Research On Multi-stage Variable Frequency Soft Start Of Mine Heavy-duty Asynchronous Motor

With the rapid development of industry,asynchronous motors have become the main power equipment,and their direct start has problems that cannot be ignored,such as large inrush current and small torque.Especially when the asynchronous motor is started in the mine,the inrush current generated by the direct start not only causes serious motor loss,but also affects the normal operation of other equipment.Although the traditional starting mode has a significant improvement on the starting current,the load capacity is limited and it is not suitable for special starting occasions such as heavy load.Based on this problem,it is necessary to study a soft start that can not only reduce the inrush current,but also increase the starting torque,and meet the requirements of heavy-load starting.Therefore,the study of practical asynchronous motor heavy-duty soft start has important theoretical significance and engineering practical value.This article first analyzes the starting characteristics of the three-phase asynchronous motor through a mathematical model,and focuses on the theories involved in the hierarchical frequency conversion.The hierarchical frequency conversion is proposed to solve the problem that soft start cannot carry heavy load.The phase sequence and phase problems of hierarchical frequency conversion are analyzed emphatically,and the optimal combination angle selection of non-positive sequence symmetrical voltage sub-bands is determined.Research on the selection method of the minimum sub-band and transition sub-band of hierarchical frequency conversion,and how each frequency band is switched,and finally determine the specific sub-band of hierarchical frequency conversion.Secondly,the control method of hierarchical frequency conversion and constant voltage frequency ratio is analyzed,and various trigger angle control strategies are studied.Based on the equivalent sine trigger,the fuzzy variable trigger angle control is proposed.The fuzzy control is used to modify the relationship between the current difference and the rate of change and the trigger angle,and the trigger angle of each frequency band that changes in real time with the input variable is obtained.The fuzzy control is used to modify the relationship between the current difference and the rate of change and the trigger angle,and the trigger angle of each frequency band that changes in real time with the input variable is obtained.The simulation results show that the fuzzy variable trigger angle control solves the torque oscillation problems in the grading frequency conversion frequency band and the switching process,and improves the starting performance of the motor.Theoretical analysis of the oscillation phenomenon during the motor starting process is carried out,and the closed-loop power factor angle adjustment control based on the gradient method is proposed for the reason of the oscillation,which can greatly reduce the torque oscillation near the rated speed.This method further optimizes the startup process.Finally,the current mainstream soft start and hierarchical frequency conversion soft start are analyzed through Matlab software modeling,and the start current,speed,and torque wave forms of the simulation model are compared.The results show that compared with other starting methods,the graded variable frequency soft start has a large initial torque and a small current,which is suitable for heavy-duty start of asynchronous motors used in mines.An experimental platform for the hierarchical frequency conversion soft start system was built,and the traditional soft start and hierarchical frequency conversion soft start modes were compared and analyzed separately,which further verified the superiority of the hierarchical frequency conversion during heavy load startup.