Mutual Induction (M)
It is the phenomenon in which change of current in a coil causes an EMF to be induced in another coil. The current in which we supply current is called primary coil and the coil in which an EMF is induced is called the secondary coil.
Mutual induction was one of Faraday's discoveries.
Lets consider the following example.
Connect a coil to an AC source. This coil is the primary coil. Because we are passing electricity through it, it will generate a magnetic field, represented by the dotted lines. Since we have used AC current, the current keeps alternating and hence the magnetic field keeps changing.
Now introduce a second coil in the magnetic field of the primary coil. According to Lenz's law the new coil will try and oppose the magnetic flux of the primary coil and hence an EMF is induced in it. This is the secondary coil.
This is the basic concept of mutual induction in which change of current in one coil induces an EMF in the second.
The concept of mutual induction is the principal of generates and transformers.
Transformer
A transformer is an electrical devise that is used to convert AC current at one voltage and deliver it at another. It delivers the same frequency of current. There is a little loss in power in transformers.
The principal of transformers is mutual inductance or some might say electromagnetic induction.
A transformer can increase the voltage and decrease the current (step-up) or it can reduce the voltage and increase the current (step-down). A transformer might look pretty intimidating but its not. It doesn't even use any moving parts!
A transformer can transform voltage with alternating current but not with direct current. This is because if we supply direct current there will be no change in magnetic flux and hence not current will be induced in the secondary coil.
The primary and the secondary coil are wrapped around a frame. The same magnetic flux flows through the two coils.
Vs - net voltage in secondary coil
vs - voltage in each secondary coil
Ns - number of loops in secondary coil
Vp - applied voltage in primary coil
vp - voltage in each primary coil
Np - number of turns in primary coil
Since the loops of the secondary coil are connected in series the net voltage across its ends is equal to voltage per turn multiplied by the number of turns.
∴ Vs= vs × Ns ------ 1
Since the same magnetic flux is passing through the primary and secondary coil, the voltage per turn is the same.
∴vp = vs --------2
The voltage per unit turn in the primary coil depends upon the number of turns and the applied voltage. The three are related as follows.
∴ vp= Vp/Np ------3
from 2 and 3 we get,
vs = Vp/Np ------ 4
Put this into 1
∴ Vs = Vp/Np × Ns
From this we draw a crucial conclusion. If the secondary coil has more turns as compared to the primary coil, then the voltage is increased and hence the transformer acts as a step-up transform.
If the secondary coil has fewer turns as compared to the primary coil, then the voltage is lowered and hence the transform is a step down transformer.
Since energy is always conserved, Vs × Is = Vp × Ip
The core of transformers are laminated. This means that is not a single block of metal, but its thickness is built by combining numerous thin sheets of aluminium or iron. The core of the transformer is not meant to conduct current. But since there is a magnetic flux in it, there will be an induced current. This current is known as eddy currents. These eddy currents heat up the system and hence waste a lot of energy. The core is thus laminated to reduce the effects of these eddy currents.
There are many ways by which heat is waster while transferring power from the primary to the secondary coil. These include: eddy current, hysteresis loss and I2R loss.
This energy is dissipated as heat. To cool the system, the transformer is immersed in cooling oil. The oil dissipates the current by convection currents which exits through the radiator fins.
Moreover, the oil expands due to this heating. To make space for this increasing volume, a tank is attached to the top of the transformer.
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