×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

300 kW Polarisation Index

300 kW Polarisation Index

300 kW Polarisation Index

(OP)
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,

RE: 300 kW Polarisation Index

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

RE: 300 kW Polarisation Index

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.

RE: 300 kW Polarisation Index

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: 300 kW Polarisation Index

(OP)
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 ?

RE: 300 kW Polarisation Index

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

RE: 300 kW Polarisation Index

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: 300 kW Polarisation Index

Values and vintage dictate either rewinding or replacement.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources