Nov 7, 2014

Self Induced EMF in Magnetic Circuit

Self induced emf
Think through a coil having number of turns and carrying current ‘I’ once switch‘s’ is in locked position. The current magnitude is varied with the assistance of variable resistance connected nonparallel (series) with battery, coil and switch as shown within the Fig.

The flux created by the coil links with the coil itself. The overall flux linkages of coil are going to be N Web-turns. currently if the present current ‘I' is modified with the assistance of variable resistance, then flux created can conjointly modification, due to flux linkages will also change.

Hence in keeping with Faraday's law, as a result of rate of change of flux linkages there'll be an induced e.m.f. within the coil thus while not physically moving coil or flux there's induced e.m.f. within the coil. The development is called self-induction.

The e.m.f. induced in a coil as a result of the modification of its own flux linked with its known as self-induced e.m.f.


The things of the coil that opposes any modification within the current passing through it's known as Self Inductance.

As per Lenz's law the direction of this induced e.m.f. are going to be thus on oppose the cause creating it. The cause is that the current I therefore the self-induced e.m.f. can attempt to come upon a current that is in opposite direction to it of current I. once current is accrued, self-induced e.m.f. reduces the current tries to stay it to its original value. If current is reduced, self-induced e.m.f. will increase current and tries to keep up it back to its original value. Thus any modification in current through coil is opposed by the coil.

Magnitude of Self-induced E.M.F.

According to Faraday's law of magnetism induction, self-induced e.m.f. is expressed as
                                    E = -N (dΦ/dt)

Negative sign designates that direction of this e.m.f. is opposing modification in current as a result of that it exists.

The flux is expressed as,

                                     Φ = (Flux / Ampere) x Ampere = (Φ/I) x I

Now for a circuit, as long as permeability ‘µ' is constant, magnitude relation of flux to current (i.e. B/H) remains constant.

Rate of modification of flux = (Φ/I) x rate of change of current.

                                    ∴      (dΦ/dt) =  (Φ/I) x (dI/dt)
                                              e = -N(Φ/I)x(dI/dt)
                                              e = -(NΦ/I)x(dI/dt)

The constant NΦ/I during this expression is nothing however the quantitative measure of the property as a result of that coil opposes any modification in current.

So this constant NΦ/I is named constant of coefficient of self-induction and denoted by 'L'.
It is outlined as aux linkages per ampere current in it. Its unit is Henry (H).

A circuit possesses a coefficient of self-induction of 1 H once a current of 1Amp through it produces flux linkages of 1Wb-tum in it.
                                           ∴ e =-L (dI/dt) volts

From this equation, the unit henry of coefficient of self-induction is outlined as below.
                                                L= (NΦ/I)

But Φ = (m.m.f/Reluctance) = NI/S
                                                L = N.NI/IS
                                                L = N2/s H
                                       Now S=l/µa
                                                L = N2 / (l/µa)
                                                L = N2 µa/l = (N2 µ0µra / l) Henries
Where  l = length of magnetic circuit
            A = area of cross-section of magnetic circuit through which flux is passing


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