P3bL4  Transformers

Key Words

Magnetic field - a force field which affects magnetic materials.

Primary coil - coil connected to the ac supply.

Secondary coil - coil connected to the output of the transformer.

Step-down transformer - transformer that turns a high voltage into a low voltage.

Step-up transformer - transformer that turns a low voltage to a high voltage

Grade E

In a transformer, there is an electromagnet making a magnetic field called the primary coil.  There is a secondary coil that converts the magnetic field into a voltage.  The two coils do not move and are NOT electrically connected in any way to each other.

The way a transformer is made up is simplicity itself.  There are three components, and no moving parts:

• the core;

• the primary coil;

• the secondary coil.

The core is the frame on which the coils are mounted.  The core is usually made of laminated soft iron.  The word "soft" does not mean that the iron is easily bent; it means that it when magnetised, it loses its magnetism as soon as the magnetism is turned off.  Electromagnets have a soft iron core.  Permanent magnets are made of hard magnetic material.

The core is made up of sheets of soft iron.  Each sheet is separated by a layer of insulating material.  This is why it's called laminated.  In the picture below, the sheets can be seen clearly.

We will look at how the transformer is made up.  The diagrams shows the demountable transformer that your physics teacher may well show you.  First the core:

The top bit of the core comes off to allow the primary and secondary coils to be changed.  Note also the laminated construction.  The laminations make the transformer much more efficient by reducing eddy currents.

Now we will put on the primary coil.

The primary coil is connected to the voltage source.  It is the coil of an electromagnet.  We could use the equipment as an electromagnet if we really wanted to.

Now we add the secondary coil.  Note that current cannot flow from the primary to the secondary because there are layers of insulating materials.

The complete transformer now looks like this:

Note that the core forms a closed loop.  This makes the transformer much more efficient.

Now suppose we connect the primary to a DC power supply.  We find the following.

• The coil acts as a very strong electromagnet.  You would find it quite hard to pull the top bit of the core off.

• There is no voltage induced in the secondary.

As it stands the transformer is really rather useless.

Now we connect the primary to an AC supply of the same voltage.

We find the following:

• The coil acts as an electromagnet, but it's much weaker than before.  You can remove the top quite easily.

• There is a voltage induced in the secondary.

• Either coil can act as the primary.

The Transformer Equation:

In Physics Code:

Vprim = Nprim

Vsec    N sec

A step up transformer has more coils on the secondary than the primary.  A step-down transformer has more coils on the primary than the secondary.

Grade C

The national grid distributes electricity at very high voltages.  The transmission lines are mostly overhead cables that are held up in the air from transmission towers (pylons).  If you look at a transmission tower, you can see the long insulators from which the cables are suspended.

Electricity is generated at about 25000 V.  The currents can be as high as 20 000 A.  The wires from the generator are very thick indeed.

The heating effect of a current is proportional to the square of the current.  If you use 10 times the current, the heating effect goes up by 100 times.  Similarly, if you reduce the current by 10 times, the heating effect is 1/100.  So step up transformers are used to raise the voltage from 25 000 V to:

• 132 000 V;

• 275 000 V;

• 415 000 V.

The step up transformers are huge, as can be seen from this picture:

You can see the thick wires going in the back and the high voltage output at the front.  The length of the insulators shows that the output voltage is high.  The output current is much lower, about 2000 A.  These high voltages are plenty enough to kill you instantly.

At the consumer end, there are substations in which there are step-down transformers to give:

• 11 000 V for big factories;

• 415 V for small factories;

• 230 V for our homes.

Grade A

The Transformer Equation:

In Physics Code:

V_prim = N_prim

V_sec    N _sec