Mechanical characteristic of an asynchronous motor formula. The equation of the mechanical characteristic of an induction motor

The mechanical characteristics of induction motors can be expressed as n=f(M) or n=f(I). However, often the mechanical characteristics of induction motors are expressed as a dependence M = f (S), where S is the slip, S = (nc-n) / nc, where n s is the synchronous speed.

In practice, for the graphical construction of a mechanical characteristic, a simplified formula is used, called the Kloss formula:

here: Mk is the critical (maximum) value of the moment. This value of the moment corresponds to the critical slip

Where λm = Mk/Mn

The Kloss formula is used in solving issues related to the electric drive, carried out using an asynchronous motor. Using the Kloss formula, you can build a graph of the mechanical characteristic according to the passport data of an induction motor. For practical calculations in the formula, when determining the critical moment in front of the root, only the plus sign should be taken into account.

Rice. 1. Asynchronous motor: a - schematic diagram, b - mechanical characteristic M \u003d f (S) - natural in motor and generator modes, c - natural mechanical characteristic n \u003d f (M) in motor mode, d - artificial rheostatic mechanical characteristics, e - mechanical characteristics for various voltages and frequencies.

As can be seen from fig. 1, mechanical characteristic of an induction motor located in the I and III quadrants. Part of the curve in the I quadrant corresponds to a positive slip value and characterizes the motor mode of operation of the induction motor, and in the III quadrant - the generator mode. The motor mode is of the greatest practical interest.

The graph of the mechanical characteristics of the motor mode contains three characteristic points: A, B, C and can be conditionally divided into two sections: OB and BC (Fig. 1, c).

Point A corresponds rated motor torque and is determined by the formula Mn = 9.55 10 3 (P n/n n)

This moment corresponds to , which for engines of general industrial use has a value in the range from 1 to 7%, i.e. Sн=1 - 7%. At the same time, small engines have more slip, and large ones have less.

High slip motors, designed to work with shock loading, have S n ~ 15%. These include, for example, engines of a single AC series.

Point C on the characteristic corresponds to the value starting torque arising on the motor shaft during start-up. This moment Mn is called the initial, or starting. The slip in this case is equal to one, and the speed is equal to zero. it is easy to determine according to the reference table, which indicates the ratio of the starting torque to the nominal Mp / Mn.

The value of the starting torque at constant voltage and current frequency depends on the active resistance in the rotor circuit. In this case, at first, with an increase in active resistance, the starting torque increases, reaching its maximum when the active resistance of the rotor circuit is equal to the total inductive resistance of the motor. In the future, with an increase in the active resistance of the rotor, the value of the starting torque decreases, tending to zero in the limit.

Point B (Fig. 1, b and c) corresponds to maximum torque, which can develop the engine over the entire speed range from n = 0 to n = n s. This moment is called the critical (or overturning) moment Mk. critical moment corresponds to the critical slip Sc. The smaller the value of the critical slip Sk, as well as the value of the nominal slip S n, the greater the rigidity of the mechanical characteristic.

Both starting and critical moments are determined through the nominal. According to GOST for electrical machines for a squirrel-cage motor, the condition Mp / Mn \u003d 0.9 - 1.2, Mk / Mn \u003d 1.65 - 2.5 must be observed.

It should be borne in mind that the value of the critical moment does not depend on the active resistance of the rotor circuit, while the critical slip S k is directly proportional to this resistance. This means that with an increase in the active resistance of the rotor circuit, the value of the critical moment remains unchanged, however, the maximum of the torque curve shifts towards increasing slip values ​​(Fig. 1, d).

The magnitude of the critical moment is directly proportional to the square of the voltage supplied to the stator, and inversely proportional to the square of the voltage frequency and current frequency in the stator.

If, for example, the voltage supplied to the motor is equal to 85% of the nominal value, then the value of the critical moment will be 0.85 2 \u003d 0.7225 \u003d 72.25% of the critical moment at rated voltage.

The opposite phenomenon is observed when the frequency changes. If, for example, a motor designed to operate with a current frequency f = 60 Hz is supplied with a current with a frequency f = 50 Hz, then the critical moment will receive a value (60/50) 2 = 1.44 times greater than at its formal frequency (Fig. 1, e).

The critical moment characterizes the instantaneous overload capacity of the engine, i.e. it shows what instantaneous (for a few seconds) overload the engine is able to transfer without any harmful consequences.

The section of the mechanical characteristic from zero to the maximum (critical) value (see Fig. 1, biv) is called stable part of the characteristic, and the BC section (Fig. 1, c) - unstable part.

This division is explained by the fact that on the increasing part of the OF characteristic with increasing slip, i.e. as the speed decreases, the torque developed by the engine increases. This means that with an increase in load, i.e. with an increase in braking torque, the engine speed decreases, and the torque developed by it increases. When the load is reduced, on the contrary, the speed increases, and the torque decreases. When the load changes over the entire range of the stable part of the characteristic, the rotation speed and torque of the engine change.

The engine is not able to develop a moment greater than the critical one, and if the braking torque is greater, the engine must inevitably stop. It happens, as they say, engine rollover.

The mechanical characteristic at constant U and I and the absence of additional resistance in the rotor circuit is called natural characteristic(characteristic of a squirrel-cage asynchronous motor with a phase rotor without additional resistance in the rotor circuit). Artificial, or rheostatic, characteristics are called those that correspond to the additional resistance in the rotor circuit.

All values ​​of starting torques are different and depend on the active resistance of the rotor circuit. The same nominal moment Mn corresponds to slips of various sizes. With an increase in the resistance of the rotor circuit, slip increases and, consequently, the engine speed decreases.

Due to the inclusion of active resistance in the rotor circuit, the mechanical characteristic in the stable part is extended in the direction of increasing slip, in proportion to the resistance. This means that the motor speed begins to change strongly depending on the load on the shaft and the characteristic becomes soft from hard.

38) Mechanical characteristic of an asynchronous motor.

Mechanical characteristic. The dependence of the rotor speed on the load (torque on the shaft) is called the mechanical characteristic of an induction motor (Fig. 262, a). At rated load, the speed for various engines is usually 98-92.5% of the speed n 1 (slip s nom = 2 - 7.5%). The greater the load, i.e. the torque that the engine must develop, the lower the rotor speed. As the curve shows

Rice. 262. Mechanical characteristics of an induction motor: a - natural; b - when the starting rheostat is turned on

in fig. 262, a, the rotational speed of an asynchronous motor only slightly decreases with increasing load in the range from zero to its highest value. Therefore, such an engine is said to have a rigid mechanical characteristic.

The engine develops the greatest torque M max at some slip s kp of 10-20%. The ratio M max / M nom determines the overload capacity of the engine, and the ratio M p / M nom determines its starting properties.

The engine can operate stably only if self-regulation is ensured, i.e., automatic equilibrium is established between the load moment M ext applied to the shaft and the moment M developed by the engine. This condition corresponds to the upper part of the characteristic until M max is reached (up to point B). If the load moment M ext exceeds the moment M max, then the motor loses stability and stops, while the current 5-7 times the nominal current will pass through the windings of the machine, and they may burn out.

When a starting rheostat is included in the rotor winding circuit, we obtain a family of mechanical characteristics (Fig. 262, b). Characteristic 1 when the engine is running without a starting rheostat is called natural. Characteristics 2, 3 and 4, obtained by connecting a rheostat with resistances R 1p (curve 2), R 2p (curve 3) and R 3p (curve 4) to the motor rotor winding, are called rheostatic mechanical characteristics. When the starting rheostat is turned on, the mechanical characteristic becomes softer (more steeply falling), as the active resistance of the rotor circuit R 2 increases and s kp increases. This reduces the starting current. Starting torque M p also depends on R 2 . You can choose the resistance of the rheostat in such a way that the starting torque M p is equal to the largest M max.

In an engine with increased starting torque, the natural mechanical characteristic approaches in its form the characteristic of an engine with the starting rheostat turned on. The torque of a double squirrel cage motor is equal to the sum of the two torques generated by the working and starting cages. Therefore, characteristic 1 (Fig. 263) can be obtained by summing characteristics 2 and 3 created by these cells. The starting torque M p of such a motor is much greater than the moment M ' p of a conventional squirrel-cage motor. The mechanical performance of the deep slot motor is the same as that of the double squirrel cage motor.

JUST A WORKING CHARACTERISTIC!!!

Operating characteristics. The performance characteristics of an induction motor are the dependences of the rotational speed n (or slip s), torque on the shaft M 2, stator current I 1 efficiency? and cos? 1, from useful power P 2 \u003d P mx at nominal values ​​of voltage U 1 and frequency f 1 (Fig. 264). They are built only for the zone of practical stable operation of the engine, i.e., from slip equal to zero to slip exceeding the nominal by 10-20%. The rotational speed n with an increase in the output power P 2 changes little, as well as in the mechanical characteristic; the torque on the shaft M 2 is proportional to the power P 2 , it is less than the electromagnetic torque M by the value of the braking torque M tr created by friction forces.

The stator current I 1 increases with increasing power output, but at P 2 \u003d 0 there is some no-load current I 0. The efficiency varies approximately in the same way as in a transformer, maintaining a fairly large value over a relatively wide load range.

The highest efficiency value for asynchronous motors of medium and high power is 0.75-0.95 (high power machines have a correspondingly higher efficiency). power factor cos? 1 asynchronous motors of medium and high power at full load is 0.7-0.9. Consequently, they load power stations and networks with significant reactive currents (from 70 to 40% of the rated current), which is a significant drawback of these motors.

Rice. 263. Mechanical characteristic of an asynchronous motor with increased starting torque (with a double squirrel cage)

Rice. 264. Performance characteristics of an induction motor

At loads of 25-50% of the nominal, which are often encountered during the operation of various mechanisms, the power factor decreases to unsatisfactory values ​​from an energy point of view (0.5-0.75).

When the load is removed from the engine, the power factor decreases to values ​​of 0.25-0.3, therefore it is impossible to allow the operation of asynchronous motors at idle and significant underloads.

