Research On Control Methods Of Variable Frequency Speed Regulation System Based On A Brushless Doubly-Fed Motor

The brushless doubly-fed machine(BDFM)is a relatively new electrical machine that has been greatly developed in recent years.The stator of a typical BDFM generally comprises two sets of windings – power winding and control winding – with different pole pairs.The two stator windings are electromagnetically non-coupled,and therefore,a specially designed rotor is necessary to complete the indirect coupling between them.The BDFM is versatile and can perform under several operation modes,wherein it exhibits certain characteristics similar to a conventional induction machine or a conventional synchronous machine.Besides,the BDFM can be used both as a motor and generator.Consequently,it is quite suitable for variable speed constant frequency generation systems and variable frequency speed regulation drive systems.This research aims to study the application of BDFM in the field of AC drive systems,and the main contents of this dissertation include several aspects as follows:Firstly,the operation principles of BDFM are analyzed,wherein the rotor speed expression under steady-state is derived,and the power allocation and power flow direction between the two stator windings are illustrated as well.A dynamic model of BDFM in power winding reference frame is introduced in order to study dynamic performance through simulation and design further control scheme.Based on the steady-state equivalent circuit of BDFM operated in synchronous mode,the characteristics of torque angle and reactive power regulation are presented.Secondly,on the basis of the above equivalent circuit,the other equivalent circuits of BDFM operated in simple induction mode and cascade induction mode are developed,whereby the mechanical characteristics of the machine under these two operation modes are analyzed in detail.As a result,the simple induction mode of BDFM is neither practical nor desirable.However,the cascade induction mode can be used to realize asynchronous starting of BDFM.In addition,once the control winding side converter fails,the motor can switch from the synchronous mode to asynchronous mode and continue running.Several asynchronous starting methods are presented based on studying the operating states of control winding side converter with the aim to meet the requirements of different applications.A simulation model of BDFM via Matlab/Simulink is built,through which the starting characteristics of BDFM by implementing different starting methods are compared.Thirdly,on the basis of mathematical model of BDFM in power winding reference frame and adopt power winding voltage orientation,the mathematical model can be simplified and a vector control scheme for brushless doubly-fed motor can be developed accordingly,whereby the decoupling control of active power,reactive power of BDFM is realized.In detail,the electromagnetic torque of BDFM can be controlled via d axis component of control winding current,while the power winding reactive power(or power winding power factor)can be controlled via q axis component of control winding current.In order to obtain the phase angle of power winding voltage required in the proposed control scheme,a software phase-locked loop is introduced.The proposed control scheme is validated through simulation.Fourthly,according to the research of magnetic field distribution in BDFM,the reason why local saturation of iron core occurs likely is studied.Local saturation results in distortion of air gap flux density,then harmonic currents are induced in the rotor windings.The harmonic magnetic fields produced by the rotor harmonic currents rotate in the machine,such that harmonic electromotive forces and currents with corresponding frequencies will be induced in stator windings accordingly.With the help of finite element method,the presented harmonic analysis is validated.Finally,an experimental rig with a 30 kW wound rotor BDFM is designed,on which a number of experiments were conducted.The experimental results verify the effectivenesses of the proposed asynchronous starting methods and vector control scheme.Through the analysis of the harmonics of the measurement stator winding currents at different speed,the previously mentioned harmonic analysis is verified.