What Is the Difference Between Inrunner and Outrunner Frameless Motors? Structure, Performance, and Applications

 In the design of drive systems for equipment such as robots, collaborative robotic arms, humanoid robots, and exoskeletons, frameless motors are attracting increasing attention from engineers because of their compact structure and flexible integration.

When selecting a frameless motor, in addition to motor size, torque, and speed, engineers also need to consider an important question:What is the difference between an inrunner frameless motor and an outrunner frameless motor? Which is more suitable for a given robotics project?

The key difference between the two lies in the relative positions of the rotor and stator. In an inrunner structure, the rotor is inside the stator, while in an outrunner structure, the rotor is outside the stator. This structural difference further affects the motor's torque output, rotational inertia, spatial layout, and system integration.

What Is a Frameless Motor?

A frameless motor is a motor typically supplied primarily as a stator assembly and a rotor assembly. Unlike conventional motors with a complete housing, output shaft, and independent support structure, a frameless motor can be integrated directly into the equipment's own mechanical structure.

This design allows engineers to design the motor mount, rotor couplings, bearing supports, and heat-dissipation structure according to the equipment's spatial layout, load requirements, and transmission structure.

The main advantages of frameless motors include:

• Flexible mechanical integration: The motor can be built directly into a robot joint or other equipment structure, reducing redundant mechanical packaging.

• Compact system design: Integrating the motor with the mechanical structure can help reduce the overall size of the drive assembly.

• Customized transmission structures: Gearboxes, encoders, bearings, and other mechanical components can be combined according to application requirements.

• Suitable for precision motion control: When paired with appropriate drives, sensors, and control algorithms, frameless motors can be used in systems requiring smooth and precise motion.

It is important to note that a frameless motor does not eliminate the need for external mechanical support. In practical applications, engineers still need to design reliable rotor retention, stator mounting, bearing support, insulation, and heat-dissipation solutions.

Inrunner vs. Outrunner Frameless Motors: What Are the Structural Differences?

Inrunner and outrunner frameless motors use similar electromagnetic operating principles, but arrange the rotor and stator differently.

1. Inrunner Frameless Motor: Rotor Located Inside the Stator

In an inrunner frameless motor, the rotor is located inside the stator. The stator windings are fixed to an external support structure, while the rotor rotates inside the stator.

This structure typically has a smaller rotor radius. Under otherwise similar conditions, a smaller rotor radius helps reduce rotational inertia, making it suitable for applications with demanding requirements for dynamic response, acceleration and deceleration, and motion control.

Inrunner motors also allow for a variety of external mounting structures and are suitable for drive systems requiring precision control, compact layouts, or relatively high speeds.

2. Outrunner Frameless Motor: Rotor Located Outside the Stator

In an outrunner frameless motor, the rotor is outside the stator and typically rotates around it in a ring-shaped or cup-shaped structure.

Because the electromagnetic interaction area can be positioned at a larger effective radius, an outrunner structure typically provides a longer lever arm. With similar motor volume and design conditions, this layout can help achieve higher output torque, especially in applications requiring high torque at low speeds.

An outrunner structure can also be used in surrounding-style joint designs, allowing compact integration of the motor with the joint housing and output structure. However, the size and inertia of the rotating outer components, as well as protective measures, must all be carefully considered during mechanical design.

3. Key Differences Between the Two Structures

These are common structural tendencies, and not every model follows exactly the same performance patterns. Actual performance still depends on electromagnetic design, windings, dimensions, materials, operating current, and heat-dissipation conditions.

Performance Comparison of Inrunner and Outrunner Frameless Motors

When selecting a motor, it is not possible to determine which type performs better solely from whether the rotor is inside or outside. For robotics applications, it is more important to analyze how structural differences affect actual operating requirements.

1. Torque Output: Outrunner Structures Typically Have a Radius Advantage

Motor torque is related to the applied force and its lever arm. For a motor, increasing the effective electromagnetic radius generally helps increase torque output when other conditions are similar.

The rotor of an outrunner frameless motor is outside the stator and can typically use a larger effective radius. It therefore has a structural advantage when achieving relatively high torque output in a compact space.

For systems that support loads, drive robot joints, or rotate at low speeds, an outrunner structure is worth evaluating closely.

However, this does not mean that outrunner motors always provide higher torque. Actual selection should compare continuous torque, peak torque, torque constant, allowable current, and thermal limits rather than relying only on structural type.

2. Dynamic Response: Consider Rotor Inertia and Load Inertia

In robot motion control, motors often need to accelerate, decelerate, and change direction. Rotational inertia is therefore an important design parameter.

The rotating components of an inrunner frameless motor are typically concentrated within a smaller radius, helping reduce rotor inertia. This is particularly important for applications requiring fast response, frequent starts and stops, and fine motion control.

