Question

A step down transformer reduces $220V$ to $11V$. The primary coil draws $5A$ current and secondary coil supplies $90A$. Efficiency of the transformer will be:

• A. $4.4\%$
• B. $20\%$
• C. $33\%$
• D. $90\%$

Answer 1

The correct option is D $90\%$

Answer is D.

The efficiency of a transformer can be calculated using Equation,
$Efficiency=\frac{Outputpowerofthetransformer}{Inputpowerofthetransformer}=\frac{P_s}{P_p}\times 100$
In this case, the output power is calculated as follows.
The output voltage is 11 V and current is 90 A
$Power=VI=11\times 90=990W$.
The input power is calculated as follows.
The input voltage is 220 V and current is 5 A
$Power=VI=220\times 5=1100W$.
So, $\eta =\frac{990}{1100}\times 100=90$.
The efficiency of the transformer is 90%.