PURPOSE:To return a motor to a normal operation with a short time interruption of the motor operation by a method wherein a fault detecting circuit, a fault discriminating circuit, a switching instruction circuit and a re-starting circuit are so provided as to switch one of first and second control units in which a fault occurs to a third control unit automatically for re-start operation. CONSTITUTION:In a normal operation, 6 pulse thyristors 4 which are connected in parallel with each other are controlled by first and second control units 12 and 17 to operate a DC motor 8. If a fault occurs in one of the first and second control units 12 and 17, the fault is detected by a fault detecting circuit 19 and the control unit in which the fault occurs is discriminated by a fault discriminating circuit 20 and is switched to a third control unit 18 which is separately provided by first, second and third switching circuits 22 - 24. After the switching is finished, the 6 pulse thyristors 4 are so controlled as to return to the normal operation by a start signal from a re-starting circuit 25.
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