Method for identifying rotor slot harmonics in motor current spectrum

Abstract

A method of identifying rotor slot harmonics (RSH) in a motor current spectrum of an AC induction motor includes receiving current data of a motor operating at two different load values, obtaining a motor current frequency spectrum, the frequency spectrum comprises a first motor current frequency spectrum corresponding to the first load value and a second motor current frequency spectrum corresponding to the second load value, and detecting the number of peak current values in the frequency range of the motor current frequency spectrum, and comparing the number of peak current values detected in the frequency range of the first motor current spectrum and the second motor current spectrum. A consecutive peak pair is determined from the number of peak current values for each of the detected first and second load values and compared. And when the comparison result is greater than a threshold value, identifying the continuous peak current value pair as RSH.

Description

Background Technology

[0001] An induction motor is an alternating current (AC) motor in which the current generating torque in the rotor is obtained through electromagnetic induction of the magnetic field of the stator windings. In an induction motor, the rotor slots (openings for placing conductors) cause harmonic components to appear in the stator current waveform. The generation of harmonics is due to the interaction between the stator and rotor magnetic fields and is related to the geometry of the rotor slots. Rotor slot harmonic components can be used to estimate the speed of the induction motor and to diagnose internal faults.

[0002] Rotor slot harmonics (RSHs) typically appear in the motor current spectrum. However, due to the presence of other frequency components unrelated to RSHs in the motor current spectrum, they are difficult to detect. Furthermore, RSHs may not even be present in the motor current spectrum. RSHs generally only exist under certain conditions, which depend on the number of poles and rotor slots. Therefore, a more accurate method is needed to identify rotor slot harmonics in the motor current spectrum. Summary of the Invention

[0003] A method for identifying rotor slot harmonics in the motor current spectrum of an induction motor is provided. The process described herein can identify rotor slot harmonics of an induction motor, which can be used to determine the rotor speed of the induction motor and diagnose faults in the induction motor.

[0004] A computer-implemented method for identifying rotor slot harmonics in the motor current spectrum of an AC induction motor includes: receiving current data of an AC induction motor operating under two different load values; acquiring a motor current frequency spectrum, the frequency spectrum including a first motor current spectrum corresponding to a first load value and a second motor current spectrum corresponding to a second load value; detecting the number of peak current values ​​within a frequency range of the motor current frequency spectrum; and comparing the number of peak current values ​​detected within the frequency range of the first motor current spectrum and the number of peak current values ​​detected within the frequency range of the second motor current spectrum. Comparing the number of peak current values ​​detected within the frequency range of the first and second motor current spectra includes determining consecutive peak current value pairs from the detected peak current value pairs, the consecutive peak current value pairs including a first peak current value for the first load value and a second peak current value for the second load value; calculating a difference between the first and second peak current values; comparing the difference with a threshold; and identifying the consecutive peak current value pairs as rotor slot harmonics in response to the difference being greater than zero and less than the threshold.

[0005] A method for determining a fault in an induction motor system includes: driving a load by an induction motor; measuring the current of the load driven by the induction motor; estimating the rotor speed of the induction motor; performing a first comparison by comparing the measured load current with a reference load current; performing a second comparison by comparing the estimated rotor speed of the induction motor with a reference rotor speed, wherein the reference rotor speed is estimated by identifying rotor slot harmonics; and determining a fault in the induction motor system based on the first comparison and the second comparison.

[0006] This invention provides a simplified overview of some concepts, which will be further elaborated in the detailed embodiments described below. This summary is not intended to identify key or essential features of the subject matter of the claims, nor is it intended to limit the scope of the claims. Attached Figure Description

[0007] Figure 1 The operating environment of an induction motor system according to one embodiment is shown.

[0008] Figure 2 The operating environment of an induction motor system according to another embodiment is shown.

[0009] Figure 3 A flowchart of a method for identifying rotor slot harmonics in the motor current spectrum of an induction motor is shown.

[0010] Figure 4 The motor current frequency spectrum of a 10 HP induction motor running using a DOL soft starter is shown.

