Servo motors require magnets that provide high torque density, accurate positioning, low rotor inertia, stable performance, and reliable operation under repeated acceleration and deceleration.
Sintered neodymium-iron-boron magnets are widely used in AC servo motors, permanent-magnet synchronous motors, robotic joints, CNC equipment, automation systems, and precision motion-control applications.
Servo motors require magnets that provide high torque density, accurate positioning, low rotor inertia, stable performance, and reliable operation under repeated acceleration and deceleration.
Sintered neodymium-iron-boron magnets are widely used in AC servo motors, permanent-magnet synchronous motors, robotic joints, CNC equipment, automation systems, and precision motion-control applications.
However, selecting a servo motor magnet involves more than choosing the highest NdFeB grade. Engineers must consider:
The correct magnet is the grade and geometry that deliver stable motor performance under the actual electrical, thermal, and mechanical operating conditions.
To select an NdFeB magnet for a servo motor:
N52 is not automatically the best grade for a servo motor. In high-temperature or high-current applications, a grade with higher intrinsic coercivity—such as H, SH, UH or EH—may provide better long-term reliability.
NdFeB magnets provide a high magnetic energy density compared with many other permanent-magnet materials.
Their advantages include:
These characteristics are particularly valuable in servo systems requiring accurate control, compact construction, and rapid changes in speed or direction.
Typical applications include:
| Selection factor | Why it matters |
|---|---|
| Remanence, Br | Influences air-gap flux and torque potential |
| Intrinsic coercivity, Hcj | Determines resistance to irreversible demagnetization |
| Maximum energy product, BHmax | Indicates magnetic energy density |
| Maximum magnet temperature | Affects grade selection and magnetic stability |
| Magnet geometry | Influences flux distribution, torque ripple and assembly |
| Dimensional tolerance | Affects air-gap consistency and rotor balance |
| Magnetization direction | Must match the rotor’s pole design |
| Eddy-current loss | Can increase rotor temperature at high electrical frequency |
| Coating | Protects NdFeB against corrosion |
| Retention method | Prevents magnet movement at high speed |
| Batch consistency | Supports stable motor torque and control accuracy |
Before selecting a magnet grade, define the motor’s operating targets.
Important parameters include:
A servo motor that operates continuously at moderate torque has different magnetic requirements from a motor that repeatedly delivers high peak current during rapid acceleration.
Peak current creates a stronger armature-reaction field. If the magnet does not have enough coercivity, this opposing field may cause partial irreversible demagnetization.
The magnet should therefore be evaluated under both continuous and worst-case transient conditions.
NdFeB grades are commonly identified by names such as:
The number is related to the material’s maximum energy product, while letters such as H, SH, UH and EH indicate progressively higher coercivity and temperature capability.
A higher numerical grade may provide greater magnetic output at room temperature, but it does not necessarily provide the best resistance to heat or demagnetization.
Servo motor selection should compare:
For demanding motors, the demagnetization curve at the maximum operating temperature is more useful than a room-temperature grade name alone.
The temperature around a servo motor magnet may be higher than the temperature measured on the outside of the housing.
Heat may come from:
Common NdFeB temperature classes are often described approximately as follows:
| NdFeB category | Common reference temperature |
|---|---|
| N | Around 80°C |
| M | Around 100°C |
| H | Around 120°C |
| SH | Around 150°C |
| UH | Around 180°C |
| EH | Around 200°C |
| AH | Up to approximately 220–230°C for certain materials |
These values are general material-category references, not guaranteed operating limits for every magnet.
Actual temperature capability also depends on:
The grade should be confirmed through magnetic-circuit analysis and testing at the maximum magnet temperature.
Servo motors may expose magnets to strong opposing fields during:
Insufficient intrinsic coercivity may result in:
A motor may continue to run after partial demagnetization, but its performance and efficiency may decline.
The engineering team should analyze the magnet’s working point under the combined worst-case temperature and demagnetizing field.
