Outrunner Motor

A brushless motor whose magnet-lined outer shell spins as the rotor, giving it high torque.

An outrunner motor is a brushless-motor layout in which the stator, carrying the windings, stays fixed in the center, while an outer shell lined with permanent magnets rotates around it as the rotor. Because the magnets sit farther from the shaft, the same electromagnetic force produces more torque, so outrunner motors naturally deliver low speed and high torque, with high torque density — but they also have higher rotational inertia, and because the windings are enclosed inside, they dissipate heat less easily. They first became popular in RC models and drones, where they're cheap and available in many sizes. In robotics, pairing one with a single low-ratio planetary reducer is the standard recipe for a quasi-direct-drive actuator: enough torque, good backdrivability, well suited to legged joints that need compliance and shock tolerance.

ExampleMIT's Mini Cheetah joint actuator is exactly this: an outrunner brushless motor with a roughly 6:1 planetary reducer built into it.

Also called

Outrunner Brushless Motor

Related

Brushless DC Motor · Quasi-Direct Drive · MIT Mini Cheetah Actuator · Motor Velocity Constant (KV Rating) · Torque Density · Axial Flux Motor

Sources

Outrunner - Wikipedia