The Last Codex
EN

Theme

Language

Transformer

Raising and lowering the voltage

Date
1885
Era
Electric age
Rebuild stage
V Electric age
Difficulty

Plate

Legend
  1. 1Primary coil: few turns of thick wire
  2. 2Secondary coil: many turns of fine wire
  3. 3Laminated iron core
  4. 4Thin iron sheets of the core
  5. 5Terminal post

Anatomy

v₁ i₁v₂ i₂ΦAC ~DC =
Magnetic flux in the core: more lines, more fluxCut windings: ⊙ current toward you, ⊗ away from youTraces: voltage solid, current dashed, flux in copperThe numbers follow the plate's example: 1,000 turns on the primary, fed at 4,600 V and 50 Hz. Every turn of both windings sees the same flux, which gives 4.6 V per turn, so the voltages are in the ratio of the turns; each band drawn on a winding stands for 50 turns. The flux peaks at 20.7 mWb (1.5 T in a limb 120 mm square of about 380 sheets 0.3 mm thick), and it peaks a quarter of a cycle after the voltage: the voltage follows how fast the flux changes, and is largest when the flux goes through zero. Each copper line on the core stands for a quarter of the peak flux. The model assumes a rating of 25 kVA and a resistive load matched to the secondary, which always takes 25 kW: a secondary with fewer turns gives a lower voltage and a larger current, in step with the voltage. It takes the losses as 600 W in the windings and 63 W in the core, close to the European limits for a transformer of this size, so 97.4 % of the power comes out. The dots on the board mark the ends that are positive at the same moment, and the currents are counted into the primary's dot and out of the secondary's. The motion is slowed 100 times. On DC the flux climbs for up to about a hundredth of a second, until the iron saturates, then barely changes: the secondary's voltage dies away, and the primary, now little more than a length of wire, heads for about 450 A, 83 times its rating, and would burn. A transformer works only on alternating current, which is why the grids adopted it; today's high-voltage DC links need converters at each end. Switching back to AC restarts the steady cycle: the surge at switch-on is left out. A transformer like this one works at a voltage that kills: never open one, climb a pole or enter a substation.

A transformer is two coils of wire on one iron core: an alternating current in the first induces a voltage in the second, higher or lower according to the ratio of their turns. It lets power travel at high voltage over long lines and arrive at the voltage appliances use, and it is the main reason alternating current became the standard for the world's grids.

What it is

A cylindrical transformer tank hung on a wooden pole, fed from the overhead line and wired to a house A pole transformer lowers the voltage of the street line to the voltage used in the house.

A transformer changes the voltage of an alternating current. It has no moving parts: two coils of insulated copper wire, the primary and the secondary, wound on a closed iron core. Power enters one coil and leaves the other at a higher or lower voltage, with the current changed the opposite way.

Power stations raise the voltage of their generators for the long lines. Substations lower it in steps, and a last transformer on a pole or in a cabin brings it down to the 230 or 120 volts of a socket. Phone chargers, doorbells and welding sets contain smaller ones.

How it works

The current in the primary makes the iron core a magnet. Because the current alternates, that magnetism grows, collapses and reverses, going through fifty or sixty full cycles a second. The changing flux passes through the secondary and induces a voltage in each turn: Faraday’s induction, which works only while the flux changes.

Every turn of both coils sees the same flux, so the voltages of the two coils are in the ratio of their turns. With 1,000 turns on the primary and 50 on the secondary, 4,600 volts in give 230 volts out. In an ideal transformer, with no losses, the low-voltage side carries twenty times the current and is wound with thicker wire.

On direct current none of this works: a steady flux induces nothing, and the primary becomes a short length of wire across the supply that overheats. Transformers need alternating current, which is why AC became the standard for grids; today’s high-voltage direct-current links use converters to turn AC into DC and back into AC at the far end.

High voltage pays because the heat lost in a wire grows with the square of the current. Send 100 kilowatts into a line whose wires total 2 ohms. At 1,000 volts the current is 100 amperes, and the wires waste 20 kilowatts, a fifth of the power. At 10,000 volts the current is 10 amperes and the loss falls to 200 watts, or 0.2 percent.

A solid iron block glowing with circling currents beside a cool block made of thin stacked sheets A solid core heats up from the currents circling inside it; thin sheets insulated from each other cut them down.

