When the conductor is carrying a steady direct-current (D.C) , the distribution of current is uniform through the conductor i.e, “The distribution of current is uniform throughout the conductor only when DC is passing through it.“
But when Alternating Current flows through the conductor , the current does not distribute uniformly. It develops a tendency to concentrate near the surface of the conductor. Therefore, current density is higher at the surface as compared to the centre.
So we can summarise the skin effect as:
The tendency of Alternating Current to concentrate near the surface of the conductor is known as Skin Effect.
EXPLANATION FOR SKIN EFFECT
A solid round conductor can be thought that it consists of large number of strands, each strand carrying a small part of the current. Since each strand is carrying current so magnetic flux will be created around each strand and this makes the inductance of each strand different. So, strands near the centre are surrounded by greater magnetic flux and hence will have the larger inductance. The higher reactance of inner strands causes the alternating current to flow near the surface of conductor.
NOTE: For Direct Current, inductance is zero so current is distributed uniformly around the conductor.
WHAT HAPPENS DUE TO SKIN EFFECT?
Due to Skin Effect, the effective cross-section area of conductor through which current is flowing is reduced. This makes the resistance of the conductor to increase slightly when carrying Alternating Current. Therefore line losses are increased.
FACTORS ON WHICH SKIN EFFECT DEPENDS
The skin effect depends upon the following factor:-
(i) Nature of the material.
(ii) Diameter of the wire-Skin Effect increases with increase in diameter of wire.
(iii) Frequency– Skin Effect increases with increase in supply frequency.
(iv) Shape of Wire– Skin Effect is less for stranded conductor than the solid conductor.