R A Z D E L 2
Job 2
STUDYING THE DESIGN AND PROTECTIVE CHARACTERISTICS OF FUSES
Goal of the work
Study the design and marking of the main types of fuses with a fuse link used to protect electrical circuits and installations in agricultural production.
Understand the methodology for calculating and selecting fuses.
Assignment for work
1. Using the guidelines and the set of fuses, study the design and marking of fuses.
2. As instructed by the teacher, calculate the fuse link and select the type of fuse for the electrical installation or distribution network.
General information
Fuses are switching electrical products used to protect the electrical network from overcurrents and short circuit currents. The principle of operation of fuses is based on the destruction of specially designed current-carrying parts (fuse links) inside the device itself when a current flows through them, the value of which exceeds a certain value.
Fuse links are the main element of any fuse. After burning out (cutting off the current), they must be replaced. Inside the fuse link there is a fusible element (it is this that burns out), as well as an arc extinguishing device. The fuse link is most often made of a porcelain or fiber body and is attached to special conductive parts of the fuse. If the fuse is designed for low currents, then the fuse for it may not have a housing, i.e. to be bodyless. The main characteristics of fuse ratings include: rated current, rated voltage, breaking capacity.
Fuse elements also include:
– fuse link holder – a removable element, the main purpose of which is to hold the fuse link;
– fuse contacts – part of the fuse that provides electrical connection between the conductors and the contacts of the fuse;
– the fuse striker is a special element whose task, when the fuse trips, is to influence other devices and contacts of the fuse itself.
All fuses are divided into several dozen types:
– according to the design of fuse links, fuses can be dismountable or non-removable. With collapsible fuses, you can replace the fuse link after it burns out; with non-removable fuses, this cannot be done;
- by the presence of filler. There are fuses with and without filler;
– according to the design of manufacturing fuse links. There are fuses with blade, bolt and flange contacts;
– according to the body of the fuse link, fuses are divided into tubular and prismatic. In the first type of fuses, the fuse link has a cylindrical shape, in the second type it has the shape of a rectangular parallelepiped;
– by type of fuse-links depending on the range of shutdown currents. There are fuses with a breaking capacity in the full range of shutdown currents - g and with a breaking capacity in part of the range of shutdown currents - a;
- in terms of speed. There are slow-acting fuses (used in most cases in transformers, cables, electrical machines) and high-speed fuses (used in semiconductor devices);
– according to the design of the base, fuses can be with a calibrated base (in such fuses it will not be possible to install a fuse-link designed to work with a rated current greater than the fuse itself) and with an uncalibrated base (in such fuses it is possible to install a fuse-link whose rated current is greater rated current of the fuse itself);
– according to voltage, fuses are divided into low-voltage and high-voltage;
– by the number of poles. There are one-, two-, three-pole fuses;
– by the presence and absence of free contacts. There are fuses with and without free contacts;
– based on the presence of a striker and a trip indicator, fuses are classified as: without a striker and without an indicator, with an indicator without a striker, with a striker without an indicator, with an indicator and a striker;
– according to the method of fastening the conductors, fuses are divided into: with front connection, with rear connection, with universal connection (both rear and front);
– according to the installation method. There are fuses on their own base and without it.
Historically, the mechanical design of fuse boxes and their overall and connection dimensions have varied from country to country. There are four main national standards for fuse mounting sizes: North American, German, British and French. There are also a number of fuse housings that are the same from country to country and are not national standards. Most often, such cases relate to the standards of the manufacturer that developed a specific type of device, which turned out to be successful and gained a foothold in the market. In recent decades, as part of the globalization of the economy, manufacturers have gradually joined the international system of fuse housing standards to simplify the conditions for the interchangeability of devices. When choosing, you should try to use fuses of international standards: IEC 60127, IEC 60269, IEC 60282, IEC 60470, IEC60549, IEC 60644.
It should be noted that according to the type of fuse-links, depending on the range of shutdown currents and operating speed, fuses are divided into usage classes. In this case, the first letter indicates the functional class, and the second indicates the object to be protected:
1st letter:
a – protection with breaking capacity in part of the range (accompanied fuses): fuse links capable of at least long-term passing currents not exceeding the rated current specified for them, and disconnecting currents of a certain multiple relative to the rated current up to the rated breaking capacity;
g – protection with breaking capacity over the entire range (general purpose fuses): fuse links capable of at least long-term passing currents not exceeding the rated current specified for them, and disconnecting currents from the minimum melting current to the rated breaking capacity.
2nd letter:
G – protection of cables and wires;
M – protection of switching devices/motors;
R – protection of semiconductors/thyristors;
L – protection of cables and wires (in accordance with the old, no longer valid DIN VDE standard);
Tr – transformer protection.
A general view of the time-current characteristics of fuses of the main categories of use is shown in Fig. 2.1.
Fuse links with the following usage classes provide:
gG (DIN VDE/IEC) – protection of cables and wires over the entire range;
aM (DIN VDE/IEC) – protection of switching devices in part of the range;
aR (DIN VDE/IEC) – protection of semiconductors in part of the range;
gR (DIN VDE/IEC) – protection of semiconductors over the entire range;
gS (DIN VDE/IEC) – protection of semiconductors, as well as cables and lines over the entire range.
Fuses with breaking capacity over the entire range (gG, gR, gS) reliably switch off both short-circuit currents and overloads.
Fuses with partial breaking capacity (aM, aR) serve exclusively for short-circuit protection.
To protect installations for voltages up to 1000 V, electric, tubular and open (plate) fuses are used.
The electrical fuse consists of a porcelain body and a plug with a fuse link. The supply line is connected to the fuse contact, the outgoing line to the screw thread. In the event of a short circuit or overload, the fuse link burns out and the current in the circuit stops. The following types of electrical fuses are used: Ts-14 for current up to 10 A and voltage 250 V with a rectangular base; Ts-27 for current up to 20 A and voltage 500 V with a rectangular or square base and Ts-33 for current up to 60 A and voltage 500 V with a rectangular or square base.
For example: electrical threaded fuses of the PRS series are designed to protect against overloads and short circuits of electrical equipment and networks. The rated voltage of the fuses is 380 VAC, 50 or 60 Hz. Structurally, PRS fuses (Fig. 2.2) consist of a body, a fuse-link PVD, a head, a base, a cover, and a central contact.
PRS fuses are produced for rated fuse-link currents from 6 to 100 A. The designation of the fuse indicates which connection it is: PRS-6-P – 6 A fuse, front wire connection; PRS-6-Z – 6A fuse, rear wire connection.
Cylindrical fuses PTSU-6 and PTSU-20 with a threaded base Ts-27 and fuse-links for currents of 1, 2, 4, 6, 10, 15, 20 amperes are produced in a plastic case. PD fuses have a porcelain base, and PDS fuses have a base material - steatite. In domestic conditions, automatic plug fuses are used, where the protected circuit is restored by a button.
Tubular fuses are produced in the following types: PR-2, NPN and PN-2. PR-2 fuses (dismountable fuse) are designed for installation in networks with voltages up to 500 V and currents of 15, 60, 100, 200, 400, 600 and 1000 A.
In the fuse holder PR-2 (Fig. 2.3), the fuse link 5, attached with screws 6 to the contact blades 1, is placed in a fiber tube 4, onto which threaded bushings 3 are mounted. Brass caps 2 are screwed onto them, securing the contact knives, which fit into fixed spring contacts installed on the insulating plate.
Under the influence of an electric arc that occurs when a fuse blows, the inner surface of the fiber tube decomposes and gases are formed that help quickly extinguish the arc.
Closed fuses with fine-grained filler include fuses of the NPN, NPR, PN2, PN-R, and KP types. Fuses of the NPN type (filled, non-removable fuse) have a glass tube. The rest have porcelain pipes. NPN type fuses are cylindrical in shape, PN type are rectangular.
The NPN fuse set consists of: fuse link – 1 piece; contact bases – 2 pcs.
NPN fuses are manufactured for voltages up to 500V and currents from 15 to 60 A, fuses PN2 (bulk fuse, collapsible) - for voltages up to 500 V and currents from 10 to 600 A. Bulk fuses have fuse links made of several parallel copper or silver-plated wires , placed in a closed porcelain cartridge filled with quartz sand. Quartz sand promotes intensive cooling and deionization of gases produced during arc combustion. Since the tubes are closed, splashes of molten metal from the fuse links and ionized gases are not emitted outside. This reduces fire hazards and increases the safety of fuse servicing. Fuses with filler, like PR type fuses, are current-limiting.
Open plate fuses consist of copper or brass plates - tips into which calibrated copper wires are soldered. The tips are connected to the contacts on the insulators using bolts.
NPR type fuses are a closed, collapsible (porcelain) cartridge filled with quartz sand for rated currents up to 400 A.
PD fuses (PDS) - 1, 2, 3, 4, 5 – with filler for installation directly on busbars for currents from 10 to 600 A.
To protect power valves of semiconductor converters of medium and high power during external and internal short circuits, high-speed fuses are widely used, which are the cheapest means of protection. They consist of contact blades and a silver foil fusible link placed in a closed porcelain socket.
The fuse link of such fuses has narrow calibrated isthmuses, which are equipped with radiators made of a ceramic material that conducts heat well, through which heat is transferred to the fuse body. These radiators also serve as arc-extinguishing chambers with a narrow slot, which significantly improves the extinction of the arc that occurs in the isthmus region. A signal cartridge is installed parallel to the fuse-link, the blinker of which signals the melting of the fuse-link and, acting on the microswitch, closes the signal contacts.
For a long time, the industry produced two types of high-speed fuses designed to protect converters with power semiconductor valves from short-circuit currents:
1) fuses of the PNB-5 type (Fig. 2.4, a) for operation in circuits with a rated voltage of up to 660 V DC and AC for rated currents of 40, 63, 100, 160, 250, 315, 400, 500 and 630 A;
2) PBV type fuses for operation in alternating current circuits with a frequency of 50 Hz with a rated voltage of 380 V for rated currents from 63 to 630 A.
galvanic coating (high conductivity and durability).
The fuse housing is made of high-strength ultra-porcelain. The design of the fuse allows the use of additional devices - trip indicator, free contact.
Structure of the symbol for fuses PNB7-400/100-X1-X2:
PNB-7 – series designation;
400 – rated voltage, V;
100 – rated current;
X1 – symbol of the type of installation and type of connection of conductors to the terminals: 2 – on its own insulating base with base contacts; 5 – on the bases of complete devices with base contacts; 8 – without base, without contacts (fuse link);
X2 – symbol for the presence of an operation indicator: 0 – without alarm; 1 – with striker and free contact; 2 – with operation indicator; 3 – with striker.
Industrial fuses of the PP series are designed to protect electrical equipment of industrial installations and electrical circuits from overloads and short circuits.
Fuses of this series are produced in the following main types: PP17, PP32, PP57, PP60S. Fuses are manufactured with a trip indicator, with a trip indicator and free contact, or without signaling. Depending on the type, fuses are designed for voltages up to 690 V and rated currents from 20 A to 1000 A. Design features allow the installation of free contacts, normally open or closed, as well as the installation method - on their own base, on the base of complete devices, on conductors of complete devices .
Designation structure for fuses of types PP17 and PP32 – Х1Х2 – Х3 – Х4 – ХХХХ:
1) X1X2 – size designation (rated current, A): 31 –100A; 35 – 250A; 37 – 400A; 39 – 630A.
2) X3 – symbol of the type of installation and type of connection: 2 – on its own base, 5 – on the base of complete devices, 7 – on conductors of complete devices (bolt connection), 8 – without a base (fuse link), 9 – without a base ( The fuse link is unified in size with fuses PN2-100 and PN2-250).
3) X4 – symbol for the presence of a trigger indicator, striker, free contact: 0 – without signaling, 1 – with striker and free contact, 2 – with trigger indicator, 3 – with striker.
4) ХХХХ – climatic version: UHL, T and placement category 2, 3.
