Mitigation of machine neutral point overvoltage in SiC motor drives by controlling antiresonance frequency and switching frequency

No descrinverter-fed motors are vulnerable to premature insulation failure due to overvoltage stress across stator windings because of antiresonance phenomenon which is characterized by two oscillatory overvoltage responses that manifest at the motor terminals and stator neutral point. While the motor terminal overvoltage has been extensively investigated in literature, the motor neutral point overvoltage has been rarely addressed. With the adoption of SiC power semiconductor devices in motor drives, the neutral point overvoltage can be more hazardous to the motor winding insulation since the drive switching frequency can match the motor antiresonance frequency. This paper proposes a novel approach to eliminate the motor neutral point overvoltage in SiC motor drives. The proposed approach shifts the motor antiresonance frequency to a lower value relative to the drive switching frequency by installing a tiny capacitor (in the motor terminal box) between the stator neutral and the grounded frame. The ratio between the shifted antiresonance frequency and the switching frequency is optimally controlled to significantly depress the motor resonance excitation, which eliminates the neutral point overvoltage. The control parameters of the proposed approach are mathematically derived for optimal performance. A three-phase 2.2kW SiC inverter-fed motor drive setup is used for experimental verification.