Research On Characteristic Analysis And Optimization Of A Soft Starter Based On Space Voltage Vector

AC asynchronous motors are the main industrial power source.However,when they are started directely,the start current is 5~8 times the rated current,which has adverse effect on the motor and the power grid.The effect of the soft starters by voltage regulated for reducing the start current is not ideal.The torque ripple of soft starter by Cycle-wave Discrete Variable Frequency(CDVF)is large.So the problem of asynchronous motor start has not been solved well.Therefore,the research on high-performance control mode of soft start is beneficial for the better application of asynchronous motor in industrial production.Supported by the National Natural Science Foundation of China(51577110),to research the control mode of soft start,in this subject,the control principle and control characteristics of the Space Voltage Vector Variable Frequency(SVVF)soft starter are analyzed and the control process is optimized.The main work are summarized as follows:(1)Principle analysis of space voltage vector supplied by sinusoidal power and control method research on frequency division.Aiming at the difference between the theory of space voltage vector in AC-AC and AC-DC-AC frequency converte,the six basic space voltage vector instantaneous functions are determined,according to the conduction characteristic of three-phase thyristor.The curved hexagon similar to circular is obtained by integrating operation.It proves that the principle of space voltage vector supplied by sinusoidal power applies to the soft start control of asynchronous motors.With the principle of space voltage vector supplied by sinusoidal power,the frequency division control method is researched and the voltage conduction waveform under the CDVF control is analyzed.The differences and connections between the SVVF and the CDVF control are summarized.(2)Analysis on the voltage harmonic and harmonic torque under the control of the SVVF and the CDVF soft starterBy the Fourier analysis of the control voltage under the SVVF and the CDVF soft starter,the three-phase equilibrium of the fundamental voltage is discussed.The influence of harmonic torque is analyzed,combining the harmonic content analyzed with the torque characteristic curve.The result shows that the operating state of asynchronous motors under the two kinds of control is mainly determined by the fundamental torque but interfered by the harmonic torque.By the comparison between the two kinds of control,it is found that the control voltage harmonic and torque ripple caused by the harmonic of the SVVF soft starter are decreased,compared with the CDVF control.(3)Comparison and analysis of start torque and start current under the SVVF soft starter and the voltage-regulated soft starter.Combining the analysis on the equivalent circuit of asynchronous motor,characteristic curves are drawn,which describes the electromagnetic torque and stator current trajectory in the asynchronous motor start process.The start torque and the start current are compared between the power frequency voltage-regulated stage of SVVF soft starter and the direct voltage-regulated start.The result proves that compared with the direct voltage-regulated start,the SVVF soft starter can promote the start torque and reduce the start current.(4)Frequency switching process optimization and simulation verification of the SVVF soft starter.Aiming at the problems of asynchronous motor’s large amplitude speed oscillation and negative torque in the frequency switching process of the SVVF soft starter,the selection of frequency switching time is optimized,according to the regular of vector action during the SVVF control period.At the same time,the initial value and the final value of the trigger angle are selected by the load condition and the principle of constant V/F,so the trigger angle control is optimizated.MATLAB is used to simulate the optimization scheme.The simulation results show that the speed of asynchronous motor controlled by SVVF soft starter can be risen smoothly and there is no speed dumping and negative torque after optimization.The characteristic analysis results are verified by simulation.The simulation results indicate that contrasting to the CDVF control,the torque ripple effect is reduce by 17% and 11% under the SVVF 7 and 4 frequency division control.And the start current of the asynchronous motor controlled by SVVF soft starter is decreased by 21%,compared with the voltage-regulated soft starter.The soft starter experiment platform is built to test.The experimental results verify that the speed oscillation amplitude of the asynchronous motor controlled by the SVVF soft starter is reduced,in the frequency switching process,after frequency switching process optimization.Therefore,the optimization scheme is feasible.Compared with the direct voltage-regulated soft start,the SVVF soft starter can reduce the start current by 20% or so,when motor load of asynchronous motor controlled is larger.To sum up,compared with the CDVF soft starter,the torque ripple situation of SVVF soft starter is improved,and compared with the voltage-regulated soft starter,the start current decreasing characteristics of the SVVF soft starter is promoted.