PURPOSE: To prevent a semiconductor switch from damaging by setting both an armature current and a field current to zero when an operation command is released and then opening a breaker.

CONSTITUTION: When an operation command is released, switches SW1a, SW1b are opened, and SW1b, SW2, SW3 are changed. Thus, a conduction rate command γa of an armature chopper is outputted in response to a value produced by subtracting a current pattern Ip gradually decreasing from an armature current Ia when an operation command outputted from a memory 1 is released by the armature current Ia. On the other hand, a conduction rate command γf of a field chopper is outputted in response to a value produced by subtracting a value mutliplied by the conduction rate (If/Ia) and the current Ia when the operation command is released by a field current If. When the currents Ia and If become zero, a breaker is opened.