Investigation Of Non-sinusoidal Power Supply Control Strategy For Seven-phase Induction Machine Based On Air-gap Flux Orientation

With the increasing demand for AC motor speed control drive power,it is generally used to increase the voltage and current,but because of the limitation of the voltage value and current value for the device,increasing the number of motor phases has become a new idea to meet the demand of high power.Compare with the traditional three-phase motor drive system,multiphase motor with more control freedom,stronger fault tolerance,higher torque density and other advantages.It is more suitable for large torque,high power and high safety level requirements of the application,such as electric vehicles,rail transportation,Marine propulsion and allelectric/more electric aircraft.In this thesis,a seven-phase induction motor is taken as the research object,and the non-sinusoidal power supply control strategy is adopted to optimize the magnetic density saturation of the motor teeth and improve the torque density of the motor.The following work is carried out:Firstly,the magnetomotive force distribution of the seven-phase induction motor is analyzed,and the mathematical model of the seven-phase induction motor in the natural coordinate system is established.By extending the Clarke transform and Park transform,the motor is decoupled to three independent harmonic subplanes,which clarifies that the harmonic plane of the seven-phase induction motor can be controlled separately and produce stable electromagnetic torque.Then,it is analyzed that the tooth density of ideal flat-top wave can improve the utilization rate of the iron core and the torque density of the motor.Aiming at the problem that the non-sinusoidal power supply control strategy based on rotor field orientation cannot generate the ideal flat-top induced electromotive force,a non-sinusoidal power supply control strategy based on air-gap field orientation is proposed in this thesis.The problem of relative position migration of air gap flux in different planes under full load is optimized,the induced electromotive force waveform of the motor is improved,and the torque current ratio of the motor is increased.At the same time,in order to measure the magnetic density waveform of the motor teeth,the induction electromotive force detection coil is buried in the prototype of the seven-phase induction motor,and the sevenphase induction motor experiment platform is built.Secondly,in the non-sinusoidal power supply control strategy with open-loop air-gap flux,there will always be a deviation between the actual stator current phase and air-gap flux,and the error in magnetic field orientation will lead to induced electromotive force waveform distortion.To solve this problem,a non-sinusoidal power supply control strategy based on the full-order airgap flux observer(FAFO)is proposed in this thesis.The feasibility and stability of the closedloop control of the proposed third harmonic plane air-gap flux observer are analyzed theoretically.A full-order third harmonic plane air-gap flux observer is established,and a feedback matrix to improve the convergence performance of the flux observer is designed.Experimental results show that the control strategy can improve the synthetic induced electromotive force waveform and the torque current ratio of the motor under various load conditions.Thirdly,the above non-sinusoidal power supply control strategy based on the full-order air gap flux observer has the problem of poor accuracy of the air gap flux observer at low speed.It is found that the nonlinear voltage drop of voltage source inverter(VSI)has a serious influence on the accuracy of air gap flux observation at low speed.In order to solve this problem,a VSI nonlinear voltage drop compensation method based on the x-y synchronous coordinate system is proposed by establishing a VSI nonlinear voltage drop model in the air gap magnetic field.The adaptive law of online identification of model reference adaptive control method is designed,and through the design of FAFO feedback matrix,the system is stable under Lyapunov’s definition.Experimental results show that the proposed method can improve the performance of the nonsinusoidal power supply control strategy based on FAFO at low speed,and has an effect on the suppression of higher harmonics in the stator current.Finally,in order to further improve the accuracy of the air-gap flux observer of the sevenphase induction motor,the steady-state model of the motor at the operating point is established,and based on this model,the motor unifies parameter sensitivity equation suitable for FAFO is established.According to the operating condition of the motor,the amplitude effect and phase deviation of the parameter error on the air-gap flux of the motor can be directly calculated by this equation,and the correctness of the sensitivity analysis of the motor parameters is verified by experiments.In addition,a simultaneous identification method of constant rotor resistance based on model reference adaptive system(MRAS)in the third harmonic plane is proposed to solve the problem that rotor resistance changes with motor temperature.This method can accurately identify the stator resistance parameters of the motor under various operating conditions,ensure the accuracy of FAFO,and improve the performance of the non-sinusoidal power supply control strategy for the motor.