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Convert a Lawn Mower into a Generator - Vertical Mount (Part 2)

19 Aug, 1999

Build a Generator using a Lawn Mower Engine

Lawnmower GeneratorMany of our visitors and customers have read our article discussing the homemade generator made from a horizontal shaft engine [part 1 of this series] and a car alternator. Now, build a generator with a lawn mower engine!

As you might remember, our first generator was built using a horizontal shaft engine. After many attempts, we have successfully completed the construction of a generator using a common lawn mower engine, the type that is sitting in your back yard right now.

You also may want to check out our article on Converting an Engine to Run on Propane. In that tip we convert a Tecumseh Carburetor for either a Horizontal or Vertical shaft engine to run on propane.

NOTE: TheEpicenter.com does not offer or provide technical support for Propane Conversion Kits!

About the Parts Needed

1. The Motor

A Briggs and Stratton lawn mower engineThe generator project will require a vertical shaft gas engine from a lawn mower. The typical lawn mower engine will be 3 to 5 horse power and will have a 7/8" shaft, with a 3/16" key way and a threaded hole in the bottom of the shaft. Most of these engines have either 3 or 4 bolts holding them down to the existing lawn mower base.

2. The Alternator

Delco-Remy AlternatorA GM 10SI or 12SI style automotive alternator.

The alternator will fall into one of three categories:

  • A) External voltage regulator type.This type of alternator does not have an internal regulator and must have one connected externally to control the alternator field intensity and thus the output voltage and current of the alternator. The disadvantage in using this type of alternator is that connection is a bit more complicated and the regulator is an added component that must be mounted and connected properly. This type of alternator is typically less expensive than the other options shown below, but like the model with an internal regulator, it too requires an external on / off switch or the alternator and regulator pair can discharge batteries when it is not charging and the switch is left on.
  • B) Single wire connection type with internal regulator.The single wire connection type alternator automatically starts producing output power when the RPM of the input shaft reaches a minimum speed. And, when the RPM drops below a preset speed the output stops. A big advantage is that it does not require a switch to isolate the alternator from the battery source to keep the alternator from draining the battery when not in use. The disadvantage in using this type of alternator is that the alternator will start to charge the batteries as soon as the minimum speed is reached, and will place a load on the engine as soon as the minimum RPM is achieved. In some cases, you might need to throttle through this minimum RPM range to insure that the motor does not bog down at low RPM when the alternator begins to produce power. Another disadvantage is that these alternators are more expensive than other options, but it provides a very simple connection method.
  • C) Internal regulator type with external control switch.Another option is to use the type we used on our last project. This alternator has an internal voltage regulator but requires an external switch to start or stop power production. The advantage in using this model is that the alternator can be switched off while the motor is still running and power output stops. This aids in connecting and disconnecting batteries or other loads.

One note in the Single wire connection type with internal regulator: We found that the single wire configuration is not ideal for this application. Because the lawn mower engine mentioned above doesn't have much throttle adjustment, if a single wire configuration is used with a 2 1/2 inch pulley on the motor the alternator doesn't kick in without manually moving the throttle butterfly to a higher setting and then releasing. This problem might be eliminated by using a 3" pulley. However, a single wire configuration can be used on our last project using a horizontal shaft engine. The problem we have right now, is that we don't have all the answers yet for this configuration. Feel free to experiment, but with the motor we used and the pulley we used, it wasn't convent in that you had to reach under the carburetor to throttle up manually to start generation.

3. Electrical Wires

Cables for building generatorA positive and a negative car battery cable is needed, as well as an alternator connector and associated wires. The alternator wires are not needed if a single wire alternator with built in regulator is used.

4. An "A" Style Industrial V Belt

V-Belt used in building a generatorThis V-Belt transfers power from the pulley (which will be mounted) on the motor to the pulley on the alternator. Various belt lengths can be used if your mounting system provides for several inches of belt length adjustment. Keep in mind that the belt length should be kept to a minimum to reduce belt slap and associated wear.

We have found that the Automotive V-belt used on a standard alternator is not compatible with the industrial "A" size belt that mates with all pulleys you will find for the motor. However, if you use an industrial "A" style V-belt, it will provide an exact fit for the motor and an "OK" fit for the alternator. Replacing the pulley on the alternator to match the pulley on the motor is an option (a more expensive option) but would be the ideal solution.

