Frameless Motor For Medical Equipment

Medical-Grade Frameless Motors are specially developed for deep integration into medical equipment. They consist primarily of two components: the stator winding and permanent-magnet rotor. Without a housing, built-in bearings, or output shaft, the motor can be directly integrated into the mechanical structure of medical equipment to achieve direct-drive transmission.

Based on extensive experience with international medical equipment projects, many R&D teams initially select standard industrial frameless motors for medical equipment prototypes. However, issues such as low-speed vibration, insufficient insulation, non-compliant materials, and the inability to provide documentation required for overseas medical certification can result in redesigns, project delays, and export restrictions. Even if a standard industrial frameless motor delivers excellent performance, it does not automatically qualify for use in medical devices.

Drawing on practical experience in engineering commissioning and overseas medical equipment projects, this article provides a comprehensive overview of frameless motors for medical equipment. It compares medical-grade and industrial-grade frameless motors, explains their major applications and selection parameters, outlines key medical compliance requirements, and highlights common integration mistakes, providing practical technical guidance for medical OEM customers worldwide.

What Is a Frameless Motor for Medical Equipment?

Basic Definition

A frameless brushless torque motor for medical equipment is a medical-customized type of permanent-magnet brushless frameless motor. The motor itself consists only of the stator and rotor and does not include a housing, bearings, encoder, or brake. It cannot operate as a standalone motor and requires mechanical integration, feedback configuration, and thermal management within the complete medical equipment system before operation.

The stator is directly mounted inside the mechanical housing of the medical device, while the rotor is directly coupled to the main rotating shaft. The equipment's own precision bearings provide rotational support. The large central hollow structure allows cables and tubing to pass through the motor, helping achieve a more compact system design.

The biggest difference from general-purpose industrial frameless motors is that medical-grade versions are specifically optimized for medical applications in terms of magnetic circuits, insulation materials, winding wires, permanent magnets, and potting processes. Particular attention is given to controlling cogging torque, torque ripple, and leakage current while meeting medical device requirements for materials, electrical safety, and environmental compliance. They can also be adapted to applications involving disinfection, continuous long-duration operation, and proximity to the human body.

Two Main Types of Medical Frameless Motors

Inner-Rotor Medical Frameless Motors

The rotor is located inside the stator. This configuration offers low rotor inertia, fast dynamic response, and a compact overall design.

It is commonly used in surgical robot end-effectors, compact rehabilitation joints, and small diagnostic devices, particularly where installation space is extremely limited.

Outer-Rotor Medical Frameless Motors

The rotor surrounds the outside of the stator, providing higher torque output, a flatter and thinner form factor, and a larger hollow bore.

This configuration is commonly used in rotating gantries for medical imaging equipment, large rehabilitation exoskeleton joints, and large-scale diagnostic and treatment platforms.

HOLRY's medical-adapted frameless motors are available in both inner-rotor and outer-rotor configurations. Windings, insulation systems, permanent magnets, and material declarations can be customized according to specific medical equipment projects and development requirements.

Medical Frameless Motors vs. Standard Industrial Frameless Motors

Standard Industrial Frameless Motors

• Design objective: Industrial automation and robotic joints, with priority given to torque, speed, and mass-production cost.

• Cogging torque: Typically ≤3–5% of rated torque, which is generally sufficient for industrial equipment.

• Insulation system: Standard industrial enamelled wire and impregnation processes.

• Materials: Basic RoHS compliance; medical-grade material declarations are generally not provided.

• Environmental resistance: Designed primarily for standard indoor industrial environments without specific consideration for repeated disinfection or high humidity.

• Compliance: Generally supported by industrial CE documentation rather than the technical documentation required for IEC 60601 medical electrical safety compliance.

• Typical applications: Industrial robots, machine-tool rotary tables, and general automation equipment.

Medical-Grade Frameless Motors

• Design objective: Extremely smooth low-speed operation, low vibration and noise, low leakage current, and enhanced safety margins.

