Impact of the Modulation Index on EDM Bearing Currents in Inverter-Fed Drives

The common-mode voltage in inverter-fed drives is a known source of EDM bearing currents. Capacitive coupling in the machine generates a bearing voltage that collapses when the bearing threshold voltage is exceeded, with the threshold voltage following a probability distribution. The shape of the common-mode voltage, and consequently the bearing voltage, is highly dependent on the modulation index, which in turn affects the EDM currents. This paper investigates the relation between the modulation index of a sinusoidal PWM (SPWM) and the rate of occurrence of EDM current pulses, and discusses the influence of several characteristics of the common-mode voltage on them. This information can contribute to the reduction of EDM bearing currents in variable speed drives.