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› Why do we have outrunners?
11-24-2006 08:54 AM  11 years agoPost 1
smudge

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Wiltshire, UK

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What is the point of outrunner motors?

I dont see how they can make more torque than an inrunner. If you hold the outside of an outrunner, the torque on the 'inside' bit will still be the same (like an inrunner)

It looks like extra complication for no real reason.

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11-24-2006 09:14 AM  11 years agoPost 2
bobkins

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Bocholt, Belgium

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Hi,

The spinning outside is not the only difference, the insides are also different.

Outrunners:
-fixed coils in the center and a rotating magnetic can on the outside
-produce more torque at lower speeds (low Kv)

Inrunners:
-rotating magnet on the inside stationary coils on the outside
-produce a higher speed (high Kv) with less torque

So we use outrunners because of the high torque at lower speeds.
If we would use a inrunner wich is also possible we would have to use different gearing ratios wich would mean that the main gear would be even bigger than we use now or use a smaller motorpinion (not always possible).

Greets Rob

Hirobo Shuttle Plus , Trex SE V2

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11-24-2006 01:46 PM  11 years agoPost 3
Gary Jenkins

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Nowhere, US

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The motor acceleration is also faster on an inrunner, because of the lower rotating mass. Race cars will use inrunners for fast acceleration. With a helicopter you want a constant head speed with a lot of torque for the variable pitch, hence the outrunner.

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11-24-2006 10:45 PM  11 years agoPost 4
busted blade

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orlando,florida

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im no motor expert yet but it seems the outrunner would run cooler because of its exposed external rotation

bling bling, cha ching......... but honey

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11-25-2006 07:12 AM  11 years agoPost 5
OICU812

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Edson, Alberta, Canada

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All you need to know is that they produce more power, I know why, but do I care,,,, not really. Yeeeah baby!

...Once upon a time there were Nitros, flybars and frequency pins...

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11-27-2006 06:13 AM  11 years agoPost 6
Wheelhaus

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Denver

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I dont see how they can make more torque than an inrunner. If you hold the outside of an outrunner, the torque on the 'inside' bit will still be the same (like an inrunner)
a proper outrunner DOES produce more torque, it doesn't really have anyhting to do with Kv, it has to do with physics and design.

I've seen inrunners with silly low Kv and they still need a gearbox. Inrunners can typically handle higher RPM because the spinning portion has less inertia and a smaller diameter. Outrunners are usually RPM limited because the spinning portion is on the outside, and "distance from center" has a huge effect on centrifugal force that is created. Magnet timing also becomes an issue as the magnets are now travelling farther than a comparable inrunner.
All you need to know is that they produce more power
Nope. They produse more torque but less speed. Inrunners produce more speed, but less torque. This equates to the same amount of "power" or "work", it's just two different ways of doing it. If you want more torque from an inrunner, you need a gearbox to slow the output and give you more torque... To speed up an outrunner you need a gearbox to speed it up, but you lose torque. See the relationship? This is a common misunderstanding and it's why power output is typically calculated in watts.

An inrunner can develop lots of torque, as can any electric motor. But the outrunner design has a trick up it's sleeve that allows it to produce more torque at the expense of rpm. An outrunner has a larger "lever arm" so to speak... Imagine using a torque wrench but when you use it, hold the handle at the halfway point. If your hand moves 3 inches, and you need 20 lb of force to turn a bolt 10° you have accomplished some work at the expense of energy. Now, hold the same wrench at the end of the handle, and you need half as much force to do the same job, but you have to move further for the same "work". Having a longer "lever arm" makes the work of the motor easier, it gives it more torque. This is the exact same idea behind a gearbox, where it allows the same "work" to be accomplished by using less force at the expense of longer actuation distance.

Simple Physics says: Work = Force x Distance. Or, translated for our topic, Power = Torque x RPM. Given a specific power requirement, a motor with higher torque needs less rpm. A motor with less torque needs higher rpm for the same power... Get it?

Another way to visualize this concept is to imagine lifting a boulder. By yourself it would be impossible. But, with a 2x4 suddenly the situation changes.. You could rest the 2x4 over a log with the shortest portion under the boulder. You pull down 3 feet and the boulder lifts 6 inches. Viola!

The reason true outrunners are limited from high rpm is because the speed at which the armatures can be energized. Smaller motore can reach MUCH higher rpm than huge motors because of several reasons. One is the magnet to armature timing.

With a moderate size inrunner, the speed at which the magnets are moving past the armature coils at 10k rpm is... Lets say 50mph for an example. Lets say at 50mph the armature is energized perfectly as the magnet passes, and you get optimal power.

