A design method of common-mode choke for reducing the effective value of motor leakage current

Abstract

The application provides a common-mode choke design method for reducing the effective value of motor leakage current, and faces the field of motor drive and electromagnetic compatibility. In view of the problem that the existing researches pay more attention to common-mode voltage and ignore leakage current caused by high switching frequency, the mechanism of leakage current is first clarified, the common-mode equivalent circuit is established, the influence of bearing current is evaluated, and the nonlinear link is simplified. The mathematical expression of the choke for suppressing the leakage current is derived, the secondary resistance is optimized based on the root locus, and the core parameters are adjusted. Through simulation and comparative evaluation verification, the optimized choke can significantly reduce the effective value of the leakage current and improve the system EMC, and provides a feasible scheme for precise suppression of high-frequency motor drive.

Description

Technical Field

[0001] This invention relates to the field of motor drive and electromagnetic compatibility technology, and in particular to a common-mode choke design method for reducing the effective value of motor leakage current. Background Technology

[0002] Modern motor drive systems have evolved from direct power frequency drive to PWM power converter drive. While this improves system performance, PWM inverters generate common-mode voltage with step characteristics, which can cause high-frequency leakage current. This leakage current forms a path through stray capacitance between the stator windings and the motor frame, with an amplitude that can reach 10% of the rated current. This can lead to corrosion and deterioration of the motor frame and bearings, shortening their lifespan. It can also cause electromagnetic interference (EMI) to surrounding electronic equipment, affecting the stable operation of the power converter.

[0003] Current research on leakage current suppression mainly revolves around common-mode voltage, starting from interference sources (optimized modulation strategies, improved topologies, soft-switching techniques, etc.) and propagation paths (passive, active, and hybrid EMI filters). However, there are some shortcomings: First, the analysis of the leakage current generation mechanism is not in-depth, and there is a lack of construction on the correlation between leakage current characteristics and suppression methods. Second, in passive filters, traditional common-mode chokes rely solely on increasing the common-mode inductance to suppress leakage current, which has limited effect and can easily cause system oscillations under some operating conditions. Active filters have poor suppression effect on high-frequency interference, and hybrid filters have complex structures. Third, the influence of nonlinear factors such as bearing current on leakage current modeling is not fully considered, or oversimplification leads to insufficient model accuracy. Summary of the Invention

[0004] This invention proposes a common-mode choke design method to reduce the effective value of motor leakage current. A series damping resistor is introduced into the secondary side of a traditional common-mode choke to form a leakage current suppression loop, improving the equivalent resistance of the path and the system damping. An expression for the choke's leakage current suppression capability is derived. The influence of the secondary resistance on the system poles is analyzed based on the root locus method to determine the optimal resistance value range, transforming the poles from oscillating complex roots to real decaying roots. The secondary resistance value is calculated in reverse based on the upper limit of the effective leakage current. At a switching frequency of 3kHz, the resistance power is less than 1W, so a low-power metal film resistor is selected. The core parameters are optimized, reducing the effective cross-sectional area of ​​the core to half that of the traditional structure, while maintaining an inductance of 17mH and a smaller volume than traditional chokes. This addresses the problems of existing PWM-based motor drive systems where increased switching frequency leads to significant conducted electromagnetic interference, leakage current affecting motor life and peripheral equipment, and the fact that existing research focuses primarily on common-mode voltage suppression, lacking in-depth analysis and effective suppression methods for leakage current. Furthermore, traditional common-mode chokes have limited suppression effects and are prone to causing system oscillations.

