GB/T 2424.1-1989 Basic environmental testing procedures for electrical and electronic products - Guidelines for high and low temperature testing
Some standard content:
National Standard of the People's Republic of China
Basic environmental testing procedures for electric and electronic productsGuidance for high temperature and low temperature testsGB2424.1-89
Replaces GB2424.181
This standard is equivalent to the international standard IEC68-3-1 "Basic Environmental Testing Procedures Part 3: Background Materials - Cold and Dry Heat Tests" (1974 Edition) and its first supplementary document IEC68-3-1A (1978). 1 Subject content and scope of application
This standard specifies the basic environmental testing procedures for electric and electronic products: Guidelines for high temperature and low temperature tests. This standard is applicable to high temperature and low temperature tests of basic environmental tests for electric and electronic products. This standard includes background knowledge materials for temperature sudden change tests and temperature gradual change tests of non-heat dissipation test samples and heat dissipation test samples (with and without cooling systems) as a guide for the application of low temperature and high temperature test methods. The test equipment (box or room) can be with forced air circulation or without forced air circulation. Generally, forced air circulation is used for non-heat dissipation test samples, while non-forced air circulation is used for heat dissipation test samples. 1.1 General block diagram of high temperature and low temperature test methods classification See the general block diagram for the classification of high temperature and low temperature test methods. 1.2 Reference environmental conditions
Product performance is generally affected and restricted by its internal temperature, and the internal temperature is determined by the heat generated by itself and the surrounding environmental conditions.
Whenever there is a temperature gradient in the system formed by the product and its surrounding environment, there is a heat transfer (heat exchange) process between them. Except for special large-scale products or complete sets of equipment, the actual environmental conditions of the product's future work are often not accurately known or accurately specified during design and manufacturing. Therefore, it is generally impossible to use actual environmental conditions as a basis for design, manufacturing or testing. Therefore, it is necessary to stipulate the reference environmental conditions of the product as the basis for design, manufacturing and testing, which are stipulated in the contents of 1.3, 1.4, 1.5 and 1.6 of this chapter.
1.3 Non-heat dissipating products
If the ambient temperature is uniform and constant, and no heat is generated in the product, the direction of heat flow is: when the ambient temperature is high, heat is transferred from the ambient atmosphere to the product; conversely, if the product temperature is high, heat is transferred from the product to the surrounding atmosphere. This heat transfer process will continue until the temperature of all parts of the product reaches the ambient air temperature. After that, unless the ambient temperature changes, the heat transfer process will stop. In this case, it is simple to determine the reference ambient temperature. The only condition is that it should be uniformly distributed and constant. However, for the situation where the product cannot reach the ambient air temperature, the determination of the reference ambient temperature is more complicated. At this time, the conclusion of Article 1.4 of this chapter should be considered. Approved by the Ministry of Machinery and Electronics Industry of the People's Republic of China on January 25, 1989. Standard industry data completed. Implementation on January 1, 1990. Sudden change of temperature between Aa and Ba. Test samples without cooling. No forced air circulation test. 1.4 Heat dissipation products. Non-heat dissipation test samples. GB2424.1-8 9
Low and high temperature tests
Ab and Bb temperature gradient
Test sample with cooling
With forced
Environment test
Cooling system
Separated from test
Cooling system
Not separated from test chamber
No forced
Environment test
Single test sample tested
Single test sample tested
Bc temperature sudden change
Heat dissipation test sample
Ad and Bd temperature gradual change
Test sample without cooling
No forced
Air circulation
Ring test
With forced
Air circulation
Ring test
Only use the induced temperature test Bd
One test catfish sample is tested
Multiple test samples are tested
High temperature and low temperature test method General block diagram of the classification method Test sample with cooling
Cooling system
Separated from the test
Box
No forced
Air circulation
Test
Cooling system
Not separated from the test box
Forced
Air circulation
Test
Heat is generated in the product. If there is no heat transfer to the surrounding atmosphere, the product temperature will continue to rise. In fact, the heat generated by the product is continuously dissipated to the surrounding atmosphere. Finally, the heat generated by the product is balanced with the heat dissipated in the surrounding cooling atmosphere, so that the product temperature reaches stability. Only when the ambient temperature rises (or falls) will the temperature inside the product rise (or fall) further until a new balance is reached.
