Aging of motor turn insulation under repetitive surges

The effect of repetitive surges on the impulse strength of interturn insulation was studied for coils in three motor stators. In each stator, the coils were divided into several groups. Breakdown voltages of coils in one group were measured for 0.1- mu s risetime impulses and the average breakdown voltage of these unaged coils was determined. Coils in the other groups were first subjected to a number of surges with magnitudes 3 to 6.5 pu (1 pu is the peak-rated line-to-ground voltage of the machine), and then the impulse breakdown voltages for these aged coils were measured and compared with those for the unaged coils. There was an indication of surge aging of the turn insulation only 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. It is suggested that there is a threshold value of surge magnitude for producing aging effects.<>