Is the PI to high 1

[eyec]

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?

 

[Skogsgurra]

That's almost twice as much as I'm used to

Sorry - couldn't resist.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[eyec]

yes, but is too much as good as not quite so much?

case in point when i drink scotch

 

[ScottyUK]

Higher is better. Looks like you machine is in good condition.


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If we learn from our mistakes I'm getting a great education!

 

[rovineye]

The variable between the 1 min and 10 min readings is the absorption current. There are many water molecules in mica or asphalt for instance, and those materials can result in high initial current and decay before the 10 min mark, for a high PI. Since your voltage is low, you have little electron trapping at laminated insulation, which is another factor that can contribute to absorption current.

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.

 

[eyec]

no trends available
first time machine has been open
machine is 13+ years old

 

[ScottyUK]

Thanks for that explanation rovineye - I may have learned something about how the insulation on older machines behaves.


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If we learn from our mistakes I'm getting a great education!

 

[eyec]

going back to basics (stitch in time) it mentions the same possibilty, but it also mentions a reading as high as 13+ while the machine was still "curing"

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?

 

[Gianoli]

The following is taken from “IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery” IEEE Std 43-2000 – March 24, 2000. Section “6.3.4 Polarization index correction” may offer an explaination as to why your readings seem high. A careful review of the entire document may be appropriate.


“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.”

 

[edison123]

Questions

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 *

 

[rovineye]

"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"

True enough, it should be considered. But there is no way to correct for it. So I would consider it invalid!

 

[eyec]

initial test PI = 5.4
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

 

[edison123]

eyec

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 *

 

[Gianoli]

“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?”

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.

 

[rovineye]

I agree with edison123, that your test voltage is high. IEEE Std 43-2000 would recommend 1000-2500 volts. You want to be below the rated voltage of the machine. This isn't a hipot.

 

[Zogzog]

I think based on the proces described lack of proper tempature correction is the most likely suspect in this case.

 

[oftenlost]

About 30 years ago, I performed a PI on a motor out of a salt mine which had wrapped and dipped coils; a pre VPI winding. The readings indicated a very high PI, on the order of 8 or so. The unit was washed and treated in a dip tank and returned to service. The megger and PI were excellent on installation. The motor failed in service after six months or so and on post failure disassembly of the machine, it was noted that the mica tapes within the coil had deteriorated and the mica had flaked and powdered over time. When the failed coil was cut and removed from the circuit, the PI was again up at very high levels. I cannot imagine a VPI winding having this problem but I never understood the reason for the high PI. I did stretch my imagination while under the influence of Kentucky spirits but never came to closure.

 

[eyec]

OK, thanks to the many replies and info. After reading them and doing a little more research I recommended another set of tests (mainly because of the uncertantity of how long between the oven and testing).

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!

 

[edison123]

Those values are more like it.

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 *