The speed of rotation of the magnetic field of the stator. Rotating magnetic field

Depends on the frequency of the supply voltage, on the power of the current load on the shaft, and on the number of electromagnetic poles of this engine. This real speed of rotation (or operating frequency) is always less than the so-called synchronous frequency, which is determined only by the parameters of the power supply and the number of poles of the stator winding of this asynchronous motor.

In this way, synchronous engine rotation frequencyi - This is the frequency of rotation of the magnetic field of the stator winding at the rated frequency of the supply voltage, and it is somewhat different from the operating frequency. As a result, the number of revolutions per minute under load is always less than the so-called synchronous revolutions.

The figure shows how the synchronous speed of rotation for an asynchronous motor with one or another pole of the stator depends on the frequency of the supply voltage: the higher the frequency - the higher the angular speed of rotation of the magnetic field. For example, when changing the frequency of the supply voltage change the synchronous frequency of the engine. This changes and the operating frequency of rotation of the engine rotor under load.

Usually, the winding of the asynchronous engine is powered by a three-phase alternating current, which creates a rotating magnetic field. And the greater the pairs of the poles - the less the synchronous frequency of rotation will be the rotational speed of the magnetic field of the stator.

Most modern asynchronous engines have from 1 to 3 pairs of magnetic poles, in rare cases 4, because the more poles are the lower the efficiency of the asynchronous engine. However, with a smaller number of poles, the rotor speed can be changed very, very smoothly, changing the frequency of the supply voltage.

As mentioned above, the real operating frequency of an asynchronous motor differs from its synchronous frequency. Why is this happening? When the rotor rotates with a frequency of less than synchronous, then the rotor conductors crosses the magnetic field of the stator at some speed and the EMF is inserted into them. This EMF creates currents in closed rotor conductors, as a result, these currents interact with the rotating magnetic field of the stator, and the torque occurs - the rotor is enjoys the magnetic field of the stator.

If the moment has a sufficient value to overcome the friction force, the rotor begins to rotate, while the solenoid moment is equal to the slowing torque, which create a load, friction force, etc.

In this case, the rotor always lags behind the magnetic field of the stator, the operating frequency cannot reach the synchronous frequency, as if it happened, the EMF would stop indulge in the rotor conductors, and the rotating moment would simply appear. As a result, the magnetization of "slip" (, as a rule, is 2-8%), in connection with which the next engine inequality is true:

But if the rotor of the same asynchronous engine is promoted using some external drive, for example, an internal combustion engine, up to such a speed that the rotor speed frequency exceeds the synchronous frequency, then the EMF in the rotor conductors and the active current will be purchased in them a certain direction, and an asynchronous motor. will turn into.

The total electromagnetic moment will be braking, S sliding will be negative. But so that the generator mode can manifest itself, it is necessary to put the asynchronous engine reactive power that would create a magnetic field of the stator. At the time of starting such a machine in the generator mode, it may be enough to break the residual induction of the rotor and capacitors, which are connected to the three phases of the stator winding supplying the active load.

As previously shown, one of the most important advantages of multiphase systems is to obtain a rotating magnetic field using fixed coils, on which the operation of the engines is based. alternating current. Consideration of this issue Let's start with the analysis of the magnetic field of the coil with a sinusoidal current.

Magnetic field coil with sinusoidal current

When overlooking the winding of the coil of the sinusoidal current, it creates

magnetic field, whose induction vector changes (pulsates) along this coil Also along the sinusoidal law Instant orientation of the magnetic induction vector in space depends on the winding of the coil and the instantaneous direction of the current in it and is determined by the rule of the right-man. So for the case shown in Fig. 1, magnetic induction vector directed along the coil axis up. After the halfer, when with the same module, the current will change its sign to the opposite, the vector of magnetic induction at the same absolute value will change its orientation in space per 1800. With regard to the above, the magnetic field of the coil with sinusoidal current is called pulsating.

Circular rotating magnetic field
two- and three-phase windings

A circular rotating magnetic field is called the field, the vector of magnetic induction of which, without changing the module, rotates in space with a constant angular frequency.

