Synchronous machines are devices with the rotor speed, in which it is always equal to or a multiple of that of the magnetic field inside the air gap, which is created due to the current passing through the armature winding. At the heart of the work of this type machines is based on the principle of electromagnetic induction.
Excitation of synchronous machines
Synchronous machines can be excited by electromagnetic action or by a permanent magnet. In the case of electromagnetic excitation, a special generator is used direct current, which feeds the winding, in connection with its main function this device called the causative agent. It is worth noting that the excitation system is also divided into two types according to the method of exposure - direct and indirect. The direct excitation method means that the shaft of the synchronous machine is mechanically connected directly to the exciter rotor. The indirect method assumes that in order to make the rotor rotate, another motor is used, for example, an asynchronous electric machine.
It is the direct excitation method that has received the greatest distribution today. However, in cases where the excitation system is expected to work with powerful synchronous electric machines, independent excitation generators are used, the winding of which is supplied with current from another DC source, called a subexciter. Despite all the heaviness, this system allows you to achieve greater stability in work, as well as more fine tuning characteristics.
Synchronous machine device
A synchronous electrical machine has two main components: an inductor (rotor) and an armature (stator). The most optimal and therefore common today is the scheme when the armature is placed on the stator, while the inductor is located on the rotor. A prerequisite for the functioning of the mechanism is the presence of an air gap between these two parts. The armature in this case is a fixed part of the device (stator). It can consist of either one or several windings, depending on the required power of the magnetic field that it must create. The stator core, as a rule, is recruited from separate thin sheets of electrical steel.
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The inductor in synchronous electrical machines is an electromagnet, while the ends of its winding are brought directly to the slip rings on the shaft. During operation, the inductor is excited by direct current, due to which the rotor creates an electromagnetic field that interacts with the magnetic field of the armature. Thus, due to the direct current that excites the inductor, a constant frequency of rotation of the magnetic field inside the synchronous machine is achieved.
The principle of operation of synchronous machines
The principle of operation of a synchronous machine is based on the interaction of two types of magnetic fields. One of these fields is formed by an armature, while the other arises around an electromagnet excited by a direct current - an inductor. Immediately after reaching the operating power, the magnetic field created by the stator and rotating inside the air gap, couples with the magnetic fields at the poles of the inductor. Thus, in order for the synchronous machine to reach its operating speed, it takes a certain time to accelerate it. After the car accelerates to required frequency, the inductor is powered by a DC source.
The most common generator excitation system is with a DC generator located on the same axis as a synchronous generator (Fig. 8.8).
The DC generator usually operates in self-excitation mode with the excitation winding connected in parallel with the armature winding. Voltage from the DC generator terminals through slip rings K 1 and K 2 is applied to the excitation winding of the generator.
To excite high-power generators, a three-phase alternating current exciter and a three-phase rectifier are mounted (Fig. 8.9).
In this case, the three-phase exciter winding is located on the rotating part of the excited generator. A three-phase rectifier is mounted on the same part. It is enough just to power the anchor of the main generator. The exciter armature can be powered by an external DC source or by an additional DC exciter mounted on the same axle.
To excite a three-phase generator, the principle of self-excitation can be used (Fig. 8.10). The conditions for self-excitation of the generator are the same as for DC generators.
The direct excitation current is obtained from the excitation transformer, since in most cases the excitation voltage is less than the mains and rectifier voltage. An excitation resistor is used to control the excitation current. To keep the generator voltage constant, excitation can be used in electronic installations automatic regulation of the excitation current.
Conclusion
The main purpose of writing the manual was to present the material of the theory and practice of operating electromechanical devices in a simple accessible language without losing the information content of the content. The study physical foundations functioning of electrical machines is a solid basis for understanding the principles of construction of other electromechanical devices that are used in enterprises of various profiles.
The rapid development of new technologies poses a number of complex scientific and technological problems for production. Energy plays a key role in solving these problems. In the conditions of the scientific and technological revolution, the pace of development of the machine-building complex and, in particular, electrical engineering, largely determines the technical progress in the field of energy, the fuel industry, transport and communications, metallurgy, machine tool building and instrument making, construction, the agro-industrial complex, etc.
