The reason why asynchronous machines are widespread is the simplicity of their design, reliability and manufacturability. Applicability for a three-phase and single-phase network, a wide range of capacities, ease of changing the direction of rotation - all this makes them indispensable as drives for a wide variety of devices, including machine tools and conveying systems.
The essential advantage possessed by asynchronous machines is their high efficiency.
The most common electric motors are kilowatt, their application is very wide, in almost every industrial enterprise they make up most of the drive devices.
Asynchronous electric machines got their name because their angular velocity depends on the magnitude of the mechanical load on the shaft. Moreover, the higher the resistance to torque, the naturally it spins more slowly. The lag of the angular velocity of the rotor from the frequency of rotation of the magnetic field created by the current passing through the stator windings is called slip. It is calculated, as a rule, as a relative value:
S = (ωn-ωp) / ωn
Where:
ωn is the rotation speed of the magnetic field, rpm .;
ωp is the rotor speed, rpm
The dependence of the relative amount of slip on the load on the shaft is manifested in particular in the fact that in idle mode S is practically zero.
The device of an asynchronous machine is the same as any other electric motor or generator. The inner surface of the stator is equipped with special grooves in which the windings are laid (in the case of three-phase power supply there are three of them, and for single-phase motors - two). The rotor is also uncomplicated, its design resembles a squirrel cage, and the windings are short-circuited or have slip rings.
In the case of a short-circuited rotor due to inductive induction from the stator currents in the rotor windings, an EMF occurs according to the rule of the right hand. Further, everything is simple: the two frames through which the electric current passes begin to interact with each other, and a torque arises.
Asynchronous machines, the rotor of which is equipped with slip rings, work even easier: power is supplied to the rotating windings directly through graphite brushes. Such rotors are also called phase rotors.
Single-phase asynchronous motors have two windings, working and starting, designed to create the initial torque and untwist the rotor to working angular speed. These motors are used where a three-phase network is not available, for example, to drive the rotating parts of household appliances.
In addition to induction motors, there are machines of the opposite purpose, generators. Their device is almost the same. To the credit of Russian electrical engineering, we can confidently talk about the priority of our country in the field of electric motors of this type. M.O. Dolivo-Dobrovolsky, as early as 1889, was the first in the world to apply three-phase power and received a rotating magnetic field. Modern asynchronous machines are fundamentally no different from the first three-phase electric motors of the great Russian inventor and scientist.