Outrunner Information Thread
 

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. :yes:

As you mentioned start can be an issue. Mainly due to the very high loads placed on the outrunner if used in direct to wheel or direct to diff setups. I found that using very high quality batteries, an esc with high ratings, and the max voltage you can will help greatly with startup issues. Since "gearing" is very limited in these applications it is very important to get a correctly sized and kv spec motor, as you really have no way to compensate.

The adjaustments available to the mamba series escs are very helpful although the quark escs (not really relevant anymore) provided better startup in every application I did. Adjusting the expo on the throttle channel was alos a big help, even when setting the startup power on high.

I never messed with timing too much, usually only went up 1 step from the default timing. A capable battery seemed to be the single most important part, todays crop of 40c batteries are very good, and have low internal resistance. Outrunner direct drive is one application where crap batteries will just not work (so leave the flaming labels in the crawler).

Hi Shonen,

Interesting thread idea. I like it.

Wow, I'm not sure which part of your post to talk about first.

First suggestion that comes to mind, if you want to learn ALOT more about outrunners and the principles behind what sets them apart from inrunners, then learn more about how to wind an outrunner.

Browse around this site, there's alot more on the homepage I think:

http://www.powercroco.de/12N10P4225kit_english.html

It was the single most educational thing that I did.

It will teach you alot about how pole count affects all the numbers and why, with both inrunners and outrunners.

Wye versus Delta winding and the differences between the two. For example: Two motors, both with the same number of winding turns, one is Wye and the other is Delta, The wye wind will make 1.73 times the amount of torque as the Delta wind and usually be more efficient doing it. While the kv of the Wye will be 1.73 times less than the Delta. There is a relationship between KV and torque, plus the pole count has a huge impact.

I'm glad you enjoyed the threads Linc and I have created over the years. Thanks for the recognition.

Outrunners do prefer higher timing. This has to do with magnetic harmonics inside the motor and impacts efficiency in a huge way.

Picture it this way:

If the next phase is fired before the magnet is readily positioned over the next pole, then magnetic forces clash. This magnet is fighting the fields produced, but is carried over by the inertia of the spinning motor.

Timing is very important. Higher timing is just favored by the outrunner. It is part of it's nature.

Does the diameter of the endbell affect torque? Absolutely.

To illustrate:

Take the doorknob off and try to open the door by just grabbing the post.

Then install a 12" diameter "doorknob". Will it be easy to turn? You bet.

It's leverage at work. :yes:

lincpimp:
The only brushless ESC's I've ever used are Castle, CCC (cheap chinese crap/copies), and two Novaks that are useless without a sensor cable. The Mamba Max Pro, Sidewinder (SV1), Mamba Monster and Phoenix ICE 100 all handle my little 6-pole (Hobbyking 500H) and my bigger 8-pole (Turnigy T600). I haven't the money to get a 'quality' outrunner yet, or test with other ESC's. :oops: There's not much of a serious RC scene on my island, everyone I know with a brushless RC either has Castle, CCC, Novak, HPI or TRX ESC so I don't have much to work with. Our only LHS is an HPI/Traxxas dealer too. :mdr:

sikeston:
thanks for the link, I'm reading it right now. I've gone through several winding tutorials, I'll go through them again. (:

As for the doorknob analogy...I get how the torque will increase by having the magnets further from the shaft, but does this translate into Kv? By making a motor 'bigger' (larger diameter) and keeping everything else the same, does Kv drop?

Thanks for the information on timing, that was the kind of response I was looking for. Physics on why it works, not 'it just does'. Will a heavier can tolerate more advance timing, or is it just down to the electro/permanent magnet configuration?

So much information, I love it! :party:

I never really understood the outrunner until I seen the right 3D animation.

This one isn't very good, but worth a look

http://www.rcgroups.com/forums/attac...mentid=2691817

The internal waveform is moving ALOT faster than the outside can is. In other words, it takes advantage of quicker fet switching inside the esc.

