A method and control system for controlling flying start of AC asynchronous motor

Abstract

The invention discloses a method and a control system for controlling flying start of an AC asynchronous motor. The method comprises the following steps: during the process of starting the motor driven by a frequency converter, the motor is controlled to perform constant current control, so that the stator and the rotor of the motor are circuit equivalent; the frequency converter applies a target current signal with a constant amplitude and a frequency that decreases step by step according to a preset step change frequency to the motor, so that the target current signal acts on the motor; the frequency converter collects the motor voltage and current values ​​at the machine end, and a controller calculates the equivalent inductance of the motor according to the motor voltage, current value, and the applied angular frequency; the frequency converter performs sweep frequency control, and when the applied angular frequency reaches the rotor frequency range of the motor, it is determined that the equivalent inductance value of the motor is the maximum, and the maximum value inductance of the motor is used as the searched target rotor frequency, and the speed tracking is successfully achieved based on the target rotor frequency, so as to achieve flying start.

Description

Technical Field

[0001] The present invention relates to the technical field of electric motors, and in particular to a flying start method and a control system for controlling an AC asynchronous motor. Background Art

[0002] The motor stator is the stationary part of the motor. When the motor stator is disconnected from the frequency converter or the industrial frequency power grid, the motor stator is "passive" and the motor rotor is still in a rotating state, but the speed is random and uncertain. If the motor is to be restarted, the current speed of the motor must be obtained first. Otherwise, there will be a large slip and an overcurrent trip will occur, or the motor will be in a generating state due to the high rotor speed, causing the capacitor direct voltage to increase and trip.

[0003] The commonly used flying start method is to control the inverter output search voltage and frequency to make the stator generate current, and then perform vector decomposition to obtain the torque current component. When the stator rotating magnetic field speed is the same as the motor speed, the torque current component is approximately zero. By observing that the torque current is at its minimum value close to zero, the rotor frequency is searched, and then the inverter output voltage is controlled at the current frequency according to the V / F curve to complete the flying start. This method has the problem that it cannot make the search frequency step extremely small or the search voltage setting value is not appropriate. The output current is uncontrollable during the search process, and there is still an overcurrent problem.

[0004] The above contents are only used to assist in understanding the technical solution of the present invention and do not constitute an admission that the above contents are prior art. Summary of the invention

[0005] The main purpose of the present invention is to provide a method and control system for controlling flying start of an AC asynchronous motor, aiming to solve the technical problem of how to avoid uncontrollable current and unstable motor starting during the flying start of a motor controlled by a frequency converter.

[0006] The method for controlling flying start of an AC asynchronous motor of the present invention comprises:

[0007] During the process of starting the motor driven by the inverter, the inverter performs constant current control on the motor so that the stator and rotor of the motor are circuit equivalent;

[0008] The frequency converter applies a target current signal with a constant amplitude and a frequency that decreases step by step according to a preset step change frequency to the motor, so that the target current signal acts on the motor; wherein the motor is an AC asynchronous motor;

[0009] The frequency converter collects the motor voltage and current value at the machine end, and the controller calculates the equivalent inductance of the motor according to the motor voltage, the current value, and the applied angular frequency;

[0010] When the applied angular frequency of the inverter reaches the rotor frequency range of the motor, it is determined that the equivalent inductance value of the motor is the largest, and the maximum inductance of the motor is used as the searched target rotor frequency, and the motor is started on the fly based on the target rotor frequency.

[0011] Preferably, the step of the frequency converter performing constant current control on the motor specifically includes:

[0012] The frequency converter collects the three-phase current on the stator side of the motor, transforms the three-phase current into the d-axis current and the q-axis current in the two-phase rotating coordinate system through abc-dq transformation, makes a difference between the d-axis current and the given current amplitude, and sends it to the PI regulator, makes a difference between the q-axis current and 0, and sends the difference result to the PI regulator, so that the PI regulator forms a current closed-loop control, wherein, when different angular frequencies are applied during the constant current control process, the stator current I is maintained S The amplitude is a given value.

