System and method for permanent magnet motor control

1. A method of operating an electric motor, the method comprising:

starting the electric motor in an open loop control mode;

using an estimator module, estimating derivatives of total flux linkages of the electric motor based on measured currents of the electric motor, an output speed of the electric motor, a commanded speed for the electric motor, and voltage commands for the electric motor;

while the electric motor is in the open loop control mode, evaluating derivatives of the total flux linkages of the estimator module;

in response to the evaluation of the derivatives of the total flux linkages, determining whether the estimator module has converged;

in response to a determination that the estimator module has not converged within a predetermined period of time after starting the electric motor, signaling a first fault condition; and

in response to the signaling of the first fault condition, at least one of: (i) shutting down the electric motor; and (ii) restarting the electric motor.

2. The method of claim 1 wherein the electric motor is a compressor motor.

3. The method of claim 1 further comprising:

switching to a closed loop control mode;

while the electric motor is in the closed loop control mode, evaluating a second parameter of the estimator module; and

in response to the evaluation of the second parameter, selectively signaling a second fault condition.

4. The method of claim 3 further comprising signaling both the first fault condition and the second fault condition using a common 1-bit signal.

5. The method of claim 3 wherein the second parameter includes magnet flux linkages.

6. The method of claim 5 wherein the evaluating the second parameter includes calculating a magnitude of the magnet flux linkages.

7. The method of claim 3 wherein the evaluating the second parameter includes:

comparing a measure of the second parameter with a threshold;

incrementing a counter in response to the measure being greater than the threshold; and

decrementing the counter in response to the measure being less than the threshold.

8. The method of claim 7 further comprising applying an upper limit to the counter, wherein the counter is incremented only up to the upper limit.

9. The method of claim 7 further comprising signaling the second fault condition in response to the counter being decremented to a lower limit.

10. The method of claim 7 further comprising, for a second predetermined period of time beginning at the switch to closed loop mode, suspending incrementing the counter and decrementing the counter.

11. The method of claim 3 wherein the evaluating the derivatives of the total flux linkages includes:

comparing a measure of the derivatives of the total flux linkages to (i) a lower threshold and (ii) an upper threshold that is greater than the lower threshold;

incrementing a counter in response to the measure being less than the lower threshold; and

decrementing the counter in response to the measure being greater than the upper threshold.

12. The method of claim 11 wherein the evaluating the derivatives of the total flux linkages further includes, in response to the measure being between the lower threshold and the upper threshold, decrementing the counter in response to the counter being decremented immediately previously.

13. The method of claim 11 further comprising determining that the estimator module has converged in response to the counter being incremented to an upper limit.

14. The method of claim 11 wherein the evaluating the derivatives of the total flux linkages further includes applying a lower limit to the counter, wherein the counter is decremented only down to the lower limit.

15. The method of claim 3 wherein switching to the closed loop control mode is performed in response to a determination that the estimator module has converged.

16. The method of claim 3 wherein switching to the closed loop control mode is performed at the predetermined time after starting the electric motor.

17. The method of claim 1 wherein the evaluating the derivatives of the total flux linkages includes calculating a variance of the total flux linkage derivatives.

18. A control system for an electric motor, the control system comprising:

an estimator module that estimates derivatives of total flux linkages of the electric motor based on measured currents of the electric motor, an output speed of the electric motor, a commanded speed for the electric motor, and voltage commands for the electric motor; and

a rotor check module that:

commands operation of the electric motor in an open loop control mode at startup of the electric motor;

while the electric motor is in the open loop control mode, evaluates the derivatives of total flux linkages of the estimator module;

in response to the evaluation of the derivatives of total flux linkages, determines whether the estimator module has converged; and

in response to a determination that the estimator module has not converged within a predetermined period of time after starting the electric motor, signals a first fault condition and at least one of: (i) shuts down the electric motor; and (ii) restarts the electric motor.

19. The control system of claim 18 wherein the rotor check module further:

commands a transition to operating the electric motor in a closed loop control mode;

while the electric motor is in the closed loop control mode, evaluates a second parameter of the estimator module; and

in response to the evaluation of the second parameter, selectively signals a second fault condition.