Discussion
The speed of a dc motor falls slightly as it is loaded because
Answer: A
Correct answer: the resistive drop in the armature grows with current
From \( N \propto (V - I_aR_a)/\Phi \), a larger load current means a larger \( I_aR_a \) drop, less voltage left for the back emf, and so a slightly lower speed. The fall is small because \( R_a \) is small - typically a few per cent from no load to full load on a shunt machine. Armature reaction weakens the flux slightly and works the other way, which is why the drop is smaller still than the resistance alone would suggest.
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