The principle of operation of the transformer. Purpose and design of power transformers
Transformer is a static electromagnetic device with two (or more) windings, most often intended to convert AC one voltage into alternating current of another voltage. Energy conversion in a transformer is carried out by an alternating magnetic field. Transformers are widely used in transmission electrical energy on long distances, its distribution between receivers, as well as in various rectifying, amplifying, signaling and other devices.
When transmitting electrical energy from a power plant to consumers, the current strength in the line causes energy losses in this line and the consumption of non-ferrous metals for its device. If, with the same transmitted power, the voltage is increased, the current strength will decrease to the same extent, and therefore, it will be possible to use wires with a smaller cross-section. This will reduce the consumption of non-ferrous metals when constructing a power transmission line and reduce energy losses in it.
Electrical energy is generated in power plants synchronous generators at voltage 11-20 kV; in some cases, a voltage of 30-35 kV is used. Although such voltages are too high for direct industrial and domestic use, they are not sufficient for economical transmission of electricity over long distances. Further increase in voltage in power lines (up to 750 kV or more) is carried out by step-up transformers.
Receivers of electrical energy (incandescent lamps, electric motors, etc.) for safety reasons rely on a lower voltage (110-380 V). In addition, the manufacture of electrical devices, instruments and machines for high voltage is associated with significant design difficulties, since the current-carrying parts of these devices at high voltage require reinforced insulation. Therefore, the high voltage at which energy is transmitted cannot be directly used to power the receivers and is supplied to them through step-down transformers.
AC electrical energy has to be transformed 3-4 times along the way from the power plant where it is generated to the consumer. IN distribution networks Step-down transformers are loaded non-simultaneously and not at full capacity. Therefore, the total power of transformers used for transmission and distribution of electricity is 7-8 times greater than the power of generators installed in power plants.
Energy conversion in a transformer is carried out by an alternating magnetic field using a magnetic core.
The voltages of the primary and secondary windings are usually not the same. If the primary voltage is less than the secondary, the transformer is called a step-up, if it is more than the secondary, it is called a step-down. Any transformer can be used both as a step-up and step-down transformer. Step-up transformers are used to transmit electricity over long distances, and step-down transformers are used to distribute it between consumers.
Depending on the purpose, power transformers, voltage measuring transformers and current transformers are distinguished
Power transformers convert alternating current of one voltage into alternating current of another voltage to supply consumers with electricity. Depending on the purpose, they can be increasing or decreasing. In distribution networks, as a rule, three-phase two-winding step-down transformers are used, converting voltages of 6 and 10 kV to a voltage of 0.4 kV. (The main types of transformers are TMG, TMZ, TMF, TMB, TME, TMGSO, TM, TMZH, TDTN, TRDN, TSZ, TSZN, TSZGL and others.)
Voltage transformers- These are intermediate transformers through which measuring instruments are switched on at high voltages. Thanks to this, the measuring instruments are isolated from the network, which makes it possible to use standard instruments (with their scale re-graded) and thereby expands the limits of the measured voltages.
Voltage transformers are used both for measuring voltage, power, energy, and for powering automation circuits, alarms and relay protection of power lines from ground faults.
In some cases, voltage transformers can be used as low-power step-down power transformers or as step-up test transformers (for testing the insulation of electrical devices).
The following types of voltage transformers are presented on the Russian market:
3NOL.06, ZNOLP, ZNOLPM, ZNOL.01PMI, 3xZNOL.06, 3xZNOLP, 3xZNOLPM, NOL.08, NOL.11-6.O5, NOL.12 OM3, ZNOL.06-35 (ZNOLE-35), ZNOL 35 , NOL 35, NOL-35 III, NAMIT-10 , ZNIOL, ZNIOL-10-1, ZNIOL-10-P, ZNIOL-20, ZNIOL-20-P, ZNIOL-35, ZNIOL-35-P, ZNIOL-35 -1, NIOL -20, NIOL-35, NOL-SESH -10, NOL-SESH -10-1, NOL-SESH-6, NOL-SESH-6-1, NOL-SESH-20, NOL-SESH-35 , 3xZNOL-SESH-6, 3xZNOL-SESH -10, NALI-SESH-10, NALI-SESH-6, NTMI 6, NTMI 10, NAMI 6, NAMI 10, NAMI 35, NAMI 110, ZNAMIT-6, ZNAMIT-10 , ZNOMP 35, NOM 6, NOM 10, NOM 35, NKF 110, NKF 150, NKF 220 and others.
