An Approach to Maximize Torque Density in a Brushless Doubly Fed Reluctance Machine

Brushless doubly fed reluctance machines (BDFRM) are an attractive option for megawatt-scale turbo-electric propulsion systems due to use of a partially rated power converter, reduced maintenance, and absence of permanent magnets. Conventional design approach of a BDFRM uses equal electrical loadings on both stators and an initial phase offset δ of π /2 between the two stator currents. However, this approach leads to poor torque density because the machine saturates even before the stator reaches the current density limit. This article proposes an approach to maximize the torque density by finding appropriate electrical excitations on both stator windings for chosen machine dimensions as well as for specific flux density, current density, and grid voltage limits. A single-objective optimization problem is formulated. The optimized solution establishes that an initial phase offset δ of 2π /3 produces 75% more torque as compared to the conventional δ= π /2 design. The new design allows a higher stator currents injection without saturating the machine. The results are verified through extensive simulations using a finite element model. Operating with the optimized design also leads to higher machine efficiency and better power factor on the secondary stator, thus reducing the converter rating.