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Question: A simple electric motor has an armature resistance of one *ohm* and runs from a dc source of 12 *vol...

A simple electric motor has an armature resistance of one ohm and runs from a dc source of 12 volts. When unloaded it draws a current of 2 amperes. When a certain load is connected, its speed becomes one-half of its unloaded value. Then the current in ampere it draws is

A

7 amp

B

6 amp

C

2 amp

D

4 amp

Answer

7 amp

Explanation

Solution

Back emf e ∝ speed, e = E – iR = 12 – 2 × 1 = 10 V

e′=e2=E−i′R⇒5=12−i′×1⇒i′=7amp.e' = \frac{e}{2} = E - i'R \Rightarrow 5 = 12 - i' \times 1 \Rightarrow i' = 7amp.