Triple Physics Topic 11 - Transformers

The Generator Effect

In the last topic we saw that if we got an electric current to interact with a magnetic field, we got movement.  Can we instead get an electric current if we move the wire through a magnetic field?  The answer is yes, as long as the wire is connected to an outside circuit.  If the wire is NOT connected to an outside circuit, there is a potential difference (voltage) instead.  This is called the generator effect.

The picture shows a carbon rod connected to a very sensitive voltmeter that can detect tiny voltages. The carbon rod is moving.

 Is there a voltage?  How can you tell?

 What would happen to the voltage if you moved the rod from right to left?

 What happens if the rod is stationary?

We could keep the rod still and move the magnet from left to right.

 Question 4 What would you see on the voltmeter this time? This time, instead of moving the carbon rod from left to right, we move it perfectly vertically up and down, as in the picture: This is easier said than done.

 Question 5 Is there a reading on the voltmeter this time? For there to be a reading on the voltmeter, the wire has to cut through magnetic field lines.  Therefore, if the rod is moved vertically, it does not cut field lines, so there is no voltage.

We can increase the voltage by:

• moving the wire quicker;

• having a stronger magnetic field;

• using a coil of wire consisting of two or more turns.

If we increase the area of the coil going through the magnetic field lines, we also increase the voltage. Both wires in the picture are travelling at the same speed.  The magnetic field lines are all going into the screen.

 Question 6 Why is the voltage increased in Wire 2? A magnet moving through a coil of wire also produces a voltage. The voltage can be increased by:

• increasing the strength of the magnet;

• increasing the area of the coil;

• increasing the number of turns in the coil;

• making the magnet move faster.

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 is hard to the feel and heavy.  It means that  when the iron is 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 7 Why can electricity not flow between the coils? 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 8 Why should we keep the voltage the same? 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 9 What simple conclusion can you draw from these findings? 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 10 Why does the magnetic field change all the time with AC? Question 11 What would happen to the voltage at the secondary if the magnetic field stopped changing? 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:

Learn this for the exam:

p.d. across primary  = number of turns on primary

p.d. across secondary           number of turns on secondary

In Physics Code:

Vprim = Nprim

Vsec    N sec

Look at the picture.  We will use it in the worked example. Worked Example An input voltage of 20 volts is applied across the terminals of the primary.  What is the secondary voltage? Equation first Vprim = Nprim Vsec    N sec Now put in the numbers 20 V = 2400 turns Vsec     240 turns   10 Vsec = 20 V   Vsec = 2 V

 Question 12 Now the primary and secondary coils are swapped over.  What is the secondary voltage now? 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 laboratory power supply (which I use in my workshop).

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 13 How can a step-down transformer be changed into a step-up transformer? 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. Switch Mode Transformers

Transformers are bulky and heavy with lots of soft iron.  Soft iron is not soft at all; it is heavy and hard.  If you drop it on your foot, you will know about it.  A standard mains adaptor has a transformer in it, and it is heavy.  There is also a certain amount of energy loss due to hysteresis.  If you leave a transformer switched on, it can be warm, even if there is no load.  Although we treat transformers as almost 100 % efficient, in reality they are not. Many electronic devices need a regulated voltage, which means a voltage that remains at a constant value.  However transformers produce a voltage that can fall when a heavy load is applied, which would be no good for an electronic device.  So the voltage is regulated to the lowest level that the transformer is at.  If the transformer has less load, the voltage is higher, but it is brought down to the regulated level by the voltage regulator.  This means that energy is lost.

Additionally larger transformers can give an audible hum, due to the movement of components because of the changing magnetic fields.

Computer supplies do not use a large transformer; it would be very heavy in order to produce the powers required in a transformer.  Instead they use a switched mode power supply that relies on electronic switching.  They have these advantages:

• Switch mode transformers operate at a high frequency, often between 50 kHz and 200 kHz.

• Switch mode transformers are much lighter and smaller than traditional transformers working from a 50 Hz mains supply.

• Switch mode transformers use very little power when they are switched on but no load is applied.

A computer power supply has a number of voltages that are required.  While these could be provided by an ordinary transformer with a differing turns ratio, there would be increased bulk and complexity of the the voltage smoothing and regulation needed for each voltage.  Therefore there would be a lot of energy lost as heat. Your light mobile phone charger is a switched mode transformer.

Transformers are very reliable, and are unlikely to fail.  A failure of the switched mode supply can result in excessive voltages that would wreck the inside of a computer.

 Question 14 Complete the space fill exercise that gets you to think about the transformer effect. Question 15 Try the Crossword which gets you to think about the motor effect, the generator effect, and the transformer effect. Summary Transformers consist of a core, the primary, and secondary coils. The core is laminated and made of soft iron. The transformer equation is: Vprim = Nprim Vsec    N sec Step-up and step-down transformers are used in distributing electricity around the National Grid