Any Electronics Whiz On Board? DC to AC Help Needed

[DougGuy]

I have an RV fridge that I would like to run off 12vdc. The compressor is a swing motor Sawafuji SK-511M that runs off 20vac 50Hz. Can't find any specs on the compressor as to it's applicable supply voltages, I am assuming it may be okay with 60Hz current. It doesn't draw much current, and doesn't require a huge startup surge like a piston or rotary compressor. I have looked all over the net, ebay, for a converter that is close but no banana.

Would it be easy to build one of these? There are tons of 12vdc to 110vac inverters around, would it be easier to use one of these and then use a step down transformer to take it to 20vac?

Not real well versed in electronics. I can follow instructions, solder pretty good, and have some experience repairing PA gear so not totally ignorant just not really good with ins and outs of making a circuit work.

[lancem]

What you suggest would probably be the easiest way to go. Problem is with all the losses between converting up and down as to whether it would be worth while doing. I'm wondering why an RV fridge would have a 20VAC compressor in it to begin with???

[DougGuy]

It's a Japan thing. We aren't supposed to understand.. You would think that since the majority of these compressors are for RV/marine/portable use that they would make them use 12 volts because it's pretty convenient. The thing is, fridge manufacturers make the circuit boards that do the inversion to 20vac, and these boards in some cases are quite problematic whereas the swing motor compressor is a really neat design with one moving part that works really good.

The problem is between the controller boards, and the compressor itself, not with the compressor. I used a $13 China PID controller wired across the on/off switch to bypass the thermostat part of it and it worked for a while but now the unit will not run so I need to completely bypass all the power supply that came built into the fridge, and be able to use the PID controller to cycle the compressor, and for this all I need is a card that will convert 12vdc to 20vac, the PID will do the switching just fine.

This is a discontinued fridge, the control boards are non existant for it, when you find a control board it is mega bucks and not worth throwing good money after bad to "try it and see if it fixes it." The compressor and the actual cooling part of the fridge work great. The expensive control boards in the power supply are problematic.

[popper]

You would basically be building a kludge controller board. Electronics failures seem to be the mode these days, cheap parts. Drive transistors are the frequent failures, maybe replace? Get another one or look into the thermo-electrics. SIL had the T/E portable, lasted a couple years. Swing motor is basically a selenoid/spring motor - 60 Hz is probably not the efficient freq to run.

[NavyVet1959]

I have an RV fridge that I would like to run off 12vdc. The compressor is a swing motor Sawafuji SK-511M that runs off 20vac 50Hz. Can't find any specs on the compressor as to it's applicable supply voltages, I am assuming it may be okay with 60Hz current.

I wouldn't be so certain that the supply voltage frequency didn't matter.

http://www.sawafuji.co.jp/english/tech/shindou.html





Resonance of the piston

To compress the gas only with the piston connected directly to the thrust generated from the coil, the amplitude is not sufficient. For this reason, to compress the refrigerant effectively, the swing motor synchronizes the frequency of input current with the specific frequency of oscillating system consisting of the piston, coil and spring so that the resonance occurs. Using this resonance phenomenon, the amplitude is increased for the compression of refrigerant.
Low starting power structure

The swing motor has a structure of directly transmitting the thrust of the coil to the piston. Therefore, it can start even if the piston does not complete the full stroke. That is, no large starting force is required.

Moreover, unlike a rotary type, there is no inrush current at the time of start, the consumption of battery power is low and it is not necessary to use wiring with thick cables.
Inverter control

In order to obtain the maximum efficiency using the resonance phenomenon of a spring, it is necessary to carry out the precise frequency control.

[Artful]

Does it require a sine wave or can it work on a square wave inverter?
And what's the maximum current draw?

You can google quite a few circuit board's for inverters....:wink:

[bangerjim]

50 vs 60 Hz DOES make a difference!!!!! Europe is all 50. And 220V in most countries.

Time for a run to the dump.

Buy something new. Forget cobbling together an inverter. Unless you have huge batteries or run your engine continuously, the extreme inefficiency of those "cheap" 12vdc to 120vac inverters will suck your system dry very fast. They are for very intermittent infrequent use for power tools and stuff you only run for a few minutes.

I personally would not attempt it and I am a well-versed engineer on most things electronic/mechanical.

12vdc fridges are not that expensive. Check out the solid state units out there. Unless you are trying to hang a side of beef, one of those smaller cooling/heating devices is what I would use.

I have two of them and the draw about 4A @12vdc And will run off a switching 120vac power supply when you are near civilization.

[DougGuy]

If this last ditch effort fails, we are looking at a Truck Fridge, 12v model draws low current and can run off the marine battery which gets charged by the alternator when driving, and the 12v inverter/battery charger when on shore power. I see no need in a dual voltage unit since it converts 110vac to 12vdc to run the compressor anyway.

