阅读说明：本技术 一种获取交流电机杂散电容的方法 (Method for obtaining stray capacitance of alternating current motor ) 是由 刘瑞芳 杨二乐 王芹芹 赵秦聪 于 2019-12-04 设计创作，主要内容包括：本发明提供了一种获取交流电机杂散电容的方法。该方法包括：建立交流电机定、转子接地时的共模等效电路；基于所述共模等效电路建立所述交流电机的杂散电容与共模电压、轴电压及定、转子接地阻抗之间的关系式；测量所述交流电机在不同定、转子接地阻抗下的共模电压和轴电压,根据测量得到的共模电压和轴电压并利用所述关系式推算出所述交流电机内部的杂散电容。本发明通过测量三组不同定、转子接地阻抗下的共模电压及轴电压,能够有效地推算出电机内部的杂散电容。可以考虑电机转速、温升等产生的影响,获取实际运行状态下的交流电机内部的杂散电容。(The invention provides a method for acquiring stray capacitance of an alternating current motor. The method comprises the following steps: establishing a common mode equivalent circuit when a stator and a rotor of the alternating current motor are grounded; establishing a relation between stray capacitance of the alternating current motor and common mode voltage, shaft voltage and stator and rotor grounding impedance based on the common mode equivalent circuit; and measuring the common mode voltage and the shaft voltage of the alternating current motor under different stator and rotor grounding impedances, and calculating the stray capacitance in the alternating current motor according to the measured common mode voltage and the measured shaft voltage and by utilizing the relational expression. The invention can effectively calculate the stray capacitance in the motor by measuring the common-mode voltage and the shaft voltage under three groups of different stator and rotor grounding impedances. The influence of the rotating speed, the temperature rise and the like of the motor can be considered, and the stray capacitance in the alternating current motor in the actual running state can be obtained.)

1. A method of obtaining stray capacitance for an ac motor, comprising:

establishing a common mode equivalent circuit when a stator and a rotor of the alternating current motor are grounded;

establishing a relation between the stray capacitance of the alternating current motor and common mode voltage, shaft voltage and stator and rotor grounding impedance based on the common mode equivalent circuit;

and measuring the common mode voltage and the shaft voltage of the alternating current motor under different stator and rotor grounding impedances, and calculating the stray capacitance in the alternating current motor according to the measured common mode voltage and the measured shaft voltage and by utilizing the relational expression.

2. The method of claim 1, wherein said establishing a common-mode equivalent circuit of the ac machine stator and rotor to ground comprises:

an alternating current motor is driven by a machine to rotate, high-frequency sinusoidal voltage is applied between the star point of a three-phase winding of the alternating current motor and the ground, and the high-frequency sinusoidal voltage is used for simulating common-mode voltage V_comThe stator and the rotor of the alternating current motor are grounded through grounding impedance respectively to establish a common mode equivalent circuit of the alternating current motor, and the common mode equivalent circuit comprises the following components: coupling capacitor C between stator winding and stator core of alternating current motor_wfAC motorCoupling capacitor C between stator core and rotor_rfCoupling capacitor C between stator winding and rotor of alternating current motor_wrStator side grounding impedance Z of alternating current motor_fgAC motor rotor side grounding impedance Z_rgAnd axis voltage V_b(ii) a Point w represents the stator winding, point r represents the axis of rotation, point f represents the casing, point g represents ground, C_bThe equivalent capacitance of the bearing is caused by an oil film between a bearing rolling body and a raceway;

in the common-mode equivalent circuit, a common-mode voltage source V_comCoupling capacitor C between stator winding and stator core of alternating current motor_wfStator side grounding impedance Z of alternating current motor_fgForm a loop l₁Common mode voltage source V_comCoupling capacitor C between stator winding and rotor of alternating current motor_wrAC motor rotor side grounding impedance Z_rgForm a loop l₂Coupling capacitor C between stator core and rotor_rfAnd the oil film between the rolling body and the raceway of the bearing makes the equivalent capacitance C of the bearing_bAC motor rotor side grounding impedance Z_rgStator side grounding impedance Z of alternating current motor_fgForm a loop l₃Loop l₁Loop l₂And loop l₃The direction of the current is recorded as the positive direction of the current flowing through the loop, and the current is positive when the current flowing direction is consistent with the direction of the loop; when the current flows in the direction opposite to the direction of the loop, the current is negative and flows through the loop₁And loop l₂Is a common mode current, flows through loop l₃The current of (2) is the shaft current.

