Analysis and simplified modelling of brushless doubly-fed induction machine in synchronous mode of operation

Similar to conventional doubly-fed induction machines (DFIMs), brushless DFIM (BDFIM) obtains variable speed operation by means of a fractionally rated converter. Due to its brushless structure, BDFIM confers higher reliability and requires lower maintenance than the conventional DFIMs. Hence, it has potential future use in offshore wind turbines. This study elaborates the most attractive mode of BDFIM operation, i.e. the synchronous mode. The BDFIM air-gap flux is complex due to the presence of spatial harmonic components with different pole numbers. To facilitate analysis of the BDFIM performance, this study aims to simplify its steady state and dynamic models. To achieve this goal, the BDFIM electromagnetic torque is analysed to induction and synchronous components. Then, considering the share of each torque components, simplifications are proposed in the steady state and dynamic models of BDFIM. Thus, the BDFIM model becomes similar to conventional DFIMs. The equivalent parameters of the proposed simplified model of BDFIM and the model of conventional DFIMs are introduced. Supplementary experiments and simulations are carried out for a prototype D180 4/8 pole BDFIM.