Comparison of synchronous and asynchronous high-voltage electric motors. Asynchronous and synchronous motors

31.03.2019

Engine

The fundamental difference between a synchronous motor and an asynchronous motor is the design of the rotor. The latter in a synchronous motor is a magnet made (at relatively low power) on the basis of a permanent magnet or on the basis of an electromagnet. Since the opposite poles of the magnets are attracted, the rotating magnetic field of the stator, which can be interpreted as a rotating magnet, drags the magnetic rotor along with it, and their speeds are equal. This explains the name of the motor - synchronous.

In conclusion, we note that, unlike an asynchronous motor, which usually does not exceed 0.8 ... 0.85, a synchronous motor can achieve a larger value and even make it so that the current will lead the voltage in phase. In this case, like capacitor banks, a synchronous machine is used to improve the power factor.

Asynchronous motors have simple design and reliable in operation. The disadvantage of asynchronous motors is the difficulty of regulating their speed.

To reverse a three-phase synchronous motor(change the direction of rotation of the motor to the opposite), it is necessary to swap two phases, that is, swap any two linear wires suitable for the motor stator winding.

That is, it is a fairly cheap motor that is used everywhere, it is extremely difficult to find a synchronous machine.

Unlike an induction motor, the rotational speed of a synchronous motor is constant under different loads. Synchronous motors are used to drive machines constant speed(pumps, compressors, fans) they are easy to control.

You can distinguish by the number of revolutions on the plate (if the type of machine is not explicitly indicated there), the asynchronous machine has a non-round number of revolutions, 950 rpm for a synchronous machine 1000 rpm.

Synchronous motors are controlled as difficult as asynchronous ones, because require frequency control of the input voltage. They are absolutely rigid. mechanical characteristic, this means that no matter how the load on the motor shaft changes, it will have the same speed. Naturally, the load must change within reasonable limits, there is a value of the critical load moment at which the engine "falls out" of the synchronous mode, which is fraught with its breakdown. The main disadvantages include the fact that the excitation winding must be powered direct current, also the presence of a sliding contact " brush slip ring", the complexity of the launch.

Most often, synchronous machines are used as generators, in general, the vast majority of generators are synchronous, starting with those installed on cars and ending with those that are at nuclear power plants. Of all the others, they are the most reliable, have the highest efficiency, and are easier to maintain than others.

machine efficiency does not depend on the cosine phi of the electrical machine. The efficiency depends mainly only on losses in the winding (losses in copper), in the magnetic circuit (losses in steel), mechanical losses and additional losses. Also, the efficiency of the machine depends on its load, while the maximum (efficiency) is observed at the point when the losses in steel and copper are equal, as a rule, this is observed when the load is 75-80% of the rated power of the machine.

Taking into account the peculiarities of the production of electrical machines, we have that with an increase in the power of the produced machine, the losses grow disproportionately, therefore powerful electrical machines can have an efficiency of up to 99%.

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. Either this is a field from a permanent magnet fixed in the rotor, or this is 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 rotor of the engine is easier to get to produce a field of the desired quality, but an electric current is required for its operation. 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 generator electric current, but working "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 between a synchronous motor and a generator is 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 in the rotor windings, as in a DC motor;
harder to start.

Despite their differences, both electric motor have found application in technology and are used in a variety of designs and sizes.

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 asynchronous 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

A 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 winding star of the phase rotor are brought to three contact 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.

The classification of engines is based on different parameters. According to one of them, a synchronous and asynchronous motor is distinguished. Differences between devices general characteristics and the principle of operation are described in the article.

This type of engine is able to work both as a generator and as an engine at the same time. Its device is similar to a synchronous generator. characteristic feature motor is the constant rotational speed of the load.

These types of motors are widely used in many fields, such as electrical wires that need constant speed.

The principle of operation of a synchronous motor

Its functioning is based on the interaction of the rotating magnetic field of the armature and the magnetic fields of the inductor poles. Usually the armature is located in the stator, and the inductor is located in the rotor. For powerful motors electric magnets are used for the poles, and permanent ones for the weak ones.