• Magnetic circuit optimization: Skewed permanent magnets and fractional-slot windings can reduce cogging torque to ≤2% of rated torque, suppress torque ripple, and minimize micro-vibrations that could interfere with medical procedures.

• Insulation system: Medical-grade enamelled wire with high resistance to temperature and humidity, combined with specialized impregnation and potting processes to improve dielectric strength and reduce leakage-current risks.

• Materials: RoHS 3 and REACH SVHC material declarations can be provided to support MDR and FDA technical documentation for complete medical devices.

• Environmental resistance: Can be adapted for disinfectant wiping and high-humidity environments, with moisture- and corrosion-resistant windings.

• Compliance support: Complete English technical documentation, thermal parameters, and electrical parameters can be provided to support IEC 60601-1 medical electrical safety certification of the complete device.

• Typical applications: Surgical robots, rehabilitation exoskeletons, medical imaging systems, and precision diagnostic and treatment equipment.

Comparison with Traditional Medical Motor + Gearbox Solutions

Traditional motor + gearbox solutions are relatively easy to integrate, but gears introduce backlash, vibration, wear, and mechanical particles. Long-term operation may generate wear debris, which is undesirable in clean medical environments.

Medical frameless direct-drive solutions eliminate mechanical gearing and associated wear particles while providing higher motion accuracy. However, they place higher requirements on the mechanical design, alignment, and assembly process of the complete medical device.

Key Advantages of Frameless Motors for Medical Equipment

Extremely Low Cogging Torque for Smooth, Vibration-Free Low-Speed Operation

Medical procedures are highly sensitive to even minor mechanical vibration. Small oscillations in surgical robots or medical imaging mechanisms can directly affect treatment accuracy and image quality.

Through magnetic circuit optimization, medical frameless motors suppress cogging torque and torque ripple, enabling smooth low-speed and micro-angle movement without sticking or jerking. This helps ensure precise medical motion and high-quality imaging.

High Torque Density for Compact and Lightweight Equipment

Medical equipment often has very limited internal space, particularly handheld instruments and portable rehabilitation devices.

By eliminating unnecessary structures such as the motor housing and integrating the motor directly into the equipment, overall volume and weight can be significantly reduced for the same torque output, supporting more compact and portable medical equipment designs.

Large Hollow Bore for Optimized Internal Routing

The hollow ring-shaped structure allows signal cables and tubing to pass directly through the center of the motor.

This reduces external cable bending and fatigue, simplifies internal system layout, and is particularly suitable for surgical robotic arms and multi-degree-of-freedom rehabilitation joints.

Zero-Backlash Direct Drive for High Positioning Accuracy

The rotor directly drives the main shaft, eliminating transmission backlash caused by gears and couplings.

This provides high repeatability and excellent force-control performance, making it suitable for precision operations in minimally invasive surgical systems.

Low Noise and No Mechanical Wear Particles

With no brushes and no gear friction, frameless direct-drive motors operate with low noise and do not generate mechanical wear particles.

This makes them well suited to operating rooms and clean medical environments while helping reduce contamination risks.

Customizable Insulation and Materials for Global Medical Compliance

HOLRY medical-adapted frameless motors can be upgraded with customized insulation systems and can be supplied with comprehensive material declarations, thermal curves, and PTC temperature protection configurations.

Wide-voltage customization is also available, helping overseas customers prepare technical documentation for complete medical devices intended for markets subject to EU MDR and U.S. FDA requirements.

Major Applications of Medical Frameless Motors

1. Surgical Robots

Frameless motors are used in joints and end-effectors of minimally invasive surgical robots.

These applications require extremely smooth motion, low vibration, high positioning accuracy, and precise force control, making them one of the most important applications for medical frameless motors.

