Polarization Index (Pi) - MarelliMotori MXB-E 225 Serie Manual Del Usuario

NOTE: The insulation resistance reported in the test report is usually considerably higher than the values
measured on site.

5.2 Polarization Index (PI)

The status of the insulation system of the electrical machine can be carried out by measuring the polarization
index according to standard IEEE 43.
The insulation resistance measurement and recording is carried out at room temperature and over different
periods of time: T1', T2', ...., T10'. The measures are spaced by a conventional time (for example, 1 minute).
Fig. 4
Figure 4 Insulation resistance qualitative performance according to the time period.
High temperatures can cause unexpected changes in the polarization index. Therefore, the test should not be
used at temperatures above 50°C.
Any dirt and humidity that accumulate in the winding generally reduce the insulation resistance and
polarization index, as well as their temperature dependency. Windings with open dispersion distances are very
sensitive to the effects of dirt and humidity.
Several rules can be applied in order to determine the lowest acceptable value at which to start the machine
safely. For the polarization index (PI), the values usually vary between 1 and 4, where 1 indicates that the
winding are humid and dirty.
Poor Acceptable Good Very Good
PI PI < 1.5 1.5 < PI < 2 2 < PI < 4 PI > 4
5.3 Reconditioning of the stator windings
Dry the active parts with a hot air flow, which must be directed towards the winding heads as much as
possible.
If the machine is equipped with anti-condensation resistors, do not use them to dry the winding. The space
heaters must only be turned on during normal and usual operation breaks of the machine in order to avoid the
formation of condensation.
The stators can be heated directly by having continuous current circulate in them (by using, for example, an
industrial welding device). In this case, the current circulating in the winding must be equal to 25% of the
current indicated on the machine's plate and anyhow adapted to reach the desired temperature.
Whenever possible, the windings of the electrical machine must be appropriately reconnected in order to adapt
their resistance to the value of the generator under the continuous current available.
The electrical machine must be covered with thermal insulating barriers to avoid complete dispersion of the
heat produced in the environment; at the same time, whenever possible, any door on the upper part of the
frame must be opened in order to allow the humidity to evaporate.
Insert a thermometer on the active parts and make sure the winding does not exceed a temperature of 100°C.
The recommended drying temperature ranges from 80 to 100°C.
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