Frameless Torque Motors: From Cogging Torque Elimination to the Three Major Technological Breakthroughs of 2026

This article begins with the principle of cogging torque in frameless motors, explains how slotless core technology eliminates torque ripple and vibration, and highlights the three major breakthroughs achieved by frameless torque motors in 2025: increasing low-speed torque through high pole-pair designs exceeding 32 pole pairs, adopting hollow frameless structures for compact and lightweight designs, and utilizing high-grade NdFeB permanent magnets to enhance torque density.

The article also summarizes the advantages of frameless torque motors, including compact size and high torque density, while discussing challenges such as magnetic circuit optimization and thermal management. Finally, it provides practical selection guidelines based on the principle of "torque first," along with key engineering considerations for installation and heat dissipation.

Understanding the Basics: What Causes Cogging Torque?

To fully understand motor operation, it is essential to first understand the three fundamental components of a motor core.

The continuous circular ring located on the outermost part of the stator is called the yoke. The tooth-shaped protrusions extending inward from the yoke are called teeth. The spaces between adjacent teeth are known as slots, while the opening at the front of each slot is referred to as the slot opening.

To enable motor operation, conductive copper windings are wrapped around the teeth. Because the teeth possess excellent magnetic permeability, they help strengthen the magnetic field.

When a rotor is placed inside the stator and begins rotating, a noticeable periodic resistance or "jerking" sensation can be felt. In motor engineering, this phenomenon is known as cogging torque.

For example, consider a six-slot, four-pole stator paired with a four-pole rotor. Since the number of magnetic poles and slots cannot align perfectly, the magnetic attraction between the rotor magnets and stator teeth fluctuates periodically during rotation. Each time the rotor passes a slot position, a torque disturbance occurs, resulting in vibration and uneven motion.

One of the most effective methods of eliminating cogging torque is the use of a slotless core structure.

As the name implies, a slotless motor contains neither slots nor teeth. Instead, copper windings are directly attached to the smooth inner surface of the iron core. Without teeth, the magnetic attraction between the rotor magnets and the stator remains constant during rotation.

As a result, slotless motors completely eliminate cogging torque, providing:

• Ultra-smooth operation

• No jerking or vibration

• Extremely low torque ripple

• High positioning accuracy

This is one of the most important performance advantages of slotless motor technology.

What Technological Breakthroughs Have Frameless Torque Motors Achieved in 2025?

Frameless torque motors are multi-pole permanent magnet synchronous direct-drive motors. Unlike conventional motors, they eliminate non-essential structures such as housings, bearings, and output shafts.

Their development focuses on three key objectives:

• Constant torque at low speed

• High torque density

• Low torque ripple

In 2025, major technological advances have been achieved across electromagnetic design, structural innovation, and material development, making frameless torque motors ideally suited for the compact, flexible, and high-precision requirements of humanoid robot joints.

1. Electromagnetic Design: Solving Low-Speed Vibration

Increasing the number of pole pairs has become the key factor driving performance improvements.

Compared with the mainstream 12-pole-pair configurations common five years ago, high-end frameless torque motors now feature 32, 64, or even more pole pairs.

The higher pole count enables the motor to deliver stable rated torque even at zero speed or ultra-low speeds as low as 0.1°/s, effectively eliminating crawling, sticking, and vibration issues associated with conventional motors.

At the same time, the industry widely adopts optimized fractional-slot concentrated winding configurations such as 48 poles and 324 slots, reducing cogging torque to less than 1% of rated torque.

This enables exceptionally smooth motion control for demanding applications such as:

• Surgical robots

• Humanoid robot joints

• Semiconductor equipment

• Precision automation systems

2. Structural Innovation: Compact Design for Humanoid Robots

Frameless torque motors have become the preferred motor architecture for humanoid robots.

Unlike integrated framed DD (Direct Drive) motors, frameless motors feature:

• No external housing

• No bearings

• No output shaft

This minimalist architecture offers significant integration advantages.

The stator can be directly embedded into the robot housing, while the rotor is mounted directly onto the load shaft.

Key benefits include:

Axial length reduced to approximately one-third of conventional drive motors

More than 30% reduction in overall weight

Significantly smaller joint dimensions

Internal routing space for cables, sensors, and fluid lines

The hollow structure is particularly well suited to the compact installation requirements of humanoid robot joints.

3. Material Advancements: Stable High-Torque Performance Across Wide Temperatures

High-end frameless torque motors commonly utilize N52H-grade and higher NdFeB permanent magnets, offering remanence values of up to 1.45 Tesla.

