To cut off the supply of a power supply voltage to a main power supply transformer when a voltage applied to an electric machine is not the same as a normally operating power supply voltage value by a method wherein the connections between an AC power supply and corresponding taps are opened and closed in accordance with the judgement result of a voltage judging means.

A power supply voltage is applied to terminals 9a and 9b by an AC power supply. After the applied voltage is transformed by a control small transformer 1, the voltage is supplied to a control rectifying/smoothing circuit 2 which outputs a DC voltage. A voltage stabilizing circuit 3 supplies the DC voltage to a voltage judging circuit 4. The voltage judging circuit 4 compares the predetermined threshold values of the upper limits and lower limits of power supply voltages which correspond to the taps 6a and 6b of a transformer 6 and with which an electric machine can be operated with the output voltage of the control rectifying/smoothing circuit 2. If the DC voltage of the control rectifying/smoothing circuit 2 is within one of the threshold values of the corresponding power supply voltages, a signal is outputted to a relay driving circuit 5a or 5b which turns on or off the power supply.