Investigation On The Harmonic Field Analysis Of Brushless Doubly-Fed Machines

Brushless Doubly-fed Machine (BDFM) consists of two different pole numbers stator windings and a special structure rotor. The machine can operate in varieties of modes, obtaining the characteristics of asynchronous and synchronous machines. In the steady state, control winding delivers a small part of the rated power and it’s brushless, having the advantages of high reliability, adjustable power factor, small rated converter and low cost, which means the machines is suitable for adjustable speed drivers and variable speed constant frequency generators, especially for off-shore wind power generation systems. However, due to its high contents of harmonics and low power density, there is few commercial applications of BDFM. In order to deal with these problems, the winding function harmonic analysis method is proposed to give a direct insight into harmonic rejection method and motor performance improvement.This paper analyze the operation principle of nested loop rotor BDFM, and give a detail insight into the operation modes, furthermore, point out the drawbacks of operating BDFM under synchronous speed. Base on the math model under d-q frame, the Matlab models of BDFM are similated. The process and concept of BDFM electronic and magnetic circuits are designed, which containing stator pole match and winding design Furthermore, winding function method and electromagnetic-circuit coupling method are used to calculate BDFM’s parameters in Maxwell2D’s transient field.To the high content of harmonics of BDFM, winding distribution function is applied to analysis of stator and rotor harmonics. The results indicate that the magnitudes of harmonics are inversely proportion to harmonic orders and closely related to winding distributions function coefficients, which gives a direct insight into the optimal design of rotor structure.Based on the analysis and design methods, an example of3/1pole BDFM is designed, whose stator winding consists of two layer conductors. The influence of different rotor bars distribution on the harmonic contents is analyzed, and an optimal design is proposed to reject harmonics. In the end, Ansoft based FEA method validates the theory and reach the conclusion that inner structure of rotor bar is more attractive in the aspect of harmonics rejection compared to the outer structure and equal distance distribution. The results confirm the value of harmonic analysis to rotor optimal design again.