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  1. #1
    Join Date
    Oct 2011
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    trane RTHD series R High Motor Current

    Hi Guys hope you can help. Im working on a RTHD UB1U ,single discharge line ,water cooled, occasionlly faulting on High motor current.
    This occurs when the chiller is almost fully loaded , Amp draw is around 95-100% RLA.
    What happens then is I hear a Knock around the discharge of compressor and amp draws jump to 110%RLA and then unloads rapidly to about 70%RLA and starts loading again.
    I have limited the RLA to 95% . It hits 93%RLA and then sometimes knocks and jumps to 107%RLA instantly and unloads again to 70%.

    My concern is the condenser approach is higher than the other two larger rthds on site. Gets up to 2.5k while operating fine , although this is high already, when I hear the knock it rapidly climbs with the discharge pressure.
    I had the condenser tubes descaled and cleaned but made no difference.Standing pressure does not indicate non condensables.

    I was leaning toward the unloaders not operating correctly so carried out the CH530 unloader test.
    They load and unload manually but I got it to knock around 94%, jumped to 107% and as I didnt unload quick enough it tripped on High motor current.
    I have checked sensor calibrations and water flows. Running condenser water in at 28C.
    Evaporator liquid level goes from -6 to 6mm EXV 30%-40%.
    Any suggestions.
    Cheers

  2. #2
    Join Date
    Aug 2006
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    The knock is certainly curious. Of course, a “knock” to you could be a “ka-blam” to me. I would start with the actual pressures (using gages) to see if there is a sudden jump in pressure when the knock happens. I would also look to see how the loading and unloading goes...is it smooth or does it jump suddenly. Perhaps you have a sticking slide valve that is being called to move 1% and it doesn’t. So the controls ask for a few more percent and then the slide valve moves suddenly causing your large changes.
    "Right" is not the same as "Wise".

    Don't step on my favorite part of the Constitution just to point out your favorite part.

  3. #3
    Join Date
    Oct 2011
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    Thread Starter
    Hi jayguy, cheers for the reply, I am thinking sticking slide valve too as I got it to knock when I was manually loading around 94%.
    The discharge goes up rapidly with the approach when it knocks.
    Can the slide valve travel more than 100% and could the knock be the slide valve jumping to the end of its travel hitting the end.
    The only other thing I can think of is the check valve banging closed due to high discharge etc..Can these screws surge..?
    Its definnately coming from the discharge /unloader end internal of the compressor..
    cheers

  4. #4
    Join Date
    May 2019
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    Are you sure its not actually the bearings your hearing? I would not allow the machine to run past 75% until a Vibration Analysis is performed. Also don’t forget to check the machines Oil Return System operation (A lot of technicians tend forget how important a role the Oil plays in this type of machine). Good Luck!

  5. #5
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    Quote Originally Posted by Dicko100 View Post
    ...Can the slide valve travel more than 100% and could the knock be the slide valve jumping to the end of its travel hitting the end....
    i don't know if they can travel more than "100%" but if it did, it wouldn't be normal. my guess is that it is just sticking and then jumping. if you take control of the loading, watch to see if it does "jump" when you control the load and unload controls. that may tell you everything you need to know.
    "Right" is not the same as "Wise".

    Don't step on my favorite part of the Constitution just to point out your favorite part.

  6. #6
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    If the puck is sticking, then freeing up, that would prolly cause those symptoms.

  7. #7
    Join Date
    Oct 2011
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    Thread Starter
    Hi Guys , I went back to site today and carried out more tests and also monitored the operation of the slightly larger rthd that runs good.
    The larger RTHD definitely loaded and unloaded smoother.
    I noticed on the larger RTHD almost every pulse of the load solenoid increased piston pressure and simultaneously increased RLA% gradulely.
    On the smaller RTHD although every pulse on the load solenoid increased piston pressure, the RLA% sometimes would remain constant for about 5 pulses and then suddenly jump 10-15%RLA.. This happens when it reaches about 80%RLA .I am convinced the slide valve is getting stuck.I also checked load and unload settings on techview (time of each pulse etc)
    I also checked oil level and was half way up on reservoir. Evap oil return working ok.
    When the chiller shutdown though I noticed the discharge gas equalize back through the compressor..Maybe check valve needs replacing too.Going down the path of slide valve puck and check valve .Might be worth doing the solenoid stems too.
    Thanks for all your help guys.

  8. #8
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    Classic “Stick-Slip” issue. It’s normal for the discharge pressure to equalize in the off cycle.

  9. #9
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    Thread Starter
    When it does shutdown and equalize though it sounds like the compressor is running backwards.I would have thought the check valve prevents this.

  10. #10
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    Quote Originally Posted by Dicko100 View Post
    When it does shutdown and equalize though it sounds like the compressor is running backwards.I would have thought the check valve prevents this.
    That’s normal. If you replace the check valve, you’ll find it doing the same thing.

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