Work at low voltage and breakage of one of the phases. Reducing the mains voltage does not have a significant effect on the rotor speed of the induction motor. However, in this case, the maximum torque that an asynchronous motor can develop is greatly reduced (when the voltage drops by 30%, it decreases by about 2 times). Therefore, with a significant voltage drop, the motor may stop, and with a low voltage, it may not start.

On e. p.s. alternating current, when the voltage in the contact network decreases, the voltage in the three-phase network also decreases, from which asynchronous motors are powered, which drive auxiliary machines (fans, compressors, pumps). In order to ensure the normal operation of asynchronous motors at reduced voltage (they should work normally when the voltage drops to 0.75U nom), the power of all motors of auxiliary machines is e. p.s. is taken approximately 1.5-1.6 times greater than is necessary to drive them at rated voltage. Such a power margin is also necessary due to some asymmetry of the phase voltages, since at e. p.s. asynchronous motors are not powered by a three-phase generator, but by a phase splitter. With voltage asymmetry, the phase currents of the motor will not be the same and the phase shift between them will not be equal to 120 °. As a result, a larger current will flow through one of the phases, causing increased heating of the windings of this phase. This forces to limit the load of the motor in comparison with its operation at a symmetrical voltage. In addition, with voltage asymmetry, not a circular, but an elliptical rotating magnetic field arises, and the shape of the mechanical characteristic of the engine changes somewhat. At the same time, its maximum and starting moments are reduced. The voltage asymmetry is characterized by the asymmetry coefficient, which is equal to the average relative (in percent) deviation of the voltages in individual phases from the average (symmetrical) voltage. A system of three-phase voltages is considered to be practically symmetrical if this coefficient is less than 5%.

If one of the phases is broken, the motor continues to operate, but increased currents will flow through the undamaged phases, causing increased heating of the windings; such a regime should not be allowed. Starting a motor with an open phase is not possible, since this does not create a rotating magnetic field, as a result of which the motor rotor will not rotate.

The use of asynchronous motors to drive auxiliary machines e. p.s. provides significant advantages over DC motors. With a decrease in voltage in the contact network, the rotational speed of asynchronous motors, and hence the supply of compressors, fans, and pumps, practically does not change. In DC motors, the rotational speed is proportional to the supply voltage, so the supply of these machines is significantly reduced.

Federal Agency for Education

State educational institution of higher professional education

Petrozavodsk State University

Kola branch

Department of High-Voltage Power Engineering and Electrical Engineering

Discipline “_Electromechanics_”

Device asynchronous machine.

Test

student __2___ course

(group AVEE - /06/3.5)

correspondence department

Faculty of Physics and Energy

speciality: 140201 - "High-voltage power industry and electrical engineering"

Vakhovsky Vladimir Alexandrovich

teacher -

prof., doc. tech. Sciences A.I. Rakaev

Apatity

Mechanical characteristics of an induction motor (IM).

1. Introduction.

2. Asynchronous machines.

3. The equation of the mechanical characteristic of an asynchronous motor.

4. Linearization of the mechanical characteristic of an asynchronous motor.

5. Mechanical characteristics of asynchronous motors in symmetrical modes

8. Device asynchronous machine.

9. Operating principleasynchronous machines.

10. Bibliography

Mechanical characteristics of an induction motor (IM).

1. Introduction.

AC electric drives are widely used in industry, transport, construction industry and other sectors of the economy. Their predominant distribution is due to: high reliability of the AC machine due to the absence of a collector, ease of control of unregulated drives, since most of them are directly connected to the network, low cost of electric machines and simple requirements for their maintenance and operating rules.

Depending on the type of motor used, not only AC and DC drives are distinguished, but also asynchronous, synchronous, stepping and other types of drives. However, one should not think that AC drives can be used everywhere instead of DC drives. For each type of drive, there are established areas of promising use. Moreover, it is difficult to unambiguously and definitely list in advance all the factors that determine the choice of the type of current for the drive. Along with traditional drives built on the basis of asynchronous and synchronous machines, in recent decades, AC drives with universal and step motors, dual-feed motors and electromagnetic speed reduction have been used.

2. Asynchronous machines.

The principle of operation of an asynchronous machine in its most general form is as follows: one of the elements of the machine, the stator, is used to create a magnetic field moving at a certain speed, and EMFs are induced in closed conductive passive circuits of another rotor element, causing the flow of currents and the formation of forces (torques) when interacting with a magnetic field. All these phenomena take place during non-synchronous-asynchronous movement of the rotor relative to the field, which gave the name to machines of this type - asynchronous.

The stator is usually made in the form of several coils located in the grooves, and the rotor is in the form of a "squirrel cage" (squirrel-cage rotor) or in the form of several coils (phase rotor), which are interconnected, brought to rings located on the shaft, and with the help of sliding through them, the brushes can be closed to external resistors or other circuits.

Despite the simplicity of physical phenomena and the constructs that materialize them, a complete mathematical description of the processes in an asynchronous machine is very difficult:

firstly, all voltages, currents, flux links are variables, i.e. characterized by frequency, amplitude, phase or corresponding vector quantities;

secondly, moving contours interact, the relative position of which changes in space;

thirdly, the magnetic flux is nonlinearly related to the magnetizing current (saturation of the magnetic circuit appears), the active resistances of the rotor circuit depend on the frequency (current displacement effect), the resistances of all circuits depend on temperature, etc.

Consider the simplest model of an asynchronous machine, suitable for explaining the main phenomena in an asynchronous electric drive.

The mechanical characteristics of the engine completely determine the quality of the electromechanical system in steady state and its performance. They also affect the dynamic modes of the electric drive, characterizing the excessive dynamic torque that determines the acceleration or deceleration of the motor.

3. The equation of the mechanical characteristic of an induction motor

In modern design practice, programs are used that take into account the magnetization of the magnetic system of the machine when calculating the mechanical characteristics. But at the same time, clarity in their study is lost. Therefore, all further dependencies will be found under this basic assumption.

The electrical power supplied to the motor from the network is used to cover losses in the magnetization circuit p μ , in copper stator p M 1 , and the rest of it is converted into electromagnetic power. Thus,

(4-12)

In its turn,

where ω 0 = 2π f 1 /p- the number of pairs of stator poles of the machine.

After minor transformations, we find

(4-14)

Therefore, the dependency M = f(s) is a complex function of slip. We examine it for an extremum by taking the derivative

(4-15)

Equating the numerator of expression (4-15) to zero, we find the value of the critical slip s K , at which the dependence M =f(s) has a maximum:

(4-16)

Physical reduction M at s s K And s > s K is explained as follows. At s s K slip reduction is associated with a decrease in motor current and torque, and at s > s K , although there is an increase in the motor current, but its active component, which determines the electromagnetic torque, does not increase, but decreases, which also leads to a decrease in the torque developed by the motor.

positive sign s K corresponds to the motor, and negative - to the generator mode of operation of the machine.

It should be borne in mind that, like a DC machine, the relative value r 1 decreases with an increase in the power of machines and already for engines with a power of 100 kW is 10-15% of the value x 1 + x 2 ". Therefore, formula (4-16) can be used in a simplified form, neglecting r 1

Where x K.Z - inductive reduced short circuit resistance.

This cannot be done for machines of medium and especially low power, in which the resistance r 1 commensurate with x K.Z.

Using formulas (4-14) and (4-16), you can get a different record of the mechanical characteristic of an induction motor if you find the values ​​of its critical moments in motor M K.D and generator M K.G operating modes:

(4-18)

Critical moment ratio

(4-19)

Here is the commonly used notation:

(4-20)

Formula (4-19) shows that the value of the critical moment of the machine in the generator mode can be significantly greater than in the motor mode (see Fig. 4-8).

For practical use, it is more convenient than in the formula (4-14), the expression of the mechanical characteristic of an induction motor. Let's find it using formulas (4-14), (4-17) and (4-20):

(4-21)

If we neglect the influence of the active resistance of the stator, then ε = 0, and formula (4-21) takes on the following form (when M K.D = M K.G = M TO):

(4-22)

For the first time, the expression (4-22) was obtained by M. Kloss, therefore it is called the Kloss formula.

Formulas (4-21) or (4-22) are more convenient for calculations than (4-14), since they do not require knowledge of the motor parameters. In this case, all calculations are made according to the catalog data. Since the value s K not specified in the catalogs, it has to be determined on the basis of other information, for example, the magnitude of the overload capacity of the machine M TO / M NOM = λ M. Then from the formula (4-21) we get:

(4-23)

whence, solving the quadratic equation, we find

where γ = λ M + (1 - λ M)ε.

In the expression (4-24), a plus sign should be taken before the root, since another value s K contradicts the physical meaning.

An approximate solution of equation (4-24) can be obtained with the coefficient ε = 0, but it is better to determine its value. The most reliable results will be obtained if, having the parameters of the machine, the value of ε is determined from the formula (4-20), a s K - from expression (4-16). For asynchronous motors with a phase rotor, expressions (4-14) and (4-21) give more reliable results, since in these machines the effects of steel saturation and current displacement in the rotor windings (skin effect) are less noticeable.

4. Linearization of the mechanical characteristic of an induction motor

On the working section of the mechanical characteristic, the slip value s much less critical s K. Therefore, in equation (4-21), we neglect the term ss K -1 and set ε = 0. Then we get

(4-25)

Thus, expression (4-25) is the linearized part of the mechanical characteristic of the engine. It can be used for slip variations within 0 s s NOM.

Rice. 4-5. Linearized mechanical characteristics of induction motors

To obtain artificial characteristics, it suffices to write down two equations of lines for the same slip values s i (Fig. 4-5):

where the indices "i" and "e" mark artificial and natural characteristics, from where it is easy to find

(4-26)

According to the formula (4-26), it is possible to construct the initial sections of any mechanical characteristic. In this case, the slip should not go beyond the specified limits.

