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Old 03-06-2017, 08:14 AM   #1
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DC vs AC Generator

This is a take off on the "Removing Inverter" thread which I followed and enjoyed.

With a 36 foot trawler, starting from scratch and you wanted simplicity why wouldn't you install a DC generator and a decent inverter? Especially if you have and enjoy a propane stove and dc lighting. A 600 amp bank that could be charged with a 150 amp dc generator seems pretty simple? That would give you 200 plus usable dc amps daily with a 2-3 hour daily re-charge / generator run time without running the main. At first look I like the size, weight, and pricing of dc over ac generators.
The only catch that I see is hot water and air conditioning with air conditioning being doable for an 8 hour stretch assuming the size of the battery bank.

Thoughts? Does anyone have 1st hand experience with a diesel / dc generator? What am I missing?
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Old 03-06-2017, 08:24 AM   #2
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Easting

Tesla and Edison had this fight 140 years ago. You are assuming AC presents problems - what are they in our small boating uses?
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Old 03-06-2017, 11:54 AM   #3
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Generators really aren't very efficient (Tesla won). The only way to get DC power is to use a commutator style design in which all of the current passes through brushes (Yuck) or an Alternator - which IS an AC power source which then rectifies the output to DC. The best data I could find is Mitsubishi claiming their 150A alternator at 56% efficiency is better than some other well known units. So at least that gives a ballpark efficiency number.

150amps at 12V is about 1800w which really isn't very much output. A 6hp Briggs and Stratton engine could power that. Are you talking about a 150A alternator added to the "generator"? Or are you looking at a generator sold specifically with a DC output?

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Old 03-06-2017, 12:15 PM   #4
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Fischer Panda builds one
Fischer Panda Generators - Fischer Panda DC AGTPM5000 Marine Generator

Also there is one built by a company in Bourne, Ma. http://www.hamiltonferris.com/catego...C_Generators/9

Seems that 150 amps would give me the correct charge rate for a 600 amp bank. I like the size, weight, and price point of the Hamilton Ferris. Just wondering why dc generators are not more common.
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Old 03-06-2017, 01:22 PM   #5
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One of the reasons that DC generators are not common is because of their limited market. Another reason is that you need specialized control circuitry to make a DC generator work efficiently. (not counting the inherent inefficiencies of a DC generator that were pointed out above.)

With a DC generator the engine throttle should be linked back to the voltage regulator on the generator. This would create a varying RPM situation where the engine RPM would be what is necessary to produce the needed current. Thisd may sound like a simple circuit to an engineer, but there is not a industry standard off the shelf unit to do this.

Speed control on a AC generator has nothing to do with the generator section of the machine, other than the number of poles. Speed regulation on a AC generator is supplied through either the injection pump, or off the shelf components. You can buy several different brands of off the shelf governors for most engines.

The next problem is physical mounting of the dc generator to the engine and it's pulley system. Few small engines have off the shelf solutions for mounting a DC generator. This means the generator manufacturer will have to custom fabricate a mounting solution.

AC generators bolt directly to the bell housing of a engine. The coupling of the crankshaft of the engine to the AC generator is via flex plates that bolt to the engines flywheel. The size of the bellhousing and the flywheel are standard measurements. Yes there are several sizes of each, but the specific measurements are standardized the Society of Automotive Engineering (SAE) guidelines. This makes it easy for manufacturers since it provides a set of standards for interfacing things like generators, pumps, transmissions to engines.

The issues go further but that's it in a nutshell.
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Old 03-06-2017, 01:28 PM   #6
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If your loads are mostly DC and you have no large, continuous AC loads then a DC generator makes some sense. But if you need to run an air conditioner, go with an AC generator. With a big enough battery bank you could run a small A/C unit in the main berth overnight from an inverter, but then you would spend many hours running your DC generator to recharge it the next day.

That Hamilton Ferris unit referenced above probably uses the NextGen marinized Kubota diesel and sound shield from their 3.5 KW A/C generator. That unit uses a belt to drive the AC generator end, so replacing it with a DC generator just takes some brackets. NextGen used to make them for others as well. It should be a solid unit, at least from the engine standpoint.

Hamilton Ferris limits the DC generator to 1,800 watts or 150 amps which is half of the NextGen's AC output, no doubt due to the poor efficiency of the DC generator- there is not enough horse power to make more. That might make overheating in the sound enclosure a problem- about 1,700 KW additional heat needs to be dissipated. That is pretty huge, more than a typical plug in electric heater.

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Old 03-06-2017, 01:31 PM   #7
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Old 03-06-2017, 02:27 PM   #8
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Charging batteries, both a DC generator and a conventional AC generator incur the same losses. KChace mentioned much of this.

With a DC generator, it's actually (typically) a 3 phase alternator being driven to generate 3-phase AC power. The AC is then rectified to create DC, the voltage is regulated, and it charges your batteries or powers DC loads. Any AC loads get powered through an inverter at approximately 90% efficiency, so additional 10% loss.