3. The method of claim 2, wherein said establishing a relationship between stray capacitance of the ac machine and common mode voltage, shaft voltage, and stator and rotor ground impedances based on the common mode equivalent circuit comprises:

let stray capacitance of the AC motor be C_wf、C_wr、C_rfCommon mode voltage of V_comAxial voltage of V_bStator ground impedance of Z_fgRotor ground impedance is Z_rg：

The loop equation of the common mode equivalent circuit is as follows:

in the formula (I), the compound is shown in the specification,

Z_wf＝1/(j2πfC_wf) (2)

Z_wr＝1/(j2πfC_wr) (3)

Z′_rf＝1/(j2πf(C_rf+2C_b)) (4)

where f is the frequency of the common mode voltage.

According to equation (1), we obtain:

4. the method of claim 3, wherein said measuring a common mode voltage and a shaft voltage of said ac machine at different stator and rotor ground impedances, and using said relation to estimate a stray capacitance inside said ac machine based on said measured common mode voltage and shaft voltage comprises:

three non-inductive resistors A, B, C with different resistance values are selected,the impedances are respectively denoted as Z_A、Z_B、Z_C；

Performing three groups of experiments, and measuring the common-mode voltage and the shaft voltage of the alternating current motor under three groups of grounding impedances;

in the first group of experiments, a tested alternating current motor is connected with an accompanying alternating current motor through an insulating coupler, a carbon brush is additionally arranged outside a rotating shaft of the tested alternating current motor, a stator casing of the tested alternating current motor is grounded through a non-inductive resistor A, and the rotating shaft of the tested alternating current motor is grounded through a carbon brush through a non-inductive resistor B;

dragging the tested AC motor to the rated speed, applying high-frequency voltage with adjustable amplitude and frequency between the star point of the stator winding of the tested AC motor and the ground, and measuring and recording the voltage between the star point of the winding and the ground by using an oscilloscope, namely: common mode voltage V_com1(ii) a The voltage between the brush and the cabinet was measured and recorded with an oscilloscope, i.e.: shaft voltage V_b1At a common mode voltage V_com1Obtaining the axial voltage V according to the recording result of the oscilloscope as the reference phasor_b1The phase of (d);

the second group of experiments are that the stator casing is grounded through a non-inductive resistor A, the rotating shaft is grounded through a non-inductive resistor C, the driving mode and the wiring mode of the alternating current motor are the same as those of the first group of experiments, and the obtained common-mode voltage and the shaft voltage are marked as V_com2、V_b2；

The third group of experiments are that the stator casing is grounded through a non-inductive resistor B, the rotating shaft is grounded through a non-inductive resistor C, the driving mode and the wiring mode of the alternating current motor are the same as those of the first group of experiments, and the obtained common-mode voltage and the shaft voltage are marked as V_com3、V_b3；

Respectively substituting the grounding impedances of the stator and the rotor of the three groups of experiments and the corresponding common-mode voltage and the shaft voltage into the relational expression shown in the formula (5) to obtain three equations, simultaneously solving the three equations to obtain Z_wf、Z_wr、Z′_rfThen, according to the formulas (2) - (4), obtaining the stray capacitance C of the alternating current motor_wf、C_wr、C_rf。

5. The method of claim 4, wherein the step of removing the metal oxide layer comprises removing the metal oxide layer from the metal oxide layerThe equivalent capacitance C of the bearing oil film in the formula (4)_bThe numerical value of (A) is calculated by the following method:

