| With the development of China’s aviation industry,aviation AC multi-stage synchronous starter/generator has become a new development direction of aviation power system.This technology not only can reduce the volume and weight of the aircraft,but also can improve the stability and reliability of the aircraft.The key problem of the starter/generator technology is how to effectively improve the starting torque in the starting stage.In order to employ the starter/generator to start the aeroengine in a better way,the maximum torque per current(MTPA)control strategy is more suitable.In this paper,the three-phase AC excited multistage starter/generator is taken as an example to analyze its starting performance in the starting mode.However,due to the difficulties for direct measurement of the excitation current of the Main Machine(MM)in this starter/generator system,inaccurate torque calculation of the MM and the variable working condition in the starting mode,achieving the MTPA control based on this system is not easy.In this paper,the MTPA control in the multi-stage synchronous starter/generator system is studied,including the calculation of the excitation current of the MM,the calculation of inductance parameters and torque characteristics of the MM and the Online-Searching MTPA(OSMTPA)control of the MM.Firstly,the equivalent circuit of the excitation system is built,including the exciter,rotating rectifier and MM field winding.According to the equivalent circuit,the excitation current of the MM is calculated mathematically,and the characteristics of the excitation current of the MM is obtained.Considering that the large commutation inductance,the high excitation frequency,the small resistance of the field winding of the MM,the rotating rectifier might be worked in the third mode.The output performance of the excitation system under the third mode is analyzed,in addition the output performance of the half bridge rectifier circuit is higher than the full bridge rectifier circuit under the third mode is found,which will extend the knowledge of the rotating rectifier in the multi-stage synchronous starter/generator applications.An experimental platform is built and the results shows that the output current in the half bridge rectifier is higher than it in the full bridge rectifier 40%.Secondly,after obtaining the excitation current of the MM,the inductance parameters and torque characteristics of the MM are analyzed.At the initial stage of starting process,since the aeroengine is a large load,the MM will work in the heavy load operation and face a strong armature reaction in the starting stage.The distribution of the magnetic permeability of the MM will be affected,then the expression of the winding inductance is different from the ideal state,which results in that the crosscoupling effects will appear and the calculation formula of the d-axis and q-axis inductance is no longer accurate.By employing the magnetic field offset angle,the calculation formula of d-axis and q-axis inductance in dq coordinate transformation are improved,and the analytical formula of cross-coupling inductance is presented.Finally,based on these formulas,the torque equation of the MM is renovated.The experiments shows that the error of the proposed method is less than it in the traditional method.Thirdly,Based on the field current characteristics of the MM,which is nearly kept unchanged in the low speed starting mode,the OSMTPA control of the multi-stage starter generator in this mode is studied.By injecting a high-frequency square wave signal into the torque angle of the machine mathematically,the first-order partial derivative of the torque is rebuilt on-line.By a Proportion Integration(PI)control,the partial derivative of the torque is fed back to the initial torque angle,then the MTPA point can be tracked on-line.Based on the above torque equation which considering the cross-coupling effects,a Partial Parameters Approach for OSMTPA control is proposed,the experiments are carried out and the accuracy of this method is verified.In the end,the load characteristics of the aeroengine might be changing after the ignition speed,the excitation mode of the exciter may be adjusted relatively,the field current of MM will gradually change and suddenly change.A Variable-Flux OnlineSearching MTPA control of the MM under the variable flux condition is presented.Based on the on-line estimation method of the excitation current of the MM and the MM inductance parameters fitting method,the above Partial Parameters Approach is extended to variable excitation current condition.The experimental results shows that no matter the excitation current is gradually variable or suddenly variable,the proposed OSMTPA control result can match the MTPA point very well.Indicating that the proposed OSMTPA method can achieve an effective MTPA control in the multistage synchronous start/generator. |