[0013] Preferably, the rotation phase θ applied during the abc-dq transformation is k , get the changing angular frequency w k Lower feedback current I d(k) ,I q(k) .

[0014] Preferably, the controller calculates the equivalent inductance of the motor according to the motor voltage, the current value, and the applied angular frequency, comprising:

[0015] The controller estimates the inductance curve of the motor, and the equivalent circuit impedance of the motor is Z = R s +L*f(w s ), terminal voltage U=I*(R s +L*f(w s ), then the equivalent inductance

[0016] Wherein, U is the terminal voltage of the motor, I is the current value, R s is the stator resistance of the motor, f(w s ) is the applied angular frequency.

[0017] Preferably, the step of estimating the inductance curve of the motor by the controller further includes:

[0018] Calculate the inductance value of the current sweep frequency segment and the change in inductance of the previous sweep frequency segment. If the change in inductance of the current sweep frequency segment is ΔL k >0, and the inductance change in the next frequency sweep band is ΔL k+1 <0, that is, when L has a peak inflection point, the current frequency is determined to be the rotor frequency.

[0019] Preferably, the preset step change frequency is 55 Hz to 0 Hz.

[0020] In addition, to achieve the above-mentioned purpose, the present invention also proposes an AC asynchronous motor flying start control system, the control system includes a frequency converter, an analog quantity sampling circuit, a controller, a drive module and a motor, and the motor is an AC asynchronous motor;

[0021] The AC input end of the frequency converter is connected to the power grid, and the output end of the frequency converter is connected to the motor through a cable, so that the frequency converter is equivalent to a constant current source, and the stator and rotor circuits of the motor are equivalent;

[0022] in,

[0023] The frequency converter is used to output a current signal to the stator side of the motor;

[0024] The analog quantity sampling circuit is connected to the frequency converter and the controller respectively, and is used to obtain the current signal and the voltage signal output by the frequency converter, perform signal conditioning on the current signal and the voltage signal, and output the current and voltage conditioning signals to the controller;

[0025] The controller is used to perform analog-to-digital conversion on the current and voltage conditioning signals, and to perform Park transformation on the three-phase currents Ia, Ib, and Ic obtained through the analog-to-digital conversion to obtain the d-axis current I d and the q-axis current I q ;

[0026] The controller is also used for the current I d and the current I q Perform frequency sweep control and current closed-loop control to synthesize a modulation wave PWM signal, and output the PWM signal to the driving module;

[0027] The driving module is used to output the PWM signal to drive the IGBT of the frequency converter, control the frequency converter to apply a target current signal with a constant amplitude and a frequency that decreases in a preset step-by-step manner to the motor, so that the target current signal acts on the motor, wherein the target current signal and the equivalent impedance of the frequency converter cause the stator side of the motor to generate a target voltage signal;

[0028] The analog quantity sampling circuit is also used to obtain the target voltage signal, and after conditioning the target voltage signal, send the target voltage signal to the controller;

[0029] The controller is used to estimate the inductance of the motor according to the target voltage signal and the target current signal, and perform a maximum inductance query through a frequency sweep control. When the maximum inductance is found, the maximum inductance found is used as the searched target rotor frequency.

[0030] Preferably, the frequency converter is provided with a plurality of bridge arms, and each bridge arm in the first direction is connected to the motor via a cable.

[0031] Preferably, the cables used to connect the motor and each bridge arm are provided with a voltage sensor and a current sensor;

[0032] The analog quantity sampling circuit includes a voltage sampling module and a current sampling module;

[0033] The voltage sensor is used to transmit the voltage signal to the voltage sampling module of the analog quantity sampling circuit for conditioning and then send it to the controller for voltage signal processing.

[0034] The current sensor is used to transmit the current signal to the current sampling module of the analog quantity sampling circuit, and then send it to the controller for current signal processing after conditioning.

