Electrical Engineering ⇒ Topic : Reluctance (S)

Maninder
 
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 ........... (1) where l = length of the magnetic circuit, A = tosssectional area of the magnetic circuit, μ_{0}= absolute permeability, and μ_{r}= relative permeability. Reluctance of a magnetic circuit is the ratio of rn.rn.f. and flux ........... (2) The unit of reluctance is AT/Wb. Since 1 AT/Wb = 1/henry, the unit of reluctance is "reciprocal henry"  
 
Sunita
 
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 crosssectional area of the specimen. If B is the magnetic flux density in the specimen, then and let A be the crosssectional 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 crosssectional 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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