What is the principle of operation of a transformer? Current transformers - operating principle and application

Transformer is a static electromagnetic device having two or more inductively coupled windings and designed to convert, by electromagnetic induction, one or more alternating current systems into one or more other alternating current systems.

Transformers are widely used for the following purposes.

For transmission and distribution of electrical energy. Typically, in power plants, alternating current generators produce electrical energy at a voltage of 6-24 kV.

To power various circuits of radio and television equipment; communication devices, automation in telemechanics, electrical household appliances; to separate electrical circuits of various elements of these devices; for voltage matching

To include electrical measuring instruments and some devices, such as relays, in high-voltage electrical circuits or in circuits through which large currents pass, in order to expand the measurement limits and ensure electrical safety. Transformers used for this purpose are called measuring. They have a relatively low power, determined by the power consumed by electrical measuring instruments, relays, etc.

Transformer operating principle

The electromagnetic circuit of a single-phase two-winding transformer consists of two windings (Fig. 2.1) placed on a closed magnetic circuit, which is made of ferromagnetic material. The use of a ferromagnetic magnetic core makes it possible to strengthen the electromagnetic coupling between the windings, that is, to reduce the magnetic resistance of the circuit through which the magnetic flux of the machine passes. Primary winding 1 is connected to an alternating current source - an electrical network with voltage u 1 . Load resistance Z H is connected to the secondary winding 2.

The higher voltage winding is called high voltage winding (HV), and low voltage - low voltage winding (NN). The beginnings and ends of the HV winding are designated by letters A And X; LV windings - letters A And X.

When connected to the network, alternating current appears in the primary winding i 1 , which creates an alternating magnetic flux F, closing along the magnetic circuit. The flow F induces alternating emfs in both windings - e 1 And e 2 , proportional, according to Maxwell's law, to the number of turns w 1 and w 2 Corresponding winding and flux rate of change d F/ dt.

Thus, the instantaneous values ​​of the emf induced in each winding are

e 1 = - w 1 d F/dt; e2= -w 2 dФ/dt.

Consequently, the ratio of instantaneous and effective EMF in the windings is determined by the expression

Consequently, selecting the number of winding turns accordingly, at a given voltage U 1 you can get the desired voltage U 2 . If it is necessary to increase the secondary voltage, then the number of turns w 2 is taken greater than the number w 1; such a transformer is called increasing If you need to reduce the voltage U 2 , then the number of turns w 2 is taken less than w 1; such a transformer is called downward,

EMF ratio E HV windings of higher voltage to EMF E Low voltage LV windings (or the ratio of their number of turns) are called transformation ratio

k= E VN / E NN = w VN / w NN

Coefficient k always greater than one.

In energy transmission and distribution systems, in some cases, three-winding transformers are used, and in radio electronics and automation devices, multi-winding transformers are used. In such transformers, three or more windings isolated from each other are placed on the magnetic core, which makes it possible to receive two or more different voltages when powering one of the windings (U 2 , U 3 , U 4, etc.) for power supply to two or more consumer groups. In three-winding power transformers, a distinction is made between high, low and medium voltage (MV) windings.

Only voltages and currents are converted in a transformer. The power remains approximately constant (it decreases somewhat due to internal energy losses in the transformer). Hence,

I 1 /I 2 ≈ U 2 /U 1 ≈ w 2 /w 1 .

When the secondary voltage of the transformer increases in k times compared to the primary, current i 2 in the secondary winding decreases accordingly k once.

The transformer can only operate in alternating current circuits. If the primary winding of a transformer is connected to a direct current source, then a magnetic flux is formed in its magnetic wire, constant in magnitude and direction over time. Therefore, in the primary and secondary windings in a steady state, EMF is not induced, and therefore, electrical energy is not transferred from the primary circuit to the secondary. This mode is dangerous for the transformer, since due to the lack of EMF E 1 primary winding current I 1 =U 1 R 1 is quite large.

An important property of a transformer used in automation and radio electronics devices is its ability to convert load resistance. If you connect a resistance to an AC source R through a transformer with a transformation ratio To, then for the source circuit

R" = P 1 /I 1 2 ≈ P 2 /I 1 2 ≈ I 2 2 R/I 1 2 ≈ k 2 R

Where R 1 - power consumed by the transformer from an alternating current source, W; R 2 = I 2 2 R≈ P 1 - power consumed by the resistance R from the transformer.