[0005] A common-mode choke design method for reducing the effective value of motor leakage current includes the following steps: S1. For motor drive systems using pulse width modulation (PWM) technology, the mechanism of motor leakage current caused by common-mode voltage of PWM converter is investigated. A high-frequency common-mode equivalent circuit containing the parasitic capacitance of stator winding to the casing is constructed. The influence of bearing current on leakage current is quantified and the nonlinear link is ignored. The main path of leakage current is simplified to an RLC series resonant circuit. S2. Improve the traditional common-mode choke by connecting a damping resistor in series on its secondary side to form a leakage current suppression loop. Derive the expression for the leakage current suppression capability of the improved common-mode choke. Use the root locus method to analyze the influence of the damping resistor on the system poles to determine the optimal range of resistor values, so that the system poles are transformed from oscillating complex roots to real decaying roots. At the same time, optimize the core parameters of the common-mode choke. S3. Compare the suppression effects of the traditional common-mode choke and the improved common-mode choke to verify the suppression performance of the improved common-mode choke on the effective value of motor leakage current and high-frequency electromagnetic interference (EMI).

[0006] Furthermore, in S1, the parameters of the high-frequency common-mode equivalent circuit are obtained as follows: the parasitic capacitance is obtained by actual measurement using an LCR meter, the loop inductance L is obtained by measuring the impedance after short-circuiting the inverter and the three-phase line on the motor side, and the loop resistance R is inferred from the damping factor of the experimental waveform. The expression for the oscillation current of the RLC series resonant circuit is:

[0007] In the formula, , , , Common-mode voltage, It is the natural angular frequency. For the damping ratio, Characteristic impedance, For time variables, The loop inductance in an RLC series resonant circuit. This refers to the loop capacitance in an RLC series resonant circuit. This is the loop resistance in an RLC series resonant circuit.

[0008] Furthermore, in S1, when the damping ratio of the RLC series resonant circuit satisfies At this time, the leakage current is approximately given by the following formula:

[0009] When using a conventional common-mode choke, assuming that both the inductance and resistance are increased... a、b If the value is multiplied by a factor of 1, then the parameter changes to:

[0010]

[0011]

[0012] The leakage current expression is then obtained as:

[0013] because ,get:

[0014] Since the amplitude and decay time are both equal to the original values... , The effective value of the leakage current changes by the following factor: .

[0015] Furthermore, in S2, the resistance value of the secondary-side series damping resistor is calculated based on the upper limit of the leakage current using the following formula:

[0016] The power of the damping resistor is:

[0017] When the switching frequency of the PWM converter is 3kHz, the power of the damping resistor is less than 1W, and a low-power metal film resistor is selected.

[0018] Furthermore, in S2, when determining the optimal range of values ​​for the damping resistor using the root locus method: The system characteristic equation including the improved common-mode choke is:

[0019] The equivalent open-loop transfer function is then derived:

[0020] By plotting the root locus diagram, the value space of the damping resistor is determined, ensuring that the system poles are transformed from oscillating complex roots to real decaying roots.

[0021] Furthermore, in S2, when optimizing the core parameters, the maximum magnetic flux of the common-mode choke secondary winding is improved to satisfy:

[0022] Furthermore, the relationship between the maximum magnetic flux and the effective cross-sectional area and common-mode inductance of the iron core satisfies:

[0023] In the formula Number of turns For effective cross-sectional area, For flux density.

[0024] Furthermore, in S2, the maximum magnetic flux With effective cross-sectional area Inversely proportional, and The common-mode inductance is also proportional to the common-mode inductance, and the improved common-mode choke's common-mode inductance satisfies:

[0025] In the formula Represents the vacuum permeability. Represents relative permeability. Represents the effective length of the magnetic circuit. Furthermore, in S2, after connecting a damping resistor in series on the secondary side of the improved common-mode choke, when the resistance value of the damping resistor is within the range that allows the system to have three real roots, the leakage current is approximately:

[0026] In a three-phase voltage-source inverter, six switching operations are performed within each PWM cycle. The average leakage current at this time satisfies the following: .

[0027] A storage medium storing a computer program, characterized in that, when the computer program is executed by a processor, it implements the above-described common-mode choke design method for reducing the effective value of motor leakage current.

[0028] A computer device includes: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the above-described common-mode choke design method for reducing the effective value of motor leakage current.

