Fig. 267.—Five coil wave winding for a four pole machine. In this winding only two brushes are used, there being only two paths through the armature.
Ques. For what service are wave windings adapted?
Ans. They are generally used on armatures designed to furnish a current of high voltage and low amperage.
An example of wave drum winding for a four pole machine is shown in fig. 267. For simplicity, very few coils are taken, there being only five as shown in the illustration. To make the winding, one strip should be removed from the wooden core and the others spaced equally around the cylindrical surface. This will give ten slots, the number required for the five coils. The winding is indicated in the following table:
| A | — | 1 | — | 4 | — | C |
| B | — | 3 | — | 6 | — | D |
| C | — | 5 | — | 8 | — | E |
| D | — | 7 | — | 10 | — | A |
| E | — | 9 | — | 2 | — | B |
Accordingly the first coil starting at segment A, is carried to the back of the drum through slot 1, thence across the back and returning through slot 4, ending at segment C the starting point of the second coil. Each coil is wound on in similar manner, the last coil ending at segment A, the starting point of the first coil. A developed view of the winding is shown in fig. 268.
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.
Fig. 268.—Developed view of the five coil wave winding shown in fig. 267.
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.
In the Siemens winding, the end of the wire used at the start is to be connected to the first commutator bar, but must be fastened to the armature core out of the way so as not to interfere with the winding of the coils.
If eight turns of wire be required to fill a slot with one layer, then the wire is carried from front to back and bent aside so as to clear the shaft; after passing across the back or pulley end of the armature, it is wound in the diametrically opposite section and brought to the front, then across the commutator end and up close to the beginning of the coil.
Fig. 269.—Series connected wave wound ring armature for a four pole machine. The coils are so connected that only two brushes are necessary.
Since eight turns are to be used, the process of winding is continued until the section is full and the end of the coil will lie in a position ready to begin the next section. Sometimes the wire is cut at this part of the coil leaving 3 or 4 inches projecting for connecting to the commutator bar 2, or next to the first bar where the winding was started.
The usual practice is, however, to make a loop of the wire of sufficient length to make the connection to the commutator and it has the advantage that since all of the coils on the armature are joined in series, the ending of one coil is joined to the beginning of the next which avoids making mistakes in making the commutator connections.
If the ends be cut they should be marked "beginning" and "end" to avoid trouble, because if they get mixed, it will be necessary to test each coil with a battery and compass needle in order to determine the polarity produced and find which is the beginning of the coil and which the end. With 32 ends of the wire projecting from the end of the armature, it is confusing and mistakes are often made in the connections, so that one or more coils may oppose each other which would reduce the voltage.
After the surface of the armature is covered with one layer it will be noticed that the number of leads from the coils to the commutator bars is only one-half the number of bars and that they lie on one-half of the armature.
In order to complete the winding the first layer should be insulated and the second layer wound on. The beginning of the new coil will be directly over the first coil put on, but the beginning of the new coil will be diametrically opposite the beginning of the first coil wound.
The winding is now continued section by section and as each coil is finished a loop or pair of leads is left to connect to each bar. When the last coil is wound, its end will be found lying next to the wire used in starting and should be joined to it and finally connected to bar number one where the start was made.
Fig. 270.—Developed view of the series connected wave wound ring armature shown in fig. 269.
With the winding and commutator connected, all of the coils are in series and the beginning of the first coil joins the end of the last coil.
If a pair of brushes be now placed on the commutator at opposite points the current will flow into the bar and then divide between the two leads connected to it, half of the current flowing around one side and the other half flowing around the other half of the armature or in other words, the two halves of the armature are joined in parallel.
Ques. What is the objection to the Siemens winding just described?
Ans. It produces an unsightly head where the wires pass around the shaft and requires considerable skill to make it appear workmanlike.
Ques. How may this be avoided?
Ans. By using the chord windings of Froehlich or Breguet, which are improvements over the Siemens in appearance and are more easily carried out.
Fig. 271.—End view of an armature, showing the distinction between Siemens' winding and chord winding.
Chord Winding.—In cases where the front and back pitches2 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.
Ques. Describe 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.
Ques. How many paths in the chord winding just described?
Ans. Two.
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. 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.
Ques. What is the effect of a multiplex winding?
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.
Ques. Does an accident to one winding disable the machine?
Ans. No, it simply reduces its current capacity.
Ques. Can multiplex windings have more than two windings?
Ans. Yes, there may be three or four windings.
Ques. What is the objection to increasing the number of windings?
Ans. It involves an increased number of inductors and commutator segments, which is undesirable in small machines, but for large ones might be allowable.
Fig. 273.—A retrogressive wave winding. If the pitches be such that in tracing the winding through as many coils as there are pairs of poles, the first segment of the commutator is not encountered or passed over, the winding is said to be retrogressive. The number of coils to consider is two, as follows: A-1-4-D and D-7-10-G. The second coil ends at G, hence, since the segment A where the start was made has not been reached or passed over the winding is retrogressive. Fig. 273 is given simply to illustrate the definition of retrogressive winding, and not to represent a practical winding.
