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  1. #1
    Join Date
    Aug 2009
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    Beatrice, NE
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    Compressor reads like a capacitor?

    I had gotten a call from these people 2 weeks ago an a Saturday and set up the appointment for the following Monday. That Monday I get a call first thing in the morning the people that live in the house had called someone else over the weekend and he filled it with R-22, house is a rental. So fine, win some lose some. I get a call Wednesday that the unit is now blowing fuses. Set the appointment for this AM and go out there. The unit is an old AM Std. with an AH5540E comp in it. The windings checked fine, normal readings winding-winding, but when I checked winding to ground is when I got the strange reading. All three terminals to ground, no wires connected would read high resistance, greater than 1M, that would continually rise as long as the meter {Fluke 289} was connected, like when you put a ohm meter across a capacitor.

    I know the compressor is dead, it needs to go anyway. I have run across this before and just wanted some clarification. I have heard that liquid in the compressor can cause this. When I have found this in the past it was always in the winter on a heat pump, so liquid in the compressor would not be a stretch.

    Could the guy ahead of me have grossly overcharge the unit and caused the failure. Or was it just time for the compressor to go? Can anyone else verify or have you seen this before?

  2. #2
    Join Date
    Jul 2013
    Posts
    63
    Sounds like he may have.

  3. #3
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,145
    Unless you completely unhooked the compressor it could be reading through the wiring.

  4. #4
    Join Date
    Nov 2006
    Location
    Athens, Ohio
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    1,694
    Quote Originally Posted by 54regcab View Post
    Unless you completely unhooked the compressor it could be reading through the wiring.
    He does state "no wires connected."

  5. #5
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,145
    Quote Originally Posted by kdean1 View Post
    He does state "no wires connected."
    I'd double verify no wired connected. Odd a compressor would read like a cap.
    Liquid makes no sense why resistance would rise like a cap, but I've seen stranger things..

  6. #6
    Join Date
    Jul 2013
    Posts
    63
    Hold on. Is refrigerant conductive? I don't quite understand that. I didn't fully read. Meter leads? If it had resistance to ground that's bad no matter what.

  7. #7
    Join Date
    Aug 2009
    Location
    Beatrice, NE
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    1,975
    If the refrigerant mixes with the oil and the oil reach the windings, isn't that similar to the old oil filled capacitors? That's the only thing I can figure. Wires were off the compressor terminals, one lead of the test meter was clipped on a terminal the other lead was clipped on the discharge line, so fingers were not involved either. Similar reading on each terminal.

  8. #8
    Join Date
    Sep 2002
    Posts
    12,195
    Try heating the compressor while evacuating it - probably has some high PPM moisture in it and maybe some acid starting.

    Why does the compressor "need to go" if the windings are OK and they are also over a million ohms to ground?

    What is it exactly about the compressor that is "dead"?

    PHM
    ------




    Quote Originally Posted by kls-ccc View Post
    I had gotten a call from these people 2 weeks ago an a Saturday and set up the appointment for the following Monday. That Monday I get a call first thing in the morning the people that live in the house had called someone else over the weekend and he filled it with R-22, house is a rental. So fine, win some lose some. I get a call Wednesday that the unit is now blowing fuses. Set the appointment for this AM and go out there. The unit is an old AM Std. with an AH5540E comp in it. The windings checked fine, normal readings winding-winding, but when I checked winding to ground is when I got the strange reading. All three terminals to ground, no wires connected would read high resistance, greater than 1M, that would continually rise as long as the meter {Fluke 289} was connected, like when you put a ohm meter across a capacitor.

    I know the compressor is dead, it needs to go anyway. I have run across this before and just wanted some clarification. I have heard that liquid in the compressor can cause this. When I have found this in the past it was always in the winter on a heat pump, so liquid in the compressor would not be a stretch.

    Could the guy ahead of me have grossly overcharge the unit and caused the failure. Or was it just time for the compressor to go? Can anyone else verify or have you seen this before?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  9. #9
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    1,975
    With the compressor out of the circuit if you energize the unit the fuse hold. You put the compressor back in the circuit and as soon as it is energized the fuse goes. So you couple that with the high resistance to ground and it's dead. Then you add in that it's a 1960 model that has not had good care and it needs to go.

