Fridge Freezer Setup

[skippydh]

Hi All

Thought I would share my success with a 1 kW Jaycar sinewave inverter and a 155 litre freezer as a fridge.  I built a small temp controller to cycle the freezer at fridge temps,  however I found that the location of the temp sensor played a larger role in fridge temperature.

Picture of fridge (not a very good one, sorry)

 

Inverter,  temp controller on top and the black box below is fusing and a current shunt.  When running it settles down at around 9 amps (varies a little based on battery volts).  The temp controller just turns the inverter on and off (which I wired in via a RJ45 thingo)

Close up of controller.

Very happy with operation.....

Skip

[tecker]

I opted for this also the main reason is I have my mind set to make an inverter for the mains that is repairable and as simple to use with taps on the inverter for diferent class power input  . I think when I have coded the power to fit the line in for sellback I'll invest in UL approved device that fits in code also . I feel sure that sell back credits will help  match EV requirements on night charging . For now I can get 800 and 1000 watt  units farly cheap  for those power digs ( refigerator, frezzer , washer and maybe a dryer when I get it worked out) Good stuff on the thermostat it looks as though your tied in for monitoring also I like those mods .

[tanner0441]

Hi

Interesting about you want to build an inverter, I could never understand why sine wave inverters are so expensive. I have a prototype design I was toying with using a crystal for frequency and into a big power amplifyer, I started it before the introduction of power mosfet devices. It was designed to use a pair of bridged output modules driven with a discreet class AB driver stage.  I wonder if anyone else has looked down this route. My main problem was I don't have the knowledge to design the output impedance matching and was looking to a toroidal transformer. I don't seem to get much beyond about 400 or 500 Watts.

As I say I wonder if anyone else has thoughts on this.

Brian

[scottsAI]

Tanner0441,

Yes, years ago.

Efficiency of AB is not very good, 70 to 80%

As it's done today: high frequency PWM simulating AC can get high 90's.

With a small transformer.

Have fun,

Scott.

[scottsAI]

Skip,

Nice work.

Now the important question.

How much power does it use each day? (total power)

What are the operating conditions. (Room temp, Fridge temp, etc.)

The example on the Web, claims 100whr/day. I do not believe, unless kept in a 55F deg room!. Now 250whr/day OK, assuming in a 72F deg room. Still a nice achievement.

My Comments based on insulation thickness differences between normal fridge and freezer used as fridge.

Have fun,

Scott.

[skippydh]

At its first run we used about 40 Ah over 3 days.  However we loaded it up will all the frozen ice packs we transported the food with and it didn't run at all on the first day.  I had to hold my hand on the temp sensor to make sure it was working. This is why I didn;t post any info regarding the performance as it is not very clear how much it would use realistically. We do not live permanently at this site and as such can only gather data whilst I'm there.

At home on the bench we seemed to use about 10 Ah (approx 130 watt hours) just maintaining about 20 litres of water at 4 degrees C.  Ambient average somewhere around 20 degrees C.

I monitor all our solar/generator, load inputs and outputs and report this data back via VHF Radio (APRS, Amateur Radio stuff).  I interface to a Plasmatronics PL20 solar controller, via a micro and report back telemetry every 20 mins.

All for now...

Skip

[tecker]

I think the core can be made of permeable ( softer ) steel . The magnetic material reluctance is slower to return and the back emf lifts and stays in a happy zone that has a proportion  with the size of the core .  That way it takes less power from each cycle .You end up with two separate toroids bonded at the neutral the main power use coming of coarse from the shunt  per cycle .The  whole system can be simplified .I 'll run it in on the board soon as I get the core to run cool. You have to separate boosting toroids  With the standard circuit used now and no induction transformer . Supper fast pwm with 60 cycle trigger.

[BigBreaker]

Great project.

Did you give any thought to swapping the motor out for a DC motor instead of keeping the existing one and using an inverter?  I thought the Sun Frost frigs used Danfoss DC motors to get around the inverter issue.

[skippydh]

I sort of explored the idea of a DC compressor.  I suspect the cost of de-gassing,  fitting the DC compressor and re-gassing would be close to the cost of an off the shelf solution.  

I spent $290AUD on the freezer and $320 AUD on the inverter. So very cheap for a 155 litre 12 volt fridge.  The control circuit was built from the junkbox. To purchase a 12 volt fridge we would be looking at least triple the cost and it would be VERY attractive to thieves. Yes we have be burgled......So a major part of our decision.  

Also a replacement is available at any whitegoods retailer,  unlike a DC version.  So in the event of failure it would be easy to run down the street and pickup another unit.

