Title: Hawkins Electrical Guide v. 02 (of 10)
Author: N. Hawkins
Release date: May 2, 2015 [eBook #48846]
Most recently updated: October 24, 2024
Language: English
Credits: Produced by Juliet Sutherland, Wayne Hammond and the Online
Distributed Proofreading Team at http://www.pgdp.net
THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER
THEO. AUDEL & CO. 72 FIFTH AVE. NEW YORK.
COPYRIGHTED, 1914,
BY
THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
| THE ARMATURE | 221 to 228 |
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| ARMATURE WINDINGS | 229 to 256 |
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| THEORY OF THE ARMATURE | 257 to 282 |
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| COMMUTATION AND THE COMMUTATOR | 283 to 302 |
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| BRUSHES AND THE BRUSH GEAR | 303 to 320 |
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| ARMATURE CONSTRUCTION | 321 to 348 |
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| MOTORS | 349 to 388 |
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| SELECTION AND INSTALLATION | 389 to 406 |
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| AUXILIARY APPARATUS | 407 to 430 |
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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 commutator is in fact a part of the armature, but is of sufficient importance to be considered in a separate chapter.
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.
Ques. Why does the elementary armature produce a pulsating current?
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.
Ques. How is a continuous current, or one of uniform pressure obtained?
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 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."
An armature for practical use has a large number of coils, suitably arranged upon an iron core, so that a large proportion of them are always actively cutting the lines of force, or moving into the positions of best action in the magnetic field.
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:
1. Ring armatures;
2. Drum armatures;
3. Disc armatures.
Each of these forms of armature has its own special advantages for particular purposes, the disc type being least in favor and not having had any extensive application in this country.
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.
Ques. What is the comparison between ring and drum armatures?
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 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.
Ques. Describe a ring armature.
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 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.
Fig. 250.—Distribution of magnetic lines of force through a Gramme ring. Since the metal of the ring furnishes a path of least reluctance, most of the magnetic lines will follow the metal of the ring and very few will penetrate into the aperture of the interior. This condition causes a serious defect in the action of ring armatures rendering the winding around the interior useless for the production of electromotive force. Hence, in ring armatures only about half of the winding is effective, the rest or "dead wire," adding its resistance to the circuit, thus decreasing the efficiency of the machine.
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.
Numerous attempts have been made to utilize this part of the winding by making the pole pieces extend around the ring in such a manner that lines of force will pass to the inside of the ring, also by arranging an additional pole piece on the inside of the armature, but mechanical considerations have shown these methods to be impractical.
Ques. Is any portion of the winding of a drum armature inactive?
Ans. Yes; the end connectors do not generate any current.
Fig. 251.—Illustrating the principle of Siemens' drum winding. In order to make the winding and connections clear, one coil and the commutator is shown assembled, although the latter is not put in place until after all the sections have been wound, the ends of the wires being temporarily twisted together until all can be soldered to the risers. The cores of these early machines were of wood overspun circumferentially with iron wire before receiving the longitudinal copper windings.
Ques. What is the chief advantage of the drum armature?
Ans. It reduces considerably the large amount of dead wire necessary with the ring type.
Ques. How is this accomplished?
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.
Fig. 252.—Elementary four coil drum winding, showing the connections with the commutator segments, and directions of currents in the several coils. The action of this type of armature is fully explained in the text.
Fig. 252 shows an elementary four coil drum armature. Starting from the point a and following the winding around without reference at first to the commutator, it will be found that the rectangular turns of the wire form a closed circuit, and are electrically in series with one another in the order of the numbers marked on them.
With respect to the connections to the four segments w, x, y, z, of the commutator it will be found that at two of these, x and y, the pressures in the windings are both directed from, or both directed toward the junction with the connecting wire. At the other two segments, z and w, one pressure is toward the junction and the other directed from it. If, therefore, the brushes be placed on x and y they will supply current to an external circuit, z and w, for the moment being idle segments.
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.
On account of the nature of the construction of a disc armature, it is necessary that the coils subject to induction occupy as small a space as possible in the direction of their axes. This requirement, as well as the connection of the inductors with each other and with the commutator, prevented the general adoption of this form of armature, and subsequent experience failed to justify the existence of the type.
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.
For example, in order to direct the winder how to make the connections for, say a four pole machine having 100 bars spaced around its armature, some plain method of representing all the connections so that they may be easily understood is necessary. From this the workman finds out whether he is to connect the front end of bar No. 1 across to 50 or across a quarter of the circumference to 24, or across three quarters of it to bar 75. Again, he ascertains to which bar he is to connect the back1 end of the bar, and how the bars are to be connected to the commutator.
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.
Fig. 254.—End of ring winding for a four pole machine. An end view is simply a view showing the arrangement of the armature inductors and connections looking from the front or commutator end. A developed view of the above winding is shown in fig. 257.
There are three kinds of winding diagram:
1. End view diagram;
2. Radial diagram;
3. Developed diagram.
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.
Fig. 255.—Partial sketch of a four pole machine laid on its side. If the observer imagine himself placed at the center, and the panorama of the four poles to be then laid out flat, the developed view thus obtained would appear as in fig. 256.
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.
If in fig. 255, which represents an armature core and a four pole field, wires a and c be placed parallel to the axis of the armature to represent two of the armature inductors, and moved along the air gap space clockwise past the S poles, they will cut magnetic lines inducing electromotive forces in the directions indicated. To attempt to show a large number of inductors in a drawing of this kind would be unintelligible. Accordingly, the observer is considered as being placed at the center of the armature, and the panorama of the four poles surrounding him to be then laid out flat or "developed" as in fig. 256.
