As the world shifts toward electric mobility and energy-efficient systems, choosing the right electric motor becomes critical. Two major types used in electric vehicles and industrial applications are the Permanent Magnet Synchronous Motor (PMSM) and the Externally Excited Synchronous Motor (EESM). Both are types of synchronous motors, they rotate at the same frequency as the supplied current, but they differ fundamentally in how the magnetic field is generated in the rotor.
Let’s explore how they work, key differences, advantages and disadvantages, real-world applications, and future potential.
1. Working Principle
PMSM (Permanent Magnet Synchronous Motor)
- Uses permanent magnets (usually rare-earth magnets) on the rotor to produce a constant magnetic field.
- The stator creates a rotating magnetic field (via 3-phase AC), and the rotor locks into synchronisation with it.
- Since the magnets are always “on,” no external excitation is needed.
EESM (Externally Excited Synchronous Motor)
- Instead of magnets, it uses an electromagnet in the rotor, powered by a separate DC excitation current, typically supplied through slip rings or brushless excitation systems.
- This allows dynamic control of rotor magnetic flux, which is not possible with fixed magnets in PMSM.
2. Comparison Table: PMSM vs EESM
| Feature | High (magnets can demagnetise) | EESM |
|---|---|---|
| Rotor Magnetic Field Source | Permanent Magnets | Electromagnetic excitation (via current) |
| Magnet Material | Rare-earth magnets (e.g., Neodymium) | Comparable; can be optimised for conditions |
| Cost | Higher (due to rare-earth materials) | Lower (no rare-earth materials) |
| Efficiency | High (at most load conditions) | Comparable; can be optimized for conditions |
| Torque Control | Limited flux control | Variable flux – better control at high speed |
| Power Density | High | Slightly lower |
| Temperature Sensitivity | High (magnets can demagnetize) | Lower |
| Regenerative Braking | Good | Better (flux can be reduced) |
| Maintenance | Low | Moderate (depends on excitation method) |
| Size and Weight | Compact | Slightly larger |
| Software Control Complexity | Medium | Higher (needs excitation control) |
3. Advantages
PMSM Advantages:
- High efficiency due to a constant magnetic field.
- Compact and lightweight, ideal for EVs where space is a premium.
- Fast dynamic response, good for acceleration and variable loads.
- Proven tech, used widely in Tesla Model 3, Nissan Leaf, Hyundai Kona, etc.
EESM Advantages:
- No rare-earth materials, cost-effective and geopolitically safer.
- Variable excitation allows better flux weakening at high speeds.
- Improved safety, since the magnetic field can be turned off in case of faults.
- More flexibility in torque-speed control, especially in high-speed operation.
4. Disadvantages
PMSM Disadvantages:
- Rare-earth dependency, expensive and vulnerable to supply chain issues.
- Thermal sensitivity, magnets can degrade if overheated.
- Limited flux control, harder to optimise across wide operating ranges.
EESM Disadvantages:
- More complex control systems, due to rotor excitation control.
- Slightly larger and heavier, due to the excitation system and slip rings (if used).
- Potentially more maintenance if brushes or slip rings are involved (though brushless excitation exists).
5. Use Cases and Applications
PMSM Applications:
- Electric Vehicles (EVs): The most common type used today due to high efficiency and size advantages.
- Drones, Robotics, Industrial automation, appliances, servo systems.
- Regenerative braking systems in hybrids and EVs.
EESM Applications:
- Commercial EVs where cost, high-speed range, or variable load is crucial (e.g., Renault ZOE).
- Heavy industrial applications: cranes, steel mills, turbines.
- Hydrogen or alternative-fuel EVs, where avoiding rare-earth is strategic.
6. Environmental and Economic Considerations
PMSM:
- Uses rare-earth elements like neodymium, dysprosium, etc., which are:
- Environmentally harmful to mine.
- Expensive and largely controlled by a few countries (e.g., China).
EESM:
- More sustainable and eco-friendly, especially in large-scale deployments.
- Offers cost stability since it avoids critical material dependencies.
7. The Future Outlook
PMSM:
- Still dominates EVs due to mature tech and compact design.
- But automakers are exploring alternatives to reduce rare-earth reliance.
- Innovations in magnet cooling, hybrid motors (PM + WRSM), and non-rare-earth magnets may keep PMSM viable.
EESM:
- Gaining traction for next-gen EVs, especially in Europe and China.
- More automakers (BMW, Renault) are investing in EESM for cost and sustainability.
- Improved control algorithms and digital excitation systems will drive wider adoption.
8. Which One Should You Choose?
| If You Want… | Choose… |
|---|---|
| High efficiency and compact size | PMSM |
| Avoid rare-earth materials and reduce costs | EESM |
| Better high-speed performance and control | EESM |
| Proven tech with lots of industry support | PMSM |
| Long-term sustainability in large-scale use | EESM |
Conclusion
Both EESM and PMSM motors are powerful solutions in electric mobility and industrial systems, each with its strengths and trade-offs. While PMSMs are more efficient and compact, EESMs offer long-term cost and environmental advantages, especially as control technology advances. The decision between them depends on your specific use case, whether that’s vehicle design, cost priorities, sustainability goals, or performance needs.
As the EV and green energy industries evolve, a hybrid approach combining the benefits of both may become more common.
