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Old 01-12-2013, 09:32 AM   #21
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Capacitors are commonly used for air conditioners run by generators in RVs. Perhaps one of the EE's here can comment on the practicality of usign a capacitor with an inverter.

RV Air Conditioner Hard Start Capacitor | ModMyRV
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Old 01-13-2013, 07:26 AM   #22
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Thanks Thomas, that was exactly the link I was looking for but could not find. My fridge compressor is about 1/10 the power of the AC units mentioned in the write-up so I'm going to contact Supco and see what they recommend. Fixing my problem for $6 is the kind of solution I was looking for!
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Old 01-13-2013, 09:59 AM   #23
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Originally Posted by MCDenny View Post
Thanks Thomas, that was exactly the link I was looking for but could not find. My fridge compressor is about 1/10 the power of the AC units mentioned in the write-up so I'm going to contact Supco and see what they recommend. Fixing my problem for $6 is the kind of solution I was looking for!
I'm not an electrical engineer, so I don't know if it will work. I'd encourage you to research it first as there may be some interaction between the inverter and capacitor that is different from similar usage with a generator. I'm sure someone else has explored it and posted, if you look long and hard enough.
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Old 01-13-2013, 12:23 PM   #24
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AC motors require a second winding producing a magnetic field out of phase with the first to start rotation. One way to achieve that phase shift is with a start capacitor.

Since the fridge is designed to work on household AC it should work just as well with a properly sized pure sine wave inverter/battery combination. Because the battery you described should be more than adequate, I would be inclined to agree that the inverter might be marginal for the surge required. Since the power rating on an inverter is a figure used to market, like horse power on lawn mowers, it's in the manufacturers best interest to inflate it's value. Perhaps 600 watts is wishful thinking??

Another possible issue could be the frequency of the AC output. Nominally it should be 60 Hz. I don't recall the tolerance but power companies stay very close to that. I don't know how tight the tolerances are on generators or inverters, especially under load, but a change in frequency would effect the phase angle of the start winding. It could be the additional capacitance (i.e. Hard Start Capacitor Kit) is intended to compensate for a decrease in frequency due to loading. Having never owned an inverter or a generator, I have never had the opportunity to take measurements. The best I can offer is an educated guess. Good luck, Raz
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Old 01-13-2013, 09:11 PM   #25
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As I understand them, the Honda EU2000i generators commonly used by dozens of posters on Thomas's modmyrv link are dc generators driving true sine wave inverters. Nearly everyone reported a marked improvement in starting. I suspect those who saw no improvement wired the cap in wrong.

How or why this works is well beyond my "electricity is like water flowing through a pipe" understanding.

I emailed Supco today and will share their response with you all.
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Old 01-14-2013, 02:28 PM   #26
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Supco said:

Dennis,

We do not have experience with this type of equipment. You might try using our SPP6 which is connected to the compressor run capacitor using 2 wires. Do not use a SSP6E style.


So I bought one and will see if I can wire it in in about 10 days when we are back at the kids house where I have good tools and an empty fridge.

I'll update you again then.
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Old 01-26-2013, 07:27 AM   #27
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The Supco cap had no effect. I don't know if the problem was my dorm fridge has the wrong kind of motor or what. It did have a tiny cap (about the size of a dime) across two wires that went into the motor. Connected the Supco cap in parallel with it as per the directions. No change, inverter still tries to start the compressor then gives up with an overload fault.

I did notice on the data plate of the fridge a LRA spec of 7.0 amps. That would 840 watts even though the unit is supposed to draw only 80 when running. My Samlex TSW inverter is capable of 600w continuous and 1000 for less than 2 sec.

