Method and apparatus for estimating a rotation speed of an electric motor

1. A method for estimating a rotation speed of an electric motor supplied by an inverter, the method comprising:

determining a time derivative of a stator current vector of the electric motor during a zero vector state of the inverter; and

determining an estimate of the rotation speed of the electric motor on the basis of the determined time derivative of the stator current vector.

2. The method of claim 1 , wherein the determining of the estimate of the rotation speed of the electric motor comprises:

determining a phase angle of the determined time derivative of the stator current vector; and

determining a time derivative of the phase angle to obtain the estimate of the rotation speed of the electric motor.

3. The method of claim 2 , wherein the time derivative of the stator current vector is filtered before the determination of the phase angle of the time derivative of the stator current vector.

4. The method of claim 2 , wherein the time derivative of the phase angle is further filtered to obtain the estimate of the rotation speed of the electric motor.

5. The method of claim 1 , wherein the determining of the estimate of the rotation speed of the electric motor comprises:

determining a stator current vector of the electric motor during a zero vector state of the inverter;

determining a product of the determined stator current vector and a stator resistance of the electric motor;

determining a product of the determined time derivative of the stator current vector and a leakage inductance of the electric motor;

determining a sum of the determined products;

determining a phase angle of the sum; and

determining a time derivative of the phase angle to obtain the estimate of the rotation speed of the electric motor.

6. The method of claim 5 , wherein the sum of the determined products is filtered before the determination of the phase angle of the sum.

7. The method of claim 1 , wherein during a zero vector state of the inverter each output phase of the inverter supplying the electric motor is set to a same potential.

8. The method of claim 1 , comprising:

comparing the estimate of the rotation speed of the electric motor to a predetermined threshold value; and

activating a safety function of the electric motor if the estimate of the rotation speed of the electric motor exceeds the predetermined threshold value.

9. The method of claim 8 , wherein comparing the determined estimate of the rotation speed of the electric motor to the predetermined threshold value is performed by using an absolute value of said estimate.

10. The method of claim 1 , comprising:

determining at least one alternative estimate for the rotation speed of the electric motor with an alternative determination method;

comparing the estimate of the rotation speed of the electric motor to the at least one alternative estimate of the rotation speed of the electric motor; and

activating a safety function of the electric motor on the basis of the comparison.

11. The method of claim 1 , comprising:

using the estimate of the rotation speed of the electric motor for control of the inverter supplying the electric motor.

12. A non-transitory computer-readable recording medium having a computer program recorded thereon that, when executed, causes a processor of a computer to carry out the steps of:

determining a time derivative of a stator current vector of the electric motor during a zero vector state of the inverter; and

determining an estimate of the rotation speed of the electric motor on the basis of the determined time derivative of the stator current vector.

13. An apparatus for estimating a rotation speed of an electric motor supplied by an inverter, the apparatus comprising:

means for determining a time derivative of a stator current vector of the electric motor during a zero vector state of the inverter; and

means for determining an estimate of the rotation speed of the electric motor on the basis of the determined time derivative of the stator current vector.

14. The apparatus of claim 13 , wherein the means for determining the estimate of the rotation speed of the electric motor comprise:

means for determining a phase angle of the determined time derivative of the stator current vector; and

means for determining a time derivative of the phase angle to obtain the estimate of the rotation speed of the electric motor.

15. The apparatus of claim 14 , wherein the means for determining the estimate of the rotation speed of the electric motor comprise:

means for filtering the time derivative of the stator current vector before a determination of the phase angle of the time derivative of the stator current vector.

16. The apparatus as claimed in claim 14 , wherein the means for determining the estimate of the rotation speed of the electric motor comprise:

means for filtering the time derivative of the phase angle to obtain the estimate of the rotation speed of the electric motor.

17. The apparatus of claim 13 , wherein the means for determining the estimate of the rotation speed of the electric motor comprise:

means for determining a stator current vector of the electric motor during a zero vector state of the inverter;

means for determining a product of the determined stator current vector and a stator resistance of the electric motor;

means for determining a product of the determined time derivative of the stator current vector and a leakage inductance of the electric motor;

means for determining a sum of the determined products;

means for determining a phase angle of the sum; and

means for determining a time derivative of the phase angle to obtain the estimate of the rotation speed of the electric motor.

18. The apparatus of claim 17 , wherein the means for determining the estimate of the rotation speed of the electric motor comprise:

means for filtering the sum of the determined products before a determination of the phase angle of the sum.

19. The apparatus of claim 13 , wherein during a zero vector state of the inverter each output phase of the inverter supplying the electric motor is set to the same potential.

20. The apparatus of claim 13 , comprising:

means for comparing the estimate of the rotation speed of the electric motor to a predetermined threshold value; and

means for activating a safety function of the electric motor if the estimate of the rotation speed of the electric motor exceeds the predetermined threshold value.

21. The apparatus of claim 20 , comprising:

means for determining an absolute value of the estimate of the rotation speed of the electric motor, wherein the means for comparing the estimate of the rotation speed of the electric motor to the predetermined threshold value are configured to use the absolute value of said estimate for the comparison.

22. The apparatus of claim 13 , comprising:

means for determining at least one alternative estimate for the rotation speed of the electric motor with an alternative determination method;

means for comparing the estimate of the rotation speed of the electric motor to the at least one alternative estimate of the rotation speed of the electric motor; and

means for activating a safety function of the electric motor on the basis of the comparison.

23. The apparatus of claim 13 , wherein the inverter supplying the electric motor is configured to use the estimate of the rotation speed of the electric motor for control of the supply of the electric motor.

24. An apparatus for estimating a rotation speed of an electric motor supplied by an inverter, the apparatus being configured to:

determine a time derivative of a stator current vector of the electric motor during a zero vector state of the inverter; and

determine an estimate of the rotation speed of the electric motor on the basis of the determined time derivative of the stator current vector.