Question

A compensated dc machine has 20000 AT/Pole. The ratio of pole arc to pole pitch is 0.8. The interpolar airgap length and flux density arc 1.2 cm and 0.3 T. For rated armature current $I_a=1000$ A, the number of turns on each interpole are

• A. 5
• B. 7
• C. 7
• D. 8

Answer 1

The correct option is B 7

$A{T}_{CW}/pole=A{T}_a\left(peak\right)\times \frac{\text{Pole arc}}{\text{Pole pitch}}$

$=20000\times 0.8=16000$

$A{T}_a\text{(peak) interpolar region}$ $=20000-16000=4000$

$A{T}_i=A{T}_a\left(peak\right)+\frac{B_i}{{\mu }_i}{l}_g$

$=4000+[\frac{0.3}{4\pi \times {10}^{-7}}\times 1.2\times {10}^{-2}]=6865AT/pole$

$N_i=\frac{6865}{1000}=6.865\simeq 7$ turns