阅读说明：本技术 电励磁双凸极电机驱动充电一体化系统前级解耦控制方法 (Pre-stage decoupling control method for driving and charging integrated system of electro-magnetic doubly salient motor ) 是由 魏佳丹 陈锦春 翟相煜 赵晓聪 周波 杨明 于 2021-08-02 设计创作，主要内容包括：本发明公开了一种电励磁双凸极电机驱动充电一体化系统前级DC/DC变换器的解耦控制方法,所述电励磁双凸极电机驱动充电一体化系统中,前级DC/DC变换器通过对开关S-(1)、S-(2)和S-(4)、S-(6)的占空比调节工作在Buck-Boost模式,采用双沿调制策略,分别实现直流侧母线和励磁电流的控制,根据励磁绕组状态将一个开关周期分为三个工作模态：励磁绕组放电模态、励磁绕组储能模态和续流模态,其中励磁绕组储能模态和放电模态分别控制励磁电流和母线电压,续流模态用于实现励磁电流控制与母线电压的解耦控制。本发明能够有效控制系统前级DC/DC变换器中励磁电流和母线电压,提高驱动充电一体化系统动稳态性能。(The invention discloses a decoupling control method for a preceding-stage DC/DC converter of an electric excitation double-salient motor driving and charging integrated system 1 、S 2 And S 4 、S 6 The duty ratio regulation works in a Buck-Boost mode, a double-edge modulation strategy is adopted, the control of a direct-current side bus and the control of exciting current are respectively realized, and one switching period is divided into three working modes according to the state of an exciting winding: the device comprises an excitation winding discharge mode, an excitation winding energy storage mode and a follow current mode, wherein the excitation winding energy storage mode and the discharge mode respectively control excitation current and bus voltage, and the follow current mode is used for realizing decoupling control of the excitation current control and the bus voltage. The invention can effectively controlExcitation current and bus voltage in the system pre-stage DC/DC converter improve dynamic and steady-state performance of the drive and charge integrated system.)

1. The decoupling control method of the pre-stage DC/DC converter of the electric excitation double-salient motor driving and charging integrated system comprises a storage battery and a DC/DC converterThe device comprises a converter, a three-phase bridge inverter and an electrically excited doubly salient motor, wherein a storage battery is connected with the input end of a DC/DC converter, the output end of the DC/DC converter is connected with the input end of the three-phase bridge inverter, an excitation winding of the electrically excited doubly salient motor is reused for the DC/DC converter, a three-phase armature winding of the electrically excited doubly salient motor is in an open structure, one end of the three-phase armature winding is connected with the output end of the three-phase bridge inverter, and the other end of the three-phase armature winding is connected with a charge-discharge changeover switch K1; the DC/DC converter adopts a buck-boost topological form and consists of six switching tubes S1-S6, two diodes D1-D2, two sections of excitation windings F1 and F2 of an electro-excitation doubly salient motor, a switch K2 and a capacitor C1, the middle points of two bridge arms formed by connecting the two switching tubes S1 and S2 and the two diodes D1 and D2 in series are respectively connected to one ends of two sections of excitation windings F1 and F2 of the electro-excitation doubly salient motor, the middle points of two bridge arms formed by the other four switching tubes S3-S6 are respectively connected to the other ends of two sections of excitation windings F1 and F2 of the electro-magnetic doubly salient motor, a switch K2 is positioned between the two bridge arms, a capacitor C1 is positioned on the output side of the DC/DC converter, the DC/DC converter is characterized in that the DC/DC converter controls the voltage of a direct-current side bus and the exciting current of an electrically-excited doubly salient motor in a system, and a double-edge modulation strategy is adopted for a switching tube S.₁、S₂And S₄、S₆Controlling a duty ratio, wherein the duty ratio is determined by the output of an exciting current loop PI regulator and the output of a bus voltage loop PI regulator;

one switching cycle is divided into three working modes according to the state of an excitation winding: (S)₁、S₂Conduction, S₄、S₆And (3) turning off, wherein an excitation current path is a storage battery-excitation winding-bus capacitor, when the corresponding excitation current decreases, the direct-current side bus voltage rises, the mode is called an excitation winding discharge mode, and the corresponding duty ratio is d₁；②S₁、S₂Conduction, S₄、S₆The circuit is conducted, the path of the exciting current is a storage battery-exciting winding, the exciting current rises and the bus voltage drops, the mode is called an exciting winding energy storage mode, and the corresponding duty ratio is d₂；③S₁、S₂Off, S₄、S₆Is conducted when the excitation current flows through S₄、S₆And a diode D₁、D₂Follow current, corresponding to the excitation current amplitude remaining constant while the DC bus voltage drops, called the follow current mode, corresponding to a duty cycle of d₃(ii) a The excitation winding energy storage mode and the discharge mode respectively control the excitation current and the direct-current side bus voltage, and the follow current mode is used for decoupling control of the excitation current control and the direct-current side bus voltage.

