Generator

Up front disclaimer notice: Working with engines, gasoline, car batteries, 115VAC, etc. can kill you if you do not follow the necessary precautions! Do not attempt anything you see on this web page unless you know what you are doing.

I had an old lawn mower kicking around the garage that I just had to put to use... so I decided to build a generator. I got the idea from this website:

It has a lot details about building you own generator. Below is the condensed version of the details that I found were important to know from their site. I have more info about adding an old UPS to allow the generator to produce 115VAC .

Parts needed	Notes
A standard 3.5 hp lawn mower engine	Make sure the engine runs well before you remove it from the mower deck. It is a lot easier to work on (and start) the engine while it is mounted. You also want to leave the blade on the mower until you have it running well. The mass of the blade helps the mower run smoothly. Once you have it tuned up and running great, remove it from the deck by removing the blade and the three bolts that hold it to the deck. Be careful to empty the gas tank before you remove it.
A suitable car alternator	I recommend a GM alternator (there are a bunch out there in the junk yards). I prefer the type with an internal voltage regulator and an external field control switch. This type of alternator has a 2 wire connector for the field switch (labeled R & F). The advantage of this type of alternator is that it presents a smaller load to the engine until the field is switched on (making it easier to start the engine). Make sure you get the R-F connector from the junked card with a suitable length of wire attached.
A 12V marine battery	Although you can use a car battery, a marine battery makes a better choice because it is designed to be run down with a steady load. Car batteries are designed to produce a lot of current for a very short duration. They do not react well if they are fully discharged.
A set of battery cables	I purchased a short (about 2 ft) set of auto battery cables from a local AutoZone. Make sure you buy a set that will attach to the type of battery you plan to use.
A toggle switch	This is a simple on/off toggle switch used when hooking up the generator circuit. See details below.
A V-belt pulley	I used a 2.5" pulley from the local Ace Hardware. The size of the pulley will determine the load and speed of the mower engine. A pulley larger that 2.5" may cause your engine to stall when the alternator asserts its load.

The easy approach is to use the mounting plate sold on the Epicenter web site link above. It has adjustable holes for mounting the alternator and engine. This is helpful to adjust and tighten the belt. Note that you will have to make an adjustment in height to align the pulleys on the alternator and engine. The Epicenter website shows how to do this with their plate using some long bolts and spacers..

I chose to mount my setup an a piece of 3/4" plywood (because I had some extra in the garage). By mounting the engine from the top and the generator from below, the proper vertical alignment was met. Once the engine and alternator are bolted to the plywood, the last step is to attach the belt to the pulleys. I used piece of rope to measure the length from pulley to pulley and then bought a fan belt of the right length. You can see in the pictures that I also recycled the wheels off the donor mower to make the whole setup easier to move around.

Using the field connector scavenged from the junk yard with the alternator, create a small cable to connect the F & R pins together and wire them to the switch. I mounted the toggle switch on a small bracket close to the alternator. Remember that that the switch is connected to the positive battery terminal and the case of the alternator is connected to the negative battery terminal. You do not want to ever allow these two items to make contact with each other as this would short the battery and possibly result in the battery exploding. Next, attach the battery cables to the alternator. When you are sure that everything is wired correctly, connect the cables to the battery (positive then negative). If you have a multi-meter, I would suggest double checking your circuit and verifying that you have no shorts in your circuit before attaching the battery.

Note that the alternator will not generate any power without a battery attached. You will need a meter to verify that the alternator is working. With the engine not running, the battery voltage will be about 12-13V. Start the engine with the field switch in the off position. Adjust the throttle to achieve a steady high rpm. Turn the field switch on. You might notice a change in engine speed as the alternator starts to put load on the engine. Adjust the throttle again if necessary. If you measure across the battery terminals now, the voltage should be higher (approx 14-15V). This indicates that the battery is charging. You now have a 12V generator!

