Pole shoes with windings are more complicated as the ux density depends
on the material of the pole shoe as well as the coil and the current owing. The
magneto-motive force (MMF) of a coil is determined as follows:
MMF = NI Ampere turns
where: N = the number of turns on the coil, I = the current owing in the coil.
Magnetic eld strength H, requires the active length of the coil to be included:
where: l = active length of the coil, H = magnetic eld strength.
In order to convert this to ux density B, the permeability of the pole shoe must
be included:
where: µ
0
= permeability of free space (4 x 10
–7
Henry/metre), µ
r
= relative
permeability ...
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