Method for automatically adjusting output voltage of frequency converter

Abstract

The invention relates to a method for automatically adjusting the output voltage of a frequency converter, which belongs to the field of power electronics and power transmission and comprises two operation modes, namely a constant voltage mode and a constant magnetic flux mode. In the constant voltage mode, after the output modulation degree is saturated, the saturation flag bit is set to be 1, the regulator stops working, the output frequency of the regulator can be continuously increased to reach the output rated frequency, and the running state is in a weak magnetic state. The zero clearing of the saturation mark position 1 is realized by hysteresis control so as to prevent the AVR regulator from frequently switching and enabling at the saturation point. And in the constant magnetic flux mode, after the modulation degree is saturated, namely when the output voltage reaches the maximum, the frequency does not rise any more, and the frequency at the moment is marked as the maximum operation frequency. And when the system is at the maximum operation frequency point, if the direct current bus voltage fluctuates and an operation point deviates from an expected constant magnetic flux VF curve, the system automatically accelerates or decelerates until a new balance point is reached and continues to operate. The two operation modes can be switched on line.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics and electric drive, and relates to a method for automatically adjusting the output voltage of a frequency converter. Background Art

[0002] The inverter is applied to shore power supply, which has the advantage of adjustable power frequency. Because foreign ships need a power supply with a frequency of 60Hz, and most shore power supplies are 50Hz, it is very convenient for the inverter to achieve 50 / 60Hz adjustable frequency. In the field of shore power, the output voltage needs to be very stable, and the shore power control methods include PQ method, VF method, etc. The first method adjusts the power factor by controlling the reactive compensation device of the shore power supply, and then adjusts the output voltage to be stable. This method is slow and has a large delay. The second method is to directly feedback the output voltage to the given voltage value under the VF method, and adjust the output voltage by controlling the inverter output modulation. The VF method has the advantages of being fast and accurate, and can automatically control the shore power output voltage to reach the desired value through the AVR function.

[0003] Inverters are used in the field of general motor control and often use VF control. VF control means that the voltage to frequency ratio is a constant value. Because in most cases, the DC bus voltage does not fluctuate much, this method is fine. However, in some applications, such as mine hoists, frequent start-stop braking causes large fluctuations in the DC bus voltage. The actual output VF curve is not a straight line, causing the actual motor voltage to fluctuate, affecting the normal operation of the system.

[0004] A main method for automatic adjustment of the inverter output voltage is to add an output voltage detection device on the output side of the inverter to obtain the actual voltage value and calculate the theoretical voltage value. The actual voltage value and the theoretical voltage value are sent to the AVR regulator, and the output modulation degree is adjusted by the regulator so that the actual voltage is equal to the expected theoretical voltage.

[0005] But in practical applications, there are some problems.

[0006] 1. In the high-speed stage, the modulation index is easy to saturate. Since the system load is relatively large in the high-speed stage, the DC bus capacitor voltage is reduced. Under the same modulation index, the output voltage is reduced. In severe cases, even if the output modulation index is the largest, the expected output voltage cannot be achieved, which makes the regulator easy to saturate.

[0007] 2. In the high-speed stage, the output voltage is low and the system is not on the VF curve, resulting in reduced motor efficiency.

[0008] 3. In the high-speed stage, the DC bus voltage is relatively low. If the output modulation index is saturated, when braking occurs, the DC bus voltage rises instantly and the modulation index needs to be desaturated, which can easily lead to overcurrent. Summary of the invention

[0009] In view of this, an object of the present invention is to provide a method for automatically adjusting the output voltage of a frequency converter (Automatic Voltage Regulator, AVR), wherein two operating modes are proposed, namely a constant voltage mode and a constant magnetic flux mode.