For voltage measuring transformers, the primary winding is 3000/√3, 6000/√3, 10000/√3, 13800/√3, 18000/√3, 24000/√3, 27000/√3, 35000/√3, 66000/√3 , 110000/√3, 150000/√3, 220000/√3, 330000/√3, 400000/√3, 500000/√3, and the secondary 100/√3 or 110/√3.
Current transformer is an auxiliary device in which the secondary current is practically proportional to the primary current and is designed to include measuring instruments and relays in alternating current electrical circuits.
Supplied with accuracy class: 0.5; 0.5S; 0.2; 0.2S.
Current transformers are used to convert current of any value and voltage into a current convenient for measuring with standard instruments (5 A), powering current windings of relays, disconnecting devices, as well as isolating devices and their operating personnel from high voltage.
IMPORTANT! Current transformers are available with the following transformation ratios: 5/5, 10/5, 15/5, 20/5, 30/5, 40/5, 50/5, 75/5, 100/5, 150/5, 200 /5, 300/5, 400/5, 500/5, 600/5, 800/5, 1000/5, 1500/5, 2000/5, 2500/5, 3000/5, 5000/5, 8000/5 , 10000/5.
Current transformers on the Russian market are represented by the following models:
TOP-0.66, TShP-0.66, TOP-0.66-I, TShP-0.66-I, TShL-0.66, TNShL-0.66, TNSh-0.66, TOL-10, TLO-10, TOL-10-I, TOL-10-M, TOL-10-8, TOL-10-IM, TOL-10 III, TSHL-10, TLSH-10, TPL-10-M, TPOL-10 , TPOL-10M, TPOL-10 III, TL-10, TL-10-M, TPLC-10, TOLK-6, TOLK-6-1, TOLK-10, TOLK-10-2, TOLK-10-1, TOL-20, TSL-20-I, TPL-20, TPL-35, TOL-35, TOL-35-III-IV, TOL-35 II-7.2, TLC-35, TV, TLC-10, TPL-10S , TLM-10, TSHLP-10, TPK-10, TVLM-10, TVK-10, TVLM-6, TLK-20, TLK-35-1, TLK-35-2, TLK-35-3, TOL-SESH 10, TOL-SESH-20, TOL-SESH-35, TSHL-SESH 0.66, Ritz transformers, TPL-SESH 10, TZLK(R)-SESH 0.66, TV-SESH-10, TV-SESH-20 , TV-SESH-35, TSHL-SESH-10, TSHL-SESH-20, TZLV-SESH-10 and others.
Classification of voltage transformers
Voltage transformers differ:
A) by the number of phases - single-phase and three-phase;
b) according to the number of windings - two-winding, three-winding, four-winding.
Example 0.5/0.5S/10P;
c) according to the accuracy class, i.e. according to the permissible error values;
d) by cooling method - transformers with oil cooling (oil), with natural air cooling (dry and with cast insulation);
e) by type of installation - for indoor installation, for outdoor installation and for complete switchgear.
For voltages up to 6-10 kV, voltage transformers are manufactured dry, that is, with natural air cooling. For voltages above 6-10 kV, oil voltage transformers are used.
Indoor transformers are designed to operate at ambient temperatures from -40 to + 45°C with relative humidity up to 80%.
IN single-phase transformers voltages from 6 to 10 kV, cast insulation is predominantly used. Transformers with cast insulation are completely or partially (one windings) filled with insulating mass (epoxy resin). Such transformers, intended for indoor installation, differ favorably from oil transformers: they have less weight and overall dimensions and require almost no maintenance in operation.
Three-phase two-winding transformers voltages have conventional three-rod magnetic circuits, and three-winding - single-phase armored ones.