Since the motor is the source of the toque, and after looking at the wear pattern of both automotive and "A" belts used in this configuration, we feel the "A" belt (which matches the motor pulley) is the better choice, and modifying the alternator by replacing the pulley is not required for most applications.

Remember that an automotive belt will work in a pinch if you can't find an "A" belt for this project. Just plan to buy the most expensive V belt you can find. Price matters! An inexpensive automotive belt usually doesn't have any nylon cords in the center, only on the outside of edge of the belt, and since the angle of the belt (mainly the inside area) doesn't match the "A" size pulley on the motor, excessive wear of the belt can result. Be sure to keep a spare around after you determine the correct size.

5. Cast Iron Pulley

Cast Iron Pulley used in building a generatorThe motor pulley needs to be high quality cast iron. The mass of a cast iron pulley tends to act as a flywheel, taking the place of the mass of the lawn mower blade. Remember that most lawn mower engines have a very light aluminum fly wheel and use the steel blade as part of the effective fly wheel mass. The added weight of the cast iron pulley (compared to the mass of an aluminum pulley) helps the engine idle smoothly and helps keep belt slap to a minimum.

Full discussion of the pulley can be found below.

6. The Mounting Bracket

Vertical Mounting BracketThe mounting bracket is the most complicated part of the project. We custom designed and manufactured one out of 1/8" steel and they are available for purchase at a very cost effective price! With this bracket the project comes together in a snap, and eliminates many hours of frustration.

The great thing about this bracket is that it's Universal in design and allows use of a wide variety of engine manufacturers and engine models. Our first prototype (shown above) was made with a 3.5 HP Briggs and Stratton engine, but we have since built one with a 3.75 HP Tecumseh, and another version with a 5 HP Tecumseh.

It not only eliminates hours of time figuring out the bolt pattern of your motor, but also eliminates the trial and error guess work in finding a belt length that will work once the motor and alternator are mounted. Additionally, this bracket can be bolted down to a simple base of your own design and the rest of the work is done!

Further discussion of the mounting bracket can be found below.

Putting It All Together

Remove motor from baseThe first step is to remove the motor from the lawn mower base. Typically, there are 3 or 4 bolts holding the motor to the base, but before you remove them, you will need to remove the mower blade and the shaft coupler that holds the blade on the motor shaft. Getting the blade and the coupling off is a bit of a pain. Removing the blade is not nearly as difficult, but still requires a bit of ingenuity to figure out a way to "jam" the blade so it won't rotate while you remove the bolt which holds it in place on the motor shaft.

We found that we had to use a "pulley puller", to remove the shaft coupler after the mower blade was removed. A "pulley puller" which can be rented or purchased at most auto parts stores. Removing the coupler is difficult to impossible without use of this tool.

Elevate MowerYou will also find that it's a bit of a pain to work under the mower to remove the blade, especially if there is still oil in the motor or gas in the tank. We found that some oil leaked when we tipped the motor over and the mower smoked like crazy for a few minutes when we started it. The ideal way to approach motor and blade removal is to elevate the mower on some type of platform and not to tip the motor at all. We ended up using a fork lift to hold the mower up in the air for the blade removal as shown in this photo.

Cast Iron PulleyThe next hassle will be in finding the required pulley. Our research indicated that nearly all the motors used in vertical shaft lawn mowers have a 7/8" shaft, and a 3/16" or 1/4" key way. However, horizontal shaft motors under about 7 HP use a 3/4" shaft. The 3/4" pulleys can be found at most hardware stores, but the 7/8" pulleys are impossible to find. What we had to do was to set up an account with a company that supplies Heating and Air Conditioning Systems, Motors, Blowers and Components to be able to order the correct pulleys. We will be making the correct pulleys available for purchase to folks who can't find them locally.

Another catch is that a 3/16" key way is not standard on cast iron pulleys. What we found was that a 1/4" key way is the norm because most high horse power AC motors use a 1/4" size key way. A pulley with a 3/16" key way is not typically available in a 7/8" shaft diameter configuration. It is possible to use a pulley with a 1/4" key way on a motor that has a 3/16" key way provided that the set screw is on top of the key on the motor shaft and NOT on the shaft itself. If close attention is paid to this detail the pulley will stay in place without vibrating or loosening.

