Design and Verification of High Power Density Electrically Excited Synchronous Machine for Traction Applications.

Doctoral thesis, 2024

The growing demand for sustainable electric vehicles (EVs) has underscored the need for alternatives to traditional Permanent Magnet Synchronous Machines (PMSMs), which rely on rare-earth materials with significant environmental and geopolitical challenges. Electrically Excited Synchronous Machines (EESMs) have emerged as a promising solution, utilizing electromagnets instead of permanent magnets. However, their design introduces complex challenges that this research has addressed through innovative engineering approaches.
This research has successfully designed, prototyped, and tested two EESMs: a 60-kilowatt model for cars and a 200-kilowatt version for trucks. Both designs have undergone rigorous analyses and experimental validation, demonstrating that EESMs are a sustainable, high-performance alternative for modern electric drivetrains.
EESMs incorporate active conductors within the rotor, creating distinctive electromagnetic design complexities. Achieving high efficiency across a broad speed range while maintaining a near-unity power factor has required precise flux control, involving optimized rotor geometry, winding configurations, and material properties to ensure both performance and reliability.
The mechanical design of EESMs poses further challenges due to substantial centrifugal forces on rotor windings at high speeds. To ensure structural integrity, robust materials and innovative topologies such as dovetail assembling, have been developed to withstand these stresses without compromising electromagnetic performance.
Thermal management is critical, as active rotor windings generate significant heat under high power densities. Advanced cooling strategies, including direct oil cooling for both rotor and stator, have been developed and validated. Rotor oil cooling has nearly doubled the machine continuous operation range, enabling high-power applications in trucks. Stator cooling strategies effectively cool laminations and slots while self-impinging oil at the machine terminals, eliminating the need for separate systems.
Although oil splashing has been considered, experiments have shown that mechanical losses due to oil viscosity become prohibitively high above one thousand rpm. Consequently, oil-level control mechanisms are necessary for higher speeds.