Practical Aspects of Brush Contact Stability [includes discussion]

The contact between a carbon brush and a revolving commutator is assumed, in the accepted theories of commutation, to be constant in position and length of arc, but it is not always so in practice. This results in unexpectedly poor machine performance in certain circumstances. It can seriously impair commutation, make it impossible to run machines in parallel, cause instability of speed and output, and selective collection of current. This paper is an account of the theories evolved to explain these undesirable phenomena, the methods of overcoming them and improving machine performance. Comparisons are made between single, tandem, and split brushes. The powerful effect of circumferential stagger is also shown, both in overcoming the instability of the arc of contact and in masking the effect of commutator surface irregularities.