An Improved Model for the Common Mode Impedance in Inverter-Fed AC Machines

In this paper, a general high frequency lumped parameter common mode impedance model is presented. The lumped parameters are derived from numerical electromagnetic finite element field simulations and analytical descriptions of the electrical machine geometry. No measurements are needed for determining the model parameters. The presented common mode impedance model also considers the frequency dependent resistances and inductances of the phase coils. The proposed model can be applied to any AC machine and is in good agreement with measurements of a 55 kW permanent magnet excited synchronous machine. The presented method enables electrical machine designers to calculate the common mode current, which causes destructive bearing currents, in the early stage of their projects and to consider the trade-off in their designs.