EDDY CURRENTS AND EDDY CURRENT LOSSEES
We have seen whenever the flux, linking with a closed electric changes, an emf is induced in the circuit and a current flows, the value of which depends on the emf around the circuit and the resistance of the circuit. It is not necessary that the circuit be a wire and that tha flux passes entirely through it. If a solid block itself, 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 loss of power, with consequent heating of the matter of power loss due to eddy currents is often a matter of considerable importance in electrical engineering. Electrical machinery usually involves varying fluxes. In dc as well as ac 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 ac 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 tend to flow in planes perpendicular to the magnetic flux, as they are induced due to variation of to the magnetic flux, as they are induced due to variation of this flux through a circuit. If the material is laminated, and these sheets are placed to the flux and insulated from one another, the eddy current loss will be reduced. When the core body is one continuous solid iron piece, as shown in Fig 1.2(a),cross-section of armature is large, therefore, resistance is very small and consequently magnitude of eddy current is large resulting in large power loss. If the same core is splitted up into thin insulated sheets, as shown in Fig.1.2.(b), the path of eddy currents is splitted 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. This is why armature of electronic machines (generators and motors) are built up of sheet-steel laminations (usually between 0.45 mm and 0.50 mm 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.
Eddy Current Loss
Eddy current loss in addition to hysteresis loss occurs in a magnetic material when it is subjected to a changing magnetic field. Like hysteresis loss, the eddy current loss results in the rise of temperature of the material. The power loss due to eddy currents is proportional to R where is the magnitude of eddy currents and r is the resistance of eddy current path, but it is difficult to determine it because the magnitude of eddy currents and resistance of eddy current path cannot be determined directly. Experiments have shown that the power loss due to eddy current is proportional to the square of the flux density and the square of number of complete magnetization cycles per second and the eddy current loss is given by the equation
where is the eddy current coefficient and depends upon the type of magnetic material used, f is the number of complete magnetization cycles per second, is the maximum flux density in tesla /, t is thickness of laminations in metres and v is the volume of material in .