Thus, the transformer changes the resistance value R to k 2 once. This property is widely used in the development of various electrical circuits to match the load resistance with the internal resistance of electrical energy sources.

Transformer is a static electromagnetic device with two (or more) windings, most often designed to convert alternating current of 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 transmitting electrical energy over long distances, distributing it 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 at power plants by synchronous generators at a voltage of 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, there are power transformers, voltage measuring transformers and current transformers

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-filled 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 during 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 switching (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 moistening 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 the operating temperature of the windings and obtain 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.

The combination of various classification characteristics is entered into the current transformer type designation, 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 an electromotive force (EMF) in it. 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 the primary network to the secondary one 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 high voltage (HV) winding, and the lower voltage winding is called the low voltage (LV) winding. 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 high-power transformers the rod has a stepped cross-section with the number of steps no more than 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, ventilation ducts are installed in magnetic cores, as well as in the windings of powerful transformers, 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, by the total 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. Short circuit voltage (%).
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.

A transformer is an indispensable device in electrical engineering.

Without it, the energy system in its current form could not exist.

These elements are also present in many electrical appliances.

Those wishing to get to know them better are invited to this article, the topic of which is the transformer: the principle of operation and types of devices, as well as their purpose.

This is the name given to a device that changes the magnitude of alternating electrical voltage. There are varieties that can change its frequency.

Many devices are equipped with such devices, and they are also used independently.

For example, installations that increase voltage to transmit current along electric highways.

They raise the voltage generated by the power plant to 35 - 750 kV, which gives a double benefit:

• losses in wires are reduced;
• smaller wires are required.

In urban electrical networks, the voltage is again reduced to 6.1 kV, again using. In distribution networks that distribute electricity to consumers, the voltage is reduced to 0.4 kV (this is the usual 380/).

Principle of operation

The operation of a transformer device is based on the phenomenon of electromagnetic induction, which consists of the following: when the parameters of the magnetic field crossing a conductor change, an EMF (electromotive force) arises in the latter. The conductor in a transformer is present in the form of a coil or winding and the total emf is equal to the sum of the emf of each turn.

For normal operation, it is necessary to exclude electrical contact between the turns, therefore they use a wire in an insulating sheath. This coil is called the secondary.

The magnetic field required to generate EMF in the secondary coil is created by another coil. It is connected to a current source and is called primary. The operation of the primary coil is based on the fact that when current flows through a conductor, an electromagnetic field is formed around it, and if it is wound into a coil, it is amplified.

How does a transformer work?

When flowing through the coil, the parameters of the electromagnetic field do not change and it is unable to cause an EMF in the secondary coil. Therefore, transformers only work with alternating voltage.

The nature of voltage conversion is influenced by the ratio of the number of turns in the windings - primary and secondary. It is designated “Kt” - transformation coefficient. The law is in force:

Kt = W1 / W2 = U1 / U2,

• W1 and W2 - number of turns in the primary and secondary windings;
• U1 and U2 - voltage at their terminals.

Therefore, if there are more turns in the primary coil, then the voltage at the terminals of the secondary coil is lower. Such a device is called a step-down device; its Kt is greater than one. If there are more turns in the secondary coil, the transformer increases the voltage and is called a step-up transformer. Its Kt is less than one.

Large power transformer

If we neglect losses (ideal transformer), then from the law of conservation of energy it follows:

P1 = P2,

where P1 and P2 are the current power in the windings.

Because the P=U*I, we get:

• U1 * I1 = U2 * I2;
• I1 = I2 * (U2 / U1) = I2 / Kt.

It means:

• in the primary coil of the step-down device (Kt > 1) a current of less strength flows than in the secondary circuit;
• with step-up transformers (Kt< 1) все наоборот: сила тока в первичной катушке выше, чем в цепи вторичной.

This circumstance is taken into account when selecting the cross-section of wires for the windings of devices.

Design

Transformer windings are placed on a magnetic core - a part made of ferromagnetic, transformer or other soft magnetic steel. It serves as a conductor of the electromagnetic field from the primary coil to the secondary coil.

