Power transformer is a transformer that is used to convert energy in electrical networks, as well as installations that are used to work with electrical energy.

A transformer with a directly connected primary and secondary winding, which provides them with both electrical and electromagnetic coupling. As a rule, the transformer winding has at least 3 terminals, connecting to which allows you to obtain different voltages. One of the main advantages of this type of transformer is its high efficiency (since only part of the power is converted). Disadvantages include the lack of electrical insulation between the primary and secondary circuits.

Instrument transformers are used in alternating current installations and serve to isolate the circuits of measuring instruments and relays from the high voltage network, as well as to expand the measurement limits of measuring instruments. If the measuring instruments were connected directly to the high voltage circuit, then each of the devices could simply become dangerous to touch. To avoid this, the design of the devices would have to be significantly more complicated, since the cross-section of the current-carrying parts would have to cope with large currents, and their insulation would have to cope with high voltages.

Instrument transformers can be divided into two types: voltage transformers and current transformers. Thanks to their use, it becomes possible to operate the same devices with standard measurement limits.

In the case of a current measuring transformer, a large current is converted into a small current, and in the case of a voltage measuring transformer, a high voltage is changed into a low one.

A transformer that is used to reduce the primary current to a level used in measurement, control, protection and signaling circuits. The secondary winding has a nominal value of 1A and 5A. The primary winding is connected to the circuit with the measured alternating current. In turn, measuring instruments are connected to the secondary. The current that passes through the secondary winding is proportional to the current that passes through the primary winding by the transformation ratio.

A transformer that is used to convert high voltage to lower voltage in circuits, in measuring circuits, as well as in relay protection and automation circuits. Thanks to the use of a transformer, it becomes possible to isolate the logical protection circuits and measuring circuits from high voltage circuits.

Pulse transformer

A transformer that is used to convert pulse signals whose duration reaches tens of microseconds, with maximum preservation of the pulse shape. It is usually used in cases where the transmission of a rectangular electrical pulse is required. It transforms short-term video voltage pulses, the periodic repetition of which is accompanied by a high duty cycle. As a rule, the main requirements for IT include the transmission of the shape of the transformed voltage pulses in an undistorted form. In addition, when the IT input is exposed to one or another type of voltage, it is necessary to obtain the same voltage pulse at the output (in extreme cases, of a different polarity or amplitude).

Isolation transformer

A transformer in which the primary winding is not electrically connected in any way to the secondary windings. The main purpose of power isolation transformers is to increase the safety of electrical networks, the requirements for which increase in the event of contact with the ground, as well as live and non-current-carrying parts that are energized as a result of insulation damage. Galvanic isolation of electrical circuits is ensured by signal isolation transformers.

Peak transformer

A transformer that converts a sinusoidal voltage into a pulsed voltage whose polarity changes every half cycle.

Transformers with minimal and normal magnetic leakage

STE transformers are characterized by the fact that their winding has minimal magnetic dissipation. In this case, the current strength is regulated due to the screw choke mechanism, which is located separately.

Transformers with normal magnetic leakage are somewhat similar to the previous ones. The main difference between them is that there is an additional reluctance coil, which is located on the main magnetic core bars and the inductor winding. The inductor is installed on a magnetic core, and the current strength is regulated in exactly the same way as when working with an STE transformer.

Transformers with increased magnetic dissipation

The main difference between transformers with increased magnetic dissipation and transformers with low and normal dissipation is the presence of a movable design of shunts and windings. Thanks to this approach, higher performance characteristics can be obtained regardless of the mass of the transformer itself.

Among transformers with increased magnetic dissipation, you can find models with moving windings, for example, transformers TSK-300, TD-300, TS-500. In addition, there are models that have movable magnetic shunts (TDM-317 and STSh-250). You can also note models with fixed magnetizing shunts and windings (TDF-2001 and TDF-1001) and designs with complex magnetic switching (VD-306 and VDU-506). Today, the most commonly used models of transformers are TD and TS, as well as their modifications TDE and TDM.