2. The decoupling control method of the pre-stage DC/DC converter of the electro-magnetic doubly salient motor driving and charging integrated system according to claim 1, wherein a double-edge modulation strategy is adopted to perform on-off S₁、S₂And S₄、S₆And performing duty ratio control, specifically:

switch tube S₁、S₂The rising sawtooth wave carrier wave is connected with d₂Comparison, d₂Greater than S when rising sawtooth carrier₁、S₂Conducting, otherwise S₁、S₂Turning off;

switch tube S₄、S₆The descending sawtooth wave carrier is connected with (1-d)₁) Comparison, (1-d)₁) Greater than S when the falling sawtooth carrier₄、S₆Conducting, otherwise S₄、S₆And (6) turning off.

3. The decoupling control method of the pre-stage DC/DC converter of the electro-magnetic doubly salient motor driving and charging integrated system according to claim 1, is characterized in that:

excitation winding energy storage mode duty ratio d₂Output d from bus voltage loop PI regulator₁And excitation current loop PI regulator output d₂' comprehensive decision:

wherein u is_cIs the DC side capacitor voltage u_bIs the battery voltage.

4. The decoupling control method of the pre-stage DC/DC converter of the electro-magnetic doubly salient motor driving and charging integrated system according to claim 3, is characterized in that: exciting current closed loop PI regulator output d₂' the corresponding amplitude is obtained by:

wherein i_LIs the load current and L is the field winding inductance.

5. The decoupling control method of the pre-stage DC/DC converter of the electro-magnetic doubly salient motor driving and charging integrated system according to claim 1, is characterized in that:

discharge mode duty cycle d of excitation winding₁Is obtained by the following formula:

wherein u is_cIs the DC side capacitor voltage i_LThe load current is C, the capacitance value of the direct current side capacitor is C, and the equivalent output resistance of the DC/DC converter is R.

6. The decoupling control method of the pre-stage DC/DC converter of the electro-magnetic doubly salient motor driving and charging integrated system according to claim 1, is characterized in that:

duty ratio d of discharge mode to excitation winding when free-wheeling mode does not exist₁And the duty ratio d of the energy storage mode of the excitation winding₂Performing overmodulation treatment, i.e. when d₁+d₂>1, ordering:

Technical Field

The invention relates to a decoupling control method for a pre-stage DC/DC converter of an electric excitation double-salient motor driving and charging integrated system, belonging to the field of motor systems and control.

Background

With the rapid popularization of electric vehicles, low-cost and high-performance motor driving systems and solutions of portable and fast vehicle-mounted chargers receive more and more attention. Due to the four-quadrant working characteristic of the motor drive converter, the electric automobile drive and charging integrated system can effectively utilize the motor drive controller and the motor winding to reconstruct a power battery charger so as to realize higher power density.

The inductance value of the armature winding of the permanent magnet synchronous motor is too small, so that the current harmonic of the grid side under a charging mode is larger when the armature winding is used as the inductance of a grid side filter, and the current harmonic of the grid side can be effectively inhibited by the permanent magnet-free motor with the larger inductance value of the armature winding. In order to ensure the safety and low noise of the electric automobile, the driving motor cannot output electromagnetic torque in the charging mode. In the solution of reconfiguring the vehicle-mounted charger by the multiplexing permanent magnet synchronous motor driver, a multi-phase winding structure is required to be adopted to eliminate the output torque in the charging mode, so that the number of devices is too many, the system is complex, and the cost is high. The driving and charging integrated system solution based on the induction motor and the switched reluctance motor also needs to design a complex armature winding structure according to the motor operation principle to eliminate electromagnetic torque. The motor system without the excitation winding needs to be additionally provided with a DC/DC converter to realize a battery charging strategy.

