Method for determining non-harmonic interference frequency of asynchronous machine, control and / or diagnostic device of asynchronous machine, and computer program product

Abstract

The invention relates to a method for determining a non-harmonic interference frequency of an asynchronous machine, a control and / or diagnostic device of an asynchronous machine and a computer program product. The invention relates to a method for determining a non-harmonic interference frequency of an asynchronous machine connected to a power supply network. In this case (210) a frequency band above the grid frequency fNetz of the three-phase grid is determined or specified, in which non-harmonic interference frequencies are expected to occur. Further, (220) generates N sampled values of the current flowing between the asynchronous machine and the power supply network over a plurality of cycles of the grid frequency at a data rate fs, and performs a first fast Fourier transform Y1 (230) with the N1 sampled values, where N1 < N. Further, a second fast Fourier transform Y2 is performed with the N1 sample values (240), where a first sample value for the second fast Fourier transform is a later sample value than a first sample value for the first fast Fourier transform. Calculating interference frequency f (250): # imgabs0 #

Description

Technical Field

[0001] The invention relates to a method for determining a non-harmonic interference frequency of an asynchronous machine, a control and / or diagnostic device for an asynchronous machine, and a computer program product. Background Art

[0002] In asynchronous machines, non-harmonic interference frequencies occur during operation, more specifically currents with a higher frequency than the grid frequency, which can be noticed as non-harmonic spectral components in the supply grid.

[0003] These disturbances are caused, for example, by the fact that the speed of the asynchronous machine lags behind the grid frequency during loaded operation. This characteristic is also called slip. The slip depends on the load of the machine and is usually less than 10% but greater than zero during rated load operation. Since the rotor of the asynchronous machine rotates slower than the grid frequency, the rotor bars and mechanical inaccuracies induce high-frequency currents in the windings, which appear in the feed network as the above-mentioned non-harmonic spectral components. Summary of the invention

[0004] It is therefore an object of the present invention to specify a method by which non-harmonic interference frequencies of an asynchronous machine can be determined.

[0005] This object is achieved by a method having the features of independent claim 1 , by a control and / or diagnostic device for an asynchronous machine having means for implementing the method according to the invention, and by a corresponding computer program product.

[0006] Advantageous developments of the invention are specified in the dependent claims.

[0007] One advantage of the present invention is that it enables accurate detection of non-harmonic interference frequencies with low computational overhead and is therefore particularly suitable for use in conjunction with devices having control and / or diagnostic devices with low computational performance. In particular, the present invention is suitable for use in situations where the computational performance (and low storage capacity) is insufficient to perform the current signature analysis.

[0008] Precise knowledge of the interference frequency(s) in turn allows conclusions to be drawn about the operating state of the asynchronous machine. For example, specific interference frequencies and / or their profiles can be used as input parameters (vectors) in so-called predictive maintenance. With the invention, different interference frequencies can be evaluated with the same calculation mechanism simply by carrying out the method several times in succession with different frequency bands.

[0009] In a preferred embodiment, the rotational speed of the asynchronous machine is determined from a specific interference frequency. Preferably, for this purpose, a non-harmonic interference frequency having a maximum spectral content in the range between 14 and 15 times the frequency of the supply network is used.

[0010] The invention advantageously does not require any special sensor arrangement, which is particularly important when determining the rotational speed, since the measuring means usually used for determining the rotational speed, such as light barriers, microphones or Hall sensors, can be omitted. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. It should be noted that the embodiments described in detail here can be combined with each other at will.

[0012] in:

[0013] Figure 1 shows an exemplary spectrum analysis of an asynchronous motor under different load conditions; and

[0014] Figure 2 A simplified flow chart showing an embodiment of the method according to the invention is shown. DETAILED DESCRIPTION

[0015] Figure 1 An exemplary spectrum analysis for four different load cases of an asynchronous machine is shown, wherein for better presentation, only the frequency range of 700 Hz to 800 Hz (corresponding to 14 to 16 times the 50 Hz grid frequency of the asynchronous machine feed network) is shown. Of course, the invention is also suitable for the precise determination of non-harmonic interference frequencies in other frequency ranges. The frequency range selected here by way of example, corresponding to 14 to 16 times the grid frequency and advantageously further restricted to 14 to 15 times the grid frequency, is particularly suitable for embodiments of the invention in which the rotational speed is calculated from the determined interference frequencies in an additional step.

[0016] In regions where the grid frequency is 60 Hz, the method according to the invention is of course also applicable. A frequency range of particular interest is then between 840 Hz and 960 Hz, and in particular between 840 Hz and 900 Hz.