[0029] The common-mode choke design method for reducing the effective value of motor leakage current according to the present invention has the following beneficial effects: (1) The improved common mode choke can effectively reduce the effective value and peak value of motor leakage current. Experiments have verified that its suppression effect is better than that of traditional common mode choke. It is stable and effective under both high and low speed conditions. At the same time, spectrum analysis shows that the scheme can maintain good attenuation characteristics for high frequency EMI noise while reducing time-domain leakage current, thus meeting the requirements of electromagnetic compatibility standards.

[0030] (2) The influence of bearing current on leakage current (accounting for about 0.3%) was quantified by experiment. The main path of leakage current was reasonably simplified to an RLC series resonant circuit to avoid design deviations caused by oversimplification or complex modeling, and to provide accurate model support for the suppression scheme.

[0031] (3) Based on the root locus method, the secondary side resistance parameters are optimized, and the core parameter design formula is combined to reduce the effective cross-sectional area of ​​the core to 1 / 2 of the traditional one. While ensuring leakage current suppression performance, the common mode choke volume is miniaturized (outer diameter is reduced by about 14.6% and height is reduced by about 18.2%), reducing installation space requirements and improving engineering applicability. Attached Figure Description

[0032] Figure 1 This is a two-level voltage source PWM motor drive system; Figure 2 It is an equivalent series resonant circuit; Figure 3 To connect the experimental system of the improved common-mode choke; Figure 4 The equivalent circuit after adding the improved common-mode choke; Figure 5 This is a root locus graph; Figure 6 The time-domain waveforms and Bode plots are shown for different resistance values. Figure 6 (1) is the spatial-time domain response; Figure 6 (2) is the spatial-temporal response; Figure 6 (3) is the spatial-temporal response; Figure 6 (4) is the spatial amplitude-frequency curve; Figure 6 (5) is a spatial amplitude-frequency curve; Figure 6 (6) is a spatial three-frequency curve; Figure 7 This is a comparison chart showing the inhibition effect; Figure 8 This is a flowchart of a common-mode choke design method for reducing the effective value of motor leakage current according to the present invention; Figure 9 The curves show the changes in resistance along with the effective value and peak value, where... Figure 9 (a) is the curve showing the change in resistance along with the effective value; Figure 9 (b) is the curve showing the change in resistance along with the peak value. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Reference Figure 8 As shown, the implementation process of a common-mode choke design to reduce the effective value of motor leakage current in this embodiment is as follows: S1. Investigate the mechanism of motor leakage current caused by common-mode voltage of PWM converter, construct high-frequency common-mode equivalent circuit, ignore bearing current (accounting for about 0.3%), and simplify the main path of leakage current to RLC series resonant circuit.

[0035] S2. Add a series damping resistor to the secondary side of the traditional common-mode choke, derive the expression for leakage current suppression capability, determine the optimal range of the resistor using the root locus method, select a small-power metal film resistor (power < 1W at 3kHz) based on the upper limit of leakage current, and optimize the core parameters (effective cross-sectional area reduced to 1 / 2 of the traditional).

[0036] S3. Comparative verification of the improved choke's suppression effect: with the same inductance value of 17mH, the suppression effect is better than the traditional model and the size is smaller than the traditional model.

[0037] Before conducting leakage current analysis, it is necessary to explain its generation principle and construct a corresponding equivalent circuit model. This invention analyzes leakage current by establishing a common-mode equivalent circuit, and at the same time, delves into the intrinsic relationship between leakage current and bearing current in the equivalent circuit.

[0038] The common-mode voltage on the motor is divided into the common-mode voltage generated on the rectifier and the common-mode voltage generated on the inverter. First, we will analyze the common-mode voltage generated on the rectifier.

[0039] Depend on Figure 1 De: (1) (2) (3) In the formula, U DC This represents the DC bus voltage output by the rectifier; u PM This represents the voltage between points P and M; u Mg This represents the voltage at the midpoint of the DC bus relative to the grounding point; u Pg This represents the voltage at point P relative to the grounding point; u Lg This represents the voltage at point L relative to the grounding point.