For this case, the base ambient temperature should be determined in such a way that simple and reproducible heat transfer conditions can be obtained. Since heat transfer is carried out by convection, radiation and conduction, clear and specific conditions must be obtained for each of these methods separately and simultaneously. For example, if multiple test samples are tested at high temperature in the same test chamber (room), it should be ensured that all test samples are at the same ambient temperature and have the same installation conditions. For low temperature tests, it is not necessary to strictly distinguish between the situation when a single test sample is tested and the situation when multiple test samples are tested.
1.5 Ambient temperature
Usually, product users require to know the maximum and minimum values of the ambient temperature allowed when the product is working, and for the purpose of testing, this should also be stipulated. Because low temperature tests are usually carried out at a temperature (level) equivalent to the minimum ambient temperature, while high temperature tests are usually carried out at a temperature (level) equivalent to the maximum ambient temperature. Since heat transfer is associated with temperature gradients, the temperature distribution of the medium surrounding the product in space must be different at each point, which brings certain difficulties to determining the "ambient temperature" of the surrounding atmosphere. The "ambient temperature" should be specifically determined (see GB2422 "Basic Environmental Test Procedures for Electrical and Electronic Products", terminology, clause 2.7). 1.6 Surface temperature GB2424.1-89 The main influence on product performance is the temperature of the product itself. From the purpose of monitoring and adjusting the test equipment, it is appropriate to monitor and adjust the test equipment by referring to the temperature of some key points on the surface of the test sample or even inside it. 1.7 Duration of test 1.7.1 If the purpose of the test is only to check the working performance of the product at high or low temperatures, the test can be carried out only until the test sample reaches temperature stability. The time required for the test sample to reach temperature stability at a certain ambient temperature is about 3 to 5 times the thermal time constant of the test sample, and generally 4 times.
(where: G is mass, g; C is specific heat, J/(g·C); S is heat dissipation area, cm2; I is heat dissipation coefficient, W/(cm\·C) of the test sample. This constant is difficult to obtain by calculation. Therefore, the time required for the test sample to reach temperature stability can only be obtained through experiments. Figure 1 is the heating or cooling curve of the test sample during the temperature mutation test. The time required for the temperature of a certain point of the test sample to reach 0.632△Tw (△Tw is the stable temperature rise) is the time required for the temperature of the test sample at that point to reach 0.632△Tw (△Tw is the stable temperature rise). Take 4 times the thermal time constant to get the time for the starting temperature to reach stability. AT
L72860
Figure 1 Heating or cooling curve
tTest time
1.7.2 In low temperature and high temperature tests, the test duration is usually calculated after the test sample reaches temperature stability. The duration should be selected from the following levels according to the characteristics of the test sample and the purpose of the test: 216, 72, 96h. 1.7.3 If the test sample is tested in connection with durability or reliability, the test duration should be separately specified by the relevant standards based on the product characteristics and actual working requirements. 1.7.4 If a certain test sample is used or stored under high or low temperature conditions for a shorter time than the time it takes to reach temperature stability, such as certain aircraft and missiles, if the test is carried out by calculating the duration after the temperature reaches stability (test A and test B), it may cause excessive stress to the test sample. To avoid such excessive stress, the test can be carried out according to the low temperature and high temperature test methods specified in test A or test B, but the test duration should be determined based on the test sample's accurate simulation of the actual situation, such as using one or two times the thermal time constant value or based on its actual experience time as the test duration. 1.7.5 Although equipment with a large thermal time constant can be compared to the situation of daily temperature changes, it is usually tested according to test A and test B where the sample temperature can reach stability. However, if it is required to accurately simulate the actual environment, the test sample can be tested without reaching temperature stability, and the duration can be in accordance with the provisions of 1.7.4 of this chapter.
