Scaling issues for common mode chokes to mitigate ground currents in inverter-based drive systems

The additional high frequency common mode current of inverter-based drive systems can cause parasitic effects such as high frequency bearing currents. This high frequency common mode current can be significantly reduced by purpose-designed common mode chokes that are inserted in the drive at the inverter-output. The paper discusses scaling issues related to the design of such common mode chokes, both of wound and feed-through toroidal types. The requirements for the design of chokes for these applications can differ from those for common mode chokes designed for use in the inverter-input, such as those used for the purpose of EMI 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 high-frequency common mode current and mitigate the resultant parasitic effects. The discussed relationships are derived analytically, and the results are supported by measurements.