Sensorless AC motor controller

1. A controller for an AC electric motor, the controller comprising:

i) a Feed Forward Torque Controller which directly derives a torque related component of applied motor voltages from a signal representing a torque command input T* and at least one motor parameter;

ii) a load model, which derives a motor speed value including a model of motor speed behaviour of the AC electric motor to provide an output signal which represents the motor speed of the AC electric motor for use in determining a frequency of rotation of an applied motor voltage vector and where an input to the load model is the signal representing the torque command input T*, the load model using the signal representing the torque command T*, at least over a part of an operating speed range of the AC motor which includes zero speed, to determine the motor speed output signal.

2. The controller of claim 1 wherein the load model uses the signal representing the torque command T* over a full operating speed range.

3. The controller of claim 1 wherein a further input of the load model receives a signal representing a torque producing component of the motor currents and used to modify the output signal of the load model for at least a part of operating speed range.

4. The controller of claim 3 , wherein the load model, includes a modelling component representing an inertial component of the load.

5. The controller of claim 4 wherein the load model receives the signal representing the torque producing component of the motor current over a full operating speed range.

6. The controller of claim 4 wherein the signal representing a torque producing component of the motor current is a correction value Δi q derived in a correction calculator which compares an applied torque current i′ q derived from the signal representing the torque command input T* and a measured torque producing component i q of the motor currents.

7. The controller of claim 4 further comprising a flux current controller which directly derives a flux related component of applied motor voltages from a flux current command input and wherein a flux component of the motor currents is increased to hold a rotor of the motor in alignment with a direction of a magnetic field produced by the flux component of the motor currents at zero motor speed and at motor speeds at which the rotor cannot align naturally from a presence of motor back EMF.

8. The controller of claim 4 wherein the Feed Forward Torque Controller does not use direct feedback information from the motor to derive the torque related component of applied motor voltages from the signal representing the torque command input T*.

9. The controller of claim 4 wherein the Feed Forward Torque Controller uses indirect feedback information comprising an applied motor speed ω′ derived by the load model using the signal representing the torque command input T*, and correction value Δi q derived in a correction calculator which compares an applied torque current i′ q derived from the signal representing the torque command input T* and a measured torque producing component i q of the motor currents.

10. The controller of claim 4 wherein the at least one motor parameter includes a value of rotor flux linkage λ r .

11. The controller as claimed in claim 10 wherein the value of rotor flux linkage λ r is an estimated value.

12. The controller of claim 4 wherein the load model incorporates a stability control component including a high speed damping component for stability control.

13. The controller as claimed in claim 12 wherein the stability control component comprises a modulator which modulates an applied motor speed signal with a signal representing a torque producing component of the motor currents.

14. The controller of claim 13 wherein a signal representing a torque producing component of the motor currents is a correction value Δi q derived in a correction calculator which compares the applied torque current i′ q derived from the signal representing the torque command input T* and a measured torque producing component i q of the motor currents.

15. The controller as claimed in claim 14 wherein the load model incorporates a 1 st order load torque correction component.

16. The controller of claim 15 wherein the 1 st order load torque correction component corrects the torque command input of the load model by subtracting a signal proportional to a signal representing a torque producing component of the motor currents.

17. The controller as claimed in claim 16 wherein the load model incorporates a 2nd order load torque correction component.

18. The controller of claim 17 wherein an integrator has an input which is a signal representing a torque producing component of the motor currents and wherein the 2nd order load torque correction component corrects the torque command input of the load model by subtracting a signal proportional to an output of the integrator.

19. The controller as claimed in claim 18 wherein, at least over a part of an operating speed range of the AC motor which includes zero speed, the integrator includes a DC gain which is limited by modifying the integrator to configure it as a single pole low pass filter.

20. The controller as claimed in claim 19 wherein the DC gain is modified as a function of motor speed with the DC gain increasing as motor speed increases.