The absorption current has two components. The first component is due to the
polarization
of the
impregnating materials because the organic molecules, such as epoxy, polyester, and asphalt, tend to change
orientation in the presence of a direct electric field. Since these molecules have to strain against the attractive
forces of other molecules, it usually takes several minutes after application of the electric field for the
molecules to become reoriented, and, thus, for the current-supplied polarizing energy to be reduced to
almost zero. A second component of the absorption current is due to the gradual drift of electrons and ions
through most organic materials. These electrons and ions drift until they become
trapped
at the mica
surfaces commonly found in rotating machine insulation systems. Usually, for clean and dry rotating
machine insulation, the insulation resistance between about 30 s and a few minutes is primarily determined
by the absorption current.