Wednesday, November 30, 2022

How do Electrical Transformers work?

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If during the first half of the nineteenth century the phenomenon of inductance nor would the first ones have been invented induction coils, there would probably be no transformer electric as we know it nor, therefore, the so vital simple handling of the alternating current which is possible thanks to the transformer. At present, transformers electrical They are essential for the transmission, distribution and use of electrical energy.

From its first commercial use in 1886, today we find electric transformers everywhere, even at home. Its size can be considerable, such as the transformers of the electrical public service networks, or very small, such as the transformers contained in the plug that we connect to the wall to recharge our cell phone, or those that are part of miniaturized electronic components.

Basis

The electric transformer has the basic purpose of transfer electrical energy from one circuit to another by inductively coupled conductors, converting that electrical energy, which has a specific voltage or current, into electrical energy with another voltage or current. These mechanisms are based on two essential principles: electromagnetism and the electromagnetic induction.

In other words, it is important that electric flow can produce magnetic fields and that these magnetic fields can change on a coil of wire e induce a voltage (or tension) at the ends of that coil.

These principles restrict transformer applications to alternating current only, but that is precisely where its advantage lies, since the DC it cannot be transformed simply or inexpensively, which explains the wide use of alternating current, which can be easily transformed.

The simplicity, reliability, and economy of transformer voltage conversion was the main factor in selecting AC power transmission in the “War of Currents” at the end of the 19th century. In electronic circuits, new circuit design methods have replaced some of the transformer applications, but electronic technology has also developed new transformer designs and applications.

Operation and parts of an electrical transformer

The basic principle of operation of a transformer is that a variable current in the primary winding creates a variable magnetic flux in the core transformer and therefore a variable magnetic flux in the secondary winding. This variable magnetic flux induces a electromotive force variable (emf) or voltage on the secondary winding.

Consequently, a simple transformer is essentially made up of three parts, as we see in the figure.

Simple electrical transformer

Primary winding:

The primary winding (or primary coil) is connected to the power source and carries the alternating current from the supply line. It can be a low or high voltage winding, depending on the transformer application.

Magnetic material core:

It is the magnetic circuit in which the windings are wound and where the alternating magnetic flux occurs. Until not long ago, all transformer cores were made up of sheet steel stacks (or laminations) held firmly together. Sometimes the laminations were coated with a thin varnish – or a sheet of insulating paper was inserted at regular intervals between laminations – to reduce eddy current losses.

A new type of core construction consists of a continuous strip of silicon steel that is tightly wound in a spiral around the insulated windings and firmly attached by spot welding at the end. This type of construction reduces the cost of manufacture and the loss of power in the core due to eddy currents.

Secondary winding:

The secondary winding (or secondary coil) is the one that supplies energy to the load and is where the electromotive force (emf) is generated by the change of magnetism in the core it surrounds. It can be a low or high voltage winding, depending on the transformer application.

Sometimes the transformer may have only one winding that will serve the dual purpose of primary and secondary coil.

Generalities

While the basic structure of electrical transformers is essentially the same across the board, the exact specifications vary widely. The cores Transformers come in a variety of shapes and materials (solid, air, steel, toroidal, etc.) and can vary considerably in size. The size of the transformer greatly affects the degree of efficiency. The energy is dissipated in the cores, windings, and surrounding structures, causing the efficiency of a transformer is never 100%. In general, the larger the transformer, the higher its efficiency. In the power transfer process, small transformers tend to lose more power than larger ones.

All transformers must include the circulation of a refrigerant to remove residual heat produced by losses. Small transformers up to a few kilowatts in size are usually adequately cooled by air circulation. Larger “dry” type transformers may have cooling fans. Some dry transformers are confined in pressurized tanks and are cooled by nitrogen or other gases.

The conductive material of the transformer must be shielded to ensure that current is carried around the core and not through a short circuit between the turns of the winding. In power transformers, the voltage difference between parts of the primary and secondary winding can be quite large, therefore between the layers of the windings a isolation to prevent arcing and the transformer can also be immersed in oil to provide additional insulation.

Classification of electrical transformers

There are a variety of ways to classify electrical transformers. One of those classifications is according to the relationship between the number of turns in the windings. These might be:

  • Winding composed of many turns of relatively fine copper wire, insulated enough to withstand the voltage applied to it. In this case we say that it is a high voltage winding.
  • Winding composed of relatively few turns of heavy copper wire, capable of carrying considerable current at low voltage. In this case we say that it is a low voltage winding.

From this point of view there are four possible combinations that give rise to different types of transformers, that is:

Step-down transformers: They are connected in such a way that the voltage delivered is less than that supplied, since the secondary winding has fewer turns than the primary, as we see in the figure below.

Step-down transformer

Step-down transformer

Step-up transformers: They are connected so that the voltage delivered is greater than the voltage delivered, since the secondary winding has more turns than the primary.

Step-up transformer

Step-up transformer

Isolating transformers: the two windings have approximately the same number of turns, although there is often a slight difference in order to compensate for the losses; otherwise instead of being the same, the output voltage would be a little lower than the input voltage. They are intended to transform from one voltage to the same voltage.

Variable transformers: the primary and secondary windings have an adjustable number of turns that can be selected without reconnecting the transformer.

Other ways of classifying electric transformers they are summarized in the following table.

According to the cooling method
  • Self-air-cooled (dry type)
  • Air-jet cooled (dry type)
  • Immersed in liquid, self-cooled
  • Immersed in oil, combined with self-cooling and air jet
  • Oil-immersed, water-cooled
  • Oil immersed, forced oil cooled
  • Oil-immersed, combination of self-cooled and water-cooled
According to the insulation between the windings
  • Windings isolated from each other
  • Autotransformers
According to the number of phases
  • Single phase
  • Polyphasic
According to the mounting method
  • On pole and platform
  • Underground
  • In vault
  • Specials
According to the purpose
  • Constant voltage
  • Variable voltage
  • Current
  • Constant current
According to the service
  • Great power
  • Small power
  • Distribution
  • Sign lighting
  • Control and signaling
  • For gas discharge lamps
  • For doorbells
  • For instruments
  • Constant current
  • Series transformers for public lighting
According to the power level
  • From fraction of a watt to thousands of megawatts
According to the voltage class
  • From a few volts to 750 kilovolts
According to the frequency range
  • For power, audio, RF, etc.

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