It is also worth noting thyristor transformers, the operation of which is based on phase regulation of current strength due to thyristors, which convert the incoming alternating current into alternating pulses. At first, such transformers, due to the instability of arc combustion, were used exclusively for resistance and slag welding. However, with the development of semiconductor technologies, thyristor welding transformers have undergone certain changes and have become one of the best devices that are excellent not only for slag and spot welding, but also for manual arc welding.

Question 1. What does a transformer consist of?
Answer. The simplest transformer consists of a closed magnetic circuit and two windings in the form of cylindrical coils.
One of the windings is connected to a source of alternating sinusoidal current with voltage u 1 and is called the primary winding. The load of the transformer is connected to the other winding. This winding is called secondary
winding

Question 2. How is energy transferred from one winding to another?
Answer. The transfer of energy from one winding to another is carried out by electromagnetic induction. Alternating sinusoidal current i 1 flowing through the primary winding of the transformer excites an alternating magnetic flux in the magnetic circuit F s, which penetrates the turns of both windings and induces EMF
And
with amplitudes proportional to the number of turns w 1 And w 2. When connected to the secondary winding of the load in it under the influence EMF e 2 an alternating sinusoidal current occurs i 2 and some tension is established u 2.
There is no electrical connection between the primary and secondary windings of the transformer and energy is transferred to the secondary winding through a magnetic field excited in the core.

Question 3. What is the secondary winding of the transformer in relation to the load?
Answer. In relation to the load, the secondary winding of the transformer is a source of electrical energy with EMF e 2. Neglecting losses in the transformer windings, we can assume that the supply voltage U 1 ≈ E 1, and the load voltage U 2 ≈ E 2.

Question 4. What is the transformation ratio?
Answer. Because EMF windings are proportional to the number of turns, then the ratio of the supply voltage of the transformer and the load is also determined by the ratio of the number of turns of the windings, i.e.
U 1 /U 2 ≈ E 1 /E 2 ≈ w 1 /w 2 = k.
Magnitude k called the transformation ratio.

Question 5. Which transformer is called a step-down transformer?
Answer. If the number of turns of the secondary winding is less than the number of turns of the primary w 2< w 1 , That k> 1 and the voltage in the load will be less than the voltage at the transformer input. Such a transformer is called a step-down transformer.

Question 6. Which transformer is called a step-up transformer?
Answer. If the number of turns of the secondary winding is greater than the number of turns of the primary w 2 > w 1, That k < 1 и напряжение в нагрузке будет больше напряжения на входе трансформатора. Такой трансформатор называется повышающим.

Question 7. Which winding of the transformer is called the high voltage winding (HV)?
Answer. The winding connected to the network with a higher voltage is called the high voltage winding (HV). The second winding is called the low voltage (LV) winding.

Question 8. Which transformers are called “dry”?
Answer. Transformers in which heat is removed by air flow are called “dry” transformers.

Question 9. Which transformers are called “oil”?
Answer. In cases where the air flow cannot remove thermal energy in such a way as to ensure limitation
winding insulation temperatures are at an acceptable level; a liquid medium is used for cooling, immersing the transformer in a tank with special transformer oil, which simultaneously acts as a coolant and electrical insulation. Such transformers are called “oil transformers”.

Question 10. How are transformers designated on electrical diagrams?
Answer.


The figure shows the symbols of single-phase two-winding (1, 2, 3) and multi-winding (7, 8) transformers, as well as three-phase transformers (12, 13, 14, 15, 16). The designations of single-phase (4, 5) and three-phase (9, 10) autotransformers and voltage (6) and current (11) instrument transformers are also shown here.

Question 11. What determines the operating conditions and properties of a transformer?
Answer. The operating conditions and properties of the transformer are determined by a system of parameters called nominal, i.e. values ​​of quantities corresponding to the design operating mode of the transformer. They are indicated in the reference data and on the plate attached to the product.

