Methods and systems for induction motor control

1. A method for controlling an induction motor having a rotor, comprising the steps of:

receiving a torque command;

comparing the torque command to a threshold torque value;

generating, with a first estimation module, a first estimated rotor resistance when the torque command is less than or equal to the threshold torque value;

generating, with a second estimation module, a second estimated rotor resistance when the torque command is greater than the threshold torque value; and

generating control signals for the induction motor based on the first estimated rotor resistance or the second estimated rotor resistance.

2. The method of claim 1 , wherein the step of generating, with the first estimation module, the first estimated rotor resistance includes

estimating a rotor temperature.

3. The method of claim 2 , wherein the induction motor is cooled by a coolant, and wherein the estimating step includes

estimating a power loss in the rotor; and

estimating a thermal resistance between the rotor and the coolant.

4. The method of step 2 , wherein the estimating step includes

estimating a rotor current.

5. The method of claim 4 , wherein the step of estimating the rotor current includes

estimating the rotor current as a function of an inductance of the rotor.

6. The method of claim 4 , wherein the step of estimating the rotor current includes

determining a correction factor based on the rotor speed.

7. The method of claim 6 , wherein the step of determining the correction factor includes

retrieving the correction factor from a look-up table based on the rotor speed.

8. The method of claim 1 , wherein the step of generating, with the second estimation module, the second estimated rotor resistance includes

calculating an estimated squared value of flux of the rotor using the torque command;

determining a resistance offset of the rotor; and

generating an updated measure of the second estimated rotor resistance using the estimated squared value and the resistance offset.

9. The method of claim 8 , wherein the step of determining a resistance offset includes

determining the resistance offset using the torque command and a look-up table.

10. The method of claim 9 , wherein the step of determining a resistance offset further includes

determining the resistance offset using the rotor speed.

11. The method of claim 10 , further comprising the step of:

determining a rotor position of the rotor, and

wherein the step of calculating an estimated squared value includes

calculating the estimated squared value using the rotor speed and the rotor position.

12. A system for controlling an induction motor having a rotor, the system comprising:

a transition module configured to receive a rotor speed of the rotor and a torque command;

a first estimation module coupled to the transition module and configured to generate a first resistance estimation;

a second estimation module coupled to the transition module and configured to generate a second resistance estimation; and

a controller module coupled to the first estimation module and the second estimation module and configured to generate control signals for the induction motor based on the first resistance estimation or the second resistance estimation as a function of the torque command.

13. The system of claim 12 , wherein the controller module is configured to

generate the control signals based on the first resistance estimation when the torque command is less than or equal to a threshold torque value, and

generate the control signals based on the second resistance estimation when the torque command is greater than the threshold torque value.

14. The system of claim 13 , wherein the first estimation module is configured to generate the first resistance estimation based on a rotor temperature.

15. The system of claim 14 , further comprising a cooling system with coolant configured to cool the induction motor, and

wherein the first estimation module is configured to generate the first resistance estimation based on a power loss in the rotor and a thermal resistance between the rotor and the coolant.

16. The system of step 15 , wherein the first estimation module is configured to generate the first resistance estimation based a rotor current.

17. The system of claim 16 , wherein the first estimation module is configured to estimate the rotor current as a function of an inductance of the rotor.

18. The system of claim 17 , wherein the first estimation module is configured to estimate the rotor current based on a correction factor that is a function of the rotor speed.

19. The system of claim 18 , wherein the first estimation module includes a look-up table with the correction factor.

20. An induction motor system, comprising:

a motor comprising a rotor and a stator;

an inverter assembly coupled to the motor and configured to drive the motor based on control signals; and

a controller coupled to the inverter assembly and configured to generate the control signals, the controller comprising:

a transition module configured to receive a rotor speed of the rotor and a torque command,

a first estimation module coupled to the transition module and configured to generate a first resistance estimation,

a second estimation module coupled to the transition module and configured to generate a second resistance estimation, and

a controller module coupled to the first estimation module and the second estimation module and configured to generate control signals for the induction motor based on the first resistance estimation or the second resistance estimation as a function of the torque command.