Lawnmower Power Generator With a Twist




About: like to work on motorcycles, especially dirtbikes. for sure like to build things and make them work..outdoors, bbq,fishing..

This is a 500 watt power generator big enough to power tools, and home electronics.  If you try plugging in kitchen appliances, heaters, window AC units will shut down, you need a 2k-3k watt power converter for large power comsumption appliances..

 It's not difficult to build this project, if you know how to wrench things apart and have a basic understanding of how things work with engines and electrical components then you'll do just fine.  If you want to save money on the build, go to junkyards, recycling centers, and landfills which is where I got most of the parts.   You'll be surprised the stuff you can find for pennies or better yet FREE.   I spent $40 for all the parts needed for this project.  

There are a few optional parts not needed to run this little power jewel.   The lawnmower tires, enclosure, vehicle battery, handle, voltage meter, 120VAC outlet, light, and on/off switch for light.   I decided to go all out with this one by adding all those extra features, just took me longer to make, 3 months exactly.  A schematic diagram will show how to wire everything up.    The AC outlet you see on the panel is tapping from the power converter which already has AC outlets.  I just wanted to make an extension of it so I wouldn't reach around the back to plug in things....

Step 1: Parts, Where, Tools, How Much$$

Here are some links for the power tools I used.....,,,                                                                       (1)  WOODEN or METAL platform-   Helps bolt down the Alternator, Engine, and DC to AC power converter.  I used aluminum sheet metal with L-frame bars for a sturdy frame.  Got the sheet metal and L-bars for FREE from recycling center.   I used a JIGSAW with a metal cutting blade to get the job done easy.(NOTE)..Do Not Try Using the jigsaw to cut  IRON or STEEL...The blade will kick back and will break sending pieces flying...If you have better cutting tools for that kind of material then by all means use it.  

Other tools include a drill press, vise grip, portable hand held drill, hacksaw, and a hand held file.   If you have better tools to get the job done quicker and safer don't hesitate....It took me three months to build the crazy thing, and a few busted knuckles.
   The platform size varies with how many components you want to throw on it, the one I made is a  2'x3'  with a top shelf to hold vehicle battery.  more details about that later.

(2)  LAWNMOWER ENGINE-   Vertical or Horizontal, 3.5HP or bigger.   I got it for FREE from landfill (vertical shaft) with a key way slot, or you can buy a new one for $100+....

(3)   ALTERNATOR-   It's a delco part# CTBBB-7127-12 rebuilt, with internal voltage regulator, 65amp, two wire connector harness.  paid $23, auto parts store.    This is the heart of the system,  I recommend using this one, it's simple to setup and comes with an internal voltage regulator.

(4)  DC to AC POWER CONVERTER-  The one I used is a 500 watt converter, got for FREE from recycling center.    The bigger the better, pure sine wave is the best. 

(5)  VARIABLE RESISTOR-  Also know as a  Rotory Switch or POT.   You can get one from an auto junkyard for $2... look for an older vehicle, 70's through late 80's any model, also known as a light dimmer switch or dashboard, panel lights just turn the knob in one direction and the lights dim or need a 2 wire pin switch....this part is used to control the amps coming from the alternator.....

(6)  ON/OFF Switch-  There are a few different kinds...rotory, toggle, rocker, and push.  I used a toggle with a built in green light.

(7)  PULLEY- 3" or 4" diameter, IRON or STEEL,   $3 from junkyard,   Make sure it has a key way to help prevent it from slipping on the engine shaft.  (NOTE) make sure the pulley is equal the weight of the cutting blade or greater,  if not the engine will not hold it's spin cycle.  Got mine for FREE from recycling center.

(8) BELT-  $12 at lawnmower repair shop or auto parts store.  If you want cheap try the  landfill which I got from, or recycling center, auto junkyard or neighbor throwing it away.  I recommend a belt from a self propelled lawnmower, makes a perfect fit.....which I used on this project.

(9) BOLTS with NUTS & WASHERS-  Paid $12 for nuts, bolts, and washers from home depot.  Try to get rust proof hardware.

