Calculation method for flux-weakening control reference current of permanent magnet motor

Abstract

The invention discloses a method for calculating flux weakening control reference current of a permanent magnet motor, and the method comprises the steps: calculating a voltage constraint coefficient according to the voltage of a DC bus, the flux linkage of a permanent magnet and the maximum no-load rotation speed, and representing the voltage limitation degree of a high-speed region; the weak magnetic expected current is directly obtained according to the coefficient, and rapid analysis without additional control links is achieved; the weak magnetic current is corrected by combining the motor resistance and the operation current, so that the adaptability of a high-speed region is improved; and the flux weakening current range is automatically constrained through current amplitude limiting judgment, and the safe operation margin is ensured. Error accumulation of torque control reference current can be avoided, and the control precision of a weak magnetic region is remarkably improved; double mechanisms of dynamic compensation and amplitude limiting protection are adopted to adapt to complex working conditions of a high-speed area, and the anti-interference capability is excellent; an additional PI controller or a torque current calculation module is not needed, the real-time performance is high, and engineering implementation is facilitated; and the voltage constraint is accurately matched, the utilization efficiency of the DC bus voltage is maximized, and higher rotation speed expansion is supported.

Description

TECHNICAL FIELD

[0001] The application relates to a method for calculating a reference current of a permanent magnet motor for field weakening control, and belongs to the technical field of field weakening control. BACKGROUND

[0002] Permanent magnet motors are widely used in new energy vehicles, electric vehicle drives, numerical control machine tools, robots, spacecraft actuators, and high-performance industrial servo systems, etc. due to their high power density, high efficiency, fast dynamic response, compact structure, and other characteristics. With the increasing demand for the expansion of high-speed operation regions for various equipment, the requirement for permanent magnet motors to operate in the field weakening region is increasingly prominent. Normally, permanent magnet motors can rely on the magnetic flux of permanent magnets to provide sufficient voltage margin to meet the excitation requirement when operating below the base speed. However, when the speed continues to rise above the base speed, the back electromotive force increases with the speed and approaches or even exceeds the DC bus voltage, resulting in that the inverter cannot provide sufficient voltage vectors to maintain normal excitation and current regulation, the system performance is reduced, and even the problem of unable to continue to increase the speed occurs. To ensure the continuous and safe operation of the motor above the base speed, field weakening control becomes a key technology in the control system of the permanent magnet motor.

[0003] The basic idea of field weakening control is to actively introduce a current that weakens the air gap magnetic field in the high-speed region to reduce the effective air gap flux and the back electromotive force amplitude, so that the inverter still has a suitable voltage modulation margin under the limited DC bus voltage, and stable high-speed operation is achieved.

[0004] In the traditional field weakening control method, the generation path of the field weakening control reference current mainly includes two types.

[0005] First, it is an automatic generation method based on voltage margin feedback, that is, by comparing the difference between the voltage vector amplitude and the DC bus voltage reference value, and using a PI controller to generate a field weakening control reference current, so as to automatically adjust the air gap flux in the high-speed region. For example, the Chinese invention patent with publication number CN102035445A discloses a high-speed field weakening closed-loop control method for permanent magnet synchronous motors, which proposes to use a closed-loop field weakening regulation structure to automatically adjust the field weakening control reference current with the change of voltage margin; the Chinese invention patent with publication number CN105450121A discloses a motor field weakening control method, which similarly uses the difference between the voltage amplitude and the DC bus voltage as a field weakening regulation signal, and automatically generates a field weakening control reference current through a PI controller to ensure that the motor obtains stable voltage margin under different high-speed conditions. This kind of method relies on PI cross-loop regulation, and the control process has self-adaptability, but the field weakening control reference current lacks direct correlation with the actual motor parameters.

