Discussion
The starting resistance of a dc motor starter is removed in steps as the machine accelerates because
Answer: D
Correct answer: the growing back emf takes over the limiting of the current
As the machine accelerates its back emf rises, so \( V - E \) falls and with it the current. Each step of resistance can then be removed to bring the current back up to the accelerating value without exceeding the limit, and by the time the machine is at speed the back emf alone is doing the limiting and no external resistance is needed. Armature resistance and flux change hardly at all during the start.
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