How Transformers Work: The Grid's Quiet Machine

Part 11 of 23 in Cobler's Electricity Fundamentals series. New here? Start with the course map.

The green metal cabinet humming quietly outside your condominium block has no moving parts. Nothing inside it spins, slides or wears out, and it will likely outlast the building. Yet it is doing something no length of copper could manage on its own: taking the 11,000 volts arriving from the street and handing your building a safe 400 volts (230 volts to each apartment), without any electrical connection between the two sides. That box is a transformer, and it is the single machine that made a national grid possible. Understanding how transformers work is understanding why the power station can sit 200 kilometres from your kettle.

How transformers work: two coils, one iron core

Two coils of wire share one iron core, and a changing magnetic field carries energy from one coil to the other.

This is the same physics that runs a generator, Faraday's law of induction, but with the motion taken out. A generator induces voltage by physically spinning a coil past a magnet. A transformer needs no spinning, because alternating current is already changing 100 times a second on our grid. Send AC through the first coil, the primary, and it magnetises the iron core, building a magnetic field that grows, collapses and reverses in step with the current. Wrap a second coil, the secondary, on the same core, and that restless field sweeps through it and induces a voltage in it. The two coils never touch. The core, not a wire, ferries the energy across.

Think of two tuning forks tuned to the same note. Strike one and the other starts to sing across the gap, with no contact between them, because a vibration in the air couples them. The primary coil "rings" the secondary through the magnetic field in the iron in exactly that way. And, as with the tuning forks, it only works while something is changing. A held note does nothing.

The turns ratio: why you get ten times the voltage but a tenth of the current

The voltage the secondary produces depends only on how many times each coil is wound.

Wind the secondary with ten times as many turns as the primary and it delivers ten times the voltage. Wind it with a tenth as many turns and you get a tenth of the voltage. That is the whole design lever, and it is why a transformer is often described as gears for electricity: gear up for voltage, gear down for current, or the reverse.

Here is the catch that stops it being magic. Power is conserved. Take a transformer fed 400 volts on a primary of 100 turns, drawing 1 amp, so 400 volt-amperes going in. Give the secondary 1,000 turns, ten times as many, and it produces 4,000 volts. But it can only supply a tenth of an amp, because 4,000 volts times 0.1 amp is the same 400 volt-amperes coming out. Step the voltage up by ten and the available current drops by ten. Nothing is created. You are trading pressure for flow, and the product, the power, stays put minus a sliver of loss. The green box outside your condo runs this backwards: it takes high-voltage, low-current power from the street and hands you low-voltage, high-current power for your air-conditioner.

Why does a transformer only work on AC?

Because a transformer runs on change, and steady direct current does not change.

Feed the primary a constant DC voltage and the core magnetises once, to a fixed level, and then sits there. The field is strong but frozen, and a frozen field induces nothing in the secondary. You would read zero volts out. The only flicker of output comes at the instant you switch the DC on or off, when the field is briefly moving. Alternating current, by contrast, is always moving, reversing direction 100 times a second, so it drives the core back and forth continuously and the secondary sees a steady changing field.

This is the fact that decided the war between direct and alternating current in the 1880s. DC could not be cheaply changed in voltage, so it could not be sent far. AC could, and the transformer is why. We tell that whole story in the war of the currents, the next part in this series.

Why do transformers hum?

The iron core physically flexes as the magnetic field cycles, and it does it 100 times a second.

Magnetise a piece of iron and it changes shape by a tiny amount, an effect called magnetostriction. In a transformer the field builds and collapses on every AC cycle, so the core's thin iron sheets (laminations) are forever stretching and relaxing by microscopic amounts. That mechanical twitch is what you hear. The pitch is the giveaway: the iron flexes the same way whether the field points one way or the other, so it strains twice per electrical cycle. On our 50 Hz grid that is a 100 Hz hum, an octave and a bit below middle C (Wikipedia, mains hum). A transformer on a 60 Hz grid hums at 120 Hz, a noticeably different note. The sound is not a fault. It is the core doing its job, audibly.

Why are transformers rated in kVA, not kW?

Because what heats and eventually kills a transformer is current and voltage, and neither cares whether the load is doing useful work.

The windings overheat from the current running through them; the insulation breaks down from the voltage across it. Both are set by the apparent power the transformer carries, measured in kVA, not the real power the load turns into work, measured in kW. A factory full of lightly loaded motors can pull a lot of current that does little useful work, a poor power factor, and the transformer feels every amp of it as heat. A 1,000 kVA transformer serving a load at 0.75 power factor delivers only 750 kW of useful power; the rest of its capacity is spent carrying reactive current it will never be thanked for. That is why the nameplate is in kVA, and why poor power factor makes a transformer run hot for no extra work. The full accounting of real, reactive and apparent power lives in the power triangle.

The transformers you see around Malaysia

Every transformer you can spot from the road is one rung on the voltage ladder between the power station and the plug.

Out in a kampung you will see the pole-mounted can, a drum bolted near the top of a concrete pole, stepping an 11 kV distribution line down to 400 volts for a street or a few houses. Outside condominiums and shopping blocks sits the kiosk substation, a locked green or grey cabinet ("pencawang") doing the same job for a bigger load. In a factory compound you will find a larger unit, perhaps 1,000 kVA or more, feeding the plant's switchboard. Each is a step-down transformer, and together they form the descending staircase we walk through in how electricity reaches you. The grid runs at hundreds of thousands of volts precisely so it can send power far at low current, then leans on transformers to bring it back down to something a wall socket can use.

Who invented the transformer?

No single person, but the machine took its modern shape in the mid-1880s.

Faraday demonstrated the underlying induction with a coil-wound iron ring in 1831. The practical device arrived in stages. Lucien Gaulard and John Dixon Gibbs showed an AC "secondary generator" in 1884, but its open magnetic path leaked badly and was inefficient. The breakthrough was the closed iron core: in 1885 three engineers at the Ganz works in Budapest, Károly Zipernowsky, Ottó Bláthy and Miksa Déri, built the efficient closed-core design and gave it the name "transformer" (IEEE Spectrum). A year later, on 20 March 1886, William Stanley ran the first full commercial AC system in Great Barrington, Massachusetts, sending about 3,000 volts down Main Street and using transformers in six shop basements to step it down to light the stores (National MagLab). George Westinghouse saw it that April. That demonstration is what armed him for the fight that opens the next article.

Modern power transformers are among the most efficient machines humans build, routinely turning 99.3% to 99.7% of the power that enters into power that leaves (OSTI). A machine with no moving parts, losing well under one percent, quietly holding the grid together. It earns the hum.

This is Part 11 of 23 in Cobler's Electricity Fundamentals series. Previous: How Generators Make Electricity: Spin Meets Wire. Next: The War of the Currents: Edison, Tesla and Why AC Won.

Curious how many kVA of transformer capacity your site is spending on reactive current instead of real work? Book a demo and CobiNeural will show you the live load and power factor on every transformer you run.