[0006] Secondly, based on motor model parameters (such as inductance, permanent magnet flux linkage, etc.) directly using mathematical relationship to calculate the required field weakening control reference current. Such methods are generally based on current limit circle and voltage limit ellipse theory, by constructing Lagrange function and solving the optimal current distribution, so as to obtain the minimum field weakening control reference current satisfying the voltage constraint in high speed area. For example, the Chinese invention patent with publication number CN103840732A discloses a driving motor field weakening control method, which establishes motor voltage equation and current, voltage limit condition, so that the field weakening control reference current is directly calculated by analytical formula, realizing the field weakening control method without additional controller. In contrast, such method has clear calculation process, strong real-time performance, does not need additional PI control structure, and the system structure is relatively simple. However, the calculation method based on current limit circle and voltage limit ellipse theory has an inherent premise: the accurate torque control reference current is required to calculate the field weakening control reference current, which is used to form the constraint equation and optimization condition. The problem brought by this is that the torque control reference current in high speed area is usually affected by motor magnetic saturation, back electromotive force distortion and parameter change, etc. It is easy to deviate, and then it will lead to the inconsistency between the field weakening calculation result and the actual optimal value, so that the error of the finally generated field weakening control reference current appears, reducing the stability of the field weakening area. SUMMARY

[0007] The purpose of the present application is to solve the above-mentioned problems of the prior art. The permanent magnet motor widely adopts field weakening strategy in high speed operation and expansion control, but the traditional field weakening control method usually depends on the pre-calculated torque control reference current, which is easily affected by magnetic saturation, back electromotive force distortion and parameter change in high speed area, thereby leading to field weakening current calculation error, and it is difficult to guarantee the system stability. In view of the above problems, a calculation method of field weakening control reference current of permanent magnet motor is proposed.

[0008] In order to achieve the above purpose, the technical scheme adopted by the present application is: A calculation method of field weakening control reference current of permanent magnet motor, comprising the following steps: S1, calculating the voltage constraint coefficient, constructing the calculation formula of the voltage constraint coefficient k1: , wherein ω max is the maximum no-load speed of the permanent magnet motor, ψ f is the permanent magnet flux linkage, V dc is the DC bus voltage; S2, calculating the expected value of the field weakening control reference current, constructing the calculation formula of the expected value i dref0 of the field weakening control reference current: , wherein L s is the winding inductance of the permanent magnet motor, ω refa desired speed of the permanent magnet motor; S3, a weak magnetic control reference current compensation, a stator resistance voltage drop is estimated according to a stator resistance Rs of the permanent magnet motor and three-phase currents ia, ib, ic measured in real time, and an effective available voltage U of the weak magnetic control is obtained ef , , the compensation formula is: , the compensated weak magnetic control reference current i drefc is calculated S4, a current limit is judged and an amplitude limiting process is performed, whether the weak magnetic control reference current initial value i max is beyond the allowable range is judged according to the maximum allowable current I dref0 of the permanent magnet motor, and a flag factor k2 is set according to the judgment result: ; S5, a weak magnetic control reference current value is set, the weak magnetic control reference current value i dref is calculated according to the flag factor k2 .

[0009] Preferably, the three-phase currents ia, ib, ic are collected in real time by a Hall current sensor on the motor stator side.

[0010] Preferably, the winding inductance L s , the stator resistance Rs, and the permanent magnet flux linkage ψ f are inherent parameters of the permanent magnet motor, and the maximum no-load speed ω max is obtained by no-load test.

[0011] The beneficial effects of the present application mainly include: 1. Avoiding error accumulation of torque control reference current, the control precision of the weak magnetic area is significantly improved; 2. Dynamic compensation and amplitude limiting protection double mechanism, adapting to complex working conditions in high speed area, excellent anti-interference ability; 3. Without additional PI controller or torque current calculation module, strong real-time performance, easy for engineering implementation; 4. Precise matching of voltage constraint, maximizing the utilization efficiency of the DC bus voltage, supporting higher speed expansion. BRIEF DESCRIPTION OF DRAWINGS

[0012] Other features, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments made with reference to the accompanying drawings: Figure 1 is a flowchart of a method for calculating a weak magnetic control reference current of a permanent magnet motor. DETAILED DESCRIPTION

