PURPOSE: To arbitrarily control the torque generated by a motor by providing a means which adjusts the base current value of a transistor between the DC- side output terminals of a full-wave rectifying circuit.

CONSTITUTION: A transistor 11 is connected between both DC-side output terminals 9 and 10 of a full-wave rectifying circuit 4, with a collector-side resistance 12 for holding the DC-side voltage being connected on the collector side. A variable and voltage dividing resistors 13 and 14 which control the electric current flowing between the collector and emitter of the transistor 11 by controlling the base current of the transistor 11 are connected in series between both output terminals 9 and 10 and the connecting point of the resistors 13 and 14 are connected to the base of the transistor 11. Therefore, when the variable resistor 13 is changed, the applied voltage of a single-phase motor 8 also changes, since the collector current Il of the transistor 11 and, accordingly, the current Il of an AC-side circuit also change.