Discussion
The armature circuit equation of a separately excited dc motor is
Answer: C
Correct answer: \( V = E + I_aR_a \)
The applied voltage must supply the back emf and drive the current through the armature resistance:
\[ V = E + I_aR_a \]The back emf takes almost all of it in normal running, the resistive term being a few per cent. At standstill \( E = 0 \), so the whole of \( V \) appears across \( R_a \) and the current would be enormous - which is why a starter is needed. The form with a minus sign describes the machine acting as a generator.
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