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case grounded compressor?

Hey everyone....I went on a service call yesterday and found the breaker tripping to the condensing unit....I condemned a faulty compressor ...I was not getting continuity from any terminal to ground...but I was getting resistance from any terminal to ground (I was getting 357 ohms)...my question is what exactly is going on in the compressor? Why want I getting the beeping from my meter on the continuity setting....and why was there resistance? Thanks Paul S
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Comments

  • Jamie HallJamie Hall Posts: 4,002Member ✭✭✭
    Somewhere in there...

    there is a moderate resistance short to ground, probably in a motor winding.  There should never -- like never! -- be a resistance less than 1 meg and preferably 10 meg from any terminal except neutral (if it's present -- it sometimes isn't, depending on the wiring) to ground.  Neutral should show at most a quarter ohm or so to ground.



    Different multimeters interpret "continuity" differently, but commonly they mean less than a few ohms.  Most would not interpret several hundred ohms as continuity.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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  • Paul SPaul S Posts: 513Member
    Meg

    So should I get a megaohmeter....would that be helpful in this situation...Paul S
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  • RJRJ Posts: 421Member ✭✭
    meter

    what setting is your meter on,   I recommend getting a megohm meter and re checking.   always check the elec circut.  for loose wiring or burnt comp. contactor contacts before condeming compressor. if it is single phase it could be a bad capacitor.    If your not getting contiuity to ground it may not be grounded
    RJ
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  • Jamie HallJamie Hall Posts: 4,002Member ✭✭✭
    Yes indeed

    burnt contacts, loose contacts -- bad cap. in single phase.  All possibilities.



    I do hope, though, that the case and frame are all properly grounded!  There must be a ground wire back to the switchboard (green or bare); this is, so far as I know, without exception a code requirement.  Neutral (white) if it's present, should show low or no resistance to ground.  Hot (black or red or both) should show one megohm plus to ground or neutral (if present) and to each other if there are both black and red when the unit is off and the fuses/breakers are pulled (don't play with this stuff unless they are!) -- but there is an exception to that: some more modern units have a computer board in them which draws power at all times, whether the unit is off or on.  That may call for a little more creativity in checking!
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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  • Paul SPaul S Posts: 513Member
    compressor

    The meter was on continuity and I wasn't getting a beep to ground....I always check contractor for voltage drop...the capacitor was a 40/5...I was getting that exactly...the breaker kept tripping...so I pulled the wires off the compressor and reset breaker it did not trip and I had voltage at the wires...them I set my meter to ohms...I got a reading from each terminal to ground of 357 ohms...so I condemned the compressor...Paul S...(condenser is six months old....what could of caused that?)
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  • TechmanTechman Posts: 2,057Member ✭✭✭
    edited July 2013
    grounded compressor

    Multi-meter that have a "continuity" checker ,that is for very low resistances. It is not for any kind of "hi resistance" readings. Many meters have two "ohm" scales ,lo & hi . The continuity setting will not work for testing for "ohms" . Then depending on the type of wire/winding problem in the comp motor the "short to ground" may be lo ohms or hi ohms, depending. Having a tripping CircuitBreaker w/ the comp wired in , and the CB not tripping with the comp wires removed sure indicates a bad comp motor.



    A Meg0meter is not an average tool for an average mechanic. So I would shy away from that meter for now.



    Jamie Hall, having a neutral  wire indicates a 115v system or a sophisticated system. Most residential comp motors are 1 phase ,220v , not using a neutral. On 115v comp motors , the Common, Start, and Run terminals are isolated from the rest of the wiring when testing ,so , the "neutral" feed of  the comp should not be grounded or shorted . The C ,S , & R terminals and the two windings ( S & R ) are isolated. Respectfully, Terry.
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  • Jamie HallJamie Hall Posts: 4,002Member ✭✭✭
    Just trying to cover all the bases

    " Most residential comp motors are 1 phase ,220v , not using a neutral.

    On 115v comp motors , the Common, Start, and Run terminals are isolated

    from the rest of the wiring when testing ,so , the "neutral" feed of

     the comp should not be grounded or shorted . The C ,S , & R

    terminals and the two windings ( S & R ) are isolated"



    Quite true.  I was just trying to cover all the bases... and perhaps confused the issue.  Sorry...
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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  • TechmanTechman Posts: 2,057Member ✭✭✭
    edited July 2013
    Grounded comp

    Jamie, or  I mistook your explanation . I read a lot of your stuff on Steam Heat. You are quite knowledgeable.
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  • unclejohnunclejohn Posts: 630Member ✭✭✭
    You weren't

    Getting a reading to ground because your compressor wasn't grounded. It was shorted. There is a difference.
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