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.