Optimal Torque Angle for a Switched Brushless Doubly Fed Reluctance Machine

A brushless doubly fed reluctance machine (BDFRM) offers several advantages over a single-port machine since it requires a partially rated power converter, provides reduced maintenance, and operates without permanent magnets. A conventional design approach for BDFRMs considers equal electrical loadings on both of its windings and a torque angle of $\textbf{90}^{\textbf{o}}$, which are an extension of typical constraints for uniform air-gap sinusoidal machines. However, these constraints in BDFRM sacrifice torque density. Accounting for effect of the rotor flux-barriers on the mean and ripple torque, the optimization framework proposes a candidate design having unequal electrical loadings and a torque angle of $\textbf{117}^{\textbf{o}}$ to maximize torque density. It is shown that the optimal design is capable of producing 84% more torque as compared to the conventional design approach. This article, first, shows that a torque angle of greater than $\textbf{90}^{\textbf{o}}$ is required to maximize the torque-producing component of secondary stator current using space phasors. Next, a switched-drive architecture is described that reconfigures one of the stator excitations on the fly to enable a wide-speed range operation. Online reconfiguration extends the use of BDFRM to many wide-speed range applications, such as propulsion systems, while preserving the benefit of fractionally rated power electronics. Using the switched-drive architecture can reduce the converter voltage rating by 47% as compared to the conventional-drive for the prototype BDFRM. The experimental results are provided to validate the proposed analytical approach.