Analysis Of Electromagnetic Coupling Mechanism And Improved Design Of Cage Rotor For Brushless Doubly-Fed Machine

Brushless Doubly-Fed Machine(BDFM)has good potentials for applications in the fields of variable-frequency-adjustable-speed motors and variable-speed-constantfrequency generators because of its high reliability and low system costs by virtue of the absence of brushes and slip rings.However,the poor operation performance,such as inefficiency,still restricts the development of BDFM with cage rotor due to its special winding structure as well as complex electromagnetic coupling mechanism.The fundamental theory and key technology of electromagnetic coupling relationship and structure design remain to be solved.The method of combining theoretical analysis,finite element(FE)simulation,prototype trial and experimental research is carried out to study the electromagnetic coupling mechanism and the improved design of BDFM with cage rotor.Firstly,the basic theories of BDFM,such as the magnetic field modulation mechanism,are briefly introduced.Based on an existing BDFM prototype of the cage rotor with public bar and public end ring,2D FE model is built up by FLUX software.The distributions of magnetic field and the waveforms of stator winding current are then obtained by the transient analysis of different operating conditions of the prototype.The results show that BDFM has the characteristics of variable-frequency-adjustable-speed motor and variable-speed-constant-frequency generator.Secondly,the opinion of “winding generalized direct coupling” is proposed.The mutually-induced electromotive force(EMF)in the coils per pole per phase of a single stator winding,induced by the rotating magnetic field of other winding,is analyzed.Detailed studies are performed to analyse the effects of the pole number combinations,parallel branch numbers,winding types,winding connections and rotor winding on the mutually-induced EMF as well as the circulating current.The correctness of theoretical analysis is verified by FE analysis and experimental studies of a prototype,and then the generation mechanism and the existing criterion of winding generalized direct coupling are revealed to extend the understanding of winding direct coupling in BDFM with dual stator winding.The results provide a theoretical base for eliminating the direct coupling phenomenon.Finally,through analyzing the generation mechanism of space harmonic magnetomotive force as well as the effects of the shorted loop on the electromagnetic coupling for the cage rotor,a new cage rotor structure,called parallel-series hybrid cage rotor,is proposed and trial-produced.And two prototypes with the different rotor structures are calculated and compared for their operation performances by FE method.The results show that the stator windings current is lower,the operating efficiency and the power factor are improved significantly for the prototype with parallel-series hybrid cage rotor.The research results in this thesis enrich the basic electromagnetic theory of BDFM,provide theoretical and technical support for eliminating the generalized direct coupling phenomenon and improve the operation performance of BDFM prototype.