Method of detecting broken bars of an induction motor and device implementing the method
By measuring the Fourier transform of the stator winding current signal and detecting rotor slip, the problem of detecting broken bars in induction motors under sensorless control was solved, achieving economical and efficient fault early warning and reducing maintenance costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SCHNEIDER TOSHIBA INVERTER EUROPE SAS
- Filing Date
- 2025-12-01
- Publication Date
- 2026-06-09
AI Technical Summary
Existing technologies struggle to effectively detect broken bars in induction motors without sensor control, leading to the risk of fault propagation and high maintenance costs and economic losses.
By measuring the current signal of the stator winding, calculating the Fourier transform and rotor slip, setting a search window, detecting whether the frequency and amplitude of the first and second peaks meet the conditions, and generating a signal indicating broken bars.
It enables inexpensive and easy-to-implement broken bar detection, reduces the risk of fault propagation and maintenance costs, and improves the reliability of induction motors.
Smart Images
Figure CN122172002A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to signal processing in the context of sensorless control of induction motors. Background Technology
[0002] Induction motors are typically powered via a variable speed drive connected to the induction motor. Classical voltage / frequency control laws are increasingly being replaced by sensorless control laws, which control the speed and torque of the motor without any mechanical speed or position sensors other than the variable speed drive.
[0003] In the context of this invention, "sensorless" does not mean the complete absence of sensors, but rather the absence of some sensors, such as rotor speed or position sensors. However, this typically relies on the measurement of motor current (or potentially motor voltage).
[0004] Induction motors are widely used in industry and are essential for industrial processes. Failures in induction motors not only lead to high repair costs but also cause significant economic losses due to unexpected downtime. Some induction motor failures are caused by cracks in the squirrel cage or broken rotor bars. Once the rotor bars are cracked or broken in the squirrel cage, the cage is usually irreparable. Squirrel cage failures can also cause shaft vibration, leading to bearing failure and air gap eccentricity. Therefore, early detection of broken rotor bars is essential not only for rotor protection but also for reducing other types of motor failures. Summary of the Invention
[0005] The purpose of this disclosure is to provide a method for detecting broken or fractured bars in an induction motor without the need for specific sensors. This method is particularly suitable for detecting a limited number of damaged bars to mitigate the risk of fault propagation.
[0006] To achieve this objective, this disclosure proposes a method for detecting broken bars in the squirrel cage of an induction motor, the induction motor including a stator and a rotor, the stator including stator windings, an AC voltage being applied to the stator windings, the method being performed by a control device and a determining device configured to measure a signal representing a current flowing through at least one stator winding, the method comprising:
[0007] a) Determine a signal representing the current flowing through at least one stator winding.
[0008] b) Calculate the Fourier transform of the determined signal.
[0009] c) Determine the fundamental frequency of the Fourier transform of the determined signal.
[0010] d) Calculate the estimated rotor slip based on the AC voltage applied to the stator windings and the signal representing the current flowing through the stator windings.
[0011] e) Set the search window based on the estimated rotor slip and fundamental frequency.
[0012] f) Search the search window for the first peak and the second peak, where the first peak has a frequency greater than the fundamental frequency and the second peak has a frequency lower than the fundamental frequency.
[0013] g) Check whether the frequency and / or amplitude of the first and second peaks meet at least one defined condition; if at least one condition is met, generate a signal indicating the presence of a break bar.
[0014] Advantageously, this method is inexpensive and easy to implement.
[0015] Optionally, the check includes comparing the difference between a first frequency interval and a second frequency interval with a defined threshold, the first frequency interval being between the frequency of the base frequency and the frequency of the first peak, the second frequency interval being between the frequency of the base frequency and the frequency of the second peak, and wherein a signal indicating the presence of a break bar is generated only if the difference between the first frequency interval and the second frequency interval is less than the defined threshold.
[0016] Optionally, the check includes comparing the amplitude of the first peak and the amplitude of the second peak with a defined amplitude, wherein a signal indicating the presence of a break bar is generated only if the amplitude of the first peak and the amplitude of the second peak are greater than the defined amplitude.
