Discussion
A dc motor of armature resistance 0.4 Ω is plugged from 220 V while running with a back emf of 200 V. The initial reverse current, with no added resistance, is
Answer: B
Correct answer: 1,050 A
Plugging reverses the supply while the machine still turns, so the applied voltage and the back emf now add instead of opposing:
\[ I = \frac{V + E}{R_a} = \frac{220 + 200}{0.4} = \frac{420}{0.4} = 1050 \text{ A} \]That is far beyond anything the machine or the converter could survive, which is why plugging is never done without series resistance. Subtracting instead of adding gives 50 A and misses the entire point of the method.
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