1.7.6 For tests where the test time is shorter than the time required for the temperature to reach stability, such as some large equipment (such as power transformers and motors with large thermal time constant values), it is required to obtain high or low temperatures in a short time. At this time, a test temperature that is higher or lower than the ambient temperature expected to be used by the equipment can be selected to accelerate the change of the test sample temperature and shorten the test time. 1.8 Air velocity
The heat exchange efficiency between the air and the test sample in the test box (room) depends on the air velocity. In high (low) temperature tests, it is expected to accurately simulate the air velocity in the actual environment, but due to the limited knowledge of the actual environment and the difficulty in providing a certain air velocity (including end flow rate, etc.) in the test box (room), this simulation is usually not possible. Therefore, it is generally necessary to conduct the test according to the "worst case" to include various possibilities. When testing non-heat dissipation test samples, a higher air velocity leads to a higher (lower for low temperature tests) test sample temperature within a certain period of time. Therefore, it is recommended to use a high air velocity (measured at no load and preferably not less than 2m/s) in the test box (room) for this test. When testing heat dissipation test specimens, if the temperature of the hottest point of the test specimen is higher than the ambient air temperature, the higher air velocity will reduce the temperature at that point. Therefore, in most cases, whenever possible, such tests should be conducted in a test chamber without forced air circulation (i.e., free air conditions). Where heating (or cooling) of the test chamber can only be achieved by air circulation, a method with forced air circulation may be used as an alternative, i.e., Method A in Test Bd (Ad) as an alternative. 1.9 Reproducibility
In order to obtain reproducibility, the temperature test must be designed so that the maximum (or minimum) temperature reached at a point on the test specimen is the same, regardless of the test chamber in which the test is conducted. In order to obtain reproducibility, the temperature-time course of the test chamber (chamber) air must be well defined during the entire conditioned test. Where accurate simulation of the actual environment is achievable, a temperature-time course may be specially designed to simulate this condition. For test designs with a duration that is relatively short relative to the time required for the test specimen to reach temperature stability, the following test temperature-time course is generally recommended (see Figure 2). Temperature change lamp
uc/min
Temperature in the box (room)
, temperature change rate
7>0.5C/min
Electrify or add load to the test sample·
Put the test sample into the temperature rise-
-specified test time
"Decrease
Figure 2 High temperature test with temperature gradient, schematic diagram of the temperature-time process of the test box and the test sample-non-heat dissipation test sample;
Heat dissipation test sample
Note: The specified test duration is calculated from the time when the air temperature of the test chamber (room) begins to reach a difference of less than 3°C from the specified test temperature. It should be noted that the temperature-time process in Figure 2 is different from Test A and Test B in the following details: a. The temperature range is narrower at the beginning (25 ± 3°C); b. The rate of change of the air temperature of the test chamber (room) during the test temperature is established; the test duration is calculated from the time when the air temperature of the test chamber (room) reaches the specified value. c.
2 Basis for the application of different test methods - heat transfer principle 2.1 Heat Convection
2.1.1 When testing in a test chamber (room), convection heat dissipation plays an extremely important role in the heat exchange of the heat dissipation test sample, especially at higher air circulation speeds.
2.1.2 The heat exchange efficiency of heat transfer from the test sample surface to the surrounding air is affected by the surrounding air circulation speed. The higher the air speed, the higher the efficiency of heat exchange. Therefore, when the ambient temperature is the same, the higher the air speed, the faster the change (increase or decrease) of the test sample surface temperature, and the shorter the time required to reach temperature stability, see Appendix B (reference). In addition to affecting the temperature of each point on the surface of the test sample, the airflow also affects the distribution of the temperature field around it. These effects of the airflow are not only related to its speed, but also have a great relationship with its direction of action. There is no simple rule to follow for the relationship between the surface temperature and temperature field distribution of the test sample and the airflow speed and direction (see Figure 3 and Appendix B). In order to simulate conditions consistent with the actual environment, a specific airflow speed must be specified for the test box (room) during the test. 2424. 1--89
and airflow direction, which will involve many problems in the design of the test chamber (room). Therefore, it is most ideal to accurately simulate the air speed and direction of the actual environment during high and low temperature tests. However, this is difficult to achieve in actual test work. Because it is very difficult to change and adjust the airflow speed and direction (including end flow, etc.) in the test chamber (room), it is usually impossible.
