Novel Characterization of Si- and SiC-based PWM Inverter Bearing Currents Using Probability Density Functions

The high frequency PWM voltage pulses from a two-level six-switch inverter produce a common-mode voltage in an electric machine’s windings, a fraction of which appears on the machine shaft due to electrostatic coupling. When the shaft voltage exceeds the dielectric strength of the bearing lubricating grease, electric discharge machining (EDM) discharges occur within the bearing, which can lead to premature failure. According to pulsed dielectric theory, the breakdown voltage across a dielectric increases with an increase in voltage rise time. Therefore, the fast voltage rise times of wide bandgap devices are expected to produce higher magnitude shaft voltages and EDM bearing currents. This paper presents circuit modeling of EDM currents and compares the shaft voltage and bearing current amplitudes of silicon- and silicon carbide-based PWM inverters through a Weibull statistical analysis.