Comparison among the PD resistance behavior of different enameled wires when subjected to PWM voltage

The electrical machines and in particular the electrical motors used in industrial applications are more and more often supplied by inverter drives, such as IGBT, so that the insulating organic materials of the inductor motor windings are subjected to voltage waveforms as PWM. In these conditions, degradation phenomena and in particular erosion due to partial discharges activity could take place, leading the insulating system to breakdown conditions. In this research work the partial discharges resistance of different insulating organic enamels has been studied: twisted pair specimens have been realized and then subjected to severe voltage waveform to simulate the real operating conditions that can take place when the electrical motors are supplied by inverter drives. The peak to peak voltage amplitude of the considered waveforms has been set always higher than the partial discharges inception voltage level in order to operate in presence of surface partial discharges activity. During the electrical tests, the specimens have been kept inside an oven at a fixed temperature level in order to study the combined effect of the thermal and electrical stress. The insulating enamel breakdown condition has been chosen as endpoint criterion: the obtained life time values have been collected and the relevant life curves plotted. Comparing the life curves it is possible to point out the different behavior of the analyzed organic materials when subjected to erosion effects due to partial discharges activity.