Impact of PWM Waveforms on Partial Discharge in SiC-Based Motor Drives

Partial discharge (PD) is a prime cause of premature failure of inverter-fed motor winding insulation. With the emergence of fast-switching wide-bandgap silicon-carbide (SiC) power devices, random-wound motors are more vulnerable to highly repetitive PDs due to the high-frequency steep-fronted switching transitions that result in overvoltage oscillations at the motor terminals and non-uniform voltage distribution within the motor winding turns. This paper investigates the impact of the applied PWM voltage waveforms on the PD behavior in SiC-based inverter-fed motor drives, including two-level and three-level PWM waveforms. The electric field distribution inside insulation defects (air cavities) is analysed for the PWM voltage waveforms to theoretically predict the number and phase of probable PD events within the fundamental cycle. An experimental PD measurement setup is used to validate the theoretical analysis by applying the PWM voltage waveforms, that are generated by SiC-based power converters, on a typical turn-to-turn motor winding insulation system created by a twisted pair of enamelled magnet wire. Phase-resolved PD patterns are generated to assess the PD behavior against the PWM characteristics of each voltage waveform and the associated overvoltage pattern when power cables are used to emulate the voltage reflections in cable-fed motor drives.