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Old 08-25-2011, 10:23 PM   #15
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OK - gotta ask - - - -exactly WHY do you not want to run the genny?
I think he answered that. He has a propane generator. Propane is expensive and limited in the amount you can carry. Even so, that is really the only viable option. Inverter/battery technology is not practical to run/start an AC unit.
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Old 08-26-2011, 12:24 AM   #16
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Well, there is the 4-70 AC unit. It always works.
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Old 08-26-2011, 09:39 AM   #17
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A 4-60 will work also
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Old 08-26-2011, 12:32 PM   #18
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With the consideration of having a propane generator and its limited run time, the question that ronspradley poses is a reasonable one.

First some details about the power needed to run one of the roof AC's is approximately 15 amps at 120 volts, with a starting rush current of almost double that (30 amps). This works out to 1800 watts running with a 3600 watt surge capabilty.

There are two possible solutions:

A. Use an inverter and upgrade the engine alternator to a larger model.

B. Replace the existing propane generator with a model that uses the same fuel as the coach.

Personally I would investigate replacing the generator. It would be more straight forward (possibly). The price tag for the 6000 watt Onan Diesel generator is over $7,000.

The inverter route would require at least a 2000 watt inverter. Note that a 2000 watt inverter will draw over 170 amps at 12 volts. A lot of power.

Aren't Diesel pusher alternators and starting battery systems based on 24 volts? This would drop the current needed to 85 amps. This would help with wire sizes, but would restrict the inverter use to when the engine is running. It would also reduce the engine power. There are 1800 ($1,300) and 3000 ($2,500) watt 24 volt inverters.

Instead of upgrading and replacing the existing alternator, why not add a second alternator just to power the AC inverter. You might have to size the alternator well above the actual need because you need the alternator to produce 85 amps (24v) or 170 amps (12v) at idle. You wouldn't need a clutch on the secondary alternator because alternators do not have any drag unless there is a load. Quality high amperage alternators can be $600 or more.

Some issues:

1. Adding a second alternator means custom brackets. There is also the issue of the belt for the new device. Do you use the existing belt, or change the pulley on one of the existing devices so that you can add another belt. I added a air compressor to my Jeep Wrangler. I replaced the standard alternator pulley with one that had grooves for the serpentine belt and a second v-belt groove for the air compressor.

2. Don't know if some batteries would be needed in this Air Conditioner circuit to smooth the power.

3. Also would need transfer switches to move the AC's from the propane generator circuit to the AC alternator circuit.

I have started to ramble......

We now have three alternatives:

A. Replace the existing propane generator with a diesel generator.

B. Upgrade the existing alternator and add an inverter to power the AC.

C. Add an additional alternator and inverter to power the AC.

I like alternative A. Good luck.
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Old 08-28-2011, 08:07 PM   #19
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ALVINC, thanks for the response. First of all, I would also prefer option A diesel generator. But the price is prohibitive.

I was thinking getting an additional alternator on the engine. Have one built for around 200 amps. I have not looked but may be able to get a mounting bracket from a salvage yard because the Cummins B5.9 has been used in so many applications from Dodge trucks to motorhomes. And then probably a 5000 inverter running thru my existing two 12v deep cycle house batteries just to smooth out the process. I would only use this when driving down the road. I do not expect to dry camp when A/C is needed, and if it happens I will run generator.

And on your mention of starting batteries for this application, caught me napping. I do not know if the starting system is 12v or 24v. I have two 12v batteries but do not know if they are parallel or in series. I will check this weekend. My MH is parked at my sister's place 85 miles from my house and this weekend will be going to visit both girls. And my Mom. They each have a house on the same 11 acres. Going to see Mom's younger brother who is having a surprise 90th birthday Saturday. Good times.........

Thanks again for all the help here. I am still looking and will report back. ronspradley
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Old 08-29-2011, 11:35 AM   #20
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Even though some inverters are rated high enough that it may seem they can run an A/C unit, their not designed for that high of continous usage. I work in a Xantrex/Magnum Repair Center and we get many of them in that have died because of over-heating. The Triac's on the power board will definitely over-heat. The control circuitry is supposed to shut down the inverter when this happens and it usally does, but repeated overheating from mis-use will cause the Triac's to fail prematurely.
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Old 08-29-2011, 12:27 PM   #21
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Even though some inverters are rated high enough that it may seem they can run an A/C unit, their not designed for that high of continous usage. I work in a Xantrex/Magnum Repair Center and we get many of them in that have died because of over-heating. The Triac's on the power board will definitely over-heat. The control circuitry is supposed to shut down the inverter when this happens and it usally does, but repeated overheating from mis-use will cause the Triac's to fail prematurely.
Call me grumpy, but I would be pretty irritated that a 3000 watt sine wave inverter would not be able to drive a 1800 watt load on a continuous basis.

Is there something about air conditioning compressors that is especially bad for sine wave inverters.
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