A method for determining parameters of AC motor high-frequency shaft current model

Abstract

The present invention belongs to the field of AC motors, and specifically relates to a method for determining parameters of a high-frequency shaft current model of an AC motor. An equivalent circuit for the high-frequency shaft current of an AC motor in a stationary state is established, and an LCR meter is used to test the motor port capacitance. A loop for measuring the shaft current is then added to the motor, and a signal generator is used in conjunction with the bearing to extract parameters. Without incurring additional economic costs, the motor can be tested using instruments such as an oscilloscope, a signal generator, and an LCR meter. Based on the existing AC motor shaft current high-frequency model, the parameters of each component of the AC motor shaft current high-frequency model are simply and quickly separated, thereby better predicting the shaft current.

Description

Technical Field

[0001] The present invention belongs to the field of AC motors, and in particular relates to a method for determining parameters of a high-frequency shaft current model of an AC motor. Background Art

[0002] PWM inverters are widely used in motor drive systems due to their advantages, including excellent speed regulation and starting and braking performance. While these inverters improve motor performance and improve the economic benefits of operators, they inevitably bring certain negative effects, such as electrical corrosion of bearings, which shortens bearing life and threatens the stable operation of the system.

[0003] The switching devices within PWM inverters widely utilize fast-switching IGBT switching power devices. This generates a common-mode voltage with a high voltage gradient at the inverter output, which in turn generates common-mode currents with frequencies ranging from tens to hundreds of kilohertz. At high frequencies, the motor exhibits capacitance between the stator core and the stator winding, between the stator winding and the rotor core, and between the rotor core and the stator core. This common-mode voltage, coupled with stray capacitance, induces shaft voltage across the bearings, generating shaft currents. When the shaft voltage exceeds the threshold voltage of the lubricating oil film, the film quickly breaks down, releasing a significant amount of heat. This can cause the metal surrounding the breakdown point on the inner and outer raceway surfaces of the bearing to melt, leading to electrical corrosion of the bearings, shortening their life and threatening the reliable operation of the motor.

[0004] Shaft current issues, such as bearing electrocorrosion, have attracted the attention of operators and scholars. Scholars and manufacturers have conducted extensive research on shaft current prediction and suppression measures. The shaft current circuit model and the corresponding component parameters play a crucial role in shaft current analysis. Currently, the shaft current high-frequency distributed parameter model has a higher accuracy in shaft current prediction than the lumped parameter model, and the shaft current high-frequency distributed parameter model is relatively mature. High-frequency shaft current models widely use impedance analyzers for parameter testing and parameter extraction, which undoubtedly increases the economic cost of parameter testing. Under limited conditions and without sacrificing shaft current prediction accuracy, motor parameter testing and model parameter extraction can be performed quickly without relying on expensive measuring instruments, greatly saving the time cost of shaft current analysis and the economic cost of renting measurement equipment. Summary of the Invention

[0005] The present invention proposes a method for determining the parameters of a high-frequency shaft current model of an AC motor to more accurately predict the shaft current, guide the research on shaft current suppression methods, increase the service life of bearings, and reduce the cost of regular maintenance and replacement of bearings.

[0006] To achieve the above object, the present invention proposes the following technical solutions:

[0007] A method for determining parameters of a high-frequency shaft current model of an AC motor comprises the following steps:

[0008] Step 1: Remove the ordinary bearing of the AC motor and support the stator housing of the AC motor with an insulating object;

[0009] Step 2: Use an LCR meter to test the capacitance of the AC motor port;

[0010] Get C wf 、C wr and C rf ; among them C wf is the stray capacitance between the stator winding and the casing at low frequency; C wr is the stray capacitance between the stator winding and the rotor at low frequency; C rf is the stray capacitance between the shaft and the casing;

[0011] Step 3: Install the AC motor's ordinary bearing back into the motor end cover and add a circuit inside the AC motor for measuring the shaft current;

[0012] Step 4: The signal generator applies square wave signals with frequencies of 1kHz and 10MHz to the three-phase winding and the housing port of the AC motor, and measures the common-mode current corresponding to the two frequencies, thereby obtaining the common-mode inductance L. CM , the inductance L of the motor's internal feed conductor and connecting wire c , additional loss resistance R g 、C wf1 、C wf0 、C wr1 and C wr2