Question 12. How does the operating frequency of a transformer affect its weight and dimensions?
Answer. Increasing the operating frequency of the transformer allows, other things being equal, to significantly reduce the weight and dimensions of the product. Indeed, the voltage of the primary winding is approximately equal to the EMF induced in it by the magnetic flux in the core Φ c, and the total power, for example, of a single-phase transformer is equal to

where and are the specified nominal values ​​of induction in the core and current density in the winding, and S c ∼ l 2 And S i– core cross-section and total cross-section w 1 winding turns. Therefore, increasing the power frequency f allows you to proportionally reduce the cross-section of the core with the same transformer power, i.e. square its linear dimensions l.

Question 13. What is the transformer magnetic circuit used for?
Answer. The magnetic core of the transformer serves to increase the mutual induction of the windings and, in general, is not a necessary structural element. When operating at high frequencies, when losses in a ferromagnet become unacceptably large, and also when it is necessary to obtain linear characteristics, transformers without a core, the so-called, are used. air transformers. However, in the vast majority of cases, the magnetic core is one of the three main elements of the transformer. By design, magnetic cores of transformers are divided into core and armored.

Question 14. What conditions must the design of the transformer windings satisfy?
Answer. The design of transformer windings must satisfy the conditions of high electrical and mechanical strength, as well as heat resistance.
In addition, their manufacturing technology should be as simple as possible, and losses in the windings should be minimal.

Question 15. What are the transformer windings made of?
Answer. The windings are made of copper or aluminum wire. The current density in the copper windings of oil transformers is in the range of 2...4.5 A/mm 2, and in dry transformers 1.2...3.0 A/mm 2. The upper limits apply to larger transformers. In aluminum windings, the current density is 40...45% less. The winding wires can be of a round cross-section with an area of ​​0.02...10 mm 2 or a rectangular cross-section with an area of ​​6...60 mm 2. In many cases, winding coils are wound from several parallel conductors. The winding wires are covered with enamel and cotton or silk insulation. Dry-type transformers use wires with heat-resistant fiberglass insulation.

Question 16. How are transformer windings divided according to the method of arrangement on the rods?
Answer. According to the method of arrangement on the rods, the windings are divided into concentric and alternating. Concentric windings are made in the form of cylinders, the geometric axes of which coincide with the axis of the rods. The low voltage winding is usually located closer to the rod, because this allows you to reduce the insulating gap between the winding and the rod. In alternating windings, the HV and LV coils are alternately positioned along the height of the rod. This design allows increasing the electromagnetic coupling between the windings, but significantly complicates the insulation and winding manufacturing technology, therefore alternating windings are not used in power transformers.

Question 17. How is the transformer windings insulated?
Answer. One of the most important design elements of transformer windings is insulation.
There are main and longitudinal insulation.
The main thing is the insulation of the winding from the rod, tank and other windings. It is made in the form of insulating gaps, electrical insulating frames and washers. At low powers and low voltages, the main insulation function is performed by a frame made of plastic or electrical cardboard, on which the windings are wound, as well as several layers of varnished cloth or cardboard that insulate one winding from the other.
Longitudinal insulation is called insulation between different points of one winding, i.e. between turns, layers and coils. Turn-to-turn insulation is provided by the winding wire's own insulation. For interlayer insulation, several layers of cable paper are used, and the inter-coil insulation is carried out either by insulating gaps, or by a frame or insulating washers.
The insulation design becomes more complicated as the voltage of the HV winding increases, and for transformers operating at voltages of 200...500 kV, the cost of insulation reaches 25% of the cost of the transformer.

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How does a transformer work?

A transformer is a static (i.e., without moving parts) electromagnetic device, single-phase or three-phase, in which the phenomenon of mutual induction is used to convert electrical energy. A transformer converts alternating current of one voltage into alternating current of the same frequency but a different voltage.

