General Analysis of power relay Contacts

Above, we start from the different working states of the power relay contacts, and focus on analyzing its contradictory essence in each working stage. However, the power relay contact itself is a whole, and the tasks specified in each stage are completed by the same power relay contact. Moreover, the characteristics of the different stages of work in the same whole are interconnected, influenced and contradicted each other. For example, the welding that may occur when the power relay contacts are working in the closed state will cause it to fail to achieve the disconnection process, so it is not possible to break the circuit; and the arc or spark that occurs when the power relay contacts open and close the circuit. The destructive effect will cause it to fail to conduct the circuit normally and reliably in the closed state. For another example, the same power relay contact material often cannot meet the requirements of good electrical conductivity in the closed state and strong plastic interference during the disconnection process (for example, silver has low resistivity and good electrical conductivity, but its baking point is low. , poor anti-isolation performance). Another example, from the point of view of the advantage of short arc and reliable insulation, it is always better to have a larger distance between the contacts of the plastic power relay and less bounce when closing. Therefore, how to solve the requirements put forward by each working stage and the hand shields between them from the overall power relay contact and relatively unify it has become the main subject to be solved when studying the overall power relay contact.

1. The overall structure, material and process of the power relay contact

In the overall structure of the power relay contact, the main point is how to unify the main measures taken under various working conditions. Shown is a typical structure of several common pocket points, which basically achieve this unification. They all consist of three parts: 1. Dynamic power relay contacts. 2. Static power relay contacts. 3. Springs or reeds. The former two are used to switch circuits, and the latter are used to generate a certain pressure on the power relay contacts. When such a power relay contact structure works in different states, the main measures and parameters required to be guaranteed in different working stages can be realized. In order to give you a more comprehensive understanding of power relay contacts, we summarize these parameters and their functions as follows:

(1) Spacing: ensure reliable arc extinguishing; ensure reliable insulation; ensure that when the gap is reduced for some reason (collision, vibration impact, power relay contact deformation, etc.), it will not cause circuit conduction or electrical breakdown to cause malfunction.

(2) Initial pressure (f) :or power relay contact reed stiffness: In bridge-type (or high-power finger-type) power relay contacts, the initial pressure is generated by the power relay contact spring. It is pre-compressed for a section during installation, so there is a certain pressure on the power relay contact cross-arm. When the contact of the moving power relay is in contact with the contact of the static power relay and the cross arm is just separated from the tie rod, the pre-pressure of the spring is immediately added to the contact of the moving power relay. This force is the initial pressure. When the power relay contact reed is used, the initial pressure is equal to zero, but it has a certain rigidity and can also reduce the bounce and bounce time.

(3) Overtravel : also known as overtravel, that is, starting from the position where the contacts of the static and dynamic power relays have just contacted, if the contacts of the static power relays are removed at this time, the distance that the contacts of the dynamic power relays can continue to move forward. Its main function is to move forward. To: obtain a certain final pressure to compensate for the power relay contact

The wear causes the contacts of the static and dynamic power relays to rub against each other within this distance (existing only in the contacts of the finger power relay) to facilitate the automatic removal of the surface film.

(4) Final pressure (F): It is a final pressure generated by the power relay contact spring or reed due to the continuous deformation of the power relay contact spring or reed after the final closing of the dynamic and static power relay contacts, ensuring that the contact resistance is small and stable, and has a certain degree of pressure. The ability of clearing film, anti-vibration and anti-welding.

(5) Pre-deformation of the contact of the moving power relay: to prevent the possible rebound of the contact of the moving power relay at the end of the disconnection, and obtain a certain anti-vibration ability.

