Step 2: A Typical Brushless Outrunner

The dominant type of motor found in the aeromodelling world today is called the outrunner motor. The first image shows literally the first result I got when I searched "outrunner" on Google Images, and is pretty damn representative of most of them.

Mounting the Outrunner

So the cool thing about outrunners if the name didn't give it away already is that the outer case of the motor is the part that spins . In the first image, only the faceplate of the motor - the part the wires exit through - is stationary, and that is the part which gets mounted to something solid. Otherwise, the entire rest of the shiny gold and silver bell spins. This tends to render the motor unsuitable for conventional "DC" mounting styles like clamp mounting or double-supported mounting. The idea of the outrunner in aeromodelling is that you can directly mount a propeller to that rotor bell end.

However, for EV usage, mounting a sprocket, pulley, or wheel directly to the motor in this fashion is a bad idea . The reason is shown in the diagram in image 2. The bell shaft bearing is effectively cantilevered, which means a strong side load (like tensioning your chain or belt, or a wheel load) can bend your whole motor . The longer the motor case, the worse this effect is.  Instead, all but the tiniest motors also hang the shaft out the other side of the motor, so it can be conventionally mounted and used. This is the end you would want to mount a radial loading component like a sprocket on - sprockets and belt pulleys, or gears for that matter, can often by purchased stock with the right bore size.

Reading the Outrunner

Outrunners are typically given a numerical designation similar to


There's two overlapping, almost indistinguishable, and sort of conflicting systems about what the letters mean. 
First is the stator-referenced system. In this system:

1. The first number AA indicates the stator diameter in millimeters. This is the active component in a motor which generates all the torque, so this is akin to selling cars by engine displacement.

2. The second number BB indicates the stator length (stack height), or the length of the magnets.

3. The third number C may be a low number (single digits to 20s), indicating the number of wire turns per stator pole. If it is a high number (high tens to hundreds) it is the motor's "Kv" constant, or voltage constant in RPMs / V

4. An optional Y or D means the windings are terminated Y or Delta - for the same TURN COUNT, Y-terminated motors rotate slower and with more torque for the same current draw, but need a higher voltage to achieve said current draw. It's a design tradeoff, but the vast majority of R/C outrunners are Delta terminated for convenience.

The second is the motor-referenced system, more common for inexpensive motors, in what I can only assume is a ploy to amplify the apparent size of the motor.

1. The first number AA now refers to the total diameter of the motor, in millimeters.

2. The second number BB is the total length of the motor case, from front to back, minus the shaft.

The third and fourth numbers typically remain the same.


So how do you tell which one is which? If its not explicitly given to you as stator diameter , it is probably the latter system. The most definitive way to tell is if you have both data points - stator and outer diameters. A list of typical stator-to-motor diameter cross correlations for typical vehicle-sized motors is given below:

42 to 45mm stator > 50 to 55mm motor case
50 to 52mm stator > 63 to 65mm motor case
68 to 70mm stator > 80 to 85mm motor case

Sizing the Outrunner

Most electric scooters will find a motor in the 60mm (motor) diameter class more than sufficient. A good seller will give at least two important specifications which you can use to determine rudimentary drivetrain parameters.

1. The Kv rating is how fast the motor will spin per applied volt. Conversely, it is how many volts the motor will generate across its terminals if you spin it . This is largely a remnant of the DC motor days when you could dump your motor on a battery and it will spin. Electronic controllers, such as BLDC controllers, can actually vary this parameter of the motor significantly, so the Kv is just a rule of thumb unless you are a motor engineer .

You can use the Kv rating in RPM/V, your system voltage, your anticipated drive ratio from motor to wheel (x to 1), and wheel diameter (in inches). to calculate a theoretical top speed for the vehicle. This is a purely theoretical number in an ideal, frictionless world. The equation goes

Speed (mph) = [( RPM/V * System voltage ) / ( Gear ratio )] * ( Diameter * pi ) * ( 60 minutes per hour / 12 inches per foot) / ( 5280 feet per mile)

A cool little resource that does all this for you and even provides you with acceleration and battery figures is the Tentacle Torque and Amp Hour calculator , written for the combat robot community by the late Steve Judd, a long time Battlebots and robot combat competitor. The website is still maintained as a resource for robot builders.  As expected, it's very "robot oriented", but to use it for vehicle calculations, just plug in your own motor statistics (or take a best guess), and use 0.5 motors per side if you have single motor drive. Note that "Average % of Peak Drain " should be turned down to 5% or 10% for EV usage - that is the amount of time you spend doing burnouts or launching from standstill. A database of cataloged motors exists for sanity checking.

If you only have a KV rating, then the only thing you can estimate is the top speed.

2. The internal resistance of the motor, also known as winding resistance, terminal resistance, etc. It will generally be a low number (less than 1) ohms . Given this value and your system voltage, you can calculate the maximum current draw the system can theoretically see based on Ohm's Law, I = V / R . Real current draw will be less (but not much less) than this value due to the inherent resistance of copper wire, semiconductors, switch contacts, etc. But again, a ballpark figure.

