Scaling Issues for Common-Mode Chokes to Mitigate Ground Currents in Inverter-Based Drive Systems

The additional high-frequency (HF) common-mode (CM) current of inverter-based drive systems can cause parasitic effects such as HF bearing currents. This HF CM current can be significantly reduced by purpose-designed CM chokes that are inserted in the drive at the inverter output. This paper discusses scaling issues related to the design of such CM chokes, of both wound and feed-through toroidal types. The requirements for the design of chokes for these applications can differ from those for CM chokes designed for use in the inverter input, such as those used for the purpose of electromagnetic interference filtering. This difference is notably due to the comparatively large magnitude of the currents involved. The developed correlations allow taking full advantage of the possibilities offered by such chokes to reduce the HF CM current and mitigate the resultant parasitic effects. The discussed relationships are derived analytically, and the results are supported by measurements.