How to determine the rotation speed of the stator magnetic field. Electrogravity is simple. Speed ​​adjustment

§ 65. ROTATING MAGNETIC FIELD

The operation of a multiphase alternating current machine is based on the use of the phenomenon of a rotating magnetic field.

A rotating magnetic field is created by any multiphase alternating current system, that is, a system with two, three, etc. phases.

It was noted above that three-phase alternating current is most widespread. Therefore, consider the rotating magnetic field of a three-phase winding of an alternating current machine (Fig. 70).

There are three coils on the stator, the axes of which are mutually shifted at angles of 120°. For clarity, each coil is depicted as consisting of one turn located in two grooves (cavities) of the stator. In reality the coils have big number turns. The letters A, B, C indicate the beginnings of the coils, X Y, Z - their ends. The coils are connected in a star, that is, the ends X, Y, Z are connected to each other, forming a common neutral, and the beginnings A, B, C are connected to a three-phase alternating current network. The coils can also be connected in a triangle.

Sinusoidal currents flow through the coils with the same amplitudes Im and frequency ω = 2πf, the phases of which are shifted by 1/3 of the period (Fig. 71).

Currents flowing in the coils excite alternating magnetic fields, the magnetic lines of which will penetrate the coils in a direction perpendicular to their planes. Consequently, the average magnetic line or axis of the magnetic field created by the coil A - X will be directed at an angle of 90° to the plane of this coil.

The directions of the magnetic fields of all three coils are shown in Fig. 70 vectors B A, B B and B C, shifted relative to each other by also 120°.

In this case, in the stator conductors connected to the starting points A, B, C, the currents accepted as positive will be directed towards the viewer, and in the conductors connected to the end points X, Y and Z, away from the viewer (see Fig. 70) .

Positive directions of currents will correspond to positive directions of magnetic fields, shown in the same figure and determined by the gimlet rule.

Figure 71 shows the current curves of all three coils, which allow you to find the instantaneous current value of each coil for any moment in time.

Without touching on the quantitative side of the phenomenon, we first determine the directions of the magnetic field created by a three-phase winding for different moments of time.

At the moment t = 0, the current in coil A - X is zero, in coil B - Y is negative, in coil C - Z is positive. Consequently, at this moment there is no current in conductors A and X, in conductors C and Z it has a positive direction, and in conductors B and Y it has a negative direction (Fig. 72, A).

Thus, at the moment t=0 we have chosen, in conductors C and Y the current is directed towards the viewer, and in conductors B and Z - away from the viewer.

With this direction of current, according to the gimlet rule, the magnetic lines of the created magnetic field are directed from bottom to top, x. That is, in the lower part of the inner circumference of the stator there is a north pole, and in the upper part there is a south pole.

At moment t 1 in phase A the current is positive, in phases B and C it is negative. Consequently, in conductors Y, A and Z the current is directed towards the viewer, and in conductors C, X and B - away from the viewer (Fig. 72, b), and the magnetic lines of the magnetic field are rotated 90° clockwise relative to their initial direction.

At moment t 2, the current in phases A and B is positive, and in phase C it is negative. Consequently, in conductors A, Z and B the current is directed towards the viewer, and in conductors Y, C and X - away from the viewer and the magnetic lines of the magnetic field are rotated at an even greater angle relative to their initial direction (Fig. 72, c).

Thus, over time, there is a continuous and uniform change in the directions of the magnetic lines of the magnetic field created by the three-phase winding, i.e. this magnetic field rotates at a constant speed.

In our case, the magnetic field rotates clockwise.

If you change the phase rotation of a three-phase winding, that is, change the connection to the network of any two of the three coils, then the direction of rotation of the magnetic field will also change. In Fig. Figure 73 shows a three-phase winding in which the connection of coils B and C to the network has been changed. From the direction of the magnetic lines of the magnetic field for the previously selected times t=0, t 1 and t 2 it is clear that the rotation of the magnetic field now occurs counterclockwise.

The magnetic flux produced by a three-phase AC system in a symmetrical coil system is a constant value and at any time is equal to one and a half times the maximum flux of one phase.

This can be proven by determining the resulting magnetic flux Ф for any moment in time.

So, for moment t 1, when ωt 1 ==90°, the currents in the coils take the following values:

Consequently, the magnetic flux F A of coil A at the selected moment has the greatest value and is directed along the axis of this coil, i.e., positive. The magnetic fluxes of coils B and C are half the maximum and are negative (Fig. 74).

The geometric sum of the flows Fa, Fw, Fs can be found by constructing them sequentially on an accepted scale in the form of segments. By connecting the beginning of the first segment with the end of the last, we obtain a segment of the resulting magnetic flux F. Numerically, this flux will be one and a half times greater than the maximum flux of one phase.

For example, for time A (see Fig. 74), the resulting magnetic flux

since at this moment the resulting flow coincides with the Fa flow and is shifted relative to the Fw and Fc flows by 60°.

