In-runner or Out-runner BLDC motors in Drones?

Drone motors are almost always outrunner BLDC (Brushless DC) motors.

Why Outrunner BLDC Motors are Used in Drones:

1. High Torque at Low RPM:

   • Outrunner motors generate higher torque at lower speeds compared to inrunner motors. This is particularly beneficial for drones, as they need to spin large propellers efficiently to generate lift.

2. Efficiency:

   • Outrunner motors are generally more efficient at converting electrical energy into mechanical energy at the speeds and torque levels needed for drone flight.

3. Cooling:

   • In outrunner motors, the rotor (which is on the outside) spins around the stator. This design allows for better cooling since the rotating outer shell helps dissipate heat more effectively, which is important during prolonged flight.

4. Compact Design:

   • Outrunner motors can deliver the required torque without the need for additional gear reduction systems, making them more compact and lightweight—crucial factors for drones where weight and space are at a premium.

Inrunner vs. Outrunner for Drones:

• Inrunner Motors:

  • Typically used in applications where very high RPMs are needed, such as in RC cars or small jet turbines, often combined with a gearbox to increase torque. They are not as suitable for direct-drive applications with large propellers, which are common in drones.

• Outrunner Motors:

  • Ideal for direct-drive applications like drones, where they can directly drive large propellers without needing a gearbox, thanks to their high torque output.

Conclusion:

The motors used in most drones are outrunner BLDC motors due to their high torque, efficiency, cooling capabilities, and suitability for direct-drive applications with large propellers.

Resources

Brushed DC motor vs Out-runner BLDC motor structural comparison 

Controlling Motor Speed with PWM