[0010] In order to achieve the above object, the present invention provides the following technical solutions:

[0011] A method for automatically adjusting the output voltage of a frequency converter comprises the following steps:

[0012] S1: The system calculates the expected voltage based on the expected VF curve;

[0013] S2: The system uses the actual output voltage detected as the AVR regulator feedback, and the expected output voltage as the AVR regulator input. The AVR regulator output is superimposed on the output modulation index, so that the actual voltage tracks the expected voltage change;

[0014] S3: Determine whether the output modulation is saturated. If not, continue to output according to the desired VF curve; if saturated, the system selects two different operating modes: constant voltage mode and constant magnetic flux mode;

[0015] In constant voltage mode, the output frequency increases until it reaches the rated frequency;

[0016] In constant flux mode, the output frequency no longer increases;

[0017] After the output modulation is saturated, it will be automatically adjusted according to the actual situation.

[0018] Furthermore, in step S2, the output voltage value is detected by a voltage detection module, and the actually obtained voltage is normalized to a per-unit value under the same reference voltage comparable to the expected voltage value, and the actual voltage per-unit value is used as the feedback quantity of the AVR regulator.

[0019] Furthermore, in the constant voltage mode, after the output modulation index is saturated, its output frequency can continue to increase to reach the rated frequency; when operating from the modulation index saturation point to the rated frequency point, the system output modulation index remains unchanged. If the DC bus voltage remains unchanged, the output voltage remains unchanged; after the frequency increases, the system is in a weak magnetic operation state.

[0020] Furthermore, in the constant voltage mode, when the output modulation index is saturated, the AVR regulator saturation flag is set to 1, and the AVR regulator stops working; when the output modulation index is desaturated, the AVR regulator saturation flag is cleared to 0, and the AVR regulator continues to work.

[0021] Furthermore, the AVR regulator saturation flag is set to 1 and cleared to 0, which is achieved by modulation hysteresis control; the hysteresis width is set to a small value; when the AVR regulator saturation flag is equal to 0 and the modulation is greater than or equal to the saturation value, the AVR regulator saturation flag is set to 1; when the AVR regulator saturation flag is equal to 1 and the modulation is less than or equal to the difference between the saturation value and the hysteresis width, the AVR regulator saturation flag is cleared to 0; under other conditions, the AVR regulator saturation flag remains unchanged.

[0022] Furthermore, in the constant flux mode, when the modulation index is saturated, that is, when the output voltage reaches the maximum, the frequency no longer increases, and the frequency at this time is marked as the maximum operating frequency; even if the set frequency is greater than the operating frequency, the frequency no longer increases, and the system is in a constant flux operating state.

[0023] Furthermore, when the system is operating stably with constant flux under modulation saturation state, if the system operating point deviates from the constant flux VF curve due to DC bus voltage fluctuations and other reasons, when the deviation of the modulation amplitude exceeds ±0.01, the system will automatically adjust the output frequency until the frequency point reaches the constant flux VF curve, mark the frequency at this time as the maximum operating frequency, and continue to operate at the new equilibrium point.

[0024] Furthermore, the two operating modes can be switched between each other.

[0025] The beneficial effects of the present invention are as follows: the present invention solves the problem of modulation saturation in the high-speed stage and the problem that the motor magnetic field does not fall on the VF curve, improves the balance of the output voltage amplitude and the motor operating efficiency, and achieves commonality in the shore power supply field and the motor control field.

[0026] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0028] Figure 1 It is the VF curve diagram of the present invention;

[0029] Figure 2 This is a schematic diagram of the hysteresis control of the modulation index saturation flag in the constant voltage mode;

[0030] Figure 3 This is the constant voltage mode program flow chart;

[0031] Figure 4 This is a diagram illustrating the VF curve of the constant flux mode;

[0032] Figure 5 This is the constant flux mode program flow chart. DETAILED DESCRIPTION

[0033] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0034] It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and thus the drawings only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0035] In the following description, numerous details are discussed to provide a more thorough explanation of the embodiments of the present invention. However, it is obvious to those skilled in the art that the embodiments of the present invention can be implemented without these specific details. In other embodiments, well-known structures and devices are shown in the form of block diagrams rather than in detail to avoid making the embodiments of the present invention difficult to understand.

[0036] The present invention provides a method for automatically adjusting the output voltage of a frequency converter (Automatic Voltage Regulator, AVR) in two operating modes, namely a constant voltage mode and a constant magnetic flux mode.

