PMSM field weakening for appliance motors

1. A motor drive system, comprising:

a motor having a rotor and a stator;

an inverter coupled to the motor, the inverter configured to apply a power signal to drive the motor; and

a controller coupled to the inverter, the controller configured to operate the motor above a base speed using a field weakening technique;

wherein the field weakening technique comprises applying a fixed field weakening phase-advance angle to the power signal relative to a rotor flux position and increasing a voltage of the power signal until a desired speed is achieved.

2. The motor drive system as in claim 1 , wherein the fixed field weakening phase-advance angle includes a fixed field weakening angle and a phase-advance angle.

3. The motor drive system as in claim 1 , further comprising a sensor to detect at least one of the current speed of the motor and the rotor flux position.

4. The motor drive system as in claim 1 , wherein the current speed of the motor and the desired speed of the motor are compared to determine a speed error of the motor.

5. The motor drive system as in claim 4 , wherein the voltage of the power signal is adjusted based on the speed error.

6. The motor drive system as in claim 1 , wherein the motor is a permanent magnet synchronous motor.

7. A method of controlling a motor using a field weakening technique, comprising:

determining a fixed field weakening phase advance angle of a power signal based on a current motor speed;

applying the fixed field weakening phase advance angle to the power signal to the motor relative to a rotor flux position using an inverter; and

increasing a voltage of the power signal until a desired speed of the motor is achieved.

8. The method as in claim 7 , wherein the fixed field weakening phase-advance angle includes a fixed field weakening angle and a phase-advance angle.

9. The method of claim 7 , wherein the fixed field weakening phase advance angle is determined when the desired speed is above a base speed of the motor.

10. The method of claim 7 , further comprising detecting the current motor speed using a speed sensor.

11. The method of claim 7 , further comprising determining a speed error of the motor based on the current speed and the desired speed.

12. The method of claim 11 , wherein the voltage of the power signal is adjusted based on the speed error.

13. The method of claim 7 , wherein the motor is a permanent magnet synchronous motor.

14. A method for operating a motor using a field weakening technique, the method comprising:

determining a current speed of the motor;

determining whether the current speed of the motor exceeds a base speed;

determining whether the current speed of the motor equals a desired speed;

when the current speed of the motor exceeds the base speed but does not equal the desired speed, performing the field weakening technique, the field weakening technique comprising:

determining a field weakening angle based on the current speed of the motor;

applying the field weakening angle to a motor drive signal used to drive the motor; and

holding the field weakening angle constant and increasing a magnitude of a voltage of the motor drive signal until the current speed of the motor equals the desired speed or a power limit is reached.

15. The method of claim 14 , wherein determining the field weakening angle based on the current speed of the motor comprises consulting a lookup table to retrieve the field weakening angle based on the current speed of the motor.

16. The method of claim 14 , wherein the field weakening technique further comprises:

when the power limit is reached, determining a new field weakening angle based on the current speed of the motor.