Correlation between shaft voltage characteristics, electrical discharge behavior and surface damage of motor bearing in EV bench tests

The electric discharge damage had emerged as the primary failure mode of bearing in electrical vehicle's (EV) driving motor, with the fundamental cause being the shaft voltage produced during motor operation. However, the shaft voltage would exhibit different characteristics depending on the bearing's operating state. Here, this paper examined a dedicated hybrid transmission (DHT) detached from a real EV to investigate the connection between shaft voltage characteristics, electrical discharge behaviors and electrical damage within the driving motor bearing. The results demonstrated that different patterns of shaft voltage were characterized, which was associated with lubricating status inside the bearing. The establishment of lubricating oil film led the bearing to undergo resistive, mixed and capacitive voltage states in sequence, with the electrical discharge behavior in the capacitive state resulting in electrical damage. The decrease in viscosity of lubricating oil caused bearing damage to change from fluting damage to frosting damage, resulting from a shift in electrical discharge mode to high discharge frequency and low voltage value. Notably, the presence of conductive ring eliminated electrical discharge behavior within the bearing by creating a low-resistance charge flow pathway, therefore preventing the bearing electrical damage. The knowledge gained herein provided a theoretical foundation for the structural design and lubricant selection of EV's DHT system.