If the total resistance is introduced into the rotor circuit R 2 NOM, then at s= 1 the current corresponding to the rated torque will flow in the rotor M NOM . Then the expression (4-26) will take the form

The last expression allows us to write the following relation for any artificial or natural characteristic:

where ρ P is the relative value of the impedance included in the rotor circuit of the machine ρ P = ρ 2 + ρ DOB; s - sliding on the corresponding mechanical characteristic.

It should be borne in mind that when R 2 = R 2 NOM nominal value of slip s H NOM =1 on this artificial characteristic .

5 Mechanical characteristics of asynchronous motors in symmetrical modes

Motor characteristics when the mains voltage or resistance in the stator circuit changes .

Symmetrical modes of operation of asynchronous motors (AM) are called, in which the supply network is symmetrical in value and phase shift of voltages, the active or reactive resistances introduced into the electrical circuits of all phases are the same and their internal parameters are symmetrical (the number of turns in the phases, angular shifts of the grooves and other factors).

First of all, consider the changes in the network. From relation (4-9) it follows that the current I 2 "is proportional to the applied voltage, and the moment is [see expression (4-14)] its square. This allows you to build the mechanical characteristics of the engine at any voltage (Fig. 4-6). Obviously, formula (4-16) confirms the constancy of the critical slip s K. Even when the voltage drops to 0.7 U NOM critical moment is

Rice. 4-6. Mechanical characteristics of an asynchronous motor at various supply voltages.

only 49% M K nominal mode. In practice, the voltage drop is even greater when starting the motor due to the large starting current. All this leads to the fact that with long power lines or for large machines with their capacities commensurate with the power of transformer substations, it is necessary to perform special calculations confirming the possibility of a normal start-up of IM and its operation with reduced voltage.

For the same reasons, a special GOST 13109-87 has been established for the quality of electrical energy, which provides for a post-accident change in voltage in an industrial network only within ± 10% of its nominal value.

Voltage reduction is especially dangerous for drives that, according to operating conditions, must be started under load (drives of conveyors, lifting devices, converters and many other mechanisms). For example, when starting without load (idle), the static moment of the conveyor does not exceed (0.2-0.3) M NOM. However, if the conveyor drive was disabled during full load operation, it will have to overcome the M C ≈ M NOM .

To limit the starting currents of large asynchronous machines or to obtain a soft start of an asynchronous drive, active or inductive resistances are used in the stator circuit, which are output at the end of the start (Fig. 4-7). A feature of such circuits is the dependence of the voltage at the motor terminals on the magnitude of the current.

The inclusion of active resistance, although somewhat increases the power factor of the drive in starting modes, but at the same time increases energy losses, compared with the "reactor" start.

Rice. 4-7. Mechanical characteristics of an asynchronous motor at rated and reduced voltage or active ( r DOB) and reactive ( x DOB) additional resistances in the stator.

In recent decades, for frequently switched on and off high power motors, "frequency" start has been used, which is more economical. For this purpose, a special converter is installed, which smoothly changes the frequency of the motor power supply at start-up, i.e., the value of ω 0. At the same time, the voltage decreases, which also limits the starting current.

Characteristics of an asynchronous motor when active resistances are included in the rotor circuit.

Asynchronous motors with a phase rotor are widely used in drives of hoisting and transport and metallurgical installations, powerful motors are used in drives of fans, wind tunnels and pumps. Due to the inclusion of active resistances in the rotor circuit, it is possible to change the critical slip of such an induction motor, the type of its mechanical characteristic, starting current and torque.

The use of phase rotor motors in pump and fan drives makes it possible to economically adjust their performance, which brings a great economic effect. Recall that the critical moment does not depend on the active resistance introduced into the rotor circuit, therefore, by choosing r DOB it is possible to change the mechanical characteristics of the AM in such a way that the drive will have the maximum torque at start-up (ω = 0), or even in the opposition mode s K > 1 (Fig. 4-8).

Increase r DOB leads to an increase in the active component of the rotor current I 2a "= I 2 "cosψ 2, since

(4-30)

Where R 2 " = r 2 " + r" DOB - total reduced active resistance of the secondary circuit of the machine.

For the same reason, motors with a phase rotor, unlike squirrel-cage motors, have large starting torques at lower currents. This property of such machines is the main condition for their predominant use in drives with heavy start-up modes (cranes, metallurgical plants, rotary machines and other energy-intensive mechanisms). It should be borne in mind that an excessive increase r DOB leads to a sharp decrease in the active component of the current I 2 ". Then the starting torque of the engine M P becomes less than the static moment when starting off M TR . As a result, the drive will not be able to start.

The artificial mechanical characteristic can be calculated using the formula (4-14) or (4-18), (4-20), (4-24) and (4-27). The method for calculating the artificial characteristics of IM with a phase rotor can be simplified based on the following relationships. Let's write expressions for equal values ​​of the moments M i on a natural and any artificial characteristic based on the formula (4-21):

The value of ε does not depend on the value of the active component of the resistance in the secondary circuit of the machine, so it remains unchanged for natural and artificial mechanical characteristics. Therefore, from formula (4-31) we have

The given values ​​can be considered: critical slips on artificial and natural characteristics s K .I And s K .E and sliding on natural characteristic s ei. Then from the expression (4-32) we get

(4-33)

Thus, the basis of the simplified calculation is the natural mechanical characteristic of the engine. As mentioned earlier for machines with a phase rotor, it can be obtained approximately from the expression (4-22) and more accurately from (4-21). Some of the machine parameters required for these calculations are indicated in catalogs or reference books, and some can be determined by the above formulas.

Rice. 4-8. Mechanical characteristics of a wound rotor motor

6. Braking modes of asynchronous motors

Braking modes for many drives with asynchronous machines are more important than starting modes in relation to the requirements for reliability and reliability in implementation. It is often required to stop exactly at a given position or brake the drive for a certain time.

For asynchronous motors, the following modes are used: regenerative braking with energy output to the network; oppositions; dynamic braking with various stator excitation systems with direct (rectified) current, when the machine works as a generator, dissipating energy in the secondary circuit; dynamic capacitor or magnetic braking with self-excitation. Therefore, braking modes can be divided into two groups according to the method of excitation of the stator magnetic field: independent excitation, carried out from an alternating or direct current network (regenerative, opposition and dynamic braking) and self-excitation, carried out as a result of energy exchange with a capacitor battery or when the motor stator is short-circuited when the magnetic flux is created by the EMF of self-induction. By definition L.P. Petrov, the latter type will be called magnetic braking.

All of these modes are used for machines with both phase and squirrel-cage rotor.

In connection with the use of powerful power semiconductor devices (thyristors and transistors), new schemes for implementing typical braking modes of asynchronous drives have appeared.

An increase in braking efficiency can be achieved by using combined methods of its implementation. It should be emphasized that most combined braking is fully controlled. This further enhances their effectiveness.

The most effective are opposition and capacitor-dynamic braking (CDT). The last method has many circuit solutions. It is recommended to use it for drives with large reduced moments of inertia, for example, exceeding twice the moment of inertia of the motor.

For low-inertia drives, capacitor-magnetic braking (CMB) can be used. Magnetic dynamic braking (MDB) will be no less effective. Rational for individual drives and other combined types of two and even three-stage braking: opposition - dynamic braking (PDT), capacitor braking and opposition (CTP), etc.

Thus, the implementation of modern methods of IM braking largely depends on the experience and knowledge of the electric drive developer. Therefore, let us consider in detail the modes of braking.

Braking with energy return to the network. The reversibility of an induction motor, like other machines using the principle of electromagnetic induction (Maxwellian type), allows it to operate in a generator mode. If there is no load on the motor shaft, then the energy consumed from the network is spent to cover losses in the stator, as well as losses in steel and mechanical losses in the rotor. By applying an external torque to the machine shaft in the direction of rotation of the rotor, synchronous speed can be achieved. In this case, the losses in the rotor are already covered by an external energy source, and only the energy going to cover the losses in the stator will be consumed from the network. A further increase in speed above the synchronous one leads to the fact that the asynchronous machine goes into generator mode.

When operating in this mode, the stator conductors are crossed by the magnetic field in the same direction, and the rotor conductors are crossed in the opposite direction, therefore the rotor EMF E 2 changes sign, i.e. E 2 "s = (- s)E 2 " ≈ - E 2 "s. The current in the rotor, respectively, will be equal to

(4-34)

Rice. 4-13. Vector diagram of an induction motor operating in generator mode

It can be seen from expression (4-34) that during the transition of the AM to the generator mode, only the active component of the rotor current changes direction, since the torque on the shaft has changed its direction compared to that which took place in the motor mode. This is illustrated by the vector diagram in Fig. 4-13. Here the angle φ 1 > π / 2, which confirms the change in the cause of the current I 1 in the form of EMF E 1 (not mains voltage U 1 , as in motoring mode), although the direction of the magnetizing current I μ remained the same. Reversal of the sign of the active component of the current I" 2a leads to the fact that the electromagnetic power becomes negative, i.e., it is given to the network, since s 0:

The sign of the reactive power of the secondary circuit remains unchanged regardless of the operating mode of the machine, which follows from the expression

Due to the presence of active static moments, braking is used in lifting installations (Fig. 4-14, a), in transport drives (Fig. 4-14, b). The difference in these braking modes lies in the fact that in the first case (Fig. 4-14, a), the engine, when lowering a large load, switches to its descent (ω 3 in the fourth quadrant for |ω| > |ω 0 |). Load Moment Limit M WITH should not exceed M NOM. When the vehicle moves "downhill", the potential energy of the transported load begins to contribute to the movement and creates an external driving moment applied to the motor shaft. Thus, in this case, due to an increase in the drive speed (ω > ω 0) and a change in the sign of the EMF E 2 , the motor directly, without switching the stator windings, goes into generator mode with energy output to the network (point 2 in fig. 4-14b).

Rice. 4-14. Mechanical characteristics of an asynchronous motor with an active static moment: a - descent of a heavy load; b - operation of the vehicle "downhill"

In the presence of a reactive static torque, regenerative braking with energy recovery to the network can be obtained in asynchronous motors with switching the number of poles or in drives with frequency, frequency-current and vector control of the speed of rotation of the IM.

