MAGNETIC CIRCUITS
Basic Electrical Theory
The permeability (µ) of a magnetic material is the ratio of B to H. Equation (1-18) is the
mathematical representation for magnetic material permeability.
(1-18)
µ
B
H
The average value of permeability is measured where the saturation point, or knee, is first
established. Figure 27 shows that the normal or average permeability for the two irons as
follows.
µ soft iron 1 =
=
= 1 x 10-4
B
H
0.2
2000
(T m)
At
µ soft iron 2 =
= 6 x 10-5
B
H
0.3
5000
(T m )
At
In SI units, the permeability of a vacuum is µo = 4 p x 10-7 H/m or 1.26 x 10-6 or T-m/At. In
order to calculate permeability, the value of relative permeability µr must be multiplied by µo.
Equation (1-18) is the mathematical representation for permeability.
µ = µr x µo
(1-18)
Example:
Find the permeability of a material that has a relative permeability of 100.
µ = µr x µo = 100 (1.26 x 10-6)
= 126 x 10-6
(T m)
At
Hysteresis
When current in a coil reverses direction thousands of times per second, hysteresis can cause
considerable loss of energy. Hysteresis is defined as "a lagging behind." The magnetic flux in
an iron core lags behind the magnetizing force.
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