Method and system for determining the speed of an asynchronous motor

Abstract

The application provides a method and system for obtaining the rotating speed of an asynchronous motor, wherein the method comprises the following steps: applying an alternating current with a preset frequency and a preset voltage amplitude to the stator winding of the asynchronous motor in a free rotating state; disconnecting the alternating current and collecting the back electromotive force voltage signal between the terminals of the asynchronous motor; obtaining the fundamental wave frequency of the back electromotive force voltage signal according to the back electromotive force voltage signal; and calculating the rotating speed of the asynchronous motor according to the fundamental wave frequency. The method solves the problem that the residual magnetism of the rotor in the free rotating state decays to cause the weak back electromotive force signal or even the unmeasurable back electromotive force signal, and realizes the high-precision and high-robustness obtaining of the rotating speed of the asynchronous motor under all working conditions, thereby laying a foundation for the smooth and impact-free starting of the asynchronous motor.

Description

Technical Field

[0001] This invention relates to the field of AC motor control and power electronics technology, and in particular to a method and system for obtaining the speed of an asynchronous motor. Background Technology

[0002] When an asynchronous motor is disconnected from the power grid and rotating freely, if the inverter directly applies a voltage of fixed frequency and amplitude to restart it, the slip frequency will far exceed the rated slip range. This will generate a huge impact component in the stator current, potentially triggering overcurrent protection or even damaging the power module. Therefore, accurately obtaining the motor's real-time speed and achieving frequency synchronization before reconnecting power is a crucial prerequisite for ensuring a safe restart.

[0003] In existing sensorless speed tracking technologies, a common method is based on back electromotive force (EMF) detection between motor terminals. After the motor is powered off, if residual magnetism remains in the rotor, the residual magnetic field will rotate with the rotor and cut the stator windings, thus inducing a back EMF signal proportional to the rotational speed. However, asynchronous motors are induction motors, and once power is off, the rotor magnetic field decays rapidly. While back EMF can be detected in the initial stages of free-stop operation, it becomes almost impossible to detect a measurable back EMF signal in the rotor when subjected to prolonged free-stop operation with a large inertia load or when the motor is driven at high speed by external machinery. Furthermore, even if residual magnetism exists, the amplitude of the back EMF is typically only in the millivolt range, resulting in an extremely low signal-to-noise ratio, making it difficult for traditional zero-crossing detection methods to operate stably. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method and system for obtaining the speed of an asynchronous motor. Under the condition that the rotor has insufficient or completely lost residual magnetism, the rotor is induced to magnetize by actively applying AC current, so as to obtain the speed of the asynchronous motor with high accuracy and high robustness under all working conditions.

[0005] To address the aforementioned technical problems, in a first aspect, the present invention provides a method for determining the speed of an asynchronous motor, comprising: applying an alternating current of a preset frequency and a preset voltage amplitude to the stator winding of the asynchronous motor in a free-rotating state; disconnecting the alternating current and acquiring a back electromotive force (EMF) voltage signal between the terminals of the asynchronous motor; obtaining the fundamental frequency of the back EMF voltage signal based on the back EMF voltage signal; and calculating the speed of the asynchronous motor based on the fundamental frequency.

[0006] Optionally, the preset frequency is a fixed frequency or an adjustable frequency during the application of alternating current; and / or the preset voltage amplitude includes: determining an initial voltage amplitude, and gradually increasing the voltage amplitude at a set increment rate during the application of alternating current.

[0007] Optionally, the preset frequency is between 10Hz and 30Hz, and / or the initial voltage amplitude is between 0.01% and 0.1% of the rated voltage of the asynchronous motor.

[0008] Optionally, disconnecting the AC power and acquiring the back EMF voltage signal between the asynchronous motor terminals includes: disconnecting the AC power and acquiring the back EMF voltage signal between the asynchronous motor terminals after a certain period of time.

[0009] Optionally, acquiring the back electromotive force voltage signal between the terminals of the asynchronous motor includes: acquiring the line voltage between any two phase terminals of the asynchronous motor using an analog-to-digital converter, and using the line voltage as the back electromotive force voltage signal.

