The rotor moves "by itself". It initially does not contain magnetic field, no electrical voltage is applied to it. It does not even have to be made of iron - a magnetic metal. Well, come on, it’s worth connecting a three-phase voltage to the motor, and the rotor spins. Without any push. But in my own way.
Two kinds of AC motors
Induction motors - naive simplicity
The rotor either catches up with the wave, or slightly lags behind, because it simply cannot run synchronously with it. This phenomenon was called "slip", having caught up with the traveling magnetic field, the rotor with the squirrel cage loses its magnetic induction and then simply slides by inertia for some time. And when friction or load forces him to lag behind the moving field, he will again “feel” in himself the changes in the lines of force of the field overtaking him and will again acquire induction, and with it the strength to move.
That is, the rotor slips slightly: either it catches up with the magnetic field running evenly in a circle, then it “forgets why it ran” and falls slightly behind, then it “recovers” again and again strives to catch up. Gradually, these deviations stabilize - depending on the friction in the bearings and the magnitude of the load on the shaft - and the asynchronous motor begins to operate simply at a rotation speed slightly lower than the frequency of the voltage on the stator. This frequency difference is called the slip frequency.
Synchronous motors: complex in simple
In order for the rotor to be connected to the traveling wave of the magnetic field of the stator coils in a rigid way, a synchronous electric motor was invented. And the problem is easily solved. In the rotor, instead of a changing magnetic field from short-circuited currents of a squirrel cage, permanent magnets and their magnetic field should be used.
There are two options. Or is this field from permanent magnet fixed in the rotor, or is it a field from electromagnets installed in the rotor instead of such a magnet.
An ordinary magnet, of course, is simpler. But then for the standard functioning of such electric motors, it is necessary that all of them - and thousands of electric motors are used - have exactly the same magnets. Otherwise, the motion parameters will be different, and the magnets still tend to demagnetize.
An electromagnet installed in the motor rotor is easier to make to produce a field of the desired quality, but it is required electricity for his work. Such a current, which is called the excitation current, in turn must be taken somewhere and somehow fed to the rotor.
1 - rotor,
2 - excitation collector
This is where a certain variety of designs of synchronous motors comes from. But most importantly, synchronous motors rotate their shaft strictly synchronously with the frequency of the stator coil field running in a circle, that is, their rotation speed is exactly equal to - or a multiple (if the stator windings are more than three) - frequency alternating current in the supply network.
However, among other things, a synchronous motor has the property of complete reversibility. Because a synchronous motor is the same electric current generator, but operating "in reverse side". In the generator, some mechanical force rotates the shaft with the rotor, and from this, an induced voltage occurs in the stator windings. electrical voltage from the rotating magnetic field of the rotor. And the difference synchronous motor from the generator in that the voltage in the stator coils generates a magnetic field running in a circle, which, interacting with the constant magnetic field of the rotor, pushes it so that the rotor also rotates.
Only if in the generator the rotation of the rotor can be mechanically given any speed, and this will change the frequency of the alternating current generated by it, then there is no such luxury in a synchronous motor. A synchronous motor rotates at the rate of change in the voltage in the network, and we maintain it strictly at 50 hertz.
Differences and disadvantages of these engines
The differences between synchronous and asynchronous motors are clear from their names. Actually, both design options have advantages. Below are the advantages that both motors differ in - synchronous and asynchronous.
An asynchronous motor differs from a synchronous motor in the following parameters:
- simplicity of design and low cost;
- no sliding contacts, reliable operation;
- voltage is applied to the fixed stator coils;
- the rotor is very simple in design;
- when starting and accelerating, it gradually increases power;
- the ability to reverse the direction of rotation by simply swapping two supply phases;
- when the movement stops (too much mechanical load on the rotor shaft), no accident occurs, the squirrel cage may overheat.
The differences between a synchronous motor and an asynchronous motor are as follows:
- stable rotation speed regardless of the load on the shaft;
- low sensitivity to voltage drops in the network;
- when the mechanical load is reduced, it is able to work as a generator by inertia, not taking energy, but giving it to the network;
- high efficiency;
- able to compensate for the reactive power of the network.
But each has its own disadvantages.
Asynchronous has the following negative traits:
- difficulty adjusting the speed;
- low speed;
- dependence of the speed lag on the axle load;
- during operation, the rotor heats up due to short-circuited currents - additional cooling is required.
Disadvantages of a synchronous motor:
- more complex in design
- in some designs, a collector is used to conduct the excitation current to the rotor windings, as in a motor direct current;
- harder to start.