In the first case (Fig. 4-15, a), switching the stator of the machine from a smaller number of poles to a larger one, the synchronous speed ω 02 decreases

With frequency regulation of speed, reducing the frequency of the stator power supply from the main f 1 to f 2 f 1 and f 3 f 2 , gradually switch the engine from one mechanical characteristic to another (Fig. 4-15, b). The drive operates in braking mode with energy output to the network while its operating point moves along the sections of mechanical characteristics located in the second quadrant. By smoothly and automatically changing the frequency of the motor supply, it is possible to obtain a braking mode of the drive with a little-changing braking torque. However, in this case, the supply voltage must also be regulated in a certain way.

Rice. 4-15. Mechanical characteristics of an asynchronous motor in the regenerative braking mode with a reactive static moment: a - switching the number of pole pairs; b - frequency regulation of speed

Reverse braking. This type of braking occurs when the motor rotor rotates under the action of a static moment in the direction opposite to the rotation of the stator field. In the presence of a reactive torque, the duration of braking is short, after which the machine again switches from the braking mode to the motor mode, (Fig. 4-16, a). Initially, the engine was running at the point 1 motor mode, and then after switching the two phases of the stator winding, the direction of rotation of the magnetic field of the machine and its electromagnetic moment (point 2 ). Drive movement decelerates to the point ABOUT, and then the rotor is reversed and the engine is accelerated in the opposite direction to a steady motion at the point 3 .

For motors with a phase rotor, in the presence of a large additional resistance, a complete stop of the drive with a braking torque is possible M TR (dot 5 in fig. 4-16a).

In the presence of an active torque (Fig. 4-16, b), if the direction of rotation of the magnetic field changes, as in the previous case, the motor also changes the mode of operation, i.e. braking by counter-switching takes place - the second quadrant, motor mode with reverse direction of rotation rotor - the third quadrant and a new mode - generator with energy output to the network - the fourth quadrant, where the point of steady long-term movement lies 3 .

For motors with a phase rotor with an active static torque, the opposition mode can be obtained without switching the stator phases, only by introducing large additional resistances into the rotor (Fig. 4-16, b). Then the machine is in motor mode from the point 1 translates to point 4 with the introduction of additional resistance r D, and then it changes its movement along an artificial mechanical characteristic, passing into the fourth quadrant. Dot 5 corresponds to a long-term steady-state movement of an asynchronous motor in the opposition mode.

Rice. 4-16. The switching circuit and mechanical characteristics of an asynchronous motor: a - in the opposition mode with a reactive static moment; b - the same, with an active static moment

The reverse current braking mode is often used in lifting and transport installations. Switching the stator phases without introducing additional resistance is used only in asynchronous motors with a squirrel-cage rotor due to the fact that the initial values ​​of the currents at the point 2 (fig. 4-16) slightly more than the starting one, which is (5-6) I NOM. For motors with a phase rotor, such current peaks are generally unacceptable. The disadvantage of the braking characteristics of the opposition is their high steepness and significant energy losses, which are completely converted into heat dissipated in the secondary circuit of the engine. Due to the steepness of the mechanical characteristics, large fluctuations in the drive speed are possible with small load changes.

If the moment is known M C, at which it is necessary to carry out braking, it is not difficult to calculate the slip value at this point using the formula (4-25), and then using the formula (4-29) to determine the additional resistance.

Electrodynamic (dynamic) braking. If the IM stator is disconnected from the network, then the magnetic flux of residual magnetization forms an insignificant EMF and current in rotor.

With independent excitation, a stationary stator flux is obtained, which induces EMF and current in the windings of the rotating rotor.

Rice. 4-17. Schemes for connecting the stator windings of an asynchronous motor to a DC (rectified) voltage network

To connect the stator windings to the DC (rectified) current network, various schemes for their connection are used, some of which are shown in Fig. 4-17.

To analyze the dynamic braking mode, it is more convenient to replace the MDS F P, created by direct current, variable equivalent MDS F~ , formed jointly by the stator and rotor windings, as in a conventional asynchronous motor. Then the synchronous generator mode is replaced by the equivalent mode of the asynchronous machine. With such a replacement, the equality must be observed: F P = F ~ .

Rice. 4-18. Connection diagrams of the beginning (H) and end (K) of the stator windings "in a star" (a), determination of the directions of the MMF of the stator windings (b), geometric addition of the MMF (c)

The interaction of small amounts of magnetic flux and current in the rotor is not capable of creating a large electromagnetic moment. Therefore, it is necessary to find ways to significantly increase the magnetic flux. This can be done by connecting the stator of the machine in dynamic braking mode to a DC or rectified voltage source. You can also create a motor self-excitation circuit by connecting capacitors to its stator winding. As a result, we obtain the modes of dynamic braking of an asynchronous machine with independent excitation and self-excitation

Determining the DC MMF for the circuit in fig. 4-17,a explains Fig. 4-18.

With a three-phase connection of the stator winding to the AC network, it is necessary to determine the maximum MMF of the machine, equal to:

(4-36)

Where I 1 - effective value of alternating current; ω is the number of winding turns of one phase of the stator.

First, consider the power supply of the stator winding with direct current. If during the operation of the machine in the motor mode, its slip and the magnetizing current change little, then in the dynamic braking mode, the slip of the rotor varies over a wide range. Consequently, with a change in speed, the EMF of the rotor changes, the current in the rotor and the MMF created by it, which has a significant effect on the resulting MMF.

Rice. 4-19. Vector diagram of an induction machine in dynamic braking mode

Obviously, the resulting magnetizing current, given to the stator, will be equal to

Using the vector diagram (Fig. 4-19), we write the following relationships for currents:

(4-37)

Taking the value of the EMF in the rotor of the machine, as before, equal to E 2 at the angular speed of rotation of the rotor ω 0 , at other speeds we have

Accordingly, the inductive resistance of the rotor

Where X 2 - inductive resistance of the rotor at a frequency ω 0 .

Now for the secondary circuit of the machine, we can write

After bringing the EMF E 2 to the parameters of the primary circuit we will have E 1 = E 2" and then

Substituting expressions (4-38) into formula (4-37), we obtain:

(4-39)

Solving equation (4-39) for current I 2 ", we find

(4-40)

The value of the electromagnetic moment of the machine is determined by the losses in its secondary circuit, namely:

(4-41)

Investigating this expression for an extremum, it is easy to obtain the critical relative rotor speed ν KP , at which there is a maximum torque:

(4-42)

(4-43)

Based on formulas (4-41) - (4-43), the following expression for the mechanical characteristic of IM can be obtained:

(4-44)

The expression (4-44) is similar to the Kloss formula, which makes it easier to understand. Analysis of formulas (4-40) - (4-44) and physical phenomena characteristic of dynamic braking of blood pressure allows us to draw the following conclusions.

1. In the dynamic braking mode, the properties of the mechanical characteristics of an asynchronous machine are similar to the properties of similar characteristics of the motor mode, i.e., the critical moment does not depend on the active resistance of the secondary circuit, and the critical speed ν KP is the same as s KP in motor mode, proportional r 2 ".

2. Parameter xμ and current I 1 can differ significantly from similar values ​​of the motor mode, since they depend on the saturation of the stator magnetic circuit.

3. The stator current of the machine in the motoring mode is a function of the rotor slip, and during dynamic braking it is constant.

4. The resulting magnetic flux during dynamic braking and low rotor speed increases, since the demagnetizing effect of the rotor reaction decreases, and in the motor mode it remains approximately constant.

Rice. 4-20. Mechanical characteristics of an asynchronous motor with dynamic braking and various values ​​of the excitation current or additional resistances in the rotor circuit

On fig. 4-20 shows the characteristics, of which 1 And 2 obtained at two values ​​of current in the stator I 11 I 12 and constant resistance r 21 , and characteristics 3 And 4 found at the same currents, but a different value r 22 > r 21 . For comparison, the mechanical characteristics of the machine operating in the motor mode are presented. If it is possible to change the resistance in the rotor circuit, then it is possible to obtain characteristics with approximately constant torque over a wide range of drive speed changes.

Reactance of the magnetizing circuit x μ is determined by the universal characteristic of the idling of the machine or experimental data. In the latter case, without taking into account the saturation of the magnetic circuit, the value x μ is found according to the formula:

Where U 0 , I 0 - phase voltage and current when the machine is idling.

More precisely, the dependence x μ = f(Iμ) can be found as follows. If an asynchronous machine, the rotor of which is rotated by an external motor at a synchronous speed, is supplied with a phase voltage varying in magnitude, then it corresponds to the EMF E 1 . Therefore, by measuring the current Iμ , it is easy to calculate the dependence x μ = E 1 Iμ -1 , which will take into account the saturation of the magnetic system of the machine. The construction of the mechanical characteristic in this case is carried out point by point. This sets the values M K.P., ν KP and calculate by formulas (4-42) and (4-43) the value r 2 " and current I 1 . Then find ν i by changing Iμi zero to I 1 at appropriate values xμi , according to the formula:

(4-45)

Expression (4-45) is obtained after operations with formulas (4-37) - (4-38). According to the formula (4-41), the mechanical characteristic can be calculated taking into account the saturation effect of the magnetic circuit of the machine.

This type of braking is used in hoisting and transport and in machine drives, powered by an unregulated AC network in frequency-controlled drives.

Capacitor braking of asynchronous motors has been used in machine drives in recent decades. The possibility of such a regime was established back in 1895 by M. Leblanc, but in the 20-40s of the XX century this type of braking was considered irrational. Only in 1944 A.T. Golovan and I.N. Barbash showed the promise of its use. However, only at the end of the 50s, thanks to the works of L.P. Petrov, practical results were achieved in the use of both capacitor and other types of combined braking. This became possible due to the reduction in the cost and dimensions of capacitors and the development of new circuits that provide intensive self-excitation of asynchronous machines in a wide range of changes in their rotation speed. Currently, various schemes for the implementation of capacitor braking are used.

