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Old 13-08-2010, 11:38   #1
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AC vs DC Genset

I'm looking for a good diesel generator for my s/v. I've got a 16k and 10k BTU A/C, plan to have a watermaker and gotta have my coffee in the morning. I've been steered in all directions but have come to some conclusions for an AC generator; 3 cylinder, low RPM, 6.5 KW. Probably an Onan, or Northern Lights. Then I read about DC generators and have talked to a few folks about them. Advantages seem to be smaller size and weight with high output and lower fuel consumption. Why aren't the DC gensets more commonplace? Are there disadvantages compared to the A/C units?

Tom
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Old 13-08-2010, 11:46   #2
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Are you planning to operate DC appliances?
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Old 13-08-2010, 12:05   #3
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There is a brand new 15K for around 5 grand at Brewers in Mystic. Long story but it is legit.
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Old 13-08-2010, 12:06   #4
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The Air Conditioner would be a big reason. I am looking in to engineering a DC powered compressor, but I am limited to 5000BTU's. Newer A/C's have smaller startup "surge" curents, but still significant. Everything else can go DC or be powered by an inverter. To date I have found no resonably priced inverter that can startup an A/C compressor. That leaves the A/C to be powered by an AC generator, sorry right now you really have no other choice.
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Old 13-08-2010, 13:25   #5
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???? I start a 5k btu hatch unit with a 3k inverter all the time!!

To the OP: I built a 12v DC genny out of an old AC one that the alternator crapped out on. I had a lesse Neville 200 amp alternator off a truck, used a tandem v belt set and the 8 hp Kubota off the old genny. 2 8d storage batts and a good inverter and all is good. Mine was manually throttled just enough to keep the batts from discharging under the steady load and any surge was covered by the reserve capacity.

Had under $500 in the whole rig. Nicest part was most time, it ran barely above idle
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Old 13-08-2010, 13:43   #6
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Are you planning to operate DC appliances?
As of right now, no DC appliances. Everything we have that generates heat (sucks power) is A/C.
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Old 13-08-2010, 13:49   #7
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The Air Conditioner would be a big reason. I am looking in to engineering a DC powered compressor, but I am limited to 5000BTU's. Newer A/C's have smaller startup "surge" curents, but still significant. Everything else can go DC or be powered by an inverter. To date I have found no resonably priced inverter that can startup an A/C compressor. That leaves the A/C to be powered by an AC generator, sorry right now you really have no other choice.
I'm lousy at the electronics math. How big an inverter do you need to start up a 16k BTU A/C unit if you're running a DC generator?
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Old 13-08-2010, 14:20   #8
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With your boat and it's A/C gear and other AC demands, you don't want a DC generator. I had one for years, but when I installed my A/C units (16.5K and 12K BTU) I installed an AC generator. That's the way to go.

With a DC genset you'd need a really hefty inverter to generate enough power to start and run your A/Cs and other AC-powered stuff.

The AC generator gives you the option of powering AC equipment directly, and/or recharging your batteries via an on-board battery charger. I have the Victron MultiPlus, with 2500watt inverter capacity and 120A DC charging capacity.

Believe you'll be happier with an AC generator.

IMO,

Bill
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Old 13-08-2010, 14:56   #9
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I'm lousy at the electronics math. How big an inverter do you need to start up a 16k BTU A/C unit if you're running a DC generator?
for a 16k assume a starting current of 25amps @ 120VAC. so we're looking at 3000 watts minimum. The DC drain for that (not counting losses) @ 12VDC is 250Amps.
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Old 13-08-2010, 15:33   #10
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I measured my 16K btu A/C units (Marine Air) to pull 57A @ 120vac inrush. Obviously, that would be half if they were 240vac. That's not to say they won't start on less current, but I would guess your going to need at least 40A.

If I were doing it again, I would consider an 'assist' type inverter that would allow you to get by with a smaller AC genset. Victron has such charger/inverter combos. Your house batteries are perfect to supply that extra 300A (@ 12V) kick for the 100 milliseconds that inrush current lasts. The reason for this is I never get the genset past 40-50% utilization, the extra capacity is there to simply start the A/C units. Smaller genset, smaller weight, lower fuel consumption, etc. My genset is a Mase 7.6is.

Good Luck!
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Old 13-08-2010, 16:11   #11
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Dotdun makes excellent points re: inrush current, but I would point out the following:

1. some A/C units, like mine (Flagship Marine), have big start capacitors and compressors which have a relatively "light" inrush current requirement; and

2. there's often need for powering other current hogs, like a big battery charger and a hot water heater and...coffee pot...etc. simultaneously.

My genset is 3.5KW. It's too small. I'd much rather have a 5KW or, better, an 8KW genset which would mean I wouldn't have to be so careful about what's switched in at the moment.

Don't believe all you read about genset loading. If your boat is reasonably decked out with AC gear, you'll find a way to load the genset enough. Especially when you consider the power factors of some inverter/chargers, A/C units, etc.

Bill
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Old 14-08-2010, 10:24   #12
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I am confused about why anybody would want a DC generator or appliances. Telsa's theories are as valid today as the were in the 19th century. Even DC generators generate AC before the output is rectified either by a comutator or silicon rectifiers.

Where on a boat does one get enough DC storage energy to operate such devices? Who sells them? Heck, even the so called DC fridges that I know of have internal inverters to make AC from the DC provided.

I thought I was abreast of technology, what am I missing?

Foggy
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Old 14-08-2010, 10:36   #13
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Its because it is easier and less expensive to convert AC to DC than the other way around. Most common marine inverters do not get over 3000 watts.
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Old 14-08-2010, 11:18   #14
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Its because it is easier and less expensive to convert AC to DC than the other way around. Most common marine inverters do not get over 3000 watts.
My guess would be there's not a market for >3kw inverters. Most house battery banks can't supply the 3kw for any sustained time. That's ~300amps!

If you have to burn fuel to generate power for the big appliances anyway, it's going to be AC natively, why would you want to convert to DC to run DC appliances?

Plus, I would guess the upcharge for the DC appliances (over a lifetime) would offset the cost of the inverter.
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Old 14-08-2010, 12:08   #15
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I have a 3.5 Kw NextGen AC generator, have 2 12K BTU A/C units and they draw 9.5 amps each. That leaves me with plenty for the battery charger, but not enough for the hot water heater (so I turn one of the A/C units off when heating water). I have a volt and amp meter wired to the genset. You do need to run a generator hard (at least 70% load) or you will score your cylinder(s). So do not buy one that is to big for your needs. I do load manage mine, and at times have to turn on other appliances that I don't need just to keep the load up. The 3.5 fits my needs, budget, space and wieght constraints.
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