A multilevel inverter system for an induction motor with open-end windings

In this paper, a multilevel inverter system for an open-end winding induction motor drive is described. Multilevel inversion is achieved by feeding an open-end winding induction motor with two two-level inverters in cascade (equivalent to a three-level inverter) from one end and a single two-level inverter from the other end of the motor. The combined inverter system with open-end winding induction motor produces voltage space-vector locations identical to a six-level inverter. A total of 512 space-vector combinations are available in the proposed scheme, distributed over 91 space-vector locations. The proposed inverter drive scheme is capable of producing a multilevel pulsewidth-modulation (PWM) waveform for the phase voltage ranging from a two-level waveform to a six-level waveform depending on the modulation range. A space-vector PWM scheme for the proposed drive is implemented using a 1.5-kW induction motor with open-end winding structure.

I. Introduction

Multilevel inverters produce a stepped output phase voltage with a refined harmonic profile when compared to a two-level inverter-fed drive system [1], [2]. The circuit complexity and cost inhibit extension of the conventional three-level neutral-point-clamped (NPC) configuration to higher levels [2] [3] [4]. Certain modifications have been suggested to extend the conventional NPC three-level inverters for a higher number of levels [5], [6]. Flying-capacitor and series-connected H-bridge configurations [7]–[10] have also been suggested as alternative circuit topologies. However, these configurations are also complex for higher number of levels. Open-end winding induction motors, obtained by removing the neutral point of the stator windings of ordinary motors, offer another approach to multilevel inversion. It has been shown that two two-level inverters, connected at either end of an open-end winding induction motor, are capable of achieving three-level inversion [11]. In this scheme, each inverter is powered by an isolated dc-link voltage, which is half compared to the conventional drive (a single two-level inverter feeding a normal motor). Open-end winding motors require either harmonic filters or isolation transformers to prevent currents of the triplen harmonic order flowing in the motor phases and the semiconductor devices. A space-vector-modulated pulsewidth-modulation (PWM) scheme has been suggested in [12] for the power circuit topology proposed in [11]. With this PWM scheme, a total of 64 space-vector combinations are possible, distributed over 19 space-vector locations. A further improvisation is suggested in [13], in which two two-level inverters with unequal dc-link voltages (which are in the ratio 2 : 1), feed an open-end winding induction motor. It has been shown that this configuration is capable of achieving four-level inversion [13]. The total number of space-vector locations produced in this scheme is enhanced to 37. A reduction in the switching ripple is achieved with this scheme, compared to the former, as the number of constituent sectors is enhanced to 54, compared to 24 with the former.