Under the influence of an alternating magnetic field, currents are also generated in the magnetic circuit - they are called eddy currents. These currents lead to energy loss and heating of the magnetic circuit. The latter, in order to reduce this phenomenon to a minimum, is made up of many plates isolated from each other.

The coils are placed on the magnetic circuit in two ways:

• near;
• wind one on top of the other.

Windings for microtransformers are made of foil with a thickness of 20 - 30 microns. As a result of oxidation, its surface becomes a dielectric and plays the role of insulation.

Transformer design

In practice, it is impossible to achieve the ratio P1 = P2 due to three types of losses:

1. magnetic field dissipation;
2. heating of wires and magnetic circuit;
3. hysteresis.

Hysteresis losses are energy costs for magnetization reversal of the magnetic circuit. The direction of the electromagnetic field lines is constantly changing. Each time you have to overcome the resistance of dipoles in the structure of the magnetic circuit, lined up in a certain way in the previous phase.

They strive to reduce hysteresis losses by using different designs of magnetic cores.

So, in reality, the values ​​of P1 and P2 are different and the ratio P2 / P1 is called the efficiency of the device. To measure it, the following operating modes of the transformer are used:

• idle move;
• short-circuited;
• with load.

In some types of transformers operating with high frequency voltage, there is no magnetic circuit.

Idle mode

The primary winding is connected to a current source, and the secondary circuit is open. With this connection, no-load current flows in the coil, which mainly represents the reactive magnetizing current.

This mode allows you to determine:

• Device efficiency;
• transformation ratio;
• losses in the magnetic circuit (in the language of professionals - losses in steel).

Transformer circuit in idle mode

Short-circuit mode

The terminals of the secondary winding are closed without load (short-circuited), so that the current in the circuit is limited only by its resistance. The voltage is applied to the primary contacts so that the current in the secondary winding circuit does not exceed the rated one.

This connection allows you to determine the heating losses of the windings (copper losses). This is necessary when implementing circuits using active resistance instead of a real transformer.

Load mode

In this state, a consumer is connected to the terminals of the secondary winding.

Cooling

During operation, the transformer heats up.

Three cooling methods are used:

1. natural: for low-power models;
2. forced air (fan blowing): medium power models;
3. powerful transformers are cooled using liquid (mainly oil).

Oil cooled device

Types of transformers

Devices are classified according to purpose, type of magnetic circuit and power.

Power transformers

The most numerous group. This includes all transformers operating in the power grid.

Autotransformer

This type has an electrical contact between the primary and secondary windings. When winding the wire, several conclusions are made - when switching between them, a different number of turns is used, which changes the transformation ratio.

• Increased efficiency. This is explained by the fact that only part of the power is converted. This is especially important when the difference between the input and output voltages is small.
• Low cost. This is due to lower consumption of steel and copper (the autotransformer has compact dimensions).

These devices are advantageous to use in networks with voltages of 110 kV or more with effective grounding at Kt not higher than 3-4.

Current transformer

Used to reduce the current in the primary winding connected to the power source. The device is used in protective, measuring, signaling and control systems. The advantage compared to shunt measurement circuits is the presence of galvanic isolation (no electrical contact between the windings).

The primary coil is connected to the alternating current circuit - being tested or controlled - with the load in series. An actuating indicator device, for example, a relay, or a measuring device is connected to the terminals of the secondary winding.

Current transformer

The permissible resistance in the secondary coil circuit is limited to scanty values ​​- almost a short circuit. For most current coils, the rated current in this coil is 1 or 5 A. When the circuit is opened, a high voltage is generated in it, which can break through the insulation and damage the connected devices.

Pulse transformer

Works with short pulses, the duration of which is measured in tens of microseconds. The pulse shape is practically not distorted. Mainly used in video systems.

Welding transformer

This device:

• reduces tension;
• designed for rated current in the secondary winding circuit up to thousands of amperes.

You can regulate the welding current by changing the number of turns of the windings involved in the process (they have several terminals). In this case, the value of the inductive reactance or the secondary open-circuit voltage changes. By means of additional terminals, the windings are divided into sections, therefore the welding current is adjusted in steps.

The dimensions of the transformer largely depend on the frequency of the alternating current. The higher it is, the more compact the device will be.

