Outrunner Information Thread

[Shonen]

I, like many others before me, have been interested and fascinated by the outrunner style brushless motor. I've taken it upon myself to do some researching (and searching...) to collect as much information as possible about them, yet some things still elude my findings. Much thanks to sikeston34m and lincpimp, whose posts (among others) have taught me a lot.

I've come across many threads talking about brushless outrunners, particularly their comparison to inrunners, but none of them contained all the information that I'd like in one place...hence my making of this thread. I will reiterate what I've found, as well as ask questions on the subjects that have not been covered comprehensively. If you find discrepancies with the information I post, please correct me.

1. Timing
Higher timing increases current draw (and thus power), decreases efficiency and raises Kv. But is it always bad to run low timing? I've read everywhere that outrunners "prefer" high timing, but how high? What is it more dependant on, pole count or efficiency?

Wye type windings can benefit from timing more than Delta type windings (or so I've heard), and the pole count vs recommended timing seems to be roughly proportional. There are also accounts of where low timing in an outrunner causes lower power AND higher current draw. Hmm...

The consensus on timing seems to be to start out at ~10° and use temperatures as your guide.

ps. For the Castle Creations crew, are the numbers in the Sensorless Motor Timing part of CastleLink equal to degrees? It goes from 0-20, yet AXi recommends 24° for their motors (and others up to 30°). If this is all done by calculations on the fly, will there be higher options in future firmwares?

Sources:
RCM: 4-Pole Timing
RCGroups: Outrunner Timing
Timing Test By Ken Myers
RCM: MM and 6 poles... (pg.2)
AXi model motors: SETTING OF SPIN CONTROLLER FOR AXI BRUSHLESS MOTORS

2. Torque
Sikeston once said that outrunners make 5-7 times the torque of inrunner motors. This statistic fascinated me and I dug around to find out why. The simple explanation is a simple equation: Kt*Kv = 1.345. Kt is the torque constant (oz-in/A) and Kv as we all know is the RPM/v constant. Outrunners have fantastically low Kv ratings, thus their Kt is huge to maintain the equation. A 600Kv outrunner does indeed, by definition, have 7x the torque of a 4200Kv inrunner that draws the same current.

At first I thought that the '5-7x torque' numbers came from the pole count of the motor, 2 pole being prevalent for inrunners and 10-14 poles being common in outrunner styles. Turns out, pole count is directly related to Kv as well.

Okay, now for a question: does the fact that the magnets being further away from the motor shaft have anything to do with the high torque? Is that factored to the Kv as well? Will an outrunner with say a 10mm diameter stator and 11mm ID can have pole-proportional torque to an inrunner with 10mm diameter rotor and 11mm ID stator (equal lengths)? There is also an effect of electromagnetic 'gearing' due to unequal numbers of electromagnets on the stator and permanent magnets on the can, is this reflected in the Kv as well?

Sources:
Aveox: Motion Control Primer
PDF: Brushless Motors
RCM: 6 Pole operation on the MMM - Long term effects?
Wattflyer: Difference Btwn Inrunner & Outrunner
Wikipedia: Outrunner
RCM: 8 Pole Heli Motor in an E Revo?

3. Other ESC Settings (Cogging prevention, PWM, etc.)
Aside from timing, I noticed that a lot of air type ESC's offer PWM frequency adjustments. Ground-based ESC's normally do not (as far as I know), thus some of them work while others don't. I don't know what PWM outrunners prefer, in fact I read that It can also be firmware-related, so if your ESC doesn't work with your outrunner make sure you try all firmware revisions (particularly with the Mamba series). Outrunners supposedly have higher inductance in their coils than inrunners, so high PWM frequencies may not work properly.

Other things to change on the Mamba ESC's include start power (high seems to work best) and drive mode to proportional w/reverse (I think they changed this in the newer beta firmwares). This helps with cogging on startup. If these don't work, try gearing down a bit...which tends to be kind of difficult when used in a direct-drive setup.

Sources:
RCM: Me and My Axi-Maxx Video's! (pg.4)
RCM: Me and My Axi-Maxx Video's! (pg.7)
RCM: outrunner'd e-revo (pg.2)
HeliFreak: Inrunner vs Out Runner? (pg.2)

4. RPM Limits
This is the one area I have not found any reliable data on. Yes, I realize that maximum RPM is highly dependant on the motor's physical size...but I rarely find that outrunners list a maximum RPM. Rather, they tend to list recommended lipo voltage. This may seem like the same thing, but is a motor under more load in an aircraft doing 60mph or a ground vehicle doing 60mph? My ERBE's 1717 was at ~92% of its maximum RPM at top speed due to driveline/bearing resistance, gear friction (10+ gears between motor and tires on the ERBE) and aero drag.

Anyone with any information about this please help...

ps. the 36mm diameter outrunner in my Revo 3.3 conversion is going at 33krpm (on 6s, its max rated), and my 49mm diameter outrunner is going at 26krpm (on 8s, max rated). From what I can see, outrunners have a max RPM of roughly 1/2 to 1/3 of what equivalent inrunners will have. I'm guessing that max RPM is inversely proportionate to how far the magnets are from the shaft of the motor (not considering bearings for this). As far as I know, maximum RPM of a motor is dependant on how many G's the magnets can take before disintegrating, minus a safety margin.



Let me know if I'm wrong about something (most of the RCM threads I used as sources are from 2008), and please add to this if you have any information I missed! The point of this thread is to be the one-stop outrunner motor reference.