Figure 3 Temperature distribution around a stable heat-generating cylinder in an airflow with a speed of 0.5, 1 and 2m/s Heat dissipation per unit surface area-1.5kW/m; AT The temperature rise of the test sample surface temperature over the ambient temperature w Air speed, m/s Air temperature 70℃: Cylinder diameter = 6mm
Note: When calculating the curve, the heat conduction in the test sample is ignored. 2.1.3 When testing non-heat-dissipating test samples, a relatively high air circulation speed can accelerate the heat exchange efficiency between the test sample and the surrounding air and shorten the time to reach thermal equilibrium (temperature stability). Therefore, it is recommended to use a test box (room) with a higher air circulation speed for this test, with an average air circulation speed of 1 to 2 m/s (the former can be used for smaller test boxes, and the latter can be used for larger test rooms). When testing heat dissipation test samples, higher air speeds will affect the test of the test sample performance. Only clearly defined and reproducible test conditions can make the test results more conveniently compared with actual conditions. Therefore, in most cases, it is hoped that the air speed during the test is as low as possible. This necessity leads to the use of "free air" conditions. 2.1.4 "Free air conditions" refer to air conditions in an infinite space. At this time, the air movement around the test sample is only affected by the heat dissipation test sample itself. The energy radiated by the test sample should be completely absorbed by the surrounding air. Therefore, it is impractical to try to reproduce free air conditions in a test box (refer to Chapter 3 of this standard). Appendix A (reference) shows that the use of simulated free air conditions does not usually require the use of expensive or large test boxes (rooms). Because test chambers (rooms) simulating free air conditions have certain technical advantages and are easier to achieve than conditions with forced air circulation, test chambers (rooms) simulating free air conditions should be used preferentially when conducting low and high temperature tests on heat dissipation test samples.
According to the instructions in Chapter 3 of this standard, some difficulties may arise when testing using the method without forced air circulation. For this reason, two methods using low forced air circulation wind speed are specified for selection: the first method is suitable for test chambers (rooms) that are large enough to meet the requirements of Appendix A, but the heating or cooling of the test chambers (rooms) requires forced air circulation;
The second method is suitable for test chambers that are too small to meet the requirements of Appendix A, or for other reasons, such as when the first method cannot be used.
2.2 Thermal radiation
2.2.1 For the test chamber (room) used for the heat dissipation test sample test, especially in the high temperature test, the heat exchange by radiation cannot be ignored. 529
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For example, when the thermal radiation is black (referred to as thermal black, the radiation coefficient is close to 1) test sample and the thermal black box (chamber) wall, almost half of the heat exchange between them is through The radiation heat transfer method is shown in Figure C1 in Appendix C (Supplement)]. If the heat dissipation test sample is subjected to a certain temperature test in a test chamber (room) with hot white or hot black walls, the surface temperature of the test sample will be significantly different. Therefore, if you want to obtain a reproducible test As a result, relevant standards should limit the radiation coefficient and temperature of the test chamber wall. 2.2.2 If there are other test samples, heating or cooling elements, mounting brackets, etc. that do not meet the thermal color and temperature requirements for the box wall between the test sample and the box wall, the heat between the test sample and the box wall will Radiation will be affected. The "seeing factor" of a certain point on the test sample is determined by the percentage of the box wall that can be "seen" at that specific point. The "viewing factor" at each point on the test specimen shall not be disturbed by devices that do not comply with the requirements for thermal color and temperature of the chamber walls. 2.2.3 Under ideal "free air" conditions, the heat transferred from the test sample to the surrounding air is completely absorbed by the surrounding air. This occurs because the heat exchanged by free convection and radiation is completely absorbed. Typically most products (including equipment and components) operate in an environment that closely approximates hot black. In fact, it is easier to make the inner wall of the test chamber (room) almost thermal black than to make it thermal white. Because most paints and (unpolished) materials are closer to hot black than hot white [see Appendix G (reference). At the same time, it will be particularly difficult to keep the walls of the box (chamber) hot white for a long time due to the aging effect of the material over time.
