PURPOSE: To reduce the voltage and capacity of a conversion device by dropping the supply voltage down to the input voltage suitable for the conversion device in a variable speed operation and by using a tapped three-winding transformer in which in a commercial bypass operation one of the two secondary windings is applied to the commercial bypass operation.

CONSTITUTION: Circuit breakers 7, 8, 9, 10 and 11 are closed when a motor 1 is operated at a variable speed up to a certain constant load less than the rated load of the motor. By a transformer 3 the commercial power supply is reduced to the voltage suitable for the input voltage of a conversion device 2 and the motor is operated at a variable speed by the conversion device 2. On the other hand, in a commercial bypass operation the tap is switched by an on-load tap changer 4 so that the secondary voltage of the transformer 3 may be the same as in the commercial voltage. Circuit breakers 7, 8 and 11 are opened, but breakers 5 and 6 are closed instantaneously and the motor is shifted to the commercial bypass operation.