Test Method

At electric machines the polarization index is a very important measured variable to determine the quality and deterioration of the insulation, which occurs with increasing age of an electric motor.

Polarization is defined as ability of the charge carrier located within the insulator to rotate and adjust to the electrical field – i.e. to polarize. The older the insulation the worse becomes the moveability of the charge carrier. Consequently the electric insulation capability decreases and there is an increasing risk that the motor wll be damaged severely.

The power that is needed to rotate the load carrier can be measured in form of a very slight current at the high-voltage test DC.

The polarization of the charge carrier does not happen apruptly when applying the test voltage but can take up to 10 minutes. It is assumed  that after one minute after applying the test objects capacity the polarization is still in full swing. Thus the moveability of the charge carrier can be determined by the ratio of the current when strongly turning at the beginning to the reduced current after the end of the turning.

• • PI =  current after 1 minute / current after 10 minutes

or alternatively

• • PI =  insulation resistance after 10 minutes / insulation resistance after 1 minute

At a good insulator the current is decreased by four to five times after 10 minutes, because all charge carriers have polarized themselves. This leads to a high good PI – for example from 4 to 5. At a bad insulator the current has hardly changed after 10 minutes because the immobilized charge carrier are not able to polarize correctly. This results in a low bad PI – for example 1,5. Such an electric machine should urgently be maintained.

In consequence the real current is only measured by the insulation resistance after the end of the polarization. In case the insulation resistance at electric motors is determined too fast, the resistance is too low due to the fact that the charging of the capacity of the test object  and subsequently the polarization is measured as well.

Standard committees

For legal reasons, in many cases, we cannot make an obligatory statement about the test conditions. Important for the application is the currently valid standard for your product to be tested.

Depending on the geographical location of the product, the standards to be applied can vary. For further information, please refer to the following institutes.