If the temperature change of the box wall is within 3% of the specified test temperature (calculated in Kelvin temperature) (for high temperature tests), and the radiation coefficient of the box wall is between 0.7 and 1 changes, the change in surface temperature of the test sample is usually less than 3K. Because the radiation heat transfer is proportional to the difference between the fourth power of the test sample surface temperature and the fourth power of the box wall temperature, the radiation heat transfer at low temperature is less significant than at high temperature, so the thermal color of the box wall during the low temperature test is And the temperature requirements are not very strict. 2.2.4 Heat exchange through thermal radiation mainly depends on the temperature of the test chamber (chamber) wall. This dependence is why when the difference between the surface temperature of the test sample and the ambient temperature is large, the test is not carried out in accordance with Appendix C. The main reason why strong pursuit of air circulation cannot be used to perform the test is to correct the temperature of the test sample (including correction for convection and radiation effects). 2.3 Thermal conduction
2.3.1 Heat exchange through thermal conduction depends on the thermal characteristics of the mounting bracket and other connections connected to the test sample. 2.3.2 There are many heat dissipation equipment and components, which are required to be installed on heat-absorbing or other devices with good heat transfer when used. At this time, a certain amount of heat will be effectively dissipated through thermal conduction, so the relevant standards should stipulate the thermal characteristics of the mounting frame, and these thermal characteristics of the mounting frame should be reproduced when testing.
2.3.3 If equipment or components can be mounted in multiple ways using mounting brackets with different thermal conductivity values, the worst case scenario should be considered during the test. Different application situations have different worst-case scenarios: a. For high-temperature tests on heat dissipation test samples, because the heat is transmitted from the test sample to the mounting bracket during the test, the worst-case scenario is at the mounting bracket. The amount of heat transfer is as small as possible, that is when the thermal conductivity of the mounting frame is small (thermal insulation). b. For high-temperature tests on non-heat dissipation test samples, when the test sample has not yet reached thermal equilibrium (thermal stability) with the environment, heat is transmitted from the box wall to the test sample through the mounting bracket. At this time, the worst case is when the thermal conductivity of the mounting frame is large, so the heat capacity (or thermal time constant) of the mounting frame should be small to avoid the mounting frame heating up for too long and delaying the flow from the box wall to the test sample. heat transfer. C． For low-temperature tests of heat dissipation test samples and non-heat dissipation test samples, heat is transmitted from the test sample to the box wall through the mounting bracket during the test. The worst case scenario (the lowest temperature of the test sample) is when the heat transfer efficiency is high, that is, when the mounting bracket When the thermal conductivity is high. 2.4 Forced air circulation
2.4.1 The test chamber (chamber) is large enough to comply with the requirements of Appendix A, but forced air circulation may be required for heating and cooling within the chamber (chamber).
In this case, the test sample should first be placed in a test chamber with room temperature and inspected with and without forced air circulation, so that the temperatures of the representative points on the surface of the test sample will not be excessively affected by the chamber. The influence of internal strong chasing air circulation speed. As a result, if the surface temperature at any point on the test sample does not decrease by more than 5°C after adding forced air circulation, the cooling effect of the forced air circulation is considered to be reasonably small, which is the same as the test conducted in the test chamber without forced air circulation. Negligible. 2.4.2 If the test chamber is too small compared to the test sample and cannot meet the test requirements of Appendix A, or it may be used in industries with or without 530
in accordance with Article 2.4.1 of this standard. Data Width
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When the surface temperature difference measured during forced air circulation exceeds 5℃, an exploratory test should be conducted outside the test chamber: first place the test sample In the test chamber (outside the test chamber), apply the load specified by the relevant standards for the test conditions, and measure the temperature of some representative points on the surface of the test sample to provide the basis for calculating the surface temperature at the specified test temperature. For a small temperature difference ΔT between the ambient temperature and the surface temperature, as long as the change in ambient temperature AT is small, it can be assumed that the temperature difference AT is the same at different ambient temperatures. If △T1<25℃ and AT<30℃, the error is within 3℃. At different ambient temperatures, the relationship between the surface temperature of the test sample and its heat dissipation power per unit time is shown in Appendix C. If the surface temperature at a certain ambient temperature is known, the surface temperature at any other ambient temperature can be calculated using the calculation chart in Appendix C. In this way, when the surface temperature of the test sample at room temperature is known, calculate through Appendix C By using the diagram, the surface temperature range under specified test conditions can be expanded and calculated. The calculation diagram in Appendix C can be used at least when AT, =80℃ and AT265℃. 2.4.3 When using any of the first and second methods in Article 2.1.4 of this standard to select representative points for inspection, the test sample (such as temperature distribution, thermal limit points, etc.) should be understood in detail, and the test The selection of representative points on a sample is primarily a matter of skilled judgment. Since the test without forced air circulation has higher reproducibility, it is recommended to give priority to the test method without forced air circulation when conducting type (qualification) testing.
