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Asynchronous motor: principle of operation and design. What types of engines are there? Types of electric motors. Asynchronous motors Types of electric motors

Electric motors are versatile units capable of converting electricity into mechanical energy. Today there are various types and classifications of electric motors used in domestic and industrial installations. Such equipment may differ in its operating principle, power supply from direct or alternating current, power and purpose.

Operating principle and design features

The electric motor design is standard, which greatly simplifies the operation and repair of equipment. The stator and rotor, which are the main elements of the technology, are located inside a cylindrical groove. When voltage is applied to the stationary stator winding, a magnetic field is excited, which drives the rotor and shaft of the electric motor.

Constant movement of the rotor is maintained by re-commutation of the windings or by creating a rotating magnetic field in the stator. If the first method of supporting shaft rotation is typical for collector modifications of units, then the formation of a rotating magnetic field is inherent in three-phase asynchronous motors.

The electric motor housing can be made of aluminum alloy or cast iron. In each specific case, the choice of body material is made based on the scope of use of the equipment and its required weight parameters.

All motors are manufactured with the same installation dimensions, which significantly simplifies their installation and subsequent operation.

Scope of use

The purpose of the electric motor is extremely wide. Such units are used to amplify the power of electrical signals; they are capable of converting direct current into alternating current and can be used in various types of electric machines. It is customary to distinguish between units intended for use in industrial equipment, mechanical engineering, on various lifting machines and special equipment. Also very popular are low-power electric motors, which are successfully used in various household tools and kitchen appliances.

Equipment classification

Today, there are various classifications of electric motors, which differ in different criteria and characteristics. Depending on the characteristics of the technology, it is customary to classify it:

In the hysteresis type modification, the rotation of the shaft is based on the magnetization reversal of the rotor. Such engines were popular in the past, but today their design is outdated, so they are practically not found. The most widespread are magnetoelectric units that can operate on alternating or direct current, as well as universal-type models that are simultaneously powered by alternating and direct current.

Magnetoelectric installations

The use of magnetoelectric modifications of DC motors allows one to obtain excellent dynamic and operational characteristics. Depending on its design, such The type of engines is divided into two main categories:

  • with permanent magnets;
  • with electromagnets.

In recent years, modifications with electromagnets, which have greater power, are more economical in operation and allow you to quickly change equipment operating parameters, have become the most popular.

In commutator motors, a brush assembly is used to connect the rotating and stationary parts of the motor. Such units can be made with independent excitation and the use of permanent magnets, but there are also those that are of a self-exciting type with a mixed, series or parallel connection. Manifold modifications have mediocre reliability indicators. They require competent and timely maintenance.

Brushless valve units have a closed system that operates on the principle of synchronous devices. High-quality brushless electric motors are equipped with a sensor for reading the rotor position and have a coordinate converter, based on the data from which the device operates.

Valve motor types can have different sizes and power. Such units are used in industrial equipment. They are also equipped with cordless tools, various toys and mobile phones.

Synchronous AC motors include modifications in which the rotor rotates synchronously with the generated magnetic field. A special feature of such units is their high power, which can reach hundreds of kilowatts. The main areas of use for synchronous equipment are powerful industrial plants, wind generators and hydroelectric power plants.

It is customary to distinguish several modifications of synchronous electric motors:

  • stepper;
  • reactive;
  • with permanent magnets;
  • reactive hysteresis;
  • valve reactive;
  • with excitation windings;
  • hybrid synchronous.

For stepper synchronous motors with discrete angular motion of the shaft, the rotor position will be fixed by applying voltage to the circuit windings. The transition to another shaft position is carried out by removing power from some windings and then applying voltage to other windings of the transformer.

Also widely used is a switched reluctance motor, the winding of which is made of semiconductor elements. Switched reluctance units are characterized by increased power, and they can be fully electronically controlled, which allows both maintaining minimum speed and quickly reaching full power at maximum speed. The advantages of synchronous motors include:

  • stable rotation speed;
  • low sensitivity to voltage changes in the network;
  • possibility of use as a power generator;
  • minimal power consumption.

However, synchronous devices still have disadvantages. These include difficulties with starting, difficulties with maintenance, and problems with adjusting the shaft speed. The main purpose of such devices is powerful industrial equipment, where the performance of units and their reliability are valued.

Asynchronous modifications

For asynchronous AC motors, the rotor speed will differ from the magnetic field. Such units are also called induction, which is explained by the principle of generating a magnetic field that arises due to the movement of the stator. Asynchronous modifications are most widespread, which is explained by the simplicity of their design, reliability, durability, as well as the ability to implement both heavy-duty industrial installations and small electric motors intended for use in household tools.

Depending on the type of electric current with which such units operate, They are usually divided into three categories:

  • single-phase;
  • two-phase;
  • three-phase.

The most widespread today are single-phase asynchronous motors that can operate from a household electrical network. A feature of single-phase motors is the presence of only one working winding and a squirrel-cage rotor on the stator. An alternating single-phase current is supplied to the stator winding, causing the rotor and motor shaft to rotate. The rotor itself has a cylindrical core with aluminum-filled cells and open ventilation blades. Single-phase squirrel cage motors are used in small power devices, water pumps and room fans.

Two-phase asynchronous motors are designed for use in single-phase AC power. Their feature is the presence of two working windings on the stator, located perpendicular to each other. During operation of the unit, alternating current is directly supplied to one winding, and to the second through a corresponding phase-shifting capacitor. A rotating magnetic field is formed at the output, which simplifies the start of the electric motor and subsequently maintains consistently high speeds.

Three-phase motors can have a squirrel cage and a wound rotor. The units are equipped with three working windings located on the stator parallel to each other. When the motor is connected to a three-phase network, the magnetic field has a spatial shift relative to the winding by 120 degrees. The presence of a short-circuited field makes it easier to put the device into operation, while subsequently maintaining stable speeds. Modifications of wound rotor motors are characterized by increased power and are used primarily in industrial equipment.

The advantages of asynchronous electric motors are their resistance to voltage surges and versatility of use. Due to the simplicity of the design, their subsequent maintenance is greatly simplified, and the equipment itself is extremely reliable and does not cause any trouble during operation. Depending on their modification, the installations can operate both from a powerful source of electricity in a three-phase network, and from a household electrical network, which allows them to be used in various household appliances and all kinds of electrical appliances.

Electric motors are the simplest and extremely reliable devices that are widely used in industry and everyday life. The currently existing types of electric motors make it possible to select a unit that will fully comply with the characteristics of its operation. With the help of such motors, powerful machines and equipment and efficient pumps can be driven. Not a single household electrical appliance can do without their use.

