Figure 8-16 shows a wave type of coil. Figure 8-17 gives a 6-pole wave winding. Study reveals that it has only two parallel paths between the positive and negative terminals. Thus, only two sets of brushes are needed

Each brush shorts p/2 coils in series. Because points a, b, and c are at the same potential (and, also, points d, e, and f ), brushes can be placed at each of these points to allow a commutator one-third as long.

The winding must progress or retrogress by one commutator bar each time it passes around the armature for it to be singly reentrant. Thus, the number of bars must equal (kp/2) # 1, where k is a whole number and p is the number of poles. The winding needs no equalizers because all conductors pass under all poles.

Although most wave windings are 2-circuit, they can be multicircuit, as 4 or 16 circuits on a 4-pole machine or 6, 12, or 24 circuits on a 12-pole machine. Multicircuit wave windings with the same number of circuits as poles can be made by using the same slot and bar combinations as on a lap winding.

For example, with an 8-pole machine with 100 slots and 200 commutator bars, the bar throw for a simplex lap winding would be from bar 1 to bar 2 and then from bar 2 to bar 3, etc. For an 8-circuit wave winding, the winding must fail to close by circuits/2 bars, or 4. Thus, the throw would be bar 1 to 50, to bar 99, to bar 148, etc.

The throw is (bars # circuits/2)(p/2), in this case, (200 # 4)/4 # 49. Theoretically such windings require no equalizers, but better results are obtained if they are used. Since both lap and multiple wave windings can be wound in the same slot and bar combination simultaneously, this is done by making each winding of half-size conductors.

This combination resembles a frog’s leg and is called by that name. It needs no equalizers but requires more insulation space in the slots and is seldom used.

Some wave windings require dead coils. For instance, a large 10-pole machine may have a circle of rotor punchings made of five segments to avoid variation in reluctance as the rotor passes under the five pairs of poles.

To avoid dissimilar slot arrangements in the segments, the total number of slots must be divisible by the number of segments, or 5 in this case. This requires the number of commutator bars to be also a multiple k of 5.

However, the bar throw for a simplex wave winding must be an integer and equal to (bars # 1)(p/2). Obviously (5k # 1)/5 cannot meet this requirement.

Consequently one coil, called a dead coil, will not be connected into the winding, and its ends will be taped up to insulate it completely. No bar will be provided for it, and thus the bar throw will be an integer. Dead coils should be avoided because they impair commutation.

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