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Old 13-01-2013, 04:47   #1
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Generator off the Main Engine Possible?

Does anybody have a generator which runs off their main large engine that works sufficiently well so that electronic equipment can be powered by AC?

My current set-up is a 130hp normally aspirated diesel to drive the prop shaft, and a separate small diesel powered generator to produce AC.

Is it possible to hook up and run a generator head off the 130hp and get rid of the small engined generator? And does it make sense?

I've read that generators need to be run at a constant 3600rpm to maintain voltage and amperage...which means that there is a problem if the rpm on the large engine is not held constant...is there a solution?
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Old 13-01-2013, 04:52   #2
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Re: Generator off the main engine possible?

Just install a large DC alternator -- like a school bus alternator, for example, Leece-Neville. Some of them are capable of producing 2 or 3 kW of power. Mine is good for 110 amps at 27 volts or around 3kW, and 2kW practically from idle.

And use a quality pure sine wave inverter.

There have been some attempts to market engine-driven AC generators, but in my opinion it is far too much trouble to be worthwhile. You get fantastic quality AC power out of a good inverter, and you have your batteries to buffer demand. I don't know why anyone would want to produce AC power with their main engines any other way -- it's a perfect solution.
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Old 13-01-2013, 05:27   #3
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Re: Generator off the main engine possible?

Yes, they exist ... but googling for them is a little difficult.

Here's a good starting point
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Old 13-01-2013, 05:49   #4
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Old 13-01-2013, 09:51   #6
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Re: Generator off the main engine possible?

The 3600RPM figure depends on how many poles in the generator. For 60hz and a 2 pole generator yeah you need a governor on the engine that will maintain 3600 RPM on generator when the load changes. You can't just set the throttle unless you have a rock steady load. But if you had a 4 pole generator you could turn the generator at 1800RPM.

Remember, the necessary RPM is at the generator. Let's say you need to run the generator at 3600RPM but your engine will run best at say 2400RPM. Let's say you are driving the generator with a belt. You want the pulley size difference that will increase by 50%. You will of course have a corresponding reduction of torque available so that has to be considered. Anyway, let's say your engine pulley is 12" in circumference. If the pulley on the generator is 8" in circumference for each foot of belt passing over the engine pulley, a foot of belt also passes over the generator pulley. One revolution of the engine will pass that foot of belt. The foot of belt will turn the generator pulley 1-1/2 revolutions. So for every 2400 revolutions of the engine you turn the generator 3600 revolutions using those pulleys. It is all about the generator RPM, and matching a pulley ratio to an engine speed that is in a favorable spot on the power/efficiency curve. Want to run your engine at 1800RPM and turn a two pole generator? No problemo. You need a 2:1 pulley ratio. Want to run it at 1200 RPM? 3:1. 900RPM? You can figure it out.

You will still need some sort of governor that will keep your output frequency reasonably stable under load changes. Over or under frequency is a bad thing for some loads.

A proper AC generator is going to be more efficient than alternator and inverter setup, but not by much. You will probably find that for loads up to a KW or so, the ease and convenience of an inverter definitely outweigh the marginally better efficiency of an AC generator. A KW at 110VAC is 9 amps. If you have an inverter rated for 1KW continuous, you have 9 amps continuous and can probably surge a bit more without tripping. For a steady 1KW load on the batteries, you need a 1KW+ alternator. At 13V that would be 77 amps so figure on at least a 100A alternator.

An alternator/inverter system would work better with a higher DC voltage. 12VDC means high current, and high current means the voltage drop is higher for a given conductor. A 36V system would give you far lower line losses and enable you to use smaller wire (lower current) but you would be looking at a higher priced alternator such as one from a trawler (fishing type trawler, not trawler style yacht) and the inverter would be expensive, too. Best bet for the inverter would be somebody like Cloud Electric or Thunderstruck. And you could go with higher voltage... 48, 72, 144, whatever. Yeah I think they do alternators too.

