A combined voltage-endurance and thermal cycling test for stator coils and bars

The authors describe an accelerated test procedure in which coils under test are subjected to rapid temperature cycling and, at the same time, the groundwall insulation has a higher-than-normal voltage applied across it as in the voltage-endurance test. The coils are deemed to have passed the test if they survive a specified number of temperature cycles without delaminating at a predetermined level of applied voltage. Preliminary tests on two coils resulted in delamination being detected in one after only 50 thermal cycles followed by eventual failure after 348 cycles. The other coil exhibited delamination after 250 thermal cycles but withstood 500 cycles, at which time the test was stopped. This test is intended to help utilities compare different coil designs and to identify those which are more likely to withstand in-service cycling