Three-phase three-winding transformer is a group of three single-phase single-pole units, the windings of which are connected according to the appropriate circuit. Three-phase three-winding voltage transformers of the old series (before 1968-1969) had armored magnetic cores. A three-phase transformer is smaller in weight and size than a group of three single-phase transformers. When operating a three-phase transformer for backup, you need to have another transformer at full power
In oil-immersed transformers, the main insulating and cooling medium is transformer oil.
Oil transformer consists of a magnetic circuit, windings, a tank, a cover with inputs. The magnetic core is assembled from sheets of cold-rolled electrical steel, insulated from each other (to reduce losses due to eddy currents). The windings are made of copper or aluminum wire. To regulate the voltage, the HV winding has branches connected to the switch. Transformers provide two types of tap switching: under load - on-load tap-changer (on-load regulation) and without load, after disconnecting the transformer from the network - off-load tap changer (non-excited switching). The second method of voltage regulation is the most common as it is the simplest.
In addition to the above-mentioned oil-cooled transformers (Transformer TM), transformers are produced in a sealed design (TMG), in which the oil does not communicate with air and, therefore, its accelerated oxidation and humidification are excluded. Oil transformers in a sealed design are completely filled with transformer oil and do not have an expander, and temperature changes in its volume during heating and cooling are compensated by changes in the volume of the corrugations of the tank walls. These transformers are filled with oil under vacuum, which increases the electrical strength of their insulation.
Dry transformer, like the oil one, consists of a magnetic core, HV and LV windings, enclosed in a protective casing. The main insulating and cooling medium is atmospheric air. However, air is a less perfect insulating and cooling medium than transformer oil. Therefore, in dry transformers, all insulation gaps and ventilation ducts are made larger than in oil transformers.
Dry transformers are manufactured with windings with glass insulation of heat resistance class B (TSZ), as well as with insulation on silicone varnishes of class N (TSZK). To reduce hygroscopicity, the windings are impregnated with special varnishes. The use of fiberglass or asbestos as insulation for windings can significantly increase operating temperature windings and get a practically fireproof installation. This property of dry transformers makes it possible to use them for installation inside dry rooms in cases where ensuring the fire safety of the installation is a decisive factor. Sometimes dry transformers are replaced by more expensive and difficult to manufacture dry transformers.
Dry transformers have slightly larger overall dimensions and weight (TSZ transformer) and a lower overload capacity than oil ones, and are used for operation in enclosed spaces with a relative humidity of no more than 80%. The advantages of dry transformers include their fire safety (no oil), comparative simplicity of design and relatively low operating costs.
Classification of current transformers
Current transformers are classified according to various criteria:
1. According to their purpose, current transformers can be divided into measuring (TOL-SESH-10, TLM-10), protective, intermediate (for including measuring instruments in the current circuits of relay protection, for equalizing currents in differential protection circuits, etc.) and laboratory (high accuracy, as well as with many transformation ratios).
2. According to the type of installation, current transformers are distinguished:
a) for outdoor installation, installed in open switchgears (TLK-35-2.1 UHL1);
b) for indoor installation;
c) built into electrical devices and machines: switches, transformers, generators, etc.;
d) overhead - placed on top of the bushing (for example, on the high-voltage input of a power transformer);
e) portable (for control measurements and laboratory tests).
3. According to the design of the primary winding, current transformers are divided:
a) multi-turn (coil, loop-winding and figure-of-eight winding);
b) single-turn (rod);
c) tires (TSh-0.66).
4. According to the installation method, current transformers for indoor and outdoor installation are divided:
a) checkpoints (TPK-10, TPL-SESH-10);
b) support (TLK-10, TLM-10).
5. Based on insulation, current transformers can be divided into groups:
a) with dry insulation (porcelain, bakelite, cast epoxy insulation, etc.);
b) with paper-oil insulation and with capacitor paper-oil insulation;
c) filled with compound.
6. According to the number of transformation stages, there are current transformers:
a) single-stage;
b) two-stage (cascade).
7. Transformers are classified according to operating voltage:
a) for rated voltage above 1000 V;
b) for rated voltage up to 1000 V.
Combination of different classification characteristics is entered into the type designation of current transformers, consisting of alphabetic and digital parts.
