Baker D30R, User Manual

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Testing Voltages, Sequence and Applicable Standards
12/3/2009 | 71-022 V6
D30R 18
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.
Unfortunately, most the insulating materials developed recently (last 20 years) do not
easily polarize. For example the newer inverter grade wires and epoxy resins do not readily
polarize. As recommended in IEEE 43-2000, if the one-minute insulation resistance is greater
than 5000Mohms, the PI measurement may not be meaningful.
To address the situation where the PI may not be meaningful, the Dielectric Absorption
(DA) is widely used instead. The DA is the IR value at 3 minutes divided by the IR value at 30
seconds. The motivation for even doing the DA test is to reduce the test time to 3 minutes
instead of 10 minutes for the PI test when the PI test may not be worthwhile. To date there
are no accepted values for the DA. However, some usefulness can be obtained by trending
the DA value over time.
5) DC HiPot test
A DC HiPot test is performed using a test voltage that is substantially higher than the
Megohm Test, but, once again, based on operating voltage of the motor and the appropriate
standards/company guidelines. Look for unusually high leakage currents or a leakage current
that doesn’t stay constant or intermittently jumps up and down. Breakdowns or high leakage
currents are an indication of damaged ground wall insulation. Inspect the motor’s slot liner,
wedges, conductors between the junction box and the coils, etc.
6) Surge test
A Surge test is performed on each phase of the motor, again using an appropriate test
voltage based on the operating voltage of the machine and the appropriate
standards/company guidelines. Look for a jump to the left of the surge waveform pattern as
the test voltage is increased. This is the signature of the turn – to – turn short. If a jump is
observed, an inspection of the motor should be made to look for damaged insulation between
adjacent conductors. The insulation may be hard to see visibly, so the motor may have to be
disassembled to find the problem. If no jump in the wave patterns is observed, the likelihood
of motor failure due to insulation failure is greatly reduced
.
Recommended test voltages – HiPot and Surge tests
Recommended test voltages for HiPot and Surge testing a motor, generator or transformer
are twice the AC line voltage plus 1000 volts. This test voltage is consistent with NEMA MG-
1, IEEE 95-1977 (for test voltage greater than 5000 volts), and IEEE 43-2000 (test voltages
less than 5000V).
View the tables below for a comparison of IEEE 95, EASA DC HiPot, IEEE522 Surge Testing,
IEC 34-15 and Baker recommended testing voltages. Note: Representations of motors are
listed. The formulas to calculate voltages are also listed so that test voltage of any size motor
can be calculated.
min) 1 (
min) 10 (
IR
IR
PI
=