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

 

[induction]

P.I. Testing on voltages you wrote of from my experience may not give you a good reading. I do not know if this is proper to say this but I believe P.I. test on any voltage higher then 480 may not be the thing to do. Most of the systems I see in plants do not have the voltage output to correctly test that high fo a voltage I once wittnessed the P.I. of a motor 4000 volt 2000 H.P. with a thousand volt output from the P.I. . The reading were not true I tested it at 9000 volts and showed there was leakage. Later I found out that the coolant had leaked into the motor the P.I. did not show this. We bake the stator and the the readings came up and there were no more fualts.

 

[DanDel]

The point of a PI test is that it is non-destructive, meaning that the test voltage is at or below the rated voltage of the equipment tested. While it will give you a good idea of the insulation quality, it may not tell you all the defects in the insulation, but it will definitely not damage the insulation further.
Testing using voltages above the equipment rating may give more information about the quality of the insulation, but it will damage the insulation. Sometimes this damage will cause the insulation to fail during the test or shortly after when in service.

 

[induction]

THis is true about P.I. testing the equipment I use shows the fualts as the voltage rises and yes you could damage insulation. It reads micro amps for leakage setting this correctly will trip before it does any harm.

 

[electricpete]

Discussion of voltage levels is sometimes a matter of opinion.

induction is right that 1kv on a 4kv motor is probably not enough. IEEE43-2000 specifies 2500vdc for a [non-destructive] insulation resistance test on 4kv motor.

Dan is right that 9kv on a 4kvdc motor is considered a Hi-pot test (potentially destructive) but still within the limits allowed by IEEE95 for hi-pot testing of service-aged windings.

I had an experience which taught me something about hi-pot testing.

We had an 8000hp 13.2kv motor refurbished. After final reassembly it was hi-potted to 18kvac and then run unloaded.

On receipt at plant we hi-potted to 24kvdc, which is a lower equivalent voltage level, considering the accepted 1.7 conversion factor between ac and dc hi-pot levels. We got to our final step 24kvdc and current was ~ 1.5 micro-amps. Without warning the test set tripped (trip setting 100 microamps). We investigated, tore down the motor, found a failure just outside the slot, required rewind. My conclusions:
#1 - Don't believe the 1.7 conversion factor. It is a thumbrule but not absolute truth.
#2 - DC hi-pot can damage a motor, no matter how careful you are.

Still we continue to do DC hi-pot on some critical motors. If it fails during test, we believe there was a probability it may have failed in service later. Overall you probably will rewind more motors if you do hi-pot testing, but you will also have less in-service failures. You have to weigh these against each other. In our case it is a no-brainer... cost of in-service failure of these particular motors can be approx 6 million dollars. Cost of a failure during test at plant outage requiring rewind is approx 1/10 of that (we have spare motor on-hand). We can afford to fail roughly 10 motors during testing if it prevents one in-service failure.

 

[jbartos]

Suggestion to the previous posting: Visit

http://www.epri.com/corporate/products_services/project_opps/gen/1007270.pdf

for motor current signature analysis and power signature analysis

 

[electricpete]

what is the relevance of motor current signature analysis and power signature analysis?

 

[electricpete]

Not to beat a dead horse, but one more data point:

EASA "Principles of Large AC Motors" (2000) page 9-13 gives the following guidelines for interpretation of PI:
<1 - dangerous
1-1.5 - poor
1.5-2 - questionable
2-3 - Fair
3-4 - Good
>4 - Excellent

So from what I can see, IEEE and EASA both tell us there is no reason to be concerned about high PI (for modern windings).

 

[gpedens]

Have you looked at the accuracy specs for your test equipment? With leakages in fractions of a microamp the error may be such that the PI will seem to vary over a wide range. I have seen the wind blow and change a PI. Also your power supply for the test set can affect the readings. If you are using a nominal 120 vac wall outlet, someone starting a drill or turning on a vacuum can cause a leakage current change and affect the PI. New motors have 10 minute winding resistance readings in the gigaohm or more range.

Thank You,Radar. Duke Power

 

[jbartos]

Suggestion to electricpete (Electrical) Apr 27, 2003 marked ///\\what is the relevance of motor current signature analysis and power signature analysis?
///If one spends $150,000 per the upcoming EPRI three-year project, then there will not be difficult to answer such a question. What if only EPRI knows?\\

 

[GusD]

Hi HYpress

I would like to if anyone out there has ever removed a motor from service, because it had a HI PI ratio?
I personally doubt it, if one did not have any other
information to correlate to the high ratio .
In the old days impregnating varnishes could become brittle with age and cracked, giving cause to the HI PI ratios.Newer insulations and varnishes don't behave in the same way and a Hi PI ratio should be no cause for alarm.
If this high ratio is asociated with other possible downgrading conditions,it should be evaluated.



GusD