Method and system for starting a brushless doubly-fed motor with power winding string reactance

Abstract

This invention discloses a method and system for starting a brushless doubly-fed motor with a series reactor in the power winding. During the motor starting phase, a reactor is connected in series in the power winding circuit to limit the starting current. Once the motor speed reaches a preset value, the reactor is disconnected and the inverter drives the control winding, causing the system to enter doubly-fed operation. This invention effectively suppresses starting shock using simple reactor components, eliminates the need for a dedicated soft starter, and is low-cost, highly reliable, and easy to implement in engineering.

Description

Technical Field

[0001] This invention relates to the field of brushless doubly fed motor starting control technology, specifically to a method and system for starting a brushless doubly fed motor with power winding series reactor. Background Technology

[0002] A brushless doubly fed motor (BDFM) is a new type of AC motor with a dual-winding stator and a brushless, slip-ring-free rotor. Its stator has two independent windings: a power winding (PW) and a control winding (CW), while the rotor typically uses a squirrel-cage design. The PW is directly connected to the mains power grid, while the CW is powered by a frequency converter. The two windings are indirectly electromagnetically coupled through the rotor, enabling energy transfer and speed control.

[0003] The significant advantage of the BDFM variable frequency speed control system lies in its matching frequency converter, which is a slip capacity type and only needs to handle a capacity equivalent to the slip power, typically 25% to 30% of the motor's rated power. During normal operation, the system speed regulation range is ±30% of the synchronous speed. This characteristic gives the BDFM system significant potential in reducing frequency converter capacity and cost, while also allowing for flexible adjustment of the grid-side power factor.

[0004] However, BDFM faces starting challenges in electric operation scenarios. Because the matching inverter is a small-capacity slip-capacity type, it cannot withstand the large current and high power demands of the control side during the motor's zero-speed start-up phase. If a direct starting method is used, i.e., directly closing the switch between the grid and the PW to start the motor at full voltage at the power frequency, the starting current typically reaches 5 to 7 times the rated current, easily causing grid voltage fluctuations and surges. Simultaneously, the sudden change in starting torque leads to mechanical shock and vibration. For BDFM, direct starting causes significant overvoltage and overcurrent in the CW due to electromagnetic coupling, which can easily damage the small-capacity inverter.

[0005] In the existing technology, the main starting methods for BDFM include: (1) starting in asynchronous mode after short-circuiting the CW. This method requires the frequency converter to participate in the bridge arm switch control to effectively insert a series resistance, which still places certain power requirements on the frequency converter; (2) using a large-capacity frequency converter to directly drive the start-up. Although this method is feasible, it increases the system cost, which contradicts the original intention of reducing the cost of BDFM by using a small-capacity frequency converter. None of the above solutions can achieve reliable starting of BDFM from zero speed while protecting the small-capacity frequency converter.

[0006] Therefore, there is an urgent need to propose a method suitable for BDFM that can achieve reliable and smooth start-up under the condition of small-capacity bidirectional frequency converters, effectively protect the frequency converter from impact during the start-up phase, and suppress the starting current on the grid side. Summary of the Invention

[0007] In view of the technical problems in the prior art, when a brushless doubly fed motor is configured with a small-capacity bidirectional frequency converter, the frequency converter is unable to withstand the power and current requirements of the control side during the zero-speed start-up stage, which easily leads to starting difficulties or frequency converter overcurrent and overvoltage protection. At the same time, the large current of direct starting will cause the grid voltage drop. This invention provides a method and system for starting a brushless doubly fed motor with a series reactor in the power winding.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a method for starting a brushless doubly-fed motor with a power winding series reactor, applicable to a brushless doubly-fed motor having a stator power winding, a stator control winding, and a squirrel-cage rotor, wherein the stator power winding is connected to the power frequency grid, and the stator control winding is connected to a bidirectional frequency converter, comprising the following steps: Step 1, Startup Preparation: The control system confirms that both main contactor K1 and bypass contactor K2 are in the open state, and confirms that the switching switch K3 connecting the bidirectional frequency converter and the stator control winding is in the open state, thus putting the stator control winding in an open circuit state and electrically isolating the bidirectional frequency converter from the brushless doubly fed motor. Before starting, the control system confirms that both main contactor K1 and bypass contactor K2 are open. The key step is to ensure that the frequency converter switching switch K3 is in the open state. This ensures that the small-capacity frequency converter will not be subjected to uncontrollable inrush current or voltage at the moment of startup.

