Resistance Commutation

Thorburn Reid considered the commutation problem in rotating machines as arising from reactance in the coils being commutated by the varying brush contact resistance. This paper extends Reid´s treatment to include the effects of reactance in the coils which are not being commutated, and shows that these noncommutating coils are an important factor in commutation performance. An equivalent circuit for a simplified commutator machine is developed and used as the basis for the analysis given in Appendixes I through VI. By means of this equivalent circuit, the average brush terminal voltage is seen to approach the value of the commutating reactance voltage under severe commutating conditions. Voltage pulses near the end of the commutation period are shown to be produced by a parallel influence of the noncommutating and the commutating coils. A testing device based on the equivalent circuit is described in Appendix VI, and consists of a mechanical inverter connected so as to separate the functions of the brushes in reversing current through the commutating coils, and in passing current through the noncommutating coils. When connected to the armature of a particular machine, the testing device is used to measure the commutating reactance and the brush contact resistance under simulated operating conditions. By means of this device, a correlation between brush electrical wear and severity of commutating conditions is established, which helps to confirm the basic theory.