Electrical Engineering ⇒ Topic : Important terms magnetic circuit
In the study of magnetic circuits, we generally come across the following terms
(1)Magnetomotive force (m.m.f.). It is a magnetic pressure which sets up or tends to set up flux in a magnetic circuit and may be defined as under :
The work done in moving a unit magnetic pole once around the magnetic circuit is called the magnetomotive force (m.m.f),It is equal to the product of cun-ent and number of turns of the coil i.e.
m.m.f. = NI ampere-turns (or AT)
Magnetomotive force in a magnetic circuit corresponds to e.m.f. in an electric circuit. The only change in the definition is the substitution of unit magnetic pole in place of unit charge.
(2)Reluctance. The opposition that the magnetic circuit offers to magnetic flux is called reluctance. The reluctance of a magnetic circuit depends upon its length, area of X-section and permeability of the material that makes up the magnetic circuit. Its unit is AT/Wb.
Reluctance in a magnetic circuit corresponds to resistance (R = ρla) in an electric circuit. Both of them vary as length ± area and are dependent upon the nature of material of the circuit. Magnetic materials (e.g. iron, steel etc.) have a low reluctance because the value of μr is very large in their case. On the other hand, non-magnetic materials (e.g. air, wood, copper, brass etc.) have a high reluctance because they possess least value of μr ; being 1 in case of all non-magnetic materials
The reciprocal of permeability μ(= μ0μr) corresponds to resistivity ρ of the electrical circuit and is called reluctivity. It may be noted that magnetic permeability (μ) is the analog of electrical conductivity.
(3)Permeance. It is the reciprocal of reluctance and is a measure of the ease with which flux can pass through the material. Its unit is Wb/AT
Permeance of a magnetic circuit corresponds to conductance (reciprocal of resistance) in an electric circuit.
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