Analysis on the selection and application of the h

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Brief analysis on the selection and application of relays

Abstract: This paper expounds that the application and selection of relays is the key to effectively improve the reliability of system operation. Combined with the working characteristics of the system, the working environment and the characteristics of relay products, this paper discusses the selection principles of relay selection, the selection of quality grade in relay application and the principle of relay derating, The installation and protection technology of relay and the problems in the application of relay are analyzed

1. Overview

in the face of complex relay products, how to choose and use them properly is a practical problem that system developers and designers pay close attention to and must give priority to. To achieve reasonable selection and correct use, we must fully study the actual use conditions and actual technical parameter requirements of the analysis system, and appropriately put forward the technical performance requirements that the selected relay products must meet according to the "value engineering principle". In the reliability design of the whole machine, it is required to select components reasonably. The selection and control of components and parts is a task that requires multidisciplinary knowledge, which should generally be completed by component engineers, reliability designers, overall and circuit designers, and failure analysts. First of all, it should be comprehensively considered and selected according to the importance of the whole system, reliability requirements, environmental conditions and costs. Specifically, it can be analyzed and studied item by item according to the following elements to confirm the required level and quantity range. When choosing, we must pay attention to the following requirements

2. Selection of environmental conditions for use

climatic stress factors mainly refer to temperature, humidity, atmospheric pressure (altitude), coastal atmosphere (salt spray corrosion), sand dust pollution, chemical gas, electromagnetic interference and other factors. Considering the general applicability of the system in various industries and natural environment all over the country, and taking into account the particularity that it must operate reliably for many years, the key parts of the system must choose fully sealed relay products (metal cover sealed or plastic sealed, metal cover sealed products are better than plastic sealed products) with high insulation and strong electrical resistance. Because only the fully sealed relay has excellent long-term resistance to harsh environment performance, good electrical contact stability, reliability and stable load switching ability (not affected by the external climate environment)

2.1 effect of temperature on relay

relay is a thermal element. High temperature can accelerate the aging of plastic and insulating materials inside the relay, oxidation and corrosion of contacts, difficulty in arc extinguishing, deterioration of electrical parameters, and reduce reliability. Therefore, it is required that the relay should not be close to the heating element during design, and there should be good ventilation and heat dissipation conditions

although the relay is a thermal element, it cannot be ignored for too low temperature (such as military aviation conditions -55 ℃). Low temperature can aggravate the cold adhesion of the contact, expose the contact surface, and produce ice film on the armature surface, which makes the contact unable to switch normally, especially for low-power relays. The test proves whether the voltage of the nixie tube is normal? The maintenance method can refer to the nixie tube maintenance section in Chapter 1. For some domestic low-power relays produced according to the ministry standard, although the low temperature is specified as - 55 ℃, in fact, the relay cannot be converted normally under this condition. It is recommended to leave sufficient margin in the selection. For important military electronic machines, it is recommended to choose national military standard products

2.2 effect of low air pressure on relay

under the condition of low air pressure, the heat dissipation condition of the relay becomes worse, and the coil temperature rises, which changes the pull in and release parameters given by the relay, affecting the normal operation of the relay; Low air pressure can also reduce the insulation resistance of the relay, make it difficult for the contact to extinguish arc, and easily make the contact burn, affecting the reliability of the relay. It is suggested that the whole machine should be sealed for the severe conditions

2.3 influence of mechanical stress on relay

mainly refers to stress action elements such as vibration, impact and collision. The main consideration of the control system is the ability to resist seismic stress and mechanical stress, and the small intermediate relay with balanced armature mechanism should be selected. The reed of the electromagnetic relay is a cantilever structure, with low natural frequency. Vibration and impact can cause resonance, resulting in the drop of the relay contact pressure, which is easy to cause instantaneous disconnection or contact jitter. In serious cases, it can cause structural damage, and the movable armature part can cause malfunction, affecting the reliability of the relay. It is suggested that anti vibration measures should be taken in the design to prevent resonance

according to the influence of the above environmental conditions on the relay, when selecting the relay, the crystallization speed of the first akulon XS in the membrane bubble is much slower. First, make the relay meet the requirements of various environmental conditions specified by the whole machine. If the environmental conditions cannot be fully considered, the developed complete machine will not meet the technical requirements specified in the contract. For example, if jrc-5m small electromagnetic relay is selected for military airborne electronic equipment, only paying attention to the ambient temperature can meet the requirements of the whole machine and ignoring the vibration and shock conditions. If these conditions cannot be met, corresponding preventive measures must be taken, otherwise the reliability cannot be guaranteed

2.4 insulation withstand voltage

the exposed insulator at the outgoing end of the unsealed or sealed relay has been polluted by dust and moisture for a long time, resulting in the decline of its insulation strength, and the insulation breakdown failure is caused by the overvoltage when switching inductive loads. According to the inherent characteristics of relay insulation, the selection must be based on the following technical characteristics of the relay:

2.4.1 sufficient creepage distance: General requirements>3 mm (working AC 220V)

2.4.2 sufficient insulation strength: between conductors without electrical connection>ac 2000V (working AC 220V), between contacts in the same group>ac 1000V

2.4.3 sufficient load capacity: DC 220 V inductive; 5~40 ms,>50W;

