【 Weak current College 】 intermediate relay reliability and application analysis 】

Summary: Describes the intermediate relay selection is effectively improve control system operation reliability of critical issues. In combination with the control system of work performance and work environment and relay product characteristics, deals with the control system of intermediate relay selection principles and application of intermediate relay questions to analysis.

Keywords: intermediate relay; reliability analysis;

1 Overview

As China's industrial production process automation level unceasing enhancement, the production process on the work of the automatic control system for higher reliability requirements. The reliability of its work not only has a direct impact on production efficiency and product quality, and directly related to production equipment and the safety of the operator. In the automatic control system uses a large number of intermediate relay, the reliability of its work on the reliability of control systems is essential. How to properly select, rational use of intermediate relay, is the control system, reliable working, control system design is also an important component. To enhance small intermediate relay inherent reliability, enhanced "small intermediate relay" design, manufacture, screening, under the control of system reliability requirements of the principles to all kinds of possible simulation of control system of the actual operating conditions, to development and develop a new generation of intermediate relay product, is the intermediate relay manufacturer before pressing issue.

2 selection of intermediate relay

Faced with complicated modern relays product, how to properly use a reasonable choice, is the control system development, designers pay close attention to and must be the actual problem solving optimization. To achieve a reasonable choice, correct use, must be fully research analysis of the control system of the actual use conditions and the actual technical parameters required, in accordance with the principle of the "value engineering", aptly put winning relay products must meet the technical performance requirements. In particular, can be broadly grouped under the following key elements of each analysis, confirm that the requirements of level and scope of the measure.

2.1 climate stress factor

Mainly refers to the temperature, humidity, atmospheric pressure (altitude), coastal atmosphere (salt spray corrosion), sand and dust pollution, chemical atmosphere and electromagnetic interference, and other elements. Consider control system throughout the industry and the general applicability of the natural environment, taking into account the need for the reliable operation of all special device key parts must use a high insulation, strong electrical performance of the entire seal type (metal cover seal or plastic, metal cover seal products better than shrink-wrapped products) intermediate relay product. Because only sealed relay have excellent long-term tolerability harsh environment performance, good electrical contact stability, reliability, and switching load capacity (from external climate environmental impact).

2.2 mechanical stress factor

Mainly refers to vibration, impact, collision, and other stress factors. On the control system is mainly to take into account the effects of seismic stress resistance, resistance to mechanical stress, to choose a balanced armature institutions small intermediate relay.

2.3 incentive coil input parameter elements

Mainly refers to the overdriven, less incentive, low motivation and high-voltage output isolation, temperature change, long-distance cable motivation, incentives, and other parameters of electromagnetic interference, which ensure the reliable operation of control systems must be carefully taken into account. According to the provisions of intermediate relay energizing motivation is to ensure that it is reliable and stable work.

2.4 contact output (exchanging circuit) parameter elements

Mainly refers to the contact load nature, such as lamp load, capacitive load, motor load, inductors, Helix, contactor (relay) coils, chokes, resistance load, etc.; contact load value (open-circuit voltage measurements, closed-circuit current values), such as low level loads, dry circuit load, small current load, high current loads, etc.

According to the relay driver device load, load capacity of the choice of suitable relay, is the reliable operation of the relay. Relay failure or reliable unreliable, main means of contact can complete the switch circuit function. If the switch is greater than the actual load capacity of the choice of relay switching load capacity, the relay is not possible and reliable work.

3 intermediate relay application problem analysis

3.1 insulation voltage

Unsealed or sealed relay's leadout exposed insulator long-term gas pollution by dust, causes it to drop, insulation strength in switching AC inductive load time of peak voltage, failure caused by insulation breakdown. The inherent characteristics of insulation for relay, selection must be based on the following technical characteristics of relay:

3.1.1 enough Creepage distance: General requirements ≥ 3mm

3.1.2 enough insulation strength: no electrical connection between conductor ≥ AC2000V (work AC220V), the same set of contacts between ≥ AC1000V;

3.1.3 sufficient load capacity: 5 ~ DC220V sensibility, 40ms, ≥ 50W;

3.1.4 long-term ability to withstand climatic stress: coil anti-mildew resistance broken, insulation level of long-term stable and reliable.

3.2 sealed relay and non-sealed relay

Part of the engineering and technical personnel considered unsealed product action state intuitive, failure analysis, and sealed product action process see eludes, both advantages and disadvantages as follows:

The advantage of non-sealed relay are: the use of bat-type armature,structure simple, the manufacturing process is simple, easy installation and maintenance, work status and intuitive, easy-to-failure analysis and cheap. The main disadvantage is: work reliability on the use of environmental (climatic stress, mechanical stress) change of sensitivity; coil easy wet gas, contamination produced electric corrosion, mildew, etc.

Fully sealed relay advantage: the use of balanced armature rotating type, sealed structure isolation external climatic stresses, rugged good performance; electrical contact performance stable reliable, corrosion resistance of the coilMoldy, long-term reliability, excellent. Disadvantage: the structure of complex, special manufacturing processes, failure analysis difficult, itself cannot be reused, costs for maintenance, prices are high.