[0035] The beneficial effects of the present invention are: the current is controllable during the flying start of the motor controlled by the inverter, and the rotor frequency can be quickly searched to achieve smooth starting of the motor; the inverter searches for the motor rotor frequency in the constant current control mode to achieve controllable current amplitude without overcurrent, and the operation is relatively safe and reliable with good characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The circuit structure connection diagram of the AC asynchronous motor flying start control system of the present invention;

[0037] Figure 2 The motor impedance equivalent circuit model of the AC asynchronous motor flying start control system of the present invention;

[0038] Figure 3 A schematic block diagram of a controller according to an embodiment of the present invention executing flying car start control;

[0039] Figure 4 The present invention is a flow chart of a method for controlling flying start of an AC asynchronous motor.

[0040] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0041] It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0042] Embodiment 1

[0043] Reference Figure 1 , Figure 1 The circuit structure connection diagram of the AC asynchronous motor flying start control system of the present invention;

[0044] The AC asynchronous motor flying start control system comprises a frequency converter 10, an analog quantity sampling circuit 20, a controller 30, a drive module 40 and a motor 50; the motor 50 is an AC asynchronous motor;

[0045] The AC input end of the inverter 10 is connected to the power grid, and the output end of the inverter is connected to the motor 50 through a cable, so that the inverter 10 is equivalent to a constant current source, and the stator and rotor circuits of the motor 50 are equivalent;

[0046] in,

[0047] The frequency converter 10 is used to output a current signal to the stator side of the motor 50;

[0048] The analog quantity sampling circuit 20 is connected to the inverter 10 and the controller 30 respectively, and is used to obtain the current signal and the voltage signal output by the inverter 10, perform signal conditioning on the current signal and the voltage signal, and output the current and voltage conditioning signals to the controller 30;

[0049] The controller 30 is used to perform analog-to-digital conversion on the current and voltage conditioning signals, and to perform Park transformation on the three-phase currents Ia, Ib, and Ic obtained through the analog-to-digital conversion to obtain the d-axis current I d and the q-axis current I q ;

[0050] The controller 30 is also used to d and the current I q Perform sweep frequency control and current closed-loop control to synthesize a modulation wave PWM signal, and output the PWM signal to the driving module 40;

[0051] The driving module 40 is used to output the PWM signal to drive the IGBT (Insulated Gate Bipolar Transistor) of the inverter 10, and control the inverter 10 to apply a target current signal with a constant amplitude and a frequency that decreases step by step according to a preset step change frequency to the motor 50, so that the target current signal acts on the motor 50, wherein the preset step change frequency is 55 Hz to 0 Hz, and the target current signal and the equivalent impedance of the inverter 10 cause the stator side of the motor 50 to generate a target voltage signal;

[0052] The analog quantity sampling circuit 20 is also used to obtain the target voltage signal, condition the target voltage signal and send the target voltage signal to the controller 30;

[0053] The controller 30 is used to estimate the inductance of the motor 50 according to the target voltage signal and the target current signal, and perform a maximum inductance query through a sweep frequency control. When the maximum inductance is found, the maximum inductance found is used as the searched target rotor frequency.

[0054] In a specific implementation, the frequency converter 10 is provided with a plurality of bridge arms, in a first direction (the first direction is Figure 1 Each bridge arm (in the horizontal direction) is connected to the motor 50 via a cable;

[0055] The analog quantity sampling circuit 20 includes a voltage sampling module 022 and a current sampling module 021;

[0056] Wherein, the cables used to connect the motor 50 and each bridge arm are provided with voltage sensors and current sensors;

[0057] The voltage sensor is used to transmit the voltage signal to the voltage sampling module 022 of the analog quantity sampling circuit 20 for conditioning and then to the controller 30 for voltage signal processing;

[0058] The current sensor is used to transmit the current signal to the current sampling module 021 of the analog quantity sampling circuit 20 , and then the current signal is sent to the controller 30 for current signal processing after conditioning.

