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 |
