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  1. #1
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    Short cycle protection - how short are we talking?


    I'm converting a circa 2000 Hotpoint side-by-side refrigerator into a kegerator and fermentation bay for my homebrew beer. The fridge's original control circuitry was very simple, and had broken anyway, so I've completely removed it.

    To cut a long story short, I'm using a programmable microcontroller to control the compressor, but I think I need to add a means of short-cycle protection to the code, as I'm not 100% sure that the compressor itself has any provision for this built into its hardware, and it makes the rest of the software somewhat more easy to write if I don't rely on any hardware mechanism.

    Right now, my code causes the controller to wait 15 minutes between switching the compressor on or off. This causes a problem once the temperature in the keg side gets close to the target temperature, because turning the compressor on means that it's committed to running for at least 15 minutes, and it thus gets way colder than intended.

    So, I have two questions:
    1. For a typical refrigerator, how short does the minimum run/off time need to be?

    2. Should the minimum run time be different from the minimum off time? For instance, can I make it sleep for at least ten minutes, but only run for a minimum of three minutes?

    Thanks in advance.



  2. #2
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    Re: Short cycle protection - how short are we talking?

    Typical is 6 to 8 starts per hour. This also answers to your second question. As long as the compressor does not start more often than 6 to 8 times per hour, it doesn't matter if it starts briefly after being stopped.

  3. #3
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    Re: Short cycle protection - how short are we talking?

    Quote Originally Posted by NoNickName View Post
    Typical is 6 to 8 starts per hour. This also answers to your second question. As long as the compressor does not start more often than 6 to 8 times per hour, it doesn't matter if it starts briefly after being stopped.
    Agreed though perhaps also setting a minimum run time of 2 minutes for ensured oil return at times of sustained low loads would be a benifit.

  4. #4
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    Re: Short cycle protection - how short are we talking?

    Awesome, thanks guys.

    Would anyone care to speculate how the logic for "6 to 8 times per hour" is usually implemented?

    I can think of two approaches that would achieve that, as stated.
    The most obvious is more or less the way I have it now, where it simply imposes an absolute delay (say 10 minutes) between starts.

    I can think of another way, though, where you'd keep track of the average interval between start events, and impose a wait if the average interval was less than the desired minimum average. The microcontroller I'm using would make this a difficult thing to implement perfectly, but it seems like it would represent the truest and most flexible approximation of "6 to 8 times per hour."

  5. #5
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    Re: Short cycle protection - how short are we talking?

    There are multiple ways to implement it.
    The most complete is using three timings:
    a) minimum ON time
    b) minimum OFF time
    c) minimum time between starts.

    e.g. 1 minute minimum ON time, 1 minute minimum OFF time, 10 minutes between starts. If elapsed a + elapsed b >= c then start is enabled, else wait for c.

  6. #6
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    Re: Short cycle protection - how short are we talking?

    If this is a domestic style fridge then it would want to be off long enough for the pressure to equalise in the system, maybe 5 to 10 minutes. Domestic stuff generally has low starting torque motors that won't start just after stopping.
    It doesn't really have a minimum run time but 5 minutes may be a sensible figure.

  7. #7
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    Re: Short cycle protection - how short are we talking?

    Short cycle protection: when the compressor starts, even if it runs for a few seconds, it builds up pressure in the high side. If it is stopped, by any reason like power failure or somebody messing with the thermostat and then tries to start again against the pressure build in the high side, the compressor will stall and draw a high current that may burn the motor, the short cycle protection kicks in and prevents the compressor from starting for 3 to 5 min, until the pressure on the high side equalizes; protecting the electric motor from burning trying to start against the pressure on the high side.

    So short cycle protection timers set the compressor off for a period that could be between 1 to 10 minutes depending on the system, starting when the compressor shuts off.

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