Magnetic material performance of transformers in common mode active EMI filters for bearing current suppression

This paper analyses the performance of two magnetic materials used in the active filters for Common Mode (CM) electromagnetic interference (EMI) reduction in induction motor drives. In particular, active filters based on CM voltage compensation, very useful in bearing current suppression, are considered. The magnetic material is exploited for the core of the common mode transformer (CMT) that it is a crucial part of such devices because it performs the injection of the compensation voltage into the power connections between the inverter and the motor. The issues regarding the choice of the CMT core magnetic material and the windings design are discussed. The problem of the magnetic saturation, related to the desired high power/weight ratio is investigated. Furthermore, the power losses in the magnetic material under non-sinusoidal supply condition are evaluated through a suitable measurement system. A comparison of two CMTs, realized with ferrite and with nanocrystalline material cores, respectively, which are designed to perform the same filtering effect, is presented.