[0037] The system output frequency is user-defined, and the expected output voltage at this frequency point is calculated by the ideal VF curve. The actual output voltage collects signals through the voltage detection device and converts them into comparable per-unit values. As we all know, the output voltage is determined by both the output modulation and the DC bus voltage. The actual output modulation is the sum of the modulation calculated by the actual VF curve and the modulation adjustment output by the AVR regulator. After the expected voltage and the actual voltage are sent to the AVR regulator, the actual output modulation is changed through the adjustment of the AVR regulator, and the actual output voltage is further changed, so that the actual output voltage is equal to the expected output voltage.

[0038] like Figure 1 As shown. The theoretical VF curve is represented by a black thick line. If there is no AVR regulator, the actual VF curve is represented by a long dashed line. For a system with an AVR regulator, the corrected VF curve is represented by a short dashed line. Assuming that the maximum output frequency of the system is f2, f2 is the rated frequency of the system, such as 50Hz or 60Hz, the corresponding system expected maximum output voltage is U2, and U2 is the rated voltage.

[0039] like Figure 1 In a system without AVR function, when the frequency rises to f1, the output voltage is U0, as shown in point ④, which is lower than the output U1 of the normal VF curve, as shown in point ①.

[0040] like Figure 1 As shown. In conventional AVR, when the frequency increases to f1, the output voltage is U1 at point ① in the figure, and the actual output voltage of the system is consistent with the expected voltage, but the actual modulation index is saturated at this time. When the frequency increases from f1 to f2, the actual maximum voltage output capacity of the system is maintained at U1, from point ① to point ② in the figure, while the maximum voltage output capacity of the normal VF curve is U2. When the frequency reaches point f2, the actual voltage is U1, as shown in point ② in the figure, and the expected voltage is U2, as shown in point ③ in the figure. The difference between the two is δU, so the AVR regulator continues to output until saturation. In addition, due to energy feedback during deceleration, the DC bus voltage will increase instantly, and the regulator needs to be desaturated, which is prone to overcurrent.

[0041] The present invention proposes the following solutions to this problem:

[0042] 1. Constant voltage without frequency reduction. The output frequency corresponding to the system modulation saturation point is defined as the turning point frequency, such as the frequency f1 at point ① in the figure. When the system moves from point ① to point ②, the frequency increases but the output modulation is saturated, the AVR regulator is not enabled, and the output voltage remains unchanged.

[0043] 2. Constant flux frequency reduction. The operating point of the actual VF curve is transferred from point ② of frequency f2 to point ① of frequency f1, and f1 is marked as the maximum output frequency.

[0044] The first solution is that after the modulation index is saturated, the output voltage is constant and the frequency is given according to the set frequency. If the frequency continues to increase at this time, the system is in a weak magnetic state.

[0045] like Figure 2As shown. When the modulation index is saturated, the modulation index is set to 1.15 in the figure, and the saturation flag is set to 1. When the saturation flag is 1, the system AVR regulator will stop working to prevent the AVR regulator from saturating. When the speed is reduced, the modulation index exits saturation. The desaturation value set in the figure is 1.14, and the saturation flag is cleared to 0. At this time, the AVR regulator continues to work.

[0046] like Figure 3 The constant voltage mode program flow chart is shown.

[0047] The second solution is that when the modulation index is saturated, that is, when the output voltage reaches the maximum, the frequency will no longer increase, and the frequency at this time will be marked as the maximum operating frequency. Even if the set frequency is greater than the operating frequency, the frequency will no longer increase.

[0048] When the DC bus voltage fluctuates, causing the output voltage to change, the actual VF curve operating point has changed, so the operating frequency is adjusted according to the direction of the bus voltage change. When the stable state is reached again, the frequency at that time is marked as the maximum frequency operation.

[0049] This control strategy is suitable for applications where the motor operating frequency is not sensitive but high efficiency is required.

