PURPOSE:To suppress the abnormal voltage low impressed by the secondary winding of a winding type induction machine to a cycloconverter in abnormality, by providing a protective device to by-pass the current induced to the secondary winding. CONSTITUTION:When the maximum voltage Vd between terminals of a motor secondary winding becomes higher than the set point, it is detected by a turn-ON control circuit 11B to turn ON a thyrister 11C. In this way, a resistor 11E is connected between the lines where the voltage is highest through a rectifier bridge 11A. On this account the rise of the voltage between the motor secondary windings can be suppressed. The voltage impressed to a cycloconverter from the motor secondary winding can therefore be suppressed.
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