Sparking in DC Machines at Brushes
Poor commutation and sparking are caused by distortion of main field flux. The sparking at the brushes results in poor commutation due to the inability of the current in the short-circuited coil to reverse completely by the end of short circuit period (Usually of the order of 1/500). The main reason to delay this quick reversal is due to the production of self-induced e.m.f in the coil undergoing commutation. This self-induced e.m.f in the coil undergoing commutation is known as reactance voltage which is of Small magnitude and produces a large current through the coil whose resistance is less due to short circuit. If this voltage exceeds more than 30 or 40 V, a spark spreads around the commutator in the form of ring fire.
