ORIGINAL: tashspop
i know this is all on paper and everything is different in real world. i guess i'm confused with the part where you said (if you keep the gearing the same, the more volts, the more amps you draw.) i always thought i motor can produce x power. ie watts. so if it takes 100amps at 10 volts to produce x, then it should only take 50amps at 20 volts to produce x. what i'm saying is electric motors are only able to produce a certain amount of power right? or does this number just go up until it tears itself apart?
Well, you know how much power you want, but what you don't know is what to gear it so that when you UP the voltage, the final result, Power (watts), stays the same, and want the amps therefore to go DOWN. I.e. 10v * 100 amps = 1000 watts. You get a 20 volt battery, so now you know that you want to KEEP the motor at 1000 watts, but with 20 volts you want 50 amps, like in your example.... So in order to keep the amp draw low (hopefully even less than before so it's ~50 amps) you will need to use a much smaller pinion... here is why:
What ordinarily will happen is this.. with the same amount of load on the motor (i.e. same exact gearing, tires, weight etc etc), the amp draw will go up proportional to the voltage increase... up to a point until the motor's wire coils reach a saturation point with the amps, beyond which it will just generate heat and become inefficient.... So what you want is to use the smaller pinion, so that with the increase in volts, you will increase the total RPM, but because of the smaller pinion the motor can turn over more easily, so it won't draw as many amps... which means with trial and error, you will find your 50 amps or so and voila, you will still be at 1000 watts peak, but now with half the amp draw, and possibly twice the runtime (assuming the 10 volt and 20 volt batteries are the same capacity).... get it?
As for that 36 volt motor you have, yes, without the factory's maximum amp rating, we can only theorize that it will draw about ~98 amps on 36 volts to reach it's max 3500 watts. But again, as you said without a data logger we won't know what it's doing (amp draw, etc) so that we can try to use our available practical voltage (battery voltage) and gearing to keep it's amp draw in check and still get good power, and all that.
Basically, the motor will continue to want to suck in more amps the more voltage you feed it. The max power rating is only there to let you know what you can expect it to perform, but you will need to gear it correctly with your particular setup to reach your performance vs. runtime goal or whatever it may be.
Hmm that help you a little?