Despite their differences, both electric motor have found application in technology and are used in a variety of designs and sizes.
Before you figure out what their difference is, you need to find out what an electric motor is? An electric motor is an electrical machine that is powered by electricity and serves as a drive for other mechanisms.
Explanation of the principle of operation of a synchronous electric motor for "dummies"
From childhood, we remember that two magnets, if they are brought closer to each other, in one case attract, and in the other they repel. This happens, depending on the fact that with what sides of the magnets we connect them, opposite poles attract, and the same poles repel. These are permanent magnets, in which the magnetic field is constantly present. There are also variable magnets.
In a school textbook on physics there is a drawing depicting an electromagnet in the form of a horseshoe and a frame with half rings at the ends, which is located between its poles.
When the frame is located in a horizontal position in the space between the poles of the magnets, due to the fact that the magnet attracts opposite poles and repels the same, a current of the same sign is applied to the frame. An electromagnetic field appears around the frame (here is an example of a variable magnet!), the poles of the magnets attract the frame, and it turns into a vertical position. When the vertical is reached, a current of the opposite sign is applied to the frame, the electromagnetic field of the frame changes polarity, and the poles of the permanent magnet begin to repel the frame, rotating it to a horizontal position, after which the rotation cycle is repeated.
This is the principle of operation of an electric motor. Moreover, a primitive synchronous electric motor!
So, a primitive synchronous electric motor works when current is applied to the loop. In a real synchronous electric motor, the role of the frame is performed by a rotor with coils of wires, called windings, to which current is supplied (they serve as sources of an electromagnetic field). And the role of a horseshoe magnet is performed by a stator made either from a set of permanent magnets, or also from coils of wires (windings), which, when current is applied, are also sources of an electromagnetic field.
The rotor of a synchronous motor will rotate at the same frequency as the current supplied to the winding terminals changes, i.e. synchronously. Hence the name of this electric motor. 
Explanation of the principle of operation of an asynchronous electric motor for "dummies"
We recall the description of the figure in the previous example. The same frame, located between the poles of a horseshoe magnet, only its ends do not have half rings, they are interconnected.
Now we begin to rotate a horseshoe magnet around the frame. We rotate it slowly and observe the behavior of the frame. Until some time, the frame remains motionless, and then, when the magnet is rotated at a certain angle, the frame begins to rotate after the magnet. The rotation of the frame is delayed compared to the speed of rotation of the magnet, i.e. it rotates out of sync with it - asynchronously. So it turns out that this is a primitive asynchronous electric motor.
In fact, the role of magnets in a real induction motor is the windings located in the stator slots, which are energized. And the role of the frame is performed by the rotor, into the grooves of which metal plates are inserted, interconnected shortly. Therefore, such a rotor is called squirrel-cage. 
What is the difference between synchronous and asynchronous motors?
If you put two modern electric motor one and the other type, then outward signs it is difficult to distinguish them even for a specialist.
In essence, their main difference is considered in the examples of the principles of operation of these electric motors. They differ according to the design of the rotors. The rotor of a synchronous motor consists of windings, and the rotor of an asynchronous motor is a set of plates.
The stators of one and the other electric motors are almost indistinguishable and are a set of windings, however, the stator of a synchronous electric motor can be recruited from permanent magnets.
The speed of a synchronous motor corresponds to the frequency of the current supplied to it, and the speed of an asynchronous motor lags behind the frequency of the current.
They differ and by application. For example, synchronous motors set to drive equipment that works with constant speed rotation (pumps, compressors, etc.) without reducing it with increasing load. But asynchronous electric motors reduce the speed with increasing load.
Synchronous motors are structurally more complex, and therefore more expensive than asynchronous motors.
Electric motors are machines that convert electrical energy into mechanical energy. The converted energy drives the motor rotor, which transmits rotation through the transmission directly to the shaft executive mechanism. The main types of electric motors are synchronous and asynchronous motors. The differences between them determine the possibilities of using in various devices and technological processes.
Work principles
All electric motors have a fixed stator and a rotating rotor. The difference between asynchronous and synchronous motors lies in the principles of creating poles. In an asynchronous electric motor, they are created by the phenomenon of induction. All other electric motors use permanent magnets or current-carrying coils to create a magnetic field.
Features of synchronous motors
Leading units of the synchronous machine - anchor and inductor. The armature is the stator, and the inductor is located on the rotor. Under the influence of alternating current, a rotating magnetic field is formed in the armature. It couples with the magnetic field of the inductor, formed by the poles of permanent magnets or coils with direct current. As a result of this interaction, the energy of electricity is converted into kinetic energy of rotation.
