Design And Starting Optimization Of Discrete Variable Frequency Soft Starting System Under Heavy Load

It is difficult for the traditional asynchronous motor starting modes to meet the requirements of the motor start under heavy load,as these modes fail to take account of starting current,starting torque,starting time and other performance indexes.On the contrary,discrete variable frequency soft start has many good characteristics such as low starting current,high starting torque and so on,which can be well applied to the heavy-load start.It is also in possession of simple topology,short starting time and other advantages.However,in the practical application,discrete variable frequency soft start exposes its shortcomings including complex control process and suboptimal starting performance,etc,so it is necessary to simplify the starting process reasonably and optimize the control strategy systematically.In summary,the research in this field has a great theoretical significance and practical value.The starting process of discrete variable frequency soft start is composed of several sub stages,according to the characteristics,it is divided into two types in this paper: single frequency operation stage and double frequency transition stage.Based on the comparison and analysis of each sub stage in the starting process above,the author puts forward two starting schemes: one is the optimal heavy-load starting scheme of dividing frequency switching to 50 Hz,the other is the optimizational starting scheme of multi-band discrete variable frequency soft start under heavy load.First of all,through the analysis of the phase sequence of discrete variable frequency,the starting frequency sequence is determined,and the optimal phase combination of each frequency band in the sequence is obtained,at the same time,the principle of selecting the trigger angles for each dividing frequency is also worked out.Secondly,according to the principle of theselection and the comparative simulation analysis of the different trigger angles obtained by each dividing frequency primaries,the optimal trigger angle combinations of 7,4,3 dividing frequency direct start under heavy load are proposed by the author.Then,combined with 3 starting modes of 50 Hz under heavy load and used the method of combinatorial analysis for optimization,the author presents 60 degree trigger angle combination of 4 dividing frequency switching to current limit soft start of 50 Hz as the optimal heavy-load starting scheme for dividing frequency switching to 50 Hz.In addition,based on the optimal trigger angle combinations of all dividing frequencies above,the author puts forward the multi-band discrete variable frequency optimizational scheme for heavy-load start by adding the trigger angles of fuzzy control in each dividing frequency period,the improvement of switching phase between dividing frequencies and the current limit soft start of 50 Hz.Finally,according to the simulation analysis of two starting schemes above and the functional requirements of discrete variable frequency soft start,the whole system platform is designed and constructed,and the experimental verification is carried out.This paper presents the optimal heavy-load starting scheme of dividing frequency switching to 50 Hz,the optimizational starting scheme of multi-band discrete variable frequency soft start under heavy load,the trigger angle selection principle and the optimal trigger angle combinations of different dividing frequencies.It effectively simplifies the starting control process,systematically optimizes each sub stage of the starting process,and also provides a reference for selecting trigger angles of all dividing frequencies,so it has a great theoretical significance and practical value.