[0013] At high frequencies, the stray capacitance C between the stator winding and the casing wf1 、C wf0 with C wf1 The sum is the stray capacitance C between the stator winding and the casing wf ; Stray capacitance C between stator winding and shaft at high frequency wr1 , C wr1 with C wr2 The sum is the stray capacitance C between the stator winding and the rotor at low frequency wr ;

[0014] Step 5: Apply a sinusoidal signal to both ends of any ordinary bearing and record the shaft current and shaft voltage on the bearing at this time;

[0015] According to the shaft current and shaft voltage, the equivalent capacitance C of the oil film of the non-drive end bearing is calculated. b,nd , driving end bearing oil film equivalent capacitance C b,d , complete parameter extraction.

[0016] Preferably, in the first step, the plain bearing includes plain bearings at the driving end and the non-driving end.

[0017] Preferably, in the second step, the specific steps are:

[0018] Use an LCR meter to measure the stray capacitance between the three-phase winding short-circuit point and the housing port, the three-phase winding short-circuit point and the shaft port, and the shaft and the housing, and record them as C1, C2, and C3 respectively;

[0019] According to the formula:

[0020]

[0021] Get: C wf 、C wr and C rf ; among them C wf is the stray capacitance between the stator winding and the casing at low frequency; C wr is the stray capacitance between the stator winding and the rotor at low frequency; C rf It is the stray capacitance between the shaft and the casing.

[0022] Preferably, the LCR meter measures the stray capacitance between the short-circuit point of the three-phase winding and the housing port, the stray capacitance between the short-circuit point of the three-phase winding and the shaft port, and the stray capacitance between the shaft and the housing at a measurement frequency of 1 kHz.

[0023] Preferably, in the third step, the plain bearing includes plain bearings at the driving end and the non-driving end.

[0024] Preferably, in the fourth step, the specific steps are:

[0025] Measure the common-mode current time interval Δt and the capacitance C1 between the winding and the casing when the motor is stationary and a 1kHz square wave signal is applied to the winding and casing ports. According to the formula:

[0026]

[0027] We can get: common mode inductance L CM .

[0028] Preferably, the fourth step further includes the following steps:

[0029] Obtain the amplitude of the common mode current I when the motor is stationary and a 10 MHz square wave signal is applied to the winding and housing ports com , oscillation angular frequency ω n , time constant τ, common mode current i corresponding to 10MHz square wave signal com (t), applied voltage value E, attenuation coefficient ζ and characteristic impedance Z0, and according to the formula:

[0030]

[0031] Get: the inductance L of the motor internal feed conductor and connecting line c , additional loss resistance R g , stray capacitance C between stator winding and casing at high frequency wf1 .

[0032] Preferably, the fourth step further includes the following steps:

[0033] According to the formula:

[0034] C wf =C wf0+ C wf1 ;

[0035] C wf1 / C wf =C wr1 / C wr ;

[0036] C wr =C wr1 +C wr2 ;

[0037] Get: C wf0 、C wr1 with C wr2 .

[0038] Preferably, in the fifth step, a signal generator is used to apply a sinusoidal signal to both ends of any ordinary bearing.

[0039] The present invention is beneficial in that:

[0040] Without the aid of expensive precision measuring instruments, the parameter measurement of the high-frequency model of shaft current can be completed by using LCR meters and signal generators, which reduces the economic cost of parameter testing and more accurately predicts the shaft current, which is beneficial to increasing the service life of bearings and reducing the cost of regular maintenance and replacement of bearings, and is popular.

[0041] When extracting the stray capacitance of a motor, there is no need to have additional knowledge of the dielectric parameters and structural dimensions. The stray capacitance parameters can be extracted simply by establishing a formula based on the circuit relationship, which greatly reduces the difficulty of parameter extraction. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The accompanying drawings, which constitute part of the present invention, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0043] Figure 1 A flow chart of a method for determining parameters of a high-frequency shaft current model of an AC motor;

[0044] Figure 2 This is a schematic diagram of the equivalent circuit of the high-frequency shaft current of the AC motor in a stationary state;

[0045] Figure 3 is a schematic diagram of the port network;

[0046] Figure 4 Schematic diagram of the equivalent circuit of the prior art.

