In case of D.C., current density is uniform over cross section of conductors. On the other hand, in a conductor carrying A C there is a tendency of current to crowd near the surface of conductor that means
alternating current flowing through a conductor doesn’t distribute uniformly and concentrate near the surface of conductor.
In AC System no current flows through the core and entire current is concentrate on surface regions. Due to this effective area of conductor is reduced so ac resistance of conductor increases.
This phenomenon is known as Skin Effect as it causes concentration of current at the skin of conductor.
To explain skin effect, we assume conductor which is made up of a number of concentric cylinders. Magnetic flux linking a cylindrical element near the centre of conductors is greater than that linking another element near the surface of conductor. It is due to the fact that former elements is surrounded by internal as well as external flux, while latter element by external flux only. So inner element will possess greater self inductance and therefore, will offer a large inductive reactance that outer element. Due to this difference in
inductive reactance current tends to crowd towards the surface or skin of conductor. So distribution of current over section of conductor will be non uniform. As show in fig.
Skin effect depends upon:
- Type of conductor material
- Diameter of conductor
- Shape of conductor.
At low frequencies, this effect is negligible , in fact it is only of importance with high frequencies on with solid conductors of large cross section.Skin effect increases with increase in frequency, conductor diameter and permeability.
In, aluminium wire effect is same as in a copper wire of equal conductivity.
Skin effect is much smaller with stranded conductor than with solid conductors.
In parctice stranded conductors are invaraibly used for transmision and distribution purpose. This is used in order to overcome adverse effect of Skin-Effect.