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Old 12-06-2018, 09:03 PM   #1
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Can someone explain to me the use of the data received when you check the motors polarization index? Is the test also relevant on transformers? I understand it’s a tell for moisture and thermal breakdown but can’t you just look at the past readings and determine the same thing?

I mean why the chart, math, and everything else?
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Old 12-06-2018, 09:21 PM   #2
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It's mainly to determine suitability of motors. Such as ones that have been sitting in a warehouse without heaters. You normally wouldn't use it for motors that are in service.
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Old 12-06-2018, 09:49 PM   #3
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Yes, it's also relevant for transformers. Same reasons as Joey stated.
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Old 12-06-2018, 10:24 PM   #4
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Quote:
Originally Posted by joebanana View Post
It's mainly to determine suitability of motors. Such as ones that have been sitting in a warehouse without heaters. You normally wouldn't use it for motors that are in service.
Suitability as in still serviceable?
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Old 12-07-2018, 01:02 AM   #5
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PI is useful because some motors show good on the standard megger test (1 minute reading, temperature corrected) and does not need temperature adjustment. It worked much better on old pre-1970 insulation. It is invalid if the megger test exceeds 5000 megaohms.

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Old 12-07-2018, 01:07 PM   #6
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When you perform a megger test you are measuring 3 different currents.

Conduction/Leakage Current - Thiscurrent passes through the surface of the insulation. The magnitude of current flow depends on theresistance of the insulation.Surfaceleakage is usually not a problem because it can be eliminated through external cleaning.

Capacitive Current - The insulatingspecimen appears, ideally, as a capacitor. As a DC voltage is applied to a capacitor initial charging current flowsuntilthevoltage drop across the component equals the source voltage. As the capacitor charges, its chargingcurrent decreases to a minimum.This iscalled its steady-state value.

Dielectric Absorption Current - Thiscurrent also appears at the initial application of test voltage the same ascapacitance current. This current isrequired to polarize the insulating medium.In other words it is energy absorbed by the insulating system.

As insulation becomes brittle its ability to absorb energy decreases, thus giving you a smaller ratio of absorption current compared to total measured current.
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Old 12-07-2018, 10:07 PM   #7
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Suitability as in still serviceable?
Maybe usability would have been a better word choice, as in, you won't have to dry it out, or send it out,(to the rewind shop) before putting it into service.
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Old 12-08-2018, 10:04 PM   #8
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Maybe usability would have been a better word choice, as in, you won't have to dry it out, or send it out,(to the rewind shop) before putting it into service.
I've heard the next edition of IEEE 43 is going to drop PI just as they've done with DAR (a faster alternative, 1 minute divided by 30 second reading). In its place they will recommend a graph of insulation resistance vs. time. Bad insulation that passes the basic megger test usually looks on a graph like one of three things. Contamination looks almost like random noise. Old, cracked insulation looks like stair steps. And compromised but still meeting 1 minute minimum insulation basically charges very quickly then plateaus. The PI test catches the first two conditions but misses severely cracked insulation where the graph shows it very easily.

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