Currently, the most common electric motor is a three-phase asynchronous motor, which differs from other types in reliability, ease of manufacture and low price. It can operate both from a three-phase electrical circuit and from a single-phase one.
Content
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Mechanism device
- The principle of operation of an asynchronous motor
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Winding and delta connection diagram
- Star
- Triangle
- Star-triangle combination
Mechanism device
An asynchronous motor is divided into two groups, which depend on the method of execution of the rotor winding:
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Phase winding motors. They have a complex rotor design, which is why the production of the device is much more expensive than other types of motors. They are used in difficult starting conditions and when necessary, smooth speed control. - Squirrel-cage winding motors. The device has a lower cost to manufacture and its speed changes by only 2-3 percent when the load changes from 0 to minimum frequency. The only drawback is the difficulty of smoothly adjusting the rotational speed over a wide range.
The device consists of a stationary cylinder - a stator, which consists of sheets of electrical steel, insulated from each other with technical varnish and assembled with staples to reduce vortex currents. In the slots of the stator there is a stator winding connected in a delta or star combination. The device also consists of a rotating part - a rotor assembled from sheets of electrical steel, where aluminum or copper is poured into the grooves under pressure. And also the end rings are poured together, on which the blades are located. They are needed to cool the rotor.
The rotor is mounted on the motor shaft, on which the bearings are fixed. This whole structure is located in the end shields.
The principle of operation of an asynchronous motor
If you apply voltage to the stator winding, then an alternating sinusoidal current begins to flow on it, creating a magnetic field. It crosses the rotor winding in which an alternating electromotive force is induced. EMF forms an alternating current in the rotor winding, and this current creates a rotating magnetic field of the rotor.
The stator and rotor fields are connected and form a common rotating magnetic field of the motor, which interacts with the current in the rotor winding and forms the force according to the left-hand rule. It turns the rotor in the direction of the rotation of the magnetic field.
The device is called asynchronous due to the fact that the rotational speed of the magnetic field is several times higher than the rotational speed of the rotor.
Winding and delta connection diagram
In practice, it is customary to use two main connections of electric motors to the network. Depending on the reliability of the network, power and engineering parameters of the device, they distinguish between the connection schemes of the motor windings with a star and a triangle. But their joint combinations are also popular.
Star
The three motor windings have start and end leads that are aligned at one neutral point. It is also called neutral. In the absence of a neutral wire in the circuit, the circuit is considered three-wire, if there is one, four-wire.
The beginning of the conclusions is attached to certain phases of the electrical network. On phases, the voltage is either 380 V or 660 V.
The scheme has several advantages:
- In the operating mode, the device case does not overheat;
- The ability to use temporary overload;
- Durability of use, safety and reliability;
- Continuous use of the electric motor for a long time.
When using such a connection, no specialized wizard work is required.
Triangle
In such a connection, the ends are not connected to one neutral point, but merged with another winding. It is a triangle in which the windings are connected in series. The difference lies in the fact that the triangular system exists only three-wire, since it does not have a common point.
Linear voltage across the windings is 220 V or 380 V.
The scheme has several advantages:
- the ability to use electrical equipment at full capacity;
- the use of a starting rheostat;
- increase in torque.
This model is most often used when working with powerful devices and if there are large starting loads.
Star-triangle combination
A similar model is used for complex mechanisms. When starting, the star-delta power rises quickly and if the motor is not designed for delta circuit, then it will quickly overheat and, most likely, burn out. To prevent fuses from blowing out, autotransformers are used.
Then the voltage becomes much less and the resulting current, respectively, also. Further, an increase in frequency and a decrease in current indicators are carried out.
The star-delta connection scheme is the maximum reliability and efficiency of the electrical equipment used.
The star-delta scheme has the following advantages:
- the possibility of using two levels of power;
- increased service life.
