PURPOSE: To avoid the short-circuit of the input side of a cycloconverter by a method wherein a bank reference signal before a fault occurs is held while the fault is continued and, after the fault is restored, a bank is selected by a bank reference signal with the held reference signal as an initial value and the output of a phase signal is started.

CONSTITUTION: If the effective value (v) of an AC voltage is out of a certain range (in this case, zero) by a fault in an AC power supply 1, a phase controller 8 stops the output of a phase signal and discontinues the output until the fault is restored. Even if the phase signal is discontinued by the fault, a cycloconverter current icc is kept on flowing while it is attenuated with a certain time constant with the inductance of a load 13. After the fault is restored, the operation of the phase controller 8 is started. A bank reference signal generator 9 holds a band reference signal Fband before the fault occurs and, after the fault is restored, outputs the held reference signal as an initial value. When the fault is restored, the cycloconverter current icc is positive and the phase signal is outputted to the positive group bank by the bank reference signal and the cycloconverter current is controlled to avoid the short-circuit of the input side of the cycloconverter 4.