The "High-Temperature Assassin" of Frameless Torque Motors: How Serious Is The Risk of Magnet Demagnetization?

In high-performance motion control scenarios such as robot joints, collaborative robotic arms, and direct-drive rotary tables, frameless torque motors are becoming increasingly common. They have no housing, no bearings—essentially a "bare rotor + stator" power core that is embedded directly into the mechanical structure, delivering high torque density, high rigidity, and a compact footprint.

However, many engineers overlook a potential risk during selection and use: irreversible demagnetization of the permanent magnets at high temperatures. Once demagnetization occurs, the motor experiences torque drop, increased heating, and may even leave the entire joint "weak."

I. What are the magnets in a frameless torque motor?

Frameless torque motors are typically permanent magnet synchronous motors, with high-performance permanent magnets—commonly referred to as "magnets"—mounted on or inside the rotor surface.

The mainstream magnet material used today is neodymium-iron-boron (NdFeB), because of its exceptional magnetic performance, capable of generating high air-gap flux in a very small volume—ideal for frameless torque motors that demand extremely high torque density.

But NdFeB has an inherent weakness: relatively poor heat resistance.

II. How does high-temperature demagnetization occur?

The magnetism of permanent magnets is not truly "permanent." When the temperature rises to a certain level, the alignment of magnetic domains inside the magnet becomes unstable, and magnetic strength gradually decreases.

Demagnetization is divided into two types:

1. Reversible demagnetization

Magnetic strength temporarily decreases as temperature rises, but recovers when temperature drops. This type is already accounted for in motor design and is usually not a problem as long as limits are not exceeded.

2. Irreversible demagnetization

If the temperature exceeds the magnet's tolerance limit, or if a strong demagnetizing field is applied at high temperatures, some magnetic domains are permanently disrupted, and the magnetism cannot fully recover even after temperature returns to normal. This is the permanent demagnetization we are most concerned about.

For ordinary NdFeB magnets, the critical temperature for irreversible demagnetization is typically between 80°C and 150°C, depending on the magnet grade. In frameless torque motors, due to their compact structure and poor heat dissipation, local temperatures can easily exceed this range during operation.

III. Why are frameless torque motors more susceptible to high-temperature demagnetization?

Compared to traditional housed motors, frameless torque motors have several "high-temperature traps":

1. Compact structure, poor heat dissipation paths

Frameless motors are embedded directly into the mechanical structure, with limited contact area between the stator and the housing, making heat extraction difficult. Many robotic joints have enclosed spaces and rely only on natural convection or simple thermal conduction, causing internal temperatures to spike.

2. High torque density means high current density

To deliver high torque within a limited volume, frameless torque motors often operate at high current densities. Copper losses are proportional to the square of the current, generating significant heat.

3. Frequent overload conditions

During startup, emergency stops, collision protection, and load-holding scenarios, motor currents can momentarily reach 2–3 times the rated value, causing rapid temperature rises.

4. Magnets are directly exposed to nearby heat sources

The rotor magnets are in close proximity to the stator windings. Heat from the stator is radiated and conducted across the air gap to the rotor, and the magnet temperature may approach or even exceed the winding temperature.

IV. Consequences of high-temperature demagnetization

Once irreversible demagnetization occurs, the motor exhibits a series of problems:

•  Torque constant decreases: For the same current, output torque drops.

• Back-EMF decreases: The motor characteristic curve changes, reducing control accuracy.

• Current increases further to maintain torque: Leading to more heating—a vicious cycle.

• System efficiency declines: Higher energy consumption, worsening battery life or overall power consumption.

• Joint rigidity weakens: For robots, positioning accuracy and load capacity are both affected.

What makes it worse is that this degradation is gradual and irreversible. Users may only notice that "the motor feels less powerful" over time, without being able to identify an obvious fault.

V. How to reduce the risk of magnet high-temperature demagnetization?

The good news is that through proper selection, design, and control strategies, the risk can be significantly mitigated.

1. Choose magnets with higher temperature grades

NdFeB magnets come in different temperature grades, commonly N, M, H, SH, UH, EH, etc. The further the letter in the alphabet, the higher the temperature capability. For example:

•  N series: maximum operating temperature 80°C

• H series: 120°C

• SH series: 150°C

•  UH series: 180°C

•  EH series: 200°C

For frameless torque motors, especially in robotic joint applications, it is recommended to select at least SH grade or higher, and where possible, UH or EH.

2. Pay attention to the "demagnetization curve" and the knee point

During motor design, the magnet operating point should stay well away from the "knee point" on the demagnetization curve. Under maximum operating temperature and maximum demagnetizing current, the magnet should still operate in the safe region. When selecting, ask the manufacturer to provide demagnetization curves and simulation data at different temperatures.

3. Enhance thermal management

• Use thermal grease or thermal pads between the stator and housing.

• Design proper heat dissipation paths, adding forced air or liquid cooling if necessary.

• Place temperature sensors inside the motor or near the windings.

• Avoid prolonged full-load operation of the motor in high-temperature environments.

4. Control current and overload duration

Limit peak current and duration via the drive to avoid prolonged overload. For applications requiring sustained holding torque, consider reducing holding current or using mechanical brakes as assistance.

5. Avoid the "stall + high current" combination

During stall, back-EMF is zero, and all current is converted to heat—this is one of the highest-risk scenarios for demagnetization. System design should avoid prolonged stall conditions or implement stall protection.

6. Regularly monitor motor condition

By monitoring parameters such as back-EMF constant, no-load current, and temperature rise, you can indirectly determine whether demagnetization has occurred. High-end systems can incorporate model-based online state estimation.

VI. Summary

High-temperature demagnetization of magnets in frameless torque motors is an "invisible but deadly" risk. It is not as obvious as a broken shaft or burnt windings, yet it stealthily robs the motor of torque and precision.

For applications such as robotics, CNC rotary tables, and direct-drive joints, spending a little more on high-temperature-grade magnets during selection, leaving extra thermal margin in design, and adding one more overload protection in control can often prevent significant performance degradation and after-sales issues down the road.