Recommended minimum insulation-resistance values for motors
- According to IEEE Standard 43, motor windings made before 1970 should have a minimum insulation resistance of 1 Megohms per kV + 1 Megohm.
- Form-wound coils made after 1970 should have a minimum insulation resistance of 100 Megohms.
- Random-wound stator coils and form-wound coils rated below 1 kV should have a minimum insulation resistance of 5 Megohms.
Motor Temperature Correction Factor
The temperature correction factors for motor windings are different than for other types of insulation as they use a 40°C base temperature.
Invalid Motor PI Values
IEEE Standard 43 recommends that when the insulation resistance is above 5 Gigaohms that the PI is not valid. This is because the leakage current is in the microampere range and too many factors can influence the measurement.
Recommended PI for Motor Insulation Classes
Class A insulation is the odd-ball of the group. All other insulation classes have a recommended PI of 2.0, but Class A has a recommended PI of 1.5.
Indications of Brittle Motor Insulation
According to IEEE Standard 43, when the PI value exceeds 8 it may indicate insulation that is dried out and brittle to the point that it could fail mechanically as well as electrically.
Reference: NETA World Summer 2004 Tech Quiz #68