[0010] Optionally, after acquiring the back electromotive force voltage signal between the asynchronous motor terminals, the method further includes: performing digital bandpass filtering on the back electromotive force voltage signal.

[0011] Optionally, obtaining the fundamental frequency of the back electromotive force voltage signal from the back electromotive force voltage signal includes: performing fast Fourier transform spectrum analysis on the back electromotive force voltage signal to extract the fundamental frequency of the back electromotive force voltage signal.

[0012] Optionally, the process of extracting the fundamental frequency of the back electromotive force voltage signal by performing fast Fourier transform spectrum analysis on the back electromotive force voltage signal includes: continuously acquiring data corresponding to multiple segments of the back electromotive force voltage signal, performing fast Fourier transform operation on each segment of data to obtain multiple frequency estimates, then determining whether the multiple frequency estimates are stably converged through consistency check, and extracting the average value of the multiple frequency estimates after stabilization as the fundamental frequency.

[0013] Optionally, calculating the speed of the asynchronous motor based on the fundamental frequency includes: according to the formula Calculate the speed of the asynchronous motor, where, Rotational speed, unit: rpm The fundamental frequency obtained from spectral analysis, in Hz. This represents the number of pole pairs of the asynchronous motor.

[0014] Secondly, the present invention provides an asynchronous motor speed determination system, comprising: an excitation module configured to apply an alternating current of a preset frequency and a preset voltage amplitude to the stator winding of the asynchronous motor in a free-rotating state; an acquisition module configured to disconnect the alternating current and acquire a back electromotive force voltage signal between the terminals of the asynchronous motor; a spectrum analysis module configured to obtain the fundamental frequency of the back electromotive force voltage signal based on the back electromotive force voltage signal; and a calculation module configured to calculate the speed of the asynchronous motor based on the fundamental frequency.

[0015] Compared with existing technologies, this invention has the following advantages: By actively applying alternating current to the stator windings before speed tracking, a magnetic field is induced in the rotor, solving the problem that the back electromotive force (EMF) signal cannot be measured after the rotor's residual magnetism decays or even disappears during free rotation. After the rotor's induced magnetic field is established, disconnecting the AC excitation yields a back EMF signal with significant amplitude and high signal-to-noise ratio. Then, the fundamental frequency of the back EMF voltage signal is obtained, and the asynchronous motor speed is calculated with high accuracy and robustness based on this fundamental frequency. Attached Figure Description

[0016] The accompanying drawings are included to provide a further understanding of this application; they are incorporated into and constitute a part of this application. The drawings illustrate embodiments of this application and, together with this specification, serve to explain the principles of this application. In the drawings: Figure 1 This is a flowchart illustrating an embodiment of the asynchronous motor speed determination method of the present invention; Figure 2 This is a schematic diagram of the process of applying alternating current in one embodiment of the present invention; Figure 3 This is a schematic diagram comparing the waveforms and spectra of the back electromotive force signals after no residual magnetism and AC excitation in one embodiment of the present invention; Figure 4 This is a schematic diagram of an asynchronous motor device including an asynchronous motor speed determination system according to an embodiment of the present invention. Detailed Implementation

[0017] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are merely some examples or embodiments of this application. For those skilled in the art, these drawings can be applied to other similar scenarios without creative effort. Unless obvious from the context or otherwise specified, the same reference numerals in the drawings represent the same structures or operations.

[0018] As indicated in this application and claims, unless the context clearly indicates otherwise, the words "a," "an," "an," and / or "the" are not specifically singular and may include plural forms. Generally speaking, the terms "comprising" and "including" only indicate the inclusion of explicitly identified steps and elements, which do not constitute an exclusive list, and the method or apparatus may also include other steps or elements.

[0019] Flowcharts are used in this application to illustrate the operations performed according to embodiments of this application. It should be understood that the preceding or following operations are not necessarily performed in exact order. Instead, various steps can be processed in reverse order or simultaneously. Furthermore, other operations may be added to these processes, or one or more steps may be removed from them.