[0040] According to equations (1)-(3), we can obtain: (4) According to the definition of common-mode voltage, voltage u Mg The common-mode voltage output by the three-phase rectifier bridge is expressed in its Fourier series expansion as follows: (5) In the formula, This represents the fundamental frequency of the AC input voltage of a three-phase rectifier. This indicates the AC input line voltage of the three-phase rectifier.

[0041] According to the analysis results of equation (5), a common-mode voltage with a fundamental frequency of 3 times is generated on the rectifier, and it also contains harmonic components of higher frequencies such as 450Hz.

[0042] Analysis of the common-mode voltage generated on the inverter: (6) In the formula u ag , u bg , u cg It is the potential of the motor input terminals a, b, and c relative to the grounding point; u CMng This represents the ground potential between the neutral point of the motor winding and the ground point, i.e., the common-mode voltage of the motor. i a , i b , i c These represent the instantaneous values ​​of the current flowing through the motor windings; R m , L m These represent the resistance and inductance of each phase winding of the induction motor, respectively.

[0043] Adding the three equations in equation (6) together, we get: (7) For three-phase symmetrical induction motors Combining equation (7), we can obtain: (8) Depend on Figure 1 From the topology, we can obtain: (9) In the formula u aM , u bM , u cM These represent the voltages at the input terminals a, b, and c of the induction motor relative to the midpoint of the DC bus, respectively.

[0044] Combining equations (8) and (9), we can obtain: (10) Then it can be made u CMThe common-mode voltage output by the inverter: (11) For a PWM inverter, the common-mode voltage output by the inverter under different switching states is as follows: (12) The common-mode motor generated on the motor is the superposition of the two.

[0045] The common-mode circuit can be equivalent to Figure 2 form.

[0046] Combination Figure 2 The common-mode voltage waveform characteristics of the motor drive system shown indicate that its amplitude exhibits a typical step-like change, and the amplitude of a single step can be quantitatively represented as the DC bus voltage. U DC One-third of it.

[0047] Figure 2 The oscillating current is shown in the following formula. (13) In the formula , ,

[0048] In the formula This refers to natural frequency. It is the damping ratio. This is the characteristic impedance. When At this time, the leakage current can be approximated by the following formula: (14) When using a traditional common-mode choke, assuming Figure 2 The inductance and resistance in the middle have each increased. a、b The change is calculated by multiplying the time by the time factor, which indicates that the parameter has changed to: (15) (16) (17) Therefore, the expression for leakage current is: (18) because We can obtain: (19) Since the amplitude and decay time are both equal to the original values... , The effective value of the leakage current changes by the following factor: (20) After adding an improved common-mode choke, the system is as follows: Figure 3 As shown, compared to the classic common-mode choke structure, this improved design introduces an additional resistor branch on its secondary side. This resistor element is specifically designed to construct a leakage current suppression loop, providing an effective damping path for system leakage current.

[0049] In transformer secondary-side modeling, the resistance of the secondary winding is often equivalent to that of the primary side using impedance reduction. After reduction, this resistance and the primary common-mode inductance form a parallel branch, and the primary input voltage is directly applied across this branch, thus effectively reflecting the electrical characteristics of the secondary load in the primary circuit. For example... Figure 3 As shown, the reduced equivalent circuit can fully express the influence of the secondary resistance on the system.

[0050] The structure employs an improved common-mode choke. This choke, together with the original resonant element, forms a composite resonant network. Under step voltage excitation, the system's leakage current response can be solved using Laplace transform, thereby obtaining its frequency domain characteristic expression.

[0051] (twenty one) In the formula, since Figure 4 middle and much smaller and Therefore, it can be ignored.

[0052] Therefore, its characteristic equation can be obtained as follows: (twenty two) After organizing, we can obtain information about the parameters. The equivalent open-loop transfer function is: (twenty three) Figure 5 This is the root locus image of the parameters, where the Z-axis represents the parameters. Values, and plane 1 and plane 2 in the figure represent respectively and The two planes divide the entire space into three parts, as shown in Table 3. The resistance values ​​of the three spaces are named Space 1, Space 2, and Space 3 in ascending order of value.

