CN115792536A - A method for online evaluation of the main insulation state of variable frequency motor - Google Patents

Abstract

The invention discloses an on-line evaluation method for the main insulation state of a variable frequency motor. First, the corresponding relationship between the main insulation and the leakage current is determined by establishing an insulation model of the stator winding of the variable frequency motor. Secondly, based on the time-frequency domain characteristics of the leakage current, the main insulation state of the variable frequency motor is evaluated online, and the degree and location of the main insulation degradation are identified by combining the common-mode and differential-mode harmonics of the leakage current, and the degradation phase is identified by the initial oscillation amplitude of the leakage current. Finally, the experimental verification is carried out on the permanent magnet synchronous motor system based on vector control. The beneficial effect of the invention is that it can effectively identify the initial degradation of the main insulation of the motor and accurately locate the degradation position.

Description

A method for online evaluation of the main insulation status of variable frequency motors

technical field

The invention relates to the technical field of variable frequency motors, in particular to an online evaluation method for the main insulation state of variable frequency motors.

Background technique

Variable frequency motors have been widely used in high-speed railways, wind power generation, and industrial machinery that require high operational reliability. As one of the most serious faults of motors, stator insulation faults account for about 30%-40% of the total faults. Therefore, in order to ensure the reliable operation of the motor system, it is necessary to evaluate the stator insulation status of the variable frequency motor in real time.

Traditional offline testing methods, such as insulation resistance and polarization index testing, AC and DC voltage testing, dissipation factor and offline partial discharge (PD) testing, have been widely used to detect the insulation status of motors. However, the monitoring period of the offline method is long, and the motor to be tested must be stopped for processing. In order to detect insulation degradation in time and avoid severe insulation faults, it is necessary to propose an online insulation state assessment method.

On-line partial discharge monitoring is one of the common methods for real-time monitoring of motor insulation conditions. However, this method is susceptible to noise and requires the installation of a specific PD current sensor. In recent years, the online monitoring method of motor insulation status based on phase current and leakage current has gradually attracted attention. For example: use the root mean square deviation of the transient phase current as the monitoring index of the insulation state of the stator winding, or further study the influence of dv/dt on the root mean square deviation of the phase current by adjusting the switching speed of the MOSFET, or establish an electromagnetic The high-frequency model of the line is used to analyze the oscillation characteristics of the transient phase current, and a method for monitoring the insulation state of the stator winding input terminal based on the amplitude-frequency characteristics of the transient phase current is proposed. However, the above-mentioned phase current monitoring methods all need to monitor the transient current up to MHz, and need to install a high-precision and high-bandwidth current sensor. In addition, the above methods do not investigate the effect of the degradation location on the degradation degree.

The leakage current of the motor system is highly coupled with the insulation resistance, and the insulation state of the motor winding can be reflected through the leakage current. For example, a differential mode leakage current measurement method proposed in the prior art is used to monitor the leakage current of each phase, and obtain the equivalent resistance and capacitance of the main insulation to evaluate the degree of insulation degradation. The differential-mode leakage current measurement method requires three high-sensitivity current transformers, which are costly and need to lead out the neutral point of the motor. Those skilled in the art also propose a method for monitoring the equivalent capacitance of the main insulation of the motor based on the common-mode leakage current and the common-mode voltage. This method can monitor the degree of degradation of the main insulation of the motor, but does not consider the influence of the location of insulation degradation. Those skilled in the art have also proposed to use the time-domain characteristics of the leakage current to identify the degradation state of the main insulation. However, this method cannot assess the degree and location of insulation degradation alone. In addition, an insulation common-mode impedance monitoring method based on leakage current is also proposed in the prior art. This method can distinguish insulation degradation at different winding positions, but it needs to monitor the common-mode leakage current above the resonant frequency, which affects the monitoring bandwidth of the leakage current. And higher sampling frequency requirements.

In order to overcome the above defects, those skilled in the art need an evaluation method that can improve the performance of the online evaluation of the motor insulation state, reduce the monitoring signal bandwidth and consider the influence of the degradation position on the degradation degree.

