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  1. #1
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    VFD on Compressor - how to control

    I have 4 10 ton package units with built-in VFD on in-door blowers. I think it's better to add VFDs on compressor> this will give the compressor the ability to unload when air flow reduced and heat load goes down.

    Half of my Units have TXV on it and others come with fixed orifice.

    • I understand VFD need a minimum speed to be safe, I will set it at 30HZ .
    • I also plan to run full speed every certain period of time, to spread oil around.



    My question is how should I control the speed? suction line temp or there is a better way.
    Any advise will be appreciated.

  2. #2
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    The Carrier 10 , 12 and 15 ton units that I have seen with VFD on them have two compressors in them.so it is a two stage unit.The VFD does a lower fan speed with one stage calling.When the second stage calls.the VFD increases the fan speed.I don't think it is variable, it is factory set aat a certain speed per stage.I believe the speed can be changed, but you have to have the VFD controller to reprogram the speed.I don't know why it would need to be reprogramed.However ,there are two different speeds for first and second stage.The older units with no VFd had only one speed, regardless of first or second stage calling.The individual compressors don't have partial loading or unloading,I don't think this is necessary.That is why there are 2 smaller compressors instead of one larger compressor.If only one larger compressor were used ,it would probably have unloaders.This is how the older units with one big semi hermetic were set up.

  3. #3
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    ramp fan and comp minimum/max speed together commission with professional HVACR technician
    Keep it simple to keep it cool!

  4. #4
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    Quote Originally Posted by windpoint View Post
    I have 4 10 ton package units with built-in VFD on in-door blowers. I think it's better to add VFDs on compressor>
    Half of my Units have TXV on it and others come with fixed orifice.

    • I also plan to run full speed every certain period of time, to spread oil around.
    Your post seems to say you have a VFD on the fan. But then the questions seem to ask about VFDs on compressors. Which situation do you have?
    Are these single stage or 2 stage equipment?
    Orifice says low tech/ low efficiency. VFD says High tech/ high efficiency. It is not usual to have both features in the same unit. Are these VFDs being applied as an add on? Or did the units come with these as part of the unit itself? The distinction is important, as I have seen these misapplied.
    Also, why are there drives? Is it to accomodate zoning? Usually variable speed fans are not there only to make the compressors happy, they are usually there for some other reason.
    Hmmmm....smells like numbatwo to me.

  5. #5
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    Thread Starter
    Thank you @gleng. They do have 2 compressors setup and single compressor also. The reason I dont like two stag setup is that the evaporator coil are split to two circuits.
    when unit runs at 1 stage, you have half coil cold and other half warm. It just doesnt work good. We basically run two compressors all the time.

    I think keep both compressor run at lower speed should be better. I have electrical background and have experience programming vfds. But I need some basically knowledge how those compressor should be controlled.

  6. #6
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    Thread Starter
    @numbawunfela , sorry for the confusions.
    The exiting system come with vfds on blowers. I am not sure aftermarket or original.

    I want to add vfds to the compressor. We have both 2 compressors and single compressor setups

  7. #7
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    yes and warranty depending on age may come into question also
    Keep it simple to keep it cool!

  8. #8
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    Thread Starter
    Quote Originally Posted by stanbyyourword View Post
    yes and warranty depending on age may come into question also
    5 years warranty on compressor only. That doesnt mean anything. We always have coils fail before that, which is not part of the warranty program.

  9. #9
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    Quote Originally Posted by windpoint View Post
    They do have 2 compressors setup and single compressor also.
    when unit runs at 1 stage, you have half coil cold and other half warm.
    I think keep both compressor run at lower speed should be better. But I need some basically knowledge how those compressor should be controlled.
    The issue is not that there are 2 circuits, it is that your evap coil is split-faced, and not interlaced. You can have 2 circuits with the right evap, but you do not have it. This tells me that your VFDs are applied as an add on, not from the factory. My reccomendation is to leave it with the mechanical guy to fix it and stay away. They chose the wrong unit to apply this way.
    Putting a drive on a compressor is ok with reciprocating and screw compressors. Scrolls do not like it. They have issues with oil getting pulled from the sump up to the scrolls. That is why variable capacity scrolls use valves and separating the scrolls to modulate, never a VFD. I am pretty sure you have scrolls, so a drive is out for you.
    Hmmmm....smells like numbatwo to me.

  10. #10
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    Pretty much exactly what this company does: http://www.bes-tech.net/digi-rtu/

    We looked at them but we are sort of sidelined at the moment as we take issue with voiding warranties like that, just superstitious.

    Why are you having coils fail inside of 5 years?!

  11. #11
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    this isnt vav Im assuming? if ramping down on load that should be doable within limitations probably around 50-60% ramping blower and compressor together. needs "yessir OK" from someone somewhere, and yes the prolonged running at low load will very likely be oil return issues, good luck
    Keep it simple to keep it cool!

  12. #12
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    Run them off of suction pressure. Temperatures will lag

    Sent from my SM-G960U using Tapatalk

  13. #13
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    What type of compressor? A discus may be slowed to around 25hz. A basic scroll 45-50hz.

    Sent from my SM-G960U using Tapatalk

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