Multi Level Voltage Space Phasor Generation for an Open-End Winding Induction Motor Drive Using a Dual Inverter Scheme with Asymmetrical DC-Link Voltages

Abstract 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 is capable of producing 8-voltage space phasors independently of the other. The proposed scheme produces 37 voltage space phasor locations while a 3-level inverter produces only 18 voltage space phasor locations. In the proposed scheme, the neutral connection of the induction motor is removed (open-end winding) and is fed from both the ends. Two 2-level inverters with isolated DC-link voltages of 2/3 Vdc and 1/3 Vdc are used for driving the motor, where Vdc denotes the DC-link voltage of an equivalent conventional (single-inverter) scheme. A combination of two 2-level inverters with 2/3 Vdc and 1/3 Vdc DC-link voltages produces a total of 64 voltage space phasor combinations in 37 space phasor locations. The voltage space phasor locations can be associated with three regions according to the speed range. In the lower speed range, while being in the PWM mode of operation, the phase voltage exhibits a six-step envelope (similar to the conventional scheme) and in the middle range the motor phase voltage, still being in the PWM mode of operation, shows a 12-step envelope. In the higher speed range (including over modulation) the motor phase voltage has an 18-step envelope. A simple and elegant space phasor based PWM scheme has been proposed in this paper, which requires only the instantaneous values of the three phase motor reference voltages in all of the sectors. 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 proposed PWM strategy has been experimentally verified on a 1HP open-end winding motor and the experimental results are presented in this paper.