Rice. 4-21. Dependence of self-excitation of an asynchronous machine during capacitor braking

The principle of self-excitation of blood pressure is illustrated by the images shown in fig. 4-21. When turning off machines with a rotating rotor from the network and connecting a capacitor bank to the stator (Fig. 4-26, a) due to residual emf E 0 capacitors begin to charge with current I μ 0 (Figure 4-21). This current raises the emf of the machine to E 1 i , which, in turn, increases the capacitor charge current to the value Iμi , and then the process would continue as shown in the figure until the point 1 (at a constant rotation speed of the motor field), where E 1 i = E 1 and Iμi = I μ .

According to the equivalent circuit (Fig. 4-22) EMF E 1 will be equal to

where φ = f X f 0 -1 and f 0 - nominal frequency in the circuit.

Assuming that at the beginning of self-excitation the current in the rotor is equal to zero and I 1 ≈ Iμ , you can find the initial relative frequency of self-excitation φ BEGINNING. Then from the formula (4-46) we find

And x μ , x 1 , x C - reactive components of the resistance of the equivalent circuit (Fig. 4-22) at the mains frequency (50 Hz).

Rice. 4-22. Equivalent circuit of an asynchronous machine with capacitor excitation

Ignoring the values IN And x 1 2 compared with xμ 2 and solving the biquadratic equation (4-47), we get:

Or (4-48)

Rice. 4-23. Static characteristics of the mode of capacitor self-excitation of an asynchronous machine Ф - magnetic flux; I 1 , I 2 " , Iμ - current in the stator, current in the rotor (reduced value), magnetization current, respectively; φ - frequency of free current oscillations in the stator; ω - angular velocity of the rotor; s - slip; M- electromagnetic moment

Thus, the initial frequency of the process of self-excitation of an asynchronous generator is approximately equal to the natural frequency of the oscillatory circuit of an unsaturated machine. This is also illustrated by the curves in Fig. 4-23 (in relative units). They allow us to draw the following conclusions.

1. The mode is limited in terms of the angular velocity of the rotor by the values ​​ω BEGINNING, where the self-excitation of the machine begins and ω K, where this process ends, and ω K > ω 0 .

2. In a significant range of changes in the rotor speed, the magnetic circuit of the machine remains saturated and the flow retains an approximately constant value (1.5-2.0) F NOM.

3. The values ​​of the currents of the rotor and stator significantly exceed the nominal values.

Considering the physical processes occurring in the machine, we can establish the following. If the rotor rotation speed exceeds ω START, then the frequency of the free component of the stator current increases due to the saturation of the magnetic system of the machine (see Fig. 4-23) and φ will be greater than φ START. The stator current vector rotates clockwise (Figure 4-24), but its amplitude increases. At the same time, the increase in current in the rotor I 2 leads to the appearance of a demagnetizing component of the magnetic flux in the air gap. At the speed of rotation of the rotor ω K, the reactive components of the currents are equal I 1 and I 2" and the process of self-excitation of the machine stops.

Considering equal I 1 and I 2 "due to the smallness of their active components, and using the expression (4-49), we find:

where φ K is the critical value of the relative frequency of the stator field.

Rice. 4-24. Vector self-excitation diagram of an asynchronous generator

The motor phase replacement circuit and its vector diagram allow you to find dependencies for electromagnetic power and torque, the latter is determined by thermal losses in the stator and rotor of the machine. However, these calculations are associated with very complex and cumbersome calculations of all the dependencies shown in Fig. 4-23. Therefore, we use a simplified method for calculating the mechanical characteristic, which is determined by the following relationship:

Where M 0 - initial (calculated) braking torque at speed ω 0 .

Value M 0 obtained experimentally as a product M NOM kC° , Where k - coefficient depending on the type of a particular engine. It can be taken equal to 0.7 for four- and six-pole machines and 0.5 for two-pole, С° - phase capacitance of capacitors in relative units from C NOM. By setting the value of φ BEGIN, one can calculate С° according to the formula

Rated capacity of the capacitor bank (phase)

Where Iμ NOM - machine magnetizing current at rated (phase) stator voltage; ω 0 - synchronous speed of rotation of the magnetic field at a network frequency of 50 Hz.

Rice. 4-25. Static mechanical characteristics of an asynchronous machine with capacitor braking: with capacitance in phase WITH 1 (curve 1), with capacitance in phase WITH 2 (curve 2 and 3) and various values ​​of the magnetizing current Iμ 2 » Iμ 3

The mechanical characteristics (Fig. 4-25) show that an increase in the capacitance of the capacitors reduces the value of the angular velocities ω START and ω K, as well as the maximum braking torque. With an increase in the magnetizing current (curve 3 ) the saturation of the magnetic circuit increases, which leads to a decrease in the inductive resistance of the machine and an increase in the maximum braking torque and angular velocity ω K.

Rice. 4-26. Combined capacitor-dynamic braking: a - schematic diagram; b - mechanical characteristics

As mentioned above, combined braking methods are effective in obtaining a complete stop of the drive. Depending on the closing times of the brake contactor contacts CT in such a system it is possible to obtain even three successively changing braking modes (Fig. 4-26, b): capacitor (curve 1 ), magnetic (curve 2 ) and dynamic (curve 3 ) or only the first and last. The transition of the drive from the motor mode to the brake mode and the switching of various braking modes is indicated in the figure by arrows. For example, if the contact closure CT occurs at the moment corresponding to the point With, then it undergoes a transition from capacitor to magnetic braking, which ends at the point d, then almost until the drive stops, dynamic braking occurs.

7. Technical implementations. Applications

An asynchronous motor with a squirrel-cage rotor has been used for about 100 years and will be used as practically the only implementation of a mass unregulated electric drive, which still makes up more than 90% of all industrial electric drives. In the last 10-20 years, many firms in America and Europe have been trying to develop and market so-called energy-efficient motors, in which, due to an increase in the mass of active materials by 30%, the nominal efficiency is increased by 1-5% with a corresponding increase in cost. In recent years there has been a major project in the UK to build energy efficient motors without increasing the cost.

In the last decade, thanks to the advances in electronics (FC), the squirrel-cage induction motor has become the basis of the variable frequency drive, successfully replacing the previously dominant DC drive in many areas. Of particular interest is the use of such an electric drive in traditionally unregulated pumps, fans, and compressors. As experience shows, this technical solution saves up to 50% of electricity, up to 20% of water and more than 10% of heat.

The transition from an unregulated electric drive to a controlled one is considered in many technologies as the main direction in the development of an electric drive, since this significantly improves the quality of technological processes and saves up to 30% of electricity. This determines the prospects for the development of a frequency-controlled electric drive.

An electric drive with motors with a phase rotor with rheostatic control is traditionally used in the crane industry, and is used in other technologies. Cascade circuits and dual-feed machines can be found in powerful electric drives of gas pumping stations with a small control range, in ship electric propulsion devices.

The device of asynchronous machines

The principle of operation of an asynchronous machine is based on the use of a rotating magnetic field, which induces an electromotive force (EMF) in the rotor winding. When the current "of the rotor interacts with a rotating magnetic field, an electromagnetic torque is created that causes the rotor to rotate (in the motor mode) or brakes it (in the braking modes)

8-The principle of operation of an asynchronous machine

The principle of operation of an asynchronous machine is based on the law of electromagnetic induction, discovered

M. Faraday, and the works of D. Maxwell and E. Lenz.

In an asynchronous machine, one of the windings is placed on the stator 1 (Fig. 1.1 a), and the second on the rotor 5. There is an air gap between the rotor and the stator, which is made as small as possible to improve the magnetic connection between the windings. Stator winding 2 is a multi-phase (or in a particular case three-phase) winding, the coils of which are placed evenly around the circumference of the stator. Stator winding phases OH,BY And cz connected according to the Y or A scheme and connected to a three-phase current network. Rotor winding 4 perform multi-phase short-circuited or three-phase and placed evenly along the circumference of the rotor.

From the course of the theoretical foundations of electrical engineering, it is known that when a three-phase sinusoidal current is supplied to a three-phase stator winding, a rotating magnetic field arises, the rotational speed (rpm) of which

П1=60f1|р Where f 1- mains frequency. R-. number of pole pairs

The rotating magnetic field induces EMF E 2 in the conductors of the short-circuited winding of the rotor and current 1 2 passes through them.

Figure 1.1, a shows (according to the right-hand rule) the direction of the EMF induced in the rotor conductors during the rotation of the magnetic flux Ф clockwise (in this case, the rotor conductors move counterclockwise relative to the flux Ф). If the rotor is stationary or the frequency of its rotation is less than the frequency n1, then the active component of the rotor current is in phase with the induced EMF; Here the symbols (crosses and dots) in fig. 1.1 show at the same time the direction of the active component of the current.

Rice. 1.1. The electromagnetic circuit of an asynchronous machine and the direction of its electrictromagnetic moment when the machine is operating in the following modes: motor(A), generatorial(b) and electr. braking(V)

Electromagnetic forces act on current-carrying conductors located in a magnetic field, the direction of which is determined by the left hand rule. The total force F pe 3 applied to all the conductors of the rotor forms an electromagnetic moment M, entraining the rotor behind the rotating magnetic field.

The electromagnetic moment arising from the interaction of the magnetic flux Phi of the current of the rotor I2

M=sFI2sosf2

where c is the coefficient of proportionality; I2cosph2 - active component of the rotor current; f2 - phase angle between current I2 and EMF E 2 in the rotor winding.

If the electromagnetic torque M is large enough, then the rotor comes into rotation and its steady rotation frequency n 2 corresponds to the equality of the electromagnetic torque to the braking torque created by the mechanism driven into rotation and internal friction forces. This mode of operation of an asynchronous machine is motor.

The frequency of rotation of the rotor P2 always differs from the frequency of rotation of the magnetic field P1, since if these frequencies coincide, the rotating field does not cross the rotor winding and no EMF is induced in it, and therefore no torque is created.