Most gensets will already be set up for pretty decent efficiency. The engine will be properly matched to the generator for a given load range. If you want continuous AC at anything over a KW or two, I really don't think you can beat a purpose-built diesel genset. Want to go cheap? Don't mind a little noise? There are some pretty cheap small aircooled diesel generators coming out of China. Don't expect one of them to last as long as something from Westerbeke or Northern Lights, though. And good luck on warranty service!
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Old 13-01-2013, 10:20   #7
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Re: Generator off the main engine possible?

The Elja Power Generator is independent of the main engines revs. Nice genarator used in The Netherlands in almost all Fire Engines, Ambulances and every where mobile 220 V AC is needed.

CeesH
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Old 13-01-2013, 20:18   #8
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Re: Generator off the main engine possible?

Quote:
Originally Posted by GrowleyMonster View Post
An alternator/inverter system would work better with a higher DC voltage. 12VDC means high current, and high current means the voltage drop is higher for a given conductor. A 36V system would give you far lower line losses and enable you to use smaller wire (lower current) but you would be looking at a higher priced alternator such as one from a trawler (fishing type trawler, not trawler style yacht) and the inverter would be expensive, too. Best bet for the inverter would be somebody like Cloud Electric or Thunderstruck. And you could go with higher voltage... 48, 72, 144, whatever.
A 115V AC output inverter with a 144V DC source is slightly more efficient than one with a lower input voltage. I would not install 144V DC on a boat just to power an inverter though. On the other hand, if I had a 12V or 24V house bank and a 144V propulsion bank and needed an inverter, I would choose a 144V inverter.
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Old 15-01-2013, 00:40   #9
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Re: Generator off the main engine possible?

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Originally Posted by Dockhead View Post
There have been some attempts to market engine-driven AC generators, but in my opinion it is far too much trouble to be worthwhile.
AFAIK these are all systems that involve inverter. The variable frequency AC output of the engine mounted alternator is rectified and then again converted to AC using a sine wave inverter.
Better to put batteries in between the rectifier and the inverter, so you can size your alternator for average loads, and only need to size your rectifier for peak loads.

An interesting option that some engine manufacturers now offer is flywheel alternators, that you mount between the engine and the gearbox. Can generate lots of power...
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Old 15-01-2013, 02:25   #10
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Re: Generator off the main engine possible?

Dockhead has it right!! theres lots of BIG alternators in wrecking yards most everywhere!! from ambulances, large trucks ect! Ive used large GM alts for years from bus as said or ambulances. Used with mulitible battery banks, and a large pure sine inverter, you can run most anything except for maybe AC altho with enough Batts ya could run it a while LOL On our colvin, we had a engine driven water maker, so when we made water we charged batts ! worked for us for years and was pretty cheap to make up! way cheaper any bal mar set up !!
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Old 15-01-2013, 02:46   #11
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Re: Generator off the main engine possible?

Quote:
Originally Posted by GrowleyMonster View Post
The 3600RPM figure depends on how many poles in the generator. For 60hz and a 2 pole generator yeah you need a governor on the engine that will maintain 3600 RPM on generator when the load changes. You can't just set the throttle unless you have a rock steady load. But if you had a 4 pole generator you could turn the generator at 1800RPM.

Remember, the necessary RPM is at the generator. Let's say you need to run the generator at 3600RPM but your engine will run best at say 2400RPM. Let's say you are driving the generator with a belt. You want the pulley size difference that will increase by 50%. You will of course have a corresponding reduction of torque available so that has to be considered. Anyway, let's say your engine pulley is 12" in circumference. If the pulley on the generator is 8" in circumference for each foot of belt passing over the engine pulley, a foot of belt also passes over the generator pulley. One revolution of the engine will pass that foot of belt. The foot of belt will turn the generator pulley 1-1/2 revolutions. So for every 2400 revolutions of the engine you turn the generator 3600 revolutions using those pulleys. It is all about the generator RPM, and matching a pulley ratio to an engine speed that is in a favorable spot on the power/efficiency curve. Want to run your engine at 1800RPM and turn a two pole generator? No problemo. You need a 2:1 pulley ratio. Want to run it at 1200 RPM? 3:1. 900RPM? You can figure it out.

