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Thread: Megging!

  1. #14
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    Jun 2009
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    It matters on the compressor I have megged many only to find inconsistent reading due to refrigerant and oil even called copeland to ask about readings on a job I did which a new building lost one leg entering power and a bunch of compressors were grunting for about 15 min and got so many different readings and supposed bad insulation readings but haven't lost a compressor and followed with acid test was ok. Also if you megged the compressor on its first start then megg after warmed up diff readings due to liquid / oil etc

  2. #15
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    Jun 2009
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    Don't meg a low pressure centrifugal lol

  3. #16
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    Quote Originally Posted by pillclinton09
    Don't meg a low pressure centrifugal
    May I ask why you wouldn't? In fact, I'll tell you why you think you shouldn't...because the motor is in a vacuum. It runs in a vauum doesn't it? When people say not to megger a motor in a vacuum they should be saying in a "deep" vacuum. Meggering a motor in it's normal conditions will not hurt it. You can megger a hermetic motor at 15" Hg and it won't hurt it. The motor is designed to run in those conditions. Introducing low current DC voltage under the same as its operating conditions won't hurt it.

  4. #17
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    The reason they say not to meg a motor in a vacuum is because vacuum there is no atmospheric insulation between the phases and from phase to ground. Arcing can occur inside the motor. Even in a low pressure machine that's in a vacuum when it's sitting idle, the refrigerant is the atmosphere that helps to insulate the phases from one another and ground.

  5. #18
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    recommend megging low pressure centrifigul raise pressure to 1 psi

  6. #19
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    Quote Originally Posted by pillclinton09
    The reason they say not to meg a motor in a vacuum is because vacuum there is no atmospheric insulation between the phases and from phase to ground.
    I agree with this statement IF there is no refrigerant in the machine. When a megger test is normally performed there is refrigerant in the machine which concurs with your next statement.
    Quote Originally Posted by pillclinton09
    Even in a low pressure machine that's in a vacuum when it's sitting idle, the refrigerant is the atmosphere that helps to insulate the phases from one another and ground.
    So why would you go to the expense and trouble of raising the pressure to 1 psig?

    Say what you will. I worked for the manufacturer long enough to know what was acceptable. Like I said before the motors run in a vacuum which is lower than the pressure normally found when the chiller is idle. Meggering the motor in those conditions will not harm it.

  7. #20
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    Feb 2011
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    Chesterfield, Virginia, USA
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    If I get a questionable meg reading I take an acid/moisture test of the refrigerant to see if the reading is caused by contamination or winding insulation beginning to deteriorate.

  8. #21
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    Jun 2009
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    If you say so likely hood nothing will happen but it can happen and cause damage. Show me where your manufacturer recommends megging a centrifugal motor in a deep vacuum.

  9. #22
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    Aug 2009
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    Quote Originally Posted by pillclinton209 View Post
    If you say so likely hood nothing will happen but it can happen and cause damage. Show me where your manufacturer recommends megging a centrifugal motor in a deep vacuum.
    I'm pretty sure he was saying NOT to megger it in a deep vacuum. But under normal PM conditions (system off, normal charge), it is just fine, and a good idea to do so. A low pressure chiller should be kept at about 0-1psig when shutdown anyway.

  10. #23
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    Quote Originally Posted by KnewYork View Post
    Really? In what way? All you are doing is inducing DC voltage into the winding. You do want to make sure that you are inducing the correct amount of voltage. You don't want to megger a 240 V. compressor with 1000 VDC. General rule is 2.1 times the name plate voltage. One exception being on medium voltage motors (4160 VAC). They should be meggered with a more sophisticated megger that can deliver 2400 VDC for 10 minutes.
    Well, as I mentioned in my post I didn't know for sure if damage could be done.
    Our engineer was one who thought the insulation could be broken down. Its good to get as many knowledgable thoughts as possible because we're in the thinking stage at this point. We didn't start this as a P/M yet. And we will refer to manufacturer's recommendations as well. But as I always say, the experience of my peers is often more helpful than those who wrote the book!

  11. #24
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    Jan 2011
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    Quote Originally Posted by pillclinton09
    Show me where your manufacturer recommends megging a centrifugal motor in a deep vacuum.
    I'm sorry you can't read, pill. I specifically have stated at least three times that meggering in the vacuum they NORMALLY RUN in won't hurt them. I also said WITH refrigerant in the system. Is 8-14" Hg a deep vacuum to you? You can't get a low pressure centrifugal into a deep vacuum with refrigerant in the system.

    But if you want to be right...OK, you're right. You obviously know far more than I do. I guess my 27 years working for the manufacturer were all for naught.

  12. #25
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    Wish I still had my old wooden " jack in the box " megger.

  13. #26
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    Jan 2012
    Location
    Central New Jersey USA
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    Never had a problem megging anything low or high pressure myself in 32 years. I don't work on many systems that run the real low pressure refrigerants either (like R-11). Any system without crankcase heat or a sump heater will run a low meg reading by nature. Medium or high pressure refrigerants without crank heaters are going to be 100 to 200 meg cold. Most scroll compressors will be at 100 to 200 cold every time. The recprocating compressors with a long run of refrig tubing are the critical ones. Most should have a crank heater to keep any migration of liquid refrigerant out of the crankcase. This keeps the oil from blowing out the discharge line when the unit starts up. When the unit starts, the TXV is full open and "floods" liquid until it "throttles back" to proper level. The liquid "Flood back" can chase the oil out of the crankcase and cause lubrication failure. Megging a motor can help determine if the windings of the motor have been "beaten up" from any flooding, overheating or moisture in the system. I reccommend all reciprocating compressors be megged once a year. The larger screws and centrifugals do not need to be done as often if they have oil heaters. Oil analasys is best used on the larger units. If you meg a larger unit with a solid state starter, you should take the starter apart or you will damage the starter.

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