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Jun 24, 2015, 01:33 PM
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Discussion

how many foot pounds of torque


can am outrunner produce?

I think it's determined by the airgap and kv and amps...but don't know the formula. How much can a typical 63mm outrunner produce?

searching through the site I find it roughly at 15
Last edited by Hummina; Jun 24, 2015 at 01:44 PM.
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Jun 24, 2015, 02:11 PM
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Hi ,
The torque belong to the of the stator . So dia is not all ,we need the lengh of the stator . Dia x pi x lengh = surface . Than there are some numbers you can use to find the torque. Formthe first aircooled outrunner we calculate with about 2,5 newton trust per square centimeter of the statorsurface. Short aircooled outrunner reach about 8 newton/cm2 for some seconds lime used for recordairplane ,maybe higher ,aske R.Okon what he currently run . Liquide cooled motors do about 8 newton continius trust and 12 newton for short periode and up to 16 and higher for very short time . Maybe someone can do moore .
Example : Stator with 63mm dia and 20 mm lengh . 6,3 cm dia x pi x 2 cm lengh = 39,6 cm2 x 2,5 newton = 99 newton trust produced x leverarm of the rotorbell ( 0,063 meter /2) = 3,1 newtonmeter torque . So with 16 newton per cm2 you can have about 20 newtonmeter Torque. This is a rule of thumb estimate ,but gives a number .
Jun 27, 2015, 06:02 PM
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To convert watts(output) to torque.

Torque(ft lbs) for a running motor= Amps(in)x Volts(in)x (motor's efficiency at the shaft

rpm it is running at) x (550 foot pounds)/ 746

The motor's efficiency can be found from that motor's graph or by calculations and simple

measurements you can look up in the (Electric motor design and construction) forum sticky.

If the motor rpm is held at zero with voltage supplied it will have its most torque

available but will burn up. At max no load rpms it will have no shaft torque available.
It is pretty much a straight graph line between those two end points(Torque,amps ,rpm).
Good luck
Jun 27, 2015, 09:25 PM
If it flies, I can crash it.
rocketsled666's Avatar
Motor torque and speed are inversely related. In a theoretical motor with no friction or other losses, when power is applied the motor produces torque and starts to accelerate. As it does, the motor generates its own electric current called "back EMF" which resists the flow of electricity being supplied to the motor. Since torque is a function of current flow, the torque starts to go down. The motor continues to speed up and the back EMF continues to increase until the back EMF and the external voltage are equal. At that point, motor torque goes to zero, the motor ceases to accelerate, and it's reached it's maximum speed as dictated by the motor's KV.

Real motors are far from lossless, so the actual maximum speed the motor runs is slightly less than what the KV dictates. But you probably also want your motor to make something turn, like a wheel or a prop. Those things require torque, which cause the motor to run slower and draw more current. As the load increases the motor runs slower and the current increases. When the motor is stalled, there is no back EMF and the maximum current flows through the motor.

So motors make the most torque when stalled and the least torque when running at top speed.

Motors come in many configurations. The size of the winding, the number and size of the magnets, the number of turns on the winding, the wire diameter, there are many mechanical features that dictate motor performance. You can't generalize based on a particular dimension. In fact, there are many motor "families" with motors that have the exact same size but significantly different performance.

I'm not certain what your application is, but also, talking about motor torque without also specifying speed basically makes any possible answer completely arbitrary. If you want a brushless motor for a camera gimbal you're going to make very difference choices than if you're looking to spin a prop...
Jun 30, 2015, 02:26 AM
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I'm simply looking to find what torque a standard hobby outrunner that's 63mm can produce. I'm reading that it will be from its first turning and roughly 15 foot pounds. That's not much. I can turn a one foot wrench with 15 pounds of force easily. Just surprised
Jun 30, 2015, 11:00 AM
If it flies, I can crash it.
rocketsled666's Avatar
"63mm" can mean a lot of different things. There is no such thing as a "standard". And how much torque the motor can produce when stalled isn't the same question as much much torque the motor will produce when stalled. RC speed controls rely on the motor to be spinning in order to understand when to commutate. If you stall it, or even load it so it runs really slowly, the ESC will have trouble and motor operation will probably produce substantially lower levels of torque than the motor specs would dictate, and possibly result in the catastrophic failure of the motor and/or ESC. For very high torque, most designs will use a gear reduction. A motor designed to spin a prop is not the right motor design for a high-torque application.

