Calculation and Analysis of the Effect of Temperature on Solenoid Valve Drive

introduction

A solenoid valve is a valve that controls the flow of fluid in a pipeline system by the electromagnetic force of an electromagnetic coil. With the acceleration of automation in the automotive industry, air conditioning, chemical equipment, aerospace and other industries, solenoid valves are increasingly used in these industries due to their fast, precise and efficient control characteristics. The electromagnetic coil is the core power source of the solenoid valve. The electromagnetic coil can also be adapted to different circuits to work and is a basic component for automated fluid control. In recent years, the design and application of solenoid valves have become more mature, but in actual applications, there are defects such as the solenoid valve's ability to open the valve is attenuated under high temperature conditions and even fails to meet the design requirements for the valve opening pressure difference.

This paper focuses on analyzing the influence of temperature on the magnetomotive force of the electromagnetic coil, the electromagnetic force and the valve opening ability of the solenoid valve, aiming to derive a direct correlation formula between temperature and the key characteristics of the solenoid valve, and provide a reference for the subsequent application of solenoid valves under different temperature environmental conditions.

1. Solenoid valve structure and operating principle

The solenoid valve is composed of an electromagnetic coil and a valve assembly to form a driving on-off function, as shown in Figure 1. The electromagnetic coil is generally composed of a coil frame, an enameled wire winding, an insulating encapsulation layer, etc. The insulating enameled wire is wound on the coil frame. The valve components include a moving iron core, a static iron core, and a reset spring. The moving and static iron cores are both made of soft magnetic materials. When the coil is not energized and there is no magnetic field, the moving and static iron cores are non-magnetic or very weakly magnetic; when the coil is energized to form a magnetic field, the soft magnetic material is excited to generate magnetism, overcoming the spring force and the pressure difference on the static iron core to attract each other.

Taking the normally closed direct-acting solenoid valve as an example, when the coil is not energized, the moving iron core and the static iron core are separated under the action of the reset spring, and the valve mouth is closed to realize the valve closing function; when the coil is energized, a magnetic field is generated, causing the moving and static iron cores to be excited and attracted to each other, the valve mouth is opened, and the valve opening function is realized.

Magnetomotive force is the main indicator for measuring the performance of electromagnetic coils. The magnetomotive force mainly depends on the number of turns of the enameled wire, the inner and outer diameters of the coil frame, and is also affected by the coil temperature and driving voltage. Electromagnetic force and valve opening pressure difference are the main indicators for measuring the performance of solenoid valves. In addition to being affected by coil factors, the electromagnetic force is also related to the air gap length and air gap area inside the valve assembly. The valve opening pressure difference calculated based on the electromagnetic force is also related to the valve port size inside the valve assembly and the reset spring force.

2 Calculation of coil magnetomotive force and electromagnetic force of solenoid valve

2.1 Calculation of magnetomotive force

The magnetomotive force is the product of the coil current I and the number of coil turns N (IN), also known as "ampere-turns". First, calculate the coil current I and the number of coil turns N respectively.

The number of coil turns N depends on the three parameters H, D1, D2 of the coil frame and the diameter d of the enameled wire wound on the frame. The calculation formula for the number of coil turns is as follows:

In the formula: H is the total winding width of the enameled wire on the coil frame (mm); D1 is the winding axis diameter of the enameled wire on the coil frame (mm); D2 is the outer diameter of the enameled wire on the coil frame (mm); d is the wire diameter of the enameled wire (mm).

The total winding length L is:

The coil resistance R is:

Where: p is the resistivity of copper (mm2/m); s is the cross-sectional area of ​​the enameled wire (mm2).

After the coil is wound, its resistance value R is also determined. When the driving voltage of the electromagnetic coil is clear, the electromagnetic driving current can be calculated by the formula I=U/R as follows:

The magnetomotive force IN is:

From the above formula, it can be concluded that when other parameters are fixed, the magnetomotive force is directly proportional to the voltage and inversely proportional to the resistivity.