Welding transformer TDM 70-460

The design of modern inverter welding machines is based on this principle. In them, the alternating current is processed before being supplied to the transformer:

• rectified by means of a diode bridge;
• in the inverter - a microprocessor-controlled electronic unit with quickly switching key transistors - it again becomes variable, but with a frequency of 60 - 80 kHz.

That’s why these welding machines are so light and small.

Switching type power supplies are also used, for example, in PCs.

Isolation transformer

This device necessarily has galvanic isolation (there is no electrical contact between the primary and secondary windings), and Kt is equal to one. That is, the isolation transformer leaves the voltage unchanged. It is necessary to improve connection security.

Touching live elements of equipment connected to the network through such a transformer will not result in a severe electric shock.

In everyday life, this method of connecting electrical appliances is appropriate in damp rooms - in bathrooms, etc.

In addition to power transformers, there are signal isolation transformers. They are installed in an electrical circuit for galvanic isolation.

Magnetic cores

There are three types:

1. Rod. Made in the form of a rod with a stepped section. The characteristics leave much to be desired, but they are easy to implement.
2. Armored. They conduct the magnetic field better than rod ones and, in addition, protect the windings from mechanical influences. Disadvantage: high cost (requires a lot of steel).
3. Toroidal. The most effective type: they create a uniform concentrated magnetic field, which helps reduce losses. Transformers with a toroidal magnetic core have the highest efficiency, but they are expensive due to the complexity of manufacturing.

Power

Power is usually denoted in volt-amperes (VA). According to this criterion, devices are classified as follows:

• low-power: less than 100 VA;
• average power: several hundred VA;

There are high power installations, measured in thousands of VA.

Transformers differ in purpose and characteristics, but their operating principle is the same: an alternating magnetic field generated by one winding excites an EMF in the second, the magnitude of which depends on the number of turns.

The need to convert voltage arises very often, which is why transformers are widely used. This device can be made independently.

The operating principle of the transformer is based on the famous law of mutual induction. If you turn on the primary winding of this one, then alternating current will begin to flow through this winding. This current will create an alternating magnetic flux in the core. This magnetic flux will begin to penetrate the turns of the secondary winding of the transformer. An alternating EMF (electromotive force) will be induced on this winding. If you connect (short-circuit) the secondary winding to some kind of electrical energy receiver (for example, to a conventional incandescent lamp), then under the influence of an induced electromotive force, an alternating electric current will flow through the secondary winding to the receiver.

At the same time, load current will flow through the primary winding. This means that electricity will be transformed and transmitted from the secondary winding to the primary winding at the voltage for which the load is designed (that is, the electricity receiver connected to the secondary network). The operating principle of the transformer is based on this simple interaction.

To improve the transmission of magnetic flux and strengthen the magnetic coupling, the winding of the transformer, both primary and secondary, is placed on a special steel magnetic core. The windings are isolated both from the magnetic circuit and from each other.

The operating principle of the transformer varies according to the voltage of the windings. If the voltage of the secondary and primary windings is the same, it will be equal to unity, and then the very meaning of the transformer as a voltage converter in the network is lost. Separate step-down and step-up transformers. If the primary voltage is less than the secondary, then such an electrical device will be called a step-up transformer. If the secondary is less, then downward. However, the same transformer can be used both as a step-up and step-down transformer. A step-up transformer is used to transmit energy over various distances, for transit and other things. Step-down ones are used mainly for redistributing electricity between consumers. The calculation is usually made taking into account its subsequent use as a voltage step-down or step-up.

As mentioned above, the principle of operation of the transformer is quite simple. However, there are some interesting details in its design.

In three-winding transformers, three insulated windings are placed on a magnetic core. Such a transformer can receive two different voltages and transmit energy to two groups of electricity receivers at once. In this case, they say that in addition to the low-voltage windings, a three-winding transformer also has a medium-voltage winding.

The transformer windings are cylindrical in shape and are completely insulated from each other. With such a winding, the cross-section of the rod will have a round shape to reduce non-magnetized gaps. The fewer such gaps, the smaller the mass of copper, and, consequently, the mass and cost of the transformer.

With the discovery and beginning of the industrial use of electricity, the need arose to create systems for its conversion and delivery to consumers. This is how transformers appeared, the principle of operation of which will be discussed.

