Electric Tractor

This page is for the BCS 201 Electric Rototiller made for Farmship Growers Cooperative/Haliburton Community Organic Farm. The design of the tiller system was inspired by Reid Allaway of Tournesol cooperative, Reid runs the DIY electric farmer facebook group, that is a good place to find out about electric tractors. The system is based around the following information The motor, the motor is permanent magnet synchronous machine motor ME1509 from Motoenergy, an american company. PMSM are highly efficient when run at the correct RPM and torque, the diagram for the me1509 is not available though this one form the me1507 shows the idea https://www.thunderstruck-ev.com/images/companies/1/ME1507dyno.pdf?1518651550248 the me1507 when run at 5000 rpm would be 100% efficient meaning that electrical power inputed would be fully applied as output power at the shaft. The BCS 201 has a reduction gear box, 12:75 or ~ 7:1 reduction. The output RPM for a tiller should be about 540 rpm so with a 7:1 gear ratio 3780. I.e. the motor used should have high efficiency at 3780 RPM the motenergy 1509 is efficient in the range of 2500-3500 rpm so an ideal motor. The power of the motor is determined by how much heat it can disappate as well as the input power. In theory since the windings of a motor are low resistance you can put as much energy into the motor as you would like, but it shouldn't overheat and melt the windings. So the motor power is essentially power in keeping in mind the rate at which the motor dissapates energy. So when making garden beds, check that the motor is not above ~70 C. It should feel warm but not feel like it is burning. It is possible to put a cycleanalyst display on the motor that can read the motor temperature as their is a sensor in the motor. The battery for the electric tractor should output sufficent power to provide the necessary torque for doing the job at the tines. The gas motors on rototillers are typically 5-10 hp, a 5000 watt motor will output approximately 5.6 hp so in the correct range. At 3600 rpm the torque of 5000 watts motor (torque = power/RPM) is 13Nm, with the 7:1 reduction gear and an efficiency factor of .8 72.8Nm so in theory the 5kw motor will deliver the torque required. Reid Allaway designed a similar tiller and found that the motor was adequate to do the job with a kelly motor controller that delivers peak amps of 200. So a 48 volt battery (potential) with 100 ah (capacity) that can run at 2C (rate) will deliver 4000-8000 watts of power to the motor drive shaft (efficiency factor of .8 included) The motor controller used in this project is a Cheetah motor controller. It is sinuoisdal brushless DC motor controller meaning it works from a computerized programming detecting the magnetic fields of the motor and not using commutator brushes. The control circuit is relatively simple as outlined Kelly Cheetah Motor Controller Guide you can see on page 13 of the manual the circuit for connecting the motor controller to the motor Important elements are a 200 amp fuse on the positive line of the controller that prevents overcurrent. The next element is a contactor that is has a coil that closes the circuit to the battery this is then connected to the battery pack and on the negative side of the pack to the low voltage.. The positive of the battery pack is also connected to a key witch then when closed powers the controller as well as the contactor coil. There's also the hall sensor connections to the motor which are the power, the three sensor signals and the ground. Their is a directional switch which is determines the direction of the motor and a throttle and diode on the contactor coil which prevents current flow in reverse direction on the power input of the controller In practice the circuit that runs the motor had two important glitches, first was proper electrical crimps, the kelly contoller connects are automotive connectors they should be crimped by having the wire at the first set of tongs and the bare wire at the second, the covered wire secures the crimp in place and the bare wire connects. One of the connectors is currently partially loose and should be plug and secured tightly to start up the tractor. This will need to be repeated all connections are secure. The motor itself is connected to the gearbox by an adaptor plate, the motor driveshaft should be centered on the bearing exactly, this is tricky if you try as I did to line up the holes with measurement of the bolt holes, what worked was to machine the motor coupler to fit between the bearing and the motor shaft thus perfectly centering the plate. The motor is bolted to the plate by machining a square hole in the plate with the milling machine sufficiently wide to accommodate the motor shaft bolting the motor to the plate, the alignment occurs by aligning the plate to the gearbox pretty much at any angle but square is sastifying. Because the electronics are currently too close to the ground a new adaptor plate will need to be machined with the plate to the side. Motor connections need to contained with a sock that is sewed.\ Battery Frame The frame was built in a bit of slipshod manner by bolting together. A key idea is to have battery swap allowing for extended work times by swapping batteries. The current situation is a 15 amp charger that can charge the 100 ah battery in ~ 7 hours a level a 32 amp level 2 charger could be installed on farmships current electrical system, this would allow for full charge in just over three hours, a system like this could also charge 4 batteries on an overnight charge that would allow for 4 hours of working time during the day. It would be possible to carry two batteries on the tractor at once. The comment was the center of mass of the tractor was too high. Rotating the battery 90 degrees would help or two batteries on either side of the tractor would also be possible Motor drive shaft You can look at the drawings from the tractor manufacturer to see inside the gearbox. In the case of the BCS 201 their was a large reduction gear that connects to the motor driveshaft. Gear The first thing to do is to calculate the module of the gear. Module is the ratio of the diameter of the gear to the number of teeth. (m=d/z) (d in millimetres) If you think about the relationship circumference = pi*d this gives you the idea that the module is essentially the inverse of the number of teeth per millimeter of circumference divided by pi. this is confusing to me as well, but what I am trying to say is that for two gears to mesh they should have the same module. I initially 3d printed a gear that I designed in cad, problem with this is that it sheared apart immediately after applying the motor torque. Theirs also the pressure angle of the gear, which is the angle between the circumference tangent and the gear tooth profile, essentially the sharpness of the gear. It should again match as close as possible. In the end I purchased a gear from aliexpress that was module 2 twelve teeth with pressure angle 20 that is standard. I'll come back to the gear Shaft The shaft is the simplest part, to extend the motor driveshaft you need a piece of keyed motor stock that is the same diameter of your motor axle, it can be to length to fit inside the gearbox, thiniking of it now I realize the original motor shaft was resting on a bearing to increase strength, something that I didn't acheive by shortening the shaft, probably should lengthen the drive shaft to rest on the bearing on the far side of gearbox. Coupler Coupler, the standard way of attaching a coupler is with a set screw from the side, pressumably into the keyed shaft. I'll get to this soon but basically we machined the coupler to fit in the bearing of the gearbox so tight that it didn't slip, put a key in the motor shaft and axle and then spot welded motor shaft to the coupler, not pretty but hey it worked.