Using RC motors on E-bikes [Archive]

[dirty_d]

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

[voicecoils]

I wonder if something like this could help for these high RPM motors:

18.75:1 gearbox for motorized (ICE) bikes:
http://www.staton-inc.com/Details.asp?ProductID=2363
$259, weight not listed.

[docnjoj]

Whats the conversion fromm radians/sec to rpm?
otherDoc

[fechter]

According to my math, Take radian/sec X 9.55 = rpm.

Those are excellent graphs since they cover the entire operating range. I wonder if they are from actual measurements?

[safe]

Dirty_D I'm glad you posted some charts.

Yes, I've spotted the "genius" of these RC motors too after Recumpence showed up. These things are freaking incredible. For something that weighs 5lbs or so you get the power of a 25lb hub motor.

I tried to get a "Developer Zone" thread going to talk about the gearing issues, but it was trashed four times in a row and then locked down after being trashed a fifth time.

Maybe we can discuss it here?

It's almost like you have to carry on discussions in a "stealth" manner these days because of the trashers.

[rkosiorek]

Whats the conversion fromm radians/sec to rpm?
otherDoc

warning for those who suffer from ADD "HIGH SCHOOL MATH LESSON AHEAD".

given : 1 radian = 180/pi degrees = 180/3.1416 = 57.296 degrees.
and : 1 revolution = 360 degress
therefore : 1 radian = 57.296/360 = 0.15915 revolutions

it follows that : 1 radian/second = 57.296 degrees/second = 0.15915 revolutions per second
given : 60 seconds = 1 minute
so: 1 revolution per second = 60 revolutions per minute.
therefore : 1 radian/second = 0.15915 revolutions/second * 60 seconds = 9.5493 rpm

so multiply radians/second by 9.5493 to get rpm.

rick

[Miles]

Here are the specs on the AXI website: http://www.modelmotors.cz/index.php?pag ... gorie=5345

They're also happy to do custom motors/windings and the cost seemed quite reasonable, to me.

[rkosiorek]

i love the machining that was done to make that RC motor work on an e-bike. but questions still remain - like - How much does all of that gearing affect the efficiency of the motor? what are the losses through the transmission?

he has a solution that qualifies more as a work of art. since he has his own machine shop he can produce these beautiful machines. now how about we ask him for an estimate of what he would charge to do a small production run of say 50 units. do you think you could afford to buy one? really how many of the readers could build something similar? they are out there but they are far from the majority.

i don't think that the idea was trashed as being impossible. i think it was criticized as impractical for most people to reproduce.

but if you do have the expertise and the equipment i would love to see your end result. in my mind i think of this as our e-bike equivalent of a custom chopper by those guys on American Chopper or a Hot Rod built by the likes of Boyd Coddington or Lil' John Buttera (may he rest in peace). lots of incredible fabricated pieces assembled into a functional machine. i do appreciate the beauty of it.

rick

[Miles]

We've covered a lot of this ground, speculatively, in this thread: viewtopic.php?f=2&t=1853

It's a pity eP doesn't contribute anymore...

[safe]

It's a pity eP doesn't contribute anymore...

It's become such a hostile place these days. In the beginning there was very little conflict, but I think the volume of people now just makes people act like monkey's. Crowd a bunch of monkeys into a tight cage and they turn into a fight club.

The RC motor is simply a wonderful idea and it's good that the person that delivered it (in a big way) as a concept wasn't me.

[safe]

More Tech Talk

I was thinking that a single gear-to-gear pair for a geardown of 1:3 combined with a secondary planetary geardown of another 1:4 would deliver a 1:12 reduction.

It's multiplication (not addition) when you combine gears right?

So:

1:3 + 1:4 = 1:12

Example being:

22:66 to 25:100 results in the equivalent of 550:6600 or 1:12

Planetary Gears

http://en.wikipedia.org/wiki/Epicyclic_gearing

One turn of the sun gear results in S / P turns of the planets

One turn of a planet gear results in P / A turns of the annulus

So, with the planetary carrier locked, one turn of the sun gear results in S / A turns of the annulus.

The annulus may also be held fixed, with input provided to the planetary gear carrier; output rotation is then produced from the sun gear. This configuration will produce an increase in gear ratio, equal to 1+A/S.

[dirty_d]

fechter, made the graphs through calculations and a program called gnuplot. the motor specs match up perfectly with the plot. the first graph isnt really that important as the motor in reality couldn't run like that because of physical limitations, but it does accurately show its electrical characteristics, the second plot should very accurately predict the motors realworld performance with a 100A limit because none of the motors physical limitations are being exceeded. well actually the amperage might be a little high, the site shows 110A as being the max current, so maybe it would be better to set the max at 60A for continuous operation, you still get a whole lot of power though.

as for the gearing, how would regular gears work? are they more efficient than belts? if they are it seems like that would be easier because you don't have to worry about anything slipping. one problem is that its hard to change gear ratios, i suppose you could just make long slots for the motor to be moved along to allow different sized primary gears.

[safe]

how would regular gears work? are they more efficient than belts? if they are it seems like that would be easier because you don't have to worry about anything slipping.

I'm assuming you meant "chains and sprockets" and not gears.

The problem with chains is that they can't be run above a certain speed before all kinds of pathological conditions develop. Basically as I recall the chain speed is not supposed to exceed about 100-200 rpm or you develop the problems.

True "gears" can be spun at any speed and the planetary gear is the most efficient of all types. The worm gear can most radically lower the gear ratio, but the efficiency is terrible. Usually a drop of about 1:3 or 1:4 can be done easily without much in the way of losses... and if you did two of them they multiply. So 1:3 plus 1:4 could yield 1:12 which is all you need. By using a straight cut gear first and then a planetary gear you make that first drop in the strongest way and the second in the most efficient. That way you use the best approach for each step.

Make it small enough and you have a "Swiss Watch" transmission for your bike.

Mass produce it and you have a manufacturing career. I would think that if someone dedicated themselves exclusively to developing this transmission they could dominate the market and maybe even have a virtual monopoly on it. If "greed is good", then this is "green greed" of both types.

("going green", "green greed", money is green... you get it)

The funny thing is that it might even be possible to convince the manufacturers of RC motors and gearboxes to build a custom adapted gearset for ebikes. It might end up staying within the existing manufacturing base and not come from the outside.

[dirty_d]

lol, no by "regular gears" i mean regular gears.

[Miles]

lol, no by "regular gears" i mean regular gears.