If you have a 12VDC device that you wish to run off the battery, simply connect it to the battery terminals and run it until the battery begins to run down. Then simply start the engine to charge the battery up again. There are a few extra electronics, however, that make this connection a little easier. I added a set of heavy gauge wires with a connector to my battery harness on the generator. I also built a small project box with a mating connector that houses a voltage meter, on/off switch and quick connect banana jacks.

Make sure you use heavy gauge wire for the 12V path to your device. A fuse is a good idea to limit the maximum current you want to pull from your battery. The size of the fuse depends on the rest of your circuit design. I designed my project box circuit to safely handle 30 amps. The banana jacks on the front make it easy to connect 12V devices. The voltage meter makes it easy to know when to start up the engine and recharge the battery. The connector on the battery harness allows me to remove this box and store it separately from generator when not in use.

If you have a 115VAC device that you wish to run off the battery, you will need to add a DC to AC converter to your setup. You can purchase "inverters" in a variety of sizes which will connect to your battery and produce 115VAC. Larger inverters (greater than 500W) will cost around $200 or more. Another possibility is to modify an old computer "uninterruptible power supply" (UPS).

Many UPS are discarded because the internal battery has failed. Often the replacement battery is near the cost of a new UPS. Internally, a UPS is a battery, battery charger and inverter. The UPS trickle charges the internal battery until it detects a power failure. Then it switches over to draw DC power from the battery to produce 115VAC. You can use the inverter circuit from your dead UPS to generate AC power from your own battery.

Do not attempt to open or modify a UPS if you do not know what you are doing! High voltages may be present that can cause injury or death. You will need to do a little detective work to determine if your dead UPS will work with your generator. The first step is to open the UPS and remove the dead battery. Verify that your UPS uses a 12V battery. If not, it will not work with your generator. Test to see if the UPS will "cold-start". Some UPSs will not start producing power unless AC power is present and then removed. Test this by connecting your battery to the UPS battery cables. If you can turn on the UPS and it starts producing power you are almost done. The UPS will likely start emitting some type of warning tone to indicate that it is generating power from the battery due to a power failure. This can easily be shut off by locating the noise maker in the UPS and disconnecting it.

If the UPS cannot cold start, you may still be able to use it. You will have to inspect the circuit board in the UPS to determine if it is possible to modify it to cold start. I modified a TrippLite 750W UPS to allow it to cold start by doing the following:

The UPS would not cold start because the inverter circuitry was initially powered by the 115VAC input to the inverter. The 115 input circuit was routed through a step down transformer and then a 4 diode full wave bridge to produce DC power. With the AC power connected to the UPS, I was able to trace this circuit and determine that the DC voltage driving the circuit was close to the 12VDC battery voltage. I added a momentary push button to connect the battery to this point in the circuit. Pushing the button and holding it for a few seconds allows the UPS circuit to start running. When the button is released, the input to the UPS circuit is removed, causing it to believe that the 115VAC input has been lost. The UPS then switches over to its failure mode and starts generating power from the battery.

A few final notes about using a UPS to generate 115VAC power: Remember that a UPS is designed to provide short duration (typically less than 10 min) power for a computer to allow it to be shut down when power fails. Continuous use of the inverter over a period of time may cause it to become very hot. Make sure you do not block any of the ventilation holes. You may need to add a 12V fan to the UPS case to help keep it cool. You should leave the original 115VAC input cable attached to the UPS. Using a pair of pliers, break off the two flat blades leaving only the ground connector.

You should plug this into a grounded home outlet when you use your inverter to generate power. This will insure that your UPS (and any devices plugged into it) are earth grounded. While this is not necessary to generate power, it provides you an additional level of safety.

Here are some pictures of the running setup. I have not completed the battery to UPS cables, so I am using a set of jumper cables to test the inverter.

Be careful about what devices you connect to an inverter. Many inexpensive inverters produce a square wave or modified sine wave output rather than the normal sine wave that the power company sends. Some equipment may have problems with this type of output.

©Copyleft Tom Vickers 2003, Last updated 03/22/2003
Send comments to VickersWeb at gee-mail-dot-com (I am sure you can figure it out)