[0011] Figure 5 The motor current spectrum of a 40 HP induction motor running using a DOL soft starter is shown.

[0012] Figure 6 A method is shown that describes the process of determining faults in an induction motor system using identified RSH.

[0013] Figure 7 A schematic diagram of computing device components is shown. Detailed Implementation

[0014] A method for identifying rotor slot harmonics (RSHs) in the motor current spectrum of an induction motor is provided. The process described herein allows for the identification of rotor slot harmonics in an induction motor, which can be used to determine the rotor speed and diagnose motor faults. The proposed method can be performed to identify the presence of RSHs in the current spectrum of an induction motor powered by a fixed frequency, such as a direct online (DOL) soft starter or a variable frequency drive. Furthermore, the proposed method can be used with any motor winding configuration, including star and delta configurations.

[0015] Currently, rotor speed estimation is accomplished using various techniques and methods. For example, rotor speed can be directly obtained using a tachometer coupled to the motor. However, using a tachometer requires proximity to the motor, which can be difficult, for example, when the motor is located in an enclosed enclosure or at a remote location. Furthermore, approximation methods, such as the linear approximation used to estimate rotor speed, are simple but not highly accurate. Other more complex techniques, while more accurate, are not useful under all conditions. This paper proposes an accurate and simple method for identifying rotor slot harmonics (RSHs) of an induction motor using motor current spectrum analysis. The identified RSHs can then be used to accurately estimate motor speed or diagnose motor faults.

[0016] Figure 1 The operating environment of an induction motor system according to one embodiment is illustrated. (Refer to...) Figure 1 The operating environment 100 includes a motor driver 110 and an induction motor 120. In the illustrated embodiment, the motor driver 110 is a variable frequency drive (VFD). The VFD 110 is coupled to a power grid 130, which provides a three-phase AC input to the VFD 110. The VFD 110 includes a power unit 140 and a control and monitoring unit 150. The VFD 110 receives the three-phase AC input, which is fed to the power unit 140. The power unit 140 converts the AC input to DC, inverts the DC, and regulates it into controlled AC for transmission to the AC induction motor 120. The VFD 110 also includes a control and monitoring unit 150, which receives current data from the induction motor 120. In one embodiment, such as Figure 1 As shown, the control and monitoring unit 150 is integrated into the VFD 110. In other embodiments, the control and monitoring unit 150 may be contained in an edge device coupled to the VFD, or located in a cloud environment. A user interface 160 may be included for communication between the control and monitoring unit 150 and the user.

[0017] Figure 2 An operating environment for implementing an induction motor control and monitoring unit according to another embodiment is shown. (Refer to...) Figure 2 The operating environment 200 includes a motor driver 210 (e.g., a DOL soft starter) and an induction motor 220. The DOL soft starter 210 is coupled to a power grid 230. The DOL soft starter 210 includes a starter 240 and a control and monitoring unit 250. The starter 240 transmits power between the power grid 230 and the induction motor 220. Similar to the VFD example described above, the DOL soft starter 210 also includes a control and monitoring unit 250, which receives current data from the induction motor 220. In one embodiment, such as Figure 2As shown, the control and monitoring unit 250 is integrated into the soft starter 210. In other embodiments, the control and monitoring unit 250 may be contained in an edge device coupled to the DOL soft starter 210, or located in a cloud environment. A user interface 260 may be included for communication between the control and monitoring unit 250 and the user.

[0018] Control and monitoring units 150 and 250 receive motor current data in the form of a current signal from the induction motor. The current data can be acquired from a current sensor positioned to receive a suitable current signal for motor current analysis. The received motor current data can be used to obtain the motor current spectrum by performing FFT (Fast Fourier Transform) or other frequency domain signal processing techniques on the received motor current data. The motor current spectrum graphically displays the individual spectral components of the current signal, providing frequency information of the current signal.

