1. What is an outrunner brushless Motor?
The outrunner bldc motor is a specially designed permanent magnet synchronous motor. Compare with the dc brushless motor, the rotor of an outrunner brushless motor consists of a housing which is connected with permanent magnets. This is completely opposite to the stator and rotor structure of the inner rotor brushless motor.
2. What is the Differences Between outrunner brushless motor and inner brushless motor.
Due to different mechanical structures, outer rotor brushless motors and inner rotor brushless motors are suited for different application fields. Choosing the right brushless motor depends on various factors, including performance requirements and application scenarios.
Doncen Motor provides both high-quality brushless inner rotor motors and brushless outer rotor motors. To assist customers in making the best choice, our team has outlined the following core differences to help select the most suitable brushless motor.
Outrunner BLDC Motor vs Inner BLDC Motor
Although both are DC brushless motors, the technology of inner rotor motors is more advanced and mature. Outer rotor motors are still in the stage of rapid development.
2.1. Comparison of Mechanical Structures
The difference between inner rotor brushless motor and outer rotor brushless motor is essentially the mechanical design.
Outer Rotor Brushless Motor
- The outer casing rotates, while the internal stator remains fixed.
- The magnets are attached to the outer casing, and the coils are positioned inside the motor.
- Due to the larger rotor diameter, it offers higher inertia, which contributes to smoother operation at low speeds.
Inner Rotor Brushless Motor
- The central shaft rotates, while the outer stator remains fixed.
- The magnets are attached to the rotating shaft, while the coils surround the rotor.
- With a smaller rotor diameter, it provides lower inertia, allowing for faster acceleration and higher speed operation.
2.2: Physical Size
Due to the different structures, we can see that the outrunner BLDC motor and the inner BLDC motor have different physical sizes.
| Comparison | Outrunner BLDC Motor | Inner BLDC Motor |
|---|---|---|
| Radial Size (Diameter) | Larger diameter as the rotor wraps around the stator. | More compact design as the stator surrounds the rotor. |
| Axial Size (Length) | Shorter axial length due to circular magnet layout. | Longer axial length to accommodate axial magnetic circuits. |
| Weight Distribution & Inertia | Mass is concentrated in the outer rotor, leading to higher inertia | Mass is centered around the shaft, resulting in lower inertia |
2.3. Rpm and Torque
Speed (RPM)
- Outer rotor motors have lower RPM due to their larger size and higher inertia.
- Inner rotor motors can reach over 50,000 RPM, as they have a smaller, lighter rotor with lower inertia.
Torque Output
- Outer rotor motors deliver higher torque at low speeds.
- Inner rotor motors produce less torque but operate at higher speeds.
Acceleration & Response
- Outer rotor motors have higher inertia, meaning slower acceleration and braking.
- Inner rotor motors respond faster.
2.4: Heat dissipation
Outer Rotor Motors – Better Cooling
- The rotor is exposed to air, allowing better natural convection.
- The rotating outer shell creates airflow, helping to dissipate heat more effectively.
Inner Rotor Motors – Limited Cooling
- The stator is enclosed inside the motor housing, trapping more heat.
- Requires heat sinks, fans, or liquid cooling for efficient heat dissipation.
3. How to choose outrunner BLDC Motor or inner BLDC Motor ？
| Feature | Outer Rotor Brushless Motor | Inner Rotor Brushless Motor |
|---|---|---|
| Length | Shorter – Compact axial design, Flat | Longer – Needs more axial space |
| Width (Diameter) | Larger – The rotor wraps around the stator | Smaller – The stator surrounds the rotor |
| Torque | Higher at low speed – Larger rotor diameter provides better leverage. | Lower at the same size – Smaller rotor means less mechanical advantage. |
| Speed (RPM) | Lower – Low KV | Higher – high Speed design |
| Cooling (Heat Dissipation) | Better – Open structure and rotating shell improve airflow. | Worse – Heat is trapped inside |
| Weight | Medium | Medium |
4. MX33 Outrunner BLDC Motor vs BL42 Inner BLDC Motor
In order to verify our theory, here we provide a comparison of two BLDC outrunner motor and BLDC inner motor of the same diameter, the MX33 series and the BL42 series. Such data comparison will be more intuitive.
4.1 Size difference
With the same diameter, the length of the MX33 series can be 32mm, while the length of the BL4259 is 59mm.
4.2 Rpm and Torque Difference between BLDC outrunner Motor vs BLDC Inner Motor
For this comparison, we chose the same voltage of 24V for testing. In order to ensure a certain degree of comparability, we chose two motors with similar no-load speeds of around 24V5000 rpm for comparison.
| Parameter | MX33KV200 | BL4259 |
|---|---|---|
| Voltage (V) | 24V | 24V |
| No-Load Speed (rpm) | 4800rpm | 5000rpm |
| Rated Speed (rpm) | 3400rpm | 4031rpm |
| Rated Torque (Nm) | 0.18N.m | 0.035N.m |
| Stall Torque (Nm) | 0.40N.m | 0.14N.m |
4.3. Selection guide for BLDC outrunner motors and BLDC inner motors
| Parameter | Outrunner BLDC | Inner BLDC | Remarks |
|---|---|---|---|
| Length | Flat Design | Longer | |
| Width/Diameter | Large | Medium | |
| Power | Large | Medium | |
| Torque | ++++ | ++ | |
| Speed (RPM) | Low Speed | High Speed | |
| Price | High | Medium | |
| Lifetime | +++++ | +++++ | |
| Heat Dissipation | ++++ | ++ |
Attention: The above comparison is based only on DONCEN MOTOR’s product series. Different product designs have different standards. Please consult our engineering team for specific motor parameter questions.