However, this is not the whole problem. The various contradictions that appear in the opening and closing process of the power relay contacts are stipulated by the general contradiction of induction and execution in the electrical receiver, and also vary according to the use conditions (circuit properties and parameters, surrounding environment, etc.) special nature. Therefore, for different power relays and switching appliances, as the main aspects of the contradiction occupied by the induction system and the execution system are different and different use conditions, the power relay contact structure, size, material and process used are also different. For example, for power relays used in small-capacity circuits and special environmental conditions, the induction system is the main aspect of its contradiction, and membrane resistance, vibration and shock are the special essence that determines the difference between the power relay contacts of this type of power relays and other electrical power relay contacts. s things. Therefore, it is determined that its power relay contacts must use smaller power relay contact pressure (several grams to tens of grams) and spacing (a few tenths of a millimeter); use simple parts such as leaf springs or wire springs, and use it as both The elastic parts are also used as conductive parts and sometimes even as magnetic conductive parts, such as reeds; point contacts with small curvature radius or split power relay contacts: sealed structure (shaft empty or filled with inert gas) and special processes, which are not easy to produce. The precious metal of the film and its high-content alloy are used as power relay contact materials or special coatings are used. For the power relay contacts of power relays and contactors used in larger-capacity circuits (currents ranging from several amps to tens of amps), the situation is different, and the things that determine its special nature have changed. At this time, there is both the harm of the surface film and the burning of the arc, so its executive function must be strengthened accordingly. Therefore, a relatively simple structure, relatively general materials, moderate power relay contact pressure (tens to hundreds of grams), moderate spacing (several millimeters), and sometimes arc shields (arc extinguishing devices are not commonly used) are used. Point contact with large radius of curvature and larger size, using power relay contact materials with good electrical conductivity and strong anti-oxidation and anti-arc ability (usually silver, silver-copper, silver-iron, silver-cadmium, etc., tungsten, molybdenum, etc. are used when the arc resistance is high ). When the circuit current is larger (tens to tens of thousands of amps), the contradictory nature of the contactor and automatic switch used in it has undergone qualitative changes, and the execution system has become the main aspect of the contradiction. Moreover, at this time, the welding of the power relay contacts in the closed state and the arc in the process of disconnection have become its main hazards, which determines its special nature. Therefore, the power relay contacts of such electrical appliances are often equipped with special arc extinguishing devices: point contact or line contact and surface contact with large curvature radius and large size; power relay contact materials with high conductivity and high arc resistance are used. Especially in some electrical appliances (such as large contactors and automatic switches), it is required that it can safely pass a large rated current without overheating and cause welding under normal conditions.

It is required that it can reliably break a large overload or short-circuit current without being burned in the event of a fault, so the structure of the step contact is often used (that is, the main contact is divided first, then the arc extinguishing contact is divided, and the arc extinguishing is closed first. The main contact after the contact is closed) to solve the contradiction that the power relay contact material cannot meet the high conductivity and high arc resistance and welding resistance at the same time.

2. Life and wear of power relay contacts

Obviously, if any power relay contact is viewed as a whole, its main technical and economic requirements are, in a nutshell: reliable work within a certain life (ie, the number of operations). However, due to various mechanical, chemical and electrical reasons,

It will cause damage (or wear) of the power relay contacts, which will reduce the service life and reduce the working reliability. The wear of power relay contacts exists in all working stages. There is mechanical wear due to mechanical impact and slippage between power relay contacts during opening and closing.

There is electrical wear due to arc and spark discharges. There is corrosion such as oxidation and sulfidation in the closed and open states. On the one hand, the wear increases with the increase of voltage and current, and also increases with the increase of the number of power relay contacts. When the power relay contacts wear out to a certain extent, the overtravel disappears, the pressure is reduced, it cannot work reliably, and its life is ended. Therefore, in order to ensure that the power relay contacts have a certain working life, so that they can still work reliably after being worn to a certain extent, it is necessary to select appropriate materials and thicknesses of the power relay contacts (greater than the maximum wear of the power relay contacts within the specified life). Stroke (generally, the excess stroke is selected as 0.25 mm). Electrical wear is often accompanied by the migration of power relay contact metal from one power relay contact to another (most pronounced at DC), resulting in pin-like bumps on one power relay contact and dimples on the other power relay contact . As the number of opening and closing times increases, this migration becomes larger and larger, resulting in severe deformation of the power relay contacts, and even needle-like protrusions hook the pits. When the breaking force is insufficient, a serious accident that cannot be opened will occur. In order to avoid this situation, materials that are not easy to form needles are often used, or the polarity of the power relay contacts is switched over after a period of use.

It can be seen from the above that although the work of power relay contacts is heavy and the internal contradictions are complex, as long as we understand and master its characteristics and laws, it is not difficult to solve various problems in its production and use.

*power relay contacts are also called contacts or contacts. Traditionally, power relay contacts or contacts are often called power relay contacts, and contacts are called contacts in large-capacity switching appliances such as contactors.