PURPOSE: To obtain an excellent braking characteristics by changing a motor braking torque to be linear in accordance with the revolving speed of a motor.

CONSTITUTION: When an acceleration switch is turned ON, a power mode instruction is delivered by a mode switching circuit 14, thereby a main contactor 5 and field relays 7 and 9 are closed onto the side (b) and the sides a1 and b2 respectively, and according to the output of an accelerating device 16, a conduction ratio instruction is delivered to an armature chopper 3 and a field chopper 12 by a power controlling circuit 15. When a brake switch is turned ON, a regeneration mode instruction is delivered by the mode switching circuit 14, and thereby the main contactor 5 and the field relays 7 and 9 are closed onto the side (a) and the sides b1 and a2 respectively. A field current instruction is delivered through a regeneration controlling circuit 13 according to the output of a motor revolving-speed detector 11, and a conduction ratio instruction is given to a field chopper 12. Meanwhile, a conduction ratio instruction is given to an armature chopper 3 so that an armature current becomes constant.