[0020] refer to Figure 1 As shown, this embodiment provides a method for obtaining the speed of an asynchronous motor, including: S110, applying an alternating current with a preset frequency and preset voltage amplitude to the stator winding of the asynchronous motor in a free-rotating state; S120, disconnecting the alternating current and acquiring the back electromotive force voltage signal between the terminals of the asynchronous motor; S130, obtaining the fundamental frequency of the back electromotive force voltage signal based on the back electromotive force voltage signal; S140, calculating the speed of the asynchronous motor based on the fundamental frequency.

[0021] This embodiment addresses the problems of weak signals and poor anti-interference capabilities in existing back EMF detection methods. By applying a brief alternating current to the freely rotating asynchronous motor for pre-magnetization before detection, the residual magnetism of the rotor is enhanced, resulting in a significant improvement in the amplitude and signal-to-noise ratio of the back EMF signal acquired after power-off. Subsequently, spectral analysis of this strong signal allows for robust and accurate extraction of the fundamental frequency that strictly corresponds to the rotational speed. The rotational speed of the asynchronous motor can then be calculated based on the fundamental frequency, achieving high-precision and robust speed determination and laying the foundation for smooth, shock-free starting.

[0022] In some embodiments, the preset frequency is a fixed frequency or the frequency can be adjusted during the application of alternating current; and / or the preset voltage amplitude includes: determining an initial voltage amplitude and gradually increasing the voltage amplitude at a set increment rate during the application of alternating current.

[0023] Figure 2 This is a schematic diagram of the process of applying alternating current in one embodiment of the present invention, for reference. Figure 2As shown, during the application of AC power, the voltage amplitude gradually increases according to a preset increment rate. During this process, the asynchronous motor current response is monitored in real time. When the current response reaches a threshold, the excitation is complete, and then the AC excitation is disconnected (all power transistors are turned off). After a delay to allow the turn-off transient to subside, the measurement relay is closed, and then the sampling channel is connected. The applied fixed-frequency AC power has a preset frequency, initial voltage amplitude, and amplitude increment rate, and is generated by the inverter using space vector modulation. In this embodiment, the inverter directly controls the voltage and current, indirectly generating a rotating magnetic field. Figure 3 The diagram shows a comparison of the waveforms and spectra of the back EMF signals after no residual magnetism and after AC excitation. It can be seen that the excitation enhances the residual magnetism of the rotor, which greatly improves the amplitude and signal-to-noise ratio of the back EMF signal collected after power failure.

[0024] For example, the preset frequency is a fixed value, preferably 10Hz to 30Hz (more preferably 20Hz), and the initial voltage amplitude is 0.01% to 0.1% of the motor's rated voltage. Subsequently, the voltage amplitude gradually increases according to a preset increment rate. The voltage amplitude increment can be 0.0002 pu (per unit) every 50 microseconds (one pulse width modulation control cycle) to ensure a smooth and controllable voltage build-up process. During the application of fixed-frequency AC power, the current response is monitored in real time. Since the current response directly reflects the establishment state of the rotor magnetic field, when the current response exceeds a preset threshold (e.g., 0.2 pu), it can be definitively confirmed that the rotor has induced a sufficient magnetic field. Alternatively, when the amplitude increases to a preset upper limit (e.g., 0.3 pu), the excitation is considered complete. Once either condition is met, the application of AC power is immediately stopped, and the inverter output is controlled to enter a high-impedance state to prepare for back EMF detection. For example, starting with a frequency of 20Hz and an initial voltage amplitude of 0.0001pu, the voltage is increased by 0.0002pu every 50μs, and the current response reaches 0.2pu after about 0.5 to 0.8 seconds.

[0025] In some embodiments, disconnecting the AC power and acquiring the back EMF voltage signal between the asynchronous motor terminals includes: disconnecting the AC power and, after a certain delay, acquiring the back EMF voltage signal between the asynchronous motor terminals. For example, the total delay time can be set to approximately 300 milliseconds, which includes: turning off the inverter power transistor drive and waiting for 100 milliseconds, disconnecting the power stage supply and waiting for 100 milliseconds, and closing the measurement relay connection to the sampling channel and waiting for 100 milliseconds. After disconnection, the inverter output is placed in a high-impedance state to prevent the back EMF from being clamped.

