AC transformer: definition, device, types of structures and designation on the diagram

Most novice radio amateurs, and just those who are fond of radio engineering, are interested in questions about what a transformer is, how it works and what it serves. In fact, everything is very simple: a transformer serves to convert an alternating current from one value with a certain frequency (parameter) to another with an identical parameter.

Transformer device

In accordance with GOST 16110−82, the definition of a transformer is as follows: a transformer is an electromagnetic device of a statistical type, which is equipped with two or more inductively coupled windings for converting one or more alternating current systems into one or more others systems.

This electromagnetic product has a simple structure, consisting of the following elements: (magnetic system), windings, winding frames, insulation (not in all transformers), cooling system. additional elements.

In practice, manufacturers use one of three basic concepts to make transformers:

1. Rod. The windings are wound on the outermost rods.
2. Armor. The side walls are left without windings.
3. Toroidal. It has the shape of a ring with uniform winding around the entire circumference.

It should be noted that the choice of a particular concept does not affect the final parameters of the transformer and does not affects the operational reliability, but, nevertheless, significantly differs in manufacturing technology.

Magnetic system

Magnetic cores for a transformer have a certain geometric shape and are made of a number of materials, to which includes electrical steel, permalloy, ferrite or other material with ferromagnetic properties. Depending on the material and design, the magnetic circuit can be collected from plates, pressed, wound from a thin tape, assembled from two, four or more "horseshoes".

The rods act as a frame for placing the main windings. They can have different spatial arrangements, depending on which several types of systems are distinguished.

1. Plane magnetic system with longitudinal axes of rods and yokes located in the same plane.
2. A spatial system where the longitudinal axes of the rods are located in different planes.
3. Symmetrical system, equipped with identical rods, which have the same relative position in relation to the yokes.
4. An asymmetrical system consisting of rods, some of which may differ in shape, design and size, with different relative positions in relation to the yokes.

Winding design

The winding is the main element of the transformer. It is a multi-turn structure made of one or more copper (less often aluminum) wires of various diameters. Typically, power transformers use square conductors that allow more efficient use of the available space, thereby increasing the fill factor (TO).

Each winding is insulated to prevent short circuits. Special paper or enamel varnish can be used as an insulating material. By the way, if two separately insulated and parallel-connected wires were used for the manufacture of the winding, then they can be equipped with a common paper insulation.

Fuel tank

The tank is one of the most important additional elements of the transformer.. It is a container designed to store transformer oil, as well as to provide physical protection for the active component. In addition, the tank body is designed for mounting auxiliary equipment and a control device.

One of the internal elements of the tank is a high-current resonator. It is susceptible to rapid and frequent overheating when the rated power and transformer currents increase. To reduce the risk of overheating, inserts made of non-magnetic materials are installed around the resonators.

The inner lining of the tank is made of conductive shields that do not allow magnetic fluxes to pass through the walls of the tank. Sometimes there is a coating that is made of a material with low magnetic resistance. This type of coating absorbs internal flows before reaching the walls of the tank.

Before changing the fuel, air is pumped out of the tank in order to prevent a decrease in the dielectric strength of the transformer insulation. From this, an additional purpose of the tank is observed, which is to withstand atmospheric pressure with minimal deformation.

Principle of operation

Transformers operate on the basis of two principles: electromagnetism - the creation of a time-varying magnetic flux under the influence of an electric current, which also changes, and electromagnetic induction - induction of EMF (electromotive force), due to a change in the magnetic flux passing through winding.

The transformer is turned on after the voltage is applied to the primary winding. Together with the voltage, an alternating current is supplied to the winding, which participates in the formation of an alternating magnetic flux in the magnetic circuit. This creates an EMF in all windings of the device.

The output voltage (secondary winding) is in a complex way related to the shape of the input voltage. These complexities have led to the creation of a line of new transformers, which have begun to be used to solve alternative tasks, for example, amplifying current, multiplying frequency and generating signals.