Other parts are optional like the voltage meter, light, on/off toggle switch for light, and extra 120VAC outlet,  which are not required to operate a fully functional generator.

Step 2: The Frame

Not much to it, I built one using dimensions 2'x3' with aluminum sheet metal, I was able to collect for free from recycling center and land fill.   The size will depend on how much you want to add to it, I decided to add a top shelf to hold a vehicle battery and lawnmower wheels to push around..also added a handle on top.

The frame is what took me the longest to make for lack of tools, I used a jigsaw with a metal cutting blade which was kicking my butt, the blade would sometimes kick back so I had to take it slow.  To smooth out the rough edges I used a hand file, also used a drill press, hand drill, and hack saw for the L -bars.   I recommend using a band saw to cut the sheet metal and L-bars, that would have saved me alot of time.

Step 3: The Engine

The engine is a 3.5HP throttle free design B&S engine, with a vertical shaft key way slotted on the shaft, got it free; the only thing I had to do to it was clean the carb and piston.  you can also use a horizontal engine meaning the shaft sticks out from the side, electric start or string pulled.  Try to get one with the shaft slotted, if you come across one with the end shaft threaded don't even bother unless you want to take the time welding the pulley to the hub....(NOTE) don't forget to remove the cutting blade.

You can use the same mounting bolts used to secure the engine to the lawnmower chassis..I used a jigsaw to cut out a hole big enough to have the pulley go through it, you first want to start a small hole with a large enough bit using a hand drill to fit the cutting blade through it

Step 4: The Alternator

The alternator is a 65 amp with a built in voltage regulator delco part# CT BBB-7127-12 which can be purchased at your favorite local parts store for $23 rebuilt including a two wire pin harness.  Alternators with built in voltage regulators are easier to work with as far as wiring goes.  Ask the desk clerk for that particular part# listed above, you shouldn't have any problems...
How to connect the alternator.  Here you want to the RED wire coming from the wire harness back into that positive nut.  The purpose of that is to trick the alternator in pushing out a steady 14VDC at any RPM.  The wire harness is included which consists of a two wire RED & WHITE.  the WHITE wire has the dimmer switch connected in series which is used to control the AMPS coming from the alternator to charge a vehicle battery.  more details about the dimmer will probably make more sense looking at the schematics shown later..   Notice the two thickest wires RED & BLACK, they are for charging a vehicle battery which is optional, not required to connect on alternator if you don't want to charge batteries.  

For a simple setup all you need to connect is 2 wires, 10 gauge size, Positive + (RED wire) connect it to the upper left nut on back of alternator,  the other end of that wire goes to the positive(+) terminal of the converter box.   The Negative - (BLACK wire) connect to the lower right nut(that is your negative) on back of alternator,  the other end of that wire connects to the Negative(-) terminal of converter box.......That is it, just start the engine and turn on the converter box you should have 120VAC.......If you want to add meter gauges, lights, dials etc....I have more details with schematics...which will take longer to build...

To install the alternator just cut out a hole big enough to adjust the belt along with two bolts to secure the alternator....if it wobbles just add brackets to make it sturdy..  nothing to it.

Step 5: The Converter Box

Here I have a 500 watt DC to AC 120VAC output.    Free from recycling center.  If you want to power kitchen appliances, heaters, window AC units, and large power tools which require alot of power comsumption, you will need at least a 2k watt or greater power converter.  I recommend a pure sine wave power box of at least 3k watts to be able to handle a large load.

Most converter boxes have built in 120VAC power outlets, just plug in directly to power up your home electronics or small power can also plug in a power strip directly into it like I did. GREY wire....Notice how I connected an outlet extending from the coverter box, that is so I wouldn't reach to the back end to plug in can see the outlet showing in front of generator..

How to connect converter box to alternator.   Simple, do you see that thick BLACK wire curving over,  that is your NEGATIVE(-) wire coming from the NEGATIVE post(nut) from back of alternator.  connect this wire to the NEGATIVE(-) terminal of converter box.

