Analysis Of Thermal Loads In Flat Forming Coil During Repeated Discharges Process

Recently,the applications of electromagnetic forming technology have been developing rapidly,while the limited and unpredictable lifetime of coil winding has become an essential problem with the application of electromagnetic forming.Partial deployed electrical energy is lost because of joule heating during the electromagnetic forming process,which will inevitably cause the rise of coil temperature and have negative effects on the physical characteristics of insulating material in the working coil.Therefore,the research of thermal loads and temperature distribution of flat forming coil during the electromagnetic forming process can be a guidance to coil design.In addition,the research of thermal loads in repeated discharges is beneficial to summarize and formulate reasonable discharge strategy for the industrial applications of electromagnetic forming.A finite element simulation model of flat forming coil in the pulse discharge process has been established based on COMSOL Multiphysics.This paper not only analyzes the characteristics of temperature trend in the single and repeated discharges process,but also introduces the influence of skin effect.In addition,this paper analyzes detailedly the radial temperature distribution and temperature balance phenomenon during repeated discharges.A kind of wire cutting type flat forming coil was made and a coil temperature real-time measurement system based on fluorescence optical fiber temperature measurement technology was established,providing a platform for the study of coil temperature features and change rules under different conditions.Several groups of repeated discharges(discharge voltage 4kV and interval 80 s with crowbar circuit,discharge voltage 4kV and interval 120 s with crowbar circuit,discharge voltage 4kV and interval 80 s without crowbar circuit)and single discharge experiments with two different coil windings were carried out.The cross section of the two flat coils are both 3x4 mm,while the wire spacing is respectively 2mm and 4mm.The experiment result shows good accordance with the simulation above,which validates the reliability of the simulation model.In addition,the difference of simulation and experiment result is explained.Moreover,the response time of fiber optic thermometer is also tested and explained.Based on the simulation and experiment results,the equilibrium temperature under different conditions are obtained,which coincide with the theoretical analysis result.This paper illustrates that enlarging the cross section of the coil can remarkably decrease the equilibrium temperature.In addition,the relationship between the equilibrium temperature and discharge energy during repeated discharges is summarized.Furthermore,this paper sufficiently proves that adding suitable crowbar circuit,extending the discharge interval and enlarging the thickness of cover board are able to effectively decrease the equilibrium temperature.Finally,the strategy of repeated discharges during electromagnetic forming is concluded.