Question

An electric metor 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. $10\Omega$

Answer 1

Answer: (C)

$12\Omega$

Input power = VI

$=200\times 10$

$=200W$

Output power $=\frac{40}{100}\times 2000=800W$

Power loss in heating of the armature

= 2000 - 800 = 1200 W

$I^2=1200$

$R=\frac{1200}{I^2}$

$=\frac{1200}{10\times 10}$

$R=12\Omega$