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Thread: megged out

  1. #1
    Join Date
    Jul 2004
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    megged out

    I have a commercial crac unit going off on hi head, internal ol. I megged out 1 comp at 500v and got 170 on the 3 legs. I megged out comp at 500v 2 and got upwards around 385 on the legs. Im used to using the push button supco meter and looking to keep it in the green. Now i have a fancy meter that shows numbers. Can someone give me a crash course on megging numbers? Now to think of i, i should have used the higher voltage. (1000v) and run the test again. I pulled the fuses and am running on 1 ckt. Any help is apreciated.
    Happiness is the only good. The time to be happy is now. The place to be happy is here. The way to be happy is to make others so.
    -Robert Green Ingersoll

  2. #2
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    I don't think megging the compressor is going to clue you on why unit is tripping head pressure. Typically we look at megg readings to trend windings.

    Is this air cooled? What are your pressures? What type refrigerant? Head pressure control type?

  3. #3
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    Didn't trip on hp, hp is tied in with iol.

  4. #4
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    Call the manufacturer to find out what they say is the safe to start on their comp. socotech is correct. Megging, at least for me, is for trending the machines I service. If your in the safe to start range the initial megging of a comp isn't going tell you much of anything else. Oil samples, refrigerant samples, pressures, temps, amps, volts, etc will get you to the issue. Log the meg data you have to compare with future data.

  5. #5
    Join Date
    Sep 2010
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    67

    Meg IOL?

    Meg +250 good. Meg will show moisture of system and integrity of motor. If moisture on compressor body at terminals meg can read bad. IOL- amps? volts? load? head pressure? Connections?Contactor?

  6. #6
    Join Date
    Jul 2005
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    Quote Originally Posted by atufano View Post
    I have a commercial crac unit going off on hi head, internal ol. I megged out 1 comp at 500v and got 170 on the 3 legs. I megged out comp at 500v 2 and got upwards around 385 on the legs. Im used to using the push button supco meter and looking to keep it in the green. Now i have a fancy meter that shows numbers. Can someone give me a crash course on megging numbers? Now to think of i, i should have used the higher voltage. (1000v) and run the test again. I pulled the fuses and am running on 1 ckt. Any help is apreciated.
    Megger readings won't help you diagnose this fault. You should probably try to determine what caused the high head pressure. Even if it was the internal overload that tripped instead, that's often due to running for an extended period with high head. If that proves not to be the issue, then at least you've ruled that out and are one step closer to finding the real problem.

  7. #7
    Join Date
    Mar 2001
    Location
    Seattle
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    384
    Its generally accepted for a good megohm reading to be 1000 ohms per volt. It should be much higher than this though. As an example, you have a 460v comp, the megohm reading should be 460,000 ohms or .46 meg ohms minimum. If its below that copeland says its too low (for their compressors). You got 230ish megohms im presuming so that should be good. You should also remove the wires from the compressor or motor you are testing as good practice.

    I don't know how to attach a file, I just use a phone to get on the internet, but someone (rundawg) may have the file or you can search for it. I recomend reading "a stitch in time". Its long and some of it is about stuff we dont deal with but it will get you going on megging.

    I usually only meg larger compressors when doing maintenance, not the typical hermetic compressor. I also have a sticker I leave on the unit and record the reading there. That way I can trend the reading. One meg ohm reading dosent really say much but watching the readings over time could show you something. You could see a trend that the meg ohms continue down and you would be able to inform your customer that an oil change, rebuild or replacement will be needed soon.

  8. #8
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    Jul 2007
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    Quote Originally Posted by patrick View Post
    I don't know how to attach a file, I just use a phone to get on the internet, but someone (rundawg) may have the file or you can search for it. I recomend reading "a stitch in time". Its long and some of it is about stuff we dont deal with but it will get you going on megging.
    For those who can't find it on the internet, send me an email, and I will get a copy off to you.

    Even though it can be found on the internet, I don't post technical info in this "OPEN" forum.
    Instead of learning the tricks of the trade, learn the trade.

  9. #9
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    check this site, plenty of info about megging. http://www.biddlemegger.com/cgi-bin/...onfig=ent-apps.

    also look at service bulletin 4-1294 http://www.hvacrinfo.com/ae_index.htm

  10. #10
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    Jul 2004
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    Poughkeepsie, Ny
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    Update. Called URI tech support and they gave me the Copeland specs to check. Compressor dead. New one on order with 407C. All is well. Thanks for the tips everyone.
    Happiness is the only good. The time to be happy is now. The place to be happy is here. The way to be happy is to make others so.
    -Robert Green Ingersoll

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