Contents of the invention

The object of the present invention is to solve the above-mentioned problems and design an online evaluation method for the main insulation state of a variable frequency motor.

To achieve the above object, the technical solution of the present invention is an online evaluation method for the main insulation state of a variable frequency motor, which includes the following steps:

Step 1: The equivalent circuit model of the main insulation degradation of the stator winding is established in the low frequency band, and the common mode frequency obtained by the harmonic analysis of the three-phase voltage in the equivalent circuit model of the main insulation degradation of the stator winding is obtained by using double Fourier integration and differential mode frequencies;

Step 2, based on the equivalent circuit model of the main insulation degradation of the variable frequency motor, the frequency-domain mathematical expression of the leakage current and the time-domain mathematical expression of the leakage current are obtained;

Step 3, according to the time-domain mathematical expression and the frequency-domain mathematical expression of the leakage current, the degradation degree, degradation position and degradation phase of the main insulation of the variable frequency motor are evaluated online.

The equivalent circuit model of the main insulation degradation of the stator winding in the step 1 is an equivalent capacitance and resistance parallel circuit, which respectively represent capacitive coupling and dielectric loss, U _{a, b, c} represent the three-phase voltage, R _s and L _s represents the stator resistance and inductance respectively, R _g and C _g represent the equivalent resistance and capacitance of the main insulation, point N is the neutral point, point D is the location where the main insulation deteriorates, x represents the number of turns from the input terminal to point D and The ratio of the number of turns of the phase winding, the three-phase voltage is replaced by an equivalent voltage source, and the three-phase branches are combined to obtain an RLC series circuit, where k , Z , and U _g represent the stator impedance coefficient and the equivalent circuit Impedance and equivalent ground voltage source, the circuit parameters are expressed as follows:

(1)

(1)

(2)

(2)

(3)。

(3).

In the first step, double Fourier integration is used to perform harmonic analysis on the three-phase voltage, wherein the Fourier expression of the A-phase voltage is as follows:

(4)

(4)

where E _d is the DC bus voltage, ω ₀ is the fundamental angular frequency, ω _c is the carrier angular frequency (the switching angular frequency of the inverter), α is the modulation depth, and J _k(x) represents the kth order Bessel function;

According to formula (3), the expression of the equivalent voltage source to ground at the common mode frequency is as follows:

(5)

(5)

Further, combining formula (3) and formula (5), the equivalent voltage source to ground at the differential mode frequency is obtained as follows:

(6)

(6)

From equations (5) and (6), it can be seen that the common-mode equivalent voltage U _{g, CM} is equal to the common-mode voltage of the C-phase winding, and is not affected by the degradation position x , but the differential-mode equivalent voltage U _g,DM decreases linearly with the increase of the degenerate position x ;

The common-mode frequency and differential-mode frequency obtained according to the fundamental frequency f ₀ and switching frequency fc of the inverter are shown in formula (7) _and formula (8) respectively:

(7)

(7)

(8)

(8)

In the formula , m =1,2,3,..., n =0,1,2,.... When n is a multiple of 3, f ₁ represents the common-mode frequency, otherwise, f ₁ represents the differential-mode frequency, when 2n +1 is a multiple of 3, f ₂ represents the common-mode frequency, otherwise, f ₂ represents the differential-mode frequency.

The frequency-domain mathematical expression of the leakage current in the step 2 is:

(9)

(9)

Where ω =2π f ₀ When f is the common-mode frequency, I _g represents the common-mode leakage current, otherwise I _g represents the differential-mode leakage current;

The time-domain mathematical expression of the leakage current is:

(10)

(10)

表示处于上升沿,当

表示处于下降沿。When in the formula

Indicates that it is on the rising edge, when

Indicates a falling edge.

In the step three, the evaluation of the degree of degradation of the main insulation of the variable frequency motor is identified by the common-mode leakage current. According to formula (5) and formula (9), the mathematical expression of the common-mode leakage current is as follows:

(11)

(11)

Where ω =2 m π f _c , m =1,3,5,…;

According to formula (11), the common-mode leakage current I _g,CM under different insulation capacitances and degraded positions is obtained. I _g,CM is highly sensitive to C _g , but is hardly affected by the degraded position, and can be identified by using the common-mode leakage current Degree of degradation of main insulation.