[0019] Identifying rotor slot harmonics in induction motors using mathematical methods is feasible; however, obtaining the necessary values ​​for calculation is challenging. To calculate rotor slot harmonics mathematically, it is first necessary to determine whether rotor slot harmonics exist in the motor current spectrum of the induction motor. All induction motors do not exhibit rotor slot harmonic components in their respective motor current spectra. Determining the presence of rotor slot harmonics in the motor current spectrum of an induction motor involves determining whether the following equation is satisfied:

[0020] (1)

[0021] Where N R p is the number of rotor bars. p Let α be the number of magnetic pole pairs, α = 1, 2, 3, ..., and = -1, 0, 1.

[0022] The motor current spectrum is related to the number of rotor blades. The RSH component in the motor current spectrum is:

[0023] (2).

[0024] Where, n d The dynamic eccentricity is R, where R is the number of rotor slots, and p is the number of rotor slots. p Let be the number of pole pairs, k be a constant, v be the harmonics 1, 3, 5, ..., f be the power supply frequency, and s be the motor slip. Under specific load conditions N1, with current i L1 Running an induction motor and generating a speed N1, it can be seen that the rotor slot harmonics in the motor current spectrum are related to the motor speed. In an induction motor, the rotor rotation speed is less than the speed of the stator rotating magnetic field, for example, the rotational speed. For example, using equation (2), in n d Under the conditions of 0, v=1, and K=1, the slot harmonic frequency can be estimated by the following equation:

[0025] ... (3).

[0026]

[0027]

[0028] ...(4).

[0029] Equation (4) shows that the sideband of a given load (such as N1) has a frequency difference of 2f. From equations (3) and (4), it can be concluded that for a given power supply frequency f, RSH changes with the increase of motor load and is therefore load-dependent.

[0030] The frequency difference between the peak values ​​of the motor current spectrum caused by different load conditions N1 and N2 can be calculated as follows:

[0032] (5).

[0033] Based on the conclusion that rotor slot harmonic frequencies are related to speed, a simplified method is proposed, wherein the only input required to execute the method is the current data obtained when the induction motor is running under two different load conditions (such as speed).

[0034] Figure 3 A flowchart illustrating a method for identifying rotor slot harmonics in the motor current spectrum of an induction motor is shown. (Refer to...) Figure 3 Method 300 begins by receiving current data (310) from two different load values ​​of the induction motor. The induction motor operates under two different load conditions, for example, current i L1 and i L2 This generates corresponding speeds N1 and N2, capturing current data for two different load values ​​in the process. The code for method 300 can be integrated into control and monitoring units 150 and 250.

[0035] Method 300 further includes acquiring a motor current frequency spectrum (320), which includes a first motor current spectrum corresponding to a first load value and a second motor current spectrum corresponding to a second load value. The motor current frequency spectrum can be obtained by performing an FFT or other frequency domain signal processing technique on current data of two different load values ​​received from the induction motor to generate a first motor current spectrum corresponding to the first load value and a second motor current spectrum corresponding to the second load value.

[0036] Method 300 further includes detecting the number of peak current values ​​within a frequency range of the motor current frequency spectrum (330). Detecting the number of peak current values ​​within a frequency range of the motor current frequency spectrum may include calculating an average value from multiple current values ​​within the frequency range and comparing each of the multiple current values ​​to the average value. A peak current value is detected when the compared current value is higher than a threshold.

[0037] For example, Figure 4 The motor current frequency spectrum of a 10 HP induction motor running using a DOL soft starter is shown. (Refer to...) Figure 4 Graph 400 shows various current values ​​displayed under four different load conditions within the illustrated frequency range. To filter meaningful values, such as peak currents, from the current values ​​on the motor current frequency spectrum within a specific frequency range in the graph, the average of all values ​​can be calculated. Values ​​close to the average, such as those below a threshold, can be removed, and only values ​​above the threshold are considered peak current values. Peak current values ​​are easily identified in... Figure 4 As seen in the graph, these values ​​are higher than other values ​​close to the average (and are indicated on the graph by their x and y values).

[0038] Figure 5 The motor current spectrum of a 40 HP induction motor operated by a DOL soft starter is shown. (Refer to...) Figure 5 Graph 500 shows the results of detecting the number of current peaks within a frequency range by removing meaningless current values ​​(such as noise). As can be seen from Graph 500, pairs of consecutive peak current values ​​are displayed.

