PURPOSE: To sustain operation stably by making a pause in the operation of an inverter circuit, when the output frequency of the inverter circuit reaches a predetermined value, and connecting a transformer during the pause of the inverter circuit operation and then operating the inverter again with the phase of the first peak voltage.

CONSTITUTION: Concerning to phase R, for example, inverters 3R, 4R are gated OFF when the output voltage reaches a predetermined level and power supply to a motor M is temporarily stopped followed by opening of a circuit breaker CB1R. Subsequently, circuit breakers CB2R1, CB2R2 are closed and the inverters 3R, 4R are gated ON to resume operation. The operation is resumed with the phase prevailing at the time when a peak voltage, i.e. a sine wave voltage of 90° or 270°, appears for the first time. By such arrangement, the operation can be sustained stably without requiring a converter having considerably high current capacity.