Author Topic: Poor man's emergency genny.  (Read 9131 times)


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Poor man's emergency genny.
« on: November 21, 2009, 06:06:16 PM »
I guess this idea should go here as I haven't actually built it.  The idea occurred to me this morning after reading some old posts over on the HSM BBS.  I posted the idea there but one fellow said I was polishing a t*rd and wasting gasoline with a totally inefficient setup.  Prolly true but hey, I don't have a grand to spend on a fancy Honda genny.

The idea is to spin a junkyard auto alternator with a lawn mower engine, charging an auto battery which in turn powers an inverter to power up misc. household items when the power goes out.

The mower could be converted in the winter and converted back in the spring or as in my case, this oldie but goody Murray could be left in the converted state since it's my #3 spare. The wheels allow it to easily moved from the shed out back up close to the house when needed. Harbor Freight has a 2000 watt inverter for $149.95 or smaller ones for much less which I think would be properly powered by the 4 hp. mower at 1/2 to 3/4 throttle. I would use a junkyard alternator that has an internal, battery sensed voltage regulator so engine speed wouldn't be critical. Later model alternators have Power Control Model voltage regulation and a pain to rig a regulator.  Should be a snap to mount it with a few scraps of strap and angle iron. The belt would come right through the side discharge chute so no major mods would be needed to the mower deck. And no, I haven't thought this through far enough to work out any alternator/mower rotation direction issues. That could possibly complicate things a little.  It would need a chunk of scrap steel turned down to make a flywheel that would bolt on in place of the blade and then a pulley grafted on.

Seems like a reasonable emergency cobble job to me.  Apologies for the crappy CAD overlay. My old steam powered version of TurboCAD doesn't allow .jpgs to show up with rendered objects....hence the wireframe objects.

Milton in Tennesee

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Offline dsquire

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Re: Poor man's emergency genny.
« Reply #1 on: November 21, 2009, 07:21:45 PM »

Definitely nothing wrong with the thought process, that's how great ideas are born. I can't see any good reason why it wouldn't work as long as your rotation was correct.  :ddb: :ddb:

Cheers  :beer:

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Re: Poor man's emergency genny.
« Reply #2 on: November 22, 2009, 12:33:14 AM »
That isn't a bad idea. I've seen lawnmower engines converted to generators before. It usually takes most of the horsepower the engine will produce, of course depending on the load. One thing you must work out is the Alternator RPM to produce sufficient voltage. A small pulley at the crankshaft and a bigger one at the alternator won't usually spin it fast enough. Engines of this size and type have a max of 3600 rpm, that is usually what you have to use to develop enough power if the alternator is under a load. You probably know this, but if you multiply the required alternator RPM by the diameter of it's pulley and divide that by the engine rpm, that will give you the required engine pulley diameter. Also, I noticed that you labeled the pulley a flywheel/pulley. The engine has a sufficient flywheel under the cowling. It will run properly with nothing attached to the crankshaft under the mower, and considering the charging application, I don't think you need to add to the flywheel to stabilize the rpm.

Interesting project, I hope that this might help. If you decide to build it I am sure that I am not the only one that would be interested reading about your progress.

« Last Edit: November 22, 2009, 12:35:31 AM by Russel »

Offline trevoratxtal

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Re: Poor man's emergency genny.
« Reply #3 on: November 22, 2009, 05:29:33 AM »
Hello Dickybird
Your concept can and will work to charge a battery.
I made something similar back in the 70s when we had power strikes in the UK.
I run lights to a milking parlor, 12v and kept the milking machines running off a tractor but that is a another story. 
The rotation of a alternator works both ways as a 3 phase AC is produced then rectified to DC for charging!.
12v to mains converters will not work with most fridges, as they use a syncro motor and to match the perfect frequency and start current is not easy.
However lights Tvs and Radios mostly will work.
Wiring to the battery from the alternator is straight forward as the regulator is in the alternator, however either disconnect the battery after charging or fit a switch between the small plug on the alternator and the battery as this is the supply line for the activating coil and if left on will flatten the battery just like the ignition switch on a vehicle.