[0050] Implementation method: Figure 4 As shown, when VF runs to point A, the modulation index is saturated and reaches the maximum output voltage capacity. At this time, the frequency at point A is marked as the maximum operating frequency, and the corresponding output voltage is the maximum output voltage. Due to the fluctuation of the DC bus voltage, the actual output voltage changes dynamically. When it rises from point A to point B, the operating point deviates from the desired VF curve. The system automatically adjusts the frequency point to point C. The system will recalculate the modulation index output according to the VF curve at the frequency of point C, and operate with point C as the new balance point. Similarly, when it decreases from point A to point D, the operating point deviates from the desired VF curve. The system automatically adjusts the frequency point to point E, and then operates with point E as the new balance point, and recalculates the modulation index output according to the VF curve at the frequency of point E.

[0051] Figure 5 Shown is the constant flux mode program flow chart.

[0052] The specific working steps of the system described in this implementation are as follows:

[0053] Step 1: If Figure 3 and Figure 5 The two modes shown in the flowchart have in common that the first step is to calculate the expected voltage value. For example, for a 10kV system, 10kV is defined as the reference voltage value, and the frequency corresponds to 50Hz, so the expected voltage value at any frequency point can be calculated. The expected voltage value is normalized and used as the input of the AVR regulator.

[0054] Step 2: Use the voltage detection module to detect the output voltage value, and standardize the actual voltage to convert it into a standard value under the same reference voltage that is comparable to the expected voltage value. The actual voltage standard value is used as the feedback of the AVR regulator.

[0055] Step 3: Determine whether the output modulation is saturated. If not, continue to output according to the desired VF curve. If saturated, the system can choose two different operating modes: constant voltage mode and constant flux mode.

[0056] In constant voltage AVR mode, when the output modulation index is saturated, its output frequency can continue to increase and reach the rated frequency. When operating from the modulation index saturation point to the rated frequency point, the system output modulation index remains unchanged. If the DC bus voltage remains unchanged, the output voltage remains unchanged. After the frequency increases, the system is in a weak magnetic operation state. When the output modulation index is saturated, the AVR regulator saturation flag is set to 1, and the AVR regulator stops working to prevent the AVR regulator from being over-saturated. When the output modulation index is desaturated, the AVR regulator saturation flag is cleared to 0, and the AVR regulator continues to work. The AVR regulator saturation flag is cleared from 1 to 0, and the modulation index hysteresis control is used. The hysteresis width value is set to a smaller value, which is 0.01 here, but not limited to 0.01. When the AVR regulator saturation flag is equal to 0 and the modulation index is greater than or equal to the saturation value, the AVR regulator saturation flag is set to 1. When the AVR regulator saturation flag is equal to 1 and the modulation index is less than or equal to (saturation value minus hysteresis width value), the AVR regulator saturation flag is cleared to 0. Under other conditions, the AVR regulator saturation flag remains unchanged to prevent the AVR regulator from frequently switching to enable at the saturation point. In the constant voltage AVR mode, the specific working steps include:

[0057] Step A1: First determine whether the modulation index is less than 1.14 (the maximum limit value of modulation index modulate is 1.15). If so, set the modulation index saturation flag to 0, which means that the modulation index is not saturated and the AVR regulator can continue to be enabled. If not, proceed to the next step.

[0058] Step A2: Determine whether the modulation index is greater than or equal to 1.15. If it is true, the modulation index reaches the maximum limit value, and the modulation index cannot continue to increase. The output voltage reaches the maximum output capacity, and the modulation index saturation flag is set to 1, which means that the modulation index is saturated and the AVR regulator is turned off. If it is not true, go to the next step.

[0059] Step A3: Determine whether the modulation index saturation flag is equal to 0. If so, it means that the modulation index is not saturated, the modulation index saturation flag remains unchanged, and the AVR regulator is enabled. If not, it means that the modulation index is saturated, the modulation index saturation flag remains unchanged, and the AVR regulator is enabled and disabled.

[0060] Step A4: Based on the above process, the output of the AVR regulator is added to the output modulation index to adjust the output voltage. This mode process ends.

