The Transformer: Example Questions

Example 1

N_s is the unknown.

We therefore have to arrange the equation as follows:

N_s / 600 = 200/20 = 10

So, N_s = 600 x 10 = 6000 (no unit as you are working out the number of turns)

In tests and examinations you 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 V_s is the unknown.

We therefore have to arrange the equation as follows:

V_s / 6 = 2000/50 = 40

So, V_s = 6 x 40 = 240 volts (don't forget the unit!)

Let's try the check again

Example 3

Now we have V_p as the unknown.

We therefore have to arrange the equation as follows:

V_p / 12 = 3500/100 = 35

So, V_p = 12 x 35 = 420 volts (unit omission loses you marks!)

A quick check:

Example 4

Finally N_P as the unknown

We must arrange the equation so N_P is at the top left

N_P / 150 = 230/12 = 19.17 (not a simple ratio here!)

So, N_p = 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.

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!