Poloraziation Index 2

[hypress]

What should the PI of new 2400V 500HP motor be?
Ihave tested 7 motors and the PI`s range from 5.8 to 12.7.
These motors are single speed some were 900 RPM and some were 1200 RPM all were from the same manufacturer and none
have been placed in service.
Our plant standard is to test 2400V motors at1000 volts
would this explain the high PI? From what I have read PI`s
should be between 2 and 6. THANKS HYPRESS

 

[jbartos]

Suggestion: Visit

http://www.usbr.gov/power/data/fist/fist3~1/3~1_2.htm

for Table I and:
IEEE Standard No. 43 indicates the recommended minimum values of polarization index for a-c (alternating current machines) are:

For class A insulation - 1.5
For class B insulation - 2.0
For class F insulation - 2.0

 

[electricpete]

I agree with jbartos. For new motors there is no upper limit on Polarization Index. The limits that he gave are consistent with IEEE43-2000. I don't think there are any new machines sold with class A so 2.0 is a good number for a minimum

I believe that a maximum limit was sometimes applied to older motors with Alsphaltic insulation (along with a minimum limit). New motors will not have that type of insulation.

 

[neps3]

Agree with Jbartos and electricpete.

If the PI on Bitumen/Alsphaltic insulation is very high then it is an indication that the system is dry. (bonding material).

Please follow IEEE 43-2000.

Regards

 

[edison123]

In my experience, I have never come across PI more than 5. Is there something I am missing in the values mentioned in this thread for they seem to be too high ?

One another thing, is there any correlation between absorption co-efficient (60 sec value / 15 second value) and PI (10 min value / 1 min value) ? In some cases, I have found absorption co-efficient higher than PI and in other cases, just the opposite.

 

[utvol]

We have found that the ratio number is less important than the shape of the curve plotted by almost continous reading. The curve should ideally be like a capacitor charge curve. It should be relatively smooth with no major negative going spikes. These show insulation breakdown. You can have this and still have a good PI number.
Electricpete is right. New motors may have very high numbers. The RTG values can run 50-60 gigohms with a PI of 15 or higher. At this level, PI numbers don't mean too much.
When the curve flattens or becomes erratic, we send the motor to the shop for service.

 

[vanskat]

Both the Polarization Index (PI) and Dielectric Absorption (DA)reflect the condition of the insulation system of the stator windings and the power circuit.

Both are ratios but the PI test allows the charging and absorption currents in an RTG test to decay so that only the actual leakage current is measured. In terms of a mathematical relationship; I have not come accross one as yet, however, it seems as if the DA provides more of a snapshot view of what is going on...in the insulation.

I would think that both tests are better than just one!

 

[utvol]

Using the PdMA MCE tester gives the DA as well as the PI numbers in the one test. It is possible toget a good DA and then the PI not be good. It also shows the profile I mentioned above. We rely on this more than any other aspect of our predictive program. Check out their website.

http://www.pdma.com

 

[GusD]

Check Kantor's advice and look at the IEEE 43.
The PI/DA ratio is important,however;one other important aspect is the PI graph.I too use the PDMA/ASI Hi voltage megger or its MCE 1000 volt megger.I have had new motors that show a terrible graph and basically no Ratio or others that show just like yours very high rations.
The PI test is only one aspect of all the tests for motor condition.I would be very careful making a call on any motor solely on a PI value or Graph.
GusD

 

[jb17136]

I agree with utvol, the PdMA tester is a good tool. My first question would be what is the resistance to ground and how quickly does it get there. If the resistance to ground exceeds 5 gigohms in the first minute and stays there througought the 10 minute test with a good smooth curve, then the PI should be 1 or nearly so. Also, with this type of reading (low PI, very high resistance to ground), the PI is not as much of an indicator as the curve of the PI.

With the higher PIs mentioned on a new motor, especially one with VPI insulation, I would suspect moisture. I would try drying the windings and running the test again.

 

[electricpete]

a specific comment to jb17136 - I believe it is a radical departure to suggest that high PI of a modern winding indicates moisture.

To all: Question about PDMA and resistance profile:
You get a lot more data when you do a PDMA check... but is it really useful? Has anyone ever seen a strange trend in the profile of resistance vs time which was:
#1 - Not reflected in 1-minute IR or 10:1 minute PI
AND
#2 - Indicative of an actual problem subsequently confirmed by internal inspection or some other reliable method.

I am evaluating PDMA for our plant. At this point I'm still skeptical.

 

[GusD]

To electricpete.

We have been using PDMA tester MCE/MCEmax since 1995.
Our plant has 5 of these testers,they were all purchased in the last few years,the last one a couple of years ago.
If I was on the market for a similar technology,I would have a look at another tester on the market(ALLPROIV)
I got a video on these tester and I was impressed by its capability and more so by the low cost compairing to the PDMA.

