help me to understand motor power.. small motors big power?

[Lynx_Gen]

First off, just to say hi, as this is pretty near my first post, and indeed my first thread.

I got into the idea of electric powered vehicles last spring when i got a chance to drive an Electric Focus.

This was just a few days/weeks after watching who killed the electric car in class. The impact of it all left me kinda sad and dissapointed with my own attitude towards transportation.

At any rate, I got into lookin into electric vehicles etc, and based on laws and feasablities, one idea turned into another and so on.

To keep things simple, I should be gettin a Magic Pie 3 ordered up here this next weekend finaly, or possibly a more reccomended motor if there is one. (Im in canada, so goldne motors is simple and easy to get for me.)

I also have another project I wana give a go, i think it'll work, but if it fails, wel then it fails, and at least I tried, but Im confident it will work.. taking offroad style rollerblades and mounting an rc motor on one of em.

so this lead me to looking into all sorts of motors... now heres the one i found that i think I'd like to give a go...

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=21969

or:

http://www.hobbyking.com/hobbyking/store/__18853__T5692_Turnigy_Pro_Comp_Brushless_Inrunner_Motor_1000kv.html

the first one has a higher kv rating, so would make sense to use a different gear ratio to get more torque to speed ratio kinda thing..

now heres the question.. it says they're 6000watts... but they're tiny lil engines.. how does 6000watts power on a small motor compare to a larger size engine? I mean 6000watts power is 6000watts power is it not?

I know it should be a feasible endevour, since I've seen skateboards n such run off of lil rc motors and the like. I just have a sneaky suspicion that things dont quite add up.. how could say a lil 1000watt rc motor compare to like a 1000wat magic pie motor? or a 5000watt rc motor compare to like the 5000watt bigger motor used in electric motor cycles?

as it is its winter here now, yay canada.. *Sighs* so theres no real hurry as of yet, but I would like to start workin on it, its gona take awhile before I even figure what i need and how to do it, I pretty much gotta start getting parts n figuring out how to get em together.. but yeah, sorry for rambling and musing out loud..

 

[Miles]

Hi Lynx,

It's better to think about motors in terms of torque.

Small motor means small torque.

Power = torque * angular velocity.

You can always get more power by spinning it faster.

 

[Punx0r]

There's also a huge difference between a rating that a motor can achieve day-in, day-out continuously without failing, and a super-optimistic one good for only a few seconds with a hurricane force wind blowing through the motor casing

 

[Farfle]

There's also a huge difference between a rating that a motor can achieve day-in, day-out continuously without failing, and a super-optimistic one good for only a few seconds with a hurricane force wind blowing through the motor casing

Exactly, model airplane use is a much different ballgame, There's a HUGE amount of airflow, and the loads can be less continuous, an ebike slogging up a hill at peak power for several minutes is much more demanding than a vertical climb in an rc airplane. If I am saying this right, a given motor can only make a given amount of torque no matter what Kv is, as long as the amount of copper is the same. Since horsepower is both torque and speed, the small outrunner can make a whopping amount of horsepower for its size, but it makes it at a crazy rpm.

 

[liveforphysics]

I used to go for tiny high power motors. Requires fairly complex gearing systems (when you get into multi-stages) to reduce them to the wheel speed you're looking to attain. Then they have a bad habit of melting down when you push them a bit, though they can work perfectly for light-duty applications. I never found a setup that was very satisfying for my needs.

Now I go with large motors that don't spin past 6-7krpm, makes the reduction simple, and they tend to survive the huge power I throw into them.

 

[Lynx_Gen]

so, to thro some rough numbers, theoreticaly, using a motor rated at 6000watts, at 1740kv, max of 44 volts, = 76560rpm...

say an 8 inch wheel, x 76560 rpm = 612460 inches per minute = 933.38904 km / hr

so, I in no way at all wanna go that fast, ever.. (at least not with todays technology)

so if I geared it down about 23:1 that would give me a top speed of about 40km/hr, which would be plenty on rollerblades for me..

I would think that even runnin it at about half power would suffice, be geared for lotsa "Torque" per say, and keep things from being overly worked..?
And its not likely I'd be starting from a dead stop..? I was thinking if I went with a bigger power rated motor, then I wouldnt have to make it work as hard towards its full potential to get satisfactory results..
maybe?

or maybe I should just look into what the electric skateboards and such use and go for something along those lines? altho I dont think theres a whole lota difference between those motors and these types...

basically to start, I just need something powerfull enough to move me, and small enough to fit on rollerblades kinda thing.. lol

perhaps i should just buy a cheaper motor to test with, see if its even feasible, then go from there getting a better one.. but meh, may as well do it right the first time.. lol

 

[gwhy!1]

the little motors should be fine for your project ( if you can get the gearing right ) , I reckon you will only need around 1 to 1.5kw to make roller blades just to un-ridable at 40kph to be honest . But there will be pleanty of spare power if you want to go faster