AC electric motors: circuit. AC and DC electric motors

In the article you will learn what AC motors are, consider their device, the principle of operation, the scope of application. It is worth noting that today in the industry more than 95 percent of all used engines are in asynchronous machines. They have become very popular due to the fact that they have high reliability, they can serve a very long time due to their maintainability.

Principle of operation of induction motors

To understand how the electric motor works, you can conduct a small experiment. Of course, this requires a special tool. Install the magnet in the shape of a horseshoe so that it is driven by a handle. As you know, a magnet has two poles. Between them it is necessary to arrange a cylinder made of copper. With such a calculation that it can freely rotate around its axis. Now the experiment itself. Begin to untwist the magnet, while creating a field that moves. Inside the copper cylinder begin to emerge eddy currents, which counteract the field of the magnet.

As a result, the copper cylinder begins to rotate in the direction in which the permanent magnet moves . And its speed is somewhat lower. The reason for this is that at equal speed the lines of force cease to intersect with the field of the magnet. The magnetic field rotates synchronously. But the speed of the magnet itself is not synchronous. And if you shorten the definition a little, it's asynchronous. Hence the name of the electric machine - an asynchronous electric motor. If it is rough, the scheme of the electric motor of an alternating current is approximately the same, as well as in the resulted experiment. Only the magnetic field is created by the stator winding.

Direct current motors

They are slightly different from AC induction motors. Firstly, it has one or two stator windings. Secondly, the way to change the rotor speed is somewhat different. But the direction of rotation of the rotor is reversed by reverse polarity (in asynchronous machines, the phases of the supply network change places). You can change the rotor speed of the DC motor by increasing or decreasing the voltage applied to the stator winding.

The DC motor can not operate without the field winding that is on the rotor. The transfer of voltage occurs by means of a brush assembly. This is the most unreliable element of the design. Brushes made of graphite, eventually erased, which leads to the failure of the motor, it needs repair. Note that DC and AC motors have the same elements, but their designs differ significantly.

Construction of the electric motor

Like any other non-static electric machine, the induction motor consists of two main parts - the stator and the rotor. The first element is fixed, three windings are placed on it, which are connected according to a certain scheme. The rotor is movable, its design is called a "squirrel cage". The reason for this name is that the internal arrangement is very similar to a wheel with a squirrel.

The latter, of course, is not in the electric motor. The rotor is centered using two lids mounted on the stator. They have bearings that facilitate rotation. An impeller is mounted on the rear of the electric motor. With its help, the electric machine is cooled. The stator has ribs that improve heat transfer. Thus, the AC motors operate in normal thermal mode.

Stator of induction motor

It should be noted that the stator of modern asynchronous electric motors has poles that are not expressed. In simpler terms, the entire surface is perfectly smooth. In order to reduce losses on eddy currents, the core is recruited from very thin steel sheets. These sheets are very closely adjacent to each other and subsequently fixed in a body of steel. The stator has grooves for laying the windings.

The windings are made of copper wire. Their connection is made in a "star" or "triangle". In the upper part of the case there is a small shield completely insulated. It contains the contacts for connecting and connecting the windings. And the windings can be connected by means of jumpers installed in this shield. The device of the electric motor of an alternating current allows to quickly connect the windings to the desired circuit.

Rotor of an asynchronous electric motor

About him was already a little told. It looks like a squirrel cage. The design of the rotor is assembled from thin steel sheets, like a stator. In the rotor grooves there is a winding, but it can be of several types. It all depends on whether the rotor is a phase or short-circuited rotor. The most common recent designs. Thick copper rods fit into grooves without insulation material. At both ends, these rods are joined by copper rings. Sometimes instead of "squirrel cage" cast rotors are used.

But there are still AC motors with a phase rotor. They are used much less often, mainly for electric motors, which have very high power. The second case in which it is necessary to use phase rotors in electric motors is the creation of a large force at the time of starting. However, for this it is necessary to use a special rheostat.

Methods for starting an induction motor

Running an asynchronous AC motor is not difficult, just connect the stator windings to a three-phase network. The connection is made using magnetic starters. Thanks to them, you can practically automate the launch. Even the reverse can be done without special difficulties. But in some cases it is necessary to reduce the voltage that is supplied to the stator windings.

This is due to the use of the "triangle" connection scheme. In this case, the start is made when the windings are connected in the "star" scheme. With increasing speed, the maximum value of the winding must be switched to the "triangle" scheme. This reduces the current consumption by about three times. But it is necessary to take into account that not every stator can function normally when connected using the "triangle" scheme.

Speed control

In industry and everyday life, frequency converters are becoming increasingly popular. With their help, you can change the speed of rotation of the rotor with an easy movement of the hand. It is worth noting that AC motors are used in conjunction with frequency converters in most mechanisms. It allows you to fine-tune the drive, without having to use magnetic starters. All controls are connected to the contacts on the frequency converter. Settings allow you to change the acceleration time of the motor's rotor, its stop, minimum and maximum speed, and many other protective functions.

Conclusion

Now you know how the AC motor works. Even studied the design of the most popular asynchronous motor. It is the cheapest of all that are on the market. In addition, for its normal operation, there is no need to use various auxiliary devices. In particular, the rheostats. And only such an addition, as a frequency converter, can facilitate the operation of an asynchronous electric motor, significantly expand its capabilities.