[0026] In some embodiments, acquiring the back electromotive force voltage signal between the terminals of the asynchronous motor includes: acquiring the line voltage between any two phase terminals of the asynchronous motor using an analog-to-digital converter, with the line voltage serving as the back electromotive force voltage signal.

[0027] For example, when acquiring voltage signals, an analog-to-digital converter (ADC) is used to acquire the line voltage between the UV and V phase terminals of an asynchronous motor. Line voltage naturally suppresses common-mode noise. Acquiring line voltage has the following advantages over single-phase neutral point voltage acquisition: line voltage naturally suppresses common-mode noise interference, which is beneficial for improving the ratio of signal to ADC quantization accuracy under millivolt-level back electromotive force conditions. In one implementation, the ADC sampling frequency is set to 10kHz, and the total sampling time for a single signal segment is approximately 420 milliseconds.

[0028] To reduce the inherent delay error between sampling channels, preferably, a three-channel synchronous sample-and-hold circuit is used to simultaneously acquire three sets of line voltages (Uab, Ubc, Uca), or a single-channel analog-to-digital converter is used in conjunction with an analog switch to acquire the voltages in a polling manner at equal intervals, and the switching delay of the analog switch is compensated in the software. During the pre-start-up phase when the asynchronous motor is freely rotating and no drive voltage has been applied, the analog-to-digital converter continuously acquires the line voltage signals and sends the acquired raw data to a digital signal processor or microcontroller to execute subsequent weak signal enhancement and frequency extraction algorithms.

[0029] In some embodiments, after acquiring the back EMF voltage signal between the asynchronous motor terminals, the method further includes performing digital bandpass filtering on the back EMF voltage signal.

[0030] Digital bandpass filtering can be implemented using either an infinite impulse response (IER) filter or a finite impulse response (FIR) filter. Its passband is configured to cover the back electromotive force (EMF) frequency range corresponding to the lowest to highest possible freely rotating speeds of the asynchronous motor. To accommodate the gradual decrease in asynchronous motor speed during free rotation, a variable center frequency tracking filtering strategy can be further employed. Before obtaining the precise speed, a wide passband filter (e.g., a passband range of 5Hz to 100Hz) is first used to preprocess the original signal to initially extract the dominant frequency component of the back EMF. After obtaining a rough estimate of the speed, the passband range is dynamically narrowed (e.g., center frequency ±5Hz) to form a narrowband tracking filter, further suppressing out-of-band noise and harmonic interference. Digital bandpass filtering is configured to execute before the zero-crossing detection or peak detection steps in timing, ensuring that the signal entering the frequency extraction stage has a sufficient signal-to-noise ratio. The back EMF voltage signal after digital bandpass filtering has a noise floor reduced by at least 10dB to 20dB compared to the original signal, providing a reliable waveform basis for subsequent frequency and phase synchronization.

[0031] For example, an analog-to-digital converter (ADC) is used to acquire the line voltage between the UV and V phase terminals of the asynchronous motor. An anti-aliasing low-pass filter is set at the front end of the ADC, and the cutoff frequency is configured to be 5 to 10 times the back EMF frequency corresponding to the highest possible speed of the motor. After acquiring the back EMF voltage signal, it is digitally bandpass filtered, and the passband range covers the back EMF frequency range (e.g., 1Hz to 300Hz) corresponding to the lowest to highest possible speeds at which the motor can rotate freely.

[0032] In some embodiments, obtaining the fundamental frequency of the back-EMF voltage signal from the back-EMF voltage signal includes: performing Fast Fourier Transform (FFT) spectral analysis on the back-EMF voltage signal to extract the fundamental frequency. Since the aforementioned AC energizing step has established a sufficiently strong rotor magnetic field orientation, the acquired back-EMF signal belongs to the strong signal category, meaning the fundamental component energy is significantly higher than the background noise and harmonic components. Based on this signal quality advantage, the method of this embodiment can robustly and accurately extract the fundamental frequency that strictly corresponds to the current speed of the asynchronous motor, providing a reliable basis for subsequent speed calculation and smooth input.

