The heat dissipation of the relay is mainly due to two parts, one part comes from the heat of the coil resistance, and the other part comes from the heat generated by the current flowing through the contact resistance of the dynamic and static contacts. In the relatively closed environment of the relay, if the internal temperature rise is too high, it will bring the impact and harm such as reed deformation and base structure damage, which is very likely to lead to the failure of the relay. Su Xiuping et al. [1] proposed to establish a relay coupling model and compare the average temperature rise under different coil voltage conditions to obtain the corresponding temperature contoursWang Wenlong [2] established a mathematical model to carry out mechanical-electrical-thermal coupling of the relay contact system, which is helpful for the optimal design of relaysMa et al. [3] established a finite element model of relays, analyzed their static characteristics, and applied the results to dynamics. According to the actual working scenario of the relay, this paper selects a certain type of DC electromagnetic relay as the research object, establishes the mathematical model of the relay, and imports ANSYS into Ansys to obtain the thermoelectric coupling finite element analysis model, and then obtains the temperature field contour diagram of the electromagnetic relay, and compares and analyzes the temperature rise data and the test temperature rise data under different coil voltage conditions, which provides data support for improving and optimizing the temperature rise of the relay.
Build a mathematical model
There are three main forms of heat transfer in relays, which are heat conduction, heat convection, and heat radiation. The heat source of the relay has two parts: one part comes from the heat of the coil resistance, and the other part comes from the heat generated by the current flowing through the contact resistance of the dynamic and static contacts. When the relay is in a stable state of thermal equilibrium, the heat generating power and heat dissipation power of the relay are equal. The following is a theoretical analysis of the three forms of heat transfer to establish a mathematical model of the relay temperature field.
Thermal conduction
In the heat transfer of relays, heat conduction is the main form. If there is a temperature difference or temperature gradient in the relay, heat conduction occurs where heat is transferred from a place with a high temperature to a place with a low temperature. Heat conduction exists between different parts of the same object and different objects that are in contact with each other. According to Fourier's law and the law of conservation of energy [4], the equation for heat conduction can be obtained as:
where: is the density of the object;c is the specific heat capacity of the object;t is the temperature of the object;t is the time;is the thermal conductivity of the object;qv is the heat generation rate of the heat source inside the object.
The internal heat of the relay is mainly conducted out to other components through the coil and contact reeds, and then conducted outward through heat dissipation holes, gaps and shells, while part of the heat is conducted outward through the pin of the relay and the wire connected to the pin, and then the heat is dissipated into the environment in the form of convection and radiation.
Thermal convection
The process of heat transfer when a fluid flows across the surface of an object is called convective heat transfer [5]. Convective heat transfer occurs between the heat exchange between the solid surface and the surrounding fluid in contact caused by temperature differences, which can be divided into natural convection and forced convection. Although the process of thermal convection is complex, convective heat transfer can be described simply by applying the Newtonian cooling formula [6], which is expressed as:
where: Heat transfer for convection;h is the convective heat transfer coefficient;a is the heat exchange area;t is the temperature difference between the solid surface and the fluid.
Thermal radiation
The phenomenon in which an object emits radiant energy due to its inherent heat content is called thermal radiation [7], which is a heat exchange process in which electromagnetic energy is emitted and absorbed by other objects or components into heat. The higher the surface temperature of the object, the more obvious the radiative heat transfer. The Stephen-Boltzmann equation [8] can be used to express radiative heat transfer:
where: q denotes the heat flow rate;denotes the emissivity;denotes the Stephen-Boltzmann constant [9];a1 is the area of surface 1;f12 is the form factor from surface 1 to surface 2;t1 and t2 are the absolute temperatures of the surface, respectively.
Finite element analysis
The material properties of the relay model were first set by using Pro E software to model the research object as a whole, and the material properties of the relay model were first set, taking the main components of the relay as an example, the shell and base were set to PBT material, the armature iron and yoke were set to DT4E material, the coil was set to pure copper material, the contact was set to AG material, and the dynamic and static reed was set to beryllium copper material. Then, in order to adapt to the condition that the relay solid model is more complex and the boundary shape is irregular, the solid70 element is selected to set the relay model to a tetrahedral shape, which can meet the requirements of the mesh accuracy, and the finite element analysis model of the relay is obtained, as shown in Figure 1. Then, the convective heat dissipation coefficient [10] on the side and top of the relay is set according to the actual application scenario, and finally the heat generated by the relay coil part and the contact reed part is converted into the heat generation rate per unit volume and loaded on the component.