The POSITIVE(+) RED wire coming from POSITIVE(+) post(nut) of alternator connects to the POSITIVE(+) terminal of converter box...I ran out of red wire so had to use black wire....

You can custom make your own wire end terminals with a little bit of solder and heat shrink to have a custom professional look....You can secure the converter box with battery j-hooks, installed one on each corner....

Step 6: Variable Resistor(light Dimmer Switch)

This pot can be found on older vehicles, 70's through late 80's maybe older.   It's purpose is to adjust the amount of amps coming from the alternator to charge a vehicle battery which is optional.    This part has a two pin wire connected in series to the back end of alternator.

Here is how you connect the dimmer.   On the back side of alternator there is a connector with two wires sticking out of it, shown in step RED and one WHITE, you can connect any one of the wires from the DIMMER to the WHITE wire, the other wire from DIMMER connects to the POSITIVE(+) nut on back of alternator nothing to it.  

That's it, the dimmer is connected in series and now you can adjust the amps by twisting the knob on dimmer, you have to play with it to have at a proper setting.  If you allow to much current to charge a battery, the alternator will slow down a bit and might cause the engine to stop.....You can install a bigger engine with more power.

Step 7: Main Power Switch

This toggle switch has a built in green LED light, costs $2 new, before you throw away the package look at the schematic on the back side, wright it down or memorize the position of the 3 legs or pins and which is positive and negative....

Here is how you connect it.   This particular switch has 3 pins on the back for attaching wires....I will refer to them as pin1, pin2, pin3.  

Step One:  connect a RED(+) wire to the POSITIVE(+) terminal of converter box, then connect the other end of wire to pin2(NOTE: do not forget to attach a 10amp fuse in series) this wire will be the main wire carrying POSITIVE voltage.

Step Two:  connect a RED(+) wire to the POSITIVE(+) terminal(nut) of alternator, then connect the other end of wire to pin1.

Step Three:  connect a BLACK(-) wire to the NEGATIVE(-) terminal of converter box, then connect the other end of wire to pin3.       Thats it.    Keep in mind! you can solder wire terminal at the end of the wires to help secure them tight and clean. 

Step 8: The Pulley

Use an iron or steel 4" pulley to help with weight differences, the cutting blade helps the engine rotate easily because of it's weight and centrifugal forces.  get one with a key way to help it lock on to the engine shaft, helps the pulley from slipping off.   not much to it there.

Step 9: The Belt

Before cutting a hole, measure the distance it will take to install the belt between the lawnmower pulley and the alternator pulley.  You don't want to end up to tight where you won't be able to install the belt, or too loose where there won't be any rotation of can make a track to adjust the alternator to release the belt or tighten...

I recommend the belt from a self propelled lawn mower, that makes a perfect fit for it.

Step 10: The Bolts

Not much to it except you can use rust proof ones....

Step 11: Optional Parts

I just added these components to add more features to it....not needed to run generator.  included in schematics with details if interested in adding them....

Step 12: Labels

You can down load labels from internet for free, just look for hazmat labels..I just printed them, cut them out, and taped with clear tape.....

Step 13: Optional Meter Gauge

Not required to operate the generator I just wanted to add extra features, the next step shows the schematics.  Not much to explain there too easy....

Step 14: Schematics

This should give a general idea of what it looks like on paper or board....will post more schematics for each section.......

Step 15: Alternator Schematic

As you go through each step, it will show you what connects where....just follow the simple steps and it will all come together...

Step 16: LED Switch Schematic

Same here.   each step will show how to connect each part, just follow the simple diagram and notes...

Step 17: Optional Meter,light,on/off Switch Schematic

Like I said before this section is not required to work or operate the generator, I just added for extra features....schematic shows how to connect if you wish to follow through...

Step 18: Final Step Optional Handle

This is the last part I put on,  once again it's optional.  Picked it out from recycling center and bolted it on...not much to it......That's pretty much it, took me 3-months to build this thing, what took the longest was building the frame because of lack of tools and finding parts at little to no cost.  That alone was a bargain. 

