from which at the back end another connector must join it to one of the ascending inductors, such as S, where it is led to b , thus making one right handed turn.

Sometimes with lap winding it is desirable to reduce the number of brushes. In fig. 276 , is shown the distribution of currents in a four pole lap wound machine having four brushes and generating 120 amperes. In each of the four circuits the flow is 30 amperes, and the current delivered to each brush is 60 amperes. If now two of the brushes be removed, the current through each of the remaining two will be 120 amperes, while 252 internally there will be only two circuits as shown in fig. 277 . It should be noted, however, that these two circuits do not take equal shares of the current since, though the sum of the electromotive forces in each circuit is the same, the resistance of one is three times that of the other, giving 90 amperes in one and 30 amperes in the other, as indicated in the figure. If no spark difficulties occur in collecting all the current with only two brushes, the arrangement will work satisfactorily, but the heat losses will be greater than with four brushes.

Ans. It is sometimes advisable to use more than two brushes with wave windings, especially when the current is very large.

Number of Armature Circuits. —It is possible to have windings that give any desired even number of circuits in machines having any number of poles.

Ans. For a simplex spirally wound ring, the number of paths in parallel is equal to the number of poles , and for a simplex series wound ring, there will be two paths . In the case of multiplex windings the number of paths is equal to that of the simplex winding multiplied by the number of independent windings .

Equalizer Rings. —These are rings resembling a series of hoops provided in a parallel wound armature to eliminate the effects of "unbalancing," by which the current divides unequally among the several paths through the armature. By means of leads, equalizer rings connect points of equal potential in the winding and so preserve an equalization of current.

Ans. Any two or more points in the winding, that during the rotation, are at nearly equal potentials.

Drum Winding Requirements. —There are several conditions that must be satisfied by a closed coil drum winding:

3. The average pitch should be approximately equal to the number of inductors divided by the number of poles.

The choice of front and back pitch for a given number of inductors should, with lap and wave windings in general, comply with the following conditions:

1. All the coils composing the winding must be similar, both mechanically and electrically, and must be arranged symmetrically upon the armature.

2. Each inductor of a simplex winding must be encountered once only, and the winding must be re-entrant.

3. Each simplex winding composing a multiplex winding must fulfill the requirement for a simplex winding.

4. A singly re-entrant multiplex winding must as a whole satisfy the requirement for a simplex winding. 256

In addition to the above requirements for lap and wave windings in general, lap windings must comply with the following conditions:

4. In multiplex windings, the front and back pitches must differ by two multiplied by the number of independent simplex windings composing the multiplex winding;

6. The number of inductors must be an even number; it may be a multiple of the number of slots;

In the case of wave windings the several conditions to be fulfilled may be stated as follows:

2. The front and back pitches may be equal or they may differ by any multiple of two.

CHAPTER XIX
THEORY OF THE ARMATURE

Current Distribution in Ring and Drum Armatures. —In studying the actions and reactions which take place in the armature, the student should be able to determine the directions of the induced currents. The basic principles of electromagnetic induction were given in chapter X, from which, for instance, the distribution of current in the gramme ring armature, shown in fig. 279 , is easily determined by the application of Fleming's rule.

Ques. In the Gramme ring armature ( fig. 279 ) what is the distribution of armature currents?

Ans. It varies according to the position of the coils, being least when vertical and greatest when horizontal in a two pole machine arranged as in fig. 279 .

Ans. Usually all the positive brushes are connected together, and all the negative brushes as in fig. 283 , giving four paths in parallel through the armature as indicated in fig. 282 .

Ans. The voltage at the terminals is equal to that of any of the sets of coils between one positive brush and the adjacent negative brush.

Ques. In general what may be said about the current paths through an armature?

Ans. The paths may be in parallel or series parallel according as the winding is of the lap or wave type.

Variation of Voltage Around the Commutator. —There are numerous ways of determining the value of the induced voltage in an armature at various points around the 262 commutator. In the method suggested by Morday, it can be measured by the use of a single exploring brush and a volt meter as shown in fig. 284 .

Ques. How does the voltage vary between successive pairs of commutator segments?

Cross Magnetization; Field Distortion. —In the operation of a dynamo with load, the induced current flowing in the armature winding, converts the armature into an electromagnet setting up a field across or at right angles to the field of the machine. This cross magnetization of the armature tends to distort the field produced by the field magnets, the effect being known as armature reaction . To understand the nature of this reaction it is best to first consider the effect of the field current and the armature current separately.

Fig. 285 represents the magnetic flux through an armature at rest, where the field magnets are separately excited. If the armature be rotated clockwise, induced currents will flow upward through the two halves of the winding between the brushes, making the lower brush negative and the upper brush positive.

Ques. If, in fig. 285 , the current in the field magnet be shut off, and a current be passed through the armature entering at the lower brush, what is the effect?

Ans. The current will divide at the lower brush, flowing up each side to the top brush. These currents tend to produce north and south poles on each half of the core at the points where the current enters and leaves the armature. Hence, there will be two north poles at the top of the ring and two south poles at the bottom.

Ques. What effect is produced by the like poles at the top and bottom of the ring?

Ans. The external effect will be the same as though there were a single north and south pole situated respectively at the top and bottom of the ring.

Ques. In the operation of a dynamo, how do the poles induced in the armature affect the magnetic field of the machine?

Ans. They distort the lines of force into an oblique direction as shown exaggerated in the diagram fig. 286 .

Ques. What effect has the presence of poles in the armature on the operation of the machine?

Ans. In fig. 286 , the resultant north pole n , n , n , where the lines emerge from the ring, attracts the south pole, s , s , s , where the lines enter the field magnet, hence a load is brought upon the engine, which drives the dynamo, in dragging the armature around against these attractions. The stronger the current induced in the armature, the greater will be the power necessary to turn it.

Ques. Why does this reaction in the armature require more power to drive the machine?

Ans. The effect produced by the armature reaction is in accordance with Lenz's law which states that: In electromagnetic induction, the direction of the induced current is such as to oppose the motion producing it.

Remedies for Field Distortion. —Since the distortion of the magnetic field of a dynamo causes unsatisfactory operation, numerous attempts have been made to overcome this defect, as for instance, by:

Normal Neutral Plane. —This may be defined as a plane passing through the axis of the armature perpendicular to the magnetic field of the machine when there is no flow of current in the armature , as shown in fig. 288 . It is the plane in which the brushes would be placed to prevent sparking when the machine is in operation were the field not distorted by armature reaction, and there were no self-induction in the coils.

Commutating Plane; Lead of the Brushes. —It has been found that in order to reduce sparking to a minimum, the brushes must be placed in certain positions found by trial and designated as being located in the neutral plane .

When the brushes are in the neutral plane, they are in contact with commutator segments connecting with coils that are cutting the lines of force at the minimum rate.

Ans. This is a plane passing through the axis of the armature and through the center of contact of the brushes as shown in figs. 289 and 300 .

Demagnetizing Effect of Armature Reaction. —In the operation of a dynamo, as previously explained, the position of the brushes for sparkless commutation must be varied with the load; that is, for light load they should occupy a position practically midway between the poles and for a heavy load they must be moved a few degrees in the direction of rotation. In other words, the commutating plane must be more or less in advance of the normal neutral plane as shown in fig. 289 .

Ans. It produces a demagnetizing effect which tends to weaken the field magnets.

Ans. Tracing the armature currents, in fig. 289 according to Fleming's rule, it will be seen that current in inductors 1 to 18 flow from the observer indicated by crosses representing the tails of retreating arrows and in inductors 19 to 36, toward the observer from the back of armature, indicated by dots representing the points of approaching arrows. In determining these current directions the inductors to the right of the neutral line are considered as moving downward, and those to the left as moving upward. The current in inductors 1 to 15 and 19 to 33, tends to cross magnetize the magnetic field of the machine, but the current in inductors 34 to 36 and 16 to 18 tends to produce north and south poles as indicated. These poles are in opposition to the field poles and tend to demagnetize them. Hence, the inductors lying outside the two upright lines are known as cross magnetizing turns , and those lying inside, as demagnetizing turns .

Eddy Currents; Lamination. —Induced electric currents, known as eddy currents, occur when a solid metallic mass is rotated in a magnetic field. They consume considerable energy and often occasion harmful rise in temperature. Armature cores, pole pieces, and field magnet cores are specially subject to these currents.

Fig. 290.—Arago's experiment illustrating eddy currents. Arago found that if a copper disc be rotated in its own plane underneath a compass needle, the needle was dragged around as by some invisible friction. The explanation of this phenomenon, known as Arago's rotations , is due to Faraday, who discovered that it was caused by induction. That is, a magnet moved near a solid mass of metal, induces in it currents, which, in flowing from one point to another, have their energy converted into heat, and which, while they last, produce (in accordance with Lenz's law) electromotive forces tending to stop the motion. Thus, in the figure, there are a pair of eddies in the part passing between the poles, and these currents oppose the motion of the disc. Foucault showed by experiment the heating effect of eddy currents, but such currents were known years before Foucault's experiments, hence they are incorrectly called Foucault currents.

Ans. In fig. 291 , a bar inductor is seen just passing from under the tip of the pole piece N of the field magnet. Noting the distribution of the lines of force, it will be seen that the edge c d is in a weaker field than the edge a b , hence, since the two edges move with the same velocity, the electromotive force induced along c d will be less than that induced along a b . This gives rise to whirls or current eddies in the copper bar as shown.

Ans. It should be noted that eddy currents are due to very small differences of pressure and that the currents are large only because of the very low resistance of their circuits.

Ques. Explain this mode of construction with respect to the bar inductor fig. 291 .

Ans. In the case of a large bar inductor such as shown in fig. 291 , it could be replaced by a number of small wires soldered together only at the ends. The layer of dirt or oxide on the outside of the wires will furnish sufficient resistance to practically prevent the eddy currents passing from wire to wire.

Magnetic Drag on the Armature. —Whenever a current is induced in an armature coil by moving it in the magnetic field so as to cut lines of force, the direction of the induced current is such as to oppose the motion producing it. Hence, in the operation of a dynamo, considerable driving power is required to overcome this magnetic drag on the armature.

A conductor carrying a current is surrounded by a circular concentric magnetic field. If now such a conductor, with current flowing toward the observer as in fig. 294 , be placed in a uniform magnetic field, a distortion of the magnetic lines will occur as shown in fig. 295 . The resulting mechanical 275 actions are easily determined by remembering that the magnetic lines act like elastic cords tending to shorten themselves . There is in fact a tension along the magnetic lines and a pressure at right angles to both, proportional at every point to the square of their density.

It is evident by inspection of the lines in fig. 295 , that there is a drag upon the conductor in the direction shown by the arrow. 276

Smooth and Slotted Armatures. —The inductors of an armature may be placed on a smooth drum or in slots cut in the surface parallel to the axis.

In the first instance, the magnetic drag comes on the inductors and in the case of slots, upon the teeth.

Comparison of Smooth and Slotted Armatures. —The slotted armature has the following advantages over the smooth type:

1. Reduced reluctance of the air gap;
2. Better protection for the winding;
3. Inductors held firmly in place preventing slippage;
4. No magnetic drag on inductors;
5. No eddy currents in inductors;
6. Better ventilation;
7. Opposition to armature reaction.

4. Leakage of lines of force through the core, especially in the case of partially enclosed slots.

Magnetic Hysteresis in Armature Cores. —When the direction or density of magnetic flux in a mass of iron is rapidly changed a considerable expenditure of energy is required which does not appear as useful work. For instance, when an armature rotates in a bipolar field, the armature core is subjected to two opposite magnetic inductions in each revolution; that is, at any one instant a north pole is induced in the core opposite the south pole of the magnet and a south pole in the core opposite 278 the north pole of the magnet as indicated in fig. 297 by n and s . Accordingly, if the armature rotate at a speed of 1,000 revolutions per minute, the polarity of the armature will be changed 2,000 times per minute, and result in the generation of heat at the expense of a portion of the energy required to drive the armature. This loss of energy is due to the work required to change the position of the molecules of the iron, and takes place both in the process of magnetizing and demagnetizing; the magnetism in each case lagging behind the force.

Core Loss or Iron Loss. —These terms are often employed to designate the total internal loss of a dynamo due to the combined effect of eddy currents and hysteresis, but as the losses due to the former are governed by laws totally different from those applicable to the latter, special analysis is required to separate them.

The eddy current loss per pound of iron in the armature core diminishes with the thinness of the laminated sheets, and may be made indefinitely small by the use of indefinitely thin iron plates, were it not for certain mechanical and economical reasons.

The loss due to hysteresis per pound of iron in the core, does not vary with the thinness of the core plates; it can be reduced only by the use of a material having a low hysteretic coefficient.

Dead Turns. —The voltage generated in a dynamo with a given degree of field excitation is not strictly proportional to the speed, but somewhat below on account of the various reactions. That is, the machine acts as though some of its revolutions were not effective in inducting electromotive force.

The name dead turns is given to the number of revolutions by which the actual speed exceeds the theoretical speed for any output.

Again, this term is sometimes used to denote that portion of the wire on an armature which comes outside the magnetic field and is therefore rendered ineffective in inducing electromotive force. The number of dead turns is about 20% of the total number of turns.

Self-induction in the Coils; Spurious Resistance. —Self-induction opposes a rapid rise or fall of an electric current in just the same way that the inertia of matter prevents any instantaneous change in its motion. This effect is produced by the action of the current upon itself during variations in its strength.

In the case of a simple straight wire, the phenomenon is almost imperceptible, but if the wire be in the form of a coil, the adjacent turns act inductively upon each other upon the principle of the mutual induction arising between two separate adjacent circuits.

Ans. It prevents the instantaneous reversal of the current in the armature coils. That is, the current tends to go on and in fact does actually continue for a brief time after the brush has been reached.

Ans. The energy of the current in the section of the winding undergoing commutation is wasted in heating the wire during the interval when it is short circuited, and as it passes on, energy must again be spent in starting a current in it in the reverse direction. There is, then, a lagging of the current in the armature coils due to self-induction.

Ans. This is an apparent increase of resistance in the armature winding, which is proportional to the speed of the armature, and due to the lagging of the current.

Armature Losses. —The mechanical power delivered to the pulley of a dynamo is always in excess of its electrical output on account of numerous mechanical and electrical losses. Mechanical losses result from:

Ans. The bearing friction varies with the load. In calculating this loss not only must the weight of the armature be considered but also the belt tension and magnetic attraction in order to get the resultant thrust on the bearing. Friction of the brushes is very small and may be neglected. A small loss of power is caused by the friction of the air on the armature. The latter, since it revolves rapidly, acts to some extent as a fan, and in some machines this fan action is made use of for ventilation and cooling.