2. Rehabilitation Robots and Exoskeletons

Applications include lower-limb rehabilitation exoskeletons, upper-limb rehabilitation robots, and dexterous hand rehabilitation joints.

Because these devices operate in close contact with the human body, they require smooth and safe operation, lightweight construction, and gentle dynamic response to protect user safety.

3. Medical Imaging Equipment

Applications include rotating gantries and scanning mechanisms for CT, DR, ultrasound, and related medical imaging equipment.

These systems often operate continuously at low speeds for extended periods and therefore require excellent thermal stability and low vibration to maintain imaging quality.

4. Laboratory and Precision Diagnostic Equipment

Applications include automated sample-processing platforms, precision drug-delivery mechanisms, and rotating worktables for analytical equipment.

These systems require highly accurate positioning and low electromagnetic interference to ensure reliable test results.

5. Portable Medical Robotic Equipment

Applications include mobile diagnostic robots and portable scanning mechanisms.

Strict weight and dimensional requirements make frameless motors particularly valuable because they can be integrated directly into the equipment to achieve a compact overall structure.

Six Key Selection Criteria for Medical Frameless Motors

Distinguish Continuous Torque from Peak Torque and Reserve Sufficient Safety Margin

Medical equipment frequently operates continuously or in repeated cycles for extended periods.

Continuous rated torque determines long-term operating reliability, while peak torque should only be used to handle short-duration transient loads.

For medical projects, it is recommended to reserve approximately 25–30% torque safety margin to prevent excessive temperature rise. In high-temperature environments, torque derating should also be evaluated.

Pay Close Attention to Cogging Torque and Torque Ripple

This is one of the most important differences between medical and conventional industrial motor selection.

For surgical and medical imaging equipment, motors with cogging torque of ≤2% of rated torque are generally preferred. Test data should be requested from the supplier to prevent low-speed vibration from affecting equipment performance.

Thermal Rise and Thermal Protection

Medical equipment typically has strict temperature-rise requirements to prevent overheating-related safety risks.

Because a frameless motor does not have its own cooling housing, heat must be transferred through the mechanical structure of the complete equipment.

It is recommended to select motors supporting PTC temperature sensors for over-temperature protection and obtain complete temperature-rise versus load curves for system-level risk analysis and documentation.

Insulation, Leakage Current, and Material Compliance

For medical equipment exported to Europe and North America, suppliers should be able to provide:

• RoHS 3 and REACH SVHC material declarations

• Winding insulation dielectric-strength data

• Leakage-current-related parameters

• Relevant electrical safety information

These documents help manufacturers complete IEC 60601-1 medical electrical safety certification for the complete device.

Standard industrial motors often lack such documentation, which can create obstacles during the medical certification process.

Mechanical Interface and Hollow Bore

Verify the outer diameter, stack length, and hollow bore diameter to ensure compatibility with internal cable routing and available installation space.

Since many medical devices use customized mechanical housings, it is important to confirm the supplier's ability to provide customized mechanical and electrical modifications.

Electrical and Feedback Component Compatibility

Medical frameless motors typically do not include encoders or brakes.

The motor drive should support torque-control mode and relevant medical equipment safety functions, such as STO (Safe Torque Off).

The supplier should provide complete winding resistance, inductance, and back-EMF parameters to facilitate system integration and commissioning.

Common Integration Mistakes with Medical Frameless Motors

Mistake 1: Assuming a standard industrial frameless motor can be directly used in medical equipment

An industrial frameless motor may meet the required torque specifications, but its vibration performance, insulation system, and material documentation may not meet medical project requirements.

Even if the prototype operates successfully, the complete device may encounter major obstacles during medical certification, creating significant risks for overseas export projects.

Mistake 2: Treating a frameless motor as a finished motor and powering it directly

A frameless motor consists only of the stator and rotor and lacks bearing support and encoder feedback.

It cannot simply be powered and operated as a complete motor. Incorrect direct-power operation can cause rotor-to-stator contact and permanent-magnet damage.

