Friday, November 18, 2011

Skin Effect

       When a conductor carries a steady or d.c. current, this current is uniformly distributed over the whole cross-section of the conductor. However the current distribution is non-uniform if conductor carries alternating current. The current density is higher at the surface than at its centre. Thus the current is concentrated near the surface of the conductor as shown in the Fig. 1. This effect becomes predominent with increase in frequency. This behaviour of alternating current to concentrate near the surface of the conductor is known as skin effect.
Fig. 1

       Because of skin effect, larger power loss is caused for a given rms value of AC than the loss when the same value of DC is flowing through the conductor. Alternatively the effective resistance of conductor is more for AC than for DC.
       Due to this skin effect, the effective cross sectional area offered to the flow of current decreases which increases resistance.
       Consider a solid, round conductor consisting of large number of strands. Each strand is carrying a small part of current. The strands near the centre are surrounded by a greater magnetic flux and hence have large inductance than that near the surface. As we move towards the outer strands, the flux linking progressively reduces for the reason that the flux inside the strand does not link it. The reactance of inner strands is greater than outer strands which cause the alternating current to flow near the surface of the conductor. With increase in the frequency, The inductive reactance of the strands becomes and more non-uniform which leads to non-uniform current distribution.
       The skin effect is quite significant for large, solid conductors even at a frequency of 50 Hz.
       The skin effect depends on following factors
1. Nature of material
2. Diameter of wire
3. Frequency of supply
4. Shape of wire.
       With increase in diameter of wire, the skin effect increases. Similarly as frequency increases, the skin effect increases. If we have stranded conductor rather than solid conductor then the skin effect is less. It can be seen that when supply frequency is less than 50 Hz and conductor diameter is less than 1 cm then skin effect is negligible. In large conductors at power frequencies the skin effect is a significant factor.

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