Design and performance of a digitally based voltage controller for correcting phase unbalance in induction machines

The case of unbalanced supply presents a uniquely difficult problem in motor applications since its causes are varied. In many situations a static starter utilizing back-to-back thyristors in series with the motor lines is already installed for the purpose of soft starting or for power factor improvement. An attractive low-cost solution for phase unbalance is simply to modify the control strategy of the static starter by adjusting the firing angles of the three thyristors pairs independently. In this manner, the series connected thyristors serve the function of unsymmetrical, variable supply impedances which can be used to balance the voltage across the motor phases. A new digital controller utilizing a static starter for balancing the phase currents of an induction motor operating with unbalanced supply is presented. Experimental results show a marked reduction in the current unbalance from approximately 40% to a few percent and clearly demonstrate the attractiveness of the new phase balancer.>