What is synchronous rotation. Rotating magnetic field of an induction machine (for non-electricians) Angular speed of rotation of the magnetic field

An important advantage of three-phase current is the possibility of obtaining a rotating magnetic field, which is the basis of the principle of operation of electrical machines - asynchronous and synchronous three-phase current motors.

Rice. 7.2. The arrangement of the coils when receiving a rotating magnetic field (a) and the wave diagram of a three-phase symmetric system of currents flowing through the coils (b)

A rotating magnetic field is obtained by passing a three-phase system of currents (Fig. 7.2, b) through three identical coils A, B, C(Fig. 7.2, a), the axes of which are located at an angle of 120 ° relative to each other.

Figure 7.2, a shows the positive directions of the currents in the coils and the directions of the induction of magnetic fields V A , V V , V WITH created by each of the coils separately.

Figure 7.3 shows the actual directions of currents for times
and directions of induction V cut the resulting magnetic field created by the three coils.

Analysis of Figure 7.3 leads to the following conclusions:

a) induction V cut the resulting magnetic field changes its direction (rotates) over time;

b) the frequency of rotation of the magnetic field is the same as the frequency of the current change. So, for f = 50 Hz, the rotating magnetic field makes five to ten revolutions per second, or three thousand revolutions per minute.

The value of the induction of the resulting V cut = 1,5B m magnetic field is constant,

where B m- the amplitude of the induction of one coil.

at different times

7.3 Asynchronous machines

7.3.1 The principle of operation of an induction motor (AM)... We place between the stationary coils (Figure 7.4) in the area of the rotating magnetic field a movable metal cylinder - a rotor - fixed on the axis.

Let the magnetic field rotate "clockwise", then the cylinder rotates in the opposite direction relative to the rotating magnetic field.

Taking this into account, according to the rule of the right hand, we find the direction of the currents induced in the cylinder.

In Figure 7.4, the directions of the induced currents (along the generatrix of the cylinder) are shown by crosses ("away from us") and dots ("towards us").

Applying the left-hand rule (Fig. 7.1, b), we find that the interaction of the induced currents with the magnetic field generates forces F, driving the rotor in rotational motion in the same direction in which the magnetic field rotates.

Rotor speed
less frequency of rotation of the magnetic field since at the same angular velocities, the relative speed of the rotor and the rotating magnetic field would be equal to zero and there would be no induced EMF and currents in the rotor. Therefore, there would be no strength F, creating a torque. The considered simplest device explains the principle of operation. asynchronous motors. The word "asynchronous" (Greek) means non-simultaneous. This word emphasizes the difference in the frequencies of the rotating magnetic field and the rotor - the moving part of the engine.

Rice. 7.4. To the principle of operation of an asynchronous motor

The rotating magnetic field created by the three coils has two poles and is called bipolar rotating magnetic field(one phase of the poles).

In one period of sinusoidal current, the bipolar magnetic field makes one revolution. Therefore, at the standard frequency f 1 = 50 Hz this field makes three thousand revolutions per minute. The rotor speed is slightly less than this synchronous speed.

In cases where an asynchronous motor with a lower speed is required, a multi-pole stator winding is used, consisting of six, nine, etc. coils. Accordingly, the rotating magnetic field will have two, three, etc. pairs of poles.

In general, if the field has R pairs of poles, then its rotation speed will be

7.3.2 Asynchronous motor design... The magnetic system (magnetic circuit) of an induction motor consists of two parts: an external stationary, in the form of a hollow cylinder (Fig. 8.5), and an internal - a rotating cylinder.

Both parts of the induction motor are assembled from 0.5 mm thick electrical steel sheets. These sheets are insulated from each other with a layer of varnish to reduce eddy current losses.

The fixed part of the machine is called stator, while rotating - rotor(from latin stare - stand and rotate – rotate).

Rice. 7.5. Diagram of an induction motor device: cross section (a);

rotor winding (b): 1 - stator; 2 - rotor; 3 - shaft; 4 - turns of the stator winding;

5 - turns of the rotor winding

A three-phase winding is laid in the grooves on the inside of the stator, the currents of which excite the rotating magnetic field of the machine. In the slots of the rotor there is a second winding, the currents in which are induced by a rotating magnetic field.

The stator magnetic circuit is enclosed in a massive case, which is the outer part of the machine, and the rotor magnetic circuit is fixed on the shaft.

