Commutation of Large D-C Motors and Generators [includes discussion]

This paper presents a method of analysis of commutation phenomena and application of the method to calculate the voltage appearing from bar-to-bar on the commutator during commutation. This is the voltage absorbed by the brush contact and which is zero for uniform contact voltage over the contact area of the brushes. The voltage is calculated as a function of time for a 2,250-horsepower 250 rpm 600-volt motor and curves are shown and compared with curves obtained by oscillograph tests. Oscillograph curves were obtained by connecting slip rings to each of a group of commutator bars and recording voltage from ring-to-ring under various conditions of load. Abrupt pulsations of voltage under the brushes appear in the calculated and test results. It is shown that perfect commutation in the sense of complete compensation of reactance voltage is associated with pulses of brush contact voltage so that nonuniform, pulsing contact voltage is a normal condition. Calculations of the type presented constitute new criteria for successful commutation.