Current transformers are characterized by rated current, voltage, accuracy class and design. At a voltage of 6-10 kV they are made as support and feed-through windings with one or two secondary windings of accuracy class 0.2; 0.5; 1 and 3. The accuracy class indicates the maximum error introduced by the current transformer into the measurement results. Transformers of accuracy classes 0.2, which have a minimum error, are used for laboratory measurements, 0.5 - for powering meters, 1 and 3 - for powering current windings of relays and technical measuring instruments. For safe operation, the secondary windings must be grounded and must not be open circuited.
When installing switchgear with a voltage of 6-10 kV, current transformers with cast and porcelain insulation are used, and for voltages up to 1000 V - with cast, cotton and porcelain insulation.
An example is the TOL-SESH-10 reference 2-winding current transformer with cast insulation for a rated voltage of 10 kV, design version 11, with secondary windings:
For connecting measurement circuits, with accuracy class 0.5 and load 10 VA;
- for connecting protection circuits, with accuracy class 10P and load 15 VA;
For a rated primary current of 150 Amperes, a rated secondary current of 5 Amps, climatic modification “U”, placement category 2 according to GOST 15150-69 when placing an order for production from JSC VolgaEnergoKomplekt:
TOL-SESH-10-11-0.5/10R-10/15-150/5 U2 - with a rated primary current - 150A, secondary - 5A.
The operation of a transformer is based on the phenomenon of mutual induction. If the primary winding of a transformer is connected to an alternating current source, then alternating current will flow through it, which will create an alternating magnetic flux in the transformer core. This magnetic flux, penetrating the turns of the secondary winding, will induce in it electromotive force(EMF). If the secondary winding is short-circuited to any energy receiver, then under the influence of the induced EMF, a current will begin to flow through this winding and through the energy receiver.
At the same time, a load current will also appear in the primary winding. Thus, electrical energy, being transformed, is transferred from primary network to the secondary at the voltage for which the energy receiver connected to the secondary network is designed.
In order to improve the magnetic connection between the primary and secondary windings, they are placed on a steel magnetic core. The windings are isolated both from each other and from the magnetic circuit. The higher voltage winding is called the winding high voltage(HV), and the lower voltage winding is the winding low voltage(NN). The winding connected to the network of the electrical energy source is called primary; the winding from which energy is supplied to the receiver is secondary.
Typically, the voltages of the primary and secondary windings are not the same. If the primary voltage is less than the secondary, the transformer is called a step-up, if it is more than the secondary, it is called a step-down. Any transformer can be used both as a step-up and step-down transformer. Step-up transformers are used to transmit electricity over long distances, and step-down transformers are used to distribute it between consumers.
In three-winding transformers, three windings isolated from each other are placed on the magnetic core. Such a transformer, powered from one of the windings, makes it possible to receive two different voltages and supply electrical energy to two different groups of receivers. In addition to the high and low voltage windings, the three-winding transformer has a medium voltage (MV) winding.
The transformer windings are given a predominantly cylindrical shape, made from round insulated copper wire at low currents, and from rectangular copper bars at high currents.
The low voltage winding is located closer to the magnetic core, since it is easier to isolate it from it than the high voltage winding.
The low voltage winding is insulated from the rod by a layer of some insulating material. The same insulating gasket is placed between the high and low voltage windings.
With cylindrical windings, it is advisable to give the cross-section of the magnetic core a round shape so that there are no non-magnetic gaps left in the area covered by the windings. The smaller the non-magnetic gaps, the smaller the length of the winding turns, and therefore the mass of copper for a given cross-sectional area of the steel rod.
However, it is difficult to produce round rods. The magnetic core is assembled from thin steel sheets, and to obtain a round rod would require a large number of steel sheets of different widths, and this would require the manufacture of many dies. Therefore, in transformers high power the rod has a stepped cross-section with the number of steps not exceeding 15-17. The number of steps in the section of the rod is determined by the number of angles in one quarter of the circle. The yoke of the magnetic circuit, i.e. that part of it that connects the rods, also has a stepped cross-section.
For better cooling in magnetic cores, as well as in the windings of powerful transformers, ventilation ducts are installed in planes parallel and perpendicular to the plane of steel sheets.
