How Magnet Shape Affects PMSM Motor Performance?+

The shape of the permanent magnets has a direct influence on PMSM motors because it determines how magnetic flux is distributed across the air gap. Parameters such as magnet arc width, thickness, curvature, segmentation, and magnetization direction affect average torque, cogging torque, torque ripple, back EMF, efficiency, vibration, and high-speed operating capability.

A larger magnet does not necessarily mean a better PMSM. Increasing magnet volume may increase magnetic flux, but it can also increase cogging torque, back EMF, leakage flux, and local iron saturation. Therefore, PMSM magnet geometry should be optimized together with the stator winding, air gap, slot-pole combination, and inverter control strategy.

Research published in Electric Power Systems Research demonstrated that changing magnet arc width could substantially reduce cogging torque and torque ripple while maintaining practical torque output.

Magnet Arc Width

The pole-arc ratio, which represents magnet arc width relative to pole pitch, is one of the most important PMSM rotor parameters.

A larger pole arc generally increases the effective magnetic coverage and can increase fundamental air-gap flux. However, excessive magnet coverage increases interaction between the rotor magnetic field and stator-slot permeance, potentially increasing cogging torque and harmonic distortion.

Published research has shown that reducing magnet arc can significantly reduce cogging torque and total torque ripple, although excessive reduction may reduce average torque and back EMF.

Therefore, the optimum pole-arc ratio is a compromise between torque density, cogging torque, and waveform quality.

Magnet Thickness

Magnet thickness determines the available magnetomotive force and strongly affects air-gap flux.

Increasing magnet thickness can increase torque and back EMF, but the relationship is not linear because the magnetic circuit contains the air gap, stator teeth, rotor iron, and leakage paths.

Once the magnetic circuit approaches saturation, additional magnet material provides progressively smaller benefits while increasing material cost and rotor mass.

For this reason, designers normally optimize magnet thickness together with:

Air-gap length
Stator-tooth width
Rotor back-iron thickness
Magnet grade
Operating temperature
Demagnetization margin

The U.S. Department of Energy’s electric-motor R&D programs emphasize efficiency, power density, cost, weight, and reduced dependence on critical materials as important motor-design objectives.

Curved and Special-Shaped Magnets

Instead of using a simple rectangular magnet, PMSM rotors can use arc-shaped, curved, bread-loaf, stepped, or specially profiled magnets.

The primary objective is to control the air-gap flux-density waveform.

A properly optimized magnet profile can reduce selected harmonic components and consequently decrease:

Cogging torque
Torque ripple
Electromagnetic vibration
Acoustic noise
Additional iron losses

Research published by Elsevier has shown that permanent-magnet geometry optimization can reduce cogging torque while maintaining required torque output.

For IPMSM designs, magnet placement and shape can also influence reluctance torque, flux concentration, mechanical strength, and field-weakening capability.

Magnet Shape and Cogging Torque

Cogging torque results from the interaction between permanent-magnet flux and the varying magnetic reluctance caused by stator slots and teeth.

Because cogging torque exists even when the stator current is zero, it is particularly important in low-speed servo systems, robotics, positioning equipment, and other applications requiring smooth rotation.

Magnet arc optimization, segmentation, skewing, and edge shaping are commonly used to reduce this effect.

However, reducing cogging torque should not be treated as the only optimization objective, because a geometry that produces very low cogging torque may simultaneously reduce average torque or increase manufacturing complexity.

PMSM Magnet Shape Comparison

Large magnet arc	Higher flux coverage	Higher torque potential	Higher cogging torque
Small magnet arc	Lower flux interaction	Reduced cogging torque	Lower torque capability
Thick magnet	Higher PM flux	Higher torque/back EMF	Cost and saturation
Curved magnet	Improved flux distribution	Lower harmonics	More complex manufacturing
Segmented magnet	Harmonic control	Reduced losses/ripple	Assembly complexity
Optimized IPM magnet	Flux concentration	Torque + field weakening	More comp

Step-by-Step Magnet Optimization

Step 1: Define the motor requirements

Start with rated torque, maximum torque, rated speed, maximum speed, DC-bus voltage, current limit, efficiency target, and operating temperature.

Step 2: Establish the baseline rotor

Define magnet thickness, pole-arc ratio, air gap, magnet material, and magnetization direction.

Step 3: Use electromagnetic FEA

Evaluate average torque, cogging torque, torque ripple, back EMF, flux linkage, iron loss, and demagnetization margin.

Step 4: Optimize the geometry

Sweep magnet arc, thickness, curvature, and edge profile rather than optimizing only one parameter.

For preliminary studies, designers may evaluate pole-arc ratios around 0.65–0.95, but the appropriate range depends heavily on the slot-pole combination and motor topology.

Step 5: Validate at multiple operating points

A magnet design that performs well at rated speed may create excessive back EMF at maximum speed or insufficient field-weakening margin, so testing should cover the complete operating envelope.

Common Engineering Problems

Problem	Possible cause	Recommended approach
High cogging torque	Excessive magnet arc	Optimize pole-arc ratio
High torque ripple	Strong magnetic harmonics	Modify magnet profile
Low torque	Insufficient effective flux	Optimize magnet arc/thickness
Excessive back EMF	Excessive PM flux	Reduce magnet volume or optimize geometry
High iron loss	Harmonic flux	Improve magnet shape and edge profile
Rotor heating	Magnet eddy-current loss	Use segmentation and reduce harmonics

Practical Engineering Insight

The key principle is that magnet shape should be optimized as part of the complete PMSM electromagnetic system, not independently.

The same magnet geometry can produce different results when used with different slot-pole combinations, air gaps, windings, or control strategies. Therefore, simply increasing magnet size is rarely the most effective way to improve PMSM performance.

For OEM motor development, a practical design process combines analytical calculations, electromagnetic FEA, thermal analysis, mechanical validation, and prototype testing before finalizing the rotor geometry.

FAQ

Does a larger magnet always increase PMSM torque?

No. Increasing magnet volume can increase magnetic flux, but saturation, leakage flux, voltage limits, and thermal constraints can reduce the additional benefit.

Which magnet shape is best for PMSM motors?

There is no universal best shape. Curved, segmented, stepped, bread-loaf, and IPM magnet geometries can each be effective depending on the motor’s torque, speed, efficiency, and noise requirements.

Can magnet shape reduce cogging torque?

Yes. Optimizing magnet arc width, edge shape, segmentation, and skew can significantly reduce cogging torque.

Does magnet shape affect PMSM efficiency?

Yes. Magnet geometry affects flux harmonics, iron losses, torque production, back EMF, and therefore overall motor efficiency.

Is FEA necessary for PMSM magnet optimization?

For production-level design, electromagnetic FEA is strongly recommended because it can evaluate saturation, leakage flux, harmonics, cogging torque, torque ripple, and demagnetization risk.