Question

An electric motor runs on $DC$ source of emf $200V$ and draws a current of $10A$. 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)

$12\Omega$

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 $R=12\Omega$.