[0039] The consecutive peak current value pairs can then be compared to determine whether the current peaks are rotor slot harmonics. Method 300 also includes determining consecutive peak current value pairs (340). These consecutive peak current value pairs are graphically adjacent to each other, for example, their frequency values ​​are close to each other. Each pair of consecutive current peaks includes a first current peak corresponding to a first load value from the first motor current spectrum and a second current peak corresponding to a second load value from the second motor current spectrum.

[0040] Method 300 further includes calculating the difference between the first current peak and the second current peak (350). This difference is then compared to a threshold (360). The threshold may depend on the number of rotor poles. In some cases, the threshold may be in the range of 1-20 Hz. When the peak current value is greater than zero and less than the threshold, the peak current value is identified as a rotor slot harmonic (370).

[0041] In some cases, once the peak current value is determined to be a rotor slot harmonic, verification calculations can be performed. Verification calculations may include determining the difference between two consecutive peak current values ​​for the same load value. Two consecutive peak current values ​​for the same load value include a first peak current value and a third peak current value for a first load value, or a second peak current value and a fourth peak current value for a second load value. When the difference between the consecutive peak current values ​​is twice the power supply frequency, the identified current peak can be verified as a rotor slot harmonic. For example, refer to... Figure 5 As can be seen, for load 2, there is a peak current value at 1253.4 and a continuous peak current value at 1373.4. The difference between these two peak current values ​​is 120Hz, which is twice the 60Hz power supply frequency.

[0042] In some cases, identified rotor slot harmonics (RSHs) can be stored in a computer-readable storage medium. The stored RSHs can be used later to estimate the induction motor speed or diagnose motor faults. In some cases, the stored RSHs can be used as a reference measurement to calculate / estimate variables indicating normal operation of the induction motor. In some cases, for example, the identified RSH components can later be used to determine the remaining useful life (RUL) of the induction motor. In some cases, the stored RSHs can be used to estimate a reference rotor speed N. rb Measure the rotor speed and compare it with the reference rotor speed N. rb The difference between the RSH frequency component and the reference measurement can also identify abnormalities in induction motors. For example, a change in the identified RSH frequency component indicates a change in the speed of the induction motor. This change in the RSH frequency component can indicate an abnormality in the induction motor relative to an induction motor operating under the same conditions. The greater the difference between the RSH frequency component and the reference measurement, the more severe the associated fault.

[0043] According to known methods, the rotor speed can be estimated using the identified rotor slot harmonics. The estimated rotor speed can be used to determine faults in induction motors or induction motor drive systems. For example, induction motors can be used to drive loads such as pumps or compressors, thus creating induction motor drive systems. Figure 6 A flowchart illustrating a method for determining induction motor system faults using identified RSH components is shown. For example, rotor slot harmonic components can be used to determine winding faults, rotor bar damage faults, cavitation faults, bearing faults, rotational faults, and pump blockage faults.

[0044] refer to Figure 6 Method 600 first measures the load current I. L (602). In this process, the reference load current I of an induction motor operating under the same power frequency and operating conditions with the same load is used. Lb and reference rotor speed Nrb These values ​​can be predetermined and stored in memory. Reference rotor speed N rb The rotor speed N can be estimated using method 300, which uses rotor slot harmonics identified from the measured reference load current. r The frequency information obtained from the motor current spectrum using conventional methods is used for estimation, for example, by using equation (2). The slip s can be derived from the frequency f, and then the slip s and synchronous speed N are used. s Calculate N r First, measure the load current I. L With reference load current I Lb Comparison (604). Furthermore, the baseline load speed N... rb With the estimated rotor speed N r A comparison is then made (the second comparison). These two comparisons can help determine the fault in the induction motor or induction motor system.