Common servo motor magnet shapes include:
Arc magnets are widely used in cylindrical rotors.
Advantages include:
Important parameters include inner radius, outer radius, arc angle, axial length, thickness and magnetization direction.
Block magnets may be used in segmented surface-mounted or embedded rotor structures.
Advantages include:
The rotor or sleeve must position each block accurately.
One-piece or multi-pole ring magnets may support compact rotor designs and reduce individual assembly steps.
However, large high-performance sintered NdFeB rings can be more difficult to manufacture, magnetize and control dimensionally.
Interior permanent-magnet motors place magnets inside rotor slots.
This structure can provide:
It also requires careful control of bridge thickness, leakage flux and local demagnetization risk.
The magnetization direction must match the rotor’s magnetic circuit.
Possible directions include:
For an arc magnet, the magnetization direction cannot be assumed from its shape alone.
The drawing should clearly show:
Incorrect magnetization may reduce torque, increase cogging, or make the rotor impossible to assemble correctly.
Pole-arc ratio and segmentation can influence:
Segmenting a large magnet can help reduce magnet eddy currents, especially in high-speed motors or motors with high electrical frequencies.
Possible solutions include:
More segments may reduce certain losses but increase assembly complexity, dimensional variation and production cost.
Electromagnetic simulation should determine whether segmentation provides enough performance benefit for the intended motor.
The air gap has a major influence on servo motor performance.
A smaller and more consistent air gap can support:
However, an excessively small gap may increase the risk of mechanical contact because of:
Important magnet dimensions may include:
For high-speed servo motors, tight control of magnet dimensions also supports rotor balance and reduces vibration.
Although NdFeB magnets are permanent magnets, they are electrically conductive. Time-varying magnetic fields can induce eddy currents inside them.
Eddy-current loss may increase with:
Possible consequences include:
Eddy-current loss can sometimes be reduced through magnet segmentation, electrical insulation between segments, optimized stator design or adjusted control strategies.
High-speed motors should evaluate this loss during electromagnetic and thermal simulation.
Sintered NdFeB contains iron and can corrode when exposed to moisture or aggressive environments.
Common protective systems include:
Coating selection depends on:
For motor applications, coating quality must also be maintained at magnet edges and corners, where damage is more likely during assembly.
Coating alone may not be sufficient in severe environments. Sealing, encapsulation or rotor overmolding may also be required.
Magnets in a rotating servo motor experience centrifugal force, vibration, thermal cycling and electromagnetic force.
Possible retention methods include:
Adhesive selection should consider:
For high-speed rotors, adhesive should not be the only retention mechanism unless the design has been fully validated.
The complete rotor should undergo mechanical stress analysis and overspeed testing.
Small differences in magnet weight, thickness or position can create rotor imbalance.
At high speed, imbalance may cause:
Quality control should cover:
Critical high-speed applications may also require nondestructive inspection of sleeves, bonds or rotor structures.
Servo systems depend on accurate and repeatable torque output.
Variation in magnet performance can cause:
Buyers should define acceptance requirements for:
The inspection method must be agreed upon because magnetic measurements depend on the fixture, distance and test equipment.
The lowest-priced magnet may increase total motor cost if it causes:
A better comparison includes:
| Cost factor | What to evaluate |
|---|---|
| Magnet material | Grade, coercivity and temperature performance |
| Machining | Shape complexity and dimensional tolerance |
| Coating | Corrosion resistance and adhesive compatibility |
| Magnetization | Direction, pole accuracy and fixture requirements |
| Assembly | Segmentation, positioning and retention |
| Inspection | Magnetic, dimensional and appearance testing |
| Reliability | Demagnetization, corrosion and mechanical risk |
| Supply | Capacity, traceability and material stability |
The most suitable magnet is the one that provides repeatable motor performance at an acceptable total cost.
