An active front-end V/Hz induction machine drive with a tiny DC link capacitor

Adjustable speed drives for induction motors that operate with V/Hz control are commonly used for high efficiency fan and pump applications where accurate torque control is not critical. While most of these applications use a passive rectifier to interface with the three phase ac line, active front end converters to improve the power quality of input waveforms are increasingly common. In these applications, the dc link capacitor is typically realized using electrolytic capacitors that form the bulk of the power converter volume. The dc link capacitor is sized to store enough energy to maintain several tens of cycles of ac output at the rated power. In this paper, we propose to reduce the size of the dc link capacitor dramatically, to store enough energy to provide just one high frequency switching cycle of ac output power. The dc bus is no longer stiff and hence classical sinusoidal pulse width modulation cannot be used. But, the stored energy modulation (SEM) concept that we propose herein synthesizes high quality sinusoidal output voltage waveforms even with such tiny dc link capacitors. This paper presents the analytical development of the stored energy modulation approach along with circuit simulation and experimental results.