In the article you will learn what AC motors are, consider their device, principle of operation, scope. It is worth noting that today in the industry more than 95 percent of all used motors are asynchronous machines. They have become 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 do a little experiment. Of course, this requires a special tool. Install the horseshoe magnet so that it is driven by the handle. As you know, a magnet has two poles. Between them it is necessary to place a cylinder made of copper. With the expectation that it can freely rotate around its axis. Now the experiment itself. You begin to spin the magnet, this creates a field thatis moving. Eddy currents begin to appear inside the copper cylinder, which counteract the magnetic field.
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 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, then it is asynchronous. Hence the name of the electric machine - an asynchronous electric motor. Roughly speaking, the AC motor circuit is approximately the same as in the above experiment. Only the magnetic field is generated by the stator winding.
DC Motors
They are somewhat 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 mains are reversed). You can change the rotor speed of a DC motor by increasing or decreasing the voltage applied to the stator winding.
A DC motor cannot operate without the excitation winding that is on the rotor. The voltage is transmitted using a brush assembly. This is the most unreliable element of the design. Brushes made of graphite wear out over time, leading to failure.engine needs repair. Note that AC and DC motors have the same components, but their designs differ significantly.
Electric 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, three windings are placed on it, which are connected according to a certain scheme. The rotor is movable, its design is called "squirrel cage". The reason for this name is that the internal structure is very similar to a squirrel wheel.
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 make rotation easier. An impeller is installed on the back of the motor. With its help, the cooling of the electric machine is carried out. The stator has ribs that improve heat dissipation. Thus, AC motors operate in normal thermal conditions.
Induction motor stator
It is worth noting that the stator of modern asynchronous electric motors has unexpressed poles. To put it simply, the inside of the entire surface is perfectly smooth. In order to reduce eddy current losses, the core is made from very thin sheets of steel. These sheets are very close to each other and are subsequently fixed in a housing made ofbecome. The stator has slots for laying 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 contacts for connecting and connecting the windings. Moreover, you can connect the windings using jumpers installed in this shield. The device of the AC motor allows you to quickly connect the windings to the desired circuit.
Induction motor rotor
A little has already been said about him. It looks like a squirrel cage. The rotor structure 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 whether the phase or squirrel-cage rotor. The most common recent designs. Thick copper rods fit into the grooves without insulating material. Both ends of these rods are connected by copper rings. Sometimes cast rotors are used instead of a squirrel cage.
But there are also AC motors with a phase rotor. They are used much less frequently, 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 launch. True, for this you need to use a special rheostat.
Methods for starting an asynchronous motor
Starting an AC induction motor is easy, just connect the stator windings to a three-phase network. The connection is made using magnetic starters. Thanks to them, you can almost automate the launch. Even the 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" connection scheme. 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" scheme. In this case, the current consumption is reduced by about three times. But it must be taken into account that not every stator can function normally when connected according to the "delta" 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 a slight movement of your 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, while there is no need to use magnetic starters. All controls are connected to the contacts on the frequency converter. The settings allow you to change the time of acceleration of the rotor of the electric motor, its stop, the time of minimum and maximum speed, as well as many other protectivefunctions.
Conclusion
Now you know how an AC motor works. We 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 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 asynchronous electric motor, significantly expand its capabilities.