To create a circular rotating field, you must execute two conditions:

1. The coil axis should be shifted in space relative to each other at a certain angle (for a two-phase system - by 90 0, for three-phase - by 120 0).
2. Casts that feed coils must be shifted in phase, respectively, the spatial displacement of coils.

Consider the preparation of a circular rotating magnetic field in the case of a two-phase Tesla system (Fig. 2, a).

When passing through the coils of harmonic currents, each of them in accordance with the foregoing will create a pulsating magnetic field. Vectors and characterizing these fields are directed along the axes of the respective coils, and their amplitudes also change according to the harmonic law. If the current in the coil in lags behind the current in the coil A 90 0 (see Fig. 2, b), then.

We will find the projection of the resultant magnetic induction vector on the X and Y axis of the Cartesian coordinate system associated with the axes of the coils:

Module of the resultant vector magnetic induction in accordance with Fig. 2, in equal

The obtained relations (1) and (2) show that the vector of the resulting magnetic field is unchanged by the module and rotates in space with a constant angular frequency, describing the circle, which corresponds to a circular rotating field.

We show that the symmetric three-phase coil system (see Fig. 3, a) also allows you to get a circular rotating magnetic field.

Each of the coils A, B and C by passing along them with harmonic currents creates a pulsating magnetic field. The vector diagram in space for these fields is shown in Fig. 3, b. For the projection of the resulting magnetic induction vector on

the axis of the Cartesian coordinate system, the Y axis, which is combined with the magnetic axis of the phase A, can be recorded

;

(3)

.

(4)

The reduced ratios take into account the spatial arrangement of coils, but they also feed on a three-phase current system with a temporary shift in phase by 1200. Therefore, for instantaneous values \u200b\u200bof induction of coils.

; ; .

Substiving these expressions in (3) and (4), we get:

;

(5)

(6)

In accordance with (5) and (6) and Fig. 2, for the module of the magnetic induction of the resulting field of three coils with a current can be written:

,

and the vector itself is with the axis of the angle A, for which

,

Thus, in this case, the vector of magnetic induction is constant in the space with a constant angular frequency, which corresponds to a circular field.

Magnetic field in an electric machine

In order to strengthen and concentrate a magnetic field in an electrical machine, a magnetic chain is created for it. The electric machine consists of two main parts (see Fig. 4): a fixed stator and a rotating rotor made according to the hollow and solid cylinders.

There are three identical windings on the stator, the magnetic axes of which are shifted by the boring of the magnetic pipeline by 2/3 of the pole division, the value of which is determined by the expression

,

where is the radius of the boring of the magnetic pipeline, and P is the number of pairs of poles (the number of equivalent rotating constant magnets, creating a magnetic field, in the presented in Fig. 4th P \u003d 1).

In fig. 4 solid lines (A, B and C) were noted positive directions of pulsating magnetic fields along the axes of the windings A, B and C.

By adopting magnetic permeability became infinitely large, we construct the magnetic induction distribution curve in the air gap of the machine generated by the phase A winding, for a certain point in time T (Fig. 5). When constructing the curve is changed by a jump in the location of the coil sides, and horizontal sites occur in areas devoid of current.

We will replace this sinusoid curve (it should be indicated that real cars due to the corresponding execution of phase windings for the resulting field such a replacement is associated with very small errors). Taking the amplitude of this sinusoid for the selected time T equal to v.We write

;

(11)

.

(12)

Having lifting relations (10) ... (12), taking into account the fact that the sum of the last members in their right-wing parts is identically equal to zero, we get for the resulting field along the air gap expression

presenting an equation of a running wave.

Magnetic induction is constant if . Thus, if you mentally choose some point in the air gap and move it along the boring of the magnetic pipeline at speeds

,

that magnetic induction for this point will remain unchanged. This means that over time, the magnetic induction distribution curve, without changing its form, moves along the circumference of the stator. Consequently, the resulting magnetic field rotates at a constant speed. This speed is made to determine in revolutions per minute:

.

