W5DXS - Heart Of Texas DX Society


- The KC5LDO Field Day Generator -

The purpose of this project was to build a low cost generator for charging 12 volt lead acid batteries. My goal was a design requiring no welding. This should simplify construction for an at home project. The entire generator was built using second hand parts with the exception of the engine pulley and drive belt. Scrounging parts is half the fun for me and cuts cost. My dimensions are based on the parts I could find and slight variations should not matter,

My engine came from a flea market lawn mower purchased for $20.00. It is a 3 H.P. vertical shaft Briggs Stratton engine. Crankshaft dimension are 7/8 in. diameter by 2 7/8 in. long. This size engine works well, but I would not recommend an engine with less horsepower.

I chose an automotive alternator with a built in voltage regulator. This simplifies construction because less parts are required and wiring is simpler. I was lucky to acquire a 60 amp. version just for the asking. Many alternators of this type were also made in 45 amp. versions which will work fine. I strongly suggest installing an off/on toggle switch in the wire going to the internal voltage regulator. This makes for easy engine warm up. Once the engine is running and the battery connected I set the throttle to about 3/4 th and turn the regulator on. The engine will load and charging starts.

The sides of the frame are made from 2" by 2" angle iron. Each side being 19" long. The ends are 1 1/4" by 1 1/4" angle iron 8 1/2" long. The end pieces are bolted to the inside of the side rails. Outer dimensions of the frame when assembled are 19" by 9 1/2". The frame can be welded if preferred.

My alternator rest against the frame without any spacers. One end of the alternator is bolted to the frame with a 3/8" bolt. I drilled a series of 5/16" holes and then used a die grinder to cut between the holes. This resulted in a nice slot to allow alternator movement for belt adjustment. I was lucky enough to have an alternator with a double grove pulley. This gave me more options on figuring out belt alignment.

For proper charging the ratio between alternator versus engine speed should be about 2 to 1. This meaning that the alternator should turn about twice as fast as the engine. I use a 5" pulley on the engine and a 3" pulley on the alternator not quite 2 to 1, but it works great. Slight variations are acceptable.

To obtain proper belt alignment with my alt,-eng. combination, I had to space the engine up 1 1/2". If my engine had a shorter shaft length, the spacer length would also have been shorter. The spacers are made from 1/2" water pipe. I also had to use two 1 1/4" by 2" pieces of angle iron as extensions because the belt pattern of the engine was smaller than the inside opening of the frame. To easily understand this arrangement, please see pictures. My combination uses a 37" drive belt.

The control box is a junk box item that houses a circuit breaker, an amp. meter, on/off switch for voltage regulator and terminal posts for battery cables. The circuit breaker is a resettable 50 amp stud type. Battery cables are #4 gauge with clamps for battery. I used a 130 amp. gauge simply because that is what I had on hand. Be sure your amp. gauge will handle the output of your alternator. The control box is mounted on four rubber mounts that rest on a bracket made from 1/8" by 1/2". Flat iron. All parts in my control box came from either my junk box or someone else's junk box. Remember scrounging can be fun!

I had safety concerns with fingers or other objects getting in the belt during operation. I fastened four angle brackets to the inside of the side rails and fastened a piece of aluminum to those to cover the bottom. This prevents dirt, grass,ect. From getting in the belts and pulleys. I then carefully cut and fit belt guards for the top of the generator. The decorative material I used was cut out of paper trays sold at office supply stories. This made a nice looking screen and is about .04" thick. Fabricate your guards carefully to prevent accidental injury.

The generator works really well with little vibration. When I charge a battery the amp. gauge shows about 40 amps. and slowly works it way down to zero. When you get close to zero the battery is charged. Please keep in mind this generator is designed for automotive type batteries. The charge rate may be too fast for some types of batteries. I would not suggest operating electronic equipment from a battery while it is being charged without some type of protection device.

Please be mindful the engine and alternator will get hot during operation. Always allow the engine to cool before refueling.

Total cost for my project was an amazing $60.00. It took about 40 hours to construct spread over three months. It is difficult to explain every procedure of construction, so please see pictures to get any ideas you may need.

This project is intended for educational purposes only. If you build something like this generator, you must build it at your own risk! YOU must determine the merits of this plan for yourself and assume all the risks associated with its construction and use. 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.

Articles written by James Tobola - KC5LDO
and reproduction, publication, or duplication of this article, or any part thereof,
in any manner is prohibited without the express written permission of the author.

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