[0033] In some embodiments, the process of extracting the fundamental frequency of the back EMF voltage signal by performing fast Fourier transform spectrum analysis on the back EMF voltage signal includes: continuously acquiring data corresponding to multiple segments of the back EMF voltage signal, performing fast Fourier transform operation on each segment of data to obtain multiple frequency estimates, then determining whether the multiple frequency estimates are stably converged through consistency check, and extracting the average value of the multiple frequency estimates after stabilization as the fundamental frequency.

[0034] For example, three independent signal segments can be continuously acquired, each approximately 420 milliseconds long, with each segment containing 4200 sampling points. 4096 consecutive sampling points are extracted from each segment as valid data frames. A Hanning window is applied to each frame, followed by a 4096-point Fast Fourier Transform (FFT). The frequency corresponding to the maximum amplitude spectral peak is searched as the fundamental frequency estimate for that frame, resulting in three frequency estimates: f1, f2, and f3. Consistency is checked on the three FFT results: if max(f1,f2,f3)-min(f1,f2,f3)≤Δfth (a preset threshold, e.g., 15Hz), the frequency is considered to have converged stably, and the average of the three values ​​is taken as the final fundamental frequency. If the deviation exceeds the threshold, all three frames are retained for subsequent processing or reacquisition. If the final obtained fundamental frequency is lower than the minimum effective frequency threshold (e.g., 15Hz), this threshold is set to avoid abnormal startup caused by excessively low frequencies.

[0035] In some embodiments, calculating the asynchronous motor speed based on the fundamental frequency includes: according to the formula... To calculate the speed of an asynchronous motor, use the following formula: Rotational speed, unit: rpm The fundamental frequency obtained from spectral analysis, in Hz. This represents the number of pole pairs for the asynchronous motor. The calculation results are then output to the inverter's main controller to set the initial output frequency and phase.

[0036] To further demonstrate the technical solution of this embodiment, an implementation process is shown below with reference to an asynchronous motor, as detailed below: Taking a certain type of high-speed asynchronous motor as an example, its main parameters are shown in Table 1, and the parameters set to achieve the speed calculation of the asynchronous motor are shown in Table 2.

[0037] Table 1 Parameters of a certain type of high-speed asynchronous motor

[0038] Table 2. Parameters set for determining rotational speed

[0039] In this example, the excitation frequency is selected as 15Hz, which is within the preferred range (10Hz~30Hz). The initial voltage amplitude is set to 0.05% of the rated voltage of the asynchronous motor (approximately 33mV), the current threshold is 0.2pu (approximately 1.7A), and the upper limit of the amplitude is 0.3pu (approximately 2.55A).

[0040] Example 1: Electrical angular frequency 250Hz (approximately 7500rpm)

[0041] 1) AC Excitation Stage: A fixed-frequency AC current of 15Hz is applied, with an initial voltage amplitude of 33mV, increasing by 0.0002pu every 50μs. Although the current electrical angular frequency of the motor is as high as 250Hz, the applied 15Hz excitation frequency is much lower than the rotor speed, and the induced current on the rotor can still stably establish a magnetic field. After about 0.5~0.6 seconds, the current response reaches the threshold of 1.7A, the rotor magnetic field is established deterministically, the application of AC current is immediately stopped, and the inverter enters a high-resistance state.

[0042] 2) Back EMF Acquisition: Disconnect the 15Hz AC excitation → delay for 300ms → close the measurement relay. At this time, the motor is still rotating at high speed, and the back EMF amplitude is approximately 250 / 255≈98% of the rated voltage, or about 64.7VAC. The ADC samples at 10kHz, with a single sampling time of 420ms, and continuously acquires three signal segments.

