As the rotor in DC machines rotates, different commutator segments become in contact with brushes. As bush moves from one commutator segment to another, the direction of current in the coil connected to the said commutator segment is reversed. Because of current reversal, there is a large rate of change of current in the coils and a high voltage is produced ( due to the phenomena called voltage kickback). This high voltage between two ends of coil produces sparking in commutator segments, thereby reducing their life and performance.
Course title: Electrical Machines
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