Asynchronous motor: device, principle of operation, purpose

An asynchronous motor is simple and reliable and therefore is very often used in production and in household appliances, from the drive of the valves to the rotation of the drum in the washing machine. In this article, we will talk in simple words about what kind of asynchronous electric motors are, what they are and how this type of electric machines works.

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Asynchronous motors (IM) are divided into two main groups:

• with a squirrel-cage rotor (SC);
• with a phase rotor.

If we omit the nuances, then the difference lies in the fact that a squirrel-cage motor has no brushes and pronounced windings, it is less demanding in maintenance. Whereas in induction motors with a phase rotor there are three windings connected to slip rings, the current from which is removed by brushes. Unlike the previous one, it is better amenable to the regulation of the torque on the shaft and it is easier to implement a soft start to reduce inrush currents.

The rest of the engines are classified:

• by the number of supply phases - single-phase and two-phase (used in everyday life when powered from a 220V network), and three-phase (most widespread in production and in workshops).
• by the method of fastening - flanged or on the legs.
• by mode of operation - for long-term, short-term or intermittent mode.

And a number of other factors that influence the choice of a particular product for use in a particular environment.

A lot can be said about single-phase electric motors: some of them are started through a capacitor, and some require both a starting and working capacitance. There are also options with a short-circuited loop, which work without a capacitor and are used, for example, in hoods. If you are interested, write in the comments and we will write an article about it.

Device

By definition, "asynchronous" is an AC motor, in which the rotor rotates slower than the stator magnetic field, that is, asynchronously. But this definition is not very informative. To understand it, you need to understand how this engine works.

An induction motor, like any other, consists of two main parts - rotor and stator. Let's decipher "For dummies" in electrics:

• A stator is the stationary part of any generator or electric motor.
• The rotor is the rotating part of the engine, which drives the mechanisms.

The stator consists of a housing, the ends of which are closed by end shields, in which bearings are installed. Plain or rolling bearings are used depending on the purpose and power of the engine. A core is located in the body, a winding is installed on it. It is called the stator winding.

Since the current is alternating, in order to reduce losses due to stray currents (Foucault currents) the stator core is recruited from thin steel plates, insulated from each other by scale and fastened with varnish. A supply voltage is applied to the stator windings, the current flowing in them is called the stator current.

The number of windings depends on the number of supply phases and the design of the motor. So a three-phase motor has at least three windings connected in a star or delta pattern. Their number may be greater, and it affects the speed of rotation of the shaft, but we will talk about this later.

But with the rotor, things are more interesting, as already mentioned, it can be either short-circuited or phase.

A squirrel-cage rotor is a set of metal rods (usually aluminum or copper), in the figure above they are indicated by the number 2, soldered or cast into the core (1) closed by rings (3). This design resembles a wheel in which domesticated rodents run, which is why it is often called a "squirrel cage" or "squirrel wheel" and this name is not jargon, but quite literary. To reduce the higher harmonics of the EMF and pulsations of the magnetic field, the rods are laid not along the shaft, but at a certain angle relative to the axis of rotation.

The phase rotor differs from the previous one in that it already has three windings, as on the stator. The beginning of the windings are connected to rings, usually copper, they are pressed onto the motor shaft. Later we will briefly explain why they are needed.

In both cases, one of the ends of the shaft is connected to the driven mechanism, it is made conical or cylindrical shape with or without grooves, for mounting flange, pulley and other mechanical drive details.

On the "back" part of the shaft, an impeller is fixed, which is necessary for blowing and cooling, a casing is put on the casing over the impeller. Thus, cold air is directed along the edges of the induction motor, if this impeller for some reason does not rotate, it will overheat.

The design of the first induction motor was developed by M.O. Dolivo-Dobrovolsky and he patented it in 1889. It has survived to the present day without any significant changes.

Principle of operation

Asynchronous electric machines are often called induction machines due to their principle of operation. Any electric motor is set in rotation as a result of the interaction of the magnetic fields of the rotor and stator, as well as due to the Ampere force. The magnetic field, in turn, can exist either around a permanent magnet or around a conductor through which current flows. But how exactly does an asynchronous machine work?

In an induction motor, unlike others, there is no excitation winding as such, then how does it get a magnetic field? The answer is simple: an induction motor is a transformer.

Let's consider the principle of its operation using the example of a three-phase machine, since it is they that are found more often than others.

In the figure below, you can see the location of the windings on the stator core of a three-phase asynchronous motor.

As a result of the flow of three-phase current in the stator windings, a rotating magnetic field appears. Due to the phase shift, the current flows through one or the other winding, in accordance with this, a magnetic field arises, the poles of which are directed according to the rule of the right hand. And in accordance with the change in current in a particular winding, the poles are directed in the corresponding direction. Which the following animation illustrates:

In the simplest (two-pole) case, the windings are laid in such a way that each of them is offset by 120 degrees relative to the previous one, as is the phase angle of the voltage in the AC network.

