Electric Motors and Drives, 5th Edition

Appendix

Solutions to review questions

Chapter 1

1. (1) The magnetomotive force or m.m.f. is simply the product of the number of turns and the current, i.e. m.m.f. = 250 × 8 = 2000 AT. The ‘AT’ stands for ampere-turns, but strictly the unit of m.m.f. is the Ampere.
2. (2) We are told that the magnetic circuit is made of good-quality magnetic steel, which is a coded way of saying that the reluctance of the steel part of the magnetic circuit is negligible in comparison with the air-gap. Under these circumstances all of the m.m.f. provided by the coil (2000 A) is available across the gap, and the flux density in the air-gap is given by Eq. (1.7) as

   $B=\frac{{\mu }_o\mathit{NI}}{g}=\frac{4\pi \times {10}^{-7}\times 2000}{2\times {10}^{-3}}=1.26\mathrm{T}.$

   The question then asks about the flux density in the iron, implying that it ...