Device of devices of protection of electric units
Devices of protection are intended in order that at emergence of emergency operation in operation of electroreceivers or network circuits automatically to disconnect the protected electric circuit.
Emergency operation is the following:
- interphase short circuit;
- short circuit of the phase on the body;
- the increase in current in network caused by the overload of the tekhnologiyochesky equipment;
- disappearance of tension or excessive lowering of the napryayozheniye (which causes dangerous increase in the consumed current).
In all listed cases protective devices have to warn the possibility of insulation failure of windings of the dviyogatel and breakdowns in the mechanical part of the drive or the working mayoshina, having timely and reliably disconnected the electric unit.
Along with it devices of electric protection have to be expected long course through them of the maximum load current and short-term action of the peak current, koyotory arises at inclusion in network of separate powerful elektroyodvigatel.
Distinguish the maximum protection, overload protection and protection of minimum voltage (or zero).
The maximum protection is called protection of the electric drive against short-circuit currents and short-term big reset. This type of protection is carried out by electromagnetic releasers of automatic air circuit-breakers, safety fuses and also the electromagnetic relays which are switched on in secondary circuits.
Overload protection of electric units long currents, for 30 — 60% exceeding rated currents is carried out by means of thermal relays or the relay of the maximum current with vyderzhyoky time.
At very considerable undervoltage and also at its total disappearance the engine can stop. If after that mains voltage is suddenly recovered, then there will be self-starting of the engine that in certain cases can lead to serious accidents and accidents. The protection working at undervoltage in network and by that the excluding possibility of self-starting (if it is inadmissible) is carried out by electromagnetic relays of tension, magnetic actuators and contactors. It is called protection of minimalyyony tension.
Let's consider the device of some devices of protection.
The automatic air vyklyuchayotel (automatic machine) is intended for automatic disconnection and short circuit of electric circuits by mechanically connected razyodvigayemy contacts. Automatic machines carry out also protection of chains at different emergency operation (the short circuit, the overload, disappearance of tension, inadmissible undervoltage, etc.).
Automatic machines happen:
- with thermal and elektroyomagnitny releasers;
- only with the thermal rasyotsepitel allowing the operation current reyogulirovka;
- only with elektromagnityony releasers;
- without releasers.
The thermal relay protects the engine from overloads over 20 — 30% of rated value of current and also of break of one phase of the feeding line as in this case in two undamaged wires the size of current is higher than rated.
The scheme of the thermal relay is provided on fig. 1. The relay consists of the heating element, for example filched 1, the bimetallic plate 2 consisting of two strips of metal with different linear expansion coefficients. The heating element turns on consistently in the power chain of the engine (on the scheme current of I). If the engine is not overloaded, then heat generated by the heating element causes the insignificant bend of the biyometallichesky plate. At the currents exceeding ratings, the bimetallic plate strongly heats up and izyogibatsya so that its loose end releases the lever 3; under the influence of the spring the 4th lever 3, turning counterclockwise, disconnects contact 5. The contact 5 joins posledovayotelno in the chain of the coil of the contactor (on the scheme i current); at the razmykayoniya of this contact there is the shutdown of the contactor who disconnects the engine from network. After operation of the relay its return to the starting position is carried out by means of the return button (on the scheme it is not shown).
For the right choice of protective devices it is necessary to know their ratings and protective characteristics.
Understand as ratings of protective devices: system of current (constant, variable); the largest tension; the greatest current, admissible in the conditions of the normal ekspluatayotion and at short circuits; the operating mode (long, short-term, repeated and short-term) and other working conditions for which the device is intended.
Understand duration of time during which the device disconnects overload currents and short-circuit currents of different size as protective characteristics. Protective harakyoteristik usually represent in the form of curves where on the gorizonyotalny axis postpone frequency rate of the overload current or short circuit in relation to rated current of the protective elemenyot of the fusible insert, and on the vertical axis — shutdown time.Top