A transformer changes the voltage of alternating current without changing the frequency, relying on electromagnetic induction between two windings on a shared iron core. The turns ratio determines whether the voltage rises or falls. Well-designed power transformers reach efficiencies above 98 percent and service lives of 30 years and more, usually limited by the ageing of the insulation: every sustained 10 kelvin above its limit temperature halves its service life. This article explains the operating principle, construction and types, and gives the insulation system of solid insulating material and insulating oil the space many explanations of the topic leave out. GOBA has supplied transformerboard, pressboard and aramid paper for it since 1959 and makes insulating parts for transformer building from them.
How does a transformer work?
Unlike a motor or generator, a transformer has no moving parts, which is why the trade calls it a static electrical machine. The energy passes from one winding to the other purely through the magnetic field. When alternating current flows through the primary winding, an alternating magnetic field builds up around it. The iron core, made of thin, mutually insulated laminations, carries this field with low losses to the secondary winding and induces an alternating voltage there, at the same frequency as the input, 50 hertz in Europe. The principle behind it, electromagnetic induction, was discovered by Michael Faraday in 1831. Because voltage can be stepped up and back down this way with low losses, electricity travels long distances at high voltage and reaches the socket at a safe level. That makes the transformer a central component of every power supply, from the power plant to the charger.
The turns ratio
How much the voltage changes depends solely on the ratio of the turns: the secondary voltage relates to the primary voltage the same way the secondary turns relate to the primary turns, and the current behaves inversely. In the loss-free ideal case, the transmitted power stays the same. If the secondary winding has more turns than the primary winding, the voltage rises, a step-up transformer. If it has fewer, the voltage falls, a step-down transformer. A real transformer never quite reaches this ideal case, part of the power is lost as heat, see the section on losses and efficiency below.
Construction of a transformer
A transformer consists of four assemblies that together make the voltage conversion safe: the iron core, the windings, the insulation system and the enclosure.
Iron core
The iron core carries the magnetic flux between the windings. It consists of thin, mutually insulated laminations of grain-oriented silicon steel, usually 0.23 to 0.35 millimetres thick. The lamination breaks up the eddy currents that would otherwise form in solid iron and be lost as heat.
Windings
The primary and secondary windings consist of insulated copper or aluminium wire, wound on or around the core. Their turns ratio sets the transformation ratio. Between the turns, the winding layers and against the core sits the winding insulation, mechanically and electrically the most heavily stressed component in the whole transformer.
Enclosure and cooling
The enclosure protects the core and windings from the environment. In an oil transformer it is also the tank for the insulating oil, which removes heat and insulates the windings. Dry transformers do without oil and cool by air, usually cast in resin or insulated with aramid paper.
The insulation system: solid and liquid insulation
How long a transformer lasts and how safely it runs depends above all on its insulation system, not on the core or windings themselves. It consists of two parts that work together: solid insulation made of cellulose or aramid materials and, in an oil transformer, liquid insulation from insulating oil.
Solid insulation: transformerboard, pressboard and aramid paper
In the oil-filled power transformer, transformerboard and pressboard provide the solid insulation: highly compressed cellulose materials in thicknesses of about 0.5 to 8 millimetres, used as barriers, angle rings, spacers and cylinders between and around the windings. Pure, additive-free cellulose stays dimensionally stable and dielectrically clean in hot oil for decades. Dry transformers and applications with higher thermal loads instead use aramid paper, which withstands significantly higher continuous temperatures than cellulose.
Liquid insulation: insulating oil
Mineral-oil-based insulating oil in the power transformer does two jobs at once: it cools the windings by convection and insulates together with the solid cellulose material. The board absorbs the oil, so solid and liquid insulation form one shared system and reinforce each other's dielectric values. Biodegradable ester oils are gaining ground in installations with strict environmental requirements, for example in water protection areas.
Thermal classes and ageing of the insulation
How hard a transformer can be loaded on a continuous basis is set by its thermal class. Pure cellulose classically belongs to thermal class A with a 105 degree Celsius limit, aramid paper reaches significantly higher classes depending on the grade. For service life, the 10-kelvin rule after Montsinger applies as a rule of thumb: every sustained 10 kelvin the limit temperature is exceeded roughly halves the insulation's service life. Whether the insulation is still intact is shown by two test values, the insulation resistance and the dielectric strength. Both fall as ageing and moisture uptake progress.
Types of transformers
Transformers differ by power, voltage level and construction.
| Type | Used in | Voltage level |
|---|---|---|
| Power transformer | power plants, substations | high voltage, several hundred kV |
| Distribution transformer | local substations, buildings | medium to low voltage |
| Autotransformer | small voltage adjustment, high efficiency | not galvanically isolated |
| Instrument transformer | current and voltage measurement, protection | all levels |
| Isolating transformer | safety isolation, construction sites, medical equipment | mostly low voltage |
Across all five types, the construction decides whether oil or air insulates and cools: power and distribution transformers almost always run in oil, smaller isolating transformers and instrument transformers are often dry. For an indoor dry transformer, for example in a hospital or a data centre, oil is ruled out for fire safety reasons in any case.
Where transformers are used
Transformers sit at every transition between voltage levels. Power transformers raise the voltage in the power plant to transmission level, substations step it back down in stages. In local substations, distribution transformers convert medium voltage to the low voltage used in households. In industry they supply production plants, in electronics small transformers do the same job in power supplies and chargers. The energy transition depends on transformers too: wind farms and solar plants feed their power into the grid through transformers, a field in which GOBA supplies insulating materials for renewable energy installations.
Losses, efficiency and maintenance
A real transformer loses part of its power as heat, in two forms. Copper losses arise from the ohmic resistance of the windings and rise with the square of the current. Iron losses arise from hysteresis and eddy currents in the core and occur even at no load. Well-designed power transformers still reach efficiencies well above 98 percent.
Regular maintenance extends the service life: oil analysis and insulation measurement for the oil transformer, visual inspection of the winding insulation for the dry transformer. Keeping temperature and moisture consistently under control brings a power transformer to 30 years of service and more, usually limited by the ageing of the insulation, not by the core or windings themselves.
GOBA Takeaway
The service life of a transformer is decided by its insulation system: solid insulation made of cellulose or aramid paper, matched to the thermal class and operating temperature. GOBA has supplied these insulation materials since 1959 and makes ready-to-fit insulation and molded parts from them for transformer building, from barriers to angle rings to cylinders made to drawing. For special formats and small to medium quantities, get in touch through custom products.
