Research On Key Technologies Of Multi-stage Brushless Synchronous Starter/Generator Based On Two-phase Exciter In The Starting Mode

With the development of large aircraft,AC power systems in aircrafts are facing more and more technical challenges.At present,multi-stage brushless synchronous machine is usually used as the generator in the AC power system of aircrafts in our country,and this generator does not have the function of starting aircraft engines.In this traditional engine-power system,a dedicated starting mechanism is used to start the aircraft engine,which makes the volume and weight larger and system more complex.The aircraft generator can operate in starting mode to start the aircraft engine first,and after the starting the generator is then driven by the engine to generate power for the aircraft electrical equipment.This aircraft start/generator system eliminates the need for dedicated starting equipment,which reduces the system volume and weight and is important for aircraft power systems.The multi-stage brushless synchronous starter/generator system faces problems of small field current for the main machine and serious electromechanical coupling between the exciter and main machine in the starting mode,so it is difficult to carry out the research on optimal starting control for the system.In order to solve these problems,key technologies of multi-stage brushless synchronous starter/generator based on two-phase exciter in the starting mode were studied in this paper,including design,analysis and control of the two-phase exciter,integrated modelling of the starter/generator,and cooperative optimal control for the main machine and exciter.Firstly,the structure and control strategy of a new two-phase exciter for the multi-stage brushless synchronous starter/generator was proposed in this paper: The two-phase exciter has two-phase symmetrical field windings,and two-phase AC excitation was used for the exciter in the starting mode and DC excitation was used in the generation mode.The operation principle of the starter/generator based on the two-phase exciter in the starting and generation modes was elaborated.Secondly,a two-phase exciter was designed based on an original single-phase exciter with its rotor structure and rotor windings unchanged.The Finite Element Analysis model of the two-phase exciter was built,and the excitation characteristics of the two-phase exciter in the starting and generation mode were analyzed,respectively,and the best excitation type for the two-phase exciter in the beginning of the start-up process was obtained.The static excitation characteristics of the two-phase exciter prototype and the original single-phase exciter were tested and compared,and the test results showed that the two-phase exciter has the advantage of high excitation efficiency in the starting mode.Thirdly,research on excitation control methods for the two-phase exciter in the starting mode was carried out in this paper to keep the main machine field current substantially constant and/or minimize the main machine armature current.Three excitation control methods for the two-phase exciter were proposed.Two-phase AC excitation was used for the exciter during the entire start-up process in excitation control Method I,and the magnetic field in the two-phase exciter and the relative speed between the rotor winding and field current were kept constant to achieve constant field current for the main machine.In excitation control Method II,which is based on Method I,two-phase AC excitation switch to DC excitation using the optimal switching method when the excitation frequency decreased to zero.In excitation control Method III,two-phase AC excitation in the same direction as the rotation direction of the rotor was used in the beginning of the start-up process,and the two-phase AC excitation switch to DC excitation gradually when the rotor speed reached a certain value.The field currents for the main machine can be kept basically constant in Method I and Method II,while Method III can achieve minimum armature current for the main machine in the beginning of the start-up process.Experimental tests for these three excitation control methods were carried out on the two-phase exciter prototype,and experimental results verified the effectiveness of these proposed methods.Fourthly,research on integrated modelling for the multi-stage brushless synchronous starter/generator was carried out in this paper.Voltage-Behind-Reactance(VBR)models of the main machine and exciter were built and connected through the rotating rectifier into the system VBR joint model.Based on this VBR joint model,improved Parametric Average Value Model(PAVM)of the rotating rectifier was proposed.And using the PAVM of the rotating rectifier as a bridge,voltage and flux linkage equations of the exciter and main machine were connected,and finally the integrated model of the multi-stage brushless synchronous starter/generator in classic state-variable formulation was obtained with field currents of the exciter,field current and armature currents of the main machine as the state variables.Computer simulation results verified that the proposed integrated model has advantages of high simulation accuracy and high simulation efficiency.Finally,research on cooperative optimal control for the main machine and exciter was carried out in this paper.A field current estimation method for the main machine from the exciter side was proposed.In this estimation method,the rotor currents and voltages of the brushless exciter were first estimated and then used to calculate the field current of the main machine through the PAVM of the rotating rectifier.Based on this estimation method,a cooperative optimal control for the main machine and exciter was proposed: The field current of the main machine was kept constant during the start-up process by closed-loop control method,and Maximum Torque Per Ampere(MTPA)control was used for the main machine to achieve optimal control for the starter/generator.Experimental results verified the effectiveness of the proposed field current estimation method and cooperative optimal control method.