What is Outrunner Motor in BLDC Motor classification？

It's defined by its fundamental mechanical configuration:

1. Fixed Stator (Core): The central part of the motor, containing the copper windings (electromagnets), is stationary. It's mounted directly to the motor's base or frame.

2. Rotating Rotor (Bell/Housing): The outer shell of the motor, which has the permanent magnets attached to its inner circumference, rotates around the fixed stator. This rotating outer shell is often called the "bell" or "can".

Key Characteristics & Advantages of Outrunners:

• High Torque, Low RPM: Due to the larger diameter where the magnets are mounted (providing a longer lever arm for magnetic forces), outrunners generate significantly higher torque at lower rotational speeds (RPM) compared to similarly sized inrunner motors.

• Direct Drive Suitability: Their high torque at low RPM makes them ideal for directly driving propellers (drones, RC planes, fans) or wheels without needing a gear reduction system. This simplifies design, reduces weight, and improves efficiency.

• Efficiency at Lower Speeds: They are generally more efficient than inrunners when operating at their optimal lower RPM range.

• Cooling: The rotating outer housing can act like a fan, aiding in air cooling. The large surface area also helps dissipate heat.

• Compact Package: While the diameter is larger, the overall package for a given power can sometimes be more compact in the axial direction compared to an inrunner + gearbox.

Disadvantages of Outrunners:

• Lower Maximum RPM: The larger rotating mass (rotor bell + magnets) creates higher centrifugal forces and inertia, limiting their maximum achievable RPM compared to inrunners.

• Higher Rotational Inertia: The higher inertia makes them slightly less responsive to rapid speed changes (acceleration/deceleration) compared to lightweight inrunners.

• Cooling the Stator: While the rotor cools easily, the fixed stator deep inside can be harder to cool effectively, especially in high-power continuous operation. Some designs incorporate cooling holes or are designed for liquid cooling.

• Potentially More Exposed Rotating Parts: The outer casing spins, which can be a safety hazard if touched and requires careful mounting considerations.

Comparison to Inrunner Motors:

How to Identify an Outrunner:

• Physically: The outer casing (bell) rotates. The base (where wires exit) is fixed.

• Specifications: They almost always have a low kV rating (e.g., 200kV - 1500kV is common for drones/RC, vs. 2000kV - 5000+kV for inrunners).

• Size Notation: Often listed with diameter first (e.g., ∅5015 means 50mm diameter, 15mm height/stator length).

In Summary: An Outrunner BLDC motor is defined by its fixed central stator and its rotating outer rotor housing containing the magnets. This configuration prioritizes high torque at low RPM, making it the dominant choice for applications like drones and RC aircraft where direct drive of large propellers is essential.