This tutorial outlines the basics of the theory, design features and modes of operation of the main types of electrical machines used in industry. At the same time, modern tendencies development of these machines, aimed at improving their reliability, energy performance, improving performance.
In general, at present, the following trends are observed in the development of domestic electrical engineering:
Improving the design of magnetic systems, windings and cooling systems in order to reduce the weight, overall dimensions of machines, energy losses in them; increase in the unit power of machines, rotational speed and rated voltage, increasing reliability by improving the quality of the insulation of the windings, eliminating, if possible, brush contacts and improving switching in collector machines; creation of new circuits of electrical machines that combine an electromagnetic system with elements of semiconductor technology (diodes, thyristors, transistors) to increase reliability, improve performance and expand the range of regulation of output parameters (current, voltage, speed, etc.), the creation of linear electric motors and reciprocating motion engines;
Development of more technological designs of machines of low and medium power and micromachines adapted for mass and serial production; improvement of methods for calculating electrical machines based on the use of computers, physical and mathematical modeling; widespread use of standardization for the main parameters of machines, their design elements, installation dimensions, cooling methods, and protection from environmental influences.
In solving the tasks set, the leading role belongs to the employees of branch research and design institutes. Scientists and teachers of higher educational institutions also provide significant assistance to workers in the electrical industry.
Electric machines used in automation and telemechanics schemes are very diverse in design, principle of operation, and in the functions they perform in various, sometimes very different automatic control, regulation and control schemes.
It is practically impossible to give a description of all the electrical machines used in one book, limited in volume by the curricula of universities. That is why the authors of this manual did not set themselves such a task, limiting themselves only to describing the device, the principle of operation, the fundamentals of the theory and the main characteristics of electrical machines that have received the most widespread use.
If you wish to become more deeply acquainted with the electrical machines presented in this tutorial, concisely, the reader can refer to the specialized literature.
Bibliography
1.Alekseev,A. E. Design of electrical machines / A. E. Alekseev. - M., 1958.
2.Armenian,E.V. Electrical micromachines / E. V. Armensky,G. B. Falk. - M., 1984.
3.Bertinov,A.I. Electrical Machines for Aviation Automation / A. I. Bertinov. - M., 1961.
4.Bruskin,D. E. Electrical Machines and Micromachines /
D. E. Bruskin,A. E. Zarokhovich,V. S. Khvostov. - M., 1981.
5.Booth,YES. Non-contact electrical machines / D. A. But. - M., 1985.
6.Vinogradov,N.V. Design of electrical machines / N. V. Vinogradov,F. A. Goryainov,P. S. Sergeev. - M., 1969.
7.Important,A.I. Electric cars / A. I. Important. - L .: Energy, 1969.
8.Vinokurov,V.A. Electric cars of railway transport / V. A. Vinokurov,D. A. Popov. - M., 1986.
9. woldek, A.I. Electric cars / A. I. Voldek. - L .: Energy, 1966.
10.Goldberg,O.D. Design of electrical machines /
O. D. Goldberg,Ya. S. Gurin,I. S. Sviridenko. - M., 1982.
11.Yermolin,N.P. Low Power Electric Machines / N. P. Ermolin.- M., 1975.
12.Ivanov-Smolensky,A.V. Electric cars / A. V. Ivanov-Smolensky. - M., 1980.
13.Katzman,MM. Electric cars / M. M. Katsman. - M., 1983.
14.Katzman,MM. Electrical machines automatic devices / M. M. Katsman,F. M. Yuferov. - M., 1979.
15.Kopylov,I.P. Electric cars / I. P. Kopylov. - M., 1986.
16.Kopylov,I.P. Electromechanical energy conversion / I. P. Kopylov. - M., 1973.
17.Kostenko,M.P. Electric cars. Part 1 / M. P. Kostenko,L. M. Piotrovsky. - L., 1973.