Ans. The loss of power due to armature resistance is easily found by Ohm's law, but the hysteresis and eddy current losses, known collectively as iron losses , are not so easily determined. If the magnetization curve of the particular quality of iron used for armature plates be known, the hysteresis loss may be calculated approximately. Eddy current losses are the most important, especially in large machines. As previously explained, in all the moving metal masses unless laminated, there will be eddy currents set up if they cut magnetic lines. Power may be lost from this cause even in the metal of the shaft if there be leakage of magnetic lines into it.

CHAPTER XX
COMMUTATION AND THE COMMUTATOR

The act of commutation needs special study. If it be incorrectly performed, the imperfection at once manifests itself by sparks which appear at the brushes. In the study of this chapter on commutation it would be advisable for the student to first review the basic principles of commutation as given in chapter XIV, which contains a brief and simple explanation of how the alternating current in the armature is converted into direct current by the action of the commutator.

Ans. The time required for commutation, or the angle through which the armature must turn to commute the current in one coil.

Ques. What is the position of the commutating plane with respect to M and S, in fig. 300 ?

Ans. An imaginary plane passing through the axis of the armature and the center of contact of the brush.

Ques. What two planes are referred to in stating the position of the brushes?

Ques. What is the difference between the normal neutral plane and the neutral plane?

Ans. This is illustrated in figs. 301 and 302 . The normal neutral plane is the position of zero induction assuming no 286 distortion of the field as in fig. 301 . The neutral plane is the position of zero induction with distorted field as in fig. 302 and as is found in the actual machine; the distortion is exaggerated in the figure for clearness.

Ans. A plane, 90° in advance of the normal neutral plane , being the position of maximum induction with no distortion of field, as in fig. 301 . 287

Ans. A plane 90° in advance of the neutral plane , being the position of maximum induction in a distorted field as in fig. 302 .

Fig. 303.—Brush adjustment for self-induction. For convenience an electric current is regarded as having weight and hence possessing the property of inertia. The current then during commutation cannot be instantly brought to rest and started in the reverse direction but these changes must be brought about gradually by an opposing force. Hence by advancing the brushes beyond the neutral plane as illustrated, commutation takes place with the short circuited coil cutting the lines of force so as to induce a current in the opposite direction; this opposes the motion of the current in the short circuited coil, brings it to rest and starts it in the opposite direction, thus preventing sparks. Figs. 301 to 303 should be carefully compared and thoroughly understood.

Ans. The commutating plane should be carefully distinguished from the normal neutral plane and from the neutral plane, as shown in fig. 303 . 288

Commutation. —In order to understand just what happens during commutation, a section of a ring armature may be used for illustration, such as shown in fig. 304 . Here the coils A, B, C, D, E, are connected to commutator segments 1, 2, 3, 4, and the positive brush is shown in contact with two segments 2 and 3, the brush being in the neutral position. Currents in the coils on each side of the neutral line flow to the brush through segments 2 and 3; the brush then is positive.

Now, as the armature turns, the commutator segments come successively into contact with the brush. In the figure, segment 3 is just leaving the brush and 2 is beginning to pass under it, hence, for an instant the coil C is short circuited.

Ans. Previous to contact with segment 2, current flowed in coil C in the same direction as in coil B.

Ans. During this brief interval, the current in C is stopped and started again in the opposite direction.

Ans. The neutral plane no longer coincides with the normal neutral plane but is advanced in the direction of rotation of the armature as shown in fig. 302 .

Position of the Brushes; Sparking. —In accordance with the laws of electromagnetic induction, if the bipolar ring armature shown in fig. 301 be rotated in the direction indicated by the arrow the armature current entering at the brush E will 290 divide, one part passing through the coils on the right half of the ring, and the other part through the coils on the left half of the ring, to the brush F, from which the total current will pass out, urged by the full value of the electromotive force induced in all the coils on both halves of the ring.

Figs. 305 to 308—Improper brush adjustment resulting in excessive sparking. When the brushes are not advanced far enough, commutation takes place before the short circuited coil reaches the neutral plane, hence, its motion is not changed with respect to the magnetic field so as to induce a reverse current till after commutation. There is then no opposing force, during commutation, to stop and reverse the current in the short circuited coil, and when the brush breaks contact with segment 1, as in fig. 308, the " momentum " of the current in coil F causes it to jump the air gap from segment 1 to segment 2 and the brush, against the enormous resistance of the air, thus producing a spark whose intensity depends on the momentum of the current in coil F. Sparking, if allowed to continue, will injure the brushes and commutator segments.

Again, if the brushes be placed at the points G and H, each half of a current entering at G, will pass through one-half of the coils on the left side and one-half of the coils on the right side of the ring, so that each half of the current will be urged forward by an electromotive force equal to the electromotive force tending to force it back, and therefore, no current will pass in or out through the brushes. From these considerations it is obvious that the proper position for the brushes would be 291 in the normal neutral plane, were it not for the disturbing effects of armature reaction and self-induction of the current .

Ans. When commutation takes place with the brushes in the neutral plane as in fig. 304 , there will be no voltage induced in the short circuited coil C. The current, therefore, which flowed in coil C before it was short circuited will cease, and as segment 3 breaks contact with the brush, it will be thrown as a perfectly idle coil upon the right hand half of the ring in which a current is flowing toward the brush. Moreover, the current which was flowing through D and 3 directly to the brush, must suddenly traverse the longer path through the idle coil C. Now, 293 on account of self-induction, the current acts in precisely the same manner as though it had weight ; that is:

It cannot be instantly stopped or started.

Figs. 309 to 313.—How sparkless commutation is obtained by advancing the brushes beyond the neutral plane; commutation progressively shown.

Fig. 310.—Segment 2 has come into contact with the brush and coil F, in which commutation is taking place, is now short circuited. The current now divides at M, part passing to the brush through segment 2, and part through coil F and segment 1. Although coil F is short circuited and having passed the neutral plane, is cutting the lines of force so as to induce a current in the opposite direction, it still continues to flow with unchanged direction against these opposing conditions. This is due to self-induction in the coil which resists any change just as the momentum of a heavy moving body, such as a train of cars, offers resistance to the action of the brakes in retarding and stopping its motion.

Fig. 311.—Segment 2 has moved further under the brush, and the opposition offered to the forward flow of the current in the short circuited coil F by the reverse induction in the magnetic field to the right of the neutral plane has finally brought the current in F to rest. The currents from each side of the armature now flow direct to the brush through their respective end segments 1 and 2.

Fig. 312.—Segment 1 is now almost out of contact with the brush. A current has now been started in the coil F in the reverse direction due to induction in the magnetic field to the right of the neutral plane; it flows to the brush through segment 2. The current has not yet reached its full strength in F, accordingly, part of the current coming up from the right divides at S and flows to the brush through segment 1.

Fig. 313.—Completion of commutation in segments 1 and 2; the brush is now in full contact with segment 2, the current in coil F has now reached its full value, hence the current flowing up from the right no longer divides at S but flows through F and segment 2 to the brush. If the current in F had not reached its full value, at the instant segment 1 left contact with the brush, it could not immediately be made to flow at full speed any more than could a locomotive have its speed instantly changed. This, as previously explained, is due to self-induction in the coil or the so called "inertia" of the current which opposes any sudden change in its rate of flow or direction. Accordingly that portion of the current which was flowing up from the right and passing off at S to the brush through segment 1 as in fig. 312, would, when this path is suddenly cut off as in fig. 313, encounter enormous opposition in coil F. Hence, it would momentarily continue to flow through segment 1 and jump the air gap between this segment and the brush, resulting in a more or less intense spark depending on the current conditions in coil F.

Therefore, when segment 3 leaves the brush, the current will not instantly change its path and flow through C, but will be urged by its " momentum ," and jump the air gap between the brush and segment 3, thus producing a spark.

Ans. If the brushes be given additional lead, that is shifted further to the right to some position as N N, fig. 304 , coil C will not remain idle during the interval it is short circuited, but will cut the magnetic lines in such a way as to induce a current in the reverse direction through it . Under these conditions, when segment 3 breaks contact with the brush, the current flowing through D does not encounter an idle coil, but one in which a current is flowing in the same direction, hence, the tendency to jump the air gap and produce a spark is reduced; with proper adjustment of the brushes, there will be no sparking.

Ans. Time must be allowed for reversal of the current, hence the brushes must not be so thin as merely to bridge the insulation between segments.

Ans. There is usually much sparking when the lead is too small; a little sparking when too great, and no sparking when just right. If the lead be excessive, there is a waste of energy due to the generation of a larger reverse current in the short circuited coil than is necessary.

Fixed Position of Brushes. —The condition for sparkless commutation is that the current in the short circuited coil be reduced to zero, and increased in the opposite direction up to the same value as that in the next coil leading. If the brushes are to remain in a fixed position, this condition will only be realized at the particular load for which the brushes are set. Thus, if the brushes be set for the average load, the reversing field will not be correct for either a weaker or stronger load. Hence, sparkless commutation with fixed brushes must be due to some other factor. 295

Ans. Since carbon possesses a high resistance, the drop will vary greatly with the contact area, thus affecting a difference of potential in the two segments passing under the brush and it is largely to this that sparkless commutation is due.

Ans. In fig. 304 during commutation, that is, while the brush contacts with any two segments, as 2 and 3, the currents coming up through the winding on either side of the neutral plane are offered two paths to the brush: 1, direct to brush through the connecting segment, or 2, across the short circuited coil and adjacent segment. Thus, on the right side: to brush through segment 3, or across coil C and adjacent segment 2.

The current will take the path of least resistance.

At the beginning of commutation, almost the entire brush area being in contact with segment 3, the contact resistance of this segment will be much less than for segment 2; hence, not only will the current at the right flow through 3, but also the current at the left after first traversing the short circuited coil. As commutation progresses, the area of contact of 3 decreases while that of 2 increases, and the respective resistances vary in inverse proportion. Likewise the tendency of the current in the left half of the winding to take the longer path through coil C and segment 3 to the brush gradually decreases, becoming zero when the two contact areas become equal. During the second half of the period of commutation, the contact area of segment 2 becomes greater and of 3, less; thus the resistance of 2 is lowered, and that of 3 increased. Accordingly, all of the current at the left will flow through segment 2, and the current at the right will flow through C and 2 rather than through 296 3. In this way the current is reversed in C, and, if the brush be broad enough to allow a sufficient time interval, the current in C is built up to its full value before segment 3 leaves the brush, thus securing sparkless commutation.

Figs. 314 to 318.—Brush contact resistance theory of commutation, neglecting self-induction and resistance in the coils. The total current is assumed to be 40 amperes made up of 20 amperes flowing toward the brush from the coils on the right and 20 amperes from the coils on the left. During commutation, that is, the interval during which the brush contacts with any two adjacent segments of the commutator, the current is assumed to vary directly as the contact area.

Fig. 314.—Beginning of commutation; segment A is entirely under the brush, and B is at the initial point of contact. For this position the currents from both sides flow to the brush through segment A.

Fig. 315.—One-quarter period of commutation. One quarter of the brush area is in contact with B and three quarters in contact with A; hence, 10 amperes will flow through B and 30 amperes through A.

Fig. 316.—Second quarter of commutation period. The brush now contacts equally with both segments, hence 20 amperes will flow through each segment.

Fig. 317.—Third quarter of commutation period. Three quarters of the brush area is in contact with segment B and one quarter with segment A; accordingly, 30 amperes will flow through B and 10 amperes through A.

Fig. 318.—Completion of commutation. The brush is in full contact with segment B and at the point of breaking contact with A, hence the entire current from both sides or 40 amperes will flow through B.

Ques. What is the effect of increasing the degree of contact of the brushes?

Ans. It lengthens the period of commutation, and permits it to start in one coil before the preceding coil has entirely passed through this stage.

Construction of Commutators. —The commutator for a closed coil armature consists of a number of segments or L-shaped bars C of drop forged hard-drawn copper assembled around a tubular iron hub as shown in figs. 324 and 325. The bars are 298 held in position by the nuts E, and washers F, screwed on the ends of the tube D. The bars are insulated from each other and from the washers by mica as shown by the heavy lines G, and they are also insulated from the tube either by a tube of mica H, or by a sufficient air space. The ends of the sections of winding are connected to the vertical portions of the bars K, by insertion in the slots L, where they are securely held in place by means of the binding screws, which for greater security are soldered together, and may be released from the slots, whenever necessary, by the application of a hot soldering iron.

It is very important that all the parts of the commutator should be fitted together perfectly and screwed up tightly, in order to prevent looseness. Commutator segments are often made with the washers E, projecting beyond the ends, but such construction reduces the effective length of the commutator, therefore the under cut form of bar is preferable.

In the construction of commutators, the conditions of operation require that there be:

Fig. 327.—Sectional view of a General Electric commutator. The segments are of rolled or forged copper and are separated by soft mica insulating sheets. This mica must wear down evenly with the copper, hence its consistency is important. The segments are wedge shaped so that when drawn radially inward they support each other like the stones of an arch. They are drawn together by hollow cone collars which bear upon lugs projecting from the ends of the segments. These lugs are turned to form a smooth cone after the segments are assembled. The collars are insulated with mica from the segments and they are held in place by nuts upon the commutator shell or by bolts passing from end to end under the segments. The segments are also provided with lugs for connection to the windings.

Points Relating to Commutators. —1. The number of commutator segments depends on the scheme of winding and on the number of sections in the armature winding.

2. Increasing the number of bars diminishes the tendency to spark, and lessens the fluctuations of the current.

Types of Commutator. —Commutators are made in various forms, but they may be grouped into two general types:

Fig. 328.—A large current low voltage bipolar dynamo built for electrolytic work and here shown to illustrate the large size commutator and brushes necessary to collect the large current. Carbon brushes would not be suitable for this class of machine because even with copper brushes, whose conductivity is much higher than carbon, the commutator must be of considerable size to give the required brush contact area. The contrast between the axial lengths of the armature and the commutator is very marked. The rocker construction is of the ordinary type, and heavy flexible cables conduct the current from the brush holders to the fixed terminals. The machine here illustrated gives 310 amperes at 7 volts when running at a speed of 1400 R. P. M., corresponding to an output of 2.17 kilowatts.

3. The segments should be of considerable depth to permit returning occasionally so that their circular form may be preserved;

4. The insulating material must be such that it will not absorb oil or moisture;

CHAPTER XXI
BRUSHES AND THE BRUSH GEAR

With respect to construction, brushes may be broadly classified as: 1, those made of metal, and; 2, those made of carbon. There are several varieties of metal brush, such as:

Gauze Brushes. —These are very flexible and yielding, their use being attended with little wear of the commutator.

