
6 Electric Motors and Drives
on the conductor will be less; and in the extreme case where
the current was in the same direction as the field, the force
would fall to zero. However, every sensible motor designer
knows that to get the best out of the magnetic field it has to
be perpendicular to the conductors, and so it is safe to
assume in the subsequent discussion that B and I are always
perpendicular.
The reason for the very low force detected in the experi-
ment with the bar magnet is revealed by equation 1.2. To
obtain a high force, we must have a high flux density, and a
lot of current. The flux density at the ends of a bar magnet is
low ...