Patent ID: 8022658

Claim:
A motor control system comprising: a rectifier circuit that is connected to an AC power supply through a circuit breaker and that includes a positive DC output portion, a negative DC output portion and a smoothing capacitor connected across the positive DC output portion and the negative DC output portion; an inverter circuit that is constituted from a plurality of arm circuits connected in parallel, each arm circuit including a pair of semiconductor switches connected in series and having an AC output portion at a connection point of the pair of semiconductor switches; and an electrical insulation deterioration detecting system that detects deterioration in electrical insulation resistance of a motor driven by the inverter circuit, the electrical insulation deterioration detecting system comprising: a voltage divider circuit that is arranged between one of the positive DC output portion and the negative DC output portion and a ground through a normally open switch circuit; a detecting operation control section that closes the normally open switch circuit and causes at least one of the semiconductor switches included in the plurality of arm circuits and electrically connected to the other of the positive DC output portion and the negative DC output portion to be in a conductive state, while the circuit breaker is opened; and a voltage comparison section that compares a divided voltage outputted from the voltage divider circuit and a reference voltage to determine if the divided voltage exceeds the reference voltage, and outputs an alarm signal indicating a possibility of occurrence of deterioration in the electrical insulation resistance if the divided voltage exceeds the reference voltage.