Currently, semiconductor converters are equipped with fuses of the PP57 (Fig. 2.5, a) and PP60S (Fig. 2.5, b) series.
The first are designed to protect converter units during internal short circuits of alternating and direct current at voltages of 220 - 2000 V for currents of 100, 250, 400, 630 and 800 A. The second - for internal short circuits of alternating current at voltages of 690 V for currents of 400, 630 , 800 and 1000 A.
Designation structure for fuses type PP57 – ABCD – EF:
Letters PP – fuse;
The two-digit number 57 is the conditional series number;
A – two-digit number – symbol of the rated current of the fuse;
B – number – symbol of the rated voltage of the fuse;
C – number – symbol according to the installation method and type of connection of conductors to the fuse terminals (for example, 7 – on the conductors of the converter device – bolted with angled terminals);
D – number – symbol indicating the presence of an operation indicator and an auxiliary circuit contact: 0 – without an operation indicator, without an auxiliary circuit contact; 1 – with operation indicator, with auxiliary circuit contact; 2 – with operation indicator, without auxiliary circuit contact;
E – letter – symbol of climatic version;
An example of a fuse symbol: PP57-37971-UZ.
PPN fuses are intended to protect cable lines and industrial electrical installations from overload and short circuit currents. The fuses are used in AC electrical networks with a frequency of 50 Hz with voltages up to 660 V and are installed in low-voltage complete devices, for example, in ShchO-70 distribution panels, VRU1 input-distribution devices, ShRS1 power distribution cabinets, etc.
Advantages of PPN fuses: 1) the fuse body and the base of the holder are made of ceramics; 2) the fuse and holder contacts are made of electrical copper; 3) the fuse housing is filled with fine quartz sand; 4) overall dimensions of fuses are ~15% smaller than PN-2 fuses; 5) power losses are ~40% less than those of PN-2 fuses; 6) presence of an operation indicator; 7) fuses are mounted and removed using a universal puller.
The design features of the PPN series fuses are shown in Fig. 2.6.
Fuses of the PPNI series (Fig. 2.7) for general use are designed to protect industrial electrical installations and cable lines from overload and short circuit and are available for rated currents from 2 to 630 A.
Used in single-phase and three-phase networks with voltages up to 660 V, frequency 50 Hz. Areas of application of PPNI fuses: input distribution devices (IDU); cabinets and distribution points (ShRS, ShR, PR); equipment of transformer substations (KSO, ShchO); low voltage cabinets (ShR-NN); control cabinets and boxes.
Due to the use of high-quality modern materials and a new design, PPNI fuses have reduced power losses compared to PN-2 fuses. The data presented in table. 2.1 show the efficiency of PPNI fuses compared to PN-2.
Table 2.1
Fuse selection example
For a rectifier valve group in a six-pulse bridge circuit, whose rated direct current is I d = 850 A, it is necessary to select fuse links in the branches. Fuse selection is given for the four typical loads listed above.
Rectifier valve group parameters:
– supply voltage
U N = 3 AC 50 Hz 400 V,
– restoring voltage
U W = 360 V = U N 0.9 (when the inverter stalls,
– thyristor T 508N (Eupec)
ultimate load integral ∫I²dt = 320·103 A2s (10 ms, cold),
– safety inserts with natural cooling, ambient temperature tu = +35°C
– connection cross-section for safety links, copper: 160 mm 2 ,
– effective value of the branch current (operating current of the fuse) I La = I d ·0.58.
Direct current I d = 850 A
I eff =I La = I d 0.58 = 493 A
Full Joule integral
∫I² tA = 360 103 0.53 = 191 103 A2s
In accordance with the nomograms given in, it is necessary to apply the following correction factors:
k u = 1.02 (tu = +35°C),
Required rated current I P of the fuse
I Р = I La ·(1/ k u · k q · k l · k i · k WL) = 493 ·(1/1.02·0.91·1.0·1.0·1.0) = 531 A
Test: 560 A > 531 A
Unknown variable load with known maximum current I MAX
I eff = I MAX = 435 A
Full Joule integral
∫I² tA = 260 103 0.53 = 138 103 A2s
Control cross-section: 400 mm 2
k u = 1.02 (tu = +35°C),
k q = 0.91 (connection cross-section on both sides 40% of the control cross-section),
k l = 1.0 (current cut-off angle l=120°),
k i = 1.0 (no intensive air cooling)
Required rated current IP fuse
I Р = ILa ·(1/ k u · k q · k l · k i · k WL) = 435 ·(1/1.02·0.91·1.0·1.0·1.0) = 469 A
Test: 560 A > 469 A
Variable load with a known load cycle.
D.C:
I d1 = 1200 A, t 1 = 20 s (Fig. 2.14),
I d2 = 500 A, t 2 = 240 s,
I d3 = 1000 A, t 3 = 10 s,
I d4 = 0 A, t4 = 60 s.
Current flowing through the fuse:
I La1 = 1200 0.58 = 696 A (Fig. 2.14),
I La2 = 500 0.58 = 290 A,
I La3 = 1000 0.58 = 580 A,
I La4 = 0 0.58 = 0 A.
Effective operating current
| |
Full Joule integral
∫I² tA = 175 103 0.53 = 93 103 A2s
Reference cross-section: 320 mm 2
We apply the following correction factors:
k u = 1.02 (tu = +35°C),
k q = 0.94 (connection cross-section on both sides 50% of the control cross-section),
k l = 1.0 (current cut-off angle l=120°),
k i = 1.0 (no intensive air cooling)
I Р = I eff ·(1/ k u · k q · k l · k i · k WL) = 317 ·(1/1.02·0.94·1.0·1.0·1.0) = 331 A
Test: 450 A > 331 A
I Р / = k u · k q · k l · k i · k WL · I Р =1.02·0.94·1.0·1.0·1.0·450 = 431 A
2. Checking the permissible duration of overload with current blocks that exceed the permissible operating current of the fuse I P /.
V = I eff / I Р / = 317/431 = 0.74
From the curve k RW1 = f (V) (Fig. 11) we determine the value of k RW1 for V = 0.74, we have k RW1 = 0.2
We determine the reduced duration of permissible load t SC for the corresponding current block by the expression:
t SC = k RW1 t S , (2.15)
where t S is the melting time of the insert for currents I La1 and I La3 flowing through the fuse (from the time-current characteristic for 3NE3 333).
We have: t S1 = 230 s, t S3 = 1200 s.
Then t S1С = k RW1 t S1 = 0.2 230 = 46 s,
t S3С = k RW1 t S3 = 0.2 1200 = 240 s
Check: t S1С = 46 s > t 1 = 20 s
t S3С = 240 s > t 3 = 10 s
Random shock load from preload with unknown shock pulse sequence
I eff = I vor , (2.16)
where I vor is the preload current (Fig. 2.15),
I Stoss – overload current,
t Stoss – duration of overload (t Stoss = 8 s).
Direct current: Current flowing through the fuse:
I dvor = 700 A I vor = I dvor 0.58 = 406 A
I dStoss = 1750 A I Stoss = I dStoss 0.58 = 1015 A
The frequency and duration of shock load pulses must satisfy the following conditions - t pausa ³ 3 · t Stoss and t pausa ³ 5 min.
Full Joule integral
∫I² tA = 360 103 0.53 = 191 103 A2s
Control cross-section: 400 mm 2
We apply the following correction factors:
k u = 1.02 (t u = +35°С),
k q = 0.91 (connection cross-section on both sides 40% of the control cross-section),
k l = 1.0 (current cut-off angle l=120°),
k i = 1.0 (no intensive air cooling)
1. Required rated current I P of the fuse
I Р = I vor ·(1/ k u · k q · k l · k i · k WL) = 406 ·(1/1.02·0.91·1.0·1.0·1.0) = 437 A
Test: 450 A > 437 A
Permissible operating current I P / selected fuse link:
I Р / = k u · k q · k l · k i · k WL · I Р =1.02·0.91·1.0·1.0·1.0·560 = 520 A
2. Checking the permissible duration of overload with peak current I Stoss.
Preliminary load factor:
V = I vor /I Р / = 406/520 = 0.78
From the curve k RW1 = f (V) (Fig. 2.11) we determine the value of k RW1 for V = 0.78, we have k RW1 = 0.18
We determine the reduced duration of permissible load t SC for shock current using the expression:
t SC = k RW1 t S , (2.17)
where t S is the melting time of the insert for a shock current I Stoss = 1015 A flowing through the fuse (from the time-current characteristic for 3NE3 333).
We have: t S = 110 s.
Then t SC = k RW1 tS = 0.18 110 = 19.8 s
Check: t SC = 19.8 s > t Stoss = 8 s
1. Name and purpose of the work.
2. The main types of fuses used to protect electrical installations and electrical circuits.
3. Calculation and selection of fuse according to individual specifications.
4. Answers to security questions.
Control questions
1. What are the design characteristics of fuses?
2. Explain the designation of fuses.
3. Describe the design of the PR-2 fuse.
4. Describe the design of the NPR fuse.
5. Describe the design of the PNB fuse.
6. What is the difference between PN fuses and PNB-7?
7. Scope of application of fuses PP57 and PP60S.
8. Scope of application of PPNI fuses.
9. What is the difference between PPNI fuses and PN-2?
10. How is the fuse-link current calculated for different loads?
11. What is selectivity of protection?
12. What is the time-current characteristic of a fuse?
13. What advantages do PPNI type fuses have over other types of fuses?
14. How to ensure selectivity of series-connected fuse-links?
15. How is the contact of the fuse blade contacts with the rack jaws checked?
Bibliography
1. Rules for electrical installations [Text]: All current sections of PUE-6 and PUE-7. Novosibirsk: Normatika, 2013. – 464 p., ill.
2. Installation of electrical equipment and automation equipment: a textbook for universities / I.R. Vladykin, A.P. Kolomiets, N.P. Kondratyeva, S.I. Yuran. – M.: Publishing house "KolosS", 2007.
3. Sibikin Yu.D. Installation, operation and repair of electrical equipment of industrial enterprises and installations: Textbook. manual for prof. textbook establishments / Yu.D. Sibikin, M.Yu. Sibikin. – M.: Higher. school, 2003.
4. Akimova N.A. Installation, technical operation and repair of electrical and electromechanical equipment: textbook. allowance / N.A. Akimova, N.F. Kotelenets, N.I. Sentyurikhin; edited by N.F. Kotelenza. – 3rd ed., stereotype. – M.: Academy, 2005
5. Kostenko E.M. Installation, maintenance and repair of industrial and household electrical equipment: practical work. manual for electricians / E.M. Kostenko. – M.: Publishing house NC ENAS, 2005.
6. EKF electrotechnica [Official website] Url: http://ekfgroup.com/produktsiya (accessed September 01, 2014).
7. KEAZ - Kursk Electrical Equipment Plant [Official website] Url: http://keaz.ru (accessed September 01, 2014).
8. IEK – Inter electro kit [Official website] Url: http://www.iek.ru (accessed September 1, 2014).
9. Siemens - Electrical products [Official website] Url: http://electrosiemens.ru (accessed September 1, 2014).
R A Z D E L 2
LOW VOLTAGE ELECTRICAL APPARATUS
Electrical devices are called electrical devices for managing energy and information flows, operating modes, monitoring and protecting technical systems and their components. Electrical devices, depending on the element base and operating principle, are divided into electromechanical and static.
TO electromechanical devices These include technical devices in which electrical energy is converted into mechanical or mechanical energy into electrical energy.
Electromechanical devices used in almost all automated systems. Some systems are built entirely on electromechanical devices. For example, automation circuits for starting, reversing and braking in an unregulated electric drive consist mainly of electromechanical devices such as relays and contactors. Electromechanical devices are used as sensors, amplifiers, relays, actuators, etc. The input and output quantities of these devices can be either mechanical or electrical. However, they must necessarily carry out the mutual conversion of mechanical energy into electrical energy and vice versa.