Mistake 3: Looking only at the torque rating while ignoring thermal rise and continuous-duty conditions

Medical equipment often operates continuously or in repeated cycles.

Selecting a motor based only on peak torque can result in excessive continuous temperature rise, accelerated winding aging, and potential safety risks.

Mistake 4: Ignoring medical compliance documentation until the certification stage

For medical export projects, material declarations, electrical safety parameters, and thermal test curves should be obtained at an early stage.

Waiting until complete-device certification to request these documents can lead to delays because many standard industrial motor suppliers cannot provide the required information.

Mistake 5: Assuming that purchasing a medical-grade motor automatically makes the complete device medically certified

A motor is only one component of a medical device.

A medical-grade component does not automatically mean that the complete medical device is certified. The medical device manufacturer must still complete risk management according to applicable requirements, such as ISO 14971, as well as complete-device testing and the applicable registration and certification procedures for the target market.

FAQ

Q1: Do medical frameless motors come with brakes and encoders?

A: Standard products do not include brakes or encoders. These are system-level components selected according to the requirements of the complete medical device. HOLRY can reserve mounting interfaces for sensors and other feedback components for OEM integration.

Q2: What documents should be requested from the motor supplier for medical equipment exported overseas?

A: Recommended documentation includes specifications, torque-speed-temperature-rise curves, cogging torque test reports, rotor inertia, electrical parameters, and PTC temperature parameters.

Material compliance documentation such as RoHS 3 and REACH SVHC declarations, as well as English assembly manuals and air-gap/concentricity tolerance documentation, should also be requested for inclusion in the complete device's technical documentation.

Q3: Can medical frameless motors withstand high-temperature and high-pressure sterilization?

A: In most applications, the frameless motor itself is not directly exposed to the sterilization environment. The motor is integrated inside the equipment housing, while the housing undergoes sterilization.

If the motor must be directly exposed to a sterilization or disinfection environment, material selection and potting processes should be specifically customized and validated in advance.

Q4: Do medical frameless motors always need to be paired with a gearbox?

A: Not necessarily.

Light-load precision mechanisms can often use direct drive without a gearbox. Surgical robots and large rehabilitation joints, however, may be combined with compact precision harmonic reducers when higher output torque is required.

Q5: What are the most common causes of failure in medical frameless motors?

A: Common causes include:

• Rotor-to-stator contact caused by misalignment

• Excessive temperature rise under continuous load

• Insulation systems that are unsuitable for the medical environment

• Excessive cogging torque resulting in low-speed vibration

• Incomplete technical documentation leading to difficulties in complete-device certification

Conclusion

A frameless motor for medical equipment is not simply a standard industrial frameless motor with a different label. It is a critical component specifically optimized for medical applications across the magnetic circuit, insulation system, materials, manufacturing processes, and technical documentation.

With advantages including high torque density, zero-backlash direct drive, low vibration, and the absence of mechanical wear particles, medical frameless motors have become an attractive drive solution for surgical robots, rehabilitation exoskeletons, and medical imaging equipment.

Medical equipment is a highly regulated industry. In addition to motor performance, thermal management, cogging torque, material compliance, and complete technical documentation are equally important.

For overseas medical equipment projects, a lack of compliance documentation during the early development stage can directly lead to certification obstacles and project delays.

HOLRY has extensive R&D and manufacturing capabilities for medical-adapted frameless motors. Backed by 15 years of practical motion-control engineering experience and 10 years of experience serving global international trade projects, HOLRY provides both standard and highly customized frameless motor solutions.

Our product portfolio covers both inner-rotor and outer-rotor configurations, with comprehensive English technical documentation and material declarations available for medical projects. We support global medical OEM customers from prototype development through mass production.

If you are selecting a motor for medical equipment, contact the HOLRY technical team with your load conditions, available installation space, and target export market to receive a free motor selection and application evaluation.