A High-Efficiency Open-Winding Induction Motor Drive Using a Constant Power Factor Control Scheme

Abstract

Power electronics based variable-frequency drives (VFDs) for electric motors are a widespread technology in industrial settings and commercial products; offering increased functionality, accurate control of speed and torque, and substantial energy savings. A myriad of converter topologies and control techniques exist for a wide range of VFD applications. A high-efficiency control scheme is presented for an open-winding induction motor (OWIM) dual inverter VFD, where primary and secondary inverters are supplied from a DC power source and a floating DC capacitor, respectively. This topology is beneficial as it can produce multilevel pulse-width modulation (PWM) waveforms, eliminates zero-sequence common-mode currents within the system, and extends the motorâs constant torque and power regions through voltage boosting. Examination of the equivalent circuit model of an induction motor (IM) reveals that very high motor efficiencies are achieved at a constant motor fundamental power factor over a wide range of motor loads and drive frequencies. Thus, the developed control scheme utilizes the driveâs topology to maintain the motorâs desired power factor angle, while also incorporating feedback control of the floating capacitorâs voltage. This approach updates the motorâs voltage automatically to ensure constant power factor operation and improves the voltage stability of the floating capacitor, while only requiring feedback measurements of the driveâs two DC link voltages. In addition, a sensorless slip compensation technique is incorporated into the control algorithm, which utilizes the correlation between the IMâs operating power factor and the machineâs slip. The inherent voltage boosting capability of this topology is especially beneficial during operation under speed range extension. Experimental testing of the proposed system has verified the predicted steady-state efficiency gains for the induction motor under constant power factor operation as compared with conventional drive control, and has demonstrated stable system performance during both load and speed transients.