An electric motor is a technical system in which electrical energy is transformed into mechanical energy. The operation of such an engine is based on the phenomenon of electromagnetic induction. The device assumes the presence of a stationary element - a stator, as well as a moving part called an armature or rotor.

In a traditional electric motor, the stator is the outer part of the structure. This element forms a stationary magnetic field. The movable rotor is placed inside the stator. It consists of permanent magnets, a core with windings, a commutator and brushes. Electric currents flow through a winding, usually consisting of many turns of copper wire.

When operating connected to an energy source, the fields of the stator and rotor interact. A torque appears. It sets the rotor of the electric motor in motion. Thus, the electrical energy supplied to the windings is transformed into the energy of rotational motion. The rotation of the electric motor shaft is transmitted to the working element of the technical system, which includes the engine.

Electric motor features

An electric motor is one of the types of electrical machines, which also include. Due to the reversibility property, the electric motor, if necessary, can act as a generator. The reverse transition is also possible. But more often than not, each electric machine is designed exclusively to perform a very specific function. In other words, the electric motor will work most efficiently in this capacity.

The conversion of electrical energy into mechanical rotation energy occurring in the engine is necessarily associated with energy losses. The reasons for this phenomenon are heating of the conductors, magnetization of the cores, and harmful friction force that occurs even when using bearings. Even the friction of the moving parts with the air affects the coefficient of the electric motor. And yet, in the most advanced engines, the efficiency is quite high and can reach 90%.

Possessing a number of undeniable advantages, engines powered by , have become extremely widespread in industry and in everyday life. The main advantage of such an engine is its ease of use and high performance characteristics. The electric motor does not produce harmful emissions into the atmosphere, so its use in cars is very promising.

Ecology of consumption. Science and technology: Why are some motors installed in a vacuum cleaner and different motors in an exhaust fan? What kind of motors are in a Segway? Which ones move the metro train?

There are many types of electric motors. And each of them has its own properties, scope and features. This article will provide a short overview of different types of electric motors with photographs and application examples. Why are some motors installed in a vacuum cleaner and different motors in a hood fan? What kind of motors are in a Segway? Which ones move the metro train?

Each electric motor has certain distinctive properties that determine its application in which it is most beneficial. Synchronous, asynchronous, direct current, commutator, brushless, switched reluctance, stepper... Why not, as in the case of internal combustion engines, not invent a couple of types, bring them to perfection and use them and only them in all applications? Let’s go through all the types of electric motors, and at the end we’ll discuss why there are so many of them and which motor is “the best.”

DC motor (DC motor)

Everyone should be familiar with this engine from childhood, because this is the type of engine found in most old toys. A battery, two wires for contacts and the sound of a familiar buzz, inspiring further design feats. Everyone did this, didn't they? Hope. Otherwise, this article most likely will not be of interest to you. Inside such an engine, a contact unit is installed on the shaft - a collector, which switches the windings on the rotor depending on the position of the rotor.

The direct current supplied to the motor flows through one or the other part of the winding, creating torque. By the way, without going too far, everyone was probably interested in what kind of yellow things were on some DPTs from toys, right on the contacts (as in the photo above)? These are capacitors - when the collector operates, due to switching, the current consumption is pulsed, the voltage can also change abruptly, which is why the motor creates a lot of noise. They are especially annoying if the DPT is installed in a radio-controlled toy. Capacitors dampen such high-frequency ripples and, accordingly, remove interference.

DC motors range from very small sizes ("vibration" in the phone) to quite large ones - usually up to a megawatt. For example, the photo below shows the traction motor of an electric locomotive with a power of 810 kW and a voltage of 1500 V.

Why aren't DBTs made more powerful? The main problem of all DFCs, and especially high-power DFCs, is the collector unit. A sliding contact in itself is not a very good idea, and a sliding contact at kilovolts and kiloamperes is even more so. Therefore, designing a collector unit for powerful DPTs is an art, and at a power above a megawatt, making a reliable collector becomes too difficult.

In consumer quality, DPT is good for its simplicity in terms of controllability. Its torque is directly proportional to the armature current, and the rotational speed (at least no-load) is directly proportional to the applied voltage. Therefore, before the era of microcontrollers, power electronics and variable frequency AC drives, the DC motor was the most popular electric motor for applications where speed or torque control was required.

It is also necessary to mention how exactly the magnetic excitation flux is formed in the DPT, with which the armature (rotor) interacts and, due to this, a torque is generated. This flow can be done in two ways: permanent magnets and field winding. In small motors, permanent magnets are most often installed, in large ones - an excitation winding. The excitation winding is another regulation channel. As the field winding current increases, its magnetic flux increases. This magnetic flux is included in both the motor torque formula and the EMF formula.

The higher the excitation magnetic flux, the higher the torque developed at the same armature current. But the higher the EMF of the machine, which means that at the same supply voltage the idle speed of the engine will be lower. But if you reduce the magnetic flux, then at the same supply voltage the no-load frequency will be higher, going to infinity when the excitation flux is reduced to zero. This is a very important property of DBT. In general, I highly recommend studying the DMT equations - they are simple, linear, but they can be extended to all electric motors - the processes are similar everywhere.

Universal brushed motor

Oddly enough, this is the most common electric motor in everyday life, the name of which is the least known. Why did this happen? Its design and characteristics are the same as those of a DC motor, so mention of it in drive textbooks is usually placed at the very end of the chapter on DC motors. At the same time, the association collector = DPT is so firmly in the head that not everyone thinks that a DC motor, the name of which contains “direct current,” can theoretically be connected to an alternating current network. Let's figure it out.

How to change the direction of rotation of a DC motor? Everyone knows this; it is necessary to change the polarity of the armature power supply. What else? You can also change the power polarity of the excitation winding if the excitation is made by the winding and not by magnets. What if the polarity is changed at both the armature and the field winding? That's right, the direction of rotation will not change. So what are we waiting for? We connect the armature and excitation windings in series or in parallel so that the polarity changes equally in both places, and then we insert it into a single-phase alternating current network! Done, the engine will spin. There is only one small detail that needs to be made: since alternating current flows through the excitation winding, its magnetic core, unlike a true DPT, must be made laminated in order to reduce losses from eddy currents. And so we got the so-called “universal commutator motor”, which by design is a subtype of DPT, but... works great on both alternating and direct current.

This type of motor is most widely used in household appliances where it is necessary to regulate the rotation speed: drills, washing machines (not with “direct drive”), vacuum cleaners, etc. Why exactly is he so popular? Because of the ease of regulation. As in the DPT, it can be regulated by the voltage level, which for the AC network is done by a triac (bidirectional thyristor). The control circuit can be so simple that it is placed, for example, directly in the “trigger” of a power tool and does not require a microcontroller, PWM, or rotor position sensor.