[0053] when At this point, there is one real root and two conjugate complex roots. The latter determines the waveform of the leakage current because the two poles near the origin are canceled out by the zero. Therefore, within this range, the leakage current waveform is an oscillating waveform, such as... Figure 6 As shown in (1).

[0054] when When there are three real roots, the oscillation of the leakage current waveform will be much smaller. Therefore, within this range, its effective value will decrease significantly, such as... Figure 6 As shown in (2).

[0055] when Then, two conjugate complex roots will appear again, and the oscillation will occur again, such as... Figure 6 As shown in (3).

[0056] from Figure 6 The analysis results (4) to (6) show that the amplitude stability margin of the system increases with the increase of the resistance value. When the resistance value is low, its suppression effect on related signals is even worse than that of a traditional common-mode choke. The mechanism of this phenomenon is that, under low resistance conditions, the resistor cannot provide an effective damping path for the system leakage current, and its equivalent impedance characteristics are close to a short-circuit state, which may lead to severe oscillations in the system, thus causing the effective value to rise. When the resistance value is as well as At these times, their phase margins are 93.37° and 126.02°, respectively. However, as the resistance continues to increase, the system amplitude gain falls below 0dB, indicating strong system stability and preventing this situation from occurring. Both space 2 and space 3 exhibit good suppression effects. Although space 3 shows oscillation again, its peak suppression effect is better than that of space 2. Figure 6 (3) It can be seen that as the resistance continues to increase, the attenuation of its peak value will also reach a limit. The curves showing the changes in RMS value and peak value along with the resistance are as follows: Figure 9 As shown, to achieve optimal amplitude attenuation characteristics under specific resistance conditions, a quantization relationship model between the resistance value and the effective signal value needs to be established. When the effective value changes, the system magnetic flux will change accordingly, which in turn will cause changes in the geometric parameters of the magnetic core, requiring appropriate adjustments. With the resistance values ​​of space 2 and space 3 where oscillations are smaller, the real root plays a major role, so the leakage current mainly flows through the [residual root]. Instead In this case, the leakage current is approximately: (twenty four) In a three-phase voltage-source inverter, six switching operations are performed per PWM cycle, resulting in: (25) Before designing, an upper limit for the leakage current needs to be determined, and then the secondary resistor is designed in reverse. According to equation (25), we can obtain: (26) It can be seen that with a switching frequency of 3kHz, the power is less than 1W, so a low-power metal film resistor can be selected. Furthermore, since the voltage applied to the magnetizing inductor is the same as the voltage across the resistor, the maximum magnetic flux of the secondary winding can be obtained as: (27) There are also (28) In the formula Number of turns For the effective cross-sectional area, For flux density. Therefore, the maximum magnetic flux can be determined. With effective cross-sectional area Inversely proportional, and It is also proportional to the common-mode inductance, as shown in equation (29). (29) Through derivation, it can be seen that as... The decrease, As the area increases, the required effective cross-sectional area... While the common-mode inductance is decreasing, the suppression effect is not weakened because the magnetic core is not saturated. By introducing a damping resistor, the frequency response characteristics of the system are effectively adjusted while maintaining optimized suppression performance. This method achieves core size reduction and structural optimization goals by optimizing the equivalent circuit parameter configuration.

[0057] from Figure 7 The comparison between the traditional choke and the improved choke clearly shows that the improved choke is effective in suppressing the effective value.

