Theory of the Sparking during Commutation on Dynamos

The lifetime of arcs at incomplete commutation is ta = FLj, where j is the current at which the arc ignites, L is the self-inductances of the commutating coil, and F is a function of inductances and resistances of the machine. Thereby, the energy consumed in the arc is found to be only a small fraction of the magnetic energy that remains uncommutated when the arc ignites. The effect of brushes contacting more than two segments is treated, and the theory of a known method to determine j is worked out.