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  1. #14
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    Dec 2008
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    Dixiana, AL
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    Quote Originally Posted by chillerguy81 View Post
    I'd like to know a answer to this too. I had heard a rule of thumb from somewhere that for accuracy you should meg motors at twice the voltage they normally run at. Never thought about it potentially causing damage to the windings though.
    "Megging at twice the operating voltage", I think, is a confusing issue and in my opinion there are some misconceptions about it. If you have a 3 phase 480 volt motor and you meg at 500 volts, you are applying twice the potential statically to each winding that is applied while operating (for all practical purposes). L1-N=277V, L2-N=277V, L3-N=277V. L1-GND during 500 volt test=500V, and so on. It don't hurt to meg a 480 volt motor at 1000 volts (or so I've studied), I just think the subject creates some confusion.

    Just my opinion also, is that meg readings need to be taken with a grain of salt. Charts and graphs will show deterioration of insulation, but they're not an end-all finding. Applying full voltage potential is a lot better than using that 9 volt ohmmeter, but even if you do go twice the applied voltage, there's still nowhere near the stress on that motor and it's components that you get when applying operating voltage to actually make that motor start and run. Some folks use the 1000 ohms/volt rule of thumb for spot checking, and there's nothing wrong with that figure. I just believe there should be a little more thought involved instead of simply using math and charts and calling it good.

  2. #15
    Join Date
    Sep 2008
    Location
    Western PA
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    25,567
    Quote Originally Posted by arcflash View Post
    I could not get access to the Copeland Thread. I am not a professional member, as of yet.

    Some Meggers have a range selection of Voltage, like 460 Volt and 220 Volt and 1000 Volt. The less expensive Megger testors adject the range automatically. If the compressor is 220 Volt, it might be easier on the compressor to use the 220 Volt Range.
    http://lvhvac.com/cope_bulletins/4-1294.pdf

    Try that link to Copeland AE bulletin.

  3. #16
    Join Date
    Jan 2009
    Location
    Indiana
    Posts
    3,228
    ok..im going to ask it...what/how is done during megging a compressor?...till i joined this site ive never herd the phrase.
    ill never stop learning

  4. #17
    Join Date
    May 2008
    Location
    Jax Fl.
    Posts
    1,943
    I have had problems with 2Es and 6Ls clearing expensive fuses, but would pass a low voltage meg/conductance test. I would borrow an ac [115v] powered "Insulation Tester" from our Motor Shop and could actually see the arcing from the internal motor leads at 750 volts [5/3 of rated voltage], which was their shop's test standard. Rerouting the leads on one actually cleared up the problem.

  5. #18
    Join Date
    Jan 2007
    Location
    Stongsville Oh
    Posts
    895
    motor would be off to meg, system would be equalized at that point.
    ckartson
    I didn't write the book I just read it!

  6. #19
    Join Date
    Sep 2008
    Location
    Delaware
    Posts
    243
    As to old farts - I resemble that remark. To prove it, my first (and only) megger was (is )a Biddle hand cranked unit that puts out 500 Volts. This is a service truck unit used for spot checks on motors. Doing saturation tests requires different equipment as does Hi- Potting. The rule of thumb I was taught is 1 meg per 1000 v of applied power. It has not failed me yet! The megging in a vacuum can be misunderstood. As mentioned in this thread we do meg motors in a vacuum on low pressure machines without any problems. So what is the issue? Megging in a deep vacuum or evacuated condition is the problem. A motor in a deep vacuum is like being in outer space - no air. Air is an insulator and if you remove all of it there is a good chance that you can get an arc between windings when power is applied. Refirerant and oil are insulators.
    Megging is another tool to help you but only if applied with an understanding of what it is telling you.

