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  1. #14
    Join Date
    Jul 2003
    Location
    Orange County N.Y.
    Posts
    1,176
    We have an oldish Biddle Megohmmeter, runs on 110v and produces up to 5,000vdc. Used it today on a big centrifugal motor, comes in handy when you want to know the condition of the insulation. It may even help make you look like you know what your doing.

    ...Ron
    Roof Rat

  2. #15
    Join Date
    Oct 2009
    Location
    aiken s.c.
    Posts
    173
    I recently used mine to find a bad condenser motor that read o k with a meter.I will admit that i don't use it often,but in this case it saved the day!

  3. #16
    Join Date
    Jul 2005
    Posts
    5,576
    Quote Originally Posted by aikenite View Post
    I recently used mine to find a bad condenser motor that read o k with a meter.I will admit that i don't use it often,but in this case it saved the day!
    Care to elaborate? Was it tripping the breaker?

  4. #17
    Join Date
    Oct 2009
    Location
    aiken s.c.
    Posts
    173
    On a carrier 50tc there are 4 condenser fans.They are fed by 15 amp fuses The fuses had blown several times and other guys would just replace the fuses and it would work for a day or two.I decided to find and correct the problem so I isolated each motor and megged the motors.1 motor gave odd readings compared to the other 3.We ordered a new motor and replaced it The unit has been working fine for a couple months.Like i said ,I don't do many start-ups so when I break out my megger I am looking for something unusual.By the way, the motor that was bad had a lot of oil inside as if the bearings were bad.

  5. #18
    Join Date
    Jul 2005
    Posts
    5,576
    Quote Originally Posted by aikenite View Post
    On a carrier 50tc there are 4 condenser fans.They are fed by 15 amp fuses The fuses had blown several times and other guys would just replace the fuses and it would work for a day or two.I decided to find and correct the problem so I isolated each motor and megged the motors.1 motor gave odd readings compared to the other 3.We ordered a new motor and replaced it The unit has been working fine for a couple months.Like i said ,I don't do many start-ups so when I break out my megger I am looking for something unusual.By the way, the motor that was bad had a lot of oil inside as if the bearings were bad.
    Repeated over amping and overheating of the windings could break down the insulation enough to show a difference in leakage current between that and the other identical motors. I wouldn't have thought of testing for bad bearings like that, but in hindsight the idea seems to have some merit despite its lack of conclusiveness. Thanks for sharing that.

  6. #19
    Join Date
    Oct 2009
    Location
    aiken s.c.
    Posts
    173
    I wasn't looking for bad bearings, that was just something that I did notice about the motor.As far as being conclusive the unit is working fine ,that's good enough for me.There are times when you're good and other times when you're lucky.I bought my megger at a pawn shop for around $40.00,but in this case I wouldn't take a gold quarter for it.When I started out I debated whether or not to get one but I couldn't pass this up.There haven't been many times that I have had to resort to this but I don't like to borrow things and it got the job done.Hope you have a great week!

  7. #20
    Join Date
    Oct 2001
    Posts
    6,966
    if the motors are 460v they have a higher rate of going nuke then 208V ones.might want to take a megger reading on a new one just to see if the others change over a season or on spring start up after a blizzard or two melts into the windings and those top bearings..
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  8. #21
    Join Date
    Feb 2011
    Location
    Orange County, in a Galaxy far far away...
    Posts
    277
    Checked grainger link. No longer in catalogue.
    Thinking of spending $600-$800 on a good one.
    Is it worth going upto a 5000vdc capable unit or not?? Will the readings be more accurate?

    Thinking Mitchell, Megger or aemc at the moment...

  9. #22
    Join Date
    Jun 2010
    Location
    Miami
    Posts
    258
    I love my old hand crank Biddle. If buying new today I'd go AEMC.

    Match the megger to the voltage classes you work with. If you work on 4,160 motors you need a 5,000 VDC unit.

    IMHO top uses for a megger are establishing trends and looking for problems that aren't obvious. Can be a time saver on shorts that come and go or are moisture related. It does require that one have an idea of what a pass/fail reading is. I would not call 100,000 ohms to ground "okay". What is okay on a 120 VAC motor isn't the same as what is okay on a 4160 motor.

    Also need to know when not to use one, such as a hermatic in a vacuum.

  10. #23
    Join Date
    Oct 2011
    Location
    Cicero, IN.
    Posts
    48
    I am required to do a Meg/Ohm test of the compressors on my annual inspections. Customer requirement. 2 Ton to 20 Ton. I usually do it at the contactor???

  11. #24
    Join Date
    Aug 2012
    Posts
    7
    Any rule of thumb on which voltage to do a test. Our megger at work has selectable voltages of 500,1000,2500,and 5000 to test with.

  12. #25
    Join Date
    Jul 2005
    Posts
    5,576
    Quote Originally Posted by LU91Tech View Post
    Any rule of thumb on which voltage to do a test. Our megger at work has selectable voltages of 500,1000,2500,and 5000 to test with.
    Typically around twice the supply voltage. For a 230 volt motor use the 500 volt setting, etc.

  13. #26
    Join Date
    Feb 2011
    Location
    Orange County, in a Galaxy far far away...
    Posts
    277
    I currently don't work on anything bigger than 460/480v.
    Is it worth getting a 5000v capable meg ohmmeter ?
    Really liking the AEMC stuff. 1050 model has my attention but not my budget :0(

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