The relative difference between the frequencies of rotation of the magnetic field and the rotor is called slip:

S=(P1- P1) | P1

It is expressed in relative units or percentages with respect to K P1 The rotor speed, taking into account

Thus, a characteristic feature of an asynchronous machine is the presence of slip, i.e. inequality of rotation frequencies P1 and P1 Therefore, the machine is called asynchronous (its rotor rotates out of sync with the field).

When an asynchronous machine is operating in a motor mode, the rotor speed is less than the rotational speed of the magnetic field P1 In the machine, electrical energy is converted into mechanical energy.

If the rotor is retarded (S=1), this is a short circuit mode. If the rotational speed of the rotor coincides with the rotational frequency of the magnetic field (synchronous frequency), i.e. S = 0, then no torque occurs.

If the rotor of an asynchronous machine is accelerated with the help of an external moment (for example, by some kind of motor) to a frequency P2, a higher frequency of rotation of the magnetic field P1, then the direction of the EMF in the conductors of the rotor and the active component of the rotor current will change. At the same time, the electromagnetic moment M will also change its direction, which will become braking, i.e., the asynchronous machine will switch to the generator mode (Fig. 1.1, b). In the generator mode, the asynchronous machine receives mechanical energy from the prime mover, converts it into electrical energy and gives it to the network, while 0>S> - ∞.

If you rotate the rotor from an external motor in the direction opposite to the rotation of the magnetic field (Fig. 1.1, c), then the EMF and the active component of the current in the conductors of the rotor are directed in the same way as in the motor mode, i.e. the machine receives electrical energy from the network . However, in this mode, the electromagnetic moment M is directed against the rotation of the rotor, i.e., it is braking. This mode of operation of an asynchronous machine is the mode of electromagnetic braking. In this mode, the rotor rotates in the opposite direction (with respect to the direction of the magnetic field), so P2

9-Design of asynchronous machines

The main types of engines. Induction motors are divided into two main types: squirrel-cage and slip ring motors (the latter are called slip ring motors). The motors under consideration have the same stator design and differ only in the design of the rotor.

Squirrel-cage motors are the most

common; the electrical industry produces tens of millions of them a year.

On fig. 1.2 A shows a general view of the most common asynchronous motor with a squirrel-cage rotor of a closed, ventilated design. The stator has a three-phase winding. The rotor winding is made in the form of a squirrel cage, i.e. it is short-circuited.

The design of the shell (hull, shields, etc.) largely depends on the design of the machine in terms of the degree of protection and on the selected cooling system. In the design under consideration, the machine body is equipped with ribs for better cooling. A centrifugal fan, located on the motor shaft outside the machine shell, blows over the ribbed motor casing. The fan is closed with an air guide casing.

Inside the machine, the air is agitated by ventilation vanes molded together with short circuit rings. A terminal box is attached to the body, in which a terminal panel is installed with the ends of the stator winding brought out.

In more powerful engines, to increase the cooling intensity, air is driven through the axial channels of the rotor by a separate fan or the same fan that blows over the outer surface of the machine. For this purpose, when using one common fan, air-conducting tubes are inserted into the axial holes of the rotor, fixed in the holes of the support disks mounted on the rotor shaft (Fig. 1.2, b). This prevents outside air, which contains moisture, from entering the windings of the machine. The end shields have louvers for the passage and exit of air.

The stator core (magnetic circuit) is assembled from stamped annular sheets of electrical steel with a thickness of 0.35 ... 0.5 mm. The sheets are stamped with grooves for placing the winding (Fig. 1.3). In large machines, the stator is assembled from sheets in the form of segments. Insulation is applied to the sheets on both sides (oxide film, varnish, etc.). The sheets in the core pack are fastened with staples, welding, or in large machines with pins. In machines over 400 kW, the cores usually have radial channels for better cooling. They are formed by dividing the core along the length into a number of packages and installing steel spacers between them, which are welded to the outer sheets of the package.

Rice. 1.2. Asynchronous squirrel-cage motors: 1-short-swinging rotor winding rings; 2, 10-bearing shields; 3 - ventilation blades; 4 - stator winding;

5 - terminal box; b - body (bed); 7 - stator core; 8-rotor core; 9-shaft; 11-fan casing; 12 - fan; 13-base disc; 14 - air supply tube

A winding made of a rectangular or round wire is placed in the grooves of the stator magnetic circuit. The windings of a rectangular wire are made in the form of rigid sections and placed in open or semi-open grooves (Fig. 1.4, a, b). Round wire windings are usually poured into semi-closed grooves through a slot in the groove (Fig. 1.5) using special stator winding machines. In high-voltage machines, the body insulation of the coils is usually made in the form of a pressed sleeve (see Figure 1.4) In modern asynchronous machines, electrical insulating materials of heat resistance classes B and F are used, and for special machines operating in difficult conditions, materials of class H

Figure 1.3 Stator core and stamped sheet

In modern asynchronous machines, electrical insulating materials of heat resistance classes B and F are used, and for special machines operating in difficult conditions, materials of class H

In machines, inter-turn and case insulation are distinguished. Inter-turn insulation (between winding turns) is provided by the insulation of the conductor itself, applied to it during the manufacturing process at cable factories or during the manufacture of an electrical machine. Case insulation separates the winding conductors from the body of the electrical machine. It uses various gaskets, sleeves or a series of layers of insulation applied to the appropriate coil before installing it in the machine

Fig 1.4Open(A)and half-open (b) stator slots for winding from rigid sections -

1.4.5-insulating pads 2-conductors 3-coil insulation (housing) 6-wedge The rotor of the machine consists of a package of electrical steel sheets with stamped grooves. In short-circuited rotaries, the grooves are filled with aluminum. In this case, the rods of a squirrel cage are formed (Fig. 1.6 a) At the same time, short-circuiting end rings and ventilation blades are cast, a general view of such a rotor is shown in Fig. 1.6, b. In larger and special machines, copper (bronze, brass) rods are inserted into the grooves of the rotor, the ends of which are soldered (welded) into short-circuiting copper rings (Fig. 1.6, c). The aluminum cage package is pressed onto the shaft. For rotors with a copper cage, the sheets are assembled

directly on the shaft, and only then copper rods are inserted into the grooves of the package .

The rotors of the motors rotate in bearings, as a rule, rolling bearings are used, in machines over 1000 kW, plain bearings are also used. If necessary, a fan is installed on the shaft. Bearings are fixed in bearing shields, bearing shields are attached to the stator housing. Motors with a phase rotor are much less used than those with a squirrel-cage rotor, and are produced by the industry mainly in the form of machines with a power of over 100 kW.

Fig 1.5 Rice. 1.5. Stator grooves for bulk odlayered(A) and two-layer(b) obmocurrent:

1 - conductors; 2 - groove insulation (case); 3 - cover - wedge; 4 - gasket

On fig. 1.7 shows a general view of an induction motor with a phase rotor of a protected design. For better cooling, the stator and rotor magnetic circuits in machines of large and medium power are divided into separate packages, between which there are ventilation ducts. Ventilation blades, reinforced

Rice. 1.6. Squirrel cage design:

/ - rotor core; 2 - rods of a squirrel cage; 3 - ventilation blades

on the frontal (external) parts of the rigid sections of the winding, they suck air into the machine through the holes in the shields and

throw it out through the holes in the case. Such ventilation is called symmetrical radial. The slip rings are located outside the machine shell.

Rice. 1.7. Induction motor with phase rotor:

7 - terminal box; 2 - shaft; 3 - ventilation blades; 4 - rotor winding; 5 - stator winding;

6.11-bearing shields; 7-stator core; 8- rotor core; 9 - radial ventilation duct; 10 - diffuser; 12 - brush traverse; 13 - casing; 14 pin rings

Rice. 1.8. Slots of a phase rotor with a random winding of round wire(A) and with hard winding(b):

1 - wedge; 2 - conductors; 3- gasket; 4 - groove insulation (case)

the output ends of the rotor winding pass through the hole in the shaft and are connected to the slip rings with bolts. Brush holders with brushes are attached to the shield with a brush traverse. In motors with a phase rotor, a loose winding of round wire (Fig. 1.8, a) or a winding consisting of rigid sections placed in the open grooves of the rotor (Fig. 1.8.6), or a winding of rods inserted into semi-closed grooves from the end. Three ends from the phase windings are connected to slip rings mounted on the motor shaft.

10. List of references

1 I.P. Kopylov - "Electric Machines" - Moscow, 2002

The most common electric motors in industry, agriculture and all other applications are induction motors. It can be said that squirrel-cage induction motors are the main means of converting electrical energy into mechanical energy. The principle of operation of an induction motor was discussed in § 1.2 and 6.1.

The electromagnetic field of the stator rotates in the air gap of the machine at a speed co = 2 nf( /r p. At a standard frequency of 50 Hz, the rated rotor speed depends on the number of pole pairs r p(Table 6.1).

Table 6.1

The dependence of the speed of rotation of asynchronous motors on the number of pairs

poles

Depending on the design of the rotor of an asynchronous motor, asynchronous motors are distinguished with phase And squirrel-cage rotor. In motors with a phase rotor, a three-phase distributed winding is located on the rotor, usually connected to a star, the ends of the windings are connected to slip rings, through which the electrical circuits of the rotor are removed from the machine for connection to starting resistances, followed by shorting the windings. In squirrel-cage motors, the winding is made in the form squirrel cage - rods short-circuited on both sides with rings. Despite the specific design, the squirrel cage can also be considered as a short-circuited three-phase winding.

Electromagnetic moment M in an asynchronous motor is created due to the interaction of the rotating magnetic field of the stator Ф with the active component of the rotor current:

Where To - constructive constant.

