One Novel Soft-Starting Control Strategy for Induction Motor Based on Space Voltage Vectors

This study presents a novel control strategy of discrete variable frequency (DVF) based on space voltage vectors for a three-phase induction motor (IM) soft-starting. The proposed strategy is developed in four steps. First, conducting states of the three-phase thyristor circuits are analysed, and the formation mechanism of space voltage vectors is also demonstrated. Then, DVF control strategy of IM based on hexagon flux linkage loci is expounded categorically, which contains frequency f/7 control, frequency f/4 control, frequency f/3 control and ramp voltage control. Next, under two-phase stationary coordinate system, stator flux equation of IM is derived, and the decrease in stator flux when zero-vector voltage working is calculated accurately. In the end, an experimental system of IM based on three-phase thyristor circuits is constructed, and the experimental results are dealt with and compared with the usual ramp voltage technique. The study results show that IM driven by the proposed strategy can steadily operate at the preset discrete frequency, and the effective value of starting current of IM can be decreased by nineteen percent. The experimental results verify the conclusions.