This paper reports observations of partial discharges (PD) occurring within the stators of random wound electric motors during surge comparison testing, and discusses criteria for limiting testing levels to avoid initiation of insulation defects. Random wound electric motors are constructed by forming coils from a number of turns of insulated wire and placing these into the stator. One common form of motor failure involves electrical breakdown between turns, which subsequently develops into a breakdown to earth or between phases. Motor manufacturers and rewinders commonly use a surge comparison tester to determine whether a new winding is free from winding defects such as shorted turns or damaged turn insulation. Partial discharges are an indication of insulation weakness and can also damage the insulation without being detected during surge comparison testing.
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