[0061] In constant flux AVR mode, when the modulation index is saturated, that is, when the output voltage reaches the maximum, the frequency will no longer increase, and the frequency at this time will be marked as the maximum operating frequency. Even if the set frequency is greater than the operating frequency, the frequency will no longer increase, and the system is now in a constant flux operating state. When the system is operating stably with constant flux in the modulation index saturation state, if the system operating point deviates from the constant flux VF curve due to reasons such as DC bus voltage fluctuations, when the deviation modulation index exceeds ±0.01, the system will automatically adjust the output frequency until the frequency point reaches the constant flux VF curve, mark the frequency at this time as the maximum operating frequency, and continue to operate at the new balance point. In constant flux AVR mode, the following working steps are specifically included:

[0062] Step B1: First determine whether the modulation index is greater than or equal to 1.15. If so, set the modulation index saturation flag to 1, turn off the AVR regulator, obtain the current actual operating frequency as the maximum operating frequency point, and keep running at this frequency point. Then proceed to the next step of judgment. If not, proceed to step B4.

[0063] Step B2: Determine whether the actual output voltage is 0.01 greater than the expected voltage. If so, it means that the DC bus voltage fluctuates, causing the output voltage to increase, and then the ramp is accelerated according to the given acceleration time. If not, proceed to the next step.

[0064] Step B3: Determine whether the actual output voltage is -0.01 greater than the expected voltage. If so, it means that the difference between the output voltage change caused by the DC bus voltage fluctuation and the expected voltage is between (-0.01, 0.01), and there is no need for adjustment, so the speed remains unchanged. If not, then decelerate according to the given deceleration time ramp.

[0065] Step B4: Determine whether the modulation index is less than 1.14. If so, it means that the modulation index is not saturated, and the modulation index saturation flag is set to 0, and the AVR regulator is enabled. If not, proceed to the next step.

[0066] Step B5: Determine whether the modulation index saturation flag is 0. If so, the modulation index saturation flag remains unchanged, the modulation index is not saturated, and the AVR regulator is enabled. If not, the modulation index saturation flag remains unchanged, the modulation index is saturated, and the AVR regulator is disabled.

[0067] Step B6: Based on the above process, adjust the speed output, add the output of the AVR regulator to the output modulation index, adjust the output voltage, and this mode process ends.

[0068] In the above embodiments, the description's reference to "this embodiment" indicates that a particular feature, structure, or characteristic described in conjunction with the embodiment is included in at least some embodiments, but not necessarily all embodiments. Multiple occurrences of "this embodiment" do not necessarily all refer to the same embodiment.

[0069] In the above-described embodiments, although the invention has been described in conjunction with specific embodiments of the invention, many substitutions, modifications, and variations of these embodiments will be apparent to those of ordinary skill in the art based on the foregoing description. For example, other storage structures (e.g., dynamic RAM (DRAM)) may use the embodiments discussed. Embodiments of the invention are intended to encompass all such substitutions, modifications, and variations that fall within the broad scope of the appended claims.

[0070] This embodiment further provides a computer-readable storage medium on which a computer program is stored. When the program is executed by a processor, any one of the methods in this embodiment is implemented.

[0071] This embodiment also provides an electronic terminal, including: a processor and a memory;

[0072] The memory is used to store a computer program, and the processor is used to execute the computer program stored in the memory, so that the terminal executes any one of the methods in this embodiment.

[0073] The computer-readable storage medium in this embodiment can be understood by ordinary technicians in this field: all or part of the steps of implementing the above-mentioned method embodiments can be completed by hardware related to the computer program. The aforementioned computer program can be stored in a computer-readable storage medium. When the program is executed, the steps of the above-mentioned method embodiments are executed; and the aforementioned storage medium includes: ROM, RAM, magnetic disk or optical disk and other media that can store program codes.

[0074] The electronic terminal provided in this embodiment includes a processor, a memory, a transceiver and a communication interface. The memory and the communication interface are connected to the processor and the transceiver and complete communication with each other. The memory is used to store computer programs, the communication interface is used to communicate, and the processor and the transceiver are used to run computer programs so that the electronic terminal executes each step of the above method.

[0075] In this embodiment, the memory may include a random access memory (RAM), and may also include a non-volatile memory (non-volatile memory), such as at least one disk memory.

