Generator Opinions

Discussions about Renewable Energy, including photovoltaics, wind, and small scale hydro.
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Lostranger
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Generator Opinions

Post by Lostranger »

I'm bringing my generator project back to the front burner. I have an Isuzu four cylinder diesel out of a Thermo King reefer for motivation. Every time I get to the selection of a generator to mate to this engine, I get confused. This generator will have to run some some shop tools occasionally (large air compressor, tig welder, contractor table saw), and it will serve as backup for an off grid solar system. It will probably eventually live in the the belly of the 40' bus we're going to live in full time and will occasionally need to start two (maybe 3) air conditioners.

I've been advised that 8 kw is too large for my needs, but I've also been advised that 8 kw is too small. Nevertheless, that's the size I'll probably go with. To spark discussion, please give me opinions on this rare earth generator currently on ebay: http://cgi.ebay.com/8-KW-Rare-Earth-Per ... 1755wt_907

If the link doesn't work, searching ebay for 8 kw rare earth generator should get you there.

Thanks in advance,

Jim in Western North Carolina
Mark R. Obtinario
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Post by Mark R. Obtinario »

When sizing a genset it is important to size it appropriately.

If it is too small it will get overworked and you will have problems.

If it is too large it won't work hard enough and you will have problems.

I am thinking that if you are going to be trying to run three roof top A/C units you might be a bit on the small size. I don't think 8KW will be enough to handle the surge when they start up.
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somewhereinusa
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Post by somewhereinusa »

My son, who is an RV engineer, tells me that you MIGHT get by with an 8 running three airs, if they never start at the same time.
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Generator Opinions

Post by Lostranger »

Mark, thanks for your reply. Your response is appreciated, and it made me realize that I rambled too much in my original post. I'll try to do better.

First off, forget the three rooftop AC units. That was a mental hiccough spurred by an opinion of one of my adult children that two roof air units are not enough to cool a 40 foot bus under hot conditions. I don't even plan to use roof air. We'll probably have one basement AC unit, but it's too soon to know what size. I have it on good authority that 6.5 kw will start two roof air units. We don't plan to spend much time in hot areas, and whatever AC we have will be for emergencies. We plan to live mostly within a "solar budget", and that means fans and open windows and doors. Still, I'm sure we'll find times when the AC is welcome.

With that said, I'm going to buy an 8 kw generator of some sort to attach to my four cylinder Isuzu. I don't know the horsepower of this motor, but I suspect it could handle a good bit more than 8 kw. No matter, it's the motor I have and the motor I'll use.

What I want to hear is opinions on types of generators. It's something I've investigated several times through the years, but I still feel like I know too little. Brushes or not, permanent magnet or wound, heavy Chinese cast iron or sophisticated European design. I'm sure I'm missing plenty of other choices, but you get the point.

Someone who knows much about generators please tell me what you would use in my situation.

Thanks again,

Jim in Western NC
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Post by Sharkey »

If I was doing this, I'd do it a lot different than you are thinking.

Don't use a generator, I'd go out and get the biggest alternator that you could lay hands on. Yes, alternator, as in automotive alternator. You can get them these days with outputs up to 350 amps.

Step #2 is to install a DC-to-AC inverter system that meets your power consumption requirements. Most good inverters will surge to four times or more their continuous output rating.

Step #3 involves some batteries.

Why go with a DC system? Simple, if you use rectified AC from the alternator to run an inverter, you are freed from any and all requirements of engine speed. With a standard generator, heavy loads will cause the engine to load down a bit, making the frequency and voltage vary some, perhaps a lot until the governor in the engine compensates. Loads disconnecting will have the opposite effect, possibly with damaging results to equipment. Allowing a modern, solid-state inverter to handle power quality for you is much more reliable.

The bonus is that with some battery capacity, you don't have to run the damned generator at all for small loads. You can consume some portion of your battery capacity during periods when you want silent power, then pound some ampere-hours back into them when you do start the generator for other purposes. You're planning on having some batteries anyway, right? I'd assume that any smart vehicle dweller would also want an inverter so that they can run AC appliances from those batteries.

Combining the systems makes a whole lot of sense, your control systems for the engine become a lot easier and power quality issues would be non-existent.

If you go this route, you'd want a 24 or 48 volt system, makes things much more modern and tidy. Lessee... 350 amps at 48 volts = 16.8 Kw. Dual 3,000 inverters (120/240 volt) with a 4x surge capacity = 24 Kw surge. I think you'd be covered, alright.
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Post by Jones'n4chrome »

I like it Sharkey! Have you built a system like that?
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Post by Sharkey »

Back in the day (1990's) when off-grid power systems would need some backup power source for long winter nights, the popular choice was to belt up a Delco alternator to a Honda 6 Hp horizontal shaft engine and connect the output of the alternator to the battery bank. One company, Feather River Stove Works even started to manufacture them, called the Genny DeeCee. The were (comparatively) cheap to build, needed no external regulator, and since the PV system owner already had the inverter sized to run their normal household loads, AC was not needed. Generating 120 volt AC in the generator, then stepping it down to 12 volts DC to charge batteries was just unnecessary conversion, with the attendant losses.

Your typical "construction" or "homeowner" grade 1.5 -5 Kw generator probably will have a Briggs & Stratton engine which needs rebuilding after about 100 hours of operation. Generators with Honda engines are available, but they cost much more. In either case, a gasoline generator will usually run flat-out at 3,600 RPM to make 60 Hz AC. This is the top end of what small engines are designed for, and running them there wears them out quickly.

Diesel engine generators nearly all run at 1,800 RPM, so if you are designing around a generator (as opposed to an alternator), it's important to pick one that specifies 1,800. Running a diesel at 3,600 will be no joy and the fuel consumption will suck.

