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I am doing some experiments in fractional freezing with a modified residential deep freezer. I have programmed my own PID controller on an Arduino setup such that it will not cycle the refrigeration pump on and off too frequently to cause damage. This has worked okay but I would like to control the temperature more precisely.

I am open to any control ideas, but one I have been contemplating is running the single phase motor/compressor unit with a single phase VFD. Definitely wont have as much turn-down as 3 phase motor but should give me enough controllable range to make it work. Thoughts? Alternatives?

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  • $\begingroup$ Incorporate a temperature sensor. If using an rtd, don't power the rtd too often as it does heat itself... VFDs are great, but you may need to incorporate more of a vfd kit due to compatibility reasons; haven't done it for refrigerators but those air conditioner mini splits do often suffer inefficiencies with mix and matching parts from different kits. $\endgroup$
    – Abel
    Sep 28, 2023 at 12:26
  • $\begingroup$ You might consider telling us what you are doing, and the precision you are after, and the fractioning heat evolution as a function of temperature, if you have computed it. It would be nice to know you trying to make ice beer and not fractionating Fentanyl. $\endgroup$
    – Phil Sweet
    Jan 13 at 1:28
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    $\begingroup$ @PhilSweet, I didn't know it was used for Fentanyl lol. Not as interesting as beer either. Evaluating if fractional freezing can economically replace the evaporators in a beet sugar factory. Not sure what resolution I need, but definitly better than the on off control with a large thermal mass. $\endgroup$
    – ericnutsch
    Jan 14 at 8:53

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Probably the simplest thing is to use a hot gas bypass valve and run it off the controller.

https://hvacknowitall.com/blog/the-hot-gas-bypass-valve-explained#google_vignette

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  • $\begingroup$ Yeah thats a good idea. I will try the VFD first since it will be cheaper and less invasive. I will give this a try if the VFD doesnt work or if i need more turn down. Thanks! $\endgroup$
    – ericnutsch
    Jan 14 at 8:55
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Let model you system as first-order system with on-off control using PWM from PID controller. As switching frequency is limited due to hardware limitation. We can make system more precise by.

  1. reduce disturbance by using good thermal insulator.
  2. Use more accurate sensor with proper installation location or using multiple sensor.
  3. make system dynamice slower by reduce cooling power or increase thermal mass.
  4. make cooling air equally distributed to reduce temperature gredient.

If you can control cooling power either via VFD or mechanical valve, it's great as it can reduce ripple effect of on-off control.

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