[0009] Step 2, series reactor reduced voltage start: Close the main contactor K1, and the power frequency grid voltage is applied to the stator power winding after being divided by the starting reactor L connected in series in the stator power winding power supply circuit. The brushless doubly fed motor uses the squirrel-cage rotor to generate asynchronous torque and accelerates from zero speed. During this stage, the switching switch K3 remains open and the stator control winding remains open. At time t1, the main contactor K1 is closed. The grid voltage is applied to the power winding PW after being divided by the starting reactor L. The motor begins to accelerate using the asynchronous torque generated by the rotor squirrel cage.

[0010] Starting reactor Constraint design: At this stage, the control winding is open-circuited, and the frequency converter does not participate. The starting reactor L is the core component for achieving smooth starting, and its impedance value... The selection must meet the condition of "limiting grid current": Constraints: Limiting the starting current on the grid side (main purpose) This is to address the issue of high direct starting current. It must be large enough to handle the starting current. Limits to grid allowable values Assuming the equivalent impedance of the motor is relatively small at startup and current limiting is mainly achieved through reactance, then in a three-phase system, the starting current is approximately: In order to satisfy The correct answer is: In the formula The effective value of the grid line voltage connected to the PW side (unit: V). This is the maximum starting current allowed by the power grid (unit: A).

[0011] In step two, the reactance value of the starting reactor L The following conditions must be met: ; In the formula, The effective value of the line voltage of the power frequency grid is given in V. This is the maximum starting current allowed by the power grid, expressed in amperes (A).

[0012] Step 3, Bypass Switching: Monitor the speed n of the brushless doubly fed motor. When the speed n reaches the preset minimum operating speed... At the same time, the bypass contactor K2, which is connected in parallel with the starting reactor L, is closed to bypass the starting reactor L, allowing the stator power winding to return to full-voltage operation of the power grid; the system monitors the motor speed. The synchronous speed can be expressed as: In the formula For the power grid frequency, This represents the number of pole pairs corresponding to PW. The motor accelerates to the preset minimum operating speed. That is, satisfying At that time, the controller is The bypass contactor K2 is closed at all times to enable the starting reactor. The circuit is bypassed, and the power winding PW resumes full-voltage operation.

[0013] In step three, the preset minimum operating speed Based on the synchronous speed of the brushless doubly fed motor Confirmed, the synchronous speed The calculation formula is: ; In the formula, The frequency of the power grid is expressed in Hz. The number of pole pairs of the stator power winding; The number of pole pairs in the stator control winding. The preset minimum operating speed. At the synchronous speed Within the nearby speed range, it meets the requirements. .

[0014] Step 4, Inverter activation: After the stator power winding is running at full voltage and the speed n is stable, close the switching switch K3 to connect the bidirectional inverter to the stator control winding. The bidirectional inverter modulates the stator control winding to pull the brushless doubly fed motor into synchronous operation and enter the doubly fed speed regulation operation state.

[0015] After the power winding is running at full voltage and the speed is stable in the minimum operating speed range (at time t3), the variable frequency speed control system composed of a brushless doubly fed motor (BDFM) can normally operate within a speed range of ±30% of the synchronous speed. At this time, the motor slip is very small, and the power and current requirements on the control side are significantly reduced, which is within the capability range of a small-capacity frequency converter.

[0016] At this point, switch K3 is closed, connecting the small-capacity frequency converter to the control winding. The frequency converter begins modulation, pulling the motor into synchronization and entering doubly-fed speed regulation operation, completing the entire starting process.

