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Test Procedures
Megohm Test
A Megohm test is performed using a test voltage based on the operating voltage of the motor and the appropriate
standards/company guidelines. Look for an unusually low Megohm value when compared to previous measurements
or industry-accepted limits for the type of insulation in the motor. If a low Megohm value is measured, the motor
should be inspected for ground wall insulation damage. Possible problems include:
Slot liner insulation or enamel wire insulation may be burned or damaged.
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The motor might be full of dirt, carbon dust, water, or other contaminants.
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Connections to the actual coils might be damaged.
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The wrong insulation might have been used to connect the coils to the motor's junction box.
No further testing is necessary until the reason for a low Megohm reading is determined and corrected.
DA/PI Test
The Polarization Index (PI) test is performed in order to quantitatively measure the ability of an insulator to polarize.
When an insulator polarizes, the electric dipoles distributed throughout the insulator align themselves with an applied
electric field. As the molecules polarize, a polarization (or absorption) current is developed that adds to the insulation
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 operating voltage of the
motor being tested. PI test duration is 10 minutes. The PI value is calculated by dividing the insulation resistance at 1
minute by the resistance at 10 minutes. In general, insulators that are in good condition will show a high polarization
index, while damaged insulators will not. (See IEEE 43-2000 for recommended minimum acceptable values for the
various thermal classes of motor insulation.)
Many insulating materials do not easily polarize. As recommended in IEEE 43-2000, if the 1-minute insulation
resistance is greater than 5000 Megohms, the PI measurement might not be meaningful.
The PI test is performed on motor of 100 Hp or greater. The PI minimum alarm should be set a 2.0 for class B, F, and
H insulation, and 1.5 for class A insulation.
It is important to note that PI should not be used as the basis for any motor acceptance criteria. It should be used as
a trending and diagnostic tool, along with other test results include the PI curve generated by the tester.
The PI test can be used to identify the following possible motor issues:
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Slot liner insulation or enamel wire insulation could be burned or damaged.
The motor might be full of dirt, carbon dust, water, or other contaminants.
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Motor windings shorted to ground.
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Poor cable insulation.
The DA is essentially the same as PI, but shorter in duration (10 min vs 3 min). The first minute for both tests is the
Megohm test.
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Baker AWA-IV 71-015 EN V16 User Guide
www.megger.com
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