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Studies on magnet wire degradation with inverter driven motors

The impetus to save electrical energy, to improve control of manufacturing processes for quality, efficiency, downtime and cost have all contributed to the proliferation of adjustable speed drives for induction motors. The use of drives, especially pulse width modulated types utilizing fast switching electronic devices such as IGBTs, has introduced another challenge to motor manufacturers, repair facilities and magnet wire manufacturers; namely, premature motor failure due to stator winding insulation degradation. Conventional testing methods for the insulation life of magnet wire do not include the combined stresses of the repetitive dielectric and thermal energy stresses imposed on the insulation due to inverter duty motor service. The intent of this paper is to provide information and data related to the causes of premature insulation failure in motors subjected to inverter duty. Standard testing methods and other alternative tests that could potentially predict motor insulation life are discussed. The data includes lifetime plots of different insulation builds and types using rectangular waveshapes, and the combined waveshape effects with different inverter drive/motor/cable combinations. The data related to the combined effects of voltage, temperature, insulation build and type are also presented and discussed.