Type of contact
Disconnectable contacts are an indispensable part of switching appliances. According to their different structures, they can be divided into the following types:
(1) Knife contact: It has a simple structure and is divided into surface contact and line contact. It is widely used in low-voltage switches and high-voltage disconnectors.
(2) Butt joint contact: It has the characteristics of simple structure and fast action, but the contact surface is unstable and varies greatly with the pressure. It is easy to rebound during operation, without self-cleaning effect, and the contact is easy to be burned by electric arc. This contact is usually used for distribution circuit breakers with rated currents below 1000 A and below 500 A
(3) Wedge contact: it consists of a pair of contact pieces and wedge contact pieces, which are pressed and fitted on the conductive seat through the stud sleeve spring. Generally, the wedge contact is used as the moving contact, but there are also contacts clamped on the conductive seat, and the wedge contact is used as the static contact in turn. This kind of contact rubs against each other when moving contact and static contact contact, and the contact surface can be cleaned automatically.
(4) Plug in contact: static contact consists of multiple trapezoidal contact fingers. It is divided into flexible conductive sheet and non flexible conductive sheet. For the socket with flexible conductive sheet, the contact finger is provided with a groove, the groove is embedded with an insulating sleeve, and a coil spring is placed to ensure the pressure of the contact finger on the conductive rod.
(5) Z-contact finger type sliding contact: similar to socket type contact in structure. It is formed by installing the Z-shaped contact finger in the conductive seat, keeping the position of the contact finger through the spring, and pressing both sides of the contact finger against the guide rod and the conductive seat respectively. Its advantages are: small height, simple assembly, no conductive sheet, stable contact, self cleaning function, so it is widely used.
(6) Rolling sliding contact: the moving contact is a circular conductive rod, and the fixed contact is composed of two circular conductive rod seats and a pair of red copper rollers installed between the conductive rod and the conductive seat. The spring is installed on both sides of the roller, and the contact between the roller and the conductive rod, the roller and the conductive seat is maintained by the pressure of the spring.
With the development of high voltage, high current, miniaturization and long life of switching devices, the electrical properties of contact materials are required to be higher and higher. Nanometer contact materials have become a hot spot in the research and preparation of contact materials.
The material property of contact is one of the key factors affecting the working characteristics and electrical life of vacuum switchgear. Almost all early vacuum switches used tungsten, molybdenum and other refractory pure metals as contact materials. This contact material has a series of advantages, such as appropriate breaking performance, good compressive strength and welding resistance, appropriate thermal conductivity and conductivity, low ablation rate during arc combustion, long contact life, etc. It is widely used in breaking small current vacuum switch. However, high melting point pure metal contact materials have many shortcomings, such as breaking current and high cut-off level, which limit the application range of vacuum switches.
The standard diode greatly extends the return time, and the connection of ordinary diode and zener diode has little effect on the return time. For inductive load, when the contact is separated, the arc generation time will be prolonged and the contact life will be shortened. For example, a relay with a diode connected to a coil takes 9.8 milliseconds to release the contacts. If the Zener diode is combined with the small signal diode, the time will be shortened to 1.9 milliseconds. The return time of the relay with no diode connected to the coil is 1.5 ms. When the service life of the relay approaches the later period, the contact resistance increases rapidly. At normal room temperature and pressure, the core dielectric breakdown voltage of air is 2