The transformer has several electrical windings isolated from one another: single-phase - at least two, three-phase - at least six.

The windings connected to the source of electricity are called primary; the remaining windings, which supply energy to external circuits, are called secondary. The figure below schematically shows the primary and secondary windings of a single-phase transformer; they are equipped with a common closed core assembled from sheet electrical steel.

The ferromagnetic core serves to strengthen the magnetic coupling between the windings, that is, to ensure that most of the magnetic flux of the primary winding meshes with the turns of the secondary winding. In Fig. on the right is the core and six windings of a three-phase transformer. These windings are connected in a star or delta configuration.

To improve cooling and insulation conditions, the transformer is placed in a tank filled with mineral oil (a product of petroleum distillation). This is the so-called oil transformer.

At an alternating current frequency above approximately 20 kHz, the use of a steel core in transformers is impractical due to large losses in steel from hysteresis and eddy currents.

For high frequencies, transformers without ferromagnetic cores are used - air transformers.

If the voltage at the terminals of the primary winding, the primary voltage U1, is less than the secondary voltage U2, then the transformer is called a step-up transformer. If the primary voltage is greater than the secondary one, then it is a step-down voltage (U1>U2). In accordance with the relative value of the rated voltage, it is customary to distinguish between the high voltage (HV) winding and the low voltage (LV) winding.

Let's take a brief look at the operation of a single-phase two-winding transformer with a steel core. Its working process and electrical relationships can be considered characteristic basically of all types of transformers.

The voltage U1 applied to the terminals of the primary winding creates an alternating current i1 in this winding. The current excites an alternating magnetic flux F in the transformer core. Due to the periodic change of this flux, an EMF is induced in both windings of the transformer.

e1= - w1 (?ф: ?t) and e2= - w2 (?ф:?t), where

w1 and w2 - the number of turns of both windings.

Thus, the ratio of EDEs induced in the windings is equal to the ratio of the number of turns of these windings:

e1: e2 = w1: w2

This is the transformation ratio of the transformer.

The efficiency of the transformer is relatively very high, on average about 98%, which makes it possible, at rated load, to consider the primary power received by the transformer and the secondary power supplied to them to be approximately equal, i.e. p1? p2 or u1i1? u2i2, on the basis of which

i1:i2? u2: u1? w 2: w 1

This ratio of instantaneous values ​​of currents and voltages is valid for both amplitudes and effective values:

L1:l2? w 2: w 1?u2: u1,

i.e., the ratio of currents in the windings of a transformer (at a load close to the rated load) can be considered the inverse of the ratio of voltages and the number of turns of the corresponding windings. The smaller the load, the more the no-load current influences, and the given approximate current ratio is violated.

When a transformer operates, the role of the EMF in its primary and secondary windings is completely different. The EMF induced by it in the primary winding arises as the circuit’s opposition to the change in current i1 in it. The phase of this EMF is almost opposite to the voltage.

As in a circuit containing inductance, the current in the primary winding of a transformer

i1=(u1 + e1) : r1,

where g 1 is the active resistance of the primary winding.

From here we obtain the equation for the instantaneous value of the primary voltage:

u1 = -e1 + i1r1 = w t(?ф: ?t) + i1r1,

which can be read as the condition of electrical equilibrium: the voltage u1 applied to the terminals of the primary winding is always balanced by the emf and the voltage drop in the active resistance of the winding (the second term is relatively very small).

Other conditions occur in the secondary circuit. Here, the current i2 is created by the emf e1, which plays the role of the emf of the current source, and with an active load r/n in the secondary circuit this current

i2= l2: (r2 +r/n),

where r2 is the active resistance of the secondary winding.

To a first approximation, the effect of the secondary current i2 on the primary circuit of the transformer can be described as follows.