Principle of action of asynchronous and synchronous engines

The device of an asynchronous engine corresponds to the image in Fig. 4. The rotating magnetic field created by the stator windings with the current interacts with the rotor currents, leading it into rotation. The most distribution currently received an asynchronous engine with a short-circuited rotor due to its simplicity and reliability. In the grooves of the rotor of such a machine there are top-ending copper or aluminum rods. The ends of all rods from both ends of the rotor are connected by copper or aluminum rings that closed the swipe rods. From here it happened such a rotor name.

In a short-circuited rotor winding under the action of the EMF caused by the rotating field of the stator, the vortex currents occur. Interacting with the field, they involve the rotor in rotation at a speed, a fundamentally lower field of rotation of the field 0. Hence the engine name is asynchronous.

Value

called relative slide. For the normal versa engines S \u003d 0.02 ... 0.07. The inequality of the velocities of the magnetic field and the rotor becomes obvious if you consider that with a rotating magnetic field it will not cross the conductive rods of the rotor and, therefore, they will not be guided by currents involved in creating a rotating point.

The principal difference between the synchronous engine from asynchronous is to perform the rotor. The latter at a synchronous engine is a magnet made (with relatively small capacity) based on a constant magnet or on the basis of an electromagnet. Since the multi-poles of magnets are attracted, the rotating magnetic field of the stator, which can be interpreted as a rotating magnet, carries the magnetic rotor behind itself, and their speeds are equal. This explains the engine name - synchronous.

In conclusion, we note that, in contrast to an asynchronous engine, which usually does not exceed 0.8 ... 0.85, a synchronous motor can be achieved with a greater value and even make it so that the current will be ahead of phase voltage. In this case, like condenser batteries, the synchronous machine is used to increase the power factor.

Literature

1. Basicschain theory: studies. For universities /G.V. Zevek, P.A.Ionkin, A.V. Nyushal, S.V.Stratov. -5-E ed., Pererab. -M.: Energoatomizdat, 1989. -528c.
2. Bessonov L.A. Theoretical foundations of electrical engineering: electrical chains. Studies. For students of electrotechnical, energy and instrument-making specialties of universities. -7-e ed., Pererab. and add. -M.: Higher. Shk., 1978. -528c.
3. Theoreticalbasics of electrical engineering. Studies. For universities. In three tons. ed. K.M. Polyivanova. T.1. KM Polyivans. Linear electrical circuits with focused constant. -M.: Energy - 1972. -240С.

Control questions

1. What field is called pulsating?
2. What field is called rotating circular?
3. What conditions are necessary to create a circular rotating magnetic field?
4. What is the principle of operation at an asynchronous motor with a short-circuited rotor?
5. What is the principle of operation at a synchronous motor?
6. What synchronous speeds are produced in our country, the engines of the alternating current of the general industrial execution?

In inductive electrical machines, the winding of the stator and the rotor are bound by a magnetic field. To communicate the rotating part of the machine with a fixed air gap through the stator winding system, create rotatinga magnetic field.

Under the rotating, we will understand such a magnetic field, the induction vector of which moves in the space (in the plane perpendicular to the axis of the rotor) at a certain angular speed. If the amplitude of the induction vector is constant, then this field is called circular.The rotating magnetic field can be created:

• alternating current in a two-phase winding system shifted in space by 90 °;
• three-phase alternating current in a three-phase winding system shifted in a 120 ° space;
• a direct current switchable in series on windings distributed by the stator of the stator of the engine;
• DC, switched by a switch on winding branches located along the surface of the rotor (anchor). Formation of a rotating magnetic field in a two-phase machine
• (Fig. 1.2). INsuch a machine axis of the windings is shifted geometrically 90 ° (the machine with one pair of poles is considered, p n \u003d one). The stator windings are powered by two-phase voltage, as shown in Fig. 1.2, I. Believing the machine is symmetric and unsaturated, we believe that currents in the windings are also shifted by 90 electrical degrees (90 ° email) and the magnetotransporting force of the windings is proportional to the current (Fig. 1 .2,6). INmoment of time, = 0 Current in Winding but equal to zero, and current in the winding b. It has the greatest negative value.

