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 a magnetic field around it, but it is not alternating, so an alternating magnetic field cannot induce a 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 the 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’s outer surface carrying alternating current is called the Skin Effect.
The ohmic resistance of the AC current-carrying conductor increases due to the skin effect. The resistance of the conductor when AC current flows through it is called the AC resistance of the conductor.
The skin effect increases with an increase in the frequency. At low frequencies, such as 50Hz, there is a slight rise in the current density near the surface of the conductor. However, at high frequencies, like radio frequency, most of the current is confined to the surface of the conductor. Thus, the skin effect in the case of radio frequency signal is more predominant, and the conductor resistance increases up to a great extent, and remedial measures are taken.
The hollow conductor is used in the RF application to reduce the weight of the conductor. In the case of power frequency 50Hz, there is a small increase in the current density near the surface of the conductor. The current density at the surface of the conductor increases with an increase in the frequency.
Reasons for Skin Effect
The conductor is considered to be made of many small 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 elements near the surface of the conductor. The central parts of the cylindrical elements of the conductor are surrounded by internal and external flux. Therefore, more flux gets linked to the cylindrical elements near the core of the conductor.
Thus, the conductors near the core have more reactance compared to the reactance offered by the current-carrying conductors near the surface of the conductor.
The difference in the self-reactance of the inner cylindrical element and outer cylindrical element causes different current concentrations 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 the current flowing through the inner cylindrical element is less.
The current density or charge concentration is more at the surface, and the current density at the center of the core is zero. The skin effect phenomenon causes a reduction in the effective cross-section area of the conductor, and thus, the resistance of the conductor increases. Frequency, the shape of the conductor, the diameter of the conductor, and material permeability are the main factors affecting the skin effect.
Factors affecting the skin effect
1. Frequency
The skin effect increases with an increase in the frequency. The conductors near the core offer more reactive impedance to the flow of current and, thus, the skin depth decreases, and the resistance of the conductor increases with an increase in the frequency.
2. The shape of the conductor
The skin effect depends on the conductor’s surface area. The skin effect in the solid conductor is greater than that of the stranded conductor because the solid conductor has more surface area.
3. Diameter of the conductor
The skin effect increases with an increase in the diameter of the conductor.
4. Permeability of the material
Permeability is the property of the material that supports the formation of the magnetic field. Materials with higher permeability tend to have a more skin effect. The higher permeability material causes the development of a more magnetic field and leads to higher self-inductance. The higher self-inductance causes more reactive impedance, which forces current to flow on the surface of the conductor.
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