Effectiveness of thermo-mechanical stress in accelerated aging of turn insulation in motor coils

Thermomechanical stresses were applied to motor coils and significant degradation of the turn insulation was observed. Two batches of five class F coils were subjected to a thermal cycling test for 200 h. The coil temperature was raised to 185°C in 15 min by circulating 60-Hz current and then lowered to about 30°C in the next 15 min by forced cooling. One of the two batches was subsequently subjected to electrical stresses applied for 500 h across the groundwall and the turn insulation. A new batch of coils was thermally cycled up to 155°C while yet another batch was heated for 100 h in an oven at 185°C. Breakdown voltages of turn insulation in all these coils were measured for 100-ns risetime impulses and compared with those for the unaged coils. The coils subjected to thermal cycling up to 185°C showed a significant drop in the impulse breakdown voltage as compared with the unaged coils