Motor-driven power steering system for a vehicle.

摘要

A motor-driven power steering system for a vehicle is disclosed which is able to prevent a motor-related element such as a motor (13) itself, power elements for controlling power supply (11) to the motor or the like from being broken due to abnormal temperature rise or overheating thereof, thereby substantially avoiding or reducing resultant unsureness or instability in the operator's steering feeling upon change in steering mode as well as improving safety in steering operation. The motor (13) operatively connected with a steering rack (16) is energizable by a power source (11) for power assisting the steering motion caused by the operation through a steering wheel (1). A temperature sensor (18) detects the temperature of the motor-related element. An electromagnetic switching clutch (16) is interposed between the motor (13) and the steering rack (6) for selectively establishing and interrupting the transmission of power assisting force from the motor (13) to the steering rack in accordance with the travelling speed of the vehicle. A control unit (9) is provided for controlling the operation of the motor (13) in a manner such that when the detected temperature of the motor-related element is higher than a prescribed level, the current supplied to the motor through the motor-related element is gradually reduced to lower the temperature of the motor-related element, preventing trouble or breakdown of the motor-related element and hence of the motor (13) due to overheating thereof.