Study On The Low Frequency Oscillation And Torque Ripple Of The Induction Motor Fed By An Inverter

This dissertation is devoted to the study on the low frequency oscillation and torque ripple of the induction motor variable frequency speed adjustment system that is fed by an inverter for the theoretical research and practical application. There are five parts are as follows:In part one, the concept of switching function is introduced first to establish the nonlinear mathematical model of the induction motor variable frequency speed adjustment system which is fed by a sine pulse width modulated (SPWM) inverter and takes the effect of the main magnetic circuit saturation into consideration, then the low frequency oscillation of the system is simulated according to the model. Next, from the view point of energy conversion of the inverter-induction system, a criterion for the low frequency oscillation is proposed to determine whether the system is in low frequency oscillation which is judged by whether the interval of the negative current component of the inverter input current is more than 1/fc(fc is the carrier wave frequency of the inverter) or not. At last, the practicability and validity of the model and the criterion is verified by the experiments.The chaotic movement of the system is clarified comprehensively in the second part. The nonlinear mathematical model for the inverter induction motor system is set up taking the effect of the dead time of the inverter into consideration. Based on the phase-space reconstructed theory of the chaotic kinetics, the nonlinear time lag method is used to determine the way to chaos, the attractors are reconstructed from steady state to chaotic state, and the essential characteristics of chaos are clarified. In addition the H.kantz method is applied to analyze the time series of the motor current, the maximal Lyapunov exponent calculation is realized, the criterion for the motor chaos is brought forward, the existence of chaotic phenomenon in the induction motor is verified by the characteristic exponents, which proves theoretically that the chaotic movement is one of the reasons that results in the low frequency oscillation of the induction motor system.The third part of the work concerns the main influencing factors of the system low frequency oscillation, the small signal mathematical model of the inverter induction motor is established in the stator reference coordinate, effects of the motor parameter on the system low frequency oscillation areresearched. Then, by the analysis of the inverter nonlinear working characteristic, the internal relationship between the inverter dead time and the low frequency oscillation is studied and clarified, and the effects on the system low frequency oscillation are taken into account, which are produced by the dead time and change of carrier wave frequency of the inverter. Finally, the rules used to design the induction motor and set the inverterV/f curve are summarized to improve the stability of the system movementIn the forth part of the dissertation, three measures are brought forward to eliminate the system low frequency oscillation. The first measure adopts pulse based dead time compensation to suppress the oscillation. As second measure, considering the motor and inverter as an integration, the discontinuous space vector modulated strategy named DPWM3 is proposed for the first time to suppress the oscillation. The third measure is adjusting parameters of the induction motor to suppress the system low frequency oscillation, and the result is verified by the experiments.The aim of the fifth part is to research how to reduce the torque ripple when the system is fed by two kinds of inverters, one is the ideal inverter, another is the inverter considering the dead time. The inverter is arranged to run in different modulated strategies as SPWM , SVPWM , THIPWM ,MTPWM, so as to seek the optimal PWM modulated strategy for reducing the torque ripple.