In the fields of new energy vehicles, industrial automation, and smart home appliances, Permanent Magnet Synchronous Motors (PMSM) are reshaping the power system landscape with their advantages of high efficiency and high power density. As a core branch of modern motor technology, PMSM has evolved into three main technical routes based on rotor magnetic circuit structure: Surface-mounted, Interior-mounted, and Hybrid. Behind these seemingly similar motors lie distinctly different electromagnetic characteristics and application scenarios. This article will unveil their technical codes for you.
1. Surface-Mounted PMSM: The Efficiency King with Simple Design
Surface-Mounted PMSM
Surface-mounted PMSM adopts a design where permanent magnets are directly attached to the rotor surface. This "externally mounted" structure gives it unique electromagnetic characteristics. When three-phase alternating current is applied to the stator winding, the generated rotating magnetic field interacts with the permanent magnet magnetic field, forming stable synchronous torque. Due to the absence of complex magnetic barrier structures, its inductance parameters exhibit isotropic characteristics, with d-axis and q-axis inductances being essentially equal (Ld ≈ Lq), making the motor control algorithm relatively simple.
Efficiency Performance
Surface-mounted PMSM demonstrates inherent advantages in efficiency. Experimental data shows that at the same power rating, its efficiency curve is typically 2-3 percentage points higher than that of interior-mounted PMSM, with the advantage being more pronounced in high-speed light-load conditions. Test data from a brand's electric vehicle drive motor shows that the surface-mounted structure achieves 96.8% efficiency at 120 km/h cruising, while the same-power interior-mounted motor achieves only 94.5%. This high-efficiency characteristic makes it the preferred solution for long-running equipment such as air conditioning compressors and servo motors.
However, the surface-mounted structure also has inherent defects. Since permanent magnets are exposed to the air gap, they face serious demagnetization risks during high-speed operation. When the motor speed exceeds the base speed, the back electromotive force rises sharply, potentially causing irreversible demagnetization of the permanent magnets. A case from a wind power converter manufacturer shows that at an altitude of 3000 meters, the temperature of permanent magnets in surface-mounted PMSM increases by 15°C compared to plain areas, with demagnetization probability increasing by 40%. Additionally, surface-mounted permanent magnets may experience mechanical deformation under high-speed centrifugal forces, limiting their application in ultra-high-speed scenarios.
2. Interior-Mounted PMSM: Performance Breakthrough with Complex Magnetic Circuits
Interior-Mounted PMSM
Interior-mounted PMSM embeds permanent magnets inside the rotor core, constructing unique magnetic barrier structures. This design breaks the magnetic circuit symmetry of traditional motors, making d-axis inductance significantly smaller than q-axis inductance (Ld < Lq), thereby generating additional reluctance torque. Tests by a new energy vehicle company show that under the same current conditions, the interior-mounted structure increases output torque by 20%-30% compared to surface-mounted structures, making it excel in scenarios requiring large torque output.
Reluctance Torque Advantage
The introduction of reluctance torque brings dual advantages: on one hand, it improves the motor's overload capability—a joint motor for industrial robots using interior-mounted design can achieve peak torque up to 3 times the rated torque; on the other hand, it extends the speed regulation range, allowing continued speed increase above the base speed through flux-weakening control. An application case for an aviation electric steering gear shows that the maximum speed of interior-mounted PMSM reaches 25,000 rpm, far exceeding the 18,000 rpm limit of surface-mounted motors.
But complex structures also bring challenges. The dq-axis inductance differences in interior-mounted motors significantly increase control algorithm complexity, requiring vector control combined with flux-weakening strategies. R&D data from a servo drive manufacturer shows that the control code volume for interior-mounted PMSM is 35% more than for surface-mounted types, with debugging cycles extended by 50%. Additionally, magnetic barrier structures increase rotor leakage flux, reducing power factor by about 5%-8%, placing higher requirements on the power grid's electrical quality.
