can i use such a small high rpm motor?

[jamesjim]

could I use an engine like this in a rc ebike build -
http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=4912

most of the low powered assists builds i have read about seam to have very large motors compared to the actual power they plan on using and I can't figure out why.

I can't find much on the limits of chain or belt drives.

please help

 

[adrian_sm]

Nah. Too small.

The power ratings are for their intended application in a plane, and even then they are generally for peak power rather than continuous.

They just don't have enough thermal mass to soak up the waste heat generated on an ebike. And something with that high a kv desn't create much torque, so you need a lot of gear reduction to make it useful.

You are better off findings something physically bigger, to be able to handle the waste heat, and low kv to make the job of gearing it easier.

- Adrian

 

[Red_Liner740]

Couple of problems with such a small motor and such a high KV.

if you intend to feed that thing any sort of decent voltage it will spin WAY too high....say 48V at 900rpm/V = 43200 RPM!! lets use 80% efficiencly thats still 34500 RPM. I'm pretty sure that a. that motors bearings cant survive such high RPM for sustainable speeds and b. the amount of reductions necessary to go from 34500 down to about 300 rpm that the wheels turn at around 35mph = 115 to 1 ratio!! You cant achieve that in a single reduction let alone two...how many more reductions can you add before you run out of room AND the driveline losses become so high that it becomes futiles to even try?

fine so you decide not to go 48V but 24V. say youre shooting for the rated 550W. 550/24V = 23Amps. If you want more then you will have to feed the motor so much amps that no normal 24 or 36V controller will be able to be used. Gonna have to spring for a lyen 12Fet or even 18Fet. Umm, youre just about to blow 160 bucks on a controller to run a 30 dollar motor?

next problem is the 5mm shaft, useless for attaching any decent sized sprockets.

do yourself a favour and go with a bigger RC motor. You dont have to go with an 80/100 like most ppl do but it does help. The reduction ratio is not as large due to slower widings and you know that the motor will take the 3, 4 KW and even higher bursts without problems.

 

[Hillhater]

It can be done , and has been done ! ... using an even smaller motor
Look up the "Gruber Assist " drive... It uses a 30mm dia motor and a planetary reduction to reduce the rpm down to BB speeds.
Now, im not saying its the best drive, and its only 100 -150 W max,........... but nothing is impossible !!
http://www.youtube.com/watch?v=wGYqKBUEelw

 

[Red_Liner740]

nobody said you cant do it, its just a question whether the time and effort and the rest of the parts cost is justifiable when going to a bigger motor for an extra 30 dollars or so would yield MUCH better performance.

we had a saying in street racing days...throw enough 100 dollar bills at ANY problem and it will go away.

 

[def215]

im using a 50mm motor that has a kv of 670. but the only reason i think im getting away with it is because im running friction drive with a tiny roller. the motor in the link is a 900kv which seems pretty high(and im the one to talk about using high kv motors...lol). im not sure what kind of drive youll be running either so thats a factor to if it can be done.

 

[Danny Mayes]

You can with sufficient reduction, however there are probably better choices like the SK3 motors for example.

http://www.hobbyking.com/hobbyking/...SK3_3542_800kv_Brushless_Outrunner_Motor.html

The SK3 motors are newer, more refined and have better quality control.

Again the key here is sufficient reduction. That is the only way a motor like that can work. I assume this is for a bike? (Edit - Duh, i can read i promise...)

What battery do you want to run? Voltage?

What topology do you want to go with or how are you planing the reduction and power transfer to the wheel?

What controller are you looking at using?

Good luck with it

D

 

[jamesjim]

Thanks for the replies

I was looking into using the motor for a 200w(state law)solar powered bike.

I have since decided to go with a hub motor as the motor would be used for coasting on dead flat roads. Hubs motors seam to be okay for this type of riding from what i have read. This allows me to concentrate on the solar battery aspects more. I may come back to these type of motors in the future as they seam so cheap.

One query i couldnt answer myself-

what is the relationship between voltage and efficiency?

from the original posts motor the specifications are - max load is 45A, max efficiency is 30A and voltage 5-15v.

am i just to assume that this means that max efficiency is 30A at 15v?

 

[neptronix]

The higher the voltage.. the higher the rpm.. the more efficient electric motors tend to be. But with high RPM comes the need for a gear reduction... and that gear reduction is going to eat some power in itself.

200w is not a lot of power. You'll go about ~15mph on it. You could definitely pedal that fast on a flat road.

You should consider a small geared motor for this task. Something like cell_man's MXUS geared kit or a bafang. Those will do 80%-85% efficiency... definitely some of the best motors for a low power situation.

 

[Lebowski]

If you do all the calculations you'll find out that:

1-efficiency goes with motor volume: the bigger the motor the more efficient it will be
2-efficiency also goes with the % of the motor volume that is copper (windings)
3-the higher the power the lower the efficiency
4-more rpm => higher efficiency
5-voltage, # of windings, Kv etc etc etc have no effect on efficiency except for when they influence point 1 or 2

 

[bobc]

I have a 35mm outrunner on my single wheel trailer. Yes it gets hot and yes it makes a racket. Good for about 14mph ;^)
I rarely use it now I've made a proper ebike.....