Electrical Engineering ⇒ Topic : Losses in magnetic materials

LOSSES IN MAGNETIC MATERIALS

When magnetic materials are subjected to an alternating flux, two types of losses occur which are:

• Hysteresis loss
• Eddy current loss.

The above losses comprise the total core loss.

Hysteresis Loss - Below curie temperature (it is the rising temperature at which the given material ceases to be ferromagnetic, or the falling temperature at which it becomes ferromdgnetic) all ferromagnetic materials exhibit the phenomenon called hysteresis which is defined as the logging ofmagnetisation or inducti on flux density (B) behind the rnagnetising force (H)or it is that quality of a magnetic substance due to which energy is dissipated in it on the reversal of its magnetism.

Eddy Current Losses

• The term "eddy currents" is applied to those electric currents which circulate within a mass of conducting material when the latter is situated in a varying magnetic field. The conducting material may be considered as consisting of large number of closed conducting paths, each of which behaves like a short circuited winding. The varying magnetic field induces eddy e.m.fs. in these closed elemental paths giving rise to eddy currents.These eddy currents produce loss in power resulting in heating of materials. This loss is of considerable importance as it affects the efficiency and heating of electrical machines.
• The eddy currents produce a magnetic field of their own which opposes the main magnetic field. As the effect of eddy currents is not uniform over the cross section of the material, this results in a flux distribution which is not uniform, the flux density in the outer portions being greater than that at the centre. Thus there is a reduction in effective cross-section.The effect of eddy currents upon flux distribution is chiefly of importance in transformers where the material otherwise would be worked at a uniform flux density. -
• The magnetic materials used for varying magnetic fields are laminated (made up of thin sheets insulated from each other) so as to reduce eddy currents and associated losses, as by laminating, the area of path of eddy currents is reduced giving rise to a large value of resistance.