Abstract
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 degrees C in 15 min by circulating 60-Hz current and then lowered to about 30 degrees 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 degrees C while yet another batch was heated for 100 h in an oven at 185 degrees 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 degrees C showed a significant drop in the impulse breakdown voltage as compared with the unaged coils. >
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