Degradation of turn insulation in motor coils under repetitive surges

Degradation of interturn insulation caused by repetitive surges is studied for coils in three motors stators. In each stator, the coils are isolated and divided into several groups. Breakdown voltages of coils in one group are measured for 0.1 μs risetime impulses and the average breakdown voltage of these unaged coils is determined. Coils in the other groups are first subjected to a number (1000 to 8000) of surges with magnitudes of 3.0 to 7.8 pu, and then the impulse breakdown voltages for these aged coils are 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 is 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