[0017] Secondly, the present invention also provides a brushless doubly-fed motor power winding series reactor starting system, comprising: a brushless doubly-fed motor having a stator power winding, a stator control winding, and a squirrel-cage rotor; a starting reactor L connected in series in the power supply circuit of the stator power winding; a main contactor K1 disposed between the power frequency grid and the power supply circuit of the stator power winding, for connecting or disconnecting the power supply from the power frequency grid to the stator power winding; a bypass contactor K2 connected in parallel with the starting reactor L, for bypassing the starting reactor L when closed; and a bidirectional frequency converter, which is a slip capacity type frequency converter; A switching switch K3 is disposed between the bidirectional frequency converter and the stator control winding, used to keep the stator control winding in an open circuit state when disconnected to electrically isolate the bidirectional frequency converter from the brushless doubly fed motor, and to connect the bidirectional frequency converter to the stator control winding when closed; a speed detection unit is used to detect the speed n of the brushless doubly fed motor; a controller is signal-connected to the main contactor K1, the bypass contactor K2, the switching switch K3 and the speed detection unit respectively, and is used to control the on / off sequence of the main contactor K1, the bypass contactor K2 and the switching switch K3 according to the starting method according to any one of claims 1 to 6.

[0018] The reactance value of the starting reactor L satisfy: ; In the formula, The effective value of the line voltage of the power frequency grid. This is the maximum starting current allowed by the power grid.

[0019] The switching switch K3 is a normally open contactor.

[0020] The controller executes the following control logic during startup: Before starting, confirm that the main contactor K1, the bypass contactor K2, and the switching switch K3 are all in the open state; issue a command to close the main contactor K1, so that the power frequency grid voltage is applied to the stator power winding through the starting reactor L; acquire the speed n fed back by the speed detection unit in real time, and when the speed n reaches the preset minimum operating speed... When the bypass contactor K2 is closed and the speed n is stable, a command is issued to close the switching switch K3 to connect the bidirectional frequency converter to the stator control winding.

[0021] The technical principle of this invention is as follows: 1. This invention is based on the unique electromagnetic characteristics of a brushless doubly fed induction motor (BDFM) when the control winding (CW) is open-circuited. When the CW is open-circuited, the power winding (PW) of the BDFM and the squirrel-cage rotor form a closed loop, and the motor is equivalent to a wound-rotor asynchronous motor. At this time, if a reactor is connected in series on the PW side, the voltage-dividing and current-limiting characteristics of the reactor can be used to significantly reduce the stator terminal voltage, thereby preventing a large drop in the grid voltage.

[0022] Under no-load or light-load conditions, the starting torque generated by this equivalent asynchronous mode, although lower than the rated value, is sufficient to overcome the load resistance torque and complete acceleration. When the speed rises to the "minimum operating speed" logic point near the synchronous speed, the reactor is disconnected and the CW circuit is closed by switching action, and the motor smoothly transitions from asynchronous starting mode to doubly-fed synchronous operation mode. This process utilizes the natural coupling characteristics of the BDFM rotor structure, and can achieve the entire process from standstill to synchronous operation without complex vector control algorithms.

[0023] 2. Core technology invention points: 1) Pioneering a "CW open-circuit + PW series reactor" BDFM starting topology: This breaks through the limitation of traditional BDFMs that must rely on large-capacity frequency converters for direct drive or complex soft starters. It is the first to propose an architecture that performs reduced-voltage starting only on the power winding side with a series reactor when the control winding is open. This architecture utilizes the squirrel-cage effect of the BDFM itself to achieve asynchronous starting, filling the gap in low-cost starting solutions in this field.

[0024] 2) A phased, smooth triggering strategy based on logical conditions: This strategy abandons the traditional approach of relying on fixed-time delays or complex synchronization detection algorithms, and innovatively adopts a "logic condition triggering" mechanism. Using "minimum operating speed" as the core criterion, it combines current state and other logical conditions to control the timing of reactor disconnection and CW activation. This strategy avoids switching shocks caused by inaccurate timing settings and simplifies the hardware cost of the control system, making it particularly suitable for no-load / light-load scenarios where dynamic response requirements are not stringent.

