Integrated Differential-Mode and Common-Mode Filter to Mitigate the Effects of Long Motor Leads on AC Drives

Pulsewidth-modulated (PWM) ac drives generate high differential-mode (DM) motor voltages due to the reflected wave phenomenon. The common-mode (CM) voltage produced by a PWM inverter also has fast transitions that result in high peak currents to ground. For low-power ac drives, this can present several issues, such as pump-up of the dc bus and failure of the power module due to high instantaneous currents that are often undetected because of the typical sampling algorithm used. In this paper, existing drive output filter solutions are reviewed, and a new filter design is presented, which has integrated DM and CM impedance with damping matched to typical cable surge impedance. This eliminates reflections of both DM and CM traveling waves and also reduces peak cable charging currents. Experimental results are provided to demonstrate that the new filter has improved performance to reduce motor DM and CM voltages and drive output CM current, thus mitigating the issues specifically seen with low-power ac drives. Complete design details for the filter and a particular implementation of the integrated DM and CM reactor are also presented.