Functional modes

Transformers can operate in three modes: idle (XX) - 1, load - 2 and short circuit - 3.

Mode 1: XX. A feature of this mode is that the secondary transformer circuit is in an open state, so no current flows through it. In this position of the circuit, the current potential is zero, which creates an idle current in the primary circuit, which has a reactive and active component. This EMF is able to fully compensate for the supply voltage. This mode is used to determine the efficiency and the level of core loss.

Mode 2: loads. In this mode, the familiar transformer winding is powered from an external power source, and the load is connected to the secondary circuit. After connecting the load, a current begins to flow through the secondary circuit, which creates a magnetic flux directed in the opposite direction from the flow of the primary winding. This provokes an inequality between the two forces - induction and power supply, which increases the current, which flows through the primary winding until the magnetic flux returns to the original meaning. This mode is the main operating mode for transformers.

Mode 3: short circuit. To obtain this mode, the secondary circuit of the transformer is short-circuited and a low AC voltage is applied to the primary winding. The value of the input voltage is chosen so that the short-circuit current is equal to the nominal one. This mode is used to determine the losses for heating the windings in the transformer circuit.

Types of products

A lot of time has passed since November 30, 1876, which is considered the date of the creation of the first transformer. During this period, the devices have been significantly changed both in terms of design and characteristics. Today there are the following types of transformers:

• AC power transformer. Such transformers are used in power supply networks and electrical installations, which are designed to receive and use electricity. These transformers are used due to the fact that there are various operating voltages along the entire length of the route, for example, on power lines (power transmission lines) it can vary from 0.035 to 0.75 MV (megavolt), and in transformer substations it is equal to 400 V, which are subsequently converted into the usual 220/380 V.
• Autotransformer. A variant of the transformer with a direct connection of the primary and secondary windings, which creates not only electromagnetic, but also electrical induction. Autotransformers are equipped with multi-output windings, the minimum number of which is three. They are used as an element connecting effectively grounded networks with a voltage of 0.11 MV and a transformation ratio of 3 to 4. Autotransformers have two key advantages and one minor disadvantage. The former include economy (due to lower costs for purchasing copper for the windings and steel for the core) and high efficiency - due to the partial conversion of the input power. The disadvantage is the lack of galvanic isolation - electrical isolation between the primary and secondary circuits.
• Current transformer. A device with a primary winding powered by an external current source, while the secondary circuit is trying to be made in such a way that it works in a mode close to a short circuit. The primary winding is connected in series with the load circuit. An alternating current flows in this circuit, which must be controlled. To approach the short circuit mode, voltmeters or indicators, for example, a relay or LED, are connected to the secondary circuit. The presence of additional elements in the secondary circuit has led to one of the areas of application of such transformers, consisting in reducing primary currents to values ​​that can be used for measurement, protection, control and signaling purposes.
• Welding transformer. It is installed in welding machines and is used to convert the 220/380 volt mains voltage to lower values, as well as to increase the current level. The current can be adjusted by changing the inductive resistance or secondary voltage XX. This is done by sectioning the number of turns of the primary or second winding, respectively.
• Isolation transformer. It differs from other devices of this type in the absence of electrical connection between the primary and secondary windings. Separation devices are used in power grids to ensure the safety of people in the event of line breaks or other emergencies that may be harmful, as well as for the purpose of providing galvanic interchanges.

Designation on the diagrams

The transformer in the diagram is indicated as follows: a thick line is drawn in the center that represents the core, to the left of it in the vertical plane the coil is depicted (turns to the core) - the primary winding, and on the right one or more coils - secondary windings.

In general, the schematic representation of the line denoting the core should correspond to the thickness of the turns of the coils shown. If it is necessary to emphasize the material or design features of the core, the center line is slightly modified in the diagram. So, a classic ferrite core is denoted by a solid bold line, and a core with a magnetic gap is denoted by a thin line with a gap in the middle. Magneto-dielectric cores are shown with a thin dashed line.