For exploratory testing, it may be necessary to use a series of similar tests (such as similar components) to check the performance of the test chamber, while in other cases (such as different equipment), a series of similar tests (such as similar components) may be needed. The test chamber needs to be evaluated before proceeding. 3 Test chamber (chamber)
3.1 General requirements
3.1.1 During the test, it is impractical to reproduce free air conditions in the test chamber, but it is still possible to simulate the effects of free air conditions .
Even in large test chambers, the air circulation and temperature distribution around the test specimen are not equivalent to those under actual free air conditions. Nonetheless, experimental results and test experience indicate that a relatively large test chamber without forced air circulation affects the temperature of the test specimen in much the same way as free air conditions. See Appendix A for the test box (chamber) size requirements related to the test sample size and heat dissipation required to simulate the effects of free air conditions.
The air in the lower half of the test chamber (room) is not greatly affected by the heat convection from the test sample, so the above requirements can be met when monitoring the ambient temperature there. In some cases, however, difficulties may arise when conducting tests without forced air circulation. In most existing test chambers, uniform heating or cooling of the chamber (chamber) cannot be achieved without forced air circulation, especially when testing large test samples or testing many products at the same time in the same test chamber (chamber). More so now. 3.1.2 Some parameters of the test chamber (chamber) that have a significant impact on the test results of the heat dissipation test sample are as follows. Heat transfer mechanism
Test chamber parameters
Free air
Dimensions, air temperature
3.2 Method flow of test chamber (chamber) to meet the requirements of test parts||tt| |Strong pursuit of air circulation
Air speed: air temperature
3.2.1. Design of test box (room) simulating the effect of free air conditions. Radiation
Box wall temperature: Box wall radiation
Emission coefficient; See the visual factor
Thermal characteristics of the mounting frame
Components for heating and cooling the box (room) body should not be placed in the working space, because the control of the temperature in the box (room) depends on the temperature change of these components, otherwise it will cause large fluctuations in the temperature in the working space. At the same time, the temperature of the box (room) wall should also avoid large fluctuations to minimize the radiation effect. In order to obtain the best results, all the walls of the test box (room) body should be heated or cooled. Using liquid circulation to heat or cool all the box (room) walls is a suitable method to prevent large fluctuations in the box (room) wall temperature. The radiation coefficient of the box (room) wall should meet the test requirements.
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For test chambers (rooms) that rely on air circulation to maintain the test temperature, the test sample can be placed in a box during the test, and then the box can be placed in the test chamber (room) for testing. At this time, the volume of the box should meet the test size requirements, and the box wall should meet the requirements of the emissivity.
3.2.2 Design of test chambers (rooms) with forced air circulation For test samples that cannot be tested in a free air condition test chamber due to large size or high heat dissipation, a test chamber (room) with air flow (with forced air circulation) should be used. Except for the test chamber (room) dimensions, all other requirements for free air test chambers (rooms) are applicable to the design of forced air circulation test chambers (rooms). The air flow rate should meet the following requirements: it should not be too small to ensure that the test sample does not overheat during the test, and it should not be too large to cause the test sample to be overcooled during the test. The effect of airflow is described in more detail in Appendix B. In practice, for non-heat dissipation test samples, it is better to have a higher airflow speed, with an average wind speed of 1 to 2 m/s. When the test box (chamber) is unloaded, the wind speed can be greater than 2 m/s; for heat dissipation test samples, it is better to have a lower airflow speed, generally less than 2 m/s. Therefore, within the scope of technical and economic permission, the airflow speed of the test box (chamber) can also be designed to be adjustable, which can expand its application range. However, although it is advantageous to change the airflow speed smoothly, it has been found in practice that a wind speed of 0.5 m/s represents a good compromise.
