Fig. 5. Armature Winding.
Armature Winding. The armature winding is what is commonly known as the series or wave winding, shown developed in the paper on Direct-Current Dynamos. This winding is shown in [Fig. 5], which is an end view of an armature and commutator. In the figure, however, the armature is shown with a much smaller number of slots than a railway armature should have in practice. One reason for the employment of the wave or series winding on railway motor armatures, is that with this winding no cross-connections are necessary when only two brushes are used, and these two brushes may be placed 90° apart in a convenient and accessible position. Another reason is that the current, in flowing from one brush on the commutator to another, must always pass through the magnetic field of all four of the motor poles. This makes it impossible for any unbalancing of the magnetic circuit to cause more current to flow through one portion of the armature than is flowing through another portion. In a railway motor it has been found quite possible to have one pole or pair of poles exerting a greater magnetic attraction on the armature than another pair, owing to differences in the iron and differences in the clearance between the armature and pole pieces, which differences cause more magnetic lines of force to flow from some pole pieces than from others. With the lap-armature or the ring-armature winding, since the various portions of the armature under different poles are in parallel with one another, any difference in the magnetic flux between different poles will cause a different amount of current to flow in the various paths through the armature.
| Type of Motor. | Horse Power. | Amperes. | Speed Full Load. | Total Field Turns. | Slots. | Conductors per Slot. | Commutator Bars. | Weight complete with Gears. | Armature Complete. | Gears and Casing. | Commutator Bearing. Inches. | Pinion Bearing. Inches. | Diameter Armature. | Length. | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
GeneralElectric 51 | 82 | 640 | 56 | 37 | 12 | 111 | 3875 | 953 | 338 | 3 | 5¾ | 3¼ | 8¼ | 16 | 10½ | |
52 | 27 | 640 | 155.5 | 29 | 24 | 87 | 1725 | 357 | 265 | 2½ | 6⅜ | 2¾ | 7¾ | 11 | 9 | |
57 | 52 | 470 | 110 | 33 | 18 | 99 | 2972 | 704 | 340 | 2⅝ | 6⅜ | 3¼ | 8¾ | 14 | 12 | |
55 | 160 | 47 | 6 | 141 | 5415 | 1550 | 490 | 3¼ | 7½ | 3¾ | 11 | |||||
67 | 40 | 110 | 37 | 18 | 111 | 2385 | 595 | 385 | 2⅝ | 6⅛ | 3 | 8 | ||||
54 | 25 | 115 | 1831 | 395 | 285 | 2½ | 6 | 2¾ | 7¾ | |||||||
74 | 65 | 113 | 70.5 | 3534 | 845 | 415 | 3⅛ | 6¾ | 3⅝ | 8¾ | ||||||
Westinghouse 68 | 40 | 55 | 12 | 109 | 2280 | 505 | 330 | 2¾ | 6¾ | 3 | 7¾ | 14 | 8 | |||
69 | 30 | 35 | 105 | 1950 | 385 | 330 | 2¾ | 6 | 2¾ | 7 | 13 | 6¾ | ||||
76 | 75 | 39 | 117 | 3840 | 505 | 860 | 3¼ | 8 | 3½ | 9 | 16½ | |||||
56 | 55 | 39 | 117 | 3000 | 315 | 720 | 3 | 7½ | 3¼ | 8½ | 14 | 12 | ||||
50c | 150 | 144 | 55 | 6 | 115 | 5550 | 1500 | |||||||||
49 | 35 | 114 | 59 | 117 | 1925 | 438 | 327 | 2¾ | 6 | 2¾ | 7½ | 13⅝ | 6½ | |||
By reference to the winding diagram given in [Fig. 5], it may be noted that a complete circuit through two coils ends at the segment adjacent to the one from which the start was made. It may also be noted in the table of motor data that all of the armatures have an odd number of segments and an odd number of slots. It is absolutely necessary in a wave winding to have an odd number of segments. Otherwise the winding could not be made symmetrical and the circuit through two coils be made to return to a segment adjacent to that from which the start was made. With equal spacing between the top and bottom leads of the two coils, an even number of segments would make the circuit return either on the segment from which the start was made or two segments from it.
The first drum-wound street railway motor armatures had as many slots in the armature as there were coils and segments. The great number of slots necessarily made the teeth very thin and consequently weak. This is very objectionable as sometimes the armature bearings wear away, allowing the face of the armature to drag on the pole pieces and thin teeth are bent out of shape.
Armatures are now almost entirely constructed with either two or three coils to a slot. When two coils are used in each slot with an odd number of slots an even number of coils results. If these were all connected to the commutator an even number of segments would be necessary. As this is not possible with a wave winding, one of the coils is “cut out.” The ends are cut short and taped and it is termed a “dead” coil. This makes the winding somewhat unsymmetrical, all the coils not bearing the same angular relation to the commutator segments to which they are connected. This difference is, however, not great enough to affect the operation of the machine.
The Westinghouse 49 motor is an example of an armature with a dead coil. By reference to the table of motor data it will be seen that this armature has 59 slots. Two coils in each slot would make 118 coils. One of these, however, is cut out, giving 117 segments.
Cutting out a coil can be avoided by putting three coils in each slot.
An odd number of coils results then no matter what the number of slots may be. In the majority of examples given in the table there are three times as many segments as slots. The sides of the slots of modern street railway armatures are straight. The coils are prevented from flying out by bands of wire extending over the tops of the coils around the armature. Steel or silicon bronze wire of about No. 14 gauge is used. Recesses are made in the armature teeth for the reception of these bands so that the wire when wound will come flush with the face of the armature. The bands are usually ¾ to 1½ inches wide. The wires are well soldered together to secure them in place. One trouble experienced with armatures is the slipping off of these bands. The heated armature expands and stretches them. When the armature cools the bands are loose and then often slip off. When they do so the coils fly out by centrifugal force, strike the pole pieces and ground the motor.