[0017] Optionally, the defined amplitude is greater than 2% of the amplitude of the fundamental frequency.
[0018] Optionally, the search window is defined by a first limiting frequency and a second limiting frequency, where the first limiting frequency is equal to the fundamental frequency minus n times the estimated rotor slip multiplied by the fundamental frequency, and the second limiting frequency is equal to the fundamental frequency plus n times the estimated rotor slip multiplied by the fundamental frequency; n is a real number included between 2 and 4.
[0019] Optionally, the determined signal can be windowed using a Hanning window before calculating the Fourier transform.
[0020] Optionally, the determined signal is sampled, and the calculated Fourier transform is a discrete Fourier transform.
[0021] Optionally, a notch filter is applied to the Fourier transform of the determined signal to suppress the narrow band centered on the fundamental frequency.
[0022] Optionally, the determined signal is the current flowing through at least one stator winding.
[0023] Optionally, the stator includes a first stator winding, a second stator winding, and a third stator winding. Steps a) to g) are performed on the first stator winding, the second stator winding, and the third stator winding, wherein a signal indicating the presence of a broken bar is generated only if at least one condition is satisfied for a signal indicating current flowing through the first stator winding, at least one condition is satisfied for a signal indicating current flowing through the second stator winding, and at least one condition is satisfied for a signal indicating current flowing through the third stator winding.
[0024] Another object of this disclosure is a variable speed drive that includes a processor, a memory operatively coupled to the processor, and an interface for coupling to an induction motor to be driven by the variable speed drive, wherein the variable speed drive is configured to perform the methods described in the above features.
[0025] Another object of this disclosure is a computer program product comprising computer program code tangibly embodied in a computer-readable medium, the computer program code including instructions that, when provided to and executed by a computer system, cause the computer to perform the methods described in the above features.
[0026] Another object of this disclosure is a dataset that represents, for example, a computer program product as described above by compression or encoding. Attached Figure Description
[0027] Figure 1 This is a schematic diagram of an electric drive assembly including a variable speed drive according to the present disclosure.
[0028] Figure 2 This is a flowchart of the method according to this disclosure.
[0029] Figure 3 This is an example of the discrete Fourier transform of the current flowing through the stator winding of an induction motor with a broken bar.
[0030] Figure 4 This is a flowchart of a method according to a second embodiment of the present disclosure. Detailed Implementation
[0031] The present invention relates to a method for detecting at least one broken bar in a squirrel cage of an induction motor.
[0032] Figure 1 An example of an electric drive assembly 2 including a variable speed drive according to the present disclosure is shown.
[0033] Figure 1 The electric drive assembly 2 includes a power grid 4, a variable speed drive 8 connected to the power grid, and an induction motor 6 connected to the variable speed drive 8.
[0034] The induction motor 6 includes a stator and a rotor. The rotor includes a squirrel cage, which is composed of two end rings connected by bars. The stator includes... Figure 1 The first, second, and third windings are not shown in the diagram.
[0035] The power grid 4 is a three-phase network that provides AC signals to the variable speed drive 8. For this purpose, the power grid 4 includes three phases 10, 12, and 14, respectively connected to the three inputs of the variable speed drive 8. The variable speed drive 8 includes three outputs 16, 18, and 20 adapted to be connected to the three stator windings of the induction motor 6, respectively.
[0036] The variable speed drive 8 is configured to provide AC voltage to the stator windings of an induction motor. AC voltage It is the control voltage of the induction motor.
[0037] exist Figure 1 In the illustrated embodiment, the electric drive assembly 2 includes a determining device 22 for measuring the instantaneous current flowing through each stator winding. The signal can be the instantaneous current itself. In this case, the determining device 22 includes, for example, an ammeter.
[0038] Alternatively, the signal can be the voltage between the terminals of the stator windings. In this case, the determining device 22 includes, for example, a voltmeter. For the following description, the determining device will be considered in a non-limiting manner to measure the instantaneous current through each stator winding.
