A static electromagnetic device designed for conversion. Types of transformers

Transformers are devices designed to convert electricity. Their main task is to change the value of alternating voltage. Transformers are used both as independent devices and as components of other electrical devices.

Quite often, transformers are used to transmit electricity over long distances. Directly at power generating enterprises, they make it possible to significantly increase the voltage generated by the alternating current source.

By increasing the voltage to 1150 kW, transformers provide more economical transmission of electricity: electricity losses in wires are significantly reduced and it becomes possible to reduce the cross-sectional area of ​​cables used in power lines.

The operating principle of the transformer is based on the effect of electromagnetic induction. The classic design consists of a metal magnetic core and electrically unconnected windings made of insulated wire. The winding to which electricity is supplied is called the primary winding. The second one, connected to devices that consume current, is called secondary.

After the transformer is connected to an alternating current source, its primary winding produces an alternating magnetic flux. It is transmitted through the magnetic circuit to the turns of the secondary winding, inducing an alternating EMF (electromotive force) in them. If there is a consumption device, an electric current arises in the secondary winding circuit.

The ratio between the input and output voltage of a transformer is directly proportional to the ratio of the number of turns of the corresponding windings.

This value is called the transformation coefficient: Ktr = W 1 /W 2 =U 1 /U 2, where:

• W1, W2 - number of turns of the primary and secondary windings, respectively;
• U1, U2 - input and output voltages, respectively.

The windings can be arranged either as separate coils or one on top of the other. For low-power devices, the windings are made of wire with cotton or enamel insulation. The micro transformer has windings made of aluminum foil with a thickness of no more than 20-30 microns. The insulating material is an oxide film obtained by natural oxidation of the foil.

TYPES AND TYPES OF TRANSFORMERS

Transformers are fairly widespread devices, so there are many varieties of them. According to design and purpose, they are divided into:

Autotransformers.

They have one winding with several taps. By switching between these taps, different voltage readings can be obtained. The disadvantages include the lack of galvanic isolation between the input and output.

Pulse transformers.

Designed to convert a pulse signal of short duration (about ten microseconds). In this case, the pulse shape is distorted minimally. Typically used in video signal processing circuits.

Isolation transformer.

The design of this device provides for the complete absence of electrical connection between the primary and secondary windings, that is, it provides galvanic isolation between the input and output circuits. It is used to increase electrical safety and, as a rule, has a transformation coefficient equal to unity.

Peak transformer.

Used to control semiconductor electrical devices such as thyristors. Converts sinusoidal AC voltage into spike-shaped pulses.

It is worth highlighting the method of classifying transformers according to the method of their cooling.

There are dry devices with natural air cooling in an open, protected and sealed housing design and with forced air cooling.

Liquid cooled devices can use different types of heat transfer fluid. Most often this is oil, but there are models where water or a liquid dielectric is used as a heat exchanger.

In addition, transformers with combined liquid-air cooling are produced. Moreover, each of the cooling methods can be either natural or with forced circulation.

CHARACTERISTICS OF TRANSFORMERS

The main technical characteristics of transformers include:

• voltage level: high voltage, low voltage, high potential;
• conversion method: up, down;
• number of phases: single or three phase;
• number of windings: two- and multi-winding;
• shape of the magnetic circuit: rod, toroidal, armored.

One of the main parameters is the rated power of the device, expressed in volt-amperes. The exact limits may vary slightly depending on the number of phases and other characteristics. However, as a rule, devices that convert up to several tens of volt-amperes are considered low-power.

Medium-power devices are considered to be devices from several tens to several hundred, and high-power transformers operate with values ​​from several hundred to several thousand volt-amperes.

Operating frequency - there are devices with a reduced frequency (less than the standard 50 Hz), industrial frequency - exactly 50 Hz, increased industrial frequency (from 400 to 2000 Hz) and high frequency (up to 1000 Hz).

APPLICATION AREA

Transformers are widely used both in industry and in everyday life. One of the main areas of their industrial application is the transmission of electricity over long distances and its redistribution.

Welding (electrothermal) transformers are no less famous. As the name suggests, this type of device is used in electric welding and for supplying power to electrothermal installations. Also, a fairly wide area of ​​application of transformers is to provide power supplies to various equipment.

Depending on their purpose, transformers are divided into:

They are the most common type of industrial transformer. Used to increase and decrease voltage. Used in power lines. On the way from power generating facilities to the consumer, electricity can pass through step-up power transformers several times, depending on the remoteness of a particular consumer.

Before being supplied directly to consumer devices (machines, household and lighting devices), electricity undergoes reverse transformations, passing through power step-down transformers.

Current.

