What Is a Transformer? What Does It Do and What Types Are There?

A transformer, also called a transformatory in some technical contexts, is an electrical machine that transfers electrical energy between two circuits through electromagnetic induction while changing voltage and current levels without changing frequency. It consists of primary and secondary windings wound on a common magnetic core and normally has no mechanical moving parts. In the shortest form, the answer to what a transformer is and what it does is this: it is the electrical machine that safely converts an AC voltage in one circuit to another voltage level and transfers energy from one circuit to another through a magnetic field. For related context, see What Tests and Maintenance Are Required for Current Transformers?.

The operating principle of a transformer is based on Faraday's law of electromagnetic induction. The alternating voltage applied to the primary winding creates a varying magnetic flux in the core. When this flux links the secondary winding on the same core, a voltage proportional to the number of winding turns is induced. In this way, energy is transferred between the primary and secondary circuits without metallic contact. The basic relationship known as the transformer formula states that the voltage ratio is approximately equal to the turns ratio: V2/V1 ~= N2/N1. For currents, the relationship I2/I1 ~= N1/N2 is used. In an ideal transformer, the input and output apparent power are approximately equal (S1 ~= S2 ~= V x I); these expressions are used in transformer power calculations and transformation ratio calculations. For related context, see What Tests and Maintenance Are Required for Voltage Transformers?.

The answer to the question of which current a transformer operates with is clear: conventional power transformers and distribution transformers are designed to operate only with alternating current (AC). Applying direct current (DC) to the primary winding can magnetize the core into saturation and cause very high magnetic flux and excessive current draw. This may overheat the transformer, damage winding insulation and lead to serious failures. Therefore, the practical answer to the question "is a transformer AC or DC?" is almost always AC. For related context, see What Tests and Maintenance Are Required in Transformer Substations?.

The reason transformers are used is to reduce losses, adapt voltage levels and transmit electrical energy safely. When voltage is stepped up and current is reduced on transmission lines, I2R losses decrease and conductor cross-sections can be reduced. In distribution systems and inside facilities, voltage is stepped down again to levels at which equipment and people can operate safely. What would happen without transformers? Electrical energy could neither be transmitted efficiently over many kilometers between cities nor reduced to safe levels such as 230/400 V for homes and workplaces. In other words, transformers form the backbone of the modern electrical grid. For related context, see What Is an Insulator? What Does It Do, How Does It Work and What Types Are There?.

In power systems, step-up power transformers are generally used at generation plants, intermediate transformers change voltage levels along transmission lines, and distribution transformers reduce voltage to low-voltage levels in residential areas. Large industrial facilities have their own transformer substations where energy is transformed from MV to LV and where transformers and MV switchgear are installed. For field testing, maintenance and repair of these substations, Bursa HV/MV testing, maintenance and repair and transformer maintenance and testing services can be planned together.

The basic components of a transformer are the magnetic core, primary winding, secondary winding, insulation materials, the tank or enclosure that protects the windings and core, and the cooling arrangements. Oil-immersed transformers use transformer oil for insulation and cooling, while dry-type transformers use epoxy resin or air cooling. This structure, represented schematically by a transformer symbol in electrical projects, is in reality a complex machine with detailed insulation, cooling and mechanical protection requirements.

Transformer types by purpose can be summarized as power and distribution transformers used in energy transmission and distribution, isolation transformers that provide galvanic isolation between the grid and the load, and instrument transformers such as current transformers and voltage transformers. Instrument transformers convert high voltage and current levels to safe values that measuring devices can read. Correct selection of transformer equivalent circuits and ratios is important for the healthy operation of protection relays and measurement systems.

By construction and cooling method, transformers are divided into two main groups: oil-immersed transformers and dry-type transformers. Oil-immersed transformers are cooled by natural or forced circulation of transformer oil, with cooling codes such as ONAN and ONAF. Dry-type transformers are generally preferred in indoor applications where fire risk is lower and maintenance demand is reduced. For this reason, the answer to "dry-type transformer or oil-immersed transformer?" is usually determined by the operating environment and fire safety requirements.

By phase number, transformers are classified as single-phase and three-phase. Because three-phase systems are widely used in the grid, transformers on transmission and distribution lines are mostly three-phase transformers. However, single-phase transformers are also used in some special applications or at small power ratings. The transformer turns calculation formula is used to calculate the required number of winding turns for the desired input and output voltages in both single-phase and three-phase transformers.

When selecting a transformer, the power rating in kVA, primary and secondary voltage levels, operating environment, indoor or outdoor installation, cooling method, short-circuit withstand capability, connection vector group and operating conditions must all be considered. An incorrectly selected transformer may cause both efficiency loss and early failures. In new transformer substation installations or power increase projects, we manage this selection and calculation process together with our LV/MV/HV project design and consultancy service.

During operation, transformer maintenance includes taking oil samples from oil-immersed transformers for laboratory analysis such as DGA, performing periodic winding resistance, TTR, insulation resistance and power factor tests, scanning hot spots with a thermal camera, and checking connections. For this type of detailed maintenance and testing, the services summarized on our Bursa transformer maintenance, testing and oil analysis page can be used.

Regular tests and maintenance extend transformer service life, reduce unexpected failures and improve operational continuity. Especially in industrial facilities, taking a transformer out of service means production loss and safety risk, so periodic transformer maintenance is critical. To manage operation and regulatory follow-up from a single point, you can work in an integrated way with our transformer operation responsibility service at your transformer substation and, when required, our general HV operation responsibility service.

On this page, we answered the questions of what a transformer is, how it works and what types it has within a general framework. For detailed engineering solutions, transformer testing and maintenance services and transformer oil analyses, you can review our Bursa transformer services page; for the operation and regulatory side, our transformer operation responsibility page; and for your HV/MV equipment, our HV/MV testing, maintenance and repair page, then contact the Pow-Sys Güç Sistemleri team.