Method for determining a state of a bearing, module for determining a state of a bearing, railway vehicle and system

1. A method for determining a state of a bearing of a three phase electric machine, the electric machine having a rotor, which is supported by the bearing, the electric machine being connected to a three phase inverter for supplying the electric machine with electric power, the three phase inverter being controlled to apply a pulse width modulation with a predefined scheme to create three phase currents, the method comprising:

determining a first phase current provided to the electric machine;

determining a second phase current provided to the electric machine, the second phase current being different to the first phase current;

determining the power spectral density of the park's vector of the current provided to the electric machine, based on the first phase current and the second phase current;

determining an amplitude of the power at a predefined frequency of the power spectral density, wherein the predefined frequency depends on the rotational speed of the rotor and the scheme of the pulse width modulation, wherein the predefined frequency is in the power spectral density at a frequency distance, which is equal the rotational speed, from a major peak of the power spectral density of the park's vector of the current provided to the electric machine;

comparing the amplitude of the power at the predefined frequency with a predefined power value; and

determining, based on the comparison, whether a failure of the bearing has occurred or the bearing is degraded.

2. The method according to claim 1 , wherein the predefined frequency is determined based on the major peak of the power spectral density of the park's vector of the current provided to the electric machine, which is based on the first phase current and the second phase current, and on the rotational speed of the rotor.

3. The method according to claim 1 , wherein the predefined power value is the power determined for the electric machine with an undamaged bearing at the predefined frequency of the power spectral density of the park's vector of the current provided to the electric machine based on the first phase current and the second phase current.

4. The method according to claim 1 , wherein the first phase current and the second phase current is determined at a first point in time, wherein the predefined power value is the power at the predefined frequency of the power spectral density of the park's vector of the current provided to the electric machine based on the first phase current and the second phase current determined for the electric machine at a second point in time being previous to the first point in time.

5. The method according to claim 4 , wherein the difference between the first point in time and the second point in time is at least one day.

6. The method according to claim 4 , wherein the difference between the first point in time and the second point in time is at least one month.

7. The method according to claim 1 , wherein the three phase electric machine is an induction machine or a synchronous machine.

8. The method according to claim 1 , further comprising: determining a time for maintenance of the electric machine based on the comparison.

9. The method according to claim 1 , further comprising: running the electric machine at a predetermined rotational speed of the rotor.

10. The method according to claim 1 , further comprising: determining the scheme of the pulse width modulation.

11. The method according to claim 10 , wherein the scheme of the pulse width modulation is determined based on the rotational speed.

12. A bearing state detector, wherein the bearing is for a three phase electric machine, the electric machine having a rotor, which is supported by the bearing, the electric machine being connected to a three phase inverter for supplying the electric machine with electric power, the three phase inverter being controlled to apply a pulse width modulation with a predefined scheme to create three phase currents, wherein the detector is adapted to:

determine a first phase current provided to the electric machine;

determine a second phase current provided to the electric machine, the second phase current being different to the first phase current;

determine the power spectral density of the parks vector of the current provided to the electric machine, based on the first phase current and the second phase current;

determine an amplitude of the power at a predefined frequency of the power spectral density, wherein the predefined frequency depends on the rotational speed of the rotor and the scheme of the pulse width modulation, wherein the predefined frequency is in the power spectral density at a frequency distance, which is equal the rotational speed, from a major peak of the power spectral density of the park's vector of the current provided to the electric machine;

compare the amplitude of the power at the predefined frequency with a predefined power value; and

determine, based on the comparison, whether a failure of the bearing has occurred or the bearing is degraded.

13. The bearing state detector according to claim 12 , wherein the predefined frequency is determined based on the major peak of the power spectral density of the park's vector of the current provided to the electric machine, which is based on the first phase current and the second phase current, and on the rotational speed of the rotor.

14. The bearing state detector according to one of the claim 12 , further adapted to determine the scheme of the pulse width modulation.

15. The bearing state detector according to one of the claim 12 , wherein the detector is further connected to a first current sensor for measuring the first phase current and to second current sensor for measuring the second phase current.

16. The bearing state detector according to one of the claim 12 , wherein the detector is further adapted to receive information from a controller about the applied pulse width modulation scheme at the inverter or wherein the detector is adapted to command the controller to apply the predetermined pulse width modulation scheme at the inverter.

17. A vehicle comprising a motor for driving the vehicle and a detector for determining the state of a bearing according to one of the claim 12 .

18. A system for supplying energy to a three phase electric machine comprising a bearing, and a rotor, which is supported by the bearing, the electric machine being connected to a three phase inverter for supplying the electric machine with electric power, the three phase inverter being controlled to apply a pulse width modulation with a predefined scheme to create three phase currents, wherein the system comprises a bearing state detector adapted to:

determine a first phase current provided to the electric machine;

determine a second phase current provided to the electric machine, the second phase current being different to the first phase current;

determine the power spectral density of the parks vector of the current provided to the electric machine, based on the first phase current and the second phase current;

determine an amplitude of the power at a predefined frequency of the power spectral density, wherein the predefined frequency depends on the rotational speed of the rotor and the scheme of the pulse width modulation, wherein the predefined frequency is in the power spectral density at a frequency distance, which is equal the rotational speed, from a major peak of the power spectral density of the park's vector of the current provided to the electric machine;

wherein the system is further adapted to:

compare the amplitude of the power at the predefined frequency with a predefined power value; and

determine, based on the comparison, whether a failure of the bearing has occurred or the bearing is degraded.