1) calculating the equivalent capacitance C of the bearing oil film through analytic calculation according to the structural parameters of the bearing and the central thickness of the lubricating oil film_bThe value of (d);

2) establishing a bearing finite element analysis model, calculating stray capacitance between each ball and the inner and outer raceways, and further calculating the equivalent capacitance C of a bearing oil film_bThe value of (d);

3) applying a sinusoidal signal with the frequency of common-mode voltage frequency between the bearing and the casing, respectively measuring the shaft voltage between the bearing and the casing and the shaft current flowing through the bearing lead-out wire by using a voltage probe and a current probe when the alternating current motor runs, and solving the equivalent capacitance C of the oil film of the bearing by using a voltammetry method_bThe numerical value of (c).

6. The method of claim 1, wherein the method is suitable for use in the acquisition of stray capacitance within three-phase ac induction motors, synchronous generators, brushless dc motors and asynchronous generators.

Technical Field

The invention relates to the technical field of alternating current motor capacitance measurement, in particular to a method for acquiring stray capacitance of an alternating current motor.

Background

Stator winding of AC motor is placed in stator slot, slot insulation in slot separates stator winding from stator core, insulation paint is filled in gap between stator and rotor, stator and rotor are separated by air gap, stator core, stator winding and rotor of motor are separated by insulation mediumAnd opening. Taking a three-phase squirrel-cage induction motor as an example, the stator core and the stator casing can be regarded as one electrode, the rotor core and the rotor bars can be regarded as one electrode, and the stator winding can be regarded as the other electrode, so that the following stray capacitances exist in the alternating current motor: stray capacitance C between stator winding and rotor_wrStray capacitance C between stator winding and stator core_wfStray capacitance C between stator core and rotor_rf. When the frequency conversion power supply is carried out, the stray capacitance of the motor provides a low-impedance path for high-frequency common-mode voltage, so that shaft voltage is induced on a bearing of the motor.

A PWM (Pulse Width Modulation) type frequency converter has advantages of good speed regulation performance, good braking performance, high efficiency, and the like, and is widely used in driving systems in various industries such as electric power, metallurgy, and the like. However, the use of high performance semiconductor switching devices inside the frequency converter has many negative effects. An Insulated Gate Bipolar Transistor (IGBT) switching power device is widely used in the PWM converter. The fast opening and closing of the switching device enables the common-mode voltage output by the frequency conversion system to have higher dv/dt.

The output voltage of the frequency converter adopting the PWM control technology is a series of pulses, and is used for controlling the three-phase alternating current motor to run, the frequency converter is connected with a three-phase winding of the motor, and a voltage difference exists between a star point of the winding of the motor and the ground, namely a common-mode voltage V_comAnd the zero sequence voltage output by the frequency converter can be regarded as the zero sequence voltage output by the frequency converter. It has a size of V_com＝(V_u+V_v+V_w) /3 wherein V_u、V_vAnd V_wIs three-phase voltage to earth of motor, because the output of frequency converter is rectangular pulse, V_u、V_vAnd V_wNot possible to be symmetrical at any time, V_comNor is it zero at any instant.

When the frequency converter supplies power to the three-phase induction alternating current motor, the common mode voltage can form a common mode loop through the stray capacitance in the alternating current motor, and high-frequency common mode current is generated. Typically, the frequency of the common mode current ranges from tens of kilohertz to hundreds of kilohertz. Stray capacitance inside three-phase induction motor has fixedStator core to stator winding capacitance C_wfStator winding group and rotor core capacitor C_wrRotor core to stator core capacitance C_rf. At high frequencies, stray capacitance of the ac machine provides a low impedance path for common mode current, which induces shaft voltage between the inner and outer races of the bearing. When the shaft voltage exceeds the threshold voltage for breakdown of the lubricating oil film, the bearing lubricating oil film breaks down, and the breakdown instantaneously generates a bearing breakdown current, which is called a shaft current. In a short time, the breakdown discharge current can generate huge heat, the temperature is increased, the performance of lubricating grease is deteriorated, metal near a breakdown point is melted, vibration and noise are generated, the bearing of the alternating current motor fails too early, and the reliable operation of the alternating current motor is damaged.

