Frameless Motor Vs Brushless Motor

With the growing global demand for automation equipment, collaborative robots, and precision machinery, overseas demand for brushless motors and frameless motors continues to increase. However, many customers directly compare the specifications of finished brushless motors with those of frameless motors while overlooking the significant differences caused by their different integration methods.

Based on hands-on commissioning experience and extensive international OEM project experience, this article explains the operating principles and structural differences between brushless motors and frameless motors. It provides a multi-dimensional comparison of their performance, advantages, disadvantages, applications, selection criteria, integration challenges, and key considerations for international projects, helping engineers make the right motor selection.

What Is a Brushless Motor?

A brushless motor is a permanent-magnet synchronous motor without mechanical DC brushes. It uses a motor drive to supply three-phase AC power to the stator windings, generating a rotating magnetic field that drives the permanent-magnet rotor. An encoder can be used to enable closed-loop control, eliminating the traditional mechanical brush-and-commutator structure.

Brushless motors are a broad motor category and can generally be divided into two main types based on their product configuration:

• Enclosed finished brushless motors: These are complete motor assemblies equipped with a housing, bearings, output shaft, and end covers. Some models also include an integrated encoder. Customers can install and use them directly after purchase. Most conventional servo motors and standard BLDC motors fall into this category.

• Frameless brushless motors: These consist only of the stator winding assembly and permanent-magnet rotor assembly. They do not include a housing, bearings, or output shaft and must be integrated directly into the customer's equipment. This configuration is commonly referred to as a frameless motor.

In simple terms:

All frameless motors are essentially brushless motors, but not all brushless motors are frameless motors.

What Is a Frameless Motor?

A frameless motor, also known as a frameless brushless torque motor, is a special configuration of a brushless permanent-magnet synchronous motor. Only two core electromagnetic components—the stator and rotor—are supplied from the factory.

• Stator: A wound iron core that is fixed inside the equipment housing.

• Rotor: A permanent-magnet rotor that is directly mounted onto the equipment's own main shaft.

The bearings, output shaft, housing, end covers, encoder, and cooling system must all be designed and integrated by the equipment manufacturer.

The motor itself is responsible only for generating torque. Mechanical support is provided entirely by the equipment structure. Frameless motors are mainly used in high-end equipment requiring high integration, lightweight construction, and a large through-hole.

A frameless motor cannot be operated simply by connecting it to power. Without proper mechanical support and feedback components, direct energization may cause rotor-stator contact, commonly known as motor rub, resulting in damage to the rotor magnets or windings.

Key Structural Differences Between Frameless and Finished Brushless Motors

Structure of an Enclosed Finished Brushless Motor

A finished enclosed brushless motor typically features:

• A metal housing for protection and heat dissipation;

• Built-in bearings to support rotor rotation;

• An integrated output shaft for connection to the external load through a coupling;

• Built-in encoders or Hall sensors;

• Complete assembly, dynamic balancing, and aging testing performed at the factory;

• Simple installation: mount the motor, connect the wiring, connect the load, and put the system into operation.

The key advantage is plug-and-play operation, with no need for extensive mechanical development.

The disadvantage is that the housing and bearing system occupy additional space, resulting in a larger overall size and weight.

Structure of a Frameless Brushless Motor

A frameless brushless motor typically features:

• No housing, bearings, or output shaft;

• Separate stator and rotor assemblies supplied to the customer;

• Optional encoders and temperature sensors rather than standard components;

• The equipment housing itself serves as the motor housing;

• The equipment's own bearings support the rotor;

• The main shaft passes directly through the rotor's inner bore, allowing the load and rotor to rotate as an integrated assembly;

• The customer is responsible for mechanical design, assembly, concentricity alignment, cooling design, and electrical matching.

The main advantages are compact size, high torque density, and support for large through-holes for cables, tubes, and other components.

The main disadvantage is the greater engineering workload and strict requirements for assembly accuracy.

Comprehensive Comparison of Performance, Advantages and Disadvantages

Advantages of Enclosed Finished Brushless Motors

Ready to use with a short development cycle

The motor is fully assembled and tested before delivery, eliminating the need for complicated mechanical design and shortening prototype development time. This makes it suitable for rapid project validation.

Low assembly requirements and controllable reliability

Dynamic balancing and aging tests are completed at the factory, reducing the risk of assembly errors and lowering the mechanical engineering requirements for downstream customers.

Complete protection options

The motor housing can provide protection levels such as IP54 and IP65, making it suitable for dusty, humid, and demanding industrial environments.