Bearing in mind that at moment t 1 the magnetic fluxes of the coils take on values, the resulting magnetic flux can be expressed as follows:

At the moment t=0, the resulting magnetic field was directed along the vertical axis (see Fig. 72, a). In a time equal to one period of change in the current in the coils, the magnetic flux will rotate one revolution in space and will again be directed along the vertical axis, the same as at the moment t=0.

If the frequency of the current is f, that is, the current undergoes f periods of change in one second, then the magnetic flux of the three-phase winding will make f (revolutions per second or 60f revolutions per minute, i.e.

n 1 - number of revolutions of the rotating magnetic field per minute.

We considered the simplest case when the winding has one pair of poles.

If the stator winding is made in such a way that the wires of each phase are divided into 2, 3, 4, etc. identical groups, symmetrically located around the circumference of the stator, then the number of pole pairs will be equal to 2, 3, 4, etc., respectively.

In Fig. 75 shows a single-phase winding, consisting of three coils symmetrically located around the circumference of the stator and forming six poles or three pairs of poles.

In multi-pole windings, the magnetic field during one period of current change rotates through an angle corresponding to the distance between two poles of the same name.

Thus, if the winding has 2, 3, 4, etc. pairs of poles, then the magnetic field during one period of current change rotates to, etc., part of the stator circumference. In the general case, denoted by the letter R number of pole pairs, we find the path traveled by the magnetic field during one period of current change equal to one R-that fraction of the stator circumference. Consequently, the number of revolutions per minute of the magnetic field is inversely proportional to the number of pole pairs, i.e.

Example 1. Determine the number of revolutions of the magnetic field of machines with the number of pairs of poles R=1, 2, 3 and 4, operating from the network with a current frequency of f=50 Hz.

Solution. Number of revolutions of the magnetic field

Example 2. The magnetic field of a machine connected to a network with a current frequency of 50 Hz makes 1500 rpm. Determine the number of revolutions of the magnetic field of this machine if it is connected to a network with a current frequency of 60 Hz.

Solution. Number of machine pole pairs

Number of revolutions of the magnetic field at the new frequency

Control questions

1. Explain the design and operating principle of a three-phase generator.
2. In what case is a neutral wire not needed when connecting the generator winding and receivers with a star?
3. What is the relationship between linear and phase values ​​of voltages and currents when connecting energy sources and consumers with a star and a triangle?
4. What are the advantages of a triangle connection between receivers?
5. What expression determines the power of a three-phase current under a symmetrical load?
6. How can you change the direction of rotation of the magnetic field of a symmetrical three-phase coil system?
7. What determines the rotation speed of the magnetic field of a symmetrical three-phase system?

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A magnetic field whose axis rotates in space with a constant angular frequency is called a rotating magnetic field. If the magnitude of the induction at any point on the magnetic field axis remains constant, then such a field is called a circular rotating magnetic field. This is due to the fact that it can be represented as a vector of constant length rotating in space, the end of which, when rotated, describes a circle.

The formation of a circular rotating magnetic field is a necessary condition operation of asynchronous and synchronous machines. To do this, three identical windings (coils), consisting of two parts located diametrically opposite in the stator package, are placed in the grooves of the stator package (Fig. 1). Moreover, the axes of the three stator windings are shifted relative to each other by 120°.

If we schematically imagine the stator windings as consisting of one turn, then there will be only six slots on the stator, each of which will contain half a turn of the winding. Let us designate the beginning of the turns of the windings with letters A, B And C, and the ends of the turns are letters X, Y And Z. Let us also denote the direction of current flow in the turns of the windings, considering the direction from the beginning to the end of the winding to be positive. Then for positive side current values A, B And C will be marked with a cross, and the sides X, Y And Z– point (Fig. 2).

When connecting the stator windings to a three-phase alternating current network, currents will flow in the windings that are shifted in time (in phase) relative to each other by 120° electrical, as shown in the figure. Let us select six moments of time within the period, spaced from each other by 60° el. and for each of them we note the directions of the currents in the windings, taking into account the signs of the currents at the corresponding moment in time. It is easy to notice that at any moment the currents in the two halves of the stator package flow in different directions and form a magnetic field, the axis of which coincides with the axis of separation of the current directions, i.e. every 60° el. The axis of the magnetic field rotates in space by 60°. Thus, with the help of this symmetrical winding system, powered by a symmetrical three-phase network system, we obtained a circular rotating magnetic field.

The angular frequency with which the magnetic field rotates in space is completely determined by the frequency of the supply network and electrical diagram windings If you double the number of turns and connect them into windings so that two alternating pairs of groups with the same direction of current are located around the circumference of the stator package, then a magnetic field with two pairs of poles is formed (Fig. 3). It will also rotate in space, moving during one period of current oscillations at an angle corresponding to the distance between poles of the same name, i.e. at 180°. This means that the angular velocity of the field rotation will be half as large.