Fig. 1.2.The formation of a rotating magnetic field in a two-phase electrical machine: A - turning on the windings: b - two-phase current system in stator windings: in - Spatial vector diagram of magnetore-breeding forces created by stator windings

Consequently, the total vector of magneto-breeding forces (MDS) windings at the time of time is t and is located in space, as shown in Fig. 1.2, in. At time from 2 \u003d 7c / currents in the windings will be TL M / And, therefore, the total MDS vector will turn to the angle to/ and_zimet in space the position indicated in Fig. 12, in, As 2 \u003d 2 + 2. In the moment

time CO 2 \u003d I / 2 The total MDS vector will be equal. Similarly, you can trace how the position of the total MDS vector changes at the time of time, etc. It can be seen that the vector rotates in space with a speed of CO \u003d 2TS, while maintaining its amplitude constant. Direction of field rotation - clockwise. We suggest make sure that if you submit to the phase but Voltage \u003d (CO -), and on the phase b. Voltage \u003d CO, then direction

rotation will change to the opposite.

Fig. 1.3.Schemes for switching on three-phase motor windings: A - Location of the engine windings at p n \u003d 1; b - connection of the windings in the star; in - Three-phase currents in engine windings

Thus, the combination of the spatial shift of the axes of windings by 90 geometric degrees (90 °) and the phase shift of the AC in the windings on (90 ° email) of electrical degrees allows you to form a magnetic field rotating along the stator circle in the air gap.

The mechanism for the formation of a rotating magnetic field in a three-phase AC machine.Machine windings are shifted in a 120 ° space (Fig. 1.3, a) and feed on the system of three-phase stresses. Currents in the winding of the machine are shifted by 120 ° EL. (Fig. 1.3, in):

The resulting MDS of the stator windings is:

Where w. - The number of turns of the windings.

Consider the position in the space of the vector at the time of time, (Fig. 1.4, o). Vector MDS winding o t is directed along the axis o in the positive direction and is equal to 0, w, those. ABOUT, . Vector MDS Winding from directed along the axis from and equal to 0 ,. The sum of the vectors J and J are directed along the axis b. In the negative direction and with this amount the vector MDS winding B, equal amount of three vectors forms vector H. \u003d 3/2, occupying at the time of time, the position that is shown in Fig. 1.4, o. After time \u003d L / SSO (at a frequency of 50 Hz through 1/300 C), the moment of time 2 will come, in which the vector MDC winding O is equal, and the vectors of MDS windings b. and from equal - 0.5. The resulting MDS 2 vector at time 2 will occupy the position indicated in Fig. 1.4.5, i.e. will move relative to the previous position w. At an angle of 60 ° clockwise. It is easy to make sure that at the time of time 3, the resulting MDS of the stator windings will occupy position 3, i.e. It will continue to move clockwise. During the period of the supply voltage \u003d 2l / CO \u003d 1 / the resulting vector MDS makes a complete turn, i.e. The speed of rotation of the stator field is directly proportional to the frequency of the current in its windings and is inversely proportional to the number of pairs of poles:

where n is the number of pairs of the poles of the car.

If the number of pairs of the engine poles is greater than the unit, then the number of windings sections disposed of stator circle increases. So, if the number of pairs of poles n \u003d 2, then three phase windings will be located on one half of the stator circle and three to another. In this case, in one period of the supply voltage, the resulting MDS vector will complete half a turn and the speed of rotation of the magnetic field of the stator will be twice as smaller than in machines with "\u003d 1-

Fig. 1.4.but - CO \u003d 7C / b. - CO \u003d l / in - CO \u003d 7C /

Based on the operation of almost all AC motors: synchronous with electromagnetic excitation (SD), with excitation of permanent magnets (SDPM), synchronous jet engines (SRD), and asynchronous engines (blood pressure) - lies the principle of creating a rotating magnetic field.

