Patent ID: 7174989

Claim:
An electric power steering apparatus comprising: a torque sensor for detecting a steering torque applied to a steering wheel; a steering assist motor which is driven to rotate based on the steering torque detected by the torque sensor; a drive circuit for driving the motor to rotate; a judgment unit for judging whether a rotational speed of the motor is within a predetermined range; and an instruction unit for supplying an instructing signal for field weakening control of the motor to the drive circuit, when it is judged that the rotational speed of the motor is within the predetermined range, wherein the motor is a brushless motor comprising a rotor having a plurality of permanent magnets which are respectively fixed in a plurality of recesses provided in a rotor core; wherein the brushless motor is set under field weakening control by controlling a phase of a motor current which is controlled with a PWM-controlled pulse signal which corresponds to a root-mean square value of an alternating signal, wherein the predetermined range is a rotational speed range where a root-mean square value of the alternating signal corresponding to the pulse signal is within a range of 90% through 100% of a maximum root-mean square value and where phase control is not performed.