Is the PI to high
Is the PI to high
(OP)
recently tested a 4160v motor and was given the results of a PI test that were 7.1
most reports from my searches only say >4 Excellent
any tnoughts?
most reports from my searches only say >4 Excellent
any tnoughts?





RE: Is the PI to high
Sorry - couldn't resist.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Is the PI to high
case in point when i drink scotch
RE: Is the PI to high
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If we learn from our mistakes I'm getting a great education!
RE: Is the PI to high
It is difficult to compare value of machines based on PI, esp if they have different insulation materials. Most newer machines with modern insulating materials have quite high initial readings and so they don't get to quite such a high PI.
But there can be cases where a high PI can be indicative of an insulation problem in older machines. If an asphaltic mica winding is overheated, the asphalt can flow away, and could result in a higher PI because there can be fewer water molecules in the mica alone.
Look at trending of the PI for this machine to guide your evaluation, if you have any that is.
RE: Is the PI to high
first time machine has been open
machine is 13+ years old
RE: Is the PI to high
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If we learn from our mistakes I'm getting a great education!
RE: Is the PI to high
so, now my question is, could the PI be high because the readings were taken shortly after being removed from the oven (after steam cleaning) and should another PI be run now the the windings have been out of the oven for a length of time?
RE: Is the PI to high
"When the polarization index is used with the insulation resistance to determine the insulation condition, it is not necessary to make a temperature correction to the P.I. Since the machine temperature does not change appreciably between the 1 min and 10 min readings, the effect of temperature on the polarization index is usually small. However, when the initial winding temperature is high, a reduction in the temperature of the insulation system during the test may result in a substantial increase in the insulation resistance between the 1 min and the 10 min readings due to the temperature effect (see 6.3.1). The resulting polarization index may be uncharacteristically high, in which case a repeat measurement at or below 40C is recommended as a check of the P.I. As stated in 6.2, if either the 1 min or 10 min measurements are taken when the winding temperature is below the dew point, the effects of moisture contamination must be considered during interpretation."
RE: Is the PI to high
1. Test voltage
2. 1 & 10 min values
3. Temperature at 1 & 10 min
4. Insulation class (B or F)
Unusually high PI's could also indicate very old and brittle windings.
Normal values I get in my shop for F class machines
New windings - 3 to 5
Old windings (afterclean up) - 2.0 to 3.5
* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *
RE: Is the PI to high
True enough, it should be considered. But there is no way to correct for it. So I would consider it invalid!
RE: Is the PI to high
test voltage = 5000
1min = 6.82G
10min = 36.4G
no temps recorded
Class F
after steam & bake PI = 7.29
1min = 5.92G
10min = 43.2G
test atmospheric conditions unknown
on a sister motor PI values were
initial = 1.86
4.09 & 7.62Gohm
after steam & bake = 7.61
8.06 & 60.9Gohm
RE: Is the PI to high
5000 V PI test for 4100 V machine is a bit steep. I would have used 2000 V at the max.
Per IEEE 43-2000
When the 1 min IR is more than 5000 Megohms, PI may or may not be an indication of insulation condition and hence not recommended as assessment tool.
Reasons: Test instrument sensitivity, small changes supply voltage, ambient humidity, test conditions.
But then, both your motors seem to have the same PI characteristics and hence a high PI is not a worrying concern - my two cents.
Are these VPI stators ? Who are the manufacturers ?
And do you do steam cleaning often and if yes, why ?
(Sorry about the more-questions times).
* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *
RE: Is the PI to high
Yes, your reading could be high because you had just removed the unit from the oven. If the unit was cooling as you did the test the results may indicate a higher value. See my previous post with the quote from the IEEE document for an explanation why. Ideally it would be a good idea to run another test with the unit at a constant temperature.
RE: Is the PI to high
RE: Is the PI to high
RE: Is the PI to high
RE: Is the PI to high
The results were:
Motor 1 (out of the overn for 5 days)
1 min = 6.70Gohm
10 min = 55.0Gohm
PI = 4.82
Motor 2 (out of the oven 3 days)
1 min = 7.53Gohm
10 min = 40.30Gohm
PI = 5.35
i now have to look into the voltage question; 5k vs 2.5k
but the numbers indicate the motors should run a couple more years after this reconditioning.
these are VPI motors by Allis
this was the first time they had been steam cleaned that I am aware of
they had ever been taken apart in 13+ years
thanks again you guys are great!
RE: Is the PI to high
I am now sure your previous PI values were chased up the tree by the temperature.
* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *