As shown in FIG. 1 a vehicle is provided with an electric power assisted steering (EPAS) system that draws current ibattery from the vehicle electrical supply across power rail 2. The supply comprises a battery 3, typically rated at 12 volts DC, which is in turn topped up by an alternator 4. The battery also provides current to other vehicle accessories 5.
The EPAS system 1 is shown schematically in FIG. 2 of the drawings. It includes a steering column 10 attached to a steering wheel 11, a torque sensor 12 which measures the torque applied to the steering column 10 by the driver as they turn the steering wheel, a motor control and drive circuit 13 and an electric motor 14. The torque sensor 12 may be attached to a quill shaft in series with the column 10, and the motor 14 may act upon the steering column or other part of the steering system, typically through a gearbox 15.
The motor 14 typically comprises a three phase wound stator element and a rotor having for example six embedded magnets within it which in this instance are arranged so as to provide six poles which alternate between north and south around the rotor. The rotor therefore defines three direct or d axes evenly spaced around the rotor and three quadrature or q axes interspaced between the d axes. The d axes are aligned with the magnetic poles of the magnets where the lines of magnetic flux from the rotor are in the radial direction, and the q axes are spaced between the d axes where the lines of magnetic flux from the rotor are in the tangential direction.
