Inverter Runs Gas Absorption Cooling Unit

Introduction:

An inverter can power your gas absorption cooling unit (GACU) providing you understand the nature of this power switching application and the requirements of the load. This article focuses up on Norcold's 1200LRIM model refrigerator with a Pines RV Refrigeration cooling unit installed and how I powered it from my Magnum Energy MS2812 inverter. It's an R&D effort on my part so accept the information I've presented in that light.

The boiler in a GACU is heated by either propane or 2 electric cartridge heaters at 225 watts each totalling 450 watts. These heaters are connected in parallel and work together to start the refrigeration process. My Magnum Energy inverter is a pure sine wave unit and I think that's the best power source for appliances designed for commercial AC power. In a normal installation, AC power to the refrigerator is provided by a shore power connection passed through a hard wired dedicated 20A breaker.

By default when travelling you have two options to keep your refrigerator cold. One is to run the generator and the other is to use the propane feature. With modifications you can use your inverter as a third option. Even when parked, this also has some value for power outages lasting up to several hours or more depending on available battery capacity.

As for propane, we don't use it when travelling for a variety of reasons. I'll mention a couple briefly here. I just don't think it's a good idea having an open propane cylinder and a lit flame burning while travelling down public highways. Something about this just doesn't sit right with me. I know people do it just the same. Proceed at your own risk is the only advice I can offer them. Before the inverter mods discussed herein, we would simply run the generator. A somewhat wasteful and expensive approach.

On an experimental basis, I rewired my circuit breaker box, added a relay and control switch to strap the refrigerator circuit over to the inverter sub-panel. This was first tested (July 1st, 2012) over the road on a six hour trip from Park City, UT to Montrose, Colorado. This battery based power source worked quite well even through the 100F degree heat we encountered in the desert. But there are some subtle things going on with this power switching circuit that you'll need to know about before wiring it up. Like so many things in life I discovered problems the hard way. I'll cover the whole story in detail here.

Our Norcold has an ice maker in it which includes a 165 watt mold heater as well as a small gear motor. You'll most likely want to turn off the ice maker to save power. Simply make the ice you'll need for your trip before going on the road. If you have the capacity to cover this load then no shut downs are required. I'm simply suggesting that it isn't needed while travelling.

Cartridge Heaters:

As mentioned, twin SS cartridge heaters (450 watts total) provide the heat energy for the boiler. The inverter will be required to power this load to keep the refrigerator operating.


Twin SS cartridge heaters power the boiler. See yellow and black wires.

These heaters are controlled through a PWM (pulse width modulation) circuit on the control board. This allows the refrigerator to vary the amount of heat input to the boiler. In hot weather this would be more and in cold weather less. A PWM circuit rapidly turns power on/off to vary the RMS power to the load. It's the most efficient way of handling this type of power management problem. Circuits like this are designed to operate on pure sine waves. Square waves and modified square waves might work fine but that's just a might. And the long term effects of using something other than a pure sine wave aren't known. In my book, it just isn't worth taking chances. Be safe and provide the device with the exact power it's designed to work with.

If you don't have a pure sine wave inverter, then upgrade to something like the Magnum Energy MS2812 or equivalent before connecting your refrigerator to it.

The Circuit:

Below is the wiring diagram I used for my implementation. There are many choices for relays and switches so take this as just one suggestion.


Inverter power to refrigerator. Click HERE for large view.

If we assume the heaters are maxed out then 450 watts is 3.75 amps on a 120 VAC circuit. Your 165 watt mold heater would be another 1.38 amps. Making the total AC amperage 5.125 which is well below the 20 amp circuit allocated for the refrigerator. Also keep in mind a GACU doesn't have a compressor so there is no inrush current to start a motor. If we remove the mold heater (turn ice maker off) then we can assume the maximum draw for the refrigerator is 4 amps. Using the simple calculation that each AC amp is 10X that on the DC side of the inverter, we should see a 40 amp DC draw on the batteries. In fact this checks out based on the reading I get back from my inverter. In reality the load will vary between 16-40 amps depending on how much cooling the control board is calling for.


Example RV circuit breaker panel. Not this sub-panel won't allow adding an extra circuit.

With a maximum draw of only 4 amps, we need to find a circuit that has at least this much spare capacity. Looking at the 4 inverter sub-panel circuits, the most under used one is the entertainment breaker. I chose this one to strap the refrigerator circuit to. In theory any one will do but why use a circuit that is (or might be) loaded to capacity.

The next challenge is to get the refrigerator circuit to switch between its native dedicated breaker and the entertainment sub-panel circuit. I used a SPDT ice cube relay with a 12 VDC coil (Grainger part # 1EHC6) for this purpose. If you're going to mount it like I did, order the socket and some DIN rail. I needed something small enough to fit inside the circuit breaker box. There are lots of options here so use what's safe and convenient for your application.


