Bearing current, EMI and soft switching in induction motor drives: A systematic analysis, design and evaluation

This dissertation proposes a unified theory to explain bearing current, EMI and other phenomena in induction motor drives. The theory is based on recognition of parasitic capacitive coupling and common mode voltage generation in a drive system. The high dv/dt differential and common mode voltages of an inverter provide excitation to the parasitic coupling circuit of the drive system and produce a significant amount of the so called coupling current in the system.;The coupling current is defined as the sum of currents flowing in all parasitic capacitors of a motor drive which is different from the well-known fundamental and harmonic currents. The motor bearing current is a common mode component of the coupling current which flows through parasitic capacitors from the motor windings to the rotor and then via the bearings to the grounded motor case. The emissions from a drive system can also be explained by flow of the coupling current into the utility AC source.;Experiments are then carried out to verify the theory. Bearing currents are measured on a modified induction motor. A model of bearing currents is also established to describe the phenomenon. Several suppression methods and circuits are proposed to reduce the conducted EMI emissions and the bearing currents in an induction motor drive. The results not only clarify some of the long suspected issues regarding existence of bearing currents and mechanisms of EMI emissions but also provide effective solutions to the problems.;Another important contribution of the dissertation is to evaluate the soft switching technology for reducing bearing currents and EMI emissions. The state of the art in soft switching inverters is reviewed. To overcome some of the problems in the conventional resonant link inverters, a novel passively clamped quasi resonant DC link inverter is proposed and built to provide a low dv/dt excitation to induction motors. The soft switching inverter is then extensively tested and evaluated in terms of bearing currents, coupling currents and other performances.