Rotor current arises due to EMF E 2, which is induced in the rotor windings by a rotating magnetic field. When the rotor is stationary, the induction motor is a three-phase transformer with windings short-circuited or loaded with starting resistance. The EMF that occurs when the rotor is stationary in its windings is called rated phase EMF rotor E 2n. This emf is approximately equal to the stator phase voltage divided by the transformation ratio to t:

When the motor is rotating, the EMF of the rotor E 2 and the frequency of this EMF (and hence the frequency of the current in the rotor windings) ^ depend on the frequency of the rotating field crossing the conductors of the rotor winding (in a squirrel-cage motor - rods). This frequency is determined by the difference in the speeds of the stator field w and the rotor w, which is called absolute slip:

When analyzing the operating modes of an asynchronous motor with a constant frequency of the supply voltage (50 Hz), the relative slip value is usually used

When the motor rotor is stationary, s= 1. The highest EMF of the rotor when operating in the motor mode will be with a stationary rotor ( E 2n), as the speed increases (slip decreases), the EMF E 2 will decrease:

Similarly, the frequency of the EMF and the rotor current / 2 with a stationary rotor will be equal to the stator current frequency /, and as the speed increases, it will decrease in proportion to the slip:

In the nominal mode, the rotor speed differs slightly from the field speed, and the nominal slip for general-purpose motors with a power of 1.5 ... 200.0 kW is only 2 ... 3%, and for motors of higher power, about 1%. Accordingly, in the nominal mode, the EMF of the rotor is 1 ... 3% of the nominal value of this EMF at 5 \u003d 1. The frequency of the rotor current in the nominal mode will be only 0.5 ... 1.5 Hz. At 5 = 0, when the rotor speed is equal to the field speed, the rotor EMF E 2 and rotor current / 2 will be zero, the motor torque will also be zero. This mode is ideal idle mode.

The dependence of the EMF frequency and rotor current on slip determines the uniqueness of the mechanical characteristics of an induction motor.

The operation of an asynchronous motor with a phase rotor, the windings of which are short-circuited. As shown in (6.16), the motor torque is proportional to the flux Ф and the active component of the rotor current / 2 "a, reduced to the stator. The flux created by the windings depends on the value and frequency of the supply voltage

The rotor current is

where Z 2 is the impedance of the phase of the rotor winding.

It should be borne in mind that the inductive resistance of the rotor winding x 2 is a variable value that depends on the frequency of the rotor current, and, therefore, on slip: x 2 \u003d 2p 2 2 \u003d 2k t 2.

With a stationary rotor at s= 1 inductive resistance of the rotor winding is maximum. As the speed increases (slip decreases), the inductive reactance of the rotor x 2 decreases and, upon reaching the rated speed, is only 1 ... 3% of the resistance at 5 \u003d 1. Denoting x 2s \u003d l \u003d x 2n, we get

Let us bring the parameters of the rotor circuit to the stator winding, taking into account the transformation ratio and on the basis of conservation

power equality:

And the active component of the rotor current has the form:

Dividing the numerator and denominator of formula (6.26) by s, we get

Mathematical operation performed - dividing the numerator and denominator by s, of course, does not change the validity of equality (6.29), but is of a formal nature, which must be taken into account when considering this relation. In fact, as follows from the original formula (6.26), the slip depends on the inductive resistance of the rotor x 2, and active resistance g 2 remains constant. The use of expression (6.29) allows, by analogy with a transformer, to draw up an equivalent circuit for an asynchronous motor, which is shown in fig. 6.4 ,A.

Rice. 6.4.Equivalent circuits of an asynchronous motor: a - complete circuit; b - scheme with remote magnetizing circuit

For the analysis of an unregulated electric drive, this scheme can be simplified by transferring the magnetization circuit to the motor terminals. A simplified U-shaped equivalent circuit is shown in fig. 6.4D based on which, the rotor current will be equal to:

Where x k \u003d x + x "2i- short circuit inductive reactance. The active component of the rotor current, taking into account (6.28), will be:

Substituting (6.22) and (6.31) into (6.16), we obtain an expression for the moment of an induction motor

The natural mechanical characteristic of an asynchronous motor oz = f(M) with a phase rotor, the windings of which are short-circuited, is shown in fig. 6.5. It also shows the electromechanical characteristic of the motor u = /(/j), determined from the vector diagram of the asynchronous motor in fig. 6.6, I x = I + / 2 ".

Rice. AT 5. Natural mechanical and electromechanical characteristics of an induction motor

Rice. V.V. Simplified vector diagram of an induction motor

Assuming the magnetizing current to be reactive, we obtain where

Equating the derivative dM/ds= , find the maximum value of the moment of the induction motor M k \u003d M n and the corresponding critical slip value s K:

Where s K- critical slip; the sign "+" means that this value refers to the motor mode, the sign "-" - to the generator mode of regenerative braking.

Taking into account (6.34) and (6.35), the mechanical characteristic formula (6.32) can be transformed into a more convenient expression for use - Kloss formula:

For motors with a power of more than 15 kW, the resistance of the stator winding r, is small and at a frequency of 50 Hz is much less x k. Therefore, in the above expressions, the value of r can be neglected:

According to the formulas obtained, it is possible to calculate the mechanical characteristic of an asynchronous motor, using its passport data, knowing the rated torque M n, nominal slip s h and motor overload capacity x.

Note that when analyzing electromagnetic processes in an asynchronous motor for a steady state, we came to the same relations (6.9) and (6.10), which were obtained in § 6.1 on the basis of differential equations of a generalized two-phase machine.

Analysis of the features of the mechanical characteristics of an induction motor (see Fig. 6.5). It is non-linear and consists of two parts. The first - the working part - within the sliding range from 0 to s K . This part of the characteristic is close to linear and has a negative stiffness. Here, the moment developed by the motor is approximately proportional to the stator current 1 X and rotor / 2 . Since on this part of the characteristic s then the second term of the denominator in formula (6.39) is much less than the first one and can be neglected. Then the working part of the mechanical characteristic can be approximately represented in a linear form, where the moment is proportional to the slip:

The second part of the mechanical characteristic of an asynchronous motor with slips large s K (s>s K) curvilinear, with a positive stiffness value (3. Despite the fact that the motor current increases as the slip increases, the moment, on the contrary, decreases. If the rotor windings of an asynchronous motor with a phase rotor in the external circuit are short-circuited, then the starting current of such a motor (with \u003d 0 and 5 \u003d 1) will be very large and exceed the rated one by 10-12 times. At the same time, the starting torque will be about 0.4 ... 5 ... 6) / n, and the starting torque (1.1 ... 1.3) A / n.

To explain this discrepancy between starting current and torque, consider the vector diagrams of the rotor circuit (Fig. 6.7) for two cases: when the slip is large (the starting part of the characteristic); when the slip is small (working part of the characteristic). At start, when 5=1, the frequency of the rotor current is equal to the frequency of the mains (f 2 = 50 Hz). Inductive resistance of the rotor winding [see. (6.24)] is large and significantly exceeds the active resistance of the rotor / * 2, the current lags behind the EMF of the rotor by a large angle φ, i.e. the rotor current is mainly reactive. Since the EMF of the rotor in this case will be large 2 \u003d 2n, then the starting current will also be very large, however, due to the small value of cp 2, the active component of the rotor current 1 2a will be small, therefore, the moment developed by the engine will also be small.

When the motor accelerates, the slip decreases, the EMF of the rotor, the frequency of the current of the rotor, the inductive resistance of the rotor decrease proportionally. Accordingly, the value of the total current of the rotor and stator decreases, however, due to the increase in f 2, the active component of the rotor current increases and the motor torque increases.

When the motor slip becomes less sK , the frequency of the rotor current will decrease so much that the inductive reactance will already be less than the active one, and the rotor current will be practically active (Fig. 6.7,6), the motor torque will be proportional to the rotor current. So, if the nominal slip of the motor is 5 n = 2%, then, compared with the starting parameters, the rotor current frequency will decrease by 50 times, and the inductive resistance of the rotor will correspondingly decrease. Therefore, despite the fact that the EMF of the rotor will also decrease by a factor of 50, it will be sufficient to create the rated current of the rotor, which provides the rated torque of the motor. Thus, the originality of the mechanical characteristics of an asynchronous motor is determined by the dependence of the inductive resistance of the rotor on slip.

Rice. AT 7. Vector diagram of the rotor circuit of an asynchronous motor: a - with large slip: b - with and small slip

Based on the foregoing, to start an asynchronous motor with a phase rotor, measures must be taken to increase the starting torque and reduce starting currents. For this purpose, an additional active resistance is included in the rotor circuit. As follows from formulas (6.34), (6.35), the introduction of additional active resistance does not change the maximum torque of the engine, but only changes the value

critical slip: , where /?" ext - reduced to

stator additional resistance in the rotor circuit.

The introduction of additional active resistance increases the impedance of the rotor circuit, as a result, the starting current decreases and the cp of the rotor circuit increases, which leads to an increase in the active component of the rotor current and, consequently, the starting torque of the engine.

Usually, a sectioned resistance is introduced into the rotor circuit of a motor with a phase rotor, the stages of which are bridged by starting contactors. Calculation of rheostatic starting characteristics can be made according to the formula (6.39), using the value sK , corresponding R2 b for each step of starting resistance. The circuit for switching on additional resistances and the corresponding rheostatic mechanical characteristics of the engine are shown in fig. 6.8. The mechanical characteristics have a common ideal idle point equal to the rotation speed of the stator electromagnetic field co, and the rigidity of the working part of the characteristics decreases as the total active resistance of the rotor circuit increases (2 + /? ext).

When starting the engine, the total additional resistance /? 1ext. Upon reaching the speed at which the engine torque L / becomes close to the moment of resistance M s, part of the starting resistance is shunted by the contactor K1, and the motor switches to the characteristic corresponding to the value of the additional resistance /? 2ext. In this case, the engine torque is increased to the value M 2 . As the motor accelerates further, contactor K2 shorts out the second stage of starting resistance. After closing the contacts of the short circuit contactor, the motor switches to natural characteristic and will operate at the speed corresponding to point 1.

To calculate the starting characteristics, you need to set the torque value M ( at which the stages of starting resistors are switched M x = 1,2M s. Torque starting values M 2(Fig. 6.8) are found by the formula, \u003d A /, where T - number of steps.