In an outrunner structure, rotating mass is distributed over a larger radius, which may result in higher rotor inertia. For joints requiring high dynamic response, engineers should verify that this inertia meets system requirements.

Actual response performance, however, is also closely related to load inertia, the gearbox, controller bandwidth, current-loop performance, and control algorithms. The response speed of an entire drive system cannot be determined from the inrunner or outrunner structure alone.

3. Speed and Torque: Evaluate Them Together Using the Motor Curves

At the same voltage and under otherwise similar conditions, a motor's speed capability depends on factors such as its back-EMF constant, winding design, and the controller's power supply capability.

Inrunner motors are often used in systems requiring relatively high speeds or fast dynamic response, while outrunner motors are often used where a larger effective radius and higher torque output are desired. These are common design tendencies, not absolute rules.

When selecting a motor, review the following together:

• No-load speed and rated operating speed

• Continuous torque and peak torque

• Torque-speed curve

• Back-EMF constant and torque constant

• Drive voltage, current, and speed-control range

If an application requires high speed and high torque simultaneously, focus on the motor's actual capability at the target operating point rather than comparing maximum speed and peak torque separately.

4. Heat Dissipation and Thermal Management: Dependent on the Overall Mechanical Design

Frameless motors typically rely on their mounting structure to conduct heat away from the stator. The contact area between the stator and housing, material thermal conductivity, mounting tolerances, ambient temperature, and cooling method all affect the motor's continuous output capability.

The inrunner or outrunner structure alone does not determine which design dissipates heat better. In practical design, focus on how the stator is secured, the heat-conduction path, and temperature rise during continuous operation.

In space-constrained applications such as robot joints, even if a motor can deliver high peak torque for a short time, it is necessary to verify whether it can operate continuously over the actual duty cycle.

Which Applications Suit Inrunner and Outrunner Frameless Motors?

1. Humanoid Robots

The hip, knee, shoulder, and elbow joints of humanoid robots have different requirements for drive-system torque, weight, space, and dynamic response.

For lightweight joints requiring fast response, an inrunner design can be considered; for joints requiring higher torque output and suited to a surrounding mechanical layout, an outrunner design is also worth evaluating.

Selection should account for peak joint load, continuous duty cycle, gearbox configuration, and overall robot weight. The motor structure should not be chosen based solely on the type of robot.

2. Collaborative Robotic Arms

Collaborative robotic arms generally need to balance compact construction, smooth motion, and control accuracy. Frameless motors can be integrated directly into joint structures, reducing the space occupied by a separate motor housing and giving mechanical designers greater flexibility.

For these applications, in addition to comparing inrunner and outrunner structures, consider:

• Smoothness at low speeds;

• Cogging torque;

• Load inertia and motion response;

• Stator heat dissipation and continuous operating capability;

• Encoder configuration and control-system performance.

3. Exoskeletons and Wearable Robots

Exoskeletons need to provide suitable assistive torque within limited space and weight budgets while minimizing interference with the wearer's movement.

Inrunner frameless motors can be candidates for lightweight drive systems with high dynamic response. Practical applications must also consider joint installation space, human range of motion, transmission efficiency, noise, and thermal comfort during extended operation.

4. Precision Rotary Platforms and Automation Equipment

In precision rotary platforms, inspection equipment, and industrial automation mechanisms, motor structure affects mounting methods, rotational inertia, and transmission-system layout.

If an application requires direct drive or high torque at low speed, an outrunner design can be evaluated; if the system places greater emphasis on rotating-component inertia and dynamic control, an inrunner design can also be considered. The final decision should be based on the actual load, precision, and motion profile.

CubeMars RI and RO Series: Understanding the Two Frameless Motor Types

CubeMars offers the RI series of inrunner frameless torque motors and the RO series of outrunner frameless torque motors, providing options for different mechanical structures and robotic drive requirements.

1. RI Series: Inrunner Frameless Torque Motors

CubeMars  RI series uses an inrunner structure and includes models such as RI20, RI30, RI50, RI60, RI70, RI80 V2.0, and RI100. The series emphasizes high torque density, low cogging torque, and a compact design; Hall sensors are optional on some models.

The RI series is suitable for balancing design space, motion response, and mechanical integration. Models can be further shortlisted according to actual torque and size requirements for lightweight robot joints, collaborative robotic arms, exoskeletons, and other precision drive systems.

When selecting a model, pay particular attention to:

• Whether the stator outer diameter and axial length meet installation-space requirements;

• Whether continuous and peak torque meet load requirements;

• Whether rotor inertia is suitable for the target dynamic response;

• Whether the winding parameters match the drive;

• Whether the stator mounting structure provides adequate heat dissipation.

