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  1. #1
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    Jan 2015
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    Liebert Computer Room HVAC Units, DS Model

    Ran into a strange one yesterday on Liebert DS model, air cooled, iCom controls, CRAC. The machine locked out both Compressor 1 and Compressor 2 due to high discharge pressure. Nothing was found abnormal. Rest the machine per vendor manual (Power Off then back on to clear the hard lock out). The machine started and ran normally for about 15 minutes then compressors locked out again. Found the Lee Temp Condensing fan motor both off. Which would explain why the compressors locked out on high discharge pressure. Check High side voltage equal all 3 phases On both AC and Condensing unit, had 24 volt control signal at condenser and AC unit, checked fan motors good. Reset power supply to condensing unit. Reset power to HAVC units everything started and has been running normal for past 24 hours. No abnormal alarms or parameters seen in the service or user menus.

    Have any of you seen the condensing fans just shut off?

  2. #2
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    Jan 2014
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    Do you have any type of head pressure control on the fans, fan cycling ( on/off through a pressure stat) or a throttle type motor master type, possibly the calibration is off. Possible one fan motor is opening on internal overload after so much run time and just the one condensor fan can not maintain pressure especially when both compressors are on. Are the condensor fan motors open type, if so are the slots all free and not blocked up, you should have physically touched the body of both motors to compare temperature. Motors run hot , but if you spit on the body of the motor and it bubbles and evaporates quickly it's running to hot and tripping internally. You say it has been running fine for 24 hours, but is the OD temperature the same or cooler that day. Possible your coil needs a good cleaning along with being slightly overcharged. Possible the High Pressure Switch is out of calibration and tripping at a little lower pressure than it's suppose to, or a combo of any of the above. You certainly could be loosing control power somewhere and dropping out the fans. The fan blades are OEM type and the correct pitch and sits on the shaft at the correct depth? Fan motor(s) been changed prior? Correct RPM of motor, correct amount of blades on the prop.? Voltage at motors within manufacture specs. All electrical connection tight, no burnt wire tips if the motors have individual contactors are the points not severely pitted. Amperage draw within motor spec.

  3. #3
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    Thread Starter
    No with the Lee Temp option the fans run at a constant speed 100% of the time. Condensing fan motors cool to touch, compressors cool to touch, not off on thermals, coils clean

  4. #4
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    Dec 2013
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    Intermittent loss of control signal @ condenser is likely.

  5. #5
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    Aux contact on contactor failing or a loose wire


    Sent from my iPhone using Tapatalk

  6. #6
    Join Date
    Jan 2002
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    I would look at the receiver head pressure control valve. If it were hanging up I think you would see issues like this.

  7. #7
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    Sounds like a classic overcharge.
    Could it be that this happened on the first high ambient day of the season, then the weather cooled off and everything is fine again?

    Sort of a common thing with flooding coil arrangements especially if the charge was "adjusted" during the cold weather.

    You have probably checked the reciever sight glasses already- Just saying.
    "Iron sharpeneth iron..."

  8. #8
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    Thread Starter
    Yes the mechanical Head pressure control could give this indication but there is no electrical connection (Circuitry) that would prevent the fan from coming back on, but will check thanks

  9. #9
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    Thread Starter
    Yes these are odd to charge (also uses 407c). It went off on high pressure at 45F outside. As I said above does not explain why the fans did not come back on. With LEE Temp (free cooling) Condensers the fans run full speed 100% of the time. Head pressure is controlled with a mechanical valve. Thanks for the input

  10. #10
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    Quote Originally Posted by Answer-Man View Post
    Yes these are odd to charge (also uses 407c). It went off on high pressure at 45F outside. As I said above does not explain why the fans did not come back on. With LEE Temp (free cooling) Condensers the fans run full speed 100% of the time. Head pressure is controlled with a mechanical valve. Thanks for the input
    I see.
    So there is not a p66 on fan number one?
    "Iron sharpeneth iron..."

  11. #11
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    Thanks Answer Man.
    How does the Lee Temp accomplish free cooling as you said?
    "Iron sharpeneth iron..."

  12. #12
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    Thread Starter
    In the winter months (like -10F) there are small pad heaters in two large accumulators on the side of the condensing unit. There is a mechanical head pressure regulator which maintains head pressure. There is enough refrigerant flow to maintain cooling in the Computer room without the compressors running. I'll see if I can get the IOM posted for you maybe a day or two with the holiday

  13. #13
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    Dec 2003
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    Hi Answerman,
    Thanks for offering to send the IOM but I think I already have a copy.

    You are in over your head and you are definitely offering wrong answers at this time.

    The Lee Temp has nothing to do with free cooling and everything to do with low ambient operation to maintain the TXV MOP.

    Please make spare time to read over the IOM on your own.

    We definitely need men with guts like you in our trade that see the value of supplying correct answers.

    Liebert offers some great classes for those of us in need that are willing to spend the time and money to be able to speak and offer assistance with gentle authority.

    Best to you.
    "Iron sharpeneth iron..."

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