Rotors of induction motors are made of two types: squirrel-cage and with slip rings. The first of them are simpler in design and are used more often.

The winding of a squirrel-cage rotor is a cylindrical cage ("squirrel wheel") made of copper tires or aluminum rods, short-circuited at the ends by two rings (Figure 7.5, b). The rods of this winding are inserted without insulation into the grooves of the magnetic circuit.

The method of filling the grooves of the rotor magnetic circuit with molten aluminum with simultaneous casting of the closing rings is also used.

7.3.3 Induction motor characteristics... The rotation speed of the rotating magnetic field is determined either by the angular frequency , n, or the number of revolutions NS per minute. These two quantities are related by the formula

. (7.3)

The characteristic quantity is the relative speed of the rotating magnetic field, called slipS:

where
- angular frequency of the rotor, rad / s;

- number of revolutions per minute, rpm.

The closer the rotor speed to the speed of the rotating magnetic field , the lower the EMF induced by the field in the rotor, and hence the currents in the rotor.

The decrease in currents reduces the torque acting on the rotor, so the rotor of the motor must rotate slower than the rotating magnetic field - asynchronously.

It can be shown that the AM torque is determined by the following expression:

, (7.4)

where , , x 1 , - the parameters of the electric equivalent circuit, which are given in the reference books for blood pressure;

- effective phase voltage on the stator winding.

In modern asynchronous motors, slip even at full load is small - about 0.04 (four percent) for small motors and about 0.015 ... .0.02 (one and a half - two percent) for large motors.

Characteristic dependence curve M from sliding S shown in Figure 7.6, a.

Maximum torque separates the curve
on a stable part of S = 0 to and the unstable part of before S = 1, within which the torque decreases with increasing slip.

On the site from S = 0 to with decreasing braking torque
the rotation speed increases on the asynchronous motor shaft, slip decreases, so that in this section the operation of the asynchronous motor is stable.

On the site from before S= 1 decreasing
the rotation speed increases, the slip decreases and the torque increases, which leads to an even greater increase in the rotation speed, so that the engine operation is unstable.

Thus, while the braking torque
, the dynamic equilibrium of the moments is automatically restored. When
, with a further increase in the load, an increase in slip leads to a decrease in the torque M and the motor stops due to the prevalence of the braking torque over the rotating one.

Meaning M To can be calculated by the formula

For practice great importance has a dependence of the motor speed from the load on the shaft
... This dependence is called mechanical characteristics(Figure 7.6, b).

As the curve in Figure 7.6, b shows, the speed of an asynchronous motor only slightly decreases with an increase in the torque in the range from zero to the maximum value
. The starting moment corresponding to S = 1 can be obtained from (7.4), taking S= 1. Typically starting torque M start = (0.8 1,2)M nom, M nom - nominal torque. This dependence is called tough.

Rice. 7.6. Dependence of the torque on the shaft of an asynchronous motor

from sliding (a); mechanical characteristic (b)

Asynchronous motors are widely used due to the following advantages: simplicity of the device; high operational reliability; low cost.

With the help of asynchronous motors, cranes, winches, elevators, escalators, pumps, fans and other mechanisms are driven.

Asynchronous motors have the following disadvantages:

regulation of the rotor speed is difficult.

One of the most common electric motors used in most electric drive devices is the induction 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 rotation speed of the magnetic field vector.

It is necessary to explain what synchronous speed is.

Synchronous speed is the speed with 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 rotation speed of the field depends on the frequency of the flowing current and the number of poles of the machine.

An induction motor always runs at a speed lower than the speed of synchronous rotation, because the magnetic field, which is formed by the stator windings, will generate a counter magnetic flux in the rotor. The interaction of this generated counter-flux with the stator flux will cause the rotor to spin. Since the magnetic flux in the rotor will lag behind, the rotor will never be able to independently reach the synchronous speed, that is, the same with which the stator magnetic field vector rotates.

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

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

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

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

The principle of operation of an asynchronous motor

By supplying voltage only to the stator winding, the asynchronous motor starts to work. It is interesting to know how it works, why is this happening? 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 the armature (rotor) not through induction, but through brushes.

When we apply voltage to the stator windings, an electric current begins to flow in them, which creates a magnetic field around the windings. Further, from many windings that are located on the stator magnetic circuit, a common stator magnetic field is formed. This magnetic field is characterized by a magnetic flux, the magnitude of which changes over time, besides this, 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 an untwisted sling with a stone.

