Motor control apparatus and method

1. An apparatus for controlling an AC power supply for an electric motor, said AC power supply being derived from a DC voltage, the apparatus comprising:

a comparer configured to provide a comparison of a modulation index of a motor control signal with a reference value; and

a current data provider, configured to provide current data based on:

a torque demand signal;

a speed signal indicating the speed of rotation of the AC motor; and

an indication of the DC voltage, modified based on the comparison;

for control of the motor control signal based on the motor current data.

2. The apparatus of claim 1 , comprising a torque demand modifier configured to modify the torque demand signal based on the modified indication of the DC voltage in the event that the modified indication of the DC voltage is outside a first selected range, and in which the current data provider is configured to provide motor current data based on the modified torque demand signal.

3. The apparatus of claim 2 comprising a current data modifier configured to modify the provided current data based on the difference between the modified torque demand signal and a boundary of a second selected range in the event that the torque demand signal is outside the second selected range.

4. The apparatus of claim 1 comprising a data store comprising a plurality of associations arranged to associate each of a plurality of current data elements with a respective:

motor speed,

torque demand, and

DC voltage;

wherein the data provider is configured to provide the current data by selecting amongst the stored data elements based on the associations.

5. The apparatus of claim 1 in which the modulation index of the control signal comprises the ratio of the motor control signal amplitude to one of: (a) a maximum amplitude value of a controller that provides the motor control signal; or (b) the amplitude of a timing signal used to determine a PWM signal from the motor control signal.

6. The apparatus of claim 1 further comprising a motor controller adapted to provide the motor control signal based on the current data.

7. The apparatus of claim 6 wherein the motor controller is configured to control an inverter adapted to derive said AC power supply from said DC power supply.

8. An apparatus for controlling an AC power supply for an electric motor, said AC power supply being derived from a DC voltage, the apparatus comprising:

a processor configured to:

compare a modulation index of a motor control signal with a reference value;

modify an indication of the DC voltage based on the comparison;

provide current data based on:

a torque demand signal;

a speed signal indicating the speed of rotation of the AC motor; and

the modified indication of the DC voltage;

for control of the motor control signal based on the motor current data.

9. The apparatus of claim 8 , in which the processor is configured to modify the torque demand signal based on the modified indication of the DC voltage in the event that the modified indication of the DC voltage is outside a first selected range, and to provide motor current data based on the modified torque demand signal.

10. The apparatus of claim 9 in which the processor is configured to modify the provided current data based on the difference between the modified torque demand signal and a boundary of a second selected range in the event that the torque demand signal is outside the second selected range.

11. The apparatus of claim 10 in which the current data comprises a direct phase current information, and quadrature phase current information.

12. The apparatus of claim 11 in which the provided current data comprises an in phase current value, I D , and a quadrature phase current value, I Q , and in which modifying the current data comprises applying a negative adjustment to the in phase current value I D .

13. The apparatus of claim 8 comprising a memory storing a plurality of associations between each of a plurality of current data elements with a respective:

motor speed,

torque demand, and

DC voltage;

wherein the processor is coupled to the memory and configured to provide the current data by selecting amongst the stored data elements based on the associations.

14. A method of controlling a power supply for an AC motor, said power supply derived from a DC voltage, the method comprising:

selecting data from stored motor current data based on:

a torque demand signal;

a speed signal indicating the speed of rotation of the AC motor; and

an indication of the DC voltage modified based on a comparison between a reference value and a modulation index of a motor control signal; and

controlling the motor control signal in accordance with the selected motor current data.

15. The method of claim 14 comprising modifying the torque demand signal based on the selection signal in the event that the selection signal is outside a first selected range, and selecting the data based on the modified torque demand signal.

16. The method of claim 15 comprising modifying the selected data based on the difference between the torque demand and a boundary of a second selected range in the event that the modified torque demand signal is outside the second selected range.

17. The method of claim 16 in which the selected data comprises an in phase current value, I D , and a quadrature phase current value, I Q , and in which modifying the selected data comprises applying a negative adjustment to the in phase current value I D .

18. The method of claim 14 in which the modulation index of the control signal comprises the ratio of the motor control signal amplitude to one of: (a) a maximum amplitude value of a controller that provides the motor control signal; or (b) the amplitude of a timing signal used to determine a PWM signal from the motor control signal.

19. An apparatus configured to perform the method of claim 14 .

20. The apparatus of claim 19 further comprising a controller adapted to provide the motor control signal based on the current data.

21. The apparatus of claim 20 wherein the controller is configured to control an inverter adapted to derive said AC power supply from said DC power supply.

22. The apparatus of claim 21 further comprising the inverter.

23. The apparatus of claim 22 further comprising the motor.

24. The apparatus of claim 23 in which the motor is a permanent magnet motor.