According to the principles of electrodynamics in all electrical engines (except for a jet), a developable electromagnetic moment is the result of the interaction of magnetic fluxes (current-shit), created in the movable and fixed parts of the electric motor. The moment is equal to the product of these threads, which is shown in Fig. 1.5, and the value of the moment is equal to the product of the flow vectors modules on the sinus of the spatial angle 0 between the flow vectors:

where to - Constructive coefficient.

Fig. 1.5.

Synchronous (SD, SDPM, SRD) and asynchronous engines There are practically the same design of states, and the rotors are different. The distributed windings of the stator of these electric motors are placed in a relatively large number of semi-closed stator grooves. If you do not take into account the influence of the teeth, the stator winding form a permanent magnetic flux by a permanent velocity determined by the current frequency. In real structures, the presence of grooves and teeth of the magnetic pipeline of the stator leads to the appearance of higher harmonics of the magnetizing forces, which leads to the ripples of the electromagnetic moment.

On the CD rotary, the excitation winding is located, which is powered by a direct current from an independent voltage source - pathogen. The excitation current creates an electromagnetic field, stationary relative to the rotor and rotating in the air gap along with the rotor at the speed of [cm. (1.7)]. For synchronous motors with a capacity of up to 100 kW, excitation from permanent magnets, which are installed on the rotor.

Magnetic power lines of the rotor field generated by the excitation winding or permanent magnets, "curb" with a rotating synchronously with a stator electromagnetic field. Interaction of stator fields H. And the rotor 0 creates an electromagnetic moment on the shaft of the synchronous machine.

In the absence of load on the shaft, the vectors of the editor fields, and the rotor 0 coincide in space and jointly rotate at a speed of CO 0 (Fig. 1.6, I).

When annexing the torque of the mode of resistance, the vectors [and 0 are diverted (stretching like a spring) to angle 0, and both vectors continue to rotate at the same rate of CO 0 (Fig. 1 .6,6). If the angle 0 is positive, the synchronous machine works in the engine. Change the load on the motor shaft corresponds to the change in the angle 0 maximum moment M. It will be at 0 \u003d l; / (0 - electrical degrees). If a

load on the engine shaft exceeds M. That synchronous mode is broken, and the engine falls out of sync. With a negative value of the angle 0, the synchronous machine will operate the generator.

Fig. 1.6.but - with perfect idling; B - when load on the shaft

Jet Synchronous Engine - This engine with explicitly pronounced poles of the rotor without winding of the excitation, where the torque caused by the desire of the rotor to occupy such a position in which the magnetic resistance between the excited stator winding and the rotor takes the minimum value.

In the SRD Rotor of the Apungous (Fig. 1.7). It has different magnetic conductivity over the axes. On the longitudinal axis d, passing through the middle of the pole, the conductivity is maximum, and on the transverse axis q. - minimum. If the axis of the magnetic forces of the stator coincides with the longitudinal axis of the rotor, the curvature of the power lines of the magnetic flux and the moment is zero. When the stream is shifted by the stator axis relative to the longitudinal axis d. When the magnetic field (MP) rotates, the stream lines are curved and an electromagnetic moment occurs. The highest moment at the same stator current is obtained at an angle of 0 \u003d 45 ° C.

The main differences in the asynchronous motor from synchronous is that the rotation speed of the engine rotor is not equal to the speed of the magnetic field created by the currents in the stator windings. The difference in the speeds of the field of the stator and the rotor is called slip \u003d CO - CO. Due to the slip, the magnetic power lines of the rotating field of the stator crosses the conductors of the rotor winding and suggest EMF and the rotor current in it. The interaction of the field of the stator and the rotor current determines the electromagnetic moment of the asynchronous motor.

Fig. 1.7.

Depending on the design of the rotor, asynchronous motors are distinguished with phase and short-circuited rotor. In engines with a phase rotor on the rotor, there is a three-phase winding, the ends of which are connected to the contact rings through which the rotor circuit is output from the machine for connecting to starting resistors, followed by wigging the windings.