Thus, the angular frequency of rotation of the magnetic field is equal to [rad/s] or [rpm], where is the frequency of the supply network, and p- number of pole pairs of the stator winding. This gives rise to a number of possible magnetic field rotation speeds for an industrial network with a frequency of 50 Hz: 3000, 1500, 1000, 750, 600, etc. [rpm]

The direction of rotation of the magnetic field is determined by the sequence of connecting the windings to the three-phase network. To change the direction of rotation, it is enough to swap the connection points of any two windings.

Basic concepts and operating principle of an asynchronous machine

The design diagram of an asynchronous machine is shown in Figure 1. It consists of a stator package 1 with slots 2 for laying the winding and a cylindrical rotor 3 in the round slots of which the conductors (rods) 4 of its winding are located. The rods are closed at the edges by rings (not shown in the figure), therefore the rotor winding is called short-circuited. This type of rotor is most common in asynchronous machines, because it is simple, reliable and technologically advanced. If you mentally remove the rotor winding from the rotor package, it will have the appearance shown in Figure 2. This type of winding is called a “squirrel cage”.

In addition to “squirrel cage” type rotors, asynchronous machines use rotors in which the same three-phase winding is laid in the slots (Fig. 3 1) as in the stator. To connect to external electrical circuits (5), the ends of the winding are brought out through contact rings (3) and brushes (4) (see figure). This type of rotor is called phase

The rotor winding has no electrical connection with external circuits and the current in it arises as a result of electromagnetic induction. This process works as follows. The three-phase stator winding is connected to the alternating current network and the winding current () forms a circular rotating magnetic field. The stator field () rotates in space relative to the axis of rotation of the rotor () and intersects the rods of its winding. As a result, induced emf () is induced in them, etc. the ends of the rotor rods are electrically closed by rings, then an electric current is formed in them under the influence of EMF (). The interaction of the current flowing in the rods with the external magnetic field causes the action of a force ( F) and the corresponding electromagnetic moment ( M), causing the rotor to rotate (). Thus, the occurrence of torque is possible only if the rotor rods intersect the magnetic field of the stator, and for this it is necessary that the rotor rotates at a speed different from the speed of rotation of the magnetic field, i.e. so that it rotates asynchronously with the field. This is where the name of this machine comes from - asynchronous.

The above can be represented in the form of a logical sequence in which there is only one conditional transition from the rotating field to the EMF and rotor current. If , then the field and the rotor rotate synchronously and the rotor EMF is not excited. This mode is called idling and it can only be created due to external torque.

If the rotor rotation speed is less than the field rotation speed, then the electromagnetic torque acting on the rotor is positive and tends to accelerate it. When the rotor speed is higher than the field speed, the directions of the emf and current in the rotor change to the opposite. The electromagnetic torque also changes sign and becomes braking.

To describe electromechanical processes in an asynchronous machine, the concept of slip s is usually used. It is equal to the difference between the speeds or frequencies of rotation of the magnetic field () and the rotor () related to the speed or frequency of rotation of the magnetic field . Hence the speed or frequency of rotation can be expressed in terms of slip. The speed or frequency of rotation of the magnetic field is also called synchronous speed or frequency.

Main magnetic flux and leakage fluxes. Inductive reactances

Currents induced by the induced emf flow in the rotor winding. They form their own rotor field rotating relative to the rotor body at a sliding frequency. Thus, the rotor field participates in two rotational movements - movement relative to the torus body and together with it relative to the stator with a frequency of . Consequently, the rotation frequency of the rotor field is equal to , i.e. The rotor field rotates in space with the same frequency as the stator field. Therefore, these fields are motionless relative to each other and form a single field of the machine. The main part of the magnetic flux field covers the stator and rotor windings, crossing the air gap. This part is called the main magnetic flux F. The other two parts are coupled with only one of the windings and form the corresponding leakage fluxes and. Leakage fluxes are formed in the windings by leakage emf or self-induction emf, which can be represented through the winding currents and the corresponding leakage inductances, taking into account that the currents in the stator and rotor windings have different frequencies ( and ): and , where and are the inductive leakage resistances at the stator frequency .

Electromotive forces of windings

The rotating magnetic field crosses the turns of the stator winding and induces an emf in them. By analogy with a transformer, we can write , where is the winding coefficient, which takes into account the design features of the stator winding (shortening the pitch, distribution of the winding among the slots, bevel of the slots). In transformers, the picture of the magnetic field is simpler, because the main magnetic flux covers almost all turns of the winding and the introduction of a winding coefficient is not required.

The rotor winding is crossed by the main magnetic flux with frequency . Hence the EMF of the winding – , where is the EMF of the rotor winding at the stator frequency, i.e. with a stationary rotor.

