Commutation and Brush Loss

C. E. Wilson
1913 Transactions of the American Institute of Electrical Engineers  
Even when made on slip rings, or on special short-circuited commutators, brush loss tests are hard to duplicate and results of tests under similar conditions vary greatly. A number of things are known to affect the brush loss at a given current density. Among these, the most important are: the direction of current in the brushes, the glaze or polish of the collector, the surface and fit of the brushes, oil or any foreign material on the collector, the temperature of the parts and the peripheral
more » ... and the peripheral speed of the collector. In direct-current machines, there are still other reasons for variations in brush loss. It is likely to be increased by high mica, unequal division of current among the brushes and brush arms, vibration or chattering of the brushes, imperfect spacing of them, and, above all, by the commutation and the strength of the magnetic fields in which the short-circuited coils are commutated. In fact, tests show that one machine may have two or three times the brush drop of another of the same type, although the grade of brushes and the apparent current density in them are the same. Even in the same machine, with the same current output, the brush drop measured at the different brush arms and at different times, may vary 100 per cent or more. Table -I gives the tests on four adjustable-speed motors of the same type, at the maximum and minimum speeds for which they were designed. These were chosen at random from a large number of tests made on this line of machines, all of which were equipped with the same grade of brushes. The voltages A, B and C were taken at different points on the brush and commutator, as shown in Fig. 1 , and were measured 577
doi:10.1109/t-aiee.1913.4765038 fatcat:u5p3ie7xarah3hunmfbk5d3y3u