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#60201 - 12/25/05 01:18 PM How To Meg A Motor?  
Old Town  Offline
Member
Joined: Nov 2005
Posts: 21
Dallas,TX USA
Merry Christmas,

Can anyone here go over with me how to properly test a motor with a megohm meter(Megger)? I have two suspect single phase 2 Hp motors on my bench. How do I go about testing them?

Thanks in advance.

Ty


Work Gear for Electricians and the Trades

#60202 - 12/25/05 04:39 PM Re: How To Meg A Motor?  
RODALCO  Offline
Member
Joined: Dec 2005
Posts: 854
Titirangi, Akld, New Zealand
Megger from windings to frame / earth lug on motor case. start and run winding.

Megger from start to run winding.

Megger capacitor terminals to earth if it got one.

Check continuity of windings

Check operation of start , centrifugal switch if it has one.

Start winding has usually a higher resistance than run winding. It also switches out when the motor is up to speed.

Hope this helps. Ray.


The product of rotation, excitation and flux produces electricty.

#60203 - 12/25/05 04:40 PM Re: How To Meg A Motor?  
RODALCO  Offline
Member
Joined: Dec 2005
Posts: 854
Titirangi, Akld, New Zealand
Sorry I forgot, Readings should be at least 1 Mega Ohm or better.


The product of rotation, excitation and flux produces electricty.

#60204 - 12/25/05 05:03 PM Re: How To Meg A Motor?  
Old Town  Offline
Member
Joined: Nov 2005
Posts: 21
Dallas,TX USA
Thanks Ray.


#60205 - 12/26/05 08:42 AM Re: How To Meg A Motor?  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
A couple of additional points to consider:

1) Check you Megger voltage. The test voltage should be 500Vdc for motors rated 600V or less.

2) Polarization: "The resistance of a good insulation system will increase when subjected to a test voltage for a period of time." The resistance 60 seconds into a particular test should be at least 1.25 times the resistance 30 seconds into the test.

Both of the above are taken from EASA Principles of Large AC Machines, a book that I have on my shelf for other reasons. I've never had to personally run these tests or deal with the results, so absorb the above information with care. Point 1 is IMHO solid, point 2 less so.

-Jon


#60206 - 12/26/05 05:52 PM Re: How To Meg A Motor?  
RODALCO  Offline
Member
Joined: Dec 2005
Posts: 854
Titirangi, Akld, New Zealand
Thanks Winnie for adding that extra info.

cheers Ray.


The product of rotation, excitation and flux produces electricty.

#60207 - 12/26/05 06:14 PM Re: How To Meg A Motor?  
AllClear  Offline
Member
Joined: Aug 2005
Posts: 44
Belmond Iowa US
Winnie, I Beleive Your second Point "resistance of a good insulation system will increase when subjected to a test voltage for a period of time" Is meant For Hi-Pot testing. Megger testing is for short Durations. Just be aware that there are Two Types of tests.


It's Not The Fall That Kills You...
It's That Sudden Stop At The End

#60208 - 12/26/05 06:24 PM Re: How To Meg A Motor?  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Allclear timed test are applicable to Mega testing.

A one minute ten minute test is common. We just had a Mega salesman at the shop giving us some lessons. [Linked Image]

Quote
The best way to quantify the results of a time resistance test is through a dielectric absorption ratio. The dielectric absorption ratio consists of two time resistance readings. A commonly used set of intervals is a 60-second reading divided by a 30-second reading. Another frequently used set is a 10-minute reading divided by a 1-minute reading. This resulting value is referred to as the polarization index.


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#60209 - 12/26/05 08:29 PM Re: How To Meg A Motor?  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
As I understand it, a 'Hi-Pot' test is used infrequently, and is a 'pass-fail' test. The voltages used are such that they are potentially destructive, and which probably cause insulation deterioration. A megger test, on the other hand, is done at lower voltage, generally in the range of what the insulation should be able to tolerate continuously.

Again looking at the EASA book, the High Potential tests are run with AC of 2xrated voltage + 1000, or with DC at 1.7 x the AC test voltage.

I presume that dielectric polarization occurs in both types of test, at least at DC.

-Jon


#60210 - 12/27/05 12:27 AM Re: How To Meg A Motor?  
macmikeman  Offline
Member
Joined: Jul 2002
Posts: 717
Honolulu, Hawaii
I keep a folder on my computer desktop that I drag and drop great threads onto. This one goes in. Thanks to all who contributed.


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