Question

A step-down transformer is connected to a 2400 volts line and 80 amperes of current is found to flow in the output load. The ratio of the turns in primary and secondary coils is 20:1. If transformer efficiency is 100%, then the current flowing in primary coil will be:

1. 8A
2. 4A
3. 5A
4. 9A

Answer 1

We have seen our parents or relatives saying that the voltages are down or the voltages are up. If they are down they use to press a button on a metallic box and the voltages come back to normal. That metallic box that makes the low voltage to high voltage is known as a transformer. There are basically two types of transformers: a step down transformer and a step up transformer. A step down transformer converts high voltage to low voltage and step up transformer is a transformer that converts low voltage to high.

Formula used:
$\dfrac{{{V_1}}}{{{V_2}}} = \dfrac{{{N_1}}}{{{N_2}}} = \dfrac{{{I_2}}}{{{I_1}}}$;
Where:
${V_1}$= Primary voltage;
${V_2}$= Secondary voltage;
${N_1}$= Number of turns in primary coil.
${N_2}$= Number of turns in secondary coil.
${I_1}$= Current in primary coil.
${I_2}$= Current in secondary coil.

Complete step-by-step answer:
Put the given value in the above formula and solve for the unknown.
$\dfrac{{{N_1}}}{{{N_2}}} = \dfrac{{{I_2}}}{{{I_1}}}$;
$\Rightarrow \dfrac{{20}}{1} = \dfrac{{80}}{{{I_1}}}$;
$\Rightarrow {I_1} = \dfrac{{80}}{{20}}$;
$\Rightarrow {I_1} = 4A$;

Final Answer: Option “2”. The current flowing in the primary coil will be 4A.

Note: Here in this question we have been given unnecessary information that is not required to solve the question. Don’t get confused by the given information, apply the basic formula $\dfrac{{{N_1}}}{{{N_2}}} = \dfrac{{{I_2}}}{{{I_1}}}$; and solve for the current in the primary wire.