Research On Soft Start Theoryand Method Of Frequency Converter With Countercurrent Resistance

As an important electromechanical equipment in modern industrial production,asynchronous motor is equipped with soft starting equipment in order to reduce the impact.In many soft starting modes,the smooth speed regulation of AC motor can be realized by the frequency converter.Since the starting current is proportional to the load torque,the torque can be increased by increasing the current,which can be called the most ideal soft start mode.Two topologies of AC/DC/AC converter are proposed in this paper,one is with switching feedback capacitance and another one is with countercurrent resistance.And using small capacity of thin-film capacitor to replace the large capacity electrolytic capacitance in the traditional converter,so as to reduce the volume of frequency converter and save costs.At the same time,the waveform of the small capacitance converter is changed into the six pulse DC voltage.By controlling the switching state of the power switch tube in the inverter bridge,to achieve the inverter output same as the grid voltage,so as to facilitate the output bypass switching of the frequency converter.But there is still a place to be discussed in theory.The following aspects in the soft start of small capacitor inverter are studied in this paper mainly.(1)The classification of inverter are summarized,and the composition of structure and principle of voltage and speed regulation are introduced.Then the topology of small capacitance AC/DC/AC voltage source converter are put forward,its working principle are analyzed.Then,according to the selection of DC link and large capacitance in traditional inverter,the selection and analysis of DC link parameters under the new topology are carried out,and the calculation method is given.At the same time,the power consumption of the countercurrent resistor is calculated,which shows that the energy efficiency of the motor will be reduced by 3%.However,the soft starting performance of the inverter is mainly suudied in this paper,so the converter can be switched by bypass after the motor start.(2)The fluctuation of DC side voltage of the converter with the small capacitance are analyzed,and its expression and the Fourier expansion analysis are given.Then the complete mathematical model of AC/DC/AC voltage source converter is established.It includes the mathematical model of rectifier side from the grid end and the mathematical model of the inverter side of the output terminal,and adopts the method of vector analysis.At the same time,the dynamic mathematical model of the asynchronous motor is established,which simplifies the complexity of the motor model effectively.(3)Under the control of SPWM control method,the harmonic analysis of inverter output of small capacitor inverter is carried out,and the adverse effect of output harmonic on the load motor is expounded.Furthermore,the SPWM control method which changes the amplitude of modulation wave in real time according to DC pulsating voltage is discussed.The simulation results show that the improved control method can improve the output harmonic characteristics of the inverter bridge.The total harmonic distortion rate of the output voltage is reduced by about 2%,and the total harmonic distortion rate of the output current is decreased by about 10%.At the same time,the fundamental amplitude of output voltage and current are increased.(4)The bypass switching mode of classified frequency converter is summarized,and the principle of synchronous switching is introduced,the control method of 120° square wave is analyzed,and the harmonic analysis of inverter output under its control is carried out.The results show that the harmonic content is too high.Then a control method of modulated square wave is proposed,its generation method is introduced and its principle is analyzed by vector analysis.At the same time,the inverter output under its control is also analyzed by harmonic analysis.The results show that the harmonic distortion rate is greatly reduced from 48.4% to 3.95%.When bypass switching is carried out under the control of this method,the switching point is selected as the zero crossing point of the stator current of the motor,and in order to avoid the intersection,the switching point is chosen as the zero crossing point of the stator current of the motor.The bypass switch are designed to avoid switching delay of the AC contactor.(5)On the platform of MATLAB/Simulink,the simulation model and bypasssimulation model of the small capacitance inverter with countercurrent resistance are built,and the composition and function of the important subsystem of the simulation model are introduced.At the same time,the observation results of the simulation module are analyzed,and compared with the results of bypass under the control of SPWM,the results show that the inverter output switching is carried out under the modulation square wave control.The impulse current of the motor is approximately equal to the peak current of its steady state operation,and the fluctuation of speed and torque is very small.Finally,the system design of this subject is done,including the main circuit,protection circuit and control system hardware and software.In this paper,the topology of a small capacitance converter with countercurrent resistance is proposed.According to the change rule and characteristic of DC voltage,the frequency conversion control modulation method is adopted to realize the soft start and bypass switch.In control theory,the function of frequency conversion is first realized.Secondly,the switching time gap and switching state generated by overmodulation are used to realize bypass switching function of common soft starter when it is close to power frequency,and frequency conversion similar to silicon controlled soft starter and no-disturbance switching of power frequency are realized.At the same time,it has stepless starting performance of frequency conversion and speed regulation,and realizes the integration of frequency converter and soft starting.