As expected, the biggest problem is figuring out how to bolt everything together. In our first attempts, we tried to figure out an easy way to reuse the lawn mower base, and somehow build something that could be attached simply to the base (so that a belt could be run to the pulley on the alternator). That proved to be an impossible task for various reasons. All the bases we looked at on the most common mowers were constructed a bit differently, so whatever would work on one mower would not fit the base for another brand of mower. In some cases, (especially with side discharge mowers) a pulley could actually be run out the discharge shoot. Some mowers did not have the required clearance, or had extra sheet metal that directed the cut grass in such a way that the metal would need to be cut in order to be able to run the belt out from under the mower.

Vertical Mounting BracketSo, what we ended up concentrating on was coming up with a universal base that just about any motor could fit on. Yes, there were some complications there, too. Some motors have the head on one side and the tank on the other and some have them set 90 degrees apart. So, our base had to allow for rotating the motor by about 30 degrees, and allow positioning the motor in any of the 90 degree quadrants. That allowed the motor to be in any position with clearance for the alternator and a method for hooking up the belt. We also wanted to allow the alternator position to be adjusted allowing for several belt sizes to be used. The bracket also has an integral belt adjustment slot which allows the alternator position to be adjusted, which also serves to tension the belt.

Let's talk a bit about lawn mower engines

lawnmower motorThe lawn mower we used had a 3.5 Horse power Briggs and Stratton 4 stroke gas engine. That particular model motor has a safety shut off lever on the lawn mower handle which has a cable attached to the motor, and the attached cable must be activated to disengage the motor shut off break and to allow spark to reach the spark plug. What we decided to do was to cut the cable off and deactivate the motor shut off feature. If your motor has this feature, you will need to spend some time looking at the cable and the levers on the motor to figure out a way to deactivate or preserve the feature. In either case the motor will not start unless something is done about the cable and levers.

We found that there was a small hole in one of the lever plates on the side of the engine, and after pulling the cable, a small nail can be inserted into the lever to keep the mechanism from retracting and shutting off the motor. Like I said, it will take a bit of time to figure out how your motor shut off mechanism works (if there is one installed on your motor).

Most of the lawn mower engines you will find have a 7/8" shaft and a 3/16" key way cut into the shaft. They also have a threaded hole in the bottom of the shaft.

The Big Issue: Pulley Size and Type

<pCast Iron PulleyThe vertical shaft motor from this lawn mower would not throttle as high as the same horse power motor we used in the horizontal shaft generator project. That motor was from a lawn edger, and could be set to a higher maximum speed. After talking with some lawn mower experts, we were told that the throttle mechanism on the lawn mower has the maximum throttle set to be about 75% of the maximum butter fly valve position of the carburetor. We were told that the lawn mower manufacturers set the throttle mechanism that way so there is extra throttle capability for when the mower hits some heavy or wet grass. The motor could then self throttle to a higher setting if necessary, then throttle back to the pre set throttle setting.

The reason we mention all this is that the pulley size we used on the horizontal shaft motor project would not work on this project. In testing this motor with the throttle set as high as possible without modifying the carburetor, and using a 4 3/4" pulley on the motor (similar size to the one on our other project), the motor would bog down and die with a 39 Amp load on the alternator. Without modifying the carburetor, we couldn't keep the thing running when the load was switched in with the large size pulley.

What size pulley works without modifying the carburetor or throttle linkage?

We had excellent results with a 2 1/2" pulley. It allowed the alternator to output voltage at even half or lower throttle settings at a slightly lower output current. So, with less than full demand, the motor speed could be reduced without killing the motor, and providing fuel savings.

Testing notes:

The largest load we had available during testing drew 39 Amps with the alternator output of 14.4 volts or about 560 Watts. Testing was conducted with an ambient outside temperature of 82 Degrees. We ran the load for 2 hours and the case temperature of the alternator only reached 148 degrees.</p<>

Another thing to note: Lawn mower engines have a rather small gas tank. We found that we could only run the generator for about an hour at a time with the standard gas tank without re-filling in use (which is not a good idea). We plan to continue using the standard tank, but other folks out there might want to figure out a way to attach a larger tank, or select a lawnmower with a large tank.

Mounting the Motor and Alternator, Made Easy!

