Skin effect is the tendency of alternating current (AC) to flow on the outer surface of the conductor. It especially takes place at a higher frequency. Skin refers to; current flowing only on the outside surface of the conductor. This means the conductor has a high current density at the surface compared to the interior of the conductor.

The current distribution for direct current (DC) over the cross-section of the conductor is uniform. The Alternating current flowing in a conductor does not flow uniformly along the conductor. This phenomenon of concentrating current at the surface of the conductor is known as the skin effect.

Due to the higher frequency of the alternating current, the current density in a conductor is no longer uniform. In fact, the higher the frequency less and less current will be flowing through the center of the conductor, and more will be flowing through the outer part of the conductor. This term is referred to as the skin effect.

Skin effect reduces the effective cross-sectional area of the conductor. Hence, the effective resistance of the conductor gets increased due to the effect. This increase in resistance introduces more loss (I^2R loss) in the line.

Cause

Conductors in the form of wires are used in electrical systems to transmit energy or signals. The energy or signals can be transmitted in the form of alternating current (AC). Electrons are the charge carriers causing the flow of current. An alternating magnetic field is produced, in and out of the conductor due to the flow of alternating current in the conductor. The current being an alternating quantity changes its value with time. The change in current in the conductor also causes a change in the induced magnetic field. Change in the magnetic field, in turn, creates an electric field that opposes the change in current according to Lenz’s law. This opposing electric field is known as back EMF or counter-electromotive force. Back EMF is strongest at the center of the conductor and it pushes the electrons to the outer surface. Due to which current flows through the outer surface of the conductor only.

Skin Effect Formula

For alternating current, current density J in a conductor decreases exponentially from its value at the surface (Js) at depth ‘d’ from the surface is given by the expression

Skin Effect Formula

Where δ is skin depth.

The depth below the surface of the conductor at which the current density has fallen to 1/e or 0.37 times that of the current density’s value at the surface (Js) is known as skin depth.

Factors Affecting Skin Effect

The factors affecting skin effects are as follows:

  • Type of material: The skin effect increases with an increase in permeability of the material.
  • Frequency: Higher the frequency higher will be the effect.
  • Diameter of conductor: With the increase in diameter of the conductor, the effect increases.
  • The shape of conductor: The effect is seen more on the solid conductor than the stranded conductor.

Mitigation or Reduction

Litz wire is used to reduce the skin effect. Litz wire consists of several insulated wire strands which are woven together in a designed pattern. These wires are used for frequencies of few kilohertz to about one megahertz.

Litz wires are used in windings of high-frequency transformers to reduce skin effect and proximity effect.

In high current applications (such as in bus bars) the solid wire is replaced by hollow tubular conductors. This reduces the weight and cost of the conductor.

Stranded conductors are used in electrical power transmission and distribution lines as it has reduced skin effect.

Skin effect is negligible for frequency levels below 50 Hz and the diameter of the conductor is less than 1cm.


Other Effects:

  1. Ferranti Effect
  2. Corona in Transmission Line

For more articles related to Electrical Engineering visit

Engineering Notes Online – Skin Effect.