PURPOSE: To realize condensation-preventing without using a space heater, an exclusive main circuit, and a control circuit by building in a safety stop mode for generating no output and stopping a motor, and a heating mode for generating voltage on specific one phase and single-phase-heating the motor under stopping.

CONSTITUTION: A perfect stop mode for generating no AC output as a motor stopping mode, and a single-phase heating mode for single-phase-operating an inverter and generating single-phase voltage just on specific one phase are built in, and each of the two modes is selected by a mode change-over switch 17. When single-phase AC voltage is applied to an AC motor 1 under stopping, then open-phase voltage is to be applied to the motor. Accordingly, rotational torque is not generated on the motor and it is set in a stopped state, and by heat generated from the winding of phase with voltage applied to, the interior section of the motor is heated.