Iron loss and saturation in brushless doubly-fed machines

The stator of a brushless doubly-fed machine (BDFM) is furnished with two sets of three-phase windings. These windings always have different pole numbers and are generally supplied at different frequencies. Investigation of the field distribution in a BDFM indicates that no polar symmetry can be associated to it. In addition the motion of the field is not a simple rotation as in a induction motor. Therefore it is not expected that the representation of saturation by means of traditional saturation factors will yield useful results. In this paper the authors describe a time-stepping finite-element model for the BDFM which can readily represent the effect of saturation. In addition, it is shown that iron losses can be calculated as the time-stepping solution proceeds. The model is verified using measurements made on a test machine. The use of saturation factors in conjunction with an analytical model is also investigated.