[0059] Understandably, referring to Figure 2 , Figure 2 The motor impedance equivalent circuit model of the AC asynchronous motor flying start control system of this embodiment; the inverter 10 and the motor 50 of this embodiment are connected to an equivalent circuit, the inverter 10 is equivalent to a constant current source, the motor 50 is a load, and its equivalent impedance is Z = R s +L*f(w s ), the motor inductance parameter is not a constant, but a nonlinear parameter. The stator induced voltage vector has a relationship with the input current, equivalent impedance, and equivalent rotor voltage vector in Equation 1, where U S is the terminal voltage, I S Machine input current, U Ψr is the equivalent rotor voltage vector, L σ is the total stator and rotor leakage inductance, R s is the stator resistance.

[0060]

[0061]

[0062] Simplify the relationship in equation 1 to the following:

[0063]

[0064]

[0065] According to Formula 4, if the motor voltage, motor input current, and applied angular velocity are known, the equivalent inductance of the motor can be estimated. A variable frequency signal is applied to the motor input end, the angular velocity is a variable, the current rotor speed is n, and the slip is s. When the difference between the applied frequency and the rotor frequency is smaller, the inductance value estimated by Formula 4 is larger, and when the applied frequency is lower or higher than the rotor frequency, the inductance value is smaller. Therefore, when a constant current is applied and the frequency scanning mode is used, the maximum inductance value can be estimated from the collected machine-end voltage and current to lock the rotor frequency.

[0066] Reference Figure 3 This is a schematic diagram of the controller of this embodiment executing the flying start control. Since there is no voltage on the stator side in the initial state of the motor, the rotor rotates freely, and the stator winding forms a current path through constant current variable frequency control. The specific control steps are as follows:

[0067] (1) Set the initial frequency to 55 Hz. The initial phase θ is obtained by integrating the angular velocity: The three-phase current on the stator side is collected, and the three-phase currents Ia, Ib, and Ic are transformed from the three-phase stationary coordinate system to the two-phase vertical coordinate system αβ. The initial angle θ1 obtained by integration is used to convert the current component i in the two-phase stationary coordinate system α 、i β Perform Park transformation to obtain the feedback current I d ,I q .

[0068] (2) Set the frequency change step Δf and time change step Δt. The inverter changes the frequency from 55Hz to 0Hz according to the set step to w k =w k-1 +Δw, integrate the transformed angular velocity to get the rotation angle required for Park transformation At this rotation angle θ k Repeat the dq transformation in step (1) to obtain the changing angular frequency w k Lower feedback current I d(k) ,I q(k) .

[0069]

[0070] (3) The inverter sets the specified current of dq axis Id=I*, Iq=0, and the frequency sweep rotation angle θ k The current Id(k) ,I q(k) Perform PI closed-loop control and detect the feedback current I d(k) The difference between the given current I* and the measured feedback current I q(k) The difference between the given current 0 and the current is sent to the PI regulator. k Next, the output values ​​of the two PI regulators are subjected to dq inverse transformation to obtain a modulation wave, which acts on the inverter IGBT to keep the current amplitude on the motor input side constant.

[0071] (4) The induced voltage U at the collector end at the sampling frequency s and current I S According to the nonlinear relationship between inductance and voltage, current and angular frequency in formula (4), the motor inductance in the sweep frequency range of 55Hz-0Hz is estimated and stored.

[0072] (5) Query the constant current frequency w applied to the motor k Estimated motor inductance L K and the applied constant current frequency w k-1 Estimated motor inductance L K-1 , calculate the inductance difference ΔL of the current frequency sweep band k =L w(k) -L w(k-1) The frequency sweep band is w k The inductance change is calculated in turn for each frequency sweep range of 55Hz to 0Hz. If the current frequency sweep range is w k Lower ΔL k >0, and the next sweep frequency band w k+1 Lower ΔL k+1 <0, that is, when L has a peak inflection point, determine the current angular frequency w k is the rotor frequency.