[0043] 3) Spectrum Analysis and Frequency Extraction: After the signal is filtered by a digital bandpass filter from 1 to 300Hz, 4096 points are extracted from each of the three 420ms signal segments to perform a 4096-point FFT. Results of the three FFTs: , , The difference between the maximum and minimum values ​​is 1.4Hz, which is less than the 15Hz threshold, indicating convergence. The average value is 249.5Hz.

[0044] 4) Rotational speed calculation: The error is approximately 0.2%.

[0045] Example 2: Electrical angular frequency 100Hz (corresponding to approximately 3000rpm)

[0046] 1) AC excitation stage: Apply a fixed frequency AC current of 15Hz, with the same parameters as in Example 1. The rotor's response characteristics to 15Hz excitation at lower speeds (100Hz electrical angular frequency) are slightly different from those at high speeds, but the judgment mechanism for the current threshold reaching 0.2pu remains unchanged, and it is still completed after about 0.5~0.6 seconds.

[0047] 2) Back EMF Acquisition: The measurement relay is closed for sampling after a 300ms delay. The back EMF amplitude corresponding to a 100Hz electrical angular frequency is approximately 100 / 255≈39% of the rated value, or approximately 25.7VAC. The ADC continuously acquires three 420ms segments of signal at 10kHz.

[0048] 3) Spectrum Analysis and Frequency Extraction: After bandpass filtering, 4096 points were extracted and a 4096-point FFT was performed. Results of 3 FFTs: , , The difference between the maximum and minimum values ​​is 1.6 Hz, which meets the consistency requirements, and the average value is 100.2 Hz.

[0049] 4) Rotational speed calculation: n The error is approximately 0.2%.

[0050] The above examples demonstrate that, under typical operating conditions of 250Hz and 100Hz, FFT analysis based on the back EMF of a 15Hz fixed-frequency AC excitation can accurately determine the motor speed with an error of less than 0.5%, verifying the effectiveness and accuracy of the method in this embodiment across the entire speed range. In particular, for high-speed asynchronous motors with a free-stop time exceeding one hour, this method eliminates the need for a speed sensor, utilizing only the inverter's own voltage and current sampling capabilities to accurately capture the rotor's free-rotation frequency at any speed. This makes it suitable for smooth, shock-free restart scenarios after long-term free-stops. It is understood that the above examples using 250Hz and 100Hz operating conditions are merely illustrative and not intended to limit the method. Those skilled in the art will understand that this method can be applied to other operating conditions, which will not be elaborated upon here.

[0051] Figure 4This is a schematic diagram of an asynchronous motor device including an asynchronous motor speed determination system according to an embodiment of the present invention. (Refer to...) Figure 4 As shown, this embodiment provides an asynchronous motor speed calculation system, mainly including: an excitation module configured to apply an alternating current with a preset frequency and preset voltage amplitude to the stator winding of the asynchronous motor in a free-rotating state; an acquisition module configured to disconnect the alternating current and acquire the back electromotive force voltage signal between the terminals of the asynchronous motor; a spectrum analysis module configured to obtain the fundamental frequency of the back electromotive force voltage signal based on the back electromotive force voltage signal; and a calculation module configured to calculate the speed of the asynchronous motor based on the fundamental frequency.

[0052] In this embodiment, by actively applying low-voltage AC current to the stator windings before speed tracking, a magnetic field is induced in the rotor, solving the problem that the back EMF signal cannot be measured after the rotor's residual magnetism decays or even disappears during free rotation. After the rotor's induced magnetic field is established, disconnecting the AC excitation yields a back EMF signal with significant amplitude and high signal-to-noise ratio. Then, the fundamental frequency of the back EMF voltage signal is obtained, and after analysis and calculation, the asynchronous motor speed can be determined with high accuracy and robustness.

[0053] In some embodiments, the preset frequency is a fixed frequency or the frequency can be adjusted during the application of alternating current; and / or the preset voltage amplitude includes: determining an initial voltage amplitude and gradually increasing the voltage amplitude at a set increment rate during the application of alternating current.

[0054] In some embodiments, the preset frequency is between 10Hz and 30Hz, and / or the initial voltage amplitude is between 0.01% and 0.1% of the rated voltage of the asynchronous motor.

