Outer rotor vs. inner rotor: A detailed explanation of brushless motor structure

 

Brushless DC motors (BLDC) have been widely used in multiple industries due to their high efficiency, low maintenance and long life, from industrial automation and smart manufacturing to consumer electronics, electric vehicles and drone systems.

 

However, in such a variety of application scenarios, a single motor structure cannot meet all performance requirements. In order to better adapt to different usage conditions, brushless motors have developed two typical structural forms: inner rotor and outer rotor.

 

There are essential differences between these two structures in terms of construction, power performance and application direction. Understanding their working principles and pros and cons will help engineers make more appropriate decisions in project design or product selection.

 

In the design of brushless DC motor (BLDC), the "rotor" refers to the rotating part containing permanent magnets, and the "stator" refers to the fixed part containing winding coils. According to the positional relationship between these two parts, brushless motors can be divided into inner rotor structure and outer rotor structure.

 

In order to better understand these two designs, we first introduce their working principles and mechanical structures respectively.

 

2.1 Inner Rotor BLDC Motor

In an internal rotor motor, the rotor is located in the center of the motor, surrounded by stator windings. When current passes through the stator coils, it creates a rotating magnetic field that drives the rotor inside to rotate.

 

Features:

· Small rotor mass, low inertia and fast response speed

· More suitable for locations requiring high speed and precise control

· Good heat dissipation performance, because the stator is close to the motor housing, which can conduct heat more easily

2.2 Outer Rotor BLDC Motor

An external rotor motor has the opposite structure: the stator is fixed inside, and the rotor containing the permanent magnets surrounds the stator like a rotating casing.

 

Features:

· Large rotor housing mass and high moment of inertia

· Suitable for low speed, high torque applications

· Since the stator is located inside, the heat dissipation performance is slightly weak, and structural optimization is needed to assist heat dissipation

 

When choosing a brushless motor, it is important to understand the performance of different structures. This section will compare the actual performance differences between outer rotor and inner rotor brushless motors from three key dimensions: stability, torque performance, and heat dissipation.

 

To make it easier to understand, we will also provide usage preference suggestions after each item.

1. Stability (smooth running)

Inner rotor brushless motor

· Small rotor mass and low rotational inertia

· Very friendly for fast start and stop and precise control

· Commonly used in control systems that require high response, such as industrial robots, servo drives, etc.

 

Outer rotor brushless motor

· Large rotor mass and high rotational inertia

· Smoother operation and less vibration

· Suitable for continuous low-speed operation, such as fans, aircraft, cooling systems, etc.

 

2 Torque Output

Outer rotor brushless motor

· Because the permanent magnet is located at a larger rotation radius, the torque generated is greater

· Particularly suitable for applications requiring high starting torque and heavy load at low speed, such as electric bicycles, power tools, etc.

 

Inner rotor brushless motor

· Small torque, but suitable for high-speed operation

· More suitable for light-load, high-speed transmission systems

3 Thermal Management

Inner rotor brushless motor

- · - The stator winding is close to the motor housing, so heat can be easily conducted and released

- · - More suitable for long-term high-power operation

 

Outer rotor brushless motor

- · - The stator is inside, so heat cannot be dissipated directly

- · - The heat dissipation design needs additional optimization, such as adding a fan or using heat-conducting structural materials

 

Performance comparison summary table:

Performance Dimension	Inner rotor brushless motor	Outer rotor brushless motor
Operation stability	⭐⭐⭐	⭐⭐⭐⭐
Response speed	⭐⭐⭐⭐	⭐⭐
Starting torque	⭐⭐	⭐⭐⭐⭐
Heat dissipation efficiency	⭐⭐⭐⭐	⭐⭐

 

 

After understanding the structure and performance characteristics of brushless motors, the core of selection is: different working scenarios require different design orientations.

The following is a comparison of the applications of external rotor and internal rotor motors in common industries, as well as the recommended structural types.

Common applications and structural selection suggestions:

Application Scenario	Recommended structure	Reason
Drones, model aircraft	Outer rotor	High starting torque, stable operation at low speed, suitable for propeller thrust control
Industrial Robots	Inner rotor	Fast response, small inertia, suitable for precise control and fast start and stop
Electric scooters, electric bicycles	Outer rotor	When large torque and compact structure are required, hub-type external rotor motors are often used.
CNC machine tools, servo systems	Inner rotor	Higher stability and control accuracy, supporting closed-loop control systems
Fan, cooling system	Outer rotor	Low speed and stable operation, small vibration, suitable for continuous operation
Medical equipment (such as syringe pumps, infusion pumps)	Inner rotor	High control accuracy and low operating noise
Power tools (screwdrivers, lawn mowers)	Outer rotor	Large starting torque, suitable for occasions with frequent load fluctuations

 

Scenario analysis points:

If the equipment requires fast response, precise positioning, and long-term stable operation, such as robots and servo systems → it is recommended to choose an inner rotor motor.

· If the equipment requires strong starting capability, stable low-speed output, and compact structure design, such as electric vehicles, fans, and drones → it is recommended to use an external rotor motor.

· Some mixed requirements (such as medical equipment) can be evaluated comprehensively based on control strategy and structural layout.

 

Whether you need a high-response inner rotor motor or a high-torque outer rotor motor , VSD can provide you with a reliable solution. As a professional brushless DC motor manufacturer , we not only provide standardized products, but also support deep customization to meet various complex application requirements.

 

Here are the important reasons why we serve our customers:

1. Complete product line: full coverage of inner and outer rotors

Provide brushless motors of different sizes, voltages and power levels

Supports compact design of outer rotor structure and also supports high-speed inner rotor products

Optional Hall sensors, encoders, gearboxes and other components for easy system integration

 

2. Deep customization capabilities to shorten project development cycles

Support customers to provide parameters or application background, customize motor size, coil parameters, interface structure

Provide 3D drawings and simulation data to speed up R&D

From sample development to batch delivery, full-process technical docking and quality management

 

3. Quality and certification assurance

The factory has passed ISO 9001 and IATF 16949 quality management system certification

Motor products comply with international standards such as CE and RoHS

Provide factory inspection reports and consistency assurance to ensure that each motor is traceable and verifiable

 

If you are looking for a brushless motor supplier with reliable performance, customization support and international certification, VSD is a trustworthy partner. Welcome to contact us for samples, technical information or development support.

 

Finally: Choose the right structure to improve performance

The structure of a brushless motor is not just a part of the engineering design, it directly affects the performance of the entire device. Choosing an outer rotor or an inner rotor is not a simple parameter comparison, but a systematic decision based on the "application goal".

· If your project requires high speed, quick response, and high control accuracy - an internal rotor motor may be a more appropriate choice.

· If you are more concerned about low-speed stability, high torque output and compact structure, the outer rotor motor has more advantages.

 

After understanding the basic principles, performance differences and typical applications of these two motor structures, we hope that you can make your selection more confidently and build a more reliable and efficient system.