METHOD AND SYSTEM FOR MONITORING A MACHINE STATE

Abstract

A method for monitoring the state of an electric machine includes determining within a defined frequency range, for example in a spectrogram of a current amplitude, a frequency position where a current amplitude has a maximum at the frequency position and where a phase relationship between a current vector and a voltage vector or between two current vectors is located in a predefined interval. The determined frequency position is characteristic; of the state of the electric machine.

Claims

1.-13. (canceled)

14. A method for monitoring a state of an electric machine, comprising: determining within a defined frequency range a frequency position that is characteristic of a state of the electric machine; and determining at the frequency position a current amplitude having a maximum value and a phase relationship between a current vector and a voltage vector or between two current vectors located in a predefined interval.

15. The method of claim 14, further comprising determining, within a plurality of different predefined frequency ranges, for each range a frequency position that is characteristic of different states of the electric machine.

16. The method of claim 15, wherein the different predefined frequency ranges are non-overlapping.

17. The method of claim 14, wherein the electric machine is a three-phase machine and the state of the machine is a fault condition or an operating state.

18. The method of claim 17, wherein the three-phase machine is an asynchronous machine and the frequency range is determined for a slip range between 0 and a breakdown slip.

19. The method of claim 18, wherein the slip range is between 5% and 10%.

20. The method of claim 17, wherein the three-phase machine is a synchronous machine and the state of the machine is a fault condition.

21. The method of claim 14, wherein the phase relationship is a phase relationship between an a current vector and a β current vector.

22. The method of claim 14, wherein the predefined interval is an interval between 40° and 90°, in particular between 40° and 60° , or between 70° and 90°.

23. The method of claim 14, wherein the predefined interval is an interval between 40° and 60°.

24. The method of claim 14, wherein the predefined interval is an interval between 70° and 90°.

25. The method of claim 14, wherein the phase relationship is determined from an admittance or an impedance.

26. The method of claim 14, further comprising measuring the current amplitude over a predefined measurement time of between approx. 0.1 second and 10 seconds.

27. The method of claim 14, further comprising measuring the current amplitude over a predefined measurement time of between approx. 1 second and 10 seconds.

28. The method of claim 14, further comprising measuring the current amplitude over a predefined measurement time of between approx. 1 second and 5 seconds.

29. The method of claim 14, further comprising measuring the current amplitude over a predefined measurement time of 1 second.

30. A computer program code stored on a non-transitory computer-readable medium and comprising commands which, when read into a memory of a computer and executed by a processor of the computer, cause the computer to perform a method as set forth in claim 14.

31. A system for monitoring a state of an electric machine, comprising a computing unit with a processor which executes a computer program code as as set forth in claim 30.

32. The system of claim 31, further comprising a measurement unit configured to measure a current or a voltage of a three-phase machine.

33. A data carrier signal, configured to transmit a computer program code as set forth in claim 30.

Description

[0031] The invention is described and explained in more detail below with reference to the exemplary embodiments illustrated in the figures, in which:

[0032] FIG 1 shows a flowchart of a computer-implemented method for monitoring the state of an electric machine,

[0033] FIG. 2 shows a detail from a spectrogram,

[0034] FIG. 3 shows a phase relationship between a current vector and a voltage vector,

[0035] FIG. 4 shows phase positions determined with and without taking the phase relationship of FIG. 3 into account,

[0036] FIG. 5 shows a phase relationship between an α and a β current vector,

[0037] FIG. 6 shows phase positions determined with and without taking the phase relationship of FIG. 5 into account, and

[0038] FIG. 7 shows a state/condition monitoring system for an asynchronous motor.

[0039] FIG. 1 shows a flowchart of a computer-implemented method for monitoring the state or condition of an electric machine, the computer-implemented method corresponding to the method according to the invention.

[0040] In a step S1 of the method, a spectrogram I(f,t) of a current amplitude can be generated (on the basis of the measured current values). An exemplary detail (frequencies between approx. 410 Hz and 450 Hz) of the spectrogram 100 is shown in FIG. 2. The detail shows a defined frequency range (f1, f2)—in this case between approx. 410 Hz (f1) and approx. 450 Hz (f2). With three-phase machines, in particular in the case of asynchronous machines, the defined frequency range (f1, f2) can comprise one of the known damage frequencies, such as e.g. a rotor bar breakage frequency, which were determined previously by means of a motor current signature analysis (MCSA) method according to the prior art.