In the asynchronous engine, in the absence of a load on the shaft on the stator windings, only magnetization currents that create the main magnetic flow occur, and the amplitude of the stream is determined by the amplitude and frequency of the supply voltage. At the same time, the rotor rotates at the same speed as the field of the stator. In the windings of the EMF rotor does not induce, there is no rotor current and, therefore, the moment is zero.

When the load is applied, the rotor rotates slower than the field, there is a slip, in the windings of the rotor, the EMF is guided proportional to slip, and the rotor currents occur. The stator current, as in the transformer, increases by the corresponding value. The product of the active component of the rotor current on the stator flow module determines the motor moment.

Combines all engines [In addition to the valve-inductor engines (view)] that the main magnetic flow in the air gap rotates a relatively fixed stator with a set-specified angular rate of CO. This magnetic floss carries a rotor, which rotates for synchronous machines with the same angular velocity CO \u003d CO, or for asynchronous machines with some lag - sliding 5. The generating power lines forming the main stream have a minimum length when the engine is operationed (\u003d). At the same time, the axis of the vector of the magnetizing forces of the stator and the rotor coincide. When a load appears on the shaft of the axis, the axis is diverged, and the power lines are twisted and extended. Since the power lines always strive to cut down the length, the tangential forces that create a torque appear.

In recent years, begin to receive application fan-inductor engines. Such an engine has an appearance stator with coil windings on each pole. The rotor is also an appearance, but with another number of poles without windings. In the winding of the stator, a unipolar current from a special transducer - switch is alternately served, and a nearby rotor shoulder is attracted to these excited poles. The next pole of the stator is then excited. The stator pole winding switches in accordance with the Rotor position sensor signals. In this, as well as the fact that the current in the windings of the stator is regulated depending on the moment of load, it is the main difference view from the stepper motor.

In the form (Fig. 1.8), the torque is proportional to the amplitude of the main stream and the degree of curvature of magnetic power lines. At the beginning, when the Pole (Teeth) of the Rotor begins to overlap the pole of the stator, the curvature of the power lines is maximum, and the stream is minimal. When the overlap of the poles is maximally, the curvature of power lines is minimal, and the flux amplitude increases, while the moment remains approximately constant. As the magnetic system is saturated, the thread increase is limited, even with increasing current in the windings, the view. Changing the moment when the poles of the rotor passes relative to the pole of the stator causes the shaft to rotate unevenness.

Fig. 1.8.

In engine direct current The excitation winding is located on the stator and the field created by this winding, motionless. Anchor is created by a rotating magnetic field, the speed of rotation of which is equal to the speed of rotation of the anchor, but is directed. This is achieved by the fact that the windings of the winding the anchor proceeds by alternating current, switching by a mechanical frequency converter - collector apparatus.

The electromagnetic moment of the DC motor determines the interaction of the main stream created by the excitation winding, and the current in the turns of the anchor winding: M \u003d K. / I

If you replace the brush-collector device of the DC motor semiconductor switch, then we get brushless DC motor. Practical implementation Such engines is a valve engine. Constructive valid engine It is a three-phase synchronous machine with electromagnetic excitation or excitation from permanent magnets. The stator winding is switched using a semiconductor controlled transducer - switch depending on the position of the engine rotor.

One of the most common electric motors, which is used in most electric drive devices is an asynchronous engine. This engine is called asynchronous (non-synchronous) for the reason that its rotor rotates at a lesser speed than the synchronous motor, relative to the speed of rotation of the magnetic field vector.

It is necessary to explain what synchronous speed is.

Synchronous speed is such a speed with which a magnetic field rotates in a rotary machine, if you are accurate, then this is an angular velocity of the magnetic field vector. The field of rotation of the field depends on the frequency of the flowing current and the number of the poles of the machine.

The asynchronous engine always operates at a speed of smaller than the speed of synchronous rotation, because the magnetic field, which is formed by the stator windings, will generate a counter magnetic flow in the rotor. The interaction of this generated oncoming magnetic flux with the magnetic flow of the stator will make it so that the rotor will start rotating. Since the magnetic flow in the rotor will lag behind, the rotor will never be able to independently reach the synchronous speed, that is, the same with which vector of the magnetic field of the stator rotates.

