PURPOSE: To maintain the voltage of a filter capacitor constant by connecting a voltage detector in parallel with the capacitor, and controlling thyristors of a rectifier by the voltage of the capacitor detected by the detector.

CONSTITUTION: A voltage detector 10 is connected in parallel with a filter capacitor 7, and the capacitor voltage 11 detected by the detector 10 is input to a voltage pattern arithmetic unit 13. The unit 13 so calculates a voltage pattern signal 14 that the voltage becomes a capacitor voltage reference value 12, and inputs the signal 14 to a phase controller 15. The controller 15 outputs the gate signals of thyristors of a rectifier 5 based on the signal 14 to control the rectifier 5. Thus, the detection delay due to a filter reactor 6 is eliminated, a controller can be instantaneously controlled to maintain the voltage of the capacitor 7 constant.