[0047] L CM is the common mode inductance; L c R is the inductance of the motor's internal feed conductor and connecting wire; e is the eddy current loss resistance; R g is the additional loss resistance; C wf1 is the stray capacitance between the stator winding and the casing at high frequency, C wf1 with C wf0 The sum is the stray capacitance between the stator winding and the casing at low frequency; C wr1 is the stray capacitance between the stator winding and the shaft at high frequency, C wr1 with C wr2 The sum is the stray capacitance between the stator winding and the rotor at low frequency; C rf is the stray capacitance between the shaft and the housing; C b,nd is the equivalent capacitance of the oil film of the non-drive end bearing, C b,d is the equivalent capacitance of the drive end bearing oil film. DETAILED DESCRIPTION

[0048] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other.

[0049] The following detailed description is an exemplary description and is intended to provide further detailed description of the present invention. Unless otherwise indicated, all technical terms used in the present invention have the same meaning as those generally understood by those skilled in the art to which the present invention belongs. The terms used in the present invention are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention.

[0050] Example 1:

[0051] See also Figure 1 As shown, the present invention provides a method for determining the parameters of the high-frequency shaft current model of an AC motor, which specifically includes:

[0052] The first step is to remove the ordinary bearings at the drive end and non-drive end of the motor, and use insulating objects such as wooden sticks to support the stator housing to avoid metallic contact between the housing and the shaft.

[0053] At this time, the equivalent circuit of the high-frequency shaft current of the AC motor in a stationary state is as follows: Figure 2As shown in the figure, R e is the eddy current loss resistance; C wf1 、C wf0 is the stray capacitance between the stator winding and the casing, where C wf1 is the stray capacitance between the stator winding and the casing at high frequency, C wf1 with C wf0 The sum is the stray capacitance C between the stator winding and the casing at low frequency wf ; C wr1 、C wr2 is the stray capacitance between the stator winding and the rotor, where C wr1 is the stray capacitance between the stator winding and the shaft at high frequency, C wr1 with C wr2 The sum is the stray capacitance C between the stator winding and the rotor at low frequency wr ; C rf is the stray capacitance between the shaft and the housing; C b,nd is the equivalent capacitance of the oil film of the non-drive end bearing, C b,d is the equivalent capacitance of the drive end bearing oil film.

[0054] When the motor is stationary, there is metallic contact between the bearing rolling elements and the inner and outer raceways, that is, there is no oil film between the bearing rolling elements and the inner and outer raceways, that is, there is no C b,nd 、C b,d .

[0055] Due to the eddy current loss resistance R e The analysis of shaft current has little effect, and the eddy current loss resistance R is removed here. e , no parameter testing and extraction are performed on it.

[0056] The second step is to use an LCR meter to test the motor port capacitance.

[0057] At a measurement frequency of 1kHz, the LCR meter measures the stray capacitance between the three-phase winding short-circuit point and the casing port, the three-phase winding short-circuit point and the shaft port, and the shaft and the casing, which are recorded as C1, C2, and C3 respectively.

[0058] At 1kHz, the common mode inductance L CM , the inductance L of the motor's internal feed conductor and connecting wire c , additional loss resistance R g The impedance is small, and the network between the short-circuit point of the three-phase winding and the casing, the short-circuit point of the three-phase winding and the shaft, and the shaft and the casing port is a capacitive network, such as Figure 3 shown.

[0059] According to the formula:

[0060]

[0061] And the formula: Cwf =C wf1 +C wf0 ,C wr =C wr1 +C wr2 ;

[0062] We can get: C wf 、C wr and C rf ; among them C wf is the stray capacitance between the stator winding and the casing at low frequency; C wr is the stray capacitance between the stator winding and the rotor at low frequency; C rf is the stray capacitance between the shaft and the casing;

[0063] The third step is to install the plain bearings at the motor drive end and non-drive end into the motor end cover, and add a circuit inside the motor for measuring the shaft current.