The principle of operation of a synchronous motor includes (short-term) and asynchronous mode, which is usually used to accelerate to the required (that is, nominal) rotation speed. At this time, the inductor windings are short-circuited or by means of a rheostat. After reaching the required speed, the inductor is fed with direct current.

Advantages and disadvantages

The main disadvantages of this type of engine are:

the need to supply the winding with direct current;
launch difficulty;
sliding contact.

Most generators, wherever they are used, are synchronous. The advantages of such engines in general are:

This type of device is a mechanism aimed at transforming AC electrical energy into mechanical energy. From the very name "asynchronous" we can conclude that we are talking about a non-simultaneous process. Indeed, the frequency of rotation of the stator magnetic field here is always higher than the rotor one. Such a device consists of a cylindrical stator and a rotor, depending on the type of which squirrel-cage induction motors can also be with a phase rotor.

Operating principle

The operation of the engine is carried out on the basis of the interaction of the magnetic stator field and the currents induced by the same field in the rotor. Torque appears when there is a difference in the frequency of rotation of the fields.

Let's recap now. What explains the widespread use of one type and the limited use of the other?

Synchronous and asynchronous motor: differences

The difference between the operation of engines is in the rotor. In the synchronous type, it consists in a permanent or electric magnet. Due to the attraction of opposite poles, the rotating field of the stator also attracts the magnetic rotor. Their speed is the same. Hence the name - synchronous.

It can be achieved, in contrast to asynchronous, even voltage advance in phases. Then the device, like capacitor banks, can be used to increase power.

Asynchronous motors, in turn, are simple and reliable, but their disadvantage is the difficulty in adjusting the speed. To reverse a three-phase asynchronous motor (that is, change the direction of its rotation in the opposite direction), change the location of two phases or two linear wires approaching the stator winding.

If we consider the speed, then they also have differences between a synchronous and asynchronous motor. In the synchronous type, this indicator is constant, unlike the asynchronous one. Therefore, the first is used where a constant speed is needed and full controllability, for example, in pumps, fans and compressors.

It is very simple to identify the presence of the types of devices in question on a particular device. An asynchronous motor will have a non-round number of revolutions (for example, nine hundred and thirty per minute), while a synchronous motor will have a round number (for example, a thousand revolutions per minute).

Both those and other motors are controlled quite difficult. The synchronous type has a rigid mechanics characteristic: with any changing load on the motor shaft, the rotational speed will be the same. In this case, the load, of course, must change so that the engine is able to withstand it, otherwise this will lead to a breakdown of the mechanism.

This is how a synchronous and asynchronous motor works. The differences between both types determine the scope of their use, when one type copes with the task in an optimal way, for the other it will be problematic. At the same time, you can meet and combined mechanisms.

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Synchronous and asynchronous motor: differences, principle of operation

In an induction motor, the rotor moves "by itself". It initially has no 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

Synchronous motors: complex in simple

There are two options. Either this is a field from a permanent magnet fixed in the rotor, or this is a field from electromagnets installed in the rotor instead of such a magnet.

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) - the frequency of the alternating current in the supply network.

However, among other things, a synchronous motor has the property of complete reversibility. Because a synchronous electric motor is the same electric current generator, but working "in the opposite direction." In the generator, some mechanical force rotates the shaft with the rotor, and from this, an induced electrical voltage arises in the stator windings from the rotating magnetic field of the rotor. And the difference between a synchronous motor and a generator is 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.

Differences and disadvantages of these engines

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 in the rotor windings, as in a DC motor;
harder to start.

Despite their differences, both electric motors have found their way into technology and are used in a wide variety of designs and sizes.

What is the difference between a synchronous motor and an asynchronous motor

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 motors of the same type side by side, 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 in 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 also differ in areas of application. For example, synchronous electric motors are installed to drive equipment that operates at a constant rotation speed (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.

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Difference between induction and synchronous motor

Electric motors can be divided into two main categories - synchronous and asynchronous (induction) motors. These two species are quite different from each other. The difference is already visible in the names themselves. The units can be distinguished by the number of revolutions stamped on the nameplate (if the type of motor is not indicated there), the asynchronous motor has an unrounded number (for example, 950 rpm), the synchronous motor has a rounded number (1000 rpm).

