System and method for commutating a motor using back electromotive force signals

1. A method for commutating a motor, comprising:

sampling back EMF values on poles in the motor at a sampling frequency that is at least two times greater than the PWM frequency; selecting from the back EMF values any two consecutive readings that have a change in sign of the rate of change in current on the poles between the any two consecutive readings; and

during a startup state, utilizing the two consecutive readings to commutate the motor,

wherein the back EMF values are sampled from a sensing coil in the motor that is not being driven by the PWM signals when two poles of the motor are being driven by the PWM signals.

2. The method of claim 1 , wherein the startup state for the motor ends when a magnitude of the filtered back EMF exceeds a specified dead-band.

3. The method of claim 1 , wherein an operating envelope for commutating said motor comprises a lower threshold defining a noise level floor and an upper threshold relating to a bus voltage for the motor.

4. The method of claim 3 , wherein the upper threshold is adjusted to account for a resistive drop across a winding in said motor.

5. The method of claim 3 , wherein when the startup state ends the method progresses to a next state for attenuating the switching noise, the next state comprising:

filtering the two consecutive readings to remove switching noise to obtain cleaned back EMF values; and

utilizing the cleaned back EMF values to commutate the motor if certain predetermined conditions are present.

6. The method of claim 5 , wherein the predetermined conditions include that the back EMF values do not cross each other twice.

7. The method of claim 5 , wherein the filtering is performed by a digital filter.

8. The method of claim 7 , wherein the digital filter has a cutoff frequency approximately half of the frequency of the PWM signals.

9. The method of claim 5 , wherein the sampling is done at regular intervals.

10. The method of claim 5 , wherein the motor is a three-phase motor.

11. The method of claim 5 , wherein the poles of the motor are provided in a delta configuration.

12. The method of claim 5 , wherein the poles of the motor are provided in a Y configuration.

13. The method of claim 1 , wherein in the startup state, the two consecutive readings used to commutate the motor comprise raw back EMF samples from said back EMF values.

14. The method of claim 13 , wherein the raw back EMF samples modulated by a bus voltage for the motor form an operating envelope having a lower threshold defining a noise level floor and an upper threshold relating to a bus voltage for the motor, the operating envelope used to commutate the motor.

15. The method of claim 1 , wherein for the two consecutive readings, a first reading is larger than a second reading.

16. The method of claim 1 , further comprising restarting the motor when said raw back EMF samples indicate a reverse rotation fault.

17. A system for commutating a motor, comprising:

a sense circuit for sampling back EMF values on poles in the motor from a sensing coil in the motor that is not being driven by the PWM signals when two poles of the motor are being driven by the PWM signals, the back EMF values being at a sampling frequency that is at least two times greater than the PWM frequency;

a selection module for identifying from the back EMF values any two consecutive readings that have a change in sign of the rate of change in current on the poles between the two consecutive readings;

a filtering module for filtering the two consecutive readings to remove switching noise to produce filtered values; and

a motor controller for commutating the motor using signals related to the two consecutives readings when the motor is in a startup state and signals relating to the filtered values when the motor is past the startup state.

18. The system of claim 17 , wherein the startup state ends when a magnitude the filtered back EMF exceeds a predefined dead-band.

19. The system of claim 18 , wherein an operating envelope for commutating said motor includes a lower threshold defining a noise level floor and an upper threshold relating to a bus voltage for the motor.

20. The system of claim 19 , wherein the envelope is adjusted by a factor to account for resistance in a winding in the motor.

21. The system of claim 19 , wherein after the startup state ends, the controller uses the filtered values to commutate the motor if certain predetermined conditions are present.

22. The system of claim 21 , wherein the predetermined conditions include that the two consecutive back EMF values do not cross each other twice.