[0064] The fourth step is to use a signal generator to apply square wave signals with frequencies of 1kHz and 10MHz to the three-phase windings of the motor and the housing port, and measure the common-mode current i corresponding to the 1kHz and 10MHz square wave signals respectively. com1 (t) and i com (t). Based on the time interval Δt of the common mode current when the motor is stationary and a 1kHz square wave signal is applied to the winding and housing ports, and the port capacitance C1 between the winding and the housing, the common mode inductance L is obtained. CM , the formula is as follows:

[0065]

[0066] Obtain the amplitude of the common mode current I when the motor is stationary and a 10 MHz square wave signal is applied to the winding and housing ports com , oscillation angular frequency ω n , time constant τ, and thus the inductance L of the motor internal feed conductor and connecting line is obtained c , additional loss resistance R g , stray capacitance C between stator winding and casing at high frequency wf1 , the formula is:

[0067]

[0068] Among them, i com (t) is the common mode current corresponding to the 10MHz square wave signal; E is the applied voltage value; ζ is the attenuation coefficient; Z0 is the characteristic impedance,

[0069] C wf C wf1 、C wf0 The equivalent capacitance after parallel connection is C wf and Cwf1 If it has been obtained, the stray capacitance C between the stator winding and the casing can be obtained. wf0 , the calculation process is:

[0070] C wf =C wf0+ C wf1 .

[0071] According to the existence of C at high frequency wf1 / C wf =C wr1 / C wr The relationship between C wr C wr1 、C wr2 The equivalent capacitance after parallel connection can be used to obtain the stray capacitance between the stator winding and the rotor to obtain C wr1 、C wr2 , the calculation process is:

[0072] C wf1 / C wf =C wr1 / C wr ;

[0073] C wr =C wr1 +C wr2 .

[0074] Step 5: When the motor is running, use a signal generator to apply a sinusoidal signal at both ends of any common bearing, and record the shaft current and shaft voltage on the bearing at this time; calculate the equivalent capacitance C of the oil film of the non-drive end bearing based on the shaft current and shaft voltage. b,nd , driving end bearing oil film equivalent capacitance C b,d ; Complete parameter extraction.

[0075] In the prior art, Oliver Magdun took a motor with a rated power of 240kW and ordinary bearings at both ends of the shaft as an example and established Figure 4 Oliver Magdun et al. used an impedance analyzer and the measured impedance characteristic curve between the short-circuit point of the three-phase winding and the housing to extract and determine C wf1 、C wf0 、L CM 、L c 、R g ; C is derived by analytical method wr1 、C wr2 、C rf 、C b,nd 、C b,dSince laboratories and motor manufacturers need to conduct relevant experiments on motors, they usually have instruments such as oscilloscopes, signal generators, and LCR meters. Impedance analyzers are precision measuring instruments and are more expensive than other measuring instruments, which increases the economic cost of parameter testing. The analytical formula for the stray capacitance inside the motor is relatively complex and involves multiple dielectric parameters and the structural dimensions of the motor. Therefore, the analytical formula is used to derive C wr1 、C wr2 、C rf The parameters make the parameter extraction process more complicated. At the same time, when the motor structure size and material properties are unknown, it is impossible to extract the stray capacitance parameters. The shaft current is a branch current of the common mode current, that is, the shaft current belongs to the parameters of the common mode loop. The eddy current loss resistance R e It is a parameter of the differential mode circuit and has a large resistance value, so it has little effect on the prediction of the shaft current. Therefore, it can be omitted.

[0076] When testing and extracting stray capacitance between winding and housing, remove the shaft from the motor housing. At this time, the capacitance C1 between the short-circuit point of the three-phase winding and the housing port measured by the LCR meter at a measurement frequency of 1kHz is the stray capacitance C between the winding and the housing. wf , and then obtain other parameters on the model according to subsequent steps.

[0077] The present invention realizes the testing of the motor using instruments such as an oscilloscope, a signal generator, and an LCR meter without adding additional economic costs. Based on the existing AC motor shaft current high-frequency model, the parameters of each component of the AC motor shaft current high-frequency model are simply and quickly separated, thereby better predicting the shaft current.

[0078] It is understood from common technical knowledge that the present invention may be implemented by other embodiments that do not depart from its spirit or essential features. Therefore, the embodiments disclosed above are, in all respects, merely illustrative and not exclusive. All modifications within the scope of the present invention or equivalent to the scope of the present invention are intended to be encompassed by the present invention.

