Aging acceleration of insulating materials for electrical machine windings supplied by PWM in the presence and in the absence of partial discharges

The speed control of ac-electric motors can be carried out efficiently by adjustable speed drives (ASD), which are constituted by an AC/DC conversion block, a DC filter and a DC/AC inverter representing the core of the whole power converter. A new generation of DC/AC converters for adjustable speed drives, the so-called PWM (Pulse Width Modulation) inverters, has been developed in recent years. It has significant advantages for motor speed control, but has been proved to overstress electrical insulation with respect to the sinusoidal supply. The increase of partial discharge (PD) activity seems to be mainly responsible for the degradation acceleration of motors fed by ASD. In order to investigate the aging behavior, the results of life tests carried out on twisted pairs subjected to 50 Hz and 10 kHz sinusoidal voltage, as well as unipolar and bipolar square pulses, are presented.