Wayne County Hose said:
That little thing isn't worth the scrap it's made out of.
First of all, the drive wheels have about 1.5 hp each after losses, it would barely climb a small grade.
Second, they have no provisions for turning. No, I don't mean the steering wheel. On a dual drive motor truck, you need to have provisions to slow down or stop the inside drive wheel when turning. Kind of like what spider gears do in a rear axle. This can be done with either limit switches on the steering or electrically thru the controller with a steering feedback sensor. And I could go on for pages.
Personally, I would have gone with a 36v controller. This way you keep the amps down prolonging the life of all the electrical components. If I were to build anything larger, it would be at least 72 volts. I would go with higher voltages except for lack of obtaining a controller. I know that 72v controllers are easily obtainable, I don't know about higher voltage.
If anyone is interested, some good motor controller companies are Curtis, General Electric, and a fairly new one on the scene is Danaher. The cheapest and most popular are the Curtis units. GE has gotten their butts kicked lately by the other 2. I've worked on some of Danaher's stuff, DC power converted to AC to power the motors. Their stuff is very reliable, but pricey too. Also, AC drive is the way to go. Most forklift manufacturers have converted over to AC drive and lift systems. Some Forklift manufacturers don't even make a DC motor anymore. I can set-up an AC powered forklift to smoke the drive tires. DC trucks could never do that. Pretty funny seeing an 8,000 lb. electric forklift smoking the tires.
Ugh, now you guys have me thinking about building an electric powered vehicle again.
Strong criticism indeed.
Hi all,
I'm the builder of the wee electric tractor in question and would like to make a few comments.
The machine isn't made out of scrap. I'm not sure why Wayne County Hose describes it as being so. It's made out of the usual range of engineering components and materials. The most notable difference is the use of timber for the structure - but timber has a long history of use as a structural material. None of it is made from scrap.
I can assure you all it can easily climb a small grade. The "rated" power output of the drive motors is as stated on the web site, although if you are used to the power ratings of IC engines beware of direct power comparisons between electric motors and IC engines - they can be very misleading. For those interested in it's hauling capacity I recently used the prototype to haul small 1/4 ton trailers of gravel when I was laying a new driveway - it had no difficulty moving these loads whatsoever. It may be a small machine but it has no difficulty hauling the loads it was designed for. The drive is designed to produce peak torques up to the limit of traction of the wheels on grippy (dry asphalt) conditions. Under more usual operation on grass, dirt etc the wheels will loose traction and spin before the drive motors stall. In my judgment this is adequate for its intended use as putting in more torque capacity than can be usefully harnessed unnecessarily adds cost to what is intended as a low cost design. (DC drive motors are capable of producing peak torques of several times their "rated" continuous use torque on which rated power is quoted - and these torques are available right down to zero output shaft speed.)
Indeed this is a dual motor drive machine. However there is no need for independent speed control of the two drive wheels to enable turning. The speed/torque characteristics of DC PM motors are such that two can be driven in parallel from a single motor controller and still provide effective differential wheel behaviour during cornering. The tractor has no difficulty turning at full steering lock and without any wheel scuffing. There is a consequential partial movement of drive torque towards the inboard wheel, however this does not noticeably affect the steering behaviour under most driving conditions. Where any effect is noticeable it is if you try to apply power fast with the vehicle stationary and the steering hard over, in which case there is some understeer as the drive resists the turn - the solution is to move off more slowly if the front wheels are at full lock and to pick up speed as you straighten up - good driving practice surely.
The satisfactory differential behaviour is a result of the way that (for a given voltage) shaft speed and torque are related in the DC PM motors - as the vehicle turns the outboard wheel runs slightly faster than its natural speed and its torque output reduces slightly, the inboard wheel runs slightly slower and its torque output increases. This difference in drive torques does tend to resist the steering, however it is a question of magnitude. The torque difference is not sufficient to overcome the steering moment generated by the front wheels over the majority of drive conditions for the vehicle. In vehicles with higher power/weight ratios the effect may well be more pronounced but this is not a design failure of this machine and should not be represented as such.
In the event that some assistance with tight turning was required the most convenient way to implement this effectively in the machine would be through independent braking on the rear wheels - not by adding further electrical complexity and cost.
Whereas a 36V system would be perfectly acceptable choice, and higher volatges will generally result in lower current draw for the same power throughput the real issue is the engineering significance of this general principal. The 24V components used are fully rated for the currents being drawn at 24V and there are no reliability, availability or cost problems with their use. In general any of a range of voltages could be designed into this machine and all would be acceptable in engineering terms, 24V is quite suitable for this size of vehicle and is cost-effective.
As it has a low power drive the AC option is not cost effective.
For higher power machines such as those more generally discussed on these forums the design issues change and the technical solutions are indeed likely to change from those I've used on this small tractor. My technical comments above are not aimed at these higher power applications but to clarify the engineering design of this machine as it has been criticised.
If the design of my wee DIY electric tractor doesn't meet the needs of users of larger machines - fair enough (and hardly surprising given its size), but this does not justify its description as being worthless - especially on a public forum.
Ian
Built For Fun EV's.
PS Happy New Year to all from here in Scotland
