Influence on commutation of brush contact drop

IN DESIGNING electrical commutating machinery, an attempt is made usually to produce during commutation a linear change of current with time. This, however, is rarely achieved, and the current density therefore usually is not constant across the brush surface, nor is the contact drop constant. The contact drop also will change with time in any small area on the brush surface, and therefore the static curve between voltage and current measured at the brush contact cannot be used.