The evaluation of the degraded position of the main insulation of the variable frequency motor in the step 3 is carried out by combining the common mode leakage current and the differential mode leakage current, and the mathematical expression of the differential mode leakage current is obtained by formula (6) and formula (9) as follows Shown:

(12)

(12)

Where ω =2 m π f _c +2π f ₀ , m =2,4,6,…;

According to formula (12) _, the differential mode leakage current Ig ,DM under different insulation capacitances and degraded positions in the frequency band from 8050Hz to 48050Hz is obtained _{. Ig} , DM decreases linearly with the increase of x , and Ig _,DM is not only related to Cg It is also affected by x , therefore, the main insulation degradation position x can be identified by I _g,DM .

In the third step, the evaluation of the main insulation degradation phase of the variable frequency motor is identified through the initial oscillation amplitude of the leakage current, and its mathematical expression is as follows:

(13)

(13)

表示PWM电压处于上升沿，当

表示其处于下降沿，由式(13)可知当E _d ，R _s 及L _s 固定时，漏电流初始振荡幅值A _mp 仅受绝缘电容C _g 和等效电路阻抗系数k影响；因此，A _mp 能够对主绝缘退化相进行评估。When in the formula

Indicates that the PWM voltage is on the rising edge when the

Indicates that it is on the falling edge. It can be seen from formula (13) that when E _d , R _s and L _s are fixed, the initial oscillation amplitude A _mp of the leakage current is only affected by the insulation capacitance C _g and the equivalent circuit impedance coefficient k ; therefore, A _mp enables the evaluation of main insulation degradation phases.

Beneficial effect

An online evaluation method for the main insulation state of a variable frequency motor produced by the technical solution of the present invention has the following advantages:

1. This method establishes the stator winding insulation model in the low frequency range, and clarifies the relationship between the time-frequency domain characteristics of leakage current and the main insulation degradation. At the same time, based on the time-frequency domain characteristics of leakage current, an online The evaluation method can separately evaluate the degradation degree, degradation location and degradation phase of the main insulation. ;

2. In this method, the common-mode and differential-mode harmonics of the leakage current in the low-frequency range are affected by the degree of insulation degradation, while the differential-mode harmonics are also affected by the location of the degradation. Compared with previous methods, this method can identify the degradation degree, degradation location and degradation phase of the main insulation at a lower monitoring bandwidth, which provides a reference for the precise maintenance of the motor insulation system.

Description of drawings

Fig. 1 is a schematic flow chart of an online evaluation method for the main insulation state of a variable frequency motor according to the present invention;

Fig. 2 is the electrical model diagram of main insulation degeneration of C-phase winding of the present invention;

Fig. 3 is a simplified electrical model diagram of phase C main insulation degradation of the present invention;

Fig. 4 is the simulation result figure of common mode leakage current under different frequencies described in the present invention, wherein (a) is along with insulation capacitance change figure, (b) is along with degraded position change figure;

Fig. 5 is the simulation result figure of differential mode leakage current under different frequencies described in the present invention, and wherein (a) is along with insulation capacitance change figure; (b) is along with degradation position change figure;

Fig. 6 is the three-phase step voltage excitation transient leakage current diagram of the present invention, wherein the degradation position is C-phase winding x =0.5;

Fig. 7 is a frame diagram of insulation state monitoring according to the present invention;

Fig. 8 is a graph of common mode leakage current experiment results under different insulation capacitance ΔC _g and degradation position x according to the present invention;

Fig. 9 is a diagram of differential mode leakage current experiment results under different insulation capacitances ΔC _g and degraded position x according to the present invention;

Fig. 10 is the K value experiment result diagram under different insulation capacitance Δ C _g and degradation position x of the present invention;

Fig. 11 is a diagram showing the variation of the three-phase leakage current ΔA _mp with the insulation capacitance ΔC _g according to the present invention.