[0039] During the operation of the induction motor, the alternating current signal A magnetic field is provided to the stator windings to create a rotational magnetic field that reciprocates in sync with the oscillation. This rotating magnetic flux induces a current in the bars of the rotor's squirrel cage. To resist the changing current in the bars, the rotor rotates in the direction of the stator magnetic field. The rotation of the rotor also induces a magnetic field in the stator windings. When the induction motor is free of defects, especially when the bars of the squirrel cage are not broken, the current flowing through the stator windings and the voltage measured at the stator winding terminals represent the rotor's rotational speed. The rotor rotates at a speed slightly slower than the stator's magnetic field. Due to the difference between the rotational speed of the stator's magnetic field and the rotor's rotational speed, the induction motor is sometimes called an "asynchronous motor." The difference between the rotational speed of the stator's magnetic field and the rotor's rotational speed, divided by the rotor's rotational speed, is commonly referred to as "slip."
[0040] s = (w s -w r ) / w s
[0041] Where s is the slip.
[0042] w sIt is the rotational speed of the stator's magnetic field.
[0043] w r It is the rotor speed.
[0044] In a sensorless control law scenario, where there is no rotor speed sensor or position sensor, slip *s* cannot be measured. Slip also varies depending on the load on the induction motor.
[0045] Using a mathematical model of an induction motor, it has been theoretically proven that the frequency of the bar breaking in an induction motor is defined by the following equation:
[0046]
[0047] in
[0048] -f BRB It is the frequency of the broken strips.
[0049] -k is a non-zero natural integer.
[0050] -s represents slip.
[0051] - It is the fundamental frequency of the alternating current signal measured between the terminals of the first stator winding. Figure 3 In the example shown, It is the current flowing through the first stator winding. The fundamental frequency.
[0052] exist Figure 1 In this context, the variable speed drive 8 includes a processor 24, such as a controller or microcontroller.
[0053] Processor 24 is configured to operate according to the methods described herein. The processor may include electronic circuitry for calculations managed by the operating system.
[0054] The variable speed drive 8 may also include a non-transient machine-readable or computer-readable storage medium. The storage medium is encoded with instructions executable by a processor, such as processor 24, to perform the example methods described herein.
[0055] The computer-readable storage unit according to this disclosure can be any electronic, magnetic, optical, or other physical storage device for storing executable instructions. The computer-readable storage unit can be, for example, random access memory (RAM), electrically erasable programmable read-only memory (EEPROM), storage drives, and optical discs.
[0056] Processor 24 includes: observer 26, configured to determine the current flowing through the stator windings as determined by determining device 22. The rotor slip is estimated by calculating the control voltage applied to the stator windings. .
[0057] Specifically, based on the current flowing through the first stator winding Current flowing through the second stator winding Current flowing through the third stator winding AC voltage applied to the first stator winding AC voltage applied to the second stator winding AC voltage applied to the third stator winding To calculate and estimate rotor slip .
[0058] The processor 24 includes a Fourier transform unit 28 connected to the determination device 22.
[0059] Fourier transform unit 28 is configured to receive a representation of the current flowing through the first stator winding. The signal is obtained and its spectrum is calculated.
[0060] In some examples, Fourier transform unit 28 is configured to sample the received signal and perform a discrete Fourier transform on it.
[0061] In some examples, the Fourier transform unit 28 is configured to also receive a representation of the current flowing through the second stator winding. The signal is analyzed and its spectrum is calculated.
[0062] In some examples, the Fourier transform unit 28 is also configured to receive a representation of the current flowing through the third stator winding. The signal is analyzed and its spectrum is calculated.
[0063] Processor 24 includes a filtering unit 30 connected to Fourier transform unit 28. Filtering unit 30 is configured to identify the fundamental frequency of the Fourier transform of the determined signal emitted from unit 28. Filtering unit 30 is configured to remove the fundamental frequency signal from the Fourier transform of the determined signal.
[0064] Processor 24 can include Figure 1 Notch filter not shown.
