Influence of parasitic brush currents on the performance of D.C. generators

The parasitic brush currents which flow in the armature coils short-circuited by the brushes at the moment of commutation modify the characteristics of a d.c. generator, and the effects are particularly noticeable when the speed is varied. The modification of the characteristics can be explained by representing the parasitic brush currents by a current flowing in an additional armature circuit subject to rotationally generated voltages. The pattern of behaviour predicated by the theory is in good agreement with the machine characteristics obtained by test. Although the theory is expressed in linear terms, it nevertheless explains phenomena which are significant in the practical performance of machines.