PURPOSE: To prevent the fluctuation of a transmission power by selecting a pulse interval phase control circuit or a power source synchronizing phase control circuit in accordance with the value of variability in the phase interval of the pulse from a power source synchronizing phase control circuit.

CONSTITUTION: When variability in the phase interval of a pulse generated by a power source synchronizing phase circuit 3 is lower than a predetermined value, the power source synchronizing phase control circuit 2 is selected. Accordingly, a constant pulse interval phase control circuit 3 is selected when a 3-phase AC system is sound. Upon the imbalance trouble of a voltage, the power source phase control circuit 2 is selected and valves of respective arms of an external exciting power inverter are provided with a trigger signals by the pulse generated in the power source phase control circuit 2. According to this method, the failure of commutation or the large fluctuation of the transmission power may be prevented.