Combined with high-conductivity copper alloy windings, these materials significantly improve:

Electromagnetic conversion efficiency

Torque density

Continuous output capability

The complete material system supports operation across a wide temperature range of -40°C to 125°C, ensuring stable torque output under demanding conditions such as:

• High-temperature environments

• Low-temperature environments

• Frequent start-stop cycles

• Mild overload conditions

This approach balances both performance and long-term reliability.

Advantages and Challenges of Frameless Torque Motors

1. Core Advantages

Compact Size

The hollow design minimizes occupied space, simplifying cable routing and system integration within robotic joints.

High Torque Density

Large torque output can be achieved even at low rotational speeds, making frameless torque motors ideal for low-speed, high-load robotic applications.

Stable Performance

Direct integration into machine structures enhances resistance to:

• High temperatures

• High voltages

• Radiation exposure

• Harsh industrial environments

• Excellent Startup and No-Load Characteristics

• Low startup voltage

• Low no-load current

• Improved energy efficiency

2. Technical Challenges

Magnetic Circuit Optimization

Engineers must carefully optimize magnetic materials and winding configurations to maximize magnetic circuit efficiency and slot fill factor.

Thermal Management

Low-voltage systems often require high current operation, generating substantial heat. Excessive temperature rise can accelerate component aging and reduce system lifespan.

Consistency Requirements

Multi-joint robotic systems require highly consistent motor performance across all joints. Variations increase commissioning complexity and negatively impact control performance.

Cost Reduction

Localization of key components remains critical for reducing manufacturing costs and enabling large-scale commercial deployment.

How to Select the Right Frameless Torque Motor？

1. Golden Selection Principle

The most important rule is:

Torque First, Speed Second

Humanoid robots frequently experience start-stop operation and rapidly changing dynamic loads.

Recommended design margins include:

• Continuous torque ≥ 1.2–1.5 × steady-state load torque

• Peak torque ≥ 2 × impact load torque

For robotic joints, inertia matching must also be carefully controlled.

The ratio of load inertia to motor inertia should remain:

≤ 5:1

to prevent vibration, instability, and oscillation.

Encoder Selection

For standard humanoid robot applications:

• 23-bit absolute encoder

• Resolution approximately 0.0001°

For ultra-high-precision applications such as:

• Medical robotics

• Semiconductor manufacturing

a 29-bit ultra-high-resolution encoder is recommended.

2. Critical Integration Considerations

Coaxiality Control

Excessive coaxiality error is one of the most common causes of frameless motor failure.

The concentricity between stator and rotor should be maintained within:

0.02 mm

Excessive misalignment can result in:

Increased torque ripple

Localized overheating

Bearing failure

Precision dial indicators should be used during assembly for accurate alignment.

Thermal Management

Because frameless motors operate at low speed and high current, heat generation can be substantial.

Under conditions such as:

• Continuous stall

• Maximum power operation

forced-air cooling or liquid-cooling systems are strongly recommended.

Advanced joint designs can employ:

• Heat pipes integrated into the joint housing

• Dielectric coolant circulation

to increase continuous torque density by up to four times.

Structural Rigidity

Direct-drive systems lack gearbox damping.

Insufficient structural stiffness can lead to resonance and vibration.

Recommended solutions include:

Hollow integrated joint structures

Reinforced cast-iron support bases

Increased system rigidity

3. Commissioning and EMI Optimization

During commissioning, three key functions should be enabled:

• Cogging torque compensation

• Harmonic suppression

• Friction feedforward compensation

The current-loop bandwidth should exceed:

• 2 kHz for standard applications

• 5 kHz for high-end precision applications

These measures effectively reduce torque ripple and improve motion smoothness.

For example, in surgical robotics applications, tuning the PI controller to:

• Kp = 0.35

• Ki = 1200

can achieve current response times as fast as 0.5 ms.

EMI Suppression

To address fixed-frequency noise sources such as 1.2 MHz interference, recommended solutions include:

• Copper foil shielding on stator windings

• Nanocrystalline magnetic shielding layers

• Conductive fabric shielding

• Ferrite cores installed on power cables

Increasing PWM frequency from 15 kHz to 18 kHz can slightly increase switching losses but helps avoid mechanical resonance frequencies and reduce electromagnetic noise peaks by approximately 8 dB.

Contact HONPINE

Contact HONPINE today to obtain more information, technical specifications, and application materials regarding frameless torque motors.