Calculation of Core and Stray Load Losses in Brushless Doubly Fed Induction Generators

The brushless doubly fed induction generator (BDFIG) has substantial benefits, which make it an attractive alternative as a wind generator. However, it suffers from lower efficiency and larger dimensions in comparison with the doubly fed induction generator. A major part of drawbacks arises from undesirable spatial harmonics of air-gap magnetic field. Calculation of core loss is an important issue in optimal design studies to improve the performance characteristics. The iron loss is higher and has a more complex nature in BDFIGs in contrast with conventional machines. Furthermore, additional losses cannot be ignored due to a high level of spatial harmonics distortion. This paper aims to formulate core loss and stray load loss (SLL) in BDFIGs based on the design data and experimental parameters provided by electrical steel manufacturers. In addition, an analytical procedure for calculating spatial harmonic components of the stator and rotor magnetic fields is presented. The acceptable accuracy of the theoretical equations is verified by comparing the calculated core loss and SLL of a D-180 prototype BDFIG with the measured data. The results show that SLL in BDFIGs is higher than the predicted per-unit values given by standards for induction machines, e.g., IEC 60034 ~ 2 and NEMA MG1.