SmCo Stator Magnets: High-Temperature Solutions for Industrial Motors

In stator magnets (magnets mounted on the stator of a PM motor or generator), the magnetic field alternates at the electrical frequency. This induces eddy currents in the magnet material, generating heat. For a 4-pole motor running at 6000 rpm, electrical frequency is 200 Hz. Eddy current losses in solid SmCo magnets can raise magnet temperature by 30-60°C above ambient, depending on magnet thickness and conductivity.

To mitigate eddy current heating, we segment SmCo magnets into thin slices (1-2mm thickness) insulated with epoxy coating between segments. This reduces eddy current loss by 80-90%. For high-frequency applications (>500 Hz), sintered SmCo with electrical resistivity 80-100 μΩ·cm is used (compared to 50-60 μΩ·cm for NdFeB).

NdFeB magnets above 220°C (even with EH or AH grades) suffer rapid irreversible demagnetization due to diffusion of the heavy rare earth grain boundaries. SmCo, with Curie temperature of 750-800°C, shows no phase transformation below 400°C. Applications that mandate SmCo include:

Aerospace actuators: continuous 250°C operation, survival at 350°C for 30 minutes.

Downhole drilling motors (oil & gas): ambient 200°C + self-heating to 280°C.

Industrial induction heating rollers: magnet temperatures >250°C.

High-speed spindle motors (20,000-60,000 rpm): windage heating + eddy currents.

In these environments, using NdFeB would require expensive liquid cooling or result in catastrophic failure within hours.

Reversible loss: the change in Br with temperature that returns when cooled. SmCo's α (temperature coefficient of Br) is -0.03 to -0.04%/°C, meaning a 200°C rise reduces Br by 6-8%. This loss is regained at room temperature.

Irreversible loss: permanent loss due to metallurgical changes or partial demagnetization. For SmCo below 350°C, irreversible loss is negligible (<0.5% per 1000 hours). For NdFeB at 200°C (grade N40UH), irreversible loss can exceed 10% per 1000 hours.

Selecting the correct SmCo grade: SmCo 2:17 (e.g., Sm2Co17, grades 26-32 MGOe) has higher Br and better thermal stability than SmCo 1:5 (18-22 MGOe). For continuous 300°C, use Sm2Co17 with Hcj > 25 kOe at 20°C, derated to >12 kOe at 300°C.

Motor thermal design must account for:

Maximum continuous magnet temperature (ambient + temperature rise).

Peak short-term temperature (overload or stall conditions).

Cooling method (air, water jacket, oil spray, or passive).

We provide a grade selection matrix:

For industrial motor manufacturers requiring SmCo stator magnets or complete rotor assemblies with SmCo arcs, please visit our Stator Magnet and Samarium Cobalt Magnets pages on our website. We supply fully segmented magnets to reduce eddy current losses.

To request a thermal simulation and grade recommendation for your motor – including expected ambient temperature, duty cycle, and cooling method – contact our motor engineering team.

Q: Can SmCo magnets be machined to tight tolerances like ±0.03mm?
A: Yes, but SmCo is harder and more brittle than NdFeB. We use diamond grinding wheels. Expect 10-15% higher machining cost than NdFeB. Tolerances ±0.05mm are standard; ±0.02mm available with premium pricing.

Q: Do SmCo magnets require coating for corrosion protection?
A: SmCo has excellent corrosion resistance in dry or mildly humid environments. For salt spray or chemical exposure, we recommend a thin Ni-Cu-Ni (10-15μm) or parylene coating. Most industrial motors with sealed housings require no coating.

Q: What is the lead time for custom SmCo arc segments (OD 80mm, ID 70mm, length 50mm)?
A: 15-20 working days for prototype quantities (10-50 pcs). 25-30 working days for production (>500 pcs). Tooling cost: $800-1200 for segment die.