Numerical Analysis of Stator Magnetic Wedge Effects on Equivalent Circuit Parameters of Brushless Doubly Fed Machines

This paper studies the effects of magnetic wedges used for closing stator open slots on the Brushless Doubly Fed Machines' (BDFM) equivalent circuit parameters. The BDFM is an attractive generator solution for offshore wind power and can replace doubly-fed slip-ring induction generators. It is shown in this paper that the use of magnetic wedges, commonly used in large induction machines, reduces the stator windings magnetising currents, reflected in the values of magnetising inductances. But they also increase the leakage flux of the stator windings and hence change the series inductance in the equivalent circuit. The series inductance significantly affects the machine performance as well as the rating of its converter. 2-D Finite element analysis of a 250 kW experimental BDFM is used to investigate the effects of magnetic wedges on the machine's magnetic field distribution and how these can alter the machine's parameters values. Experimental tests have also been carried out to validate the analysis.