Turn insulation aging of motors exposed to fast pulses of inverter drives

The effects of fast pulses from PWM inverter AC drives on motor insulation are not well understood and their impact on long term reliability is not known. Turn insulation stresses have been simulated on small "layer-to-layer" specimens subjected to typical pulses of 120 ns rise-time at a repetition rate of 10 kHz. Voltage pulse overshoots reached 920 V. Under these conditions no partial discharge was detected. Measurements of capacitance, dielectric losses, relaxation currents, return voltage and thermally stimulated current (TSC) have been used to characterise insulation modifications resulting in the cumulative effects of multiple pulses. Preliminary results indicated that changes in the dielectric properties after a short exposure (1310 hours) to voltage alone were negligible. When specimens were exposed to both temperature and voltage, TSC results showed that some modifications of the dielectric properties could be initiated after short exposure time, even below the partial discharge inception voltage.