300 kW Polarisation Index

[maypot]

Hi,

I performed a PI Test on one 300 kW 400 V electric motor and surprisingly got a value of 0.75 which is considered as a worst result. Is there any standard for comparing results,

 

[GTstartup]

What were the 1 minute and ten minute values? What was the test voltage?

 

[edison123]

maypot,

IEEE std 43-2000 states that with modern insulation system (in random wound and form wound windings) the absorption current becomes zero very fast. So, it recommends in such modern insulation, a 1 min/30 sec value may be taken as PI.

 

[electricpete]

IEEE 43-2000 is the relevant standard for the US.

It specifies a minimum PI of 2.0.

Some exceptions are:
1 - Asphaltic windings. Not present if your machine was built in the 1980's or later.
2 - If the I.R. reading gets too close to the top of your resistance scale it may not be possible to obtain a good pi reading. The standard states PI is not needed if IR exceeds 5,000 megaohms. Depending on your equipment you may have difficulty measuring PI even at lower IR's.

Most often in the field when I see high PI it is an indication the windings are wet. Oh another thing, if you are measuring a long run of cable along with the motor the cable may interfere with your measurement. P.I. is not a standard test for cable.

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[maypot]

The insulation resistance measured is around 100 k and this value dropped to 83 k after ten minutes. We applied heat to the winding for a period of 24 hours but the insulation did not improve.It`s a rather old motor , 1970`s.I am rrequesting the management of the company to drop the motor.
Any comment ?

 

[elechris]

Did you take note of the ambient temp. whilst performisng the test, and the temp. of the stator itself?
Sounds like the winding is very dry and a good steam clean, varnish and bake may do the job. Good Luck

 

[electricpete]

In the older version of IEEE43 applicable in 1970's, minimum 1-minute insulation resistance would have been kv+1 i.e. 1.5 megaohms. First you must apply a temperature correction as Edison mentioned. Factor of 2 for every 10C deviation from 40C. One other factor that can interfere with measurement on occasion is high relative humidity.

I am not familiar with older windings, but if these readings are taken directly at the motor (not through long run of cable), in low-humidity conditions, with winding temeprature 40C or below, with no change after 24 hours of drying, I would think the windings are suspect.

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[edison123]

Values and vintage dictate either rewinding or replacement.