Research On Circuit Structure And Control Method Of An AC-DC-AC Converter Without Electrolytic Capacitor

Three phase asynchronous AC motor has been widely used in more practical occasions everywhere because of its simple structure,convenient production,low cost.At the same time,the new electronic transistor devices and programmable control chip technology are developing rapidly.Frequency conversion technique is also used in more and more occasions.The laboratory project is funded by the National Natural Science Foundation of China(51577110),in view of the shortcomings of the traditional soft starter such as low starting torque and large starting current,combined with the theory of discrete wavelet transform and flux linkage trajectory tracking.Through a large number of simulation and experimental data,we can see that the starting torque is increased by 20%,the starting current is reduced by 21%,and the torque ripple is also significantly reduced compared with the traditional voltage regulation soft start.Through further research,it is found that based on the control theory of thyristor soft starter circuit,because the thyristor is a half-control device,it can't control its turn off,and the motor's starting performance has been improved to the maximum.Based on this research,team proposes a new topological structure based on switched feedback capacitance and counter-current resistance,adopting fully controlled devices in 2017.On this basis,the topology structure is applied to the frequency converter,which can reduce the volume,save the cost and achieve the purpose of multi-purpose machine,and the main research contents are as follows:(1)Analysis of the working principle and reactive energy feedback process of AC-DC-AC converter without electrolytic capacitor.Through the understanding of the circuit structure and working principle of the traditional AC-DC-AC converter,the working principle of AC-DC-AC converter without electrolytic capacitance circuit structure is analyzed.The difference between the two lies in the capacitance capacity of the DC bus,which leads to the change of DC voltage output to the converter bridge.According to the research of the current flow direction of reactive power feedback generated by the traditional converter with motor load,it analyses the energy flow direction of the new structure under PWM pulse width modulation based on sinusoidal law,finds out the rule and summarizes the difference between them.And,through the equivalence rule,the AC side motor output is equivalent to the DC load.Using the formula of capacitance discharge voltage and the feedback energy and comparing with the traditional frequency converter,the bus switched capacitor with the new topology can reduce the capacitance.(2)Under different control methods of the AC-DC-AC converter without electrolytic capacitance,comparing and analyzing the content of the output voltage and the content of the harmonic wave.By analyzing the control method of constant voltage frequency ratio,the method of injecting third harmonics and reconstructing SPWM modulation wave is designed.Based on MATLAB simulation software,the whole circuit structure is built,and the operation mode of AC-DC-AC converter without electrolytic capacitance is experimented.The feasibility of the new topology is judged by the waveform of the starting and stable operation of the motor at different frequency.Finally,according to the simulation waveform and FFT analysis,the running performance of the motor is compared under the different modulation ratios of the traditional SPWM,the third harmonic injection and the reconstructed SPWM modulation wave control method.(3)Software and hardware design of the AC-DC-AC converter system without electrolytic capacitance.Through the construction work rule,simulation,and combining hardware and software,the whole circuit is divided into the following parts,voltage and current detection system,signal modulate circuit,overvoltage,overcurrent and undervoltage protection circuit,drive and protection circuit,keyboard and LED display circuit and switched small capacitance in the DC circuit.According to the circuit parameters,the related calculation is carried out and the selection of the device is designed.The control code is written,and the flow chart of the system software is drawn.(4)Set up an experimental platform for AC-DC-AC converter without electrolytic capacitance and test the performance and functionality.For the traditional frequency converter,the large capacitance in parallel to the DC bus is replaced by a small capacitor.The experiment records the waveform of motor line voltage and DC bus voltage based on different power supply modes.The waveform data of the oscilloscope are used to verify the correctness of the theoretical analysis.In this paper,a new type of AC-DC-AC frequency conversion topology with small switched capacitance is adopted.The capacitance of bus capacitor can be calculated by equivalent circuit of capacitance discharge and analysis of feedback energy.In order to eliminate the influence of six-pulse wave on output voltage of DC bus,reconstructing SPWM and third harmonic injection method are designed on the basis of traditional SPWM to improve the output characteristics of motor and realize the integration of frequency conversion and soft start.