PROBLEM TO BE SOLVED: To select most suitable semiconductor switches for the detection of arm short-circuiting based on output rating by monitoring each arm current, when one of the semiconductor switches to bridge-connected arms is kept on and the other is kept off for a specified period, based on direct-current voltage after an inverter device is actuated. ;SOLUTION: After the start-up of an inverter device 30 has been completed, an on signal is given from a control circuit 31 only to a phase-U arm for a certain period, and then only to a phase-X arm, a phase-V arm, a phase-Y arm, a phase-W arm and a phase-Z arm for a certain period, respectively. If the semiconductor switch to the phase-W arm, for example, is brought into short-circuit breakdown state for some reason, the voltage produced across both the ends of a shunt resistance is monitored through the control circuit 31. If a judgment level is exceeded, the arm is discriminated to be in a short-circuit state. That is indicated on an indication panel or the like and the subsequent operation is prevented from being performed. Thus, any short circuit can be detected without breakdown of the semiconductor switches of the other sound arms, by keeping on the faulty semiconductor switch for a specified period based on a direct-current input voltage to a power converter 15, when the inverter is started.;COPYRIGHT: (C)1999,JPO
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