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Re: Poor man's emergency genny.
« Reply #4 on: November 22, 2009, 07:00:34 AM »
Wot's that big red thing with all that green stuff under it?  :lol:

I went concrete and cement years ago.


Offline John Hill

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Re: Poor man's emergency genny.
« Reply #5 on: November 22, 2009, 02:06:02 PM »

I went concrete and cement years ago.


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Offline sbwhart

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Re: Poor man's emergency genny.
« Reply #6 on: November 22, 2009, 04:00:52 PM »

I went concrete and cement years ago.


Where does Bandit go for his number ones?

Where ever I'm going to step

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Offline John Rudd

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Re: Poor man's emergency genny.
« Reply #7 on: November 22, 2009, 04:41:03 PM »

Where does Bandit go for his number ones?

I think someone has miscounted....

1 = (pee) wee....... :scratch:

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Offline crabsign69

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Re: Poor man's emergency genny.
« Reply #8 on: November 23, 2009, 10:35:01 AM »
ive got something simler but its a welder ill see if i can find that though i dont know how to get it on here yet . maybe you all could guide me in the right direction


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Re: Poor man's emergency genny.
« Reply #9 on: November 23, 2009, 09:19:58 PM »
There is a photo posting guide:

Offline jatt

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Re: Poor man's emergency genny.
« Reply #10 on: November 25, 2009, 02:28:13 AM »
Your backup genny doesnt have to be a Honda.

Picked up a new 3500W (peak) so what's that about 3000W in real terms in Oz for $850 about 18 months ago.  Yes it is a chop stick import with a Honda copy for an engine, but it is doing the job out on site nicely.

I'm sure that with some thought and trial and error u will achieve roughly what u set out to do.  In the past folks who tinker have created some terrific stuff, so I'm not gunna put down ur idea.

I know how it sounds, call me what u like, but I would at least price up what the bits u will need to do the job before going further.

Its only my opinion.
From an early age my father taught me to wear welding gloves . "Its not to protect your hands son, its to put out the fire when u set yourself alight".

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Offline Fred Bloggs

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Re: Poor man's emergency genny.
« Reply #11 on: December 03, 2009, 09:08:54 AM »

Just a comment on petrol generators and small plant engines in general, I have in the past played/modified and bodged these engines to run off LPG (liquid petroleum gas) which is 20% of the price of petrol in the UK, I even ran a old mini digger off LPG as the carb was B***ered and I refused to spend 200 quid on a new engine (there was no spares available for the original engine). All I did to try it out was get a small LPG tank (5Kg I think) fitted a variable pressure regulator (hiding in the junk box) and fitted 10 feet of gas hose to the reg, then I removed the air filter, wired the free end of the gas hose into the carb inlet cracked open the gas at a very low output pressure (approx 1/4 psi) and kept pulling the starter rope whilst playing with throttle settings and choke , after 10 or 15 pulls the engine started and ran!!
Once I got it to run I just set the pressure and throttle so that the engine would run well at 3/4 full revs and started to use the digger. Once I'd proved the engine would drive the digger whilst running on LPG I removed the carb , bored out the fuel jet to a size that would take a piece of small bored copper pipe (6mm) (glued in with epoxy) and attached it to the gas hose. Digger proceeded to digger the footings for the extension (SWMBO was please as total cost 0) and project only delayed by a day.
I have made up adapters in the past that fitted between the carb and the air cleaner so that an engine could be started on petrol then turned over to gas (easier and more flexible); normally a piece of 30mm diameter ally bored out to the internal diameter of the carb with a piece of 6mm copper pipe at 90degrees to the bore, is good enough.

I/we have also run diesels on gas (they actual still require diesel for ignition purposes, the gas will give more power or just enable you to reduce the amount of diesel you burn for a give power output) !! Note-you can blow the engine up, stick with petrols until youve had some practise and thought!!

Sorry about the poor grammar english was never my strong point at school.