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Removal of the retaining rings exposes the end windings and shows how the insulation has crept out from between the turns.

Rotor Turn Insulation Migration

Problem: Since the early 1990s, a number of rotors have been experiencing turn insulation migration. The insulation between the copper turns slips out of position because the adhesive bond between the insulation and the copper breaks down. With the bare copper exposed, shorted turns result. The coils with shorted turns run cooler. This causes a temperature imbalance, which if enough shorted turns exist, can increase rotor vibration to unacceptable levels. A boroscope slipped up and under the retaining ring can be useful in identifying the problem. Removal of the retaining rings provides solid confirmation.