Additionally, as described in my just-build-your-own-damned-motor-already writeup , given the Kv of a motor in RPM / V, you can also find the torque produced per amp of current draw. RPM/V is not a SI unit, but a little math will get you to the SI definition of an electric motor's voltage constant, V / (rad/s) ; that is, volts per (radian per second). In short, the voltage constant in V / rad/s is also the torque constant in Nm / A, or newton-meters per ampere

If you're that inclined, Nm/A can be directly back-converted into ft-lb/A or in-oz/A, as they are all units of torque.

Therefore, if you know the IR of the motor, and your system voltage, you can find a theoretical peak torque value for the system, which is useful for calculating maximum accelerations: Torque (Nm) = ( Nm/A ) * ( System Voltage / Motor Resistance ). This number is, indeed, very theoretical. I'll address special considerations for R/C motors near stall in a little while.

More Sizing of the Outrunner

Some times, you will also see a power rating - usually in the hundreds or thousands of watts. It's important to remember here that the value given will almost invariably be power input - that is, the power your battery is feeding into the motor. If you are familiar with DC motor principles, you know that the motor can only ever deliver 50% of this value back out as mechanical output power - torque times speed. (If you're not, read this ). And that's if it's a ideal motor -  at this operating point, 50% or more of the input power is being dissipated as heat. Essentially, the "power rating" figure is not very helpful, since if the motor is operated at anywhere near half of the figure, it will quickly overheat.

Ultimately, the way to size a motor by power is to roughly calculate your total drag force using the Drag Equation, and assuming Cd is about 1.0 (for a person standing up and moving forward), and multiply that by your desired cruising speed - in SI units, the result is the power the motor needs to output to keep you going at that speed. In other words, Pmotor in watts = (Drag Force in newtons * Cruising Velocity in meters per second).

As a rule of thumb, this should be less than 15% of the maximum motor input power.

Why not an Inrunner?

An "inrunner" is the back-constructed word for a conventional brushless motor. In the aircraft domain, they are much less suited to vehicle propulsion because they spin significantly faster i.e. have very high Kv values. Subsequently, they require much more geardown to achieve the same torque levels. While inrunner drives are definitely possible, the added mechanical complexity is suboptimal. However, they're definitely easier to mount and less susceptible to getting dirt and road junk in the motor.

