Scheme of control of the asynchronous electric motor
Let's consider very widespread scheme of control of the asynchronous engine by means of the magnetic actuator.
The scheme of management without the possibility of veering of rotation (reversal) is provided on fig. 1. The irreversive magnetic actuator consists of the triple-pole contactor and the thermal relay.
Let's trace the device and work of power (main) chains, and then control circuits in the beginning.
Power chains. The three-phase current to the stator электродвигателяД arrives via the triple-pole breaker P. The breaker gives the chance to switch-off the electric motor in case of repair or failure of the magnetic actuator. Further in the power chain there are safety locks 1P which are located usually on the group distribution guard; they protect chains from korotyoky short circuits. The main contacts of L of the triple-pole linear contactor include or disconnect the winding of the statoraelektroyodvigatel. The main contacts are connected so that mobile contacts were located from the engine, and motionless, always being energized, - from network, such connection increases safety of service. Thermal relays turn on in two phases as excessively big current is possible not less than in two wires, they serve for protection of the engine against long overloads and against work on two phases.
Application in the scheme along with thermal relays of fusible predoyokhranitel is explained by the fact that power contacts of magnetic actuators allow the rupture of overload currents no more semiyokratny size of rated current of the electric motor which power is admissible in this actuator; and these contacts are not expected the rupture of currents of korotyoky short circuit. The power chain turns on heating elements of the relay.
Control circuits. The power supply of the control circuit osushchestyovlyatsya via the breaker and safety locks of the main chain here. Besides, control circuits are protected by one predokhrayonitel 2P, he protects the control circuit from short circuits. Apparently from the scheme, the control circuit eats tension of the same size, as the power chain.
The control circuit switched on the "feet" and "start-up" buttons.
The coil L of the linear contactor with block-contact L1 by means of the main contacts of L in the power chain carries out inclusion and shutdown of the electric motor D. Further the chain of the upravyoleniye included the disconnecting contacts (with manual return) thermal relays 1PT and 2PT which heating elements are switched on in the main chain. At some types of thermal relays two heating elements and only one disconnecting contact which each of bimetallic plates can influence by means of lever system imeyotsya.
The scheme works as follows. For launch of the engine lodge turning on of the breaker P it is necessary to press the start-up button. At the same time the chain of the coil of the contactor L. Tok becomes isolated goes on the sleduyuyoshchy chain: the phase L1 - the safety lock 2P - the disconnecting feet button - the start-up button - the contactor L's coil - the disconnecting contacts of thermal relays 1PT and 2PT - the phase L3. Because on the coil of the contactor there passes current, its core namagniyochivatsya, the anchor is involved and includes the main contacts. Vyyovoda windings of the C1C2C3 stator join network of the power supply L1, L2, L3, and the engine turns on. Along with the main contacts also interlock contacts so become isolated that the chain of the coil of the contactor becomes isolated via the L1 interlock contact shunting the start-up button. Now it is not necessary to hold the button in the pressed state more; due to action of the spring it returns to the starting position. For shutdown of the engine it is necessary to press the feet button; at the same time the power supply of the coil of the contactor L preryyovatsya, and the main contacts under the influence of the weight or the spring razmyyokatsya and disconnect the stator winding from network.
The considered scheme carries out also so-called "nuleyovy" (or minimum) protection: at disappearance or considerable undervoltage of network up to 35 — 40% of rated value the contactor is disconnected and disconnects the electric motor from network.
At voltage recovery of the self-starter of the engine will not occur any more as the start-up button is released, and block контакт L1 is opened.
In case of the long overload the disconnecting contact of the teployovy relay 1PT (2PT) is turned off by the contactor and consequently, and the engine. After operation of the relay of thermal protection (if the thermal relay is executed by the principle of forced return) for the vozyovrat of contact of the relay in the starting position it is necessary to press the knopyoka which is located on the actuator cover; return of contacts of the relay 1PT (2PT) after shutdown is possible only through time necessary in order that bimetallic plates cooled down.
