1. UNIT-II Magnetic Circuits
Basic definition of MMF, flux and reluctance, analogy
between electrical and magnetic circuits, Faraday’s laws of
electromagnetic induction – concept of self and mutual
inductance, Dot convention – coefficient of coupling and
composite magnetic circuit, analysis of series and parallel
magnetic circuits.
2. Magnetic flux and flux density
Magnetic flux is the amount of magnetic field (or the number of
lines of force) produced by a magnetic source. The symbol for
magnetic flux is (Greek letter ‘phi’). The unit of magnetic flux is
the weber, Wb.
Magnetic flux density is the amount of flux passing through a
defined area that is perpendicular to the direction of the flux:
Magnetic flux density = magnetic flux/area
The symbol for magnetic flux density is B. The unit of magnetic flux
density is the tesla, T, where1 T = 1Wb/m2
3. • Magnetomotive force (m.m.f.) is the cause of the existence of a
magnetic flux in a magnetic circuit,
m.m.f. Fm = NI amperes
where N is the number of conductors (or turns) and I is the current in
amperes. The unit of m.m.f is sometimes expressed as ‘ampere-
turns’. However since ‘turns’ have no dimensions, the S.I. unit of
m.m.f. is the ampere.
• Magnetic field strength (or magnetising force), H = NI/l ampere per
metre
where l is the mean length of the flux path in metres. Thus m.m.f. = NI = Hl
amperes
• For air, or any non-magnetic medium, the ratio of magnetic flux density to
magnetising force is a constant,
i.e. B/H =a constant. This constant is μ0, the permeability of free space (or
the magnetic space constant) and is equal to 4π×10−7 H/m,
B/H = μ0
4. • Reluctance
– The measure of “opposition” the magnetic circuit
offers to the flux
– The analog of Resistance in an electrical circuit
– Symbol, R
– Definition, R = F/Φ=NI/ Φ=Hl/BA=l/A
– Units, (A-t/Wb)
5.
6.
7. Faraday’s laws of electromagnetic induction:
• An induced e.m.f. is set up whenever the
magnetic field linking that circuit
changes.
• The magnitude of the induced e.m.f. in
any circuit is proportional to the rate of
change of the magnetic flux linking the
circuit.
Faraday’s law of electromagnetic induction (referred to as Faraday’s
law) is a basic law of electromagnetism predicting how a magnetic field
will interact with an electric circuit to produce an electromotive force
(EMF). This phenomenon is known as electromagnetic induction.
8. • Self and Mutual Inductance:
• When the e.m.f. is induced in the same circuit as that in
which the current is changing, the property is called self
inductance, L
• When the e.m.f. is induced in a circuit by a change of flux
due to current changing in an adjacent circuit, the property
property is called mutual inductance,M. The unit of
inductance is the henry, H.
