Lecture 3 Magnetic Circuits

This mmf creates a magnetic field in the core having an intensity of H c ampereturn/meter
along the length of the magnetic path. According to Ampere’s law
∮ (1)
If H c is constant, and l c is the mean path length of the core, then (1) gives,
H c l c =NI=F= mmf (2)
The core is usually made of ferromagnetic material. The magnetic flux density B
(weber/m 2 ) in the core is related to the magnetic field H according to the saturation
curve, or B-H curve (Fig 3). The slope of this is designated as µ, the permeability of
the material, thus giving relationship between B and H as
B = µH (3)
As seen from Fig 3, the slope of the curve that µ is not constant and depends of the
operating value of magnetic flux density ( ). The permeability of a magnetic material
is usually given relative to the permeability of the free-space, µ 0 . Thus
µ= µ 0 µ r . (4)
where, µ r is the relative permeability. In SI units, the permeability of the free-space is
µ 0 = 410 -7 H/m.
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