Diagram of the temperature field
Using ANSYS software, the coil excitation voltage was set to a rated voltage of 12 V and a contact resistance of 06. Set the current of the contact load circuit to be 10 A, and obtain the temperature field distribution diagram of the relay coil part and the contact reed part respectively under the two cases of retaining the shell and not retaining the shell, as shown in Figure 2 and Figure 3. In order to facilitate the observation of the temperature field distribution, the shell is hidden in Figure 2. The housing of this type of relay is equipped with heat dissipation vents, and if the housing is not retained, it means that the relay is directly exposed to the environment, so there is a significant difference between the maximum and minimum temperatures in this case. The heating of the internal coil of the relay and the heat of the contact load circuit when the housing is retained is significantly higher than that when the housing is not retained, which is mainly caused by air convection heat dissipation, and the relay housing prevents most of the internal heat from being exchanged with the external environment.
As can be seen from the above temperature field contour diagram, after the first circle is energized and the contact contact is connected to the load circuit, the heat of the relay mainly gathers the first ring body and the contact reed part, and the temperature rise of the two parts of the relay is low without retaining the shell, indicating that the temperature rise of the relay is greater with or without the shell, but the temperature rise of the relay is still within the acceptable range when the shell is retained. There are two main directions for the heat dissipation of the coil of the relay, one is to transfer heat energy to the yoke and armature iron through the coil body, and the other is to transfer heat to the surrounding environment and air through the coil, that is, there are two main forms of convection and conduction. The heat of the armature mainly comes from the heat transferred by the yoke, so the heat dissipation of the coil is mainly transferred through the yoke, armature, and coil pins, and then transferred to the base, and finally the heat is transferred to the air, and the ambient temperature is slightly increased by this. The heat dissipation of the contact load loop of the relay is mainly through the reed pins, the base and the plastic push block, which is also mainly in the form of convection and conduction. As a result, coil heat dissipation and contact load circuit heat dissipation interact and influence, resulting in the overall temperature field distribution of the relay.
Analysis and conclusions
By adding different excitation voltages to the coil and using ANSYS for thermoelectric coupling analysis, the temperature rise value of the coil can be obtained. On the premise of retaining the relay housing, the classical resistance method is used to carry out the temperature rise test, and the data of the corresponding resistance change when the coil is heated is collected, and the average temperature rise value is calculated, and the test value is obtained. The range of the excitation voltage set in the test is 96～18 v。Table 1 is the average temperature rise data of the coil of the relay with the case of retaining the shell, and at the same time, the absolute value of the difference between the temperature rise value and the test value is divided by the percentage of the test value, that is, the error rate in Table 1.
A comprehensive comparison of the average temperature rise value of the coil and the test test value shows that the temperature rise value is smaller than the test value, and the maximum difference between the two is 44 k, the error rate is within 8%, so the simplified internal structure model of the relay is effective, and the error rate is within a reasonable range.
The main error lies in:
1) **The ambient temperature is constant, but the ambient temperature will be affected by external factors during the actual test
2) Simplify the parts that have less impact on the heat generation power and heat dissipation power
3) There are some small deviations in setting the material properties of the components.
Therefore, in the relay application and design analysis, we should focus on the part with high temperature rise, so as to avoid the failure of the relay structure due to its high temperature rise. In addition, when developing and designing relays, on the premise of ensuring mechanical strength and dielectric strength, plastic materials with large conductivity should be selected as much as possible to make bases, shells and plastic blocks.
Conclusion
In this paper, the heating process of the relay is analyzed in detail, and the mathematical model is established based on the principle of heat conduction, thermal convection, and thermal radiation, and then the finite element analysis model of the DC electromagnetic relay is established by ANSYS software, the model is meshed, the parameters such as material, convection coefficient, and conduction coefficient are set and the thermoelectric coupling temperature field analysis is carried out, and the temperature rise value of the first temperature rise value and the test temperature rise value are compared, and it is found that the error of the two is small, which verifies the effectiveness of the model and the correctness of the temperature field analysis. From the temperature contour diagram, it is found that the heating of the relay is mainly concentrated in the first circle part and the contact contact part, and special attention needs to be paid to the temperature rise change of these two parts in the design and development of the relay to ensure the normal performance index of the relay. The analysis results can provide some basis and ideas for the optimal design of the same type of DC electromagnetic relay, and can also provide theoretical and data support for the development and design of new relay products.
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