Discussion
If the brushes of that machine are shifted forward by 9 degrees, the demagnetising ampere-turns per pole are
Answer: B
Correct answer: 250
\[ AT_d = AT \times \frac{2\theta}{180} = 2500 \times \frac{18}{180} = 250. \] So a nine-degree shift converts a tenth of the armature ampere-turns into demagnetising ones, leaving 2,250 still cross-magnetising. That is the cost of using a brush shift for commutation, and it is why interpoles are preferred.
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