[Petrol & Powder]

I'm not sure what your application is but if the control board costs more than the fridge is worth that wouldn't my fix. I also agree that using an inverter to convert 12VDC to 120VAC and 60Hz and then stepping that AC voltage down to 20 VAC would be extremely inefficient and you'd still be struck with 60Hz and not 50.
I think a replacement fridge is a better option. The other possibility would be to trouble shoot the control board if possible and replace the faulty components but that would be a lot of work assuming it could even be repaired.

[DougGuy]

There are two boards in the power supply. One takes 110vac and runs it out at 12vdc that feeds the other board which does the 12vdc to 20vac. I took the second board and hooked it to a 12 battery and troubleshot it for an hour and never could get anything out of it. The problem is on that second board but none of the components I tested measured bad and there is no visible damage. This board also has the thermistor and the temperature controller connected to it, which I connected for the purpose of troubleshooting so who knows now where the problem is.

I just need a 12vdc to 20vac converter OR a whole new fridge.

[Petrol & Powder]

I'd go with the whole new fridge idea

[NavyVet1959]

I had a fridge that was in good shape other than the controller board being bad. It was a lare Amana side-by-side. It would just not defrost, so it iced up. It needed a new controller board and it was expensive enough that I just decided to get my wife a new refrigerator instead. The ice maker also didn't work and I had determined that I didn't really like the side-by-side refrigerators anymore. So, after getting her a new refrigerator, I wanted to see what I could do with the old one since it definitely did not have a problem getting cold. I rigged up a temperature probe and relay to turn the entire unit on and off. I cut a hole between the sides and installed a muffin fan so that I could keep both sides at 33F. I then bought the parts necessary to make it into a kegerator. A full size keg is just slightly narrower than the right side of that refrigerator. I already had a 20-lb CO2 tank, so the kegerator conversion ended up costing about $100. The freezer side is now just a refrigerator side and I keep other alcoholic beverages on that side.

[NavyVet1959]

When searching for a power supply, you need to know not just the voltage and frequency, but also the amps.

[SGTM9]

If I am not mistaken the fridge should work with 24 vac at 60 hz. That way you do not need to get to 50 hz. I work on electric motors for a living and we run 200vac 50hz motors at 240vac 60hz.

[DougGuy]

Normal draw is about 3.5a, the board is fused with a 5a fuse. The compressor on this one runs really good, and with a China made PID running it before the controller went tits up it only had a 5° swing between cycles. Wish I could find a 12vdc PID that output 24vac that might run the compressor good enough right there. Those PIDs are cheap, and usually no problem running a 10a load off one. Gotta remember, this is not a rotary motor, it is a solenoid that bounces back and forth between springs. I can't find specs on it that say what it's allowable input current is.

[MaryB]

Well an inverter that will last and be reasonably efficient will run $100++ is it worth that? Add a 120 to 24vac transformer(common size) for another $30-50...

Or go on craigslist and find someone parting out an RV...

[NavyVet1959]

If I am not mistaken the fridge should work with 24 vac at 60 hz. That way you do not need to get to 50 hz. I work on electric motors for a living and we run 200vac 50hz motors at 240vac 60hz.

From my reading, this is a different type of system that relies on harmonics, so I wouldn't necessarily bet on it working with 60 Hz. Personally, I would contact the manufacturer to see if it will work on 60 Hz and what the acceptable voltage range is for it. It seems like an interesting concept. I have to think that if they could develop a portable air-conditioner that used the same principle and could work on 12VDC, it would have a market in small aircraft, sailboats, or small vehicle / trailer camping where you either *can't* or don't want to run a normal genset to power an air-conditioner.

[NavyVet1959]

If it will work with 24VAC, your options are probably going to be better since that is a voltage that is used by irrigation systems and such. Of course, you could probably put a load on it in series to drop the 24V down to 20V. Hopefully, the manufacturer will tell you that it can run off of 24VAC.

[Cmm_3940]

Here's a page with some circuit schematics for 12V DC to 50Hz AC inverters. I'd try starting with one of these and choose a transformer for the output to produce 20V instead of line voltage.

http://circuit-zone.com/?cat=AC_DC_Inverters

[MtGun44]

Wait a minute. 20VAC or 220VAC? Never heard of 20VAC for anything and in Japan,
220VAC 50 Hz is standard. And YES, 50 hz vs 60 hz is critical, 50 Hz motor will overheat
and fail on 60 Hz, and vice versa.

If this is, as I suspect, 220VAC, look for a European inverter, should not be hard to find on the web.

