A new stator back iron design for brushless doubly fed machines

The brushless doubly-fed machine (BDFM) has been under investigation in its modern forms for 50 years. The reason for this interest is the machine's ability to operate as a brushless variable speed motor or generator through the use of a partially-rated power electronic converter connected to its second stator winding. Interest has increased in the last 20 years because of greater penetration of wind turbines with Doubly-Fed induction Generators (DFIGs) where brush gear and slip-ring maintenance and reliability have been a major issue. The BDFM appears in two forms, the brushless doubly fed induction machine (BDFIM), with two stator windings and a third closed rotor winding or the brushless doubly fed reluctance machine (BDFRM), with two stator windings and a reluctance rotor with no winding. This paper sets out to provide a rotor-centric view of BDFM operation, which shows the basis of its rotating flux pattern, aligning it with the known Natural Speed, and clarifying the synchronous and induction modes of operation of the BDFIM and the synchronous BDFRM. Based on rotor-centric view of the BDFM, it is shown that the conventional design methods for the BDFM stator back iron can be modified, leading to a lighter and smaller machine. The proposed design concepts are supported by analytical methods and their practicality is verified using 2-D Finite Element (FE) modeling and analysis of three experimental BDFMs.