Magnetomotive forces and currents of the stator and rotor

Optimal energy conversion in an asynchronous machine is possible provided that the magnetomotive forces (MFF) of the windings are distributed along the gap circumference according to a sinusoidal law. However, the stator windings are coils that create an MMF with a distribution close to rectangular. Therefore, they are divided into sections and laid out along the gap into adjacent grooves. As a result, the MMF acquires a distribution close to sinusoidal, but if we isolate the main spatial harmonic, which is actually required for the operation of the machine, it turns out that the calculation of the MMF according to the expression valid for a concentrated winding, where w And I– the number of turns and the current in the winding will be overestimated. Therefore, to calculate the MMF of an asynchronous machine, the so-called winding coefficient, which takes into account the design features of the windings - distribution along the gap, bevel of the grooves and shortening of the pitch. As a result of the introduction of this coefficient, the real distributed winding is, as it were, transformed into a lumped winding, which, with a current equal to the current in the real winding, creates an MMF with a sinusoidal distribution corresponding to the MMF of the fundamental harmonic of the real winding.

is the stator current reduced to the parameters of the rotor winding, and is the transformation ratio of the currents of the asynchronous machine.

It should be noted that the number of phases of the squirrel cage rotor winding is equal to the number of rods, and the number of turns is 0.5.

A feature of multiphase systems is the ability to create a rotating magnetic field in a mechanically stationary device.
A coil connected to an alternating current source produces a pulsating magnetic field, i.e. a magnetic field that varies in magnitude and direction.

Let's take a cylinder with an internal diameter D. On the surface of the cylinder we will place three coils, spatially displaced relative to each other by 120 o. We connect the coils to a three-phase voltage source (Fig. 12.1). In Fig. Figure 12.2 shows a graph of changes in instantaneous currents forming a three-phase system.

Each of the coils creates a pulsating magnetic field. The magnetic fields of the coils, interacting with each other, form a resulting rotating magnetic field, characterized by the vector of the resulting magnetic induction
In Fig. 12.3 shows the magnetic induction vectors of each phase and the resulting vector constructed for three moments in time t1, t2, t3. The positive directions of the coil axes are designated +1, +2, +3.

At the moment t = t 1, the current and magnetic induction in the coil A-X are positive and maximum, at coils B-Y and C-Z are identical and negative. The vector of the resulting magnetic induction is equal to the geometric sum of the vectors of the magnetic induction of the coils and coincides with the axis of the coil A-X. At the moment t = t 2, the currents in the coils A-X and C-Z are equal in magnitude and opposite in direction. The current in phase B is zero. The resulting magnetic induction vector rotated clockwise by 30 o. At the moment t = t 3, the currents in the coils A-X and B-Y are equal in magnitude and positive, the current in the C-Z phase is maximum and negative, the vector of the resulting magnetic field is located in the negative direction of the axis coils C-Z. During the period of alternating current, the vector of the resulting magnetic field will rotate 360 ​​o.

Magnetic field rotation speed or synchronous rotation speed

where P is the number of pole pairs.

The coils shown in Fig. 12.1, create a two-pole magnetic field, with the number of poles 2P = 2. The field rotation frequency is 3000 rpm.
To obtain a four-pole magnetic field, it is necessary to place six coils inside the cylinder, two for each phase. Then, according to formula (12.1), the magnetic field will rotate twice as slowly, with n 1 = 1500 rpm.
To obtain a rotating magnetic field, two conditions must be met.

1. Have at least two spatially offset coils.

2. Connect out-of-phase currents to the coils.

12.2. Asynchronous motors.
Design, principle of operation

The asynchronous motor has motionless part called stator , And rotating part called rotor . The stator contains a winding that creates a rotating magnetic field.
There are asynchronous motors with squirrel cage and wound rotor.
Aluminum or copper rods are placed in the slots of the short-circuited rotor. The ends of the rods are closed with aluminum or copper rings. The stator and rotor are made of electrical steel sheets to reduce eddy current losses.
The phase rotor has a three-phase winding (for a three-phase motor). The ends of the phases are connected into a common unit, and the beginnings are brought out to three slip rings placed on the shaft. Fixed contact brushes are placed on the rings. A starting rheostat is connected to the brushes. After starting the engine, the resistance of the starting rheostat is gradually reduced to zero.
Let's look at the operating principle of an asynchronous motor using the model shown in Figure 12.4.

Let's imagine the rotating magnetic field of the stator in the form of a permanent magnet rotating at a synchronous rotation speed n 1.
Currents are induced in the conductors of the closed rotor winding. The poles of the magnet move clockwise.
To an observer placed on a rotating magnet, it seems that the magnet is stationary, and the conductors of the rotor winding are moving counterclockwise.
The directions of rotor currents determined by the right-hand rule are shown in Fig. 12.4.

Rice. 12.4

Using the left-hand rule, we find the direction of the electromagnetic forces acting on the rotor and causing it to rotate. The motor rotor will rotate at a rotation speed n 2 in the direction of rotation of the stator field.
The rotor rotates asynchronously, i.e. its rotation frequency n 2 is less than the rotation frequency of the stator field n 1.
The relative speed difference between the stator and rotor fields is called slip.

