Fig. 276.—Distribution of armature currents in a four pole lap wound dynamo having four brushes and generating 120 amperes.
Ques. Are more than two brushes ever used with wave winding?
Ans. It is sometimes advisable to use more than two brushes with wave windings, especially when the current is very large.
For instance, in the case of a singly re-entrant[3] simplex wave winding for an eight pole machine, whenever any brush bridges adjacent bars of the commutator, it short circuits one round of the wave winding and this round is connected at three intermediate points to other bars of the commutator. Hence, if the short circuiting brush be a positive brush, no harm will be done by three other positive brushes touching at the other points. If these other brushes be broad enough to bridge across two commutator bars, they may effect commutation, that is, three rounds instead of one undergoing commutation together.
Number of Armature Circuits.—It is possible to have windings that give any desired even number of circuits in machines having any number of poles.
Fig. 277.—Showing effect of removing two of the brushes in [fig. 275]. If no spark difficulties occur in collecting the current with only two brushes, the arrangement will work satisfactorily, but the heat losses will be greater than with four brushes.
Ques. How many paths are possible in parallel?
Ans. For a simplex spirally wound ring, the number of paths in parallel is equal to the number of poles, and for a simplex series wound ring, there will be two paths. In the case of multiplex windings the number of paths is equal to that of the simplex winding multiplied by the number of independent windings.