Dielectric Heating

Dielectric  losses  occur in  insulating materials. These are same  as hysteresis losses  in magnetic materials. When  an alternating  electric  field  is applied to an insulating materials due to inter atomic  friction the atoms  get stressed and  heat is produced. This inter atomic friction caused by repeated deformation and rotation of  atomic  structure constituting the dielectric due to which  dielectric  loss occurs.

An atom of any  materials  has  a nucleus with a net positive charge at the  centre and negatively charged electrons surrounding the nucleus. Shown that when  there is no electric field the centred of positive and negative charge  are coincided. As the electric field  is applied to the atom the positive  charge of the  nucleus is forced in the  direction of field  and the negative charge  is forced  in the opposite direction of field  as shown in figure .

                 figure  (a) An atom without  electric field(b) an atom  with electric field and (c)  equivalent  of

Let  the two  centres are displaced by a distance d as shown  in figure. This gives  rise to  the electric dipole moment of magnitude P= qd. This state is known  as polarization. With increase in electric field strength the degree of polarization is increased. At a certain value  of electric fields  all  electric  dipoles align themselves. In this condition the dielectric material is said to be saturated. This state  is shown in figure .

                                                                      Figure  Alignment  of electric dipoles

In dielectric  heating it is desirable to use high  frequency instead of high  voltage. If very  strong  electric field  is applied to insulating material then it  may possible.

That the strong  electric  fields may brings  out electron form  the orbit  resulting in a rupture of the dielectric  medium. Therefore  the applied  voltage  should always below  the breakdown  values.

The  principle  of operation of  dielectric heating can be explained by  an example. Consider  a piece of insulator. When an alternating voltage  is applied across this an  electric current  flows. Which may have two component  capacitive  current  and resistive current.  Obviously  leads applied voltage  by 90⁰ and i_R will be phase with  v as shown in figure.

                       Figure (a) Dielectric  heating of a insulating material (b) equivalent circuit (c) phase or diagram

It behaves like a  insulating  resistance  parallel to capacitor as shown  in figure  the dielectric  loss  appears in the form  of heat  in the  dielectric of the capacitor.