Shaft current induced damage has attracted extensive attention in the industry. When analyzing the shaft current, an equivalent circuit of the shaft current needs to be established. And establishing an equivalent circuit involves determining the parameters of stray capacitance inside the alternating current motor.

When the shell of the alternating current motor and the ground, and the rotating shaft and the ground have grounding impedance, the change of the grounding impedance can change the potential distribution at two ends of the bearing, and further change the shaft voltage. Therefore, the corresponding common mode voltage and the shaft voltage can be measured under different grounding impedances, and then the stray capacitance inside the alternating current motor can be estimated.

Disclosure of Invention

Embodiments of the present invention provide a method for obtaining stray capacitance of an ac motor to overcome the disadvantages of the prior art.

In order to achieve the purpose, the invention adopts the following technical scheme.

A method of obtaining stray capacitance for an ac motor, comprising:

establishing a common mode equivalent circuit when a stator and a rotor of the alternating current motor are grounded;

establishing a relation between stray capacitance of the alternating current motor and common mode voltage, shaft voltage and stator and rotor grounding impedance based on the common mode equivalent circuit;

and measuring the common mode voltage and the shaft voltage of the alternating current motor under different stator and rotor grounding impedances, and calculating the stray capacitance in the alternating current motor according to the measured common mode voltage and the measured shaft voltage and by utilizing the relational expression.

Preferably, the establishing of the common-mode equivalent circuit when the stator and the rotor of the alternating current motor are grounded comprises:

an alternating current motor is driven by a machine to rotate, high-frequency sinusoidal voltage is applied between the star point of a three-phase winding of the alternating current motor and the ground, and the high-frequency sinusoidal voltage is used for simulating common-mode voltage V_comThe stator and the rotor of the alternating current motor are grounded through grounding impedance respectively to establish a common mode equivalent circuit of the alternating current motor, and the common mode equivalent circuit comprises the following components: coupling capacitor C between stator winding and stator core of alternating current motor_wfCoupling capacitor C between stator core and rotor of alternating current motor_rfCoupling capacitor C between stator winding and rotor of alternating current motor_wrStator side grounding impedance Z of alternating current motor_fgGrounding of the rotor side of an AC motoranti-Z_rgAnd axis voltage V_b(ii) a Point w represents the stator winding, point r represents the axis of rotation, point f represents the casing, point g represents ground, C_bThe equivalent capacitance of the bearing is caused by an oil film between a bearing rolling body and a raceway;

in the common-mode equivalent circuit, a common-mode voltage source V_comCoupling capacitor C between stator winding and stator core of alternating current motor_wfStator side grounding impedance Z of alternating current motor_fgForm a loop l₁Common mode voltage source V_comCoupling capacitor C between stator winding and rotor of alternating current motor_wrAC motor rotor side grounding impedance Z_rgForm a loop l₂Coupling capacitor C between stator core and rotor_rfAnd the oil film between the rolling body and the raceway of the bearing makes the equivalent capacitance C of the bearing_bAC motor rotor side grounding impedance Z_rgStator side grounding impedance Z of alternating current motor_fgForm a loop l₃Loop l₁Loop l₂And loop l₃The direction of the current is recorded as the positive direction of the current flowing through the loop, and the current is positive when the current flowing direction is consistent with the direction of the loop; when the current flows in the direction opposite to the direction of the loop, the current is negative and flows through the loop₁And loop l₂Is a common mode current, flows through loop l₃The current of (2) is the shaft current.