The slip cannot be equal to zero, since at the same speeds of the field and the rotor the induction of currents in the rotor would cease and, therefore, there would be no electromagnetic torque.
The rotating electromagnetic torque is balanced by the counteracting braking torque M em = M 2.
As the load on the motor shaft increases, the braking torque becomes greater than the rotating torque, and slip increases. As a result, the EMF and currents induced in the rotor winding increase. The torque increases and becomes equal to the braking torque. The torque can increase with increasing slip up to a certain maximum value, after which, with a further increase in the braking torque, the torque decreases sharply and the engine stops.
The slip of a stalled motor is equal to one. The engine is said to be running in short circuit mode.
The rotational speed of an unloaded asynchronous motor n 2 is approximately equal to the synchronous frequency n 1. Slip of an unloaded engine S 0. The engine is said to be running in idle mode.
The slip of an asynchronous machine operating in motor mode varies from zero to one.
An asynchronous machine can operate in generator mode. To do this, its rotor must be rotated by a third-party motor in the direction of rotation of the stator magnetic field with a frequency n 2 > n 1. Slip of an asynchronous generator.
An asynchronous machine can operate in electric machine brake mode. To do this, it is necessary to rotate its rotor in the direction opposite direction rotation of the stator magnetic field.
In this mode, S > 1. Typically, asynchronous machines are used in motor mode. The induction motor is the most common type of motor in industry. The field rotation frequency in an asynchronous motor is strictly related to the network frequency f 1 and the number of stator pole pairs. At frequency f 1 = 50 Hz, there is the following series of rotation frequencies.

One of the most common electric motors, which is used in most electric drive devices, is the asynchronous motor. This motor is called asynchronous (non-synchronous) for the reason that its rotor rotates at a lower speed than that of a 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 the speed at which the magnetic field rotates in a rotary machine; to be precise, it is the angular speed of rotation of the magnetic field vector. The speed of rotation of the field depends on the frequency of the flowing current and the number of poles of the machine.

An asynchronous motor always operates at a speed lower than the synchronous rotation speed, because the magnetic field that is formed by the stator windings will generate a counter magnetic flux in the rotor. The interaction of this generated counter magnetic flux with the stator magnetic flux will cause the rotor to start rotating. Since the magnetic flux in the rotor will lag behind, the rotor will never be able to independently achieve synchronous speed, that is, the same speed as the stator magnetic field vector rotates.

There are two main types of induction motor, which are determined by the type of power supplied. This:

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

It should be noted that a single-phase asynchronous motor is not capable of independently starting 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-up. Unlike a single-phase asynchronous motor, a three-phase motor is capable of starting independent movement (rotation) without making any changes to the design or starting conditions.

From engines direct current(DC) alternating current (AC) induction motors are structurally different in that power is supplied to the stator, in contrast to a direct current motor, in which power is supplied to the armature (rotor) through a brush mechanism.

Operating principle of an asynchronous motor

By applying voltage only to the stator winding, the asynchronous motor begins to operate. Interested to know how it works, why this happens? This 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, you have to separately create a magnetic field in the armature (rotor) not through induction, but through brushes.

When we apply voltage to the stator windings, an electric current begins to flow through them, which creates a magnetic field around the windings. Further, from many windings that are located on the stator magnetic circuit, a common magnetic field of the stator is formed. This magnetic field is characterized by a magnetic flux, the magnitude of which changes over time; in addition, the direction of the magnetic flux changes in space, or rather, it rotates. As a result, it turns out that the stator magnetic flux vector rotates like a spun sling with a stone.

In full accordance with Faraday's law of electromagnetic induction, in a rotor that has a short-circuited winding (short-circuited rotor). An induced electric current will flow in this rotor winding since the circuit is closed and it is in short circuit mode. This current, just like the supply current in the stator, will create a magnetic field. The motor rotor becomes a magnet inside the stator, which has a magnetic rotating field. Both magnetic fields from the stator and rotor will begin to interact, obeying the laws of physics.

Since the stator is motionless and its magnetic field rotates in space, and a current is induced in the rotor, which actually makes it a permanent magnet, the movable rotor begins to rotate because the magnetic field of the stator begins to push it, dragging it along with it. The rotor seems to mesh with the magnetic field of the stator. We can say that the rotor tends to rotate synchronously with the magnetic field of the stator, but this is unattainable for it, since at the moment of synchronization the magnetic fields cancel each other out, which leads to asynchronous operation. In other words, when an asynchronous motor operates, the rotor slides in the magnetic field of the stator.

Sliding can be either delayed or advanced. If there is a delay, then we have a motor mode of operation, when electrical energy is converted into mechanical energy; if sliding occurs with the rotor advancing, then we have a generator mode of operation, when mechanical energy is converted into electrical energy.

The torque generated on the rotor depends on the frequency of the alternating current supply to the stator, as well as on the magnitude of the supply voltage. By changing the frequency of the current and the magnitude of the voltage, you can influence the rotor torque and thereby control the operation of the asynchronous motor. This is true for both single-phase and three-phase asynchronous motors.

