How to Choose the Right NdFeB Magnets for Electric Motors
Choosing the right neodymium magnets (NdFeB magnets) for an electric motor involves more than simply selecting a grade or looking at magnetic strength. Motor speed, operating temperature, magnetic circuit design, operating environment, and cost all affect the final choice. For permanent magnet motors in particular, magnetic performance, temperature resistance, and resistance to demagnetization must be evaluated together.
1. Determine the Motor's Operating Temperature
Temperature is one of the most important factors when selecting NdFeB magnets. As temperature rises, their magnetic performance decreases. If the magnets do not have sufficient resistance to demagnetization, they may also suffer irreversible losses in magnetic performance. Temperature capabilities vary by grade and performance class.
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Common Grade Suffix |
Typical Maximum Operating Temperature |
Key Considerations |
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N series (no suffix) |
Approx. 80°C |
Room-temperature applications and applications with low temperature rise |
|
M |
Approx. 100°C |
Moderate temperature requirements |
|
H |
Approx. 120°C |
Greater resistance to demagnetization |
|
SH |
Approx. 150°C |
Motors operating at relatively high temperatures |
|
UH |
Approx. 180°C |
High-temperature operating conditions |
|
EH |
Approx. 200°C |
More demanding high-temperature applications |
*These values are general reference points for common grades, not guaranteed limits for every product. The actual maximum operating temperature depends on the specific magnet, its operating point in the magnetic circuit, the demagnetizing field, and cooling conditions. Always refer to the supplier's datasheet and validate the selection through motor design and testing.
2. Select a Grade Based on the Required Magnetic Properties
NdFeB grades are usually referred to as N35, N42, N48, N52, and others alike. Generally speaking, the higher the number, the higher the maximum energy product (BHmax). However, it doesn’t mean that the magnet would work better in each specific motor.
- Remanence (Br): Shows how much magnetic flux the magnet will be able to produce in the external magnetic circuit.
- Maximum energy product (BHmax): Shows the magnet’s capacity to produce magnetic energy per unit volume.
- Intrinsic coercivity (Hcj): Shows the magnet’s resistance to demagnetization. It becomes critical for high-temperature operation or operation under the influence of powerful opposite magnetic fields.
An increase in resistance to demagnetization does not always accompany an increase in maximum energy product. So selecting a high-grade magnet (N52 grade and others) is not enough.
Use the Stanford Magnet Calculator to estimate the Pull Force of neodymium magnets
3. Consider the Motor Type and Operating Conditions
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Issues to examine include torque, rotational speed, dimensional constraints, heating effects, and operational stability. The grade of the magnet material should be compatible with the rotor design and the operating conditions dictated by the motor control circuit. |
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Traction Motors for Electric Vehicles Issues to be evaluated include high temperature, high opposing magnetic field, rotational speed variation, and reliability. The material selection should not be made considering only magnetic characteristics at room temperature. |
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Servo Motors and Precision Motors The parameters of torque density, torque ripple, dimensional tolerance, and dynamic performance should all be looked into simultaneously. The magnet geometry, magnetization orientation, and assembly accuracy are equally important. |
Various motors have different magnetic circuits, rotational speeds, and thermal stress levels. Thus, the proper choice of magnet material should be made for a specific motor design.
4. Don't Overlook Coatings, Magnet Geometry, and Manufacturing Precision
- Coatings: NdFeB magnets are susceptible to corrosion. In humid or corrosive environments, protective options such as nickel plating and epoxy coatings should be selected according to the actual operating conditions.
- Shape and dimensions: Arc-shaped, segment-shaped, and block magnets must match the rotor geometry and magnetic circuit design.
- Magnetization direction: Confirm that axial, radial, or other specified magnetization directions meet the motor's design requirements.
- Manufacturing tolerances: Consistency in dimensions, angles, surface quality, and magnetic properties affects assembly accuracy and motor performance.
How to Choose a Coating for Humid or Corrosive Environments
Ni-Cu-Ni (triple-layer nickel plating) is a common baseline option. It can be suitable for applications where the coating remains intact, and the level of environmental corrosion is manageable. However, edges, coating defects, and damage during assembly can increase the risk of corrosion.
Epoxy coatings and composite coatings are also worth considering when moisture exposure or corrosion risks are higher. Their coating thickness, adhesion, chemical resistance, and integrity after thermal cycling should be verified.
If there is a possibility of exposure of the motor to salt spray, coolants, oils, or solvents, the actual nature of those substances needs to be known in advance before choosing a coating.
Conclusion
A high-grade magnet does not always represent the best choice. It is the magnet that can satisfy the motor's performance and life criteria at the actual motor operating temperature, the magnetic forces acting opposite to it, and the magnetic circuit environment.
Prior to making a final selection, it should be ensured that the following parameters correspond to the chosen magnet: magnet size, magnet orientation, speed of rotation, air gap distance, required torque, and the full set of magnetic characteristics provided by the manufacturer.