[0065] Processor 24 includes a broken bar detection unit 32. The broken bar detection unit 32 is connected to observer 26 and to filter unit 30. The broken bar detection unit 32 is configured to check whether the induction motor 6 includes at least one broken bar. The broken bar detection unit 32 is configured to generate a signal BB indicating the presence of a broken bar when at least one broken bar is detected.
[0066] exist Figure 1In the example, each of the observer 26, Fourier transform unit 28, filter unit 30, and broken bar detection unit 32 is implemented—that is, executed—as software that can be executed by processor 24.
[0067] In step 40, the method according to this disclosure includes determining, by the determining device 22, the current representing the current flowing through the first stator winding. The signal. This signal is determined, for example, by measurement.
[0068] The following description is intended to be considered in a non-limiting manner, determining that device 22 measures the instantaneous current through the first stator winding. Alternatively, the determining device 22 can also determine the voltage between the terminals of the first stator winding.
[0069] In step 42, the method includes sampling the determined current. The determined current For example, it can be sampled at 4 kHz during a 20-second time window. The time window can be applied to the sampling current. To enhance its spectral resolution, for example, the Hanning window can be applied.
[0070] In step 44, the method includes calculating the determined current. The spectrum. Calculate the determined current. The Discrete Fourier Transform (DFT). Figure 3 This shows the determined current in the first stator winding of an induction motor with a broken bar. An example of the magnitude of the Discrete Fourier Transform (DFT).
[0071] Alternatively, the method does not include step 42. In this case, a time window is applied to the determined current in analog form, and the determined current is... Calculate the Fourier transform (FFT).
[0072] In step 46, the method includes transferring the baseband frequency. Identified as the determined current The highest peak of the Discrete Fourier Transform (DFT). For example, this identification can be made by comparing the amplitudes of the Discrete Fourier Transform (DFT) pairwise in a table. This can be determined from the current... Removing the fundamental frequency in the Discrete Fourier Transform (DFT) .exist Figure 3 In the example, the base frequency It has a frequency of 53 Hz.
[0073] In step 48, to increase recovery accuracy, the method may include applying a notch filter to attenuate the fundamental frequency. Surrounding lobes.
[0074] In step 50, the method includes calculating the estimated rotor slip. Observer 26 uses the current flowing through the first stator winding... and the alternating voltage applied to the terminals of the first stator winding To estimate the rotor slip The estimated rotor slip is biased because the difference between the physical system and the mathematical model increases significantly when a broken bar failure occurs.
[0075] In step 52, the method includes determining the rotor slip based on the estimated rotor slip. and base frequency To set the search window SW. The search window SW is defined by the first limiting frequency and the second limiting frequency.
[0076] In some examples, the first limiting frequency is equal to the fundamental frequency minus n times the estimated rotor slip. Multiply by the fundamental frequency. The second limiting frequency equals the fundamental frequency plus n times the estimated rotor slip. Multiply by the base frequency.
[0077] Search window SW equals The quantity n is a real number between 2 and 4.
[0078] For example, the quantity n equals 2.5.
[0079] In another example, the quantity n equals 3. In this case, the search window SW equals... .
[0080] The closer the value of the quantity n is to 2, the faster the broken strip will be detected. If the value of the quantity n is higher, the chance of detecting broken strips increases even when the slip deviation is significant.
[0081] In step 54, the method includes searching in the search window SW for frequencies higher than the base frequency. The first peak of the frequency is 55 and has a frequency lower than the fundamental frequency. The second peak of the frequency is 57. This search can be accomplished using a peak-finding algorithm.
[0082] Theoretically, if the rotor includes a broken bar, then the first peak should have The frequency of the second peak. The frequency. However, due to the slip of the induction motor with broken bars and the estimated rotor slip. To find the differences, search in the larger search window SW.
[0083] In step 56, the method includes checking that the frequency and / or amplitude of the first peak 55 and the frequency and / or amplitude of the second peak 57 satisfy at least one defined condition.