Static devices are performed on the basis of electronic components (diodes, thyristors, transistors, etc.), as well as controlled electromagnetic devices in which the input and output are connected through a magnetic field in a ferromagnetic core. Examples of such devices are a conventional transformer made of electrical steel and a magnetic amplifier.
The basis for the functioning of most types of electrical devices (circuit breakers, contactors, relays, control buttons, toggle switches, switches, fuses, etc.) are switching processes (switching on and off) of electrical circuits.
Another large group of electrical devices designed to control operating modes and protect electromechanical systems and components consists of regulators and stabilizers of electrical energy parameters (current, voltage, power, frequency, etc.). Electrical devices of this group operate on the basis of continuous or pulsed changes in the conductivity of electrical circuits.
Let's look at some types of electrical devices.
Contactor is an electrical device designed for switching power electrical circuits both at rated currents and at overload currents.
Magnetic switch is an electrical device designed to start, stop, reverse and protect electric motors. Its only difference from a contactor is the presence of a protection device (usually a thermal relay) against thermal overloads.
The uninterrupted operation of asynchronous motors largely depends on the reliability of the starters. Therefore, high demands are placed on them in terms of wear resistance, switching ability, precise operation, reliability of motor overload protection, and minimal power consumption.
In crane mechanisms, controllers that control low- and medium-power motors and command controllers (high-power motors) are widely used.
Controller is a device with the help of which the necessary switching is carried out in the circuits of AC and DC motors. Switching is carried out manually by turning the flywheel.
Command controller the operating principle is no different from the controller, but has a lighter contact system designed for switching in control circuits.
Relay An electrical device is called in which, with a smooth change in the control (input) quantity, an abrupt change in the controlled (output) quantity occurs.
Electromagnetic relays are widely used in various automated electric drive systems. They are used as current and voltage sensors, time sensors, for transmitting commands and multiplying signals in electrical circuits. They are used as actuators in sensors of technological parameters of various machines and mechanisms.
Magnetic contact (reed switch) is a contact that changes the state of an electrical circuit by mechanically closing or opening it when a control magnetic field is applied to its elements. Reed switches have increased speed and, due to their design features, operational reliability, which is why they are widely used in automatic systems. On their basis, relays for various purposes, sensors, buttons, etc. are created.
Actuator- this is a device that moves the executive body or exerts a force on this body in accordance with specified functions and when appropriate signals are supplied to the control windings. Most often, electromechanical actuators are used to convert an electrical signal into movement of the moving part of the device. Examples are solenoid valves, solenoid couplings, solenoid latches, gate valves, etc.
All elements of the devices have established graphic images and names, some of which are given in table.
Symbols of device elements
| Name | Designation |
| Push-button switch: with normally open contact | |
| with break contact | |
| Single pole switch | |
| Switching device contact: normally open | |
| opening | |
| switching | |
| Contact for switching high-current circuit: normally open | |
| opening | |
| closing arc extinguishing | |
| breaking arc extinguishing | |
| Normally closed contact with a retarder that operates when triggered | |
| Electrical relay with normally open, normally closed and switching contacts |  |
The position of the contacts of the devices shown on the control diagrams, in the absence of external influence, corresponds to their normal state. Device contacts are divided into making, breaking and switching. In electric drive control circuits, a distinction is made between power or main circuits through which electric current is supplied to electric motors, as well as auxiliary circuits, which include control, protection and alarm circuits.
Electric drives of pumps,
Fans, compressors
In modern technology, a large class consists of machines designed to supply liquids and gases, which are divided into pumps, fans and compressors. The main parameters characterizing the operation of such machines are the flow (performance), pressure and pressure they create, as well as the energy imparted to the flow by their working parts.
Typically, these electric drive systems are divided into several groups:
1) Pumps, fans, compressors of the centrifugal type, the static power on the shaft of which varies in proportion to the cube of the speed, if no-load losses can be neglected and there is no back pressure, i.e. these are mechanisms with the so-called fan characteristic. This is the most common group;
2) Various piston-type pumps and compressors, the shaft power of which varies sinusoidally depending on the angle of rotation of the crank. For single-acting piston pumps, supply occurs only when the piston moves forward; during reverse stroke, there is no supply;
3) Various double-acting piston-type pumps and compressors. Feed is carried out when the piston moves in both directions.
Adjustable electric drive of mechanisms with fan torque
In installations requiring smooth and automatic feed control, the electric drive is used adjustable.
The characteristics of centrifugal-type mechanisms create favorable operating conditions for an adjustable electric drive both in terms of static loads and the required speed control range. Indeed, as the speed decreases, at least quadratically, the moment of resistance on the motor shaft also decreases. This facilitates the thermal regime of the engine when operating at reduced speed. From the laws of proportionality it follows that the required range of speed control in the absence of static pressure 
does not exceed the specified feed change range
If the static head is not zero, then to change the flow from zero to the nominal value 
speed control range required
where is the pressure developed by the mechanism at .
On average, for adjustable centrifugal-type mechanisms, the required speed control range usually does not exceed 2:1. The noted features of these mechanisms and low requirements for the rigidity of mechanical characteristics make it possible to successfully use simple schemes of a controlled asynchronous electric drive for them.
For low-power installations (7...10 kW), the problem is solved using a voltage regulator system - an asynchronous motor with a squirrel-cage rotor. Thyristor switches are most often used as voltage regulators. Such systems have found application in fan equipment complexes designed to ensure the required air exchange and create the necessary temperature conditions in livestock and poultry buildings in accordance with veterinary standards.
In installations where the operating conditions allow the use of an asynchronous motor with a wound rotor, the capabilities of a controlled electric drive are expanded. The mechanical characteristics of this drive ensure stable operation over a fairly wide speed range with an open-loop electric drive system.
In some cases, speed control of mechanisms driven by asynchronous or synchronous motors is used. In this case, a fluid coupling or asynchronous slip clutch is installed between the engines and the production mechanism, which allows you to change the speed of the production mechanism without changing the speed of the engine.
For example, consider Electrical diagram of fan installation automation.
Control circuit for an asynchronous squirrel-cage motor M a fan located in the machine room and designed for independent ventilation of large electrical machines is shown in Fig. 4.13. The fan is controlled from the control panel using a control key K1 , having four contacts and a self-resetting handle. Key K2 serves to allow or prohibit the inclusion of the fan at the installation site when there is no need for its operation.
The scheme works as follows. Key K2 set to position R (allowed). The machine turns on AT 2 control circuits and automatic IN 1 main circuits (its contact in the starter self-locking circuit closes). The green lamp lights up L3 (engine switched off). To start the engine M key K1 moves from zero position 0 to starting position P . this turns on the magnetic starter TO, It is placed on self-power and uses the main contacts to connect the motors to the network. Green lamp LZ goes out, red light OK lights up - the engine is on.
Key handle K1 is released and the key returns to the zero position, at which the contact 2 the key closes and the contact 1 remains closed.
The diagram provides for testing the fan at the installation site using a button KnO . Blocking is also provided (using a normally open block contact TO ), which does not allow the ventilated machine to be turned on before the fan starts. Protection against short circuits or motor overload M carried out automatically IN 1 with combined release. And zero protection - by starter TO (restarting the engine is not possible until the key handle K1 will not be placed in the starting position P) . When the fan is turned off as a result of the protection, a warning signal is activated, since the contacts 3 And 4 key K1 while closed. When manually turning off the fan by moving and then releasing the key handle K1 pregnant WITH no warning signal is given because the contact is open 4 .
Electrical Basics
Electricity supplycalled the generation, transmission and distribution of electrical energy between consumers.
The generation of electrical energy is created by electrical stations. Almost all industrial power plants have a synchronous three-phase sinusoidal voltage generator as their final element. As the unit power of the generator increases, its efficiency increases, which is why modern stations have generators of very high power.
Electrical stations can be classified as follows:
thermal, hydraulic, nuclear, wind power plants, solar power plants, geothermal, tidal, etc. more common than others thermal power plants, which burn coal, peat, gas, oil, etc. These stations generate electrical energy with an efficiency of about 40%. Thermal stations pollute the air due to incomplete combustion of fuel and insufficient filtration of exhaust gases.
Hydraulic stations use the energy of water flow. Such stations produce significantly cheaper electrical energy. A high-capacity hydroelectric power plant has an efficiency approaching 90%. Hydraulic stations disrupt the water balance of rivers and also worsen the environment.
Nuclear power plants convert the fission energy of the atomic nucleus into electrical energy. The efficiency of a nuclear power plant reactor is 25…35%. In the event of an accident at a nuclear power plant, there is a threat of radiation contamination of the environment.
The operation of any source of electrical energy may cause environmental disturbances. Therefore, in developed countries, much attention is paid to the technology of generating electrical energy. Using modern technology, some countries safely generate over 60% of their electricity from nuclear power plants.
The use of wind and solar power plants begins. Low power electricity is provided by geothermal (in Kamchatka) and tidal (on the Kola Peninsula) stations.
Synchronous generators of power plants induce a three-phase sinusoidal EMF of 18 kV. To reduce losses in power lines at step-up substations, the voltage is transformed to 110 and 330 kV and supplied to the Unified Energy System. Losses in transmission lines are proportional to the square of the current, so electricity is transported at increased voltage and decreased current.
Power lines There are overhead and cable. Overhead power lines (power lines) are much cheaper than cable (underground) ones and are therefore more widely used. Power lines are connected to transformers with special high-voltage switching devices.
Typically, industrial enterprises consume electrical energy at a voltage of 380 V. Therefore, distribution points and transformer substations are installed in front of the consumer, reducing the voltage to 6...10 kV and 380...220 V.
There are three main schemes for power supply to consumers: radial, main, mixed.
Radial power supply circuit provides for the use of a transformer substation for each consumer. This is a very reliable power supply scheme, but requires a large number of substations.
Trunk circuit provides only a few substations that are included in the power transmission line. Many consumers are connected to each substation.
Mixed scheme provides sections with radial and main inclusions. Consumers are connected differentially. This scheme is used more often.
The power supply circuit of an autonomous energy unit can be quite original. Features of power supply depend on the functional tasks of actuators, operating conditions, special requirements regarding weight, dimensions, efficiency of electrical devices, etc.
Power supply for industrial enterprises. About two thirds of all electricity is consumed by industry. The power supply scheme for industrial enterprises is built on a stepwise principle, the number of steps depends on the power of the enterprise and the layout of individual electricity consumers. At the first stage, the power system voltage is supplied to the main substation, where it is reduced from 110-220 kV to 10 -6 kV. Second-stage networks supply this voltage to workshop transformer substations, where it is reduced to the consumer voltage. The third stage consists of networks that distribute the voltage of the workshop substation between individual consumers.
In large enterprises with high electricity consumption, consumers can be powered at a voltage of 660 V. Most enterprises use three-phase networks 380/220 V. In areas with increased danger, the permissible supply voltage for consumers should not exceed 36 V. In particularly hazardous conditions (boilers, metal tanks ) – 12 V.
According to the required power supply reliability, electrical energy consumers are divided into three categories. The first category includes those consumers whose interruption in the supply of electricity is associated with danger to people or entails great material damage (blast furnace shops, industrial steam boiler houses, lifting and ventilation installations of mines, emergency lighting, etc.) they must work continuously. For consumers of the second category (the most numerous), food breaks are allowed for a limited time. Consumers of the third category include auxiliary workshops and other facilities for which a break in power supply is allowed for up to one day.
To increase the reliability of power supply, consumers are supplied from two independent networks and an automatically switched on backup power source. There are “hot” and “cold” backup sources. A “hot” backup source provides immediate emergency power and is used for trouble-free shutdown of the consumer.
Further improvement of power supply systems for industrial enterprises is associated with an increase in supply voltage (from 220 to 380 V, from 6 to 10 kV, etc.) while bringing the high voltage as close as possible to consumers (deep input) and reducing the number of transformation stages.
