Step 2: A Typical Brushless Outrunner

Picture of 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.

m0ng00se4 years ago

Is a good piece of software to calculate speed etc, I got the link from http://www.jeromedemers.com/