What is an Outrunner Motor?
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. As a result, the rotor lacks any mechanical reduction nor gearbox. In other words, the motor directly receives its torque from the stator by means of the Electromagnetic fields generated by the stator coils.
Magnetic Innovations designs and builds configurations where the rotor rotates around the stator lamination stack and therefore is commonly called an outrunner topology. Another common used motor configuration is the "inrunner" motor. Consequently, the rotor is situated inside the stator for an inrunner motor.
Advantages of an Outrunner motor
Notably, an advantage of an outrunner motor compared to an inrunner motor is that the air gap surface is substantial larger. In other words, the surface area through which the electromagnetic field lines pass from rotor to stator and vice versa is for an outrunner motor larger than for an inrunner motor.
Moreover, the torque of both inrunner and outrunner motors is generated in the air gap and the maximum achievable torque level is therefore also depending of the total available air gap surface area. Specifically, the torque is a product of the total force generated in the air gap multiplied by the radius of the air gap. In other words, due to physical constraints the airgap force per square mm is limited. You can think of constraints such as the magnetic circuit, maximum (thermal) current density of electrical conductors and materials used. Consequently, outrunner motors have an advantage over inrunner motors because they benefit from a per building volume larger air gap surface area and a consequently larger air gap radius (which is the torque arm). Therefore, a larger surface area can yield a higher total air gap force. As a result, the larger force combined with a larger radius of the air gap results in higher achievable torque levels.
Because of building volume constraints in most applications, the rotor assembly has a maximum allowed diameter determined by the stator diameter plus the magnet height (between 3 and 10 mm for smaller motors) and the thickness of the circular back iron enclosing the magnets (between 4 and 12 mm for smaller motors). As a result, this limited combined diameter in case of an outrunner configuration yields an air gap diameter only 2 times the magnet height plus back iron thickness smaller than the outer diameter of the rotor which is significantly larger than that of an inrunner motor with the same maximum allowed diameter.