Mature supporting components and stable supply

Drives, gearboxes, couplings, and other components are widely standardized, while spare parts are readily available worldwide. This can significantly reduce after-sales pressure in international projects.

Limitations

The housing and bearings occupy additional axial space, resulting in a larger and heavier motor for the same torque output. The use of output shafts and couplings also introduces intermediate transmission components that may create backlash. In addition, conventional enclosed motors generally cannot provide a large hollow through-hole for internal cable routing.

Typical applications

• Standard automation production lines

• Conveyor systems

• CNC engraving equipment

• Packaging machinery

• Standard AGVs

• Small-batch prototype projects

Advantages of Frameless Brushless Motors

Higher torque density and lightweight design

By eliminating the housing and unnecessary mechanical components, frameless motors can achieve a smaller size and lower weight for the same torque output, making them ideal for equipment with strict space and weight limitations.

Direct drive with zero transmission backlash and high rigidity

The rotor is mounted directly onto the load shaft, eliminating couplings, gears, and other intermediate transmission components. This removes transmission backlash and can significantly improve positioning accuracy and response speed.

Large through-hole design

Cables, oil lines, and pneumatic tubes can pass through the motor's central bore, greatly simplifying the internal layout of robotic joints, rotary tables, and other hollow-axis mechanisms.

Flexible cooling solutions

Because the stator can be directly integrated with the equipment housing, natural cooling, water cooling, or oil cooling can be implemented according to application requirements. This makes frameless motors suitable for continuous heavy-load operation.

Highly customizable mechanical configuration

The motor diameter, thickness, winding parameters, and other specifications can be customized according to the equipment's internal space, making frameless motors suitable for medical equipment, semiconductor equipment, robotics, and other specialized applications.

Limitations

Higher development cost and longer development cycle

The mechanical engineering team must design the motor cavity, bearings, cooling system, and assembly tooling. Therefore, frameless motors are generally not the most economical choice for small-batch prototype projects.

Strict assembly requirements

The air gap between the stator and rotor is very small. Poor concentricity can cause rotor-stator contact, vibration, or permanent-magnet damage. High-precision manufacturing and assembly processes are therefore essential.

Protection depends on the overall equipment design

Since the motor itself does not have a protective housing, dust and moisture protection must be provided by the equipment enclosure. Poor sealing or insufficient protection can result in moisture entering the windings.

Higher engineering requirements for international projects

If an overseas customer lacks sufficient integration and precision assembly capabilities, incorrect installation can easily lead to motor failure and increased after-sales costs.

Typical applications

• Collaborative robot joints

• Humanoid robots

• Medical surgical equipment

• Direct-drive rotary tables

• Semiconductor rotary platforms

• Specialized gimbals

• High-volume OEM equipment

Five Key Factors for Motor Selection

Development Cycle and Production Volume

For prototypes and small-batch projects, enclosed finished brushless motors are generally preferred because they reduce development complexity and enable faster implementation.

For high-volume OEM production, where a longer development cycle is acceptable and equipment miniaturization and weight reduction are priorities, frameless motors can be considered.

Equipment Space and Structural Requirements

If the equipment has limited internal space, requires weight reduction, needs hollow-axis cable routing, or requires zero-backlash direct-drive performance, a frameless motor is usually the better choice.

If sufficient installation space is available and deep mechanical integration is not required, an enclosed finished brushless motor is generally more practical.

Customer Engineering Capability

For overseas small and medium-sized customers with limited mechanical engineering and precision assembly capabilities, it is generally recommended to supply finished brushless motors directly. This can reduce the risk of installation errors and subsequent after-sales problems.

For OEM manufacturers and robotics companies with complete R&D and manufacturing capabilities, frameless motor kits can be supplied together with detailed assembly instructions, torque-speed curves, and temperature protection parameters.

Cooling and Operating Conditions

For continuous heavy-load applications, an enclosed brushless motor can dissipate heat through its own housing.

For frameless motors, the entire equipment system must be designed with an appropriate thermal management path.

When supplying motors to tropical or high-temperature markets, sufficient thermal margin should be considered and appropriately increased.

Protection and Operating Environment

For applications involving high levels of dust, humidity, or other harsh environmental conditions, enclosed finished brushless motors are generally preferred.

If a frameless motor is used, the overall equipment housing must provide adequate sealing and environmental protection.

Common Motor Selection Mistakes

Misconception 1: Frameless Motors and Brushless Motors Are Two Equivalent Motor Categories

A frameless motor is a sub-category of brushless motors. Their basic operating principles are essentially the same. The main differences lie in their physical configuration and integration method.