Wires and cables. Various types of bare wires are used to lay overhead lines. Single-wire steel wires are made with a diameter of no more than 5 mm. The most common are stranded wires, which have high strength and flexibility. They are made from identical wires, the number of which can reach 37. The diameter of the wires and their number are selected in such a way as to ensure the greatest packing density of the wires in the wire. Usually 6, 11, 18 wires are placed around one central one and twisted loosely. Stranded wires are made of steel, aluminum, steel-aluminum and bimetallic wires. In steel-aluminum wires, some of the wires are steel, some are aluminum. This ensures mechanical strength with increased electrical conductivity. Bimetallic wires are produced using the electrolytic method: a steel core is coated with a layer of copper or aluminum.
For electrical wiring indoors, as a rule, insulated wires made of copper or aluminum are used. Insulated single-wire wires have greater rigidity and a cross-sectional area of no more than 10 mm 2.
Stranded wires are made from tinned copper or aluminum conductors. They are convenient for installation and operation.
Electric cables are used to lay hidden unsupported lines, as well as to channel electricity supplied to moving objects. In the cable, the wires of a two or three-phase line are enclosed in a durable hermetically sealed multilayer sheath, which increases the reliability of power lines. Cables can be laid underground and underwater. Underground cables are the main means of channeling electricity in large cities. The disadvantage of cable lines is their high cost.
Electrical Safety Basics
→ Basic definitions
1. Basic definitions and classification of electrical apparatus
1.1. Basic definitions
Electrical devices (EA) are called electrical technical devices for control. flows of energy and information, modes of operation, control and protection of technical systems and their components.
Electrical devices are used for switching, signaling and protecting electrical networks and power receivers, as well as controlling electrical and technological installations and are extremely widely used in various areas of the national economy: in the electric power industry, in industry and transport, in aerospace systems and defense industries, in telecommunications, in public utilities, in household appliances, etc. Moreover, in each of the areas, the range of used range of devices is very wide. It can definitely be said that there is no area related to the use of electrical energy where electrical devices are not used.
The functioning of most types of electrical devices is based on the processes of switching (turning on and off) electrical circuits. The main phenomena that accompany the operation of any electrical apparatus include: processes of switching electrical circuits, electromagnetic and thermal processes. Electromagnetic processes mean electromechanical and induction phenomena, electromagnetic interactions of apparatus elements, etc.
Thermal processes have a direct impact on the operation of the device and depend on the operating mode of the device. Three types of operating modes are established for electrical devices:
- long-term (in this mode, with a long current flow, the device heats up to a steady temperature value);
- short-term (in this mode, when switched off, between separate switching ons, the heating temperature of the device is reduced almost to the ambient temperature);
- repeated-short-term (the heating temperature does not have time to drop to the ambient temperature during the current pause).
The last two modes are characterized by the relative duration of PV switching on, %. Standard PV values: 15; 25; 40; 60%.
1.2. Classification of electrical apparatus
The exceptionally wide range of applications of electrical devices determines the variety of types of their classification.
Electrical devices are classified according to the following criteria:
1) according to the operating voltage - low voltage (up to 1000 V) and high voltage (more than 1000 V);
2) according to the magnitude of the operating or switching current - low-current (control, protection, alarm devices) and high-current used in power circuits;
3) according to the function performed:
- switching devices: switches, disconnectors, contactors, magnetic starters;
- control, protection, alarm: relays of various types, travel and limit switches (contact and non-contact);
- command: control buttons, keys, command controllers and command devices;
- protection devices: arresters, fuses. Electrical devices also include ballast resistances.
Based on switching and element base, electrical devices are divided into:
- electromechanical
- static
- hybrid.
Electromechanical devices are distinguished by the presence of moving parts. Electromechanical devices have movable and fixed contact systems that switch electrical circuits.
Static devices are made on the basis of power semiconductor devices: diodes, thyristors, transistors, as well as controlled electromagnetic devices: magnetic amplifiers, saturation chokes, etc. Devices of this type usually belong to power electronic devices, as they are used to control the flow of electrical energy.
Hybrid electrical devices are a combination of electromechanical and static devices.
According to their functional purpose they are distinguished:
- NI and HV control devices;
- low voltage switchgear devices;
automation devices.
Electrical devices are also classified:
by voltage: LV devices - low (up to 1000 V) and HV devices - high (from units to thousands of kilovolts) voltage;
According to the value of the switched current: low-current devices (up to 5 A) and high-current devices (from 5 A to hundreds of kiloamperes);
by type of current: direct and alternating;
by power source frequency: devices with normal (up to 50 Hz) and devices with increased (from 400 Hz to 10 kHz) frequency;
by the type of functions performed: switching, regulating, monitoring, measuring, limiting current or voltage, stabilizing;
- according to the design of the switching element: contact and non-contact (static), hybrid, synchronous, without arc.
1.3. High voltage devices
High voltage devices are divided into the following types according to their functionality:
- switching devices (switches, load switches, disconnectors);
- measuring devices (current and voltage transformers, voltage dividers);
- limiting devices (fuses, reactors, arresters, nonlinear surge suppressors);
- compensating devices (controlled and uncontrolled shunt reactors);
- complete distribution devices.
Electrical devices also include various types of sensors that have a complete design. The purpose of most sensors related to electrical devices is to convert parameters of physical quantities of different nature into electrical signals of an informational nature. Such sensors are widely used in various automatic control systems.
1.4. Electrical control devices
Electrical control devices are designed to control the operating mode of electrical equipment and are divided into the following types:
- contactors;
- starters;
- controllers;
- electrical control relays;
- command devices;
- switches;
- control electromagnets
- electrically controlled clutches.
Contactors are used for repeated switching on and off of an electrical circuit at load currents not exceeding the rated one, as well as for rare shutdowns at overload currents (usually 7-10 times the rated current). The type of current determines the design features of contactors. Therefore, AC and DC contactors are usually not interchangeable. However, there are contactors that combine the capabilities of switching both direct and alternating currents.
Starters are designed to turn motors on and off and differ from contactors mainly by the presence of a built-in system that protects motors from overload currents.
A controller is a manually controlled electrical device designed to change the connection diagram of an electric motor to the power supply system, as well as to switch transformer windings.
Electric control relays operate in automatic control circuits for electric drives. Switched currents do not exceed 10 A, and therefore arc extinguishing devices are not used in them.
Command devices are designed for switching in control circuits of power electrical devices (contactors, starters).
Switches are designed for almost the entire range of rated currents. Disabling the electrical circuit with a switch is usually done in a de-energized state or at low currents.
Control electromagnets are used in actuators for various industrial purposes, and also as an independent functional unit.
Electrically controlled clutches are designed to transmit a flow of mechanical energy or torque
og the driving part of the coupling to its driven part.
Depending on the type of connection between the master and slave
coupling parts are divided into three main types:
- electromagnetic couplings with mechanical coupling;
- electromagnetic powder couplings;
- induction couplings.
1.5. Switchgear devices
Low voltage switchgear devices (up to 1000 V) are designed to protect electrical equipment from various emergency conditions associated with the occurrence of overload and short circuit currents, unacceptable voltage drops, the occurrence of leakage currents to the ground when insulation is damaged, reverse currents, etc.). These devices are divided into circuit breakers and low-voltage fuses.
Automatic switches (circuit breakers) turn on and off relatively rarely. Automatic devices with different rated currents are capable of disconnecting large short-circuit currents (up to 150 kA). In this case, shutdown occurs with a pronounced current-limiting effect. Automatic machines usually have complex contact-arc devices.
Low-voltage fuses are used to protect electrical equipment from large overload currents and short circuit currents. There are fuses with an open fuse-link, closed (the fuse-link is placed in a socket) and fuses with a filler, which is quartz sand, chalk, etc.
1.6. Electrical automation devices
Electrical automation devices are technical means with the help of which various operations with signals are performed (reception and collection, reading, formation, processing, conversion, addressing, comparison, storage, reproduction, level changes, logical operations, etc.), if at least one of the signals (at the input or output of the device) is electrical.
The corresponding operations with non-electrical or electrical signals are performed in the information processing path.
A signal is information about a material or energy parameter perceived or transmitted by a device. The real parameter is understood as size, density, color, etc. The energy parameter is speed, pressure, temperature, voltage, current, abbr., efficiency.
Signals can be periodic and non-periodic, continuous and discrete.
The information processing path usually includes the following devices:
- primary converters (sensors) that convert the controlled (input, usually non-electrical) quantity into an output electrical signal;
- distributors (switches), distributing information in the form of electrical signals through various communication channels;
- adders, logical elements, regulatory bodies that process information received through various channels (inputs) in the form of electrical signals and generate a command (signal) for actuators;
- executive apparatus.
The last type of devices includes the actual electric automation relays, electric hydraulic valves, electric hydraulic valves, electric valves, magnetic supports and suspensions, valves, etc.
Electrical automation relays are devices for protecting electrical systems, networks and circuits, as well as other objects from unauthorized operating modes; to generate signals notifying about the approach of emergency situations and their occurrence; to enhance, reproduce, process, encode and remember incoming information.
Varieties of electrical automation relays include reed relays, which are based on sealed magnetically controlled contacts (reed switches), as well as relay devices with mechanical control (input) and electrical output: buttons, keys, keyboards, toggle switches, microswitches.
3.1 GENERAL PRINCIPLES OF CREATION OF DC SEMICONDUCTOR DEVICES
The thyristor is switched to a conducting state by applying a control signal with a certain duration and amplitude to its input. After removing the control pulse, the thyristor remains on indefinitely unless the current in its anode circuit decreases to a value less than the holding current I H, therefore, when using thyristors as switching elements, not only for closing, but also for opening DC circuits, it is necessary to resort to artificial measures to ensure short-term interruption of the current in the anode circuit of thyristors or its reduction to values
I A> I H.
In practice, this can be implemented using simple circuits shown in Figure 3.1. In the diagram (in Figure 3.1, A) the load current is switched off by opening the mechanical contact S 1, connected in series with the thyristor VS. After a time sufficient to restore controllability of the thyristor, the contact S 1 may be closed again. In this case, the circuit remains open, since the thyristor is in the off state. The circuit works similarly when the thyristor is short-term bypassed with a closed contact S 2, the connection of which is shown in figure 3.1 with dashed lines.
In both cases, the mechanical contacts carry the full load current and must be rated for it. The disadvantage of such circuits is also that the thyristors in them, when the contacts return to their original state, are exposed to direct voltage with high values du/dt.
Figure 3.1 - Schematic diagrams of thyristor
DC devices
An improved version of the switching device is the circuit shown in Figure 3.1, b. The sequence of its work is as follows. In the initial state, the thyristor is closed, the voltage at the load RH and capacitor C K absent.
The circuit is turned on by a control signal, which must be applied to the input of the thyristor (control electrode-cathode). In this case, simultaneously with the load current IH =U/RH, the capacitor charging current flows through the thyristor C K. The capacitor is charged with the polarity indicated in the figure in a time determined by the circuit time constant τ=R 1 C K.
By subsequent closing of contact S, the capacitor C K, charged almost to the voltage of the power source, is connected in parallel with the thyristor. It starts to discharge. Moreover, the discharge current flows through the thyristor in the direction opposite to the anode current.
If the current exceeds iC anode current I H conditions are created for turning off the thyristor and, consequently, de-energizing the load. This method of turning off the thyristor, called forced (artificial), capacitive, is preferable, since it allows you to reduce the time to restore the controllability of the thyristor and the speed of applying voltage in the forward direction, immediately after switching the current.