In low-power transformers, the cross-sectional area of the wire is small and the windings are simplified. The magnetic cores of such transformers have a rectangular cross-section.
Transformer ratings
The useful power for which a transformer is designed according to heating conditions, i.e. the power of its secondary winding at full (rated) load is called the rated power of the transformer. This power is expressed in units of apparent power - volt-amperes (VA) or kilovolt-amperes (kVA). The active power of a transformer is expressed in watts or kilowatts, i.e. the power that can be converted from electrical to mechanical, thermal, chemical, light, etc. Cross-sections of the wires of the windings and all parts of the transformer, as well as any electrical apparatus or an electrical machine, are determined not by the active component of the current or active power, but by the total current flowing through the conductor and, therefore, full power. All other values that characterize the operation of a transformer under the conditions for which it is designed are also called nominal.
Each transformer is equipped with a shield made of material that is not subject to atmospheric influences. The plate is attached to the transformer tank in a visible place and contains its rating data, which is etched, engraved, embossed or in another way to ensure the durability of the signs. The following data is indicated on the transformer panel:
1. Manufacturer's brand.
2. Year of manufacture.
3. Serial number.
4. Type designation.
5. Number of the standard to which the manufactured transformer corresponds.
6. Rated power (kVA). (For three-windings, indicate the power of each winding.)
7. Rated voltages and branch voltages of windings (V or kV).
8. Rated currents of each winding (A).
9. Number of phases.
10. Current frequency (Hz).
11. Diagram and connection group of transformer windings.
12. Voltage short circuit (%).
13. Type of installation (internal or external).
14. Cooling method.
15. Total mass of the transformer (kg or t).
16. Mass of oil (kg or t).
17. Mass of the active part (kg or t).
18. Switch positions indicated on its drive.
For a transformer with artificial air cooling, its power is additionally indicated when cooling is turned off. The serial number of the transformer is also stamped on the tank under the shield, on the cover near the HV input of phase A and on the left end of the upper flange of the yoke beam of the magnetic circuit. The transformer symbol consists of alphabetic and digital parts. The letters mean the following:
T - three-phase,
O - single-phase,
M - natural oil cooling,
D - oil cooling with blast (artificial air and with natural oil circulation),
C - oil cooling with forced oil circulation through a water cooler,
DC - oil with blast and forced oil circulation,
G - lightning-proof transformer,
H at the end of the designation - transformer with voltage regulation under load,
H in second place - filled with non-flammable liquid dielectric,
T in third place is a three-winding transformer.
The first number after the letter designation of the transformer shows the rated power (kVA), the second number - the rated voltage of the HV winding (kV). Thus, type TM 6300/35 means a three-phase two-winding transformer with natural oil cooling with a power of 6300 kVA and a HV winding voltage of 35 kV. The letter A in the transformer type designation means autotransformer. In the designation of three-winding autotransformers, the letter A is placed either first or last. If the autotransformer circuit is the main one (the HV and MV windings form an autotransformer, and the LV winding is additional), the letter A is placed first; if the autotransformer circuit is additional, the letter A is placed last.
The operation of the transformer is based on two basic principles:
1. Changing over time electric current creates a magnetic field (electromagnetism)
2. A change in the magnetic flux passing through the winding creates an EMF in this winding (electromagnetic induction)
The alternating current flowing in the primary winding creates an alternating magnetic flux in the magnetic core, changes in which, in turn, passing through the secondary winding, create an alternating EMF in it.
Rice. 1 Schematic structure of the transformer. 1 - primary winding, 2 - secondary
Faraday's law
The emf created in the secondary winding can be calculated by Faraday's law, which states that:
N2 - number of turns in the secondary winding,
Φ is the total magnetic flux through one turn of the winding. If the turns of the winding are located perpendicular to the magnetic field lines, then the flux will be proportional magnetic field B and the area S through which it passes.
The EMF created in the primary winding is, respectively:
U1 - instantaneous voltage value at the ends of the primary winding,
N1 is the number of turns in the primary winding.
Dividing the equation U2 by U1, we get the ratio:
Ideal Transformer Equations
If the secondary winding is connected to a load, then electrical energy will be transferred from the primary circuit to the secondary. Ideally, a transformer transforms all incoming energy from the primary circuit into a magnetic field and then into the energy of the secondary circuit. In this case, the incoming energy is equal to the converted energy.