[0017] As mentioned earlier, Figure 1In the same graph, an excerpt from the frequency spectrum of the current flowing between an asynchronous machine and its feeder network is shown for four load cases. In addition to different smaller local maxima or peaks which are less important for the invention, local maxima or peaks stand out for each load case, which are marked M1, M2, M3, M4, wherein the maximum value M1 associated with the first load case is at about 725 Hz, the maximum value M2 associated with the second load case is at about 735 Hz, the maximum value M3 associated with the third load case is at about 740 Hz, and the maximum value M4 associated with the fourth load case is at about 749 Hz.

[0018] As mentioned above, the slip increases with increasing machine load. This is manifested in the spectral analysis by the deviation of certain maximum values ​​from integer multiples of the grid frequency, where Figure 1 15 times the grid frequency deviating from 50 Hz is shown as an example. Figure 1 In the first load case shown in with the associated maximum value M1 , the deviation or lag relative to the grid frequency multiple is greatest and therefore in this load case the load is highest, whereas in the fourth load case with the associated maximum value M4 the deviation is smallest and therefore the load is lowest.

[0019] With the help of the next combination Figure 2 The described method allows these non-harmonic interference frequencies to be determined very accurately with low computational effort. Without limiting the generality, we consider Figure 1 The frequency range of 700 Hz to 750 Hz shown in , ie between 14 and 15 times the grid frequency. In addition, in the following, likewise without limiting the generality, a first load case is considered in which the maximum value M1 is approximately 725 Hz.

[0020] In a first step 210, the frequency band to be investigated is defined or determined. For example, the frequency band to be investigated can be determined by means of a spectrum analysis, in which a larger spectrum with a lower resolution is investigated and frequency bands are selected in which the non-harmonic components are particularly concise, i.e., whose amplitude is particularly high relative to the remaining frequency components, such as Figure 1 This is the case for the peak values ​​M1-M4 in the corresponding load case. Alternatively, the frequency band can be specified by the operator of the device by means of an input at a corresponding input device. In another alternative, the frequency band is specified based on the characteristics of asynchronous machines generally known to the person skilled in the art and is stored in a non-volatile memory, for example, in the form of upper and lower limits of the frequency band. In the example considered here, as mentioned above, a frequency band of 700 Hz to 750 Hz is selected and specified, i.e. between 14 and 15 times the grid frequency.

[0021] The spectrum analysis is performed within this frequency band. Here, the nominal frequency to be studied is defined as the grid frequency fNetz k times that of, and hereinafter referred to as the frequency multiple f p , where applicable:

[0022] f p = kf Netz

[0023] In a preferred embodiment, k = 15 and f p = 750 Hz.

[0024] Spectrum analysis is preferably carried out by means of a first fast Fourier transform Y1 on N1 sampled values of the current flowing between the asynchronous motor and the power supply network, where these sampled values are determined at a sufficiently high data rate. Preferably, in step 220, these data are determined over a plurality of power grid cycles, and in a preferred embodiment, at least three and preferably five power grid cycles are selected. Here, as is well known to those skilled in the art, the data rate must be at least twice the maximum frequency to be studied, i.e., 2*f p , which in this case is at least 1,500 data values per second. A data rate of 4,000 data values per second, which can be easily achieved using a common and inexpensive sampling circuit and can be used for the present invention, and a slightly more complex device can also easily provide 12,000 data values per second.

[0025] From the number of power grid cycles l (preferably l = 5) specified for data acquisition and the data rate f s (preferably f s = 4,000 / s), the number N of available sampled values can be obtained:

[0026]

[0027] Using the preferred values, N = 400 is thus obtained.

[0028] From these values, N1 < N values are used in step 230 to calculate the first fast Fourier transform Y1. Here, the sampled values are preferably also weighted by means of a window function, for example by means of a Hanning window:

[0029] Weighting.

[0030] From this, it follows that:

[0031]

[0032] Subsequently, in step 240, a second fast Fourier transform Y2 is calculated from N1 sampled values. Here, a later sampled value m is used as the starting value than for Y1. In the above formula for Y1, it is assumed without limitation that the calculation of Y1 starts from subscript n = 0. Then, m > 0 applies, and the preferred value is m = 1 and the calculation of Y2 is as follows:

[0033]

[0034] In a preferred embodiment, N1=Nm applies, whereby the generated N values ​​are optimally utilized for the method. Of course, however, more N values ​​than the N1+m values ​​required for the method can also be generated, for example, when the generation of data values ​​for the current flowing between the asynchronous machine and the power supply network is carried out continuously for other calculation or monitoring tasks.

