Influence of different types of supply cables on shaft currents in VSI-fed induction motors

The effects of bearing-currents in variable speed drives have been reported during the last few years. This paper examines the influence of different types of motor supply cables on shaft currents due to the high du/dt of the inverter voltage of about 2 to 10 kV/mus. Voltage source inverters with pulse-width-modulation produce a common mode voltage at the motor terminals. Due to the parasitic capacities from stator windings to the stator, the rotor body etc. common mode currents occur. At the best, the common mode current flows back to the converter via the screening of the supply cable. The worst case is if the current flows back via the grounding of the working machine. In this case, the bearings would be damaged in a relatively short time