Brush contact losses due to load and parasitic currents in polyphase commutator motors

After brief reference to present methods of measurement, the interdependence of the brush losses due to load and parasitic currents is discussed and the need for measuring them simultaneously is argued. Owing to the impossibility of doing this satisfactorily on a motor, an apparatus for simulating brush parasitic currents is described and test data obtained with this apparatus are used to establish the argument. The effects of rotor power factor and speed on the losses are also discussed and established experimentally. Finally, a new test is described by means of which the total rotor I2R losses of a polyphase commutator motor can be measured with reasonable accuracy. Curves obtained by this method of test using plain and sandwich brushes show the dependence of these losses on the coil e.m.f. between segments.