Question

What is increased in a step-down transformer
A.) Voltage
B.) Current
C.) Wattage
D.) None of these

Answer 1

Hint: Study the types of transformers. The transformer works on Faraday’s principle of mutual induction where emf is induced in the secondary coil through the voltage or current flowing through the primary coil. Try to derive the formulas involving different types of transformers.

Complete step by step answer:
A transformer is a passive electrical device which transfers one electrical circuit to another. It consists of two electrically isolated coils and works Faraday's principle of mutual inductance. It transforms electrical energy to the magnetic energy on the primary side and again from magnetic energy to the electrical energy on the secondary side.

A step-down transformer can be defined as an object which transforms the high voltage and low current to low voltage and high current. The number of turns of wire in the coil on the primary side is high and the number of turns of wire in the coil on the secondary side is less.
Voltage across the secondary coil is,

${{V}_{s}}=-{{N}_{s}}\dfrac{d\phi }{dt}$
Voltage across the primary will be,
${{V}_{p}}=-{{N}_{p}}\dfrac{d\phi }{dt}$

Equating the above formulas the formula for a step-down transformer is,

$\dfrac{{{N}_{s}}}{{{N}_{p}}}=\dfrac{{{V}_{s}}}{{{V}_{p}}}$

Where, ${{N}_{s}}$ is the number of turns in the secondary coil;
${{N}_{p}}$ is the number of turns in the primary coil;
${{V}_{s}}$ is the voltage in the secondary and;
${{V}_{p}}$ is the voltage in the primary

Voltage and current are inversely proportional,

${{i}_{p}}{{v}_{p}}={{i}_{s}}{{v}_{s}}$

So, we can write

$\dfrac{{{N}_{s}}}{{{N}_{p}}}=\dfrac{{{V}_{s}}}{{{V}_{p}}}=\dfrac{{{i}_{p}}}{{{i}_{s}}}$
As the number of turns in the secondary is less than the number of turns in the primary, the secondary current will be more than the primary current.

There are three types of step-down transformer based on tapping in the secondary coil – the single phase step-down transformer, centre tapped step-down transformer and the multi tapped step-down transformer.

So, in a step-down transformer current is increased.

The correct option is (B)

Note: From the above relations we can see that in a step-down transformer voltage will always decrease as voltage and current are inversely proportional.
Again, if the transformer is assumed to be 100 percent efficient the input power and the output power will be equal. So, the wattage will neither increase or decrease.