Testing electric motors at time of installation and periodically thereafter can reveal hidden problems and prevent unnecessary failure. IEEE and NETA standards provide several guidelines for the testing of electric motor insulation in the field.
The purpose of the insulation resistance and polarization index tests is to determine whether or not an insulation system is suitable for operation or overvoltage testing. Insulation resistance is conducted while the motor is offline, therefore, the test will not detect problems due to rotation, such as loose coils, or vibration.
- 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.
The temperature correction factors for motor windings are different than for other types of insulation as they use a 40°C base temperature. A motor rated for 40°C is suitable for installation where the normal surrounding air temperature does not exceed 40°C (104°F).
IEEE Standard 43 Section 12.2.2 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.
Class A insulation is the odd-ball insulation in IEEE 43. All other insulation classes have a recommended PI of 2.0, except Class A which has a recommended PI of 1.5. NETA Acceptance Testing Specifications note that polarization index values shall not be less than 2.0 and dielectric absorption ratios shall not be less than 1.4 should be investigated.
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. Equipment susceptible to this condition include varnished cambric, shellac mica-folium, or asphaltic stator windings