18.Kostenko,M.P. Electric cars. Part 1. - Ed. 2nd /
M. P. Kostenko,L. M. Piotrovsky.-
L.: Energy, 1964.
19.Kostenko,M.P. Electric cars. Part 2. - Ed. 2nd /
M. P. Kostenko,L. M. Piotrovsky. - L.: Energy, 1965.
20. Petrov,G. N. Electric cars / G. N. PETROV - M., Gosenergoizdat, 1956. - Part I.
21.Petrov,G. N. Electric cars / G. N. Petrov. - M., 1963. - Part II; 1968. - Part III.
22. Special electrical machines / ed. A. I. Bertinova.- 1982.
23.Khrushchev,V.V. Electric machines of automation systems / V. V. Khrushchev. - L., 1985.
Preface. 3
Introduction. four
Chapter 1. Basic physical laws of functioning
electrical machines. 9
Chapter 2. General questions of DC machines. 13
2.1. The principle of operation of DC machines. 13
2.2. Design of DC machines. 17
2.3. Armature windings of DC machines. eighteen
2.4. Equipotential connections of armature windings. 31
2.5. Methods for creating a magnetic field or methods of excitation
DC machines. 34
2.6. EMF of the armature winding of DC machines. 36
2.7. Mechanical torque on the shaft of a DC machine. 39
2.8. The magnetic field of a DC machine running
in idle mode. 41
2.9. The magnetic field of a loaded DC machine.
anchor reaction. 42
2.10. Switching of the armature winding of DC machines. 45
Chapter 3. DC motors. 49
3.1. The principle of operation of DC motors. 49
3.2. Basic equations of a DC motor. 51
3.3. Losses and efficiency of engines
direct current. 51
3.4. Characteristics of DC motors. 54
3.5. Starting DC motors. 65
3.6. Speed control of DC motors. 71
Chapter 4. DC generators. 80
4.1. Classification of DC generators according to the method of excitation. 80
4.2. Energy diagram of DC generators. 81
4.3. The main characteristics of DC generators. 86
4.4. Characteristics of the generator with independent excitation.. 86
4.5. Working point of the loaded generator. 94
4.6. Characteristics of the generator with parallel excitation.. 95
4.7. Generators with serial excitation.. 100
4.8. DC generators with mixed excitation.. 101
4.9. Use of DC generators. 105
4.10. Parallel operation of generators. 106
Chapter 5. Transformers .. 109
5.1. The principle of operation of transformers. 110
5.2. Design of single-phase transformers. 112
5.3. Losses of electrical energy in the transformer and the efficiency of the transformer. 114
5.4. Transformer idle mode. 118
5.5. The operation of the transformer in load mode. 121
5.6. The reduced transformer and its equivalent circuit. 124
5.7. Experimental determination of transformer parameters. 129
5.8. Changing the output voltage of the transformer
when the load current changes. External characteristic
transformer. 132
5.9. External characteristics of transformers. 135
5.10. Three-phase transformers. The principle of operation of three-phase transformers 137
5.11. Schemes and groups for connecting three-phase windings
transformers. 141
5.12. Special transformers.. 145
5.13. Parallel operation of transformers. 150
Chapter 6. Asynchronous machines .. 154
6.1. Magnetic fields of asynchronous motors. rotating
a magnetic field. 154
6.2. Elliptical and pulsating magnetic fields. 160
6.3. The principle of operation of an asynchronous motor. 165
6.4. Construction of an asynchronous motor. 168
6.5. Windings of asynchronous machines. 170
6.6. Electromotive forces of the stator and rotor windings. 177
6.7. Magnetic flux of asynchronous machines. 178
6.8. Vector diagram of an induction motor. 181
6.9. Wiring diagram substitution of an asynchronous motor. 184
6.10. Energy processes of an asynchronous machine.. 186
6.11. Energy diagram of an induction motor. 188
6.12. The general equation of the torque of an asynchronous machine.. 189
6.13. The equation of the mechanical characteristic of the asynchronous
engine. 191
6.14. Kloss formula. 194
6.15. Equivalent equivalent circuit of an asynchronous machine
with magnetizing circuit connected to mains terminals.. 196
6.16. Pie diagram of an asynchronous machine. Building a chart.. 198
