What is an Outrunner Motor?

Typically an outrunner motor is a Permanent Magnet Synchronous Motor (PMSM). Therefore, the motor consists of a stator with coils and a rotor with permanent magnets. With conventional motors, the magnets are attached to a shaft that rotates on the inside of the stator coils. This is called an inrunner motor. With an outrunner motor the magnets are attached to a ring or sleeve on the outside of the stator coils. As a result, the motor has a rotating ring or sleeve, instead of a rotating shaft.

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. 

Adventages outrunner motor

Adventages of an outrunner motor, in short, are:

      • Maximum torque achieved for minimal build volume 
      • No need for mechanical transmission or gearboxes
      • Rotor can directly drive the application
      • Highly accurate
      • No risk of contamination
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