With an equivalent outrunner (lets say twice the armature diameter as an example) the same 10k rpm will mean the magnets are flying past the armature at 100mph! That's no good, way too fast. Slow the outrunner down to 5k rpm and the magents are now moving 50mph past the armature. Now we have the same timing, but the lever arm is now longer because the magnets have to travel a farther distance to achieve half the rpm of the inrunner. I believe an outrunner will typically have more poles, meaning more points of armature energization because the magnets are covering more distance, so you have more room for armature windings. This is the reason outrunners are considered to run so smoothly. This extra distance gives you more torque, just like 1st gear in your gar compared to 4th gear. Same power, but different output speed and different output torque.
im no motor expert yet but it seems the outrunner would run cooler because of its exposed external rotation
Not necessarily.
Brushless inrunners typically run more efficiently and can cool themselves better because the armature (winding) is against the wall of the can which acts as a heatsink.

Many people think outrunners are a godsend, not really. They are just a simpler way to get the same work done. They usually aren't as efficient, but they are lighter because there's no need for a gearbox. This functional simplicity and lighter weight make them the choice for most small-mid sized electric airplane pilots who are willing to lose a few % efficiency for the benefit of light weight and quiet, simple operation. Many people think inrunners with a proper gearbox are more efficient. They'd be right, however there's more parts to break. It's a tradeoff.

Wow that was way more than I wanted to type... If any of that doesn't make sense let me know and I'll try ti straighten it out...

..........
Dave

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11-27-2006 03:55 PM  11 years agoPost 7
stickyfox

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Rochester, NY - US

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I concur. But I wouldn't say that it has nothing to do with kV; kV is just a numerical way of expressing the "physics and design" at work in that particular motor. It lets you compare it to other motors.

The power you get out of a motor depends on the size and efficiency of the motor. That doesn't have anything to do with kV. It's kind of like comparing Vin Diesel's muscle car with Lucas Black's rice rocket. One has tons of horsepower at low RPMs. The other has the same amount of horsepower when you wind it up to 6000 RPM. They both make for fast cars, but the transmissions are very different.

-fox

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11-27-2006 09:22 PM  11 years agoPost 8
ozace

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melbourne, australia

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one important point is outrunners are much cheaper and easier to make, thats probably the big reason for their popularity.

we can never have too many, can we ?

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11-28-2006 04:44 AM  11 years agoPost 9
Wheelhaus

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Denver

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The power you get out of a motor depends on the size and efficiency of the motor. That doesn't have anything to do with kV. It's kind of like comparing Vin Diesel's muscle car with Lucas Black's rice rocket. One has tons of horsepower at low RPMs. The other has the same amount of horsepower when you wind it up to 6000 RPM. They both make for fast cars, but the transmissions are very different.
thats a much better example... I was about to fall asleep at the keyboard.

..........
Dave

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12-09-2006 04:31 AM  11 years agoPost 10
BobOD

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New York- USA

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I don't think anyone has really answered the original question here so I'll give it a try.
If you took an outrunner and reversed it again, and changed nothing else, you would have an inrunner with the exact speed and torque characteristics as the outrunner. Similarly, if you had reversed an inrunner, and changed nothing else, you would have an outrunner with the exact speed and torque characteristics as the inrunner. The original point was, for a given diameter at the air gap, the outer part will feel the same torque as the inner. This is absolutely true, so what's the real difference?
The difference is that the mechanical design of an outrunner allows for a much bigger diameter at the air gap in a much more compact and lighter arrangement than an inrunner. Basically, you move the magnets way out and place them on the inside of a rotating can then fill the middle with copper and iron. As you move the magnets out on an inrunner, you just create excess space on the inside and make for a bulky external stator.Now, if it's RPM's you're after, then you want to decrease the diameter and all the above arguments reverse.
Hope that answers it.

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12-09-2006 12:47 PM  11 years agoPost 11
PJRono

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Minnesota, Ya!

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The part about lower Kv isn't exactly true either. If you change how the motor is wound you can get higher Kv. By wiring the stator in parallel Wye or parallel Delta instead of regular Wye or Delta the Kv is tripled (or more) resulting in much higher Kv. I have made outrunners that turn 60,000rpm unloaded. The motor I made(480 size) hade the same Kv as a Mega 16/15/3 but with more torque. Much of this also depends on how many poles the motor has. You can also trick an outrunner into higher KV by changing the magnet configuration. instead of using 12 magnets NSNSNSNSNSNS you can alter this and go NNSSNNSSNNSS. Or you can use less magnets that are bigger.

