SHENZHEN TENKAI GROUP LIMITED 深圳市腾凯特种机电有限公司
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The inner rotor motor and the outer rotor motor are two common structures of permanent magnet synchronous motors. The core difference between them lies in the relative positions of the rotating part (the rotor) and the stationary part (the stator).Next, I will elaborate on the differences among them from multiple perspectives in detail.
Comparison Table
| Item | inner rotor motor | external rotor |
| basic structure | The rotor is inside, and the stator surrounds the rotor on the outside. | The rotor is on the outside and the stator is inside. The rotor housing encloses the stator. |
| rotational inertia | Small (with a small rotor radius and concentrated mass) | Large (rotor radius is large, mass is distributed on the outer edge) |
| Speed | High | Low |
| torque | relatively low | relatively high |
| Heat dissipation conditions | Good (The stator is located outside and directly contacts the casing, facilitating heat dissipation) | Poor (The stator is located inside, making it difficult for heat to dissipate. Special design is required.) |
| structural strength | The rotor shaft directly outputs power, with a simple and sturdy structure. | The rotor housing requires high strength and has a relatively complex structure. |
| application scenarios | Electric vehicles, industrial servos, model aircraft (high-speed), household appliances, etc. | Drones, fans, disc motors, direct drive applications, etc. |
1. Basic Structure
Inner rotor motor:
This is the most common and intuitive form of motor we have.
The rotor (the permanent magnet part) is located at the center of the motor, supported by bearings and fixed to the motor shaft.
The stator (a core with copper wires wound around it) is fixed to the motor housing and surrounds the rotor.
During operation, the rotor of the center rotates and transmits power through the shaft.
Outer rotor motor:
This structure can be understood as a "flipping inside out".
The stator is fixed to the central fixed shaft of the motor (and does not rotate).
The rotor (which is usually in a shell shape and has permanent magnets on its inner wall) surrounds the stator and is supported on a fixed shaft by bearings.
During operation, it is the entire outer rotor housing that rotates, directly driving the external load (such as fan blades, or the propellers of an unmanned aircraft).
| inner rotor motor | external rotor |
 |  |
2. Performance Feature Comparison
Rotational Inertia and Dynamic Response
Inner rotor: The rotor mass is concentrated at the rotation center, resulting in a low moment of inertia. This means it can start, stop, accelerate, and decelerate very quickly, with excellent dynamic response. It is highly suitable for applications requiring rapid and precise position control, such as industrial robots and servo systems.
Outer rotor: The rotor's mass is distributed on the outer edge, resulting in a large moment of inertia. Starting and stopping require greater force, and the response is slower. However, once it starts rotating, the operation is more stable, with greater inertia, which helps to smooth out speed fluctuations.
Speed and torque
Inner rotor: The rotor has a smaller diameter, allowing it to reach a higher rotational speed under the same linear velocity. However, due to the short torque arm (radius), the output torque is relatively smaller under the same magnetic force.
Outer rotor: The rotor has a large diameter, equivalent to a large torque disc motor. Its long torque arm enables it to generate greater torque at a relatively lower rotational speed. This is very suitable for direct drive applications, eliminating the need for a reduction mechanism.
Heat dissipation performance
Inner rotor: It has obvious advantages in heat dissipation. The stator windings that generate heat are directly in contact with the external casing. The heat can be easily dissipated to the environment through the casing and the heat sink. Therefore, it can withstand a higher power density.
Outer rotor: Heat dissipation is the main challenge. The stator that generates heat is enclosed inside, making it difficult for the heat to escape and potentially causing the motor to overheat. Special designs are usually required, such as designing cooling channels within the inner stator.
Structure and Installation
Inner rotor: Classic structure, sturdy output shaft, easy installation.
Outer rotor: The external rotor housing itself serves as the installation base for the load (for example, the fan impeller is directly mounted on the rotor housing). The structure is compact and can achieve direct drive. However, the mechanical strength and dynamic balance of the housing are very demanding.
Summary of application scenarios
Internal rotor motors are suitable for:
Electric vehicle drive motor: Requires high rotational speed and is used in conjunction with a reducer.
Industrial robots and CNC machines: Require high dynamic response and precise control.
High-speed model aircraft: Requires extremely high no-load rotational speed.
Household appliances (such as washing machines, vacuum cleaners): Traditional and mature applications.
The external rotor motor is suitable for:
Unmanned aerial vehicle (multi-rotor): Its low rotational speed and high torque characteristics make it highly suitable for directly driving large-sized propellers. It is highly efficient, has a simple structure, and requires no maintenance.
Cooling fan/blower: The blades are directly mounted on the rotor housing, resulting in a very compact, efficient and quiet structure.
Direct-drive washing machine: Without the traditional belts and gearboxes, it directly uses an external rotor with low speed and high torque to drive the inner drum. It has low noise and low vibration.
Disc motors and hub motors: Integrating the motor directly into the wheel is a typical application of the external rotor structure.
