Patent ID: 8401738

Claim:
An electric power steering system comprising: a motor that applies assist force for assisting steering operation to a steering system; a driving circuit that supplies a driving current to the motor on the basis of a power supply voltage, and that is formed by connecting switching arms, each of which is formed of a pair of switching elements connected in series, in parallel with each other; a controller that controls operation of the driving circuit; a relay circuit that turns on or off a relay contact provided in a first power supply path that connects the driving circuit and a power supply to each other; and a fault detector that detects a fault of the relay contact, wherein a capacitor is interposed between the relay contact and the driving circuit, the relay circuit is activated after the capacitor is charged through a second power supply path that bypasses the relay contact when an ignition switch is turned on, the fault detector determines whether the relay contact has a stuck-open fault, in which the relay contact remains turned off, on the basis of a voltage difference between both terminals of the relay contact after the relay circuit is activated, a resistor is interposed in the second power supply path, and the controller turns on both of the switching elements that constitute one of the switching arms when whether the relay contact has the stuck-open fault is determined.