Key takeaways
- IPM rotors can combine permanent-magnet and reluctance torque through saliency. SPM is often approximated as non-salient.
- Field weakening uses negative d-axis current. Both IPM and SPM can use it within their voltage, current and demagnetization limits.
- N42SH and N45SH share the listed 20 kOe room-temperature HcJ minimum. Changing the numeric grade does not automatically change coercivity class.
- Retention depends on radius, speed, geometry and material stresses; there is no universal 12,000 rpm sleeve threshold.
- Rotor topology and grade require model-specific evidence. A brand name alone does not identify the motor design.
Overview
Overview
IPM magnets sit inside rotor cavities; SPM magnets are mounted at the rotor surface. Rotor saliency and the control strategy influence torque and field weakening. Evaluate the voltage and current limits, magnetic circuit, losses and retention together before comparing the two architectures.
Side-by-side
Side-by-Side Comparison
Reference properties and design questions. A count of comparison points does not establish a material or supplier recommendation.
| Criterion | Interior PM (IPM) Rotor | Surface PM (SPM) Rotor |
|---|---|---|
| Magnet location | Inside rotor cavities | At the rotor surface |
| Torque | Permanent-magnet torque plus a possible saliency contribution | Permanent-magnet torque; often modeled with low saliency |
| Field weakening | Negative d-axis current within motor limits | Negative d-axis current within motor limits |
| Speed range | Depends on saliency, flux linkage and inverter limits | Depends on flux linkage, inductance and inverter limits |
| Demagnetization | Check local hot curves and opposing fields | Check local hot curves and opposing fields |
| Grade selection | Specify Br and hot demagnetization resistance separately | Specify Br and hot demagnetization resistance separately |
| Retention | Analyze cavities, bridges and magnet movement | Analyze bond, support and any retaining sleeve |
| Torque ripple | Complete rotor/stator/control design | Complete rotor/stator/control design |
Confirm source conditions and the actual part requirements before using reference values as acceptance limits.
Use cases
Conditions for considering each option
When Interior PM (IPM) Rotor wins
- The saliency and magnetic design meet the required torque-speed envelope.
- The rotor cavities and bridges meet the structural and manufacturing requirements.
When Surface PM (SPM) Rotor wins
- The surface-magnet arrangement meets the torque-speed and loss requirements.
- The retention and assembly approach suit the rotor and production plan.
How to choose
Decision Framework
Decision framework
Model both candidates over the required torque-speed envelope with the proposed inverter. Include hot material curves, local opposing fields and faults. Analyze retention at the actual radius and speed. Select Br/BHmax separately from coercivity; replacing N45SH with N42SH does not by itself establish greater or lower demagnetization margin.
Specs
Related NdFeB Grades
N42SH
150°C classN42SH reference: 40–43 MGOe at 20°C, with a nominal 150°C temperature class.
Reference; drawing review required
N42UH
180°C classN42UH reference: 40–43 MGOe at 20°C, with a nominal 180°C temperature class.
Reference; drawing review required
N45SH
150°C classN45SH reference: 43–46 MGOe at 20°C, with a nominal 150°C temperature class.
Reference; drawing review required
N48SH
150°C classN48SH reference: 45–48 MGOe at 20°C, with a nominal 150°C temperature class.
Reference; drawing review required
Industries
Related Applications
EV Motors
NdFeB magnet design inputs for permanent-magnet traction motors, auxiliary drives and e-axles.
Robotics
NdFeB design inputs for robot joint motors, compact actuators and servo drives.
Industrial Automation
NdFeB magnets for stepper motors, servo drives, linear actuators, magnetic couplings, and factory automation equipment across European and North American manufacturing.
Questions
Frequently Asked Questions
How does field weakening work in an IPM motor?
+
The controller applies negative d-axis stator current to reduce the effective flux linkage and stay within the voltage limit above base speed. IPM saliency can support the torque-speed tradeoff. The relevant quantity is current, not negative inductance; SPM motors can also field-weaken.
Does SPM always have higher torque density?
+
No. Compare the complete motor at the intended speed, current, cooling and package limits. Rotor topology alone does not establish torque per amp or torque density.
Does IPM allow a lower numeric magnet grade?
+
Local fields can differ between IPM and SPM designs, but the actual hot working point must be evaluated. N42SH and N45SH share the listed HcJ minimum. Their numeric grades concern energy product, not a different coercivity class.
What magnet shape do IPM and SPM use?
+
IPM cavities often use blocks shaped to fit the rotor. SPM designs may use arcs or a one-piece ring. Provide the rotor drawing, orientation, pole pattern and retention requirements. A radial ring does not automatically give sinusoidal air-gap flux.
Does rotor topology determine the coating?
+
Exposure and assembly requirements determine the protective system. Qualify the coating with the actual adhesive, cure, fluids, temperature and retention loads; cavities should not be assumed sealed without an enclosure design.
Need help deciding?
Share your drawing, operating conditions and acceptance requirements for review. Human response within 1 business day.