Influence of parasitic brush currents on the performance of d.c. motors

The brushes in d.c. machines bridge the micas between the commutator segments, and the coils thereby short-circuited may carry parasitic currents. As described in a previous paper on d.c. generators, the short-circuited loops may be represented, for many calculations, by an auxiliary-armature circuit. In the present paper, it is shown, by calculation and test, that the parasitic currents may have marked effects on the speed-regulation properties of motors under linear conditions, and that these effects may partly persist in the nonlinear régime. It is suggested that many anomalies, commonly ascribed in an undefined way to brush effects and armature reaction, are amenable to a stricter treatment in terms of the concepts proposed.