6.17. Analysis pie chart.. 202
6.18. Starting three-phase asynchronous motors. 207
6.19. Starting motors with a phase rotor .. 207
6.20. Starting a squirrel-cage motor .. 210
6.21. Motors with special rotor winding and improved starting characteristics. 214
6.22. Ways to control the speed of a three-phase asynchronous motor 216
6.23. Performance characteristics of asynchronous motors. 222
6.24. The operation of an asynchronous motor in various modes. 226
6.25. The operation of an asynchronous machine with a phase rotor in the mode
three-phase voltage regulator. 227
6.26. Single-phase asynchronous motors. 228
6.27. Marking the conclusions of an asynchronous motor. 232
Chapter 7. Synchronous generators .. 234
7.1. The principle of operation of synchronous machines. 234
7.2. The design of the synchronous machine.. 237
7.3. Generator idle mode. 238
7.4. Armature reaction of a synchronous machine.. 240
7.5. Vector voltage diagrams of a three-phase synchronous generator 245
7.6. Change in voltage at the output of a synchronous generator. 249
7.7. Main characteristics of a synchronous generator. 253
7.8. Inclusion in the network of three-phase generators or parallel
generator operation alternating current. 257
7.9. Angular characteristics of synchronous generators. 261
7.10. Synchronization power and synchronization torque. 264
7.11. Influence of the excitation current on the mode of operation of the synchronous
generator. 264
7.12. Energy loss and efficiency
synchronous generator. 266
Chapter 8. Synchronous motors. 269
8.1. The principle of operation of synchronous motors. 269
8.2. Vector voltage diagram of a synchronous motor. 270
8.3. Power and mechanical torque of a synchronous motor. 271
8.4. V-shaped characteristics of synchronous motors. 272
8.5. Characteristics of a synchronous motor. 274
8.6. Starting methods for synchronous motors. 275
8.7. Synchronous compensators.. 277
8.8. Ways of excitation of synchronous machines. 277
Conclusion. 280
References.. 282
Educational edition
Goryachev Vladimir Yakovlevich
Jazz Nikolai Borisovich
Nikolaev Elena Vladimirovna
Electromechanics
Editor V. V. Chuvashova
Technical editor N. A. Vyalkova
Corrector N. A. Sidelnikova
Computer layout N. V. Ivanova
Put into production 07.12.09. Format 60x841/16.
Conv. oven l. 16.74. Uch.-ed. l. 19.98.
Circulation 100. Order No. 643. "C" 164.
_______________________________________________________
PSU publishing house
440026, Penza, Red, 40.
Electric drives with synchronous motors can be divided into three classes from the conditions of load formation: electric drives with a constant or slowly changing load, electric drives with a pulsating load, electric drives with a sharply variable load. Main specifications synchronous electric drives, depending on the type of load that occurs, are given in Table. 6.1.
As follows from Table. 6.1, in electric drives with a pulsating and sharply variable load, it is necessary to carry out automatic control of the excitation of a synchronous motor. Automatic excitation control systems ensure stable operation of a synchronous motor during load surges or when the mains voltage drops. In these cases, automatic excitation control systems increase the excitation current, thereby increasing the maximum torque of the synchronous motor. In addition, changing the excitation current of the synchronous motor allows you to adjust the reactive power of the stator circuit of the motor.
Table 6.1
|
Load types |
Mechanisms |
Range capacities |
Automatic control of the excitation current |
|
Unchanging |
Fans Blowers Compressors |
Yuch-YuOO kW |
Not required |
|
Pulsating |
Pumping units Reciprocating compressors |
Necessary |
|
|
Sharply changeable |
Crushers Mills Rolling mills Shears Saws |
1004-10000 kW |
Necessary |
The possibility of reactive power control in the synchronous motor stator circuit by changing its excitation current is illustrated by the vector diagrams shown in fig. 6.14.