P1 is the instantaneous value of the power supplied to the transformer coming from the primary circuit,
P2 is the instantaneous value of the power converted by the transformer entering the secondary circuit.
Combining this equation with the ratio of the voltages at the ends of the windings, we obtain the equation of an ideal transformer:
Thus, we find that as the voltage at the ends of the secondary winding U2 increases, the secondary circuit current I2 decreases.
To convert the resistance of one circuit to the resistance of another, you need to multiply the value by the square of the ratio. For example, resistance Z2 is connected to the ends of the secondary winding, its reduced value to the primary circuit will be 
. This rule This is also true for the secondary circuit: 
.
The operation of a transformer is based on the phenomenon of electromagnetic induction. One of the windings, called the primary winding, is supplied with voltage from external source. The alternating current flowing through the primary winding creates an alternating magnetic flux in the magnetic core, shifted in phase, with a sinusoidal current, by 90° relative to the voltage in the primary winding. As a result of electromagnetic induction, an alternating magnetic flux in the magnetic circuit creates in all windings, including the primary, an induction emf proportional to the first derivative of the magnetic flux, with a sinusoidal current shifted by 90° in reverse side in relation to magnetic flux. When the secondary windings are not connected to anything (no-load mode), the induced emf in the primary winding almost completely compensates for the voltage of the power source, so the current through the primary winding is small and is determined mainly by it inductive reactance. The induction voltage on the secondary windings in no-load mode is determined by the ratio of the number of turns of the corresponding winding w2 to the number of turns of the primary winding w1:
When the secondary winding is connected to a load, current begins to flow through it. This current also creates a magnetic flux in the magnetic circuit, and it is directed opposite to the magnetic flux created by the primary winding. As a result, compensation of the induced emf is disrupted in the primary winding and EMF source power supply, which leads to an increase in the current in the primary winding until the magnetic flux reaches almost the same value. In this mode, the ratio of the currents of the primary and secondary windings is equal to the inverse ratio of the number of turns of the windings
the stress ratio to a first approximation also remains the same. As a result, the power consumed from the source in the primary winding circuit is almost completely transferred to the secondary.
Schematically, the above can be depicted as follows:
U1 → I1 → I1w1 → Ф → ε2 → I2
The instantaneous magnetic flux in the magnetic circuit of the transformer is determined by the time integral of the instantaneous value of the emf in the primary winding, and in the case of a sinusoidal voltage, it is shifted in phase by 90° with respect to the emf. The EMF induced in the secondary windings is proportional to the first derivative of the magnetic flux, and for any current shape it coincides in phase and shape with the EMF in the primary winding.
Transformer– a static electromagnetic device for converting alternating current of one voltage into alternating current of another voltage, the same frequency. Transformers are used in electrical circuits in the transmission and distribution of electrical energy, as well as in welding, heating, rectifying electrical installations and much more.
Transformers are distinguished by the number of phases, the number of windings, and the cooling method. Power transformers are mainly used to increase or decrease voltage in electrical circuits.
Design and principle of operation
The diagram of a single-phase two-winding transformer is presented below.
The diagram shows the main parts: a ferromagnetic core, two windings on the core. The first winding and all the quantities that relate to it (i1-current, u1-voltage, n1-number of turns, Ф1 – magnetic flux) are called primary, the second winding and the corresponding quantities are called secondary.
The primary winding is connected to a network with alternating voltage, its magnetizing force i1n1 creates an alternating magnetic flux Ф in the magnetic circuit, which is coupled to both windings and induces an emf in them e1= -n1 dФ/dt, e2= -n2dФ/dt. With a sinusoidal change in the magnetic flux Ф = Фm sinωt, the emf is equal to e = Em sin (ωt-π/2). In order to calculate the effective value of the EMF, you need to use the formula E = 4.44 f n Фm, where f is the cyclic frequency, n is the number of turns, Фm is the amplitude of the magnetic flux. Moreover, if you want to calculate the value of the EMF in any of the windings, you need to substitute the number of turns in this winding instead of n.