[0073] The beneficial effects of this embodiment are: the current is controllable during the flying start of the motor controlled by the inverter, and the rotor frequency can be quickly searched to achieve smooth starting of the motor; the inverter searches for the motor rotor frequency in the constant current control mode to achieve controllable current amplitude without overcurrent, and the operation is relatively safe and reliable with good characteristics.

[0074] Embodiment 2

[0075] In this embodiment, the method for controlling the flying start of an AC asynchronous motor mainly includes the following technical links, which are: constant current control of the motor, frequency sweep control of the frequency converter, detection of the terminal voltage, estimation of the motor inductance value, and search for the rotor frequency. First, during the process of starting the motor driven by the frequency converter, the frequency converter is equivalent to a constant current source, the stator and rotor of the motor are circuit equivalent, and the frequency converter applies a current signal with a constant amplitude and a frequency decreasing from 55 to 0Hz according to the set step to the motor; then the frequency converter collects the terminal voltage and estimates the equivalent inductance of the motor based on the voltage value. When the applied frequency is close to the rotor frequency, the equivalent inductance value of the motor is the largest, and the inductance value is smaller when it is farther away from the rotor frequency. The maximum value of the inductance is recorded during the frequency sweep. At this time, the rotor frequency search is completed, the current frequency is recorded, and the motor is started by flying.

[0076] refer to Figure 4 , Figure 4 The flowchart of the method for controlling flying start of an AC asynchronous motor according to the present invention is as follows:

[0077] Step S10 (continuous current control of the frequency converter): during the process of starting the frequency converter-driven motor, the frequency converter performs constant current control on the motor so that the stator and the rotor of the motor are circuit equivalent; the motor is an AC asynchronous motor;

[0078] It is understandable that when the motor rotor does not stop rotating and the inverter detects that the motor has no voltage, the inverter executes a constant current control strategy and outputs a current signal with a constant amplitude and a starting frequency of 55Hz. The constant current control strategy includes collecting the three-phase current on the stator side, transforming the three-phase current into the d-axis current and q-axis current in the two-phase rotating coordinate system through the Park transformation of abc-dq. The initial value of the Park transformation angle is the angle θ1 obtained by integrating the angular frequency of 55Hz. The d-axis current is subtracted from the given current amplitude and sent to the PI regulator. The q-axis current is subtracted from 0 and sent to the PI regulator, so that the PI regulator forms a current closed-loop control, and the stator current I is always maintained during the constant current control process. S The amplitude is a given value.

[0079] Step S20 (sweep frequency control of the motor): the frequency converter applies a target current signal with a constant amplitude and a frequency that decreases in steps according to a preset step change to the motor, so that the target current signal acts on the motor;

[0080] Preferably, the preset step change frequency of this embodiment is 55 Hz to 0 Hz;

[0081] It can be understood that the frequency converter performs frequency sweep control from 55Hz to 0Hz according to the set frequency step Δf and time step Δt, and the frequency and time change evenly, then the angular velocity w kUniform change, integrating the angular velocity to obtain the rotation angle required for Park transformation The d-axis and q-axis components at different frequencies are calculated in turn and sent to the PI controller respectively to realize variable frequency constant current control in the sweep frequency mode.

[0082] Step S30 (estimating motor inductance): the frequency converter collects the motor voltage and current value at the machine end, and the controller calculates the equivalent inductance of the motor according to the motor voltage, the current value, and the applied angular frequency;

[0083] It is understandable that when the signals of step S10 and step S20 are applied, the motor equivalent circuit impedance is Z=R s +L*f(w s ), terminal voltage U=I*(R s +L*f(w s )),but The motor voltage, current and frequency are nonlinear functions. If the voltage and current are known, the motor inductance can be estimated. The inverter collects the terminal voltage and current, and estimates the motor equivalent inductance L from the voltage and current values;

[0084] In the specific implementation, the induced voltage U at the acquisition machine end is collected at the sampling frequency s and current I S According to the nonlinear relationship between inductance and voltage, current and angular frequency in formula (4), the motor inductance in the sweep frequency range of 55Hz-0Hz is estimated and stored.