Rice. 6.14. Vector diagrams of a synchronous motor at different field winding currents: a - the excitation current is less than the nominal; b - the excitation current is equal to the nominal; c - excitation current is greater than the nominal
Vector diagram fig. 6.14, a corresponds to the field winding current less than the nominal one, while the stator current vector /, lags behind the mains voltage vector L.J.X at the angle cf. Reactive power is active-inductive. With an increase in the excitation current (Fig. 6.14 , b) EMF E ) , induced in the stator windings, increases and can reach such a value at which the stator current / will be in phase with the voltage (/, that is, costp = 1. The reactive power is zero. If the excitation winding current is further increased, then the stator current vector / , will lead in phase the voltage vector 6/, (operation with leading coscp) and the synchronous motor will be equivalent to an active-capacitive load connected in parallel with the network (Fig. 6.14, in).
On fig. 6.15 shows ^/-shaped characteristics. They show the dependence of the stator current /, synchronous motor on the excitation current / in at various loads on the motor shaft (M s! With numerical values of the parameters, the 67-shaped characteristics allow you to correctly select the excitation current in order to provide the required operating mode of the synchronous motor.
Currently, automatic excitation control systems are used in practice. Depending on the circuit solutions, automatic excitation current control systems can perform the following main functions:
- ensure stable operation of the synchronous motor under given load conditions;
- maintain the optimal voltage in the load node to which the synchronous motor is connected;
- ensure a minimum of energy losses in the synchronous motor and power supply system.
Rice. 6.15.
When choosing schemes for automatic control of the excitation current, they are guided by the following provisions:
- in electric drives with a constant load and slight fluctuations in the supply voltage, the installation of devices for automatic control of the excitation current, as a rule, is not provided;
- in electric drives with a pulsating load or shock load, it is necessary to install devices for automatic control of the excitation current. The excitation current of such motors is regulated as a function of the active stator current, which makes it possible to significantly increase the overload capacity of the motor, and in some cases reduce its installed power;
- when operating a synchronous motor with a rapidly changing load, it is also necessary to install devices for automatic control of the excitation current, however, in this case, the control system must respond not only to a change in load, but also to the rate of this change.
The simplest diagram of the system for automatic control of the excitation current for electric drives with a pulsating load is shown in fig. 6.16. The system allows excitation of the synchronous motor in all normal modes of its operation. When the load on the motor shaft changes, the stator winding current / also increases, which
leads to an increase in the positive current feedback signal Uoc[
and, as a consequence, to an increase in the voltage of the controlled rectifier and an increase in the excitation current of the synchronous motor.
Rice. 6.16.
Taking into account the proportionality between the EMF and the magnetic flux Ф, and, consequently, the current of the excitation winding / in, equation (1.71) can be written in the following form:
where to in - coefficient of proportionality between the flux F and the excitation current 1 a.
Analysis (6.10) shows that an increase in the excitation current causes an increase in the maximum torque of the synchronous motor. Consequently, automatic excitation control leads to an increase in the dynamic stability of the synchronous motor when the load on its shaft changes and damping of the rotor oscillation.
It is also possible to maintain the optimal voltage in the load node to which the synchronous motor is connected using automatic excitation current control systems.
To improve the performance of an extensive industrial network, reactive power is compensated by installing synchronous motors or synchronous compensators. On fig. 6.17 shows a diagram of the load node, to which consumers are connected, generating and consuming reactive power.
Rice. 6.1 7.
Inductive reactive current / p is equal to the sum of reactive currents P
consumers (transformers; asynchronous motors; DC motors powered by adjustable converters) and is determined by the expression
where / . - reactive current /-th load.
For full compensation of reactive power in the network, it is necessary to fulfill the condition
The reactive current of the synchronous machine required to compensate for the voltage drop in the network:
where X p- equivalent phase reactance of the network, taking into account all consumers: 
AU C- network voltage drop; 
- phase voltage of the network;
- total phase resistance of all consumers of electrical energy, except for a synchronous motor; p, - electrical conductivity of the circuit section; U, t- line voltage of the network; S K With -
network short circuit power.