The airflow should be as uniform as possible and the airflow direction should be vertically upward to minimize the airflow changes caused by convection from above. If the fan generates positive pressure in the front box (chamber), the air can escape from the front box (chamber) through a filter (such as a glass fiber grid), so that a uniform airflow can be obtained. A heater to control the temperature of the box (room) can also be installed in the front box (room), or a mesh heater can be used instead, combining the heater and the filter together
3.3 Emissivity of the box (room) wall
If the free air condition of an infinite space is to be simulated, the box wall should be thermally black. Table G2 in Appendix G lists the emissivity values of some materials. It can be seen from the table that materials with an emissivity of more than 0.7 are easy to obtain. For the treatment of the walls of the test box (room) operating at medium temperatures, most matte coatings can fully meet the requirements. 3.4 Thermal characteristics of the mounting frame
For the requirements on the thermal characteristics of the mounting frame, please refer to Article 2.3 of this standard. The thermal conductivity of various materials can be seen in Table D1 of Appendix D (reference). The characteristics of the influence of the material and size of some component wire ends on the surface temperature can be seen in Figure D1 of Appendix D. If the thermal conductivity of the mounting frame or the connecting wire (such as the lead wire) has a significant impact on the test results, the thickness (diameter or area) and length of the lead wire should be fixed in all tests, or in accordance with the relevant standards.
4 Measurement
4.1 Temperature
The results of the heat dissipation test samples under non-free air conditions indicate that it is necessary to measure the temperature at different points on (or in) the test sample. Www.bzxZ.net
Temperature measurement is the most general and common measurement. However, in order to obtain the high accuracy required by the test, the corresponding measuring instruments and measurement methods should be specified in the relevant standards.
The methods for measuring temperature include: direct measurement method (such as thermometer method, color change effect or melting effect method, etc.) and indirect measurement method (such as thermocouple method, resistance method, infrared sensor method, etc.). For the selection of temperature measurement methods, please refer to Appendix E (reference). 4.2 Air velocity
Knowing the air velocity in the test chamber (room) is not the main factor for the test specification, but it is still very useful. For example, when testing multiple test samples in a test chamber (room) with forced air circulation, a certain air velocity is required to ensure the uniformity of the conditions in the chamber (room). The air velocity can be measured by cup anemometer, catarrhal thermometer, hot bulb anemometer and hot wire anemometer. For the selection suggestions for their use, please refer to Appendix F (reference).
4.3 Emissivity
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When testing heat dissipation test samples, especially for heat dissipation test samples with high heating power or high test temperature, radiation heat transfer occupies a very important position. At this time, the emissivity of the box wall should be measured and should be regularly checked. Check. For suggestions on the measurement of the emissivity, see Appendix G. 533
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Appendix A
The influence of box size on the surface temperature of the test sample during the test without forced air circulation
(reference)
A1 The results of a series of tests conducted to determine the minimum allowable size of the high-temperature test chamber (room) are shown in Figure A1. In this test chamber, for a certain test sample, its surface temperature is close to the value under "free air" conditions. When test samples of different sizes and different unit surface heat dissipation are subjected to high temperature tests with an ambient temperature (according to the relevant provisions in GB2422) of 70°C in high-temperature test chambers of different sizes, the qualified condition for the test chamber used is: the surface temperature of the test sample measured in this test chamber is the same as The deviation of the surface temperature measured in the largest test chamber, which is much larger than the size of the test sample, should not be greater than 5°C, and the difference between the chamber wall temperature and the test temperature should not exceed 5°C.