What are you trying to do?
Jun 30, 2015, 12:57 PM
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make a 100kv outrunner hub motor for a skateboard. I appreciate the math given to me and I'll go over it. 100kv seems to be a good number given the wheel size at 85mm diameter and the 1:1 gearing on it. Most people push off when using electric skateboards unless they are sensored and that's fine.
Jun 30, 2015, 07:49 PM
If it flies, I can crash it.
rocketsled666's Avatar
I assume we're talking about more than one motor? Two or Four?

You should figure out how much HP you think you want the Skateboard to have, convert that to Watts, and divide by the number of motors. Then you'll know how much power you're going to need each motor to put down, and then you can start shopping for motors that will fit inside the diameter of a wheel.

You're not going to try and carry the load on these motors, right? The bearings in our hobby motors aren't designed for a high side load, I doubt they'd last (if your idea was to build the motor directly in to the wheel).
Jun 30, 2015, 08:02 PM
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there are some hub motors that someone is selling now on endless-sphere. they are simply some low kv turnigy motors that are slid on the skate axle and with a bit of lathe work voila. Then glue on some polyurethane. they run on the same bearings that come with the motor. they've been doing well so far. Until the polyurethane wears down and then it's a glue removal job and getting messy. He's been selling a pair of them on an axle and seemingly they have tons of torque and speed. Other people have used smaller turnegy motors, with 20mm stators, two of them, and gotten almost as good efficiency as with electric bikes. these two 20mm stators suffered efficiency a bit going up steep hills. He'd also converted them to wye dropping the kv to about 120. the motors I was writing about at the beginning are sold at 130kv I think
Jun 30, 2015, 08:29 PM
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Do post when you manage a "Back to the Future" Hoverboard
Jun 30, 2015, 09:37 PM
If it flies, I can crash it.
rocketsled666's Avatar
Quote:
Originally Posted by Hummina
there are some hub motors that someone is selling now on endless-sphere. they are simply some low kv turnigy motors that are slid on the skate axle and with a bit of lathe work voila. Then glue on some polyurethane. they run on the same bearings that come with the motor. they've been doing well so far.
OK. You made me think. I hope you're happy!

A Skateboard uses an 8mm axle, I think. Motors with 8mm diameter shafts are motors like the Turnigy G160 and G110, which are 2-3HP motors. The G110 comes in a 210KV, which is the lowest in a motor with an 8mm shaft, it's 63mm in diameter, and at 2KW/2.6HP is probably about right for a skateboard that you want to be able to go uphill under power.

G110 210KV Motor

Are you saying they wrap the motor housing with poly-u, and run *that* as the wheel? If so, I don't understand how they'd get away with that. The motor "bell" housing, the part that turns in an Outrunner, is only supported on the shaft at one end. It can't take a high radial load at the unsupported end, it's intended to transfer torque to the shaft, only. Push hard enough sideways near the bottom, I'd expect the housing to deflect so that the magnets will hit the armature.
Jul 01, 2015, 11:02 AM
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Yea normally the motor bell/can/rotor is supported at one end only but the one he uses has another skirt bearing at the wire end which fits nicely over a lathed truck. There's a fan in there too I think. Can't search for it now for you but it's front and center in the skate section on endless-sphere. "Got a lathe and made hub motors" or the like. Thought they'd have died by now too. The eternal quest for hub motors on skateboards is relentless..and eternal.
Think g110. 2 of them. Said to have lots of dangerous power to rip you off
Jul 01, 2015, 05:47 PM
If it flies, I can crash it.
rocketsled666's Avatar
After doing a little more poking around, it looks like the 8mm motors probably use an 808 bearing, which also happens to be the bearing that's favored for skates. So I retract my earlier statement (I was thinking "much smaller motors"). The bearings in a G110 are probably identical to the bearings in a skate truck.

I did go look at endless-sphere, but without having any idea what to search for, it was too daunting a task, I didn't find anything in the short time I poked around. But I'll go look again!

As for the OP's query, the G110 210KV looks like the exact motor he's looking for...
Jul 01, 2015, 06:30 PM
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I'm not trying to convert a motor. I did that once and it was a failure in the end. Trying to make motors...get them made in China. Wondering if the torque would be too much to have the motor design such that it could simply be epoxied on the skate axle. The strength of epoxy, from the numbers, is strong enough. Beats having to make some kind of clamp or something to secure it.
Jul 01, 2015, 07:34 PM
If it flies, I can crash it.
rocketsled666's Avatar
If the motor is going to carry the load, I would be surprised if Epoxy would hold it on. It's a question of surface area for the bond. The back of the motor isn't going to present a large mating contact area. If you're going to have the motors made, though, you can change that maybe. Though screws would still be preferable I'd think...


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