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  1. #1
    Join Date
    Jan 2010
    Location
    Toronto,Ontario Canada
    Posts
    355

    Using a megger for hvac/r

    How many people here actually have a megger in their trucks and how often is it being used. i have a family member that says he uses it all the time in his jobs (of course hes a electrician) is it really worth buying one. or is ohming out a motor just as easy?

    thanks in advance
    Akelesis
    Get er Done!

    Do what has to be done
    when it has to be done
    as well as it has to be done
    And doing it all the time.

  2. #2
    Join Date
    Feb 2011
    Location
    Orange County, in a Galaxy far far away...
    Posts
    277
    I'm keen to hear the input on this thread. I could've used a megohmeter on 3 separate occasions this week, anybody on here got thoughts on brand, price functions etc?
    You cannot cheat an honest man. But that doesn't stop people trying!

  3. #3
    Join Date
    Aug 2012
    Location
    Hot Springs, Arkansas, United States
    Posts
    55
    A good megger test can give you a heads up about a potential future problem. A good multi-meter is great for checking motor windings, bad connections, caps ect... I have seen a compressor fail a megger test and run for years.

  4. #4
    Join Date
    Oct 2001
    Posts
    6,966
    ohm'n out a motor is a different animal the megging out one.if you held the lead on a megger and pushed that red but it would take you off you feet,meggers test the insulation on the windings and the reading is the leakage of that charge in testing.big tonnage OEMs design their chiller for so many starts when the equipment sees 10X the rated amperages on start.the windings actually move and rub together...number of starts/hours run are a major log input with building engineers...100,000 ohms to ground is mint... solid...tight...new windings on any motor.as that number drops over the years 75,000>50,000 a burn out situation is in the future..and there is nothing a tech can do to fix that reading it being useage run time,to many restarts..add air within the system.. burn out clean up...water into chiller and that drops that numer also...
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  5. #5
    Join Date
    Feb 2011
    Location
    Orange County, in a Galaxy far far away...
    Posts
    277
    Maxster, any input as to a good "megger" for starting out? I work on everything up to 50tonnes at the moment.
    You cannot cheat an honest man. But that doesn't stop people trying!

  6. #6
    Join Date
    Oct 2001
    Posts
    6,966
    Grainger item # 3T243 is an industry standard that all the manufacturer tech have for the big tonnage stuff...how important are the readings always remeber calling the factory engineers witha start problem or tripping...you better have that megger reading written on you hand or they will blow you off as a clown...very important.if you have windngs breaking down it will show up on that contactor especially if your doing a PWS the start will take the lock rotor starting amps.this item is a company suplied item.go cheaper if your doing the buying it will give you good results i only used Amprobes for years.that 5ZK90 is a good price and it is Amprobe also there.rooftop companies rarely use them but if you do any chillers it is critical with the possibility of water within the lines..getting on those windings...i had a hellical screw air cooled chiller in the dead of the winter and the CCH took a crap the compressor read a dead short building engineer pushed the start contactor in and ba boooom...started a fire in the panel.replaced the CCH the compressor read 100K to ground good to go...cold frames as with hellicals or actual cold oil sumps..can also give you a dead shorted compressors heat the oil up and your back up in that 80K range to ground....great for large fan motors.WARNING never ever meg with a compressor in a vacuum...you'll short out the termistor winding sensors...that is a bad call to your SM...
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  7. #7
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,664
    I use 1 daily. You have to learnwhat the readings are, for example a wet fam motor or pump motor may read bad, but its not.

  8. #8
    Join Date
    Oct 2001
    Posts
    6,966
    some construction sites especially basements are so humid you can read a dead short but is the soaked air in and around the motor that your reading.i had to go out side of a site to read fuses with just a OHM meter cause they were reading good but were blown with the humidity within the place.another Megger tip bigtime is if you have a burn out install a new compressor the acid tripping back to the oil sump and collecting can cause a wierd megger reading 30K to ground..then as you clean it up just flipping oil changes...the reading will climb up into that 75K acceptable area...
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  9. #9
    Join Date
    Aug 2012
    Location
    FL
    Posts
    18
    Hello guys Quanchai sent me to ARP he said that you are a very good group. My personal favorite Fluke 1587, It will test insulation from 250v to 1000vdc. It is important to know the range that a motor it would be consider bad, for example I saw that a costumer tagged a 50 hp motor grounded, I megged it it had 1 ohm to ground reading on all legs, there was no burned smell. I decided to brake out the heat gun and warm up the windings, after 30 minutes the motor readings past 100 mega ohms to ground all legs.That is a good motor ,the moisture in the motor made it read grounded. There is heaters that can be added to maintain temperature in the windings and trickle voltage if the motor has a vfd. Don't meg a motor under deep vacuum or with scr's connected.

  10. #10
    Join Date
    Jul 2005
    Posts
    5,571
    Quote Originally Posted by akelesis View Post
    How many people here actually have a megger in their trucks and how often is it being used. i have a family member that says he uses it all the time in his jobs (of course hes a electrician) is it really worth buying one. or is ohming out a motor just as easy?

    thanks in advance
    Akelesis
    If your job doesn't demand the use of a megger, then you most likely don't need one. If your intent is to use it like an ohmmeter, then just use an ohmmeter.

    BTW, yes I have one, but only use it on electrical jobs. I'm an hvac tech/electrician. I don't use it on compressors.

  11. #11
    Join Date
    Jul 2012
    Location
    Western KY
    Posts
    1,221
    I have 2 on the truck, old and new, and I use it regularly. Part of PM checklist on most industrial accounts. Most industrial clients would rather spend the money to replace a motor before it fails then wait and replace a motor or comp when it fails and shuts down production. Keeping track of starts, runs, and megging will let you know when it's time to start budgeting for that motor during the next main shut down.

  12. #12
    Join Date
    Aug 2006
    Location
    Mixing oil and fire with a big spoon.
    Posts
    4,388
    i have a personal one and the company owns one (it's a little nicer!). maxster...i think that your numbers are low. 100k is mint? my Amprobes smallest measurement is 0.1 Megohms (100,000). I use for air cooled motors, >100M (100,000,000 ohms) is ok, under 100M then you should really start paying attention. for refrigerant cooled compressors, the number is closer to 5M because the oil will mess with the numbers.

    below the above numbers does NOT mean that you need to replace the motor as others have said...it means that you need to start paying attention. the generally accepted 'GO/NO GO' numbers are: Voltage times 1000....on a 460 VAC motor, the GO/NO GO number is 460 x 1000 = 460,000 ohms or about 0.5 Megohms...below this number, I would not even think about starting the motor.

    i use a megger on ALL motor PM's...it is very easy and tells so much.
    When I am late for work, I usually make up for it by leaving early.

  13. #13
    Join Date
    Jan 2005
    Location
    Houston TX
    Posts
    875
    I use mine any time the need arises, such as annuals, motor issues, checking out wire runs in conduit and the presence of moisture to any of the above. On annuals it goes along with an oil sample and vibration analysis. This way you can trend a system, compressor or motor.-GEO
    Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO

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