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GAPPED TOROIDAL CORES
The design of a storage choke or transformer which also carries direct current
normally requires air gaps which prevent the core from being magnetized by the
dc current above 1/2 B max.
An air gap in a core has the effect of flattening/
shearing the B-H loop of the material, lowering the residual flux and the
permeability, extending the incremental
a.c. permeability to higher values of B and H. Precise control of the inductance
L, or the remanence Br is
therefore possible with gapped toroids.
Figure 4 shows a comparison of hysteresis loops for a core with and without
gaps.
For larger air gaps,
the B field fringes over a larger area. The actual μg
or Lg is then significantly greater than calculated. The deviation increases
with lg, with 1/Ac and the more Ac deviates from a
square area.
When an unbalanced dc current flows in a winding, the core is subjected to
a dc magnetizing force. This results in a flux density which may be
expressed as
Bdc = .4π x N Idc / Ig + Im/μm
Ig and Im in cm.
If the sum of Bac and Bdc are over the maximum operating
flux density of the core material, the incremental permeability and inductance
are greatly reduced; to reduce Bdc the Ig increases the
inductance will decrease, therefore, the designer should
balance Ig to produce maximum inductance. See Hanna curvers.
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