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  1. #1
    Join Date
    Jan 2004
    Location
    Fort Worth, TX
    Posts
    204
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    I have a compressor for RTU unit that is tripping its internal over load protection.
    Checked amp draw (7.2, 7.0, 6.8 / RLA 10.0)
    Checked ohms windings (4, 5, 5)
    Checked to ground ( Neg.)

    R22
    20 S.H. / 15 S.C.
    Discharge 220 psi / Discharge Temp. 160F
    Suction Pressure 65 psi / Suction temp 57F
    Liquid Line 198psi / Liq. Line Temp 114F
    Compressor shell exterior temp.....140F

    The only thing I didnt do so far is meg it out yet.
    Here is my question.........Little or no insulation on windings could cause the heat within the windinigs right?
    Please explain why if so. If not.......what else could cause the Internal Protection to trip?


  2. #2
    Join Date
    Nov 2004
    Posts
    2,928
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    Originally posted by brettln
    ....what else could cause the Internal Protection to trip?

    Low voltage and/or faulty starter , poor connections.

  3. #3
    Join Date
    May 2004
    Posts
    52
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    Talking

    Check the contacts on the contactor, they may be pitted causing intermittent unbalanced voltage.

  4. #4
    Join Date
    Nov 2002
    Location
    NJ - WORK IN NYC AREA
    Posts
    1,985
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    Wink

    As stated already, could be low volts. Ceck and tighten all connections and check contactor as stated.
    When checking amperage, watch it for a few minutes, does amperage go up and then down?? Is there a posibility compressor is starting to seize up or sticking ounce in a while?
    "My hands are for sale"

  5. #5
    Join Date
    Jan 2004
    Location
    Oregon
    Posts
    1,963
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    Maybe the bearings are shot.

  6. #6
    Join Date
    Nov 2004
    Location
    North Florida/South Georgia
    Posts
    1,001
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    more info please

    How long does it take for it to fail? What kind of unit? Do you find the overload open yourself or is there an interface thats telling you it failed? Is it the type that is incorporated in the windings or the type that is in the control circuit yet still in the terminal box? Is it a scroll or recip.? If its a recip. & you suspect its locking up from time to time you can reverse it. The overload works off of heat & heat alone. I doubt megging the comp. is going to tell you anything applicable to whats happening. If you do meg it you want 1 meg ohm per 100 volts min. Does the unit have a hp control? If so does it work properly? If there is no hp or you conclude that its not opening you could be having trouble with the cnd. fan.
    All my leon freaked out!

  7. #7
    Join Date
    Aug 2000
    Location
    Austell, Ga.
    Posts
    1,307
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    If that is an accurate Ohm reading on the compressor motor windings, you have some shorted turns.
    NEMA states an "Absolute" maximum resistance deviation of 5% difference on any "New" Poly-Phase motor, and 2.5% on a rewound motor.
    Assuming that 5% is the "Proper" resistance, then the "Maximum" allowable ohmic deviation would therefore be 1/4 ohm on a "New" motor.
    I assume you are using a wheatsone bridge or another source of obtaining accurate ohm readings.
    If so..."Ding-Dong the Witch is Dead"...
    You needs another compressor.
    If resistance readings were in order, and current imbalance was as stated, You should roll the phases and find the minimum current imbalance.

    Also..The 20f suction superheat is is excessive on this hermetic compressor...

    Also...To this lowly scribe....It surely does not seem fair that a "Rewound" motor from the local repair shop, be held to twice the precision as the OEM....
    Ain't "None" of us as smart as "All" of us..

  8. #8
    Join Date
    Nov 2004
    Location
    Anchorage Alaska / Seattle WA
    Posts
    205
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    Looks like Richard has pretty well hit it on the head. A 20% difference is ALOT when looking at the difference in ohms in winding in the same 3 phase motor. Id be pricing compressors - It probably would be a good idea to go ahead and change it out before it turns into a real bad burn down and is hard to clean up. Now it wont be bad - If you wait it will be a mess. She is talking to you - better listen...

  9. #9
    Join Date
    May 2002
    Posts
    43
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    I take it there is a good chance this is a scroll compressor. If the resolution of your ohm meter is not very discrete, the difference in 4 to 5 ohms may not be telling you very much. The possible cause is internal breakdown of thermistor circuitry wiring that is not totally open, but, the gas turbulence inside the compressor, or, shake at start up is enough to cause high ohms through the circuit with trip of motor protector. I have found this on a lot of Copelands. If this is somewhat erratic, try by-passing the motor protect module long enough toprove this out. If its an internal line break device, then you have a different story.

  10. #10
    Join Date
    Aug 2000
    Location
    Austell, Ga.
    Posts
    1,307
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    Okay Brer' Brettlin....It's been a week now, Howzzabout'
    letting us know what you found or did to correct your problem...
    Ain't "None" of us as smart as "All" of us..

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