Thanks GusD

 

[DKB1943]

I suspect PI >5 could be caused by evaporation of moisture from VPI insulation surface by heating caused by leakage current. It appears to be abnormally high.

 

[electricpete]

Thanks for responding dkb. I meant no disrespect and I don't claim to have all the answers.

One thing I do know is that IEEE43-2000 does not apply any maximum PI limit to modern (epoxy or polyester) windings, which my knowledge includes all VPI windings.

The only thing that IEEE43-2000 mentions about high PI applies to older insulation systems. Specifically, section 11.2 contains the following text: "For varnished shellac mica-follium, or asphaltic stator windings, a very high PI (for example >8) may indicate that the insulation has been thermally aged and may have a high risk of failure. If physical inspection indicates dry or brittle insulation, it is best not to attempt cleaning or overpotential test. Failure may occur any time after machine is placed in service".

So in summary, IEEE43-2000 does not recommend rejecting modern windings based upon high PI (only older windings). Your discussion appears to contradict this. Your explanation seems like a plausible explanation for very high PI on a modern winding, but leaves me questioning whether a dryout would be necessary... haven't we accomplished dryout of the surface moisture during the 10-minute test.

I am curious if you have any additional info to suggest that a motor should be rejected upon high PI until drying creates a lower pi (rather than ignoring high PI on a modern winding as the standard would suggest)

 

[GusD]

For anyone that would like to evaluate Electric Motor Testing technologies such as PDMAs MCA/MCEmax, try a
product sold by BJM Corp.ALL-TEST IV PROtm2000.
I have only seen a brochure and a video of this tester and i was impressed with its simplicity and cost.
I have been using PDMA MCE since 1995,and the MCEMax for Dynamic testing since 1999.I like the tester but I dislike the cost.I bilieve the ALL- TEST IV PRO sells for about 25% of what PDMA sell for .
I cannot tell if this tester has the same capabilities as the MCE,I have not used it myself.


Thanks
GusD GusD

 

[jb17136]

To Electricpete.

I would agree that suspecting moisture due to a high PI for a modern winding would be a departure except the initial conditions were: new motors, never in service. I assumed storage of some kind, and while dirt, contamination, and deterioration and other normal enemies of insulation and causes of high PI are possible, I would expect moisture to be the most probable and easiest to check. Just performing a PI would help drive some moisture off.

We have had some experience with out PdMA test equipment and yes, we have seen things on the traces that have been correlated and confirmed by motor repairs. Our database is still pretty small, but we have had some good successes. Since we are a nuclear facility, if we see deterioration, we like to plan to perform maintenance rather than just be ready to do it when a motor fails. In that respect, it is much easier to justify our testing and trending costs.

 

[electricpete]

"while dirt, contamination, and deterioration and other normal enemies of insulation and causes of high PI are possible, I would expect moisture to be the most probable and easiest to check"

My point is that high PI is NOT a condition that needs to be investigated for modern windings (it was for old windings), according to IEEE. I don't think that dirt, contamination, deterioration will give high pi (they may give low pi). Possibly surface moisture as you say, but if so, it is of a nature that is removed by the test itself and not of concern.

I work at a nuc plant to. I'm curious if you have been to LEMUG meetings... maybe we know each other.

 

[GusD]

Hi Hypress.
I don't know if meant to say that the PI test would help
drive out the moisture.I don't think so.A PI for 10 min
at either a 1000v or higher,does not generate any heat to
drive out anything.
You have a better chance to get rid of moisture if you
hook up the Anti-condensation heaters.
If the amount of moisture is excessive you might have to use additional heat.
As for the PI levels you got I don't think it necessarily mean anything.
If you like, I could send you the PIs I did on 8 brand new
1500 hp motors /4160 volts.I did the test around 1996 and I too was using the PDMA tester.The PIs I got are definitely different.My E-mail =dalmeida.gus@syncrude.com GusD

 

[jbartos]

Suggestion: Theoretically, there are two ways to dry the machine. The first way is to provide heat externally to the machine and let heat to propagate from outside of the machine to inside. The second way is to heat the machine internally by electrical current. This way heat propagates from the inside of the machine to the outside, e.g. winding outside of the machine. The second way appears to be more effective and faster drying approach since the heat source is closer to the hidden moisture.

 

[jb17136]

To electricpete
Yes I go to LEMUG, are you going in August?

I think we are dancing around the whole PI issue here. Hypress was looking for a good PI acceptance number for a new, unused motor. The ones he tested seemed to be high. Moisture seemed to be a possibility, especially for a modern winding. Hope to see you in Minneapolis.