Remote measuring current transformers are used to ensure the operability of electricity metering circuits for the protection of power lines and power autotransformers. They have different sizes and performance characteristics. They can be placed in the housings of small devices or be separate, large devices.

Depending on the functions performed, the following types are distinguished:

• measuring - supplying current to measuring and control devices;
• protective - connected to protective circuits;
• intermediate - used for repeated conversion.

Voltages.

They are used to convert voltage to the required values. In addition, such devices are used in galvanic isolation circuits and electrical and radio measurements.

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It is difficult for a person who is little familiar with electrical engineering to imagine what a transformer is, where it is involved, and the purpose of its design elements.

General information about the device

A transformer is a static electromagnetic device designed to convert a variable frequency current with one voltage into an alternating current with a different voltage, but with the same frequency, based on the phenomenon of electromagnetic induction.

Devices are used in all spheres of human activity: electric power industry, radio engineering, radio-electronic industry, household sphere.

Design

The transformer design assumes the presence of one or more individual coils (tape or wire), located under a single magnetic flux, wound on a core made of a ferromagnet.

The most important structural parts are as follows:

• winding;
• frame;
• magnetic circuit (core);
• cooling system;
• insulation system;
• additional parts necessary for protective purposes, for installation, to provide access to the output parts.

In devices you can most often see two types of winding: the primary, which receives electric current from an external power source, and the secondary, from which the voltage is removed.

The core provides improved return contact of the windings and has reduced magnetic flux resistance.

Some types of devices operating at ultrahigh and high frequencies are produced without a core.

The production of devices is established in three basic winding concepts:

• armored;
• toroidal;
• core.

The design of core transformers involves winding the winding onto the core strictly horizontally. In armor-type devices it is enclosed in a magnetic circuit and placed horizontally or vertically.

Reliability, operational features, design and principle of operation of the transformer are taken without any influence from the principle of its manufacture.

Principle of operation

The operating principle of the transformer is based on the effect of mutual induction. The supply of variable frequency current from a third-party electricity supplier to the inputs of the primary winding forms a magnetic field in the core with a variable flux passing through the secondary winding and inducing the formation of an electromotive force in it. Short-circuiting the secondary winding at the electricity receiver causes an electric current to pass through the receiver due to the influence of the electromotive force, and at the same time a load current is generated in the primary winding.

The purpose of the transformer is to move converted electrical energy (without changing its frequency) to the secondary winding from the primary winding with a voltage suitable for the functioning of consumers.

Classification by type

Power

An AC power transformer is a device used to transform electricity in supply networks and electrical installations of significant power.

The need for power plants is explained by the serious difference in operating voltages of main power lines and city networks coming to end consumers required for the operation of electrically powered machines and mechanisms.

Autotransformers

The design and principle of operation of a transformer in this design implies direct coupling of the primary and secondary windings, thanks to which their electromagnetic and electrical contact is simultaneously ensured. The windings of the devices have at least three terminals, differing in their voltage.

The main advantage of these devices should be called good efficiency, because not all the power is converted - this is significant for small differences in input and output voltages. The downside is that the transformer circuits are not isolated (lack of separation) from each other.

Current transformers

This term usually denotes a device powered directly from the electricity supplier, used to reduce the primary electric current to suitable values ​​for those used in measuring and protective circuits, alarms, and communications.

The primary winding of electric current transformers, the design of which provides for the absence of galvanic connections, is connected to a circuit with an alternating electric current to be determined, and electrical measuring instruments are connected to the secondary winding. The electric current flowing through it approximately corresponds to the current of the primary winding divided by the transformation ratio.

Voltage transformers

The purpose of these devices is to reduce voltage in measuring circuits, automation and relay protection. Such protective and electrical measuring circuits in devices for various purposes are separated from high voltage circuits.

Pulse

These types of transformers are necessary for changing short-term video pulses, which, as a rule, are repeated in a certain period with a significant duty cycle, with a minimum change in their shape. The purpose of use is the transfer of an orthogonal electrical pulse with the steepest cut and front, a constant amplitude indicator.

The main requirement for devices of this type is the absence of distortion when transferring the shape of the converted voltage pulses. The action of a voltage of any shape on the input causes the output of a voltage pulse of an identical shape, but probably with a different range or changed polarity.

Separating

What an isolation transformer is becomes clear based on the definition itself - it is a device with a primary winding that is not electrically connected (i.e., separated) from the secondary windings.

There are two types of such devices:

• power;
• signal.

Power ones are used to improve the reliability of electrical networks in the event of an unexpected synchronous connection with the ground and live parts, or non-current carrying elements that are energized due to an insulation failure.

Signal signals are used to ensure galvanic isolation of electrical circuits.

