Discussion
A dc motor drawing 40 A at 220 V with an armature resistance of 0.5 Ω and negligible field current develops a back emf of
Answer: A
Correct answer: 200 V
From the armature circuit equation,
\[ E = V - I_aR_a = 220 - 40 \times 0.5 = 220 - 20 = 200 \text{ V} \]The back emf takes 91 per cent of the applied voltage. Note that this same machine at standstill would draw \( 220/0.5 = 440 \) A, eleven times its running current, which is why it needs a starter.
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