2. RO Series: Outrunner Frameless Torque Motors

The CubeMars RO series uses an outrunner structure and includes models such as RO40 Lite, RO50 Lite, RO60, RO80, and RO100. The series is intended for robotics and other drive applications requiring flexible integration; torque, dimensions, weight, and speed performance should be evaluated for each specific model.

The outrunner structure provides another option for rotary-drive designs requiring a larger effective radius. The RO series is worth comparing for joints or drive assemblies that require torque output and whose spatial layout allows the outer rotor to rotate.

When selecting a model, pay particular attention to:

• Whether the rotating outer components interfere with surrounding structures;

• Whether the motor outer diameter, axial length, and mounting interfaces meet requirements;

• Whether continuous and peak torque meet load requirements;

• Whether rotor inertia meets the system's dynamic-response requirements;

• Whether rotor retention, mechanical protection, and heat-dissipation solutions are appropriate.

3. How Should You Choose Between the RI and RO Series?

The two series should not simply be viewed as better or worse in performance; the choice should be based on the mechanical structure and operating conditions of the robot joint.

• If a project prioritizes lower rotor inertia, dynamic response, and an inner-rotor structure, consider evaluating the RI series first.

• If a project prioritizes torque output through a larger effective radius and its mechanical structure suits an outrunner layout, consider evaluating the RO series first.

• If a project requires high torque, fast response, and compact size at the same time, compare the continuous torque, rotational inertia, overall dimensions, speed range, and temperature-rise data of specific models.

The final choice should be based on the official technical specifications and actual test results for the relevant model.

Which Parameters Should Be Checked When Selecting a Frameless Motor?

Whether selecting an inrunner or outrunner frameless motor, it is recommended to define the following parameters early in the design process.

1. Continuous Torque and Peak Torque

Continuous torque determines the motor's ability to operate over the long term under specified thermal conditions, while peak torque usually refers to short-duration output subject to time limits. Calculate each based on the load profile and duty cycle.

2. Speed and Operating Point

Confirm that the motor can deliver the required torque at the target speed, and check drive voltage, current, and back-EMF limits. Do not treat no-load speed as the operating speed under load.

3. Rotational Inertia and Dynamic Response

For systems with frequent starts and stops, rapid reversals, or high-bandwidth control, include both motor rotor inertia and load inertia in the calculations.

4. Installation Space and Mechanical Interfaces

In addition to the motor's outer diameter and axial length, allow space for rotor rotation, stator mounting, bearings, cables, and maintenance. For an outrunner design in particular, confirm that the rotating outer components will not interfere with surrounding structures.

5. Heat Dissipation and Continuous Operating Capability

Check the thermal conductivity of the stator mounting surface, ambient temperature, cooling method, and permitted temperature rise to ensure that the motor meets the requirements of the actual duty cycle.

6. Drive and Position Feedback

Confirm that the motor's electrical parameters are compatible with the drive, and select a suitable encoder or other position-feedback solution according to control requirements. For high-precision robot joints, also verify the overall performance of the controller, current loop, and position loop.

Conclusion

Inrunner and outrunner frameless motors each have their own characteristics. The choice of structure depends on the specific requirements of the robotic system, rather than a simple judgment about which type performs better.

Inrunner frameless motors place the rotor inside the stator and, in some designs, help control rotor size and rotational inertia. They are suitable for applications where dynamic response, motion control, and compact structural layout are important.Outrunner frameless motors place the rotor outside the stator and can use a larger effective radius, giving them the potential to produce higher torque under otherwise similar design conditions. However, actual torque output still depends on electromagnetic design, dimensions, winding parameters, operating current, and heat dissipation; performance cannot be judged from rotor structure alone.

For applications such as humanoid robots, collaborative robots, exoskeletons, and precision automation equipment, the advantage of frameless motors is that they can be integrated into the equipment's own mechanical structure, providing more flexible integration options for joints, rotary components, and compact drive systems. In actual selection, however, in addition to comparing the structural differences between inrunner and outrunner motors, it is also necessary to evaluatecontinuous and peak torque, target speed, rotational inertia, installation space, thermal management, drive compatibility, and position-feedback requirements. The motor's continuous output capability and heat-dissipation design are equally important, especially under high loads or during extended operation.

When choosing CubeMars frameless motors, the RI inrunner series and RO outrunner series can serve as references for different structural approaches. It is recommended to compare the specifications of specific models according to the equipment's mechanical layout, motion-response requirements, and target operating point, then validate them against the actual load and installation conditions rather than making a decision based only on the series name or a single performance metric.

Overall, a suitable frameless motor should strike a balance among torque output, motion response, structural integration, and thermal management. Clearly defining application requirements and evaluating the complete motor specifications alongside the drive-control solution and system design will help identify a more suitable drive solution for robot joints and other precision motion systems.