If you skip me I can't play!

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12-09-2006 02:53 PM  11 years agoPost 12
GyroFreak

rrProfessor

Orlando Florida ...28N 81W

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Wheelhaus

Thanks for the excellent discussion of the diffrent types. Your explanation is great. While I know the diff in OUT vrs IN runners, I hadn't thought thru the torque / rpm diffs.
Thanks to all the others who put in some good info on this subject.
Now for my question. I still don't understand the kV rating (other than its a type of power rating). I have been told its the volts per rpm .. But isn't the brushless motor a type of stepper motor where the rpm is controled by the ESC ? I guess that rating worked for brushed DC motors and has been applied to brushless design as a carry over ??.
I'll reread this thread and pick some more good stuff out, thanks guys.

I think about the hereafter. I go somewhere to get something, then wonder what I'm here after ?

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12-09-2006 07:00 PM  11 years agoPost 13
Wheelhaus

rrVeteran

Denver

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I still don't understand the kV rating (other than its a type of power rating). I have been told its the volts per rpm .. But isn't the brushless motor a type of stepper motor where the rpm is controled by the ESC ?
Kv is not a power rating. It's only an rpm rating. You can have two identical motors that have different Kv, meaning at the same voltage you will have two different rpm. This doesn't make one more efficient than the other, but it helps to select a motor that will spin your junk at just the right speed.

Rpm IS controlled by the ESC, but If I'm correct on this, the ESC pulses voltage on and off many thousands of times each second to lower rpm below 100%. This is the high freqnency whine you hear at partial throttle. Old brushed motors make the same sound at low throttle. This whine disappears when you go full throttle because there's no pulsing, it's sending 100% voltage. The ESC can't run efficiently at partial throttle because the chips are going insane trying to pulse on and off tens of thousands of times each second.

On a related note...
The amperage that the motor draws is related to the amount of work it's trying to do. At 100% voltage, the ESC and motor can run at top efficiency and the least heat is built up because the voltage just flows right through. Lets use an example for a governor running at 100%. At 100% voltage, if there is very little load on the head the motor needs very few amps to spin the rotor at full speed. Now when you punch out the motor suddenly needs 40 amps to maintain that rpm. If the motor is not powerful enough, or the battery can't sustain that high discharge, you will hear bogging. This is why you hear some people say the ESC should be set to 100%. this does NOT mean 100% POWER, it only means 100% voltage. The motor will draw the current it needs from the battery. This is why some people don't notice any change whatsoever in rpm anywhere from 50% to 100% gov settings, the ESC can so easily spin the head that it just goes 100% voltage anyways.

Think of the governor as cruise control. it maintains a speed, that's it. If you're rolling down a hill, it's super easy and you use little power. If you go up a hill, suddenly the engine is cranking out power like crazy to keep the speed you requested. Speed stays the same. So, for a governor ESC, it ideally applies 100% voltage in order to run efficiently, but the motor will draw as many amps as it needs to get the job done because it's Kv rating determines it's RPM.
ok that wasn't so related after all... I was just talking about governors with someone else... (:

BobOD almost has it perfectly right. Now that I've thought about this more I can make better sense of it.
An outrunner develops more torque because the magnet/armature field (air gap) has a greater distance from the center axis. This makes a bigger lever arm, like using a longer torque wrench. This air gap has to be as thin as possible, the closer the magnets are to the armature (electromagnet) the more powerful the magnetic fields are. Cheap motors can have larger tolerances, expensive motors often have much tighter tolerances. This "distance from center" creates a larger lever to create more torque.

PJRono described in better detail what I was talking about with magnet timing. The magnet flies past the armature at a given speed, so for a high rpm outrunner you need longer (or paired) magnets to increase the interaction time. An inrunner has a lower magnet speed because the "air gap" distance from center is shorter. This increases the interaction time of the magnet and armature. It can also naturally spin at a higher rpm because it's small center has little inertia, the mass doesn't move through space as fast because it has a shorter distance to cover for 1 rotation.

Outrunners designed for low rpm have a benefit of having room for more armature windings to be packed around the core. Packing more armature windings allows the ESC to time them just right so the magnets are ALWAYS being pushed. This creates a smoother rotation with more torque. This is why most low rpm outrunners don't need a gearbox. High rpm outrunners usually still need a gearbox, but depending on the appliction and motor's design, they may or may not run as well/better as a tight inrunner.

..........
Dave

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