Abstract— DC insulation resistance (IR) and polarization
index (PI) testing is very widely used to assess the condition of the
rotor and stator windings in motors and generators. The IR/PI
tests are good at identifying contaminated windings; however,
such issues as thermal deterioration or abrasion of the insulation
due to coil vibration, are not detected by the IR/PI test in modern
insulation. The polarization/depolarization current (PDC) test,
where the charging current is measured for several minutes and
then the test object is grounded and the discharging current is
measured for several minutes, may provide more diagnostic
information. This paper presents PDC data from a stator as well
as individual stator coils and bars and compares it to results from
other off-line tests including the low- and high-frequency PD,
dissipation factor tip-up, and DC ramp tests. It seems that partial
discharge tests are more sensitive to some insulation problems
than PDC, however, more results are needed.