Ans. A gauze brush is made up of a sheet of copper gauze, folded several times, with the wires running in an oblique direction, so as to form a solid flat strip of from ¼ to ½ inch in thickness, increasing with the volume of the current to be collected.

Ques. What is the object of folding the gauze with the wires running in oblique directions?

Ans. It is to prevent the ends of the brush fraying or threading out, which would be the case if the gauze were folded up in any other manner.

Ans. They make good contact, but are quite expensive. They may be set either tangentially or radially, the latter preferably, since the point of contact remains the same as the brushes wear away.

Wire Brushes. —This class of brush, which was extensively used before the invention of the gauze brush, is made up of a bundle of brass or copper wires, laid side by side and soldered together at one end. Since wire brushes are harder than the gauze brush, they are more liable to cut or score the commutator, and are also more troublesome to trim. 305

Laminated or Strip Brushes. —These probably represent the simplest form of brush, but are not extensively used owing to the lack of flexibility. They consist of a number of strips of copper or brass, laid one upon the other and soldered at one end, as in fig. 330 .

Fig. 333.—General Electric brush holder. The brush holder yoke consists of a cast iron ring of elliptical section, supported from the bracket of the end shield in such a manner as to facilitate the shifting of the brushes. It is provided with a suitable handle, and may be fastened in any position by means of a thumb nut on the outside of the bracket. It is so constructed that the tension on the individual brush can be adjusted without lifting the brush from the commutator and without the use of tools. The brush can be removed while the machine is running, without moving the holder on the stud and without disturbing any other brush. Removal of the brushes for inspection can be accomplished without permanent change in the adjustment of the tension of the brush holder spring. The connection between the brush and stud is made through a flexible copper connection.

Ans. They are commonly and erroneously called tangential brushes , but they are really beveled at the end and set inclined to the line of tangency so that the ends of all the sheets will make contact.

Fig. 334.—Crocker-Wheeler brush holder. The carbon brush B is firmly clamped in the "box" C by two screws which bear on a sheet of brass to protect the carbon from being broken by the ends of the screws. The box C is carried by four flexible springs S S, one at each corner and formed of hard copper leaves. These are fixed at one end to the box and at the other to the solid base which is in one piece with the spoke attached to the rocker ring. An adjusting screw passes through appropriate lugs on the box C and loosely through the head A of a fixed arm a . Between the lower surface of a and the upper lug on the box C is placed the pressure spring.

Carbon Brushes. —When metallic brushes are used upon the commutators of high tension machines, they frequently give rise to excessive sparking and also heating of the armature, the metallic dust given off appearing to lodge between the segments of the commutator, thus partially short circuiting the 307 armature. To obviate this, carbon brushes are extensively used in such dynamos, this material being found very effectual in the prevention of sparking.

Ans. They are set "butt" end on the commutator, and fed forward as they wear away by means of a spring holder.

Ans. Because they are the only form of brush that will give good commutation with fixed lead.

Ques. What may be said of the different grades of carbon in use for brushes?

Ans. The very soft carbon leaves a layer of graphitic matter on the commutator, and at high voltages, this may cause sparking; such grade of carbon should only be used on low voltage machines.

Fig. 336.—Western Electric box type brush holder. The box which holds the brush is broached to allow the brush to slide freely, but not loosely, to and from the commutator against which it is normally held by a lever acting directly upon the brush head. This avoids the possibility of uneven bearing on the commutator, as the brushes are allowed very slight lateral or angular motion. The adjustment of a brush is also simplified after it has been removed and then replaced. Tension on the brush head is obtained by a special spring which maintains any given tension for which it may be set. An auxiliary flat steel spring on the lower side of the lever acts as a shock absorber between the lever and the brush head, absorbing all minor vibrations caused by a worn commutator. Side contact between brush and brush holder is not relied upon to carry the current, flexible copper pigtails performing this function to the exclusion of sliding contacts or tension springs, in order to reduce the brush loss. It is not necessary to take the brush rigging apart or loosen cable connections when it is desired either to remove or reverse the brushes to change the direction of armature rotation.

Ans. They are usually covered at their upper part with a coating of electro-deposited copper to insure good contact with the holder.

Comparison of Copper and Carbon Brushes. —Copper brushes tend to tear and roughen the surface of the commutator, while carbon brushes tend to keep the surface smooth. Copper causes more wear of the commutator than carbon. With carbon brushes, the armature may be run in either direction. The resistance of carbon being greater than copper, there is less short circuiting caused by carbon particles than by those of copper.

Ans. Since, for minimum sparking, it is only necessary that the brush have high resistance in the region near its edge, 310 attempts have been made to increase the conductivity of the other portions by combining with the carbon, copper sheets or wires.

Ans. They are easily broken and not being flexible, vibration, or any roughness of the commutator will cause bad contact.

Fig. 338.—Holzer Cabot multiple brush holder. Each brush is fastened securely to a machined surface by one or two machine screws, making a positive contact. Several strips of flexible copper of ample section to carry the current are interposed between the part of the holder carrying the brush and the portion clamping the stud, no sliding contact or spring being therefore required to carry any current. The brushes are proportioned for a carrying capacity of not more than 25 amperes per square inch of brush surface. The brush can be adjusted to any degree of tension during the operation of the machine if necessary. Each holder is insulated in such a manner that no short circuit can occur if the holder be accidentally tipped backward while the operator is changing a brush or cleaning the commutator during a run.

Size of Brushes. —The number of brush sets depends upon the number of poles of the machine, but there may be several brushes in each set. It is usual, except in the smallest machines, to place at least two brushes exactly similar side by side instead of one broad brush, thus allowing one brush to be removed for trimming or renewal while the machine is running. Moreover, better contact is secured by this sub-division, because a slight elevation in the commutator surface at one point may slightly raise one brush of a set at each revolution without much harm, while with one broad brush, the entire brush would be lifted, causing bad sparking.

Ans. It depends upon the current capacity, size of machine, and judgment of the designer.

Ans. No general rule can be given for breadth and thickness of brush. The contact face must clearly be wider than the thickness of the insulation between commutator segments, since the period of commutation must last an appreciable interval of time on account of self-induction.

Ans. It may be taken as one and one-half times the thickness of the commutator segments.

Ans. The brush should be thick enough to cover two and one-half commutator segments. The thickness should in no case be excessive on account of the loss due to heating, which results from the difference of potential at the forward and rear edge of the brush.

Contact Angle of Brush. —This may be defined as the angle which the brushes make with the commutating plane as shown in fig. 339 . The several kinds of brush, together with the varied conditions of operation require different contact angles ranging from zero to 90°.

Brush Contact. —The relation between contact pressure, contact resistance , and friction of brushes varies greatly for different kinds of brush. Copper brushes will carry from 150 to 200 amperes per square inch of contact surface; and carbon brushes from 40 to 70 amperes per square inch. The usual contact pressure is 1.25 to 1.5 pounds per square inch for copper brushes, and 1.5 to 2 pounds per square inch for carbon brushes. The rim velocities of commutators vary from 1,500 to 2,500 feet per minute, the velocity usually increasing with the size of the machine.

Ans. For carbon brushes it is about 0.8 to 1.0 volt at each contact, or 1.6 to 2.0 volts for the two, positive and negative, contacts of a machine.

Ans. The coefficient of friction of brushes is about .2 to .25 for copper and .3 for carbon.

Ans. The watt loss is equal to 1.6 to 2. volts for carbon multiplied by the total current carried.

Brush Holders. —These are devices employed to hold the brushes against the commutator with the proper pressure. They differ considerably in various types of machine, hence, no general rules can be given with respect to their construction or use, but any brush holder must fulfill the following requirements:

1. It must hold the brush securely and at the same time feed it forward as it wears away so as to maintain a proper contact;

3. It must be capable of being raised from the commutator, and held out of contact by some form of catch;

4. It must be so constructed that the brush can be easily removed for cleaning or renewal;

6. The brush holders themselves must be carried on a rocker arm, or rocker ring.

It is desirable that brush holders be capable of individual adjustment, so that each may be set at its own point of minimum sparking. A few forms of brush holder are illustrated in figs. 342 to 345.

In the arm or lever type the brush is firmly attached to the extremity of a rigid arm capable of movement about the brush spindle, except in so far as it is restrained by a spring as in fig. 342 .

Fig. 343 shows a brush holder of the spring arm type. The brush is firmly attached to the extremity of a spring arm, the other end of which is secured to the brush spindle, and when once adjusted is not capable of movement about the brush spindle.

In the box type of brush holder as illustrated in fig. 344 , the brush is free to move up and down in the brush box, so far as it is not restrained by a spring rigidly secured to the arm which carries the brush box at its extremity.

Fig. 346.—Fort Wayne type MPL dynamo; view showing details of armature, commutator and brush rigging of large machine. The laminations of the armature core are punched from thin sheet steel, annealed and japanned. Spacing ribs are built into the core at proper intervals forming air passages for ventilation. In addition, there are recesses in the inside of the flanges which permit the passage of air from the interior around the ends of the core to the openings in the end flanges. The armature coils are constructed of round or bar copper on standard forms. The coils are laid in slots in the surface of the core. The commutator is constructed of bars of hard-drawn copper of uniform size and shape, supported and clamped at either end between beveled rings and securely seated on the commutator drum. The drum is connected by radial arms to the commutator sleeve which is mounted and keyed on the armature hub extension.

Fig. 345 shows the reaction type of brush holder, in which the movement of the brush is constrained in one direction by the surface of a part rigidly secured to the brush spindle, and is further constrained by a spring controlled arm, the pressure of which is capable of ready adjustment.

Fig. 347.—Western Electric brush holder. This holder consists of a rugged iron casting, elliptical in section, and supported from the commutator end bearing bracket in such a manner as to provide for the shifting of the brushes. A handle attached to the yoke aids in this shifting and a thumb nut on the outside holds the whole brush gear in the desired position. The brush is fed through an accurately broached slot by a spring which maintains uniform pressure against the commutator throughout the wearing length of the brush. The long lever arm of the spring is sufficiently flexible to take up any minor vibrations of the brush. The tension of the brush may be adjusted without lifting it from the commutator or disturbing any of the other holders. The brush, may be removed for inspection by throwing the spring out of notch. The brush is connected to the holder by flexible copper pig tails of ample current carrying capacity.

Ans. They are carried by a rocker arm for bipolar, and by a rocker ring for multipolar machines, which is mounted upon one of the main bearings, or upon a support specially provided for it, being pivoted to revolve from the same center as the shaft, to permit shifting the brushes.

Ans. There is sometimes trouble resulting from the current passing through the spring which heats it and destroys its elasticity.

Ans. By insulating one end of the spring, and carrying the entire current directly from the brush itself to the main conductors by a flexible copper strip or cable firmly connected to both.

Ques. What may be said with respect to brush construction on machines for electrolytic work?

Ans. The collection of large currents at low voltage, generated by comparatively small machines, requires careful design of brushes and brush holders. The commutator is longer than the 320 commutators on machines of equal capacity at higher voltages, and as a rule the commutator segments are thicker and fewer in number. Each brush set is made up of numerous narrow brushes rather than two abnormally wide ones.

Multipolar Brush Gear. —The brush gear which includes the holders and carrier arm or ring, becomes more complicated as the number of poles and magnitude of the current is increased.

Ans. They are held at the proper points of commutation by arms offset from a cast iron rocker ring, which is itself supported by brackets projecting from the magnet yoke as shown in fig. 346 .

CHAPTER XXII
ARMATURE CONSTRUCTION

The armature of a dynamo has been defined as: a collection of coils of wire wound around an iron core, and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field .

From the mechanical point of view the armature may be said to be made up of the following parts:

1. Shaft;
2. Core;
3. Spider
(in large machines);
4. Winding;
5. Commutator
(broadly speaking).

Of the two types of armature, ring and drum, the latter is almost universally used, hence the examples of construction which follow will be confined chiefly to this type.

Shaft. —A typical armature shaft is shown in fig. 349 . It is made of steel and, except in the smaller machines, is thicker in the middle than at the ends for stiffness to withstand the 322 strong magnetic side pull on the core when the latter is slightly, nearer one pole piece than the other.

Ques. What is the object of providing shoulders on the shaft as in fig. 349 ?

Ans. They serve to keep the armature in the proper position with respect to the bearings.

Ans. If it be proportioned to secure the proper stiffness, it will be found of ample size to resist the twisting strain.

Core. —In the small and medium size dynamos, the core is attached direct to the shaft. There are two kinds of core:

Ans. It has become obsolete, except in special cases, as for machines used for electrolytic work where a large current at low voltage is required.

Ans. One having a series of parallel slots, similar to the spaces between the teeth of a gear wheel, and in which the inductors are laid.

Ans. The core is made of stampings of thin wrought iron or mild steel. The numerous discs stamped from the sheet metal are threaded on the shaft as in fig. 350 , forming a practically solid metal mass.

Ans. The thickness ranges from .014 inch to .025 inch, corresponding to 27 and 22 B and S gauge respectively, 27 gauge being mostly used.

Ans. By two end plates pressed together either by large nuts screwed directly on the shaft as in fig. 351 , or by bolts 325 passing through the core from end to end, as in fig. 352 , holes being punched in the discs for the purpose.

Ans. They are insulated from the core by tubes and washers of mica or other insulating material.

Ans. Since the core has the full torque exerted upon it by the drag of the inductors, it must be firmly connected to the shaft by means of a key, as shown, so that it may be positively driven.

Insulation of Core Discs. —When the discs are stamped from very thin metal, the mere existence of a film of oxide is sufficient insulation. It is usual, however, to apply a quick drying varnish that will give a hard tough coat and not soften with heat or become brittle and crumble under vibration. The varnish may be applied either by dipping or with a japanning machine; it must be very thin, and the solvent employed should be a very volatile spirit.

Forms of Armature Teeth. —The teeth stamped in the core discs are made in various shapes, depending largely on the 327 method of securing the inductors in the slots against electromagnetic drag and centrifugal force. The teeth may be cut with their sides:

Ans. A tooth of this type is shown in fig. 356 , being slightly narrower at the root than at the top, the resulting slot having parallel sides.

Ans. The projecting type is shown in figs. 357 and 358 in which the tops project; this gives a larger core area around the circumference of the armature which reduces the reluctance of the air gap, and provides projecting surfaces for retaining the inductors in the slots by the insertion of wedges.

Ans. They are provided for the insertion of retaining wedges, as in fig. 361 ; this results in less area at the top of the teeth.

Ans. The width of the tooth should be about equal to the width of the slot minus twice the thickness of the slot insulation; that is, the cross sectional area of the teeth should be equal to that of the slots.