Asynchronous electric motor

Even more common than brushed motors is the asynchronous motor. It is only widespread mainly in industry - where there is a three-phase network. In short, its stator is a distributed two-phase or three-phase (less often multiphase) winding. It is connected to an alternating voltage source and creates a rotating magnetic field. The rotor can be thought of as a copper or aluminum cylinder, inside of which there is an iron magnetic circuit. Voltage is not explicitly supplied to the rotor, but it is induced there due to the alternating field of the stator (which is why the motor in English is called an induction motor). The resulting eddy currents in the squirrel-cage rotor interact with the stator field, resulting in the generation of torque.

Why is the asynchronous motor so popular?

It does not have a sliding contact like a brushed motor and is therefore more reliable and requires less maintenance. In addition, such a motor can be started from an AC network by “direct start” - it can be turned on with a switch “on the network”, as a result of which the engine will start (with a high starting current of 5-7 times, but permissible). A relatively high power DC motor cannot be turned on like this; the inrush current will cause the collector to burn out. Also, asynchronous drives, unlike DPTs, can be made with much higher power - tens of megawatts, also due to the absence of a collector. At the same time, an asynchronous motor is relatively simple and cheap.

An asynchronous motor is also used in everyday life: in those devices where it is not necessary to regulate the rotation speed. Most often these are so-called “capacitor” motors, or, what is the same, “single-phase” asynchronous motors. Although in fact, from the point of view of an electric motor, it is more correct to say “two-phase”, simply one phase of the motor is connected directly to the network, and the second through a capacitor. The capacitor phase shifts the voltage in the second winding, which creates a rotating elliptical magnetic field. Typically, such motors are used in exhaust fans, refrigerators, small pumps, etc.

Disadvantage of an asynchronous motor compared to DBT in that it is difficult to regulate. An asynchronous electric motor is an alternating current motor. If you simply lower the voltage of an asynchronous motor without lowering the frequency, then it will slightly reduce the speed, yes. But its so-called slip will increase (the lag of the rotation speed from the frequency of the stator field), losses in the rotor will increase, which is why it can overheat and burn out. You can think of this as regulating the speed of a passenger car solely with the clutch, applying full throttle and engaging fourth gear. To properly regulate the rotation speed of an asynchronous motor, you need to proportionally regulate both frequency and voltage.

It would be better to organize vector control altogether. But for this you need a frequency converter - a whole device with an inverter, microcontroller, sensors, etc. Before the era of power semiconductor electronics and microprocessor technology (in the last century), frequency regulation was exotic - there was nothing to do with it. But today, an adjustable asynchronous electric drive based on a frequency converter is already a de facto standard.

Synchronous motor

There are several subtypes of synchronous drives - with magnets (PMSM) and without (with field winding and slip rings), with sinusoidal EMF or trapezoidal (brushless DC motors, BLDC). This also includes some stepper motors. Before the era of power semiconductor electronics, the destiny of synchronous machines was to be used as generators (almost all generators of all power plants are synchronous machines), as well as as powerful drives for any serious load in industry.

All these machines were made with slip rings (can be seen in the photo); of course, there is no talk of excitation from permanent magnets at such powers. At the same time, a synchronous motor, unlike an asynchronous one, has big problems with starting. If you connect a powerful synchronous machine directly to a three-phase network, then everything will be bad. Since the machine is synchronous, it must rotate strictly at the network frequency. But in 1/50 of a second, the rotor, of course, will not have time to accelerate from zero to the mains frequency, and therefore it will simply jerk back and forth, since the moment will be alternating. This is called “the synchronous motor has not entered into synchronism.” Therefore, in real synchronous machines, asynchronous starting is used - they make a small asynchronous starting winding inside the synchronous machine and short-circuit the excitation winding, simulating the “squirrel cage” of an asynchronous machine, in order to accelerate the machine to a frequency approximately equal to the field rotation frequency, and after that the direct current excitation is turned on and the machine is drawn into synchronism.

And while with an asynchronous motor it is at least somehow possible to regulate the rotor frequency without changing the field frequency, then with a synchronous motor it is absolutely impossible. It either spins with a frequent field, or falls out of synchronization and stops with disgusting transient processes. In addition, a synchronous motor without magnets has slip rings - a sliding contact - to transfer energy to the field winding in the rotor. In terms of complexity, this is, of course, not a DPT collector, but it would still be better without a sliding contact. That is why in industry, less capricious asynchronous drives are mainly used for unregulated loads.

But everything changed with the advent of power semiconductor electronics and microcontrollers. They made it possible to generate any desired field frequency for a synchronous machine, linked through a position sensor to the motor rotor: to organize a valve mode of motor operation (autocommutation) or vector control. At the same time, the characteristics of the entire drive (synchronous machine + inverter) turned out to be the same as they are obtained from a DC motor: synchronous motors began to sparkle with completely different colors. Therefore, starting around 2000, a “boom” of synchronous motors with permanent magnets began. At first they timidly crawled out in cooler fans as small BLDC motors, then they got to aircraft models, then they climbed into washing machines as direct drive, into electric traction (Segways, Toyota Prius, etc.), increasingly displacing the classic brushed motor for such tasks. Today, permanent magnet synchronous motors are gaining more and more applications and are advancing by leaps and bounds. And all this thanks to electronics. But how is a synchronous motor better than an asynchronous motor, if we compare the converter + motor set? And what's worse? This issue will be discussed at the end of the article, but now let's go through several more types of electric motors.

Self-excited switched reluctance motor (VID SV, SRM)



It has many names. Usually it is briefly called a switched reluctance motor (SMR) or a switched reluctance machine (VIM) or a drive (VIP). In English terminology, this is switched reluctance drive (SRD) or motor (SRM), which translates as a machine with switchable magnetic resistance. But a little lower we will consider another subtype of this engine, which differs in its principle of operation.

In order not to confuse them with each other, the “ordinary” TYPE, which is discussed in this section, we at the Department of Electric Drives at MPEI, as well as at the company NPF Vector LLC, call a “self-excited switched reluctance motor” or briefly SV TYPE, which emphasizes the principle of excitation and distinguishes it from the machine discussed next. But other researchers also call it a type with self-magnetization, sometimes a reactive type (which reflects the essence of the formation of torque).

Structurally, this is the simplest motor and its operating principle is similar to some stepper motors. The rotor is a gear piece of iron. The stator is also geared, but with a different number of teeth. The simplest way to explain the principle of operation is this animation:

By supplying direct current to the phases in accordance with the current position of the rotor, the motor can be made to rotate. There can be a different number of phases. The real drive current waveform for the three phases shown in the figure (current limit 600A):


However, the simplicity of the engine comes at a price. Since the motor is powered by unipolar current/voltage pulses, it cannot be connected directly “to the network”. A converter and a rotor position sensor are required. Moreover, the converter is not a classic one (like a six-switch inverter): for each phase, the converter for SRD must have half-bridges, as in the photo at the beginning of this section.

