26
GMC-I Messtechnik GmbH
6.5
Polarization Index Measurement – PI
Polarization index testing is recommended for electric machines.
This procedure involves expanded testing of insulation resistance.
DC measuring voltage from the multimeter is applied to the insu-
lation for a duration of 10 minutes. Measured values are docu-
mented after one minute, and after ten minutes. If the insulation is
good, the value measured after ten minutes is higher than the
value measured after one minute. The relationship between the
two measurement values is the polarization index. Charged mate-
rial within the insulation is aligned due to the application of mea-
suring voltage over a long period of time, resulting in polarization.
The polarization index indicates whether or not the charged mate-
rial contained in the insulation can still be moved, i.e. whether or
not polarization is possible at all. This, in turn, is an indication of
the condition of the insulation.
The following rules apply in general:
PI values < 1:
Troubleshooting is required.
PI values = 1
2
Maintenance is recommended
PI values = 2
4
DUT is OK,
no immediate action is required
Preventive maintenance can be planned
according to workload.
PI values > 4
DUT in flawless condition
Applications: Determination of moisture and contamination levels
The following applies to the insulation of electric drive units:
Intact insulation
PI
2
Very good insulation
PI > 4
➭
Set the rotary switch to “
RISO
”.
➭
Repeatedly press the
Func
softkey until the measurement view
for
PI
appears at the display.
➭
Contact the device under test with alligator clips which have
been plugged onto the test probes.
➭
Interference voltage measurement (V AC+DC TRMS) is con-
ducted in this switch position.
➭
Measurement may only be started when the device under test
is voltage-free (U
ext
= 0000 V).
➭
Activate the measurement by pressing the
Start
softkey. The
high-voltage symbol next to the battery level display blinks
during measurement.
➭
You can interrupt the measurement at any time by pressing
the
Stop
softkey.
The measurement is recorded and you can observe its progress
which is displayed as a curve over the time axis.
The result for R
60
is displayed after reaching the 60 s mark. Mea-
surement is stopped automatically and results for R
600
and
DAR
are displayed after reaching the 600 s mark.
The following measured values are displayed digitally next to the
graphic representation:
PI
Polarization index after 600 s
Riso
Momentary measured value in M
/G
U
ext
Momentary measuring voltage (actual
value)
U
set
Test voltage (target value)
t
act
Measured time since beginning of mea-
surement
R
60
Measured value after 60 s in M
/G
R
600
Measured value after 600 s in M
/G
After measurement has been completed, any
remaining residual voltage U
ext
is displayed which
may result from cable capacitance and a capaci-
tive device under test. The instrument’s internal
1 M
resistor causes rapid discharging.
Contact to the insulation resistance must be retained to
this end.
+
–
0 V!
Discharge
Func
Func
Func
Zero:
RISO
COIL
RISO
Coil
DAR
PI
The above symbol
blinks when test volt-
age is connected.
Func
