Eddy Current loss | What is Eddy Current | Eddy current application | Eddy current Loss and Mathematical Expression

EDDY CURRENT LOSS

When a magnetic material is subjected to a changing (or alternating) magnetic field, an emf is induced in the magnetic material itself according to Faraday’s laws of electromagnetic induction. Since the magnetic material is also a conducting material, these EMFs. circulate currents within the body of the magnetic material. These circulating currents are known as eddy currents. As these currents are not used for doing any useful work, therefore, these currents produce a loss (i 2 R loss) in the magnetic material called eddy current loss. 

 The hysteresis and eddy current losses in a magnetic material are called iron losses or core losses or magnetic losses. A magnetic core subjected to a changing flux is shown in Fig. 1. For simplicity, a sectional view of the core is shown. When changing flux links with the core itself, an emf is induced in the core which sets up circulating (eddy) currents (i) in the core as shown in Fig. 1. (a). These currents produce eddy current loss (i2 R), where i is the value of eddy currents and R is the resistance of the eddy current path. As the core is a continuous iron block of large cross-section, the magnitude of I will be

very large, and hence greater eddy current loss will result. 

To reduce the eddy current loss, the obvious method is to reduce the magnitude of eddy currents. This can be achieved by splitting the solid core into thin sheets (called laminations) into the planes parallel to the magnetic field as shown in Fig. 1(b). Each lamination is insulated from the other by a fine layer of insulation (varnish or oxide film). This arrangement reduces the area of each section and hence the induced emf It also increases the resistance of eddy currents path since the area through which the currents can pass is smaller. This loss can further be reduced by using a magnetic material having a higher value of resistivity (like silicon steel).

Useful Applications of Eddy Currents

It has been seen that when the affects of eddy currents (production on heat) are not utilised, the power or energy consumed by these currents is known as eddy current loss. However, there are the places where eddy currents are used to do some useful work e.g., in case of Induction heating. 

In this case, an iron shaft is placed as a core of an inductive coil. When high frequency current is passed through the coil, a large amount of heat is produced at the outer most periphery of the shaft by eddy currents. The amount of heat reduces considerably when we move towards the centre of the shaft.

This is because outer periphery of the shaft offers low resistance path to eddy currents. This process is used for surface hardening of heavy shafts like axils of automobiles.

Mathematical Expression for Eddy Current Loss

Although it is difficult to determine the eddy current power loss from the current and resistance values. However, experiments reveal that the eddy current power loss in a magnetic material can be expressed as:

where,

• Ke = co-efficient of eddy current, its value depends upon the nature of magnetic material;
• Bm = maximum value of flux density in Wb/m2;
• t = thickness of lamination in m;
• f = frequency of reversal of magnetic field in Hz;
• V = volume of magnetic material in m3.