When a magnetic material is subjected to a changing magnetic field, an e.m.f. induced in the magnetic material itself according to Faraday’s laws of electromagnetic induction. Since the material is conducting, these induced voltages circulates currents within the body of the material. These circulating current is known as eddy currents. As these currents are not used for doing any useful work, therefore, these currents develop i2R loss in the material. This loss is known as eddy current loss. Like hysteresis loss, this loss also increases the temperature of the magnetic material. 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 shown in the figure. When changing flux links with the core itself, an e.m.f. is induced in the core which circulating currents in the core. These currents produce eddy current loss i2R, 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 piece of large cross-section, the magnitude of i will be very large and hence greater eddy current loss will result.
The obvious method of reducing this loss is to reduce the magnitude of eddy current. This can be achieved by splitting the solid block into this sheets (called laminations) in the planes parallel to the magnetic field. Each lamination is insulated from the other by a layer of varnish. This arrangement reduces the area of each section and hence the induced e.m.f. it also increases the resistance of eddy current paths since the area through which the currents can pass is smaller.
The only drawback of laminated core is that the total cross-sectional area of the magnetic material is reduced by the total thickness of the insulation. This generally taken into account by allowing about 10% reduction in the thickness of core when making the magnetic calculations.
Applications of Eddy Currents
It has been seen that when the affects of eddy currents are not utilized, the power or energy consumed by these currents is known as eddy current loss. There some applications of eddy currents that are given below;
- Induction heating
- Eddy current damping torque.