Improved Low-Frequency Operation of Hybrid Inverter for Medium-Voltage Induction Motor Drive Under V/f and Vector Control Mode of Operation

In this paper, an improved control method has been proposed for hybrid inverter to drive medium-voltage induction motor. This hybrid inverter consists of high-voltage three-level inverter that is operated in quasi-square-wave mode. Low-voltage high-frequency H-bridge cells are connected in series with the three-level inverter. They eliminate harmonic voltages generated by the high-voltage quasi-square-wave inverter. This paper proposes systemic choice of harmonic voltages that the series cells need to generate. The choice of these harmonic voltages is dynamically modified based on operating frequency of the drive. This method ensures that the quality of motor current remains same at all operating frequency of the drive. The naturally regulated dc bus voltage of the series cells are kept low for the pulsewidth modulation (PWM) operation of the series cells. Several issues of adopting vector control strategy for hybrid drive have been addressed in this paper. Based on this analysis, suitable motor current control strategy has been proposed without speed sensor. Experimental results are produced for open-loop and closed-loop operations of this drive. The results show the satisfactory operation of this hybrid inverter-based induction motor drive operating from 5 to 50 Hz.