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  1. #1
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    Controlling compressor speed based on suction temperature?

    http://www.youtube.com/watch?v=b4xb6EgEATg
    It seems to be a roundabout way of doing it when the more logical way is to have the thermostat adjust the compressor speed (as well as a timer for oil return) and have the evaporator fan adjust based on the combination of humidity and evaporator temperature.

  2. #2
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    I watched a few of your videos and just gotta say make sure you tell your Dad thank you for the opportunities and knowledge he has passed on to you. Pretty incredible the things you have already messed with and the knowledge gained. Pretty cool experiments.

    While I admit I haven't pulled up a chart just yet on all the 'standard' refrigerants, my gut tells me a fixed setpoint of 59 deg F would be a problem. Maybe I missed it on what refrigerant exactly you're using? While this is so simple it makes it very nice I would think for longevity of the system I would actually want to run the speed based on superheat. Granted if one is just going to leave it at a fixed setpoint with relatively stable conditions, etc then it may not matter much, especially given you have other safeties in place.
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  3. #3
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    http://youtu.be/2dg8J9TS7ps.

    Your idea is in the right place but as you can see this technology has been around for awhile at least on the mini splits systems but as things move forward I believe it will spread more into the central air larger systems there are already variable speed furnaces and air side units also solar equipment units. Most homes are all single phase so 3 phase compressor just ain't doing it for most homeowners and most don't have the assistants of an on site technician to help them thru the installation process , I could customize my own system with controls technology but once I sell my home the next buyer would have a hell of a time trying to make it work let alone fix it.
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  4. #4
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    Not to burst anyones bubble, saturated suction temperature as a value to control the variable speed of the compressor(s) has been a long and widely used practice in commercial refrigeration.

  5. #5
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    And controlling fan speed of an evaporator to dehumidify as well.

  6. #6
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    Gotta control condenser fan as well

  7. #7
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    and yes the condenser fans.

  8. #8
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    Quote Originally Posted by crab master View Post
    I watched a few of your videos and just gotta say make sure you tell your Dad thank you for the opportunities and knowledge he has passed on to you. Pretty incredible the things you have already messed with and the knowledge gained. Pretty cool experiments.

    While I admit I haven't pulled up a chart just yet on all the 'standard' refrigerants, my gut tells me a fixed setpoint of 59 deg F would be a problem. Maybe I missed it on what refrigerant exactly you're using? While this is so simple it makes it very nice I would think for longevity of the system I would actually want to run the speed based on superheat. Granted if one is just going to leave it at a fixed setpoint with relatively stable conditions, etc then it may not matter much, especially given you have other safeties in place.
    That wasn't my video. (Unless you were actually addressing the creator of the video, in which case he should join here and comment!)

  9. #9


    I see you have found my videos. I'm the creator of the video posted.

    This is the latest of the same unit. I show the pressures and temperatures this time and the air temperatures from the air handler

    I'm not trying to promote this as "new technology". I'm well aware that variable speed control and saturated evaporator/suction control technology has already been in place. I make videos just to show others how it's done (and how I prefer to do it). It's not very common in households, I know it's available in mini splits but there is no way I'd have one of those. The VFD takes care of generating 3 phases, so pretty much any 3 phase compressor will do. I originally did use a VFD for the condenser fan as well, but dropped it. I didn't really see much of a difference with it. The air handler already has a variable speed blower and a 2 stage gas valve.

    I did this primarily because I wanted to see how it would work. I've read so many places that VFDs and compressors don't work well in a residential installation. I just wanted to see for myself if that was true or not. So far it has saved a ton on the electric bill. That's what counts to me. Thanks for watching

  10. #10
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    Beware - oil return.

  11. #11
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    I would have at least added a 555 timer or something to force full speed for something like a minute every 10 minutes. Adding a few percent of hydrocarbons to the charge also helps oil return, but complicates recovery.

    Without seeing the whole setup, it's hard to tell just how much of an oil issue there is. It's pretty much all going to be on the suction gas side, since liquid refrigerant is a good solvent and hot discharge gas thins out the oil so that it carries easily. The evaporator tubes are relatively small, so while oil can definitely trap in them, it's far less likely than inside the large suction line. In that case, if much of the run is downhill to the compressor (as is common in residential systems), gravity would return the oil for you. Uphill runs are the most troublesome, and some rather unusual arrangements like doubled up suction lines and oil traps are used to fix such problems.

    There's a Shannon Liu quadrature drive out there that can effectively VFD a "single phase" motor by driving it as the two phase motor it really is. But since the run and start windings in such a motor are very different, the torque isn't as constant as a proper two phase or three phase motor, where the torque is theoretically constant.

  12. #12
    I should make this clear, the system does run from a thermostat, and the speed control is based on the suction line temperature right at the outdoor unit. So it follows a fairly normal on/off cycle.

    So each time the thermostat starts the VFD, the compressor slowly ramps up to 70 Hz, and stays there until the suction line temperature pulls down to the setpoint (usually 14 or 15 C ) This should help the oil return. This system also uses a piston type refrigerant control, which is intentionally oversized to allow greater refrigerant/oil flow

    If the outdoor or indoor temperature is high, that is reflected on the suction line, so the VFD will speed up accordingly. When it starts getting into the 80s, the VFD will usually run between 35 and 40 Hz, in the 90s, from 40 to 50 Hz. Never will it use full capacity (except at start up) unless a temperature of 10 C is commanded, which I find unnecessary. minimum frequency is set to 30 Hz.

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