An electromagnetic switch is a high-voltage switching device in which the electric arc is quenched by the interaction of the arc plasma with a magnetic field (so-called magnetic blast) in arc chambers with narrow slits (straight or tortuous) or with chambers with arcing lattices.
The principle of magnetic arc extinction is widely used in low-voltage switching equipment ( circuit breakers , contactors ).
Content
- 1 device
- 2 Principle of operation
- 3 Benefits
- 4 disadvantages
- 5 Application
- 6 Manufacturers
- 7 Literature
- 8 Notes
Device
An electromagnetic switch consists of a frame on which three poles are mounted on insulators and are connected to the switching shaft by dielectric rods. Each pole consists of movable and fixed contacts, each of them has a pair of main and arcing contacts made of cermet. An arcing chamber made of arc-resistant material (asbestos) and ceramic or steel arcing plates coated with a layer of copper is installed on top of the contacts. Outside, the chamber is covered by a U-shaped magnetic circuit, on which an arcing coil is mounted, connected on one side to a power bus with a fixed contact, and the other to an arcing horn located inside the arcing chamber in front of a packet of lattice plates; the second arcing horn is also located inside the chamber on the other side of the arcing lattice and is connected to the second terminal of the switch. For better arc extinction when switching low currents, a device for auto-pneumatic blowing (consisting of a piston and cylinder mechanically connected to power contacts) may be provided in the design. In addition, the switch is equipped with a drive (usually a spring-motor type), which, according to the signals in the secondary circuit, commutes the power contacts by rotating the switching shaft.
Principle of Operation
When a shutdown signal is applied, the shaft rotates and the movement through the rods is transmitted to the moving contacts, while the main contacts open first, then the interrupter contacts. The resulting electric arc, under the influence of its own electrodynamic forces, moves upward along the arcing horns, while being squeezed out to the side of the grating, in addition, the arc closes the circuit of the arcing coil (through the arcing horns), the magnetic field of which accelerates the arc even more (the so-called “magnetic blast” ) Once in the lattice, the arc is divided into many small arcs (between the plates of the lattice), which begin to move up independently and quickly go out (due to heat transfer to the plates and deionization; due to the near-cathode voltage drop, the electric field strength in the arcs increases - see more details Arc chamber ) . At the top of the arcing chamber there may be plates of the second lattice, (the so-called “flame arrester”), to exclude the exit of ionized plasma outside the apparatus and the overlapping of live parts to it. At low tripping currents, the electrodynamic forces can be small and not able to squeeze the arc into the lattice, and for this purpose autopneumatic blowing is sometimes used in the form of a stream of compressed air, which carries out cooling and deionization of the electric arc.
Benefits
Full explosion and fire safety (unlike, for example, from oil circuit breakers), a complex pneumatic system is not used, low wear of the arcing contacts, the possibility of use in installations with frequent switching, relatively high breaking capacity.
Weaknesses
The complexity of the arcing chamber with a magnetic blast system, a limited range of rated voltages (up to 15-20 kV), limited suitability for outdoor installation [1] .
Application
Electromagnetic circuit breakers are produced mainly for use in 6-10 kV networks with a rated current of up to 2000 A (with switched power up to 400-200 MVA, respectively) for indoor installation of mounted and withdrawable type - for switchgear cubicles (complete switchgear).
Manufacturers
- Rivne High Voltage Equipment Plant
- Chimkent factory "Electoapparat" [2]
Literature
- Rodstein A. Electrical Appliances, L. Energoizdat, 1981