[0076] The above-mentioned processor can be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it can also be a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components.

[0077] The present invention can be used in many general or special computing system environments or configurations, such as personal computers, server computers, handheld or portable devices, tablet devices, multiprocessor systems, microprocessor-based systems, set-top boxes, programmable consumer electronic devices, network PCs, minicomputers, mainframe computers, distributed computing environments including any of the above systems or devices, and the like.

[0078] The present invention may be described in the general context of computer-executable instructions executed by a computer, such as program modules. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform specific tasks or implement specific abstract data types. The present invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices connected through a communication network. In a distributed computing environment, program modules may be located in local and remote computer storage media, including storage devices.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A method for automatically adjusting the output voltage of a frequency converter, characterized in that: The following steps are involved: S1: The system calculates the expected voltage based on the expected VF curve; S2: The system uses the actual output voltage detected as the AVR regulator feedback, and the expected output voltage as the AVR regulator input. The AVR regulator output is superimposed on the output modulation index, so that the actual voltage tracks the expected voltage change; S3: Determine whether the output modulation is saturated. If not, continue to output according to the desired VF curve; if saturated, the system selects two different operating modes: constant voltage mode and constant magnetic flux mode; In constant voltage mode, the output frequency increases until it reaches the rated frequency; In constant flux mode, the output frequency no longer increases; After the output modulation is saturated, it will be automatically adjusted according to the actual situation.

2. The method for automatically adjusting the inverter output voltage according to claim 1, characterized in that: In step S2, the output voltage value is detected by a voltage detection module, and the actual voltage is normalized to a per-unit value under the same reference voltage comparable to the expected voltage value, and the actual voltage per-unit value is used as the feedback of the AVR regulator.

3. The method for automatically adjusting the inverter output voltage according to claim 1, characterized in that: In constant voltage mode, when the output modulation index is saturated, the output frequency can continue to increase to reach the rated frequency; when operating from the modulation index saturation point to the rated frequency point, the system output modulation index remains unchanged. If the DC bus voltage remains unchanged, the output voltage remains unchanged; after the frequency increases, the system is in a weak magnetic operation state.

4. The method for automatically adjusting the inverter output voltage according to claim 3, characterized in that: In constant voltage mode, when the output modulation index is saturated, the AVR regulator saturation flag is set to 1 and the AVR regulator stops working; when the output modulation index is desaturated, the AVR regulator saturation flag is cleared to 0 and the AVR regulator continues to work.

5. The method for automatically adjusting the inverter output voltage according to claim 4, characterized in that: The AVR regulator saturation flag position 1-clearing operation is implemented by modulation hysteresis control; the hysteresis width value is set to a small value; when the AVR regulator saturation flag is equal to 0 and the modulation is greater than or equal to the saturation value, the AVR regulator saturation flag position is 1; when the AVR regulator saturation flag is equal to 1 and the modulation is less than or equal to the difference between the saturation value and the hysteresis width value, the AVR regulator saturation flag is cleared to 0; under other conditions, the AVR regulator saturation flag remains unchanged.

6. The method for automatically adjusting the inverter output voltage according to claim 1, characterized in that: In the constant flux mode, when the modulation index is saturated, that is, when the output voltage reaches the maximum, the frequency will no longer increase, and the frequency at this time will be marked as the maximum operating frequency; even if the set frequency is greater than the operating frequency, the frequency will no longer increase, and the system is in a constant flux operating state.

7. The method for automatically adjusting the inverter output voltage according to claim 6, characterized in that: When the system is running stably with constant flux under modulation saturation state, if the system operating point deviates from the constant flux VF curve due to DC bus voltage fluctuation and other reasons, when the deviation modulation amplitude exceeds ±0.01, the system will automatically adjust the output frequency until the frequency point reaches the constant flux VF curve, mark the frequency at this time as the maximum operating frequency, and continue to run at the new equilibrium point.

8. The method for automatically adjusting the inverter output voltage according to claim 1, characterized in that: The two operating modes can be switched between each other.

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