You will still need some sort of governor that will keep your output frequency reasonably stable under load changes. Over or under frequency is a bad thing for some loads.

A proper AC generator is going to be more efficient than alternator and inverter setup, but not by much. You will probably find that for loads up to a KW or so, the ease and convenience of an inverter definitely outweigh the marginally better efficiency of an AC generator. A KW at 110VAC is 9 amps. If you have an inverter rated for 1KW continuous, you have 9 amps continuous and can probably surge a bit more without tripping. For a steady 1KW load on the batteries, you need a 1KW+ alternator. At 13V that would be 77 amps so figure on at least a 100A alternator.

An alternator/inverter system would work better with a higher DC voltage. 12VDC means high current, and high current means the voltage drop is higher for a given conductor. A 36V system would give you far lower line losses and enable you to use smaller wire (lower current) but you would be looking at a higher priced alternator such as one from a trawler (fishing type trawler, not trawler style yacht) and the inverter would be expensive, too. Best bet for the inverter would be somebody like Cloud Electric or Thunderstruck. And you could go with higher voltage... 48, 72, 144, whatever. Yeah I think they do alternators too.

Most gensets will already be set up for pretty decent efficiency. The engine will be properly matched to the generator for a given load range. If you want continuous AC at anything over a KW or two, I really don't think you can beat a purpose-built diesel genset. Want to go cheap? Don't mind a little noise? There are some pretty cheap small aircooled diesel generators coming out of China. Don't expect one of them to last as long as something from Westerbeke or Northern Lights, though. And good luck on warranty service!
The slight theoretical efficiency advantage of a pure AC alternator is offset many times over by the big advantage of using an inverter in that the batteries can "shave peaks" off your demand.

So if you need 3kW of power (similar to what you get out of a regular shore power connection), you can easily cover this with a large inverter, and you don't need 3kW of generating capacity. You can use a much smaller DC alternator, school bus size. It just needs to cover your average power consumption.

So this will be more efficient than a larger AC alternator which is run at a small percentage of its capacity all the time.

Carrying 2kW with 12 volts will require quite large cables. Of course, 24 volts is better for this, but if the rest of our DC system are 12v, it's probably not worth creating a separate 24v system.

One more tip -- if you try to invent some unusual system, chasing some small theoretical advantage, you will anyway be sorry, because you will lose the benefit of all the R&D done by all the manufacturers in creating the conventional systems. A very well trodden path, and thus highly developed, reliable, and relatively cheap equipment, is a regular school bus alternator, battery bank, inverter.

Now you mentioned that you have a separate diesel genset on board. Don't toss that out if it works well! If you're using a lot of AC power for some reason, you can't beat a separate diesel genset. No matter what kind of generating equipment you have on your main engine, you will not want to run your main engine for hours and hours at a time just to generate electricity. It's harmful to run it for long periods of time without a good load.

Separate diesel gensets are great things to have! I have on my boat both a Kohler 6.5kW low speed 1500 rpm genset, driven by a 3-cylinder Yanmar engine, as well as a school bus alternator & inverter. These two ways of producing AC power are complementary to each other-- it's good to have both. I keep my boat on a mid-river mooring without shore power, and I have a large complex boat with a lot of AC electrical equipment on board, including a washer/dryer. So the genset is almost indispensible -- I run it once or twice a day when I'm not moving around, running the main engine. I do this at times when I need a lot of AC power, and so I charge my batts at the same time.

The school bus alternator puts a charge in the batts very rapidly when I'm running the main engine to move somewhere. After a long sail, it will just about get the batts back up during the time I am manuevering in harbour or anchoring. While the main engine is running, there is abundant AC power through the inverter.

And, when at anchor on battery power, there is also AC power, from the inverter powered by the batts, plenty for most ordinary purposes like running a microwave or toaster, boiling water in a kettle, running power tools, etc. It's great not having to crank up the generator or main engine, just to use the microwave.

So I can recommend to you the following scheme, which covers all your bases: 1. keep your diesel genset; 2. add a second alternator, high capacity, school bus type, dedicated to powering your house bank (mine is 110 amps x 24v); 3. add a good charger/inverter, powerful enough to cover your expected maximum AC loads (mine is 3kW and has always been more than enough for all my loads).
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Old 15-01-2013, 03:33   #12
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Re: Generator off the main engine possible?

