Field Testing of Motors, IEEE 43
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
Last edited by SecondGen; March 14, 2016 at 04:25 PM.
ieee standard 43
Why is 1970 used as a reference for different results?
The standard that we generally reference within industry is the IEEE 43, which went through a major revision in May, 2000. It was updated because post 1970 insulation systems went through a series of changes in their chemical makeup.
The insulation systems of newer insulation systems are very different from the older systems, including how they react through testing methodologies. The revised standard drastically changed a number of traditional testing programs for insulation resistance that had been in place for over 50 years, including the Polarization Index (PI), insulation to ground tests and AC vs DC testing of insulation systems.
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