New sinusoidal pulsewidth-modulated invertor

Abstract

Sinusoidal p.w.m. invertors depend intrinsically for their operation on quite complex modulation processes. An in-depth study of these modulation processes is presented and the sources of possible operational problems are identified. These problems are shown to be manifest in low-frequency harmonic distortion and the possible existence of subharmonics at noninteger frequency ratios, particularly at low-frequency ratios. These problems restrict the viable invertor-output frequency range. A new method of control is presented which overcomes these problems and allows the possibility of low-frequency-ratio operation, resulting in significantly wider invertor-output frequency ranges. An analytic comparison of both the existing and new methods of control is presented, using a 3-dimensional modulation-model approach. The results of this analysis provide the system designer with generalised frequency-spectra characteristics for any particular frequency ratio and modulation depth by direct inspection.