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12-15-2017, 09:42 AM
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#1
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Senior Member
Spartan Chassis
Join Date: Jan 2014
Location: Oregon coast
Posts: 141
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Stupid question: Why can't our 300+ hp DP alternator run house a/c?
Rather than hijacking one of the recurring threads about running our generator while driving in order to have house a/c, I'll put a stupid question out there for the mechanics, electricians, engineers or generally well-informed types: Why isn't the alternator on that big powerful engine back there enough to run our house a/c?
My on-board generator is 5.5 kW, which I think translates to something around 10 hp and it's enough to run everything in my 36' (not all-electric) coach. Seems like a lot of the alternators on motorhome main engines are rated in the neighborhood of 200 amps or more, so obviously I'm suffering from some kind of brain block about why the availability of that much amperage apparently can't run an a/c pump.
Would someone please explain it in somewhat simple terms?
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12-15-2017, 09:52 AM
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#2
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Senior Member
Newmar Owners Club Spartan Chassis
Join Date: Jul 2016
Posts: 665
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The engine alternator is 12 volt DC for running 12 volt systems and charging the batteries. Your genny makes 110 volt AC which is required for your A/C units.
__________________
Terry & Sue
2015 Newmar Essex 4553
2023 Bronco OBX
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12-15-2017, 09:59 AM
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#3
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Senior Member
Spartan Chassis
Join Date: Jan 2014
Location: Oregon coast
Posts: 141
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Thanks Coral, yes I get that, but what about a separated dedicated inverter just for the a/c?
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12-15-2017, 10:05 AM
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#4
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Senior Member
Country Coach Owners Club Solo Rvers Club iRV2 No Limits Club
Join Date: May 2011
Location: Vancouver, WA
Posts: 37,725
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It can be done. One option on the books for my rig was the ability to run the front heat pump while on the road and not need the generator.
It took an add'l two 8D AGM's (total of 6), an add'l 3000 watt psw inverter and a bigger or second alternator. Not a cheap upgrade just to avoid running the generator.
__________________
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft, HWH Active Air
Charter Good Sam Lifetime Member, FMCA,
RV'ing since 1957, NRA Benefactor Life, towing '21 Jeep JLU Rubicon Ecodiesel
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12-15-2017, 10:15 AM
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#5
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Senior Member
Join Date: Apr 2003
Location: Rochester, MN, USA
Posts: 374
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At 30 amps @ 120volts AC is 3600 Watts.
3600 Watts from inverted DC volts would be 300 amps DC.
I believe that my Engine alternator is 185 amps. Yours may be different.
Have a nice Darrel
PS - I always use the generator and both AC units on hot travel days. About 1/2 gallon per hour and much more comfort than Dash AC.
__________________
2007 3912 KSDP w/350HP ISL Cummins
2003 Grand Cherokee toad w/M&G Brake
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12-15-2017, 04:26 PM
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#6
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Senior Member
Tiffin Owners Club
Join Date: Aug 2016
Location: Bastrop Texas
Posts: 1,343
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Quote:
Originally Posted by Coral
The engine alternator is 12 volt DC for running 12 volt systems and charging the batteries. Your genny makes 110 volt AC which is required for your A/C units.
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Not So A Alternator on a vehicle engine produces AC current which goes to a rectifier to change it to DC,
__________________
2000 Allegro Bus 35R 3126 Cat 300 Allison 3060MD 6 speed
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12-15-2017, 04:37 PM
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#7
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Senior Member
Newmar Owners Club
Join Date: Oct 2009
Posts: 10,308
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The Newmar King Aire is set up to run the middle a/c unit off the "16 house batteries " with a 350 amp alternator, plus big charger/inverters.
One of the only one I know of till you move up to a Prevost conversion or Newell
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2012 Essex 4544 2011 Jeep JK, M&G Braking, 2014 MTI 27' Hog Hauler, Wireless brake control, 2006 Ultra & 1989 Springer, 2003 Harley-Davidson
FLHR Road King Anniversary
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12-15-2017, 04:39 PM
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#8
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Senior Member
Foretravel Owners Club
Join Date: Apr 2016
Location: Home is Where WE PARK IT...
Posts: 6,007
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IF You have the $$$....It CAN be done...
But it mostly boils down to simple economic's for a Coaches pre-determined price point..