[0055] In some embodiments, disconnecting the AC power and acquiring the back EMF voltage signal between the asynchronous motor terminals includes: disconnecting the AC power and acquiring the back EMF voltage signal between the asynchronous motor terminals after a certain period of time.

[0056] In some embodiments, acquiring the back electromotive force voltage signal between the terminals of the asynchronous motor includes: acquiring the line voltage between any two phase terminals of the asynchronous motor using an analog-to-digital converter, with the line voltage serving as the back electromotive force voltage signal.

[0057] In some embodiments, the system further includes a processing module, which performs digital bandpass filtering on the back electromotive force voltage signal after acquiring the back electromotive force voltage signal between the asynchronous motor terminals.

[0058] In some embodiments, obtaining the fundamental frequency of the back electromotive force voltage signal from the back electromotive force voltage signal includes: performing fast Fourier transform spectrum analysis on the back electromotive force voltage signal to extract the fundamental frequency of the back electromotive force voltage signal.

[0059] In some embodiments, the process of extracting the fundamental frequency of the back EMF voltage signal by performing fast Fourier transform spectrum analysis on the back EMF voltage signal includes: continuously acquiring data corresponding to multiple segments of the back EMF voltage signal, performing fast Fourier transform operation on each segment of data to obtain multiple frequency estimates, then determining whether the multiple frequency estimates are stably converged through consistency check, and extracting the average value of the multiple frequency estimates after stabilization as the fundamental frequency.

[0060] In some embodiments, calculating the speed of the asynchronous motor based on the fundamental frequency includes: according to the formula To calculate the speed of an asynchronous motor, use the following formula: Rotational speed, unit: rpm The fundamental frequency obtained from spectral analysis, in Hz. This represents the number of pole pairs of the asynchronous motor.

[0061] Details of other operations performed by each module in this embodiment can be found in the foregoing embodiments, and will not be elaborated here.

[0062] The basic concepts have been described above. Obviously, for those skilled in the art, the above disclosure is merely illustrative and does not constitute a limitation of this application. Although not explicitly stated herein, those skilled in the art may make various modifications, improvements, and corrections to this application. Such modifications, improvements, and corrections are suggested in this application, and therefore remain within the spirit and scope of the exemplary embodiments of this application.

[0063] Furthermore, this application uses specific terms to describe embodiments of the application. For example, "an embodiment," "one embodiment," and / or "some embodiments" refer to a particular feature, structure, or characteristic related to at least one embodiment of the application. Therefore, it should be emphasized and noted that "an embodiment," "one embodiment," or "an alternative embodiment" mentioned twice or more in different locations in this specification do not necessarily refer to the same embodiment. In addition, certain features, structures, or characteristics in one or more embodiments of the application can be appropriately combined.

[0064] Some aspects of this application can be executed entirely by hardware, entirely by software (including firmware, resident software, microcode, etc.), or by a combination of hardware and software. The aforementioned hardware or software may be referred to as a "data block," "module," "engine," "unit," "component," or "system." The processor may be one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DAPDs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, or combinations thereof. Furthermore, aspects of this application may manifest as computer products residing in one or more computer-readable media, including computer-readable program code. For example, computer-readable media may include, but are not limited to, magnetic storage devices (e.g., hard disks, floppy disks, magnetic tapes, etc.), optical discs (e.g., compressed CDs, digital multifunction DVDs, etc.), smart cards, and flash memory devices (e.g., cards, sticks, key drives, etc.).

[0065] In some embodiments, numbers describing the quantity of components and attributes are used. It should be understood that such numbers used in the description of embodiments are modified in some examples with the terms "approximately," "approximately," or "generally." Unless otherwise stated, "approximately," "approximately," or "generally" indicates that the numbers are allowed to vary by ±20%. Accordingly, in some embodiments, the numerical parameters used in the specification and claims are approximate values, which may be changed depending on the characteristics required by individual embodiments. In some embodiments, numerical parameters should take into account specified significant digits and employ a general method of digit reservation. Although the numerical ranges and parameters used to confirm their breadth of scope in some embodiments of this application are approximate values, in specific embodiments, such values ​​are set as precisely as feasible.

