A multi level space phasor based PWM strategy for an open-end winding induction motor drive using two inverters with different DC link voltages

An open-end winding induction motor drive fed from two inverters with different DC link voltages is proposed in this paper. A total of 64 voltage space phasor combinations are possible in this scheme as each inverter produces 8 voltage space phasors. The proposed scheme produces voltage space phasor locations more than that of a 3-level inverter. A total of 37 space phasor locations are possible from the proposed scheme as compared to 18 voltage space phasor locations in a 3-level inverter. In the proposed scheme, the induction motor neutral connection is separated (open-end winding) and fed from both the ends. Two 2-level inverters with DC link voltages of 2/3 VDC and 1/3 VDC are used for driving the motor. Here VDC is the conventional 2-level inverter DC link voltage for identical power. A combination of two 2-level inverters with 2/3 VDC and 1/3 VDC DC-link voltages will produce a total of 64 voltage space phasor combinations in 37 space phasor locations. For the present scheme a space phasor based PWM scheme will produce stepped voltage waveforms for the motor phases in different speed ranges. The whole voltage space phasor locations can be divided into three regions according to the speed range. In the lower speed range the PWM phase voltage will have a six-step envelope (similar to the conventional scheme) and In the middle range the PWM motor phase voltage will have an 12-step envelope and the outer speed range (including the over modulation) the PWM motor phase voltage will have an 18-step envelope. A simple and elegant space phasor based PWM scheme is proposed in all sectors based on three phase motor reference voltages only. The sector identification is achieved by comparing the components of the motor reference voltages, using simple hysteresis comparators, along the three orthogonal axes to the a,b,c phase axes. The whole scheme is studied for a 1 hp open-end winding motor and the results of the study including the over modulation ranges are presented in this paper.