Coordinating

How this type of transformer works is also clear from its name. Matching devices are devices that are used to match the resistance of individual elements of electrical circuits with each other with a minimum change in the signal shape. Also, devices of this type are used to eliminate galvanic interactions between individual parts of circuits.

Peak transformers

The principle of operation of peak transformers is based on transforming the nature of the voltage, from input sinusoidal to pulsed. The polarity after the transition changes after half the period.

Twin throttle

Its purpose, structure and principle of operation as a transformer are absolutely identical to devices with a pair of similar windings, which, in this case, are absolutely identical, wound counter-winding or coordinated.

You can also often find this device called a counter inductive filter. This indicates the scope of application of the device - input voltage filtering in power supplies, audio equipment, and digital devices.

Operating modes

Idling speed (XX)

This operating procedure is implemented by opening the secondary network, after which the flow of electric current in it stops. An no-load current flows in the primary winding; its component element is the magnetizing current.

When the secondary current is zero, the electromotive force of induction in the primary winding completely compensates for the voltage of the supply source, and therefore, when the load currents disappear, the current passing through the primary winding corresponds in its value to the magnetizing current.

The functional purpose of idle operation of transformers is to determine their most important parameters:

• transformation indicator;
• losses in the magnetic circuit.

Load mode

The mode is characterized by the operation of the device when voltage is applied to the inputs of the primary circuit and a load is connected to the secondary circuit. The loading current flows through the “secondary”, and in the primary - the total load current and the no-load current. This operating mode is considered predominant for the device.

The question of how a transformer operates in the main mode is answered by the basic law of induced emf. The principle is this: applying a load to the secondary winding causes the formation of a magnetic flux in the secondary circuit, which forms a loading electric current in the core. It is directed in the direction opposite to its flow, created by the primary winding. In the primary circuit, the parity of the electromotive forces of the electricity supplier and induction is not respected; in the primary winding, the electric current is increased until the magnetic flux returns to its original value.

Short circuit (SC)

The device switches to this mode when the secondary circuit is briefly closed. A short circuit is a special type of load, the applied load - the resistance of the secondary winding - is the only one.

The principle of operation of a transformer in short-circuit mode is as follows: a slight alternating voltage comes to the primary winding, the terminals of the secondary are short-circuited. The input voltage is set so that the magnitude of the closing current corresponds to the rated current of the device. The voltage value determines the energy losses due to heating of the windings, as well as to the active resistance.

This mode is typical for measuring type devices.

Based on the variety of devices and types of purposes of transformers, we can confidently say that today they are indispensable devices used almost everywhere, thanks to which stability is ensured and the voltage values ​​​​required by the consumer are achieved, both in civil networks and industrial networks.

Transformer is an electromagnetic device that transfers electrical energy from one circuit to another through inductively coupled wires. In other words, if two coils of wires are placed close to each other, without touching, the magnetic field from the first coil (called the primary winding) affects the other coil (called the secondary winding). The property is called "induction". Induction was discovered by Joseph Henry and Michael Faraday in 1831.

How does a transformer work?

A transformer is used to drive the voltage up or down in an AC electrical circuit. A transformer can be used to convert alternating current to direct current. They can be very large, as in national utility systems, or they can be a very small device built inside the electronics. It is an inseparable part of all electrics today.

Now, if you want to change the voltage in the circuit, you can do so by changing the current flowing in the primary winding (the voltage remains high). In this case, the current level affects the induced voltage on the secondary winding. An alternating magnetic field induces a change in electromagnetic force or "voltage".

Types of transformers

Welding transformer

Voltage stabilizer (the main component of the device is a transformer)

Current transformers

Electronic transformer for halogen lamps 220V/12V

Who invented the transformer?

Otto Blati, Miksa Dery, Károly Cypernovsky, engineers of the Austro-Hungarian Empire, first developed and used the transformer, both in experimental and commercial systems. Later, Lucien Gaulard, Sebastian de Ferranti, and William Stanley improved the design. See the next question for more details.

When was the transformer invented?

The property of induction was discovered in the 1830s, but the device did not exist until 1886, when William Stanley, working for Westinghouse, assembled the first redesigned, commercial transformer. His work was built on some rudimentary construction by Ganz & Co., in Hungary, and by Lucien Gaulard and John Dickson Gibbs, in England. Nikola Tesla did not invent the transformer, as some dubious sources claim. The Europeans mentioned above did the first work in this area, George Westinghouse and Stanley developed a transformer that was cheap to manufacture and easy to use in the end.

Where were the first transformers used?