The electrically excited doubly salient machine motor is similar in structure to the switched reluctance motor, but has separate field and armature windings, which provides more possibilities for reconfiguring the on-board charger. The electric excitation double-salient-pole motor driving and charging integrated system based on the split excitation winding is used for multiplexing the excitation winding as a filter inductor of a front-stage DC/DC converter, under the charging mode, a rear-stage inverter and a motor armature winding are reconstructed into a three-phase PWM rectifier to realize network side power factor correction, the front-stage DC/DC converter controls two sections of excitation windings to have equal current and opposite directions to realize constant current charging, and torque output is eliminated. In the driving mode, the preceding-stage DC/DC converter needs to control the exciting current and the bus voltage simultaneously, so the control mode is different from that of the traditional buck-boost converter. The invention patent with the patent number ZL20171445250.6 proposes a control strategy that a front-stage buck-boost converter in the system is feasible from the power perspective, but the method cannot completely decouple excitation current control and bus voltage control, and the control response has hysteresis, influences the control effect and has the possibility of instability under the dynamic working condition of the system. Therefore, how to decouple the excitation current control and the bus voltage control in the preceding-stage DC/DC converter is important for the system stability and the motor output performance.

Disclosure of Invention

The invention aims to solve the problems in the prior art, and provides a decoupling control method for a pre-stage DC/DC converter of an electric excitation doubly-salient motor driving and charging integrated system, so that the stable control of the pre-stage DC/DC converter on excitation current and bus voltage is realized, and the dynamic and steady-state performance of the driving and charging integrated system is improved.

In order to achieve the purpose, the technical scheme provided by the invention is as follows:

the electro-magnetic doubly salient motor driving and charging integrated system comprises a storage battery, a DC/DC converter, a three-phase bridge inverter and an electro-magnetic doubly salient motor, wherein the storage battery is connected with the input end of the DC/DC converter, the output end of the DC/DC converter is connected with the input end of the three-phase bridge inverter, an excitation winding of the electro-magnetic doubly salient motor is reused for the DC/DC converter, a three-phase armature winding of the electro-magnetic doubly salient motor is set to be an open structure, one end of the three-phase armature winding is connected with the output end of the three-phase bridge inverter, and the other end of the three-phase armature winding is connected with a charging and discharging changeover switch K1; the DC/DC converter adopts a buck-boost topological form and comprises six switching tubes S1-S6, two diodes D1-D2, two sections of excitation windings F1 and F2 of an electro-magnetic doubly salient motor, a switch K2 and a capacitor C1, the middle points of two bridge arms formed by connecting the two switching tubes S1 and S2 and the two diodes D1 and D2 in series are respectively connected to one ends of the two sections of excitation windings F1 and F2 of the electro-magnetic doubly salient motor, the middle points of two bridge arms formed by the other four switching tubes S3-S6 are respectively connected to the other ends of the two sections of excitation windings F1 and F2 of the electro-magnetic doubly salient motor, the switch K2 is positioned between the two groups of bridge arms, and the capacitor C1 is positioned on the output side of the DC/DC converter.

The decoupling control method of the pre-stage DC/DC converter of the electric excitation double-salient-pole motor driving and charging integrated system comprises the following steps: in the integrated system for driving and charging the electro-magnetic doubly salient motor, a front-stage DC/DC converter and a rear-stage three-phase inverter are cascaded, the front-stage DC/DC converter realizes the control of the voltage of a direct-current side bus in the system and the exciting current of the electro-magnetic doubly salient motor, and a dual-edge modulation strategy is adopted to control a switch S₁、S₂And S₄、S₆And performing duty ratio control, and dividing a switching period into three working modes according to the state of the excitation winding: the device comprises an excitation winding discharge mode, an excitation winding energy storage mode and a follow current mode, wherein the excitation winding energy storage mode and the discharge mode respectively control excitation current and bus voltage, and the follow current mode is used for decoupling control of the excitation current control and the bus voltage. The decoupling control method effectively realizes decoupling control of the exciting current and the bus voltage in the pre-stage DC/DC converter, and improves dynamic and stable performance of the doubly salient electro-magnetic motor driving and charging integrated system.

Furthermore, the preceding-stage DC/DC converter adopts a dual-edge modulation strategy to the switch S₁、S₂And S₄、S₆And performing duty ratio control, namely:

switch tube S₁、S₂The rising sawtooth wave carrier wave and the duty ratio d₂Comparison, duty ratio d₂Output d from the excitation current loop₂', bus voltage ring output d₁And decoupling the compensated output d₁Is' added to obtain the duty ratio d₂S when greater than carrier₁、S₂On, S is smaller than carrier wave when control loop output₁、S₂Turning off;

switch tube S₄、S₆The descending sawtooth wave carrier wave and the duty ratio (1-d)₁) Comparison, duty cycle (1-d)₁) Output d from bus voltage loop₁To obtain the duty ratio (1-d)₁) S when greater than carrier₄、S₆On, S is smaller than carrier wave when control loop output₄、S₆Turning off;

due to the switch S₁、S₂And S₄、S₆When the switches are all turned off, the change rate of the exciting current is larger, and the double-edge modulation strategy aims to reduce the switch S₁、S₂And S₄、S₆The duty ratio of the off state to realize low ripple excitation current control.

