Mechanical Load Aspects of High-Current Brush System Design

Selection of proper mechanical loading is necessary to assure satisfactory brush operation in rotating electrical machines. Irregularities in the moving surface produce forces such as brush inertia and holder friction which alter the contact force. The range of force variation depends upon controllable factors in the brush system design. For high metal-content brushes operating in an inert atmosphere, the sliding contact is modeled using theory applicable to metallic contacts with high conductivity films. Based on this model and under conditions of fluctuating contact force, the brush power loss is higher than that which would occur for a steady force equal to the average value. The results may be used to determine the sensitivity of a brush system design to irregularities in the dimensions of the slip ring or commutator and in the selection of proper brush loading. In addition, the results may explain differences in performance between laboratory brush tests and machine operation, which are caused by differences in acceleration forces and current redistribution.