The flux also induces currents circling inside the iron, called eddy currents, and a solid core would heat up like a stove plate. So the core is a stack of sheets a few tenths of a millimetre thick, each insulated from the next by varnish or oxide. The eddy loss falls with the square of the sheet thickness. Early builders used bundles of iron wire for the same reason; modern cores use silicon steel.

Grids use three-phase current: three alternating currents shifted by a third of a cycle, on three wires, transformed by three sets of coils on one core.

How to build

A bench demonstration at a few volts, never a mains project. You need a steel bolt about 10 cm long, a second bare bolt and two flat steel strips, 20 to 30 m of enamelled copper wire 0.3 to 0.5 mm thick, insulating tape, fine sandpaper, a multimeter that reads AC volts and, for the last test, a centre-zero galvanometer.

Wind the coils. Over a layer of tape, wind a primary of 100 turns on the bolt. Tape it, wind a 100-turn secondary over it, tape again and wind a 50-turn secondary on top. Sand the enamel off each end.

Choose a safe source. Use a bought plug-in adapter whose label reads “AC output” at 12 volts or less, or a function generator giving a sine wave of a few volts at about 1 kHz with no DC offset. Most adapters give DC and will not work. A 12-volt incandescent car bulb of about 5 watts in series with the primary limits the current.

Measure. Read the AC voltage across the primary and each secondary, then disconnect: the solid bolt warms up. With the adapter, expect a few tenths of a volt, since most of the voltage drops across the bulb. The 50-turn coil gives about half the voltage of the 100-turn one, and both read less than the primary, because the flux of a single bolt closes through the air. Lay the bare bolt beside the wound one and bridge their heads and their tails with the steel strips, so the iron forms a closed frame around the coils: the readings rise.

Try a battery. With a 4.5-volt battery in place of the adapter, bulb still in series, connect a centre-zero galvanometer to the secondary on its least sensitive range, or through a resistor of a few kilohms. Its needle kicks one way when you close the circuit, the other way when you open it, and rests at zero in between.

Read a rating plate. On a doorbell transformer, find the input and output voltages, such as 230 V / 12 V, and the power in volt-amperes (VA). Large units add frequency, cooling and impedance; read those from outside the fence.

Safety. Never give a secondary more turns than the primary, and remember that an adapter’s output with no load can be a fifth above its label. Opening a circuit through a coil makes a brief voltage spike, so switch with a switch or by an insulated wire, never with fingers on bare ends. Never wind coils for the mains, and never open, modify or rewire mains-powered equipment, even unplugged: microwave-oven and neon-sign transformers deliver lethal currents. A step-down transformer fed backwards raises the voltage, and capacitors in a device can stay charged after it is unplugged. Coils stay hot after use. Never climb poles, enter substations or touch fallen lines. Mains wiring is work for a licensed electrician.

History and evolution

Michael Faraday showed induction with his two-coil iron ring in August 1831, Joseph Henry found it independently in America, and around 1850 Heinrich Ruhmkorff in Paris made the induction coil a standard instrument.

In 1884 Lucien Gaulard and John Dixon Gibbs lit lamps along a 40 km line from Turin to Lanzo with their “secondary generators”, wired in series on open iron cores, so switching one lamp disturbed the others.

A ring of copper windings completely wrapped in iron wire, standing on a wooden support An early Budapest transformer: the iron wire wraps the copper ring and closes the magnetic path.

At the Ganz works in Budapest, Károly Zipernowsky, Miksa Déri and Ottó Bláthy saw that a closed iron core and transformers connected in parallel would hold the voltage steady. They filed patents between January and March 1885, named the device “transformer”, and lit the Budapest National Exhibition with their system that May.

George Westinghouse bought the Gaulard–Gibbs rights, and his engineer William Stanley built closed-core transformers that lit the main street of Great Barrington, Massachusetts, in March 1886. Edison’s direct current at about 110 volts reached only a few streets around each station, and his companies campaigned on the dangers of AC. Westinghouse lit the 1893 Chicago fair with AC, and in 1896 power from Niagara Falls reached Buffalo.

By 1888 Galileo Ferraris and Nikola Tesla had used currents out of step to make a rotating field for motors; in 1891 a three-phase line carried 300 horsepower 175 km from Lauffen to Frankfurt. Today’s main lines run at hundreds of thousands of volts.

Type a name, a date, a material, a domain…