We have seen that whenever the flux, linkage with a closed electric circuit changes, an e.m.f. is induced in the circuit and a current flows the values of which depends on the e.m.f around the circuit and the resistance of the circuit. It is not necessary that the circuit be a wire and that the flux passes entirely through it. If a solid block of metal is traversed by a varying flux, metallic circuits in the block it self which are linked by the flux will carry current. If the magnetic circuit is made up of iron and if the flux in the circuit is variable, currents will be induced by induction in the iron circuit itself. All such currents are known as eddy currents.



Eddy currents result in a los of power, with consequent heating of the material. The magnitude of power loss due to eddy currents is often a matter of considerable importance in electrical engineering.

Electrical machinery usually involves varying fluxes. In d.c. as well as a.c. machines, we have an armature built of iron, which is either revolved in a permanent magnetic field or through which a revolving magnetic field passes. In a.c. engineering the flux linking the core of transformer also varies. In either case there will be losses in the iron due to hysteresis as well as due to eddy currents.

Eddy current always lend to flow in lanes perpendicular to the magnetic flux, as they are induced due to variation of this flux through a circuit. If the material of the magnetic circuit is sub-divided into then sheets i.e. if the material is lamented and these sheets are placed parallel to the flux and insulated from one another, the eddy current loss will be reduced. When the core eddy is one continuous solid-iron plece, as shown in, cross-section of armature s large, therefore resistance is very small and consequently magnitude of eddy current is large resulting in large poser loss. If the same core is splinted up into thin insulated sheets, as shown in the path of eddy current sis spitted up into several paths of high resistance because cross-section of such paths is very less. Hence magnitude of eddy currents and eddy current loss is considerably reduced. That is why armature of electrical machines (generators and motors) are built up of sheet-steel laminations (usually between 0.45 m m and 0.50 m m thick) assembled on a shaft so that the armature is subdivided into insulated sheets which are parallel to the working flux of the machine. The core of transformer is also built up of thin laminations due to the same reasons.

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