Nat,
Okay, here's a stab at the Bradley drive.
Bear with me - I get there in the end!
A rough estimate can be obtained of the limit of traction of the two wheeler from its weight and typical coefficients of adhesion for wheels operating on soil/dirt/clay etc. Knowing this would let us estimate the drive forces/torques that the tractor is capable of generating at it's slip limit - and this, together with a desired operating speed, would let us estimate the mechanical power (at the wheels) that can be usefully harnessed by the machine for land use.
Three sources I've managed to find suggest that the limit of slip for tractor type wheels on land use (ie not on asphalt/concrete type surfaces but when working land) may be at or below coefficients of adhesion of about 0.6. Ie the wheel might be able to produce forward drive force at the ground contact point up to about 60% of the down-load on the wheel - wheel torques that try to produce drive forces bigger than this will just spin the wheel as the traction is lost. This is a 1st approximation there are many factors in play here so the numbers might vary for different tires, pressures and soil conditions but the 60% seems to be a reasonable start point. Note also all of this drive force may not be available for haulage - some will be used to overcome rolling resistance etc.
If the machine weights in at about 350 lbs (160 kgf) this suggests an estimated maximum total drive force at the limit of slip of the wheels of about 350 x 0.6 = 210 lbs (95 kgf or 930N).
Nat mentioned that the original machine was too fast, if we assume a working speed of the machine when tilling say, of 2 mph (4 mph is a brisk walking pace and many walk-behind lawn mowers do about 2.5 mph) then we can get an estimate of the mechanical power at the wheels when the tractor is operating just below the torque limit of slip and at 2 mph of, P = 932N x 0.9m/s = 840 Watts (2 mph = 0.9 m/s). If I knew the wheel diameter this calc could also be expressed in torque and shaft speed terms - but the answer would be the same.
So, this is the mechanical power at the wheel consumed when the wheels are just below slipping and the tractor is moving at 2 mph. The drive motor will need to provide this power plus whatever is lost in the transmission to the wheels. If the transmission is still in good condition then you shouldn't loose too much power - however check it out, is everything turning freely?
If the machine is to be used for a reasonable period of continuous use at this power output eg used at the limit of its hauling capacity to plough an area of land for an hour say without stopping, then we are effectively saying that the drive motor needs to have a continuous use power rating that matches or exceeds the above figure + losses. An hour's continuous use is easily long enough for the temperature in the motor to saturate and reach a steady level and it will overheat if the current draw is greater than its continuous use rating.
If I was having a go at converting the tractor I'd probably look at a 1000W motor and build a transmission with a speed reduction ratio that ties the rated speed of the motor to the rotational speed of the wheels at 2 mph - this will probably be more than can be achieved in a single stage reduction - but the existing transmission may well do much of it for you. Getting this speed reduction ratio right is important it's the way the motor is properly matched to the duty.
By way of example the 1000W 36V motor on this link (towards the page bottom) is an example of a inexpensive DC drive motor that might be used - difficult to look past these when you see the price - there are other suppliers.
TNC Scooters (Scooter Sales and Service)
Batteries. This motor draws about 36 Amps at 1000W and for it to run at full rated power for 1 hour it would need a 36V battery pack with at least a 36AmpH 1 hour rating. A 12V 38 Amph (20 hour rating) deep cycle battery weights in at about 33 lbs 3 would be 100 lbs or so. (Note the 1 hour rating can be as little as 65% of the 20 hour rating). The real battery life depends though on the real current draw and this is where our estimates become less useful perhaps. The above numbers are based on an estimate of the machine's operation right at it's maximum useful hauling capacity. If through experience you know that is is quite comfortable doing the type of work asked of it and it is not always working right at the limit of slip then the torque demand might be less than we calculated and so will the current demand made by the motor and the battery charge will last longer. Without doing the conversion and setting it to work we can't say for sure how long the batteries will last - but it might be reasonable to assume the above sums are conservative if the machine was comfortable in the past with the duty asked of it.
Controllers - you would need a controller that can also handle the continuous rated current of the motor. Trouble is many of them are rated on short term duty - check the user documentation before buying. For example a 75 Amp controller rated for 1 minute might happily carry our 36 Amps continuously whereas a 35 Amp controller would not.
Again just for the purposes of examples this supplier in the US has a wide range of DC speed controllers of this size and you can get some good tech info on them from the site.
Robot MarketPlace - Electronic Motor Speed Controllers (ESC)
I have tended to use 4QD units for vehicles as they are manufactured here in the UK but there are several competent alternatives and Wayne County Hose has mentioned a few already, he may also be able to help with local sources for suitable batteries. Also his advice on buying a boxed/enclosed controller is good - look at one intended for vehicle use, they are usually better protected than some robot type units.
If I have the weights, speeds, or other data wrong then these numbers may also be wrong - so please use with caution. The best test is to build it and see!
While I remember, a quick note on cooling - don't fully enclose the motor or the speed controller. Both will be happier with a flow of cooling air over them some weather protection may be appropriate but still allow for good air cooling.
Hope this all helps and makes sense, sounds an interesting project.
Ian
Built For Fun EV's
