Introduction
Behind every nimble turn and precise grasp of a humanoid robot lies a group of silently working "muscles"—the frameless torque motor. These motors shed the bulky housing of traditional motors, retaining only the stator and rotor as their core components. Like bare "prime movers," they are embedded directly into a robot’s joint structure, taking on the critical tasks of driving key joints such as the shoulder, hip, and knee with extreme compactness and ultra-high torque density.
However, frameless torque motors are not a one-size-fits-all solution. Depending on the relative position of the rotor and stator, they can be divided into two major schools: outer rotor and inner rotor designs. The two differ structurally, each has its own performance strengths, and they demonstrate a clear division of labor in application. The rotary joints of Tesla's Optimus and the proprioceptive actuators of the MIT Cheetah quadruped robot both make deliberate choices between these two configurations.
01 Basic Understanding: What is a Frameless Torque Motor?
To understand the difference between outer and inner rotors, we first need a fundamental understanding of the frameless torque motor itself.
A traditional motor is a complete, packaged unit: it comes with a housing, end caps, bearings, and a shaft—a self-contained power module that can spin once connected to power. The frameless torque motor completely overturns this concept: it consists of only two independent components, the stator and the rotor, with no housing, no bearings, and no output shaft.
This minimalist design transforms the frameless torque motor from a standalone device into a "power cell" that can be directly integrated into a mechanical structure. Engineers can fix the stator into a robot’s joint housing and mount the rotor directly onto the load shaft, enabling a "zero-transmission-chain" transfer of power from the motor to the joint.
The core advantages of this design are substantial: it dramatically increases space utilization (volume reduction of over 30%), eliminates transmission backlash, achieves transmission efficiency of over 95%, and allows for a high degree of customization based on the specific dimensions and torque requirements of the joint.
Given that both are combinations of a stator and rotor, what exactly distinguishes an outer rotor from an inner rotor?
02 Structure Decoded: When the Rotor Differs "Inside" and "Outside"
The fundamental difference between outer and inner rotor motors can be summed up in one phrase: the spatial relationship between the rotor and stator is completely inverted.
The inner rotor configuration represents the more "traditional" design approach. In an inner rotor frameless motor, the rotor (containing the permanent magnets) sits at the motor’s center, while the stator windings surround and wrap around the outside of the rotor. The rotor is connected to the load via an output shaft, giving the overall structure a slender, elongated form. This configuration follows the lineage of common industrial motors, for which engineers possess deep design experience.
The outer rotor configuration is an "inside-out" design. In an outer rotor frameless motor, the stator windings are fixed to a central base, while the rotor, resembling a hollow cup-shaped shell, envelops the entire stator from the outside. The rotor shell itself is the rotating part, connecting directly to the equipment load, resulting in a flatter overall structure.
Simply put: take an inner rotor motor and turn it "inside out"—move the originally outer stator to the inside, and flip the originally inner rotor to the outside, and you get an outer rotor motor. This structural inversion leads to a comprehensive divergence in everything from performance to application.
The structural "inversion" directly determines the starkly different performance characteristics of outer and inner rotor motors. Here is a detailed comparison across six core dimensions:
1. Torque Output: The Outer Rotor’s "Herculean Strength"
Torque capability is the most prominent performance label of the outer rotor motor. Given the same volume and current, an outer rotor frameless motor delivers 30%-50% higher torque output than an inner rotor one. The reason is simple: Torque = Force × Lever Arm. The outer rotor has a larger rotation radius and a longer lever arm, naturally generating greater torque for the same electromagnetic force. This advantage is particularly pronounced in low-speed, heavy-load scenarios.
2. Speed and Dynamic Response: The Inner Rotor’s "Quick Step"
The rotor of an inner rotor motor is located at the center, resulting in low rotational inertia. This makes it more agile during start, stop, and acceleration, enabling a faster dynamic response. Additionally, inner rotor motors typically have fewer pole pairs and higher speeds, making them suitable for applications requiring high-speed operation and frequent starts and stops. Due to the rotor’s larger mass and higher inertia, an outer rotor motor has a relatively slower dynamic response but operates more smoothly with less speed fluctuation.
3. Heat Dissipation: The Outer Rotor’s "Built-in Radiator"
The rotor shell of an outer rotor motor is in direct contact with the air, offering a large heat dissipation area. Heat can be quickly released to the external environment, making it suitable for long-duration, high-power operation. In an inner rotor motor, the stator windings are enclosed by the outer rotor, trapping heat inside and making it difficult to dissipate. This requires reliance on the motor base or additional thermally conductive structures for thermal management. This difference becomes critical under continuous high-load conditions.
4. Control Precision: Each Has Its Strengths
Regarding positioning accuracy, the two present an interesting complementarity. The inner rotor motor, with its fast dynamic response, is better suited for applications demanding high positioning response speed. The outer rotor motor, with its smooth operation and low torque ripple, is more suitable for scenarios requiring stringent positioning accuracy and motion smoothness.
5. Structural Complexity and Installation Difficulty
The outer rotor’s shell must simultaneously perform multiple functions: magnetic flux conduction, heat dissipation, and bearing the permanent magnets. This places higher demands on materials and manufacturing processes, leading to relatively higher costs. Installation also requires precise control of air-gap uniformity and coaxiality between the stator and rotor, making it more challenging than an inner rotor motor. Inner rotor motors have a relatively simpler structure and lower cost, and are currently the mainstream choice in the humanoid robot field.