Modern systems for automatic control of the excitation current of synchronous motors intended for reactive power compensation are built on the principle of subordinate coordinate control and provide for the regulation of three variables: the excitation current, the voltage drop across the equivalent phase reactive resistance of the network, and the stator reactive current of the synchronous motor. The functional diagram of such a system is shown in fig. 6.18.
Rice. 6.18.
The internal circuit provides regulation of the excitation current using the excitation current regulator PTB. The reference for the excitation current of a synchronous motor is the output signal U pj regulator
reactive current PPT. The synchronous motor excitation current feedback voltage is subtracted from this signal. The output signal? / RTV of the excitation current controller affects the controlled
rectifier SW, changing the excitation current / in the synchronous motor.
The reactive current regulator is included in the second circuit - the reactive current control circuit I. Signals are summed at its input.
negative feedback on the reactive current (7 ort and the command signal for the reactive current - from the output of the voltage regulator PH.
Negative voltage feedback signals are summed at the input of the voltage regulator PH U on . Feedback voltage is formed from the reactive current and the equivalent phase resistance of the network: U0H = I X C1. The voltage regulator is adaptive, proportional type, changing the gain when the voltage of the supply medium drops below (0.8 - 0.85) U H .
The transfer functions of control loops and current controllers are obtained under the following basic assumptions:
The saturation of the magnetic circuit of the synchronous motor is not taken into account;
Controlled rectifier - aperiodic link of the first order with a transfer function
where k. w- gain of the controlled rectifier (thyristor converter); 
- delay time constant
thyristor converter; t in- the number of voltage ripples of the thyristor converter for the period of the mains voltage; co e -
the angular frequency of the supply network, equal to 314.15 s" 1, at the frequency of the supply network / s \u003d 50 Hz; all filter time constants and small inertias are summed up and replaced by one time constant.
Transfer functions of regulators in accordance with the modular optimum:
Excitation current regulator
Reactive current regulator
where T- time constant of the excitation current control loop; 7j ipp - time constant of the reactive current control loop; to jap- transmission coefficient of the excitation current sensor; R B - active resistance of the excitation winding of a synchronous motor; to yarya- transfer coefficient of the reactive current sensor; to xia- transmission coefficient of a synchronous motor controlled by a voltage change in the excitation winding circuit.
Compensation of the booster link 7^ ptv R+1 in the numerator of the transfer function of the excitation current controller WpTB(p) is carried out inside the object of regulation - a synchronous motor. Thus, there is no time constant in the reactive current control loop that needs to be compensated, therefore, the implementation of the controller with a proportional-integral characteristic makes it possible to eliminate the disadvantage of the slave control system.
Using a synchronous motor with automatic adjustment excitation allows you to maintain reactive power and voltage at the load node at a given level. The assignment to the automatic excitation controller to generate reactive power is a variable value that depends on the parameters and load of the supply network.
According to the method of excitation, synchronous machines are divided into two types:
Excitation of an independent type.
Self-excitation.
With independent excitation, the circuit implies the presence of a sub-exciter that feeds: winding of the main exciter, rheostat for adjustment, control devices, voltage regulators, etc. In addition to this method, excitation can be carried out from a generator that performs an auxiliary function, it is driven by a synchronous or asynchronous type motor.
For self-excitation , the winding is powered through a rectifier operating on semiconductors or ionic type.
For turbo and hydro generators, thyristor excitation devices are used. The excitation current is adjustable in automatic mode, with the help of an excitation regulator, for low-power machines, the use of adjusting rheostats is typical, they are included in the excitation winding circuit.
27. Advantages and disadvantages of a synchronous motor.
A synchronous motor has several advantages over an asynchronous one:
1. High power factor cosФ=0.9.
2. The possibility of using synchronous motors in enterprises to increase the overall power factor.
3. High efficiency, it is more than that of an asynchronous motor by (0.5-3%), this is achieved by reducing losses in copper and large CosФ.