The problem is that in order to reduce the cost of components and improve the layout of converters, power switches and diodes are often not manufactured separately: ready-made modules are usually used, containing simultaneously two switches and two diodes - the so-called racks. And it is precisely them that most often have to be installed in the converter for VID SV, simply leaving half of the power switches unused: this results in a redundant converter. Although in recent years, some IGBT module manufacturers have released products designed specifically for SRDs.

The next problem is torque ripple. Due to the gear structure and pulsed current, the torque is rarely stable - most often it pulsates. This somewhat limits the applicability of engines for transport - who wants to have pulsating torque on the wheels? In addition, the engine bearings do not feel very good from such pulling impulses. The problem is somewhat solved by special profiling of the phase current shape, as well as by increasing the number of phases.

However, even with these shortcomings, the motors remain promising as a variable speed drive. Thanks to their simplicity, the motor itself is cheaper than a classic asynchronous motor. In addition, the motor can be easily made multi-phase and multi-sectional by dividing the control of one motor into several independent converters that operate in parallel. This allows you to increase the reliability of the drive - turning off, say, one of the four converters will not lead to stopping the drive as a whole - three neighbors will work for some time with a slight overload. For an asynchronous motor, such a trick cannot be accomplished so easily, since it is impossible to make stator phases unrelated to each other that would be controlled by a separate converter completely independently of the others. In addition, the VIDs are very well regulated “up” from the fundamental frequency. The rotor iron can be spun up to very high frequencies without problems.

At NPF Vector LLC, we have completed several projects based on this engine. For example, we made a small drive for hot water pumps, and also recently completed the development and debugging of a control system for powerful (1.6 MW) multiphase redundant drives for the processing plants of AK ALROSA. Here is a 1.25 MW machine:



The entire control system, controllers and algorithms were made by us at NPF VECTOR LLC, the power converters were designed and manufactured by NPP CIKL+ LLC. The customer of the work and the designer of the engines themselves was the company MIP Mechatronics LLC SRSTU (NPI).

Switched reluctance motor with independent excitation (VID NV)

This is a completely different type of engine, differing in operating principle from the usual TYPE. Historically, switched reluctance generators of this type are known and widely used, used on airplanes, ships, and railways, but for some reason little attention is paid to engines of this type.



The figure schematically shows the geometry of the rotor and the magnetic flux of the field winding, and also shows the interaction of the magnetic fluxes of the stator and rotor, while the rotor in the figure is set to a consistent position (the torque is zero).

The rotor is assembled from two packages (of two halves), between which an excitation winding is installed (shown in the figure as four turns of copper wire). Despite the fact that the winding hangs “in the middle” between the rotor halves, it is attached to the stator and does not rotate. The rotor and stator are made of laminated iron, there are no permanent magnets. The stator winding is distributed three-phase - like a conventional asynchronous or synchronous motor. Although there are options for this type of machine with a concentrated winding: teeth on the stator, like an SRD or BLDC motor. The turns of the stator winding cover both rotor packages at once.

In simplified terms, the operating principle can be described as follows:: the rotor tends to rotate to a position in which the directions of the magnetic flux in the stator (from the stator currents) and the rotor (from the excitation current) coincide. In this case, half of the electromagnetic moment is formed in one package, and half in the other. On the stator side, the machine implies multi-polarity sinusoidal power supply (EMF is sinusoidal), the electromagnetic torque is active (the polarity depends on the sign of the current) and is formed due to the interaction of the field created by the current of the excitation winding with the field created by the stator windings. According to the principle of operation, this machine differs from classical stepper and SRD motors, in which the torque is reactive (when a metal blank is attracted to an electromagnet and the sign of the force does not depend on the sign of the electromagnet current).

From the control point of view, the NV type turns out to be equivalent to a synchronous machine with slip rings. That is, if you do not know the design of this machine and use it as a “black box”, then it behaves almost indistinguishably from a synchronous machine with an excitation winding. You can make vector control or autocommutation, you can weaken the excitation flow to increase the rotation speed, you can strengthen it to create more torque - everything is as if it were a classic synchronous machine with controlled excitation. Only VID NV does not have a sliding contact. And it doesn't have magnets. And a rotor in the form of a cheap iron blank. And the moment does not pulsate, unlike SRD. Here, for example, are the sinusoidal currents of VID NV during vector control operation:

In addition, the NV VIDE can be created multi-phase and multi-section, similar to how this is done in the SV VIEW. In this case, the phases turn out to be unconnected with each other by magnetic fluxes and can work independently. Those. it’s as if there were several three-phase machines in one, each of which is connected to its own independent inverter with vector control, and the resulting power is simply summed up. In this case, no coordination between the converters is required - only a general setting of the rotation speed.
This motor also has disadvantages: it cannot spin directly from the mains, since, unlike classic synchronous machines, VID NV does not have an asynchronous starting winding on the rotor. In addition, it is more complex in design than the conventional SRD.

We have also made several successful projects based on this engine. For example, one of them is a series of pump and fan drives for district heating stations in Moscow with a capacity of 315-1200 kW.

These are low-voltage (380V) type of NV with redundancy, where one machine is “broken” into 2, 4 or 6 independent three-phase sections. Each section is equipped with its own converter of the same type with vector sensorless control. Thus, it is possible to easily increase power based on the same type of converter and motor design. In this case, some of the converters are connected to one power input of the district heating station, and some to another. Therefore, if there is a “blinking power supply” on one of the power inputs, then the drive does not stop: half of the sections operate briefly in overload until the power is restored. As soon as it is restored, the resting sections are automatically put into operation while moving. In general, this project would probably deserve a separate article, so for now I’ll finish about it by inserting a photo of the engine and converters:

Conclusion: which electric motor is the best?

Unfortunately, two words are not enough here. And general conclusions about the fact that each engine has its own advantages and disadvantages. Because the most important qualities are not considered - weight and size indicators of each type of machine, price, as well as their mechanical characteristics and overload capacity. Let's leave the unregulated asynchronous drive to spin its pumps directly from the network; it has no competitors here. Let’s leave collector machines to turn drills and vacuum cleaners; here it’s also difficult to compete with them in ease of regulation.

Let's look at an adjustable electric drive whose operating mode is long-term. Collector machines are immediately excluded from competition here due to the unreliability of the collector unit. But there are still four left - synchronous, asynchronous, and two types of switched inductor. If we are talking about the drive of a pump, fan and something similar that is used in industry and where weight and dimensions are not particularly important, then synchronous machines fall out of the competition. The field winding requires slip rings, which is a finicky element, and permanent magnets are very expensive. The competing options remain the asynchronous drive and switched reluctance motors of both types.