In full accordance with Faraday's law of electromagnetic induction, in a rotor that has a squirrel-cage winding (squirrel-cage 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 rotor of the motor becomes a magnet inside the stator, which has a rotating magnetic field. Both magnetic fields from the stator and rotor will begin to interact, obeying the laws of physics.

Since the stator is stationary 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 stator magnetic field begins to push it, dragging it along. The rotor, as it were, engages with the stator magnetic field. We can say that the rotor tends to rotate synchronously with the stator magnetic field, 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 induction motor is running, the rotor slides in the stator magnetic field.

Sliding can be either lagging or leading. If a lag occurs, then we have a motor mode of operation, when electrical energy is converted into mechanical energy, if slip occurs ahead of the rotor, then we have a generator mode of operation, when mechanical energy is converted into electrical energy.

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

Types of asynchronous motor

A single-phase asynchronous motor is divided into the following types:

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

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

Squirrel cage induction motor;
With slip rings, phase rotor (Slip ring induction motor);

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

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

In a three-phase system, each phase has an angle of 120 degrees relative to the other two. All three phases, thus, are evenly spaced around the 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 can see 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 after the first, and the third phase will follow the second. So we have an alternation order phases A-B-C as their value increases and a different order in descending order is possible 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 carried away by the stator magnetic field and slides in it, it is quite obvious that the rotor will move in the direction of the stator magnetic field vector. Which way will the stator magnetic field rotate? Since the stator winding is three-phase and all three windings are evenly located on the stator, the generated field will rotate in the direction of the phase rotation of the windings. From here we draw a conclusion. The direction of rotation of the rotor depends on the phase sequence of the stator windings. By changing the order of the alternation of the phases, we will get the rotation of the motor in the opposite direction. In practice, to change the motor rotation, it is enough to swap any two supply phases of the stator.

Why doesn't a single-phase induction motor start spinning on its own?

For the reason that it is powered by 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 the pulsating field. This problem is solved by creating a phase offset in another stator winding using capacitors, inductors and the spatial arrangement of the windings in the motor structure.

It should be noted that single-phase asynchronous motors are effective in the presence of a constant mechanical load. If the load is less and the engine is running below its maximum load, then 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.

When designing equipment, it is necessary to know the number of revolutions of the electric motor. There are special formulas for calculating the speed, which are different for AC and DC motors.

Synchronous and asynchronous electrical machines

There are three types of AC voltage motors: synchronous, the rotor angular speed of which coincides with the angular frequency of the stator magnetic field; asynchronous - in them, the rotation of the rotor lags behind the rotation of the field; collector, the design and principle of operation of which are similar to DC motors.

Synchronous speed

The rotational speed of an alternating current electric machine depends on the angular frequency of the stator magnetic field. This speed is called synchronous. In synchronous motors, the shaft rotates at the same speed, which is the advantage of these electric machines.

For this, in the rotor of high-power machines there is a winding to which constant pressure creating a magnetic field. In low-power devices, permanent magnets are inserted into the rotor, or there are pronounced poles.

Slip

In asynchronous machines, the shaft speed is less than the synchronous angular frequency. This difference is called “S” slip. Thanks to sliding, an electric current is induced in the rotor and the shaft rotates. The larger S, the higher the torque and the lower the speed. However, when the slip exceeds a certain value, the electric motor stops, begins to overheat and may fail. The rotational speed of such devices is calculated using the formula in the figure below, where:

n is the number of revolutions per minute,
f - network frequency,
p is the number of pole pairs,
s - slip.

There are two types of such devices:

Squirrel cage rotor. The winding in it is cast from aluminum during the manufacturing process;
With phase rotor. The windings are made of wire and are connected to additional resistances.

Speed control

In the process of work, it becomes necessary to adjust the number of revolutions of electrical machines. It is done in three ways:

Increase of additional resistance in the rotor circuit of electric motors with a wound rotor. If it is necessary to greatly reduce the speed, it is allowed to connect not three, but two resistances;
Connection of additional resistances in the stator circuit. It is used for starting high-power electric cars and for adjusting the speed of small electric motors. For example, the speed of a desktop fan can be reduced by connecting an incandescent lamp or capacitor in series with it. The same result is obtained by reducing the supply voltage;
Changing the network frequency. Suitable for synchronous and asynchronous motors.

Attention! The speed of rotation of collector electric motors powered by an alternating current network does not depend on the frequency of the network.