Advantages and Defects of Slotted Armatures. —The slotted armature, sometimes called the Pacinotti armature, after its inventor, has the following advantages over the smooth type: 329

1. The inductors are held more firmly in place to resist stresses due to electromagnetic drag and centrifugal force;

4. The intermittent induction due to the presence of the teeth prevents the formation of eddy currents.

5. When the teeth are saturated they oppose the shifting of the lines due to armature reaction.

2. Generation of eddy currents in the polar faces when the latter are not of laminated construction;

Slotted Cores; Built Up Construction. —In the case of large dynamos, the core discs are built up in order to reduce the cost of construction; the following parts are used:

Ans. The core should be of the built up construction to avoid waste of material in the stampings.

Ans. Ring sections stamped, from sheet metal are fastened to a central support or spider , which consists of an iron hub 331 with radiating spokes and a rim with provision for fastening the rings. The rim of the spider is provided with dovetail notches into which fit similarly shaped internal projections on the core segments. These features are shown in figs. 362 to 364. Each layer of core sections is placed on the spider so as to break joints and the core thus formed is firmly held in place by end clamps as shown. The manner of fastening the rings to the spider is an important point, for it must be done without reducing the effective cross section of the core in order not to choke the magnetic flux.

Ventilation. —In the operation of a dynamo more or less heat is generated, depending on the load; hence it is desirable that provision be made to carry off some of this heat to prevent excessive rise of temperature.

Ans. They heat from these causes: eddy currents, hysteresis, and heat generated in the inductors.

Ans. The spider is constructed with as much open space as possible through which air currents may circulate. The core 333 is divided into several sections with intervening air spaces D as shown in fig. 363 , the discs being kept apart at these points by distance pieces. These openings between the discs are called ventilating ducts ; they are usually spaced from 2 to 4 inches apart.

Ans. In some machines a forced circulation of air is secured by means of a fan attached to one end of the armature as shown in fig. 366 .

Insulation of Core. —Before the winding is assembled on the core, the latter should be thoroughly insulated. Japan or enamel insulation is not sufficient because it is liable to have bubbles or minute holes in it, or be pierced by particles of metal or by the rough edges of the core discs. Two or more layers of strong paper, fibre, canvas or mica, should be applied to the 334 core before placing the inductors in position. The ends of the core should be insulated with thicker material, since the strain upon it is greater, especially at the edges.

Armature Windings. —The subject of windings has been fully treated from the theoretical point of view in chapter XVIII. It remains then to explain the different methods employed in the shop and the mechanical devices used to construct the scheme of winding adopted.

Ans. They are made of copper; the ordinary form consists of simple copper wire, insulated with a double or triple covering of cotton, and in some cases copper bars are used for large current machines.

Ans. They are liable to have eddy currents set up in them as illustrated in fig. 291 .

Ques. What may be said with respect to the sizes of wire used for inductors?

Ans. Wire larger than about number 8 B and S gauge (.1285 inch diameter) is not easily handled, hence for large inductors, two or more wires may be wound together in parallel.

According to the mechanical features and manner of assembling on the core, drum windings may be divided into several classes, as follows:

1. Hand winding;
336 2. Evolute or butterfly winding;
3. Barrel winding;
4. Bastard winding;
5. Former winding.

Hand Winding. —The first windings were put on by hand and proved objectionable on account of the clumsy overlapping of the wires at the ends of the armature, which stops ventilation and hinders repairs, while the outer layers overlying those first wound, bring into close proximity inductors of widely varying voltage. The method is still used in special cases and for small machines. Such a winding has rarely, if ever, been made with one continuous wire.

Evolute or Butterfly Winding. —This mode of winding, was introduced by Siemens for electroplating dynamos to overcome the objections to hand winding. It takes its name from the method of uniting the inductors by means of spiral end connectors as shown in fig. 374 , also in figs. 369 and 370, which show more modern forms.

Ans. The fork shaped strips used to connect bars at different positions on the armature, as shown in fig. 369 .

Ans. In fig. 373 , it will be seen that the ends of the evolute connectors lie in two planes, hence the inductors must project to different distances beyond the core. Accordingly, one long and one short bar may be conveniently placed in each slot, side by side. In large machines, especially where the teeth are wide, 339 the connectors may be straight as in fig, 370 . Evolute connectors may be used for either lap or wave windings.

Barrel Winding. —This is a form of drum winding in which the inductors are arranged in two layers and carried out obliquely on an extension of the cylindrical surface of the drum to meet and connect with radial risers .

Bastard Winding. —In this type of winding, the end connectors project from the inductors in straight lines parallel to the shaft and then are bent inward. It has the 341 effect of being somewhat shorter than the barrel winding. In order to secure better ventilation, it is usual to combine a bastard winding at the rear end of the armature with a barrel winding at the commutator end. This class of winding is used only with bar armatures.

Former Winding. —This relates to a method of winding coils, and not to any particular type; that is, mechanical winding as distinguished from hand winding. While hand winding is necessary for ring armatures, a drum armature is wound better and more easily by the aid of machinery.

Ans. A former coil, as its name suggests, is one that is wound complete upon a former before being placed upon the armature.

Ans. By the use of formers much time is saved, thus reducing the cost, and also by their use all the coils are symmetrical which improves the appearance of the finished winding.

Ques. How is the required shape of the template or former for winding the coils determined?

Ans. By winding one coil on the armature in order to ascertain its dimensions and shape; it is then removed from the armature and used as a pattern in constructing the former.

Types of Former Coil. —Of the numerous shapes of former coil, mention should be made of:

Ans. The evolute coil is wound around eight pins inserted in a board as shown in fig. 381 . The required number of turns are taken around these pins and their ends G and H left projecting. The coil thus formed is now covered with tape and after removal from the board, is put into a clamp at C and F, and opened up as shown in fig. 382 , which is the form required for insertion in the proper slots of the armature.

Fig. 381.—Method of winding evolute coils. In preparing the former, it is necessary to know the dimensions of the coil, hence, a pattern coil must first be made, from which the spacing of the pins can be taken so that the completed coil will fit into the slots for which it is intended. After the pins have been properly spaced on the board, the wire is wound around them as indicated, as many turns being taken as decided on for each coil. When the coil is thus completely wound, it is taken from the pins, and the lower ends, C and F, placed in a suitable clamp. The two halves of the coil are then spread apart, the coil assuming the shape illustrated in fig. 382.

Fig. 382.—Appearance of an evolute former wound coil opened out. The points A, B, C, etc., correspond to similar points in fig. 381 .

Ans. The two sides of the evolute coil have unequal dimensions. The part marked AB, in fig. 381 which is an upper layer inductor is longer than the part DE, which constitutes a lower layer inductor. The portions DC and EF act as parts of an inner layer of evolutes, and the portions AF and BC as parts of an outer layer of evolutes. These features are shown in fig. 382 .

Ans. They are placed in position as shown in figs. 372 and 373, continuing around the core until all the slots are filled. To complete the operation it is necessary to raise some of the first laid coils and insert the last ones below them. The winding is thus completed and is symmetrical.

Ques. Describe the method of winding the "straight out" type of former coil.

Ans. The straight out coil may be wound on a former such as shown in fig. 384 . This consists of a board having four upright pins, A, B, D, E, properly spaced and two horizontal pins C, F, attached to extensions at each end of the board. A coil of the required number of turns is wound around these pins and then opened out as in fig. 385 . After varnishing and baking it is ready to be placed on the armature.

Ans. In the same manner as described for evolute coils; when in position straight out coils appear as in fig. 372 .

Ans. Considerable time is saved by the use of a machine designed for the purpose, such as shown in fig. 387 .

Coil Retaining Devices. —In the operation of a dynamo there are two forces which tend to throw the inductors out of position:

Both of these forces are present with smooth core armatures, but only centrifugal force with slotted armatures. The devices used to hold the inductors in position against these forces are:

Ans. They are simply pins or strips projecting from the surface of a smooth core as shown in fig. 251 .

Ans. They require several binding ribbons or brass bands placed around the winding to prevent the inductors being thrown off the core by centrifugal force.

Ques. With slotted armatures what provision must be made for retaining the inductors in position?

Ans. Retaining wedges must be inserted into the notches or between the projecting tops of the teeth.

Ans. They are usually made of well baked hard wood, such as hornbeam, or hard white vulcanized fibre. Sometimes a springy strip of German silver is used.

CHAPTER XXIII
MOTORS

An electric motor is just the reverse of a dynamo; it is a machine for converting electrical energy into mechanical energy .

The electrical energy delivered by the dynamo must be obtained from a steam engine, gas engine, or other power; the mechanical energy obtained from the motor comes from the energy of the current flowing through its armature.

Principles of the Motor. —All the early attempts to introduce motors failed, chiefly because the law of the conservation of energy was not fully recognized. This law states that energy can neither be created nor destroyed .

Early experimenters discovered, by placing a galvanometer in a circuit with a motor and battery, that, when the motor was running, the battery was unable to force through the wires so strong a current as that which flowed when the motor was standing still . Moreover, the faster the motor ran, the weaker did the current become.

Ques. Why does less current flow when the motor is running than when standing still?

Ans. Because the motor, on account of its rotation acts as a dynamo and thus tends to set up in the circuit a reverse electromotive force , that is, an electromotive force in opposite direction to the current which is driving the motor.

Ques. What is the real driving force which causes the armature of a motor to rotate?

Ans. The propelling drag , that is, the drag which the magnetic field exerts upon the armature wires through which the current is flowing, or in the case of deeply toothed cores, upon the protruding teeth.

The Propelling Drag. —In fig. 389 is shown the condition which prevails when a conductor carrying no current is placed in a uniform magnetic field. The magnetic lines pass straight from one pole to the other. The field is not distorted whether the conductor be at rest or in motion, so long as there is no flow of current. This represents the condition in the air gap of a motor or dynamo, when no current is flowing in the armature.

Ans. It distorts the original field ( fig. 389 ) in which the conductor lies, making the magnetic lines denser on one side and less dense on the other as in fig. 390 .

Ans. It produces a force on the conductor tending to push it in the direction indicated by the arrow, fig. 390 .

Ans. They are: 1, a magnetic field, 2, conductors placed perpendicular to the field, and 3, provision for motion, of the conductors across the field in a direction perpendicular to both themselves and the field.

The Reverse Electromotive Force. —When an electric current flows through some portion of a circuit in which there is an electromotive force, the current will there either receive or give up energy, according to whether the electromotive force acts with or against the current.

Fig. 393.—Force exerted on a current carrying conductor placed across a magnetic field. Let N, S, be the pole pieces of an ordinary electromagnet, having their faces flat and with only a narrow air gap between. In this gap is stretched the vertical copper wire A B, kept taut by a strong spring at A; current can be passed into the wire from the leads C and D. Attached to the wire in the middle of the gap is a horizontal cord passing over a pulley P and kept taut by a weight W; the pulley carries a pointer F which moves in front of a scale s s . If the electromagnet be now excited and have the polarity indicated, it will be found that on passing a strong current down the wire, the index F moves toward the right , showing a similar movement in the wire. The index returns to zero when the current in the wire ceases, and moves in the opposite direction if the current in the wire be reversed and sent up instead of down. The experiment can be further varied by reversing the magnetizing current of the electromagnet.

Fig. 394.—Showing relative directions of armature current and reverse electromotive force of a motor. When a motor is in operation, the wires around the periphery of its armature "cut" the magnetic lines of force produced by the field magnet exactly as in the case of the dynamo. Consequently, an electromotive force is induced in each wire, as in the dynamo armature. This induced electromotive force is in opposition to the flow of current due to the electromotive force of the supply circuit, and tends, therefore, to keep down the flow of current. The figure shows a single loop of wire, on the armature core connected directly to the source of electricity. With current flowing in the loop in the direction indicated by the arrows marked c , a magnetic field is set up in the direction indicated by the large arrow marked "direction of armature flux." With the field magnet energized so as to produce a field in the direction indicated by the large arrow F, the reaction between the two fields will turn the armature core in the direction indicated by the arrow R. As the core turns, the upper wire of the loop will cut the flux under the south pole of the field magnet, and the other side of the loop will cut the flux under the north pole. The result will be the induction of a reverse electromotive force in the loop, the direction being indicated by the small arrows marked e . The actual flow of current in the armature is that due to the difference between the impressed and reverse voltage; the latter is proportional to the speed of the armature, the number of armature wires and the strength of the magnetic field in the air gaps between the armature and the pole faces. The speed of a motor supplied with current at constant voltage varies directly with the reverse electromotive force, also with other conditions fixed, the stronger the field, the slower the speed. Weakening the field will increase the speed up to the point where the increase in reverse electromotive force due to the increased speed cuts down the armature current below the value necessary to give the requisite pull at the armature periphery. When this point is reached, any weakening of the field will reduce the speed of the armature. The pull or torque of a motor armature is directly proportional to the strength of the magnetic field, and to the strength of the armature current, the number of armature inductors being fixed. In a field of constant strength, therefore, the pull of the armature depends on the amount of current passing through the winding. The torque must be just sufficient to overcome the load; if in excess, the speed will increase until the increase of the reverse electromotive force reduces the current and the increase of speed increases the load to the point of equilibrium between load and torque. If the torque be insufficient for the load, the speed will diminish until equilibrium is established, assuming the motor is running on constant voltage circuit.

Ques. Describe similar conditions which prevail in the operation of a dynamo.

Ans. When no current is being generated by the dynamo, little power is required to drive it, but when the external circuit is closed and current is forced through it against more or less resistance, work is being done, hence more power is required. In other words, there is an opposition to the mechanical force applied at the pulley which is proportional to the electric power delivered by the dynamo. An opposing reaction or reverse force then is set up in a dynamo when it does work.

Fig. 395.—Circuit with generator and motor. Whenever current flows through some portion of a circuit in which there is an electromotive force, the current will there either receive or give up energy according to whether the electromotive force acts with the current or against it. In the figure, the generator and motor are rotating clockwise, and hence each generates an electromotive force tending upwards from the lower brush to the higher. In each case the upper brush is the positive one. In the dynamo, where energy is being supplied to the circuit, the electromotive force is in the same direction as the current, while in the motor where work is being done and energy is leaving the circuit, the electromotive force is in a direction which opposes the current.

Ques. In the operation of a motor what is the nature of the reverse electromotive force?

Ans. It is proportional to the velocity of rotation, the strength of the magnets, and to the number and arrangement of the wires on the armature, that is, the reverse voltage depends on the rate at which the lines of force are cut.

In the diagrams:
The pump	corresponds	to	the	dynamo.
The high level pipe	"	"	"	positive conductor.
The low level pipe	"	"	"	negative conductor.
The valve	"	"	"	switch.
The water motor	"	"	"	electric motor.
The water pressure (called head)	"	"	"	electric pressure (called voltage).
The flow in gallons per minute	"	"	"	amperes.
The size of pipe	"	"	"	size of conductor.
The foot pounds	"	"	"	watts.