To calculate the stages of starting resistance, we find the nominal resistance of the rotor R 2h \u003d 2n.lin /\u003e / 3 2n

Step resistances:

In squirrel-cage induction motors, the introduction of additional resistance into the rotor circuit is impossible. However, the same result can be obtained by using the effect of displacement of current on the surface of the conductor. The essence of this phenomenon is as follows. According to the law of electromagnetic induction, when an alternating current flows through a conductor, an EMF of self-induction is induced in it, directed against the current:

The value of this EMF depends on the current I , its frequency and inductance, determined by the characteristics of the medium surrounding the conductor. If the conductor is in the air, then the magnetic permeability of the medium is very small, therefore, the inductance is small L. In this case, at a frequency of 50 Hz co = / s, the influence of the self-induction EMF is insignificant. Another thing is when the conductor is placed in the body of the magnetic circuit. Then the inductance increases many times and the EMF of self-induction, directed against the current, plays the role of an inductive resistance that prevents the current from flowing.

Rice. AT 9. The design of the rotor of an asynchronous squirrel-cage motor: A- with a deep groove; b - with a double cage; V- diagram explaining the effect of current displacement

Consider the manifestation of the action of the EMF of self-induction for the case of a conductor (rotor winding rod) placed in a deep groove of the magnetic circuit of the motor rotor (Fig. 6.9 ,A). We conditionally divide the section of the rod into three parts, which are connected in parallel. The current flowing through the lower part of the rod forms a flux Ф, the magnetic lines of force of which are closed along the magnetic circuit. In this part of the conductor, a large EMF of self-induction occurs eLV opposing current 1 2y

Current / 23 (Fig. 6.9, V), flowing along the upper part of the rod of the rotor winding forms a flow Ф 3, but since the lines of force of this flow are closed in air for a significant part of their length, the flow Ф 3 will be much less than the flow Ф. Hence the EMF e 1b will be many times less than eLV

The specified distribution of the EMF of self-induction along the height of the rod is typical for the mode when the frequency of the rotor current is high - close to 50 Hz. In this case, since all three parts of the rotor bar are connected in parallel (see Fig. 6.9, V), then the rotor current / 2 will go along the upper part of the rod, where there is less back EMF e L . This phenomenon is called displacement of current to the surface of the groove. In this case, the effective cross section of the rod through which the current flows will be several times less than the total cross section of the rod of the rotor winding. Thus, the active resistance of the rotor increases g 2 . Note that since the EMF of self-induction depends on the frequency of the current (i.e., on slip), then the resistance g 2 And x 2 are slip functions.

At start-up, when the slip is large, the resistance r 2 increases (an additional resistance is introduced into the rotor circuit, as it were). As the motor accelerates, the motor slip decreases, the current displacement effect weakens, the current begins to propagate down the conductor cross section, the resistance g 2 decreases. When the operating speed is reached, the frequency of the rotor current is so small that the phenomenon of current displacement no longer affects, the current flows through the entire cross section of the conductor, and the resistance g 2 minimum. Due to this automatic change in resistance g 2, the start of asynchronous squirrel-cage motors proceeds favorably: the starting current is

5.0 ... 6.0 nominal, and the starting torque is 1.1 ... 1.3 nominal.

It is possible to vary the parameters of the starting characteristics of an asynchronous motor during design by changing the shape of the groove, as well as the resistance of the material of the rods (alloy composition). Along with deep grooves, double grooves are used, forming a double squirrel cage (Fig. 6.9,6), and also use pear-shaped grooves, etc.

On fig. 6.10 shows the typical mechanical characteristics of various modifications of asynchronous squirrel-cage motors.

Rice. AT 10 O'CLOCK. Approximate mechanical characteristics of asynchronous squirrel-cage motors: a - normal version; 6 - with increased slip; V- with increased starting torque; g- crane and metallurgical series

Normal squirrel-cage motors used to drive a wide class of working machines and mechanisms, primarily for drives operating in continuous mode. This design is characterized by high efficiency and minimum nominal slip. The mechanical characteristic in the region of large slips usually has a small dip, characterized by a minimum torque M t (p.

High slip motors have a softer mechanical characteristic and are used in the following cases: when two or more engines operate on a common shaft, for mechanisms (for example, cranks) with a cyclically changing load, when it is advisable to use the kinetic energy stored in the moving parts of the electric drive to overcome the resistance to movement , and for mechanisms operating in intermittent mode.

Motors with increased starting torque Designed for machines with difficult starting conditions, such as scraper conveyors.

Engines for crane and metallurgical series designed for mechanisms operating in intermittent mode with frequent starts. These motors have a large overload capacity, high starting torque, increased mechanical strength, but worse energy performance.

The analytical calculation of the mechanical characteristics of squirrel-cage induction motors is quite complicated, therefore, the characteristic can be approximately built using four points: at idle (5 = 0), at maximum M k, launcher M p and minimum M t[n moment at the start of the launch. The data of these characteristic points are given in catalogs and reference books for asynchronous motors. The calculation of the working part of the mechanical characteristic of a short-circuited asynchronous motor (with slips from 0 to 5 k) can be made using the Kloss formula (6.36), (6.39), since the effect of current displacement in the operating mode is almost not manifested.

Full mechanical characteristic of an induction motor in all quadrants of the field Ms, shown in fig. 6.11.

The asynchronous motor can operate in three braking modes: regenerative and dynamic braking and reverse current braking. A specific braking mode is also capacitor braking.

Regenerative regenerative braking possible when the rotor speed is higher than the rotation speed of the stator electromagnetic field, which corresponds to a negative slip value: oo>co 0 5

A slightly larger value of the maximum torque in the generator mode is explained by the fact that the losses in the stator (at the resistance G () in the motor mode, the torque on the shaft is reduced, and in the generator mode, the torque on the shaft must be greater to cover the losses in the stator.

Note that in the regenerative braking mode, the asynchronous motor generates and delivers active power to the network, and in order to create an electromagnetic field, the asynchronous motor in the generator mode must also exchange reactive power with the network. Therefore, an asynchronous machine cannot work as an autonomous generator when disconnected from the network. It is possible, however, to connect an asynchronous machine to capacitor banks as a source of reactive power.

Dynamic braking method: stator windings are disconnected from the AC mains and connected to a DC voltage source (Fig. 6.12). When the stator windings are powered by direct current, an electromagnetic field that is stationary in space is created, i.e. rotation speed of the stator field with dt = . The slip will be equal to 5 DT = -co/co n, where co n is the nominal angular velocity of rotation of the stator field.

Rice. 6 .12 A- inclusion of dynamic braking; b - when connecting the windings into a star; V- when connecting the windings in a triangle

The type of mechanical characteristics (Fig. 6.13) is similar to the characteristics in the regenerative braking mode. The starting point of the characteristics is the origin of coordinates. You can adjust the intensity of dynamic braking by changing the excitation current / dt in the stator windings. The higher the current, the more braking torque the motor develops. In this case, however, it must be taken into account that at currents / dm > / 1n, the saturation of the magnetic circuit of the engine begins to affect.

For asynchronous motors with a phase rotor, the braking torque can also be controlled by introducing additional resistance into the rotor circuit. The effect of the introduction of additional resistance is similar to that which occurs when starting an asynchronous motor: due to the improvement of f, the critical slip of the motor increases and the braking torque increases at high rotation speeds.

In the dynamic braking mode, the stator windings are powered by a DC source. It should also be borne in mind that in the dynamic braking circuit, the current / d t flows (when the windings are connected to a star) not through three, but through two phase windings.

To calculate the characteristics, it is necessary to replace the real / equivalent current /, which, flowing through three phase windings,

creates the same magnetizing force as the current I. For the scheme in fig. 6.12 ,6 1 =0.816/ , and for the circuit in fig. 6.12 ,in I =0,472/ .

A simplified formula for an approximate calculation of mechanical characteristics (not taking into account the saturation of the engine) is similar to the Kloss formula for the motor mode:

Where - critical moment in dynamic braking mode;

It should be emphasized that the critical slip in the dynamic braking mode is significantly less than the critical slip in the motor mode, because . The voltage of the DC power supply will be significantly less than the rated voltage and approximately equal to dt = (2, ... 4) / eq.

Energetically, in the dynamic braking mode, the asynchronous motor operates as a synchronous generator, loaded by the resistance of the rotor circuit of the motor. All mechanical power supplied to the motor shaft during braking is converted into electrical power and is used to heat the resistance of the rotor circuit. Reverse braking can be in two cases:

• when, during engine operation, it is necessary to urgently stop it, and for this, the order of the phase alternation of the power supply of the stator windings of the engine is changed;
• when the electromechanical system moves in a negative direction under the action of the descent load, and the motor is turned on in the ascent direction to limit the descent speed (pull load mode).

In both cases, the electromagnetic field of the stator and the motor rotor rotate in different directions. Engine slip in pro-

anti-inclusion is always greater than one:

In the first case (Fig. 6.14), the motor operating at point 1, after changing the order of the motor phase sequence, goes into braking mode at point G, and the drive speed quickly decreases under the action of the braking torque M T and static M s. When decelerating to a speed close to zero, the motor must be switched off, otherwise it will accelerate in the opposite direction of rotation.

Rice. 6.14.

In the second case, after the release of the mechanical brake, the engine, turned on in the upward direction, under the action of the gravity of the lowered load, will rotate in the opposite direction at a speed corresponding to point 2. Operation in the opposition mode under the action of the pulling load is possible when using motors with a phase rotor. In this case, a significant additional resistance is introduced into the rotor circuit, which corresponds to characteristic 2 in Fig. 6.14.

Energetically, the opposition mode is extremely unfavorable. The current in this mode for asynchronous squirrel-cage motors exceeds the starting current, reaching a 10-fold value. Losses in the rotor circuit of the motor are the sum of the losses of the motor short circuit and the power that is transferred to the motor shaft during braking: A P n = L/T co 0 + M t (about.

For squirrel-cage motors, the anti-switching mode is only possible for a few seconds. When using motors with a phase rotor in the opposition mode, it is mandatory to include an additional resistance in the rotor circuit. In this case, the energy losses remain the same significant, but they are carried out of the engine volume into rotor resistances.