3. Hybrid PMSM: Innovative Attempt at Balanced Approach
Hybrid PMSM
Hybrid PMSM attempts to integrate the advantages of both previous types by arranging both surface-mounted and interior-mounted permanent magnets on the rotor, achieving synergistic effects between reluctance torque and permanent magnet torque. An innovative design from a university laboratory shows that a hybrid structure combining V-shaped interior magnets with surface magnets can increase torque density by over 25%. This composite structure is particularly suitable for applications requiring wide speed regulation ranges, such as electric vehicle drive systems.
Practical Application Value
In practical applications, hybrid motors demonstrate unique value. In power system testing of a hybrid vehicle, a model using hybrid PMSM relies on reluctance torque for additional power during low-speed hill climbing, while utilizing the high-efficiency characteristics of surface-mounted structures during high-speed cruising, reducing comprehensive fuel consumption by 12% compared to single-structure motors. In the industrial field, a CNC machine tool spindle motor using hybrid design improved machining accuracy from ±5μm to ±2μm, meeting precision manufacturing requirements.
However, hybrid structures also face manufacturing process challenges. Permanent magnets with different installation methods require precise positioning and assembly—statistical data from a motor production line shows that the assembly defect rate for hybrid rotors is 18% higher than for single structures. Additionally, the magnetic field coupling effect of two types of magnets increases the complexity of electromagnetic design, requiring multi-physics joint simulation optimization.
Feature Comparison of Three PMSM Structures
| Feature | Surface-Mounted PMSM | Interior-Mounted PMSM | Hybrid PMSM |
|---|---|---|---|
| Magnetic Circuit Structure | Permanent magnets attached to rotor surface | Permanent magnets embedded inside rotor | Both surface and interior magnets |
| d/q-axis Inductance | Ld ≈ Lq (Isotropic) | Ld < Lq (Anisotropic) | Ld < Lq with enhanced effects |
| Torque Characteristics | Mainly permanent magnet torque | Permanent magnet + reluctance torque | Enhanced permanent magnet + reluctance torque |
| Efficiency | Highest (96-98% typical) | High (94-96% typical) | High to highest (95-97% typical) |
| Speed Range | Limited by demagnetization risk | Wide (good flux-weakening capability) | Very wide (optimal flux-weakening) |
| Control Complexity | Lowest (simpler algorithms) | High (requires advanced control) | Highest (complex control needed) |
| Manufacturing Cost | Lowest | High | Highest |
4. Application Scenarios for Different PMSM Types
Electric Vehicles
Interior-mounted PMSM dominates due to high torque density and wide speed range. Tesla Model 3 rear drive motor uses this structure.
Industrial Servo Systems
Hybrid PMSM is gaining traction in robot joints requiring fast response and precision control.
HVAC Compressors
Surface-mounted PMSM holds over 90% market share in home air conditioning due to high efficiency.
5. Three-Dimensional Decision Model for Technology Selection
Key Factors for PMSM Selection
When facing diverse PMSM types, engineers need to systematically evaluate based on specific application scenarios. Cost factors cannot be ignored either. Surface-mounted motors have simple structures, with material costs 15%-20% lower than interior-mounted types, suitable for mass production; interior-mounted motors, due to complex magnetic circuit design, increase mold development costs by over 30%.
Simple structure, lower manufacturing cost, ideal for high-volume applications
Higher cost due to complex magnetic circuit design and manufacturing process
Highest cost with complex assembly and specialized manufacturing requirements
6. Future Development Trends
Future development trends show that exploration of new topological structures is accelerating. Innovative designs such as Axial Flux PMSM and Transverse Flux PMSM continue to emerge—a dual-rotor PMSM prototype developed by a research institution achieves power density of 10 kW/kg, a 50% improvement over traditional structures. Meanwhile, advances in intelligent control algorithms are blurring the performance boundaries of different motor types. Deep learning-based parameter identification technology can compensate for inductance parameter changes in real-time, paving the way for widespread application of hybrid motors.
Conclusion
In this evolutionary race of motor technology, there is no absolute winner, only the most suitable solution. From the simplicity and efficiency of surface-mounted types to the performance breakthrough of interior-mounted types, and further to the balanced approach of hybrid types, each structure shines in specific scenarios. With the continuous emergence of new materials and processes, the PMSM family will continue to evolve, injecting stronger power into energy transformation and intelligent manufacturing.
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