[0025] 3. Simplified Protection Design for Small-Capacity Frequency Inverters: Addressing the weakness of small-capacity frequency inverters in overcurrent capability, this invention does not pursue full-range torque optimization, but rather precisely matches the requirements of no-load / light-load conditions. Starting current can be clamped within a safe threshold through calculation of a single reactor parameter. This resolves the conflict between large starting current and small frequency inverters with minimal hardware cost (only adding contactors and reactors), achieving an optimal balance between system cost and reliability.

[0026] 4. In the implementation of the technical solution of the present invention: Start-up process: After the system is powered on, the CW circuit is disconnected, and the reactor is connected in series in the PW circuit. The motor accelerates asynchronously, and the current is limited to below the set value by the reactor.

[0027] Switching process: The control system monitors in real time whether the speed has reached the preset logic threshold (minimum operating speed), and connects the CW control loop after the speed has stabilized further.

[0028] Operation process: The motor enters the standard doubly fed operating mode and speed regulation control is performed.

[0029] Parameter basis: The inductance value of the reactor is calculated and determined based on the current limiting formula; the switching timing is determined based on the speed logic, without the need for specific millisecond-level timing, ensuring the robustness of the project implementation.

[0030] Compared with the prior art, the present invention has the following beneficial effects: 1. Starting current suppression: By connecting a starting reactor L in series in the stator power winding power supply circuit, the voltage applied to the power winding during the starting phase is reduced, the starting current is effectively limited, the impact of starting current surge on grid voltage fluctuations is suppressed, and grid friendliness is improved. 2. Inverter protection: During the initial startup phase, the stator control winding is kept in an open circuit state, which completely isolates the small-capacity bidirectional inverter from the electromagnetic shock during the startup phase, thus avoiding damage to the inverter caused by startup overcurrent and overvoltage. 3. Smooth transition: The bypass switching time is selected in the operating range where the motor has completed the main acceleration and the starting current has dropped significantly, reducing the risk of current surges and voltage jumps when the bypass is engaged; the inverter is engaged at high speed and low slip conditions, achieving a smooth transition from asynchronous start-up to doubly fed speed regulation. 4. Cost advantage: This method only requires the addition of a starting reactor, a bypass contactor, and a switching switch, without the need for a large-capacity frequency converter or an additional soft starter, which significantly reduces the system cost. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the topology of the brushless doubly fed motor power winding series reactor starting system of the present invention; Figure 2 This is a timing diagram of each switch during the startup process of the present invention; Figure 3 This is a control flowchart of the starting method of the present invention.

[0032] In the diagram: stator power winding PW; control winding CW; starting reactor L; main contactor K1; bypass contactor K2; switching switch K3. Detailed Implementation

[0033] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the implementation of the present invention is not limited thereto.

[0034] Example 1: System Topology like Figure 1 As shown, this embodiment provides a brushless doubly-fed motor power winding series reactor starting system, which includes the following components: (1) Brushless doubly-fed motor: It has a stator power winding PW, a stator control winding CW, and a squirrel-cage rotor. The number of pole pairs of PW is The number of extreme logs of CW is The two sets of stator windings are indirectly electromagnetically coupled through the rotor.

[0035] (2) Main contactor K1: It is installed between the three-phase power frequency grid and the power supply circuit of the stator power winding PW, and is used to connect or disconnect the power supply of the grid to PW.

[0036] (3) Starting reactor L: A three-phase starting reactor connected in series in the power supply circuit between the main contactor K1 and the stator power winding PW. The reactance value of starting reactor L... Determined based on the grid line voltage and the maximum allowable starting current.

[0037] (4) Bypass contactor K2: It is connected in parallel with the starting reactor L. When closed, it short-circuits the starting reactor L to bypass it, so that PW can be restored to full voltage operation.

[0038] (5) Bidirectional frequency converter: It is a slip capacity type frequency converter with a capacity of 25% to 30% of the rated power of the motor.

