| Abstract |
The case of unbalanced supply is a uniquely problem in motor application since its causes are multivaried. 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 to modify the control strategy of the static starter by adjusting firing the angles of the three thyristor pairs independently. ln this manner the series connected thyristors serve the function of unsymmetrical, variable supply impedances which can be used to balance the voltage across the motorphases. A new digital controller utilizing a static starter with a 'losses calculator loop' for balancing the phase currents of an induction motor operating with unbalanced supply is the topic of this paper. Experimental results show a marked reduction in the current unbalance and in the related negative sequence of the currents. As a consequence, the motor efficiency is approximately constant around the motor rated efficiency, in the whole range of unbalanced supply operation ( -10% to + 10%). |
