Brushless doubly-fed reluctance machine drive for turbo-electric distributed propulsion systems

Turbo-electric distributed propulsion systems are considered to be a critical enabler for low-carbon emission in the aircraft industry. A brushless doubly-fed reluctance machine (BDFRM) is an attractive option to drive the distributed propeller fans for these megawatt-scale turbo-electric propulsion systems due to use of a partially-rated power converter, reduced maintenance, and absence of permanent magnets. This thesis investigates the torque production in BDFRM, discusses machine modeling and drive control architecture, and reports on an initial sizing of a 1.5 MW motor. However, the BDFRM has inherently poor torque density because of machine saturation, even at low current-density, that offsets all the benefits. This thesis proposes an approach to maximize the torque density by finding appropriate electrical excitations on the two stator windings for a given machine dimension while remaining within flux- and current-density limits. A single-objective optimization problem is formulated. The obtained results prove that while designs with equal electrical loadings on both stators, and an initial current phase offset of pi/2 between the two stators, may seem a good design approach, they are far from optimal. Our optimized solution establishes that the phase offset of 2pi/3 provides maximum torque capability for an identical dimension. This procedure is validated using FEA simulations. Operating with this design also leads to higher machine efficiency and better power factor on the secondary stator, thus reducing the converter rating. Finally, the thesis concludes with a summary of findings and suggestions for future work in this field