When the DC current flows through the conductor the charge distribution throughout the conductor cross section is uniform and the current gets evenly distributed.Direct current also produces magnetic filed around it but it is not alternating, so an alternating magnetic field cannot induce current in a stationary object. So there is no skin effect.However, in the case of alternating current this is not true.
The current distribution in the conductor cross section area is non-uniform with flow of AC current through the conductor. The charge concentration near the conductor surface is much more than the charge concentration at the center of the conductor.The phenomenon of non-uniform distribution of the electric current on the conductor outer surface carrying alternating current is called Skin Effect.
The ohmic resistance of the AC current carrying conductor gets increased due to skin effect. The resistance of the conductor, when ac current flows through it, is called ac resistance of the conductor.
The skin effect increase with an increase in the frequency. The skin effect in case of radio frequency signal is more predominant and the conductor resistance increases up to a great extent and its remedial measures are taken.
The hollow conductor is used in the RF application to reduce the weight of the conductor. In case of power frequency 50Hz, there is small increase in the current density near the surface of the conductor. The current density at the surface of the conductor increase with an increase in the frequency.
Reasons of Skin Effect
The conductor is considered to be made of many small elements of cylindrical elements. When AC current flows through the conductor, the magnetic flux is set up in the entire cross section area of the conductor and, this flux gets linked to the conductor.
The magnetic flux linking to cylindrical elements near the center is more than the flux linking to the cylindrical element near the surface of the conductor. The central parts of cylindrical elements of conductor is surrounded by the internal and external flux, therefore more flux gets linked to the cylindrical elements nearby to the core of the conductor.
Thus, the conductors nearby the core has more reactance compared to the reactance offered by the current carrying conductors nearby the surface of the conductor.
The difference in the self reactance of the inner cylindrical element and outer cylindrical element cause different current concentration throughout the cross section area of the conductor. The self inductance offered by the cylindrical element near the core is more and, hence it offers more reactive impedance and current flowing through the inner cylindrical element is less.
The current density or charge concentration is more at the surface and, current density at the center of the core is zero.The skin effect phenomenon cause reduction in the effective cross section area of the conductor and thus,the resistance of the conductor gets increased.Frequency, shape of the conductor, diameter of conductor and material permeability are the main factors affecting skin effect.
Factors affecting skin effect
The skin effect increases with increase in the frequency. The conductors nearby to the core offers more reactive impedance to flow of current and, thus the skin depth gets decreased and the resistance of the conductor increase with an increase in the frequency.
The shape of conductor
The skin effect depends on the surface area of the conductor. The skin effect in the solid conductor is more than the stranded conductor because the solid conductor has more surface area.
Diameter of the conductor
The skin effect increases with increase in the diameter of the conductor.
Permeability of the material
The permeability is the property of the material which support the formation of magnetic field. The materials which have higher permeability tends to have more skin effect.The higher permeability material cause to develop more magnetic field and leads to higher self inductance. The higher self inductance cause more reactive impedance which force current to flow on the surface of the conductor.