[0085] Step S40 (searching for the maximum inductance value and tracking the rotor frequency): When the applied angular frequency of the inverter reaches the rotor frequency range of the motor, it is determined that the equivalent inductance value of the motor is the maximum, and the maximum inductance of the motor found is used as the searched target rotor frequency, and the motor is started on the fly based on the target rotor frequency.

[0086] It should be noted that under constant current, when the inverter scanning frequency is near the rotor frequency, the motor inductance value becomes larger, and when the scanning frequency is higher or lower than the rotor frequency, the motor inductance becomes smaller.

[0087] It is understandable that under constant current frequency sweep, the motor inductance curve is estimated based on the nonlinear functional relationship between voltage, current and frequency, and the difference ΔL between the inductance value at the current frequency sweep and the inductance value at the previous frequency sweep is determined. k =L w(k) -L w(k-1) , if the current sweep frequency range is ΔL k >0, and the next sweep frequency segment ΔL k+1 <0, that is, when L has a peak inflection point, the current frequency is judged to be the rotor frequency. Therefore, when the peak inflection point of the machine-end inductance is detected, it is considered that the rotor frequency is searched.

[0088] The beneficial effects of this embodiment are: the current is controllable during the flying start of the motor controlled by the inverter, and the rotor frequency can be quickly searched to achieve smooth starting of the motor; the inverter searches for the motor rotor frequency in the constant current control mode to achieve controllable current amplitude without overcurrent, and the operation is relatively safe and reliable with good characteristics.

[0089] The serial numbers of the embodiments of the present invention are only for description and do not represent the advantages and disadvantages of the embodiments. The above are only preferred embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for controlling flying start of an AC asynchronous motor, characterized in that: The method comprises: During the process of starting the motor driven by the inverter, the inverter performs constant current control on the motor so that the stator and rotor of the motor are circuit equivalent; The frequency converter applies a target current signal with a constant amplitude and a frequency that decreases step by step according to a preset step change frequency to the motor, so that the target current signal acts on the motor; wherein the motor is an AC asynchronous motor; The frequency converter collects the motor voltage and current values ​​at the machine end, and the controller calculates the equivalent inductance of the motor according to the motor voltage, the current value, and the applied angular frequency; When the applied angular frequency of the frequency converter reaches the rotor frequency range of the motor, the frequency converter determines that the equivalent inductance value of the motor is the largest, takes the maximum inductance of the motor found as the searched target rotor frequency, and implements flying start of the motor based on the target rotor frequency; The controller calculates the equivalent inductance of the motor according to the motor voltage, the current value, and the applied angular frequency, including: The controller estimates the inductance curve of the motor, and the equivalent circuit impedance of the motor is Z = R s +L*f(w s ), terminal voltage U=I*(R s +L*f(w s )), then the equivalent inductance Wherein, U is the terminal voltage of the motor, I is the current value, R s is the stator resistance of the motor, f(w s ) is the applied angular frequency; The step of estimating the inductance curve of the motor by the controller further includes: Calculate the inductance value of the current sweep frequency segment and the change in inductance of the previous sweep frequency segment. If the change in inductance of the current sweep frequency segment is ΔL k >0, and the inductance change in the next frequency sweep band is ΔL k+1 <0, that is, when L has a peak inflection point, the current frequency is determined to be the rotor frequency.

2. The method for controlling flying start of an AC asynchronous motor as claimed in claim 1, characterized in that: The step of the frequency converter performing constant current control on the motor specifically includes: The frequency converter collects the three-phase current on the stator side of the motor, transforms the three-phase current into the d-axis current and the q-axis current in the two-phase rotating coordinate system through abc-dq transformation, makes a difference between the d-axis current and the given current amplitude, and sends it to the PI regulator, makes a difference between the q-axis current and 0, and sends the difference result to the PI regulator, so that the PI regulator forms a current closed-loop control, wherein, when different angular frequencies are applied during the constant current control process, the stator current I is maintained S The amplitude is a given value.