Fig. 398.—Fairbanks-Morse standard TR type motor. This type is built in the smaller sizes and the design is such that the motor can be installed upon the floor, wall or ceiling, the bearing yokes being attached to the frame by four equally spaced bolts so that they can be turned to provide for proper operation of the oiling devices in either position. A substantial base is provided with a thrust screw for adjusting the belt tension. This base has clamping bolts which permit adjusting the position of the motor while suspended. There is a cast ring type frame having steel side pieces which press firmly together, the steel laminations making up the pole pieces. The field coils armature, and armature coils are illustrated in detail in figs. 399 to 401. The commutator bars are of drawn copper, insulated with mica. The lugs which extend outward from the bars to receive the lead wires from the armature windings are formed in one piece with the bars, and are of the full width of the bars with the insulator extending outward between them, so that when assembled a solid flange is formed to receive the armature connections. Self-oiling bearings are provided and the location of the bearing sleeves in the housing is adjustable so that the armature may be centered in the magnetic field. The brush rigging is carried on a skeleton rocker supported in a groove, turned in the edge of the frame. The brush holders are of the box type with independently adjustable tension spring for each. Standard shunt windings are for 115, 230 and 550 volts. The compound wound motors operate at approximately the same full load speeds as the shunt wound, but the no load speeds will be about 20 per cent. higher than the full load speeds. They have, however, the ability to exert a more powerful starting effort than shunt motors without drawing such a heavy current from the line, and are, therefore, especially adapted for driving apparatus that has to be frequently started and stopped under load and where close speed regulation is not required.

Ques. Describe an experiment which shows the existence of a reverse electromotive force in a motor.

Ans. The apparatus required consists of a small motor, battery, and ammeter. They should be connected in one circuit and the deflection of the ammeter observed when the armature is held stationary, and when it rotates with various loads.

Ques. Explain the action of the current supplied to a motor for its operation.

Ans. The motor current passing through the field magnets polarizes them and establishes a magnetic field, and entering the armature, polarizes its core in such a way that the positive 360 pole of the core is away from the negative pole of the magnetic field, and the negative pole is away from the positive pole of the magnetic field. The magnetic repulsions and attractions thus created cause the armature to rotate in a position of magnetic equilibrium or so as to bring its positive and negative poles opposite the negative and positive poles respectively of the magnetic field. It is evident that unless suitable means were provided to reverse the polarity of the armature core at the instant it reached the position of the magnetic equilibrium, the armature would not rotate any further. The construction is such that the polarity of the armature core, or the direction of the current in the armature coils is reversed at the proper instant automatically by the commutator, thus giving continuous rotation.

Fig. 400.—Fairbanks-Morse armature for 7½ H. P., 1300 R. P. M., TR type motor. The armature core is built up of thin sheet steel laminations with notches in the circumference, which, when the discs are placed together, form grooves or slots to receive the armature coils. The armature cores for the larger machines are mounted on a cast iron spider, which also carries the commutator, making the two parts entirely self-contained, and with this construction, it is possible to remove the armature shaft, without disturbing the core, commutator or windings. Cores of all sizes are provided with ventilating spaces, running from the surface to the central opening of the core, so that air is drawn through the core and blown out over the windings by the revolution of the armature.

Direction of Rotation of Motors. —In the case of either a motor, or a dynamo used as a motor, the direction in which the armature will rotate is easily found by the left hand rule, as 361 illustrated in fig. 411 , when the polarity of the field magnets and the direction of currents through the armature are known.

Ans. By reversing either the current through the fields, or the current through the armature.

Ques. What is the result of reversing the direction of current at the terminals of a series motor?

Ans. It will not change its direction of rotation, since the current still flows through the armature in the same direction as through the field.

Figs. 402 to 410.—Diagrams showing relative direction of rotation of motors and dynamos. From figs. 391 and 392, it is seen that the direction of the current in a motor armature must be such as will increase, by the flux it produces, the intensity at the leading polar edge and decrease the intensity at the trailing polar edge . In a dynamo, the armature has to be moved by mechanical force, against a magnetic force, hence the leading polar edge is weakened, while the trailing edge is strengthened. The magnetomotive force in a motor armature is, therefore, opposed to the direction of that in a generator armature, when the direction of rotation and the direction of the field magnetomotive force are the same. Upon this depends all the relations existing between the direction of rotation of a machine when acting as a motor or as a dynamo.

Ans. Because if the connections be such that the current supplied will flow through the armature in the same direction as when the machine is used as a dynamo, the current through the field will be reversed, since the field windings are in parallel with the brushes.

Armature Reaction in Motors. —In the operation of a motor the reaction between the armature and field magnets distorts the field in a similar manner as in the operation of a dynamo. A current supplied from an outside source magnetizes the armature of a motor and transforms it into an 364 electromagnet, whose poles would lie nearly at right angles to the line joining the pole pieces, were it not for the fact that negative lead must be given to the brushes.

Fig. 412.—Principle of the electric motor as illustrated by experiment showing effect of a magnetic field on a wire carrying an electric current. Let a vertical wire ab be rigidly attached to a horizontal wire gh , and let the latter be supported by a ring or other metallic support as shown, so that ab is free to oscillate about gh as an axis. Let the lower end of ab dip into a trough of mercury. When a magnet is held in the position shown and a current from a cell is sent through the wire as indicated, the wire will move in the direction shown by the arrow f , that is, at right angles to the direction of the lines of magnetic force. Let the direction of the current in the wire be reversed, then the direction of the force acting on the wire will be found to be reversed also. The conclusion is that a wire carrying a current in a magnetic field tends to move in a direction at right angles both to the direction of the field and to the direction of the current . The relation between the direction of the magnetic lines, the direction of the current, and the direction of the force, is often remembered by means of the following rule, known as the motor rule , and which differs from the dynamo rule only in that it is applied to the fingers of the left hand instead of to those of the right. Let the forefinger of the left hand point in the direction of the magnetic lines of force and the middle finger in the direction of the current sent through the wire, then will the thumb, at right angles to the other two fingers, point in the direction in which the wire is urged.

Ans. It tends to shift the field around in a direction opposite to that of the rotation.

Fig. 413.—Current commutation in a motor. Considering the coil W which is ascending, current is flowing through it from the top brush, while it is itself the seat of an electromotive force that tends to stop or reverse its current. The condition for sparkless commutation requires that during the interval the coil is short circuited by the brush, the coil should be passing through a field that is not only sufficiently strong but one that tends to reverse the direction of its current. The coil is already in such a field, hence, commutation must take place before it passes put of this field. To accomplish this the brushes must be shifted backward, that is, given negative lead , to overcome sparking. In other words, the commutating plane must be shifted back of the neutral plane in a motor instead of being placed in advance as in a dynamo.

Ans. The same conditions must obtain as in a dynamo, that is, the current in the coil undergoing commutation must be brought to rest and started again in the opposite direction. This involves that while the coil is short circuited by the brush, it should be passing through a field that tends to reverse the direction of the current. Since the coil is already in such a 366 field, the act of commutation must take place before it passes out of this field. Accordingly, a negative lead must be given the brushes.

Method of Starting a Motor. —Although motors and dynamos are practically similar in general construction and either one of them will act as the other when suitably traversed by an electric current, there are certain differences between the connections and accessories of a machine operated as generator and one employed as a motor. For instance, when a machine is operated as a dynamo, it is first driven up to speed until it has excited itself to the right pressure, and then it is connected to the circuit; but when a machine is used as a motor it will not start until it has been connected to the circuit, and this must not be done until the proper precautions have been taken to ensure that the current, which will pass through it when so connected, will not be excessive and thereby result in serious injury to the motor. For this reason a rheostat or variable 367 resistance, commonly called a starting box is usually inserted in the armature circuit of a motor to prevent an undue rush of current before the motor attains its speed, and subsequently the speed is regulated by the cutting in or out of the circuit of certain extra resistances which constitute the controller used on a series motor requiring variable torque at variable speed, as in the case of elevator or electric traction service.

Fig. 415.—View of railway motor, open. The frame is of cast steel for lightness, and which serves as magnetic circuit and protecting case. It is circular or octagonal in form except in very large motors. Four short magnets project from the case. The armature is large in order to secure the required torque. It is always series wound, requiring two brushes. The brush holders are mounted upon a frame of insulating material which is attached to the upper half of the case. The brushes are adjustable radially, but usually it is not necessary to provide for shifting as they remain in the neutral plane. In motors which receive so little attention as these, special attention must be given to the design of devices for keeping oil and grease out of the case. These would injure the insulation of the coils and produce sparking at the commutator. Oil rings are, therefore, placed on the shaft, and these discharge into chambers connected to the oil wells or allow the oil to overflow on the track. The bearings are made self-oiling or self-greasing by means of rings or wicks and will run for weeks without attention.

Classes of Motor. —Motors are classified in the same manner as dynamos. The fields may be either bipolar or multipolar, and with respect to the type of armature winding employed, motors are classed as:

Series Motors. —A series motor is one in which the field magnet coils, consisting of a few turns of thick wire, are connected in series with the armature so that the whole current supplied to the motor passes through the field coils as well as the armature. Fig. 416 is a diagram of a series motor showing the connections and rheostat.

Ans. The field strength increases with the current, since the latter flows through the magnet coils. If the motor be run on a 369 constant voltage circuit, with light load, it will run at a very high speed; again, if the motor be loaded heavily, the speed will be much less than before.

Fig. 417.—General Electric type CL-B motor for slow and moderate speeds. It is of multipolar construction, having six pole pieces. The advantages of slow speed machinery are generally understood, and in motors the additional outlay to secure slow speeds is warranted, inasmuch as it results in diminished wear and friction losses in gearing, belting, bearings, and commutators, and decreased brush renewals. The comparatively slow speeds of these motors are of importance in that they permit belting or gearing the motors directly to ordinary slow speed line shafting without employing intermediate counter shafting. When motors are geared to heavy duty machines, it is considered better practice to supply an outboard bearing to take up the additional strain that would otherwise be put on the gearing and bearing.

Ans. Series motors should not be employed where the load may be entirely removed because they would attain a dangerous speed. They should not be used for driving by means of belts, 370 because a sudden release of the load due to a mishap to the belt would cause the motor to "run away."

Fig. 418.—Shunt motor connections. A shunt motor runs at constant speed on a constant voltage circuit. In connecting the motor in circuit, the field coils must be placed in circuit first, so that there is a certain amount of field strength to produce rotation of the armature and thus prevent excessive current through the armature. If the field magnets were not put in the circuit first, the armature, at rest on receiving current, would probably burn out, because it is of low resistance, and would take practically all the current supplied, especially since no reverse voltage is generated in the armature at rest. The method of starting is shown in the illustration. To start, the switch is closed, and the rheostat lever pushed over so as to make contact with A and B, thus first exciting the magnets. On further movement of the lever, the rheostat resistances R, R ₁ , R ₂ , R ₃ , etc., are gradually cut out as the speed increases, until finally all the resistance coils are cut out. To stop, the lever is brought back to its original position.

Ques. What advantage is obtained with series motors with respect to the connections?

Ans. A single wire only proceeds from the rheostat to the motor, so that, with the return wire, only two wires are required.

Ans. Series motors are used principally for electric railways, trolleys, and electric vehicles, and similar purposes where an attendant is always at hand to regulate or control the speed. They are also used on series arc light circuits in which the current is of constant strength. Very small motors are generally provided with series windings.

Fig. 419.—Speed regulation of a shunt motor. The speed of a motor depends on the voltage of the current supplied and the field strength. The motor tends to rotate so fast as to produce a reverse voltage nearly equal to that supplied to the brushes; hence, the speed varies with the voltage supplied. By decreasing this voltage then, the speed is decreased. Accordingly, the speed may be reduced by inserting, by means of a rheostat, a resistance in series with the motor. By inserting this resistance in the field circuit, the voltage at the terminals of the motor is lowered, thus giving the condition necessary to reduce the speed. The arrangement for speed regulation shown in the figure includes a starting regulator and a shunt regulator.

Shunt Motors. —A shunt motor may be defined as one in which the field coils are wound with many turns of comparatively fine wire, connected in parallel with the brushes. The 372 373 current then is offered two paths: one through the armature, and one through the field coils.

Figs. 420 to 422.—Reversing the direction of rotation of a series motor. Fig. 420 shows the connections for counter clockwise rotation. The motor may be reversed: 1, by allowing the current to flow in its original direction (from D to C) in the field magnet coils, and altering the direction of the armature current by changing the two connections on the brushes A and B, thus connecting C to A and B to the return wire as in fig. 421, or 2, by leaving the direction of the current in the armature in its original direction, and reversing that of the field current, as in fig. 422. If the wires leading to the rheostat and motor directly, were reversed there would be no reversal of the motor, because by so doing, both the armature and field magnet currents would be reversed.

Influence of Brush Position on Speed. —In the case of a shunt motor supplied with current at constant pressure, the speed is a minimum when the brushes are in the neutral plane, and the effect of giving the brushes either positive or negative lead is to increase the speed, especially with little or no load.

Ans. When the brushes are shifted from the neutral plane, the reverse voltage between the brushes is decreased, speed remaining unchanged. Accordingly, the pressure in the supply mains forces an increased current through the armature thus producing an increased armature pull which causes the speed to increase until the reverse voltage reaches a value sufficiently large to reduce the current to the value required to supply the necessary driving torque.

Compound Motors. —This type of motor has to a certain extent, the merits of the series motor without its disadvantages, and is adapted to a variety of service. If the current flow in the same direction through both of the field windings, then the effect of the series coil strengthens that of the shunt coil; this strengthening is greater, the larger the armature current.

Fig. 426.—Compound motor connections for starting from a distant point. A compound winding may be used on motors for many different purposes. If the current flow in the same direction through both windings, then the effect of the series coil strengthens that of the shunt coil. This strengthening increases with the load. Thus the motor gets, at increasing load, a stronger magnetic field, and will therefore, if the voltage remain constant run slower than before. Accordingly, for a given current, the starting power will be greater than that of a shunt motor. With a decreasing load the motor will run faster. The compound motor has, to a certain extent, the merits of the series motor without its disadvantages. By means of compound motors the starting at a distance with only two mains may be effected, just as in the case of the series motor. The connections are shown in the diagram. If the motor be regarded as being without the shunt coil, then it is connected up exactly as the series motor in fig. 416 . The current coming from the starter enters the series coil at F, flows through the series coil and leaves it at E, flowing from there to the armature brush B, through the armature to brush A, and from there through the second main back to the generator. The shunt winding is connected directly with the armature brushes A and B, and gets at starting, therefore, only a very small voltage, hence its field is nearly ineffective. But on account of the series winding, the motor starts as a series motor. Obviously such a motor will not develop a very large starting power like a real series motor, for, on account of the large space occupied by the shunt coils, there is less space available for the series coils than with a series motor. A compound motor may, however, even with this arrangement, be easily started, provided the load on starting be not too heavy. When once running the armature will produce a reverse voltage and the shunt coil will be supplied with nearly the full terminal voltage.