Experience shows that all three types of machines are successfully used. But - an asynchronous drive is impossible (or very difficult) to section, i.e. break a powerful car into several low-power ones. Therefore, to provide high power to an asynchronous converter, it is necessary to make it high-voltage: after all, power is, roughly speaking, the product of voltage and current. If for a sectionalized drive we can take a low-voltage converter and set several of them, each for a small current, then for an asynchronous drive there must be one converter. But why not make a converter for 500V and a current of 3 kiloamps? These wires are needed as thick as an arm. Therefore, to increase power, the voltage is increased and the current is decreased.

A high voltage converter– this is a completely different class of problem. You can’t just take 10 kV power switches and make a classic 6-key inverter out of them, as before: there are no such keys, and if there are, they are very expensive. The inverter is made multi-level, using low-voltage switches connected in series in complex combinations. Such an inverter sometimes pulls behind it a specialized transformer, optical key control channels, a complex distributed control system that works as one... In general, everything is complicated with a powerful asynchronous drive. At the same time, a switched reluctance drive, due to sectioning, can “delay” the transition to a high-voltage inverter, allowing you to make drives up to several megawatts from a low-voltage supply, made according to the classical scheme. In this regard, VIPs become more interesting than an asynchronous drive, and even provide redundancy. On the other hand, asynchronous drives have been operating for hundreds of years, and the motors have proven their reliability. VIPs are just making their way. So here you need to weigh many factors in order to choose the most optimal drive for a particular task.

But everything becomes even more interesting when it comes to transport or small-sized devices. There you can no longer be careless about the weight and dimensions of the electric drive. And now you need to look at synchronous machines with permanent magnets. If you look only at the parameter of power divided by weight (or size), then synchronous machines with permanent magnets are unrivaled. Some examples can be several times smaller and lighter than any other “magnetic-free” AC drive. But there is one dangerous misconception here, which I will now try to dispel.

If a synchronous machine is three times smaller and lighter, this does not mean that it is better suited for electric traction. The whole point is the lack of regulation of the flux of permanent magnets. The flux of magnets determines the emf of the machine. At a certain rotation speed, the EMF of the machine reaches the inverter supply voltage and further increasing the rotation speed becomes difficult.

The same applies to increasing torque. If you need to realize more torque, you need to increase the stator current in a synchronous machine - the torque will increase proportionally. But it would be more effective to increase the excitation flux - then the magnetic saturation of the iron would be more harmonious, and the losses would be lower. But again, we cannot increase the flux of magnets. Moreover, in some designs of synchronous machines, the stator current cannot be increased above a certain value - the magnets may become demagnetized. What happens? A synchronous machine is good, but only at one single point - at the nominal one. With rated speed and rated torque. Above and below - everything is bad. If you draw this, you will get this characteristic of frequency versus moment (in red):

In the figure, the horizontal axis shows the engine torque, and the vertical axis shows the rotation speed. The point of the nominal mode is marked with an asterisk, for example, let it be 60 kW. The shaded rectangle is the range where regulation of a synchronous machine is possible without problems - i.e. “down” in torque and “down” in frequency from the nominal.

The red line indicates what can be squeezed out of a synchronous machine beyond the nominal value - a slight increase in rotation speed due to the so-called field weakening (in fact, this is the creation of excess reactive current along the d-axis of the motor in vector control), and also shows some possible torque boost, so that it is safe for magnets. All. Now let's put this car in a passenger vehicle without a gearbox, where the battery is designed to deliver 60kW.

The desired traction performance is shown in blue. Those. starting from the lowest speed, say 10km/h, the drive must develop its 60kW and continue to develop it up to the maximum speed, say 150km/h. A synchronized car was not even close: its torque is not enough even to drive onto the curb at the entrance (or onto the curb at the front door, for political correctness), and the car can only accelerate to 50-60 km/h.

What does this mean? Is a synchronous machine not suitable for electric traction without a gearbox? It fits, of course, you just need to choose it differently. Like this:

It is necessary to choose a synchronous machine such that the required traction control range is entirely within its mechanical characteristics. Those. so that the machine can simultaneously develop high torque and operate at high speed. As you can see from the figure... the installed power of such a machine will no longer be 60 kW, but 540 kW (can be calculated by divisions). Those. in an electric car with a 60 kW battery, you will have to install a synchronous machine and a 540 kW inverter, just to “pass” the required torque and rotation speed.

Of course, no one does it as described. Nobody puts a car at 540kW instead of 60kW. A synchronous machine is being modernized, trying to “smear” its mechanical characteristic from the optimum at one point, up in speed and down in torque. For example, they hide magnets in the iron of the rotor (make them incorporated), this allows you not to be afraid of demagnetizing the magnets and weakening the field more boldly, as well as overloading the current more. But as a result of such modifications, the synchronous machine gains weight, size and becomes no longer as light and beautiful as it was before. New problems arise, such as “what to do if the inverter turns off during field weakening mode.” The EMF of the machine can “pump up” the DC link of the inverter and burn everything out. Or what to do if the inverter breaks down while running - the synchronous machine will close and can kill itself, the driver, and all the remaining living electronics with short-circuit currents - protection circuits, etc. are needed.

That's why synchronous machine good where a large range of regulation is not required. For example, in a Segway, where the speed from a safety point of view can be limited to 30 km/h (or whatever it is?). A synchronous machine is also ideal for fans: the fan’s rotation speed changes relatively little, at most twice as much - there’s no point in doing more, since the air flow weakens in proportion to the square of the speed (approximately). Therefore, for small propellers and fans, a synchronous machine is what you need. And it is precisely there that, in fact, it is successfully placed.

The traction curve, shown in blue in the figure, has been implemented since time immemorial by DC motors with controlled excitation: when the field winding current is changed depending on the stator current and rotation speed. As the rotation speed increases, the excitation current also decreases, allowing the machine to accelerate higher and higher. Therefore, DPT with independent (or mixed) excitation control has classically stood and still stands in most traction applications (metro, trams, etc.). What AC electric machine can compete with it?

This characteristic (constant power) can be better approached by motors whose excitation is controlled. This is an asynchronous motor and both types of VIPs. But the induction motor has two problems: first, its natural mechanical characteristic is not a constant power curve. Because the excitation of an asynchronous motor is carried out through the stator. And therefore, in the field weakening zone at a constant voltage (when it has ended at the inverter), a doubling of the frequency leads to a twofold drop in the excitation current and a twofold drop in the torque-forming current. And since the torque on the motor is the product of current and flux, the torque drops by 4 times, and the power, respectively, by two. The second problem is losses in the rotor during overload with a large torque. In an asynchronous motor, half of the losses are generated in the rotor, half in the stator.