3. The method for controlling flying start of an AC asynchronous motor as claimed in claim 2, characterized in that: The rotation phase θ of the transformation applied during the abc-dq transformation process k , get the changing angular frequency w k Lower feedback current I d(k) ,I q(k) .

4. The method for controlling flying start of an AC asynchronous motor according to any one of claims 1 to 3, characterized in that: The preset step change frequency is 55 Hz to 0 Hz.

5. An AC asynchronous motor flying start control system, characterized in that: The control system includes a frequency converter, an analog quantity sampling circuit, a controller, a drive module and a motor, and the motor is an AC asynchronous motor; The AC input end of the frequency converter is connected to the power grid, and the output end of the frequency converter is connected to the motor through a cable, so that the frequency converter is equivalent to a constant current source, and the stator and rotor circuits of the motor are equivalent; in, The frequency converter is used to output a current signal to the stator side of the motor; The analog quantity sampling circuit is connected to the frequency converter and the controller respectively, and is used to obtain the current signal and the voltage signal output by the frequency converter, perform signal conditioning on the current signal and the voltage signal, and output the current and voltage conditioning signals to the controller; The controller is used to perform analog-to-digital conversion on the current and voltage conditioning signals, and to perform Park transformation on the three-phase currents Ia, Ib, and Ic obtained through the analog-to-digital conversion to obtain the d-axis current I d and the q-axis current I q ; The controller is also used for the current I d and the current I q Perform frequency sweep control and current closed-loop control to synthesize a modulation wave PWM signal, and output the PWM signal to the driving module; The driving module is used to output the PWM signal to drive the IGBT of the frequency converter, control the frequency converter to apply a target current signal with a constant amplitude and a frequency that decreases in a preset step-by-step manner to the motor, so that the target current signal acts on the motor, wherein the target current signal and the equivalent impedance of the frequency converter cause the stator side of the motor to generate a target voltage signal; The analog quantity sampling circuit is also used to obtain the target voltage signal, and after conditioning the target voltage signal, send the target voltage signal to the controller; The controller is used to estimate the inductance of the motor according to the target voltage signal and the target current signal, and perform a maximum inductance query through a frequency sweep control, and when the maximum inductance is found, use the found maximum inductance as the searched target rotor frequency; The controller calculates the equivalent inductance of the motor according to the motor voltage, current value, and applied angular frequency, including: The controller estimates the inductance curve of the motor, and the equivalent circuit impedance of the motor is Z = R s +L*f(w s ), terminal voltage U=I*(R s +L*f(w s )), then the equivalent inductance Wherein, U is the terminal voltage of the motor, I is the current value, R s is the stator resistance of the motor, f(w s ) is the applied angular frequency; The controller estimates the inductance curve of the motor, further comprising: Calculate the inductance value of the current sweep frequency segment and the change in inductance of the previous sweep frequency segment. If the change in inductance of the current sweep frequency segment is ΔL k >0, and the inductance change in the next frequency sweep band is ΔL k+1 <0, that is, when L has a peak inflection point, the current frequency is determined to be the rotor frequency.

6. The AC asynchronous motor flying start control system as claimed in claim 5, characterized in that: The frequency converter is provided with a plurality of bridge arms, and each bridge arm in a first direction is connected to the motor via a cable.

7. The AC asynchronous motor flying start control system according to claim 5, characterized in that: The cables used to connect the motor and each bridge arm are provided with voltage sensors and current sensors; The analog quantity sampling circuit includes a voltage sampling module and a current sampling module; The voltage sensor is used to transmit the voltage signal to the voltage sampling module of the analog quantity sampling circuit for conditioning and then send it to the controller for voltage signal processing; The current sensor is used to transmit the current signal to the current sampling module of the analog quantity sampling circuit, and then send it to the controller for current signal processing after conditioning.

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