A Novel Configuration of DC Link EMI Filter Capacitors in Variable Frequency Drives

Direct Current (DC) link electro-magnetic interference (EMI) suppression filter capacitors are normally connected between the DC bus positive and negative nodes to the Earth node in variable frequency drives (VFDs) to minimize voltage overshoots from power device switching action as well as to suppress the electromagnetic interference (EMI) generated by the drive. With long motor leads, the switching of the inverter power devices generates high-frequency common-mode (CM) currents and overshoots in the DC bus to ground voltage. The DC link EMI filter capacitors not only provide a low impedance path for the high-frequency CM currents to return to the inverter, which significantly reduces conducted emissions of the VFD, but also reduce the peak DC bus to ground voltage. In this paper, a novel DC link EMI filter capacitor configuration is proposed for VFDs. This configuration can meet regulatory agency requirements for safety capacitors in VFDs with commercially available safety rated metallized polypropylene film (MPPF) capacitors. The configuration is shown to have similar suppression of EMI as prior art configurations but with reduced number of parts.