With an AC generator, you get single or split phase AC directly from the generator end with no rectifiers. That power can be used for AC loads with no loss penalty. To charge your batteries or power DC loads, the AC gets rectified and regulate just like in a DC generator, just by an external charger. But the losses are comparable.

So the bottom line is that when you look at conversion losses, generating DC power is the same from a DC or AC generator. Yet generating AC power incurs less loss when generated from an AC generator vs a DC generator.

But there is another factor at play which is the loading on the motor that is driving the generator. If lightly loaded - say 25-30% or less, the efficiency pretty much goes to hell because the diesel is operating inefficiently. But I would presume that you would equip a boat with roughly the same capacity generator whether AC or DC, so also presume both diesels would be similarly loaded.

You might find this article interesting

Adventures of Tanglewood: Engine Alternator or Generator: Which is More Efficient?

It's looking at a similar, but slightly different question. Regardless, many of the same principals and considerations apply equally to what you are considering.

There are also some equipment considerations. With an AC generator, you need a battery charger to get DC for battery charging. But you arguably need that same charger anyway for charging off shore power. So that's a wash.

With a DC generator you probably need an inverter for whatever AC appliances you want to run. I would personally have an inverter under any circumstance, but if you have an AC generator you could possibly get away without having an inverter.

So assuming you are going to have a battery charger and an inverter no matter what, the cost difference comes down to the difference in cost between say a 5kw DC gen vs the same size AC gen. If you are comparing different capacity machines then the savings comes from a smaller machine, not from DC vs AC. And beware of some of the low cost generators. Lots work fine, but if you read around you will also find a lot of nightmares. And consider serviceability, especially if you will be in remote areas.
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Old 03-06-2017, 04:14 PM   #9
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Interesting to note that the 5kw Fischer Panda specs show output of 12V @ 272A which is actually 3264 watts. Also the 5kw is listed at $15,535 list price where for example a Northern Lights 6kw (which should do 6kw continuous) is listed at $13,295 list price. So it looks like a significant price premium for the DC generator. I also can't find any fuel consumption figures for the Fischer Panda even though they tout using "less fuel".

Depending on your AC load, maybe you could run it off an inverter, but in my case I have 3 AC's that total about 20A @ 120V - not counting start surge. To power even 10A @ 120V from an inverter for 8 hours would be 100 amps for 8 hours or 800AH out of the batteries not counting the 90% best case efficiency from the inverter. To not go below the normal 50% of battery charge, that would require a very large battery bank. Also keep in mind that to recharge the battery bank you still would have to deal with the way batteries accept charge. So for example a 200A charger would take much longer than 4 hours to replace a battery bank that needed 800AH replaced.

This may be why you don't see more of these around.

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Old 03-06-2017, 04:31 PM   #10
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DC to AC inverters also are from 75 to 90% efficient, so you lose there too.
I have two inverters on boat for convenience sake, and some AC power incase the gen breaks down. I also have the 6500 watt generator.

I have run my stove top on my 3000 watt gen and an electric grill one time when the gen shut off to finish cooking.
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Old 03-06-2017, 07:06 PM   #11
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Speaking about this, I have a little Yamaha portable generator aboard. Barely use it but I keep it in case y batteries go flat or if I want to stay more than 1 day at anchor without moving. I have 2 type of outlet, 110V AC and 12V DC. Curently I have a switch at on the main panel to select either dock or generator source and the power goes through the batteries charger to charge them.
Wouldn't it be more efficient to connect directly the 12v output to the batteries instead of burning fuel to make 110v and transform it to 12v?
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Old 03-06-2017, 07:22 PM   #12
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You're not going to get 8 hrs of air cond off an inverter and battery bank alone unless the bank is huge or its a small window unit. Since I installed a Magnum 2000 watt pure sine wave inverter/charger I get to see what the actual power use is for things like freezer boxes and microwave ovens. For example my little 900 watt microwave uses 140 amps DC when running off the inverter! My smallest air cond unit uses double that when you add in the blower and water pump.
I suggest the smallest AC genset that can run your air cond needs in addition of the next largest load (in my case a 1200 Watt water heater) and a large inverter/charger which can handle the smaller loads and perform battery charging while the genset is running. My 2000 w inverter/charger uses 15.6 amps AC to produce 100 amps DC for battery charging. Thats too much load for a Honda EU2000i that I'm currently using because my Westerbeke is dead.
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Old 03-06-2017, 07:48 PM   #13
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Quote:
Originally Posted by Lou_tribal View Post
Speaking about this, I have a little Yamaha portable generator aboard. Barely use it but I keep it in case y batteries go flat or if I want to stay more than 1 day at anchor without moving. I have 2 type of outlet, 110V AC and 12V DC. Curently I have a switch at on the main panel to select either dock or generator source and the power goes through the batteries charger to charge them.
Wouldn't it be more efficient to connect directly the 12v output to the batteries instead of burning fuel to make 110v and transform it to 12v?
Generally with those small gensets...you can get more amps out of a battery charger for just charging than their 12V output.