Their appearance was preceded by the discovery of the phenomenon of electromagnetic induction by the great English physicist Michael Faraday almost 200 years ago. Later, he and his American colleague D. Henry drew a diagram of the future transformer.

Faraday transformer

The first embodiment of the idea in iron took place in 1848 with the creation of an induction coil by the French mechanic G. Ruhmkorff. Russian scientists also made their contribution. In 1872, Moscow University professor A.G. Stoletov discovered the hysteresis loop and described the structure of a ferromagnet, and 4 years later, the outstanding Russian inventor P.N. Yablochkov received a patent for the invention of the first alternating current transformer.

How a transformer works and how it works

Transformers are the name of a huge “family” that includes single-phase, three-phase, step-down, step-up, measuring and many other types of transformers. Their main purpose is to convert one or more alternating current voltages to another based on electromagnetic induction at a constant frequency.

So, briefly, how the simplest single-phase transformer works. It consists of three main elements - the primary and secondary windings and the magnetic circuit that unites them into a single whole, on which they are, as it were, strung. The source is connected exclusively to the primary winding, while the secondary winding removes and transmits the already changed voltage to the consumer.

The primary winding connected to the network creates an alternating electromagnetic field in the magnetic circuit and forms a magnetic flux, which begins to circulate between the windings, inducing an electromotive force (EMF) in them. Its value depends on the number of turns in the windings. For example, to lower the voltage, it is necessary that there be more turns in the primary winding than in the secondary. It is on this principle that step-down and step-up transformers work.

An important feature of the transformer design is that the magnetic core has a steel structure, and the windings, usually cylindrical in shape, are isolated from it, are not directly connected to each other and have their own markings.

Voltage transformers

This is perhaps the most numerous type of transformer family. In a nutshell, their main function is to make the energy produced in power plants available for consumption by various devices. For this purpose, there is a power transmission system consisting of step-up and step-down transformer substations and power lines.

First, the electricity produced by the power plant is supplied to a step-up transformer substation (for example, from 12 to 500 kV). This is necessary in order to compensate for the inevitable losses of electricity during transmission over long distances.

The next stage is a step-down substation, from where electricity is supplied via a low-voltage line to a step-down transformer and then to the consumer in the form of a voltage of 220 V.

But the work of transformers does not end there. Most of the household electrical appliances around us - PCs, TVs, printers, automatic washing machines, refrigerators, microwave ovens, DVDs and even energy-saving light bulbs have step-down transformers. An example of an individual “pocket” transformer is a mobile phone (smartphone) charger.

The huge variety of modern electronic devices and the functions they perform correspond to many different types of transformers. This is not a complete list of them: power, pulse, welding, separating, matching, rotating, three-phase, peak transformers, current transformers, toroidal, rod and armor.

What are they, transformers of the future?

The transformer industry is considered to be quite conservative. Nevertheless, it also has to reckon with revolutionary changes in the field of electrical engineering, where nanotechnology is making itself known more and more loudly. Like many other devices, they are gradually getting smarter.

An active search is underway for new structural materials – insulating and magnetic – that can provide higher reliability of transformer equipment. One direction could be the use of amorphous materials, which will significantly increase its fire safety and reliability.

Explosion- and fire-proof transformers will appear in which chlorinated biphenyls, used to impregnate electrical insulating materials, will be replaced by non-toxic liquid, environmentally friendly dielectrics.

An example of this is SF6 power transformers, where the function of the coolant is performed by non-flammable SF6 gas, sulfur hexafluoride, instead of the far from safe transformer oil.

It’s a matter of time to create “smart” power grids equipped with semiconductor solid-state transformers with electronic control, with the help of which it will be possible to regulate the voltage depending on the needs of consumers, in particular, connect renewable and industrial power sources to the home network, or, conversely, turn off unnecessary ones when they are not necessary.

Another promising area is low-temperature superconducting transformers. Work on their creation began back in the 60s. The main problem faced by scientists is the enormous size of the cryogenic systems required to produce liquid helium. Everything changed in 1986, when high-temperature superconducting materials were discovered. Thanks to them, it became possible to abandon bulky cooling devices.

Superconducting transformers have a unique quality: at high current densities, losses in them are minimal, but when the current reaches critical values, the resistance from zero level increases sharply.