DC motors

In addition to AC voltage machines, there are electric motors connected to the DC network. The number of revolutions of such devices is calculated using completely different formulas.

Rated speed of rotation

The number of revolutions of the DC apparatus is calculated using the formula in the figure below, where:

n is the number of revolutions per minute,
U - mains voltage,
Rя and Iя - resistance and armature current,
Ce - motor constant (depends on the type of electric machine),
Ф - stator magnetic field.

These data correspond to the nominal values of the parameters of the electric machine, the voltage on the field winding and the armature or the torque on the motor shaft. Changing them allows you to adjust the speed. It is very difficult to determine the magnetic flux in a real motor, therefore, for calculations, they use the strength of the current flowing through the excitation winding or the voltage at the armature.

The number of revolutions of AC brushed motors can be found using the same formula.

Speed regulation

Adjustment of the speed of an electric motor powered by a DC network is possible within a wide range. It is available in two ranges:

Up from the nominal. For this, the magnetic flux is reduced using additional resistances or a voltage regulator;
Down from par. To do this, it is necessary to reduce the voltage at the armature of the electric motor or include a resistance in series with it. In addition to reducing the number of revolutions, this is done when starting the electric motor.

Knowing which formulas are used to calculate the speed of rotation of an electric motor is necessary when designing and setting up equipment.

Video

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 generates a pulsating magnetic field, i.e. a magnetic field that varies in magnitude and direction.

Let's take a cylinder with an inner diameter D. On the surface of the cylinder we 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. 12.2 shows a graph of the change 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 the resulting rotating magnetic field, characterized by the vector of the resulting magnetic induction
In fig. 12.3 shows the vectors of the magnetic induction of each phase and the resulting vector constructed for three times t1, t2, t3. The positive directions of the coil axes are indicated by +1, +2, +3.

At the moment t = t 1, the current and magnetic induction in the A-X coil are positive and maximum, in the B-Y and C-Z coils they are the same and negative. The vector of the resulting magnetic induction is equal to the geometric sum of the vectors of the magnetic inductions 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 the same in magnitude and opposite in direction. Phase B current 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 the same 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 of the C-Z coil. During the alternating current period, the vector of the resulting magnetic field will rotate 360 o.

Magnetic field rotation frequency or synchronous rotation frequency

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 frequency of rotation of the field 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 two times slower, 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 induction motor has motionless the part called stator , and rotating part called rotor ... The stator contains a winding that creates a rotating magnetic field.
Distinguish between asynchronous motors with a squirrel cage and a phase rotor.
In the grooves of the rotor with a short-circuited winding, there are aluminum or copper rods. At the ends, the rods are closed by aluminum or copper rings. The stator and rotor are assembled from 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 to a common unit, and the beginnings are brought out to three slip rings placed on the shaft. Stationary contact brushes are applied to 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 consider the principle of operation of an induction motor on the model shown in Figure 12.4.

We represent the rotating magnetic field of the stator in the form of a permanent magnet rotating with a synchronous rotation frequency n 1.
Currents are induced in the conductors of the closed rotor winding. The poles of the magnet move clockwise.
To an observer sitting on a rotating magnet, it seems that the magnet is stationary, and the conductors of the rotor winding move counterclockwise.
The directions of the rotor currents, determined according to the rule of the right hand, 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 making it rotate. The rotor of the motor will rotate at a 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 difference between the stator and rotor field speeds 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 moment is balanced by the opposing braking moment M em = M 2.
With an increase in the load on the motor shaft, the braking torque becomes greater than the torque, and the 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 to a certain maximum value, after which, as the braking torque increases further, the torque decreases sharply and the motor stops.
The slip of the decelerated motor is equal to one. The motor is said to operate in short circuit mode.
The rotational speed of an unloaded induction motor n 2 is approximately equal to the synchronous frequency n 1. Unloaded engine slip S 0. The engine is said to be idling.
The slip of an induction 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. Asynchronous generator slip.
The asynchronous machine can operate in the electric machine brake mode. To do this, it is necessary to rotate its rotor in the direction opposite to the direction of rotation of the stator magnetic field.
In this mode S> 1. As a rule, asynchronous machines are used in motor mode. The induction motor is the most common type of motor in the industry. The frequency of rotation of the field in an asynchronous motor is rigidly connected with the frequency of the network f 1 and the number of pairs of stator poles. At a frequency of f 1 = 50 Hz, there is the following series of rotational speeds.