To reduce weight and size parameters in transport, liquid cooling is often used. But the water jacket will effectively cool only the stator due to the phenomenon of thermal conductivity. It is much more difficult to remove heat from a rotating rotor - the path for heat removal through “thermal conduction” is cut off, the rotor does not touch the stator (bearings do not count). What remains is air cooling by mixing the air inside the engine space or radiating heat from the rotor. Therefore, the rotor of an asynchronous motor turns out to be a kind of “thermos” - having overloaded it once (by dynamically accelerating the car), you need to wait a long time for the rotor to cool down. But its temperature cannot be measured yet... you just have to predict it using a model.

Here it should be noted how skillfully both problems of an asynchronous motor were circumvented by Tesla in their Model S. They solved the problem of heat removal from the rotor... by introducing liquid into the rotating rotor (they have a corresponding patent, where the rotor shaft is hollow and it is washed inside by liquid, but I don’t know for sure if they use this). But they didn’t solve the second problem with a sharp decrease in torque when the field weakens. They supplied a motor with a traction characteristic almost like the one I drew for the “excess” synchronous motor in the figure above, only they have 300 kW instead of 540 kW. The field weakening zone in a Tesla is very small, about two times. Those. they installed an engine that was “excessive” for a passenger car, essentially making a sports car with enormous power instead of a budget sedan. The disadvantage of the asynchronous motor was turned into an advantage. But if they tried to make a less “performance” sedan, with a power of 100kW or less, then the induction motor would most likely be exactly the same (at 300kW), it would just be artificially strangled with electronics to suit the battery’s capabilities.

And now the VIPs. What can they do? What is their traction characteristics? I can’t say for sure about the VID SV - by its operating principle it is a nonlinear motor, and its mechanical characteristics can vary greatly from project to project. But in general, it is likely to be better than an induction motor in terms of approaching the desired traction characteristic with constant power. But I can say more about VID NV, since we work on it very closely at the company. See that desired traction characteristic in the picture above, which is drawn in blue, that we want to strive for? This is actually not just a desirable characteristic. This is a real traction characteristic, which we measured point by point using a torque sensor for one of the NV TYPES. Since the NVID type has independent external excitation, its qualities are closest to the NVD DPT, which can also form such a traction characteristic by regulating the excitation.

So what? VIID NV - the ideal machine for traction without a single problem? Not really. He also has a lot of problems. For example, its field winding, which “hangs” between the stator packages. Although it does not rotate, it is also difficult to remove heat from it - the situation turns out almost like an asynchronous rotor, only a little better. You can, if necessary, “throw” the cooling tube from the stator. The second problem is the overestimated weight and size indicators. Looking at the drawing of the rotor VIEW NV, you can see that the space inside the motor is not used very efficiently - only the beginning and end of the rotor “work”, and the middle is occupied by the excitation winding. In an asynchronous motor, for example, the entire length of the rotor, all the iron, “works.” The difficulty of assembly is that you still have to be able to insert the excitation winding inside the rotor packages (the rotor is made dismountable, so there are problems with balancing). Well, it’s just that the weight and size characteristics so far are not very outstanding compared to the same Tesla asynchronous motors, if you superimpose the traction characteristics on top of each other.

And there is also a common problem with both types of VIEW. Their rotor is a steamship wheel. And at high rotation speeds (and high frequencies are needed, since high-speed machines with the same power are less than low-speed ones), losses from mixing the air inside become very significant. If up to 5000-7000 rpm VID can still be done, then at 20000 rpm it will turn out to be a large mixer. But an asynchronous motor at such frequencies and much higher can be made using a smooth stator.

So what is the best overall solution for electric propulsion? Which engine is the best?
I have no idea. All are bad. We need to keep inventing. But the moral of the article is this - if you want to compare different types of adjustable electric drives, then you need to compare them on a specific task with a specific required mechanical characteristic in all parameters, and not just in power. Also, this article does not cover a lot of comparison nuances. For example, such a parameter as the duration of work at each point of the mechanical characteristic.

At maximum torque, usually no machine can work for a long time - this is an overload mode, and at maximum speed synchronous machines with magnets feel very bad - they have huge losses in steel. Another interesting parameter for electric traction is the loss when coasting when the driver releases the gas. If VIPs and asynchronous motors spin like blanks, then a synchronous machine with permanent magnets will have almost nominal losses in steel due to magnets. And so on and so forth…

Therefore, you can’t just pick and choose the best electric drive. published

Of the entire range of currently produced electric motors, the most widely used is the three-phase asynchronous motor. Almost half of the world's electricity is used by these machines. They are widely used in the metalworking and woodworking industries. An asynchronous motor is indispensable in factories and pumping stations. You cannot do without such machines in everyday life, where they are used in other household appliances and in hand-held power tools.

The scope of application of these electric machines is expanding every day, as both the models themselves and the materials used for their manufacture are being improved.

What are the main parts of this machine?

Having disassembled a three-phase asynchronous motor, you can observe two main elements.

1. Stator.

One of the most important details is stator.In the photo above this part of the engine is located on the left. It consists of the following main elements:

1. Frame. It is necessary to connect all parts of the machine. If the engine is small, then the housing is made of one piece. The material used is cast iron. Steel or aluminum alloys are also used. Sometimes the housing of small engines combines the functions of the core. If the engine is large in size and power, then the housing is welded from separate parts.

2. Core. This engine element is pressed into the housing. It serves to improve the quality of magnetic induction. The core is made of electrical steel plates. In order to reduce losses inevitable when eddy currents appear, each plate is coated with a layer of special varnish.

3. Winding. It is placed in the grooves of the core. Consists of coils of copper wire that are assembled into sections. Connected in a certain sequence, they form three coils, which together form the stator winding. It connects directly to the network, which is why it is called primary.

Rotor- this is the moving part of the engine. In the photo it is on the right. It serves to convert the force of magnetic fields into mechanical energy. The rotor of an asynchronous motor consists of the following parts:

1. Shaft. Bearings are attached to its shanks. They are pressed into shields that are bolted to the end walls of the stator box.

2. A core that is assembled on a shaft. It consists of special steel plates, which have such valuable properties as low resistance to magnetic fields. The core, having the shape of a cylinder, is the basis for laying the armature winding. The rotor, or, as it is also called, the secondary winding receives energy thanks to the magnetic field that appears around the stator coils when electric current passes through them.

Engines by type of manufacturing of the moving part

The engines are distinguished:

1. Having a short-circuited rotor winding. One of the versions of this part is shown in the figure.

A squirrel-cage induction motor has a winding made of aluminum rods that are located in slots in the core. At the end part they are short-circuited with rings.

2. Electric motors having a rotor made with slip rings.

Both types of asynchronous motors have the same stator design. They differ only in the design of the anchor.

