Magnetic Circuits
A magnetic circuits can be defined as the path which is followed by magnetic flux.
An example of magnetic circuit is given below:-
The Core is made of iron. Iron being a good magnetic material (high value of relative permeability), flux produced by the iron core will entirely pass through the iron core and thus magnetic field intensity(H) can be assumed to remain fixed at every point within the iron core.
Magnetic field intensity developed by the iron core, Hc = Ni /lc AT/m
Flux density within the iron core, Bc = μo μr Hc Wb/m^2
where μr is the relative permeability of iron material used in the iron core.
The amount of flux flowing through the iron core,
Ⲫc = B.Ac
or, Ⲫc = μo μr Hc Ac
or, Ⲫc = μo μr (Ni /lc) Ac
or, Ⲫc = Ni /(lc /μo μr Ac)
or, Ⲫc = Ni /S
Product of number of turns (N) in the coil and current (i) passing it i.e. (F=Ni) is known as the Magneto Motive Force (MMF).
MMF is the source that produces magnetism in a magnetic circuit.
MMF is analogous to EMF in an electric circuit and can be defined as the magnetic potential difference that forces magnetic flux to flow through a magnetic circuit.
S = lc /μo μr Ac is called reluctance of the magnetic flux path that is a measure of how much opposition the flux faces while flowing through the magnetic circuit.
unit of reluctance is AT/Wb
Reluctance is inversely proportional to relative permeability, it is obvious that a good magnetic material (high value of μr) will offer less opposition (low reluctance) to the flux flowing through it.
The quantity 1/μo μr is called Specific reluctance or reluctivity of the magnetic material.
Sometimes, energy conversion device like generators or motors may have air gap between stator & rotor, when air gap is much smaller than the dimensions of adjacent core faces, magnetic flux will follow the path defined by air gap & core.
We know that,
Flux, Ⲫc = MMF(F=NI) / Reluctance(S)
F = Ⲫc.S
F = Ⲫc.[ Sc + Sg ]
Sc = lc / μc.Ac , Sg = g / μo.Ag
Where, g = air gap length
Ag = area of cross section of air gap
So, introduction of air gap increases the reluctance in the path of flux. So, to maintain same flux machines draw more amount of magnetizing current.
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