Equivalent circuit parameters calculation of a wound rotor brushless Doubly-Fed Machine based on finite element analysis

The Brushless Doubly-Fed Machine (BDFM) has two sets of three-phase stator windings with different pole numbers and only one rotor which contains three phases windings. The two sets stator windings are named Power Winding (PW) and Control Winding (CW), but they cannot couple with each other directly. The Rotor Winding (RW) is particularly designed to couple with both of them. The BDFM can be conceived as two distinct induction machines with the same frame, intended for operation in the self-cascaded mode. Therefore a fundamental equivalent circuit, using the standard per-phase model can be developed to represent the BDFM as two connected induction machines. The equivalent circuit is a simple method of representing the steady-state performance of the machine. This method, however, does not give enough accuracy because it assumes linearity of the iron core and derived from the analytical solutions under basic simplified assumptions. Thus, many correction factors are needed to adjust the design solution to fit the testing results, and significant errors may be caused. With the development of numerical modeling techniques, the finite element analysis (FEA) is well suited for the precise prediction of electromagnetic device. A method based on the use of complex 2-D finite element solutions for accurately predicting the steady-state performance of BDFM is presented, and the saturation effect is considered by introducing a new method named frozen permeability. Based on the new method, the magnetizing inductance and leakage inductance per phase can be worked out precisely.