In a final stage, a drive circuit 27 converts the d-q axis currents output from the current controller 22 into three current demand components in a static reference frame, one for each phase of the motor 17, 18 or 19. These demand currents a, b, c are then converted by the drive stage 27, in combination with an estimate of the rotor position, into suitable PWM signals that are supplied to the switching motor phases of the drive stage 27 by PWM of the switches. A range of PWM switching strategies are known in the art and so will not be described in detail here. The switch arrangement is well known and described in such documents as EP 1083650A2. To provide control feedback a measurement of the phase currents is fed into the apparatus 13 from a current monitor 34, shown in FIG. 3 as a current shunt in the common ground path from the motor back to the battery.

The application of the torque demand limit causes the torque of the motor to deviate from the ideal torque demanded by the torque demand generator, but as will be apparent optimal setting of the limits ensures that at all times the motor is producing the maximum possible torque whilst ensuring that the current demanded from the battery and alternator do not exceed system limits. Importantly, the modification of the torque demand makes the implementation of the current controller simpler than prior art arrangements as all limiting is performed prior to the current controller. The controller, the torque demand generator and the torque demand limiter may be implemented using an electronic control unit running software that is stored in an area of memory.