[0039] (6) Switch K3: It is a normally open contactor, which is installed between the bidirectional frequency converter and the stator control winding CW. When it is open, CW is in a completely open circuit state, and the bidirectional frequency converter is electrically isolated from the motor; when it is closed, the bidirectional frequency converter is connected to CW.

[0040] (7) Rotor speed detection unit: used to detect the rotor speed n of the brushless doubly fed motor in real time and feed the speed signal back to the controller. The rotor speed detection unit can use an encoder, a rotary transformer or a sensorless speed estimation method based on voltage and current signals.

[0041] (8) Controller: It is connected to the main contactor K1, bypass contactor K2, switching switch K3 and speed detection unit respectively, and is used to issue on / off commands of each switch according to the specified timing to realize automatic control of the starting process.

[0042] Example 2: Start-up Method like Figure 2 and Figure 3 As shown, this embodiment provides a method for starting a brushless doubly-fed motor by series reactor of the power winding. Based on the system described in Embodiment 1, it is divided into the following four stages in chronological order: Step 1, Start-up Preparation Phase: Before startup, the controller performs an initialization check, confirming that the main contactor K1, bypass contactor K2, and switching switch K3 are all in the open state. At this time, the stator power winding PW is not connected to the power grid, the stator control winding CW is completely open, and the bidirectional inverter is completely electrically isolated from the motor. The key purpose of this step is to ensure that the small-capacity inverter will not be subjected to uncontrollable inrush current or voltage at startup.

[0043] Step 2, Series Reactor Voltage Reduction Start-up Stage: The controller issues a command to close the main contactor K1. After K1 closes, the three-phase power grid voltage is applied to the stator power winding PW after being divided by the starting reactor L. Due to the impedance of the starting reactor L, the voltage applied to PW is lower than the grid line voltage, effectively limiting the starting current. During this stage, the switching switch K3 remains open, the stator control winding CW remains open, and the bidirectional frequency converter does not participate in the starting process. The brushless doubly-fed motor generates asynchronous torque through the interaction between the squirrel-cage rotor and the magnetic field of PW, accelerating from zero speed.

[0044] Reactance value of starting reactor L The selection of the starting current must meet the condition of limiting the starting current on the grid side. In a three-phase system, the starting current... Approximately: ; To make the starting current Not exceeding the maximum starting current allowed by the power grid The reactance value of the starting reactor L should meet the following requirements: ; In the formula, The effective value of the power frequency grid line voltage connected to the PW side, in V; This is the maximum starting current allowed by the power grid, expressed in amperes (A).

[0045] Under CW open-circuit conditions, the PW side of BDFM is equivalent to a pole pair number of An asynchronous motor. The asynchronous torque generated by the interaction between the rotor squirrel cage and the PW magnetic field drives the motor to accelerate. As the speed increases, the rotor current gradually decreases, and the PW side current also decreases accordingly.

[0046] Step 3, Bypass Switching Phase: The controller monitors the speed n of the brushless doubly-fed motor in real time through a speed detection unit. The synchronous speed of the brushless doubly-fed motor... Determined by the following formula: ; In the formula, The power grid frequency is expressed in Hz. This represents the number of pole pairs in the stator power winding. This represents the number of pole pairs in the stator control winding.

[0047] When the rotational speed n reaches the preset minimum operating speed At this time, the controller issues a command to close the bypass contactor K2. After K2 closes, the starting reactor L is short-circuited and bypassed, and the stator power winding PW resumes full-voltage operation of the power grid.

[0048] The preset minimum operating speed Located at synchronous speed Within the nearby speed regulation range, the following conditions are met: ; The selection criteria for bypass switching time are as follows: at this time, the motor has completed the main acceleration process, the speed is close to the synchronous speed, the starting current has dropped significantly to close to the rated current level, the risk of current change and voltage jump when the bypass is engaged is low, and a smooth transition can be achieved.