In Figure 3.1, V Another diagram of a thyristor device is shown, illustrating the use of capacitive artificial switching. Unlike the diagram in Figure 3.1, b The capacitor C K in it is initially charged to the voltage of the power source. Therefore, when the thyristor is turned on VS with a control pulse, the load current and capacitor discharge current C K begin to flow through it (Figure 3.2). During the second half-cycle of oscillatory recharging of the capacitor, when the current iC, directed counter to the anode current in the thyristor (load current), becomes larger in value, the thyristor turns off (Figure 3.2). From this point in time, the residual voltage on the capacitor C K acts in accordance with the voltage of the power supply, so the load current increases sharply and then decreases as the capacitor is recharged. The final equalization of the current in the circuit occurs at the moment of time t z, which corresponds to the end of the capacitor recharging.
The reverse voltage across the thyristor is maintained for a period of time t c = t 2 - t 1. This time is called circuit time, since it is determined by the parameters of the circuit elements - in this case, the capacitance of the switching capacitor C K and coil inductance LK.
In the considered circuits (except for Figure 3.1), current interruption is provided essentially by traditional contact devices. Therefore, the presence of thyristors in them does not provide any advantages. As for the switching mode, it is carried out by thyristors, and in this case their capabilities in terms of speed, readiness for work, etc. are realized.
The main purpose of such devices is to connect loads with high time accuracy, as well as to change circuit parameters ( R, L, C) for various experimental studies of transient processes, automatic connection of power supplies. Switching device (Figure 3.1, V) along with interrupting the current in the circuit, generates current (power) pulses. This can be used to regulate the output power according to a given program, which is set by the thyristor control system.
Figure 3.2 - Timing diagrams of the circuit operation,
shown in Figure 3.1
The parameters of current pulses (amplitude, duration, shape) can be changed by changing the voltage of the power source and the parameters of the switching circuit elements.
3.2 HIGH SPEED DC THYRISTOR SWITCH
Forced switching (switching off) of thyristors is the basis for the operation of semiconductor DC devices and a means of increasing the speed when switching off AC devices. There are various circuit solutions that provide a short-term reduction in the current in the circuit with thyristors to zero and turn them off. But only capacitor forced switching circuits have found practical application in electrical devices, the principle of operation of which is discussed in the example of Figure 3.1, b and c. Options for constructing forced switching circuits and methods for their calculation are discussed in the works. Here we note that in terms of the structure that determines the connection of the elements of the switching circuit and its connection to the switchable thyristors, the forced switching units in alternating current devices and in direct current devices have significant differences. However, the principle of their operation, tasks and methods for calculating contour elements are common. Therefore, the main tasks of designing such devices are discussed below, using the example of a simple DC switch circuit (Figure 3.3). In terms of the nature of the processes occurring, it practically does not differ from the already discussed scheme in Figure 3.1, b. However, replacing the mechanical contact with an additional thyristor VS2 can significantly improve the switching characteristics of the device and makes it more sensitive to control.
The figure shows that the auxiliary (switching) thyristor VS 2 can be turned on either from the anode voltage (by closing the “Stop” button), or from the voltage taken from the measuring resistor - shunt R w. In the latter case, the voltage on the shunt must exceed a value equal to U=U GT +U F +U C, Where U GT− control voltage sufficient to reliably turn on the thyristor VS 2; U F− voltage drop across the diode VD 2 And U C− stabilization (switching) voltage of the zener diode VD 1.
In emergency operating modes, accompanied by a multiple increase in current relative to the rated one, the circuit is switched off automatically when the thyristor is turned on VS 2. By adjusting the resistance R w and selecting a zener diode according to the parameter U C You can preset the value of the overload current or short circuit current (SC) at which the circuit breaker will trip. Moreover, the high speed of the switch allows you to interrupt the short-circuit current long before the moment when it reaches its maximum value.
In online mode, switching on and off rated currents is carried out by closing the control circuits of the thyristors VS 1 And VS 2, respectively, with the “Start” and “Stop” control buttons. Current limitation in the control circuits of thyristors is carried out by resistors R Y. The operation of the circuit in this mode with an active load is illustrated by timing diagrams in Figure 3.4
Figure 3.3 - Automatic thyristor
switch
To reliably turn off the thyristor VS 1 it is necessary that the circuit time t s, shown on the voltage change graph u VS 1 =f(t), there was more time to turn off the thyristor. Otherwise, the thyristor may again go into a conducting state under the influence of direct voltage, which is applied to it during the process of recharging the capacitor (see also Figure 3.2).
The minimum capacitance of the capacitor to maintain the reverse voltage across the thyristor VS 1 for a time t s, can be determined from the analysis of switching processes occurring immediately after turning on the thyristor VS 2.
Assuming that the turning off capacity of the thyristor VS 1 in the opposite direction is restored instantly (reverse current i R missing), the equation for discharging the capacitor after turning on the thyristor VS 2 let's write it in the form
Where U− power supply voltage ;
i− current through series-connected R n C K , VS 2.
Figure 3.4 - Electromagnetic processes during shutdown
DC switch
The solution to this equation is known:
Capacitor voltage From to, which is also the voltage on the thyristor VS 1, is found by integrating expression (3.1):
At a moment in time t = t 2 - t 1 = t c thyristor voltage VS 1 is equal to zero, and, therefore, from expression (3.2) we obtain
By taking the logarithm of this expression, we can determine the relationship between the capacitance of the capacitor From to and circuit time
Considering that the relationship between resistance R H and current in the switched circuit I K at source voltage U is expressed by the formula U= R H I K, the last equation can be rewritten as follows
Reliable thyristor shutdown VS 1, having a turn-off time equal to t q, will be at t s >> t q k q, Where k q= 1.5...2 – coefficient taking into account the change t q in case of temperature mismatch PN-structure, switching current, reverse voltage and speed of application of forward voltage with classification values. Therefore, the minimum capacitance of the switching capacitor must satisfy the condition
If the load is active-inductive, then to ensure dissipation of the energy stored in the inductive elements at the moment the current is interrupted, it must be shunted with a diode, as shown in Figure 3.3 with a dashed line. Calculation S K in this case it is based on the assumption that the load current remains unchanged throughout the entire switching interval. Capacitor S K in this case it will be discharged at a constant speed, and the voltage across it changes according to the equation
As with a resistive load, the circuit time t s is determined by the period of time after which the reverse voltage on the thyristor decreases to zero. Therefore, when substituting into expression (3.5) t s instead of t we have t with t K /C K = 0. Subject to the condition t с ≥ t q k q From this expression the formula for determining the minimum capacitance of the capacitor directly follows:
It should be emphasized that expressions (3.4) and (3.6) were obtained without taking into account the inductances and active resistances that the circuit elements, including connecting wires, have. These resistances limit the maximum current value in the switching circuit and the rate of its increase.
However, if the device is designed to switch off emergency currents, the self-inductance of the circuit elements is insufficient to limit di/dt to values withstandable by low-frequency thyristors. In this case, it is necessary to connect an additional inductance reactor in series with the switching thyristor L K(in Figure 3.3 this corresponds to switch S being moved to position 2). Parameters of switching circuit elements when shunting a power thyristor VS 1 reverse-connected diode are determined by the expressions
Where Uc 0− capacitor precharging voltage.
The maximum rate of current rise in the switching circuit, which determines the choice of thyristor group VS2 according to (di/dt) crit, is determined by the formula
di/dt = (Uc 0 /L K) 10 6.
Turning to the diagrams of transient processes (Figure 3.4), we highlight the features characteristic of switches with capacitive switching of thyristors.
1. When the switching thyristor is turned on, the power source and the capacitor charged to the source voltage are connected in series. This causes a sudden increase in the current in the circuit to a value I N =2U/R N, which has an adverse effect on the load, especially when emergency currents are turned off.
2. Time interval t = t 3 – t 1, during which the capacitor S K recharges, determines the speed of the circuit breaker when turned off and the frequency of switching. When the thyristor is turned on again VS 1 the capacitor must be recharged again and thereby ensure readiness for the subsequent shutdown of the device. To reduce the recharging time of the capacitor C K, which is important when the circuit breaker operates in automatic reclosure mode, it is necessary to reduce the charging circuit constant τ = R 1 C K. Considering that the capacity S K due to circuit time t s, this can be achieved by reducing the resistance of the resistor R 1.
3. The process of turning off the current in the load circuit ends with turning off the thyristor VS 2. To do this, it is necessary to ensure that the current is limited by a resistor R 1(after recharging the capacitor S K) to values I ≤ I N thyristor. Due to the fact that the holding current of powerful thyristors is tens or hundreds of milliamps, the resistor resistance R 1 should be large enough, which contradicts the requirement formulated in paragraph 2. Therefore, in order not to reduce the switching frequency of the switch, charging the capacitor S K is usually carried out using an additional charging circuit with a small time constant τ from an autonomous power source.
4. An important task when creating switches with capacitive switching of thyristors is to limit the overvoltages that occur on the capacitor S K. Depending on the parameters of the switched circuit and the short circuit mode, they may exceed the value (1.5...2)U. To limit the level of overvoltages to acceptable values, it is necessary to use various damping circuits, semiconductor or zinc oxide (varistors) nonlinear limiters. In some developments, it becomes advisable to use double-circuit or two-stage switching units, with the help of which a reduction in the rate of current decay during its shutdown and a significant reduction in overvoltages is realized.
If the device (see Figure 3.3) is designed to operate only with rated currents at constant load parameters, problems with overvoltages and short-term increases in current do not arise. In this case, without fundamental changes, the circuit can be used to implement many other functions. For example, when replacing the charging resistor R 1 with the second load, it is capable of performing the functions of a high-speed switch, i.e., connecting the loads one by one to the power source. If the load resistances are equal, the same circuit is a symmetrical trigger that can be used to control electromagnets, relays or any other actuators. At the same time, the principle of operation of the circuit, regardless of what functions it performs, remains unchanged.
3.3 WAYS TO REDUCE SWITCHING OVERVOLTAGES IN DEVICES
DIRECT CURRENT
Overvoltages during the shutdown of the device are caused mainly by the oscillatory nature of recharging the switching capacitor. Their level depends on the parameters of the disconnected circuit and the dynamic characteristics used in the power circuit of the SPP. Since overvoltages determine the requirements for the insulation of the protected equipment and the insulation of the devices themselves, and affect the dimensions, cost and reliability of power supply systems as a whole, it is necessary to strive to reduce them.
In capacitively switched thyristor devices, overvoltage limitation can be achieved in various ways. The simplest of them is to connect a linear or nonlinear resistor in parallel with the capacitor, at a certain stage of its recharging. The essence of this approach is to dampen vibrations by increasing their attenuation coefficient. The effectiveness of this method is shown based on the analysis of switching processes in an AC switch. In DC switches, the use of linear resistors for shunting capacitors C K is associated with the need to introduce into the circuit an additional switching unit (usually a thyristor), which interrupts the current in the resistor.
One of the possible designs of switches with two-stage current switching is shown in Figure 3.5. Readiness for shutdown in the circuit of this device is ensured by preliminary charging of the capacitor C K from the network with the polarity indicated in Figure 3.5. To do this, you need to turn on the thyristors VS 2 And VS 5, applying control signals to their input circuits (control electrode - cathode). The charging current of the capacitor C K flows through the circuit elements L 1, L 2, R l, VS 5, S K, jumper P, VS 2, L 3. As the capacitor charges, the current in the thyristor circuit VS 2, VS 5 decreases and, when it becomes less than the holding current, the thyristors turn off on their own. During long-term nominal operation, the voltage on the capacitor C K gradually decreases due to the imperfection of its own insulation and due to charge leakage through the circuits with thyristors connected to the capacitor. To prevent a significant decrease in voltage, the control system must ensure periodic switching on of the thyristors VS 2 And VS 5. As a result, on the capacitor S K a constant voltage will be automatically maintained, almost equal to the mains voltage. Reactors L 1, L 2, L 3 in the circuit are necessary to limit the rate of current rise when the thyristors are turned on and to implement the oscillatory mode of transient processes.
