PURPOSE: To improve reliability by using reverse switches for the transfer of driving and braking, too, while a driving and braking transfer switch is omitted by reversing the functions of main thyristors and auxiliary thyristors by driving and braking.

CONSTITUTION: A filter circuit FIL consisting of a filter reactor FL and a filter capacitor FC is connected to the armature A of a motor through a thyristor switching circuit CHG and a motor main circuit MMC consisting of the series field system F of the motor and a main smoothing reactor MSL. Reverse switches RV1 and RV2 are connected across the field system F, and the circuit is formed so that the current direction into the field system F when a switch f is ON may reverse itself in case a switch r is ON.