Baker D30R User Manual - page 26
Principles of High Voltage DC Testing
12/3/2009 | 71-022 V6
D30R
26
As mentioned above, a suggested test voltage for the Megohm test is 1.7 times the
applied/operating line voltage for the motor. For example a 480 volt motor would be tested at
480V*1.7=816VDC. Recommended test voltages can also be found in IEEE 43-2000, NEMA
MG-1-1993 and EASA technical manuals (see chapter 2).
When first applying the voltage to a motor or when increasing the voltage, an unusually high
current is observed. This high current is not a leakage current, but the charging current of the
capacitor formed by the motor’s copper coils, the ground wall insulation and the motor’s steel
core. This capacitor is usually called the “machine capacitance”.
The
polarization index test (PI test)
is performed to quantitatively measure the ability of the
ground wall insulation to polarize. The PI test is the most confusing DC test in use due to the
subtleties in the interpretation of the results. When an insulator polarizes, the electric dipoles
distributed in 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. The test results become confusing when attempting to
attribute variations in the PI value to the polarizability of the insulator or other affects such as
humidity, moisture and instrument error.
The PI test is typically performed at the same voltage as the Megohm test and takes 10
minutes to complete. The PI value is calculated by dividing the IR 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 recommends minimum acceptable values
for the various thermal classes of motor insulation:
NEMA Class A
1.5
NEMA Class B
2.0
NEMA Class F
2.0
NEMA Class H
2.0
The tester will automatically calculate the PI value at the end of a 10-minute test. At the
test’s conclusion, the PI value may be stored in one of the 10 memory locations in the Baker
tester for later recall.
Note:
Some insulating materials developed in recent years for wire insulation do not readily
polarize. For example the newer inverter grade wire insulation do not significantly polarize. As
recommended in IEEE 43, if the one-minute insulation resistance is greater than
5000Mohms, the PI measurement may not be meaningful. In these situations the leakage
current is often very low – almost zero. Such low leakage currents are difficult to accurately
measure and as a result, instrument errors become very evident. However, the operator must
use judgment before declaring the PI test to be meaningless. The indication of damaged
insulation based on the PI test can be a very low leakage current and a low PI value.
The
dielectric absorption test (DA test)
is often substituted for the PI test for the following
reasons:
1)
Some insulation systems do not polarize or polarize so fast the process is not observed
2)
Some motors are so small that doing a PI test offers no useful information
3)
Some motors have such a small total leakage current, it is not possible to resolve the
polarization current
min)
1
(
min)
10
(
IR
IR
PI
=