Motor-driven power steering system

The assistive torque generated by a motor or the torque transmitted by a clutch when the speed of travel of a motor vehicle is increased beyond a prescribed value while a steering torque is being applied, is reduced with time from a predetermined value. The assistive torque generated by the motor or the torque transmitted by the clutch when the speed of the motor vehicle is reduced below the prescribed value while the steering torque is being applied, is increased with time up to the predetermined value. Therefore, when the speed of the motor vehicle changes from a high speed to a low speed, the motor-driven power steering system is smoothly stopped and enters a manual steering mode. When the speed of the motor vehicle changes from a low speed to a high speed, the motor-driven power steering system is smoothly started from the manual steering mode. The motor-driven power steering system is continuously operated when the speed of the motor vehicle fluctuates in the vicinity of the prescribed value. The motor-driven power steering system can therefore be stopped and operated smoothly and stably as the motor vehicle speed reaches the prescribed value, so that the steering feeling and stability can be improved.