Novel PWM modulation strategy for 3-phase inverters with reduced distortion at low switching frequency

This paper presents an improved PWM strategy for three-phase inverters, especially suited for use at low ratios of switching frequency to fundamental output frequency. The method of third-harmonic injection is used to ensure maximum utilisation of the DC bus voltage. Implementation issues for the new modulation strategy are discussed. A comparison with a conventional, regularly sampled PWM is presented, based on simulation and experiment. The advantage of the new modulation strategy is evident in the frequency band from zero to half the switching frequency. While distortion below half the switching frequency exists in conventional PWM due to intermodulation, the new modulation strategy can completely avoid any distortion below half the switching frequency, even at low switching frequency. The minimum switching frequency is in the range of twelve times the output frequency, which is only four times the frequency of the injected third harmonic.