In the article you will learn what AC electric motors are, consider their device, principle of operation, and scope. It is worth noting that today in the industry more than 95 percent of all engines used are asynchronous machines. They have become very widespread due to the fact that they have high reliability, they can serve for a very long time due to their maintainability.
The principle of operation of induction motors
To understand how an electric motor works, you can conduct a small experiment. Of course, this will require a special tool. Mount the horseshoe-shaped magnet so that it is driven by the handle. As you know, a magnet has two poles. Between them it is necessary to arrange a cylinder made of copper. In such a way that it can rotate freely around its axis. Now the experiment itself. You begin to unwind the magnet, while creating a field that moves. Inside the copper cylinder eddy currents begin to occur , which counteract the field of the magnet.
As a result of this, the copper cylinder begins to rotate in the direction in which the permanent magnet moves . Moreover, its speed is somewhat lower. The reason for this is that at equal speed the lines of force cease to intersect with the magnet field. The magnetic field rotates synchronously. But the speed of the magnet itself is not synchronous. And if you slightly reduce the definition, then it is asynchronous. Hence the name of the electric machine - an asynchronous electric motor. If roughly, the circuit of the AC motor is approximately the same as in the given experiment. Only a magnetic field is created by a stator winding.
DC motors
They are slightly different from AC induction motors. Firstly, it has one or two stator windings. Secondly, the method of changing the rotor speed is somewhat different. But the direction of rotation of the rotor is changed by polarity reversal (for asynchronous machines, the phases of the supply network are reversed). You can change the rotor speed of a DC motor if you increase or decrease the voltage supplied to the stator winding.
A DC motor cannot operate without a field winding, which is located on the rotor. Voltage transmission is carried out using a brush assembly. This is the most unreliable structural element. Brushes made of graphite wear out over time, which leads to failure of the motor, it needs repair. Note that DC and AC motors have the same elements, but their designs differ significantly.
Motor design
Like any other non-static electric machine, an induction motor consists of two main parts - a stator and a rotor. The first element is fixed, it contains three windings that are connected in a certain pattern. The rotor is mobile, its design is called the "squirrel cage." The reason for this name is that the internal structure 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 covers mounted on the stator. They have bearings that facilitate rotation. An impeller is mounted on the back of the electric motor. With its help, the cooling of an electric machine is carried out. On the stator are made ribs that improve heat transfer. Thus, AC motors operate in normal thermal conditions.
Asynchronous motor stator
It is worth noting that the stator of modern asynchronous electric motors has unexpressed poles. Simply put, inside the entire surface is perfectly smooth. In order to reduce eddy current losses, the core is composed of very thin sheets of steel. These sheets fit very tightly together and are subsequently fixed in a steel casing. The stator has grooves for laying the windings.
The windings are made of copper wire. Their connection is made into a "star" or "triangle". In the upper part of the case there is a small shield, completely insulated. It contains contacts for connecting and connecting the windings. Moreover, the windings can be connected using jumpers installed in this shield. The AC electric motor device allows you to quickly connect the windings to the desired circuit.
Induction motor rotor
A little has been said about him. It looks like a squirrel cage. The rotor design is assembled from thin steel sheets, like the stator. There is a winding in the grooves of the rotor, but it can be of several types. It all depends on the phase or squirrel-cage rotor. The most common recent designs. Thick copper rods fit into grooves without insulating material. At both ends, these rods are connected by copper rings. Sometimes instead of a "squirrel cage" cast rotors are used.
But there are also 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 lot of force at the time of launch. True, for this it is necessary to use a special rheostat.
Ways to start an induction motor
It is not difficult to start an asynchronous AC motor, it is only enough to connect the stator windings to a three-phase network. Connection is made using magnetic starters. Thanks to them, you can almost automate the launch. Even a reverse can be done without much difficulty. But in some cases it is necessary to reduce the voltage that is supplied to the stator windings.
This is done through the use of a “triangle” type connection circuit. In this case, the start is made when the windings are connected according to the "star" scheme. With an increase in the number of revolutions, reaching the maximum value of the winding, it is necessary to switch to the "triangle" circuit. In this case, the current consumption decreases by about three times. But it must be borne in mind that not every stator can function normally when connected according to the "triangle" scheme.
Speed control
In industry and everyday life, frequency converters are becoming increasingly popular. With their help, you can change the rotor speed with a slight 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, with no need 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 rotor of the electric motor, its stop, the time of the minimum and maximum speed, as well as many other protective functions.
Conclusion
Now you know how the AC electric motor works. They even studied the design of the most popular induction motor. It is the cheapest of all that are on the market. In addition, for its normal functioning there is no need to use various auxiliary devices. In particular, rheostats. And only such an addition as a frequency converter can facilitate the operation of an induction motor, significantly expand its capabilities.