If a short circuit occurs and the current reaches the set value I(Figure 3.6) the control system turns on the thyristors VS 3 And VS 4. As a result, as in all previously discussed circuits, the thyristor turns off VS 1. After changing the polarity of the voltage on the capacitor and increasing it to the specified value U m 1 the control system issues a signal to turn on the thyristor VS 5. In this case, a resistor is connected in parallel with the capacitor R 1, helping to limit further increase in voltage across the capacitor S K. From this point on, the voltage across the capacitor decreases along with a decrease in the switching current.
The capacitor is discharged through a thyristor VS 3, and after turning it off - through a diode VD 1. The second stage of switching processes begins immediately after the thyristor is turned off VS 3 and reducing the current to a value determined by the total resistance of the external circuit and the resistor R 1.
Figure 3.5 – Thyristor switch with
two-stage current switching
Figure 3.6 – Switching processes in the circuit (Figure 3.5)
At this moment in time ( t 2, in Figure 3.6), the control system turns on the thyristor VS 2, and current begins to flow through the circuit R l, VS 5, S K, P, VS 2 And VD 2. As a result, the voltage across the capacitor changes polarity again. When it reaches the value U m 2 at a point in time t 3 the load current is completely interrupted.
Since the polarity of the voltage on the capacitor after switching off corresponds to the initial state, the switch is ready to operate again. Moreover, in the case under consideration, which corresponds to the inductive nature of the load, the voltage on the capacitor significantly exceeds the network voltage. With an active load, the voltage across the capacitor does not reach the value U m 1 so there is no need to turn on the thyristors VS 5 And VS 2. In this case, even after turning off the current, the residual voltage on the capacitor U C< U . To ensure readiness for operation, the capacitor must be recharged.
The advantages of circuit diagrams with two-stage current switching include optimal use of capacitors, higher performance and switching frequency. However, this is achieved by significantly complicating the switching unit and control system, which must respond to many parameters of the transient process and provide a certain sequence of switching on the thyristors.
Another possibility of creating direct current devices with low switching overvoltages and a simple structure is associated with the development and development of turn-off thyristors. The main distinguishing feature of these devices compared to conventional thyristors is their ability to be switched off by a current pulse in the control circuit. The fundamental possibility of developing such devices was substantiated back in the 50s, and already in the 60s the industry mastered devices capable of switching currents up to 5A at a voltage of 100...200V. Rapid progress in the creation of high-power turn-off thyristors has been observed since the early 80s. Currently, a number of foreign companies and in Russia produce devices of this type for currents of hundreds of amperes and voltages over 1000 V. Literary sources report the development of turn-off thyristors with maximum current and voltage parameters comparable to the parameters of conventional thyristors.
The schematic diagram of a direct current device based on a turn-off thyristor is shown in Figure 3.7. Its shutdown is illustrated by oscillograms of changes in the anode current I A, voltage on the thyristor U A and pulse current control of negative polarity i G(Figure 3.8).
An important advantage of the circuit in Figure 3.7 over those previously discussed is that it contains only one high-current device - a turn-off thyristor VS 1. It is controlled by multi-polar voltage pulses. When turned on, a pulse of positive polarity (relative to the cathode) is supplied from an external pulse generator to terminals 1. Through a current-limiting resistor R 2 this pulse arrives at the control electrode of the thyristor VS 1. The process of turning on a lockable thyristor proceeds in the same way as with a conventional thyristor (non-lockable).
To turn off the thyristor, a voltage pulse of negative polarity must be applied to its control electrode. In the diagram shown in Figure 3.7, it is formed by an electrical circuit based on a low-power thyristor VS 2. When a voltage pulse from an external pulse generator arrives at the control electrode of the thyristor, it turns on. In this case, the capacitor pre-charged from the power source E G C 2(charging polarity is indicated in the figure) is discharged to the input circuit of the turn-off thyristor VS 1 in the direction from the cathode to the control electrode.
Parallel connected to a thyristor VS 1 circuit consisting of a diode VD 1, resistor R 1 and capacitor C 1, performs protective functions. In circuits with active loads, it is designed to limit the rate of rise of the recovery voltage. As can be seen from the oscillogram i A = f(t)(Figure 3.8), a current approximately equal to 200A is interrupted by a thyristor in less than a microsecond. Without taking special measures, this would cause an almost instantaneous restoration of the mains voltage at the thyristor.
Turn-off thyristors, like other SPPs, are sensitive to the effect (du/dt) cr it, therefore it is necessary to limit the rate of voltage rise to values permissible for the device used. In the circuit in Figure 3.7, the increase in voltage across the thyristor when it is turned off is determined by the charging rate of the capacitor C 1, i.e., a time shift is provided between the decrease in current in the circuit and the increase in voltage across the device.
Figure 3.7 - Schematic diagram of a direct current device based on a turn-off thyristor
Figure 3.8 − Diagrams of current and voltage changes
when the turn-off thyristor is turned off
Resistor R 1 During the charging process, the capacitor is bypassed (shorted) by a diode, which in this case is biased in the forward direction. Therefore, the charging time constant of the capacitor is determined only by the resistance of the connecting wires, the intrinsic resistance and inductance of the capacitor and the differential resistance of the diode. On the oscillogram (Figure 3.8), the change in the differential resistance of the diode and the inductance of the elements of the protective circuit are manifested by a short-term surge of the recovering voltage at the time corresponding to the beginning of the decline of the anode current.
When the turn-off thyristor is turned on, the capacitor C 1, which is charged to the voltage of the power supply, is discharged through a resistor R 1, since the diode VD 1 in this case it turns out to be shifted in the opposite direction. This ensures that the thyristor is protected from exceeding its permissible current rise rate when turned on. Note that the capacitance of the protective circuit capacitor, which ensures the normal operation of a turn-off thyristor in a circuit with an active load, amounts to units of microfarads. In particular, the oscillograms shown in Figure 3.8 were obtained with the following circuit parameters:
U A = 200 V; R H =2 Ohm; U G = 12 V; R 1 = 20 Ohm; C 1 = 2 10 -6 F.
An abrupt interruption of the current by a turn-off thyristor when the inductive load is turned off is accompanied not only by a high speed of voltage recovery, but also by multiple overvoltages. To limit overvoltages, a protective circuit with the same structure can be used (see Figure 3.7). However, the capacitor capacity C 1 in this case it can be tens and even hundreds of microfarads.
If the active resistance of the load is small and the energy dissipation in it during the charging of the capacitor can be neglected, then approximately the capacitance of the capacitor can be determined from the equality of energies
Where L H− load inductance, H;
I− switched current, A;
Umax− maximum permissible voltage, V.
For comparison with the mode of switching off the active load, we calculate the capacitance of the capacitor C 1 necessary to limit the recovery voltage at the level Umax = 1.5U when disconnecting a circuit with inductance L H =10 -3 H:
Using expression (3.6), we determine the capacitance of the capacitor that would be required to disconnect the same circuit with a switch with capacitive artificial switching (see Figure 3.3), made on the basis of the T123-200 thyristor ( t q= 250∙10 -6 s):
Comparing the obtained values C 1 And S K, we can draw a conclusion about their commensurability. But we must keep in mind that expression (3.6) determines only the condition of sufficient capacitance of the capacitor to reliably turn off the thyristor. It does not take into account the resulting overvoltages. If the choice of capacity S K produced taking into account the limitation of overvoltages, its numerical value will be much greater. On the other hand, when calculating the capacity S K energy losses in the circuit elements during the charging of the capacitor and the real rate of change of current when the thyristor is turned off were not taken into account ( -di/dt< ∞ ). These factors contribute to a decrease in the amplitude of the recovering voltage.
3.4 MAIN OPTIONS FOR USE OF AC SEMICONDUCTOR DEVICES
Compared to DC switching devices, AC semiconductor devices have a more complex structure. Their schematic diagram and design are determined by their purpose, requirements and operating conditions. With the wide application that contactless devices find, there is a wide variety of options for their implementation. However, all of them can be represented by a general block diagram that shows the required number of functional blocks and their interaction. Figure 3.9 shows a block diagram of a single-pole AC semiconductor device. It includes four functionally complete units.
Power block 1 with surge protection elements ( RС-circuit in Figure 3.9) is the basis of the switching device, its executive body. It can be made on the basis of only controlled valves - thyristors or using diodes. When designing a device for a current exceeding the current limit of one device, a parallel connection is required. In this case, special measures must be taken to eliminate the uneven distribution of current across individual devices, which is not due to the identity of their current-voltage characteristics in the conducting state and the spread of the on-time.
Control unit 2 contains devices that select and store commands coming from control or protection elements, generate control pulses with specified parameters, and synchronize the arrival of these pulses at the thyristor inputs with the moments when the current in the load crosses zero. The control unit circuit becomes significantly more complicated if the device, in addition to the function of switching circuits, must regulate voltage and current. In this case, it is supplemented with a phase control device, which ensures a shift of control pulses by a given angle relative to current zero.
The device operating mode sensor block 3 contains current and voltage measuring devices, protection relays for various purposes, a circuit for generating logical commands and signaling the switching position of the device.
Forced switching unit 4 combines a capacitor bank, its charging circuit and switching thyristors. This block is contained in AC devices only if they are used as protection (circuit breakers). The power part of the device can be made according to a circuit with back-to-back connection of thyristors (see Figure 3.9), based on a symmetrical thyristor (triac) (Figure 3.10, A) and in various combinations of thyristors and diodes (Figure 3.10, 6 And V). In each specific case, when choosing a circuit option, the following factors must be taken into account: the voltage and current parameters of the device being developed, the number of devices used, load capacity in long-term mode and resistance to current overloads, the degree of complexity of thyristor control, requirements for weight and dimensions, cost.
Figure 3.9 – Block diagram of a thyristor device
alternating current
Figure 3.10 − Power blocks of AC devices
A comparison of the power blocks shown in Figures 3.9 and 3.10 shows that the circuit with back-to-back thyristors has the greatest advantages. This scheme contains fewer devices and is characterized by smaller dimensions, weight, energy losses and cost. Compared to triacs, thyristors with one-way (unidirectional) conductivity have higher current and voltage parameters and are capable of withstanding significantly higher current overloads. Thyristors of tablet design have higher thermal cycling. Therefore, a circuit using triacs can be recommended for switching currents that, as a rule, do not exceed the classification value of the current of a single device, i.e., when their group connection is not required. Note that the use of triacs helps to simplify the control system of the power unit, which must contain an output channel to the pole of the device.
The circuits shown in Figure 3.10 b, V, illustrates the possibility of designing AC switching devices using diodes. Both of these schemes are easy to control, but have disadvantages due to the use of a large number of devices. In the diagram in Figure 3.10, b The alternating voltage of the power source is converted into a full-wave pulsating voltage of the same polarity using a rectifying diode bridge. As a result, only one thyristor, connected at the output of the rectifier bridge (in the diagonal of the bridge), becomes capable of controlling the current in the load during both half-cycles, if at the beginning of each half-cycle control pulses are received at its input. The circuit is switched off when the load current passes zero the next time after the generation of control pulses ceases.
It should be borne in mind, however, that reliable shutdown of the circuit is ensured only with a minimum circuit inductance on the rectified current side. Otherwise, even if the voltage at the end of the half-cycle drops to zero, current will continue to flow through the thyristor, preventing it from turning off. The danger of emergency operation of the circuit (not switching off) also appears when the frequency of the supply voltage increases. In this case, it may turn out that the circuit time tC is not enough to restore controllability by the thyristor, i.e. tC< .
In the circuit in Figure 3.10, the load is controlled by two back-to-back thyristors, each of which is shunted in the opposite direction by an uncontrolled valve. Since with this connection the cathodes of the thyristors are at the same potential, this allows the use of control pulse generators with one output or two outputs with a common ground. The circuit diagrams of such generators are greatly simplified. In addition, the thyristors in the circuit in Figure 3.10c are protected from reverse voltage and, therefore, must be selected only for forward voltage.