Detailed ways

The present invention is described in detail below in conjunction with accompanying drawing, as shown in Figure 1;

In order to analyze the coupling relationship between the leakage current and the main insulation, an equivalent circuit model of the degradation of the main insulation of the stator winding is established in the low frequency band (about 0Hz to 100kHz). The stator winding electrical model of phase C winding main insulation degradation is shown in Figure 2, where the main insulation model is a parallel circuit of equivalent capacitance and resistance, which represent capacitive coupling and dielectric loss respectively, and U _a,b,c represent three-phase Voltage, R _s and L _s represent the stator resistance and inductance, respectively, R _g and C _g represent the equivalent resistance and capacitance of the main insulation. Point N is the neutral point, point D is the location where insulation deteriorates, and x represents the ratio of the number of turns from the input terminal to point D to the number of turns of the phase winding.

The three-phase voltage is replaced by the equivalent voltage source, and the three-phase branches are combined to obtain the RLC series circuit shown in Figure 3, where k , Z , and U _g represent the stator impedance coefficient, the equivalent circuit impedance, and the voltage source to ground. The specific circuit parameters are as follows:

(1)

(1)

(2)

(2)

(3)

(3)

Double Fourier integration is used to analyze the harmonics of the three-phase voltage of the variable frequency motor, and the Fourier expression of the A-phase voltage is as follows:

(4)

(4)

where E _d is the DC bus voltage, ω ₀ is the fundamental angular frequency, ω _c is the carrier angular frequency (the switching angular frequency of the inverter), α is the modulation depth, and J _k(x) represents the kth order Bessel function.

The pulse width modulation (PWM) voltage can be divided into common mode voltage and differential mode voltage. The common-mode voltage is evenly distributed in the three-phase windings, and has the same amplitude and phase angle at the same frequency, which only contributes to leakage current and is not used for power transmission. In contrast, the differential-mode voltages of each phase have the same amplitude at the same differential-mode frequency, and the phase angles are 120 degrees different from each other, which not only contributes to the leakage current but also is the only excitation of the load current.

The common-mode voltage can be calculated from the average value of the three-phase voltage. According to formula (3), the expression of the equivalent voltage source to ground at the common mode frequency is as follows:

(5)

(5)

Further, combining formula (3) and formula (5), the equivalent voltage source to ground at the differential mode frequency is obtained as follows:

(6)

(6)

From equations (5) and (6), it can be seen that the common-mode equivalent voltage U _{g, CM} is equal to the common-mode voltage of the C-phase winding, and is not affected by the degradation position x , but the differential-mode equivalent voltage U _g,DM decreases linearly with the increase of degenerate position x .

The common-mode frequency and differential-mode frequency obtained according to the fundamental frequency f ₀ and switching frequency fc of the inverter are shown in formula (7) and formula (8 ₎ , respectively.

(7)

(7)

(8)

(8)

In the formula , m =1,2,3,..., n =0,1,2,.... When n is _a multiple of 3, f1 represents the common-mode frequency, otherwise, f1 represents the _differential -mode frequency. When 2n +1 is a multiple of 3, f2 represents the common-mode frequency, otherwise, f2 represents _the differential- _mode frequency.

According to the circuit in Figure 4, the frequency-domain mathematical expression of the leakage current can be obtained as follows:

(9)

(9)

Where ω =2π f ₀ When f is the common-mode frequency, I _g represents the common-mode leakage current, otherwise I _g represents the differential-mode leakage current.

The leakage current is mainly driven by the high voltage change rate (dv/dt) of the rising and falling edges of the PWM voltage. Therefore, the equivalent voltage source Ug to ground can be equivalent to a series of square wave pulses, so the time-domain mathematical expression of the leakage _current can be regarded as the step response of the RLC circuit, expressed as follows:

(10)

(10)

表示处于上升沿,当

表示处于下降沿。When in the formula

Indicates that it is on the rising edge, when

Indicates a falling edge.

According to the time-domain and frequency-domain characteristics of the leakage current, the degradation degree, degradation location and degradation phase of the main insulation can be evaluated respectively.

The degree of degradation of the main insulation can be identified by the common mode leakage current. According to formula (5) and formula (9), the mathematical expression of the common mode leakage current is as follows:

(11)

(11)

Where ω =2 m π f _c , m =1,3,5,….

