Discussion
A dc motor runs at 1,500 rpm on 240 V with an armature resistance of 0.5 Ω, drawing 20 A. Its back emf is
Answer: A
Correct answer: 230 V
Rearranging the armature circuit equation,
\[ E = V - I_aR_a = 240 - 20 \times 0.5 = 240 - 10 = 230 \text{ V} \]The back emf takes 96 per cent of the applied voltage, which is typical in normal running - and the reason the starting current would be so large, since at standstill \( E \) is zero and the whole 240 V appears across the 0.5 ohm resistance.
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