Analysis and Suppression of Shaft Voltage in SiC-Based Inverter for Electric Vehicle Applications

SiC mosfets allow a higher frequency, lighter weight inverter over their Si counterparts for electric vehicle (EV) applications. However, using SiC-based EV traction inverters is likely to increase the shaft voltage in an ac motor drive system, which significantly affects the reliability of the system. This paper investigates the impact of high-speed switching of SiC devices and high switching frequency of SiC-based inverter on the common-mode voltage (CMV) and the shaft voltage. It is theoretically illustrated and experimentally demonstrated that the high switching frequency increases the amplitude of the shaft voltage, while the fast switching speed has limited influence on both CMV and the shaft voltage. Afterward, active zero state pulsewidth modulation (AZSPWM) and common-mode chokes are employed to mitigate the shaft voltage. The experimental results show that either the AZSPWM modulation scheme or common-mode chokes have the capability to minimize the shaft voltage. Better performance can be achieved by utilizing both techniques simultaneously. It is also observed that the high switching frequency enabled by the fast-speed SiC device is able to be helpful to the use of reduced shaft voltage technologies.