Harmonic Distortion Analysis and Suppression for Asynchronous SVPWM at Low Sampling- to-Fundamental Frequency Ratios

Operation at low sampling-to-fundamental frequency ratios is challenging, but inevitable for high-speed electric drives due to the limited switching frequency. Synchronous modulation is commonly used to replace the asynchronous one to reduce harmonic distortion at these operation states. However, synchronous modulation faces the severe problems of poor dynamic response and intensive computation. To avoid falling into this dilemma, this article studies the improvement of asynchronous space vector pulsewidth modulation (SVPWM) at low sampling-to-fundamental frequency ratios, instead of following the traditional way of applying synchronous modulation. The conventional asynchronous SVPWM schemes are analyzed from the point of view of their stator flux trajectories, by which the reason for their poor harmonic characteristics at low sampling-to-fundamental frequency ratios is revealed. Improved asynchronous SVPWM schemes are proposed accordingly. The comparison results show that proposed modulation schemes have much less harmonic distortion than conventional schemes. Besides, the high dynamic response ability and simple implementation are preserved for the improved schemes. The effectiveness of the proposed modulation schemes is experimentally verified on an 18-kW interior permanent magnet synchronous machine (IPMSM) drive test rig.