A magnetic circuit is defined as the route or path which is followed by a magnetic flux. Some important terms connected with magnetic circuits are discussed below:
1. Permeability (µ). Permeability of any material is a measure of ease with which the atoms can be arranged. The S.I. unit for permeability is henry/metre (H/m).
Mathematically, µ = µ0µr = H/m … (10)
=4π × 10-7 H/m (in S.I. units)
µ0 = permeability of free space
= 4π × 10-7 H/m (in S.I. units)
ur = relative permeability (= 1 for air and non-magnetic materials)
Relative permeability (µr) is simply a numeric which expresses the degree to which the material is a better conductor of magnetic flux as compared to free space.
Note: µr for air (and non-magnetic materials) =1
µr, for diamagnetic materials e slightly less than one
µr, for paramagnetic materials e slightly higher than one
µr, for ferromagnetic materials e in the hundreds or thousands
(µr for ferromagnetic materials changes with the magnetic field strength)
2. Magnetomotive force (m.m.f.). Magnetomotive force drives or tends to drive flux through a magnetic circuit and corresponds to electromotive force (e.m.f.) in a circuit. M.m.f. is equal to the work done in joules in carrying a unit magnetic pole once through the entire magnetic circuit.
M.m.f. is measured in ampere-turns (AT). Ampere-turns are found by multiplying number of turns(N) of a magnetic circuit and the current in amperes (I) in those turns.
3. Reluctance. Just as the current in an electric circuit is limited by resistance, the flux in the magnetic circuit is limited by the reluctance (symbol S) of the magnetic circuit. Thus reluctance is a measure of opposition offered by a magnetic circuit to the setting up of the flux.
The reluctance of a magnetic circuit is given by,
Where l = length of the magnetic circuit,
A = Cross-sectional area of the magnetic circuit,
= absolute permeability, and
= relative permeability.
Reluctance of a magnetic circuit is the ratio of m.m.f. and flux.
In other words the reluctance of a magnetic circuit is the number of ampere-turns required
per weber of magnetic flux in the circuit. The unit of reluctance is AT/Wb. Since 1 AT/Wb = l/henry, the unit of reluctance is “reciprocal henry”.
4. Permeance. The reciprocal of reluctance is known as permeance.
It is measured in Wb/AT on henry.
5. Reluctivity. It is the specific reluctance and corresponds to resistivity which is specific resistance
Relation between flux density (B) and magnetic field strength (H):
Comparing eqns. (12) and (13), we get
Thus, permeability is the ratio afflux density to magnetic field strength.