Electrical Engineering ⇒ Topic : Eddy Current Loss

Eddy Current Loss

When a magnetic material is subjected to a changing magnetic field, in addition to the hysteresis loss, another loss that occurs in the material is the eddy current loss. The changing flux induces voltages in the material according to Faraday 's laws of electromagnetic induction. Since the material is conducting, these induced voltages circulate currents within the body of the material. These induced currents do no useful work and are known as eddy currents. These eddy currents develop i²R loss in the material. Like hysteresis loss, the eddy current loss also results in the rise of temperature of the material. The hysteresis and eddy current losses in a magnetic material are sometimes called core losses or iron losses

                                                                                          figure (a)

Fig. a (i) shows a solid block of iron subjected to a changing magnetic field. The eddy current power loss in the block will be i²R where i is the eddy cun-ent and R is the resistance to the eddy current path. Since the block is a continuous iron piece of large X-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 thin sheets (called laminations) in planes parallel to the magnetic flux as shown in Fig.a (ii).

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. Both effects combine to reduce the eddy current and hence eddy current loss. Further, reduction in this loss can be obtained by using a magnetic material of high resistivity (e.g. silicon steel).The only drawback of laminated core is that the total cross-sectional area of the magneticmaterial is reduced by the total thickness of the insulation. This is generally taken into account by allowing about 10% reduction in the thickness of core when making the magnetic calculations.