[0045] In some cases, cavitation or leakage faults in the drive load can be identified. Cavitation in a pump refers to the formation and collapse of bubbles in the pumped liquid due to insufficient pressure. Cavitation and leakage can be identified in all types of pumps (such as centrifugal pumps, displacement pumps, etc.). Air leakage can be identified in all types of compressors. Under the same power frequency and operating conditions, when the measured load current I... L Less than the reference load current I Lb (608) and the rotor speed Nr is greater than the reference load speed N rb (612) can then be used to determine the cavitation / leakage fault (614). If the load current I L Equal to the reference load current I Lb If the rotor speed N is normal, then the system is operating normally (610). r Not greater than the reference rotor speed N rb If so, the cavitation / load fault (616) is not determined.

[0046] In some cases, under the same power supply frequency and operating conditions, the load current I L Greater than the reference load current I Lb (606) It can indicate a winding fault (618) or a rotor bar damage fault (622). After the first comparison indicates a fault (606), a technician can use existing methods to determine the winding fault or rotor bar damage fault (620, 624).

[0047] After confirming that there are no winding faults or rotor bar damage, the pump blockage or rotation system fault can be determined. Under the same power frequency and operating conditions, when the load current I... L Greater than the reference load current I Lb(606), and the rotor speed N r Less than the reference rotor speed N rb (626) If the rotor speed is N, then a rotary / pump blockage fault can be identified (628). Rotary faults may include bearing problems, such as roughness issues or excessive bearing wear. r If the speed is greater than the reference rotor speed, it can be determined that there is no fault related to motor bearings, motor rotation system, or pump blockage (630).

[0048] Figure 7 A schematic diagram is shown to illustrate components of a computing device that can be used in some embodiments described herein. This computing device may represent the control and monitoring unit described herein. (Refer to...) Figure 7 The computing device 700 can represent a personal computer, reader, mobile device, personal digital assistant, wearable computer, smartphone, tablet, laptop, gaming device or console, entertainment device, hybrid computer, desktop computer, or smart TV. Therefore, a specific computing device can be implemented using more or fewer descriptive elements related to the computing device 700.

[0049] Computing device 700 may include at least one processor 710, memory 720, software 730 including an operating system 740 and an application program 750, a network interface 760, and a user interface 770. The processor 710 processes data according to instructions from the software 730. Instructions for the application program 750 may be loaded into the computing device 700 and run on or in association with the operating system 740. The application program 750 may include the methods described above. The memory 720 may include any computer-readable storage medium readable by the processor 710 and is capable of storing software including the application program 750.

[0050] The computing device 700 may also include a user interface 770, which may include input / output (I / O) devices and components that enable communication between the user and the computing device 700. The computing device 700 may also include a network interface 760, allowing the system to communicate with other computing devices (including server computing devices and other client devices) via a network.

[0051] Although the subject matter has been described in language specific to structural features and / or behavior, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or behaviors described above. Rather, the specific features and behaviors described above are disclosed as examples of implementing the claims, and other equivalent features and behaviors that can be recognized by a person skilled in the art are also within the scope of the claims.

Claims

1. A computer-based method for identifying rotor slot harmonics in the motor current spectrum of an AC induction motor, comprising: Receive current data of the AC induction motor operating under two different load values; Obtain the motor current frequency spectrum, which includes the first motor current spectrum corresponding to the first load value and the second motor current spectrum corresponding to the second load value; The number of peak current values ​​within the frequency range of the motor current frequency spectrum is detected; as well as The number of detected peak current values ​​within the frequency range of the first motor current spectrum and the second motor current spectrum is compared using the following steps: Determine consecutive peak current value pairs from the number of detected peak current values, the consecutive peak current value pairs including a first peak current value of the first load value and a second peak current value of the second load value; Calculate the difference between the first peak current value and the second peak current value; The difference is compared with a threshold. as well as In response to the difference being greater than zero and less than the threshold, the consecutive peak current value pairs are identified as rotor slot harmonics.

2. The method of claim 1, further comprising estimating the speed of the motor using the identified rotor slot harmonics.

3. The method of claim 2 further includes using the identified rotor slot harmonics to diagnose faults.

4. The method according to claim 1, wherein the first load value is a first current i L1 And the second load value is the second current i L2 .