4. Possesses the big durability caused by the increased air gap.
The torque of a synchronous motor is directly proportional to the voltage to the first power. That is, the synchronous motor will be less sensitive to changes in the magnitude of the mains voltage.
Disadvantages of a synchronous motor:
1. The complexity of the launch equipment and the high cost.
2. Synchronous motors are used to drive machines and mechanisms that do not need to change the speed, as well as for mechanisms in which the speed remains constant with a change in load: (pumps, compressors, fans.)
Starting a synchronous motor.
In view of the absence of a starting torque in a synchronous motor, the following methods are used to start it:
2. Asynchronous motor start.
1. Start with auxiliary motor.
The start-up of a synchronous motor with the help of an auxiliary motor can only be carried out without a mechanical load on its shaft, i.e. practically idle. In this case, for the start-up period, the motor temporarily turns into a synchronous generator, the rotor of which is driven by a small auxiliary motor. The stator of this generator is connected in parallel to the network in compliance with all the necessary conditions for this connection. After the stator is connected to the network, the auxiliary drive motor is mechanically switched off. This starting method is complex and has an auxiliary motor in addition.
2. Asynchronous motor start.
The most common way to start synchronous motors is asynchronous start, in which the synchronous motor turns into an asynchronous motor for the duration of the start. To enable the formation of an asynchronous starting torque, a starting short-circuited winding is placed in the grooves of the pole pieces of a salient-pole motor. This winding consists of brass rods inserted into the grooves of the tips and short-circuited at both ends with copper rings.
When the engine is started, the stator winding is connected to the AC network. The excitation winding (3) for the start-up period is closed to some resistance Rg, fig. 45, key K is in position 2, resistance Rg = (8-10) Rv. At the initial moment of starting at S=1, due to the large number of turns of the field winding, the rotating magnetic field of the stator will induce an EMF Ev in the field winding, which can reach quite of great importance and if at start-up the excitation winding is not turned on for resistance Rg, insulation breakdown will occur.
Rice. 45 Fig. 46.
The process of starting a synchronous motor is carried out in two stages. When the stator winding (1) is connected to the network, a rotating field is formed in the motor, which will induce an EMF in the short-circuited rotor winding (2). Under the action, which will flow in the rods current. As a result of the interaction of a rotating magnetic field with a current, a torque is created in a short-circuited winding, as in an asynchronous motor. Due to this moment, the rotor accelerates to slip close to zero (S=0.05), fig. 46. This ends the first stage.
In order for the motor rotor to be drawn into synchronism, it is necessary to create a magnetic field in it by turning on the DC excitation winding (3) (by switching the key K to position 1). Since the rotor is accelerated to a speed close to
to synchronous, then the relative speed of the stator and rotor fields is small. The poles will smoothly find each other. And after a series of slips, the opposite poles will be attracted, and the rotor will be drawn into synchronism. After that, the rotor will rotate at a synchronous speed, and its rotational speed will be constant, fig. 46. This ends the second stage of the launch.
On the rotor of the synchronous generator there is a source of MMF (inductor), which creates a magnetic field in the generator. With the help of a drive motor (PD), the generator rotor is set in rotation with a synchronous frequency n 1 . In this case, the magnetic field of the rotor also rotates and, mating with the stator winding, induces an EMF in it.
The main way to excite synchronous machines is electromagnetic excitation, the essence of which is that the excitation winding is located on the poles of the rotor. When a direct current passes through this winding, an MMF of excitation occurs, which induces a magnetic field in the magnetic system of the machine.
Until recently, special independent excitation DC generators, called exciters B, were used to power the field winding (Fig. 82, a), the excitation winding of which (OV) received DC power from another generator (parallel excitation), called a subexciter (PV). The rotor of the synchronous machine and the exciter and subexciter armatures are located on a common shaft and rotate simultaneously. In this case, the current enters the excitation winding of the synchronous machine through slip rings and brushes. To control the excitation current, adjusting rheostats are used, which are included in the excitation circuit of the exciter ( r 1) and subexciter ( r 2).