During the operation of the variable frequency motor, the DC bus voltage E _d and the modulation depth α are usually fixed, and the stator resistance R _s and stator inductance L _s can be obtained by an online parameter identification method. According to formula (11), the common-mode leakage current Ig _{, CM} under different insulation capacitances and degraded positions is obtained, as shown in Figure 4. It can be seen from Figure 4(a) that I _g.CM increases linearly with the insulation capacitance C _g from 0 to 1nF, reaches the maximum at 4000Hz (switching frequency), and tends to decrease. Furthermore, as shown in Fig. 4(b), Ig _,CM almost remains unchanged when the degenerate position x is changed. Therefore, Ig _,CM is highly sensitive to Cg , but hardly affected by the location of the degradation, and the degree _of degradation of the main insulation can be identified using the common-mode leakage current.

The location of degradation in the main insulation is assessed by combining common and differential mode leakage currents. The mathematical expression of the differential mode leakage current obtained through formula (6) and formula (9) is as follows:

(12)

(12)

Where ω =2 m π f _c +2π f ₀ , m =2,4,6,….

According to formula (12), the differential mode leakage current Ig _,DM under different insulation capacitances and degraded positions in the 8050Hz to 48050Hz frequency band is obtained, as shown in Figure 5. It can be seen from Fig. 5(a) that as C _g increases from 0 to 1nF, I _{g, DM} increases gradually, and the maximum value of I _{g, DM} is obtained at 8050 Hz. Figure 5(b) shows that Ig _,DM decreases linearly with increasing x and is almost zero near the neutral point. It can be seen from the above that I _{g, DM} are not only related to C _g , but also affected by x . Therefore, the main insulation degradation location x can be identified by Ig _,DM .

The main insulation degradation phase can be identified by the initial oscillation amplitude of the leakage current, and its mathematical expression is as follows:

(13)

(13)

表示PWM电压处于上升沿，当

表示其处于下降沿。由式(13)可知当E _d ，R _s When in the formula

Indicates that the PWM voltage is on the rising edge when the

Indicates that it is on a falling edge. From formula (13), it can be seen that when E _d , R _s

And when L _s is fixed, the initial oscillation amplitude A _mp of the leakage current is only affected by the insulation capacitance C _g and the equivalent circuit impedance coefficient k . Therefore, the state _{of insulation to ground can be evaluated using Amp} .

Use MATLAB/Simulink to simulate the RLC circuit shown in Figure 3, where C _g is set to 220pF, and the degradation position is C phase ( x = 0.5). It can be seen from Figure 6 that the leakage current starts to oscillate on the rising edge of the PWM voltage, and decays to zero after several oscillation cycles. Since the winding insulation conditions of phase A and phase B are the same, the A _mp of the leakage current at the moment of PWM voltage step is 58mA. In addition, the A _mp of the C-phase winding is 150mA, which is obviously larger than the initial oscillation amplitude of the corresponding leakage current of the other two phases. therefore,

The main insulation degradation phase can be evaluated by comparing the leakage current A _mp corresponding to the three-phase PWM voltage step moment.

Example 1

The monitoring framework of the insulation state evaluation method proposed by the present invention is shown in Figure 7, wherein the motor to be tested is a 3kW vector-controlled permanent magnet synchronous motor. Taking the I _{g, CM} , I _{g, DM} and A _mp in the healthy state of the variable frequency motor as reference values, real-time online monitoring of the leakage current signal characteristic quantity increment to evaluate the degradation state of the main insulation.

Connect an adjustable capacitor in the range of 0.1 to 1nF between the phase C winding tap and earth ground to simulate the degradation of the motor's main insulation. The common-mode leakage current increment obtained under different insulation capacitance ΔC _g and degradation position x is shown in Fig. 8 . It can be seen that ΔIg _,CM increases with the increase _of ΔCg , and the change slope of ΔIg _,CM is almost the same at different degradation positions x . Therefore, ΔIg _,CM is not affected by x and can be used to assess the degree of degradation of the main insulation.

