The polarization index (according to IEEE 43-2000) represents one of the most significant parameters to monitor the health of an motor, with reference to the condition of its insulators.
It is directly related to the insulation resistance of the insulators themselves, defined as the resistance value of an insulator when subjected to a continuous electric field.
The resulting current is called insulation current and, in essence, the resultant of 3 components:
When direct voltage is applied to an insulator, the insulation current immediately peaks, then gradually decreasing over time to eventually become stable.
This behavior is partly due to the capacitive current (which, however, becomes negligible after a few seconds), and, in part, to the absorption current. In fact, in order to reach a negligible value, it requires a longer time, from 10 minutes to several hours (however, for purposes related to insulation tests, the decrease in the current value after 10’ can be neglected).
The curve that describes the trend of the insulation current (or insulation resistance) over time, is known as dielectric absorption curve.
The steepness of this curve, measured for a given temperature value, is an indication of the conditions of the insulation (in terms of humidity, pollution, aging, etc.).
This steepness is also called “POLARIZATION INDEX”, which is defined as follows:
(“INDICE DI POLARIZZAZIONE” = POLARIZATION INDEX
“a tensione costante” = constant voltage)
Where: R 10 = Insulation resistance after 10 minutes
R 1 = Insulation resistance after 1 minute
R 1 = Insulation current after 1 minute
I 10 = “ = Insulation current after 10 minutes
It can be verified how, in the case of new insulators, the value of the polarization index is higher than the value found on aged insulators, or on insulators that work in particularly stressful or polluting conditions, both from a temperature and chemical point of view.
It is evident how this test must be performed in order to evaluate the state of the insulators of electrical machines that have worked for a certain time and determine their probable residual life, in order to schedule special maintenance in advance, thus avoiding the possibility of sudden failures.
The POLARIZATION INDEX test must be carried out by applying the same continuous voltage values used to check the insulation resistance (500 Vdc for machines with rated voltage up to 1000 V., 1000 Vdc for those with rated voltage higher than 2500 V.), for a period of 10 minutes.
In practice, the test voltage is applied between the windings connected to each other, and the grounding; after 1 minute, the instrument reads and stores the MWvalue. The test then continues for 10 minutes and the final measured value is compared to the initial one.
The minimum value recommended by the standards for the POLARIZATION INDEX on Class B, and higher, insulation is 2 (see IEEE Std. 43, sec. 9.2 and IEEE Std. 43, App A.2).
In summary, it can be concluded that:
Finally, it should be noted that, for resistance values, after 1’, higher than 5,000 Mohm, the test is not significant (see IEEE 43) as such high values are difficult to assess, and, in any case, the object being tested is definitely
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