PURPOSE: To rapidly detect faults by monitoring the rectified value of a potential difference between the potential of an arbitrary output terminal of a PWM inverter and the intermediate point potential of its DC power source.
CONSTITUTION: A DC power output from a DC power source 2 is converted by a PWM inverter 3 to an AC power of desired voltage and frequency to operate an induction motor 4 in a quadrant at an arbitrary speed. A loading and unloading machine 6 is connected to the motor 4 to move up or down a load 7, a brake unit 5 is further connected to the motor 4 to operate it in emergency, thereby preventing the load 7 from dropping. In this case, a fault detector 10 of the inverter 3 is provided. The intermediate point potential of the power source 2 and the phase potential of the inverter output are input to the detector 10 to rectify the potential difference. When it does not coincide with the half of the DC power source voltage, a fault signal is output to the unit 5 to operate the unit 5.