Our Solar-Powered Tractor

Reader Contribution by Kathryn Griffin
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A little over a year ago, my husband, Terry, started talking about building a solar tractor. Since we were planning a move to a thirty-acre farm in Arkansas, the tractor would have to actually be the workhorse we expected we would need rather than a conversation piece shown off in parades. The original tractor — and I use that expression loosely — was a Ford 1950 8n bound for the scrap metal pile. When Terry told me he paid $200 for the rusty tractor pieces he proudly showed me I admit I wondered about his sanity. The front tires were lying off to one side and there was no motor, seat or fenders. Despite my doubts, his enthusiasm never waned and he began building his solar tractor.

For those who are mechanically-minded and are thinking about building their own solar tractor I will

attempt to relay the information given to me by Terry—also sometimes known as “MacGyver” due to his ability to fix anything and make something out of nothing! So, according to my mad scientist…

First, the crank shaft was removed and cut at the last rod bearing, leaving the rear main, flywheel attachment and collection pressure plate. Next the crank shaft was machined to accept the end cap of a universal joint after the drilling and tapping on the crank shaft was completed. The crank shaft was re-installed into the tractor, leaving the two rear main bearings. A grease fitting was installed into the block of the tractor. A rubber bladder was located—an automatic lubricator which keeps the rear main lubricated.

A trip to the welding shop for steel—including sixteen inches of a 16” pipe to cradle the 36-48 volt Hyster forklift motor purchased in Dallas at a rewinding shop for $600—came next. The pipe was cut in half, leaving two “cradles,” one of which was attached by welding a bracket to hold the pipe. The bracket was attached to the front frame of the tractor and a chain was welded over the top of the forklift motor once it was set inside its cradle. The chain basically works as a strap which holds the motor in place, preventing it from slipping backwards or forwards or rotating under torque. Next I searched (and searched and searched) on the Internet to find a spline to fit on the 21-tooth motor drive shaft. Eventually we ended up ordering a 21-spline clutch and taking it apart. (Price for steel and spline—approximately $250)

A bolt was run through a link of the chain with a nut threaded onto the bolt which pulls the chain down tight against the forklift motor, creating a tensioning chain bracket. A steel bracket was then fabricated and bolted to the original cast iron oil pan. Two block bearings with a drive shaft between them were installed, then a pulley was installed on the drive shaft and a universal attached to the end of the drive shaft to mate with the universal joint on the crank shaft. Terry next installed a double-belt pulley on the 21-tooth spline onto the motor drive shaft. After another Internet search we ordered an Alltrax Performance Controller ($700) to control the voltage which goes to the motor and to allow operation of the electronic foot pedal.

The electronic foot pedal was then attached to the original tractor throttle lever. The Alltrax Performance Controller is programmable; it works in a 300-650 amp range, 12-72 volts DC. Terry selected a 48-volt system so it would work with the 48-volt solar photovoltaic system installed at our home. In this way, when the batteries on the tractor are running low it can be plugged into the household battery bank.

The tractor is mostly kept at a working amperage rate of 100-200 amps per hour. It presently has eight-250 amp batteries installed, but we intend to add four more deep cycle batteries (purchased at Sam’s for $95 each) by the time we are ready to begin plowing and need longer operating times.

We bought a new seat from Tractor Supply, two new wheels, two front tires and a new steering wheel (approximately $600 for all) Terry later found the original two rear fenders at the same shop we purchased the tractor. We ordered an amp meter in order to be able to easily see how many amps were being drawn from the batteries as well as a voltage gauge which displays the voltage remaining in the batteries. ($100) I think of these two additions in the same way a gas gauge tells you when it’s time to pull into the gas station and re-fuel.

The tractor was painted Terry’s favorite color—green—before the motor was strapped in. After the motor was installed a battery stand was welded over the top of the DC drive motor which is large enough to accommodate twelve 250-amp deep-cycle batteries. Once the performance controller was installed battery cables were run, linking the batteries together. A frame was welded to hold the three 240-watt Canadian Solar panels. ($700) The panels and a Morningstar Solar MPPT 45 amp charge controller ($150) were installed. As with any solar photovoltaic application the charge controller is connected to the batteries, working like the regulator on a car. The batteries are charged but prevented from over-charging.

The total cost of the tractor was approximately $4,100. We purchased a brush hog, box blade, scoop and a five-blade cultivator after searching on the local Craigslist for the best prices for good used equipment. The size of the PTO had to be enlarged a bit to run the newer equipment. The tractor’s top speed is approximately 20 mph. It will run for nearly two hours with the eight batteries however that time would be extended with the addition of four more batteries. Because there is no oil pump to keep the back main lubricated it is necessary to grease the automatic lubricator each time the tractor is used which takes about 3-5 minutes. There is no difference in power as compared to a typical gas or diesel tractor — Terry has graded our rough driveway, slashed the weeds in one field and tilled up our garden area.

The tractor requires from 8-10 hours for a total recharge, therefore those farmers who spend all day on their tractor probably would not be able to use only a solar tractor. For our small farm applications the solar tractor is perfect. Terry has no desire to spend more than a couple of hours at a time on the tractor anyhow so the time limitations don’t really affect us. Aside from being a great little workhorse on our farm, the solar tractor is — as you can probably imagine — a great conversation piece!

I put a couple of short clips of the tractor working on YouTube here and here.

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