How Transformers Work
A plain-English explanation of how a transformer changes voltage using electromagnetic induction, the turns ratio, and the difference between distribution and dry-type units, plus what they mean for buildings and substations in the UAE.
A transformer is a static electrical machine that transfers energy between two or more circuits through a shared magnetic field, changing the voltage and current along the way without any moving parts. It is the device that allows electricity to be generated, transmitted across long distances at high voltage, and then delivered to a building at a safe, usable level.
Almost every supply in a modern building has passed through several transformers before it reaches a socket. In the UAE, where demand is dominated by air conditioning and where master developments draw power from medium-voltage networks, transformers in packaged substations and building electrical rooms are a core part of every distribution scheme.
How it works
A transformer relies on electromagnetic induction. When an alternating current flows through the primary winding, it sets up a continuously changing magnetic flux in an iron core. That changing flux passes through the secondary winding and induces a voltage in it. Because the magnetic coupling depends on a changing field, transformers work only on alternating current (AC), not direct current.
The voltage relationship is set by the turns ratio — the number of turns on the primary winding compared with the secondary. If the secondary has fewer turns than the primary, the output voltage is lower: this is a step-down transformer. If it has more turns, the voltage rises: a step-up transformer. The ratio of voltages equals the ratio of turns.
Energy is broadly conserved, so when voltage is stepped down the available current rises in roughly the same proportion, and apparent power (in kVA) stays about the same on both sides minus the losses. This is why a transformer is rated in kVA rather than kW — its limit is set by voltage and current heating, not by the power factor of the load.
The iron core is built from thin, insulated laminations to reduce eddy-current losses, and a low-loss magnetic steel reduces hysteresis losses. These core losses occur whenever the transformer is energised, while copper (winding) losses rise with load current. The heat produced must be removed, which is why transformers are cooled either by oil and radiators or by air over the windings.
A transformer also provides electrical isolation between primary and secondary circuits and helps define the earthing arrangement of the supply through the way its windings are connected and earthed. In three-phase units the windings are commonly connected in delta or star (wye) configurations, which sets the available phase and line voltages and whether a neutral is provided.
Main types
In the UAE
- In Abu Dhabi, medium-voltage connections and packaged substations are delivered to the local distribution company requirements (such as ADDC/AADC under the Department of Energy), and transformer ratings and arrangements must be coordinated with the utility.
- Transformers and electrical equipment are subject to UAE federal standards and conformity assessment under ESMA, and indoor units are often specified as dry-type to suit fire-safety expectations in occupied buildings.
- Efficient transformers reduce no-load and load losses, supporting the energy-efficiency goals of Abu Dhabi's Estidama Pearl Rating System, and substation rooms must be coordinated with the UAE Fire and Life Safety Code of Practice for ventilation, access and fire separation.
How GPR applies this
GPR coordinates and installs the low-voltage side of packaged substations and building transformers across Abu Dhabi, from the LV main switchboard and metering to earthing and cable terminations. We design substation and electrical-room layouts for adequate ventilation and access, select dry-type units where the building demands it, and align ratings, protection and earthing with the local distribution company and UAE Fire and Life Safety requirements.
Frequently asked questions
Why do transformers only work on AC and not DC?
A transformer needs a continuously changing magnetic field to induce voltage in the secondary. A steady DC current produces a constant field, so no voltage is induced once it is established.
What is the turns ratio of a transformer?
It is the ratio of primary winding turns to secondary winding turns, and it equals the ratio of primary to secondary voltage. Fewer secondary turns means a lower output voltage.
Why is a transformer rated in kVA and not kW?
Its heating and limits depend on voltage and current, which together give apparent power (kVA). The real power (kW) drawn depends on the load power factor, which the transformer cannot control.
What is the difference between oil and dry-type transformers?
Oil-immersed units use insulating oil and radiators for cooling and are common outdoors; dry-type units are air-cooled with resin-encapsulated windings and are preferred indoors for lower fire and spill risk.
Do transformers lose energy?
Yes. Core (no-load) losses occur whenever the unit is energised, and copper (load) losses rise with current. Efficient designs minimise both, which lowers running cost and heat.