Ans. Since it is a combination of the shunt and series types, it partakes of the properties of both. The series winding gives it strong torque at starting (though not as strong as in the series motor), while the presence of the shunt winding prevents excessive speed. The speed is practically constant under all loads within the capacity of the machine.

Ans. Control at a distance can be effected with only two wires, just as in the case of a series motor. In the diagram fig. 426, the current coming from the rheostat enters the series coil at F, and leaves it at E, thence it flows to the armature brush B, through armature to brush A, and from here back to the dynamo. The shunt winding, which is connected across the brushes, gets a very small voltage at starting and is accordingly very ineffective. The motor then starts as a series motor. The starting effect is smaller than in a series motor because of the fewer turns in the series winding, most of the available space being occupied by the shunt coils.

Power of a Motor. —The word "power" is defined as the rate at which work is done , and is expressed as the quotient of the work divided by the time in which it is done, thus:

Work is the overcoming of resistance through a certain distance. It is measured by the product of the resistance into the space through which it is overcome, thus:

The unit of work is the foot pound , which is the amount of work done in overcoming a pressure or weight equal to one pound through one foot of space .

The unit of power is the horse power which is equal to 33,000 foot pounds of work per minute, that is :

In order to measure the mechanical power of a motor, it is necessary to first determine the following three factors upon which the power developed depends:

Ans. The net horse power developed by a machine at its shaft or pulley; so called because a form of brake is applied to the pulley to determine the power.

Fig. 428.—Prony brake for determining brake horse power. It consists of a friction band ring which may be placed around a pulley or fly wheel, and attached to a lever bearing upon the platform of a weighing scale in such a manner that the friction between the surfaces in contact will tend to rotate the arm in the direction in which the shaft revolves. This thrust is resisted and measured in pounds by the scale. In setting up the brake the distance between the center of the shaft and point of contact (knife edge) with the scales must be accurately measured, the knife edge being placed at the same elevation as the center of the shaft . An internal channel permits the circulation of water around the interior of the rim as shown, to prevent overheating.

Ans. Tests of this kind are usually made with a Prony brake as shown in fig. 428 . It consists of a band of rope or strip iron—the latter is the arrangement shown—to which are fastened a number of wooden blocks, several carrying shoulders to prevent the contrivance from slipping off the wheel rim. The brake band is drawn tight, as shown, so that the blocks press against the surface all around. The brake thus formed is restrained from revolving with the pulley by two arms attached near the 379 top and bottom centers of the wheels, and joined at the opposite ends to form a lever which bears upon an ordinary platform scale, a suitable leg or block being arranged to keep its end level with the center of the shaft. By this arrangement the amount of friction between the brake band and the revolving wheel is weighed upon the scales. Since the brake fits tightly enough to be carried around by the wheel, but for the arms bearing upon the scale, the amount of frictional power exerted by the wheel in turning free within the blocks may be transmitted and measured, just as would be the case were a machinery load attached, instead of a friction brake.

Ques. Why must the point of contact of the brake with the scales be level with the center of the shaft?

Ans. In order to determine the force acting at right angles to the line joining the point of contact and center of the shaft.

Ques. What is the distance between the center of the shaft and point of contact with the scales called?

Ques. What three quantities must be determined in a test in order to calculate the brake horse power?

Ans. The lever arm, the force exerted on the scales, and the revolutions per minute.

Now, if the voltmeter and ammeter readings be 220 and 65 respectively, what is the efficiency of the motor at this load?

Speed of a Motor. —The normal speed at which any motor will run is such that the sum of the reverse electromotive force and the drop in the armature will be exactly equal to the electromotive force applied at the brushes. The drop in the armature is the difference between the applied voltage and the reverse voltage.

Mutual Relations of Motor Torque and Speed. —The character of the work to be done not only determines the condition of the motor torque and speed required, but also the suitability of a particular type of motor for a given service. There are three general classes of work performed by motors, and these require the following conditions of torque and speed:

Speed Regulation of Motors. —The speed of motors connected to constant voltage circuits is usually regulated by the two following methods:

The first method is sufficiently explained under fig. 418 and the second method is illustrated in fig. 430 . The controller switch S is so arranged that a greater or lesser number of field coils can be inserted in the field circuit. When the switch arm is 383 on point 1, the motor current will flow through all the field windings, and the strength of the field will be at its maximum. When the switch arm is moved so as to successively occupy positions 2, 3, and 4, thus cutting out of circuit a greater and greater number of field coils the strength of the field will be gradually decreased until practically all of the motor current is led or wired through the armature. Under these conditions, when the field of a motor is at its maximum strength, the motor torque will be at a maximum for any given strength of current, and the reverse electromotive force will also be at a maximum for any given speed, therefore, when the field strength is increased the speed will decrease and vice versa .

Ans. The speed of a series motor may be nearly doubled, that is, if the lowest permissible speed of the motor be 250 revolutions per minute it can be readily increased to 500 revolutions per minute by changing the field coil connections from series to parallel. It is on this account, as much as on their powerful starting torque, that series motors have been until recently almost exclusively employed for electric traction purposes.

Series Parallel Controller. —When two motors are used in electric railway work, their armatures are connected in series with each other and an extra resistance which prevents the passage of an excessive current through the armature before the motor starts. As the speed of the car increases, the extra resistance is gradually cut out of circuit and the field winding connections changed from series to parallel by means of a series 384 parallel controller, which finally connects each motor directly across the supply mains, or between the trolley line and the track or ground return.

Efficiency of a Motor. —The commercial efficiency of a motor is the ratio of the output to the input. As a rule, the power developed by a motor increases as the reverse voltage generated by it decreases, until this voltage equals one-half of the voltage applied at the brushes. After this point is reached, the power developed by the motor decreases with the decrease of the reverse voltage. Therefore, a motor performs the largest amount of work when its reverse voltage is equal to one-half the impressed voltage.

The efficiency of a motor as just stated is the ratio of the output to the input; this is equivalent to saying that the efficiency 385 of a motor is equal to the brake horse power divided by the electrical horse power.

The electrical horse power is easily obtained by multiplying the readings taken from volt meter and ammeter, which gives the watts, and dividing the product by 746, the number of watts per horse power. That is:

Fig. 432.—Wiring diagram, showing electrical connections between the armature, field, and interpoles of an interpole motor. As the name implies, an interpole motor has in addition to the main poles, a series of interpoles which are placed between the main poles, and whose function is to assist in the reversal of the current under the brushes. They provide a separate commutating field of a correct value at all loads and speeds, and their windings are for this purpose connected in series with the armature. The proper functioning of the interpoles is independent of the direction of rotation of the armature, also of the load carried over the whole speed range. In an ordinary motor without interpoles, commutation is assisted by a magnetic fringe emanating from the main poles, but as the value of this fringe is altered by the load of the motor and by rheostatic field weakening, if higher speeds be desired from such a machine, commutation becomes imperfect and sparking results, making a readjustment of the brushes necessary.

Interpole Motors. —An interpole motor has in addition to the main poles, a series of interpoles, placed between the main poles. The object of these poles is to provide an auxiliary flux or "commutating" field at the point where the armature coils are short circuited by the brush.

Ques. What is the object of the commutating field produced by the interpoles?

Ans. Its object is to assist commutation, that is, to help reverse the current in each coil while short circuited by the brush, and thus reduce sparking.

Ans. The excitation of the interpoles being produced by series turns, the field will vary with the load, and will, if once adjusted to give good commutation at any one load, keep the same proportion for any other load, provided the iron parts of the circuit be not too highly saturated.

Ques. State briefly how sparking is reduced or prevented by the action of the interpoles.

Ans. Sparking is due to self-induction in the coil undergoing commutation, which impedes the proper reversal of the current. The action of the interpoles corrects this in that they set up a field in a direction that causes a reversal of the current in the coil while it is short circuited. Thus, the coil at the instant it leaves the brush, is not an idle coil, but has a current flowing in it in the right direction to prevent sparking.

Ans. Constant or adjustable speed, and momentary overloads without sparking; constant brush position; operation at adjustable speeds on standard supply circuits of 110, 220, and 500 volts; constant speed with variable load; reversal without changing the position of the brushes.

CHAPTER XXIV
SELECTION AND INSTALLATION OF DYNAMOS AND MOTORS

General Conditions Governing Selection. —In any particular case, the voltage, current capacity, and type of dynamo selected will depend upon the system of transmission or distribution to which it is to be connected, and the character of the work which it is required to perform. The suitability of the different types of dynamo for various kinds of work has already been considered to some extent, but there are certain general conditions which are applicable to almost all cases, such as:

Construction. —This should be as simple as possible and of the most solid character. All parts should be interchangeable, and have a good finish. All machines should be provided with eye bolts or other means by which they can be lifted or moved, as a whole or in parts, easily and without injury. These features are so carefully attended to and guaranteed by the manufacturers as to leave little choice in this direction.

Operation. —The considerations relating to the operation of a machine involve an examination of the details of its construction, in order to determine the amount of attention it will require, the character of its regulating device, its capacity , form , and weight .

Ans. Dynamos and motors should not be overloaded, because the efficiency is greater when the working load does not exceed the rated capacity of the machine.

Form. —As a rule, there is not much choice in the matter of form between standard machines, as they are uniformly symmetrical, well proportioned and compact. It is a mistake, however, to select a light machine for stationary use, as the weight of a machine increases its strength, stability and durability.

Cost. —In some cases, the matter of first cost is important and deserves careful consideration. It should be remembered, however, that high grade electric machinery cannot be built out of low grade materials and with poor workmen; therefore, when necessity compels the selection of a cheap machine, it should not be expected that its service will be as satisfactory as that of a first class machine.

Number and Size of Units. —The best number and size of units for an electrical plant is usually governed by the requirements of the driving engines. As a rule, dynamos and motors are not much less efficient at quarter-load than at full load, and the smaller dynamos are fully equal to the larger machines in this respect, therefore, a generating plant can be subdivided, and if so desired, without any detrimental results except those to a multiplicity of units.

Ans. Efficiency at maximum load is not so important as efficiency at average load.

Fig. 439.—Efficiency curves for 100 K. W. dynamos. The efficiency of a dynamo at maximum load is not so important as at average load. For instance, if in the figure the curve O B C represent the efficiency of a 100 K. W. dynamo and O A D, that of another machine, it would be in accordance with common practice to compare them at rated load, at which the efficiency of the first is only 91%, while the other is 93%. The first machine, however, is far better than the second, since its average efficiency is much higher, being nearly 91% between half-load and 25% overload. It should be noted that full load is a limit which should be but occasionally reached, and then only for short periods of time.

Ans. The different classes of field winding have already been discussed, but in general the conditions governing selection are as follows: The series dynamo is used where a constant current at variable voltage is desired, as in series arc lamp circuits. A shunt dynamo is used on constant voltage circuits, where the distance from the machine to the load is not great, that is, where there is small line loss. With a compound dynamo there is compensation for line loss, that is, it can be constructed so that the voltage at its terminals, or at the load can be maintained constant or allowed to increase or decrease with a change in load. It can thus operate lamps at constant voltage though they be located at some distance, or the voltage at the end of the line can be made to increase with an increase of load, as is frequently the case in railway work.

Ans. They are used on constant current circuits, and also on constant voltage circuits as in railway work and similar purposes where an attendant is always at hand to regulate the speed.

Ans. They are easily started even under heavy loads, the winding is cheaper than the other types and the speed is nearer constant than shunt motors when operated on constant current circuits. When used on constant pressure circuits, such as is employed for incandescent lighting, the speed will depend on the load.

Ans. They start less easily under a heavy load than do series motors, and the speed cannot be varied through any wide range without considerable loss. The shunt motor requires more attention than the series type and is more liable to be burnt out.

Location. —The place chosen for the dynamo or motor should be dry, free from dust, and preferably where a cool current of air can be had. It should allow sufficient room for a belt of proper length when a belt drive is used.

Foundations. —It is most important to secure a good foundation for every dynamo, and great care should be taken to have them entirely separate from those of the walls of the building in which the machine is installed, and if the dynamo be directly driven, but not on the same bed plate as the engine, a foundation large enough for both together should be laid down. Stone or concrete may be used, or brick built with cement, having a large thick stone bedded at the top.

For small machines the holding down bolts may be set with lead or sulphur in holes in the stone top, but for large machines the bolts should be long enough to pass down to the bottom, where they should be anchored with iron plates.

Setting up of Dynamos and Motors. —In unpacking the machines care should be taken to avoid injury to any part, and in putting the parts together, each part should be carefully cleaned, and all the parts put together in exactly the right way. The shafts, bearings, magnetic joints, and electrical connection should receive especial attention and be thoroughly cleaned of every particle of dirt, grit, dust, metal clippings, etc.

Ans. Whenever possible, they should be assembled by someone thoroughly familiar with the construction; but if the services of such a person cannot be had, no one should attempt to put a machine together unless he has a drawing or photograph of the same for a general guide.

Ans. It should be handled carefully to avoid any injury to the wires of the winding and their insulation.

Connecting Up Dynamos. —The manner in which the connections of the field magnet coils, brushes, and terminals, are connected to one another depends entirely upon the type of machine. The field magnet shunt coils of shunt and compound wound dynamos, are invariably arranged in series with one another, and then connected as a shunt to the brushes or terminals of the machine. The series coils of series and compound wound machines are arranged either in series or in parallel with one another, according to conditions of operation, and then connected in series to the armature and external circuit.

Coupling Up Field Magnet Coils. —In coupling up the coils of either salient or consequent pole field magnets, assume each of the pole pieces to have a certain polarity (in bipolar dynamos two poles only, a north and south pole respectively, are required; in multipolar dynamos the poles must be arranged in alternate order around the armature, the number of N and S poles being equal), then apply Flemming's rule as given under fig. 132, to each of the coils, and ascertain the direction in which the magnetizing current must flow in each in order to produce the assumed polarity in each of the pole pieces. Having marked these directions on the coils, they can be coupled up in either series or parallel connection according to requirements, so that the current flows in the proper direction in each.