The faces of the N and S poles are shaded obliquely for distinction. By choosing the proper directions for these oblique lines, a piece of paper having a narrow slit to represent the wire may be laid over the drawing of the pole and when moved, as indicated by the dotted arrows to the right, the slit in passing over the oblique lines will cause an apparent motion in the direction in which the current in reality tends to flow. It is easily remembered which way the oblique lines must slope, for those on the N pole slope parallel to the oblique part of the letter N.
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.
Fig. 256.—Developed view of the four pole field shown in perspective in fig. 255.
Ques. What is lap winding?
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.
Ques. What is a wave winding?
Ans. One in which the coil ends diverge and go to segments widely separated, the winding to a certain extent resembling a wave.
Fig. 257.—Development of ring winding of four pole machine shown in fig. 254. The dead wire or inactive inductors on the inside of the ring are shown in dotted lines, the full lines representing the active portion of the winding.
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 pitch of the poles, represented by the angle M, which is the angle between the pole centres.
Figs. 258 and 259.—Wooden armature core and winding table for practice in armature winding. By using strings of different colors to represent the various coils, the path of each coil is easily traced when the winding is completed, as in fig. 263.
For instance, on a four pole machine the pitch would be 90°, on a six pole machine, 60°, etc. Usually the angular pitch of the coil is made just a little less than the pole pitch of the machine, in order to shorten the end connections of the coils from slot to slot. However, if the angular pitch be made too small trouble will be encountered in commutation.
In addition to the angular pitch there is the commutator pitch which relates to the distance around the commutator bridged by the ends of the coil. Thus, if the commutator segments were numbered consecutively 1, 2, 3, etc., and the commutator pitch say is 10, it would signify that one end of the coil was connected to segment 1 and the other end to segment 11; the ends of the next coil in order then would be connected to segments 2 and 12, in each case there would be ten segments between the two segments connecting with the coil ends.
Fig. 260.—End view of drum armature of a multipolar machine showing one coil in position to illustrate the angular pitch or spread of drum coils.
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.
In making the wooden core, the slots may be formed by nailing a series of thin strips around a cylindrical piece of wood, thus avoiding the trouble of cutting grooves. In the illustrations the commutator segments are shortened (leaving no room for brushes) in order to show the connections as clearly as possible.
Fig. 261.—Developed view of a typical lap winding. From the figure it is seen that at the back of the armature each inductor is united to one five places further on, that is, 1 to 6, 3 to 8, etc., and at the front end of the winding, after having made one "element," as for example d-7-12-e, then forms a second element e-9-14-f which "laps" over the first, and so on all around until the winding returns on itself.
Ques. Describe the simple lap winding 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 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.
Ques. What is the path of the first 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.
Fig. 262.—Skeleton view of wooden armature core showing in position the first two coils of the winding indicated in the table fig. 259.
Ques. Describe the path of the second coil.
Ans. The second coil, having the block insulation, is wound
according to the table, in the order:
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.
The completed winding of the first two coils are shown in fig. 262, the drum being shown in dotted lines so that all of each coil may be visible.
Fig. 263.—View of completed winding as indicated in the table fig. 259. Thus the path of the first coil, according to the table is A-1-6-B which means that the coil begins at segment A of the commutator, rises to slot 1, and proceeds through the slot to the back of the drum; thence across the back to slot 6, through the slot and ending at segment B. The other coils are wound in similar order as indicated in the table.
Ques. How are the remaining coils wound on the drum?
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.
Ques. What is the general form of the completed winding?
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.
Fig. 264.—Developed view of the winding shown in perspective in fig. 263.
Ques. What condition must obtain in winding an even number of coils?
Ans. The wire must not be wound around the drum to diametrically opposite positions, as for instance 1 to 7 in fig. 265.
Ques. Why is this?
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:
| A | — | 1 | — | 7 | — | B |
| B | — | 3 | — | 9 | — | C |
| C | — | 5 | — | 11 | — | D |
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.
Fig. 265.—Lap winding for bipolar machine, with uneven number of coils; in this case the rear connectors may be made directly across a diameter as shown.
Ques. How is a symmetrical winding obtained having an even number of coils?
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.
In the example here shown there are six coils, comprising twelve inductors and six commutator segments; it should be noted, however, that if there were an uneven number of coils, the rear connections could be made directly across a diameter as shown in fig. 265, which would give a symmetrical winding.
With ten slots as shown in the figure, the drum would be wound, for a bipolar machine, according to the following table:
| A | — | 1 | — | 6 | — | B |
| B | — | 3 | — | 8 | — | C |
| C | — | 5 | — | 10 | — | D |
| D | — | 7 | — | 2 | — | E |
| E | — | 9 | — | 4 | — | F |
Fig. 266.—Developed view of a typical wave winding. This winding, instead of lapping back toward the commutator segment from whence it came, as in lap winding, turns the other way. For instance, d-7-12 does not return directly to e, but goes on to i, whence another element i-17-4-e continues in a sort of zigzag wave.
Ques. Are coils such as shown in figs. 263 and 265 used in practice?
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 is, the coil ends instead of connecting with adjacent segments of the commutator, are attached to segments more or less remote.
Ques. Describe the circuits of a simple or simplex wave winding.
Ans. Only two sets of brushes are required for such a winding, but as many brushes as there are poles can be used.