Question

An electric motor runs on DC source of emf $200 \,V$ and draws a current of $10\, A$. If the efficiency be $40\%$, then the resistance of armature is

• A. $ 2\,\Omega $
• B. $ 8\,\Omega $
• C. $ 12\,\Omega $
• D. $ 16\,\Omega $

Answer 1

Answer: (C)

Input power $ =VI=200\times 10 $
$ =2000W $
Output power $ =\frac{40}{100}\times 2000=800W $
Power loss in heating the armature
$ =2000-800 $
$ =1200W $
$ \therefore $ $ {{I}^{2}}R=1200 $
or $ R=\frac{1200}{{{I}^{2}}} $
$ =\frac{1200}{10\times 10} $
or $ R=12\,\Omega $