Question

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

• A. 4.4%
• B. 20%
• C. 33%
• D. 90%

Answer 1

Answer: (D)

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%.