The two secondary windings (N 21 , N 22 ) of a transformer the primary winding (N,) of which is connected to an AC power supply (P s ) are each provided with a controlled rectifying circuit (R f1 , R f2 ). The DC sides of the rectifying circuits (R f1 , R f2 ) are connected in series to supply power to DC motors (M, M 1 -M 6 ). One of the secondary windings (N 21 , N 22 ) is detachably connected with a capacitor bank (CLC). The capacity of the capacitor bank (CLC) is selected so that the power factor of the converter can be improved to a satisfactory value when the two rectifying circuits (R f1 , R f2 ) are both controlled to produce output voltages. The capacitor bank (CLC) becomes connected in response to the fact that the current in the primary winding (N,) of a transformer (T r ) has arrived at a predetermined value.