Factors affecting contact resistance of power relay

The contact resistance of power relay is an objective phenomenon, which can not be avoided at any contact. However, when we understand its essence and grasp its characteristics, we can further analyze its influencing factors and take corresponding measures to reduce or eliminate its impact.

(1) Contact pressure: a certain contact pressure must be applied when any contact works in the closed state. Its function, on the one hand, is to crumple and deform the contacted points, so that the contact area of these points is increased, and at the same time, more points are contacted, thus reducing the contact resistance. In this way, when the same current flows, the heating of the contact will be greatly reduced, or the current allowed to pass through the contact can be greatly increased under the same heating condition. This effect of contact pressure can also be called heat resistance or fusion welding resistance. Obviously, the greater the contact pressure, the smaller the contact resistance, and the larger the allowable current. Generally, the contact pressure of large capacity contacts is determined based on this principle. That is, ensure that the contact does not soften due to heating when passing through the rated current (with a certain safety factor), and does not melt due to overheating when passing through the specified overload current (with a certain safety factor). For example, the contact pressure of high-power power relays for general control is usually 0 ~ 100g, while the contact pressure of contactors or automatic switches is hundreds of grams to tens of kilograms. Another function of the contact pressure is to crush the surface facial mask and make the contact metal contact directly, thus reducing and stabilizing the contact resistance. This effect is called film clearing ability. The third function of contact pressure is that it can resist external vibration and impact, so as to ensure that the contact resistance will not increase or even the contact will leave instantaneously and cause power failure due to these factors. This effect is called anti vibration ability. However, things are always in two. Excessive pressure will increase the operating force, which will increase the size of the electromagnetic system and reduce the sensitivity of the power relay. Therefore, in a high-sensitivity power relay, since the contact pressure cannot be too large, the contact capacity is very small. This is the result of the contradiction between induction and execution.

(2) Contact material (including surface coating): it is obvious that if the hardness of the material is low, the contact is easy to be crumpled and deformed: if the chemical stability of the material is high and the ability to resist pollution and corrosion is strong, it is not easy to produce a chemical film; Good thermal and electrical conductivity of materials is conducive to the improvement of heat generation. Therefore, we can choose different contact materials according to different requirements for electrical appliances. For example, gold and other precious heavy metals and their alloys are not easy to be oxidized and sulfurized, and are mostly used in weak and small current contacts; Silver and non precious metals such as copper (and their alloys) can form various films, so they are mostly used in the contact of medium current or large current.

(3) Contact structure: there are three main contact modes (Fig. 2-2): point contact, line contact and surface contact. Obviously, there are more actual contact points in surface contact than in line contact, and there are more line contacts than point contacts. Among the power relays with medium and small power, the contact capacity is small, so the point contact type is used most to increase the pressure per unit area and improve the film cleaning ability. When the contact current is large and the number of operations is frequent, the contact form of surface contact or large arc spherical surface should be used to improve its anti fusion welding, anti-wear and other capabilities. In small power relays, split contacts, leaf springs or wire springs are sometimes used (Fig. 2-3) to reduce the possibility of breaking due to dust falling at the same time. power relays that specially use leaf spring or wire spring contacts are called leaf spring power relays or wire spring power relays. In addition, the processing condition of the contact surface, that is, whether it is rough and fine, also has a great influence on the contact resistance. In general, the contact resistance of rough surface processing is larger than that of fine processing, and dust is easy to adhere. However, excessively fine processing and polishing are unfavorable to increase the compressive stress per unit area and damage the surface facial mask. At the same time, it also increases the surface hardness and thus the contact resistance. Therefore, the machining accuracy of the contact surface is usually between 7 and 8.

(4) Sealing structure, process and surrounding environment: in small power relays, they are often sealed in a metal cover and evacuated or filled with inert gas (such as chlorine, hydrogen, etc.) to prevent the pollution of the surrounding atmospheric conditions to the junction point, thus greatly reducing and The contact resistance of the contact is stabilized. This is the biggest feature of sealed power relay. However, there is another world in this small sealing cover: the insulating materials in the coil and other components will precipitate organic vapor at high temperature (that is, the commonly called insulating materials The mechanical wear and contact wear of the material weight-loss square structure will also produce various particles: all kinds of dirt and residues adhered to the parts during processing will fall off again under the action of later operation or external vibration and impact. All this surrounds me，The contact forms a so-called "mini weather", which will have new and even worse effects on the contact. Therefore, some power relays seal the contact parts separately (which brings complexity and variability in structure) in order to avoid these effects. However, no matter whether it is sealed or not, attention must be paid to the processing technology of the joint, and new processes (such as flux free welding, organic material exhaust treatment, good cleaning, constant heating vacuum baking, and the establishment of ultra clean assembly rooms) must be adopted as far as possible. Similarly, different surroundings will have different effects on contact resistance. For example, under the conditions of high temperature and humidity, the contact metal is easy to be corroded, oxidized or sulfurized, and the organic material is easy to volatilize harmful gases; When there is impact vibration, the reliability of the contact will be reduced.

In short, there are many factors affecting contact resistance. Although there are many measures that can be used to control and reduce the contact pollution, reduce and stabilize the contact resistance, it is still a major problem in the current power relay production and use, which needs to be further studied and solved.