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Saturday, December 6, 2014
Dielectric Phenomena and Polarization
The dielectrics have the property of both temporary and permanent absorption of electrical charges and property of conduction. When a voltage is applied to a dielectric, forces on the positive and negative charges inherent in the particles which make up the dielectric tend to orient the particles in line with the applied field. Some dielectric materials have molecules that have uneven number of atoms, that is, having asymmetrical arrangement of charges.
When such a molecule is placed in an electrical field, it will migrate in an electric field, thus become polarized with the electric field.
Such a molecule is called a dipole. Dipoles play an important role in the electrical characteristics of the insulation. A dipole may be represented by a particle having small positive charge at one end and a small negative charge at the other end. When these dipoles are subjected to DC voltage, they are polarized and become aligned with respect to positive and negative polarity of the DC voltage. This phenomenon is known as dipole polarization. Polarization phenomenon is influenced strongly by the material properties, structure, and condition of the insulation.
On the other hand, charged particles, that is, particles with positive and negative charges, which are not interrupted by interfacial barriers, and can travel through the dielectric from one electrode to the other, constitute the leakage current, and are not part of the polarization phenomenon.
After a time when the applied voltage is removed from the dielectric, the polarized molecules will eventually revert to their initial random arrangement so that the polarization approaches zero. The time it takes for the polarization to drop to zero when the dielectric is short-circuited is known as relaxation time. It should be noted that the large dielectrics have a much longer relaxation time, and appropriate measures should be taken to discharge the released energy (voltage and current) to ground, which is given by the polarized molecules when they revert to their original state.