Author Topic: refrigerator defrost cycle and load-sense inverter  (Read 7229 times)

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vtpeaknik

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refrigerator defrost cycle and load-sense inverter
« on: March 12, 2013, 10:20:44 AM »
The days are getting longer now so I decided to start running the fridge on solar power again.  I had it and the chest freezer do that most of last summer.  This is with a small PV system with batteries and a true sine wave inverter.

After two days of normal running, I got up in the morning and found that the freezer section of the refrigerator was thawing.  When I opened the fridge door the light went on (i.e., inverter was working) but the compressor was not running.  I did hear a faint ticking/buzzing/rumbling sound inside the fridge with the door open, like the sound of a small timer motor.

Switched it back to grid power but it still wasn't running.  Thought the fridge died (but it's only about 3 years old).  Just as I was getting ready to move the food elsewhere, it came back on.

So what's going on?  My initial guess is that it got into a point in its auto-defrost cycle where it tries to run a timer (only) for some minutes, and that load was not enough to keep my inverter running, in its auto-load-sensing mode.  Thus that cycle could not complete.  Could that be correct?  I don't really understand the auto-defrost cycle.  Perhaps I should disable the auto-defrost, but how?

Last year I had the freezer too running on the inverter, perhaps that's what kept this from happening: the fridge timer could advance while the freezer came on and kept the inverter active?

OperaHouse

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Re: refrigerator defrost cycle and load-sense inverter
« Reply #1 on: March 12, 2013, 11:11:10 AM »
"When I opened the fridge door the light went on (i.e., inverter was working)"

That may be true, but maybe it wasn't working till the refrigerator light came on.  A timer probably consumes only about 3W and between switching the heater off and enabling the compressor to run again there is likely a time period where the water is allowed to drain out before you refreeze it.  That time period may be longer than the inverters automatic off with no load time.   Assume that during the day there  are other things that keep the inverter on.  That this occurred at night is key.  It is only a matter of chance that it took this long for you to see this happen.   Other times it may have happened just before morning and it wasn't obvious. 

The only simple work around is to disable it.  Does this have a wiring schematic that you could post?

« Last Edit: March 12, 2013, 11:17:46 AM by OperaHouse »

OperaHouse

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Re: refrigerator defrost cycle and load-sense inverter
« Reply #2 on: March 12, 2013, 03:09:28 PM »
If I remember how these work, the timer works anytime the thermostat is over temp.  It is like a 24-36 hour timer.  When it times out the defrost heater is turned on for like 30 minutes.  In series with the defrost heater is  a 35-40 degree klixon NC thermostatic switch.  This opens up once the ice is melted on the coils and prevents overheating.  If you don't have a lot of ice there will be no load except the timer motor.  This is likely where the inverter shuts off.   So if you found the wires going up to the heater you could add a lamp or resistor to that circuit for just enough load to keep the inverter on.

vtpeaknik

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Re: refrigerator defrost cycle and load-sense inverter
« Reply #3 on: March 13, 2013, 03:20:12 PM »
So are you saying that the timer runs ALL the time, and activates the defrost every 24 hours or so?  If so, then there are consequences for running the frig on an auto-sense inverter:
* The interval between defrost cycles will be longer, since the timer is not running when the compressor is not running.  Probably not a problem except in rather humid environments.
* Setting the threshold load (for the inverter to turn on) low enough for the timer to trigger the inverter will mean that the inverter will run ALL the time, not good.

Probably my best solution is to do what I did last year: run the freezer on the inverter along with the frig.  That guarantees that the inverter will turn on every once in a while, even when the frig is only running the timer.

Note: I am not running anything else off the inverter, since this is an on-grid house. The main purpose of the solar system is as a backup in case of power outage.  It's been "working" perfectly for 5 years now, in the sense that there hasn't been a significant outage ;-)  But I'm trying to make some use of the solar power even when the grid is working.  Thus the frig and freezer use.  I also run some lights on it but not through the inverter.  A little Xantrex inverter dedicated to a single lamp (15W CFL) has been handy and reliable - model 813-0400-01 - was only $25 back then.  Doubles as a handy way to monitor the battery voltage right from the living room since it has a digital voltage display.




OperaHouse

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Re: refrigerator defrost cycle and load-sense inverter
« Reply #4 on: March 14, 2013, 12:58:27 AM »
No, the timer only runs ONLYwhen when the compressor motor should be running.   That time period would take about a week or so of run time.  When it finally hits the defrost cycle the refrigorator is already at a high turn on temp. 

The defroster time cycle is designed to get rid of the worst ice buildup  during the summer high moisture conditions.  Since it is dry right now there is minimal ice.  Defroster heats up quickly and the over temp switch is series with the heater turns off.  With only the timer load the inverter now turns off. 

In five minutes or so the over temp switch cools off and and connects the heater again.   This cycle keeps repeating so it could be that the refrigerator is effectivly off for four hours.

You say this is the only thing on this inverter.  If you run this on the inverter only in the simmer when there is lots of ice buildup it is possible that the defrost cycle only gets extended a half hour. This could explain why you havent't noticed this.

Another possibility is that the over temp switch is now defective.  In that case the fridge will never turn on the inverter( unless you open the door).   You can check the resistance of the heater.  Or you can run it on line power and in a couple months the cooling coil will turn into a solid block of ice and it won't cool the fridge.   That happened to my fridge.  The $8 temp isn't hard to get at.

Placing a load on the defrost wires like a 60W lamp only during defrost cycles should work if that is the real problem.  I think putting it on the same inverter as the freezer would be disasterous,  Diagnosis will take a lot of effort so you will probably wait and see for more evidence.

vtpeaknik

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Re: refrigerator defrost cycle and load-sense inverter
« Reply #5 on: March 14, 2013, 09:23:18 AM »
Thanks for the info, Operahouse!   The part I'm still  not sure about is, how does it decide when the defrost cycle is complete?  Does that depend on the timer (but not the compressor) running during the defrost?  If that's the case, the behavior I saw (cycle never finished, frig getting way too warm meanwhile) could happen without any malfunction in the frig nor the inverter, simply because when the temp sensor cuts out the defrost heating element, the inverter goes to sleep.

And why do you say that "putting it on the same inverter as the freezer would be disasterous"?  Or do you only mean that it would interfere with diagnosis?  Other than diagnosis, it seem advantageous, since the freezer would turn on once in a while, causing the inverter to be on, which would at the same time advance the defrost cycle in the frig.

OperaHouse

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Re: refrigerator defrost cycle and load-sense inverter
« Reply #6 on: March 14, 2013, 01:49:37 PM »
This is a simple defrost schematic. 

vtpeaknik

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Re: refrigerator defrost cycle and load-sense inverter
« Reply #7 on: March 14, 2013, 02:01:38 PM »
Thanks!  One picture worth 1000 words...

So it seems that if the frig is the only load on the inverter it should still eventually advance through the defrost cycle, as long as the temperature at the hi-temp sensor (where is it?  In the freezer section presuambly?) keeps periodically falling down low enough to reconnect the heater load thus activating the inverter.  "Coffin corner" would be if the process took so long that the temperature in the sensor area gets high enough (from ambient room heat) to keep the heater off "forever".  I wonder if my frig got into that state, since the foods in the freezer section were definitely thawing.

XeonPony

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Re: refrigerator defrost cycle and load-sense inverter
« Reply #8 on: March 20, 2013, 12:20:50 PM »
get your self a good old style plate evaporatour fridge, no defrost times to mess things up, just never use a tool to defrost them!
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

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