3-phases Induction AC motor
Three-phase induction motor is the most common type of AC motors in the world. Its main feature is that three-phase windings are installed on the stator.
Three-phase asynchronous motor design
The main task of the engine is the conversion of electrical energy into mechanical one. Its design consists of two main elements such as a rotor (moving part) and a stator (stationary part).
Between them is an air gap. Both of these elements have own cores in which special windings are placed. They are located directly on shaft in rotor, and in special slots on housing in stator.
The stators winding is mounted on special grooves, which have an angular distance of 120 degrees. The most popular is a system with a short-circuited rotor or also called "squirrel cage". In this case, the coil fixed to the cylindrical frame and the rods are connected with the rotor core and short-circuited by phases ends.
In addition to the squirrel cage, a phase-rotor motor is also used. In this case, phases of winding are attached to special contact rings, and their ends are isolated from each other and from shaft. Both types can have the same stators structure.
There are several schemes of connecting windings to each other. Main methods are so-called "star" and "delta". Sometimes both of them are installed on the same motor.
In first case, the phase’s ends of windings are all connected at one point. In second type end of each phase is alternately connected with the beginning of the next one.
Selection of suitable scheme depends on the supply voltage in electrical network.
How it works?
The operation of induction motor is based on revolving magnetic fields.
Current in stator windings creates a moving magnetic field, which affects the contour of the rotor and induces in it an electromotive force. If this parameter is higher than friction force, the shaft is set in motion.
Rotor starts to increase the frequency of rotation trying to achieve the speed of stator`s winding revolving magnetic fields. However, when shaft speed reaches magnetic field rotation speed, the electromotive force is equal to zero.
This means that the magnetic effect will disappear and the rotor speed will decrease. It`s the endless process. Therefore, the frequency of shaft rotation never coincides (not synchronous) with the frequency of revolving magnetic fields. That effect is the main basis of induction motors operation.
Three-phase induction electric motor has several possible operation modes:
- Motor mode.
- Generator mode.
- Electromagnetically brake.
Start is the initial stage of operation in any engines. In this mode, the coil engages the current and creates a rotating magnetic. When the friction force is less than the electromotive, rotor begins to motion.
Motor mode performs the main task of electric motor. That is, converts the electromotive force into mechanical shaft rotation.
Idling occurs when it is no load on the shaft. That is, it is not connected to other devices.
Generator mode is activated when shaft speed is forcibly, for example, with help of another engine, exceeding the rotation speed of electromagnetic field. In this case, electromotive force has a reverse vector and the engine become a source of active energy.
Electromagnetic braking occurs when artificially changing the direction of rotation of the electromagnetic field and the rotor on the opposite. This leads to fast deceleration. This method are used only in emergency cases, because a huge amount of heat is released.
Advantages of a three-phase asynchronous motor
The three-phases motor can also operate in single-phase mode when required. However, rated power at the same time decreases approximately twice.
If one phase disappears, the engine can continue to work. Even it will be possible to launch a new cycle but with reduced power.
Motors of this type have relative cheapness, good efficiency and reliability. These qualities have made three-phase motors the most popular type, used in the world.