inner rotor vs. outer rotor

Differences Between Inner Rotor and Outer Rotor Permanent Magnet Motors

Permanent magnet synchronous motors (PMSMs) are categorized into inner rotor and outer rotor structures based on the position of the rotor within the motor. The fundamental disparity lies in whether the rotating component is located inside or outside the motor. This article delves into the variances between inner rotor and outer rotor permanent magnet motors, shedding light on their distinctions and practical implications.

What Is Permanent Magnet Synchronous Motor?

The permanent magnet synchronous motor uses permanent magnets to provide excitation, which makes the motor structure simpler, reduces processing and assembly costs, and eliminates the problem-prone collector rings and brushes, improving the reliability of the motor operation; and because no excitation is required winding current, without excitation losses, increasing the efficiency and power density of the motor.

motor components

Differences Between External Rotor Motor and Internal Rotor Motor

Rotational Position:

Inner Rotor: In this configuration, the driving part of the motor resides internally, while the casing remains static. The rotation occurs inside the motor.

Outer Rotor: The outer rotor motor rotates externally. In an external rotor motor, the rotor rotates together with the motor casing, the main shaft of the motor is fixed, the casing serves as the rotor, and the interior and the main shaft serve as the stator. The distinction between outer and inner rotors can also be discerned based on the embedding technique and position of the permanent magnet.

Volume of Permanent Magnet:

Outer Rotor: Outer rotor PMSMs feature a smaller volume of permanent magnet, resulting in reduced axial dimensions and enhanced system stability.

Inner Rotor: Conversely, inner rotor PMSMs have a larger volume of permanent magnet, corresponding to larger axial dimensions and higher center of gravity, leading to comparatively poorer operational stability.

Rotational Speed:

In the context of identical brushless DC motors, inner rotor motors exhibit significantly higher rotational speeds compared to their outer rotor counterparts.


Internal Rotor: These motors find application in various power machinery such as motors, generators, gas turbines, and compressors. The permanent magnet rotor application area includes medical infusion pumps, respirators, ventilators, surgical hand tools, and industrial power tools.

External Rotor: On the other hand, external rotor motors are typically installed in applications such as impellers, where they facilitate heat dissipation. It is suitable for low speed and high torque, direct drive, and constant speed motors such as  drones, fans, pumps, surgical robots, electrical grippers, E-bikes, and premium tattoo machines.

outer rotor vs. inner rotor structure


In conclusion, the disparity between the inner rotor and outer rotor permanent magnet motors encompasses rotational position, volume of permanent magnet, rotational speed, and application areas. While inner rotor motors offer higher rotational speeds and find utility in diverse power machinery, outer rotor motors, with their compact dimensions and effective cooling capabilities, are favored in applications necessitating efficient heat dissipation. Understanding these discrepancies aids in the selection of the most suitable motor configuration for specific industrial or commercial requirements.


What are the advantages of Inner Rotor Permanent Magnet Motors?

Inner rotor permanent magnet motors typically offer higher power density and torque output compared to outer rotor motors. They are often preferred in applications requiring high-performance characteristics, such as electric vehicles, industrial automation, and renewable energy systems.

What are the advantages of Outer Rotor Permanent Magnet Motors?

Outer rotor permanent magnet motors are known for their effective cooling capabilities due to the outward placement of the rotor assembly. This configuration allows for improved heat dissipation, making them suitable for applications where thermal management is critical, such as HVAC systems, refrigeration units, and automotive cooling systems.

How do I choose between Inner Rotor and Outer Rotor Permanent Magnet Motors for my application?

The choice between inner rotor and outer rotor permanent magnet motors depends on various factors such as power requirements, space constraints, cooling needs, and operational environment. Inner rotor motors are preferred for high-performance applications, while outer rotor motors are suitable for applications prioritizing efficient cooling and ventilation. Consulting with us can help determine the best option for your specific requirements.

What materials are used in Internal Rotor and External Rotor Permanent Magnet Motors?

Both inner rotor and outer rotor permanent magnet motors utilize similar materials for their core components. These typically include high-quality permanent magnets such as neodymium (NdFeB) or samarium-cobalt (SmCo), which provide the magnetic field required for motor operation. Additionally, the stator and rotor cores are commonly made from laminated electrical steel to minimize eddy current losses and enhance efficiency.

Why Is The High Cost Of Permanent Magnet Synchronous Motors?

The high cost of Permanent Magnet Synchronous Motors (PMSMs) primarily stems from the expense of rare earth materials used in their magnets, intricate manufacturing processes, sophisticated control systems, and the demand for precision engineering. These factors collectively contribute to their elevated price compared to conventional motors.

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