Lifetime Evaluation of High-frequency Voltage Endurance of Sheet Insulation Materials for electric-vehicle Motors

Recently, the converter fed motors of electric vehicles are driven at higher voltages to increase their power density. The voltage supply from a converter has a surge pulse waveform with high repetition rate and short rise time causing partial discharges in the insulation of the motor winding. The partial discharges accelerate the deterioration of the insulating materials. This paper focuses on a mica based wire insulation for form wound motor coils as an alternative to the conventional enamel-coated insulation of random wound coils, and presents the results of the partial discharge inception voltage measurements and the electrical endurance tests of mica sheet materials. Mica sheets were manufactured by lamination of a polyimide film and a muscovite calcined mica paper. In the experiment, simple polyimide films were also evaluated as references representing the enamel-coating. Mica sheets and polyimide films were subjected to high repetition pulsed voltages using flat plate-cylindrical electrodes at room temperature and at an elevated temperature. The results show that the mica significantly improves the partial discharge resistance of the insulation and may contribute to longer service life of a motor or may enable feeding higher voltage.