What is the working principle

The armature of a three-phase asynchronous motor, designed in a similar way, is driven into rotation due to the effect of the appearance of an alternating magnetic field in the stator coils. To understand how this happens, it is necessary to recall the physical law of self-induction. It states that a magnetic field arises around a conductor through which a stream of charged particles passes. Its value will be directly proportional to the inductance of the wire and the intensity of the flow of charged particles flowing in it. In addition, this magnetic field generates a force with a certain direction. It is this that interests us, since it is the reason for the rotation of the rotor. For efficient operation of the motor, it is necessary to have a powerful magnetic flux. It is created thanks to a special method of installing the primary winding.

It is known that the power source has alternating voltage. Consequently, the magnetic field around the stator will have the same characteristic, which directly depends on the change in current in the supply network. It is noteworthy that each phase is shifted relative to each other by 120˚.

What happens in the stator winding

Each phase of the power supply is connected to the corresponding stator coil, so the magnetic field arising around them will be shifted by 120˚. has an alternating voltage, therefore, an alternating magnetic field will arise around the stator coils that the asynchronous motor has. The asynchronous motor circuit is assembled so that the magnetic field arising around the stator coils gradually changes and sequentially passes from one winding to another. This creates the effect of a rotating magnetic field. You can calculate its rotation frequency. It will be measured in revolutions per minute. Determined by the formula: n=60f/p, where f is the frequency of the alternating current in the connected network (Hz), p corresponds to the number of pairs of poles mounted on the stator.

How does a rotor work?

Now it is necessary to consider what processes occur in the secondary winding. An asynchronous motor with a squirrel-cage rotor has a design feature. The fact is that voltage is not supplied to its armature winding. It appears there due to magnetic induction coupling with the primary winding. Therefore, a process occurs that is the opposite of what was observed in the stator, in accordance with the law, which states that when crossing a conductor, and in our case this is a short-circuited rotor winding, an electric current arises in it by a magnetic flux. Where does the magnetic field come from? It appeared around the primary coil when connecting a three-phase power source.

Let's connect the stator and rotor. What will happen?

Thus, we have an asynchronous squirrel-cage motor with a rotor in the winding of which electric current passes. This will be the cause of the appearance of a magnetic field around the armature winding. However, the polarity of this flux will be different from that created by the stator. Accordingly, the force generated by it will counteract the one caused by the magnetic field of the primary winding. This will set the rotor in motion, since the secondary coil is assembled on it, and the armature shaft shanks are fixed in the motor housing on bearings.

Let us consider the situation of interaction of forces arising from the magnetic fields of the stator and rotor over time. We know that the magnetic field of the primary winding rotates and has a certain frequency. The force it creates will move at a similar speed. This will make the asynchronous motor work. And its rotor will rotate freely around its axis.

Sliding effect

The situation when the rotor's power flows seem to be repelled by the rotating magnetic field of the stator is called slip. It should be noted that the frequency of an asynchronous motor (n1) is always less than that with which the stator magnetic field moves. This can be explained this way. In order for a current to arise in the rotor winding, it must be crossed by a magnetic flux with a certain angular velocity. And therefore, the statement is true that the shaft rotation speed is greater than or equal to zero, but less than the intensity of movement of the stator magnetic field. The rotor has a rotation speed that depends on the friction force in the bearings, as well as on the amount of power taken from the rotor shaft. Therefore, it seems to lag behind the magnetic field of the stator. It is because of this that the frequency is called asynchronous.

Thus, the electrical energy from the supply source was converted into kinetic energy of the rotating shaft. The speed of its rotation is directly proportional to the frequency of the supply network current and the number of pairs of stator poles. To increase the armature rotation speed, frequency converters can be used. However, the operation of these devices must be coordinated with the number of pole pairs.

How to connect the motor to a power source

To start an asynchronous motor, it must be connected to a three-phase current network. The asynchronous motor circuit is assembled in two ways. The figure shows the connection diagram of the motor leads, in which the stator windings are assembled in a star manner.

This figure shows another connection method called a "triangle". The circuits are assembled in a terminal box attached to the housing.

You should know that the beginnings of each of the three coils, they are also called phase windings, are called C1, C2, C3, respectively. The ends, which are named C4, C5, C6, are signed similarly. If there are no pin markings in the terminal box, then you will have to determine the beginnings and ends yourself.

How to reverse

If there is a need to start an asynchronous motor by changing the direction of rotation of the armature, you just need to swap the two wires of the connected three-phase voltage source.

Single-phase asynchronous motors

In everyday life, it is problematic to use three-phase motors due to the lack of the required voltage source. Therefore, there is a single-phase asynchronous motor. It also has a stator, but with a significant design difference. It lies in the number and method of arrangement of windings. This also determines the startup pattern of the machine.

If a single-phase asynchronous motor has a stator with two windings, then they will be located with a circumferential offset at an angle of 90˚. The coils are called starting and working. They are connected in parallel, but in order to create conditions for the appearance of a rotating magnetic field, an active resistance or capacitor is additionally introduced. This creates a phase shift of the winding currents close to 90˚, which creates the conditions for the formation of a rotating magnetic field.

If the stator has only one coil, then a single-phase power source connected to it will cause a pulsating magnetic field. An alternating current will appear in the short-circuited rotor winding. It will cause the emergence of its own magnetic flux. The resultant of the two resulting forces will be equal to zero. Therefore, to start an engine with this design, an additional push is required. You can create it by connecting a capacitor starting circuit.

Connect the motor to a single-phase circuit

An electric motor made to operate from a three-phase power source can also operate from a single-phase home network, but this will significantly reduce its characteristics, such as efficiency and power factor. In addition, power and starting performance will decrease.

If you cannot do without a connection, then you need to assemble a circuit from three stator windings where there will be only two of them. One is working and the other is starting. For example, there are three coils with beginnings C1, C2, C3 and ends C4, C5, C6, respectively. To create the first (working) winding of the motor, we combine the ends C5 and C6, and connect their beginnings C3 and C2 to a single-phase current source, for example, a 220-volt household network. The role of the second, starting winding, will be performed by the remaining unused starter coil. It is connected to the power source through a capacitor connected in series with it.

Asynchronous motor parameters

When selecting such machines, as well as during their further operation, it is necessary to take into account the characteristics of the asynchronous motor. They can be energetic - this is the efficiency factor, the power factor. It is also important to take mechanical indicators into account. The main one is the relationship between the speed of rotation of the shaft and the working force applied to it. There are also starting characteristics. They determine the starting, minimum and maximum torques and their ratio. It is also important to know what the starting current of an asynchronous motor is. To use the engine most efficiently, all these parameters must be taken into account.

