Discussion
The starting current of a 250 V motor with an armature resistance of 0.25 Ω, connected directly to the supply, is
Answer: B
Correct answer: 1,000 A
At rest there is no back emf, so the armature resistance alone limits the current:
\[ I = \frac{V}{R_a} = \frac{250}{0.25} = 1000 \text{ A} \]That is many times the rated current of such a machine, and it would destroy the armature and the commutator in seconds. It is the whole reason a dc motor cannot be switched directly onto the supply and needs a starter.
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