[0084] In some examples, the defined condition is that the first peak 55 and the second peak 57 are related to the fundamental frequency. The frequency ranges are roughly the same.
[0085] To check this condition, the method includes determining a first frequency interval I1 and a second frequency interval I2 during step 58.
[0086] The first frequency interval I1 includes the fundamental frequency. The frequencies between the first peak (55) and the fundamental frequency. The first frequency interval I1 is obtained by the difference between the frequency of the first peak 55 and the frequency of the second peak 55. The second frequency interval I2 includes the fundamental frequency. The frequency between the second peak (57) and the third peak (57). This can be calculated by determining the fundamental frequency. The second frequency interval I2 is obtained by the difference between the frequency of the second peak 57 and the frequency of the second peak 57.
[0087] The method also includes calculating the difference between a first frequency interval I1 and a second frequency interval I2. At least during step 58, the difference is compared with a defined threshold S1.
[0088] For example, the threshold S1 is equal to 0.1 Hz.
[0089] According to the first embodiment, when the difference between the first frequency interval I1 and the second frequency interval I2 is less than the first defined threshold S1, a signal BB indicating the presence of at least one broken bar is generated in step 60.
[0090] According to the second embodiment, when the difference between the first frequency interval I1 and the second frequency interval I2 is less than the first defined threshold S1, the method includes checking the second defined condition in step 62.
[0091] In some examples, the second defining condition is that the amplitudes of the first peak 55 and the second peak 57 are greater than the defined amplitude A.
[0092] To check this condition, the method includes comparing the amplitude of the first peak 55 and the amplitude of the second peak 57 with a defined amplitude A during step 62.
[0093] Preferably, the defined amplitude A is equal to or greater than the fundamental frequency. The range is 2%.
[0094] Alternatively, the defined amplitude A is a predefined value that is independent of the fundamental frequency. The amplitude, and more generally, its independence from the determined signal. .
[0095] If the amplitude of the first peak 55 and the amplitude of the second peak 57 are greater than the defined amplitude A, then in step 64 a signal BB indicating the presence of at least one broken bar is generated.
[0096] The method according to the invention can be applied only to the first stator winding. In this case, only the current flowing through the first stator winding is considered. The signal implementation steps are 40 to 64.
[0097] Alternatively, steps 40 through 64 are repeated for the second stator winding. In step 40, the current flowing through the second stator winding is determined. The signal. For representing current. The signal is implemented in steps 42 to 64.
[0098] In this case, only for the signal representing the current flowing through the first stator winding The signal representing the current flowing through the second stator winding. A signal indicating the presence of a broken bar is generated only when at least one condition is met.
[0099] If only for signals representing the current flowing through the first stator winding The signal representing the current flowing through the second stator winding. If one of the signals satisfies at least one condition, a signal indicating a problem on the stator is generated.
[0100] Alternatively, refer to Figure 4 Steps 40 to 64 are performed once for the first stator winding. Then, in step 66, steps 40 to 64 are repeated for the second stator winding. In this case, in step 40, the current flowing through the second stator winding is determined. The signal.
[0101] In step 66, steps 40 to 64 are repeated again for the third stator winding. In this latter case, in step 40, the current flowing through the third stator winding is determined. The signal.
[0102] according to Figure 4 The embodiment shown applies only to signals representing the current flowing through the first stator winding. For signals representing the current flowing through the second stator winding And for the signal representing the current flowing through the third stator winding A signal BB indicating the presence of a broken bar is generated in step 68 only when at least one condition is met.
[0103] If only for signals representing the current flowing through the first stator winding This indicates the signal that represents the current flowing through the second stator winding. And represents the signal of the current flowing through the third stator winding. If one of the signals satisfies at least one condition, a signal indicating a problem on the stator is generated.
