Eddy Current Loss

              Eddy Current Loss

     Consider a coil wound on a core.  If this coil carries an alternating current i.e. current whose magnitude varies with respect to time, then flux produced by it is also of alternating nature.  So core is under the influence of the changing flux and under such condition according to the Faraday's law of electromagnetic induction, e.m.f. gets induced in the core.  Now if core is solid, then such induced e.m.f. circulates currents through the core.  Such currents in the core which are due to induced em.f. in the core are called as eddy currents. Due to such currents there is power loss(I²R) in the core.  Such loss is called as eddy current loss.  This loss, similar to hysteresis loss, reduces the efficiency.  For solid core with less resistance, eddy currents are always very high.

     Eddy current loss depends on the various factors which are

 

i)Nature of the material

ii)Maximum flux density

iii)Frequency

iv)Thickness of lamination are used to construct to core

v)Volume of magnetic material

     It has been found that loss can be considerable reduced by selecting high resistivity

magnetic material like silicon.  Most popular method used to reduce eddy current loss is to use laminated construction by stacking thin pieces known as lamination.  The lamination are insulated from each other by thin layers of insulating material like varnish, paper, mica.  This restrict the paths of the eddy currents, to respective lamination only.  So are through which current gets reduced considerably.

 

     The loss may also be replace by grinding the ferromagnetic material to a powder and ixing it witg a binder that effectively insulates the particles  one from other.  The mixture is then formed under pressure into the desired shape and heat treated.  Magnetic cores for use in communication euqioment are frequently made by this process.

     This loss is quantified by using the expression,

    Eddy current loss=K_e(B_m)²f²t²*Volume watts

where,      Ke =   A characteristic constant of material

                Bm =  Maximum flux density

                  f   =  Frequency

                  t   =  Thickness of the lamination

What is Hysteresis loss?

           Hysteresis loss

      According to the molecular theory of magnetism groups of molecules acts like elementary magnets, which are magnetized to saturation. This magnetism is developed because of the magnetic  effect of electron spins, which are known as 'domains'.

     When the material is demagnetized, the axis of the different domains are in various direction.  Thus the resultant magnetic effect is zero.

      When the external magneto motive force is applied the axes of the various domains are oriented. The axes coincide with the direction of the magneto
motive force.  Hence the resultant of  individual magnetic effects is a strong magnetic field.

     When a magnetic material is subjected to repeated cycles of magnetization and demagnetization, it results into disturbance in the alignment of the various domain.  Now energy gets stored when magnetic field is established and energy is returned when field collapses.  But due to hysteresis,all the energy is never returned though field completely collapses. This loss of energy appears as heat in the magnetic material.  This is called as hysteresis loss.  So disturbances in the alignment of the various domains causes hysteresis loss to take place.  This hysteresis loss is undesirable and may cause undesirable high temperature rise due to heat produced.  Due to such loss overall efficiency also reduces.

     Such hysteresis loss depends on the following factors:

1.  The hysteresis loss is directly proportional to the area under the hysteresis curve i.e., area of the hysteresis loop.

2.  It is directly proportional to frequency i.e. number of cycles of magnetization per second.

3.  It is directly proportional to volume of the material.  It can be shown that quantitatively the hysteresis loss in joules per unit volume of the material in one cycle is equal to the are of the hysteresis loop.