My little Honda only puts out 6 or 8 amps DC but ran a 25 amp Battery charger no sweat.
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Old 03-06-2017, 08:36 PM   #14
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Generally with those small gensets...you can get more amps out of a battery charger for just charging than their 12V output.

My little Honda only puts out 6 or 8 amps DC but ran a 25 amp Battery charger no sweat.
indeed I just checked the doc and the dc output is 6.5A.
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Old 03-06-2017, 08:54 PM   #15
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You could power your vessel hotel loads with a DC diesel generator. It would require a high power inverter to run HVAC loads and water heater, etc. And at such high loads, 12v is not optimum as the amperage will require huge diameter cables. So you could go to 36vdc or 48vdc or higher to resolve the amperage issue, but then you end up having to separate large loads from the typical 12v loads like lighting, engine, pumps etc.

So to optimize the high load stuff, you end up making things more complicated.

Yes, it all can be done with various flavors of DC. But since shore power is AC, and small DC loads 12v, it really does not make sense to power the big loads with DC. Once you start looking at HVAC and water heater loads, AC makes sense. You feed with AC on shore power, the gennie fits well as AC too.

But it certainly can be done either way.
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Old 03-06-2017, 10:31 PM   #16
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My 2000 w inverter/charger uses 15.6 amps AC to produce 100 amps DC for battery charging. Thats too much load for a Honda EU2000i that I'm currently using because my Westerbeke is dead.
If you have the remote control panel for the Magnum inverter / charger, you can set the maximum charge rate to a lower level which should reduce the AC amp requirement to a level your Honda can handle. A short term solution till you get the Westerbeke fixed or replaced.

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Old 03-06-2017, 11:26 PM   #17
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I ran tugs, built in WWII, with a 250 volt DC ship service power supply. They used a motor generator for AC power. Everything in the 250 service was massive. Tugs were diesel electric with twin GM 8268A diesels driving generators that provided 250v DC for propulsion and ship service. A pair of Detroit 671 provided DC when mains weren't running. Lights, reefer, all were 250v. No air conditioning. DC motors are expensive, but usually rebuildable. Many smaller WWII built ships were all DC.
I use Magnum inverters for 120/240 AC, 8kw, on my current boat and can charge battery banks with an alternator on the mains when running, but could use solar, etc. Normally high power items are run only when on a generator or shore power, but I have used the water heater, water maker and always have reefer and freezers on line. I have several banks of 8dd batteries and can go a couple days or more w/o charging. Plan to add solar. But little sun in the winter here I am.
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Old 03-07-2017, 03:44 AM   #18
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Another issue if you raise DC volts so you can use smaller gauge wires is the DC switches.
As AC power goes through zero as it cycles up and down, when opening or closing a switch, the resultant arcs get quenched. But DC being on all the time, the arcs with higher DC voltages will destroy the switches.
Car makers years ago were looking at using 48 volt DC systems for efficiency reasons, but the switches were breaking down, so we can be glad they gave up this idea.

And another thing, 12vdc being low voltage system, does not need special human protection circuit breakers etc.
They have a cutoff point as higher DC volts would need to meet greater circuit protection requirements. At some level is no longer a low voltage system.

Imagine requiring DC based ELCI or GFCI or AFCI protection.
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Old 03-07-2017, 05:41 AM   #19
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I would hardly be worried about theoretical "efficiency" .DC would be my choice.

A good sized DC alternator like a DN 50 can create 200+A in the oldest 12v model, newer can do 250A of 24V.

These are available rebuilt fairly inexpensivly.

The ability of DC is two fold for the folks on board.

It can charge the Largest batt bank in the least amount of time .

It can also operate at low speeds when huge amps is not required

"With a DC generator the engine throttle should be linked back to the voltage regulator on the generator. This would create a varying RPM situation where the engine RPM would be what is necessary to produce the needed current. This may sound like a simple circuit to an engineer, but there is not a industry standard off the shelf unit to do this."

Folks have used std welding generator controls , yes, with some fiddeling.

The DC in higher output with a sine wave inverter can power air cond when required.

The same (expensive) inverter can boost power at a crummy dock to help air cond or laundry voltages.

I do value QUIET so a fast recharge and a slow running noisemaker would be first choice.Less stink too.

The new fancy Honda gas units operate by creating DC volts and use an inverter for good AC , and the partial load low RPM saves fuel too.
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Old 03-07-2017, 07:06 AM   #20
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This whole DC-AC discussion is great when we are talking about theory.

It is not as easy of a discussion when we talk about actually implementing it.

There are what...two choices in a off the shelf DC generator on the market. Maybe three???

How many brands of AC generators do we have to choose from? Pick your poison there.

I myself thought about building a DC generator...

But...

When the time came to make a real decision about my boat I bought a AC generator. No engineering, no trouble, just buy, install, and move on to the next problem.
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