Will A Frameless Torque Motor Demagnetize After Long-Term Low-Speed Operation?

In scenarios such as robot joints, collaborative robotic arms, and direct-drive rotary tables, frameless torque motors are becoming increasingly common. They have no housing and no bearings; in essence, they are a "bare rotor + stator" power core embedded directly into the mechanical structure, achieving extremely high torque density and compact space usage.

However, many engineers have the same question when selecting and using them: If the motor runs at low speed for a long time, or even operates close to stall for a long time, will the magnets demagnetize?

This concern is not unfounded. The space available for integrating a frameless torque motor is often very small, and heat dissipation conditions are far worse than those of a traditional housed motor. "Demagnetization" is one of the most fatal failure modes of permanent magnet motors. Once irreversible demagnetization occurs, torque will permanently decrease, and the driving capability of the entire joint will be "discounted."

But the truth is more complicated than "yes" or "no." Demagnetization requires specific conditions, and long-term low-speed operation is precisely a high-risk scenario for triggering them.

There are two types of demagnetization: which one really matters?

The magnetism of a permanent magnet is not truly "permanent." When the temperature rises to a certain level, the arrangement of magnetic domains inside the magnet becomes unstable, and magnetic performance decreases. This process falls into two categories.

Reversible demagnetization is "temporary." When the temperature rises, magnetic force decreases; when the temperature drops back, the magnetic force recovers. This type of demagnetization has already been taken into account in motor design. As long as the temperature rise does not exceed the design limit, it usually does not cause problems.

Irreversible demagnetization is the real trouble. When the temperature exceeds the limit that the magnet can withstand, or when a sufficiently strong reverse magnetic field is superimposed at high temperature, the arrangement of some magnetic domains is permanently damaged. Even when the temperature returns to normal, the magnetic force cannot fully recover.

For commonly used NdFeB magnets, the critical temperature for irreversible demagnetization is usually between 80°C and 150°C, depending on the magnet grade. High-temperature grades can push this threshold to nearly 150°C or even higher, but at the cost of a trade-off between magnetic performance and cost.

Why does low-speed operation make it easier to "cook" the magnets?

Intuitively, one might think that a motor running at high speed generates more heat. But with frameless torque motors, the logic is exactly the opposite.

At low speed, the motor's back EMF is very low, and the drive must continuously output a relatively large current to maintain torque. All this current flows through the stator windings, and copper loss (I⊃2;R loss) is proportional to the square of the current, becoming the main source of heat. Because the motor speed is low, there is almost no air convection cooling caused by rotor rotation—many frameless torque motors have only a very small air gap between the rotor surface and the stator, and at low speed there is almost no effective air cooling.

More critically, the structure of a frameless torque motor itself is not conducive to heat dissipation. It is embedded directly into a robot joint or equipment housing, and the contact area between the stator and the external housing is limited. Many joints are also closed spaces, so heat can only be dissipated slowly through natural convection and conduction. If the joint housing itself is metal, some heat can be conducted away; but if there are other heat sources nearby (such as an adjacent drive or reducer), the local temperature may be higher than expected.

A study by Zhejiang University specifically examined the "magneto-thermal coupling" problem of low-speed, high-torque direct-drive permanent magnet synchronous motors. It pointed out that during long-term continuous load operation, temperature rise can cause reversible demagnetization of the permanent magnets, thereby affecting the motor's operating performance. Although reversible demagnetization itself is recoverable, its presence means that the magnet's operating point is moving toward a dangerous region. If the temperature continues to climb and crosses the critical point, the reversible will become irreversible.

Beyond temperature, there is another "invisible killer"

Temperature is only one dimension of demagnetization. Another factor that is easily overlooked is the armature reaction magnetic field.

When a motor passes a large current, not all of the magnetic field generated by the stator windings is used to drive the rotor. Part of it acts on the permanent magnets in a "demagnetizing" direction. During normal high-speed operation, the current is relatively small, and this demagnetizing field is also weak. But under low-speed, high-torque conditions, the current remains high, and the demagnetizing field also increases.

An IEEE study clearly pointed out that the causes of permanent magnet demagnetization include chemical corrosion, physical damage, reverse armature magnetic field, and temperature rise. The first two are usually caused by harsh environments and are irreversible; whether the latter two are reversible depends on whether the permanent magnet's operating point is below the "knee point" of its demagnetization curve.

In other words, the superposition of high temperature and a strong demagnetizing field is the real trigger for irreversible demagnetization. High temperature alone or a large current alone may not immediately cause permanent damage, but when both occur at the same time, the risk rises sharply.

Long-term low-speed operation provides exactly both conditions at once: a large current brings a strong demagnetizing field, and poor heat dissipation brings high temperature. This is why low-speed conditions require more vigilance against demagnetization risk than high-speed conditions.

Protection mechanisms: engineers have already done a great deal at the design level

However, demagnetization risk does not mean that frameless torque motors cannot be used for long-term low-speed operation. Engineering practice in the industry has already formed a fairly mature protection system.

Magnet selection is the first line of defense. For high-temperature scenarios, motor manufacturers choose high-coercivity NdFeB grades. For example, when a wafer-handling robot customer faced demagnetization risk in a high-temperature vacuum environment, the motor manufacturer selected high-temperature-resistant NdFeB magnets and designed a special heat-dissipation channel structure, ensuring that the motor's magnetic performance remained stable at 120°C. Some products also use rare-earth NdFeB magnets and can operate continuously and stably at winding temperature ratings of up to 155°C.

Protection at the drive level is equally critical. During the motor design stage, engineers use finite element analysis to calculate the magnet's "maximum demagnetization operating point," ensuring that under the harshest operating conditions, this operating point remains above the knee point of the demagnetization curve. The drive then limits the amplitude and duration of peak current to prevent demagnetization impact caused by transient overcurrent.

Heat dissipation design is the last barrier. Frameless torque motors generate significant heat when operating at low speed and high current. Industry practices include epoxy encapsulation and potting, forced air cooling, or liquid cooling systems, and even integrating heat pipes or dielectric coolant circulation into the joint housing. The potting process fills the gaps between the windings and the stator core with thermally conductive material, which both improves the heat dissipation path and enhances the insulation reliability of the windings.

Back to the question itself

So, will a frameless torque motor demagnetize after long-term low-speed operation?

The answer is: under correct design and reasonable use conditions, no. But if heat dissipation design is inadequate, magnet selection is improper, or drive protection parameters are set unreasonably, long-term low-speed operation is indeed a high-risk condition.

Demagnetization is not a fate that "will definitely happen after running for a long time"; it is a coupling problem between temperature and magnetic field. By controlling these two variables well—keeping the magnet's operating temperature far from the critical point and ensuring the demagnetizing field does not exceed the knee point of the magnet's operating point—frameless torque motors can be used safely in long-term low-speed operation scenarios.

For users, the most practical approach is: when selecting a motor, clearly inform the manufacturer of the low-speed, long-term operating conditions and confirm the magnet grade and temperature rise limit; during use, regularly monitor the motor's magnetic performance status through parameters such as the back-EMF constant, no-load current, and temperature rise; and set reasonable overcurrent protection thresholds in the controller to avoid irreversible impact on the magnets from transient large currents.

The "frameless" design of frameless torque motors brings extreme integration and torque density, at the cost of smaller heat dissipation margin and greater sensitivity to operating conditions. Understanding the boundary conditions of demagnetization is more meaningful than simply worrying about "whether it will demagnetize."