And alternator only needs to spin as fast as needed to make the desired output power (in watts). The little Honda "inverter" generators are popular for this reason. They have a built-in inverter, and only throttle up to whatever RPM is needed to supply the power output being consumed. Under light loads, they almost idle. Load them more heavily and they open up and run at higher RPM.

In comparison, a "normal" generator runs 3,600 RPM all day and all night to make the 60 Hz needed for AC power requirements. If the engine runs faster or slower, you don't get 60 Hz and things can get wonky if the equipment you are using cares about frequency.

Using the alternator/inverter gets around all this, it's a system with merits.
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Post by Jones'n4chrome »

Thanks Sharkey.

I've been thinking about this more lately because I want to put a workshop in the back of my bus.
I bought an Onan 4kw at pick-a-part auto recycling. It was in a wrecked motorhome. I happened to be there when they set it there to be "picked" It was a Friday about 20 minutes before they closed, and the next day was the 1/2 price sale. :D

So I went there right when they opened the next morning and pulled it out. The hour meter say 103 hours. It cost me $83.00 after taxes.
It started right up and ran good with a load. The problem is that I would rather sweat then listen to it run just to run the AC.
But now I'm thinking more about running power tools. I'm not sure if I even want to use it in the bus though.
Sharkey
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Post by Sharkey »

$83 is a good price for a running generator, and the Onan equipment is mostly good stuff.

Consider, however, that running a generator to run air conditioning is kind of backwards, why not run the a/c compressor from a belt driven by a pulley on the hybrid engine/alternator setup? It's bound to be more efficient to run a compressor directly off mechanical rotation rather than making electricity to run a motor to turn the compressor shaft. It's also likey to require lower RPM from the engine as well. Auto a/c runs at varible speeds all the time.
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Post by graydawg »

Chuck do you feel at least some guilt on that score, thats a fantastic little acquisition you made. I have a 7kw propane genny and it works everything well for me, I sure do want some solar to go with it. MRSHARKEY thats why we are following you, to learn and quite possibly live a better Life ourself, I have so much on the wants list and needs list, but it just has to wait for now. I also respect your judgement on things as well, I can't afford to do the tip jar to show my appreciation in monetary value, but one day it will come. James in da GRAYDAWG
PS I love the way you changed everything on the site it makes it so easy and faster to keep up with everything, THANKS again for your work and knowledge.
I ONCE WAS A MIGHTY GREYHOUND
I THEN GOT OLD AND RETIRED
I LOST MY SEATS AND GOT A NEW GIG
I AM NOW A HAULIN SOME OLD DAWGS &
I BECAME THE GRAYDAWG
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Lostranger
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Alternator Backup (was Generator Opinions)

Post by Lostranger »

Sharkey, I'm intrigued by your alternator idea. We'll be running a 24v system with a battery bank and an inverter. I was figuring on a singe inverter rated at 2 kw, but I can change those plans. We'll have shore power part of the time, but I don't want to be dependent on it. A large alternator and more inverter capacity might be the ticket, but I'll bet that much inverter will cost lots of bucks, especially if I go sine wave. (I like what I read about Magnum inverters.)

Sounds like I could use my Isuzu diesel to run this large alternator AND a compressor for our basement air. Where should I look for such an alternator? Truck salvage? Heavy equipment? Our Flex Metro that we've been using as a motor home for almost ten years has a large gear driven alternator on its 6v92, but I think it's only 225 amps. That's probably large enough, but I've never had it off, and I'm not sure how difficult the gear drive would be to adapt to a different form of motivation.

Should I go direct drive with a Lovejoy coupling, or would belt drive make more sense. Of course I'm working with 1800 rpm on the diesel.

Thanks everyone, especially Sharkey, for the input.

Jim in Western North Carolina
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Post by Jones'n4chrome »

Sharkey
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Post by Sharkey »

The first place I went to look at high output alternators was Wrangler NW: http://www.wranglernw.com/ShowCategory. ... goryid=770 they manufacture their own units in their factory in Portland.

Any alternator you buy should come with specifications describing the RPM required to produce the rated output. If that speed is equal to or below 1,800, you can direct drive it, otherwise, you'll need to use a belt or gearbox. Note that alternators can produce output when spun in either direction, but they come with fins to move air through them for cooling, and those fins only work in one direction. It might be possible to specify rotation direction when purchasing the alternator. This will matter, depending on which end of the engine you mount the alternator, and which way it faces. You don't get a choice of which way it faces in direct drive.
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Post by Jones'n4chrome »

Sharkey wrote:The first place I went to look at high output alternators was Wrangler NW: http://www.wranglernw.com/ShowCategory. ... goryid=770 they manufacture their own units in their factory in Portland.

Any alternator you buy should come with specifications describing the RPM required to produce the rated output. If that speed is equal to or below 1,800, you can direct drive it, otherwise, you'll need to use a belt or gearbox. Note that alternators can produce output when spun in either direction, but they come with fins to move air through them for cooling, and those fins only work in one direction. It might be possible to specify rotation direction when purchasing the alternator. This will matter, depending on which end of the engine you mount the alternator, and which way it faces. You don't get a choice of which way it faces in direct drive.
Good info, thanks.
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High Output Alternator (was Generator Opinions)

Post by Lostranger »

The problem I'm running into involves voltage. 12v high output is everywhere. 24v, not so much. Wrangler Power Products has one, part number 200-004146. It's in a Delco-type frame, and they only make it for one customer. Availability might be a problem. Rated at 115 amp, but that should be plenty at 24v. They get $489 for the alternator and a pulley.

Thanks for the lead, Sharkey.

Jim
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