  7. #20
    Join Date
    Dec 2008
    Location
    Dixiana, AL
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    2,609

    Thumbs up

    Quote Originally Posted by Chiller Guy View Post
    I was taught is 1 meg per 1000 v of applied power. It has not failed me yet! The megging in a vacuum can be misunderstood. As mentioned in this thread we do meg motors in a vacuum on low pressure machines without any problems. So what is the issue? Megging in a deep vacuum or evacuated condition is the problem. A motor in a deep vacuum is like being in outer space - no air. Air is an insulator and if you remove all of it there is a good chance that you can get an arc between windings when power is applied. Refirerant and oil are insulators.
    Megging is another tool to help you but only if applied with an understanding of what it is telling you.
    My first was also a Biddle handcrank...........aaahhhh for the old days. I still carry a Biddle tach, none of that fancy non-contact stuff for me!! The megger gave up years ago.

    Your comments on megging in vacuum are exactly what I was driving at in post 12 (by the way, that was an excellent explanation). I'm not sure there is a simpler, yet more misunderstood task in this industry than what we're discussing here. And megging is akin to eddy current testing and vibration analysis - it's only a tool, not a rule.

    To yellowwirenut: megging is the act of taking an ohmmeter that will apply a higher voltage (250, 500, 1000>) than a standard ohmmeter (normally 9 volts, depending on the meter), and applying that higher voltage to your motor windings to test for breakdown of the winding insulation that would cause a short circuit under load. You can "meg" from winding to ground, or from winding to winding if you disconnect and isolate them from each other, such as in a 6, 9, or 12 lead motor.

  8. #21
    Join Date
    Mar 2004
    Location
    ottawa canada
    Posts
    2,045
    Chiller Guy explained the megging and vacuum thing well . The only other tip I can add is If its a solid state starter or speed drive make sure you dissconnect the motor from the starter or drive before megging or it will be expensive ?????
    The 64 roars to life Whoo hoo ...shes a rolling chassis .
    You bend em" I"ll mend em" !!!!!!!
    I"m not a service tech.. I"m a thermodynamic transfer analyst & strategic system sustainability specialist
    Best Austin Healey In Show twice in 2013 .....All those hrs paid off .

  9. #22
    Join Date
    Oct 2004
    Location
    ca
    Posts
    69

    Smile

    old fart rule of thumb is 1million ohms per 100volts to ground,80% of the time this will get you close to a good reading on most equipment, old fart say its close enough--pat

  10. #23
    Join Date
    Mar 2008
    Location
    philadelphia
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    347
    http://www.biddlemegger.com/biddle/Stitch-new.pdf
    This is a great little book to help anyone understand megging.

  11. #24
    Join Date
    Nov 2002
    Location
    NJ - WORK IN NYC AREA
    Posts
    1,447

    Question

    Chiller,
    I understand about the deep vacuum. At what vacuum do you check the larger machine's, and how low a vacuum before any damage can be caused?
    "My hands are for sale"

  12. #25
    Join Date
    Mar 2004
    Location
    ottawa canada
    Posts
    2,045
    Its not the level of vacuum that is the issue . It is dependant on the purity of the vacuum. If there is refrigerant in the machine then the refrigerant is the insulator .As has been stated its when the vacuum is pure there is no insulation from the refrigerant so you could fry the motor . You can megger a centravac motor if it is open to the atmosphere because the air is the insulator . In short you need to have a pure vacuum to create a problem when megging motors
    The 64 roars to life Whoo hoo ...shes a rolling chassis .
    You bend em" I"ll mend em" !!!!!!!
    I"m not a service tech.. I"m a thermodynamic transfer analyst & strategic system sustainability specialist
    Best Austin Healey In Show twice in 2013 .....All those hrs paid off .

  13. #26
    Join Date
    Jul 2006
    Location
    The Great country of Texas
    Posts
    429
    Megging is great for spot checking but is not good long readings. Temperature, the type megger and voltage play a big part in the readings. I try to sell my customers a AWA or Baker test on chiller motors. It is a complete test which megging is a part of. This type of test IS a way to trend a motors condition. How many of you have megged a motor in a moist environment and showed it grounded with a megger? I don't understand Copeland's thought of megging a scroll will damage the windings. it is a passive test. Do they have it confused with hi-pot testing which can be destructive?
    "I'm from Texas, what country are you from?"

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