Megohm
A Megohm test uses a test voltage close to the motor’s operating voltage and appropriate
standards and/or company guidelines. This voltage will be held for 60 seconds during which the
analyzer watches for overcurrent conditions and microarcs or insulation resistance values below the
minimum megohm setting.
Look for an unusually low Megohm value when compared to previous measurements or industry
accepted limits. If a low Megohm value is measured, inspect the motor for ground wall insulation
damage.
PI/DA
The Polarization Index (PI) test measures the insulation’s ability to polarize. When an insulator
polarizes, the electric dipoles distributed throughout the insulator align with the applied electric
field. As the molecules polarize, a polarization (absorption) current develops and adds to the
insulation’s leakage current.
The additional polarization current decreases over time and drops to zero when the insulation is
completely polarized.
The PI test is typically performed at 500, 1000, 2500, or 5000 volts depending on the motor’s
operating voltage. The duration of the PI test is 10 minutes (600 seconds). The PI value is calculated
by dividing the insulation resistance at 1 minute by the resistance at 10 minutes. In general,
damaged insulators will show a low polarization index.
Many insulating materials do not easily polarize. If the 1-minute insulation resistance is greater than
5000 Megohms, the PI measurement may not be meaningful. The PI test is performed on motor
100 Hp or greater. Motors less than 100 Hp should use the Dielectric Absorption (DA) test. On the
AWA, set the PI minimum alarm at 2.0 for class B, F, and H insulation, and 1.5 for class A insulation.
The PI test should not be used as the basis for any motor acceptance criteria. It should be used as a
trending and diagnosis tool along with other test results, including the PI curve generated.
If the Test ID was set up for a DA test, the test duration will be three minutes (180 seconds).
Basic Testing Procedures
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