From the above formulas we can conclude that the EMF lags behind the magnetic flux by a quarter of a period and the ratio of the EMF in the transformer windings is equal to the ratio of the number of turns E1/E2=n1/n2.
If the second winding is not under load, then the transformer is in no-load mode. In this case, i2 = 0, and u2 = E2, the current i1 is small and the voltage drop in the primary winding is small, so u1≈E1 and the EMF ratio can be replaced by the voltage ratio u1/u2 = n1/n2 = E1/E2 = k. From this we can conclude that the secondary voltage can be less or more than the primary, depending on the ratio of the number of turns of the windings. Ratio of primary to secondary voltage at idling transformer is called the transformation ratio k.
As soon as the secondary winding is connected to the load, current i2 appears in the circuit, that is, energy is transferred from the transformer, which receives it from the network, to the load. The transfer of energy in the transformer itself occurs due to the magnetic flux F.
Typically, the output power and input power are approximately equal, since transformers are electric machines with quite high efficiency, but if you need to make a more accurate calculation, then the efficiency is found as the ratio active power at the output to the active power at the input η = P2/P1.
The transformer magnetic core is a closed core assembled from sheets of electrical steel with a thickness of 0.5 or 0.35 mm. Before assembly, the sheets are insulated on both sides with varnish.
Based on the type of construction, a distinction is made between rod (L-shaped) and armored (W-shaped) magnetic cores. Let's consider their structure.
A rod transformer consists of two rods on which there are windings and a yoke that connects the rods, in fact, that’s why it got its name. Transformers of this type are used much more often than armored transformers.
Armor transformer It is a yoke inside which contains a rod with a winding. The yoke seems to protect the rod, which is why the transformer is called armored.
Winding
The design of the windings, their insulation and methods of fastening to the rods depend on the power of the transformer. For their manufacture, copper wires of round and rectangular cross-section are used, insulated with cotton yarn or cable paper. The windings must be strong, elastic, have low energy losses and be simple and inexpensive to manufacture.
Cooling
Energy losses occur in the windings and core of the transformer, resulting in heat generation. Therefore, the transformer requires cooling. Some low-power transformers release their heat to the environment, and the steady-state temperature does not affect the operation of the transformer. Such transformers are called “dry”, i.e. with natural air cooling. But at medium and high power, air cooling does not cope, instead they use liquid, or rather oil. In such transformers, the winding and magnetic circuit are placed in a tank with transformer oil, which enhances the electrical insulation of the windings from the magnetic circuit and at the same time serves to cool them. The oil receives heat from the windings and magnetic circuit and transfers it to the walls of the tank, from which the heat is dissipated into the environment. At the same time, layers of oil with a difference in temperature circulate, which improves heat transfer. For transformers with a power of up to 20-30 kVA, cooling a tank with smooth walls is enough, but at higher powers, tanks with corrugated walls are installed. You also need to take into account that when heated, oil tends to increase in volume, so reserve tanks and exhaust pipes are installed in high-power transformers (if the oil boils, vapors will appear that need an outlet). In transformers of lower power, they are limited by the fact that oil is not poured all the way to the cover.
Generators that are located at power plants produce a very powerful EMF. In practice, such tension is rarely needed. Therefore, such voltage must be converted.
Transformers
Devices called transformers are used to convert voltage. Transformers can either increase the voltage or decrease it. There are also stabilizing transformers that do not increase or decrease the voltage.
Consider the transformer design in the following figure.
picture
Transformer design and operation
The transformer consists of two coils with wire windings. These coils are placed on a steel core. The core is not monolithic, but is assembled from thin plates.
One of the windings is called the primary. Connect to this winding alternating voltage, which comes from the generator and which needs to be converted. The other winding is called the secondary winding. A load is connected to it. Load is all the devices and devices that consume energy.
The following figure shows symbol transformer.
picture
The operation of a transformer is based on the phenomenon of electromagnetic induction. When alternating current passes through the primary winding, an alternating magnetic flux is created in the core. And since the core is common, the magnetic flux induces a current in the other coil.
There are N1 turns in the primary winding of the transformer, its total induced emf is equal to e1 = N1*e, where e is the instantaneous value of the induced emf in all turns. e is the same for all turns of both coils.
