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Joined: Nov 16 2016
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Yesterday I bought a new petrol lawnmower from B&Q for £140. Got to agree with my dad's engineer mate than a complete lawnmower with petrol engine and all the other bits for that money seems ridiculously good vfm when you consider what went in to making it... Probably couldn't even buy the engine separately for that much. So I wondered about buying another just for the engine and using the engine to drive an alternator to charge a battery to power an invertor to make my own invertor generator. Alternators are cheap from scrapyards and the lawnmower engine would have enough power to drive several alternators.. So how about a high power invertor and several alternators in parallel... But are there any issues with wiring alternators in parallel?

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Bolt is your man to answer this one but would you need more than one? The average modern alternator can give 100A so to require more than that would need a pretty big inverter. You'd probably also need a battery to float across the alternator output as I can't see the alternator and/or inverter being that happy driven directly. I've got a kilowatt UPS that had two 12V jelly cells in it and have toyed with the idea of connecting it to a couple of car batteries in series and charging them from a 24V truck alternator. My mate in France has moved into an old water mill that has the mill race still there so we were thinking about making a paddle wheel and driving the alternator from that. Then discovered that the water level has dropped over the years and there's only any flow through it for a couple of months a year.

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I don't actually need a jenny, just a matter of interest really and got to wondering after looking at the cheap petrol lawnmowers. Thought if I ever did need one a self build one might make sense and might as well be 2kw rather than 1kw.. Could probably make one quieter than most stuff on the market by experimenting with exhausts, cheaper and to own spec.

I've seen quite a few big RV's with 3kw invertors installed but wired up to just a single 100Ah battery, even with the capable invertor that sort of setup isn't going to run at fill tilt for long before the battery is flat (knackered even) even with the engine running. So also imagined a lawnmower engine on the RV blowing it's exhaust into the big engine exhaust, alternators obviously local to the lawnmower engine but the battery(s) and invertor could be a bit more remote. A lot of big RVs have standard fit jennys but they're 110v 60hz, some campsites don't allow generators but nobody need know one is running if it's quiet enough.

I know a couple of alternators feeding individual batteries and a 24v invertor would be a fall back option if there were issues with connecting 2 alternators in parallel... as long as one or both were electrically insulated from the chassis of the setup.

Lots of torque from a water wheel but not much power?

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Joined: Jan 07 2016
Posts: 99

Someone call?
Well, you have just re invented the wheel!
This has been done many times with highly variable results over the years. Remember that it will take about 1.4hp to get 1kw of power
out the wires.....This is in a perfect world, with no friction......Your results will vary.....
I built a rig like this to power up my "Bach" in NZ about 30 years ago. I had a 2 cyl Kawasaki motor from a "Mule 500" (500cc) that
I converted to electric for the customer. Brand new, and I converted it to LPG as well!
Using an 150 amp marine cruising alternator with precision 3 stage regulator, I was able to get about 75 amps consistently. into
400ah worth of flooded batteries. 150 was a real reach for it!
The main issue with automotive alternators and small engines is that, being designed to be small and light, they get their high output
only at high rpm. Not a problem under the bonnet. As they reach their rated HP at about 3600 Rpm or "Flat screech"
as it is called by those around them.......Also, being air cooled, lawn mower engines are difficult to sound proof. Muffler is only 1/2 of the issue.

Lead Acid batteries (Flooded, Gel, and AGM) all share the same basic characteristics in that they will all accept a high amperage charge initially, but their ability to accept the current will fall off rapidly. As the voltage rises, the current will fall.
If you try to shove in more current you will need to raise the voltage to do so. If you exceed about 14.6 (Avg) on a sealed
battery (AGM/Gel) you will damage the battery by causing gassing in excess of what the cells can re absorb. Battery will vent, and the
lost moisture cannot be replaced. Do this enough, and the battery will dry out, and die. Use a quality regulator with sealed batteries!
Mechanically, some of the best ones I have seen are direct drive. If you have a deck type mower, just remove blade and couple up alternator.
Belts are tricky unless you go with flat or cogged types they will usually slip. Also, the main bearings on a lawnmower engine are
not designed for side loading, and there is a LOT of side loading if you want to transfer 3-5hp. so straight shaft will last longer as well.
Running 2 in parallel is not really a problem, aside from the mechanical complexity. In your case here, Richard is correct. There will need to be a battery in parallel with the inverter. If you tried to run directly, the noise alone (Electrical) would probably do in a cheepo high frequency
transformer-less switching inverter. Further, as the alternator will need B+ to make power, if there is no battery it will not excite the field.
The "Inverter" generators you see on the market today are high voltage DC generators that use a trick inverter to hold frequency and
voltage stable over a surprisingly wide RPM range. Honda does this best..........If I wanted a small, efficient, quiet, stable genset,
I would buy one of those.....
Having said that....I would be happy to share any tips on a home brew! It's fun, if not really practical......
Hmmmmm There are those who say that about P-38s!

