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Old 05-04-2007, 14:28   #1
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Any way to convert a DC Generator to an AC Generator?

Here's the deal, the new boat has a Kubota 100 amp DC Generator, basically it's a kubota engine with a 100 amp alternator on it. As we'd like to have a genset for running a/c from time to time, I would like to know if anyone has ever heard of converting a DC generator to an AC generator.

I don't mean using the 100amp alt to a inverter, my question is if there is a method to remove the dc output alltogether and go ac?
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Old 05-04-2007, 14:33   #2
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If you remove the alternator diodes you can get 3-phase ac. Other than Hall-effect generators virtually all generators are basically ac.
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Old 05-04-2007, 14:35   #3
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Captain Scott,

Yes, anything is possible. The question is, do you really want to do it? It would be a lot of work, and a considerable expense (you'd still need a way to regulate the AC, after doing the mod suggested by Rick).

Much better, from several points of view, would be to simply get yourself a good inverter. For many applications, a modified sine wave inverter would do fine. For some, with sensitive loads, a pure sine wave inverter would be indicated.

This would allow you to have 110V AC available WITHOUT RUNNING THE ENGINE OR THE GENERATOR.

And, the 100A DC generator is a very efficient way to charge batteries.

Bill
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Old 05-04-2007, 14:42   #4
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I'm not too worried about battery charging, it has 4 85 watt solar panels and a KISS wind generator. About the only thing I need AC for is the airconditioner, which is nice every once in a whie here in south florida and the water heater. I had given some thought to going with possibly a flash water heater like we had in Costa Rica, however that still requires a/c (more efficient as you aren't heating when you aren't using.)

As it sits, the PO never used the genset, the solar and wind were enough, however he never used the a/c unless he was at the dock. About the max inverter i've seen (haven't looked much) is a 3kw, which isn't bad, but i don't know how efficient it would be to convert ac to dc to ac, which is effectively what i would be doing.
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Old 05-04-2007, 14:55   #5
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Yep, A/C is the one application where you've almost got to have an A/C generator, since if it's of any size it would draw a LOT of DC current on an inverter. I ran the numbers for just the smaller of my two A/C units, and they convinced me to install an A/C generator when I removed my old Kubota-powered DC generator.

Guess you're stuck. You need to consult a local engineering shop re: the conversion of your generator.

Or, of course, go without A/C when away from the marina :-)

Good luck,

Bill
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Old 05-04-2007, 15:30   #6
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If we ever went to a marina i would prolly consider that but we keep it on the ball and we anchor out whenever we travel, soooo its pretty much gotta be self sufficient.

Sounds like converting the dc genset is more trouble then it's worth, I'll have to start shopping for a new genset.
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Old 05-04-2007, 15:43   #7
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Well, I wouldn't write off the present one quite yet. A new genset is gonna cost you very big bucks.

Consider two things:

(1) depending on the size of your A/C (and, I assume it's not too big if you're on a 30-footer), you might be able to fit a big inverter to handle the load, and you'd have to run the genset most of the time; or

(2) a small Honda genset might do the trick; lots of folks using them; very cost efficient; can be dangerous if you don't take care to avoid carbon monoxide problem. The EU2000i would probably run a small A/C with no problem, and it's under $1,000.

Either way, it seems you're looking at spending a minimum of a boat unit or two; and, if you do opt for a new genset, think 4-6 boat units :-((

Bill
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Old 05-04-2007, 16:23   #8
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We all have Carbon Monoxide detectors in our boat! I hope! Don't we?

One of mine even went off once. Scared me sily!
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Old 05-04-2007, 16:52   #9
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This is actually on the new boat which is a CAL 3-46, so its a little bigger. Definetely aware of the costs associated with a new genset, as we are living aboard/cruising on the new boat I want to have it well outfitted. I use a portable on the 30 foot, but it's a hassle for day to day use, ie throwing it on for an hour in the morning to take showers, etc... In the c-30 i do have a combo co/smoke detector, the new boat has two and it also has an auto extinguisher system in the engine room. I think a 5.5kw will do all we need, just a matter of picking on out and installing it, i actually have a line on an 8.3 westerbeke with 1000 hours, so i need to see if i can make that deal happen as it would save me a bundle.

btw, the ac is a 18,000 btu unit. which draws around 15amps.
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Old 06-04-2007, 00:40   #10
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Quote:
Originally Posted by Latitude9.5
About the max inverter i've seen (haven't looked much) is a 3kw, which isn't bad, but i don't know how efficient it would be to convert ac to dc to ac, which is effectively what i would be doing.
I usually pull 80% out of my butt when I need to know the efficiency of an inverter. That is, I expect the AC output power to be about 80% of the DC input power. If you have a specific inverter in mind, the manufacturer's specifications should include a more accurate number.

Here is a quickie estimate, ignoring efficiency. You said it is a 100 amp DC generator, right? 100 A * 12 V = 1200 watts. Let's say the "12 volt generator" output is really 14 volts: 100 A * 14 V = 1400 watts. So we can assume your generator is making somewhere around 1200 to 1400 watts.

So, if you want an inverter just to convert the DC generator output to AC, that 1200 - 1400 watts is as big as you need. (Again, the manufacturer's specifications for your generator may have more accurate numbers.) If you use 3000 watts through an inverter while you have the generator on, more than half of the power will have to come from the batteries.

Unless your generator was very badly designed in the first place, it isn't a honking big engine attached to a tiny alternator -- the engine power should be about the right amount to produce the electric power you get out of the generator, and no more. Otherwise, the generator manufacturer would be paying for a bigger engine than it needs. I wouldn't expect any possible modification would get you much more power.

Quote:
btw, the ac is a 18,000 btu unit. which draws around 15amps.
1400 watts at 120 volts is 11.6 amps. If your air conditioner really needs 15 amps, I doubt you could run it from this generator under any circumstances.

Quote:
I think a 5.5kw will do all we need, just a matter of picking on out and installing it, i actually have a line on an 8.3 westerbeke with 1000 hours, so i need to see if i can make that deal happen as it would save me a bundle.
If you're going diesel, be careful not to get a generator that is too big. This gets into the whole thing about not running diesel engines under light load.
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Old 06-04-2007, 08:00   #11
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Hi,
Just for what it's worth, I have an A/C unit that nominally draws 11 amps. I tried to run it off my inverter (ProSine 2000). No go. Turns out that the starting amperage for the compressor and pumps is over 40 amps! We had to use meters to measure the draw. I had hoped to use the A/C on the hook, but it's just for marinas now. Thank God for fans.
Richard
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Old 06-04-2007, 08:41   #12
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why not switch ac unit

It seems that it would be easier and less expensive to switch your air conditioner to a DC powered unit. check out dcbreeze.com
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Old 06-04-2007, 08:57   #13
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It seems that it would be easier and less expensive to switch your air conditioner to a DC powered unit. check out dcbreeze.com
@$2500 plus installation kit, not a cheap alternative either!!!
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Old 06-04-2007, 12:06   #14
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Perhaps you should clarify--I don't think you have a generator at all, just a heavy duty alternator. If that is the case, it is totally unsuitable for powering an air conditioner for the reasons coot cited. A dc air conditioner like the one linked above is way too small for your boat, plus it will light-load your main engine. I think you are going in the right direction with a dedicated genny. Try to size it so it remains loaded at around 80%, plus has surge capacity for the starting load on your a/c.

Brett
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