Patent ID: 8798863

Claim:
An electrically driven power steering system, comprising: a steering mechanism that steers steerable wheels along with a steering operation of a steering wheel; a motor providing a steering force for the steering mechanism; a control unit drivingly controlling the motor; a command signal calculating section disposed in the control unit to calculate a vector control command signal for drivingly controlling the motor in accordance with a driving situation of a vehicle; a two-phase to three-phase converting section disposed in the control unit to convert the vector control command signal into a voltage command signal for each phase of the motor; a PWM control section disposed in the control unit to output a PWM duty signal (Du, Dv, Dw) of each phase of the motor in accordance with the voltage command signal of a corresponding one of the phases of the motor; an inverter, disposed in the control unit, constituted by a switching circuit controlled by means of the PWM duty signals (Du, Dv, Dw), and drivingly controlling the motor; a current sensor disposed in a direct current bus bar of the inverter to detect a direct current bus bar current; a phase current detecting section disposed in the control unit to detect or estimate a current value Iu, Iv, Iw of each phase of the motor on the direct current bus bar current and the PWM duty signals (Du, Dv, Dw); an estimated primary current calculating section disposed in the control unit to calculate an estimated primary current as follows: (Iu×Du+Iv×Dv+Iw×Dw)/100% on a basis of the current value (Iu, Iv, Iw) of each phase of the motor and the PWM duty signals (Du, Dv, Dw); and an abnormality monitoring section configured to detect an abnormality of the control unit on a basis of the direct current bus bar and the estimated primary current.