There are other important differences, in this article we will consider the most significant of them: constructive, working and price.

Differences in work and cost

Any engine consists of two elements: stationary and rotating. The stator has axial slots - grooves, on the bottom of which current-carrying copper or aluminum wires are laid. At the electric motor, a rotor with an excitation winding is attached to the shaft.

The fundamental difference between synchronous and asynchronous motors are rotors, or rather, their design.

For synchronous models at low powers, they are permanent magnets.

Alternating voltage is applied to the stator winding, the rotor is connected to a constant power source. The direct current passing through the excitation winding induces a magnetic field in the stator. Torque is generated due to the angle of retardation between the fields. The rotor has the same speed as the stator magnetic field.

Units are used in practice both as generators and as engines.

Asynchronous models are fairly inexpensive motors that are used often and everywhere. They are easier to constructively, despite the fact that the fixed parts, in principle, are similar for all motors.

An alternating electric current is passed through the stator winding, which interacts with the rotor winding. Two fields rotate at the same speed in the same direction, but cannot be equal, otherwise the induced EMF and, especially, the torque would not be created. This causes an induced current in the rotor winding, the direction of which, according to Lenz's rule, is such that it tends to oppose the cause of its production, i.e., the sliding speed.

The speed of rotation of the rotor does not coincide with the speed of the magnetic field, it is always less. Thus, the rotor tries to catch up with the speed of the rotating magnetic field and reduce the relative speed.

Main advantages and disadvantages

Asynchronous units do not require any additional power supply. Synchronous requires an additional DC source to supply voltage to the windings.
Synchronizers have a relatively low sensitivity to mains voltage fluctuations and rotational stability, regardless of the load.
Induction motors do not require slip rings, except for slip ring motors, which have slip rings for soft start or speed control. There are more vulnerabilities in synchronous motors, as they use slip rings with brushes. Consequently, the parts wear out faster and the contact between them weakens.
Synchronizers need auxiliary triggers, as they do not have a self-start function. For induction motors with their own starting torques, such a mechanism is not required.

Which unit is better

In conclusion, it should be noted that it is impossible to say that supposedly one motor is better than another. However, asynchronous models are more reliable in operation, they are simple in design. If the units are not overloaded, then they long term service user can be satisfied.

The advantage of the synchronous model is that a high power factor can be easily set. Therefore, the model is much more efficient, but at a price it will be correspondingly more expensive. The machines are used in systems with a required power of 100 kW or more.

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Synchronous and asynchronous motor differences

Exist different kinds electric motors, and very often the question arises, what are the differences between a synchronous and asynchronous motor. IN asynchronous winding, located in the stator, create a rotating magnetic field that interacts with the currents generated in the rotor, due to which it comes into a rotating state. Therefore, at present, the simplest and most reliable asynchronous electric motor with a squirrel-cage rotor is considered the most popular.

In its grooves there are conductive rods made of aluminum or copper, connected at their ends with rings of the same material, which produce short circuit these rods. Therefore, the rotor is called short-circuited. Eddy currents interacting with the field cause the rotor to rotate at a speed less than the rotation speed of the field itself. Thus, the whole motor was called asynchronous. This movement is called relative slip, since the speeds of the rotor and the magnetic field are not equal and the magnetic field does not intersect with the current-carrying rods of the rotor. Therefore, they do not create a rotating moment.

The fundamental difference between both types of engines is the design of the rotor. In synchronous, it is a permanent magnet with respect to high power or the same electromagnet. A rotating magnet, which creates a magnetic field in the stator, drives the magnetic rotor. The speed of movement of the stator and rotor, in this case, is the same. Therefore, this motor is called synchronous.

Features of a synchronous motor

A synchronous motor is characterized by the possibility of a significant phase advance of the voltage by the current. Increasing the power factor by the type of capacitor banks.

Asynchronous motors are simple in design and reliable in operation. The only drawback of these units is the sufficient difficulty in adjusting the frequency of their rotation. Three-phase asynchronous motors can be easily reversed, i.e. the rotation of the motor can be reversed. To do this, it is enough to change the location of two linear wires or phases that are closed to the stator winding. Unlike a synchronous motor, this is a simple and cheap motor that is used everywhere.

