Baker D30R, User'S Manual & Operating Instructions

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D30R Users Manual
26
Do Not Touch Test Leads while test is in Progress!
71-023 Rev C 5/8/2007
3. Principles of the Dielectric Absorption (DA) Test
The Dielectric Absorption (DA) Test is essentially a short-duration PI test and is
usually intended for smaller motors. Larger motors whose insulation does not
easily polarize are also good candidates for the DA Test. Other than the shorter
test time, all other principles are the same as the PI test, explained in the next
section.
While the PI test is recommended only for motors 200 horsepower or greater,
the DA test is useful for motors in approximately the 50 to 200 horsepower
range. The DA value is the ratio of the ground wall insulation resistance (IR) at
3 minutes to the IR value at 30 seconds.
4. Principles of the Polarization Index (PI) Test
The
Polarization Index Test (PI test)
is the most confusing HVDC test in use
due to the subtleties in the interpretation of the results. The 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 current”, also called an absorption current, is developed that adds
to the insulation leakage current. This additional polarization current decreases
over time and drops to zero when the insulation is completely polarized.
The PI result becomes confusing when attempting to attribute variations in the
PI value to the polarizability of the insulator or other affects such as humidity or
moisture, surface leakage or instrument error. The result is even more confusing
when attempting to reconcile a PI of 1 when one is expecting some other PI.
The PI test is typically performed at 500, 1000, 2500 or 5000 volts, depending
on the operating voltage of the motors being tested and takes 10 minutes to
complete. The PI value is calculated by dividing the insulation resistance at 10
minutes by the resistance at 1 minute as shown below:
In general, insulators that are in good condition will show a “high” polarization
index while insulators that are damaged will not. IEEE 43-2000 recommends
minimum acceptable values for the various thermal classes of motor insulation.
min) 1 (
min) 10 (
IR
IR
PI
=