Bill

[popper]

First board is just a 110AC/12DC supply, no problem. Second is the actual control board. Without an Oscope to check waveforms, pretty hard to trouble shoot. If you have a good DVM, check for bad capacitors, hi AC on them (dc volts should be > 60% of rating). Solenoid driver is probably a flyback type - charge a cap then dump into the solenoid. A trick is to charge the cap to < rated voltage, through a resistor (2-3Kohms) to see if it will hold a charge. Unit powered off of course. Fixed the kitchen oven that way. Another easter egg approach is to replace all electrolytic caps, starting with the big ones. Good luck.

[MaryB]

Electrolytic capacitors dry out and drift out of tolerance and most switching supplies will shut down because of it. Usually easy to spot, look for a bulged up top or bottom and a wet looking spot around them(not always there). Probably 80% of the repairs at the casino were swapping them out for new ones.

[DougGuy]

Yup, there was this scandal a number of years back where the "formula" for electrolyte was "stolen" and sold to a *major* cap maker, but the formula was incomplete, and they made MILLIONS of caps with the incomplete formula, and all the big motherboard mfgrs were having failures by the thousands after about 2yrs of use on the caps. I re-capped an Abit BH6 with some Rubycon caps and it ran it's behootie off for years afterwards. Replaced a bulged cap in a Stinksys gigabit router and it fixed it permanently..

This one has no bulged caps and no visible damage of any kind, no blown fuses. It won't run if the thermistor is not connected to it so I soldered it directly to the terminals where it would connect, it just does not want to play nicely.

Found an H bridge mosfet inverter schematic that looks promising (and can also output 50Hz):

http://www.homemade-circuits.com/2012/04/how-to-design-h-bridge-circuit-for.html

Sent the author a friend request on FB, if this one will work with using a different transformer I may give it a shot.

When the unit ran, I measured the current at the compressor while it was running, at 20vac. The current on the control board (not the power supply board) was 24vdc, which is industry standard for thermostats and HVAC controls afaik. Could the AC side once it has a load on it drop down to 20vac from 24vac?

[popper]

If you didn't look at it with an Oscope, it might actually be 24 vdc pulsed which could measure 20vac on your meter. I'm guessing it rins from 24 pulsed to compress, then spring return, like a jack hammer. If so, don't need a bridge, just a switch. Solenoid should have a catch diode across it, probably a big cap. also.

[DougGuy]

Here is the compressor motor:

http://www.sawafuji.co.jp/english/tech/shindou.html

[Cmm_3940]

Just to be clear, you measured the voltage at the terminals in parallel with compressor while it was running? If so, your compressor has internal impedance, and what you are actually seeing is the voltage drop across a load.

ETA: you got me looking into this to try to give a clearer answer, and I was reminded why I never liked doing AC circuit analysis in school... Math. Math with lots of letters and squiggles in it.

http://fourier.eng.hmc.edu/e84/lectures/ch3/node4.html

[DellAnderson]

I have an RV fridge that I would like to run off 12vdc. The compressor is a swing motor Sawafuji SK-511M that runs off 20vac 50Hz. Can't find any specs on the compressor as to it's applicable supply voltages, I am assuming it may be okay with 60Hz current. It doesn't draw much current, and doesn't require a huge startup surge like a piston or rotary compressor. I have looked all over the net, ebay, for a converter that is close but no banana.

Would it be easy to build one of these? There are tons of 12vdc to 110vac inverters around, would it be easier to use one of these and then use a step down transformer to take it to 20vac?

Not real well versed in electronics. I can follow instructions, solder pretty good, and have some experience repairing PA gear so not totally ignorant just not really good with ins and outs of making a circuit work.

I registered specifically to reply to this post (old as it is!) Did you solve the power problem? Altho no whiz kid, I had a similar issue and have done a lot of research on this type of fridge and compressor. I might have some information that could be useful to you although I suspect the fridge is either running or trashed by now:x:!:. Otherwise, would like to compare notes.

[popper]

Some LG fridges use the same type compressor, now changed to a different type. If your fridge ices up, usually the defrost switch (or timer). Timer is in series with temp switch to cut off the heater when defrosted. Samsung fridge I replace used standard compressor but 2 computer boards ( >200$ each replacement) and a cheap computer fan for airflow.

[karel]

Hi DougGuy

I have done it, same setup/problem Sawafuji SK-511M camping fridge

Step 1 (living in Namibia, our supply voltage is 220V 50Hz)

I use a 220V to 24V ac transformer, from an old Hansa gate motor,on the old PC board of the fridge is a big capasitor, in series just before the motor, I have solder it in my sircuit (in series) and works 100%. voltage if freezer has stabilised is 28V, 1.7 Amps, startup runs at 22V 2.4 amps (even without the big cap it sill works fine)I guest the cap is only for protection for volt drop or spikes

Step 2 (and this is your question)

working on building a 12V DC to 20 - 30V AC inverter, I think it can be done with some of the components from the old board, but will keep you in the loop (the compressor can run between 20 - 30 V ac) if you have some luck that work, please let me know
Thanks