    I have had other compressors with this type of reading, after you get it out and it sits in the shop for a while and recheck it test defiantly bad. But it still leaves the question as to why it tests this way in the unit.

  10. #10
    Join Date
    Nov 2011
    Location
    St. Louis
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    3,208
    Yes, liquid refrigerant in the compressor can give a false reading when checking for a grounded compressor. But if you have a reading of over 250k (as in a quarter million ohms), are you sure it's grounded?

    Could be a mechanical lock up blowing the fuse?

  11. #11
    Join Date
    Sep 2002
    Posts
    12,195
    I told you the likely reasons why it may test as you state.

    What I want to know is what about it exactly is opening the fuses? If the windings show the proper ohms and the resistance-to-ground is within the limits you state - what is wrong with the compressor?

    PHM
    ------




    Quote Originally Posted by kls-ccc View Post
    With the compressor out of the circuit if you energize the unit the fuse hold. You put the compressor back in the circuit and as soon as it is energized the fuse goes. So you couple that with the high resistance to ground and it's dead. Then you add in that it's a 1960 model that has not had good care and it needs to go.

    I have had other compressors with this type of reading, after you get it out and it sits in the shop for a while and recheck it test defiantly bad. But it still leaves the question as to why it tests this way in the unit.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  12. #12
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    1,975
    With the compressor in the circuit it will clear the fuse before the fan can start. Take the compressor out of the circuit and the fan runs fine. If it's not the compressor what is it? That's the only component in the system that even shows any kind of reading to ground. We're talking simple 1965 A/C, compressor, fan, 2 run caps, and a contactor. Even if the readings are suspect the process of elimination says the compressor is bad.

    On a unit this old is it really worth recovering the refrigerant, pulling a vacuum while heating the compressor, putting in filter driers assuming moisture and then recharge with fresh R-22 so as not to re-introduce moisture. And that's not counting doing the repair for why refrigerant was added in the first place. There either has to be a leak or a restriction i.e. bad TXV, plugged cap tubes, etc. I don't know why refrigerant was added in the first place, the owners of the rental said they didn't think he knew what he was doing. I don't know who it was and really don't care. The thing is broke now and the cost of digging into this thing would mean putting a lot of money into an old unit that's seen better days.

  13. #13
    Join Date
    Nov 2011
    Location
    St. Louis
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    3,208
    I don't disagree at all that any significant repairs to a '65 system is likely unwise. I was born in '65 and I'd have to think about any significant repairs to myself. I'm sure my wife would opt for a new unit; even if she could get a pump on the side for cheap.

    Hell I'd urge replacing a 48 year old system even if it was working ok. Again, so would my wife.

    I only raised an eyebrow over the ohm readings. Assuming incoming voltage is within 10% of data plate: A million ohms from winding to ground would indicate that the winding is not shorted to ground. The old RSES ROT is anything over 1k ohms per volt is passable. At 240 volts give or take, and 1k per; the compressor would pass the short to ground test at a quarter of a million ohms from winding to ground.

    If there is measurable resistance from common to run and common to start, and start to common is 3-6 times as much as the run winding it would pass the winding to winding, shorted and open winding tests.

    Which leaves mechanical failure. The two most common causes of breakers or fuses tripped immediately upon start up are windings shorted to ground and mechanical lock up. An amp clamp on the start winding might not be fast enough to catch it if it's blowing right away. But I'd hazard the latter based on the resistance readings you provided.

    That said, I just read an entry in the MSAC technical Q&A of the July RSES mgazine. The tech replaced a Carlyle compressor in a Flotronic chiller and it blew up the contactor in a few days. He got the replacement compressor, took the resistance readings and they were identical to the suspect compressor. So he just changed the breaker and the contactor - which he pushed in with a 4' pole and it immediately blew up. He then went ahead and changed the compressor and everything ran fine.

    So if incoming voltage is within spec, winding to winding and winding to ground all check ok, and all wiring to and from is corrosion free and not providing any meaningful resistance...it has to be mechanical seizure of some nature.

    Right? I'm really asking...not telling.

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