Asynchronous motor - design and operation principle

The induction motor is an electric motoralternating current motor. Asynchronous, this electric machine is named because the frequency with which the moving part of the motor rotates does not equal the frequency with which the magnetic field rotates, which is created due to the flow of alternating current along the winding of the immovable part of the engine-the stator. Asynchronous motor - the most common of all electric motors, it has received the widest popularity in all branches of industry, machine building and so on.

Asynchronous motor in its constructionnecessarily has two most important parts: the rotor and the stator. These parts are separated by a small air gap. Active parts of the engine can also be called windings and magnetic circuit. The structural parts provide cooling, rotor rotation, strength and rigidity.

The stator is a cast steel or cast iron housingcylindrical shape. Inside the stator housing there is a magnetic circuit, the stator winding is laid in special cut-off grooves. Both ends of the winding are connected to the terminal box and connected by either a triangle or a star. From the ends, the stator housing is completely covered by bearings. Bearings on the rotor shaft are pressed into these bearings. The rotor of an asynchronous motor is a steel shaft, on which the magnetic circuit is also pressed.

Constructively, the rotors can be divided into twomain groups. The engine itself will carry its name in accordance with the principle of the rotor design. An induction motor with a squirrel-cage rotor is the first type. There is a second one. This is an asynchronous motor with a phase rotor. In the grooves of the motor with a short-circuited rotor (it is also called a "squirrel cage" due to the similarity of the appearance of such a rotor with a cell in a protein), aluminum rods are poured in and close them at the ends. The phase rotor has three windings in it, which are connected to each other in a star. The ends of the windings are attached to the rings fixed to the shaft. When starting the engine, special stationary brushes are pressed against the rings. These resistors are connected to these brushes, designed to reduce the starting current and smoothly start the induction motor. In all cases a three-phase voltage is applied to the stator winding.

The principle of operation of any induction motoris simple. The basis is the famous law of electromagnetic induction. The magnetic field of the stator, created by a three-phase voltage system, rotates under the action of a current passing through the stator winding. This magnetic field crosses the winding and conductors of the rotor winding. From this, an electromotive force (EMF) is created in the winding of the rotor according to the law of electromagnetic induction. This EMF causes the alternating current to flow in the rotor winding. This rotor current subsequently creates the magnetic field itself, which interacts with the magnetic field of the stator. This process also starts the rotation of the rotor in magnetic fields.

Often to reduce the starting current (and it isasynchronous motor can many times exceed the operating current) start capacitors are used, connected in series to the starting winding. After start-up, this capacitor turns off, keeping the performance unchanged.