How does an electric transformer work?

Look up at almost any Indian street and you'll find one: a grey-green box on a platform between two poles. It is a distribution transformer. It takes 11,000 volts from the wires above and turns it into the 433 volts (230 V per phase) that goes to about a hundred homes and shops.

It has no moving parts. Inside the steel tank sits a core of thin, stacked sheets of special steel called CRGO (cold-rolled grain-oriented). Round each of its three legs are two coils: a thin LV winding of fat copper strip close to the core, and a tall HV winding of thousands of turns of fine wire outside it.

Everything is soaked in mineral oil. The oil insulates the coils and carries their heat to the radiator fins, where the air cools it. Oil swells when it's hot, so a drum on top, the conservator, gives it room. As the oil shrinks and swells the transformer "breathes" through a jar of silica gel that dries the air. Blue gel is dry; pink means it's full of water and needs changing.

The tall HV bushings on top take the 11 kV in, through fuses and lightning arresters on the cross-arm. The short LV bushings send out three phases and a neutral.

A transformer is two coils of wire wound on one iron core. They never touch. Feed the first coil, the primary, with AC and its current makes a magnetic flux in the core. Because the current keeps reversing, 50 times a second in India, the flux keeps growing, shrinking and reversing too.

That changing flux runs round the core and through the second coil, the secondary. Faraday found that a changing flux makes a voltage in every turn of wire it passes through (see FaradayClear). Every turn on the core feels the same volts. So the voltages go as the turns: V₂ ÷ V₁ = N₂ ÷ N₁.

Energy is conserved, so if the voltage goes down, the current goes up by the same factor: I₂ ÷ I₁ = N₁ ÷ N₂. Power in ≈ power out. A good transformer wastes only 1 to 2%.

How much flux does the core carry? The EMF equation says E = 4.44 × f × N × Φmax. For the pole transformer: 250 V ÷ (4.44 × 50 × 56 turns) = 0.02 webers. Squeeze that through too small a core, or feed it too low a frequency, and the iron saturates: it can't carry any more flux.

And on DC? The flux is steady, so nothing changes and the secondary gives zero volts. Only the wire's resistance holds back the current, which becomes huge. The coil would cook.

A big power station generator makes about 21,000 volts. Right beside it, a huge generator transformer steps that up to 400,000 volts (or 765,000) for the long-distance lines.

Why so high? The power is volts × amps. At 400 kV the same power needs 19 times less current than at 21 kV. The heat lost in the wires goes as the current squared, I²R, so it falls by about 360 times. High voltage is how India moves power from coal fields in Chhattisgarh and solar parks in Rajasthan to cities a thousand kilometres away (see PowerLineClear).

Near the cities, substations step it down in stages: 400 → 220 → 132 → 33 → 11 kV. Each lower voltage is safer and cheaper to run down smaller lines and streets. The pole transformer makes the last big step: 11 kV to 433 V, which reaches your wall socket as about 230 V.

The chain doesn't stop there. Your phone wants 5 volts. An old-style adapter used a small 50 Hz iron transformer, heavy for its size. A modern charger is a switch-mode power supply (SMPS): it turns mains into DC, chops it at about 65,000 times a second, and uses a transformer as small as a fingernail. Higher frequency means far less core is needed.

A distribution transformer is about 98 to 99% efficient. But it runs every hour of every year, and India has millions of them, so the lost 1–2% adds up to a lot of coal.

Core loss (no-load loss) happens whenever the transformer is switched on, even with nothing connected. Two things cause it. Hysteresis: the steel's tiny magnetic domains flip 100 times a second, and each flip wastes a little energy. Eddy currents: the changing flux also drives little swirls of current inside the steel itself, heating it.

That's why the core is made of thin laminations, each about 0.27 mm thick and coated with insulation. The swirls are trapped inside each thin sheet, and eddy loss falls with the square of the thickness. Amorphous steel, a glassy ribbon ten times thinner, cuts core loss by about three-quarters.

Copper loss is the I²R heat in the windings. It grows with the square of the load: double the load, four times the heat. Efficiency peaks where copper loss equals core loss.

The heat warms the oil. Hot oil rises past the coils, flows into the radiator fins, cools and sinks back down. No pump, no fan: this is called ONAN (oil natural, air natural) cooling. India's BEE star label rates distribution transformers by their total losses at half and full load; more stars, less waste.

The grid doesn't carry one AC voltage but three, called phases (in India, R, Y and B: red, yellow, blue). Each one peaks a third of a cycle after the last, 120° apart, like three people pedalling a bicycle in turn. That gives smooth, steady power and lets motors start by themselves.

The pole transformer has three legs, one per phase. On the 11 kV side the three HV coils are joined in a triangle, called delta (Δ). On the LV side the three coils meet at one point, a star (Y). That star point is brought out as the neutral and connected to earth.

Between any phase and the neutral you get about 230 V: that's what most homes get. Between two phases you get √3 × 230 ≈ 400 V. You may hear "415 V": that's the older 240 V standard times √3. Shops, pumps and big air conditioners often take all three phases.

Why the neutral? Houses along the street are shared out among R, Y and B. If each phase carried exactly the same load, the three return currents would cancel and the neutral would carry nothing. Real streets are never balanced, so the neutral carries the difference back to the transformer.

On a hot summer night, with every fan, cooler and AC running, a street transformer can be asked for far more than its rating. The copper loss grows with the square of the load, the oil heats up, and the hottest spot in the windings climbs.

That hot spot matters because the windings are wrapped in paper soaked in oil. Paper slowly turns brittle with heat. Engineers use a rule from IEC 60076-7: at 98 °C the paper ages at its normal rate, and every 6 °C hotter doubles the rate. A week of heavy overload can use up months of a transformer's life.

Old transformers can also fail from within: damp oil, cracked insulation or a lightning surge can start an arc inside the tank. The arc breaks the oil down into gas. Bigger transformers have a Buchholz relay in the pipe to the conservator. Slowly collecting gas lowers a float and sounds an alarm. A sudden arc pushes a surge of oil through it and trips the breaker in a fraction of a second. A leak that drains the oil sets it off too.

Transformer fires do happen in Indian cities, often in summer, and burning oil is hard to put out. Stay safe: never climb a transformer structure or open its fence or box. Keep well clear of the lines: electricity can jump a gap at 11 kV. If you see sparks, smoke or leaking oil, keep away and call your electricity company's helpline. Only licensed electricians and lineworkers should touch it.