Types of asynchronous motor

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

• With separate windings (Split-phase motor);
• With a starting capacitor (Capacitor start motor);
• With start capacitor and run capacitor (Capacitor start capacitor run induction motor);
• With a displaced pole (Shaded-pole motor).

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

• With a squirrel cage induction motor;
• With slip rings, wound rotor (Slip ring induction motor);

As mentioned above, a single-phase asynchronous motor cannot start moving (rotating) on ​​its own. What should be understood by independence? This is when the machine starts working automatically without any influence from the external environment. When we turn on a household electrical appliance, such as a fan, it starts working immediately upon pressing a key. It should be noted that in everyday life a single-phase asynchronous motor is used, for example a motor in a fan. How does such an independent start occur, if it was said above that this type of engine does not allow it? In order to understand this issue, you need to study methods of starting single-phase motors.

Why is a three-phase asynchronous motor self-starting?

In a three-phase system, each phase relative to the other two has an angle of 120 degrees. All three phases are thus evenly spaced in a circle; the circle has 360 degrees, which is three times 120 degrees (120+120+120=360).

If we consider three phases, A, B, C, then we will notice that only one of them at the initial moment of time will have the maximum value of the instantaneous voltage value. The second phase will increase its voltage value following the first, and the third phase will follow the second. So we have the alternation order phases A-B-C as their value increases and another order is possible in descending order voltage C-B-A. Even if you write the alternation differently, for example, instead of A-B-C, write B-C-A, the alternation will remain the same, since the alternation chain in any order forms a vicious circle.

How will the rotor of an asynchronous three-phase motor rotate? Since the rotor is entrained by the stator's magnetic field and slides in it, it is quite obvious that the rotor will move in the direction of the stator's magnetic field vector. In which direction will the stator magnetic field rotate? Since the stator winding is three-phase and all three windings are located evenly on the stator, the generated field will rotate in the direction of the phase alternation of the windings. From this we draw a conclusion. The direction of rotation of the rotor depends on the phase sequence of the stator windings. By changing the alternation order of the phases, we get the engine rotating in the opposite direction. In practice, to change the rotation of the motor, it is enough to swap any two supply phases of the stator.

Why doesn't a single-phase asynchronous motor start rotating on its own?

For the reason that it is powered from one phase. The magnetic field of a single-phase motor is pulsating, not rotating. The main task of the launch is to create a rotating field from a pulsating field. This problem is solved by creating a phase shift in the other stator winding using capacitors, inductors and the spatial arrangement of the windings in the motor 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 less and the engine is running below its maximum load, its efficiency is significantly reduced. This is a disadvantage of a single-phase asynchronous motor and therefore, unlike three-phase machines, they are used where the mechanical load is constant.

In inductive electrical machines, the stator and rotor windings are connected by a magnetic field. In order to connect the rotating part of the machine with the machine stationary in the air gap through a system of stator windings, they create rotating a magnetic field.

By rotating we mean a magnetic field whose induction vector moves in space (in a plane perpendicular to the rotor axis) with a certain angular velocity. If the amplitude of the induction vector is constant, then such a field is called circular. A rotating magnetic field can be created:

• alternating current in a two-phase system of windings shifted in space by 90°;
• three-phase alternating current in a three-phase system of windings shifted in space by 120°;
• direct current switched sequentially through windings distributed along the motor stator bore;
• direct current, switched using a commutator along winding branches located along the surface of the rotor (armature). Formation of a rotating magnetic field in a two-phase machine
• (rice. 1.2). IN In such a machine, the axes of the windings are shifted geometrically by 90° (a machine with one pair of poles is considered, r p = 1). The stator windings are powered by two-phase voltage, as shown in Fig. 1.2, i. Assuming the machine is symmetrical and unsaturated, we assume that the currents in the windings are also shifted by 90 electrical degrees (90° el.) and the magnetomotive force of the windings is proportional to the current (Fig. 1 .2,6). IN moment of time, = 0 winding current A is equal to zero, and the current in the winding b has the greatest negative value.

Rice. 1.2. Formation of a rotating magnetic field in a two-phase electric machine: a - winding connection diagram: b - system of two-phase currents in the stator windings: V- spatial vector diagram of magnetically moving forces created by the stator windings

Consequently, the total vector of magnetic motion forces (MFF) of the windings at an instant of time is equal to t and is located in space, as shown in Fig. 1.2, V. At the moment of time c 2 = 7s/ the currents in the windings will be Tl m / and, therefore, the total MMF vector will rotate by an angle To/ and will occupy the position in space indicated in Fig. 12, V, like 2 = 2 + 2. In the moment

time with 2 = i/2 the total vector of the MMF will be equal. Similarly, you can trace how the position of the total MMF vector changes at moments of time, etc. It can be seen that the vector rotates in space at a speed of co = 2ts, keeping its amplitude constant. The direction of field rotation is clockwise. We suggest making sure that if you apply to the phase A voltage = (co -), and per phase b voltage = co, then direction

rotation will be reversed.