The test sample is cubic and almost thermally white (emissivity is close to 0), giving the worst case where almost all heat is dissipated by convection, and the test chamber walls are close to thermally black (emissivity is close to 1). W/m
d=7.2od=3.7
The test should be carried out with
d>20cm in the upper part of the curve
The test should be carried out with
d>10cm in the lower part of the curve
Test sample volume m*×10
Figure A1 Test of heat dissipation per unit surface area when the difference between the surface temperatures of the test sample in the large test chamber and the small test chamber reaches 5°C Data; d The distance between the test sample surface and the box wall.cm Appendix B
The influence of airflow on the test box conditions and the surface temperature of the test sample (reference)
B1 Calculation of the influence of airflow on the test sample temperature During stable heating, the heat dissipation P on the surface of the heating element is calculated by formula (B1): P=A()FT
Wherein: P The heat dissipated from the surface of the heating element to the surrounding medium per unit time, W; F-the heat dissipation surface area of the heating element, m\;
T-the stable temperature rise of the heating element, K;
(Y) Heat dissipation coefficient, W/(m2·K)
From formula (B1), the stable temperature rise of the heating element is obtained: X()
Wherein (Y) is a complex parameter, which is related to many factors, mainly related to the shape, position and airflow conditions of the heat dissipation surface of the heating element, and should be determined by experiments based on actual conditions. Under the test conditions of Figure B1, the relevant test results show that the heat dissipation coefficient and the air flow velocity have the following relationship:
Wherein: v---air flow velocity, m/s;
GB 2424.1-89
a(r)--a+bu
value related to air flow velocity, α10; 6--value related to air flow velocity. The test results show that when the air flow velocity is low, 6~3, the b value increases with the increase of air flow velocity. When the air flow velocity is 3m/s, b~8. According to the test results of Figure B1, there is the following relationship: ub
is equal to 0.When the speed is 3m/s, the error of the stable temperature rise T obtained by applying the above relationship is equal to or less than 10%. Some test results of electrical product test samples B2
Due to the complex structure and shape of electrical products and different powers, it is difficult to find a unified relationship for the effect of air flow velocity on their heat dissipation. When some electrical products recover after low temperature testing, changing the air flow velocity can obtain the effect of air flow velocity on the "heating" of the test sample, and obtain its relationship with the thermal time constant of the test sample, as shown in Figure B2. B3 The temperature difference between the inlet and outlet of the test chamber should keep the air pressure in the test chamber unchanged, and the inlet and outlet volumes should be equal. The temperature change between the inlet and outlet is the heat taken away by the air per unit time:
AT=CGCpSur
Where: P—the heat dissipated from the surface of the heating element to the surrounding medium per unit time, W; C.-the specific heat capacity of air at constant pressure, 1o00J/(kg·K); G the inlet or outlet volume per unit time, kg/s; S-—the cross-sectional area of the test chamber, m\;
the average air flow velocity in the test chamber, m/s; air density, 1.29kg/m2.
For a cubic test chamber with an air flow velocity of 0.3m/s, a heat dissipation power of 100W per unit time in the chamber, and an inner side length of 0.5m, substitute into formula (B3) to obtain:
AT1000×0. 5080.3X1.2~1C
The temperature change of the inlet and outlet air is only 1℃. This shows that for test samples with heat dissipation power equal to or less than 100W, it is feasible to use a cubic test box with an inner side length of 0.5m to test. For test samples with higher power (1kW), if the inlet and outlet air temperature change value is to be maintained at 1℃, a larger test box (such as a cubic box with each side of 1.5m) must be used for testing. Otherwise, to maintain the allowable temperature difference, a higher air flow rate must be used. 685
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GB 2424.1—89
Actual size of transverse wound resistors
Radial airflow
Axial airflow
Air flow velocity m/s
Measured data on the effect of air flow velocity on the surface temperature of wound resistors Radial airflow;． Axial airflow
5 Wind speed m/s
Figure B2 Measured data of the relationship between the thermal time constant of the test sample and the air flow velocity 1--CJ0-75 coil (resistance method); 2-QC810-60 contactor coil surface: 3.·Z2-41 DC motor magnetic field winding (resistance method); 4--Z2-11 DC motor magnetic field paper winding (resistance method): 5CZKN2-2 Internal combination switch terminal surface Appendix C
Heat exchange calculation and calculation diagram
(Supplement)
C1 Symbol explanation
Heat transferred by the test sample per unit time, W; A2
Test sample surface area, m;
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