OH! as for the French mill?? Let me know, I will fly over to help! Hydro is the BEST form of renewable energy, and a typical 1800's grain mill will give
you about 10kw/hr of energy, with a LOT of torque, so it will not stall! If the power happens during the winter, do the mill
wheel generator, and run electric heat. For FREE!
Cheers!
Tom

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Bolt wrote:

OH! as for the French mill?? Let me know, I will fly over to help! Hydro is the BEST form of renewable energy, and a typical 1800's grain mill will give > you about 10kw/hr of energy, with a LOT of torque, so it will not stall! If the power happens during the winter, do the mill wheel generator, and run
electric heat. For FREE!

The mill was originally built in 1737 according to the stone set into the wall. The mill pond is still there but the small river/large stream that is supposed to feed it is dry most of the year. The hole where the paddle would have been is there, complete with big bits of stone with a hole in them for the axle to run through. The paddle would have been horizontal for some strange reason but we've no idea on the sort of design so any advice on that would be welcome. I'm up for it just for the hell of it and to prove it can be done, my mate isn't so sure..... All this is in a lower level which these days is used as a garage at one end and general storage at the other separated by these big bits of stone where the mill workings would have been so the actual house is on the first floor.

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If it isn't practical to use the original paddle location, you could pipe the flow and use an Archimedes screw instead...

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Joined: Aug 07 2019
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I used to do this for power at my shack 30 odd years ago. we used 4 to 5 hp stationary engines of all types , we used pulleys so we could get the right speed for the alternators and charge battery's with them, not direct power out of the alternator. the smart guys tweeked their regulators so they charged better (that's old type regulator not new electronic regulators)don't even know if you can still buy them?
PS you had to tickle them to start charging (put power across the alternator so it would load up)
hope that give you some ideas.

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Joined: Jan 07 2016
Posts: 99

There are all sorts of possibilities......Horizontal mills were used when there was not enough vertical fall for an overshot wheel.
They were not very efficient.....
Ducted screws likewise need some vertical fall to work well, and tend to be higher speed. I have seen a few very good ones in Idaho.
We need pictures!! Sounds like a seriously useful waste of time and effort! (and a great excuse to go to France!)
Since you guys are being so well surveiled over there, perhaps a google earth shot of the site?....Just wear your hoodie!

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Doesn't help you a lot. The mill pool is in the mucky looking bit below the house. As the shot dates back to 2017 and he didn't move in until November last year, you can't see the P38 parked outside or the new road that has been laid from the back lane to the barn (to the right of the house) that has now become a nice big workshop.

enter image description here

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Joined: Nov 16 2016
Posts: 890

Thanks for all the tips / advice. I did realise the need for a battery and about side loading the mower bearing, thought about a CV perhaps from a steering joint to turn a pulley and usual belts between the pulley and alternators.

Looks like Gilbert's under surveillance too... Dunno whether to blame davew or someone here with one or two Russian ex's (ahem) for bringing this attention upon us lol.

Maybe a lot of digging out of the pond etc there Gilbert.. Pity it isn't 1986 or you could've got job centres to send you some unemployed youths as they did for digging canals out.. Probably a few takers too, 'work experience in France' sounds fun.

As a kid I made a wind turbine (first with mecano blades then with thermoplastic got from school) to turn an electric motor direct drive with a diode to charge a 6v lead/acid bike battery I found at the local tip and partly revived. It did charge when it was very windy, powered bulbs in the summer house that used to be at the bottom of the garden.

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Joined: Jan 16 2016
Posts: 237

I have got about 5 small generators dotted around the place. I never throw anything out as my missus will testify.
I have an unused Petter diesel engine dating from 1960's that I need to get going, a French Bernard engine from 1950's, a couple of far east suitcase type generators and a small Honda clone made in China .... that is until Honda stopped them.
I also have a Briggs and Stratton 2 stroke lawnmower engine lying under a bench in one of my sheds, which you are welcome to have. I am not paying postage. Ha Ha. The lawnmower rusted away but the engine is OK. It runs horizontal though.
I have made the small Honda clone ultra quiet for use on camp sites. Enclosed it in an accoustic enclosure with a cooling fan and I run the exhaust into a bucket of water.

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Joined: Nov 16 2016
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Thanks for your very kind offer Dave, it's not something I intend on having a mess with just yet so I won't take you up on that.. Thanks again though.