Topic 9

How do transformers work?

The Transformer Effect

In the last Topic we saw how a voltage can be induced in a wire by:

• moving a wire in a magnetic field;

• moving a magnet past a wire.

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:

It is important to understand that electricity cannot flow from the primary to the secondary.  The secondary has a voltage induced in it by the magnetic field made by the primary.

Question 1  Why can electricity not flow between the coils?  ANSWER

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

Question 2  Why should we keep the voltage the same?  ANSWER

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.

Question 3  What simple conclusion can you draw from these findings?  ANSWER

The reason for this is that the strong magnetic field made by the DC is constant.  The magnetic field made by the AC is changing all the time.  It's the change in the magnetic field that induces the voltage.  The induced voltage is changing all the time, so it's an alternating voltage.

Question 4  Why does the magnetic field change all the time with AC?  ANSWER

Question 5  What would happen to the voltage at the secondary if the magnetic field stopped changing?  ANSWER

There are three ways of getting a changing magnetic field:

• Moving a wire through a magnetic field;

• Moving a magnetic field past a wire;

• Changing the magnetic field with an AC supply.

The Transformer Equation

The output of the secondary is related to the input of the primary by the following equation:

Look at the picture.  We will it in the worked example.

Worked Example

Question 6  Now the primary and secondary coils are swapped over.  What is the secondary voltage now?  ANSWER

Examples of Transformers

Practical transformers are constructed slightly differently to the example we have looked at above.  The primary is mounted onto the core, with the secondary surrounding it.  This is shown in the picture below.

Transformers are found in a wide range of electronic devices.  This one above is a labpack. 

Some transformers can be fitted into a plug as shown below:

Transformers that convert a high voltage to a lower voltage are called step-down transformers.  Transformers that convert a low voltage into a higher voltage are called step-up transformers.  These are found widely in power stations to convert the 25 000 V produced by a power station to 275 000 V (or 415 000 V) used in the grid.  This is shown in the picture:

The picture shows a large step-up transformer.

The step up transformer increases the voltage, but reduces the current.  A smaller current leads to a lower heating effect in the wires, so less energy is lost in the wires.

Although transformers are very efficient, some energy is lost as heat.  A large transformer like this is cooled by oil and you can see the large number of fans that blow cool air across the heat exchanger.

Question 7  How can a step-down transformer be changed into a step-up transformer?  ANSWER

A radio transmitter and receiver work using the transformer effect.  In the transmitter there is a long wire carrying current that acts as the primary.  In the receiver the aerial acts as the secondary.  Since there is no core, the process is extremely inefficient.  The induced voltage is tiny, but is boosted by a process called electrical resonance.  When you tune a radio in, you alter the resonance.  Then the signal is boosted by amplifiers to the sound that you can hear.

Now answer Question 8 that gets you to think about the transformer effect.

Try the Crossword which gets you to think about the motor effect, the generator effect, and the transformer effect.

Now try the Topic Quiz

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