In terms of dimensions, technical characteristics and economic indicators of the device, made according to the diagrams shown in Figure 3.10, b, V, inferior to switching devices, the circuits of which are shown in Figure 3.9 V, 3.10, A. However, they are widely used in automation and relay protection devices, where the switching power is measured in hundreds of watts. In particular, they can be used as output devices of pulse shapers to control thyristor blocks of more powerful devices.
Through each thyristor in the circuits shown in Figures 3.9 and 3.10, V, half the load current flows. The relationship between the average current through the thyristor (classification current of the SPP, indicated in the technical specifications) and the effective current in the load circuit is equal to
Accordingly, the average current flowing through the thyristor, if expressed through the current in the load, will be written as follows
Similarly, the average current flowing through the thyristor in the circuit in Figure 3.10 is b, is determined by the equality
Symmetrical thyristors, which conduct current in both directions, are classified according to the effective current. Therefore, for the circuit in Figure 3.10, A
3.5 AC THYRISTOR CONTACTOR WITH ANODIC VOLTAGE CONTROLLED
The peculiarity of semiconductor switching devices is that they can perform various functions without fundamental changes in the power part. Thus, a thyristor unit made according to the circuit in Figure 3.9 can equally successfully work both as a contactor and as a switch. Only by replacing thyristors (changing the type, voltage class or group of the device according to dynamic parameters) can the scope of application of devices be expanded in terms of current or voltage. You can significantly influence the operation of the circuit using the control system, which will be shown using the example of the operation of a thyristor contactor
The power block of the contactor is made according to a circuit with an anti-parallel connection of thyristors VS 1 And VS 2. It is controlled using a circuit consisting of resistors R1, R2, R 3 and mechanical contact S. This circuit is connected in parallel to the thyristors, therefore, when the switch S is closed, the voltage on its elements, and in particular on the resistors R 1 And R 3, changes synchronously with the anode voltage on the thyristors. And since these resistors are connected in parallel to the control circuits of the thyristors, the voltage of the same polarity simultaneously increases both at the anode of the thyristor and at its control electrode.
If this voltage is positive, for example with respect to a thyristor VS 1 and removed from the resistor R 1 voltage exceeds the unlocking voltage value, thyristor VS 1 turns on. When the voltage polarity changes, the thyristor turns on in the same way VS 2, diodes VD 1 And VD 2 in the circuit are necessary to protect the control circuits of thyristors from reverse voltage when the voltage at their anodes is negative.
Adjustable resistor R 2 in the control circuit is selected from the condition of limiting the amplitude of the control current pulse to a value permissible for the thyristors used I Gmax.
Figure 3.11 − AC Contactor
Considering that contact S can be closed in the half-cycle interval at any time, including at the moment the network voltage reaches its amplitude value U m, the resistor resistance is determined from the expression
Where R G− own resistance of the thyristor control circuit.
Changing the resistance of the resistor R 2 it is possible to control the current in the input circuits of the thyristors and, therefore, the moment they are turned on in relation to the beginning of the voltage half-cycle. As a result, the contactor becomes capable of performing one more function - regulating the current in the load. Limit angle of thyristor turn-on delay αmax, which can be provided by a resistor control circuit, is 90º.
The process of regulating the current (voltage, power) in a circuit by changing the thyristor turn-on delay angle is called phase control. The dependences of the change in voltage on the active load and the current in it on the angle for the circuit under consideration are determined by the expressions.
where 0< ≤90 °.
The minimum delay angle for turning on thyristors with an active load is ≈2 °. This is explained by the fact that all thyristors have a sensitivity threshold for the control circuit and, in addition, the anode voltage, varying according to a sinusoidal law, must also exceed the threshold value U(TO) by at least twice. These factors lead to the appearance of dead pauses in the load current curve t p.
Due to the variation in the control characteristics of thyristors, these pauses may not be the same in duration, which leads to the appearance of a constant component in the load current. If necessary, the turn-on delay angles of the thyristors are leveled by adjusting the control currents by changing the resistance of the construction resistors R 1 And R 3(See Figure 3.11).
If there is a need to expand the range of current regulation in the load, then control circuits are performed using RС-chains (Figure 3.12 A).
When the anode voltage across the thyristor becomes positive, the capacitor WITH recharged through a variable resistor R and load from a voltage equal to – U m, to tension U GT, at which the thyristor turns on VS 1(Figure 3.12 b). By changing the capacitor charging circuit constant τ = (R+RH)C By means of an adjustable resistor R, it is possible to provide a delay in turning on the thyristor relative to the maximum anode voltage, i.e. to an angle > 90 ◦.
Expressions that determine the change in the average and effective voltage across the load, depending on the angle of the thyristor switch-on delay, respectively have the form
a − regulation scheme; b – time characteristic
regulation
Figure 3.12 − Operating principle of the control circuit on RС- chains
The method of controlling thyristors used in the considered circuits is one of the simplest and most reliable, since it is implemented by a minimum number of elements in the control circuits. At the same time, the direct connection between the control electrode and the anode of the thyristor makes it possible to ensure the fulfillment of other requirements that apply to control systems: strict synchronization of the receipt of control signals with the moment of possible switching on of the thyristors is automatically carried out; power losses for control are insignificant, since the duration of exposure to the control current is regulated by the thyristor itself.
As soon as it switches to the conducting state, the control circuit is shunted with a small resistance (the resistance of the thyristor in the conducting state) and the current in it decreases to almost zero.
Due to the noted factors, thyristor control circuits powered by anode voltage are widely used in low voltage devices. In particular, using this control principle, domestic industry produces thyristor control stations of the BSE type, dimmers for incandescent lamps, thyristor starters of the PT type in a three-pole design with a rated current of up to 63 A.
3.6 COMBINED CONTACT-SEMICONDUCTOR DEVICES
Combined electrical devices (also called hybrid) are devices that simultaneously contain a contact system of traditional electromechanical devices and a power circuit based on an SPP, connected in parallel with an open contact. As a result of this essentially mechanical combination of contact and non-contact switching devices in one design, a successful combination of the advantages of both types of devices is achieved and at the same time many of their disadvantages are eliminated.
Let's consider the principle of operation of combined devices using simple devices (Figure 3.13), which use diodes and thyristors. In all of the above power blocks, the SPP are connected in parallel with one of the open contacts. Let us recall that in electromechanical devices the voltage drop across closed contacts at rated currents does not exceed tenths of a volt. At such voltages, SPPs connected in parallel with the contacts do not go into a state of high conductivity and the load current practically does not flow through them.
Figure 3.13 - Power blocks of combined devices
During the process of switching off the device, the ratio of the resistances of the contact and semiconductor circuits changes, which leads to a redistribution of current between them.
Let us consider the essence of this phenomenon using the example of turning off the device, performed according to the diagram in Figure 3.13, A. Opening the arcing contacts S 1 in the circuit it is necessary to provide at the beginning of the half-cycle a current whose polarity coincides with the conducting direction of the diode VD (in the time interval t 2< t< t 3 , in Figure 3.14). In this case, the voltage on the resulting electric arc is direct to the diode. As the distance between the contacts increases and the intensity of the impact on the electric arc, for example, due to its movement in the air at high speed under the influence of an electromagnetic field, the resistance of the intercontact gap increases and, consequently, the voltage on the diode increases. As a result, conditions are created for switching it to a conducting state.
In practice, the transition of the diode to the conducting state in low-voltage devices occurs already at the stage of formation of the electric arc, since the near-electrode voltage drop across it is much higher than the threshold voltage of the arc.
From this point in time, the current in the contact circuit begins to rapidly decrease, and the current in the semiconductor circuit increases. The duration of the transient process during which the switched current completely passes into the diode circuit and the electric arc goes out is determined mainly by the inductance of the circuits, the dynamic characteristics of the diode used, and the method of influencing the electric arc.
In the time remaining until the end of the half-period t = t 4 − t 3 deionization processes in the intercontact gap are completed and its electrical strength is restored.
The final interruption of the current in the circuit is carried out by a diode immediately after the moment of time t 4, corresponding to a change in the direction of the current. During the time that the voltage is reversed for the diode, it is necessary to open the auxiliary contacts S 2.
Note that for the illustrated case (Figure 3.14) of disconnecting a circuit with an active-inductive load, this time is less than a half-cycle. In the limit, it can be equal to 5 ms, which leads to the need to use high-speed drives.
Figure 3.14 – Diagrams of switching processes
in a contact-diode device
When turning on the device, the sequence of closing the contacts must be reversed: during the half-cycle of voltage that is non-conductive for the diode, it is necessary to close the contacts of the separator S 2, and during the next half-cycle - arc-extinguishing contacts S 1.
Characteristic of the switching mode is the closure of contacts S 1 at low voltages, determined by the voltage drop across the conducting diode. As a result, preliminary breakdown of the gap when the contacts approach each other and the associated phenomena of erosion and welding of the contacts are eliminated.
But it must be borne in mind that in combined devices there is a danger of these same effects manifesting themselves due to the high rate of current increase in the contacts after their contact. Therefore, the designs of the contact device and drive must provide a forced increase in contact pressure to the final value.
Devices made according to the diagram in Figure 3.13, b, according to the principle of action and the nature of the processes occurring, do not differ from those discussed above. However, the presence of two diode circuits with counter-oriented conductivity allows switching off at any half-cycle of the current. As a result, the device shutdown time is reduced.
The disadvantages of this option include doubling the number of SPPs and significantly complicating the design of the mechanical part of the device. Since the synchronized opening of the contacts is carried out in a sequence determined by the direction of the current at the moment the switching command is given, the device must contain two independent and high-speed drives.
Strict requirements are also imposed on the stability of the drives: they must have a small time spread. Obviously, achieving a high level of functional reliability with such a design of the power part of the device is a difficult task.
A significant simplification of the drive mechanism and the apparatus as a whole can be obtained by refusing to synchronize the opening of contacts with the corresponding half-cycle of the current. In this case, both contacts, controlled by a common drive, open simultaneously and in any phase of the current. As a result, an electric arc occurs on both contact pairs, but on one of the pairs it goes out due to the shunting effect of the diode circuit. On other contacts, the direction of the current in which does not coincide with the conducting direction of the diodes in the shunt circuit, the electric arc is maintained until the end of the half-cycle (until the current direction changes).
The maximum duration of the arc on the contacts, equal to approximately 11 ms, corresponds to the most unfavorable mode, when the contacts open in a relatively narrow time interval before the current passes through zero.
In this case, the process of current transition from the contact circuit to the diode circuit is not completed or the electrical strength of the intercontact gap does not have time to recover; it breaks through again at the beginning of the next half-cycle.
In case of a large number of shutdowns, contact opening S 1 And S 2 occurs with equal probability both in the interval of positive and in the interval of negative half-cycles; the same law determines the distribution of the contact opening moment within each half-cycle. As a result, the duration of exposure of the contacts to the electric arc decreases and, as a result, the switching life of the device increases. Moreover, in comparison with similar devices without shunt diode circuits, in which extinguishing the electric arc is ensured in one half-cycle, the increase in service life is at least 150%.
The capabilities of combined devices can be significantly expanded by replacing uncontrolled SPP with thyristors (Figure 3.13, V).
The semiconductor circuit in this device, made according to a circuit with an anti-parallel connection of thyristors (see Figure 3.9), is connected in parallel with only one arc-extinguishing contacts. But the ability of thyristors to be in a closed state at a voltage of positive polarity allows switching operations to be performed at any half-cycle of voltage (current).
Figure 3.15 - Transient processes in a contact-thyristor device
Let's consider the interaction of the contact unit and the thyristor unit in the device switching mode. Considering the large difference in the speed of operation of the contact circuit and the ignition switch, commands to turn them on should not be issued simultaneously. First, a command must be received to turn on the contact drive. After a certain time, equal to the proper switching time of the contact device, its contacts S 1 are closed. In Figure 3.15, the moment of contact of the contacts corresponds to the time t 2.
With the necessary advance of this moment in time, the control system issues a control impulse I G 1 to thyristor VS 1, for which the voltage in the half-cycle under consideration is direct. As a result of turning on the thyristor, the voltage at the converging contacts is reduced to the value of the voltage drop across the thyristor in the conducting state, i.e., to 1.5...2.5 V.