There are two main types of asynchronous motor, which are determined by the type of supply. It:

• single-phase asynchronous engine;
• three-phase asynchronous engine.

It should be noted that a single-phase asynchronous motor is not able to independently begin the movement (rotation). In order for it to start rotating, it is necessary to create some displacement from the equilibrium position. This is achieved different ways, With the help of additional windings, capacitors, switching at the time of start. Unlike a single-phase asynchronous engine, a three-phase engine is able to start an independent movement (rotation) without making any changes to the design or starting condition.

From DC motors (DC), asynchronous AC motors (AC) are constructively characterized in that the power is supplied to the stator, unlike the DC motor, in which an anchor (rotor) is supplied through the brush mechanism.

Principle of operation of an asynchronous engine

Feeding voltage only on the stator winding, an asynchronous engine begins to work. It is interesting to know how it works, why is it going on? It is very simple, if you understand how the induction process occurs when a magnetic field is induced in the rotor. For example, in DC machines, it is necessary to separately create a magnetic field in anchor (rotor) not through induction, but through brushes.

When we feed the stress on the winding of the stator, the electric current starts in them, which creates a magnetic field around the windings. Next, from many windings that are located on the magnetic power line of the stator are formed by the general magnetic field of the stator. This magnetic field is characterized by a magnetic flux, the value of which varies in time, besides this, the direction of the magnetic flux changes in space, or rather it rotates. As a result, it turns out that the vector of the magnetic stream of the stator rotates as a promoted routine with a stone.

In full accordance with the law of electromagnetic induction of Faraday, in the rotor, which has a short-circuited winding (short-circuit rotor). In this rotary winding, an injected electric current will be protected, as the chain is closed, and it is in a short circuit mode. This current exactly as well as the supply current in the stator will create a magnetic field. The engine rotor becomes a magnet inside the stator, which has a magnetic rotating field. Both magnetic fields from the stator and the rotor will begin to interact, submitting to the laws of physics.

Since the stator is still motionless and its magnetic field rotates in space, and the current is induced in the rotor, which actually makes a permanent magnet from it, the movable rotor begins to rotate because the magnetic field of the stator starts to push it, fascinating. The rotor as if clips with a magnetic field of the stator. It can be said that the rotor seeks to rotate synchronously with the magnetic field of the stator, but it is unattainable for him, since at the moment of synchronization magnetic fields compensate each other, which leads to asynchronous work. In other words, when operating an asynchronous engine, the rotor slides in the magnetic field of the stator.

Slide can be both with delay and ahead. If a lag occurs, we have a motor mode of operation, when electrical energy is converted into mechanical energy if the sliding occurs with a rotor protrusion, then we have a generator mode of operation when the mechanical energy is converted into electrical.

The generated torque on the rotor depends on the frequency of the alternating current of the stator supply, as well as the value of the supply voltage. By changing the frequency of the current and the magnitude of the voltage can be affected by the rotor torque and thereby drive the operation of the asynchronous motor. This is true for both single-phase and three-phase asynchronous engines.

Types of asynchronous engine

Single-phase asynchronous engine is divided into the following types:

• With separate windings (Split-Phase Motor);
• With a starting condenser (Capacitor Start Motor);
• With a starting capacitor and the working capacitor RUN INDUTION MOTOR);
• With a displaced pole (Shaded-Pole Motor).

Three-phase asynchronous engine is divided into the following types:

• With a short-circuited rotor in the form of a squirrel Cage Induction Motor;
• With contact rings, phase rotor (Slip Ring Induction Motor);

As mentioned above, a single-phase asynchronous engine cannot start moving independently (rotation). What should be understood under independence? This is when the car begins to work automatically without any influence from the external environment. When we turn on the household appliance, such as a fan, then it starts to work immediately, from keystrokes. It should be noted that a single-phase asynchronous motor is used in everyday life, such as an engine in the fan. How does such an independent launch occur, if it says that this type of engines does not allow it? In order to understand this issue, it is necessary to study the ways of starting single-phase motors.

