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Old 01-09-2011, 10:28   #46
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Re: DC Generator - The Smart Way to Go

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Originally Posted by typhoon View Post
I can't really understand why you would have an AC generator on a small sailboat. As small, I mean anything under 75 feet , bigger then that and you just leave the ac generator on all the time. But us, without deep pockets, have to conserve our energy and our diesel. So that means storing your energy in batteries .

So I ask, why use an AC generator to make power then change it to dc to charge your batteries, its nuts! And to just fire up a big genset to make toast? Thats nuts too.
99% of gensets on cruising sailboats are AC gensets for two very simple reasons: (1) the main load which people want to run when they buy or specify a genset is air conditioning, and the motors which drive air conditioning systems are AC motors; and (2) the world market for gensets for all purposes is 99% AC, so that's what's available and well-proven.

My boat doesn't have air conditioning, and we have a Victron 3kW inverter, so your system would work ok for us. AC loads are not that big a deal -- coffee machine, electric kettle, microwave, power tools, vacuum cleaner, washing/drying machine. I have even run the washer/dryer off the inverter while motoring, just for kicks. Probably wouldn't make a habit of that, since it uses 2.2kW, which is a big load for the 24 volt 110 amp (nominal) alternator.

But the advantages of your system over my Kohler 6.5kW genset are few. Mainly slightly better efficiency at charging batteries and running DC loads. At the expense of considerably less efficiency at running AC loads. And the big disadvantage of your system is that it is not a standard, off-the-shelf deal which you can buy already fully developed, and with a warranty. That's a BIG disadvantage.

Your system provides no advantage in reducing genset run times in order to spend more time on batteries -- you can just as well have a big charger and an inverter with an AC genset, as you can have a big alternator and an inverter, with a DC genset.

And if I ever thought about putting in air conditioning -- and I might well, if I ever get a chance to wander off the to Med or the tropics -- your system would be poor for that.

That's kind of a long answer to your question "why is anyone crazy enough to use an AC genset on a sub-75 foot boat."
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Old 01-09-2011, 10:29   #47
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Re: DC Generator - The Smart Way to Go

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Originally Posted by David M View Post
... DC is also less efficient over distance. It also requires larger gauge wire for the amount of power carried.
Not at the same voltages. In fact, DC may be more efficient in transmission.
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Old 01-09-2011, 10:35   #48
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Re: DC Generator - The Smart Way to Go

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Originally Posted by Beausoleil View Post
Not quite - most diesel marine gensets (5kW and up) run at 1800 (60Hz) or 1500 (50Hz) rpm to save fuel and wear. Northern Lights, Westerbeke, Kohler, Beta Marine, Norpor - all run at 1500/1800 rpm, not 3000/3600. See Kubota Diesel Generators ,kubota diesel generator,isuzu diesel generator, yanamr diesel gensets for some specs.
+1

My 6.5kW Kohler runs at 1500 rpm inside a sound enclosure, which in turn is inside a sound-insulated engine compartment. It is practically inaudible and wouldn't bother anyone in an anchorage.
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Old 01-09-2011, 10:36   #49
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Re: DC Generator - The Smart Way to Go

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Not at the same voltages. In fact, DC may be more efficient in transmission.
Sure, but that's a purely theoretical point, since no one I have ever heard of has 110 volt or 230 volt DC power on his boat.

I think what he meant was that 110volt (or better yet, 230 volt) AC power -- the kinds we use in real practice -- are much more efficient to transmit than 12 v (or better, 24v) DC power. And in that he would be correct.
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Old 01-09-2011, 11:16   #50
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Re: DC Generator - The Smart Way to Go

"So I ask, why use an AC generator to make power then change it to dc to charge your batteries, its nuts! "
No, it's physics. While the terms generator and alternator are often confused, a generator puts out DC and an alternator puts out AC. Alternators are also easier to control (by feedback) and regulate, while a true generator can only be regulated by dumping excess power or slowing it down. Alternators with integral DC conversion are often called generators these days, which arguably the entire unit now is.

So as long as there's an alternator in the machine, you have AC, and as long as you have AC, making it available as 120VAC or 240VAC isn't such an odd idea since many folks find a use for that. Continuing to convert that into 12VDC (14.4 actually) doesn't add much expense or engineering trouble to the design.

So there's 12v gensets, and 120v gensets, and combo units. And if you ever had to work on your boat using some power tools, it is much simpler to run a 50' 120V extension cord, than the same 50' of heavy gauge battery cable.

Of course if all you ever will need is battery charging, the whole AC section is a waste of time and money.
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Old 01-09-2011, 11:19   #51
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Re: DC Generator - The Smart Way to Go

Gord-
"Not at the same voltages. In fact, DC may be more efficient in transmission. "
Yes, but we don't use them at the same voltages.

And if we did, surely you haven't forgotten that when an AC transmission line has voltage losses, a simple transformer boosts them back up again. Can't do that for DC, you need a motor-genset regenerator, or an incredible boost converter that's gonna cost a bloody fortune for high power.

AC won over DC, in part because it is simpler and cheaper to send over long distance wires.
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Old 01-09-2011, 15:35   #52
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Re: DC Generator - The Smart Way to Go

The term generator is generic, it can mean AC ,DC etc. an alternator is a generator that produces alternating current output

While there are some advantage to a high power DC generator, especially one based on an alternator ( over say rare earth permanent magnets), its a specialist field and there's little off the shelf available.

It could be argued that a AC generator and a big battery charger ( or several) is basically achieving the same and its certainly cheaper, if one looks at the only real commercial unit, the Fisher Panda AGM.