The rotation speed of the stator magnetic field is usually called synchronous. Learn more about how it rotates and why you will find out in the following video. Note that in two-phase (capacitor) and single-phase electric motors, it is not rotating, but elliptical or pulsating, and the windings are not 3, but 2.

If we consider an asynchronous electric motor with a squirrel-cage rotor, then the magnetic field of the stator induces an EMF in its rods, since they are closed, then a current begins to flow. This also creates a magnetic field.

As a result of the interaction of two fields and Ampere forceacting on the rotor, it begins to rotate following the rotating magnetic field of the stator, but at the same time it is always slightly lagging behind the rotation speed of the stator MF, this lag is called slip.

If the speed of rotation of the magnetic field is called synchronous, then the speed of rotation of the rotor is already asynchronous, from which it got its name.

For an AD with a phase rotor, things are similar, except that they connect to its rings rheostat, which, after the engine enters the operating mode, is removed from the circuit and the windings are closed shortly. This is shown in the diagram below, but instead of a rheostat, constant resistors are used, connected or shunted by the KM3, KM2, KM1 contactors.

This approach allows for a smooth start and reduce starting currents by increasing the active electrical resistance of the rotor.

Let's summarize:

1. The current in the stator windings generates a magnetic field.
2. The magnetic field generates a current in the rotor.
3. The current in the rotor creates a field around it.
4. Since the stator field rotates, because of its field, the rotor begins to rotate behind it.

Glide and rotation speed

The stator magnetic field speed (n1) is greater than the rotor speed (n2). The difference between them is called slip, and is denoted by the Latin letter S and is calculated by the formula:

S = (n1-n2) * 100% / n1

Slipping is not a disadvantage of this electric motor, since if its shaft rotated at the same frequency, as the magnetic field of the stator (synchronously), then no current would be induced in its rods, and it would simply not become rotate.

Now about a more important concept - the rotor speed of an induction motor. It depends on 3 quantities:

• supply voltage frequency (f);
• number of pairs of magnetic poles (p);
• slip (S).

The number of pairs of magnetic poles determines the synchronous rotation speed of the field and depends on the number of stator windings. The slip depends on the load and the design of a particular electric motor and lies in the range of 3-10%, that is, the asynchronous speed is quite a bit less than the synchronous one. Well, the frequency of the alternating current is fixed at us and equals 50 Hz.

Therefore, the frequency of rotation of the shaft of an asynchronous motor is difficult to regulate, you can only influence the frequency of the supply network, that is, by setting a frequency converter. It is possible to lower the stator voltage, but then the power on the shaft decreases, nevertheless, such a technique used when starting up the IM with switching the windings from star to delta to reduce starting currents.

The frequency of rotation of the stator field (synchronous speed) is determined by the formula:

n = 60 * f / p

So in a motor with one pair of magnetic poles (two poles), the synchronous speed is:

60 * 50/1 = 3000 rpm

The most common options for electric motors with:

• one pair of poles (3000 rpm);
• two (1500 rpm);
• three (1000 rpm);
• four (750 rpm).

The real rotor speed will be slightly lower, on a real asynchronous motor it is indicated on the nameplate, for example, here - 2730 rpm. Despite this, the people will call such an asynchronous motor according to the synchronous speed or simply "three thousandth".

Then its slip is equal to:

3000-2730*100%/3000=9%

Scope of application

The asynchronous electric motor has found application in all areas of human activity. Those that are powered from one phase (from 220V) can be found in low-power actuators or in household appliances and tools, for example:

• in a "baby" type washing machine and other old Soviet models;
• in a concrete mixer;
• in the fan;
• in the hood;
• and even in high-end lawn mowers.

In production in three-phase networks:

• automatic latches;
• hoisting mechanisms (cranes and winches);
• ventilation;
• compressors;
• pumps;
• wood and metalworking machines and more.

Also, AD is used in electric transport, and recently an induction motor has been actively advertised on the Internet. with a winding of the "Slavyanka" type and the so-called Duyunov wheel motor, which you can learn from the video developer.

The field of application of induction motors is so extensive that the list alone will be longer. than this article, so every electrician should know how it works, what it is for and where applies. Let's summarize and list the pros and cons of these devices.

Pros:

1. Simple construction.
2. Low cost.
3. Almost maintenance free.

The main disadvantage is the difficulty of adjusting the speed, in comparison with the same DC motors or universal collector machines. Accordingly, it is difficult to organize a smooth start of large machines, and more often this is done with the help of an expensive frequency converter.

This is where we end the review of asynchronous electric motors and their areas of application. We hope that after reading the article, it became clear to you what it is and how this electric machine works!

Related materials:

• How to choose a frequency converter for power and current
• The difference between alternating current and direct current
• Phase and line voltage