[0041] In a step S2 of the method, a phase relationship (in degrees) is calculated for example between a current vector and a (previously measured) voltage vector (I and U). The calculated phase angle P.sub.UI(f,t) between the current vector and the voltage vector for the spectrogram 100 in a frequency range (f1, f2) between approx. 350 Hz (f1′) and approx. 450 Hz (f2) is apparent from FIG. 3. The phase relationship P.sub.UI(f,t) can be determined for example from or on the basis of or using admittance or impedance.

[0042] A current and/or voltage measurement which serves to generate the spectrogram 100 and/or the phase relationship can be conducted over a predefined measurement time, A current and/or voltage measurement which has a measurement time per measurement of approx. 0.1 second to 10 seconds, in particular one second, is particularly advantageous. This allows a dynamic evaluation of operating states and/or fault conditions. In this case the measurement time is shorter than the measurement time for a typical motor current signature analysis (MCSA), in which the measurement time lies in the region of approx. 30 seconds.

[0043] In a step S3 of the method, a frequency position f.sub.L within the defined frequency range (f1, f2) (cf. the spectrogram 100 or 101) is determined in such a way that, at the frequency position f.sub.L, the current amplitude I(f,t) is at a maximum and the phase relationship P.sub.UI(f,t) lies in a predefined interval.

[0044] The frequency position f.sub.L can correspond for example to a principal slot harmonics (PSH) frequency of a three-phase asynchronous machine, the frequency f.sub.PSH(7n) being for example approx. 427 Hz.

[0045] The PSH frequencies are a function of the number of rotor bars R, the number of pole pairs p and the slip s (f.sub.PSH=func(R,p,s)). If a PSH frequency is now determined more precisely/more reliably, the slip, for example, can also be determined more precisely/more reliably.

[0046] In particular in the case of three-phase machines, the frequency range (f1, f2) can be given by a slip range between 0 and breakdown slip, in particular by a slip range between approx. 5% and approx. 10% (a slip range can be converted into a frequency range). The slip range between approx. 5% and approx. 10% contains typical slip values for asynchronous machines with power ratings between approx. 5 KW and approx. 30 KW.

[0047] In other words, when the current amplitude I(f,t) is being determined, a phase filter is applied in order to distinguish between actually measured currents and fault-induced current signature amplitudes which can result due to interferences/noise caused for example by grid voltage components of other power-consuming loads in the power supply grid, and to exclude the fault-induced current signature amplitudes.

[0048] The predefined interval can comprise for example angles between approx. 40° and approx. 90°. In the example of the method shown here (FIGS. 2 to 4), in which the phase relationship P.sub.UI between a current vector and a voltage vector is determined, the phase filter can be set for angles between approx. 70° and approx. 90°, for example between approx. 80° and approx. 90 . Generally, the interval should be chosen such that the angles included in the interval describe a phase which is possible between the actually measured current vectors and voltage vectors or which makes sense physically.

[0049] In the case of the aforementioned fault-induced current signature amplitudes, which are roughly equal in magnitude to the measured current amplitudes, the phase relationship in the phase image shown in FIG. 3 lies in the range of approx. 20° to 30°, with the result that the fault-induced current signature amplitudes are filtered out by means of the phase filter.

[0050] The determined frequency position f.sub.L is characteristic of a state of the electric machine or, as the case may be, the determined frequency position f.sub.L is assigned to a (specific) state of the electric machine.

[0051] Once the state of the machine has been determined on the basis of the identified frequency position f.sub.L, the machine can be controlled accordingly. For example, should it be detected that the state of the machine is critical due to a broken rotor bar, the machine can be switched off. Should the state of the machine still be acceptable but it is detected that the critical state is likely imminent, a corresponding warning message can be output, for example.

[0052] In the case of three-phase machines, the frequency position f.sub.L can be assigned for example to one of the following fault conditions: air gap eccentricity, rotor bar breakage (in the case of asynchronous machines), bearing breakage/failure, stator winding fault.

[0053] Furthermore, a load state of a three-phase machine can be assigned to the frequency position f.sub.L.