When there are two independent windings the arrangement is called duplex, with three windings, triplex, and with four, quadruplex.
Ques. What loss is reduced with multiplex windings?
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.
It should be noted that the number of brush sets does not necessarily show the number of circuits through the armature.
Ques. How many brushes are required for wave windings?
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.
Ques. What is the angle between these two brushes?
Ans. It is the same as the angle between any north and south pole.
For instance, in a ten pole machine with wave winding the pitch between the brushes may be any of the following angles:
| 360 / 10 | = | 36° |
| 3 * 36° | = | 108° |
| 5 * 36° | = | 180° |
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 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.
Fig. 276.—Distribution of armature currents in a four pole lap wound dynamo having four brushes and generating 120 amperes.
Ques. Are more than two brushes ever used with wave winding?
Ans. It is sometimes advisable to use more than two brushes with wave windings, especially when the current is very large.
For instance, in the case of a singly re-entrant3 simplex wave winding for an eight pole machine, whenever any brush bridges adjacent bars of the commutator, it short circuits one round of the wave winding and this round is connected at three intermediate points to other bars of the commutator. Hence, if the short circuiting brush be a positive brush, no harm will be done by three other positive brushes touching at the other points. If these other brushes be broad enough to bridge across two commutator bars, they may effect commutation, that is, three rounds instead of one undergoing commutation together.
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.
Fig. 277.—Showing effect of removing two of the brushes in fig. 275. If no spark difficulties occur in collecting the current with only two brushes, the arrangement will work satisfactorily, but the heat losses will be greater than with four brushes.
Ques. How many paths are possible in parallel?
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.
In large multipolar dynamos it is, as a rule, inadvisable to have more than 100 or 150 amperes in any one circuit, except in the case of special machines for electro-chemical work. Such considerations are factors which govern the choice of number of circuits.
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.
Fig. 278.—Rear view of armature of a large dynamo built by the General Electric Co., showing equalizer rings.
Ques. In multipolar machines what points are connected by equalizer rings?
Ans. Any two or more points in the winding, that during the rotation, are at nearly equal potentials.
If there were perfect symmetry in the field system, no currents would flow along such connectors; however, owing to imperfect symmetry, the induction in the various sections of the winding may be unequal and the currents not equally distributed.
Drum Winding Requirements.—There are several conditions that must be satisfied by a closed coil drum winding:
1. There cannot be an odd number of inductors;
An odd number of inductors would be equivalent to not having a whole number of coils. The even numbered inductors may be regarded as the returns for the odd numbered inductors.
2. Both the front and back pitches must be odd in simplex windings.
3. The average pitch should be approximately equal to the number of inductors divided by the number of poles.
This condition must obtain in order that the electric pressures induced in inductors moving simultaneously under poles of opposite sign, will be added. The smallest pitch meeting this condition would stretch completely across a pole face, while the largest would stretch from the given pole tip to the next pole tip of like polarity.
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.
In addition to the above requirements for lap and wave windings in general, lap windings must comply with the following conditions:
1. The front and back pitches must be opposite in sign;
2. The front and back pitches must be unequal;
If they be equal, the coil would be short circuited upon itself.
3. The front and back pitches must differ by two;
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;
5. The number of slots on a slotted armature may be even or odd;
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:
1. The front and back pitches must be alike in sign;
2. The front and back pitches may be equal or they may differ by any multiple of two.
They are usually made nearly equal to the number of inductors divided by the number of poles.
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.
Tracing the current from the negative to the positive brush, it will be seen that it divides, half going through coils 1, 2, 3, and half through coils I, II, III, these two currents ascend to the top of the ring, uniting at the positive brush.
Ques. In the Gramme ring armature (fig. 279) what is the distribution of armature currents?
Ans. There are two paths in parallel as indicated in fig. 279.
Ques. How does the voltage vary in the coils?
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.
The upper and lower coils in the right hand half of the ring armature, fig. 279, will have about the same electromotive force induced in them, say 2 volts each, while the two coils between them will have a higher electromotive force, at the same instant, say 4 volts each, since they occupy nearly the positions of the maximum rate of change of the magnetic lines threading through them. These eight coils may be represented by two batteries connected in parallel, each battery consisting of two 2 volt cells and two 4 volt cells as shown in fig. 280. The voltage of each battery then will be
2 + 4 + 4 + 2 = 12 volts
Fig. 279.—Current distribution in a gramme ring armature. There are two paths for the current between the brushes, half going up each side of the ring as indicated by the arrows, thus giving two paths in parallel as indicated in fig. 281.