Claims

1. A method for determining parameters of a high-frequency shaft current model of an AC motor, characterized in that: The steps include: Step 1: Remove the ordinary bearing of the AC motor and support the stator housing of the AC motor with an insulating object; Step 2: Use an LCR meter to test the capacitance of the AC motor port; Get C wf 、C wr and C rf ; among them C wf is the stray capacitance between the stator winding and the casing at low frequency; C wr is the stray capacitance between the stator winding and the rotor at low frequency; C rf is the stray capacitance between the shaft and the casing; Step 3: Install the AC motor's ordinary bearing back into the motor end cover and add a circuit inside the AC motor for measuring the shaft current; Step 4: The signal generator applies square wave signals with frequencies of 1kHz and 10MHz to the three-phase winding and the housing port of the AC motor, and measures the common-mode current corresponding to the two frequencies, thereby obtaining the common-mode inductance L. CM , the inductance L of the motor's internal feed conductor and connecting wire c , additional loss resistance R g 、C wf1 、C wf0 、C wr1 and C wr2 ; C wf1 is the stray capacitance between the stator winding and the casing at high frequency; C wf0 with C wf1 The sum is the stray capacitance C between the stator winding and the casing at low frequency wf ; C wr1 is the stray capacitance between the stator winding and the shaft at high frequency, C wr1 with C wr2 The sum is the stray capacitance C between the stator winding and the rotor at low frequency wr ; Step 5: Apply a sinusoidal signal to both ends of any plain bearing and record the shaft current and shaft voltage on the corresponding plain bearing at this time; According to the shaft current and shaft voltage, the equivalent capacitance C of the oil film of the non-drive end bearing is calculated. b,nd , driving end bearing oil film equivalent capacitance C b,d , complete parameter extraction.

2. The method for determining the parameters of the AC motor high-frequency shaft current model according to claim 1, wherein: In the first step, the plain bearing includes plain bearings at the driving end and the non-driving end.

3. The method for determining the parameters of the AC motor high-frequency shaft current model according to claim 1, wherein: In the second step, the specific steps are: Use an LCR meter to measure the stray capacitance between the three-phase winding short-circuit point and the housing port, the three-phase winding short-circuit point and the shaft port, and the shaft and the housing, and record them as C1, C2, and C3 respectively; According to the formula: Get: C wf 、C wr and C rf ; among them C wf is the stray capacitance between the stator winding and the casing at low frequency; C wr is the stray capacitance between the stator winding and the rotor at low frequency; C rf It is the stray capacitance between the shaft and the casing.

4. The method for determining the parameters of the AC motor high-frequency shaft current model according to claim 3, wherein: At a measurement frequency of 1kHz, the LCR meter measures the stray capacitance between the three-phase winding short-circuit point and the casing port, the three-phase winding short-circuit point and the shaft port, and the shaft and the casing.

5. The method for determining the parameters of the AC motor high-frequency shaft current model according to claim 1, wherein: In the third step, the plain bearing includes plain bearings at the driving end and the non-driving end.

6. The method for determining the parameters of the AC motor high-frequency shaft current model according to claim 1, characterized in that: In the fourth step, the specific steps are: Measure the common-mode current time interval Δt and the capacitance C1 between the winding and the casing when the motor is stationary and a 1kHz square wave signal is applied to the winding and casing ports. According to the formula: We can get: common mode inductance L CM .

7. The method for determining the parameters of the AC motor high-frequency shaft current model according to claim 6, characterized in that: The fourth step also includes the following steps: Obtain the amplitude of the common mode current I when the motor is stationary and a 10 MHz square wave signal is applied to the winding and housing ports com , oscillation angular frequency ω n , time constant τ, common mode current i corresponding to 10MHz square wave signal com (t), applied voltage value E, attenuation coefficient ζ and characteristic impedance Z0, and according to the formula: Get: the inductance L of the motor internal feed conductor and connecting line c , additional loss resistance R g , stray capacitance C between stator winding and casing at high frequency wf1 .

8. The method for determining the parameters of the AC motor high-frequency shaft current model according to claim 6, wherein: The fourth step also includes the following steps: According to the formula: C wf =C wf0 +C wf1 ; C wf1 / C wf =C wr1 / C wr ; C wr =C wr1 +C wr2 ; Get: C wf0 、C wr1 with C wr2 .

9. The method for determining the parameters of the AC motor high-frequency shaft current model according to claim 1, wherein: In the fifth step, a signal generator is used to apply a sinusoidal signal to both ends of any plain bearing.

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