[0054] FIG. 4 shows a spectrogram 101 on which the determined frequency position f.sub.L is plotted. FIG. 4 illustrates an improvement in slip detection 102 (crosses) compared to the conventional evaluation 103 (dashed line) in a permitted slip range, for example between 0 and breakdown slip, preferably between 5% and 10%, which is affected by interference frequencies. This improvement is achieved by adding the aforementioned phase information during the determination of the frequency position.

[0055] FIG. 5 shows a further possible phase relationship P.sub.αβ(f,t), which can be calculated during the aforementioned step S2 of the method. The phase angle P.sub.αβ(f,t) shown in FIG. 5 is a phase angle between two phase currents I.sub.αand f.sub.β, which can be obtained from three phase currents I.sub.U, I.sub.V, I.sub.W, of a three-phase machine by means of a Clarke transformation.

[0056] The phase relationship P.sub.αβ(f,t) can be calculated for example in a frequency range (f3, f4) between approx. 1140 Hz (f3) and approx. 1151 Hz (f4).

[0057] The frequency position f′.sub.L corresponds to a PSH frequency (7n). The predefined interval (of the phase filters) comprises angles between approx. 40° and approx. 60°.

[0058] FIG. 6 shows the determined frequency position f′.sub.L. Also to be seen in FIG. 6 is a slip 104 (crosses) determined on the basis of the identified frequency position f′.sub.L compared to the conventional evaluation 105 (dashed line) in a permitted slip range which is affected by interference frequencies that are attributable for example to existing sidebands 106 or voltage changes.

[0059] It is also clear from FIG. 6 that the identified frequency position f′.sub.L also provides information about a load state L1, L2, L3 and allows this to be determined very much more precisely.

[0060] The above-described method can also be performed within a plurality of different, preferably non-overlapping, predefined frequency ranges. Here, a frequency position can be identified in each frequency range in each case, wherein different frequency positions or lines can be characteristic of different states of the electric machine.

[0061] It is furthermore apparent from the overall view provided by FIG. 5 and FIG. 6 how the predefined interval is used in order to select the right frequency position. One frequency position (the frequency line of interest=the “main line”) for the description of a state (the approximate position of which can be determined e.g. with the aid of a conventional MCSA evaluation) has a well-defined absolute phase position (in FIG. 5 e.g. 60-80°, dark), whereas interference ones, e.g. sideband lines (clearly visible in FIG. 6), have a phase position that deviates therefrom (in FIG. 5 0-20°, bright).

[0062] The selection of a “predefined” phase interval in a frequency band around the main line can therefore be made in such a way that the ability to differentiate phase positions from interfering lines is made possible, is preferably increased, in particular is maximized,

[0063] The (phase) filter can be realized e.g. in such a way that Y(f,t)=X(f,t)*phase filter(f,t),

where phase filter(f,t)=0 if P(f,t) is outside, phase filter (f,t)=1 if P(f,t) is inside the predefined interval. This enables interferences in the amplitude to be filtered out, the equation Y(f,t)=X(f,t) remaining unchanged where the phase position of the main line lies.

[0064] FIG. 7 shows a system 1 for monitoring the state of an electric machine which is embodied for example as a three-phase U, V, W asynchronous machine 2. The system 1 comprises a measurement unit 3 for measuring currents and/or voltages at the three-phase asynchronous machine 2 and a computing unit 4. The computing unit 4 has a computer program 40. The computer program 40 can be resident on a computer-readable volatile or non-volatile medium of the computing unit 4.

[0065] The computer program 40 can comprise two modules 41, 42, wherein the first module 41 can comprise instructions which, when the first module 41 is executed by the computing units 4, cause the latter to evaluate spectral amplitudes on known damage frequencies for example by means of Fourier or wavelet transform. The second module 42 can in this case comprise instructions which, when the second module 42 is executed by the computing units 4, cause the latter to identify a frequency position f.sub.L, f′.sub.L in accordance with the above-cited method steps S1 to S3 and preferably determine the slip and/or the load state of the asynchronous machine 2.

[0066] Each of the modules 41 and 42 can also be embodied as a computer program. In this case it can be beneficial to make a corresponding spectrogram 100 available to the computer program 42.

[0067] Although the invention has been illustrated and described in greater detail on the basis of exemplary embodiments, the invention is not limited by the disclosed examples. Variations hereof may be derived by the person skilled in the art without leaving the scope of protection of the invention as defined by the following claims. In particular, the features described in connection with the method can also find application in the system or, as the case may be, complete the latter, and vice versa.