Claims
1. A method for detecting broken bars in an induction motor, the induction motor comprising a stator and a rotor, the stator comprising stator windings, and an AC voltage... A current is applied to the stator winding, the method being performed by a control device (24) and a determining device (22), the determining device (22) being configured to measure a signal representing the current flowing through at least one stator winding, the method comprising: a) Determine (40) the signal representing the current flowing through at least one stator winding. ), b) Calculate the Fourier transform of the signal determined in (44), c) Determine the signal determined by (46) The fundamental frequency of the Fourier transform of ) ), d) Based on the AC voltage applied to the stator winding ( ) and the signal representing the current flowing through the stator winding ( ) Calculate (50) the estimated rotor slip ( ), e) Based on the estimated rotor slip ( ) and fundamental frequency Set up the search window (SW) f) Search in the search window (SW) for (54) the first peak (55) and the second peak (57), the first peak (55) having a frequency greater than the fundamental frequency ( The frequency of the second peak (57) is lower than the fundamental frequency ( ). The frequency of ) g) Check whether the frequency and / or amplitude of the first and second peaks (56, 58, 62) meet at least one defined condition; if the at least one condition is met, generate a signal (BB) indicating the presence of at least one break bar.
2. The method according to claim 1, wherein, The check includes comparing the difference between a first frequency interval (I1) and a second frequency interval (I2) with a defined threshold (S1) (58), the first frequency interval including the base frequency ( The second frequency interval (I2) is between the frequency of the first peak (55) and the frequency of the second peak (55), and includes the fundamental frequency (55). The frequency of the first frequency interval is between the frequency of the second peak and the frequency of the second peak, and a signal indicating the presence of a break bar (BB) is generated only when the difference between the first frequency interval and the second frequency interval is less than a defined threshold (S1).
3. The method according to any one of claims 1 and 2, wherein, The inspection includes comparing the amplitude of the first peak (55) and the amplitude of the second peak (57) with a defined amplitude (A) (62), wherein a signal (BB) indicating the presence of a break bar is generated only if the amplitude of the first peak and the amplitude of the second peak are greater than the defined amplitude (A).
4. The method according to claim 3, wherein, The defined amplitude (A) is greater than 2% of the fundamental frequency amplitude.
5. The method according to any one of claims 1 to 4, wherein, The search window (SW) is defined by a first limiting frequency and a second limiting frequency. The first limiting frequency is equal to the fundamental frequency minus n times the estimated rotor slip multiplied by the fundamental frequency, and the second limiting frequency is equal to the fundamental frequency plus n times the estimated rotor slip multiplied by the fundamental frequency; n is a real number included between 2 and 4.
6. The method according to any one of claims 1 to 5, wherein, Before calculating the Fourier transform, the determined signal is subjected to a Hanning window. ) to window (42).
7. The method according to any one of claims 1 to 6, wherein, The determined signal ( ) is sampled (42), and the calculated Fourier transform is a discrete Fourier transform.
8. The method according to any one of claims 1 to 7, wherein, Notch filter (48) is applied to the determined signal ( The Fourier transform of ) to suppress the fundamental frequency ( A narrow band centered on ).
9. The method according to any one of claims 1 to 8, wherein, The determined signal is the current flowing through at least one stator winding. ).
10. The method according to any one of claims 1 to 9, wherein, The stator includes a first stator winding, a second stator winding, and a third stator winding. Steps (66) a) to g) are performed on the first stator winding, the second stator winding, and the third stator winding, and wherein, only when for a signal representing the current flowing through the first stator winding ( () satisfies at least one condition for a signal representing the current flowing through the second stator winding. () satisfies at least one condition for a signal representing the current flowing through the third stator winding. When at least one condition is met, generate (68) a signal (BB) indicating the presence of a broken bar.
11. A variable speed drive, the variable speed drive including a processor, a memory operatively coupled to the processor, and an interface for coupling to an induction motor to be driven by the variable speed drive, wherein, The variable speed drive is configured to perform the method according to any one of claims 1 to 10.
12. A computer program product comprising computer program code tangibly embodied in a computer-readable medium, the computer program code including instructions that, when provided to and executed by a computer system, cause the computer to perform the method according to any one of claims 1 to 10.
13. A dataset representing the computer program product of claim 12 by compression or encoding.