Polarization Index values

Status
Not open for further replies.

vinod

Senior Member
Location
Saudi Arabia
What is PI values?
How important these values in an inductive circuits like morors, transformers?
What are acceptable values for a good PI?
As per my knowledge , 'TWO' is the accepted value for a good PI. Please give me the standerd If any?

Please help..
 

arjun_athi

Member
Location
SAUDI ARABIA
P.I

P.I

Polarization Index of Machine Windings
For many years, the polarisation index of an electric motor or generator has been regarded as a useful tool in evaluating its windings for:
- Build up of dirt or moisture
- Deterioration of the insulation (this is done by comparing results of tests made earlier on the same machine, at the same temperature)
- Fitness for over potential tests
- Suitability for further operation

The recommended minimum value of polarisation index for form wound or random wound ac and dc motors and generators is 2.0. Therefore if you know the polarisation index of a motor or generator you can determine if the machine is fit for further use. If your machine has an index of less than 2, then it is worth considering changing out.


IEEE Std. 43
IEEE Standard No. 43 covered the procedure for determining Polarisation Index (PI) in detail. This was superceded by IEEE St No. 43-2000 which addresses some new issues regarding modern insulation systems. In particular, it identifies the complexities of performing PI tests and notes their limitations. Notwithstanding this, polarisation index is still regarded as a useful tool by many.


How to Measure the Polarisation Index
IEEE Standard No. 43-2000 covers the procedure for determining Polarisation Index in detail. However, the following provides a summarised guide to carrying out a polarization index test :

Caution: Before proceeding with this test, the windings MUST be discharged against the frame.

- Remove all external connections to the machine and completely discharge the windings to the grounded machine frame
- For machines rated 500 volts and over, apply 1000 volts dc between the winding and ground using a direct indicating, power driven megohmmeter. (If your machine is rated less than 500 volts then apply 500 volts)
- Continue to apply this voltage for 10 minutes
- Measure the insulation resistance
- Completely discharge the windings to the grounded machine frame
- Reapply the voltage for 1 minute
- Measure the insulation resistance
- Calculate the polarisation index by dividing the 10 minute insulation resistance by the 1 minute insulation resistance.
 

arjun_athi

Member
Location
SAUDI ARABIA
P.I

P.I

POLARISATION INDEX (PI) MEASUREMENT

The PI measurement is a ratio metric test used to assess the performance of insulation. Moisture may be absorbed within the insulation and/or condensed on the connection surfaces, which is often dirty. For very moist and dirty windings, the relatively constant surface leakage component of the current will predominate over the time varying components, so that the total current will rapidly reach a near steady-state. Thus, to help determine how dry and clean the insulation is, the IR is usually measured after 1 minute and after 10 minutes. The PI is the ratio of ten minutes reading to one-minute reading. Since this being a ratio, it is independent of temperature and no temperature correction is required.

The value of PI generally shall be above 2. In general, a high value of PI indicates the good condition of the insulation system. If the PI values are less than normal value or shows large deviations from the previous measured values then immediate corrective actions need to be taken. Steps should be taken to dry the insulation and remove the contamination before proceeding with other tests.

In some cases the measured PI values may be misguiding. For example, if multi-layered insulation fails in one of the layers, while the others retain high resistance, the effect on the test current will tend to increase the PI value, masking the possible problems from surface leakage caused by dirt and contamination. Also, for older asphalted windings, a PI greater than six or seven may indicate that it is thermally degraded. If the IR value is more than 1000MW , the PI becomes an erratic indicator.
 

zog

Senior Member
Location
Charlotte, NC
INSULATION CONDITION (PI)
Dangerous - Less than 1
Poor - Less than 1.5
Questionable - 1.5 - 2
Fair - 2 - 3
Good - 3 - 4
Excellent- Greater than 4
 
Status
Not open for further replies.
Top