Electric power steering system

An electric power steering system comprising:

• a permanent magnet synchronous motor disposed in a steering mechanism to generate steering assist torque, and including a motor stator provided with multiple sets of three-phase coils with different drive systems;
• a motor drive unit that includes drive circuits provided so as to respectively correspond to the multiple sets of three-phase coils, and that energizes the multiple sets of three-phase coils independently through the respective drive circuits to drive the permanent magnet synchronous motor;
• an electrical angle estimator that computes an estimated electrical angle of the permanent magnet synchronous motor;
• and
• a motor control unit that controls the drive circuits of the motor drive unit based on the estimated electrical angle computed by the electrical angle estimator to drive the permanent magnet synchronous motor,
• wherein the electrical angle estimator includes
• multiple three phase-two phase voltage vector converters each of which converts a voltage vector of voltages supplied to the three-phase coils, the voltage vector being expressed in a three-phase fixed coordinate system, into a voltage vector expressed in a two-phase fixed coordinate system, and which are provided respectively for the multiple sets of three-phase coils,
• multiple three phase-two phase current vector converters each of which converts a current vector of currents passed through the three-phase coils, the current vector being expressed in a three-phase fixed coordinate system, into a current vector expressed in a two-phase fixed coordinate system, and which are provided respectively for the multiple sets of three-phase coils,
• multiple voltage vector adders each of which adds together the voltage vectors of the multiple sets of three-phase coils, the voltage vectors being expressed in the two-phase fixed coordinate system,
• multiple current vector adders each of which adds together the current vectors of the multiple sets of three phase coils, the voltage vectors being expressed in the two-phase fixed coordinate system, and
• an induced voltage computing unit that computes an induced voltage generated at the permanent magnet synchronous motor based on an added voltage vector computed by the voltage vector adders and an added current vector computed by the current vector adders,
• wherein the electrical angle estimator estimates an electrical angle of the permanent magnet synchronous motor based on the computed induced voltage.