Why is a three-phase asynchronous engine self-missing?

In a three-phase system, each phase relative to the other two has an angle of equal 120 degrees. All three phases are thus located evenly in a circle, the circle has 360 degrees, and it is three times 120 degrees (120 + 120 + 120 \u003d 360).

If we consider the three phases, a, b, c, then it can be noted that only one of them in the initial moment of time will have the maximum value of the instant voltage value. The second phase will increase the value of its voltage after the first, and the third phase will follow the second. Thus, we have an alternation of phase phases A-B-C as their value is raised and another order is possible in descending order. c-B-A voltage. Even if you record alternation otherwise, for example, instead of A-B-C, write B-C-A, then the alternation will remain the same, since the chain of alternation in any order forms a vicious circle.

How will the asynchronous rotor rotate three-phase engine? Since the rotor is fond of the magnetic field of the stator and slides in it, it is quite obvious that the rotor will move in the direction of the magnetic field of the stator. Which way will the magnetic field of the stator be rotated? Since the stator winding is three-phase and all three windings are located evenly on the stator, then the formed field will rotate in the direction of the alternation of the phases of the windings. From here we conclude. The direction of rotation of the rotor depends on the order of the alternation of the phases of the stator winding. By changing the order of alternation, the phases we get the rotation of the engine in the opposite direction. In practice, to change the engine rotation, it suffices to change two any feed phases of the stator in places.

Why is a single-phase asynchronous engine starting to rotate independently?

For the reason that it is powered by one phase. The magnetic field of a single-phase engine is pulsating, not rotating. The main task of launch is to create a rolling field from the pulsating field. This problem is solved by creating a phase offset in another stator winding with capacitors, inductance and spatial windings in the engine design.

It should be noted that single-phase asynchronous motors are effective in use in the presence of a constant mechanical load. If the load is smaller and the engine works without reaching its maximum load, its effectiveness is significantly reduced. This is a disadvantage of a single-phase asynchronous engine and therefore, in contrast to three-phase machines, they are used there, where the mechanical load is constant.

In the previous paragraph, it was shown that the speed of rotation of the magnetic field is constant and is determined by the current frequency. In particular, if the three-phase engine winding is placed in six grooves on the inner surface of the stator (Fig. 5-7), then, as shown (see Fig. 5-4), the axis of the magnetic flux will turn

over half of the AC period for half a turn, and for the full period - one turn. The speed of rotation of the magnetic flux can be represented as follows:

In this case, the stator winding creates a magnetic field with one pair of poles. Such a winding was called bipolar.

If the stator winding consists of six coils (two successively connected phase coils), laid in twelve grooves (Fig. 5-8), then as a result of constructions similar to the bipolar winding, it is possible to obtain that the axis of the magnetic flux for the half period will turn on Quarter turnover, and for the full period - on half a turn (Fig. 5-9). Instead of two poles at three

the windings of the stator field now has four poles (two pairs of poles). The speed of rotation of the magnetic field of the stator in this case is equal to

By increasing the number of grooves and windings and producing similar arguments, it can be concluded that the speed of rotation of the magnetic field in the general case in pairs of poles is equal to

Since the number of pairs of poles can only be integer (the number of coils in the stator winding is always multiple of three), then the speed of rotation of the magnetic field may not have any arbitrary, but quite defined values \u200b\u200b(see Table 5.1).

Table 5.1.

In practice, to obtain a constant value of the torque acting on the rotor for one turn, the number of grooves in the stator significantly increases (Fig. 5-10) and each side of the coil is placed in several grooves, and each winding consists of several sections connected between by itself consistently. Winding, as a rule, make two-layer. In each groove stacked one over the other two sides of the sections of two different coils, and if one active side lies at the bottom of one groove, then the other active side of this section lies at the top of the other groove, section and coils are connected to each other so that in most part conductors Each groove direction of currents was the same.