The rotor of a synchronous machine has the same speed as that of the stator field. Advantages of synchronous motors:
- Structurally, it is used both as an engine and as a generator.
- Load-independent speed.
- Great efficiency.
- Low labor intensity in repair and maintenance.
- High degree reliability.
Synchronous machines are widely used as electric motors. high power for low speed and constant load. Generators are used where an independent power source is required.
The synchronous machine also has disadvantages:
- Requires a DC source to power the inductor.
- There is no initial starting torque; starting requires external torque or asynchronous start.
- Brushes and collectors quickly fail.
Modern synchronous units contain in the inductor, in addition to the winding supplied with direct current, also a starting short-circuited winding, which is designed to start in asynchronous mode.
Distinctive features of asynchronous motors
The rotating magnetic field of the stator of an induction motor induces induction currents in the rotor, which form their own magnetic field. The interaction of the fields causes the rotor to rotate. In this case, the rotor speed lags behind the magnetic field speed. It is this property that is reflected in the name of the engine.
Asynchronous motors are of two types: with a squirrel-cage and with a phase rotor.
Household appliances, such as a fan or a vacuum cleaner, are usually equipped with squirrel-cage motors, which is a "squirrel wheel". All rods are closed with discs welded on both sides. The interaction of the stator magnetic field with the induced currents in the rotor forms an electromagnetic force that acts on the rotor in the direction of rotation of the stator field. The torque on the motor shaft is created by all the electromagnetic forces from each conductor.
In the electric motor with a phase rotor, the same stator is used as for the motor with a squirrel-cage rotor. And windings are added to the rotor three phases connected in a star. When starting the engine, they can be connected to rheostats that regulate starting currents. With the help of rheostats, you can also adjust the engine speed.
The advantages of asynchronous motors include:
- Powered directly from AC mains.
- Simplicity of the device and relatively low cost.
- Possibility of use in household appliances using a single-phase connection.
- Low energy consumption and economical.
Serious disadvantages- complex speed control and large heat losses. To prevent overheating, the body of the unit is ribbed, and an impeller for cooling is installed on the motor shaft.
The difference in the characteristics of electric motors
Design features and performance characteristics of electric motors are crucial when choosing units. The design of transmissions and all power units mechanisms. When choosing an engine, you need to rely on the commonality and main differences in the properties of the machines:
Synchronous and asynchronous electric motors each find their own application. Synchronous motors are recommended for use everywhere high power, where there is a continuous manufacturing process and you do not need to frequently restart the units or adjust the speed. They are used in conveyors, rolling mills, compressors, stone crushers, etc. A modern synchronous motor has the same quick launch, like asynchronous, but it is smaller and more economical than asynchronous, equal in power.
Asynchronous motors with a phase rotor are used where a large starting torque is needed and frequent stops aggregates. For example, in elevators and tower cranes. Asynchronous electric motors with a squirrel-cage rotor are widely used due to the simplicity of the device and ease of operation.
Using the advantages of different units and how a synchronous motor differs from an asynchronous one, you can make a reasonable choice of one or another motor when designing machines, machine tools and other equipment.
All electric motors are based on the principle of electromagnetic induction. The electric motor consists of a fixed part - a stator (for asynchronous and synchronous AC motors) or an inductor (for DC motors) and a moving part - a rotor (for asynchronous and synchronous AC motors) or an armature (for DC motors). as an inductor small engines DC often used permanent magnets.
All engines, roughly speaking, can be divided into two types:
DC motors
AC motors (asynchronous and synchronous)
DC motors
According to some opinions this engine can also be called a synchronous DC machine with self-synchronization. A simple engine, which is a DC machine, consists of a permanent magnet on an inductor (stator), 1 electromagnet with clearly pronounced poles on the armature (two-prong armature with pronounced poles and one winding), a brush-collector assembly with 2 plates (lamellas ) and 2 brushes.A simple motor has 2 rotor positions (2 "dead spots"), from which self-starting is not feasible, and uneven torque. In the first approximation, the magnetic field of the stator poles is uniform (homogeneous).
These motors with the presence of a brush-collector assembly are:
Collector - electrical device, in which the rotor position sensor and the current switch in the windings are the same device - a brush-collector assembly.
Brushless- a closed electromechanical system consisting of a synchronous device with a sinusoidal distribution of the magnetic field in the gap, a rotor position sensor, a coordinate converter and a power amplifier. A more expensive option compared to collector engines.
