Evidence of charge injection resulting from fast pulse operation of inverter drives

Laboratory specimens consisting of random wound motor turn insulation have been subjected to aging below Corona Inception Voltage (CIV), at a maximum temperature of 120°C. Four specimens were either exposed to thermal stresses alone, to unipolar fast voltage pulses or to both thermal and electrical stresses. Thermally Stimulated Current (TSC) measurements showed measurable currents only for those specimens exposed simultaneously to electrical and thermal stresses suggesting that charge traps are being created within the insulation as a result of this type of voltage pulse operation. Results from consecutive TSC measurements performed up to 150°C showed good reproducibility. By increasing the polarization temperature to 200°C, an additional much larger current peak associated with the glass transition temperature (Tg) was detected. This peak was not observed with reference specimens only aged thermally