Comparison of different inverter-fed AC motor types regarding common-mode bearing currents

A comparison of three different inverter-fed AC motor types concerning the common-mode bearing voltage and currents is given both by measurements and calculations. Two cage induction motors (IM), a doubly-fed induction motor (DFIM) and a permanent magnet synchronous motor (PMSM) with surface mounted magnets, the IM and PMSM stator-fed, the DFIM rotor-fed by an IGBT-voltage source inverter at V_DC = 560 V DC link voltage, are compared. According to measurements the DFIM (14 kW) has the highest bearing voltage ratio (BVR) of 66 % with big rotor-to-ground currents, the IM (11 kW, 110 kW) have a much lower BVR of 2.5 % with small discharge bearing currents, whereas the PMSM (90 kW) with a very small BVR of only 0.8 % shows no bearing currents at all. Calculating the BVR by the parasitic capacitances of the machines confirms the experimental results in general.