PURPOSE: To prevent a DC circuit from getting to overvoltage by regenerative mode by being equipped with a rectifier to supply DC power to a transistor inverter group and a thyristor inverter device to regenerate the regenerating energy from a DC circuit to an AC system.

CONSTITUTION: DC power is supplied from a rectifier 2 in a common power device 1 to a common bus 10 of a transistor inverter group 4 each of which drives plural induction motors 7. a thyristor inverter device 3 is further provided in the common power device 1 and by a voltage detector 5 of a DC bus, DC voltage is always detected. With an inverter phase controller 4 the phase of the thyristor inverter device 3 is controlled so that the DC voltage may be constant. The inverter phase controller 4 controls the phase so that minute current may always flow to the thyristor inverter device 3. The regenerating energy generated to the DC circuit can thereby be regenerated to the AC power source side to prevent the DC circuit voltage from getting to overvoltage.