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)

$12\,\Omega$

Answer 2

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 $R=12\,\Omega$