[0066] Although this application has been described with reference to specific embodiments, those skilled in the art should recognize that the above embodiments are only used to illustrate this application, and various equivalent changes or substitutions can be made without departing from the spirit of this application. Therefore, any changes or modifications to the above embodiments within the essential spirit of this application will fall within the scope of the claims of this application.

Claims

1. A method for determining the speed of an asynchronous motor, characterized in that, include: An alternating current with a preset frequency and preset voltage amplitude is applied to the stator winding of the asynchronous motor, which is in a free-rotating state. Disconnect the AC power and collect the back electromotive force voltage signal between the terminals of the asynchronous motor; The fundamental frequency of the back electromotive force voltage signal is obtained based on the back electromotive force voltage signal; The rotational speed of the asynchronous motor is calculated based on the fundamental frequency.

2. The method for determining the speed of an asynchronous motor as described in claim 1, characterized in that, The preset frequency is a fixed frequency or an adjustable frequency during the application of alternating current; and / or the preset voltage amplitude includes: determining an initial voltage amplitude, and gradually increasing the voltage amplitude according to a set increment rate during the application of alternating current.

3. The method for determining the speed of an asynchronous motor as described in claim 2, characterized in that, The preset frequency is between 10Hz and 30Hz, and / or the initial voltage amplitude is between 0.01% and 0.1% of the rated voltage of the asynchronous motor.

4. The method for determining the speed of an asynchronous motor as described in claim 1, characterized in that, Disconnecting the AC power and acquiring the back EMF voltage signal between the asynchronous motor terminals includes: disconnecting the AC power and, after a certain delay, acquiring the back EMF voltage signal between the asynchronous motor terminals.

5. The method for determining the speed of an asynchronous motor as described in claim 1, characterized in that, Acquiring the back electromotive force voltage signal between the terminals of the asynchronous motor includes: using an analog-to-digital converter to acquire the line voltage between any two phase terminals of the asynchronous motor, and the line voltage is used as the back electromotive force voltage signal.

6. The method for determining the speed of an asynchronous motor as described in any one of claims 1 to 5, characterized in that, After acquiring the back electromotive force voltage signal between the asynchronous motor terminals, the method further includes: performing digital bandpass filtering on the back electromotive force voltage signal.

7. The method for determining the speed of an asynchronous motor as described in claim 1, characterized in that, Obtaining the fundamental frequency of the back electromotive force voltage signal from the back electromotive force voltage signal includes: performing fast Fourier transform spectrum analysis on the back electromotive force voltage signal to extract the fundamental frequency of the back electromotive force voltage signal.

8. The method for determining the speed of an asynchronous motor as described in claim 7, characterized in that, The process of extracting the fundamental frequency of the back electromotive force voltage signal by performing fast Fourier transform spectrum analysis on the back electromotive force voltage signal includes: continuously acquiring multiple data segments corresponding to the back electromotive force voltage signal, performing fast Fourier transform operation on each data segment to obtain multiple frequency estimates, then determining whether the multiple frequency estimates are stably converged through consistency check, and extracting the average value of the multiple frequency estimates after stabilization as the fundamental frequency.

9. The method for determining the speed of an asynchronous motor as described in claim 1, characterized in that, According to the formula Calculate the speed of the asynchronous motor, where, Rotational speed, unit: rpm The fundamental frequency obtained from spectral analysis, in Hz. This represents the number of pole pairs of the asynchronous motor.

10. A system for determining the speed of an asynchronous motor, characterized in that, include: The excitation module is configured to apply an alternating current of a preset frequency and a preset voltage amplitude to the stator windings of the asynchronous motor in a free-rotating state; The acquisition module is configured to disconnect the AC power and acquire the back electromotive force voltage signal between the terminals of the asynchronous motor; The spectrum analysis module is configured to obtain the fundamental frequency of the back electromotive force voltage signal based on the back electromotive force voltage signal; The calculation module is configured to calculate the rotational speed of the asynchronous motor based on the fundamental frequency.

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