Therefore, they should not be treated as two completely separate motor technologies.

Misconception 2: Frameless Motors Always Perform Better Than Finished Brushless Motors

Under the same electromagnetic design conditions, a frameless motor can achieve higher overall system performance. However, this depends on the mechanical structure, bearings, cooling system, and assembly process of the complete machine.

If the integration design is inadequate, the actual performance of a frameless motor can be significantly worse than that of a standard finished brushless motor.

Misconception 3: A Frameless Motor Can Work Simply by Connecting It to a Drive

A frameless motor consists primarily of the stator and rotor and does not include bearings, mechanical support, or an encoder.

It therefore cannot operate independently and must be fully integrated into the equipment.

Many overseas customers mistakenly assume that a frameless motor can be wired and operated in the same way as a conventional BLDC motor, which can result in serious mechanical damage during initial operation.

Misconception 4: Motors Can Be Directly Substituted If Their Torque Ratings Are the Same

Even if the rated torque and peak torque are identical, an enclosed brushless motor includes its own bearings and shaft system, while a frameless motor relies on the equipment structure.

Their inertia, thermal conditions, mounting constraints, and mechanical support requirements can be completely different.

Therefore, the two types of motors cannot simply be substituted based on torque specifications alone.

Misconception 5: Frameless Motors Reduce Costs for Small-Batch Prototypes

Although the purchase price of a frameless motor kit may be lower, the total cost of the project can increase after adding the costs of motor cavity development, assembly tooling, encoders, bearings, cooling systems, and precision assembly.

The cost advantages of frameless motors are generally more evident in high-volume production.

FAQ

Q1: Can a Frameless Motor Use a Standard Brushless Motor Drive?

A: Yes, provided that the drive supports brushless torque-control operation. However, the motor resistance and inductance parameters must be correctly matched, and the current loop must be tuned accordingly.

Many general-purpose brushless motor drives cannot fully utilize the torque performance of a frameless torque motor without proper configuration and tuning.

Q2: For a Direct-Drive Project, Should I Choose a Frameless Motor or a Finished Brushless Torque Motor?

A: If the equipment has enough internal space to integrate the motor, and the application requires a lightweight, hollow-axis structure and the customer has sufficient R&D capabilities, a frameless motor is a suitable option.

If the customer wants a ready-to-use solution, a finished enclosed brushless torque motor is generally the better choice.

Q3: What Technical Documents Should Be Provided for Frameless Motor Kits Exported Overseas?

A: Recommended documentation includes:

• Stator and rotor assembly instructions;

• Air-gap requirements;

• Torque-speed curves;

• Temperature sensor specifications;

• Insulation class;

• Bearing requirements;

• Encoder requirements;

• Cooling requirements.

Customers should also be clearly informed that the frameless motor requires appropriate bearings, encoders, mechanical support, and thermal management as part of the complete system.

Q4: Can a Standard Brushless Motor Be Modified for Frameless Motor Applications?

A: It is generally not recommended.

The stator and rotor of a standard brushless motor are designed to operate together with its original housing and bearing system. Removing the housing can compromise permanent-magnet retention and air-gap control, significantly increasing the risk of rotor-stator contact and motor damage.

Q5: In Which Applications Are Frameless Motors Not Recommended?

A: Frameless motors are generally not recommended for:

• Small-batch rapid prototypes;

• Customers without precision mechanical integration capabilities;

• Dusty, humid, or harsh environments without adequate enclosure protection;

• Projects with very limited development budgets.

In these cases, an enclosed finished brushless motor is usually the more practical choice.

Conclusion

A brushless motor is a broad category of permanent-magnet motors, while a frameless motor is a specialized integrated configuration of a brushless motor.

Although the two share the same fundamental operating principle, their real differences come from their product configuration, integration method, engineering requirements, and development complexity.

HOLRY has comprehensive R&D and manufacturing capabilities for both brushless motors and frameless torque motors. Backed by 15 years of hands-on experience from motion-control engineers and 10 years of international OEM project experience, HOLRY can provide both standardized finished brushless motors and standard or customized frameless motor kits.

Our solutions are designed to meet the needs of automation equipment, robotics, medical equipment, precision machinery, and other industrial applications worldwide.

If you are unsure whether to choose a brushless motor or a frameless motor, contact the HOLRY technical team and provide your load conditions, available installation space, production volume, and target export market.

Our engineers can provide a free motor selection and application evaluation to help you identify the most suitable solution.