                                                                                                                                                                 Good luck with this project, if you have any questions about the schematic let me know, some of the pics are far away.  The battery is used for backup in case the motor runs out of fuel which is not being used indoors.....That would be to loud!

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60 Discussions


8 years ago on Introduction

Nice, I'm looking at building one of these. I want to add a circut to recharge or jump car batteries and maybe add field control to convert mine to a light-duty welder.

The 3.5 HP engine is limited to maybe 2500 watts total.

You are using a GM Delco 10si alternator, which can only put out about 900 watts at 14volts times 63 amps. I assume the unit is self-excited, so if you pulled one out of a junked car, you would probably have to install a "one wire" kit. Although the 10si is  ubiquitous, with that engine I guess you have headroom to use a larger alternator.

Technically, that cooling fan on the alternator is running the wrong way, but I'll bet it works fine because it's not located in a hot under hood area of a car. The alternator itself doesn't care which way it spins.

"Battery" is not in your parts list. I'd recommend a deep cycle type. If you use the less expensive wet-cell type, get a battery box for the battery and bolt it down. Acid burns.

Good job with the safety-nazi stickers. No one could fault you there!

4 replies

Reply 1 year ago

I use an old ford alternator with internal field removed and just some wires added. I have then built a regulator with an lm358 and a couple off high power relays to connect an disconnect the alternator output from the battery and to connect the field directly to the output. This way I have a toggle switch to switch between battery charging and welding. Works great


Reply 3 years ago

Harbor Freight has a cheep jump box with two 12 volt light plugs to add to this configuration for a cheep battery and possible starter circuit. The great part about this box is you can connect it to the 12 volt side and charge the battery directly or you can use the provided power adapter for a trickle charge. pickup and cheep 10 dollar drill for a starter moter and volla, you built something you could by for 150 already complete but it is nice to customize these sort of projects.

watch the coupons and you can get it for 39 bucks. add a cheep 12 to 120 converter zip tied to the back and you can actually carry it around and run about anything you want.


Reply 8 years ago on Introduction

I bought the alternator at a parts store with a 2 wire harness connector. I didn't mention the battery because it is not required to start the generator, it's just an optional feature.


confirmed with a search: 7127 is "self-exciting" or "one-wire". The "-12" means that alternator is "clocked" at the "12 o'clock" position.

Looking at the back of the alternator, think of the field winding contacts as the hour hand and the pivot bolt hole (the unthreaded mounting bolt hole) as "12" on a clock. By assembly, the alternator can be in four positions. 3, 6, 9, and 12 o'clock. If you know how to restrain the brushes, you can take apart the alternator and "re-clock" it for different applications in different cars.


1 year ago

The only thing I would change on your design is turn the engine 180 degrees to keep the hot air from the head away from the alternator


4 years ago on Introduction

Hi, I tried your setup, but my engine shaft is turning to the left, and the alternator turns right (otherwise no power), had to turn it upsidedown. Nice project !

7 replies

Reply 4 years ago on Introduction

just ran across your post. you can turn the alternator in either direction, it will work

fine either way. hope u haven't given up on this project. good luck


Reply 1 year ago

True and not true... The alternator doesn't care which rotation it runs in as far as power generation, however the cooling fan IS directionally sensitive and operating near the top end of it's power generation and with the reduced air flow caused by operating in reverse rotation will shorten the life of the components with the internal voltage regulator usually being the first to fail, potentially killing the inverter.


Reply 1 year ago

In view of the fact that this setup is running in more or less an open environment (under 110 degrees Fahrenheit), the reduced air flow that you are suggesting, will keep the alternator well within operating temperatures, as opposed to under the hood of a vehicle where temperatures can exceed 300 degrees Fahrenheit. Have you ever had a setup similar to this, and actually run it both ways just to see the difference in amount of air that is moved? I have and although i don't have a degree engineering or anything else for that matter, it isn't that hard figure out that there is plenty of air movement to keep the alternator cool enough to do it's job. I also think that a 3.5 B&S engine would probably self destruct trying to burn up the alternator. Just my personal opinion. Any comments are welcomed.