Preferably, the establishing a relationship between the stray capacitance of the ac motor and the common mode voltage, the shaft voltage, and the ground impedances of the stator and the rotor based on the common mode equivalent circuit includes:

let stray capacitance of the AC motor be C_wf、C_wr、C_rfCommon mode voltage of V_comAxial voltage of V_bStator ground impedance of Z_fgRotor ground impedance is Z_rg：

The loop equation of the common mode equivalent circuit is as follows:

in the formula (I), the compound is shown in the specification,

is a loop l₁The loop current of (a) is,

is a loop l₂The loop current of (a) is,

is a loop l₃Loop current of, Z_wfFor reactance of stator winding to stator core, Z_wrIs the reactance of the stator winding to the rotor, Z'_rfThe equivalent reactance of the capacitance of the rotor to the stator core and the capacitance of the bearing is as follows:

Z_wf＝1/(j2πf C_wf) (2)

Z_wr＝1/(j2πf C_wr) (3)

Z′_rf＝1/(j2πf(C_rf+2C_b)) (4)

where f is the frequency of the common mode voltage.

According to equation (1), we obtain:

preferably, the measuring the common mode voltage and the shaft voltage of the ac motor under different stator and rotor ground impedances, and the estimating the stray capacitance inside the ac motor by using the relational expression based on the measured common mode voltage and shaft voltage includes:

three non-inductive resistors A, B, C with different resistance values are selected, and the impedance is recorded as Z_A、Z_B、Z_C；

Performing three groups of experiments, and measuring the common-mode voltage and the shaft voltage of the alternating current motor under three groups of grounding impedances;

in the first group of experiments, a tested alternating current motor is connected with an accompanying alternating current motor through an insulating coupler, a carbon brush is additionally arranged outside a rotating shaft of the tested alternating current motor, a stator casing of the tested alternating current motor is grounded through a non-inductive resistor A, and the rotating shaft of the tested alternating current motor is grounded through a carbon brush through a non-inductive resistor B;

dragging the tested AC motor to the rated speed, applying high-frequency voltage with adjustable amplitude and frequency between the star point of the stator winding of the tested AC motor and the ground, and measuring and recording the voltage between the star point of the winding and the ground by using an oscilloscope, namely: common mode voltage V_com1(ii) a The voltage between the brush and the cabinet was measured and recorded with an oscilloscope, i.e.: shaft voltage V_b1. With common mode voltage V_com1Obtaining the axial voltage V according to the recording result of the oscilloscope as the reference phasor_b1The phase of (d);

the second group of experiments are that the stator casing is grounded through a non-inductive resistor A, the rotating shaft is grounded through a non-inductive resistor C, the driving mode and the wiring mode of the alternating current motor are the same as those of the first group of experiments, and the obtained common-mode voltage and the shaft voltage are marked as V_com2、V_b2；

The third group of experiments are that the stator casing is grounded through a non-inductive resistor B, the rotating shaft is grounded through a non-inductive resistor C, the driving mode and the wiring mode of the alternating current motor are the same as those of the first group of experiments, and the obtained common-mode voltage and the shaft voltage are marked as V_com3、V_b3；

Respectively substituting the grounding impedances of the stator and the rotor of the three groups of experiments and the corresponding common-mode voltage and the shaft voltage into the relational expression shown in the formula (5) to obtain three equations, simultaneously solving the three equations to obtain Z_wf、Z_wr、Z′_rfThen, according to the formulas (2) - (4), obtaining the stray capacitance C of the alternating current motor_wf、C_wr、C_rf。

Preferably, the equivalent capacitance C of the bearing oil film in the formula (4)_bThe numerical value of (A) is calculated by the following method:

1) calculating the equivalent capacitance C of the bearing oil film through analytic calculation according to the structural parameters of the bearing and the central thickness of the lubricating oil film_bThe value of (d);

2) establishing a bearing finite element analysis model, calculating stray capacitance between each ball and the inner and outer raceways, and further calculating the equivalent capacitance C of a bearing oil film_bThe value of (d);

3) in thatApplying a sinusoidal signal with the frequency of common-mode voltage frequency between the bearing and the casing, measuring the shaft voltage between the bearing and the casing and the shaft current flowing through the bearing lead-out wire by using a voltage probe and a current probe respectively when the alternating current motor runs, and solving the equivalent capacitance C of the oil film of the bearing by using a voltammetry method_bThe numerical value of (c).

