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  1. #1
    Join Date
    May 2020
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    Driving an old AC using VFD

    Hello guys

    I am thinking of driving an old 3.55 tons On/Off non inverter outdoor AC unit using a VFD. The unit has a scroll type compressor with 16 A rated current.

    What do you guys think, and how energy efficient would it be in comparison with the other brands systems that work on inverter.

  2. #2
    Join Date
    Sep 2005
    Location
    Atlanta GA area
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    A Scroll compressor will not pump efficiently when running too slowly (not sure what the base point it)...

    I do know the variable speed scrolls in residential units run up well above the 3600 RPM of fixed speed units...

    AND... not all motors are designed to work efficiently (or for that matter, mechanically work for extended periods) at different ROM's.

    Perhaps a good idea in theory, not a good idea in practice.
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *Cheap is not good, good is not cheap; however expensive is not a guarantee of quality!
    Choose your contractor wisely!

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  4. #3
    Join Date
    Jun 2012
    Location
    Northern NV
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    I looked at doing this also and biggest VFD I could find that'd run on single phase would only handle a 2T unit.

    If ya have 3 phase? Golden. Experiment away (on your own stuff) and let us know what you discover!

    And how are you planning on modulating the frequency according to load? My thought was a pressure transducer on the suction side. Kinda like the Bosch ODU does.
    “A dying culture invariably exhibits personal rudeness. Bad manners. Lack of consideration for others in minor matters. A loss of politeness, of gentle manners, is more significant than is a riot.”
    ― Robert A. Heinlein

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  6. #4
    Join Date
    Jul 2000
    Location
    Guayaquil, EC
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    14,366
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    Copeland's ZPV variable speed R410A scrolls have a brushless permanent magnet (BPM) motor and also have scrolls optimized for variable speed operation from 900-7000 RPM.

    https://climate.emerson.com/CPID/GRA...AEB/ae1407.pdf

    Some Copeland refrigeration scrolls like the ZS and ZF can be used with a VFD but are limited to 45-60 hz operation.

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  8. #5
    Join Date
    Sep 2005
    Location
    Atlanta GA area
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    This is un-related to HVAC...

    I have a metal working machine shop in my garage... both the lathe and mill have 3PH motors, run off separate Hitachi WRF200 VFD's... input is single phase 240V, output is 230V variable frequency.

    Now for the interesting part: Learned this when studying VFD's: Most motors are designed for a specific speed of operation... they do NOT like being run at variable speeds. Most 3PH motors on small machines will 'tolerate' 45-75 Htz... however they do not really like anything beyond 50-70 Htz...
    There are motors specifically designed to run variable speeds... they can run from 10-120 Htz... however they cost more...
    Leeson and Marathon make most of them in small HP ranges (1.5-2-2.5-3, etc)...

    Sooo... perhaps not a good idea to hang a VFD onto a single speed motor and expect it to work long term.
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *Cheap is not good, good is not cheap; however expensive is not a guarantee of quality!
    Choose your contractor wisely!

  9. Likes shamooooot liked this post.
  10. #6
    Join Date
    Sep 2018
    Location
    Mojave Desert California
    Posts
    142
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    Quote Originally Posted by ga-hvac-tech View Post
    This is un-related to HVAC...

    I have a metal working machine shop in my garage... both the lathe and mill have 3PH motors, run off separate Hitachi WRF200 VFD's... input is single phase 240V, output is 230V variable frequency.

    Now for the interesting part: Learned this when studying VFD's: Most motors are designed for a specific speed of operation... they do NOT like being run at variable speeds. Most 3PH motors on small machines will 'tolerate' 45-75 Htz... however they do not really like anything beyond 50-70 Htz...
    There are motors specifically designed to run variable speeds... they can run from 10-120 Htz... however they cost more...
    Leeson and Marathon make most of them in small HP ranges (1.5-2-2.5-3, etc)...

    Sooo... perhaps not a good idea to hang a VFD onto a single speed motor and expect it to work long term.
    Yes they are called inverter duty motors. That is the only type of motor rated for VFD use.
    The other issue with a VFD on a compressor is oil return. If the system runs at very low RPMs for an extended time you must drive it to a higher RPM for a minimum 15 minutes for oil return regardless of the cooling/heating demand. Also if the compressor is on a VFD the indoor blower motor must also modulate to match the refrigerant flow. So there are more controls and programming to think about.
    I was formally trained on the XV21 modulating Trane System. It would be fun to play with just keep in mind the additional programming required for long term use. Most of this could be achieved through PLC. Sounds super expensive but fun.

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  12. #7
    Join Date
    Sep 2005
    Location
    Atlanta GA area
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    I have the 20 SEER variable capacity AC systems on my home (AlliedAir, a Lennox clone)...
    The compressor, inside blower, and outside fan are all VFD driven, with lots of software and a proprietary thermostat to keep it all working.
    They are GREAT... however VERY complicated...
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *Cheap is not good, good is not cheap; however expensive is not a guarantee of quality!
    Choose your contractor wisely!

  13. #8
    Join Date
    Sep 2018
    Location
    Mojave Desert California
    Posts
    142
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    Nice... I think I will go all ductless once it’s time to retire my goodturd.

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