To study the principles and operations of various electric machines it is very important to understand some basic laws. Faraday's law of electro-magnetic induction is one among such law. This law deals with the emf induced in an electric circuit and you know every machine works on the principle of emf induction except d.c. Machines.
Faraday's law of electro-magnetic induction states that the emf induced in a circuit is equal to the rate of change of flux linkages.
Flux linkage is give by product of NΦ( number of turns of coil*Φ, flux linking with all of them ).
This is theoretical explanation but in most of the cases this flux Φ, does not link with all the turns or in other words all turns do not link with the same flux. Under such conditions the summation of the products of magnetic flux with complete turns of the magnetic circuit gives the total value of flux linkage.
Now according to Faraday's law of electro-magnetic induction if there is any change in flux linking with a conductor, or induction coil, an emf will be induced and magnitude of this induced emf is given by : -
e ∝ dΦ/dt.
Where e is induced e.m.f. in the coil and dΦ/dt is the rate of change of flux with respect to time.
I know you are thinking about the negative sign that you usually see in a standard Faraday's law but that is not a part of Faraday's law. That negative sign is explained by Lenz law.
According to lenz law the direction of induced emf is always in opposition to that of main or parent emf. This is the reason we put a negative sign to calculate the magnitude and direction of the induced emf.
After applying both laws viz. Lenz and Faraday's law the exact formula will be
e ∝ - dΦ/dt.
Now when you learn basics about Faraday's law of electromagnetic induction I would like to tell you the causes of emf induced in an electric circuit according to Faraday's law of electromagnetic induction.
The emf in a circuit is induced by change in flux linking ( discussed above ) and this flux linking can be changed by any of the following actions or by a combination of these : -
- The coil is stationary with respect to flux and the flux varies in magnitude with respect to time.
- The flux is constant with respect to time and is stationary and the coil moves through it.
- Both the changes mentioned above occur together i.e. The coil moves through a time varying field but relative motion should not be zero.
If coil is stationary and flux is time varying, an emf called transformer emf or pulsation emf is produced.
If the flux is stationary w.r.t. time and conductor is moving the emf induced is give by the relation : -
e = B*l*v volts
where v is linear velocity of conductor in m/s
l is te length of conductor
and B is the magnetic field density.
In such case the generated emf is called motional emf.
So friends this was all about the Faraday's law of electromagnetic induction or emf generation.