Generalised sinusoidal PWM with harmonic injection for multi-phase VSIs

Application of power electronics in electric drives enables utilisation of ac machines with a phase number higher than three. Such multi-phase motor drives are nowadays considered for various applications. Multi-phase drives are invariably supplied from multi-phase voltage source inverters (VSIs) and adequate methods for VSI pulse width modulation (PWM) are therefore required. This paper deals with carrier-based PWM, where harmonic injection enables an improvement in the dc bus utilisation without moving into the over-modulation region, while still providing sinusoidal output phase voltages. A generalised concept of n(th) harmonic injection for n-phase inverters is presented. The viability of the proposed concept is proved by simulation taking five-phase VSI as an example. The method of fifth-harmonic injection in five-phase VSIs enables operation with pure sinusoidal voltages up to a certain reference voltage value which is smaller than the maximum achievable with the given dc link voltage. Sinusoidal PWM is further modified, by injecting other harmonics, in order to enable full utilisation of the dc bus voltage. The correlation of this PWM scheme with space vector PWM, based on utilisation of large vectors only, is established in this way. Some experimental results, illustrating application of the sinusoidal PWM in series-connected live-phase two-motor drives, are included.