DEGRADATION OF TURN INSULATION IN MOTOR COILS UNDER REPETITIVE SURGES

Degradation of interturn insulation caused by repetitive surges was studied for coils in three motor stators. In each stator, the coils were isolated and divided into several groups. Breakdown voltages of coils in one group were measured for 0.1 μs risetime impulses and the average breakdown voltage of these " unaged" coils was determined. Coils in the other groups were first subjected to a number (1000 to 8000) of surges with magnitudes 3.0 to 7.8 pu, and then the impulse breakdown voltages for these " aged" coils were measured, and compared with those for the " unaged" coils. The results of the measurements on two stators showed no evidence of degradation of turn insulation by surges. There was an indication of surge aging of the turn insulation for one stator. Surge magnitudes capable of causing detectable aging of turn insulation appear to be higher than those likely to occur at normally operating utility motors. The existence of a threshold value of surge magnitude required to produce aging effects is a possible hypothesis. © 1990 IEEE