The issue of energy saving cannot be ignored. Recently, it has been considered not only from the standpoint of reducing operating costs. The efficiency of electric motors reduces the environmental problems associated with electricity production.

Manufacturers are constantly tasked with developing and producing energy-saving engines, increasing service life, and reducing noise levels.

Energy-saving performance can be improved by reducing operating losses. And they directly depend on the operating temperature of the machine. In addition, improving this characteristic will inevitably lead to an increase in engine life.

The temperature of the windings can be reduced by using an external fan mounted on the rotor shaft shank. But this leads to an inevitable increase in the noise produced by the engine during operation. This indicator is especially noticeable at high rotor speeds.

Thus, it is clear that the asynchronous motor has one significant drawback. It is not able to maintain a constant shaft speed under increasing loads. But such an engine has many advantages compared to electric motors of other designs.

Firstly, it has a reliable design. The operation of an asynchronous motor does not cause any difficulties when using it.

Secondly, an asynchronous motor is economical to manufacture and operate.

Thirdly, this machine is universal. It is possible to use it in any devices that do not require precise maintenance of the armature shaft rotation speed.

Fourthly, a motor with an asynchronous operating principle is also in demand in everyday life, receiving power from only one phase.

An electric motor is an electrical device for converting electrical energy into mechanical energy. Today, electric motors are widely used in industry to drive various machines and mechanisms. In the household, they are installed in a washing machine, refrigerator, juicer, food processor, fans, electric shavers, etc. Electric motors drive the devices and mechanisms connected to it.

In this article I will talk about the most common types and operating principles of AC electric motors, widely used in the garage, household or workshop.

How does an electric motor work?

The engine works based on the effect, discovered by Michael Faraday back in 1821. He made the discovery that when an electric current in a conductor interacts with a magnet, continuous rotation can occur.

If in a uniform magnetic field Place the frame in a vertical position and pass current through it, then an electromagnetic field will arise around the conductor, which will interact with the poles of the magnets. The frame will repel from one, and attract to the other.

As a result, the frame will rotate to a horizontal position, in which the effect of the magnetic field on the conductor will be zero. In order for the rotation to continue, it is necessary to add another frame at an angle or change the direction of the current in the frame at the appropriate moment.

In the figure, this is done using two half-rings, to which the contact plates from the battery are adjacent. As a result, after completing a half-turn, the polarity changes and the rotation continues.

In modern electric motors Instead of permanent magnets, inductors or electromagnets are used to create a magnetic field. If you disassemble any motor, you will see wound turns of wire coated with insulating varnish. These turns are the electromagnet or, as they are also called, the field winding.

At home Permanent magnets are used in battery-powered children's toys.

In others, more powerful Motors use only electromagnets or windings. The rotating part with them is called the rotor, and the stationary part is the stator.

Types of electric motors

Today there are quite a lot of electric motors of different designs and types. They can be separated by type of power supply:

  1. Alternating current, operating directly from the mains.
  2. Direct current that operate on batteries, rechargeable batteries, power supplies or other direct current sources.

According to the operating principle:

  1. Synchronous, which have windings on the rotor and a brush mechanism to supply electric current to them.
  2. Asynchronous, the simplest and most common type of motor. They do not have brushes or windings on the rotor.

A synchronous motor rotates synchronously with the magnetic field that rotates it, while an asynchronous motor rotates slower than the rotating magnetic field in the stator.

Operating principle and design of an asynchronous electric motor

In the asynchronous case motor, stator windings are laid (for 380 Volts there will be 3 of them), which create a rotating magnetic field. Their ends are connected to a special terminal block for connection. The windings are cooled thanks to a fan mounted on the shaft at the end of the electric motor.

Rotor, which is one piece with the shaft, is made of metal rods that are closed to each other on both sides, which is why it is called short-circuited.
Thanks to this design, there is no need for frequent periodic maintenance and replacement of current supply brushes, and reliability, durability and reliability increase many times over.

Usually, main cause of failure of an asynchronous motor is the wear of the bearings in which the shaft rotates.

Principle of operation. In order for an asynchronous motor to work, it is necessary that the rotor rotates slower than the electromagnetic field of the stator, as a result of which an EMF is induced (an electric current arises) in the rotor. An important condition here is that if the rotor rotated at the same speed as the magnetic field, then, according to the law of electromagnetic induction, no EMF would be induced in it and, therefore, there would be no rotation. But in reality, due to bearing friction or shaft load, the rotor will always rotate more slowly.

Magnetic poles are constantly rotating in the motor windings, and the direction of the current in the rotor constantly changes. At one point in time, for example, the direction of currents in the stator and rotor windings is depicted schematically in the form of crosses (current flows away from us) and dots (current towards us). The rotating magnetic field is shown as a dotted line.

For example, how does a circular saw work. It has the highest speed without load. But as soon as we start cutting the board, the rotation speed decreases and at the same time the rotor begins to rotate more slowly relative to the electromagnetic field and, according to the laws of electrical engineering, an even larger EMF begins to be induced in it. The current consumed by the motor increases and it begins to operate at full power. If the load on the shaft is so great that it stops, then damage to the squirrel-cage rotor may occur due to the maximum value of the EMF induced in it. That is why it is important to select an engine with suitable power. If you take a larger one, then the energy costs will be unjustified.

Rotor speed depends on the number of poles. With 2 poles, the rotation speed will be equal to the rotation speed of the magnetic field, equal to a maximum of 3000 revolutions per second at a network frequency of 50 Hz. To reduce the speed by half, it is necessary to increase the number of poles in the stator to four.

A significant disadvantage of asynchronous motors is that they can adjust the speed of rotation of the shaft only by changing the frequency of the electric current. And so it is not possible to achieve a constant shaft rotation speed.

Operating principle and design of an AC synchronous electric motor


This type of electric motor is used in everyday life where a constant rotation speed is required, the ability to adjust it, and also if a rotation speed of more than 3000 rpm is required (this is the maximum for asynchronous ones).

Synchronous motors are installed in power tools, vacuum cleaners, washing machines, etc.

In a synchronous housing In the AC motor there are windings (3 in the figure), which are also wound on the rotor or armature (1). Their leads are soldered to the sectors of the slip ring or collector (5), to which voltage is applied using graphite brushes (4). Moreover, the terminals are located so that the brushes always supply voltage to only one pair.

Most common breakdowns commutator motors are:

  1. Brush wear or their poor contact due to weakening of the pressure spring.
  2. Collector contamination. Clean with either alcohol or grit sandpaper.
  3. Bearing wear.

Principle of operation. The torque in an electric motor is created as a result of the interaction between the armature current and the magnetic flux in the field winding. With a change in the direction of the alternating current, the direction of the magnetic flux in the housing and armature will also change simultaneously, due to which the rotation will always be in one direction.