Device and a method for estimating inductances of an electric machine

1. A device for estimating inductances of an electric machine when a rotor of the electric machine is stationary, the device comprising a processing system configured to:

control stator voltages of the electric machine to constitute a balanced multi-phase alternating voltage,

estimate rotational position of the rotor based on a negative sequence component of stator currents of the electric machine, the negative sequence component being caused by saliency of the rotor,

control direct-axis current of the electric machine to be direct current and quadrature-axis voltage of the electric machine to be alternating voltage,

estimate quadrature-axis inductance of the electric machine based on the quadrature-axis alternating voltage and on quadrature-axis alternating current caused by the quadrature-axis alternating voltage,

control direct-axis voltage of the electric machine to be alternating voltage and the quadrature-axis voltage of the electric machine to be substantially zero, and

estimate direct-axis inductance of the electric machine based on the direct-axis alternating voltage and on direct-axis alternating current caused by the direct-axis alternating voltage,

wherein the processing system is configured to set a ratio of amplitude of phase-voltages of the balanced multi-phase alternating voltage and frequency of the balanced multi-phase alternating voltage to be in a range from 5% to 15% of a ratio of amplitude of a nominal phase voltage of the electric machine and a nominal supply frequency of the electric machine so as to produce a rotating magnetic flux whose amplitude is in a range from 5% to 15% of a nominal magnetic flux of the electric machine.

2. The device according to claim 1 , wherein the processing system is configured to determine a direction in which a length of a space-vector of the stator currents reaches a minimum or a maximum and to determine the rotational position the rotor based on the determined direction of the minimum or the maximum.

3. The device according to claim 1 , wherein the processing system is configured to set the frequency of the balanced multi-phase alternating voltage to be equal to or less than the nominal supply frequency of the electric machine.

4. The device according to claim 1 , wherein the processing system is configured to set the frequency of the balanced multi-phase alternating voltage to be greater than the nominal supply frequency of the electric machine.

5. The device according to claim 1 , wherein the processing system is configured to vary, when estimating the quadrature-axis inductance, a value of the direct-axis direct current so as to obtain an estimate of the quadrature-axis inductance as a function of the direct-axis direct current.

6. The device according to claim 1 , wherein the processing system is configured to set, when estimating the quadrature-axis inductance, a ratio of amplitude of the quadrature-axis alternating voltage and frequency of the quadrature-axis alternating voltage to be in a range from 1% to 5% of a ratio of amplitude of a nominal phase voltage of the electric machine and a nominal supply frequency of the electric machine so as to produce an alternating quadrature-axis flux linkage whose amplitude is in a range from 1% to 5% of a nominal flux linkage of the electric machine.

7. The device according to claim 1 , wherein the processing system is configured to vary, when estimating the direct-axis inductance, amplitude of the direct-axis alternating voltage so as to obtain an estimate of the direct-axis inductance as a function of the direct-axis current.

8. The device according to claim 1 , wherein the processing system is configured to set, when estimating the direct-axis inductance, frequency of the direct-axis alternating voltage to be less than a nominal supply frequency of the electric machine.

9. A power electronic converter comprising:

a converter stage for forming stator voltages for an electric machine,

a controller for controlling the stator voltages at least partly based on stator currents of the electric machine, and

a device for estimating the direct-axis and quadrature-axis inductances of the electric machine when a rotor of the electric machine is stationary,

wherein the device comprises a processing system configured to:

set the controller to control the stator voltages to constitute a balanced multi-phase alternating voltage,

estimate rotational position of the rotor based on a negative sequence component of the stator currents, the negative sequence component being caused by saliency of the rotor,

set the controller to control direct-axis current of the electric machine to be direct current and quadrature-axis voltage of the electric machine to be alternating voltage,

estimate the quadrature-axis inductance of the electric machine based on the quadrature-axis alternating voltage and on quadrature-axis alternating current caused by the quadrature-axis alternating voltage,

set the controller to control direct-axis voltage of the electric machine to be alternating voltage and the quadrature-axis voltage of the electric machine to be substantially zero,

estimate the direct-axis inductance of the electric machine based on the direct-axis alternating voltage and on direct-axis alternating current caused by the direct-axis alternating voltage, and

set a ratio of amplitude of phase-voltages of the balanced multi-phase alternating voltage and frequency of the balanced multi-phase alternating voltage to be in a range from 5% to 15% of a ratio of amplitude of a nominal phase voltage of the electric machine and a nominal supply frequency of the electric machine so as to produce a rotating magnetic flux whose amplitude is in a range from 5% to 15% of a nominal magnetic flux of the electric machine.

