Use of thermal cycling as type test for turn insulation in motor coils

Costs associated with premature failures of stator insulation in critical large motors in utility generating stations and large industrial plants are very high. Type tests or accelerated aging tests are used to check the long term reliability of insulation systems in power apparatus. However, such type tests exist only for the groundwall insulation in motors. No such test exists for the turn insulation which is responsible for many stator failures. Ontario Hydro has been trying to develop a type test for turn insulation in motor coils and this paper reports promising results from tests conducted on three sets of coils with similar groundwall insulation but three different types of turn insulation. Thermal cycling of coils, i.e., repetitive alternate raising and lowering of the temperature, subjects the insulation system of coils to thermo-mechanical stress which can degrade the turn insulation. Many coils with class F insulation system were tested. All these new coils from one manufacturer had the same (polyester-mica) groundwall insulation, but three different types of turn insulation, enamel plus daglass, double daglass, and micatape. The coils were thermal cycled 400 times. Each cycle was about 30 minutes long with 15 minutes for each of the heating and cooling periods. The peak temperatures were 155°C and 185°C. The degradation of turn insulation was measured as the decrease of breakdown voltage under fast fronted (0.1/50 μs) surges. For the three sets of coils the decreases in impulse breakdown voltages (in comparison with new coils) were different. Aging coils in an oven at the same constant temperatures produced no or little decrease in impulse breakdown voltages. The tests demonstrate clearly that thermal cycling may be used as a type test for the long term reliability of turn insulation in motor coils. Further work for optimizing the test parameters is outlined