Quote:
Originally Posted by Honey Ryder View Post
Does anybody have a generator which runs off their main large engine that works sufficiently well so that electronic equipment can be powered by AC?

My current set-up is a 130hp normally aspirated diesel to drive the prop shaft, and a separate small diesel powered generator to produce AC.

Is it possible to hook up and run a generator head off the 130hp and get rid of the small engined generator? And does it make sense?

I've read that generators need to be run at a constant 3600rpm to maintain voltage and amperage...which means that there is a problem if the rpm on the large engine is not held constant...is there a solution?
I am working on a 100Kw off the main engine, it's a variable speed during propulsion, my big issue is bringing a second 100KW online and syncing them up so they share the load. When I started this job, no problems with the 1st gen, it all cascaded when the 2nd gen came
online!!!

Lloyd
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Old 15-01-2013, 04:22   #13
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Re: Generator off the main engine possible?

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Originally Posted by FlyingCloud1937 View Post
I am working on a 100Kw off the main engine, it's a variable speed during propulsion, my big issue is bringing a second 100KW online and syncing them up so they share the load. When I started this job, no problems with the 1st gen, it all cascaded when the 2nd gen came
online!!!

Lloyd
Good God, Lloyd, what are you doing with all that power?! Making steel on board?

Is that DC or AC power? If it's DC power you won't have any problem synching them-- just dump it all onto a common bus and the inverter (or in this case, gang of paralleled inverters) will sort it out.

If it's AC power, then you are in for some commercial power generating type control systems, costing probably millions.
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Old 15-01-2013, 04:52   #14
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Re: Generator off the main engine possible?

And always remember, that a big alternator will apply big loads on the belts.
This may give you trouble, while manouvering, i.e. when changing from
forward into reverse gear, cause when doing so, you need low revs in order to keep the gearbox alive, but low revs might not be enough to withstand the load of the alternator(s), and the engine will stop....
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Old 15-01-2013, 06:40   #15
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Re: Generator off the main engine possible?

OMG Do NOT try to run two AC generators in parallel unless you are equipped to do so and know what you are doing!!!!

How can your engine be varying speed while producing 60Hz AC? Is it not an actual AC Generator? Is it a DC generator and electronically controlled Motor/Generator, or DC gen and integral inverter? With an AC generator, frequency is determined by speed.

If you need to run two generators at the same time and you do not have any kind of electronic speed control, load meters and a synchroscope, you need to run in split plant mode. Simply put, you have a switchboard for each generator. Some loads are fed by one board, some by the other, and you can have loads with either board available as an alternate power source. You need a breaker between the two )or more) switchboards. In split plant, each generator powers one isolated part, i.e. one switchboard, of the entire system. You don't have to worry about phase angle differences. When running just one generator, close the breaker between the two (or more) switchboards so the bus feeds everything. Want to switch generators? If you can't run them in parallel, you start the offline generator with the breaker open, then trip the online generator breaker (you will be dark ship for a moment) and close the oncoming generator breaker.

To parallel two AC generators, you first match the frequency as closely as possible, referring to a synchroscope or other instrumentation. The phase angle should slowly advance or retard. Be ready to adjust the frequency control of the oncoming generator. When the two generators are about 5 degrees before being in phase, close the oncoming generator breaker and watch your loads. Immediately dial up the frequency of the oncoming generator which actually will not change, but it pushes the oncoming generator into the load. You will see the load balance between the generators. THis is not for the fainthearted or the underequipped. Just saying how it is accomplished so you know you probably shouldn't try. You can totally fry a generator trying to parallel two generators out of phase. You can trash your whole AC electrical system. Start a fire? Sure. Your local electrician won't respect you but he will love you cause you are looking at a big bill.

To shut down one generator after running in parallel, retard the frequency control for that generator so that it sheds the load onto the other generator. Then when the load is near zero trip the generator breaker and go through your shutdown procedure.
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