10. A method for estimating inductances of an electric machine when a rotor of the electric machine is stationary, the method comprising:

controlling stator voltages of the electric machine to constitute a balanced multi-phase alternating voltage,

estimating rotational position of the rotor based on a negative sequence component of stator currents of the electric machine, the negative sequence component being caused by saliency of the rotor,

controlling direct-axis current of the electric machine to be direct current and quadrature-axis voltage of the electric machine to be alternating voltage,

estimating quadrature-axis inductance of the electric machine based on the quadrature-axis alternating voltage and on quadrature-axis alternating current caused by the quadrature-axis alternating voltage,

controlling direct-axis voltage of the electric machine to be alternating voltage and the quadrature-axis voltage of the electric machine to be substantially zero, and

estimating direct-axis inductance of the electric machine based on the direct-axis alternating voltage and on direct-axis alternating current caused by the direct-axis alternating voltage,

wherein a ratio of amplitude of phase-voltages of the balanced multi-phase alternating voltage and frequency of the balanced multi-phase alternating voltage is in a range from 5% to 15% of a ratio of amplitude of a nominal phase voltage of the electric machine and a nominal supply frequency of the electric machine so as to produce a rotating magnetic flux whose amplitude is in a range from 5% to 15% of a nominal magnetic flux of the electric machine.

11. The method according to claim 10 , wherein the method comprises determining a direction in which a length of a space-vector of the stator currents reaches a minimum or a maximum, and the method comprises determining the rotational position of the rotor based on the determined direction of the minimum or the maximum.

12. The method according to claim 10 , wherein the frequency of the balanced multi-phase alternating voltage is equal to or less than the nominal supply frequency of the electric machine.

13. The method according to claim 10 , wherein the frequency of the balanced multi-phase alternating voltage is greater than the nominal supply frequency of the electric machine.

14. The method according to claim 10 , wherein the method comprises varying, when estimating the quadrature-axis inductance, a value of the direct-axis direct current so as to obtain an estimate of the quadrature-axis inductance as a function of the direct-axis direct current.

15. The method according to claim 10 , wherein, when estimating the quadrature-axis inductance, a ratio of amplitude of the quadrature-axis alternating voltage and frequency of the quadrature-axis alternating voltage is in a range from 1% to 5% of a ratio of amplitude of a nominal phase voltage of the electric machine and a nominal supply frequency of the electric machine so as to produce an alternating quadrature-axis flux linkage whose amplitude is in a range from 1% to 5% of a nominal flux linkage of the electric machine.

16. The method according to claim 10 , wherein the method comprises varying, when estimating the direct-axis inductance, amplitude of the direct-axis alternating voltage so as to obtain an estimate of the direct-axis inductance as a function of the direct-axis current.

17. The method according to claim 10 , wherein, when estimating the direct-axis inductance, frequency of the direct-axis alternating voltage is less than a nominal supply frequency of the electric machine.

18. The method according to claim 10 , wherein the electric machine is one of the following: a synchronous reluctance machine, an interior permanent magnet machine, a permanent magnet assisted synchronous reluctance machine.

19. A non-volatile computer readable medium encoded with a computer program for estimating inductances of an electric machine when a rotor of the electric machine is stationary, the computer program comprising computer executable instructions for controlling a programmable processor to:

control stator voltages of the electric machine to constitute a balanced multi-phase alternating voltage,

estimate rotational position of the rotor based on a negative sequence component of stator currents of the electric machine, the negative sequence component being caused by saliency of the rotor,

control direct-axis current of the electric machine to be direct current and quadrature-axis voltage of the electric machine to be alternating voltage,

estimate quadrature-axis inductance of the electric machine based on the quadrature-axis alternating voltage and on quadrature-axis alternating current caused by the quadrature-axis alternating voltage,

control direct-axis voltage of the electric machine to be alternating voltage and the quadrature-axis voltage of the electric machine to be substantially zero, and

estimate direct-axis inductance of the electric machine based on the direct-axis alternating voltage and on direct-axis alternating current caused by the direct-axis alternating voltage,

wherein a ratio of amplitude of phase-voltages of the balanced multi-phase alternating voltage and frequency of the balanced multi-phase alternating voltage is in a range from 5% to 15% of a ratio of amplitude of a nominal phase voltage of the electric machine and a nominal supply frequency of the electric machine so as to produce a rotating magnetic flux whose amplitude is in a range from 5% to 15% of a nominal magnetic flux of the electric machine.