<p>This 4 year old instructable is STILL a treasure trove of useful relevant information. Thanks for sharing!</p>
A fun build! Thanks for the awesome instructable!
<p>Actually instructive. Though the term is common I had no idea what a brushless motor was or how it worked, and I am an electrical engineer, an old one. Sixty years ago I was wondering why the brushes of the dead motor that I was rewinding were not semiconductor devices. The transistor was then 10 years old. Life took me in other directions so the idea never matured - and suddenly I know. Thanks! Further I now understand something about the DC/AC drive of the Tesla Auto. My interest is, at the moment, in electric propulsion for a nearly finished Sampan sitting in my back yard. Though off the shelf devices are readily available I am hoping for something better. Your remarks about RC devices and propellers seem to apply. The battery info is also helpful. I am struggling with the tradeoff of a low HP gas outboard, at less than 40lb and a 20lb trolling motor with a 50lb 12 volt lead acid battery. You advice about how to purchase motors, batteries and controllers is most pertinent. - Boatmakertoo</p>
<p>Hi My gfs bike has a 12mm rear axle and she hates the 6kg front hub. Id like to use this motor to drive the back wheel. It looks very similar to my internal motor on a mac 10t geared hub motor ie lots of turns. Could I not just use the infineon sensorless controller? http://www.hobbyking.com/hobbyking/store/__54888__9235_100KV_Turnigy_Multistar_Brushless_Multi_Rotor_Motor.html</p><p>Anyway thanks for the article, Dan</p>
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<p>Hi, your<br>article is sooo good!</p><p><a href="http://www.instructables.com/id/The-New-and-Improved-Brushless-Electric-Scooter-Po/all/?lang=es" rel="nofollow">http://www.instructables.com/id/The-New-and-Improv...</a></p><p>I created<br>some links to it at my site <a href="http://www.avdweb.nl" rel="nofollow"> www.avdweb.nl</a></p><p>You've put all<br>topics under each other so they are difficult to localize, it is sooo long now.<br>Can you create a table of contents on the top with links to the separate<br>topics?</p><p>Or create<br>separate items on instructables.</p><p>Kind regards,</p><p>Albert</p>
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What is the pinout for the standard, RC esc sensor connector?
Wonderful instructable. Have been giving it some thought for a bit and wondering why you haven't considered building your own ESC. From your descriptions a sensorless controller suitable for our purposes seems very doable, but not available within the R/C airplane realm. We need a custom, self-starting, sensorless ESC. Other's have made them. <br> <br>Once you have the basics together fine-tuning becomes possible. Seems like the perfect solution to a host of problems. Perhaps the springboard to designs otherwise out of reach.
Thanks for your instructable. It's very interesting and complete but I have a, may be, stupid question. I'm planning to put sensors in an outrunner as well and wanted to know how to tie the sensors to the ESC. There seems to be a standard 6 pin connector shared between the different ESC models. How do I wire the sensors to this connector?
This reminds me of a really cool scooter my best friend made once. While I was just looking for <a href="http://www.voloscooters.com" rel="nofollow">electric scooters for sale</a> he decided to make one himself, he even made it run off of a vacuum motor! I really didn't think it would work, but it sure did! I always admire those who are creative enough to figure out how to do this kind of thing.
This is a pretty sweet little scooter, might make one for my son if he starts behaving again. lol <br> <br>I am legitimately trying to find that gas motor which you showed but did not use. <br> <br>Can you tell me where you found it? I can only find electric models, which are great but not appropriate for my current project. <br> <br>Thanks, <br>-Grey
I found a good place to get thick very flexible 8 and 4 gauge wire is the local car stereo center. Th 8 gauge I used cost $1 per foot. They had 4 gauge but it was much more expensive.
Wow after reading this article now I'm building a scooter. <br>Bad influence I think.. <br>Welding and machining I get but the electrics are not so much my thing :( <br> <br>I bought 2 motors <br> <br>KA63-18L <br>Constant: 259Kv <br>Battery: 10Cell Lipo <br>Operating Current: 25-60A <br>Peak Current: 72A(15sec) <br> <br>Here is the problem.. Do I run one and have a spare or use them both? <br>If both what would be the best way? <br>
If you run both you need a controller for each. Now this may sound easy but you need to be careful to set up each esc the same and use the exact same model of esc and motor or one motor will do more work. Now unless you want to push more than 200 lbs around at 20 mph or more one motor should be sufficient as long as you use the gearing equation from this article as to not burn up your motor. Good luck and feel welcome to ask any questions you want to.
Hobbyking now sells 20C four cell 5 Ah lipo packs from the US warehouse for about $25 per pack: <br>http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18631
Could I maybe use a large pvc pipe to hold the hall effect sensors?
How exactly do you back install hall effect sensors in a motor? I am trying to make an electric motorcycle and want to use a kelly controller. I have a Turnigy 80-100-A 180 KV brushless outrunner (the same class that is on the scooter in this instructable) and have had a really hard time figuring out how to install sensors. I am also having a hard time figuring out how this motor is wound. <br>Any help would be greatly appreciated, thanks in advance.
It is extremely difficult to install *internal* sensors onto a mystery motor (of which you do not know the internal winding pattern). The C80 series is also very difficult to discern because they require so much current. <br><br>Usually, you would run current through two phase wires (&quot;line to line&quot;) and use a test magnet to observe which poles are polarized in which directions - they should always be alternating from tooth to tooth and there should always be 8 of them on in total. Using this and the process of elimination you can separate the 12 teeth of the stator into 3 groups of 4 teeth which you can call &quot;A&quot;, &quot;B&quot;, and &quot;C&quot;, which are associated with one of the three phase wires. Then, the Hall sensors are placed *between* 2 teeth of *different* phases.<br><br>Because the C80s have a very low resistance and few turns, you need either a ton of current or a sensitive magnetometer to determine the direction of the stator field. <br><br>Using external Hall sensors on the C80s is possibly the easiest solution. <br><br>
Thank you for the info, I have tried to run current through the different phases but I could not get enough current to get discernible results. I am thinking of just rewinding the motor so I know where the different phases are. Unfortunately I do not have access to anything to make an external mount for hall effect sensors. Do you know of a website that could make a mount for me? I also do not have any experience using CAD software. Another question I have is do all hall effect sensors have the same wiring scheme? The website I got mine from did not tell me which lead is which.
hi <br>a bit for the sake of accuracy: when you refer to &quot;convection&quot;, you actualy mean free convection(no air speed), as opposed to forced convection. <br>other than that petty note, its a great article and im learning why i shouldnt have bought the esc that i have...
Just wanted to say a quick thanks for this instructable. It's the detail all in the one place that i have been searching for, for years. Thanks for confirmation of R/C parts as well and the technical detail.
Can you use the sensored kelly controllers with a sensorless motor?
Not the KBS - Kelly does have the KSL line which is sensorless, but I have not heard vehicle stories about them nor know what settings they can manipulate (e.g. ramp-up time, initial current, etc.). And they're huge - they're full-size Kelly cases.
Great tips, especially about bore size changing! Thanks for taking time and explaining this!
If I'm going to use LiFePo4 batteries from HK, will I need to include a battery management system when discharging and/or charging? Great instructable, by the way.

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