After contact of the contacts, the thyristor circuit is quickly de-energized, since the resistance of the contact circuit is much less than the differential resistance of the thyristor.
When the device is turned off, the sequence of operation of the contact and thyristor circuit is the same as in contact-diode devices. The only difference is that at the moment of opening the contacts ( , in Figure 3.15) to the thyristor VS 2 a control current pulse must arrive I G 2. In practice, it is very difficult to strictly synchronize the operation of the thyristor unit control system with the contact drive mechanism. Therefore, in most switching devices of this type, control pulses are supplied to the thyristor inputs with anticipation of contact opening, taking into account the instability of the mechanical part of the device over time.
As with the use of diodes, in contact-thyristor devices, the opening of contacts and restoration of the electrical strength of the intercontact gap must be completed before the end of the half-cycle. If the design of the device does not provide synchronized shutdown, the contacts can open at any time, including in the critical half-cycle zone before the current passes through zero, in which the current does not have time to pass from the contact circuit to the semiconductor circuit. In this case, it is necessary that at the beginning of the next half-cycle the control system ensures that a thyristor with a different direction of conduction is turned on.
Summarizing the considered possibilities for creating combined devices, we will highlight their most important characteristics.
In all variants of combined SPP devices (diodes or thyristors), they do not conduct current during long-term nominal operation, therefore, relatively large power losses characteristic of semiconductor devices are eliminated. Consequently, in terms of this indicator, combined devices do not differ from conventional contact devices.
In modes of changing the switching positions by the device, using the SPP, the intercontact gaps are shunted with low resistance, characteristic of diodes and thyristors in a conducting state. This ensures rapid extinguishing of the electric arc that occurs during switching on due to contact bounce and when the device is turned off. Experience in operating combined devices shows that when switching currents up to 500A, the duration of the arc does not exceed 100 μs. As a result, combined devices have switching wear resistance that is 20–50 times greater than that of contact devices.
Since SPPs in combined devices are exposed to short-term current, it is possible to make maximum use of their pulse overload capacity. At the initial temperature of the structure (20±5)°C, most devices can be loaded with a half-wave current pulse of a sinusoidal shape, lasting 10 ms with an amplitude exceeding the value of the average (classification) current in
8 − 10 times. For example, diodes of the D253-1600 type are capable of withstanding a current with an amplitude of 12 kA without deterioration. As the pulse duration decreases to 2 ms, the permissible current amplitude increases approximately threefold. In emergency modes, the number of which during the operation of the SPP should be limited to a few, the current amplitude increases accordingly to 28 kA at a pulse duration of 10 ms and to 44 kA at 2 ms.
In many cases, the specified overload capacity is sufficient to create combined devices without parallel connection of devices in power units. By ensuring the opening of contacts immediately before the critical zone of the current half-cycle, the best use of the pulse load capacity of the SPP is achieved.
An important circumstance is that during short-term current influences, the generated heat in the structure of the SPP does not spread beyond the boundaries of the structural elements directly adjacent to it. Therefore, there is no need not only for the use of forced cooling, but also for the coolers themselves. As a result, the design of the semiconductor unit is significantly simplified, its weight and dimensions are reduced.
The noted positive features of combined devices determined their intensive development. To date, several versions of such devices have been developed and are produced by industry, differing both in the design of contact and semiconductor parts, and in the method of controlling thyristors. The diagram of one of the options for a combined contactor with a control system powered by a current transformer is shown in Figure 3.16.
Figure 3.16 - Schematic diagram of a combined
contactor
The semiconductor block in it is connected in parallel to a circuit consisting of contacts S and the primary winding of the current transformer connected in series with them TA. Two secondary windings of the transformer are connected through diodes that match the polarity of the control and anode voltages to the thyristor control circuits. When the contacts S are turned on, a sinusoidal current flows through them, therefore through the primary winding of the current transformer
In the secondary windings of the transformer, the current will generally be non-sinusoidal due to the nonlinearity of the resistance of the thyristor control circuit and the influence of the zener diodes, which protect these windings from exceeding the permissible voltage. At the rated current in the contactor circuit, the thyristors should not turn on. This is ensured by choosing parameters in such a way that the total voltage drop on the primary winding of the transformer and closed contacts does not exceed the threshold voltage U(TO) used power thyristors.
When through short circuit currents flow, the voltage between the points of connection of the thyristor unit to the main circuit increases significantly and conditions are created for switching on
Electrical apparatus is a device that controls electrical consumers and power sources, and also uses electrical energy to control non-electrical processes.
Electrical devices for general industrial use, electrical household appliances and devices are produced with voltages up to 1 kV, high-voltage - over 1 kV. Up to 1 kV they are divided into manual and remote control devices, protection devices and sensors.
Electrical devices are classified according to a number of criteria:
1. according to its intended purpose, i.e. the main function performed by the device,
2. according to the principle of operation,
3. by the nature of the work
4. kind of current
5. current value
6. voltage value (up to 1 kV and above)
7. execution
8. degree of protection (IP)
9. by design
Features and areas of application of electrical devices
Classification of electrical devices depending on their purpose:
1. Control devices, designed for starting, reversing, braking,speed regulationrotation, voltage, current of electrical machines, machine tools, mechanisms, or for starting and regulating the parameters of other electricity consumers in power supply systems. The main function of these devices is to control electric drives andother consumers of electrical energy. Features: frequent switching on, switching off up to 3600 times per hour i.e. 1 time per second.
These include electrical manual control devices- controllers and command controllers, rheostats, etc., and electrical remote control devices- , contactors, etc.
2. Protection devices are used for switching electrical circuits, protecting electrical equipment and electrical networks from overcurrents, i.e. overload currents, peak currents, short circuit currents.
These include, etc.
3. Control devices, are designed to monitor specified electrical or non-electrical parameters. This group includes sensors. These devices convert electrical or non-electrical quantities into electrical ones and provide information in the form of electrical signals. The main function of these devices is to control specified electrical and non-electrical parameters.
These include sensors for current, pressure, temperature, position, level, photosensors, as well as relays that implement sensor functions, for example, voltage, current.
Classification of electrical devices according to operating principle
According to the principle of operation, electrical devices are divided depending on the nature of the impulse acting on them. Based on the physical phenomena on which the operation of the devices is based, the most common categories are:
1. Switching electrical devices for closing and opening electrical circuits using contacts interconnected to ensure the passage of current from one contact to another or remote from each other to break the electrical circuit (switches, switches, ...)
2. Electromagnetic electrical devices, the action of which depends on the electromagnetic forces arising during operation of the device (contactors, relays, ...).
3. Induction electrical devices, the action of which is based on the interaction of current and magnetic field ().
4. Inductors(reactors, saturation chokes).
Classification of electrical devices by nature of operation
By the nature of their operation, electrical devices are distinguished depending on the mode of the circuit in which they are installed:
1. Devices that operate for a long time,
2. intended for short-term operation,
3. working under conditions of repeated short-term load.
Classification of electrical devices by type of current
By type of current: direct and alternating.
Requirements for electrical devices
The design varieties of modern devices are especially diverse, and therefore the requirements for them are also different. However, there are some general requirements, regardless of the purpose, application or design of the devices. They depend on the purpose, operating conditions, and the required reliability of the devices.
The insulation of an electrical apparatus must be calculated depending on the conditions of possible overvoltages that may arise during the operation of the electrical installation.
Devices intended for frequent switching on and off of the rated load current must have high mechanical and electrical wear resistance, and the temperature of the current-carrying elements must not exceed permissible values.
During short circuits, the current-carrying part of the device is subjected to significant thermal and dynamic loads, which are caused by high current. These extreme loads should not interfere with the further normal operation of the device.
Electrical devices in the circuits of modern electrical devices must have high sensitivity, speed, and versatility.
The general requirement for all types of devices is the simplicity of their design and maintenance, as well as their efficiency (small size, light weight of the device, minimum amount of expensive materials for the manufacture of individual parts).
Operating modes of electrical devices
The nominal operating mode is a mode when an element of the electrical circuit operates at the values of current, voltage, power specified in the technical data sheet, which corresponds to the most favorable operating conditions in terms of efficiency and reliability (durability).
Normal operation- mode when the device is operated with mode parameters slightly different from the nominal ones.
Emergency operation- this is a mode when the parameters of current, voltage, power exceed the rated ones by two or more times. In this case, the object must be disabled. Emergency modes include the passage of short circuit currents, overload currents, and a decrease in voltage in the network.
Reliability – trouble-free operation of the device throughout its operation.
The property of an electrical apparatus to perform specified functions, maintaining over time the values of established operational indicators within specified limits, corresponding to specified modes and conditions of use, maintenance and repairs, storage and transportation.
Design of electrical devices according to degree of protection
Determined by GOST 14254-80. In accordance with GOST, 7 degrees are established from 0 to 6 from the ingress of solids and from 0 to 8 from the penetration of liquids.
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Designation of degrees of protection |
Protection against penetration of solid bodies and contact of personnel with live and rotating parts. |
Protection against water penetration. |
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There is no special protection. |
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A large area of the human body, such as the arm and solid bodies larger than 50 mm. |
Drops falling vertically. |
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Fingers or objects no longer than 80 mm and solid bodies larger than 12 mm. |
Drops when the shell is tilted up to 15 0 in any direction relative to the normal position. |
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Tools, wires and solids with a diameter of more than 2.5 mm. |
Rain falling on the shell at an angle of 60 0 from the vertical. |
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Wires, solids larger than 1 mm. |
Splashes falling on the shell in any direction. |
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Dust in an amount insufficient to disrupt the operation of the product. |
Jets thrown in any direction. |
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Full protection against dust (dustproof). |
Waves (water should not get inside during waves). |
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When immersed in water for a short time. |
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During prolonged immersion in water. |
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The abbreviation “IP” is used to indicate the degree of protection. For example: IP54.
In relation to electrical devices, the following types of execution exist:
1. Protected IP21, IP22 (not lower).
2. Splash-proof, drop-proof IP23, IP24
3. Waterproof IP55, IP56
4. Dustproof IP65, IP66
5. Closed IP44 - IP54, these devices have internal spaces isolated from the external environment
6. Sealed IP67, IP68. These devices are made with particularly tight insulation from the environment.
Climatic performanceelectrical apparatus determined by GOST 15150-69. In accordance with climatic conditions, it is designated by the following letters: U (N) - temperate climate, HL (NF) - cold climate, TB (TH) - tropical humid climate, TC (TA) - tropical dry climate, O (U) - all climatic conditions areas, on land, rivers and lakes, M – temperate maritime climate, OM – all areas of the sea, B – all macroclimatic areas on land and sea.
1. Outdoors,
2. Rooms where fluctuations in temperature and humidity do not differ significantly from fluctuations in the open air,
3. Closed spaces with natural ventilation without artificial regulation of climatic conditions. There is no exposure to sand and dust, sun and water (rain),
4. Premises with artificial regulation of climatic conditions. There is no exposure to sand and dust, sun and water (rain), outside air,
5. Rooms with high humidity (prolonged presence of water or condensed moisture)
Selection of electrical devices
The choice of electrical devices is a task in which the following must be taken into account:
- currents, voltages and powers switched by an electrical apparatus;
- parameters and nature of the load - active, inductive, capacitive, low or high resistance, etc.;
- number of switched circuits;
- voltages and currents of control circuits;
- electrical apparatus coil voltage;
- operating mode of the device - short-term, long-term, intermittent;
- operating conditions of the device - temperature, humidity, pressure, vibration, etc.;
- methods of mounting the device;
- economic and weight and size indicators;
- ease of pairing and electromagnetic compatibility with other devices and devices;
- resistance to electrical, mechanical and thermal overloads;
- climatic modification and placement category;
- degree of IP protection,
- safety requirements;
- height above sea level;
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