A synchronous and asynchronous motor also has such an important difference as a constant speed for the first under various loads. Therefore, they are used in the drives of machines requiring constant speeds, such as compressors, pumps or fans, because they are very easy to control.

Classification of electric motors

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What is the difference between a synchronous motor and an asynchronous motor

An engine is a device that converts energy into mechanical type work. Only knowing the functions and specifications motor, we can correctly summarize how a synchronous motor differs from an asynchronous type of device.

The principle of operation of synchronous and asynchronous motors

The functioning of synchronous electric motors is based on the interaction of the poles of the stator and the inductor. At the starting moment, the motor accelerates to the indicators of the rotational speed of the magnetic flux. Under such conditions, the device operates in synchronous mode, and a special intersection is formed by the magnetic fields, resulting in synchronization.

Synchronous motor in section

Asynchronous motors have a rotor speed different from the frequency at which the magnetic field rotates, created as a result of the supply voltage. Such motors do not have automatic control of current excitation.

Asynchronous motor in section

Main differences

The presence of armature windings is one of the main differences between the two types of motors.

Despite the external similarity, asynchronous motors and synchronous type devices have several fundamental differences:

the rotor of asynchronous motors does not need current supply, and the induction of the poles depends on the magnetic field of the stator;
the rotor in a synchronous motor has an excitation winding under conditions of independent power supply;
revolutions in an asynchronous motor under load lag behind the magnitude of slip from the rotations of the magnetic field inside the stator;
revolutions in synchronous motors correspond to the frequency of "turns" of the magnetic field in the stator and are constant under different loads.

Stators in asynchronous and synchronous motors are characterized by the same device and create a rotating magnetic field.

Synchronous motors are capable of operating with the simultaneous combination of the functions of a motor and a generator.

Such devices fall into the category modern engines having high efficiency and constant speed. Asynchronous motors are more difficult to regulate, and their ratio useful action not high enough. However, the second option is more affordable.

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principles of operation and differences in characteristics

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

The leading units of a synchronous machine are an armature and an 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 high power motors 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 electric motors are of two types: squirrel-cage and 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 single-phase connection.
Low energy consumption and economical.

Serious disadvantages are 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 nodes mechanisms. When choosing an engine, you need to rely on the commonality and main differences in the properties of the machines:

The main difference between a synchronous and asynchronous motor is the design of the rotor. It is a permanent or electric magnet. At the asynchronous magnetic fields in the rotor are induced using electromagnetic induction.
For synchronous motors, the shaft speed is constant, for asynchronous motors, it can change when the load changes.
Synchronizers have no starting torque. To enter synchronization, you need to use asynchronous start.

Synchronous and a synchronous electric motors each find their own use. Synchronous motors are recommended to be used everywhere at high powers, 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.

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Three-phase AC machines. They are of two types - asynchronous and synchronous. This article explains the similarities and differences between both types of machines and their scope.

The principle of operation and the device of electric machines of different types

Asynchronous and synchronous motors are similar in design, but there are differences.

The device and principle of operation of asynchronous electric motors

These are the most common AC machines. Such electric motors consist of three main parts:

Housing with end shields and feet or flange.
The case contains a magnetic circuit made of iron plates with windings. This magnetic circuit is called a stator.
Shaft with bearings and magnetic wire. This design is called a rotor. In electric motors with a squirrel-cage rotor, aluminum rods are interconnected in the magnetic circuit, this design is called the "squirrel cage". In machines with a phase rotor, windings are wound instead of rods.

Three windings are wound in the stator slots with a shift of 120°. When connected to a three-phase network, a rotating magnetic field is induced in the stator. The speed of rotation is called "synchronous speed".

Reference! IN single-phase electric motors a rotating field is created by an additional winding or design features stator.

This field induces an EMF in the rotor, the resulting current creates its own field, which interacts with the stator field and sets it in motion. The rotor speed is less than the synchronous speed. This difference is called slip.

The slip is calculated using the formula S=(n1-n2)/n1*100%, where: · n1 - synchronous speed; · n2 - rotor speed.