2.2 Calculation of magnetic induction intensity and electromagnetic force

Most of the magnetomotive force drop of the solenoid valve occurs at the air gap. Except for the air gap, the magnetic conductivity of the rest of the material of the solenoid valve is good. The air gap length refers to the running length of the moving iron core from the reset position to the attracted position, that is, the stroke [5]. The longer the stroke, the greater the decrease in magnetic field strength and electromagnetic force. The formula can be converted into:

In the formula: H0 is the air gap magnetic field intensity (A/m); 6 is the air gap length (mm); B0 is the air gap magnetic induction intensity (T); μ0 is the magnetic permeability, which is 4m×10-7H/m.

Calculation of magnetic induction intensity:

Simplified electromagnetic force algorithm:

Where: s0 is the air gap area (mm2).

From the above formula, it can be concluded that when other parameters are fixed, the electromagnetic force is directly proportional to the square of the voltage and inversely proportional to the square of the resistivity.

2.3 Calculation of the relationship between magnetomotive force, electromagnetic force and temperature

When the coil is energized, it will heat up, and the resistivity of the enameled wire will change with the temperature. The calculation formula is:

Where: 7 is the coil temperature (℃)

Substituting formula (9) into the magnetomotive force calculation formula (5), the calculation formula of magnetomotive force and temperature is obtained:

In the formula: U is the working voltage (v); d is the diameter of the enameled wire (mm); 7 is the coil temperature (℃); D1 is the diameter of the enameled wire winding shaft on the coil frame (mm); D2 is the outer diameter of the enameled wire winding on the coil frame (mm).

Substituting formula (9) into the electromagnetic force calculation formula (8), the calculation formula of electromagnetic force and temperature is obtained:

In the formula: U is the working voltage (v); d is the diameter of the enameled wire (mm); μ0 is the air gap magnetic permeability, which is 4m×10-7H/m; s0 is the air gap area (mm2); 7 is the coil temperature (℃); D1 is the diameter of the enameled wire winding axis on the coil frame (mm); D2 is the outer diameter of the enameled wire winding on the coil frame (mm); 6 is the air gap length (mm).

2.4 Calculation of valve opening pressure difference

The solenoid valve design needs to be able to overcome the pressure difference and achieve the normal valve opening requirements at different temperatures. Therefore, the valve opening pressure difference of the solenoid valve is equivalent to the valve opening capacity. To open the valve, it is necessary to overcome the reset spring force Fs and the pressure difference force Fp acting on the moving iron core. The pressure difference force Fp is the product of the size of the moving iron core valve port do and the pressure difference p above and below the valve port. The conversion formula of electromagnetic force and valve opening pressure difference is as follows:

In the formula: Fs is the return spring force (N); Fp is the pressure difference force (N); do is the valve port diameter (mm); p is the valve opening pressure difference (MPa).

3 Calculation Example

The parameters of the solenoid valve are shown in Table 1. The specific data of the magnetomotive force, electromagnetic force and valve opening pressure difference are calculated when the temperature changes in the range of 0~120℃, and the data analysis is carried out using two typical temperature points of normal temperature 2o℃ and high temperature 12o℃ as examples.

When the temperature changes in the range of 0~120℃, the magnetomotive force changes as shown in Figure 2. At room temperature 2o℃, the magnetomotive force is about 691At. When the temperature rises to 120℃, the magnetomotive force decreases to 496At, and the calculated attenuation amplitude is 28%.

When the temperature changes in the range of 0~120℃, the electromagnetic force changes as shown in Figure 3. At room temperature 20℃, the electromagnetic force is about 8.5N. When the temperature rises to 120℃, the electromagnetic force decreases to 4.4N, and the calculated attenuation is 48%.

When the temperature changes in the range of 0~120℃, the change of the valve opening pressure difference is shown in Figure 4. At 20℃, the valve opening pressure difference p is 3.7MPa. When the temperature rises to 120℃, the valve opening pressure difference decreases to 1.4MPa, and the calculated attenuation amplitude is 62%.

According to the above calculations, the magnetic motive force, electromagnetic force and valve opening pressure difference all drop significantly when the temperature rises. When the temperature rises to 12o℃, the valve opening pressure difference is already lower than the design requirement. There are two solutions: