Hi Nat,
If you could be patient and educate up this Glaswegian lad could you let me know what a PV is? Also do you have a photo or could you post a link to a photo of the tractor?
An issue that frequently arises is the choice of electric motor power to replace an IC engine in your case what might be needed to replace a 1.5 HP unit. It may be worth making just a few points here that might help others who may be pondering the possibilities of electric drives.
IC power ratings are usually of peak power output they are quoted at the particular combination of shaft speed and shaft torque that produces the biggest power output (Power = Torque x Speed, Watts = Nm x rad/s). Broadly speaking the engine isn't capable of producing more torque than that implied by the peak power figure and attempting to load the engine with a higher torque stalls the engine. (Okay, the peak power and peak torque conditions might not be exactly the same point but they are usually not too far apart).
Electric motors are rated differently however, and a number of different rating methods can be used. One common one is the motor's continuous use rating (s1) this is usually based on the full loaded shaft speed of the motor and the continuously rated shaft torque. This torque is however a bit different from that for the IC engine in that it is not the maximum torque that the motor can produce - nothing like it, it is the torque that matches the current flow through the machine which does not produce overheating in the motor (in DC machines torque is approximately proportional to the current drawn). There are other ratings s2, s3 etc and these are power ratings based on shaft speed and the various higher torque (hence current) levels that can be sustained by the motor for more limited time periods. Electric motors can typically produce torques several times their continuous rating but for limited time periods and are routinely used in various applications in this way.
So where does this get us? To get a feel for what size of electric motor is needed to replace a known IC engine power we might start with the questions - how long do I want the machine to run continuously at the peak torque level of the IC engine being replaced, and how well do I need it to hold top speed under this loading? If you need the machine to operate continuously at what was the IC engine's peak torque output and to sustain its higher operational speeds at that torque level you will need to fit a motor with a continuous rating close to the peak rating of the IC engine. (This motor will substantially out-perform the IC engine in other circumstances however because it can produce much higher torques levels for short term bursts watch the strength of the transmission.)
If, however, as is the case in many types of vehicles, the peak torque is only really needed for short term bursts of activity and won't be sustained for lengthy periods then an electric motor with a continuous rating much less than the peak rating of the IC engine could be used. This will be able to provide the short term intermittent peak torques from within the intermittent use area of it's performance envelope so long as they are not sustained long enough to cause overheating. The normal lower power duties will fall within the continuous use area of its operation and won't be a problem.
If these sums are done for normal road-going passenger cars for example then numbers come out indicating that an electric motor with a continuous rating of roughly 25% of the peak power rating of the IC engine might be a good place to start (see the AC drive site from my previous post). I suspect that working tractors are going to be higher than this because they will spend more time using more of the power available but just how much I couldn't say, that's an interesting question for all you tractor users. How fully utilised is the available power on your machine (and for what periods of time)? This might not be a very easy question to answer but I have a feeling that many experienced tractor users will have quite a good feel for how often their machines get close to stalling and under what circumstances. If so it might be possible to define a rough duty cycle to use to select the electric motor size.
I would guess maybe a power rating for the electric motor of 30 to 40% of that of the IC engine being replaced might be a start point but that really is a guess not to be relied on.
Off course for smaller power drives the question may be less important than for bigger machines where there is more money at stake. For example 1200W (1.6HP) continuously rated DC PM motors designed for vehicle use can be bought in the US for less than $100 and agonising about whether you can get away with a smaller cheaper motor might not be worth the time.
The cost of these types of motors (and the cost of DC motor controllers) contributes to the popularity of DC drives on lower power applications. They are also simple systems to implement in that the electronics is all packaged in the controller and there does not in most vehicle applications need to be any informational feedback between the driven motors and the controllers. 4 quadrant (fwd/rev & regenerative braking) DC controllers up to 300 Amp capacity are widely available and can pack quite a punch.
I'm afraid I don't have many movie clips of the wee tractor in use, but for readers interested in what simple DC drives in vehicles can do have a look at this link.
Movie clips
The vehicle in the photos/clips isn't a tractor (obviously) but a replica veteran car. It uses exactly the same drive motors and controller in the same dual drive setup as the tractor (although the reduction ratio of the transmission is different). I ended up reducing the power output for the final version because there was more power there in the 1300W drive than was probably good for it. Movie clips 1 through 4 are probably of most interest and show it just recently out the workshop and having its first test runs. (The clunking was my dodgy front suspension design which I subsequently changed.)
Apologies if I'm teaching folks to suck eggs here, I hope all this info will be helpful.
Jeff,
Thanks very much for the plans order, it's much appreciated.
I'm sorry about the metric units this is a perennial problem for me as I have many US based customers. We too in the UK have an imperial measurement past but all education and most engineering practice is now metric in common with the rest of Europe and other parts of the world - time for you guys to change over?
Interestingly many of our materials and components dimensions give away their imperial past and are just metric conversions, and there are several close equivalents that can be used 1/2 ply instead of 12mm, 1/4 fastening hardware instead of M6, your #35 roller chains and sprockets will do instead of our 3/8 pitch UK specs etc etc. Much of the model engineering world here uses imperial measurement and some inch sized materials and components are available but not enough to spec up a whole vehicle. The main irritant is matching bearings to shafts to drill sizes for the bearing housings. We can get whole number mm sized bright steel shafting which matches standard metric bearings whose OD's match standard wood drill sizes which all helps to reduce the extent of any machining required. It's more difficult for me to do this for inch components and I have to trust my US customers can look at the plans and adapt them to suit their sources of supply.
If it helps I put together this equivalence chart for US customers of the antique style vehicle, perhaps I should include it with the tractor design also.
conversions.pdf
Off course if you find in the end you can't use the plans as they are I would be happy to give you a refund.
Cheers all for now.
Ian
Built For Fun EV's
By the way, try to get a controller that's one box, not a system with a bunch of wires and components. The one module controller is much easier to work with. You might see something like "EV-100." That was probably the best system ever made, but not for a rookie to work on. Good luck and let me know if you need anything, Andy.
