Patent ID: 7845459

Claim:
An electric power steering apparatus, comprising: a steering mechanism that steers steering wheels in response to a steering operation of a driver's steering wheel; an electric motor that is fitted to the steering mechanism and that rotates using electric power supplied from an electric power supply device to generate a force used to assist the steering operation; and a motor control unit that controls an operation of the electric motor based on a manner in which the steering operation of the driver's steering wheel is performed, wherein the electric power supply device includes at least two electric power supply units connected in parallel, which are a main electric power supply unit and a sub-electric power supply unit that outputs a voltage lower than a voltage output from the main electric power supply unit, the electric power supply device being structured such that electric power is supplied to the electric motor from one of the main electric power supply unit and the sub-electric power supply unit, and the motor control unit includes: a vector control unit that controls rotation of the electric motor by executing vector control indicated by a two-phase rotating flux coordinate system where a direction in which the electric motor rotates is used as a q-axis and a direction perpendicular to the direction in which the electric motor rotates is used as a d-axis; and a motor speed compensation unit that increases, when electric power is supplied to the electric motor from the sub-electric power supply unit, a d-axis current used as a field-weakening control current by a larger amount than when electric power is supplied to the electric motor from the main electric power supply unit so that a decrease in a motor speed due to a decrease in a voltage output from the electric power supply device is suppressed.