Insulation resistance and polarization of rotating machines

This contribution explains insulation resistance and polarization of rotating machines through measurement results of Polarization Depolarization Current (PDC), or charging and discharging current, of machines having different types and conditions of insulation aging. It demonstrates why the Polarization Index (P.I.) cannot be used individually to judge insulation dryness and why the product of insulation resistance and capacitance is a better measure of insulation quality than the insulation resistance alone. But it is the combination of PDC, Capacitance ratio and Dielectric Dissipation Factor (DDF) which provides a decisive key in the diagnosis of global problems in machine insulation.