2.4.4 long term ability to withstand climatic stress: the mold breaking resistance of the coil and the electrical resistance level of the insulation are stable and reliable for a long time

2.5 sealed relay and non sealed relay

some engineering and technical personnel believe that the action state of non sealed products is intuitive and convenient for failure analysis, while the action process of sealed products is invisible and opaque. The advantages and disadvantages of both are as follows:

the advantages of non sealed relay are that it adopts multiple snap type armature, which has simple structure, simple manufacturing process, convenient installation and maintenance, intuitive working state, convenient for failure analysis, and low price. The main disadvantage is that the working reliability is highly sensitive to the changes of the use environment (climatic stress, mechanical stress); The performance of long-term tolerance to climatic conditions is vulnerable to environmental pollution and damage with time; Poor electrical contact stability and reliability; The coil is susceptible to electric corrosion and mildew caused by moisture and impurity pollution

the advantages of sealed relays are that balanced rotating armature is often used, and the fully sealed structure isolates the effect of external climate stress, with excellent performance against harsh environment; The electrical contact performance of the contact is stable and reliable, the coil is resistant to corrosion and mildew, and the long-term reliability is excellent. The disadvantages are complex structure, special manufacturing process, difficult failure analysis, unable to repair and reuse itself, and high cost and price

therefore, considering the long-term weather stress resistance, harsh environment resistance and electrical contact stability and reliability, sealed relays are significantly better than non sealed relays. For aerospace, aviation and military series with high reliability requirements, metal cover sealed relay products are mainly selected. For the production process automation control system in important occasions, the particularity of requiring long-term stable and reliable work should be based on the selection of sealed relay products

3. Excitation coil input parameters

mainly refer to overexcitation, underexcitation, isolation between low-voltage excitation and high-voltage (220V) output, influence of temperature change, long-distance wired excitation, electromagnetic interference and other factors, which must be carefully considered to ensure the reliable operation of the system. Excitation according to the specified excitation amount of the relay is a necessary condition to ensure its reliable and stable operation

the technical conditions of the relay generally give the working voltage, pull-in voltage and release voltage to the coil voltage. To ensure the normal operation of the relay, when the circuit is connected, it must be ensured that the given three voltages meet the values specified in the technical conditions in any case. Otherwise, the relay cannot be converted normally

3.1 about the series power supply excitation mode

many users use the series partial voltage power supply mode to apply excitation to the relay coil to drive the relay action. This kind of incentive is generally undesirable. Because the pull in time of the relay mainly depends on the time constant T of the circuit, and T = l/r. When the series resistance R1 supplies power to the relay coil, r=r1+ R2, then l/r2>l/(r1+r2); Obviously, after connecting R1 in series, t decreases and the pull in time of the relay accelerates. Especially when R1 ≥ R2 and the voltage is very high, the pull in time will be greatly reduced. The too fast action of the moving parts will increase the impact, collision and rebound when the moving parts are connected, so as to increase the contact bounce, accelerate the mechanical wear, and reduce the load capacity and mechanical life of the contact. Therefore, the series power supply excitation mode changes the normal working state specified by the original design of the relay, which is generally undesirable. When the contact bounce and mechanical wear do not pose a stake to the actual use, and it is particularly necessary to speed up the action speed, the excitation voltage can be increased or the series resistance power supply excitation mode can be adopted

3.2 relay line is the use of series connection of Hongwang group's important regional layout circle in North China

after multiple relay coils are connected in series, DC 220V power supply is used for excitation. This excitation method must be used with caution

3.2.1 for relay products of the same type and specification, the impedance (including DC resistance and instantaneous reactance) of each coil is roughly the same, and the difference is small, so it is not a problem to use the series partial voltage excitation method. Practice has proved that it is also feasible

3.2.2 for relay products of different types or specifications, because the impedance of different relay coils is inconsistent, and the difference is very different with the instantaneous reactance, so at the moment of series excitation, the difference of excitation voltage (determined by the instantaneous partial voltage ratio) distributed on each relay coil must be very large, and some relays are bound to be in overvoltage excitation state, while others are in undervoltage excitation state, The switching sequence and speed of each relay contact will change substantially, and it is inevitable that the action will be reversed first, then, fast and slow, and the switch is unreliable. Therefore, different types and specifications of relay coils should not be excited by series voltage division

3.3 about the parallel use of relay coils

in complex control circuits, it often happens that two (or more) coils of different types of relays (such as contactor K1 and small sensitive relay K2) are used in parallel. In this case, there may be practical problems such as delayed release of K1, decrease of contact arc breaking capacity, reverse repeated excitation of K2, contact misoperation and so on. In the DC control circuit, the magnetic energy stored in K1 and K2 coils may vary greatly. When the coil power supply loses power, the stored energy of K1 (high magnetic energy) will be discharged through the coil of K2 (low magnetic energy), generating reverse current. As a result, the release time of K1 is prolonged, the arc breaking speed of contacts is slow, and the arc burning time between contacts is prolonged; The release time of K2 is short, and then it is excited by the reverse discharge current, and even repeatedly pulls in instantly after release, resulting in malfunction. In practical application, attention should be paid to avoid the above unreliability caused by neglect of research

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