There can be two types of circulating currents in paralleled high power adjustable speed drives (ASDs) with common DC link: (a). It varies with motor operating speed such as at 50Hz, caused by unbalanced output inductors and hardware variation; (b). It occurs at inverter switching frequency, associated with pulse width modulation (PWM) carrier shift between paralleled ASDs. This paper models their equivalent circuit, analyzes the effects, and introduces unified controller solutions mitigating effectively both types of circulation currents. Simulation results on two 500HP (370kW), 590A ASDs with induction machine (IM) are obtained to verify theoretical derivations, both are further validated experimentally using two 3.6kW paralleled ASDs including rectifiers, DC chokes, inverters, output inductors, and a three-phase RL load.