Ice cube relay in socket on DIN rail. It fits inside circuit breaker box.

I wired the NC contact back to the refrigerator's dedicated breaker. The NO contact straps onto the entertainment breaker. The refrigerator line goes to common. Follow the circuit diagram presented earlier.


Toggle switch controls refrigerator power. You can put this almost anywhere.

Opening and closing the relay thus switches the refrigerator's power from its default circuit to the entertainment circuit. This effectively allows easy selection of shore or inverter power. I labelled my switch travel/park for easy understanding of which position it should be in. It just as well could have been labelled Inverter/AC power. Pick what you want. The toggle switch powers the relay's coil from +12 VDC derived from a convenient point. Use appropriate fusing, wire routing and sizing.

Inside the breaker box, simply wire the relay socket according to the pin out functions of the relay. Use 12 gauge solid copper wire (black) for AC runs. Install a coil ground wire to the box and run an appropriate 12 DC leg back to your control switch. All this is a pretty easy wiring job. Obviously, don't wire anything hot. Shut down the power before playing around inside the breaker box.

VERY IMPORTANT INFORMATION:

PLEASE READ CAREFULLY!! Owing to some subtle circumstances, I had a surprise arc over and short out which proceeded to blow the inverter's 300A DC fuse. I trashed the relay and had to replace the DC fuse. Luckily there wasn't any additional damage. It took a bit of thinking to figure out what happened. Let me share that information here with you so you can avoid this from happening with your implementation.

There isn't a lot of extra room in my breaker box. The only relay that would fit in the space allotted was a miniature ice cube type. I found one with a 120 VAC 20A contact rating. On the surface these ratings seemed fine. If you look at the circuit breaker panel, you'll notice the refrigerator is on line 2 and the inverter is on line 1. This actually is significant.


AC wave relationships. Click HERE for large image.

If you look at the diagram above you'll notice the phase relationships between line 1 and line 2 are at 180 degrees. These are the outer legs of the power company's transformer and represent 240 VAC. When the inverter is in charge mode (not inverting) its AC is passed through to the sub-panel. Since the relay's NO and NC contacts connect to a line 1 and line 2 circuit, the relay sees 240 VAC. The miniature relay that I used is fairly compact meaning minimal space exists between its contacts and armature. During a transition an arc develops which is basically a dead short across line 1 and line 2. If the space between the contacts was physically greater (as would be the case in a larger open frame relay) an arc would self extinguish during a transition.

If you look at the inverter's AC wave (see diagram above) you'll notice it has an arbitrary phase relationship with the shore power waves. The algebraic sum of these two waves is thus unpredictable. It could be anywhere from 0 to 240 volts. Arcing at the contacts during a relay transition that involves inverter power is thus unpredictable. Meaning it may or may not happen. It all depends on where the sine waves are at the instant the relay changes states. To complicate matters even more, the inverter has a lag time between when it loses its AC feed and begins to produce its own AC from the batteries. Results of this on contact arcing are hard to predict.

Once you understand what's going on there are two easy ways to avoid short circuits due to arcing. 1) You could use a larger relay mounted perhaps in its own enclosure. More physical space between the contacts would prevent arcing. 2) Use a simple transition procedure that removes power before switching the relay thus preventing any arcing. Since I've already installed a small ice cube relay, I've opted for the transition procedure. It's explained in the next paragraph.

When you are ready to switch your system from "Park" to "Travel" follow the steps below:

1) Turn OFF the refrigerator.
2) Switch the refrigerator breaker OFF.
3) Switch the entertainment breaker OFF.
4) Switch modes from "Park" to "Travel".
5) Establish inverter AC power.
6) Switch the entertainment breaker ON.
7) Turn ON the refrigerator.
8) Observe refrigerator pulling power from the inverter.

When you reach your destination and before establishing shore power follow the steps to change the mode switch from "Travel" to "Park" below:

1) Turn OFF the refrigerator.
2) Switch the entertainment breaker OFF.
3) Establish shore power.
4) Wait for inverter to go into charge mode.
5) Switch modes from "Travel" to "Park".
6) Switch the refrigerator breaker ON.
7) Turn ON the refrigerator.
8) Switch the entertainment breaker ON.

Using the procedures above will ensure the relay circuit is completely dead during any state changes. No power means no arcing of the contacts. If you plan on using a miniature relay you'll need to follow this syllabus.

If you can afford the extra space of a dedicated enclosure, it's best to go for an open frame power relay with a good amount of space between its contacts. This would allow operating the mode switch without any regard for how the breakers are set. The presence or absence of shore/inverter power won't matter either. In other words, circuits could be switched when hot. The advantages of not having to think about anything first might outweigh the extra effort involved to mount a large relay. It's your call as to which approach is best.

