Taking equation 4.3 into consideration, the relationship described by equa-
tion 4.10 can be rewritten as,
B c L c
+ B g g
: fl ux density in the core,
: fl ux density in the airgap,
: permeability of the core, and
: permeability of air.
Since the fl ux lines are continuous and the cross-section areas of the core
and the airgap are same, it can be assumed that
B c = B g .
The permeability of steel is very large compared to that of airgap making
the fi rst term in the left hand side of equation 4.11 negligible. With that
assumption, equations 4.11 and 4.12 can be combined into
B c = B g = NI µ 0
This equation can also be used to de fi ne the fl ux,
φ g = φ c = B c A c = NI µ 0 A c
in which, A c is the area of cross section of the core, and g is de fi ned as the
reluctance of the magnetic circuit,
µ 0 A c .
De fi ning the Magnetromotive force (MMF) as
F = NI,
the MMF consumed by the reluctance g can be expressed as,
F g = φ g .
referring to the circuit shown in Figure 4.5, as there is only one signi fi cant
reluctance g , the MMF consumed in the airgap reluctance is equal to the
MMF generated by the winding current, that is
F = F g .