6. The method of claim 1, wherein the method is suitable for use in the acquisition of stray capacitance within three-phase ac induction motors, synchronous generators, brushless dc motors and asynchronous generators.

According to the technical scheme provided by the embodiment of the invention, the stray capacitance in the motor can be effectively calculated by measuring the common-mode voltage and the shaft voltage of three groups of different stator and rotor grounding impedances. The influence of the rotating speed, the temperature rise and the like of the motor can be considered, and the stray capacitance in the alternating current motor in the actual running state can be obtained.

Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

Fig. 1 is a processing flow of a method for obtaining stray capacitance of an ac motor according to an embodiment of the present invention;

fig. 2 is a schematic diagram of a common-mode equivalent circuit when both a stator and a rotor of an ac motor have ground impedances according to an embodiment of the present invention.

Detailed Description

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the accompanying drawings are illustrative only for the purpose of explaining the present invention, and are not to be construed as limiting the present invention.

As used herein, the singular forms "a", "an", "the" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may also be present. Further, "connected" or "coupled" as used herein may include wirelessly connected or coupled. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

For the convenience of understanding the embodiments of the present invention, the following description will be further explained by taking several specific embodiments as examples in conjunction with the drawings, and the embodiments are not to be construed as limiting the embodiments of the present invention.

According to the embodiment of the invention, the forming condition of the shaft voltage during the grounding impedance is considered, and the stray capacitance in the alternating current motor is calculated by changing the grounding resistance according to the circuit relation of the common-mode voltage, the shaft voltage and the grounding impedance, so that the shaft current is accurately analyzed and predicted. According to the embodiment of the invention, the stray capacitance in the motor can be calculated by measuring the common-mode voltage and the shaft voltage under three groups of different stator and rotor grounding impedances.

The shaft voltage in the embodiment of the invention is the voltage difference between the rotating shaft and the shell. The method for acquiring the stray capacitance of the alternating current motor in the embodiment of the invention is suitable for various types of alternating current motors, and a three-phase alternating current induction alternating current motor is taken as an example for description.

The processing flow of the method for obtaining the stray capacitance of the alternating current motor in the embodiment of the invention is shown in fig. 1, and comprises the following processing steps:

step S10: and establishing a common mode equivalent circuit when the stator and the rotor of the alternating current motor are grounded.

The AC motor is driven by other machines to rotate, a high-frequency sinusoidal voltage is applied between the star point of the three-phase winding of the AC motor and the ground, and the high-frequency sinusoidal voltage is used for simulating a common-mode voltage V_comAnd respectively grounding the stator and the rotor of the alternating current motor through grounding impedance to establish a common mode equivalent circuit of the alternating current motor. Fig. 2 is a schematic diagram of a common-mode equivalent circuit when both a stator and a rotor of an ac motor have ground impedances according to an embodiment of the present invention.