In a 2 pole motor, the rotor spins 120 degrees. In a 12 pole motor, the outside can only spins 30 degrees for each phase shift. For me, this explains alot about the outrunner motor's lower kv compared to the inrunner.

It also says alot about the increased torque.

For timing, it is just a matter of finding that motors sweet spot. For the ESC, this is a relationship between the switching timing and the emf feedback that the esc receives from the motor. The emf feedback is what the esc uses to track rotor position and know when to "flip the switch" and fire the next phase.

If you notice in the winding information, all three legs of any 3 phase motor ARE connected to each other. The chopped DC pulse from the ESC ONLY goes through 2 legs at any given time. The pulse travels the path of least resistence and ignores the longer path. EMF is "sensed" and measured coming back through the "dead" leg. This applies to all sensorless motors.

There's ALOT to understanding an outrunner motor winding. Then check it out, there are many different ways to wind one.

For as many pole configurations as you can come up with, you can multiply that number times 2, since you can do any wind in either Delta or Wye.

Adding sensors manually seems to help the startup a lot. Would be nice if there were some sensored outrunners available.
http://endless-sphere.com/forums/vie...edf29c2960f750

[YOUTUBE]3EiDExMfxq0[/YOUTUBE]

I have been wating a few sensored outrunners for a while. They would be great to have for many direct to diff/wheel applications as well as being great for crawling. Imagine a moa rig with a pair of 22 series sensored outrunners run by a pair of mmpro escs on 6s. Unstopable!

I may have to look into adding sensors myself, looks like a fun project.

Me too Linc.

Sensors would be nice. This would allow the user to take full advantage of the ultra low speed increased torque.

Sensorless start up routines just don't cut it when starting under a big load (high geared) direct drive setup.

Looks like Holmes Hobbies is getting close to releasing one.
http://www.rccrawler.com/forum/showt...136315&page=13

John Rob is a regular here.

Perhaps he could join in here and shed some light on this.

I would buy if they were large enough.

I wonder what sizes are going to be offered?

I think a kit that would allow for the sensors to be placed outside the can would be a good starting point. Offer mount plates that are the correct diameter, and pre-wire the sensors. I am guessing the sensors are the only part needed to be added? They rely on the rotating magnets to provide something to sense? I have no experience or technical knowledge of how a sensored motor works.

Would adding sensors to a motor have any affect on the braking performance? It looks like the ones on the bikes don't use motor brakes, so I'm curious if the mod would work as well in a car with motor brakes.

I can imagine it would, as the esc needs to know where the rotor is in order to juggle the 3 phases to produce braking. My understanding is that the motor then acts like a generator and the esc places it under load in a certain manner to slow the rotation. Might be way off on that, though.

I think you're right about the process. I think what I was trying to say is; will the fixed timing in a sensored setup have any adverse effects on the motor braking response? Do the CC ESCs vary the brake timing with sensorless motors, as they do while powering them? Do they work the same way while running a sensored motor?

I am asking because I want to try to add sensors to a CC outrunner and use a MM Pro.

EDIT: I found the ROAR sensor pinout in case anyone was curious...

The pinout for the motor sensor wire is as follows (as per ROAR 8.8.1.3 specs):
Pin #1- ground potential
Pin #2- phase C
Pin #3- phase B
Pin #4- phase A
Pin #5- temp control, 10 k Thermistor referenced to ground potential
Pin #6- + 5.0 volts =/- 10%

I was wondering why the harness had 6 wires in it... Pin #5 can be omitted.

I've seen that animation before, and I must say it is quite informative. I can stare at it forever :mdr: it helped when I looked at it frame-by-frame as well.

It does seem to work very well for low-speed control, I will read up more on sensored outrunners.

I hope he sees this thread, cause the one on RCC has been dead since January...

As for the DIY sensor projects, it seems to be viable since apparently sensored RC outrunners are quite rare. I wonder if it's due to the higher pole count? Or maybe just because outrunners were typically air-type motors, and startup was not as important due to the small loading vs. ground startup?