Current i2, passing through the secondary winding, tends to create a magnetic flux in the transformer core, determined by the magnetizing force (MF) i2w2. According to Lenz's principle, this flow should be in the opposite direction to the direction of the main flow. Otherwise, we can say that the secondary current tends to weaken the magnetic flux inducing it. However, such a decrease in the main magnetic flux F t would disrupt the electrical equilibrium:

u 1 = (-е 1) + i1r1,

since e1 is proportional to the magnetic flux.

A predominance of the primary voltage U1 is created, therefore, simultaneously with the appearance of the secondary current, the primary current increases, moreover, so much as to compensate for the demagnetizing effect of the secondary current and, thus, maintain electrical equilibrium. Consequently, any change in the secondary current should cause a corresponding change in the primary current, while the current of the secondary winding, due to the relatively small value of the component i1r1, has almost no effect on the amplitude and nature of changes over time in the main magnetic flux of the transformer. Therefore, the amplitude of this flow Ft can be considered almost constant. This constancy of Ft is typical for the transformer mode, in which the voltage U1 applied to the terminals of the primary winding is maintained constant.

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.

A transformer is an electromagnetic device designed to convert alternating current of one voltage into alternating current of another voltage at the same frequency.
The operation of the transformer is based on the use of the phenomenon of electromagnetic induction.

Alternating electric current (current that varies in magnitude and direction) induces an alternating magnetic field in the primary coil. This alternating magnetic field induces an alternating voltage in the secondary winding. The magnitude of the EMF voltage depends on the number of turns in the coil and on the rate of change of the magnetic field.

The ratio of the number of turns of the primary and secondary windings determines the transformation ratio:
k = w1 / w2; Where:
w1 - number of turns in the primary winding;
w2 is the number of turns in the secondary winding.
If the number of turns in the primary winding is greater than in the secondary winding, this is a step-down transformer.
If the number of turns in the primary winding is less than in the secondary, this is step-up transformer.

The same transformer can be both downward and upward, depending on which winding is supplied with alternating voltage.

Transformers without a core or with a core of high-frequency ferrite or alsifer are high frequency transformers(frequency above 100 kilohertz).
Transformers with a ferromagnetic core (steel, permalloy, ferrite) are low frequency transformers(frequency below 100 kilohertz).

High-frequency transformers are used in telecommunications equipment, radio communications, etc. Low-frequency transformers are used in audio frequency amplification technology and in telephone communications.
Transformers with a steel (set of steel sheets) core occupy a special place in electrical engineering.

The development of the electric power industry directly depends on powerful power transformers.
The power of power transformers ranges from several watts to hundreds of thousands of kilowatts and above.

Power transformer - what is it?

Two or more windings are put on a closed core (magnetic core), made of steel sheets, one of which is connected to an alternating current source. The other (or other) winding is connected to the consumer of electric current - the load.

The alternating current passing through the primary winding creates a magnetic flux in the steel core, which induces an alternating voltage in each turn of the winding - coil. The voltages of all turns add up to the output voltage of the transformer.

The shape of the core - magnetic circuit, can be W-shaped, O-shaped and toroidal, in the form of a torus. Thus, in a power transformer, electrical power from the primary winding is transferred to the secondary winding through the magnetic flux in the magnetic core.

There are a lot of consumers of electrical energy: electric lighting, electric heaters, radio and television equipment, electric motors and much more. And all these devices require different voltages (AC and DC) and different powers.

This problem is easily solved using a transformer. From a household network with an alternating voltage of 220 volts, you can obtain alternating voltage of any magnitude and, if necessary, convert it into direct voltage.

The efficiency of the transformer is quite high, from 0.9 to 0.98 and depends on losses in the magnetic circuit and on magnetic stray fields.
From the amount of electrical power P depends on the cross-sectional area of ​​the magnetic circuit S.
Based on the value of the area S, when calculating the transformer, the number of turns w per 1 volt is determined: