Megger Test 3

[jncza]

I need to megger a 6,6KV motor.I will be using a 5000/10000 Volt megger.What scale shoud I use and what must be the minimum ohm value be.Thanks

 

[laundry]

Hi
The megger should be set to 10k and the reading to earth should be 0.ie no leakage.If you are testing between windings then you should have equal readings between windings.Iam not sure that you should be doing this test as you do not sound competent to do this,as the motor is a HV class system.No disrespect intended.

Barry.

 

[jncza]

Hi Laundry.
Thanks . None taken.When I megger between phases ,should it also be on 10000V scale

 

[stardelta]

Owing to the nature of the insulation systems used in HT machines such as this, a simple megger test will prove inconclusive. What is required is a Polarization (Or PI) test. The issue is covered by IEEE standard No43 and is a lengthy and involved subject. As Laundry says its best left to the experts. The link will provide you with a lot more information than I can.
th

http://www.ecmweb.com/mag/electric_review_polarization_index/.

 

[stardelta]

This site explains in simple terms what a PI test is and how it should be performed. Anything over 2 is accecptable.

http://www.joliet-equipment.com/determining_polarization_index.htm

 

[electricpete]

In addition to PI>2, look for a bare minimum of 100 megaohms on the temperature-corrected 1 minute insulation resistance.

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[edison123]

jncza,

For a 6.6 KV motor, 2.5/5 KV megger would suffice.

For class B winding, PI > 1.5

For class F winding, PI > 2.0

One minute IR values of > 100 megohms at room temp preferred but not a must.

Whether you can measure phase-phase IR depends on all 6 leads being brought out separately to the terminal board and star/neutral shorting done there.

 

[BigWallyB]

As far as I know, megger readings are more of a reference as to motor condition than an absolute failure indicator. I have seen some motors (low voltage) that have read off the scale and still ran for years. I've learned some new (high voltage) stuff here myself.

Life is what happens while we're making other plans.

Wally

 

[aolalde]

Get IEEE STD 43 and study it in detail. That is the safe way to go. Do not follow past experiences not based on scientific bases. I work on a repair shop and the bad experiences of negative results due to mistakes made by technicians with 20+ years of practical experience are countless.

 

[Skogsgurra]

I tend to agree heartily, aolalde. Many myths and "obviuos truths" walking around out there.

Gunnar Englund

http://www.gke.org

 

[petronila]

Hello JNCZA,

Don`t perform the test at 10 KV is better at 5 KV because you only needs to take an insulation reading.

The Recommended minimun Insulation resistances(IRmin)values corrected at 40 C as Defined by IEEE 43-2000 like:

"IR min= KV + 1 ,For most windings made before about 1970,all field windings"

"IRmin= 100 MOhm, for most DC armatures and AC Windings built after about 1970 (Form wound coils)"

Best Regards,

PETRONILA


A 500 V Megger is suitable for machines rated up to 2.4 KV above this this voltage a 1000 V Megger is recommended.