Page 22 - Basics of Electrical, Electronic and Communication

36 Chapter 3. Magnetic circuits

3.6 Magneto motive force

Magneto motive force (mmf) is the force which establishes the mag-
netic flux in a magnetic circuit. When a current is passed through a
coil, a magnetic flux is set up around it. The product of current (I) and
number of turns (N) of the coil gives MMF.
MMF F is given by the expression,

F = NI

Its unit is Ampere-turns (AT).

3.7 Magnetic field intensity

Magnetic field intensity is the magnetic motive force per unit length of
the magnetic circuit. It is denoted by H and its unit is Ampere-turns/
meter. Magnetic field intensity is also called magnetic field strength or
magnetizing force.

mmf F
H = =
l l
NI
H =
l
Or
F = NI = Hl (3.1)

3.8 Permeability

The ability of a material to conduct magnetic flux through it is called
permeability of that material. It is represented by the letter µ and the
unit Henry/meter. The flux and hence the flux density B are directly
proportional to the magnetizing force H.
i.e.,
B ∝ H

36