In fig. 2, point w represents the stator winding, point r represents the rotation axis, point f represents the casing, point g represents the ground, C_wfIs a coupling capacitor between the stator winding and the stator core of the AC motor, C_wrFor coupling capacitance, C, between stator winding and rotor of AC motor_rfIs a coupling capacitance between the stator core and the rotor, C_bCapacitance equivalent to the oil film between the rolling elements and the raceways of a bearing, Z_fgIs the stator side ground impedance of the AC machine, Z_rgIs the grounding impedance of the rotor side of the alternating current motor. Common mode voltage source V_comCoupling capacitor C between stator winding and stator core of alternating current motor_wfStator side grounding impedance Z of alternating current motor_fgForm a loop l₁. Common mode voltage source V_comCoupling capacitor C between stator winding and rotor of alternating current motor_wrAC motor rotor side grounding impedance Z_rgForm a loop l₂. Coupling capacitor C between stator core and rotor_rfAnd the oil film between the rolling body and the raceway of the bearing makes the equivalent capacitance C of the bearing_bAC motor rotor side grounding impedance Z_rgStator side grounding impedance Z of alternating current motor_fgForm a loop l₃. Loop l₁Loop l₂Loop l₃The direction of the detour is defined as the positive direction of the loop current. When the current flow direction is consistent with the bypassing direction of the loop, the current is positive; when the current flows in the opposite direction to the direction of the loop, the current is negative. Flow through the circuit l₁And loop l₂Is a common mode current, flows through loop l₃The current of (2) is the shaft current.

When the AC motor rotates, the oil film between the rolling bodies and the rolling way of the bearing makes the bearing equivalent to a capacitor C_b。Z_fgRepresenting the stator-side earth impedance, Z, of the AC machine_rgRepresenting the rotor-side ground impedance of the ac machine. V_bThe shaft voltage represents the voltage between the rotating shaft and the shell, and also represents the voltage between the inner ring and the outer ring of the bearing. Assuming that the equivalent capacitance of the bearings at the driving end and the non-driving end is the same, the capacitance of the bearings at the two ends can be considered as parallel connection, and 2C is used_bAnd (4) showing.

Step S20: obtaining stray capacitance C of AC motor_wf、C_wr、C_rfAnd common mode voltage V_comShaft voltage V_bAnd stator and rotor ground impedance Z_fg、Z_rgThe relation between them.

According to the reference loop in FIG. 2, the column write-back loop equation:

in the formula (I), the compound is shown in the specification,

is a loop l₁The loop current of (a) is,

is a loop l₂The loop current of (a) is,

is a loop l₃Loop current of, Z_wfFor reactance of stator winding to stator core, Z_wrIs the reactance of the stator winding to the rotor, Z'_rfThe equivalent reactance of the capacitance of the rotor to the stator core and the capacitance of the bearing is as follows:

Z_wf＝1/(j2πf C_wf) (2)

Z_wr＝1/(j2πf C_wr) (3)

Z′_rf＝1/(j2πf(C_rf+2C_b)) (4)

where f is the frequency of the common mode voltage.

From equation (1), one can obtain:

it can be seen from equation (5) that when three stray capacitance parameters in the ac motor need to be obtained, the stray capacitance parameters can be obtained by simultaneously solving equations by using three groups of common mode voltages and shaft voltages under three groups of different stator and rotor ground impedances.

Step S30: three non-inductive resistors A, B, C with different resistance values are used, and their impedance values are respectively recorded as Z at high frequency (e.g. 1MHz)_A、Z_B、Z_C. The three non-inductive resistors need to consume no more than rated power when the alternating current motor runs.

Step S40: three sets of experiments were performed to measure the common mode voltage and the shaft voltage at three sets of ground impedances.

And in the first group of experiments, the tested alternating current motor is connected with the accompanying alternating current motor through an insulating coupler. The carbon brush is additionally arranged on the rotating shaft of the alternating current motor to be detected. The stator casing of the alternating current motor to be detected is grounded through a non-inductive resistor A, and the rotating shaft of the alternating current motor to be detected is grounded through a carbon brush and a non-inductive resistor B.

The measured ac motor is driven to its rated speed. A high-frequency (such as 1MHz) voltage with adjustable amplitude and frequency is applied between the star point of the stator winding of the alternating current motor to be tested and the ground. Use the showWave filter measuring and recording common mode voltage V_com1And the axis voltage V_b1. The common-mode voltage is the voltage between the star point of the three-phase stator winding of the alternating-current motor and the ground, and the shaft voltage is the voltage between the rotating shaft and the shell. The magnitude of the applied common mode voltage ensures that the amplitude and the waveform of the shaft voltage have no obvious sudden change and do not generate discharge breakdown current. With common mode voltage V_com1Obtaining V according to the recording result of the oscilloscope for reference phasor_b1The phase of (c).

