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#60201 - 12/25/05 09:18 AM How To Meg A Motor?
Old Town Offline
Member
Registered: 11/24/05
Posts: 21
Loc: 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
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#60202 - 12/25/05 12:39 PM Re: How To Meg A Motor?
RODALCO Offline
Member
Registered: 12/08/05
Posts: 854
Loc: 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.
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The product of rotation, excitation and flux produces electricty.
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#60203 - 12/25/05 12:40 PM Re: How To Meg A Motor?
RODALCO Offline
Member
Registered: 12/08/05
Posts: 854
Loc: 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.
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#60204 - 12/25/05 01:03 PM Re: How To Meg A Motor?
Old Town Offline
Member
Registered: 11/24/05
Posts: 21
Loc: Dallas,TX USA
Thanks Ray.
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#60205 - 12/26/05 04:42 AM Re: How To Meg A Motor?
winnie Offline
Member
Registered: 09/15/03
Posts: 649
Loc: 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
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#60206 - 12/26/05 01:52 PM Re: How To Meg A Motor?
RODALCO Offline
Member
Registered: 12/08/05
Posts: 854
Loc: Titirangi, Akld, New Zealand
Thanks Winnie for adding that extra info.

cheers Ray.
_________________________
The product of rotation, excitation and flux produces electricty.
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#60207 - 12/26/05 02:14 PM Re: How To Meg A Motor?
AllClear Offline
Member
Registered: 08/22/05
Posts: 44
Loc: 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.
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It's Not The Fall That Kills You...
It's That Sudden Stop At The End
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#60208 - 12/26/05 02:24 PM Re: How To Meg A Motor?
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: 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.

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.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#60209 - 12/26/05 04:29 PM Re: How To Meg A Motor?
winnie Offline
Member
Registered: 09/15/03
Posts: 649
Loc: 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
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#60210 - 12/26/05 08:27 PM Re: How To Meg A Motor?
macmikeman Offline
Member
Registered: 07/16/02
Posts: 717
Loc: 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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