Leakage current and common mode voltage issues in modern AC drive systems

Due to rapid developments of IGBT technology, switching time and frequency are dramatically increased. At higher carrier frequencies, IGBTs induce more capacitive coupled current into a rotor and a stator frame and lead to faster bearing damage. Common mode voltage enables motor to create shaft voltage through electrostatic couplings between the rotor and the stator windings and between the rotor and the frame, and it can caused bearing currents when the shaft voltage exceeds a breakdown voltage level of the bearing grease. Also, high frequency leakage current occurs through stray capacitors between stator winding and the motor frame due to a high rate of the common mode dv/dt at motor terminals which can produce induced shaft voltage. Conducted and radiated Electromagnetic Interference (EMI) emissions are major problems in recent motor drives that produce undesirable effects on electronic devices. In modern power electronic systems, increasing power density and decreasing cost and size of system are market requirements. Switching losses, Harmonics and EMI are the key factors which should be considered at the beginning stage of a design to optimize a drive system. In most of power electronic designs, EMI issues have not been taken into account as one of the main factors; and mitigation techniques for EMI are considered at the last stage of design. This paper also presents a relationship between these three factors and how a motor drive system can be optimised when EMI is taken into consideration with respect to leakage current, shaft voltage and bearing current.