CIGRÉ 1996 : 15/21/33-02

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ENSURING THE LIFE TIME PERFORMANCE OF TURN INSULATION IN MOTOR COILS THROUGH ACCELERATED AGING TESTS B.K. Gupta* Ontario Hydro Technologies (Canada) W.T. FinkConsolidated Edison Co. of New York (USA)

1.0 SUMMARY Accelerated aging tests or type tests are now routinely used to ensure reliable life time performance of ground-wall insulation in stator coils andbars. However, many groundwall failures in multiturn coils are initiated by a breakdown of the turn insulation. At present no accelerated aging test is available for checking the long term performanceof the turn insulation. The need for such a test is becoming even more important with the increasing trend to apply variable speed drives to many motors. Results from extensive work on developing atype test for turn insulation in motor coils are presented here. In the accelerated aging tests, new motor coils were subjected to enhanced stresses and then tested to breakdown under fast fronted(0.1/50 µs) surges after aging. The aging or degradation of turn insulation was measured as the decrease of breakdown voltage compared to unaged coils. Coils aged up to 1000 hours under enhanced voltagestress at 3 kHz and moderate thermal stress showed no degradation of the turn insulation. However, thermal cycling of coils, i.e., repetitive fast alternate raising and lowering of the temperature, whichsubjects the insulation system of coils to thermo-mechanical stress, degraded the turn insulation significantly. Class F coils with polyestermica groundwall insulation and three different types ofturn insulation (enamel + daglass, double daglass, and micatape) were subjected to 400 thermal cycles, each cycle having 15 minutes for heating and 15 minutes for cooling. Two peak temperatures of 155°Cand 185°C were tried. The decreases in impulse breakdown voltages (compared with unaged coils) were different for the two peak temperatures and for the three types of turn insulations. Aging coils... [continua]

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