I'd love to get a 12v unit like Norcold or NovaKool but really reluctant to spend $1000. A $100 MSW 1000w inverter might be my next move.
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Old 01-26-2013, 10:24 AM   #28
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I have a 69$ Black & Decker (rebadged Haier) from Walmart and 1500watt (2000 surge) pure sine wave inverter on a 6' 1/ought leash to a group 27 battery. The starting amperage of the compressor motor has NEVER been a problem for this inverter even starting with the fridge at ambient temp., but I must make clear that I do normally cool the fridge from a warm start on 120VAC shore power and plug into inverter on the road and off grid when conditions are good for solar recharging with an 80watt panel. Not a perfect system. Knowing how, knowing how much, and knowing when to switch to a cooler with ice is something you learn and learn to plan around in terms of days on the road, interval between on grid plug ins, and daily hrs. of sun. I suspect that your inverter's peak watt output is inadequate.

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Old 03-30-2013, 12:54 PM   #29
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Update. I finally decided to upsize my inverter. Had a nice chat with the Samlex tech support guy, he said my 600 watt was very marginal at best to start a small fridge. When I tried the larger capacitor (above post) I noticed a sticker on the motor that said 8.4 LRA. That's locked rotor amps and means it will draw 1000w the moment of start up.

He and I agreed a 1500w unit would be best as that would provide a little headroom for other loads too. I stayed with a true sine wave model as that is about 30% more efficient when I driving a motor. I want to have the fridge work for a couple of days of dry camping so the efficiency is important to me.

A tip of the hat to Samlex, they answered the phone right away, had lots of helpful advice and even agreed to take back my old 600w unit as a credit toward the new one.

The new unit is currently in route. I'll update again when I get it installed.
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Old 03-30-2013, 01:12 PM   #30
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Update. I finally decided to upsize my inverter. Had a nice chat with the Samlex tech support guy, he said my 600 watt was very marginal at best to start a small fridge. When I tried the larger capacitor (above post) I noticed a sticker on the motor that said 8.4 LRA. That's locked rotor amps and means it will draw 1000w the moment of start up.

He and I agreed a 1500w unit would be best as that would provide a little headroom for other loads too. I stayed with a true sine wave model as that is about 30% more efficient when I driving a motor. I want to have the fridge work for a couple of days of dry camping so the efficiency is important to me.

A tip of the hat to Samlex, they answered the phone right away, had lots of helpful advice and even agreed to take back my old 600w unit as a credit toward the new one.

The new unit is currently in route. I'll update again when I get it installed.
Great follow up. Thanks for completing the story.

I'm impressed with the service from Samlex in this age of labyrinthine telephone menus that usually end up hanging up on you.
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Old 03-30-2013, 02:48 PM   #31
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I stated that my Ramsond is 1500/2000surge. It is actually 1500/3000 and the inverter is happy with a hot start to "ON." It is not happy with a hot start to midway (4) on the dial. I have optimized the solar panel leads to my battery this winter and want to do some more duration tests this spring when the sun gets higher. I have never gone beyond 10 hrs. (cool down on shore power and no solar) with the battery discharge limit at 12.2V. 1.5 amps (15amps DC) running means my 100amp hr. battery SHOULD only allow approximately 3-1/2 hrs. run time. I've done 16 with daytime solar charging in high summer. Obviously, the insulation factor is good enuf to avoid running full time. I don't know how much better I can expect to do. It has worked great on tow vehicle alternator current getting from one hookup site to another. At least I have a refrigerator (rather than a chiller)when I have hookups and an old Coleman ice chest when I don't. I also have the RM2202 if I should ever decide to stand it on its head for decrystallization.

Please give us some numbers when you're up and running, Denny.

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Old 03-30-2013, 02:55 PM   #32
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Will do. I've got a 220ah battery bank so I believe I can get a couple of days.

I also have a victron battery meter so I can see the actual amps consumed.

Looks like the new inverter will be here Mon or Tues and I hope to get in quickly. We are still camping until mid April so l can give it a workout right away.
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Old 04-11-2013, 09:11 PM   #33
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Well, here's the rest of the story.

The 1500w inverter beeped every time the fridge started and sometimes wouldn't start it at all. I noticed the overload light was not coming on, just the alarm tone noise. Finally read the manual very carefully and realized a beep without the overload light meant low voltage on the 12v side, not overload as I had incorrectly assumed. My battery meter showed 12.8v with no load but less than 11v when the inverter was drawing 85 amps. The 12v cables are 4 gage and the manual calls for 1/0 for runs of 4' or less so I first thought the wire was undersized causing the voltage drop. I checked a wire size calculator though and it showed I should should only see a 2% drop at that amperage with my 4 gage wire.