After the double-edge modulation strategy, the front-stage DC/DC converter is divided into three working modes in one switching period, wherein S₁、S₂Conduction, S₄、S₆Off, corresponding to a duty cycle of d₁At the moment, the excitation current path is a storage battery-excitation winding-bus capacitor, and when the corresponding excitation current decreases, the voltage of a direct-current side bus rises, which is called as an excitation winding discharging mode; when S is₁、S₂Conduction, S₄、S₆On with a corresponding duty cycle of d₂The path of the exciting current is a storage battery-exciting winding, the exciting current rises and the bus voltage drops, and the exciting winding is in an energy storage mode; when S is₁、S₂Off, S₄、S₆On with a corresponding duty cycle of d₃When the excitation current flows through S₄、S₆And a diode D₁、D₂And the follow current is kept constant corresponding to the amplitude of the exciting current, the voltage of the direct current bus is reduced, the follow current mode is adopted, and the existence of the working mode enables the exciting current control and the bus voltage control to be independent.

Duty ratio d of excitation winding energy storage mode₂The method is divided into two parts:

wherein d is₁Is the discharge mode duty ratio of the excitation winding.

Then the excitation current state space equation is simplified as:

d is thus regulated by the PI controller₂The closed-loop control of the exciting current is irrelevant to the bus voltage and the control thereof, and the decoupling is realized.

And (3) enabling the excitation current to preferentially reach the given value, and considering that the excitation current is constant in the bus voltage state space equation, simplifying the bus voltage state space equation into:

regulation of d by PI controller₁The bus voltage is controlled in a closed loop.

The decoupling control method is characterized in that excitation current control and bus voltage control are mutually independent due to the existence of a follow current mode, and the energy storage mode duty ratio d of an excitation winding is required to be adjusted when the follow current mode does not exist₂And discharge mode d₁The duty ratio is subjected to overmodulation. Therefore, when d₁+d₂>1, time:

compared with the prior art, the invention has the following beneficial effects:

1. compared with the control strategy of the traditional double-tube buck-boost converter, the decoupling control strategy of the exciting current and the bus voltage provided by the invention can simultaneously control the stability of the exciting current and the bus voltage.

2. Compared with a control strategy provided from the power perspective, the decoupling control strategy provided by the invention can decouple the exciting current control and the bus voltage control, greatly inhibit the cross influence of the motor load change on the exciting current, and improve the dynamic and steady-state performance of the system.

Drawings

FIG. 1 is a schematic diagram of a decoupling control method of a pre-stage DC/DC converter of an electric excitation doubly salient motor driving and charging integrated system;

FIG. 2 is a rotation speed waveform of an electro-magnetic doubly salient motor;

FIG. 3 is a current waveform of an A-phase armature winding of an electro-magnetic doubly salient motor;

FIG. 4 is an electromagnetic torque waveform of an electro-magnetic doubly salient motor;

fig. 5 is a waveform of the excitation current on the excitation winding F1;

fig. 6 is a detailed waveform of the excitation current on the loading instant excitation winding F1;

fig. 7 is a detailed waveform of the excitation current on the unloading instant excitation winding F1;

fig. 8 is a waveform of the excitation current on the excitation winding F2;

FIG. 9 is a bus voltage waveform;

FIG. 10 is a detail waveform of the bus voltage at the moment of loading;

fig. 11 is a detail waveform of the bus voltage at the moment of unloading.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The conventional double-tube buck-boost converter usually works in a buck working mode, a boost working mode and a buck-boost working mode, but the common control modes are all controlled according to the output voltage, and the current processing of the inductive energy storage element is only used for optimizing the system performance. In the electro-magnetic doubly salient motor system applied to the patent, a pre-stage buck-boost converter needs to control both bus voltage and exciting current, and a special design control strategy is needed for implementing control.