The differential mode leakage current increment _{ΔIg ,DM} under different insulation capacitance ΔCg and degradation position x is shown in Figure 9. It can be seen that when x is fixed, ΔIg ,DM increases with the increase of ΔCg , but Its growth slope gradually decreases from the maximum value and tends to zero with the increase of x . Therefore, ΔI _g,DM contains degenerate position information. In this application, the degradation position is determined by the ratio K of the common-mode leakage current increment to the differential-mode leakage current increment, as shown in equation (14), where the superscript * indicates that the variable is the leakage current measurement in the degraded state result.

(14)

(14)

The slope K values under different ΔC _g and x are shown in Figure 9. It can be seen that the slope K is not affected by the insulation capacitance at any degraded position x , but it increases linearly with the increase of x . Therefore, the K value is only related to x and can be used to assess the location of main insulation degradation.

Insert ΔCg of different capacitance values at the phase C winding x = 0.5 to simulate the degree of degradation of the phase winding insulation, and obtain the initial oscillation amplitude increment Δ A _mp of the leakage current as shown in Figure 11, where Δ A _{mp - A , B , C} respectively represent the initial oscillation amplitude increment of the leakage current excited by the three-phase voltage. As can be seen,

The degenerated phase Δ A _mp-C gradually increases with the increase of Δ C _g , while the healthy phases Δ A _mp-A and Δ A _mp-B have smaller values than Δ A _mp-C and are hardly affected by Effect _of ΔCg . Therefore, the main insulation degradation phase can be identified by comparing the amplitude difference between Δ A _mp-A,B,C .

It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is a relationship between these entities or operations. There is no such actual relationship or order between them. Furthermore, the term "comprises", "comprises" or any other variation thereof is intended to cover a non-exclusive inclusion such that a process, method, article, or apparatus comprising a set of elements includes not only those elements, but also includes elements not expressly listed. other elements of or also include elements inherent in such a process, method, article, or device. without further restrictions. The phrase "the inclusion of an element defined by ... does not preclude the presence of additional identical elements in the process, method, article, or apparatus comprising said element".

The above-mentioned technical solutions only reflect the preferred technical solutions of the technical solutions of the present invention, and some changes that those skilled in the art may make to certain parts reflect the principles of the present invention and fall within the protection scope of the present invention.

Claims (7)

1. The method for on-line evaluation of the main insulation state of the variable frequency motor is characterized by comprising the following steps:

establishing an equivalent circuit model of the stator winding main insulation degradation at a low frequency stage, and performing harmonic analysis on three-phase voltage in the equivalent circuit model of the stator winding main insulation degradation by adopting double Fourier integration to obtain common mode frequency and differential mode frequency;

obtaining a frequency domain mathematical expression of leakage current and a time domain mathematical expression of the leakage current based on an equivalent circuit model of the main insulation degradation of the variable frequency motor;

and step three, respectively carrying out online evaluation on the degradation degree, the degradation position and the degradation phase of the main insulation of the variable frequency motor according to the time domain mathematical expression and the frequency domain mathematical expression of the leakage current.

2. The method for on-line estimation of main insulation state of inverter motor according to claim 1, wherein the equivalent circuit model of the stator winding main insulation degradation in the first step is an equivalent capacitor and resistor parallel circuit, which represents capacitive coupling and dielectric loss, respectively,U _a,b,c the voltage of the three phases is represented,R _s andL _s the stator resistance and the inductance are shown separately,R _g andC _g representing the equivalent resistance and capacitance of the main insulation, point N is the neutral point, point D is the deterioration position of the main insulation,xrepresenting the ratio of the number of turns from the incoming line end to the D point to the number of turns of the phase winding, replacing the three-phase voltage by an equivalent voltage source, and combining three-phase branches to obtain an RLC series circuit, whereink、Z、U _g Respectively representing the stator impedance coefficient, the equivalent circuit impedance and the equivalent earth-groundVoltage source, circuit parameters are specifically expressed as follows:

(1)

(2)