Fig. 442.—Comparison of space occupied by direct and belt connected dynamos. In office buildings space is of value and the room required by belt connected dynamos can always be put to profitable use. For this reason the direct connected unit has become generally adopted in the best type of office buildings. In large factories the direct connected unit is generally adopted also to save space. Where these conditions do not obtain, belted type of dynamo can be used to advantage as a given output can be obtained with a smaller size machine than where it is direct connected to the engine. This is due to the limited rotative speeds at which engines can be run. The illustration shows the relative space required by the two types.

The Drive. —Various means are employed to connect the engine or other prime mover with the dynamo, or the motor with the machinery to driver. Among these may be mentioned the following:

1. Direct drive;
2. Belt drive;
3. Rope drive;
4. Gear drive;
5. Friction drive.

Fig. 443.—General Electric type M P, marine generating set with tandem compound engine. The requirements of such units are compactness, light weight, simplicity, freedom from vibration and noise at high speed, perfect regulation and durability. By adopting a short stroke for the engines and a special armature winding for the dynamos, the height and length of the sets have been reduced. The bed is carried out to the full width of the dynamo frame, making an ample base surface for foundation without increasing the floor space required. While the construction gives a massive appearance, the bed has been cored out and the various parts so designed that the complete sets have an approximate capacity of 3½ watts per pound. All of the moving parts are enclosed by the engine column, excluding dust and reducing wear and attention to a minimum. The bearing are oiled automatically under pressure. These sets are made in sizes from 25 K. W. to 75 K. W., the cylinder dimensions for the smallest size being 6½ and 10½ by 5, and for the largest size 10½ and 18 by 8. Single cylinder sets are made in sizes ranging from 2½ K. W. to 50 K. W., the cylinder dimensions ranging from 3½ x 3 to 12 x 11. See fig. 730.

Ans. One in which the driving member is connected direct to the driven member, without any interposed gearing.

Ans. It is the simplest method and the space required is less than with belt drive. With direct drive the engine and dynamo must run at the same speed; this is a disadvantage because the desirable speeds of the two machines may not agree.

Ans. Greater flexibility in the original design of a plant is possible and new arrangements of old apparatus can be made at any time. It gives conveniently any desired speed ratio and permits the use of high speed dynamos and motors.

Ans. Considerable space is required and the action is not positive. Belts exert a side pull on the bearings which results in wear, also loss of power by friction.

Figs. 445 and 446.—Two methods of lacing a belt. In fig. 445 two rows of oval holes should be made with a punch, as indicated. The nearest hole should be ¾ inch from the side, and the first row ⁷ / ₈ inch from the end, and the second row 1¾ inches from the end of the belt. In large belts these distances should be a little greater. A regular belt lacing (a strong, pliable strip of leather) should be used, beginning at hole No. 1, and passing consecutively through all the holes as numbered. In fig. 446 the holes are all made in a row. This method has the advantage of making the lacers lie parallel with the motion on the pulley side. The lacing is doubled to find its middle, and the two ends are passed through the two holes marked "1" and "1 a " precisely as in lacing a shoe. The two ends are then passed successively through the two series of holes in the order in which they are numbered, 2, 3, 4, etc., and 2 a , 3 a , 4 a , etc., finishing at 13 and 13 a , which are additional holes for fastening the ends of the lacer.

Ans. A single belt travelling 1,000 feet per minute will transmit one horse power per inch of width; a double belt will transmit twice this amount.

Points Relating to Belts.

Rules for Calculating Speed and Sizes of Pulley. —When two pulleys are working together connected by a belt, the one which communicates the motion is called the driver and the other which receives it, the driven pulley .

Rope Drive. —In this method of power transmission, rope is run in V-shaped grooves in the rims of the pulleys; this form of drive, in some cases, is more desirable than others.

Ans. More power can be transmitted with a given diameter and width of pulley, on account of the increased grip in the grooves. Rope drive can be employed for long or short distances by reason of its lightness and the action of the grooves.

Gear Drive. —This method is used where a positive drive is desired, as for elevator or railway motors. It admits of any degree of speed reduction without attending difficulties as would be encountered with belt drive.

Friction Drive. —This is a very simple mode of transmitting power and has the advantages of simplicity and compactness. In operation, the driving wheel is pressed against the wheel to be driven, transmitting motion to the latter by the frictional 405 grip. The drive is thrown out of gear by slightly moving the machine on its sliding base. In construction, the friction may be increased by making one wheel of the pair of wood, compressed paper, or leather.

Electrical Connections. —Circuits for dynamos and motors should be carefully planned so as to secure the simplest arrangement, and to avoid unnecessary expense and delay, the wiring should be installed in accordance with the requirements of the National Electrical Code.

Ans. Exposed wiring is cheap and accessible; a short circuit or ground is easily located and repaired. Concealed wiring, especially when placed under the floor, has the advantage of being out of the way, and thus protected from injury.

Ques. In wiring a dynamo what are the considerations with respect to size of wire?

Ans. All conductors, including those connecting the machine with the switchboard, as well as the bus bars on the latter, should be of ample size to be free from overheating and excessive loss of voltage. The drop between the generator and switchboard should not exceed one-half per cent. at full load, because it interferes with proper regulation and adds to the less easily avoided drop on the distribution system.

CHAPTER XXV
AUXILIARY APPARATUS

There are numerous devices that must be used in connection with dynamos and motors for proper control and safe operation. Among these may be mentioned:

Switches. —A switch is a device by means of which an electric circuit may be opened or closed. There are numerous types of switch; they may be either single or multi-pole, single or double-throw and either of the "snap" or knife form.

Ans. A single-pole switch controls only one of the wires of the circuit, while a double-pole switch controls both.

Ques. What is the difference between a single-break and a double-break switch?

Ans. The distinction is that the one breaks the circuit at one point only, while the other breaks it at two points.

Ans. If the circuit be opened at two points in series at the same instant, the electromotive force is divided between the two breaks and the length to which the current will maintain an arc at either break is reduced to one-half; thus there is less chance of burning the metal of the switch. Another reason for providing two breaks is to avoid using the blade pivot as a conductor, the contact at this point being too poor for good conductivity.

Ans. When the capacity of the circuit in which it is to be placed exceeds 10 amperes.

Ans. Fig. 461 illustrates a knife switch of the double-pole, single-throw type. It consists of the following parts: base, hinges, blades, contact jaws, insulating cross bar, and handle, as shown.

Ans. The minimum area of the contact surfaces should not be less than .01 square inch per ampere, and in those used on arc lighting or other high voltage circuits where the current is usually small, the area of the contact surfaces are usually from .02 to 410 .05 inch per ampere. Since dirt or oxidation would prevent good contact under a simple pressure between the contact surfaces, the mechanism of a switch provides a sliding contact.

Ques. What difficulty is experienced in opening a circuit in which a heavy current is flowing?

Ans. It is impossible to instantly stop the current by opening the switch, consequently the current continues to flow and momentarily jumps the air gap, resulting in a more or less intense arc which tends to burn the metal of the switch.

Ans. The contact pieces are so shaped that they open along their whole length at the same time, so as to prevent the concentration of the arc at the last point of contact. This feature is clearly shown in fig. 461 .

Ans. They are used on circuits containing lamps in comparatively small groups, and other light duty service.

Ans. A form of switch in which the contact pieces are snapped apart by the action of the springs, as shown in fig. 463 , so as to make the duration of the arc as short as possible.

Fig. 467.—Spool of fuse wire. The wire is usually made of an alloy of tin and lead, such as half and half solder. Bismuth is frequently added to the alloy to lower the melting point For half and half solder the melting point is 370° Fahr. The quickness with which a fuse will melt after the current has reached the limit depends upon the specific heat and latent heat of the metal. The current required to "blow" a fuse increases somewhat with the age of the fuse owing to oxidation and molecular changes. Fuses are sometimes rated according to the number of amperes to be taken normally by the circuit they are to protect. Thus, a 10 ampere fuse is supposed to protect a circuit whose regular current should not exceed 10 amperes, and to blow if the current rise to say 12 amperes. The Underwriters' rule requires that the rating be about 80% of the maximum current it can carry indefinitely, thus allowing about 25% overload before the fuse melts. The fusing current varies considerably according to circumstances. The temperature of the surrounding air or other substances affects the melting current greatly, because the rate at which heat from the fuse will be transferred to the surroundings depends upon the difference of temperance between them and the fuse. Hence a fuse in a warm place will be melted by a smaller current than a similar fuse in a cold place. For a similar reason, a fuse in an enclosed place where there is little chance for the heat to be dissipated, will melt with a smaller current than the same in an open place. If the current increase gradually to that which would ordinarily melt the fuse, the high temperature makes the fuse wire oxidize rapidly; this sometimes makes a sort of tube of oxide which will not break even after the fuse wire inside has melted, and so the fuse carries more than its rated current. Open fuses are so unreliable that circuit breakers are preferable for large currents; when fuses are used, the enclosed type as shown in figs. 468 to 470, is usually the more desirable.

Fuses. —All circuits subject to abnormal increase of current which might overheat the system, should be protected by fuses which will melt and thus open the circuit. A fuse is simply a strip of fusible metal, often consisting of lead with a small percentage of tin, connected in series in the circuit.

Ans. They should be inserted wherever the size of wire changes or wherever there is a branch of smaller size wire connected, unless the next fuse on the main or larger wire is small enough to protect the branch or small wire.

Figs. 471 to 478.—Interior construction of D. & W. fuses. In the manufacture of these fuses, four types of fuse link are used according to capacity of fuse, and classified as: 1, air drum link; 2, flat link; 3, multiple link; 4, cylinder link. In the air drum link, figs. 471 and 472, a capsule provides an air space about the center of the link, the rate of heat conduction through the confined air being very slow, the temperature of that portion of the link rises rapidly with increasing current, rendering the blowing point practically constant; fig. 473 shows a section through the complete fuse. In the flat link, fig. 474, the section is reduced in the center, cutting down as far as possible the volume of metal to be fused. Figs. 475 to 478 show various form of multiple link construction. By subdividing the metal, increased radiating surface is obtained which permits a reduction in the volume of fusible metal necessary, and the metal vapor formed when the fuse blows on heavy overload is more readily dissipated. Figs. 477 and 478 show two forms of the cylinder link, the plain cylinder fig. 477, being used for low voltage and large current, and fig. 478, for certain high tension service. The corrugated cylinder presents more surface to the fuse filling than the plain type and secures a maximum radiating surface with resulting minimum volume of metal for a given current.

Ans. They should be placed on a base of slate, porcelain, marble, or other incombustible material.

Ans. They heat perceptibly soon after their rated capacity is passed. The melting temperature is higher than lead alloy.

Ques. Upon what consideration does the choice between switches and circuit breakers depend?

Ans. Simple knife switches are suitable for use when the circuit is not liable to be opened while carrying large current. A circuit breaker, operated automatically or by hand should be used for interrupting heavy currents.

Circuit Breakers. —A circuit breaker is a switch which is opened automatically when the current or the pressure exceeds or falls below a certain limit, or which can be tripped by hand.

Ans. It is composed of a switch and a solenoid in the main circuit. When the current, flowing through the circuit, exceeds a certain value, the core of the solenoid is drawn in and trips a trigger which allows the switch to fly open under the action of a spring.

There are numerous kinds of circuit breaker to meet the varied conditions of service of which may be mentioned the following:

1. Maximum circuit breaker;
2. Minimum circuit breaker;
3. Reverse current circuit breaker;
4. Maximum and reverse circuit breaker;
5. No voltage circuit breaker.

Figs. 489 and 490.—Front and top views of I-T-E automatic overload circuit breaker. In fig. 489 the current in the circuit enters at A, passes through the solenoid coil B (which in its iron jacket becomes a powerful magnet), through the copper terminal C, to the contact blades D, across the bridge at E to the contact blades F, and out into the line at G. The path of the current as indicated above is more clearly indicated in the top view fig. 490. When the current in the solenoid coil produces sufficient magnetism to overcome the weight of the plunger, the latter is drawn up with constantly increasing velocity until it strikes a restraining latch or trigger which forces the arm out of the switch, thus automatically opening the circuit. The device is so constructed that in opening the circuit the arc is broken on the carbon contacts instead of the copper contacts.

Ans. This type of circuit breaker is arranged to open a circuit in the event of current flowing in the circuit in a direction reverse to the normal. This is sometimes effected by winding the electromagnet of the circuit breaker with two coils, one connected as a shunt across the main circuit and the other in series with the main circuit, the two coils being so arranged that when the main current flows in the normal direction their effects assist one another, whereas, when the main current reverses, the effects of the coils are neutralized and the breaker opens.

Fig. 491.—Roller-Smith "S.E." plain overload circuit breaker. In operation, current entering through the lower studs flows through the laminated strap windings C, from this into the arm D, through the contact plate E, into the stationary brush F, and finally out through the upper stud Q. In its passage through the laminated windings C, the square core A is of course magnetized to a degree dependent on the current strength. When this magnetization reaches a predetermined value, the attraction exerted on the ends K of the pivoted armature causes the same to rise with great and increasing velocity, finally bringing the finger D which forms part of the armature into violent contact with the face R of the corresponding projection on the housing which carries the handle and the roller H. This heavy blow causes H, in its rotation about the shaft J, to go over the center and consequently allows the strong outward pressure of the brush F and the resilient coil C to throw the arm outward with a high velocity and so break the circuit, first between the brush fingers and the contact plate and finally between the carbons S and F, the one of which is rigidly secured to the arm and the other of which is resiliently mounted on its supporting spring. To reset the breaker, the handle, which the act of opening has raised, is pulled down, thus bringing roller H into engagement with roller G once more and in that way forcing the arm back into its initial position.

Fig. 492.—Roller-Smith "S.E." combination overload and underload circuit breaker. Attached to the supporting frame B is the extension Z, which like B, is of non-magnetic material and carries a rectangular magnetic core around which there are wrapped laminated copper conductors. Hinged at U is a heavy cup-shaped mass of magnetic material, and hinged at V is a flat lever X which bears against the extension Y secured to the housing which carries the operating handle. The circuit through the breaker conveys the current around the windings of this underload coil carried by frame Z and passes from it to the regular overload winding C from which it pursues the same course and exercises the same function as in a plain overload breaker. The core of Z being thus magnetized, the cup-shaped member W is held in firm contact therewith and the lever X hangs free. Should, however, the current fall below the minimum value, W is no longer sustained by the magnetic attraction but drops away, swinging on its hinge U until the projection on the heel thereof strikes the lever X, which blow is transmitted through Y to the handle and thus trips the breaker. When closing to reset the breaker, the handle is manipulated just as in the case of a plain overload breaker, that is, it is pulled down, thus not only closing and locking the breaker as before but through the pressure exerted by Y on X and by X on W, putting the latter into contact with its rectangular core to which it will adhere if the necessary current be present.