The dual-edge modulation strategy also has application in conventional buck-boost converters with the goal of increasing the direct transmission power ratio to reduce device stress and increase system efficiency. In the system, the modulation strategy can eliminate the mode that the upper pipe and the lower pipe are both switched off, and the excitation current change rate of the model is the largest, so that the dual-edge modulation strategy is favorable for stabilizing the excitation current. After the double-edge modulation strategy, one of the remaining three modes of the system is a follow current mode, the exciting current in the mode is kept unchanged, the bus voltage is reduced, and the bus voltage and the exciting current are respectively increased by the remaining two modes. Therefore, the possibility that the excitation current and the bus voltage are respectively controlled according to modes exists after the front-stage buck-boost converter is subjected to double-edge modulation, feasibility is provided for decoupling of excitation current control and bus voltage control due to the existence of the follow current mode, stable control of the excitation current and the bus voltage is finally achieved, stability of the system is improved, and the method has good research significance.

The decoupling control method of the pre-stage DC/DC converter of the electric excitation doubly salient motor driving and charging integrated system is applied to the electric excitation doubly salient motor driving and charging integrated system based on the split excitation winding, the pre-stage DC/DC converter and the post-stage electric excitation doubly salient motor driving are cascaded by using a three-phase inverter, the pre-stage DC/DC converter realizes the control of the voltage of a direct-current side bus in the system and the exciting current of the electric excitation doubly salient motor, and a dual-edge modulation strategy is adopted to control a switch S₁、S₂And S₄、S₆And performing duty ratio control, and dividing a switching period into three working modes according to the state of the excitation winding: the device comprises an excitation winding discharge mode, an excitation winding energy storage mode and a follow current mode, wherein the excitation winding energy storage mode and the discharge mode respectively control excitation current and bus voltage, and the follow current mode is used for decoupling control of the excitation current control and the bus voltage. A schematic diagram of the system architecture and control strategy is shown in fig. 1.

Wherein, the front-stage double-tube buck-boost converter (DC/DC converter) adopts a double-edge modulation strategy, and the switching tube S₁、S₂The rising sawtooth wave carrier wave and the duty ratio d₂Comparison, duty ratio d₂Output d from the excitation current loop₂', bus voltage ring output d₁And decoupling the compensated output d₁Is' added to obtain the duty ratio d₂S when greater than carrier₁、S₂On, S is smaller than carrier wave when control loop output₁、S₂Turning off;

switch tube S₄、S₆The descending sawtooth wave carrier wave and the duty ratio (1-d)₁) Comparison, duty cycle (1-d)₁) Output d from bus voltage loop₁To obtain the duty ratio (1-d)₁) S when greater than carrier₄、S₆On, S is smaller than carrier wave when control loop output₄、S₆Turning off;

due to the switch S₁、S₂And S₄、S₆When the switches are all turned off, the change rate of the exciting current is larger, and the double-edge modulation strategy aims to reduce the switch S₁、S₂And S₄、S₆The duty ratio of the off state to realize low ripple excitation current control.

After the double-edge modulation strategy, the front-stage DC/DC converter is divided into three working modes in one switching period, wherein S₁、S₂Conduction, S₄、S₆Off, corresponding to a duty cycle of d₁At the moment, the excitation current path is a storage battery-excitation winding-bus capacitor, and when the corresponding excitation current decreases, the voltage of a direct-current side bus rises, which is called as an excitation winding discharging mode; when S is₁、S₂Conduction, S₄、S₆On with a corresponding duty cycle of d₂The path of the exciting current is a storage battery-exciting winding, the exciting current rises and the bus voltage drops, and the exciting winding is in an energy storage mode; when S is₁、S₂Off, S₄、S₆On with a corresponding duty cycle of d₃When the excitation current flows through S₄、S₆And a diode D₁、D₂And the follow current is kept constant corresponding to the amplitude of the exciting current, the voltage of the direct current bus is reduced, the follow current mode is adopted, and the existence of the working mode enables the exciting current control and the bus voltage control to be independent.

The state space average equation of the buck-boost converter after column write double edge modulation is as follows:

d is contained in the excitation current differential equation₁And u_cThe quantities in the two voltage control loops and thus in fact there is also coupling.

When the excitation current differential equation is collated, it is found that:

can be combined with₂Is divided into two parts, one part is used for controlling exciting current, the other part is used for compensating bus voltage and d₁The effect of the change of (c):

d₂＝d₂'+d₁'

the excitation current differential equation can be simplified to a first order system:

the principle is that the bus voltage drops to cause d₁Increasing the discharge time of the exciting current, and correspondingly increasing d₂The energy storage of the excitation winding is compensated by partial duty ratio, namely the average value of the excitation current is not influenced. D is thus regulated by the PI controller₂The closed-loop control of the exciting current is irrelevant to the bus voltage and the control thereof, and the decoupling is realized.

