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| The Transformer |
 Most
of the electronic devices that we use need a transformer if they are to
run when they are plugged into the mains. Microchip circuitry does not
need a big voltage to operate, in fact a big voltage will simply cause
the chip circuitry to burn out. Such circuits run on voltages of between
5V and 12V. Therefore a transformer is necessary to 'step-down' mains
voltage (230V) to this level. Any device you have that can run off batteries
or via a mains connector will have a transformer incorporated into that
connector. Sometimes they 'hum' and they always get warm after being switched
on for a while.
What is a transformer? It is a laminated (made up in layers - see the photo below) soft iron core with two insulated coils of wire wrapped around it.
Click here for Faraday's early experiment The windings of the core are made of low resistance copper. The first coil is called the primary coil. This is the coil that has the supply voltage applied across its ends. The second coil is called the secondary coil. This is the coil that has the output voltage across its ends. This is the coil that is connected to your appliance.
The core is not designed to have any currents induced
in it. It is however a conducting loop that experiences a changing magnetic
field, it will therefore have small currents induced in it - these are
called 'eddy currents'. The core is laminated to reduce these to a minimum
as they interfere with the efficient transfer of energy from the primary
coil to the secondary one. The eddy currents cause energy to be lost
from the transformer as they heat up the core. Laminated means 'made
up of insulated layers of iron 'glued' together' rather than being in
a single solid 'lump'. A laminated core has a higher resistance than
a non-laminated one with the same number of domains. It therefore does
not get such big a currents induced in it How a transformer works An alternating
voltage (VP) is applied across the primary coil. This
causes a changing magnetic field to be formed around the primary
coil. (The key words you must mention when explaining this are in bold type)
Electromagnetic induction only happens when a wire loop experiences a changing magnetic field . The wire and field lines must move with respect to each other so that the wire can 'cut the lines of flux'. How can we get a different voltage out of a transformer from the one we put in? This is achieved by varying the ratio of the number of turns of wire on the primary and secondary coils. If there are more turns on the secondary then the output voltage will be bigger by the same ratio and vice versa. A step-up transformer has more turns on the secondary than on the primary. It therefore produces a bigger output voltage than input voltage.
A step-down transformer has less turns on the secondary than on the primary. It therefore produces a smaller output voltage than input voltage.
Although transformers lose some of the energy input as heat (that is why the transformer gets warm), they are pretty efficient and about 99.9% of the input electrical energy is converted to output electrical energy. Therefore in examination questions at this level you can assume that the transformer is 100% efficient. This means that: power in = power out power of primary = power of secondary PP = PS IPVP = ISVS As the voltage is stepped up, so the current is stepped down!
In the above photograph the school transformer was used.
10.05V (rounded by the meter to 10.1V) across the primary coil of 2,000
turns resulted in an output of 100.5V across the secondary coil of 20,000
turns. The Symbol
The symbol consists of two curled lines, representing the coils. These are unconnected, as there is no connection between the coils on a real transformer. The line (sometimes two or three are drawn) in the middle represent the magnetic link between the coils that is made by the soft iron core. The Equation There is a mathematical link between the number of turns and the voltages on each coil.
This ratio equation must be known for examination purposes. It doesn't matter which way up you write it, or which way round. Therefore always start with the unknown quantity in the top left position. Example questions: Example 1
Ns is the unknown. We therefore have to arrange the equation as follows:
Ns / 600 = 200/20 = 10 So, Ns = 600 x 10 = 6000 (no unit as you are working out the number of turns) In tests and examinations y ou must show your calculation in full in an examination to show that you understand how to use it. There is a quick way to analyse this circuit and it is handy to do this as a quick check. Look at the ratio relationship between the voltages and turns they will be the same for each. Example 2
This time Vs is the unknown. We therefore have to arrange the equation as follows:
Vs / 6 = 2000/50 = 40 So, Vs = 6 x 40 = 240 volts (don't forget the unit!) Lets try the check again
Example 3
Now we have Vp as the unknown. We therefore have to arrange the equation as follows:
Vp / 12 = 3500/100 = 35 So, Vp = 12 x 35 = 420 volts (unit omission loses you marks!) A quick check:
Example 4 Finally NP as the unknown
We must arrange the equation so NP is at the top left
NP / 150 = 230/12 = 19.17 (not a simple ratio here!) So, Np = 150 x 19.17 = 2876 (to the nearest turn… you can't have part turns!) This is more difficult to do our 'quick-check' way
but still possible! Try some questions or try to complete the transformer crossword. Here is a link to School Science for more background reading on transformers and a little interactive quiz. See the National Grid for how transformers are used in power distribution. You will also be used to using one to charge up your mobile phone from the mains!.
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