Time-harmonic finite element model for brushless doubly-fed induction machine in natural operating mode

Abstract

The natural mode of operation for the brushless doubly-fed induction machine is a particular instance of synchronism at a so-called natural rotor velocity when one stator winding is powered by an AC and the other by a DC voltage source. Consequently, in addition to the rotating magnetic field, there exists a magnetic field that is fixed to the stator frame of reference. Analysis in this specific mode is essential as the natural velocity arises from the choice of pole numbers, thereby determining machine efficiency. However, this presents a significant challenge when it comes to mathematical modeling using complex-valued steady-state models through either equivalent-circuit or finite element analysis. This paper presents a study on the extension of the recently-proposed steady-state complex-valued finite element model for the brushless doubly-fed induction machine to enable its application in the natural operating mode. A high correlation with the data obtained from a time-stepping model is obtained for the extended model when subjected to both low and high levels of saturation of the magnetic circuit. This extension makes the whole approach applicable in all operating conditions and modes of the brushless doubly-fed induction machine. Considering the nearly two orders of magnitude lower computational costs associated with analysis via the proposed model compared to time-stepping analysis, it is particularly useful in scenarios that involve extensive computations and require multiple cases to be considered such as design sensitivity analysis, topology optimization or a connection with machine learning techniques.