(3)。

3. the method for on-line estimation of the main insulation state of the variable frequency motor according to claim 1, wherein in the first step, double Fourier integration is adopted to perform harmonic analysis on the three-phase voltage, wherein the Fourier expression of the A-phase voltage is as follows:

(4)

in the formulaE _d Is a voltage of the direct-current bus,ω ₀ is the angular frequency of the fundamental wave,ω _c is the carrier angular frequency (switching angular frequency of the inverter),αin order to modulate the depth of the light,J _k(x) is shown askAn order Bessel function;

the equivalent voltage source expression to the ground at the common mode frequency is obtained according to the formula (3) as follows:

(5)

further, combining equations (3) and (5) yields the equivalent voltage source to ground at the differential mode frequency as follows:

(6)

as is clear from the expressions (5) and (6), the common mode is to the ground or the likeEffective voltageU _g,CM Equal to common mode voltage of C phase winding and not subject to degradationxBut differential mode to ground equivalent voltageU _g,DM Following the location of degenerationxThe increase in (a) linearly decreases;

according to the fundamental frequency of the inverterf ₀ And switching frequencyf _c The obtained common mode frequency and differential mode frequency are respectively shown as formula (7) and formula (8):

(7)

(8)

in the formulam=1,2,3,...，n=0,1,2,...

When the temperature is higher than the set temperaturenIn the case of a multiple of 3,f ₁ indicating the frequency of the common mode, otherwise,f ₁ represents the frequency of the differential mode when2nWhen +1 is a multiple of 3,f ₂ indicating the frequency of the common mode, otherwise,f ₂ representing the differential mode frequency.

4. The on-line evaluation method for the main insulation state of the variable frequency motor according to claim 1, wherein the frequency domain mathematical expression of the leakage current in the second step is as follows:

(9)

in the formulaω=2πf ₀ When in usefIn the case of a common-mode frequency,I _g indicating common mode leakage current, otherwiseI _g Representing differential mode leakage current;

the time domain mathematical expression of the leakage current is as follows:

(10)

in the formula

Indicating on a rising edge when

Indicating a falling edge.

5. The method for online evaluation of the main insulation state of the inverter motor according to claim 1, wherein the evaluation of the degradation degree of the main insulation of the inverter motor in the third step is identified by a common-mode leakage current, and a mathematical expression of the common-mode leakage current is obtained according to equations (5) and (9) as follows:

(11)

in the formulaω=2mπf _c ，m=1,3,5,…；

According to the formula (11), the common mode leakage current under different insulation capacitances and degradation positions is obtainedI _g,CM ， I _g,CM To pairC _g Highly sensitive, but hardly affected by the location of degradation, the use of common mode leakage current allows the degree of degradation of the main insulation to be identified.

6. The method for online evaluation of the main insulation state of the inverter motor according to claim 1, wherein the evaluation of the degradation position of the main insulation of the inverter motor in the third step is performed by combining the common mode leakage current and the differential mode leakage current, and the mathematical expression of the differential mode leakage current obtained by the equations (6) and (9) is as follows:

(12)

in the formulaω=2mπf _c +2πf ₀ ，m=2,4,6,…；

According to the formula (12), the differential mode leakage current under different insulation capacitances and degradation positions in the frequency range of 8050Hz to 48050Hz is obtainedI _g,DM ，I _g,DM With followingxThe increase of (a) is in a linear decrease, I _g,DM not only withC _g In connection with, also receivexInfluence, therefore, location of main insulation degradationxCan pass throughI _g,DM To identify.

7. The method for on-line estimation of the main insulation state of the variable frequency motor according to claim 1, wherein the estimation of the main insulation degradation phase of the variable frequency motor in the third step is identified by the initial oscillation amplitude of the leakage current, and the mathematical expression of the method is as follows:

(13)

in the formula

Indicating that the PWM voltage is on the rising edge when

Indicates that it is at the falling edge, and is known from the formula (13)E _d ，R _s AndL _s at fixed, leakage current initial oscillation amplitudeA _mp Capacitor only by insulationC _g And equivalent circuit impedance coefficientkInfluence; therefore, the temperature of the molten metal is controlled,A _mp the main insulation degradation phase can be evaluated.

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