The first AC system to use the new transformer was in Great Barrington, Massachusetts in 1886. Previously, the devices were used in Austria-Hungary in 1878-1880, and in 1882 in England. Lucien Gaulard (French) used an alternating current system for the revolutionary Lanzo, at the electrical exhibition in Turin in 1884 (Northern Italy). In 1891, Mikhail Dobrovsky developed and demonstrated a three-phase transformer at an electrical exhibition in Frankfurt, Germany.

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

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 turns of the windings 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

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,

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

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.

What is a transformer. Beginning in the 1830s, transformers became an important component in electrical and electronic circuits. And, despite the fact that new advanced technologies in the field of electronics have reduced the need for transformers, they are still in demand in various devices.

Transformer operating principle

The operation of a transformer is based on the principles of electromagnetism, and this allows alternating current voltages to be reduced or increased. Experiments by Michael Faraday in the 19th century showed that changes in current in a conductor (for example, the primary winding of a transformer) affects changes in the magnetic field around that conductor. If another conductor (secondary winding) is located directly in the area of ​​the changing magnetic field, then voltage induction will occur in it.

Transformation ratio

Faraday also calculated that the voltage induced in the secondary winding would have a magnitude that depended on the transformation ratio of the transformer itself. That is, if the secondary winding has half the number of turns of the primary winding, then the voltage on the secondary winding will be two times lower than the voltage on the primary winding. Conversely, if the secondary winding has twice as many turns as the primary winding, the secondary voltage will be twice the primary voltage.

Winding power ratio

Since the transformer is a passive circuit component (does not have any external power source), it cannot supply more power than it receives. Therefore, if the secondary voltage is greater than the primary voltage by a certain amount, then the secondary current will be less than the primary current by the same amount. That is, if the voltage of the secondary winding is twice the voltage in the primary, then the current in the secondary will be two times lower than in the primary.

The operation of a transformer can be described by two formulas that relate the transformation ratio to the ratio of turns of the transformer windings.

• U1 = primary voltage.
• I1 = primary current.
• U2 = secondary voltage.
• I2 = secondary current.
• N1 = number of turns in the primary winding.
• N2 = number of turns of the secondary winding.

Transformer power loss

The formulas given above refer to an ideal transformer. An ideal transformer does not have any power losses, that is, the power of the primary winding (U1*I1) is equal to the power of the secondary winding (U2*I2).

While real transformers can be extremely efficient, some losses will still occur because not all of the magnetic flux coming from the primary winding reaches the secondary winding. Power losses that occur in a transformer are of three types:

Power losses in windings

These losses can occur in windings made of metals other than copper. Losses manifest themselves in the form of heat that occurs in the wires of the windings. The power loss in the transformer windings can be calculated based on the current in the winding and its resistance using the following formula: P = I2*R2. To minimize losses, winding resistance should be kept low by using winding wires of suitable cross-section.

Hysteresis losses

Every time alternating current causes magnetization and demagnetization of the transformer core (once in each cycle), the magnetic field strength vector changes its direction and a certain amount of energy is expended.

In this case, the amount of energy used depends on the magnetic resistance of the core material. In large power transformer cores, where hysteresis losses are a big problem, this is solved by using special crystallized steel, which creates minimal magnetic resistance.

Eddy current losses

Since the iron or steel core is the electrical conductor in a magnetic circuit, a change in current in the primary winding will tend to generate an emf in the core as well as in the secondary winding. The current will resist the change in the magnetic field generated in the core. For this reason, these eddy currents must be reduced.

Therefore, the iron core is not made from a single piece of iron, but is assembled from thin sheets or plates, each plate having an insulating layer in the form of varnish or an oxide film. Multilayer cores significantly reduce the formation of eddy currents without compromising the magnetic properties of the core.

Transformer Ferrite Cores

In high-frequency transformers, eddy current losses are reduced by using cores made of ceramic material containing a large number of small particles of iron, zinc or manganese powder. Ceramic isolates metal particles from each other, which gives the same effect as thin plates and is more effective at high frequencies.

Due to the applied loss reduction techniques described above, real transformers approach transformers with ideal performance. In large power transformers, efficiency is about 98%. Therefore, for most practical calculations we can assume that the transformer is “ideal”.

Ratio of volts per turn of winding

A transformer having 1000 turns in the primary winding and 100 turns in the secondary winding has a transformation ratio of 1000:100 or 10:1. Therefore, 100 volts applied to the primary winding will produce a secondary voltage of 10 volts.

Another way to calculate transformer voltage is the volts/turn ratio. If 100 volts are applied to the primary winding containing 1000 turns, then 1 turn accounts for 0.1 volt (100/1000). Therefore, every ten turns on the secondary winding will create 1 volt of voltage.