[0035] In step 250 , the sought interference frequency f is calculated from the angular difference of the two fast Fourier transformations Y1 and Y2 and the sampling frequency:

[0036]

[0037] exist Figure 1 In the first load case shown in , this value may be approximately 725 Hz. This value can be displayed to the user in step 255 and / or transmitted to a computing device for further processing. Additionally or alternatively, the speed of the asynchronous machine (in revolutions per minute) can be calculated from the interference frequency f in step 260 as follows:

[0038]

[0039] Where p is the number of poles of the asynchronous motor. p The ratio is the slip rate.

[0040] The speed can then be displayed on a suitable display device and / or transmitted to a computing device for further processing in step 265. Since the speed and / or slip are important operating parameters, it is proposed in this application to execute the method continuously starting from step 220, if necessary with a delay 270 in order not to overload the computing unit. In an alternative embodiment, the method ends with step 250 / 255 or step 260 / 265 and is restarted at the request of the operator.

[0041] Unlike the above-described preferred frequency band between 700 Hz and 750 Hz, of course any frequency band occurring during the operation of the asynchronous machine and thus any (non-)harmonic spectral components can also be investigated and allow conclusions to be drawn regarding specific parameters or disturbances.

[0042] The above-described method can be completely executed by a motor controller or an evaluation and / or diagnostic unit (eg a microcontroller) coupled to the asynchronous motor and the results can be transmitted to a display device via a wireless or wired interface.

[0043] It should be noted that the term "control and / or diagnostic device" used herein includes processors and processing units in a broad sense, such as general-purpose processors, digital signal processors, application-specific integrated circuits (ASICs), programmable logic circuits such as FPGAs, discrete analog or digital circuits, and any combination thereof, including all other processing units known to those skilled in the art or developed in the future. The processor may be composed of one or more devices. If the processor is composed of multiple devices, these devices may be configured to process instructions in parallel or sequentially. The instructions required to execute the method may be present in a fixed or volatile memory and may be available by means of a storage medium or by downloading through a network system.

Claims

1. A method for determining the non-harmonic interference frequency of an asynchronous motor connected to a power supply network, comprising the following steps: a) Determine or specify a grid frequency f higher than the three-phase grid frequency Netz a frequency band in which non-harmonic interference frequencies (210) are expected to appear; b) At data rate f s Generating N sample values ​​(220) of the current flowing between the asynchronous motor and the power supply network over a plurality of cycles of the grid frequency; c) Performing a first fast Fourier transform Y1 (230) using N1 sample values, where N1 <N; d) performing a second fast Fourier transform Y2 (240) using N1 sample values, wherein a first sample value used for the second fast Fourier transform is a later sample value than a first sample value used for the first fast Fourier transform; e) Calculate the interference frequency f(250) according to the following formula:

2. The method according to claim 1, wherein f) The speed of the asynchronous motor in revolutions per minute is determined from the interference frequency f, the grid frequency f Netz , the number of poles p of the asynchronous motor and the grid frequency f in or above the frequency band Netz The frequency multiple f p With the help of the following formula: to be sure. 3 . The method according to claim 1 , wherein during the calculation of the first and / or second fast Fourier transform, the sampled values ​​are filtered by means of a window filter function, in particular by means of a Hanning window. 4 . The method according to claim 1 , wherein the N current sampling values ​​are generated over at least three, preferably five, periods of the grid frequency.

5. The method according to any one of the preceding claims, wherein a frequency band is selected whose lower limit is greater than or equal to 14 times the grid frequency and whose upper limit is less than or equal to 16 times the grid frequency, preferably less than or equal to 15 times the grid frequency, and wherein the frequency multiple f p Equal to 15 times the grid frequency.

6. The method according to claim 5, wherein the data rate f s At least 2*f per second p A sample value.

7. A control and / or diagnostic device for an asynchronous machine having means suitable for carrying out the method according to any of the preceding claims.

8. A control and / or diagnostic device for an asynchronous machine, comprising means for displaying an interference frequency f and / or a rotational speed of the asynchronous machine.

9. A computer program product comprising instructions which, when executed by a processor, cause the processor to perform the method according to any one of claims 1 to 6.

10. A computer-readable medium comprising instructions which, when executed by a processor, cause the processor to perform the method according to any one of claims 1 to 6.

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