A space vector PWM technique for symmetrical six-phase voltage source inverters

Six-phase a.c. motor drives are usually supplied from two-level six-phase voltage source inverters (VSIs), which are controlled using appropriate PWM techniques. Most of the existing work applies to the asymmetrical six-phase VSI, supplying an asymmetrical a.c. machine (two three-phase windings shifted in space by 30 degrees degrees) with two isolated neutral points. However, it has been shown recently that a symmetrical six-phase induction machine (equidistant spacing of all six-phases, with 60 degrees spatial displacement between any two consecutive phases) offers the same quality of performance as the asymmetrical machine,, provided that good quality current control and an adequate PWM technique are utilised for the symmetrical six-phase VSI (Fig. 1) control. Since this requires sinusoidal VSI output voltages (neglecting the PWM ripple), a space vector PWM technique (SVPWM) is described in this paper, such that sinusoidal or at least near-sinusoidal output voltages are generated across the whole range of the available output voltage fundamental for the six-phase load with a single neutral point. Operation of the SVPWM scheme is investigated by extensive experimentation and time-domain waveforms and spectra are given for inverter leg, phase-to-neutral, line-to-line and common mode voltages, as well as for the load current, for a variety of operating conditions. Total voltage harmonic distortion (THD) is also calculated using experimental results and is used as a figure of merit in evaluation of the quality of performance of the SVPWM schemes.