Question

A transformer having efficiency of $90\%$ is working on $200\text{V}$ and $3\text{kW}$ power supply. If the current in the secondary coil is $6\text{A}$, the voltage across the secondary coil and the current in the primary coil respectively are

• A. $300\text{V},15\text{A}$
• B. $450\text{V},15\text{A}$
• C. $450\text{V},13.5\text{A}$
• D. $600\text{V},15\text{A}$

Answer 1

The correct option is B $450\text{V},15\text{A}$

Given Power input to primary coil is $3\text{kW}$
current in secondary coil is $6\text{A}$
potential across primary coil $200\text{V}$
Let $V_s$ be potential drop in secondary coil

Efficiency of the transformer is $\eta =\frac{P_{out}}{P_{in}}$
$\eta =\frac{V_s{I}_s}{V_p{I}_p}$
$0.9=\frac{V_s\left(6\right)}{3\times {10}^3}$
$V_s=450\text{V}$

Using $P_{in}=V_p{I}_p=3000$
$I_p=\frac{3000}{200}=15\text{A}$