Fig. 280.—Battery analogy illustrating current distribution in a ring armature. The eight coils of the armature, fig. 279, are represented by two batteries of four cells each. The action of the two units thus connected is indicated by the arrows. In the external circuit the voltage is equal to that of one battery and the current is equal to the sum of the currents in each battery.
The two batteries being connected in parallel, the voltage at the terminals will be the same, but the current will be the sum of the currents in each battery.
Ques. How may the number of paths in parallel be increased?
Ans. By increasing the number of poles.
For instance, in a four pole machine, as in fig. 283, there are four paths in parallel. In this case the armature may be used to furnish two separate currents, though this is not desirable.
Fig. 281.—Diagram showing distribution of current in the gramme ring armature of fig. 279. The current flows in two parallel paths as indicated.
Fig. 282.—Diagram showing current distribution through armature of a four pole machine with like brushes connected. There are four paths in parallel, hence the induced voltage will equal that of one set of coils, and the current will be four times that flowing in one set of coils.
Ques. How are the brushes connected?
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.
Fig. 283.—Brush connections for four pole dynamo. It is usual to connect all the positive brushes to one terminal and all the negative brushes to the other which gives four parallel paths as shown in the diagram, fig. 282. In a four pole machine, two separate currents can be obtained by omitting the parallel brush connections.
Ques. How does this method of brush connection affect the voltage?
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.
Thus in the four pole machine, fig. 283, the coils of the four quadrants are in four parallels, which gives an internal resistance equal to one-sixteenth that of the total resistance of the entire ring.
When the coils are connected in two circuits or series parallel, it requires only two brushes at two neutral points on the commutator, for any number of poles; this arrangement is shown in fig. 269.
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.
Fig. 284.—Morday's method of measuring the variation of voltage around the commutator by use of a single exploring brush and volt meter. It consists in connecting one terminal of the volt meter (preferably an electrostatic one) to one brush of the machine, and the other terminal to the exploring brush, which can be moved from point to point, readings being taken at each point.
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 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.
In this method, one terminal of the volt meter is connected to one of the brushes of the dynamo, and the other terminal is joined by a wire to a small pilot brush which can be pressed against the commutator at any desired part of its circumference. With the machine running at its rated speed, the exploring brush is placed in successive positions between the two brushes of the machine. In each position a reading of the volt meter is taken and the angular position of the exploring brush noted.
Fig. 285.—Cross magnetization. This is defined as lines of magnetic force set up in the windings of a dynamo armature which oppose at right angles the lines of force created between the poles of the field magnet. The figure shows this cross flux which is due to the armature current alone.
Ques. How does the voltage vary between successive pairs of commutator segments?
Ans. The variation is not constant.
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.
Fig. 286.—Distortion of magnetic field due to cross magnetization. For clearness, the effect is shown somewhat exaggerated. A drag or resistance to the movement of the armature is caused by the attraction of the north and south poles on the armature and pole pieces respectively.
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.
Fig. 287.—Actual distortion of field resulting from cross magnetization, as shown by iron filings.
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:
1. Experimenting with different forms of pole piece;
The reluctance of the pole piece should be increased in the region where the magnetic flux tends to become most dense. The trailing horn of the pole piece may be made longer than the advancing horn and cut farther from the surface of the armature, so as to equalize the distribution of the magnetic flux.
2. Lengthening the air gap;
This increases the reluctance, and also necessitates more ampere turns in the field winding. The field distortion, however, will not be so great, as it would be if the magnetic field of the machine were weaker.
3. Slotting the pole pieces;
Both longitudinal gaps and oblique slots have been tried. The reduction of cross section of the pole piece causes it to become highly saturated and to offer large reluctance to the cross field.
4. The use of auxiliary poles.
These are small poles placed between the main poles and so wound and connected that their action opposes that of the cross field.
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.
Ques. Define the term "commutating plane."
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.
Ques. What is the angle of lead?
Ans. The angle between the normal neutral plane and the commutating plane.
In the operation of a dynamo since the field, on account of armature reaction, is twisted around in the direction of rotation, the proper position for the brushes is no longer in the normal neutral plane, but lies obliquely across, a few degrees in advance. Hence, for sparkless commutation, the commutating plane is a little in advance of the normal neutral plane, the lead being measured by the angle between these planes, as stated in the definition.
Fig. 288.—Normal neutral plane. This is a reference plane from which the lead is measured. As shown, the normal neutral plane lies at right angles to the lines of force of an undistorted field.
Ques. What may be said with respect to the angle of lead?
Ans. For sparkless commutation, the angle of lead varies with the load.
If the field be much altered at full load, it is evident that at half or quarter-load it will not be nearly so much twisted, hence the necessity for mounting the brushes on some kind of rocking device which will allow them to be shifted in different positions for different loads. A desirable point, then, in dynamo design is to make the angle of lead at full load so small that it will not be necessary to shift the brushes much for variation of load. This can be accomplished by making the field magnet field considerably more powerful than the armature field.