It should be noted that the number of brush sets does not necessarily show the number of circuits through the armature.
Ques. How many brushes are required for wave windings?
Ans. If arrows be drawn marking the direction of the induced electromotive forces to determine the number of brushes, it will be found that only two brushes are required for any number of poles.
Ques. What is the angle between these two brushes?
Ans. It is the same as the angle between any north and south pole.
For instance, in a ten pole machine with wave winding the pitch between the brushes may be any of the following angles:
| 360 / 10 | = | 36° |
| 3 * 36° | = | 108° |
| 5 * 36° | = | 180° |
Figs. 274 and 275.—Right and left hand windings. These consist respectively of turns which pass around the core in a right or left handed fashion. Thus in fig. 274, in passing around the circle clockwise from a to b, the path of the winding is a right handed spiral. In fig. 275, which shows one coil of a drum armature, if a be taken as the starting point, in going to b, a must be connected by a spiral connector across the front end of the drum to one of the descending inductors such as M, from which at the back end another connector must join it to one of the ascending inductors, such as S, where it is led to b, thus making one right handed turn.
Sometimes with lap winding it is desirable to reduce the number of brushes. In [fig. 276], is shown the distribution of currents in a four pole lap wound machine having four brushes and generating 120 amperes. In each of the four circuits the flow is 30 amperes, and the current delivered to each brush is 60 amperes. If now two of the brushes be removed, the current through each of the remaining two will be 120 amperes, while internally there will be only two circuits as shown in [fig. 277]. It should be noted, however, that these two circuits do not take equal shares of the current since, though the sum of the electromotive forces in each circuit is the same, the resistance of one is three times that of the other, giving 90 amperes in one and 30 amperes in the other, as indicated in the figure. If no spark difficulties occur in collecting all the current with only two brushes, the arrangement will work satisfactorily, but the heat losses will be greater than with four brushes.