Motor Bearing Current Characterization in SiC- based Variable Frequency Drive Applications

Variable frequency drive (VFD) motor controllers have many applications in commercial and industrial facilities for pumps, fans, compressors, etc., as well as in electrified transportation. VFDs employ pulse-width modulated (PWM) inverters sending high frequency square wave pulses to the motor for efficient speed and torque control. The high dv/dt PWM inverter output pulses can have adverse effects on the motor including motor insulation stress and damaging bearing currents. These adverse effects in Si-based VFDs have been studied in detail. Next generation power electronics based on wide bandgap (WBG) devices, such as SiC and GaN, have much higher switching frequency, efficiency and power density compared with traditional Si-based power electronics. This paper presents trends in SiC inverter characteristics including rise/fall times (dv/dt), common mode voltages, shaft voltages, bearing currents and damage in SiC vs. Si-based inverters, in addition to circuit models and theoretical analyses. Finite element modeling (FEM) of the bearing is presented, as well as scanning electron microscope (SEM) images of new and pulsed bearing surfaces.