How do transformers work?

What are transformers and what are they used for?

Electrical transformers are devices that allow the voltage level of alternating current to be increased or reduced via electromagnetic induction, without modifying its frequency.

Thanks to them, electricity can be transmitted over long distances with lower energy losses and distributed at safe and suitable voltage levels for use in homes, businesses and industries.

How do transformers work?

The operation of transformers is based on a fundamental physical principle: electromagnetic induction, discovered by Michael Faraday.

A transformer is made up of two coils of wire wound around a magnetic core, whose function is to concentrate and guide the flow of the magnetic field:

• Primary coil: where the current enters.
• Secondary coil: where the transformed current exits.

The relationship between the input and output voltage depends on the number of turns of wire in each coil. If the secondary coil has more turns than the primary, the voltage increases; if it has fewer, the voltage is reduced.

In an ideal transformer the following applies:

• V2/V1 = N2/N1 (voltage is proportional to the number of turns).
• I2/I1 = N1/N2 (if voltage rises, current falls, and vice versa).

In practice, there are small energy losses, so the relationships are approximate.

Thanks to this operation, transformers allow voltage to be raised to transmit electricity over long distances, from generation plants to our homes, reducing losses in the form of heat.

Understanding why these losses occur and how they influence the efficiency of the electrical system leads us to a key physical phenomenon: the Joule effect.

The Joule effect: why electricity is transmitted at high voltage

One of the most common questions is: why is electricity not transmitted at the same voltage we use at home? The answer lies in the Joule effect, a physical phenomenon whereby part of the electrical energy circulating through a conductor is dissipated in the form of heat due to its resistance.

The higher the current circulating through a conductor, the greater the energy loss in the form of heat. This implies that transmitting large amounts of electricity at low voltage would generate enormous losses.

This is where step-up transformers come into play. Increasing the voltage reduces the current needed to transmit the same electrical power, thereby minimising losses associated with the Joule effect. For this reason, electricity is transmitted at high voltage and subsequently reduced, via other transformers, before reaching consumption points.

Main components of a transformer

Internally, a transformer is made up of elements that work together. The most important ones are:

• Magnetic core: channels the magnetic flux.
• Coils or windings: carry the electrical current.
• Insulation: prevents short circuits between windings.
• Dielectric oil or cooling systems: remove the heat generated.
• Metal tank: protects against moisture, dust and impacts.
• Bushings (pass-through insulators): allow the internal parts of the transformer to connect to the outside while maintaining insulation.
• Radiators and, in some cases, fans: help dissipate heat.
• Tap changer: adjusts the output voltage in small steps (on-load or off-load, depending on the design).
• Protection devices in oil-filled transformers, such as the Buchholz relay and the pressure relief valve (depending on type and size).
• Depending on the application, transformers may be oil-immersed or dry-type (for example, resin-cast) transformers.

How were electrical transformers developed?

The history of transformers is also the history of modern electricity. In 1831, Michael Faraday described the principle of electromagnetic induction, laying the theoretical foundations that would make their operation possible.

At the end of the 19th century, Lucien Gaulard and John Dixon Gibbs developed the first practical transformers, capable of stepping up and stepping down voltage in alternating current systems. Later they were perfected by Károly Zipernowsky, Miksa Déri and Ottó Bláthy (Ganz) who also promoted key designs such as the closed core and more practical and efficient alternating current systems.

In parallel, Nikola Tesla and George Westinghouse promoted the industrial adoption of alternating current, demonstrating its viability compared to other systems.

Thanks to these advances, transformers became an essential piece for transmitting electricity over long distances and the consolidation of modern electrical grids.

Types of transformers and most common applications

Power transformers

These are used to step up or step down voltage in electrical grids, facilitating both the transmission and distribution of electricity.

• Step-up transformers: increase voltage. They are mainly used in generation plants to send electrical energy over long distances with fewer losses.
• Step-down transformers: decrease voltage. These allow electricity to reach our homes at safe voltage levels.
• Autotransformers: change voltage when the difference between the input and output level is relatively small. They are more compact and efficient because they use a single coil.
• Three-phase: used in electricity transmission grids and industrial environments. They are designed to work with three-phase systems and are more compact and efficient than using several independent single-phase transformers.

Instrument transformers

Serve to measure high currents and voltages safely.

• Current transformers: reduce very high currents to standardised values so that instruments can measure them without risk.
• Voltage transformers: reduce very high voltages to safe and manageable levels for measurement and control.

What are they used for in day-to-day life?

In the electrical grid, transformers allow energy to be transmitted over long distances and adapted so that we can use it safely. Furthermore, almost all electronic devices incorporate systems that adjust the mains voltage to the values they need to function correctly. They are also present in home safety elements. For example, the RCD (Residual Current Device) uses a current transformer to detect current leakage and cut the supply before damage can occur.

From the electricity system to mobile chargers

Although we tend to associate them with large infrastructures, transformers are present in many everyday objects: household appliances, electronic equipment, air conditioning systems, solar installations, electric vehicles and, of course, mobile chargers.

The same logic that allows electricity to be transmitted over long distances is also present in many devices for daily use.