[0013] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0014] The present application will be described in further detail below with reference to the drawings and embodiments. It can be understood that the specific embodiments described herein are only used to explain the related application, and not to limit the application. In addition, it should be noted that, for the convenience of description, only the parts related to the application are shown in the drawings. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0015] The present application provides a kind of permanent magnet motor weak magnetic control reference current calculation method, according to DC bus voltage, permanent magnet flux linkage and motor maximum no-load speed constructs voltage constraint coefficient, accurately characterizes the voltage limit degree in high speed area;Second, according to the constraint relationship directly analytical obtains weak magnetic desired current, avoids the influence of traditional torque control reference current on weak magnetic calculation;Third, combining motor resistance and operating current, preliminary weak magnetic current is corrected, improve the adaptability of calculation result to high speed working condition;Finally, through current limiting mechanism, the range of weak magnetic current is automatically limited, to ensure electromagnetic safety and system stability.Dependence on torque control reference current is got rid of, directly based on motor inherent parameters (inductance, permanent magnet flux linkage etc.) And real-time operating parameters, construct voltage constraint, analytical solution, dynamic compensation, complete process of limiting protection, realize the accurate, fast calculation of weak magnetic control reference current.

[0016] Specifically, a kind of permanent magnet motor weak magnetic control reference current calculation method as shown in Figure 1 It includes the following steps: S1 calculates voltage constraint coefficient, constructs the calculation formula of voltage constraint coefficient k1: Wherein ω max is the maximum no-load speed of permanent magnet motor, ψ f is permanent magnet flux linkage, V dc is DC bus voltage; S2 calculates weak magnetic control reference current expected value, constructs the calculation formula of weak magnetic control reference current expected value i dref0 : Wherein L s is the winding inductance of permanent magnet motor, ω refa desired speed of the permanent magnet motor; S3 weak magnetic control reference current compensation, according to the stator resistance Rs of the permanent magnet motor and the three-phase current ia, ib, ic measured in real time, the stator resistance voltage drop is estimated, and the effective available voltage U of the weak magnetic control is obtained ef , the three-phase current ia, ib, ic is generally collected in real time through the Hall current sensor on the motor stator side, the winding inductance L s , the stator resistance Rs, the permanent magnet flux ψ f is the inherent parameter of the permanent magnet motor, the maximum no-load speed ω max is obtained by no-load test, , through the compensation formula: , the compensated weak magnetic control reference current i drefc is calculated; S4 judges the current limit and performs amplitude limiting processing, according to the maximum allowable current I max of the permanent magnet motor, whether the initial value i dref0 of the weak magnetic control reference current exceeds the allowed range is judged, and the flag factor k2 is set according to the judgment result: ; S5 sets the weak magnetic control reference current value, according to the flag factor k2, the weak magnetic control reference current value i dref is calculated, .

[0017] In one embodiment, the permanent magnet flux ψ f =0.083Wb, the maximum no-load speed ω max =560rpm, the DC bus voltage V dc =200V, the maximum allowable current I max of the permanent magnet motor =150A. The calculation method of the weak magnetic control reference current of the permanent magnet motor is as follows: Calculate the voltage constraint coefficient: The maximum no-load speed ω max =560 of the permanent magnet motor, the permanent magnet flux ψ f =0.083 and the DC bus voltage V dc =200 are substituted into the following formula to calculate the voltage constraint coefficient k1: .

[0018] Calculate the weak magnetic control reference current expected value: Assuming that the desired motor speed is ω ref =560, the k1=4.174 obtained in step S1 and the winding inductance L s= 0.0125 and permanent magnet flux linkage ψ f = 0.083, the expected value of the field weakening control reference current i dref0 : .

[0019] The field weakening control reference current compensation: According to the stator resistance R s = 0.5 Ω of the permanent magnet motor and the real-time measured three-phase current i a , i b and i c The estimated stator resistance voltage drop is obtained, and the effective available voltage U ef during field weakening control is obtained: .

