Electrical Engineering ⇒ Topic : Reluctance (S)
Reluctance (S). Reluctance is a measure of opposition offered by a magnetic circuit to the setting up of flux.
The reluctance (S) of a magnetic circuit is given by
where l = length of the magnetic circuit,
A = toss-sectional area of the magnetic circuit,
μ0= absolute permeability, and
μr= relative permeability.
Reluctance of a magnetic circuit is the ratio of rn.rn.f. and flux
The unit of reluctance is AT/Wb. Since 1 AT/Wb = 1/henry, the unit of reluctance is "reciprocal henry"
RELUCTANCE OF A MAGNETIC CIRCUIT
In the previous section, it has been shown that the magnetic field intensity in the ring specimen = I N/l. Let (to be the magnetic flux in the ring specimen.and let A be the cross-sectional area of the specimen. If B is the magnetic flux density in the specimen, then
and let A be the cross-sectional area of the specimen. If B is the magnetic flux density in the specimen, then
H = I N I L ............... (1)
Φ =B x A ............... (2)
But we also know that
B = μH ................ (3)
where, μ is the permeability of the material of the specimen.
From Eqs (1) and (2), we get
Φ = μ H A .............(4)
We know that for an electric circuit, current = emf resistance.
In a similar manner, in a magnetic circuit, the magnetic flux can be defined as magnetic flux = mmf/reluctance
Therefore, reluctance of a magnetic circuit
(S) = mmf / flux
i.e. S = H .l/ μHA [from Eqs (1) and (2)]
i.e. S = l /μ .............. (5)
Reluctance has the units of l l henry, as seen from Eq. (5) and has the same dimensions as ampere/weber. The reluctance is expressed in units of ampere/weber and it is directly proportional to the length of the magnetic path and is inversely proportional to the cross-sectional area of the specimen. From the above expression, it can be seen that for a given set of dimensions,the reluctance of the air path is many times more than the reluctance of a magnetic material. The following table gives a comparison between resistance and reluctance.
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