Insulation Resistance Measurement For Auxiliaries; The Polarization Index - ABB AMA Series Manual

Manual for Induction Motors and Generators

7.6.5 Insulation resistance measurement for auxiliaries

7.6.6 The polarization index

84 - Maintenance
Figure 7-8 Insulation resistance measurement of the rotor winding
In the figure above the rotor is star-connected.
To ensure correct operation of the machines protections and other auxiliaries, their condition
can be determined by an insulation resistance test. The procedure is described in detail in
Chapter 7.6 Maintenance of stator and rotor windings. The test voltage for the space heater
should be 500 VDC and for other auxiliaries 100 VDC. The insulation resistance measurement
for Pt-100 detectors is not recommended.
For the polarization index test the insulation resistance is measured after the voltage has been
applied for 15 seconds and 1 minute (or 1 minute and 10 minutes). The polarization index test is
less dependent on the temperature than the insulation resistance. When the winding temperature
is below 50ºC (122ºF), it may be considered independent of temperature. High temperatures can
cause unpredictable changes in the polarization index, therefore the test should not be used at
temperatures above 50ºC (122ºF).
Dirt and humidity accumulating in the winding normally reduces the insulation resistance, and
the polarization index, as well as their dependence on temperature. Thus, the line in Figure 7-6
Correlation between the insulation resistance and the temperature becomes less steep.
Windings with open creepage distances are very sensitive to the effects of dirt and humidity.
There are several rules for determining the lowest acceptable value with which the machine can
be safely started. For the polarization index (PI), the values usually range between 1 and 4.
Values close to 1 indicate that the windings are humid and dirty.
The minimum PI value for class F stator windings is more than 2.
NOTE: If the insulation resistance of the winding is in the range of several thousands of MΩ,
the polarization index is not a meaningful criterion of the condition of the insulation,
and it can be disregarded.
⎛
R R
⎜ ⎜
=
1 min 10 min
PI or
⎝
R R
15 s 1 min
MΩ
⎞
⎟ ⎟
⎠
3BFP 000 050 R0101 REV E