Brushless Inrunner vs Outrunner motor?

There are many options that we have for RC brushless motors regardless of the application. Motor come in many sizes to fit certain power ranges. One of the biggest factors that could effect your brushless motor purchase, is a brushless inrunner option or a brushless outrunner. Which motor option would you choose, inrunner or outrunner?

Key Differences – Brushless Inrunner vs Outrunner Motor

Take a look at the image below. You can see that the brushless outrunner motor has the output shaft, connected to a propeller in this case attached to the case of the motor. This would suggest that the motor shaft when spun would also spin the outer motor case.   This is exactly what happens. The permanent magnets on the outrunner are placed on the rotor and the rotor spins on the outside case. On the inside of the motor are the stator windings which do not rotate, they are fixed in position.

Brushless Outrunner Motor vs Inrunner Motor

Brushless Outrunner Motor vs Inrunner Motor. Outrunner on right side

On the inrunner motor, you essentially have the complete opposite true for how it is built. On the outer side of the motor is the case. The case in this situation does not rotate and is fixed. The stator windings are placed on the inside face of the case. When you spin the motor shaft of an inrunner, you are spinning the rotor which also contains the permanent magnets much like the outrunner. The difference of course being that they are now at the center of the motor.  For most, this would be the more conventional type of electric motor, especially if you are familiar with large AC motors or even old brushed DC motors.

Performance Differences – Brushless Inrunner vs Outrunner Motor

This can be easily debated as to which motor has the best performance when you dive deep in to the specifics. For simplicity let’s loosely consider motors of equal size and weight in order to compare the possible performance differences.

Physical Size differences

Generally speaking brushless outrunner motors will have a larger diameter and a smaller length vs a comparable inrunner motor of similar weights.  Conversely, Inrunners are smaller in diameter and typically larger in length.   Physical size is one area that your application may be limited in, however there are other trade offs that would have to be considered as we will get in to below.

RPM / Volt (Kv)

When you consider the RPM per volt of a brushless motor, (rotation speed per one volt applied) this is one of the biggest factors in choosing the correct motor for your application. Often times when one does not correctly select the appropriate Kv motor, risk of burning a power system component out is greatly increased. An inrunner motor of equal size to a brushless outrunner motor will have higher Kv. Although different motor wind selections (same size motor with Kv options) provides a decent range, outrunner motors will typically have a lower Kv value. This is key in your selection of a brushless motor to directly fit your application.

How does an outrunner produce lower Kv? Well, we already did speak about physical size difference. Physical size does represent a primary factor that effects kv. The larger can diameter of the outrunner allows a higher quantity of magnets to be used in the outer case. More magnets alternating magnetic poles forces the ESC to switch more rapidly slowing down the overall speed as there is more work to be done by the ESC. You could also more simply look at it as the larger diameter creates a larger circumference for the motor to travel in one rotation. The larger can diameter also represents a larger moment arm for an outrunner that is a good segue in to the next topic.

 

Torque Comparison of a Brushless Outrunner vs Inrunner motor

The larger moment arm that we have spoken about above converts directly in to more torque being created. Therefore the brushless motor will generate more torque as a general comparison against an inrunner motor. The relationship ties in with the fact that outrunners do have a lower RPM per volt. The relationship with Kv and torque are inversely proportional. As RPM per volt (Kv) increases, torque of the motor decreases.

Efficiency the same between inrunners and outrunners?

This can be a tough question to answer actually as the true answer has many dependencies. These dependencies can be anything from the effective size comparison to quality of the motor, manufacturer of the motor, power output of the motor and several more. In general for most applications in RC, a brushless inrunner motor has the potential to be more efficient than a brushless outrunner motor.

Waste Heat of Inrunner vs Outrunners

Let’s consider waste heat that a motor outputs. If we look at the brushless outrunner motor, heat must be dissipated through the center of the motor. The source of heat generation is of course the stator windings of the motor. The stator windings of the outrunner is located at the core of the motor. heat must transfer from the windings to the center of the motor being the motor shaft. Heat is then conducted outward through the motor shaft. An outrunners solution to heat is to  have large accessible cooling vents in the case to allow air to flow over the windings directly.

Brushless Outrunner Motor Windings

Brushless Outrunner Motor with large vents to the stator windings

An inrunner motor on the other hand has the stator windings directly on the inner face of the outer can. This direct point of contact provides an excellent surface with a very large area for heat to transfer. Air is a very poor conductor of heat. Increasing the amount of surface area for heat to gather in order to dissipate in to the air is how an inrunner can get rid of excess heat energy.

Being able to get rid of waste heat allows the inrunner motor to run cool being more efficient for similar power outputs on a motor of equal size/weight.

Common Brushless Outrunner vs Inrunner motor Applications

Motors that can be fit as direct drive offer simplicity which increases reliability, reduces weight that a transmission would add, and reduces amount of component wear items. The amount of load or torque required also plays in to the overall equations of an inrunner vs outrunner choice. Lastly required RPM is considered.

Here’s a chart to outline the most common approach for motor selection. Note that not in all cases this chart is followed. Deviating from it is entirely possible and really depends on other factors that we didn’t speak about. These can be availability, cost and other similar factors.

In general when I am selecting a motor, I am looking for simplicity and reliability. My first pick would be an inrunner motor and if this does not fit, then I select an outrunner.

Application Motor Choice
3D Airplane Outrunner
Trainer Airplane Outrunner
Pylon Racer Airplane Inrunner
Electric Ducted Fan Jet Inrunner
RC Car, Stadium Truck, Monster Truck Inrunner
RC Fast Electric Boat Inrunner
RC Scale Electric Boat Outrunner
Drone or Quadcopters, Hexacopters, Octacopters Outrunner
RC Helicopter Outrunner

Summary of Differences – Brushless Inrunner vs Outrunner Motor

Parameter for comparison Inrunner Outrunner
Can Diameter Smaller Larger
Can Length Larger Smaller
RPM per Volt Higher Lower
Torque Lower Torque Higher Torque
Efficiency Best
Heat Dissipation Best

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