6. Space Utilization and Integration Method
The inner rotor motor has a compact, elongated structure, suitable for embedding into narrow joint spaces. The outer rotor motor has a flat, pancake-like structure, making it easy to connect directly to load rollers or flanges, offering unique advantages in applications like hub drives and winding equipment.
For an intuitive comparison, the summary table below is clear at a glance:
Comparison Dimension | Outer Rotor Frameless Torque Motor | Inner Rotor Frameless Torque Motor |
Torque Output | High (30%-50% higher for same volume) | Relatively lower |
Speed | Lower | Higher |
Dynamic Response | Slower (high inertia) | Fast (low inertia) |
Heat Dissipation | Good (direct shell cooling) | Dependent on base cooling |
Operational Smoothness | High (low speed ripple) | Lower |
Positioning Accuracy | High precision (low torque ripple) | Fast response |
Structural Complexity | Higher | Lower |
Cost | Relatively higher | Relatively lower |
04 Application Mapping: Role Division in Robot Joints
If performance differences are the "hard power," then the division of application scenarios vividly projects these differences into practice. In robotics, inner and outer rotor motors each play their distinct roles.
Inner Rotor: The "Main Force" for Agile Motion
In humanoid robots, inner rotor frameless torque motors, with their low inertia and fast response, are the preferred choice for joints requiring frequent starts, stops, and rapid posture adjustments, such as the waist and shoulders. They currently account for over 70% of frameless torque motor selections in humanoid robots.
The rotary joints of Tesla Optimus extensively use inner rotor frameless torque motors, paired with harmonic reducers and torque sensors, to provide a power output that combines explosive force and precision for large joints like the shoulders and hips. In the realm of quadruped robots, the original MIT Cheetah also chose an inner rotor configuration for its proprioceptive actuator design.
Outer Rotor: The "Powerhouse" for Load-Bearing and Impact Resistance
The high torque and superior smoothness of outer rotor motors make them irreplaceable in heavy-load joints. Domestic companies have achieved industrial breakthroughs with outer rotor frameless motors, achieving a maximum output torque of 285 Nm (for comparison, mainstream inner rotor models peak at 50-150 Nm). These motors can pass impact resistance tests at 5 times the rated torque, calmly handling high-intensity actions like jumping and load-bearing.
In the industrial robot sector, outer rotor motors are widely used in waist and wrist joints that demand high torque and precision. Among quadruped robots, the MIT Cheetah Mini adopted an outer rotor configuration, fully utilizing its flat structure and high torque advantages to achieve a compact joint design.
Cross-Over Applications: From Robotics to a Broader World
The application landscape of these two motor types extends far beyond robot joints. The outer rotor motor, with its flat structure and high torque characteristics, excels in hub drives (e-bikes, e-scooters), medical imaging equipment (CT scanner rotating components), and precision gimbals. The inner rotor motor, leveraging its high-speed response advantage, is widely used in high-speed spindles (CNC machines, engraving machines), drone propulsion systems, and various small servo systems. In collaborative robots and exoskeletons, both have their own strengths—exoskeleton scenarios tend to use outer rotor motors with integrated planetary gearboxes, while collaborative robots mostly adopt frameless torque motors integrated with harmonic reducers.
05 Technology Trends and Market Outlook
Frameless torque motors are in a golden era of rapid development. According to QYResearch, global sales of frameless torque motors reached RMB 5.461 billion (approximately USD 803 million) in 2025, and are projected to grow to RMB 9.63 billion (approximately USD 1.416 billion) by 2032, with a compound annual growth rate of about 8.4%.
The core driver of this growth is the explosion of the humanoid robot industry. One study predicts that by 2030, the global market space for humanoid robot motors could reach RMB 91.76 billion, with the frameless torque motor segment alone for humanoid robots reaching USD 2.397 billion.
In terms of technological evolution, outer and inner rotors are on separate development paths: inner rotor motors continue optimizing for higher power density and lower cogging torque, consolidating their mainstream position in humanoid robot joints. Outer rotor motors are breaking through towards higher torque output and better thermal design. Meanwhile, their costs are gradually decreasing as manufacturing processes mature, promising to replace traditional solutions in more heavy-duty joints and industrial scenarios.
06 Summary and Selection Recommendations
There is no absolute superiority between outer and inner rotor frameless torque motors. The key is "tailoring the motor to the joint." The following selection principles can serve as a reference:
Consider the Load: For heavy-load, low-speed, high-torque joints (like the hip and knee), prioritize an outer rotor motor. For light-load, high-speed, frequent start/stop joints (like the shoulder and wrist), an inner rotor motor is more suitable.
Consider the Space: For slender joints with ample axial space but tight radial space, an inner rotor motor fits well. For scenarios with relatively loose radial space requiring a flat design, the outer rotor motor has a clear advantage.
Consider Cooling Conditions: For long-duration, heavy-load operation where cooling relies on natural convection, an outer rotor motor is more reliable.
Consider Cost and Installation: On a limited budget or when rapid integration is needed, the inner rotor motor is the more pragmatic choice. For applications with extreme demands for torque smoothness and impact resistance, the outer rotor motor is worth the investment.
Consider Precision Requirements: Choose an inner rotor motor for fast positioning response; choose an outer rotor motor for motion smoothness and positioning accuracy.
As humanoid robots move from the lab to mass production, the technological iteration and industrialization of frameless torque motors are accelerating. Understanding the core differences between outer and inner rotors will help engineers find the optimal solution in complex selection decisions—just like choosing the right "muscle" for joints in different positions; each has its most suitable way of exerting force.