In synchronous generators of medium and high power, the process of regulating the excitation current is automated.
In high-power synchronous generators - turbogenerators - sometimes inductor-type alternators are used as a pathogen. At the output of such a generator, a semiconductor rectifier is turned on. The adjustment of the excitation current of the synchronous generator in this case is carried out by changing the excitation of the inductor generator.
Has been used in synchronous generators contactless electromagnetic excitation system, at which the synchronous generator does not have slip rings on the rotor.
In this case, an alternating current generator is also used as a pathogen (Fig. 82, b), in which the winding 2, in which the EMF is induced (armature winding), located on the rotor, and the excitation winding 1 located on the stator. As a result, the armature winding of the exciter and the excitation winding of the synchronous machine turn out to be rotating, and their electrical connection carried out directly, without slip rings and brushes. But since the exciter is an alternating current generator, and the excitation winding must be supplied with direct current, then a semiconductor converter is turned on at the output of the armature winding of the exciter 3, fixed on the shaft of the synchronous machine and rotating together with the excitation winding of the synchronous machine and the armature winding of the exciter. DC supply of the field winding 1 the exciter is carried out from the subexciter (PV) - a direct current generator.
Rice. 82. Contact (a) and non-contact (b) systems of electromagnetic
excitation of synchronous generators
The absence of sliding contacts in the excitation circuit of a synchronous machine makes it possible to increase its operational reliability and increase efficiency.
In synchronous generators, including hydrogenerators, the principle self-excitation(Fig. 83, a), when the AC energy necessary for excitation is taken from the stator winding of the synchronous generator and is converted into DC energy through a step-down transformer and a rectifier semiconductor converter (PP). The principle of self-excitation is based on the fact that the initial excitation of the generator occurs due to the residual magnetism of the magnetic circuit of the machine.
Rice. 83. The principle of self-excitation of synchronous generators
On fig. 19.2, b structural diagram of an automatic self-excitation system a synchronous generator (SG) with a rectifier transformer (VT) and a thyristor converter (TP), through which AC power from the SG stator circuit, after being converted to direct current, is fed into the excitation winding. The thyristor converter is controlled by automatic regulator excitation of the ARV, the input of which receives voltage signals at the output of the SG (through the voltage transformer VT) and the load current of the SG (from the current transformer TT). The circuit contains a protection unit BZ, which provides protection of the excitation winding and the thyristor converter of the TP from overvoltage and current overload.
In modern synchronous motors, excitation is used thyristor exciters, included in the alternating current network and carrying out automatic control of the excitation current in all possible modes of operation of the engine, including transient ones. This excitation method is the most reliable and economical, since the efficiency of thyristor exciters is higher than that of DC generators. The industry produces thyristor exciter devices for various excitation voltages with valid value DC 320 A.
Excitatory thyristor devices of types TE8-320/48 (excitation voltage 48 V) and TE8-320/75 (excitation voltage 75 V) are most widely used in modern series of synchronous motors.
The power spent on excitation is usually from 0.2 to 5% useful power machines (lower value applies to machines of high power).
In synchronous machines of low power, the principle is applied excitation by permanent magnets, when permanent magnets are placed on the rotor of the machine. This method of excitation makes it possible to save the machine from the excitation winding. As a result, the design of the machine becomes simpler, more economical and more reliable. However, due to the scarcity of materials for the manufacture of permanent magnets with a large supply of magnetic energy and the complexity of their processing, the use of excitation by permanent magnets is limited only to machines with a power of no more than a few kilowatts.
test questions
1. What are the ways to excite synchronous machines?
2. Explain the purpose of a thyristor converter in the self-excitation system of a synchronous generator?
3. Explain the device of salient and implicitly plus rotors?
4. Explain the design of a synchronous motor of the SDN2 series?
5. What methods of fixing the poles are used in synchronous salient-pole machines?
6. What provides an uneven air gap in a synchronous machine?