[0020] In this embodiment, it can be assumed that the three-phase current of the permanent magnet motor satisfies , then the effective available voltage U ef during field weakening control is 189.91 V. Then, the compensated field weakening control reference current i drefc is calculated using the following formula: .

[0021] Determine the current limit and perform amplitude limiting processing: According to the maximum allowable current I max = 150 of the permanent magnet motor, it is determined whether the compensated field weakening control reference current i drefc obtained in step S3 exceeds the allowable range using the following formula, and the value of the flag factor k2 is set according to the determination result: .

[0022] In this embodiment, k2 = 1, and the calculated field weakening control reference current compensation value can avoid the risk of excessive demagnetization.

[0023] Set the field weakening control reference current value: According to the value of the flag factor k2 in step S4, the field weakening control reference current value i dref = -0.45 A is obtained using the following formula: .

[0024] As can be seen from the above description, the error accumulation of the torque control reference current is avoided, the control accuracy of the field weakening region is significantly improved; the dynamic compensation and amplitude limiting protection double mechanisms adapt to complex working conditions in the high-speed region, and have excellent anti-interference ability; without additional PI controller or torque current calculation module, the real-time performance is strong, and it is convenient for engineering implementation; the voltage constraint is accurately matched, the utilization efficiency of the direct current bus voltage is maximized, and higher speed expansion is supported.

[0025] The term "comprising" or any other similar word is intended to encompass the inclusion of one or more steps, features, or elements but not to the exclusion of any other steps, features, or elements. The term "comprising" therefore indicates that the inclusion of one or more steps, features, or elements is not a requirement for the process, method, article, or apparatus / apparatus to fall within the scope of the present application.

[0026] So far, the technical solutions of the present application have been described in combination with the preferred embodiments shown in the drawings, but it is easy for those skilled in the art to understand that the protection scope of the present application is obviously not limited to these specific embodiments. Those skilled in the art can make equivalent changes or replacements to the related technical features without deviating from the principles of the present application, and the technical solutions after these changes or replacements will all fall within the protection scope of the present application.

Claims

1. A method for calculating the reference current for field weakening control of a permanent magnet motor, characterized in that... Includes the following steps: S1 calculates the voltage constraint coefficient and constructs the formula for calculating the voltage constraint coefficient k1: , where ω max The maximum no-load speed of the permanent magnet motor, ψ f For permanent magnet flux, V dc This is the DC bus voltage; S2 calculates the expected value of the field weakening control reference current and constructs the expected value i of the field weakening control reference current. dref0 The calculation formula is as follows: L s For the winding inductance of a permanent magnet motor, ω ref This represents the desired rotational speed of the permanent magnet motor. S3 field weakening control reference current compensation estimates the stator resistance voltage drop based on the stator resistance Rs of the permanent magnet motor and the real-time measured three-phase currents ia, ib, and ic, thus obtaining the effective usable voltage U during field weakening control. ef , Through the compensation formula: The compensated field weakening control reference current i is calculated. drefc ; S4 determines the current limit and performs limiting processing based on the maximum allowable current I of the permanent magnet motor. max Determine the initial value i of the field weakening control reference current. dref0 Whether it exceeds the allowed range, set the flag factor k2 based on the judgment result: ； S5 sets the reference current value for field weakening control, and calculates the reference current value i for field weakening control based on the flag factor k2. dref , 。 2. The method for calculating the reference current for field weakening control of a permanent magnet motor according to claim 1, characterized in that: The three-phase currents ia, ib, and ic are acquired in real time by Hall current sensors on the stator side of the motor.

3. The method for calculating the reference current for field weakening control of a permanent magnet motor according to claim 1, characterized in that: The winding inductance L s Stator resistance Rs, and permanent magnet flux linkage ψ f These are inherent parameters of the permanent magnet motor, with the maximum no-load speed ω being... max The results were obtained through actual measurements during no-load testing.

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