The second set of experiments is that the stator casing is grounded through a non-inductive resistor A, the rotating shaft is grounded through a non-inductive resistor C, and the driving mode and the wiring mode of the alternating current motor are the same as those of the first set of experiments. The obtained common mode voltage and the axis voltage are recorded as V_com2、V_b2；

The third group of experiments are that the stator casing is grounded through a non-inductive resistor B, the rotating shaft is grounded through a non-inductive resistor C, and the driving mode and the wiring mode of the alternating current motor are the same as those of the first group of experiments. The obtained common mode voltage and the axis voltage are recorded as V_com3、V_b3。

Step S50: obtaining internal stray capacitance parameter C of alternating current motor_wf、C_wr、C_rf。

And respectively substituting the grounding impedances of the stator and the rotor of the three groups of experiments, the common-mode voltage and the shaft voltage corresponding to the grounding impedances into a relational expression shown in a second step (5) to obtain three equations. By simultaneous solution of three equations, Z can be obtained_wf、Z_wr、Z′_rf. According to the formulas (2) to (4), the stray capacitance C of the alternating current motor can be obtained_wf、C_wr、C_rf。

Formula (4) includes C_b，C_bThe determination can be made by the following method:

1) according to the structural parameters of the bearing and the central thickness of the lubricating oil film, the equivalent capacitance of the bearing oil film can be obtained through analytic calculation;

2) establishing a bearing finite element analysis model, calculating stray capacitance between each ball and the inner and outer raceways, and further calculating a bearing oil film equivalent capacitance;

3) the alternating current motor is reformed, a sinusoidal signal with the frequency of common-mode voltage frequency is applied between the bearing and the shell, when the alternating current motor runs, the voltage probe and the current probe are used for respectively measuring the shaft voltage between the bearing and the shell and the shaft current flowing through the bearing outgoing line, and the equivalent capacitance of the bearing oil film is solved by using a voltammetry method.

The invention discloses a method for acquiring stray capacitance in a motor by taking a three-phase alternating current induction motor as an example, but the method is not limited to the three-phase induction motor, and is also suitable for acquiring stray capacitance parameters of other types of alternating current motors, such as a synchronous motor, a synchronous generator, a brushless direct current motor, an asynchronous generator and the like.

In summary, in the embodiments of the present invention, the stray capacitance inside the motor can be effectively calculated by measuring the common mode voltage and the shaft voltage under three different sets of stator and rotor ground impedances. The influence of the rotating speed, the temperature rise and the like of the motor can be considered, and the stray capacitance in the alternating current motor in the actual running state can be obtained.

The method of the embodiment of the invention can measure the stray capacitance of the common bearing motor and the insulation bearing motor; when the common bearing motor is used for measurement, the motor does not need to be disassembled to replace a plastic bearing. An LCR meter is not needed, so that the measurement frequency and the built-in connection mode of the LCR meter are not needed to be selected by predicting internal stray capacitance; errors caused by stray parameters introduced by adopting connecting wires are avoided.

Those of ordinary skill in the art will understand that: the figures are merely schematic representations of one embodiment, and the blocks or flow diagrams in the figures are not necessarily required to practice the present invention.

The embodiments in the present specification are described in a progressive manner, and the same and similar parts among the embodiments are referred to each other, and each embodiment focuses on the differences from the other embodiments. In particular, for apparatus or system embodiments, since they are substantially similar to method embodiments, they are described in relative terms, as long as they are described in partial descriptions of method embodiments. The above-described embodiments of the apparatus and system are merely illustrative, and the units described as separate parts may or may not be physically separate, and the parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the present embodiment. One of ordinary skill in the art can understand and implement it without inventive effort.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.