Design of a Passive Common-Mode Transformer for Reduction of the Common-Mode Current in a Drive System with Long Cables

The usage of variable frequency drives (VFDs) to control induction motors is extremely relevant for industrial applications because of their low cost. The switching behavior of the inverter in the VFD generates common-mode voltages which can cause a common-mode current to flow in the system. This common-mode current can propagate through the protective earth connection and causes the unwanted trigger of safety devices such as residual current breakers. Because of the relevance of the common-mode current, many different filter topologies have been developed to reduce the common-mode on the motor side. A suggested topology is the so-called commonmode transformer, which adds a fourth winding to a classic common-mode choke. In this work, a passive common-mode transformer is proposed with a simulation model to evaluate the reduction of the common-mode current in a system with a long cable. The developed topology is then validated with a simulation in the frequency domain and its effect on the common-mode current and common-mode voltage is analyzed. The simulation model is developed by using a lumped element representation of the motor to simulate the common-mode impedance of the motor. The cable model is obtained through a finite element model to represent the capacitive and inductive parameters of the cable. The proposed simulation model allows a good evaluation of the common-mode current flowing in the system and tests the effectiveness of the filter topology. The application of the common-mode transformer ensures the reduction of both the common-mode voltage and common-mode current in the system.