[0049] Step 4, Inverter commissioning stage: The stator power winding operates at full voltage PW and the speed n is stable at the minimum operating speed. After the above steps (for a variable frequency speed control system consisting of a brushless doubly fed motor (BDFM), the normal operating speed range is ±30% of the synchronous speed), the controller issues a command to close the switching switch K3. After K3 is closed, the small-capacity bidirectional frequency converter connects to the stator control winding CW. At this time, the motor slip is very small, operating under low slip conditions. The voltage and current requirements on the CW side have been significantly reduced and are within the rated capacity range of the small-capacity frequency converter. The frequency converter begins to modulate CW, applying a voltage of appropriate frequency and amplitude to pull the motor into synchronous operation, entering the doubly fed speed control state, and completing the entire starting process.

[0050] Example 3: Parameter Design The parameter design is explained using a 200kW rated power BDFM as an example. The motor parameters are as follows: PW, number of pole pairs. =1, CW pole pair number =3, rated voltage is 10kV, rated current is 15A, grid frequency =50Hz.

[0051] (1) Calculation of synchronous speed: According to the formula, =60×50 / (1+3)=750r / min.

[0052] (2) Minimum operating speed setting: Take ≥0.7×750=525r / min. This embodiment uses... =600r / min.

[0053] (3) Calculation of starting reactor parameters: Assume the grid line voltage =10000V, maximum allowable starting current of the power grid =45A (3 times the rated current), then ≥10000 / (1.732×45)≈128.3Ω. When selecting a specific type, take... =130Ω, to leave a margin.

[0054] Example 4: Controller Control Logic like Figure 3 As shown, the controller executes the following control logic during startup: S1: Controller power-on initialization, reads preset parameters, including minimum operating speed. The set value; S2: Check the status of main contactor K1, bypass contactor K2 and switching switch K3, and confirm that all three are in the open state; S3: After receiving the start command, it sends a control signal to close the main contactor K1; S4: Enter the speed monitoring cycle and obtain the speed n fed back by the speed detection unit in real time; S5: Determine whether the rotational speed n has reached the preset minimum operating speed. If n < Then return to S4 to continue monitoring if n ≥ Then proceed to S6; S6: Issues a control signal to close the bypass contactor K2, and the starting reactor L is bypassed; S7: Wait for the rotation speed to stabilize, and determine whether the rotation speed n remains stable within the preset range; S8: Sends a control signal to close the switching switch K3, and the bidirectional frequency converter is connected to the stator control winding CW; S9: The frequency converter executes synchronous pull-in control, the motor enters the doubly fed speed regulation operation state, and the starting process is completed.

[0055] It should be noted that the above embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention. For example, the starting reactor L can be replaced by a starting resistor or an autotransformer or other step-down and current-limiting components; the switching switch K3 can be a circuit breaker, a solid-state relay, or other controllable switching devices; and the specific implementation of the speed detection unit is not limited to the scheme described herein.

Claims

1. A method of starting a brushless doubly-fed motor power winding string reactance, characterized by: This invention relates to a brushless doubly-fed induction motor with a stator power winding, a stator control winding, and a squirrel-cage rotor, wherein the stator power winding is connected to the power frequency grid, and the stator control winding is connected to a bidirectional frequency converter. The invention includes the following steps: Step 1, Start-up preparation: The control system confirms that both the main contactor K1 and the bypass contactor K2 are in the open state, and confirms that the switching switch K3 connecting the bidirectional frequency converter and the stator control winding is in the open state, so that the stator control winding is in the open circuit state, and the bidirectional frequency converter is electrically isolated from the brushless doubly fed motor. Step 2, series reactor reduced voltage start: Close the main contactor K1, the power frequency grid voltage is applied to the stator power winding after being divided by the starting reactor L connected in series in the stator power winding power supply circuit. The brushless doubly fed motor uses the squirrel-cage rotor to generate asynchronous torque and accelerates from zero speed. During this stage, the switching switch K3 remains open and the stator control winding remains open. Step three, bypass switch: monitoring the speed of brushless doubly-fed motor n, when the speed n reaches the preset minimum working speed , closing the bypass contactor K2 in parallel with the starting reactor L, bypassing the starting reactor L, and restoring the stator power winding to full voltage operation of the power grid; Step 4, Inverter activation: After the stator power winding is running at full voltage and the speed n is stable, close the switching switch K3 to connect the bidirectional inverter to the stator control winding. The bidirectional inverter modulates the stator control winding, pulling the brushless doubly fed motor into synchronous operation and entering the doubly fed speed regulation operation state.

