Discussion
The back emf of a running dc motor
Answer: B
Correct answer: opposes the applied voltage and limits the current
A rotating armature generates an emf of its own, and by Lenz’s law it opposes the applied voltage:
\[ I_a = \frac{V - E}{R_a} \]Since \( E \) takes most of \( V \) in normal running, the current is small - and at standstill, where \( E \) is zero, it is enormous. The back emf is present whenever the machine turns, not merely while accelerating.
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