Cheers for now, Skip

[Opera House]

I have a freezer fridge that I am less than amazed with.  It is a 5 CF new GE unit with a modified HF 2000/4000W inverter.  The start up current is over well over 90A even with the compressor modified with capacitive start.  Makes me think you are really on the edge with a 1KW inverter.  I built a temperature controller but abandoned it in favor for operating it as a dump load instead.  When the battery reaches a set voltage the fridge comes on for 10 minutes.  This worked well till temperatures inside the house dropped below 45F.  As an active use fridge the 14# of ice last about 3 days in the summer.  150W of solar panels is not sufficient to sustainably run the system.

[skippydh]

The 1kw inverter has a surge capacity of 2kW.  I haven't measured the startup surge yet.

I see your 90A to be well within the capabilities of your inverter of 2 kW.  So a 90A startup,  is only 1080 watts (12v * 90a).  

What issues do you have with your configuration?  

Skip

[Ungrounded Lightning Rod]

Has anybody looked into just tweaking the thermostat that came with the freezer so it works OK around 40F, rather than kludging up a homebrew replacement?

[vtpeaknik]

I set up a 9 cubic feet chest freezer as a fridge some months ago.  Bought a new plain-vanilla Kenmore "energy star" model (about $270).  It uses about 90 watts when running and perhaps 1.2 KW when starting up.  When not using the grid I run it off a Prosine 2.0 so that's no problem.  I measured the average power usage at about 7 watts with the ambient temp about 60F (basement) and the internal temp about 35F.  That's about 170 watts-hours per day.  I used an external add-on thermostat so that the freezer itself is un-modified and I can use it as a freezer if need be by plugging it into power directly.

For the external thermostat I got (on ebay for $20) a device that was described as intended for beer brewing - I don't know anything about beer brewing but this worked out for my purpose, barely.  It's a solid state design (no moving parts other than the temp-adjustement potentiometer) with the main part in a small metal box and the temp sensor a long (2 feet?) thin (1/8-inch?) metal "wire".  A mystery to me is whether that sensor is only sensing near its tip or all along the "wire".  I mounted it so that most of the sensor wire is inside the freezer (but insulated from the interior wall).  About 8 inches of the sensor are still outside the freezer.  In the summer that resulted in the highest temp achievable (adjusted as warm as it can go) being not much above freezing.  Perhaps that's because the part of the sensor that is in the warmer ambient air is "fighting" the part that is inside the colder freezer?  Now that the basement has cooled off (summer is over, alas) the temp inside the freezer rose to about 42F so I adjusted the thermostat to be a bit colder.

Observations regarding the actual usage of the freezer as a fridge: the inside walls get very cold (the machanism's cooling pipes are embedded inside the walls, in the top foot or so), I have to arrange so that vegetables don't touch the wall or they get frost-burned.  I can't use an insulating layer against the wall since that would impede the coldness from getting from the walls to the interior, i.e. would reduce the efficiency.  Another issue is that I got quite a bit of condensation inside - with the temp above freezing you get water rather than frost.  It collects on the bottom which already shows some signs of rusting.  Perhaps that will shorten the life of the device.  I sponged the water out a couple of times but it's always wet anyway.  Now that the basement as a whole is nearly cold enough (with the root cellar portion of it now down to fridge temp) I am thinking of not using the freezer as a fridge during the winter - can use it as a freezer or not at all.

[skippydh]

Just an update to how the fridge performed over a 9 day period, during fairly high temperatures.  

To give some idea of the environment the fridge operated in,   I gleaned the daily maximum temperatures from the closest weather station.  As you can guess,  as we operate from solar,  we do not have AC,  so the internal temperatures ( where the fridge is located) followed the outside temp within a few degrees C.  

    Daily Max   

    C    F

Day 1    27    80.6

Day 2    32    89.6

Day 3    36    96.8

Day 4    43    109.4

Day 5    38    100.4

Day 6    36    96.8

Day 7    32    89.6

Day 8    28    82.4

Day 9    34    93.2

Day 10    33    91.4

12 volt system

Total Ah used        320 Ah over 9 complete days   

Therefore Average per day    35.6 Ah   

Family of 4,  used as a normal fridge.       

Cooling lots of drinks,  from ambient and keeping beer cold.

Fridge set to nominal 4 deg C.       

I feel that this is close to the worst case power usage we would expect to see from this system.

Regards Skip

[scottsAI]

I just stopped by to see if there was an update!

450whr/day for Fridge, very good. My 24cuft combo was using 4kwhr/day. Found an "energy saver" turned on... which is a 90(?)w heater to keep doors from sweating. Turned off. Think the heater was on all the time, accounting for half the power. Need to retest.

Have fun,

Scott.