i thought this was crazy until i saw recumpent's build log of his e-bike, i did a little more research and these motors are actually pretty spectacular. i got the info for the motor he used here http://www.espritmodel.com/index.asp?Pa ... rodID=5421
. then i used that info to plot the motors performace. all i needed was the no-load speed at a certain voltage and the resistance(0.027 Ohms) of the windings, with that you can find all the other characteristics of the motor.
here is the plot of the motor taking all the amps it wants at a constant 48V from 0 - 10000rpm
WTF! those are some big numbers, these motors are designed to be grossly underpowered(electrically i mean, not construction wise). at stall this motor will draw 1777A at 48V, thats 85kW!, the max output power would be about 21kW. of course this isnt possible because the motor windings would melt instantly. its designed to be run at very conservatively which allows it to still produce a lot of power while being very efficient. you can see the site does seem to publish the real peak efficiency as it matches up exactly on the graph, its hard to see, but in the graph on my computer i could see exactly and it was 92.8% at peak.
heres how it runs with a 100A motor current limit.
now this is where the motor shows its beauty, with a 100A limit the motor is over 85% efficient over more than half of the rpm range, and its already 70% efficient at about 1/10th no-load speed. and there is still a respectful amount of power available. between 1500-4500W the motor is running 80-90% efficiency that is pretty amazing.
ive known for a while that using a more powerful motor than you need is more efficent because you can run it under max power and get good efficiency while still having all the power you need. i always though if the motor was to be more powerful it needed to be physically larger, but it seems you can get the same effect by using low resitance windings and strong magnets in a tiny motor. of course the motor wont actually be able to run at more than maybe 10% of the stall current without burning up, but it will run more efficiently at low power over a wide rpm range.
the only problem is the high rpms, its doable but there are problems, recumpent had problems with the belts slipping and breaking. and with 2+ chains/belts you do lose efficiency, how much you lose i really have no idea but im sure he will post his findings after analyzing the data from his logger. and you need a lot of skill and tools to do this stuff yourself stuff has to be precise and perfect, its not like by bike with a 10lb motor with a #35 chain going straight to the rear wheel, i didnt need anything more than a welder, grinder, and a saw for that. i might have a go at it someday though