Reply 1 year ago

That may be true Sidecar, however as set up with no storage battery, the electronic regulator will be seeking constantly to maintain a controlled output, forcing the electronics to run hotter and making it more likely to fail prematurely and after spending $50 on the alternator, I would prefer not to have to change it early. The storage battery also limits the current need to far less than the full current by smoothing the power demands to the system, where this system can jump from near zero use to near full load in a split second with the same effect. It's not the mechanicals I am concerned about in the cooler air with a reversed cooling fan limiting the flow, but the device on the inside of all those fins on the back...


Reply 1 year ago

I guess that big yellow thing sitting on top in the first picture isn't a battery. What was I thinking?


Reply 4 years ago on Introduction

Damn, an old mechanic told me there was a 'direction' in which it has to turn ... so I had to make A LOT of adaptations .. Anyways, here's a pic of the ongoing project. (finished now, but no pic)
Not at all as perfect as yours, but got everything for free, so no complain about the 'look' from my part. ;-)
Thanks for the idea and the share of your project !


Reply 3 years ago

all you have to do is offset the pullies and flip the belt to a 8 configuration it will reverse the direction and will just slide past it self. you might have to set the alternator a few degrees off center but vbelts are very forgiving over chain drive. This also gives more surface area contact between the pulley and the belt for less slippage.


3 years ago

An Easy to build generator,but one thing that the author didn't consider...Since the engine came off a lawnmower,(Vertical shaft), it has an aluminum flywheel and the engine needs the blade and blade adapter on it for the engine to be balanced...Without those components the engine will fail prematurely...A side shaft ,(or Hortizontal),has a cast steel flywheel which keeps the engine balanced...So my recommendation would be to use a "Hortizontal shaft engine for longevity and reliability...

3 replies

Reply 2 years ago

Using a cast iron pulley on the engine shaft makes up for the missing blade.


Reply 2 years ago

With every respect, that is simply not true. It depends greatly on type of engine used yes, but is more dependent on whether or not the engine is internally balanced. 99% of all of these OPE engines are, which means as long as it has some load at the crankshaft/pto end of the engine, it'll be fine. Type of flywheel is irrelevant in this case, as a common retrofit for older engines to gain performance (ie above rated rpm output) is to convert with an aluminum aftermarket flywheel. In the case of what the author has suggested, this setup is fine.


Reply 2 years ago

My former position,before retirement,was at Briggs and Stratton as a engine/ power train design engineer I guess lends nothing then...Not!!!

Tecumseh,Briggs and Stratton,Wisconsin,Milwaukee,and Kohler all utilized what I had mentioned for the point of engine balance,but on newer engines that are vertical shaft,the newer flywheels although Aluminum still require the blade and adapter for the purpose of engine balance...

As far as your statement of "but is more dependent on whether or not the engine is internally balanced. ",All engines are internally balanced to a degree because of the crankshafts on all engines have counterweights,or balanced crank wheels,(as in a Harley engine),but still require more to balance them to reduce bearing and seal wear......i.e. Vibration dampener on the front of a car engine...


2 years ago

Hi...and first, BIG kudos on a great setup with what looks like fairly easy construction to boot! You got me thinking: I have a sorta high-end audio system made up of a number of older components (mostly '70s vintage stuff). I've long pined for an AC regenerator like PS Audio makes, but $5K is a lot to spend. Can you tell me how to construct such a machine as yours to power my audio with CLEAN, REGULATED power like the PS unit? Generously estimated, it consumes about 4,000 watts, not counting bass notes and surges in the musical signal that cause the amps to draw more power towards output. I may be overstating or oversimplifying, but I'd like to know how to make this happen, including type and number of batteries, how many solar collectors, etc. Besides being free, what could be 'cleaner' electrically than solar energy dedicated to the one use? THANKS in advance, and I hope to hear from y'all soon.

Sincerely, Chuck K

Norman, OK

ay be overstating and or oversimplifying, but it


3 years ago

Cool project