Compensation Method for Neutral-Point Voltage Fluctuation in Four-Leg Inverter Dual-Motor System

Abstract

In multimotor driving systems, inverters with a reduced number of switches have been used to reduce costs and decrease volume and weight. This article considers a three-phase four-leg inverter (FLI) to drive a dual-induction motor (IM). In this system, since one of the three-phase windings in each motor connects to the neutral point of the dc link, neutral-point voltage fluctuation in the two dc-link capacitors occurs. The neutral-point voltage fluctuation, caused by common leg current, leads to voltage utilization reduction and unbalanced three-phase currents. Therefore, this article proposes a method of minimizing the common leg current and it consists of phase difference compensation control during initial magnetization and torque current compensation control over the entire speed range. The phase difference compensation control maintains the electrical phase difference between two IMs as $\pi $ during magnetization. Torque current compensation based on a PI controller eliminates neutral-point voltage fluctuation in the two dc-link capacitors at steady state. In addition, the proposed method is combined with an existing angle controller to maintain the electrical phase difference between two IMs as $\pi $ and, with the slip controller, to synchronize the speed of the two IMs. The validity of the proposed method is verified by experimental results.