Convert Lawn Mower To Generator
My old Craftsman lawn mower still starts on the first pull, but the years have taken it's toll on the mower deck, which seems to be growing rust holes. So, I started thinking about how to build a generator from an old lawn mower.
Aside from a suitable engine, you'll need a car alternator, belt and pulleys, DC to AC power converter, and a platform.
Small Generator Uses
In its simplest form, a small engine-powered generator can be a DC charging system. By adding a DC to AC power converter, it also becomes an AC generator system with battery back-up.
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Engine Selection
Most gas-powered lawn mower engines are in the 5 to 10 horsepower range, which is enough to run a 12-volt car alternator. But the important thing to know is, the alternator has to be spinning above 3000 rpm for any useful power to be made. This is why pulley ratios are so important.
Vertical-Shaft Engine
Many DIY generator projects start with a used vertical-shaft engine from a lawn mower. These are cheaper and easier to find than horizontal-shaft engines.
Pictured is my 22" Craftsman mower, powered by a 7-hp Briggs & Stratton Intek engine. This one is self-propelled, so there is already a pulley to drive the alternator.
Horizontal-Shaft Engine
If you have a horizontal-shaft engine for your DIY generator project, the mounting will be a little different, as the two pulleys and belt will be mounted vertically.
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Generator Platform
After a suitable engine, the next major part to consider is the base or platform onto which everything is to be mounted. There are several platforms you can use for a DIY generator:
Lawn Mower Deck
The least expensive option is to re-purpose the mower deck as a base. It already has wheels so it's easy to move around, but will it have room for an alternator and an inverter, and possibly a battery?
When using a lawn mower deck for a platform, the cutting blade is removed and replaced with a pulley. The alternator gets mounted on the deck nose down, so that the alt pulley and engine pulley can line up underneath.
If your donor lawn mower wasn't self-propelled, the engine shaft needs to stick out at least 1-1/2" from the bottom. This is so that, when you mount a pulley, it will clear any obstacles (like engine mounting bolts).
NOTE: Some mower decks will not have the required clearance for an alt mount.
Pressure Washer Frame
If you have an old pressure washer that the motor stills runs but the pump doesn't work, the pump can be removed and an alternator fitted to the frame.
Perhaps because there are so many brands and styles out there, no company has yet offered a kit to build a DIY generator from a pressure washer. This means all the required fabrication is up to you.
Steel Base
A common platform for a homemade generator is a flat metal plate. Ideally, this would be rectangular in shape, between 12" x 24" and 24" x 36".
As an alternative to fabricating one, you can buy a steel mounting plate from a company called the EpiCenter.
The Epicenter bracket allows use of either three or four-bolt engines. It can be mounted on lengths of 2x4's to make a standing base.
Wood Base
If you're going the inexpensive route, a heavy (3/4") piece of plywood will also work. But, to best hold up to the vibrations of the motor, a 1/8" piece of steel or thicker is preferred. I've heard that some folks rubber-mount the engine to the frame to help reduce the vibration.
On either a wood or metal base, the engine and alternator need to be mounted so that their pulleys are parallel. Plus, there needs to be alternator mounting holes and a belt adjustment slot.
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Alternator
Alternators can be mounted in any position, and will work in either direction. However, cooling can be an issue. Notice the metal fan behind the pulley and see which way it cools. The best direction of rotation would be the one that augments it's cooling fan function.
Read: Best Alternator for DIY Generator
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Pulleys
A vertical-shaft lawn mower engine relies on the weight of the mower blade to help keep the engine running smoothly. To replace the missing weight of the blade, a cast-iron pulley is a better choice over a light-weight alloy pulley. The new pulley should be equal (or greater) the weight of the removed cutting blade.
The pulley size and ratio must be considered. To put out acceptable wattage, the engine needs to spin the alternator in the 3000 to 3500 RPM range. There is no 'standard' size that will work for all applications; you may have to try several pulleys before optimum alt output is seen.
Pulleys are available at industrial supply stores and through equipment suppliers. A common pulley shaft ID is 7/8". If you cannot find one locally, they can be found online.
Shop: 7/8 bore V-Belt Pulley
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Belt
You don't need or want a belt any longer than 30". An 'A-style' industrial belt is preferred over a thinner passenger car belt, as they will have less tendency to slip.
Automotive "V" belt on left, Industrial "A" belt on right. A typical V-belt is 3/8" wide. An industrial A-belt is 1/2" wide.
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Power Inverter
An inverter is an electronic device that converts low voltage DC (direct current) electricity from a power source into a standard 120 volt AC (alternating current).
Inverters are sized by the amount of wattage they can output. They are offered from as low as 50 watts to upwards of 4000 watts.
Look for at least a 1500 or 2000 watt inverter.
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Battery/Charging Station
Gasoline-powered generators can also double as an AC-powered DC battery charger. This would allow you to recharge the 12-volt DC batteries of portable equipment.
If you want the generator to also serve as a battery charging station, you'll need a battery and an on/off switch.
A deep cycle battery is preferred, but any good 12 volt automotive battery will do the job.
Wiring Tip: Use 4 gauge cable to connect alternator to battery and inverter.
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Is an On/Off Switch Necessary?
If you are using a two-wire GM alternator with an integral voltage regulator, an on/off switch must be added.
Why? Because with power applied, you're pull-starting against the magnetic resistance of the alternator. When the switch is off, there is little to no resistance, making the engine easy to start. Once started, the switch can be turned on.
Also consider the vapors coming off the batteries and gas tank, so for safety's sake, install and use an on/off switch!
There are several styles of switches to choose from; toggle, rocker, rotary, and push. Consider one with a built-in green light, so you can tell at a quick glance that's it's on.
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Notes and Misc
Lawn mower engines have a small gas tank, and generally allow about an hour of run time.
An engine any larger than 10 horsepower will require more fuel to run, plus it will be heavier and louder.
Consider the noise factor when the generator is running. How close to your house will it be?
On an old, used lawn mower, the original muffler has likely burned out already. You may want to consider adding a quieter exhaust. With some adapter pipes, a small car muffler will help reduce the noise.
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Related Articles:
Best Alternator For DIY Generator
Convert Gas Generator To Propane
Best Fuel For Small Engines
