Outrunner and inrunner motors are two common brushless DC (BLDC) motor configurations used in drones. The main difference is the position of the rotor: an outrunner has a rotating outer shell, while an inrunner has a rotor spinning inside the stator.

The two configurations differ in torque and speed characteristics, as well as stator layout and cooling arrangements.

Key Takeaways

Outrunner motors are commonly used for direct-drive propellers, while inrunners are often selected for high-speed applications such as electric ducted fans (EDFs).
Rotor position affects torque characteristics, mechanical design, and heat transfer, while motor speed and Kv also depend on winding and magnetic circuit design.
Both configurations use laminated stator cores, with electrical steel requirements influenced by magnetic loading, operating frequency, and stator geometry.

How Outrunner and Inrunner Motors Are Built

Both motor types have a stationary stator and a rotating rotor carrying permanent magnets. Their stator and rotor arrangement is what distinguishes the two configurations.

Outrunner Motors

In an outrunner motor, the stator is fixed near the center, while a cylindrical outer shell containing permanent magnets rotates around it.

The magnets sit at a relatively large radius from the shaft. This arrangement allows a larger effective air-gap radius, making outrunners well suited to direct-drive applications requiring substantial torque.

Outrunners are widely used in multirotor drones, including FPV quadcopters, aerial photography platforms, and industrial UAVs.

Inrunner Motors

An inrunner places the rotor and magnets inside the stationary stator. The shaft rotates, while the surrounding stator remains fixed.

The smaller rotor diameter can be advantageous for high-speed operation, particularly where compact rotating components are required.

Inrunners are commonly found in electric ducted fan systems and other propulsion designs that operate at high RPM.

Torque, RPM, and Kv: What Changes?

Outrunners and inrunners are often described as high-torque and high-speed motors, respectively.

The larger effective air-gap radius of an outrunner helps it produce torque in a relatively compact axial length. This is one reason outrunners work well with direct-drive propellers, where the motor must turn the propeller without a reduction gearbox.

Inrunners typically have smaller-diameter rotors and can be designed for high rotational speeds. Their construction suits applications such as EDF units, which use rapidly spinning impellers to generate thrust.

Motor Kv describes the approximate no-load speed in RPM per volt, rather than the maximum operating speed.

A high-Kv motor generally turns faster per volt than a low-Kv motor. Kv is influenced by winding design, magnetic flux, and pole count, rather than rotor position alone.

How Rotor Configuration Affects Stator Design

In an outrunner, the laminated stator sits inside the rotating bell. In an inrunner, the stator surrounds the internal rotor and can be connected to a stationary outer housing.

This difference influences the available space for stator teeth, windings, and the magnetic circuit. It also affects the path through which heat can leave the windings and core.

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Outrunner and inrunner BLDC motor cutaway comparison showing stator positions and illustrated cooling paths

Stator Geometry and Magnetic Loading

Inrunners typically have a smaller rotor diameter and may use a longer active stack to meet torque requirements.

These geometric differences affect magnetic flux distribution and the dimensions of the stator teeth and back iron.

Both configurations commonly use non-grain-oriented electrical steel (NGOES) for their laminated stator cores.

Operating Frequency and Lamination Thickness

Electrical frequency depends on mechanical RPM and rotor pole count, rather than rotor position alone.

An outrunner with more pole pairs can operate at a higher electrical frequency than an inrunner running at a greater mechanical speed. This makes electrical frequency an important factor when considering core loss and electrical steel lamination thickness.

Cooling and Heat Transfer

In an inrunner, the stationary outer stator can transfer heat directly to the surrounding motor housing. This can be useful where the housing is designed to conduct heat away from the windings and core.

An outrunner has its stator inside the rotating bell, so the heat-transfer path is different. Ventilation openings, internal airflow, and the motor's mounting structure can all influence cooling.

Choosing Outrunner or Inrunner Motors for Drone Applications

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Outrunner motor applications in multirotor drones compared with inrunner motors used for electric ducted fan propulsion

Outrunners for Multirotor Drones

Quadcopters, aerial photography drones, agricultural UAVs, and many heavy-lift platforms commonly use outrunner motors.

These aircraft often need direct-drive propulsion with sufficient torque to turn their propellers. Larger agricultural and industrial drones may use low-Kv motors with large propellers, while compact FPV drones often use smaller outrunners designed for rapid throttle response.

Inrunners for High-Speed Propulsion

Inrunner motors are frequently used in electric ducted fan systems, where an impeller rotates at high speed inside a surrounding duct.

These systems appear in high-speed model aircraft and certain specialized UAV designs.

Outrunner vs Inrunner Motor Comparison

Feature	Outrunner Motor	Inrunner Motor
Rotor configuration	Rotating outer shell	Internal rotating rotor
Stator position	Inside the rotor	Surrounds the rotor
Typical design strength	Direct-drive torque	High-speed capability
Common drone applications	Multirotors, direct-drive propellers	EDF systems, specialized high-speed UAVs
Kv characteristics	Often lower in large-propeller designs	Often higher in high-speed designs
Thermal design	Heat transfer from the inner stator	Potential direct heat path to the outer housing
Stator material	Commonly NGOES laminations	Commonly NGOES laminations

What Electrical Steel Buyers Should Consider

Drone motor manufacturers sourcing NGOES coils should provide their stator dimensions, operating RPM, pole count, and magnetic performance requirements when discussing material specifications.

Project Parameter	Relevance to Stator Manufacturing
Outrunner or inrunner configuration	Identifies the intended stator arrangement and core geometry
Stator dimensions and lamination design	Helps determine material dimensions and downstream processing requirements
Operating RPM and rotor pole count	Establishes the fundamental electrical frequency range
Magnetic flux density and core-loss targets	Helps identify suitable NGOES magnetic properties
Lamination thickness and insulation coating	Relates to the customer's cutting and stacking process
Required coil width and dimensional tolerances	Affects material utilization and production consistency

1792634830051-HWHG Non-Grain-Oriented Electrical Steel Coils.webp

Illustration of HWHG non-grain-oriented electrical steel coils for downstream BLDC motor stator core manufacturing

Both outrunner and inrunner motors require electrical steel with magnetic properties suited to their stator designs and operating conditions. As an electrical steel manufacturer, HWHG supplies NGOES coils for downstream motor-core production, with available grades and specifications that can be discussed according to customer requirements.

Conclusion

For drone motor manufacturers, outrunner and inrunner configurations present different requirements for propulsion design, stator geometry, and thermal management. These differences should also be considered when selecting electrical steel for the stator core.

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