WHAT IS AN OUTRUNNER MOTOR?

OUTRUNNER AND INRUNNER TORQUE MOTORS

There are two types of direct drive frameless torque motors: the outrunner and the inrunner torque motors. In case of an inrunner motor, the rotor is located on the inside of the stator.  In case of the outrunner motor, the rotor is located on the outside of the stator. 

Outrunner motors produce more torque for the same build volume compared to inrunner motors. Magnetic Innovations is specialised in the outrunner typology. But both type of torque motors, each with their own advantages, still function in a similar way.

DIFFERENCES BETWEEN AN OUTRUNNER MOTOR VS INRUNNER MOTOR

An advantage of an outrunner motor compared to an inrunner motor, is that the air gap surface is substantially larger. In other words, the surface area through which the electromagnetic field lines pass from rotor to stator, is much larger. This way more electromechanical force is generated.

Additionally the torque arm is longer for an outrunner motor, as the force is generated further from the centre of rotation. Consequently a larger air gap surface area and a longer torque arm both lead to higher torque. Hence, outrunner motors can achieve much higher torque levels than inrunner motors with the same build volume. 

To compensate for the lower torque, inrunner motors are often equipped with transmissions or gearboxes. But adding these mechanics lead to even higher build volume and mechanical losses. Furthermore it requires more maintenance, increases risk of contamination (oil, grease) and leads to less accuracy. So when build volume is restricted and high torque levels are required, outrunner motors are the best option. 

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.

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.