And (3) enabling the excitation current to preferentially reach the given value, and considering that the excitation current is constant in the bus voltage state space equation, simplifying the bus voltage state space equation into:

regulation of d by PI controller₁The bus voltage is controlled in a closed loop.

The decoupling control method is characterized in that excitation current control and bus voltage control are mutually independent due to the existence of a follow current mode, and the energy storage mode duty ratio d of an excitation winding is required to be adjusted when the follow current mode does not exist₂And discharge mode d₁The duty ratio is subjected to overmodulation. Therefore, when d₁+d₂>1, time:

examples

Matlab/Simulink simulation is carried out on the driving and charging integrated system of the electro-magnetic doubly salient motor and the corresponding working condition of the system according to a specific implementation mode. The parameters of the electro-magnetic doubly salient motor are as follows: the resistance value of each section of excitation winding is 0.4 omega, the inductance value of each section of excitation winding is 13mH, the resistance value of the armature winding is 0.1 omega, and the inductance value of the armature winding is 5.6 mH. The simulation working condition is as follows: the voltage of a storage battery is 72V, the voltage of a bus is 120V, the given rotating speed of the motor is 200rpm, the load torque is 1 N.m, the motor is accelerated at 0.5s to improve the loading of the output power equivalent preceding-stage DC/DC converter, and the motor is decelerated at 0.7s to reduce the unloading of the output power equivalent preceding-stage DC/DC converter.

The motor state waveform of the electric excitation double-salient motor driving charging integrated system is shown in fig. 2-4, and fig. 2 is a motor rotating speed waveform which meets the working condition setting that the motor starts accelerating for 0.5s and starts decelerating for 0.7 s; FIG. 3 is a waveform of the current of the A-phase armature winding, which shows that the armature current increases at 0.5s, and rapidly decreases at 0.7s, corresponding to the load current state when the preceding DC/DC converter is loaded or unloaded; FIG. 4 shows the electromagnetic torque waveform of the motor, which reaches the rated torque at 0.5s, and rapidly decreases due to the speed reduction at 0.7s, and meets the setting of the working condition.

Excitation current waveforms and bus voltage waveforms in the pre-stage DC/DC converter of the electro-magnetic doubly salient motor driving and charging integrated system are shown in fig. 5-11, fig. 5-7 are excitation current waveforms on an excitation winding F1 and detailed waveforms at the moment of loading and unloading, it can be seen that the average value of the excitation current before loading is 6A, the peak value is 0.0275A, and the steady-state performance is good; the exciting current is increased by about 0.09A and quickly recovers to a stable state during loading, the average value of the exciting current after loading is 6A, and the peak value is 0.0525A; the exciting current is almost unchanged during unloading, the average value is still 6A, and the peak-to-peak value is 0.0293A. Therefore, the excitation current control can keep good control effect in both the steady state and loading and unloading dynamic processes. The field winding F2 is symmetrical to the field winding F1, and the waveform is quite similar, and the field current waveform is shown in fig. 8.

Fig. 9 to 11 are a bus voltage waveform and an instant detail waveform of loading and unloading, respectively, and it can be seen that the average value of the bus voltage before loading is 120V, the peak-to-peak value is 0.116V, and the steady-state performance is better; the bus voltage drops by about 1.6V and rapidly recovers to a stable state when being loaded, the average value of the bus voltage after being loaded is 120V, and the peak value is 0.454V; when the motor is unloaded, the voltage of the bus is increased by about 1.3V due to the energy feedback of the motor, the steady state is recovered after 0.1s, the average value of the voltage of the bus after unloading is still 120V, and the peak value is 0.0853V. Therefore, the bus voltage control can keep better control effect in the steady state and loading and unloading dynamic processes.

The influence on the bus voltage is inevitably generated in the loading and unloading process of the front-stage DC/DC converter, the fluctuation of the bus voltage almost has no influence on the exciting current, and the decoupling of the exciting current control and the bus voltage control is verified.

The test example verifies that the bus voltage variable working condition switch table optimization direct torque control strategy provided by the invention can enable the motor to be normally driven in a wide bus voltage variation range, and compared with a motor system adopting a traditional off-line switch table control strategy, the motor system adopting the control strategy of the invention has more stable output performance.

The above description is only a preferred embodiment of the present invention, and it should be understood that various equivalent substitutions and modifications within the spirit and scope of the present invention, which are obvious to those skilled in the art, are also included in the protection scope of the present invention.