Research On Brushless Doubly-fed Machine:Magnetic Field Theory Of Tooth Standing Wave

Brushless doubly-fed machine(BDFM)is evolved from two induction machines with cross-connected rotor windings on the same shaft.It can operate in the asynchronous and synchronous mode as a generator or a motor according to the connection method of control winding.Two magnetic fields produced by stator windnings with different pole pairs will rotate oppositely with respect to rotor in the air-gap.Because of the reduced capacity requirements of converters and elimination of slip ring and brush,the BDFM has become an attractive alternative to the doubly-fed induction machine in variable-speed constant-frequency generation systems and variable-speed drive applications.This dissertation aims to study the variation law of resultant magnetic field and to investigate the electromagnetic performance of the BDFM.These researches may solve problems in design and analysis of the BDFM and promote its industrial popularization.Main contents and innovations of this dissertation are as followed:1.The "tooth standing wave" feature of the resultant magnetic field in a BDFM is proposed.The variation law of resultant MMF is found to be similar with the standing wave through summation of analytical equations for two magnetomotive forces(MMFs)with different pole pairs and opposite revolution directions.The special distribution of magnetic field is further verified by finite element(FE)method.2.Based on the feature of tooth standing wave,this disseration investigates the structural design of the BDFM.Harmonic analysis results show that the novel wound rotor which accords with the principle of tooth harmonic wave can eliminate undesired harmonics and maintain constant proportional relationship with different load conditions.Therefore,the wound rotor,which is suit for the magnetic field of tooth standing wave,is one of ideal rotor structures for the BDFM.3.In the steady state,the principle of tooth standing wave is applied to research the origin of unbalanced magnetic pull and electromagnetic force.The pole number combination of |pp-pc|=2k is derived to form a symmetric magnetic field with respect to rotor center.Through FE simulation,the proposed combination can toughly eliminate asymmetrical magnetic forces.In addition,results from no load experiments and full load simulations are compared with data calculated by analytical methods.The good agreement validates the proposed torque angle expression and the torque angle curve.4.This dissertation established a unified dq model of the BDFM to study the transient behavior.Three-phase symmetric short circuit currents of the BDFM are calculated by analytical method and then are verified by dynamic simulation and experimental tests.The small impulse current demonstrates that the BDFM has a good ability of low voltage ride through.5.The application of the BDFM in stand-alone power generation systems is explored by experimental tests.Although decoupled voltage control strategy shows fast dynamic response and high control precision,the unexpected voltage amplitude fluctuation still exists in a certain speed range with specific loads.After analyzing the formation mechanism of harmonics,a voltage compensation method is proposed to eliminate the amplitude fluctuation.