There are significant losses in both methods.

Dave
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Old 01-09-2011, 16:26   #53
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Re: DC Generator - The Smart Way to Go

To further complicate this discussion...

Note that some boats with electric propulsion have a lot of high-voltage DC power available (360VDC or more). The whole equation changes for running a lot of the high loads...some run better under high VDC than 120/240 AC. For instance, water heaters, etc. And the wiring to inverters/etc. is MUCH smaller than 12V or 24V. This is pretty new stuff, however could wind up saving a lot of weight on larger yachts and change things quite a bit.

Add in the regen capability of the propulsion system and it gets even more interesting.

Still pretty expensive tho...
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Old 01-09-2011, 16:28   #54
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Re: DC Generator - The Smart Way to Go

I forgot to point out the obvious...that the high-voltage DC generator only runs at optimum efficiency, saving fuel. Otherwise it's off and the high-voltage house DC batteries run everything.
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Old 01-12-2011, 11:18   #55
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Re: DC Generator - The Smart Way to Go

I've been considering (future project) switching out my 16kw AC genset for a DC setup. I, like Mike, have an EP boat and unlike him have a 144V DC system. Presently I have to convert to DC to run my 2 motors and charge the Propulsion bank. Power is then fed through a 144v to 12v charger (house bank) or directly to a 6kw Inverter (120V AC). Since all power from the genset could go directly to the distribution board (motors, batteries, or Inverter) it would make sense to go DC. The genset I would go with would be a PolardcMarine PDC-8340VP-40. Smaller and less weight.

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Old 20-04-2012, 08:20   #56
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Re: DC Generator - The Smart Way to Go

One thing also to remember is that while charging the house bank of batteries I am also making 40 gallons of fresh water. As the Hydracell pump is belted and clutched to same output shaft as the alternator. All in about one hour running time .

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Old 20-04-2012, 21:55   #57
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Re: DC Generator - The Smart Way to Go

The overheated hot water can be controlled with a automatic mixer valve, I believe in some places they are called anti-scald valve. The side load problem could be cured a couple of ways. One is with a jack shaft, another way would be to mount to equal alternators 180° apart, good for small engines where you can use to small alternators in place of one large one.

Reference old generator regulators. Most had three relays in them one relay control the voltage, another relay controlled maximum amperage and the third relay was a cutout relay its job was to keep the generator from trying to become a motor. The primary reason and alternator charges more than the generator is that the alternator has a smaller pulley generator the reasoning being in alternator spins the field where is the generator spins the armature. Just my two cents, Mike.
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Old 22-04-2012, 09:45   #58
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Re: DC Generator - The Smart Way to Go

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Originally Posted by goboatingnow View Post
........
It could be argued that a AC generator and a big battery charger ( or several) is basically achieving the same and its certainly cheaper, if one looks at the only real commercial unit, the Fisher Panda AGM.

There are significant losses in both methods.

Dave
We are full time liveaboards and have a 7 year old 4KVA Fischer Panda DC genset with 1050 Ah Lifelines and a Victron inverter. We run the water heater - 750 watt element in a 29 litre tank - and we run the air-con for a couple of hours, all from the batteries. We have AC all the time without having to run a generator. The genset is under the bed and is quiet enough to watch TV when it is running. We think this is much more "efficient" and friendly than running an AC genset.

A 12 volt diesel genset is quieter and more efficient because it runs at a variable speed to deliver the load needed. It uses less fuel and requires less maintenance than an AC genset running at a constant speed, no matter what the load. A DC generator is much more efficient at providing 12 volts, an AC generator has to convert back down to DC. An inverter is up to 95% efficient in converting to AC, so together the DC option is a better all round solution for both DC or AC. A major plus point is a DC genset can charge batteries at a very high current, up to 120 amps from the Mastervolt, 280 amps from the Fischer Panda. With AGMs this is a very good option.

For a very detailed and in-depth look at “electricity on board” see this booklet from Victron – sections 7 and 8:

http://www.victronenergy.com/upload/...yUnlimited.pdf
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Old 22-04-2012, 11:16   #59
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Re: DC Generator - The Smart Way to Go

Quote:
Originally Posted by goboatingnow View Post
The term generator is generic, it can mean AC ,DC etc. an alternator is a generator that produces alternating current output

While there are some advantage to a high power DC generator, especially one based on an alternator ( over say rare earth permanent magnets), its a specialist field and there's little off the shelf available.

It could be argued that a AC generator and a big battery charger ( or several) is basically achieving the same and its certainly cheaper, if one looks at the only real commercial unit, the Fisher Panda AGM.

There are significant losses in both methods.

Dave
Actually there are at least two more making DC units: Aqua marine, and Ample power. A direct drive unit would be an excellent way to go. I know there are people who will say you are not spinning the alternator fast enough, but it does work. If you put a 150 amp or larger unit you are beltless and running the alternator well below it's maximum design limits. Win win situation. The nice thing about DC is that you can throttle back after the initial hi amp output time.... quiet running. Aquamarine makes a unit with watermaker and alternator....
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Old 22-04-2012, 20:06   #60
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Quote:
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Actually there are at least two more making DC units: Aqua marine, and Ample power. A direct drive unit would be an excellent way to go. I know there are people who will say you are not spinning the alternator fast enough, but it does work. If you put a 150 amp or larger unit you are beltless and running the alternator well below it's maximum design limits. Win win situation. The nice thing about DC is that you can throttle back after the initial hi amp output time.... quiet running. Aquamarine makes a unit with watermaker and alternator....
That horizontal kubota has a reputation for being very loud.

Dave
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