polarization index of LVSCIM 50hp motor

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we are doing an insulation resitance test and at the same time the polarization index for our LVSCIM 50hp motor.we are getting less than 2.0 PI which is an accptable value set by our client's but the insulation resistance is ranging from 25 Giga-ohm up to 40-45 Giga-ohm.my question is, is the motor under test acceptable to use and if yes what is the reason we must tell to the client?
 
Are you sure your PI value is ok?

Are you sure your PI value is ok?

One of the Ref's I use, Motor System Management 97 - Table 4-6, page 103, states a PI of 1.0 to 2.0 is questionable. Where as If your reading was actually the DAR anything above 1.6 would be excellent.
 
INSULATION CONDITION DAR PI
Dangerous Less than 1
Poor Less than 1.1 Less than 1.5
Questionable 1.1 - 1.25 1.5 - 2
Fair 1.25 - 1.4 2 - 3
Good 1.4 - 1.6 3 - 4
Excellent Greater than 1.6 Greater than 4


Dont know why these are all mashed together, looked goog when I posted it.

Anyways, 2.0 is questionable
 
VinceS said:
One of the Ref's I use, Motor System Management 97 - Table 4-6, page 103, states a PI of 1.0 to 2.0 is questionable. Where as If your reading was actually the DAR anything above 1.6 would be excellent.
Click to expand...

yes we are getting less than 2.0 but as per IEEE std 43-2000 it states that "if the 1 minute insulation resistance is above 5000 Meg-Ohms, the calculated PI may not be meaningful.In such case, the PI may be disregarded as a measure of winding condition". By the way we are using Megger MT520 model which can read up to 1Terra-Ohms and we can get also the DAR value.
Is this explanation from the reference standard enough to prove our cause,please advise as i am a neophyte on this matter.Thanks
 
Sorry, didnt look at your IR values, yes you are correct, when the readings are that high, PI becomes useless as long as it is >1.0.

I think you answered your own question and can justify your answer with those references.
 
zog said:
Sorry, didnt look at your IR values, yes you are correct, when the readings are that high, PI becomes useless as long as it is >1.0.

I think you answered your own question and can justify your answer with those references.
Click to expand...

1-minute reads 25.7G-Ohm, 10-minute reads 47.6 G-ohm which give us around 1.85 and by the way DAR is the same as PI reading. thanks!
 
zog said:
Huh?

DAR=60 sec/30 sec

PI=10 min/1min
Click to expand...

Yes you're correct sir!DAR = T2/T1 and it so happen that we set T1 and T2 1-minute and 10-minute respectively. That's why i like this forum i guess i can learn a lot from here..Thanks zog:grin:
 
PI and DAR are pre-determined times and measure different charateristics of insulation. DAR is always 60sec/30sec and PI is always 10min/1min
 
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