Rice. 1.3. Schemes for connecting the windings of a three-phase motor: a - location of the motor windings at p p = 1; b - star connection of windings; V- diagrams of three-phase currents in the motor windings

Thus, the combination of a spatial shift of the winding axes by 90 geometric degrees (90°) and a phase shift of the alternating current in the windings by (90° el.) electrical degrees allows the formation of a magnetic field rotating along the circumference of the stator in the air gap of the machine.

The mechanism of formation of a rotating magnetic field in a three-phase alternating current machine. The windings of the machine are shifted in space by 120° (Fig. 1.3, a) and are powered by a three-phase voltage system. The currents in the machine winding are shifted by 120°el. (Fig. 1.3, V):

The resulting MMF vector of the stator windings is equal to:

Where w- number of turns of windings.

Let's consider the position in space of the vector at the moment of time (Fig. 1.4, o). The winding MMF vector o t is directed along the o axis in the positive direction and is equal to 0, w, those. ABOUT, . Vector MDS winding With, directed along the axis With and equals 0, . The sum of vectors j and j is directed along the axis b in the negative direction and with this sum the winding MMF vector is added b, equal The sum of three vectors forms a vector X= 3 /2, occupying at the moment of time the position shown in Fig. 1.4, o. After time = l/30 (at a frequency of 50 Hz after 1/300 s) there will be a moment in time 2 at which the MMF vector of the winding o is equal, and the MMF vectors of the windings b And With equal - 0.5. The resulting MMF vector 2 at time 2 will take the position shown in Fig. 1.4,5, i.e. will move relative to the previous position at at an angle of 60° clockwise. It is easy to verify that at time 3 the resulting MMF vector of the stator windings will take position 3, i.e. will continue to move clockwise. During the period of supply voltage = 2l/co = 1/ the resulting MMF vector will complete a full revolution, i.e. the speed of rotation of the stator field is directly proportional to the frequency of the current in its windings and inversely proportional to the number of pole pairs:

where n is the number of pole pairs of the machine.

If the number of motor pole pairs is greater than one, then the number of winding sections located around the circumference of the stator increases. So, if the number of pole pairs n = 2, then three phase windings will be located on one half of the stator circumference and three on the other. In this case, during one period of the supply voltage, the resulting MMF vector will make half a revolution and the rotation speed of the stator magnetic field will be half that in machines with „=1-

Rice. 1.4.A- с = 7с/ b- co = l/ V- с = 7с/

The operation of almost all alternating current motors: synchronous with electromagnetic excitation (SM), with excitation from permanent magnets (PMSM), synchronous reluctance motors (SRM), and asynchronous motors (IM) is based on the principle of creating a rotating magnetic field.

According to the principles of electrodynamics, in all electric motors (except reactive ones), the developed electromagnetic torque is the result of the interaction of magnetic fluxes (flux linkages) created in the moving and stationary parts of the electric motor. The moment is equal to the product of the vectors of these flows, as shown in Fig. 1.5, and the value of the moment is equal to the product of the modules of the flow vectors and the sine of the spatial angle 0 between the flow vectors:

Where To - design factor.

Rice. 1.5.

Synchronous(SD, SDPM, SRD) and asynchronous motors They have almost identical stator designs, but the rotors are different. The distributed stator windings of these electric motors fit into a relatively large number of semi-closed stator slots. If we do not take into account the influence of tooth harmonics, then the stator windings form a magnetic flux of constant amplitude, rotating at a constant speed determined by the frequency of the current. In real structures, the presence of grooves and teeth in the stator magnetic circuit leads to the appearance of higher harmonics of magnetizing forces, which leads to pulsations of the electromagnetic torque.

On the rotor of the LED there is an excitation winding, which is powered by direct current from an independent voltage source - the exciter. The excitation current creates an electromagnetic field, stationary relative to the rotor and rotating in the air gap together with the rotor at a speed of [see. (1.7)]. For synchronous motors with power up to 100 kW, excitation from permanent magnets is used, which are installed on the rotor.

The magnetic force lines of the rotor field, created by the excitation winding or permanent magnets, “couple” with the electromagnetic field of the stator rotating synchronously with it. Interaction of stator fields X and rotor 0 creates an electromagnetic torque on the shaft of a synchronous machine.

In the absence of load on the shaft, the field vectors of the stator and rotor 0 coincide in space and rotate together at a speed of 0 (Fig. 1.6, i).

When a moment of resistance is applied to the motor shaft, the vectors [ and 0 diverge (stretch like a spring) at an angle of 0, and both vectors continue to rotate at the same speed from 0 (Fig. 1 .6,6). If angle 0 is positive, then the synchronous machine operates in motor mode. A change in the load on the motor shaft corresponds to a change in angle 0 Maximum torque M will be at 0 = l;/ (0 - electrical degrees). If

the load on the motor shaft exceeds M then the synchronous mode is disrupted and the engine falls out of synchronism. If the angle is negative 0, the synchronous machine will operate as a generator.

Rice. 1.6.A- at ideal idle speed; b - with load on the shaft

Synchronous reluctance motor - This is a motor with pronounced rotor poles without an excitation winding, where the torque is determined by the desire of the rotor to occupy a position in which the magnetic resistance between the excited stator winding and the rotor takes on a minimum value.

In the RDS, the rotor is salient-pole (Fig. 1.7). It has different magnetic conductivity along its axes. Longitudinal axis d, passing through the middle of the pole, the conductivity is maximum, and along the transverse axis q- minimal. If the axis of the stator magnetizing forces coincides with the longitudinal axis of the rotor, there is no curvature of the magnetic flux lines and the torque is zero. When the flow of the stator axis is displaced relative to the longitudinal axis d When the magnetic field (MF) rotates, the flux lines of force become bent and an electromagnetic torque arises. The highest torque at the same stator current is obtained at angle 0 = 45°el.

The main difference between an asynchronous motor and a synchronous one is that the speed of rotation of the motor rotor is not equal to the speed of the magnetic field created by the currents in the stator windings. The difference in speed between the stator and rotor fields is called sliding= co - co. Thanks to sliding, the magnetic field lines of the rotating stator field cross the conductors of the rotor winding and induce emf and rotor current in it. The interaction of the stator field and rotor current determines the electromagnetic torque of an asynchronous motor.

Rice. 1.7.

Depending on the rotor design, asynchronous motors are distinguished with phase And short-circuited rotor. In slip-ring motors, a three-phase winding is located on the rotor, the ends of which are connected to slip rings, through which the rotor circuit is removed from the machine for connection to starting resistors with subsequent short-circuiting of the windings.

In an asynchronous motor, when there is no load on the shaft, only magnetizing currents flow through the stator windings, creating the main magnetic flux, and the amplitude of the flux is determined by the amplitude and frequency of the supply voltage. In this case, the rotor rotates at the same speed as the stator field. No EMF is induced in the rotor windings, there is no rotor current and, therefore, the torque is zero.

When a load is applied, the rotor rotates slower than the field, slip occurs, an EMF proportional to the slip is induced in the rotor windings, and rotor currents arise. The stator current, as in a transformer, increases by an appropriate value. The product of the active component of the rotor current and the stator flux modulus determines the motor torque.

What all motors have in common [except switched reluctance motors (SMR)] is that the main magnetic flux in the air gap rotates relative to a stationary stator at a given frequency and angular velocity co. This magnetic flux carries along the rotor, which rotates for synchronous machines with the same angular speed co = co, or for asynchronous machines with some lag - slip 5. The power lines forming the main flow have a minimum length when the engine is running idle (=). In this case, the vector axes of the magnetizing forces of the stator and rotor coincide. When a load appears on the motor shaft, the axes diverge, and the force lines bend and lengthen. Since lines of force always tend to shorten in length, tangential forces appear, creating torque.

In recent years, they have begun to be used switched reluctance motors. This type of motor has a salient pole stator with coil windings on each pole. The rotor is also salient pole, but with a different number of poles without windings. A unipolar current is alternately supplied to the stator windings from a special converter - a commutator, and a nearby rotor tooth is attracted to these excited poles. Then the next stator pole is excited in turn. The stator pole windings are switched in accordance with the signals from the rotor position sensor. This, as well as the fact that the current in the stator windings is regulated depending on the load torque, is the main difference between VID and a stepper motor.

In VIEW (Fig. 1.8), the torque is proportional to the amplitude of the main flow and the degree of curvature of the magnetic field lines. At the beginning, when the rotor pole (teeth) begins to overlap the stator pole, the curvature of the power lines is maximum and the flux is minimum. When the pole overlap is maximum, the bending of the field lines is minimal, and the amplitude of the flow increases, while the torque remains approximately constant. As the magnetic system of the VID becomes saturated, the increase in flux is limited, even with an increase in the current in the windings of the VID. The change in torque as the rotor poles pass relative to the stator poles causes uneven rotation of the VID shaft.

Rice. 1.8.

In a DC motor, the field winding is located on the stator and the field created by this winding is stationary. A rotating magnetic field is created in the armature, the rotation speed of which is equal to the speed of rotation of the armature, but is directed in the opposite direction. This is achieved by the fact that alternating current flows through the turns of the armature winding, switched by a mechanical frequency converter - collector apparatus.

The electromagnetic torque of a DC motor determines the interaction of the main flux created by the field winding and the current in the turns of the armature winding: M = k/ I

If we replace the brush-commutator apparatus of a DC motor with a semiconductor switch, we get brushless DC motor. A practical implementation of such motors is a valve motor. Structurally valve motor is a three-phase synchronous machine with electromagnetic excitation or excitation from permanent magnets. The stator windings are switched using a semiconductor controlled converter - a commutator, depending on the position of the motor rotor.