I then thought I might have a bad connection and removed, sanded and retightened the connectors one at a time. Son of a b..., I had a loose battery connection. Tightened it up and the inverter works like a charm. I put the old 600w unit back in and it starts the fridge perfectly. Don't I feel stupid for overlooking such a basic fault! Since the 12v system worked fine with low loads like lights and the inverter powering phone and computer chargers I figured it must be good. Wrongo!

I've run the fridge now for 24 hrs starting with a fully charged 220 ah bank. Started the test in the afternoon with a warm empty fridge (at least 90 degrees in the trailer), low was around 65 at night and a high of 80 today. The fridge plus inverter overhead drew 69 ah, meter indicated 71% of battery capacity was left. This gives me a conservative two day run time on battery alone. If I dry camp with my Honda generator, two hours a day charging will take care of 24 hours of refrigeration.

Over the course of the winter we have had several 4 to 8 hour drives and now that the inverter works, we can keep the fridge cold while on the road. Even 4 hours without power will cause the frost on the freezer compartment to melt.
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Old 04-12-2013, 05:20 PM   #34
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Update after 48 hours:

I noticed at about 46 hours a very short low voltage beep when the fridge kicked on. With inverter draw of 9 amps voltage was down to 11.8 volts. 12.1 with no load.

At 48 hrs fridge was still starting fine. Usage over 48 hrs was 133 ah. Victron meter said 45% state of charge. Turned it off and started recharging the battery.

BTW, this means the fridge was actually running 1/3 of the time.
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Old 04-12-2013, 05:26 PM   #35
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Interesting.
Should the stock Snoozy run the fridge on battery as set up
or did you make certain changes on yours?
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Old 04-12-2013, 07:42 PM   #36
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Denny, should I be taking an offline voltage reading rather than shutting down at 12.2V while under load? Maybe I've short changed myself on runtime tests. Could I assume that 11.8V under load would correlate to 12.2 without?

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Old 04-12-2013, 09:05 PM   #37
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Interesting.
Should the stock Snoozy run the fridge on battery as set up
or did you make certain changes on yours?
I think the stock Snoozy has a GP 24 battery? Maybe 60 or 80 ah capacity? Not sure but you could check yours.

I have two 6v 220 ah golf cart batteries and a 30 amp Iota charger. If the stock battery is one third my capacity it would run the fridge a little less than one third as long.
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Old 04-12-2013, 09:22 PM   #38
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Denny, should I be taking an offline voltage reading rather than shutting down at 12.2V while under load? Maybe I've short changed myself on runtime tests. Could I assume that 11.8V under load would correlate to 12.2 without?

jack
It's tricky correlating voltage to charge state. To use the theoretical voltage vs charge state relationship, you should let the battery 'rest' for a couple of hours before taking a voltage measurement at the battery terminals. Measuring voltage under load introduces ankther variable as the greater the current the lower the voltage will be no matter what the state of charge is. The only practical way I know is to measure the current flow with a meter like the Victron, Link 10, etc. The Victron is the best as well as least expensive but it's still around $150.

The voltage will differ depending on where you make the measurement. The battery terminals will always show a higher voltage than further downstream due to the resistance of the wires and connectors. Wire and connector resistance is a function of current so they will have almost no effect on voltage with little or no load but can have a big effect with large amp draw.

With my slightly loose battery connector there was no significant voltage drop when drawing a few amps to run the lights or charge the computers but a two volt drop (18%!!) at 85 amps running the inverter with a hair dryer as the 120v load.
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Old 04-14-2013, 12:07 PM   #39
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Unpredictable chemistry? I'll say. Thanks for the eval, Denny. Some more tests coming soon for my rig.

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Old 04-14-2013, 01:13 PM   #40
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Some of the battery monitoring tools mentioned by Denny:
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