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MV motor winding PI

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power2020

Electrical
Aug 18, 2005
127
Dear Folks,
Recently we have measured polarisation Index (PI) of one MV motor-11kV, 5100MW, we got the following values.
R -E --- 4
Y -E ---- 4
B -E ----4
R+Y+B-E --2.5

Anyone pl. discuss the reason for less PI if we measure all windings with respect to ground.
Thank you..
 
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It is more like 5100 kW, isn't it?

Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
Sorry for wrong unit. It is 5100kW, 11kV motor.
Thank you Mr.Skogsgurra.
 
Above 2 we are usually not concerned.

Can you give the values upon which the PI was calculated (1 minute and 10 minute readings)?

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I've seen PI's skewed by failure to allow individual windings to adequately discharge before subsequent tests. That might not be your reason, but it's an observable phenomenon.

My schedule: Ground windings B & C. Test winding A. Then ground winding A. Go get cup of coffee and sip thoughtfully.

Then remove ground from winding B and test it. After testing, ground winding B. Finish up old reports. Chat with co-workers on jobsite.

Unground winding C. Test it. Ground winding C. Carefully fill out more reports. Go get second cup of coffee. Ponder some problem that's bugging a co-worker.

Unground and connect all three windings, and do your overall PI. Bring the results to the end user and have a cup of coffee from his pot.

Seriously, don't get in a hurry between tests. Allow insulation to fully discharge. A rule of thumb for this is to ground the winding for at least the amount of time that voltage was applied. Some operators say "twice the time".

Also, keep in mind that the Polarization Index test is not a precision event. Good care will produce reasonably useful results. Any technician who brings me a PI calculated to two decimal places is due for some retraining.

NOTE: This procedure also explains my predisposition to kidney stones...

old field guy
 
I couldnt have said it better myself oldfieldguy
 
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