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Comments on " meggers ".


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#1 Northern-Tech

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Posted 23 May 2006 - 03:44 PM

  I have seen several comments on meggers, in regards to testing compressors, motors, etc. From my experience using meggers, here is some general info, as well as some pre-cautions when testing compressors.

" Megger " is a general name for a meg - ohm meter, used to measure resistance of a motor, or compressor winding. (  By the way, there is also a company named megger, that makes the same. )  Generally, what you are doing, is imposing a voltage on the winding, to see if there are any breaks, or bad spots, which may arc to ground. These tests are done,  with power off, of course. The meter I  use , is capable of putting out 250 volt, 500 volts, and 1000 volts dc. You want to use a higher voltage, than the tested equipment normally runs at, so you can stress the winding and see if there are any bad spots. So if you are testing a 120 volt motor, 250 volt would probably suffice,....... on a 240 volt motor, 500 v,................ and a 600 volt motor  would be 1000 volts.  You would attach one test lead to ground wire, or  casing of the motor / compressor, and the other lead to the one of the motor  leads, compressor leads, or if a small compressor, then the pins on the compressor themself.  Sometime if the service tech. does not have a megger, he will try with his multi meter, which puts out 9 volts. If he can read that the winding is grounded, by putting 9 volts,  ( which is the meters output,) across the winding,  ....... its pretty safe to say....yup, its shot. Now what should a  good reading be ??

Motor manufacturers typically recommend that for each 1000 volt ( KV ) that the motor runs at, you have one meg ( meg - ohm ) of resistance, with 1 meg being the MINIMUM.  The higher reading , the better.....so infinity, ( a symbol, looks like  8 turned sideways ) would be the ideal reading, meaning that no voltage was able to arc to ground. Excellent !! :)If I am testing a 120 volt motor, then the one meg rule applies. If I am testing a 4600 volt motor ( using a high - pot, which is like a high voltage megger ), then 4.6 meg would be my minimum.

  However, taking one reading on a motor, and making a decision could be questionable, unless the reading is dead to ground. and there is no question that the winding is shot.  A borderline motor could continue to run normally. Practice in industry, is to take a test, every so often, and " trend " these readings. if there is a downward trend in the resistance, then the winding is starting to fail, and should be replaced.

 

Compressor manufacturers will have their own recommendations as to what to their specs should be. However, there are some cautions to be wary of when testing compressors:

1 - Do not power up, or megger  test a compressor while in a vacuum. :yikes:I have seen this caution many times, so its good practice to listen to. Apparently you can damage the winding while in a vacuum, and applying power.:shock:

Hoping this assists someone in their trouble shooting !!;)


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#2 Dan Webster

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Posted 25 May 2006 - 05:46 PM

I always use a big old hammer and slam the hell out of it if all else fails;)
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#3 kdog

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Posted 29 May 2006 - 05:02 AM

i had the pleasure when i attended my technical apprenticeship training years ago to be instructed by a gentleman who had spent most of his carreer as a motor rewind mechanic,for those of us who attended his classes,it was a great deal of insight into motor theory,and to boot; the guy had a natural ability to put all of these theories into laymans terms and understanding. we were treated to some pre ww2 equipment which included meggers,these units were actually operated by a hand crank! the pmmc meter movement would freeze at the breakdown voltage.  his take on it was this- meggers are to be used when winding the motors and the purpose is to test the actual epoxy/varnish coating on the windings,both to ground and to adjacent windings- the coating should be able to insulate for at least 2 1/2 times the designed voltage for extended periods; he was also quick to add that each time you applied a "megger voltage" you were comprimising the insulation value of the varnish,once you start a leak in the dam .................   for service purposes,use of a multimeter should be sufficient,yes most meters use a 9v sampling source,but this is not the factor that limits their ability to measure ground leaks; this is the meters "sensitivity" and is often overlooked and misunderstood by the owners of the tools- for basic motor servicing,one requires a meter with a minimum 20,000 ohms per volt of sensitivity,the higher the better(more sensitive),also ones understanding of using the highest scale and dialing down to verify readings is crucial. i personally bought the fluke 77 meter which is 50,000 o/p/v and is auto scaling,has never lied to me yet,have found ground leaks in a/c compressors to ground as high as 16k/ohms- any ground leak is unacceptable as the condition will only worsen and the windings should be totally isolated from ground. sensitive meters will also pickup small turn to turn winding leaks.
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#4 Northern-Tech

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Posted 29 May 2006 - 06:50 AM

  Ah yes.........the old hand cranks. Im sure most electrical shops, still have one tucked somewhere in the tool cabinet.  Still serves the purpose though. Probably not as accurate as some of the newer ones.




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