5. The method according to claim 1, further comprising determining, before obtaining the motor current frequency spectrum, that the rotor slot harmonics exist in the motor current frequency spectrum of the AC induction motor.

6. The method of claim 5, wherein determining that the rotor slot harmonics exist in the motor current spectrum of the induction motor includes determining that the equation satisfies... , where N R It is the number of rotors, p p It is the number of magnetic pole pairs, α = 1, 2, 3, ..., and = -1, 0, 1.

7. The method of claim 1, wherein obtaining the current frequency spectrum of the two different load values ​​comprises performing an FFT on the received current data of the induction motor operating under the two different load values ​​to generate a first motor current spectrum corresponding to the first load value and a second motor current spectrum corresponding to the second load value.

8. The method according to claim 1, wherein detecting the number of peak current values ​​within the frequency range of the motor current frequency spectrum includes: Calculate the average value from multiple current values ​​within the frequency range; Each of the plurality of current values ​​is compared with the average value; as well as In response to the comparison where the current value is higher than a threshold, the current value is detected as a peak current value.

9. The method of claim 1 further includes storing the identified rotor slot harmonics as a reference measurement in a memory of a computer-readable storage medium.

10. The method of claim 1, further comprising verifying that the identified peak current value is a rotor slot harmonic by determining a difference between two consecutive peak current values ​​from the same load value, wherein the two consecutive peak current values ​​from the same load value include a first peak current value and a third peak current value of the first load value or a second peak current value and a fourth peak current value of the second load value, and verifying that the identified current peak value is a rotor slot harmonic in response to the difference between the consecutive peak current values ​​being twice the power supply frequency.

11. The method of claim 1, wherein the threshold ranges from 1 to 20 Hz.

12. The method of claim 1, wherein the motor driver of the AC induction motor is coupled to one of the frequency converter and the DOL soft starter.

13. A method for determining a fault in an induction motor system, comprising: The load is driven by an induction motor; Measure the current of the load driven by the induction motor; Estimate the rotor speed of the induction motor; The first comparison is performed by comparing the measured current of the load with a reference load current; A second comparison is performed by comparing the estimated rotor speed of the induction motor with a reference rotor speed, wherein the reference rotor speed is estimated using identified rotor slot harmonics; as well as The fault of the induction motor system is determined based on the first comparison and the second comparison.

14. The method of claim 13, wherein the rotor slot harmonics are identified by: Receive current data from an induction motor operating under two different load values; Obtain the motor current frequency spectrum, which includes the first motor current spectrum corresponding to the first load value and the second motor current spectrum corresponding to the second load value; The number of peak current values ​​within the frequency range of the motor current frequency spectrum is detected; as well as The number of detected peak current values ​​within the frequency range of the first motor current spectrum and the second motor current spectrum is compared using the following steps: Determine consecutive peak current value pairs from the number of detected peak current values, the consecutive peak current value pairs including a first peak current value of the first load value and a second peak current value of the second load value; Calculate the difference between the first peak current value and the second peak current value; The difference is compared with a threshold. as well as In response to the difference being greater than zero and less than the threshold, the consecutive peak current value pairs are identified as rotor slot harmonics.

15. The method of claim 13, wherein determining a fault in the induction motor system comprises: A cavitation / leakage fault is determined in response to the measured current of the load being less than the reference load current from the first comparison, and the estimated rotor speed of the induction motor being greater than the reference rotor speed from the second comparison.

16. The method of claim 13, wherein determining a fault in the induction motor comprises: In response to the measured current of the load being greater than the reference load current from the first comparison, and the estimated rotor speed of the induction motor being less than the reference rotor speed from the second comparison, a rotation / blockage fault is determined.

17. The method of claim 13, wherein determining a fault in the induction motor system comprises: In response to the measured current of the load being greater than the reference load current from the first comparison, a winding fault test is performed to determine a winding fault.

18. The method of claim 13, wherein determining a fault in the induction motor system comprises: In response to the measured current of the load being greater than the reference load current from the first comparison, a rotor bar damage fault test is performed to determine rotor bar damage faults.

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