__________________
Retired truckdriver,
'02 Foretravel... "This Shack will do"
being pushed by an '06 Scion xB
SKP's of Box Elder, South Dakota
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12-15-2017, 04:48 PM
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#9
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Moderator Emeritus
Join Date: Jan 2000
Location: West Palm Beach, FL. USA
Posts: 27,519
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Quote:
Not So A Alternator on a vehicle engine produces AC current which goes to a rectifier to change it to DC,
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Sure, but the rectifier is part of the alternator itself. You only get 12vdc (actually more like 14.5v) on the alternator output wire, and that won't run a 120v a/c unit. You would need to build/but a different alternator to produce 120vac. The same sort of alternator that is in a mechanical genset. Otherwise, use the existing DC power and add an inverter.
__________________
Gary Brinck
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL
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12-15-2017, 04:54 PM
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#10
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Moderator Emeritus
Join Date: Jan 2000
Location: West Palm Beach, FL. USA
Posts: 27,519
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An additional point: using the main engine to power the a/c has no advantage over using a separate smaller engine. You still need to burn more fuel to produce extra horsepower - it ain't free just because the engine is running. There might be a purchase cost advantage if there was never a need to have a separate genset, but if you are gonna have one anyway, it is probably more efficient for powering things than a much bigger engine that does 120vac as a sideline.
__________________
Gary Brinck
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL
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12-15-2017, 06:04 PM
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#11
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Senior Member
Tiffin Owners Club
Join Date: Aug 2016
Location: Bastrop Texas
Posts: 1,343
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Quote:
Originally Posted by Gary RVRoamer
Sure, but the rectifier is part of the alternator itself. You only get 12vdc (actually more like 14.5v) on the alternator output wire, and that won't run a 120v a/c unit. You would need to build/but a different alternator to produce 120vac. The same sort of alternator that is in a mechanical genset. Otherwise, use the existing DC power and add an inverter.
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I did not say how much it produced I just corrected the statement that the alternator produced DC, and some older alternators did not have internal rectifiers.
__________________
2000 Allegro Bus 35R 3126 Cat 300 Allison 3060MD 6 speed
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12-15-2017, 07:00 PM
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#12
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Senior Member
Spartan Chassis
Join Date: Jan 2014
Location: Oregon coast
Posts: 141
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Thanks for the replies everyone.
I realize there's no such thing as a free lunch and I'm not really seriously considering trying to do some kind of retrofit on my 23 year old coach, it's just that like many, my dash air is anemic at best and even though I have a generous 150 gallon diesel tank my generator is propane powered so running it while traveling ultimately cuts into my boondocking endurance.
No big deal, just thought I'd put it out there.
On a somewhat related tangent, while we were cruising in New Mexico last winter we met an RVer in a Safari Trek of roughly the same vintage as our Barth who had cleverly devised a way to run a small ductless mini-split a/c unit directly from solar panels. Sorry, I don't recall exactly how big his array was but I think it was just a couple residential panels in the 300 watt range. The mini-split system had a built-in inverter.
Obviously this would only work during a few hours of direct sunlight and would be limited to fairly small coaches at that but I was impressed with his ingenuity and thought it was pretty cool.
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12-15-2017, 07:17 PM
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#13
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Senior Member
Join Date: Jul 2014
Posts: 35,317
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Keep in mind the the alternator will only produce its rated output at crusing RPMs. In traffic and driving slow, it drops significantly.
The make, or used to make 120 volt belt driven generators that worked with variable speed.
You could also design a constant speed hydraulic driven generator.
There are tractor driven generators that require 540 RPMs.
Last but not least, some motor coaches use a large crankshaft driven AC compressor. You use a PTO to the front of the crank pulley.
Here is a site with some stuff. http://www.fabcopower.com/generat/bgen.htm
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12-16-2017, 12:18 PM
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#14
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"Formerly Diplomat Don"
Newmar Owners Club
Join Date: Apr 2005
Location: Moorpark, Ca.
Posts: 23,914
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To answer your initial question, the horsepower of your coach and your generator have nothing to do with how much electrical power, be it AC or DC they put out. Your coach engine is using the 300 HP to power the drivetrain. Maybe one or two HP of that 300 is used to turn your alternator, which provides your DC charging.
__________________
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor
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