Bottom viewShown here is a bottom view of the bracket, motor, alternator and 2x4 base.

Mounting all this stuff is the tricky part of this project. But like last time, we designed and manufactured a bracket to make the task simple!

The bracket is made from 1/8" steel and has provisions for mounting the motor and alternator, and additional holes for mounting the plate to a base of your own design.

What we did for the base was cut two 2x4's the length of the bracket, and another 2x4 as a cross brace to be installed under the bracket at the bottom of the long 2x4's. Imagine the base as being an H. The two long pieces were installed so that the base was 4 inches in height, and the cross brace was installed on the two length wise 2x4's at the bottom, and on it's side so that it stood 2" in height. That provided the necessary clearance for the belt and provided stabilization of the two side pieces. A further improvement would be to install two more short 2x4 at each end of the assembly to completely box in the rotating pulleys (for added safety).

Mounted ProjectAs in the last project, we elected to mount the alternator in such a manor that it actually runs backwards. This simplifies the hook up and it still works. Many astute readers questioned the alternators rotation direction and what affect would be seen if the alternator fan (which is attached to the alternator input shaft) also runs backwards. Well, the fan still functions, but instead of pulling air through the back and exhausting through the front, the air flow direction is reversed. Also, the fan blades are not as efficient when running backwards so air flow is reduced slightly. But, remember that like in the other project, the alternator is mounted to a steel plate which also serves as a large heat sink. And from the two hour test run at 39 Amp output, the alternator case temperature was only 148 degrees (ambient temperature was 80 degrees). So, I guess what I'm saying is that it really doesn't matter. These alternators normally spend most of their lives under the hood of cars stuck in traffic jams on hot days, and see temperatures much higher than this.

Now, getting back to the mounting issues: The lawn mower engine has a longer shaft than the alternator, and if the pulley is installed in the ideal location on the motor shaft the two pulleys do not align. So, what we found on our project was that the alternator needs to be mounted flush on top of the bracket but the motor needs to be spaced 1" above the mounting plate. This is easily carried out by using 1" longer bolts, and 1" long spacer tubes. Then the alignment of the pulleys is correct.

Take a look at the photo of the generator in operation charging a bank of three deep cycle marine batteries. A spacer tube is visible at the bottom of the motor, and provides the necessary height adjustment to keep the pulleys aligned. The pulley is also visible beneath the mounting plate. This motor had three mounting bolts, so three extension tubes were needed.

So What Can This Thing Be Used For?

DC charging systemIn its simplest form this is a high current DC charging system. With the addition of a DC to AC power converter, it also becomes an AC generator system with battery back up.

This charging system can be used to recharge a battery bank which can be used later to power a DC to AC inverter for your household AC power needs like the 21" TV as shown. Imagine being able to charge your batteries during the day, and then to be able to silently extract power at night for entertainment, lighting or cooking needs without disturbing your neighbors! This system can also be used in conjunction with other alternative energy system components like solar panels or wind generators as back up power for when the sun isn't shining or the wind isn't blowing!DC AC Power Converter

The addition of a DC to AC power converter allows 120 Volt AC devices (like the television above) to be powered either from the lawn mower DC generator or by the batteries the system can charge.

Wiring It Up

Alternator Wiring Diagram

The wiring depends on which alternator you choose. All three alternator types are shown.

Do not wire the alternator unless you are sure about what type you are using. If you make a mistake in the selection of the alternator or wiring diagram you run a very high risk of damaging your battery, electronic devices, or worse yet causing personal injury! Consult a parts professional for additional information!

This tip is intended for educational purposes only. No guarantees are expressed or implied as to the accuracy of information presented here! Consult with an automotive wiring expert before attempting to carry out any wiring.

One final note:

If you are using an alternator that requires an external switch, you will need to turn off the switch prior to attempting to start the generator. If the switch is on, the generator will try to output voltage while you are pulling the starting cord on the motor. You will find that it will be nearly impossible to pull the cord! If the switch is off, then there is little to no resistance from the alternator. Once the motor is running, the switch can be set to the on position.

To Recap

lawnmower motor  +  gm alternator Vertical Mounting Bracket Lawnmower Generator


Many of these parts are available on our DIY Vertical Shaft Belt Drive Generator page here @ TheEpicenter.com.