The Transformer: Structure and How It Works

1The drum on the pole

Look along the electricity poles beside a road and every so often you see a grey metal drum or box fixed high on a pole, with thick insulated wires going in at the top and other cables coming out. It is a transformer. In Rwanda, the medium-voltage lines that feed it run at 30 kV in rural areas and 15 kV in some urban areas, and the transformer gives out the 400 V and 230 V that homes and schools use: 230 V between a live wire and neutral, 400 V between two of the three live wires of a three-phase supply (REG's reticulation standard). Without it, a 15 000 V line could never be connected to a phone charger or a lamp.

A transformer changes the voltage of an alternating current, and it does so with no moving parts. To see how, you first need a new idea: that a changing magnetic field can produce a voltage.

2Electromagnetic induction

Connect a coil of insulated wire to a sensitive meter (a galvanometer) and push a bar magnet into it.

• While the magnet moves in, the needle kicks one way.
• While the magnet is held still, even deep inside the coil, the needle reads zero.
• While the magnet is pulled out, the needle kicks the other way.
• Moving the magnet faster, using a stronger magnet or a coil with more turns gives a bigger reading.

A voltage (an electromotive force, emf) is produced in the coil only while the magnetic field through it is changing. This is electromagnetic induction, discovered by Michael Faraday in 1831. It is how every generator in Rwanda's power stations produces electricity.

3From one coil to another

Now wind two separate coils on the same iron ring. Connect the first coil to a battery through a switch, and the second to a galvanometer.

• At the moment the switch is closed, the current in the first coil grows, its magnetic field grows, and the galvanometer on the second coil kicks.
• While a steady d.c. flows, the field is steady and the meter reads zero.
• At the moment the switch is opened, the field collapses and the meter kicks the other way.

Now replace the battery with an a.c. supply. The current in the first coil changes all the time, so its magnetic field changes all the time, and an alternating emf is induced in the second coil continuously. This is mutual induction, and it is exactly how a transformer works. It also explains why a transformer works only with alternating current.

4The structure of a transformer

• The primary coil ($N_{p}N\_p$ turns) is connected to the a.c. supply of voltage $V_{p}V\_p$.
• The secondary coil ($N_{s}N\_s$ turns) is connected to the load and gives the output voltage $V_{s}V\_s$.
• The core of soft iron, built from thin insulated sheets (laminations), links the two coils so that almost all the changing magnetic field made by the primary passes through the secondary. Lesson 2 explains why it is laminated and made of soft iron.
• The coils are made of insulated copper wire, so the turns do not short-circuit one another or the core.

In circuit diagrams a transformer is drawn as two coils side by side with two parallel lines between them, standing for the iron core.

5Why the voltages are in the ratio of the turns

The same changing magnetic field passes through every turn of both coils, so each turn has the same emf induced in it. The turns of a coil are connected in series, so their emfs add up: a coil with twice as many turns has twice the voltage. Hence $V_{s}V\_s$ and $V_{p}V\_p$ are in the ratio $N_s:N_{p}N\_s\; :\; N\_p$.