Magnetic actuators are produced for control of electric motors up to 75 — 100 kW. The considered scheme can be collected as well with the contactor. Triple-pole kontakyotor of the KT alternating-current series with the coil of the variable bale are usually applied to asynchronous engines up to 500 V.
Either the reversivyony magnetic actuator, or the scheme of management with two kontakyotor, a little different from the scheme of the reverse actuator is used to control of the mechanisms demanding veering of rotation (reversal).
The scheme of control of the asynchronous korotyokozamknuty engine with the possibility of reversal is provided on fig. 2. As well as the scheme of management with the magnetic actuator, this scheme allows distance control as buttons of management whom in this scheme three - "forward", "back" and "stop", it is possible to place at some distance from the engine. By means of the scheme, izoyobrazhenny in fig. 2, it is possible to launch the engine (and, sledovayotelno, the related mechanism), to change the direction of the vrayoshcheniye, to stop it; besides, the scheme carries out protection of installation against short circuits, against the overload, against voltage drop in network (zero protection) and from the self-starter. In this scheme two schemes of irreversive start-up are combined and there are some features. The scheme is supplied by two contactors: the contactor "forward" (the coil and its three main contacts are designated by the letter B, and block контактыB1и B2) and the contactor "back" (the coil and three main contacts are designated by the letter N, and H1 and N2 interlock contacts). The main contacts of contactors In and N are included in the power chain in such a way that when contacts become isolated In (contacts of N at the same time are opened), on the winding of the stator three phases of network in one order and when contacts of N become isolated move, two phases from three are interchanged the position. In this regard magnetic field of the stator of the engine begins to rotate in the opposite direction, and the engine is reversed.
Really, at inclusion of contacts In the phase L1 of network the phase L2 - on C2, the phase L3 - on C3 is eaten on the C1 stator winding. If contacts of N become isolated, then the phase L1 moves on the obyomotka of C3, the phase L2 - on C2 (without change), the phase L3 - nas1, sledoyovatelno, the phases L1 and L3 are interchanged the position.
The scheme works as follows. For inclusion of the dvigayotel in the direction the button "forward" "forward" is pressed; at the same time current from the phase L2 goes on the chain: 1 - 3 - 5 - 7 - 6 - 4 - 2 - phase L3; the coil B closes the main contacts In, and the engine vklyuyochatsya on the movement "forward". For veering the vrashcheyoniya joins the feet button, and then the button turns on "back"; at the same time current goes on the chain: the phase L2 - 1 - 3 - 9 - 11 - 6 - 4 - 2 - the phase L3. Now current already goes on the coil N which zamyyokat the contacts, and the engine is reversed. Simultaneous inclusion of both contactors in the considered scheme can priyovest to the short circuit in the power chain. If to turn on the engine in the direction, for example, "forward" and by mistake to press the button "back", then the coil N will also include the contacts (konyotakta In were included earlier as the engine worked in the direction "forward"), in the power chain will be included in all six main contacts that will lead to the short circuit in two phases (L1 and L3). That it did not occur, in the scheme two-chain buttons "forward" and priyomenyatsya "back"; when pressing the knopyoka the contact in the coil N chain and vice versa if to press the button "back", then it is disconnected контакт coils B is at the same time disconnected "forward". This device is called the mechanical blokiyorovka. For increase in reliability of work of the scheme as mechanical blocking are supplied also the anchor of coils of contactors which have the special lever: retraction of the anchor of one kayotushka makes impossible simultaneous retraction of the anchor of the second coil.
Except mechanical also electric bloyokirovka is applied. In fig. 2 buttons of management "forward" and "back" normal; however the coil chain included the disconnecting contact of the contactor "forward" "back" and vice versa, the coil chain included the disconnecting contact of the contactor "back" "forward". If to press, for example, the button "back", then current will pass on the contactor's coil "back", the contactor will close the closing contacts and razomkyont the disconnecting contact of H2 in the chain of the coil of V. Sledovatelyyono until the coil of the contactor of N is switched on, the kontakyotor coil chain In will be opened, and it is impossible to turn on the coil B along with the coil N. This device is called elektricheyosky blocking. Odnoyovremenno from electric apply mechanical blocking to increase in reliability of work of the scheme.Top