Therefore, from a long-term resistance to climatic stress performance, rugged performance and electrical contact reliability considerations, fully sealed relay outgo non-sealed relay. High reliability requirements for aerospace, aviation, military series, the main selection of the shroud sealed relay product. In view of the production process automation control system requires long-term, stable and reliable work of particularity, ought to use sealed relay products.

3.3 load on-contact

Relay contact failure is at the core of relay failure, when the actual load switch contact voltage is less than arc voltage, electric current is less than lA, especially secondary currents (testing standards for DC28V, 0.1A), low level (10 ~ 30mV, 10 ~ 50mA) or dry circuit (relay contact first closed, plug in the MV µA-load), contact the hours actually worked, the failure mode of failure mechanism and the actual switch rated power load completely different. It is designed to meet the different requirements of different load, different products in the design, manufacture, testing, testing requirements vary. Therefore, the actual selection relay products, must not be mistakenbelief: relay contact switch applies to the from zero to provide all the load rating, the more you cannot view the actual through-contact product standard load ratio of rated load smaller and more reliably. For example: reliable switching 220V, 10A load contacts and may not be able to reliably switch 10mA, the actual load is unavailable it to exchanging low level or dry circuit load. Thus, the medium-current, low level, it is proposed to adopt dry circuit load contact reliability fine metal cover seal products.

3.4 capacitance load

Relay contacts as switch capacitive load circuit of self-insurance, easily cause contact adhesive and cannot release, which is due to the capacitors in the charging process is similar to a capacitor resistance spot welding process. Further analysis revealed: μ to 22 F capacitor adequate DC220V voltage, and encouraging the direct contact of relay circuit discharge, 10-, silver-contact can be produced without welding.

In theory, the capacitor discharge current

i=-(UR)e-t/τ

-U — is the capacitor voltage;

R-loop resistance for discharge;

The time constant τ-;

T — to discharge time.

Since R is about contacts and the contact resistance, which is close to zero. I start to discharge moment U/R ≈ → ∞, in other words: all of the storage capacitor energy, in a very short time all bleed through contacts, which directly led to the failure of spot welding.

Therefore, the length of transmission lines, to eliminate electromagnetic interference filters, power supplies, etc. are all strong capacitive. For this type of load relay selection should be considered in conjunction with the device characteristics.

3.5 incentive series power supply

Many user-volt power supply to the relay coil, driving imposed energizing relay. This incentive is generally not desirable. Pick-up time for relay depends primarily on the circuit's time constant τ, and τ = L/R. When an in-line resistorR1 when power to the relay coil, R = R1 + R2, R2 > L/L/(R1 + R2); obviously, in-line after the τ decrease R1, relay for pick-up time acceleration. Especially when R2 R1 >>, voltage is high, pick-up time will be significantly reduced. Moving parts of the fast movements, will increase the moving parts of shock when engaged, collision, rebound, thereby increasing the contacts back to jump, speed mechanical wear, reduces the load capacity and the contacts of the mechanical life. Therefore, in-line power excitation mode changes the relay original design by normal working condition, is generally not desirable. When the contacts on the back, the mechanical wear on the actual use does not constitute an interest, and in particular need to accelerate action speed can only be used to improve the excitation voltage or power incentive series resistance.

3.6 relay coil series with the power to motivate DC220V, this incentive parties must carefully consider.

3.6.1 on the same type, the same specification relay products, because the coil resistance (including DC resistance and instantaneous gankang) largely the same, the difference is small, in-line pressure stimulation modes use is not a big problem. Practice has proven are also possible.

3.6.2 on different type or different specifications of relay products, since different relay coil impedance is inconsistent, and the difference with instantaneous inductive reactance of vary greatly, so the series excitation instant relay coil on the share of excitation voltage (instantaneous pressure ratio) is a big difference will inevitably have the relay is in a State of overvoltage, motivate others in the incentive, undervoltage relay contacts for switching timing and speed of the essential changes will occur, will appear before and after an action, fast and slow reverse, switch is not reliable.

Therefore, the different types and different sizes of relay coil unsuitable for use in-line pressure stimulation modes.

3.7 relay coil parallel use

In the complex control loop, only the 2 (or more) different types of relays (e.g. contactor relays K1, small sensitive K2) coil parallel use of time, in this case, it is possible to produce K1 delayed release, contact broken arc decline, K2 is reverse repeat incentives, contact malfunction, etc.

In DC control circuit, K1, K2 coils storage of magnetic energy may vary greatly. When the coil power outage, K1 (magnetic energy large) of storage through K2 (magnetic energy small) coil relief, reverse current, resulting in the release time stretch, K1 contact broken arc speed slow contacts arcing time betweenExtended between; K2 release time is short, then be the reverse discharge current, even after release of incentive instantaneous repeat pick-up, produces incorrect operation breakdown. Avoid these studies due to neglect leads to unreliable.