2. The brushless doubly-fed motor power winding series reactor starting method according to claim 1, characterized in that: In step two, the reactance value of the starting reactor L The following conditions must be met: ； In the formula, This represents the effective value of the line voltage in the power frequency grid. This is the maximum starting current allowed by the power grid.

3. The brushless doubly-fed motor power winding series reactor starting method according to claim 2, characterized in that: In step three, the preset minimum operating speed Based on the synchronous speed of the brushless doubly fed motor Confirmed, synchronous speed The calculation formula is: ； In the formula, The frequency of the power grid, This represents the number of pole pairs in the stator power winding. This represents the number of pole pairs in the stator control winding.

4. The brushless doubly-fed motor power winding series reactor starting method according to claim 3, characterized in that: The preset minimum operating speed satisfy: ； in, This is the synchronous speed.

5. The brushless doubly-fed motor power winding series reactor starting method according to claim 4, characterized in that: In step four, the prerequisite for closing the switching switch K3 is that the stator power winding is already in full-voltage operation and the rotational speed n has stabilized at the preset minimum operating speed. The above ensures that the slip of the brushless doubly fed motor is in a low slip condition.

6. The brushless doubly-fed motor power winding series reactor starting method according to claim 5, characterized in that: Before step one, there is also a parameter tuning step: the reactance value of the starting reactor L is determined according to the effective value of the line voltage of the power frequency grid and the maximum starting current allowed by the grid, and the preset minimum operating speed is determined according to the pole pair parameters of the brushless doubly fed motor and the frequency of the power frequency grid.

7. A brushless doubly-fed motor power winding series reactor starting system, characterized in that: include: A brushless doubly fed motor has a stator power winding, a stator control winding, and a squirrel-cage rotor; The starting reactor L is connected in series in the power supply circuit of the stator power winding; The main contactor K1 is located between the power frequency grid and the power supply circuit of the stator power winding, and is used to connect or disconnect the power frequency grid from the stator power winding. Bypass contactor K2 is connected in parallel with starting reactor L and is used to bypass starting reactor L when closed; Bidirectional frequency converter, which is a slip capacity type frequency converter; The switching switch K3 is located between the bidirectional frequency converter and the stator control winding. When it is open, it keeps the stator control winding in an open circuit state to electrically isolate the bidirectional frequency converter from the brushless doubly fed motor. When it is closed, it connects the bidirectional frequency converter to the stator control winding. The speed detection unit is used to detect the speed n of the brushless doubly fed motor; The controller is connected to the main contactor K1, the bypass contactor K2, the switching switch K3 and the speed detection unit respectively, and is used to control the on-off sequence of the main contactor K1, the bypass contactor K2 and the switching switch K3 according to the starting method described in any one of claims 1 to 6.

8. The brushless doubly-fed motor power winding series reactor starting system according to claim 7, characterized in that: The reactance value of the starting reactor L satisfy: ； In the formula, This represents the effective value of the line voltage in the power frequency grid. This is the maximum starting current allowed by the power grid.

9. The brushless doubly-fed motor power winding series reactor starting system according to claim 8, characterized in that: The switching switch K3 is a normally open contactor.

10. The brushless doubly-fed motor power winding series reactor starting system according to claim 9, characterized in that: The controller executes the following control logic during startup: Before starting, ensure that the main contactor K1, bypass contactor K2 and switching switch K3 are all in the open state; The command to close the main contactor K1 is issued, so that the power frequency grid voltage is applied to the stator power winding through the starting reactor L; The rotational speed n is acquired in real time from the speed detection unit. When the rotational speed n reaches the preset minimum operating speed... At that time, a command to close the bypass contactor K2 is issued; After the bypass contactor K2 is closed and the speed n is stable, a command is issued to close the switching switch K3 to connect the bidirectional frequency converter to the stator control winding.

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