Postby hyve » Tue May 20, 2008 8:48 am
Hi Malcolm,
Going back to your response to my offering: not only does torque fall off to nothing at the no-load speed, horsepower does too, being a product of torque and revs. Since at top speed you need a lot of power, the gearing, or electricity utilisation at this point will need to be rather different.
These questions get to the heart of the whole problem of trying to use electric motors for vehicle propulsion: their characteristics are so different to IC engines, which increase power along with revs to a much more marked extent. This rather suits vehicle propulsion at the higher end, but not starting from rest.
But there are so many variations on electric motor configuration that a lot more is possible and getting to grips with all this is taking me some time. Your idea of switching power for 2 motors is a fine example, though as you say it sounds like a big bunch of contactors will be needed.
Returning to the physics, you really need power curve graphs for the different motors you're looking at. The only way to gear the vehicle is to get maximum output power/kw at max. road speed, if you're trying to keep the motor size, battery pack and finished weight/cost within reason.
What actual power you'll need at 70mph is the hard question ! Examples of similar machines is about your best hope for arriving at this figure, but frontal area has a lot to do with it.
Having worked this out you will then have a gear ratio to multiply the torque figure by. It's important to remember that torque versus weight is what governs acceleration, not horsepower. There is a lot of misinformation in the popular motoring jargon, talking about horsepower per ton. This ratio is fairly irrelevant. Torque is what accelerates the vehicle; bhp comes at just one point on the rev band and affects top speed mainly, unless you have an infinitely variable transmission and can keep the motor at that speed constantly.
By comparing the result of your sums with some figures for ordinary IC cars with known gear ratios, torque figures and allup weight, you should be able to predict whether your planned vehicle will have enough torque to cope without a gearbox.
There is a complication in this approach, of course: a lot depends on what type of motor you use. This is where my query about tipping point comes in. So far my feeling is that we need a series wound DC motor to give that huge start-off torque, then change it's wiring to an AC motor for high speed ! Such a motor would compare with the IC version so very differently that making this comparison will not really help.
Whether anything like this is possible I leave to you electric boffins, because I just don't yet understand or know enough about what can be done. I'm just drawing conclusions from the requirements vs. the characteristics. Someone else has put it well in saying that electric cars use clever electronics instead of old fashioned mechanical gearboxes to create the same result.
Good luck with it anyway.
Peter Ph