Boondocking:

I've measured the DC load for the CAGU heaters at between 16-40 amps. While travelling and having the advantage of a 165 amp alternator there are no problems with the house batteries covering this load. Running the refrigerator strictly off the batteries could be a problem, however. I've not done any testing with this. For us the primary impetus was to power the refrigerator and avoid running the generator and using propane while travelling. This simple circuit achieves this goal and saves money in the process. Power outages of several hours could also be handled with a good set of 4 8D house batteries.

As for boondocking, you might get by if you had a good solar system to recharge your batteries during the day. Solar would need to cover all the DC loads and have extra power to complete a charge. As mentioned, I've not done any testing of a boondocking situation. I would be interested in any information you have about this.

Conclusions & Summary:

Running your GACU off the inverter is a practical alternative to propane and the generator while travelling. If your alternator has the spare capacity (and it easily should) you don't need really strong house batteries. Obviously a good powerful house battery bank helps the cause and extends possibilities.


My Lifeline batteries and Magnum Energy MS2812 pure sine wave inverter. Perfect together.

In my case, no spare breaker capacity exists within the inverter sub-panel. Some vehicles may allow a full width breaker to be replaced with a 1/2 width breaker thus allowing you to gain an extra circuit. With an extra circuit you can simply wire your frig there which is much simpler. If your sub-panel is like mine, you'll need to install a relay (or other means of switching) to allow the refrigerator access to inverter power away from its default 20A circuit. The default 20A circuit will go dead when the shore power connection if removed. Small ice cube relays may present an arcing problem as outlined above. Be careful with this and follow an orderly set of transition steps (outlined above) to change power sources. Doing this will avoid any problems.

The Norcold 1200LRIM heaters draw 450 watts. The ice maker mold heater draws 165 watts. Combined these are well below the dedicated 20A circuit allocated for the refrigerator. Heater power is a function of cooling demand. The refrigerator's control board will draw power as required to satisfy the temperature setting. Expect this to create a 16-40 amp DC load (ice maker turned off) through the inverter.

Reasonable protection from long power outages is also a benefit of this system. Assuming your battery bank is fully charged and robust enough to carry the load. Boondocking performance has not been tested at this time. Theoretically it's possible but needs to be proven out before drawing any conclusions. My opinion is boondocking would make sense only if you have a strong solar system. Something few vehicles have at this time.

I hope you've found this article useful, interesting and informative. Please feel free to add your comments, suggestions, corrections and experiences.

Second Road Test:

This is just a follow up to the material presented above. Jeannie and I left Montrose, CO on November 1 to relocate to St. George, UT. This was a 460 mile trip which took us 7.5 hours. Temperatures were in the low 40s when we left and hit highs in the low 60s. All under mostly cloudy skies.

I used the procedures outlined above and ran the refrigerator off the inverter for the second time while traveling. Owing to cooler temperatures I observed a nominal 12 amp DC load for the inverter. This covered both the refrigerator and music amplifier. We enjoy listening to music while driving. There were no problems during the trip and the refrigerator worked fine. Once we got parked and plugged in, I returned the refrigerator to its default circuit.

This trip I observed much less of a power drain. Last time in hot weather I was seeing 40+ amps on the DC load. My explanation is the refrigerator needed to work harder to keep things cool in the elevated temperatures which reached 100F going through desert roads in the open sun.

Wire Code Violations:

As presented, the implementation above straps two circuits to one breaker. Although this doesn't exceed or come close to 15 amps, it may violate NEC's wiring guidelines. Some may feel this presents safety or possible insurance concerns. Proceed at your own risk if you concur with the above concerns. I would suggest a simple solution to the two circuits on one breaker situation as follows: Use a DPDT relay and wire the extra set of contacts so as to disconnect the entertainment circuit from its breaker when the refrigerator gets connected. This electrically substitutes the refrigerator load for the entertainment load and ensures that only one circuit (frig) emanates from the breaker. It also means your entertainment components will lose power during your trip. Although less convenient, they could be plugged into another inverter powered outlet with an extension cord.

Other solutions involve jockeying and snaking extension cords around which would allow plugging your refrigerator into an existing inverter powered outlet. Use what works best for you and creates the desired comfort. My approach is one idea and its focus is on inverter power and how well it works with gas absorption cooling units. In particular, the Norcold 1200 LRIM outfitted with the Pines RV Refrigeration aftermarket cooling unit. Based on 14 hours of testing, my pure sine wave inverter has worked perfectly as an AC power source while traveling. More details will need to come forward to adapt this idea to all vehicles and address any applicable safety requirements.

Feel free to provide comments, suggestions, corrections and constructive criticisms on the material presented.