Rated value

for slip in conventional electric motors 1-8%. With an increase in the load on the motor shaft, the slip and torque increase to a critical value, upon reaching which the motor stops.

In electric motors with a phase rotor, instead of a squirrel cage, three windings are wound in the grooves of the rotor. Through slip rings and brushes, they are connected to additional resistances. These resistances limit the current and the magnetic field in the rotor. This increases slip and reduces motor speed.

Such devices are used in heavy starting and variable speed applications such as overhead cranes.

The principle of operation of synchronous electric motors

These motors are more complex and more expensive than asynchronous machines. Their advantage is a constant rotation speed that does not change under load.

The stator of a synchronous machine is no different from an asynchronous one. The difference is in the rotor. Unlike an asynchronous motor, rotation is carried out due to the interaction of the rotating magnetic field of the stator and the constant field of the rotor. To create it, there are electromagnets in the rotor. Voltage is supplied to the coils using slip rings and graphite brushes.

Reference! In the rotor of synchronous machines low power instead of electromagnets, permanent ones are installed or simply the magnetic circuit has pronounced poles. There is no slip, as in asynchronous machines, and the rotational speed is determined only by the frequency of the supply voltage.

Starting electric motors

Asynchronous electrical machines with a power of up to 30-50 kW are started by direct supply of electricity. With high power motors and synchronous machines, the situation is more complicated.

Starting high power asynchronous motors

To start such machines, different methods are used:

Inclusion of additional resistances in the stator circuit. They limit the starting current, and after acceleration they are shorted by the starter.
In devices designed to operate on a network with a phase voltage of 660 volts, the windings in a 380-volt network are connected by a triangle. At the time of launch, they switch to a star.
In electric machines with a phase rotor, additional resistances are included in the rotor circuit to start. After acceleration, they short out.
If there is a speed control, switching windings or changing the frequency, the engine turns on at minimum speed. After the start of rotation, the speed increases.

Starting synchronous electric machines

Unlike asynchronous machines, which are started by the interaction of the stator field and the windings or the squirrel cage of the rotor, a synchronous machine must first be accelerated to a speed close to synchronous.

With an additional asynchronous motor. This is how cars start permanent magnets in the rotor. When a speed close to synchronous is reached, the asynchronous circuit is switched off and voltage is applied to the stator of the synchronous motor.
Asynchronous start. In the rotor, in addition to the electromagnet, there is a "squirrel cage". With its help, the device accelerates, after which it is fed into the winding constant pressure, and the motor starts to work as a synchronous one.
The rotor windings are short-circuited directly or through additional resistance. After acceleration, a constant voltage is applied to them.
With help TFC (thyristor frequency converter) the frequency of the supply voltage and the rotation speed smoothly rises to the nominal. This method is used in mechanisms with speed control.

Features and application of different types of electric motors

Each type of engine has advantages and disadvantages compared to others. This determines the scope of their application. Application different types electric machines depends on their design features and principle of operation.

Advantages and uses of induction motors

Such machines have advantages over synchronous devices:

simplicity of design and low price; devices with a phase rotor allow you to adjust the speed of rotation and carry out smooth start without the use of frequency converters;
a wide variety of powers - from a few watts to tens of kilowatts.

In addition to the advantages, there are disadvantages:

drop in rotation speed with increasing load;
lower efficiency and large dimensions than synchronous devices of the same power;
in addition to active, such devices consume reactive (inductive) power, which leads to the need to install compensators or additionally pay for reactive electricity.

Such machines are used almost everywhere where it is necessary to set the mechanism in motion and there is a three-phase voltage of 380 volts.

Application of synchronous machines

Adjustment by changing the excitation current cos φ. This allows you to reduce the current consumption, dimensions and cross section of the supply cable, as well as increase the efficiency. In addition, such devices are used as reactive power compensators.
They are less sensitive to voltage fluctuations and have a greater overload capacity, especially to shock loads. The overpower capability is increased by overexciting the rotor windings. Due to this, such engines are used in excavators, guillotine shears and other similar mechanisms.
The rotational speed does not change when the load changes. Therefore, synchronous machines are used in precision machine tools in metallurgy, mechanical engineering and the woodworking industry.