[Wiredsim]

What kind of batteries are available in the local market? Lead Acid deep cycle like Marine or better yet Golf cart batteries? I don't see the weight of Lead acid as being as much of a negative for this usage. Looking at that basket like design you could balance the cart with one battery on either end and the motor in the middle.

You could definitely use Parallel to Series switching as a method to control speed. If you used solenoids as mentioned above it would only be limited by your number of batteries, such as 4 x 6, 3 x 8 or 2 x 12. 2 speeds would probably be enough if you designed full speed to be just faster then a normal walking gait.

What kind of electric motors are available? Golf cart motors? Motors from
electric forklifts? Motors are heavy to ship and not easy to build locally unless there is already an established manufacturer. Golf Cart motors will easily give you the power you need with a fairly low RPM. You're going to want a motor that runs in the 2-4k rpm (lower the better) at the full voltage and can handle the amps needed at the lower voltages.

I'm surprised there isn't more interest in the Rototiller functionality. Surely pulling objects isn't the only need?

Aftermarket car parts available? Motorcycles or scooter parts? I think Chain or Belt drive parts common from other purposes is going to be your best bet for replacement parts and get the reduction you need. If the motor runs at 2000rpm for example, you will need an ultimate reduction of roughly 50:1 to get to walking speed (~3mph). Scooter gearboxes often give you a 5:1 reduction, than another 10:1 on chain drive gets you the rest of the way.

 

[JRP3]

Might need to use a jack shaft setup to get the gear reduction needed, though there are some motors built for lower RPMs.

 

[Wiredsim]

After thinking about your challenge, I suspect the only readily sourced motors on the local market with enough power will be used Alternators. This is actually a reasonable solution, as they are quite powerful depending on the model.

Here are a couple of resources about how to convert:

http://www.rcgroups.com/forums/showthread.php?t=905411

http://www.endless-sphere.com/forums/viewtopic.php?f=30&t=16328

A couple of cliff notes for you:

You need a Sensorless Brushless controller, such as a RC ESC to drive the stator. This is extremely simple to setup, you just disconnect the voltage regulator and connect the three wires to the controller. You can also add Hall sensors easily to these to improve the low speed torque and starting ability. I think it would be fine without if you are able to get the motor up to speed without a load, using a clutch of some kind (even a belt clutch using a tensioner could work).

You need to supply voltage to the rotor, the torque you get at a given RPM is greatly dependent on your rotor voltage/current. If you use a simple Brushed controller for the Rotor, you can vary this dynamically. However I think a simple DC-DC converter will get you what you need, it looks like the rotors usually saturate at around 5-9volts. If it is a switchable DC-DC converter all the better (or you could use two and switch one in and out of the circuit to get two speeds). A Rheostat is another way to adjust the voltage to the rotor.

You will want to run this at a higher speed then I mentioned before, depending greatly on the particular model of Alternator used. Truck, Marine, Bus and other large commercial Alternators are better then car units. However just about any unit will get you a decent amount of power.

The nice thing about this is that you can easily bring a large amount of controllers and DC-DC converters in your luggage, as they are small and light. All that remains would be the batteries as a local challenge. However even car or truck batteries will work as a worst case scenario.

Good luck!

*Here is a good example picture*

 

[Wiredsim]

Here are a couple of links about electric tractors that already exist:

General info:

http://www.eeevee.com/tractors/TNF_article.html

GE Elec-Trak tractor:

http://www.builditsolar.com/Projects/Vehicles/ElecTrak/ElecTrak.htm

http://www.crystel.com/Elec-Trak/E-20 Tractor.html

The Sunhorse:

http://www.freepowersys.com/suhorse.htm

 

[jk1981]

It seems to me your design has to use readily available parts that are simple to modify and will work so long as they're nearly right since you're unlikely to find the same part twice if something needs replacing.

To me that says something simple like a lightly modified car or light truck starter motor (possibly with an alternator fan grafted on for cooling) for power, possibly one per wheel to make turning easier and to give more go and a simple chain reduction drive from motorcycle parts, gear it for slow walking speed. It's robust, easy to clean/service and there are no oil seals to get right. Use the starter solenoids from a car for on-off forward control, they're plentiful and simple.

Mechanically you need to look at where the load attaches and how the thing balances if you're to get decent traction since it'll be pretty light (100-150kg at a guess). You may need to have the man-handle in one position for maneuvering around the yard or load carrying (well balanced) and another for pulling (unbalanced until the load is applied). To make best use of the battery's weight.

A parallel (or series if you can find 24V motors) set of 2 or 3 similar sized car batteries or a large tractor/truck battery are likely to be the cheap option (care needed to balance them before connection). Ok it's crude and the they wont last forever but you have the lead to trade against your next ones once they're done and again, they're going to be much easier to find locally than proper deep cycle batteries. So long as it's well balanced (low but giving sufficient ground clearance) the weight is a good thing for traction. Low voltage is safer. Wherever you put the batteries if they're flooded wet cells they'll need to be kept as near level as possible and any spills that may occur need to be able to drain away safely.

I guess you can get agricultural tyres but if you can't then angle iron paddles bolted to bald car tyres or welded direct to steel rims would work fine on soft ground (car wheels allow you to easily use self contained hub-bearing assemblies for simplicity). Maybe sand fill the tyres for low maintainance (no tubes, no pump needed, no punctures) and more weight over the contact patch.

All just ideas, I hope some of them are of help.

 

[dirtmover]

I build and sell plans to a machine that should do what you want. although I am afraid that it would be difficult to get a tractor to Africa. Check out the video: http://www.youtube.com/watch?v=EJGvYYP_wz0

There is more info on my website http://www.thegreensmachines.com/

thank you , Lonnie.

 

[Salty9]

Another site that discusses alternators used as motors without changing the rotor: http://www.rcgroups.com/forums/showthread.php?t=1123956&highlight=reem

The posts by Ineptone were the most useful to me.

 

[JRP3]

I build and sell plans to a machine that should do what you want.

Neat machine you have there.