The secondary winding has N2 turns. EMF e2 = N2*e is induced in it.
Hence:
We neglect the winding resistance. Consequently, the values of induced emf and voltage will be approximately equal in magnitude:
When the secondary winding circuit is open, no current flows in it, therefore:
Instantaneous emf values e1, e2 oscillate in one phase. Their ratio can be replaced by the ratio of the values of the effective emfs: E1 and E2. And we replace the ratio of instantaneous voltage values with effective voltage values. We get:
E1/E2 ≈U1/U2 ≈N1/N2 = K
K – transformation coefficient. At K>0 the transformer increases the voltage when K<0 – the transformer reduces the voltage. If a load is connected to the ends of the secondary winding, an alternating current will appear in the second circuit, which will cause another magnetic flux to appear in the core.
This magnetic flux will reduce the change in the magnetic flux of the core. For loaded transformer, the following formula will be valid:
U1/U2 ≈ I2/I1.
That is, when the voltage increases several times, we will reduce the current strength by the same amount.
The operating principle of a transformer is related to the principle of electromagnetic induction. The current entering the primary winding creates a magnetic flux in the magnetic circuit.
The operation of a transformer is based on the phenomenon of electromagnetic induction. One of the windings, called the primary winding, is supplied with voltage from an external source. The alternating current flowing through the primary winding creates an alternating magnetic flux in the magnetic core, phase-shifted, with a sinusoidal current, by 90° relative to the current in the primary winding. As a result of electromagnetic induction, an alternating magnetic flux in the magnetic circuit creates in all windings, including the primary, an induction emf proportional to the first derivative of the magnetic flux, with a sinusoidal current shifted by 90° relative to the magnetic flux. When the secondary windings are not connected to anything (no-load mode), the induced emf in the primary winding almost completely compensates for the voltage of the power source, so the current through the primary winding is small and is determined mainly by its inductive reactance. The induction voltage on the secondary windings in no-load mode is determined by the ratio of the number of turns of the corresponding winding w2 to the number of turns of the primary winding w1: U2=U1w2/w1.
When the secondary winding is connected to a load, current begins to flow through it. This current also creates a magnetic flux in the magnetic circuit, and it is directed opposite to the magnetic flux created by the primary winding. As a result, the compensation of the induced emf and the power source emf is disrupted in the primary winding, which leads to an increase in the current in the primary winding until the magnetic flux reaches almost the same value. In this mode, the ratio of the currents of the primary and secondary windings is equal to the inverse ratio of the number of turns of the windings (I1=I2w2/w1), the voltage ratio, to a first approximation, also remains the same.
Schematically, the above can be depicted as follows:
U1 > I1 > I1w1 > Ф > ε2 > I2.
The magnetic flux in the magnetic core of the transformer is shifted in phase with respect to the current in the primary winding by 90°. The emf in the secondary winding is proportional to the first derivative of the magnetic flux. For sine signals, the first derivative of sine is cosine, and the phase shift between sine and cosine is 90°. As a result, when the windings are turned on in agreement, the transformer shifts the phase by approximately 180°. When the windings are connected in opposite directions, an additional phase shift of 180° is added and the total phase shift by the transformer is approximately 360°.
Idle experience
To test the transformer, use the open-circuit test and the short-circuit test.
When the transformer is in idle mode, its secondary winding is open and there is no current in this winding (/2-0).
If the primary winding of the transformer is connected to the network of an alternating current electrical energy source, then the no-load current I0 will flow in this winding, which is a small value compared to the rated current of the transformer. In high-power transformers, the no-load current can reach values of the order of 5-10% of the rated current. In low-power transformers, this current reaches 25-30% of the rated current. The no-load current I0 creates a magnetic flux in the transformer magnetic circuit. To excite the magnetic flux, the transformer consumes reactive power from the network. As for the active power consumed by the transformer during idle operation, it is spent to cover power losses in the magnetic circuit caused by hysteresis and eddy currents.
Because reactive power when the transformer is no-load, there is significantly more active power, then its power factor cos φ is very small and is usually equal to 0.2-0.3.