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  1. #1
    I encontered something strange today. Upon arriving at a sevice call on a 6 ton trane condensing unit, (208/230 3 phase w/ a CRHL compressor, unit model #' N/A) the compressor appeared to be on overload. This unit from past experiences has a history of high head pressure issues limmiting the correct charge of refrigerant the system requires. Not bothering to ohm out the compressor, I cooled the shell down briefly, and ohmed the windings shortly after. Once it appeared to be off overload the unit was turned on, nothing, I measured voltage after the contactor and had full potential. An amp draw reading was taken when the unit was energized to assume a possible locked rotor... nothing, in fact the amp reading was odd, 6.5, 4.5 at start. My general question would be is their some type of limmiting overload that isnt showing me an indication at the windings? Unfortunatelly The meter I was using doesnt have a detailed ressistance scale and couldnt propperly ohm the compressor out.I encountered this a while back on another system and decided to return the next day once the unit was off overnight, the unit started back up properlly and I'm begining to assume the same for this unit. Any info would be greatly appreciated. Thanks

  2. #2
    Join Date
    Sep 2000
    Location
    Miami, Fla. USA
    Posts
    417
    Those amp readings are questionable. Are you sure the comp. did infact start or were you reading the fan motors?

    " I cooled the shell down briefly, and ohmed the windings shortly after. Once it appeared to be off overload."
    Did you actually read a resistance value or you assumed that the internal T/stat had reclosed?

    Why is the head pressure high?

  3. #3
    Join Date
    Sep 2000
    Location
    Miami, Fla. USA
    Posts
    417
    Oh, I see where you had the wrong meter. Let me guess you have one of the ampmeter that does everything except critical values?

  4. #4
    I dont see much advice here. the head pressure is an issue beside itself which I am not concerned about at the time. The compressor is not starting. There is no audible straining sound from anywhere withing the compressor. I am aware I am using a multi-meter without an adjustable resistance scale just as I indicated in my initial post. My question once again.. is there some type of limmiting thermal overload incorperated into the windings that is showing me a resistance to all three but at the same time preventing the compressor from starting?.. It is a CRHL compressor from around 89'. Any info would be greatly appreciated... Thx

  5. #5
    Join Date
    Feb 2004
    Location
    Madison, WI
    Posts
    292
    I'm not 100% sure what you're asking here but you should try to ohm all 3 legs phase to phase and to ground to ensure proper readings.
    On the amp draw it is a little suspicious that one phase read 6.5 and another 4.5, but what did the 3rd phase read?
    What are your voltage readings between phases? You could have high resistance on one pole in the contactor or some other unbalanced voltage condition.
    Compressor overloads typically pop open due to high motor winding temperatures, deactivating all phases until it cools down. I've never heard of a "limiting" overload protector.
    Good luck.

  6. #6
    Join Date
    Oct 2003
    Location
    edgewood nm
    Posts
    73

    compressor

    High pressure is an issue here. Those trane units are designed on very tight specs. Those coils have to be clean or all kinds of problems occur. Dirty coil leads to high head, to high superheat, to no cooling to windings, to high temps in mechanical region of compressor, which makes windings work harder, which lowers capacity, which makes superheat higher, etc, adnausium. Then, ba da bing you have an open overload or a thermistor embedded in the windings that won't cool off no matter what you do. Those compressors have a thermistor embedded inside each winding. And I found that nothing will cool them down but time. That's why they run the next day for you. Another cause besides dirty coils or low charge would be cheap installs without cch's. This causes eventual mech. failure which will cause the above problem also.Cleaning coils; it's a crappy job but some one's got to do it.

  7. #7
    I am well aware of the theoretic balance of high head pressure and its effects. However I'll say again, I was more concerned about the ressistance reading across each winding, but if there is a internal thermistor opening the circuit as explained in one the previous posts, that may lead me to where I was going/thinking. The unit is past its prime and we will be doing a change out very soon, the fact that I read ressistance through the windings but had no success starting the machine had me wondering. I had a suspicion there was something going on. not being able to check its true ohm ressistance value had me at a disadvantage, but suspected aomething out of the ordinary. Can anyone tell me generally if an internal thermistor was standard on other equiptment? from what time frame was it popular? was this type of protection used on other compressors besides oem trane's?

  8. #8
    Join Date
    Jul 2002
    Posts
    897
    You've probably got a blown internal pressure relief (you said it had head problems) . Sounds like the comp is starting but not pumping. Feel it and put a screwdriver to it to listen. Could also be either a broken crankshaft or wasted valves. I've seen all three.

  9. #9
    Join Date
    Aug 2004
    Posts
    37

    Maybe this can be of some help.

    In that situation I would of checked the supply Voltage From 1 phase to the next. Givin a 5% tolerance of one another would confirm normality of supply. As far as there being an internal overload, that is very possible. I've worked on a 3phase motor where there was an external plus the internal overload in series with the windings. Sounds to me that you need to get a megometer and check to see if there is an insulation burn out on the windings which is causing different amp readings on your windings.

  10. #10
    Join Date
    Feb 2004
    Posts
    534
    I see you took amps draws did you happen to install a set of gauges, what are the pressures sounds like compressor is running but not pumping also is the compressor loud while running.

  11. #11
    Join Date
    Aug 2004
    Location
    washington dc
    Posts
    13

    Lightbulb

    first you need to decifer what is overloading compressor.
    is it liquid flooding,starving the compressor of suction
    gas needed for cooling the motor,high superheat? Then you
    need to invest in a megohm meter used to properly ohm out
    a compressor motor windings.Trane compressors have a
    reliable overload installed in the motor and would not cause
    such a reading. you may have readings down to 0 ohms on any
    trane compressor and still have a good compressor.trane recomends that you run the compressor until the oil sample
    shows signs of wear and tear. most meters on the market are
    not equiped to perform proper megohm readings unless you go
    analog or use a higher priced fluke meter designed for this.
    I think you might have another problem in the system you may
    have overlooked.

  12. #12
    Join Date
    Aug 2004
    Posts
    7
    In most semi herm. comp. there are intenal winding temp safetys.

  13. #13
    Join Date
    Oct 2002
    Posts
    970
    Pristine, are you jusy getting started in this trade?
    I work with the Chiller Whisperer...

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