[0058] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A common-mode choke design method for reducing the effective value of motor leakage current, characterized in that, Includes the following steps: S1. For motor drive systems using pulse width modulation (PWM) technology, the mechanism of motor leakage current caused by common-mode voltage of PWM converter is investigated. A high-frequency common-mode equivalent circuit containing the parasitic capacitance of stator winding to the casing is constructed. The influence of bearing current on leakage current is quantified and the nonlinear link is ignored. The main path of leakage current is simplified to an RLC series resonant circuit. S2. An improvement is made to the traditional common-mode choke by connecting a damping resistor in series on its secondary side to form a leakage current suppression loop. The expression for the improved common-mode choke's ability to suppress leakage current is derived. The root locus method is used to analyze the effect of the damping resistor on the system poles to determine the optimal range of resistor values, transforming the system poles from oscillating complex roots to real decaying roots. Simultaneously, the core parameters of the common-mode choke are optimized. When determining the optimal range of values ​​for the damping resistor using the root locus method: The system characteristic equation including the improved common-mode choke is: in, L For loop inductance, This refers to the loop capacitance in an RLC series resonant circuit. The circuit resistance is the loop resistance in the RLC series resonant circuit. The equivalent open-loop transfer function is then derived: By plotting the root locus diagram, the value space of the damping resistor is determined, ensuring that the system poles are transformed from oscillating complex roots to real decaying roots; When optimizing the core parameters, the maximum magnetic flux of the improved common-mode choke secondary winding satisfies: in, E Common-mode voltage, This is the RMS value of the common-mode leakage current. Furthermore, the relationship between the maximum magnetic flux and the effective cross-sectional area and common-mode inductance of the iron core satisfies: In the formula Number of turns For the effective cross-sectional area, Flux density; Maximum magnetic flux With effective cross-sectional area Inversely proportional, and The common-mode inductance is also proportional to the common-mode inductance; the improved common-mode choke's common-mode inductance satisfies: In the formula Represents the vacuum permeability. Represents relative permeability. Represents the effective length of the magnetic circuit; With a damping resistor connected in series on the secondary side of the improved common-mode choke, when the resistance value of the damping resistor is within the range that allows the system to have three real roots, the leakage current is approximately: In a three-phase voltage-source inverter, six switching operations are performed within each PWM cycle. The average leakage current at this time satisfies the following: in, It is a time variable; S3. Compare the suppression effects of the traditional common-mode choke and the improved common-mode choke to verify the suppression performance of the improved common-mode choke on the effective value of motor leakage current and high-frequency electromagnetic interference (EMI).

2. The common-mode choke design method for reducing the effective value of motor leakage current according to claim 1, characterized in that, In S1, the parameters of the high-frequency common-mode equivalent circuit are obtained as follows: the parasitic capacitance is obtained by actual measurement with an LCR meter, and the loop inductance is... L The impedance was measured by short-circuiting the three-phase lines on the inverter side and the motor side respectively, and the loop resistance R was inferred from the damping factor of the experimental waveform. The expression for the oscillation current of the RLC series resonant circuit is: In the formula, , , , Common-mode voltage, It is the natural angular frequency. For the damping ratio, Characteristic impedance, The loop inductance in an RLC series resonant circuit. This refers to the loop capacitance in an RLC series resonant circuit. This is the loop resistance in an RLC series resonant circuit.

3. The common-mode choke design method for reducing the effective value of motor leakage current according to claim 2, characterized in that, In S1, when the damping ratio of the RLC series resonant circuit satisfies At this time, the leakage current is approximately given by the following formula: When using a conventional common-mode choke, assuming that both the inductance and resistance are increased... a、b If the value is multiplied by a factor of 1, then the parameter changes to: The leakage current expression is then obtained as: because ,get: Since the amplitude and decay time are both equal to the original values... , The effective value of the leakage current changes by the following factor: 。 4. The common-mode choke design method for reducing the effective value of motor leakage current according to claim 3, characterized in that, In S2, the resistance value of the secondary-side series damping resistor is calculated based on the upper limit of leakage current using the following formula: The power of the damping resistor is: When the switching frequency of the PWM converter is 3kHz, the power of the damping resistor is less than 1W, and a low-power metal film resistor is selected.

5. A storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the common-mode choke design method for reducing the effective value of motor leakage current as described in any one of claims 1-4.

6. A computer device, characterized in that, include: The memory, the processor, and the computer program stored in the memory and executable on the processor, the processor executing the program to implement the common-mode choke design method for reducing the effective value of motor leakage current as described in any one of claims 1-4.

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