Outrunner vs. Inrunner Brushless Motors in Robotic Actuators

When designing a robotic powertrain, engineering teams must choose between two distinct structural layouts for brushless permanent magnet motors: outrunners and inrunners. The physical location of the rotating magnets completely dictates the motor's torque profile and thermal behavior.

### 1. Outrunner Motors: High Torque, Low RPM

In an outrunner configuration, the motor’s external housing (the bell) contains the permanent magnets and spins *around* the internal, fixed stator windings. 

*  **The Advantage:** Because the magnets sit further away from the central shaft, the motor benefits from a much larger mechanical lever arm. This results in massive native torque density ($\text{N}\cdot\text{m}/\text{kg}$), allowing engineers to build direct-drive or low-reduction robotic joints without relying on heavy, complex gearboxes.

*  **The Trade-off:** Outrunners generally operate at lower maximum RPMs and can be harder to seal against environmental dust and moisture.

### 2. Inrunner Motors: High Speed, Low Native Torque

In an inrunner configuration, the rotating magnet core sits directly on the central shaft *inside* the stationary outer windings.

*  **The Advantage:** Because the spinning mass is concentrated tightly around the center axis, inrunners can achieve staggering operational speeds (often exceeding 30,000 RPM). Additionally, because the windings sit directly against the outer metal shell, heat can be dissipated directly into an external aluminum chassis or liquid-cooling jacket.

*  **The Trade-off:** They possess very little native torque. To drive a heavy robotic joint, an inrunner *must* be paired with a high-reduction planetary or strain-wave (harmonic) gearbox, which introduces backlash, weight, and mechanical complexity.

**Engineering Verdict:** Opt for premium outrunners (like T-MOTOR or KDE Direct) if your robotic application demands a compact, high-torque, direct-drive system. Choose inrunners if your design requires extreme RPMs or closed-loop liquid cooling.