AC motors
According to the type of operation, these motors are divided into synchronous and asynchronous motors. The fundamental difference is that in synchronous machines the 1st harmonic of the stator magnetomotive force moves at the speed of rotation of the rotor (therefore, the rotor itself rotates at the speed of rotation of the magnetic field in the stator), while in asynchronous machines there is and remains a difference between the speed of rotation of the rotor and the speed of rotation of the magnetic field in the stator (the field rotates faster than the rotor).Synchronous- an alternating current motor, the rotor of which rotates synchronously with the magnetic field of the supply voltage. These engines are traditionally used at huge power (hundreds of kilowatts and above).
There are synchronous motors with discrete angular motion of the rotor − stepper motors. For them, this position of the rotor is fixed by supplying power to the corresponding windings. The transition to another position is performed by removing the supply voltage from some windings and transferring it to other motor windings.
Another type of synchronous motors is a valve reluctance motor, the power supply of the windings of which is formed using semiconductor elements.
Asynchronous- an alternating current motor, in which the rotor speed differs from the frequency of the torsional magnetic field created by the supply voltage, the second name of asynchronous machines - induction is justified by the fact that the current in the rotor winding is induced by the rotating stator field. Asynchronous machines now form a huge part of electrical machines. Basically, they are used in the form of electric motors and are considered key converters of electrical energy into mechanical energy, and asynchronous motors with a squirrel-cage rotor are mainly used.
By the number of phases, motors are:
- single-phase
- two-phase
- three-phase
The most popular and popular engines that are used in production and households:
Single-phase asynchronous squirrel-cage motor
A single-phase asynchronous motor has only 1 working winding on the stator, to which alternating current is supplied during motor operation. Although, to start the motor, there is also an auxiliary winding on its stator, which is connected to the network for a short time through a capacitor or inductance, or is short-circuited by the starting contacts of the knife switch. This is necessary to create an initial phase shift so that the rotor starts to spin, otherwise the pulsating magnetic field of the stator would not move the rotor from its place.
The rotor of such a motor, like any other a synchronous motor with a squirrel-cage rotor, is a cylindrical core with aluminum-filled grooves, with immediately cast ventilation blades.
Such a rotor is called a squirrel-cage rotor. Single-phase motors are used in low-power devices, including room fans or small pumps.
Two-phase asynchronous squirrel-cage motor
Two-phase asynchronous motors are more efficient when operated from a single-phase AC mains. They contain two working windings on the stator that are perpendicular, while one of the windings is connected to the AC network directly, and the second through a phase-shifting capacitor, this is how a rotating magnetic field comes out, but without a capacitor, the rotor would not move.
These motors, among other things, have a squirrel-cage rotor, and their use is even more extensive than that of single-phase ones. Here already washing machines and various machines. Two-phase motors for power supply from single-phase networks are called capacitor motors, because a phase-shifting capacitor is often considered an indispensable part of them.
Three-phase asynchronous squirrel-cage motor
three-phase asynchronous motor has three working windings on the stator, shifted relative to each other so that when connected to a three-phase network, their magnetic fields are shifted in space relative to each other by 120 degrees. When turned on three-phase motor to a three-phase AC network, a rotating magnetic field appears, leading to the movement of a squirrel-cage rotor.
The stator windings of a three-phase motor can be connected according to the “star” or “triangle” scheme, while powering the motor according to the “star” scheme will require a voltage higher than for the “triangle” scheme, and therefore 2 voltages are indicated on the engine, for example: 127/220 or 220/380. Three-phase motors are indispensable for driving various machine tools, winches, circular saws, cranes, etc.
Three-phase asynchronous motor with phase rotor
A three-phase asynchronous motor with a phase rotor has a stator similar to the types of motors described above, a laminated magnetic circuit with 3 windings laid in its grooves, but duralumin rods are not poured into the phase rotor, but a real three-phase winding is already laid, in the star connection. The ends of the phase rotor winding star are brought out to three slip rings mounted on the rotor shaft and electrically separated from it.
By means of brushes, three-phase alternating voltage is supplied to the rings, among other things, and the inclusion can be carried out both directly and through rheostats. Of course, slip-ring motors are more expensive, although their starting torque under load is much higher than that of squirrel-cage motors. It is in consequence of the overestimated force and huge starting torque, this species engines found use in the drives of elevators and cranes, in other words, where the device starts under load and not at idle, like engines with a squirrel-cage rotor.
