How do asynchronous short-circuited engines work?

  Asynchronous short-circuited engines of small and medium power are launched most often by direct turning on of the stator of the engine in network.

Устройство асинхронных машин

Device of asynchronous machines.

The most often applied scheme  of engine management is provided on fig. 1.

Protection of the power chain is carried out by safety fuses P  and two thermal   relays 1PT and 2PT. Safety fuses P  protect the power chain from short circuits, and thermal  relays 1PT and 2PT    protect the engine from the overloads exceeding the rated load for 10 — 20%.

Рис. 1 Схема управления асинхронным короткозамкнутым двигателем

Fig. 1 Scheme of control of the asynchronous short-circuited engine.

Inclusion and shutdown of the engine is made by the linear contactor L, N / about power contacts of which are in the power chain of the engine. Engine management are made remotely from the push-button station consisting of two buttons, Start-up and Stop.

The linear contactor L  and thermal   relays 1PT and 2PT represent one device — the magnetic actuator. When pressing the Start-up button  the coil of the contactor L  receives the power supply and attracts the anchor with the mobile contacts strengthened on it. By means of power contacts the engine turns on in network, and the blocking N / about contact of L shunts the Start-up button  that allows to release this button, without interrupting the power supply of the coil L.

The stop of the engine is carried out by the Feet button, and in case of the engine overload — disconnection of contacts of thermal relays 1PT and 2PT  .

The magnetic actuator carries out also so-called "zero protection". At undervoltage on the coil L  up to the size of 0,8 U/number. the anchor of the contactor will be released and the engine will be switched-off.
If under the terms of technology process it is necessary to change the rotation sense of the production mechanism, then engine management is exercised by means of the reverse magnetic actuator consisting of two contactors: In — "forward" and N — "back" which are run by the corresponding buttons of Vp, Nz and Feet (fig. 2).

Рис. 2 Схема управления асинхронным короткозамкнутым двигателем для двух направлений вращения

The fig. 2 Scheme of Control of the Asynchronous Short-circuited Engine for two rotation senses.

Protection of power chains is similar to the scheme provided on fig. 1. For prevention of simultaneous inclusion of contacts In and the N (that can lead to the short circuit in the power chain) are provided in the scheme two blocking. One of them is executed by N / з contacts of N and In in chains of coils of contactors and excludes the possibility of simultaneous inclusion of contactors of N and Century. The second blocking executed by contacts of buttons of management provides disconnection of the chain of the disconnected contactor before there is circuit closing of included.

The scheme of control of the asynchronous short-circuited engine with the pure resistance in the stator chain is presented on fig. 3. This scheme is applied to restriction of fluctuations of tension in low-power network circuits at considerable starting currents.

Рис. 3 Схема управления асинхронным короткозамкнутым двигателем с активным сопротивлением в статорной цепи

The fig. 3 Scheme of Control of the Asynchronous Short-circuited Engine with the pure resistance in the stator chain.

By pressing the Start-up button  the contactor At turns on  the engine stator in network through the limiting resistance. At the same time the RU pendular relay attached to the contactor comes into action. This relay will be turned on by the contactor L  later that time which is necessary for running start of the engine till rated speed.

 The contactor L  will work and the main contacts zashuntirut the limiting resistance.

In all above-stated schemes braking of the drive happens at the expense of friction forces. Let's provide some schemes providing electric braking.

The scheme with dynamic braking is presented on fig. 4. When pressing the Start-up button   the contactor L's coil which the main contacts connects the engine stator to  the alternating current main turns on. The interlock contact of the contactor L turns on  RV time relay coil in direct current mains, and the RV relay, working, the N / about contact in the chain of the coil of the contactor of braking of T closes. On it start-up operation comes to an end.

Рис. 4 Схема управления асинхронным короткозамкнутым двигателем с динамическим торможением

The fig. 4 Scheme of Control of the Asynchronous Short-circuited Engine with dynamic braking.

By pressing the Feet button  the chain of the coil of the contactor L  which contacts disconnect the engine from alternating current is disconnected. The normal closed interlock contact of the contactor L  prepares the chain for inclusion of the contactor T, and the N / about the interlock contact L disconnects RV time relay coil chain.

At the same time through the live contact of the timing circuit of RV and N / з  the interlock contact L  will receive the power supply the coil of the contactor T which will rise on the automatic interlock by means of N / about the interlock contact T and thus will connect the engine stator to direct current mains. The engine at the same time works in the mode of dynamic braking.

Рис. 5 Рис. Схема управления асинхронным короткозамкнутым двигателем с торможением

Fig. of 5 Fig. The scheme of control of the asynchronous short-circuited engine with braking.

The timing circuit of RV is adjusted so that its endurance was  slightly more time of braking of the electric motor therefore the relay the contacts will open the contactor T's chain after there is the engine stop.

In fig. 5 the scheme of control of the asynchronous short-circuited engine in the countercurrent mode is represented. Start-up is carried out to similarly above-stated schemes. When pressing the Feet button  the coil of the contactor L  loses the power supply and the engine is switched-off from network. The normal closed interlock contact L will become isolated, and the coil of the contactor T will receive the power supply. During rotation of the engine contacts of the relay of control of speed of RKS are closed and are disconnected at the speed of the engine close to zero.

Thus, before disconnection of contacts of RKS, i.e. almost to the full stop of the drive, the engine works in the countercurrent mode that provides the fast stop it.

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