What is Eddy Current?
If a coil is wound on a ferromagnetic core (such as in a transformer) and alternating current is passed through the coil, an alternating flux is set up in the core. The alternating flux induces an e.m.f. e in the coil given by e D N d,/dt However, in addition to the desirable effect of inducing an e.m.f. in the coil, the alternating flux induces undesirable voltages in the iron core. These induced e.m.f.s set up circulating currents in the core, known as eddy currents.
Since the core possesses resistance, the eddy currents heat the core, and this represents wasted energy. Eddy currents can be reduced by laminating the core, i.e., splitting it into thin sheets with very thin layers of insulating material inserted between each pair of the laminations (this may be achieved by simply varnishing one side of the lamination or by placing paper between each lamination). The insulation presents a high resistance and this reduces any induced circulating currents.
The eddy current loss may be determined as follows. Let Figure represent one strip of the core, having a thickness of t metres, and consider just a rectangular prism of the strip having dimensions tm*1m*1m as shown. The area of the front face ABCD is( t*1) m2 and, since the flux enters this face at right angles, the eddy currents will flow along paths parallel to the long sides.
From equation, it is seen that eddy current loss is proportional to the square of the thickness of the core strip. It is therefore desirable to make lamination strips as thin as possible. However, at high frequencies where it is not practicable to make very thin laminations, core losses may be reduced by using ferrite cores or dust cores. Ferrite is a ceramic material having magnetic properties similar to silicon steel, and dust cores consist of fine particles of carbonyl iron or permalloy (i.e. nickel and iron), each particle of which is insulated from its neighbour by a binding material. Such materials have a very high value of resistivity.
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