| Servo motor condition | Magnet-selection priority |
|---|---|
| Compact low-temperature servo | High energy density and dimensional accuracy |
| Industrial robot joint | Torque density, low torque ripple and stable batch performance |
| High-peak-current servo | High intrinsic coercivity and demagnetization resistance |
| High-temperature servo | SH, UH or EH-class material evaluation |
| High-speed spindle motor | Eddy-current control, segmentation and mechanical retention |
| Precision positioning motor | Pole consistency, air-gap accuracy and low cogging |
| Corrosive environment | Improved coating, sealing and adhesive compatibility |
| Cost-sensitive automation motor | Balanced grade, manufacturable geometry and stable supply |
This table provides selection priorities, not final grade recommendations. The complete magnetic circuit and operating conditions must be evaluated.
Provide the following information when requesting custom servo motor magnets:
If the magnet grade has not been selected, provide the motor operating conditions and magnetic-circuit design. A magnet supplier can then recommend candidate grades for simulation and sample validation.
Application: Industrial robot servo motor
Rotor type: Surface-mounted permanent-magnet rotor
Magnet shape: Sintered NdFeB arc segment
Candidate grade: ___
Maximum magnet temperature: ___°C
Maximum speed: ___ rpm
Peak current and duration: ___ A for ___ seconds
Dimensions: According to approved drawing
Magnetization direction: Radial / parallel / customized
Coating: ___
Dimensional tolerance: ___
Magnetic requirement: Br ___; Hcj ___; flux ___
Retention method: Adhesive plus sleeve/banding
Inspection: Dimensions, polarity, flux, coating and appearance
Sample quantity: ___ pieces
Annual demand: ___ pieces
AIM Magnet develops custom NdFeB magnets and rotor magnetic assemblies for servo motors, high-speed motors, industrial robots and intelligent motion systems.
Engineering support can include:
A motor magnet project should begin with the operating conditions and rotor design—not only a request for an N52 arc magnet.
Send AIM Magnet your rotor drawing, maximum magnet temperature, speed, peak current, pole configuration and annual demand for an engineering review.
There is no single best grade. The correct choice depends on torque, air gap, magnet dimensions, maximum temperature and opposing magnetic field. High-temperature or high-current motors often require greater intrinsic coercivity rather than the highest room-temperature energy product.
N52 may be suitable for compact motors operating under controlled temperatures and demagnetizing fields. It may not be suitable for motors exposed to high temperature, field weakening or strong peak-current demagnetization.
These letters generally represent different coercivity and temperature categories. H, SH and UH grades provide progressively greater resistance to high-temperature demagnetization, although exact performance must be confirmed from the supplier’s datasheet and demagnetization curves.
Arc magnets fit cylindrical rotors and can provide a suitable air-gap field, while block magnets may simplify manufacturing and segmentation. The better option depends on rotor design, cost and electromagnetic performance.
Segmentation can reduce magnet eddy-current loss and provide design flexibility. It may also increase assembly complexity, so its value should be confirmed through simulation.
Nickel-copper-nickel and epoxy are common options. The correct coating depends on corrosion exposure, temperature, adhesive, insulation requirements and dimensional tolerance.
It may be sufficient in some validated designs, but high-speed rotors often require an additional sleeve, banding, slot or mechanical retention system.
Select sufficient intrinsic coercivity, analyze the magnet’s load line, control temperature, avoid excessive opposing fields, and validate the motor under peak-current and fault conditions.
Recommended validation includes dimensional inspection, magnetic-property testing, polarity verification, coating inspection, thermal cycling, peak-current testing, overspeed testing, dynamic balancing and complete motor-performance testing.
Selecting NdFeB magnets for servo motors requires more than choosing the strongest available material.
A reliable selection process should evaluate:
The correct magnet should maintain stable flux, torque and mechanical integrity under the motor’s actual thermal, electrical and high-speed operating conditions.
Prototype evaluation, electromagnetic simulation, thermal analysis and complete rotor testing are essential before mass production.
