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

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 Ω  (B)  8 Ω  (C)  12 Ω  (D)  16 Ω

Tardigrade Admin · Accepted Answer

Input power =VI=200× 10 =2000W Output power =(40/100)× 2000=800W Power loss in heating the armature =2000-800 =1200W ∴ I2R=1200 or R=(1200/I2) =(1200/10× 10) or R=12 Ω

Q. 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

$2 Ω$

$8 Ω$

$12 Ω$

$16 Ω$

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

Q. 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

2Ω

8Ω

12Ω

16Ω