Everyone knows that the main purpose of electric motors is the conversion of electrical energy into mechanical energy. This was discovered already in 1821 by Michael Faraday, who conducted experiments with magnets and a magnetic field. Since then, a lot of time has passed, and electric motors have taken their main place in industry and everyday life. Without them today nowhere. Motor manufacturers currently offer a large number of models differing in design and principle of operation. These are DC and AC motors, synchronous and asynchronous. Today we are interested in exactly the synchronous and asynchronous motor - the differences.
To understand the differences, we need to consider design features each type of motor and understand how it works.
Asynchronous motor
So, we need to start by looking at the design of the asynchronous model. The main difference from synchronous is the presence of three windings in the stator, the ends of which are brought out for connection to the terminal box. The second main part of the motor is a one-piece type rotor, the ends of which are closed to each other, hence, in principle, the name is short-circuited.
An addition to the design is the impeller, with which the engine is cooled. The impeller is mounted on the shaft (rotor) of the electric motor. The rotor itself is supported and rotated in bearings mounted in two housing covers. Please note that it is the bearings that are the most vulnerable point of the unit. They are the ones that fail the most. True, it is not very difficult to replace them.
Principle of operation
How does an asynchronous motor work? Inside the motor housing, where the stator windings are located, a magnetic field arises that acts on the rotor, causing it to rotate under the action of the resulting electromotive force. But the rotation of the rotor can only be if the speed of rotation of the magnetic field is faster than the rotation of the motor shaft itself. If the speeds are the same, then electromotive force will not appear.
But in any case, this cannot happen, because there are several reasons that restrain the speed of rotation of the rotor.
- friction in bearings.
But most importantly, the magnetic poles in an induction motor are constantly changing, which affects the change in current directions in the stator of the electric motor. That is, at a certain time, the current begins to rotate "on us", and in the next interval "away from us". That is why such motors are called asynchronous, they simply do not have a stable current direction.
As for the speed of rotation of the rotor, one remark must be made here. This indicator will depend on how many poles are simultaneously connected to the power supply. Eg, maximum speed shaft rotation will be with two connected poles. To reduce this figure, you need to add two more poles, that is, double them.
And one more drawback. Asynchronous motors during operation have different speed shaft rotation. For example, on Idling it can be one value, it drops sharply under load. In fact, it turns out that changing the frequency of the current affects the speed of the shaft. There is no other way to change the rotation speed.
Synchronous motor
So, a synchronous electric motor is a motor with a constant rotor speed, plus the ability to adjust this speed. The device of a synchronous motor is quite complicated. To understand it, you need to consider the photo below.
It clearly shows that the motor windings are located on the armature or rotor of the unit. The ends of the windings are brought out and fixed to the current collector ring, or rather, to its sectors. The current itself is supplied to the same ring only through graphite brushes that are connected to the mains.
Attention! The ends of the windings are connected in such a way that when the motor is running through the brushes, the electric current always falls on only one pair.
The engine of this model has more vulnerabilities than asynchronous.
- Graphite brushes wear out.
- Poor contact between the slip ring and the brushes due to the weakening of the spring, which presses the latter against the ring (collector).
- Bearings wear out.
- The formation of dirt deposits on the surface of the slip ring.
Now let's move on to another position - the principle of operation of a synchronous electric motor. The torque inside the motor is formed due to the interaction of the magnetic field, which is formed in the excitation windings, and the current passing through the armature of the unit. But there is one point - the changing direction of the current (alternating) will also change the direction of rotation of the magnetic field of the engine. True, the change of rotation will change both in the body of the apparatus and at anchor at the same time. That is why the rotation of the motor rotor always occurs at the same speed.
That is why this value can be changed only by changing the voltage of the electricity supplied to the brushes. Remember vacuum cleaners, where the suction power is changed by a switch that is simply connected to a rheostat. And the power of the vacuum cleaner depends on the speed of rotation of the impeller shaft, that is, the motor shaft. How more speed, topics more power suction.
But synchronous electric motors have not found their main place in industry. Here, asynchronous models are mainly used.
Which is better
So, in the article the device and the principle of operation of two types of electric motors were disassembled. It is impossible to say that one of them is better. But we note that asynchronous models are simpler in a constructive aspect. They are more reliable in operation. If they are not overloaded, the service life can be very long. Unfortunately, synchronous species cannot boast of this. Graphite brushes wear out quickly and need to be replaced. But if you do not keep track, and the graphite is completely erased, then the metal brush holders will begin to abrade the slip ring. And its failure is not only a complete failure of the engine, it is a large number of sparks (metal-to-metal friction) and the possibility of more serious troubles.