Ans. If one current reverse very rapidly, and soon reach a large value in the opposite direction, it is possible the cut out may not open at the desired instant, and thereafter the effect of the heavy reverse current will be so great that the breaker will be held in more and more strongly; a second disadvantage is that should the supply fail, the breaker will open in any case, and have to be reset before the supply can be resumed, though in certain cases, as, for instance where there is a motor load, this feature is an advantage and not a disadvantage, since the breaker acts as a no-voltage cut out as well as a reverse current cut out.

Ans. Devices which are fitted to circuit breakers and which act as dampers and prevent the too sudden operation of the breakers on what may be only a temporary overload or reverse current.

Ans. There are numerous types. It may consist of a clockwork device, a weight acting on a small drum or pulley, 421 a modified dash pot arrangement, or a device operating by the expansion of a conductor due to the heat generated by a current passing through it.

Ans. It should be so arranged that the heavier the overload the quicker the device acts, until with a short circuit the device is almost instantaneous in its action.

Fig. 493.—Diagram showing connections of a rheostat. The various resistance coils are connected to brass buttons or "contacts." The rheostat is connected in series in the circuit that it is to control. In operation when the lever is on contact 1, the current is opposed by all the resistance of the rheostat so that the flow is very small. As the lever is moved over contacts 1, 2, 3, etc., the coils are successively cut out, thus diminishing the resistance, and when contact 11 is reached all the resistance is short circuited allowing the full current to flow. In some types of rheostat the wire is wound around an iron framework which has been previously dipped into a fireproof insulating enamel. The advantage of this construction is that the heat from the wire is dissipated much more rapidly, so that a much smaller wire can be used to carry a given current. The size of such an enameled rheostat required for absorbing a given amount of energy is much smaller than one made of coils of wire stretched between an iron supporting framework.

Rheostats. —These devices consist of conductors inserted into a circuit for the purpose of diminishing, either constantly or in a variable degree, the amount of current flowing, or to develop heat by the passage of a current through them. Rheostats designed to be used in starting electric motors are frequently called "starting boxes."

Ans. In fig. 493 , resistance coils, A, B, C, etc., are mounted in a frame or box, and are connected at intervals to the contacts 1, 2, 3, etc. The rheostat arm or lever L is pivoted at S, and when moved over the contacts, inserts more or less of the resistance in the circuit thus regulating the flow of the current. One terminal M of the rheostat is connected to the first contact and the other terminal O, to the lever at S.

Fig. 494.—Starter with no voltage release for a series motor. A helical spring coiled around the lever pivot P, and acting on the lever A, tends to keep it in the off position against the stop S. This lever carries a soft iron armature I, which is held by the poles of the electromagnet E, when, in starting the motor, the arm has been gradually forced over as far as it will go. Should anything happen to interrupt the current while the motor M is running, E will lose its magnetism and A will be released, and will fly over to the off position. E is usually shunted by a small resistance R, so that only a portion of the main current flows through it. This device constitutes the no voltage release , and ensures that all the resistance is in circuit every time the motor is started.

Ans. If the line voltage should be applied directly to the terminals of the armature when not running, an excessive flow of current will result, on account of the low resistance. 423 424 Accordingly, to prevent injury to the winding, a variable resistance or starting box is inserted between one supply terminal and the armature so that the pressure may be applied gradually while the motor is coming up to speed.

Fig. 495.—Starter with no voltage release for a shunt motor. The terminals of the motor are at M, M', m , and those of the starter at S, S', s . The lever SA is shown in the "on" position. The current enters the motor at the terminal M, and there divides, part going through the field coil F, and the main current through the motor armature A. The armature current enters the starter at the terminal S', and traversing the lever SA, leaves by the terminal S. The field current enters the starter at the terminal s , traverses the coil of the magnet E (which holds up the armature a linked to the lever) and thence completes its journey through the whole of the resistance R, and through the lever SA, to the terminal S. When the supply is cut off by opening Sw , or should the field circuit be accidentally broken, the magnet E will release a and the lever, which will thereupon fly to the "off" stop O. It should be noticed that when SA is off, A and F form a closed circuit with the resistance R and magnet E. The inductance of F has consequently no chance of causing destructive sparking when the current is shut off. In starting the motor, Sw is first closed, and then, as the lever is slowly moved, the resistance R, which at first is all in circuit with A, is gradually transferred from A to F. The resistance of R is too small to affect appreciably the current in F, which necessarily consists of a comparatively large number of turns of fine wire. The arrangement is adopted to render the breaking of the shunt circuit unnecessary and is rendered clearer by the diagram fig. 496. It should be noted that E may be provided with a short circuiting key or push if required.

Fig. 497.—Starter with no voltage release and overload release connected to a compound motor. With a shunt motor, the only difference in the diagram would be that the series winding Se would be absent, and the armature A would then be connected straight across between the main terminals M and M'. When switch Sw is closed, the current will enter the starter at its terminal S, and pass through the magnet coil m' of the overload release to the switch lever L, which is shown in the off position. As soon as L is moved up to make contact with the first contact S the current divides; part going through the resistance R and the terminals S' and M' to the series coil Se (if a compound motor) and armature A; and part through the no voltage magnet E to the shunt winding Sh . As the lever L is moved up toward E, the effect is to take R out of the armature circuit and put it into the shunt circuit. When the iron armature a , fixed on the switch lever, comes against the poles of E, the laminated copper brush C bears against the blocks B, B, and so affords a better path for the current than through the spindle s . Should the supply voltage fail, either temporarily or permanently, E will release a , and L will fly off under the tension of a helical spring coiled round s . If there should be an overload on the motor, tending to pull it up and cause an excess of current to flow through the armature; this excess current, passing through m' , will make it attract its armature, so bringing two contacts together at K which will short circuit E, and allow the switch to fly off. The connections between E and m' are not shown in the figure, but they are indicated at C in fig. 498 , which is a simplification of fig. 497 , and which should be carefully compared therewith. When only the normal current is flowing, the attraction between m' and its armature is not sufficient to pull the latter up. The actual forms and arrangement of parts on the starters are well shown in some of the figures.

Ans. The overload release is an electromagnetic circuit breaker that opens the circuit if the motor become greatly overloaded. A no voltage release may consist of an electromagnet in series with the shunt field circuit; it holds the rheostat arm in the operating position as long as current flows through the shunt field from the line. If the line switch be opened or the shunt field circuit accidentally broken, the device becomes demagnetized and releases the arm, which returns to its starting position by the action of a spring.

The general arrangement of switches, cut outs and starting boxes should be in accordance with the requirements of the National Electrical Code as follows:

Switchboards. —A switchboard consists of a panel or series of panels of slate, marble, soapstone or brick tile erected in an electric plant for the purpose of mounting in a convenient group the instruments for controlling and distributing the current and safeguarding the system. Switchboards may be divided according to operation into two classes:

A direct control switchboard has all its apparatus mounted directly on the board and controlled by hand, while in the remote control type, the main current carrying parts are at some distance from the operating board, the control being effected by mechanical devices or by electric motors or solenoids. When the control system of a plant is very extensive, it sometimes occupies a separate building known as the switch house .

Ans. In order to avoid danger of fire from short circuits, the panel should be made of some non-combustible material, such as marble, slate, glass plates or earthenware tiles. If slate be used, care should be taken to have it free from conducting veins, or it should be marbleized, that is, subjected to a treatment that will fill up the pores of the veins and thus prevent the absorption of moisture.

Ques. How should the instruments and connections be arranged on a switchboard?

Ans. They should be arranged so as to provide the shortest possible path for the current, and preferably always in the same direction, that is, from right to left or from top to bottom, the connecting wires being brought in on one side and out on the other, and the crossing of wires avoided as far as possible.

Ans. Fig. 500 shows one suitable for two dynamos. At the top is a voltmeter and two ammeters. Immediately below is a row of feeder switches serving to connect and disconnect the 429 various feeders with and from the bus bars which are mounted behind the board. Below are two rheostat handwheels, and two large switches connecting the dynamos with the bus bars. V S is a voltmeter switch connecting the voltmeter with various parts of the system. Below the voltmeter switch is a double-throw switch to transfer the bus bars from connection with the dynamo switches to one with some other source of current such as a street circuit, in the event of a breakdown. At the bottom are two circuit breakers.

Ans. Fig. 501 shows the connections, from which it can be seen that the voltmeter can be connected with the terminals of either dynamo or with the bus bars, or with either a central or remote part in the lamp circuits.

Fig. 502.—Roller-Smith, single-pole, plain overload circuit breaker. As its name indicates, the function of the plain overload circuit breaker is to automatically interrupt the circuit in which it is placed when the flow of current through it exceeds the predetermined limit for which the apparatus is set. It is the most common of all of the types and is utilized for the protection of dynamos and motors and all other electrical apparatus which, by reason of the conditions of operation, may become subject to loads in excess of the normal. The single-pole type may be used separately for the protection of a single wire of a given circuit or grouped to protect the two or more wires of one circuit, becoming in the latter case the so called independent arm multi-pole apparatus. The action of this type of circuit breaker is fully explained in fig. 491 .

HAWKINS PRACTICAL LIBRARY OF
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FOOTNOTES

1 NOTE:—The "front" end means the end at which the commutator is located. Armatures are most conveniently regarded from this end, the opposite end being known as the "back" end.

2 NOTE—The term back pitch means the number of spaces between the two inductors of a coil. For instance, in fig. 267 , the pitch is 3; that is, there are three spaces between say inductors 1 and 4 which form part of the coil A—1—4—C. It is called the back pitch in distinction from the front or commutator pitch, which in this instance is 2.

3 NOTE—A re-entrant winding is one in which both ends re-enter or lead back to the starting point; a closed winding.

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360 / 10	=	36°
3 * 36°	=	108°
5 * 36°	=	180°

Revolutions per minute	0	50	100	160	180	195
Amperes	20	16.2	12.2	7.8	6.1	5.1

horse power =	foot pounds per minute
horse power =	33,000

B. H. P. =	2 π L N W
B. H. P. =	33,000

B. H. P.	= brake horse power;
L	= lever arm, in feet ;
N	= number of revolutions per minute;
W	= force in pounds at end of lever arm as measured by scales.

The Project Gutenberg eBook of Hawkins Electrical Guide v. 02 (of 10)

HAWKINS
ELECTRICAL GUIDE
NUMBER
TWO
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
A PRACTICAL TREATISE
by
HAWKINS AND STAFF

TABLE OF CONTENTS
GUIDE NO. 2.

CHAPTER XVII
THE ARMATURE

CHAPTER XVIII
ARMATURE WINDINGS

CHAPTER XIX
THEORY OF THE ARMATURE

CHAPTER XX
COMMUTATION AND THE COMMUTATOR

It cannot be instantly stopped or started.

The current will take the path of least resistance.

CHAPTER XXI
BRUSHES AND THE BRUSH GEAR

CHAPTER XXII
ARMATURE CONSTRUCTION

CHAPTER XXIII
MOTORS

CHAPTER XXIV
SELECTION AND INSTALLATION OF DYNAMOS AND MOTORS

Points Relating to Belts.

CHAPTER XXV
AUXILIARY APPARATUS

HAWKINS PRACTICAL LIBRARY OF
ELECTRICITY
IN HANDY POCKET FORM PRICE $1 EACH

FOOTNOTES

Transcriber's Note:

THE FULL PROJECT GUTENBERG LICENSE

A	—	1	—	6	—	B
B	—	3	—	8	—	C
C	—	5	—	10	—	D
D	—	7	—	2	—	E
E	—	9	—	4	—	F

A	—	1	—	4	—	C
B	—	3	—	6	—	D
C	—	5	—	8	—	E
D	—	7	—	10	—	A
E	—	9	—	2	—	B

efficiency =	output	=	brake horse power	=	17.1	= 89%
	input		electrical horse power		19.16

Electrical horse power =	volts × amperes	=	watts
Electrical horse power =	746	=	746

4,000	× 2 = 8
1,000	× 2 = 8

4,000 x 12	= 50.31 inches
954	= 50.31 inches

A	—	1	—	6	—	B
B	—	3	—	8	—	C
C	—	5	—	10	—	D
D	—	7	—	2	—	E
E	—	9	—	4	—	F

A	—	1	—	4	—	C
B	—	3	—	6	—	D
C	—	5	—	8	—	E
D	—	7	—	10	—	A
E	—	9	—	2	—	B

The Project Gutenberg eBook of Hawkins Electrical Guide v. 02 (of 10)

HAWKINS ELECTRICAL GUIDE NUMBER TWO 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 A PRACTICAL TREATISE by HAWKINS AND STAFF

TABLE OF CONTENTS GUIDE NO. 2.

CHAPTER XVII THE ARMATURE

CHAPTER XVIII ARMATURE WINDINGS

CHAPTER XIX THEORY OF THE ARMATURE

CHAPTER XX COMMUTATION AND THE COMMUTATOR

It cannot be instantly stopped or started.

The current will take the path of least resistance.

CHAPTER XXI BRUSHES AND THE BRUSH GEAR

CHAPTER XXII ARMATURE CONSTRUCTION

CHAPTER XXIII MOTORS

CHAPTER XXIV SELECTION AND INSTALLATION OF DYNAMOS AND MOTORS

Points Relating to Belts.

CHAPTER XXV AUXILIARY APPARATUS

HAWKINS PRACTICAL LIBRARY OF ELECTRICITY IN HANDY POCKET FORM PRICE $1 EACH

FOOTNOTES

Transcriber's Note:

THE FULL PROJECT GUTENBERG LICENSE

HAWKINS
ELECTRICAL GUIDE
NUMBER
TWO
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
A PRACTICAL TREATISE
by
HAWKINS AND STAFF

TABLE OF CONTENTS
GUIDE NO. 2.

CHAPTER XVII
THE ARMATURE

CHAPTER XVIII
ARMATURE WINDINGS

CHAPTER XIX
THEORY OF THE ARMATURE

CHAPTER XX
COMMUTATION AND THE COMMUTATOR

CHAPTER XXI
BRUSHES AND THE BRUSH GEAR

CHAPTER XXII
ARMATURE CONSTRUCTION

CHAPTER XXIII
MOTORS

CHAPTER XXIV
SELECTION AND INSTALLATION OF DYNAMOS AND MOTORS

CHAPTER XXV
AUXILIARY APPARATUS

HAWKINS PRACTICAL LIBRARY OF
ELECTRICITY
IN HANDY POCKET FORM PRICE $1 EACH

A	—	1	—	6	—	B
B	—	3	—	8	—	C
C	—	5	—	10	—	D
D	—	7	—	2	—	E
E	—	9	—	4	—	F

A	—	1	—	4	—	C
B	—	3	—	6	—	D
C	—	5	—	8	—	E
D	—	7	—	10	—	A
E	—	9	—	2	—	B