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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
2Ω
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B
8Ω
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C
12Ω
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D
10Ω
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Solution

The correct option is D 12Ω
Input power = VI
=200×10
=200W
Output power =40100×2000=800W
Power loss in heating of the armature
= 2000 - 800 = 1200 W
I2=1200
R=1200I2
=120010×10
R=12Ω

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