Applicability Of Low Frequency Transverse Flux Induction Heating For Continuos Welded Rails Under Track Laying

High speed, Heavy load railway is a common trend for world railway and has become an important sign of railway modernization. In order to guarantee the safety and stability of high-speed train, the study of continuous welded rail (CWR) tracks has become more and more important.The variability of weather and climate is a cause of considerable stress to CWR track which has influence on the stability of rail. Therefore, rigid jointless track laying must be carried out under specific thermal conditions to avoid dangerous tensile and compressive stress in the rails during extreme weather. The specific thermal condition so called neutral temperature is the temperature where no thermal forces are acting upon the CWR track. In north of China, it is common that the daily mean temperature is lower than the required neutral one (25～30℃). Thus, in order to guarantee the service life and safety of rail track, it is necessary to heat the rigid CWR track during track-laying to control stress-free temperature of rail. The control technology of stress-free temperature (CSFT), as one of the key technologies on CWR track laying, needs to be considered and researched seriously.In Europe, CSFT is very frequently done with propane-butane gas heaters which use heat transfer method to heat the rail. This technology is easy to operate and do not have any complex process. The main disadvantage of this method is the high thermal stress gradient generated in the rail cross-section and overheating of the surface.In China, stretching method is the most popular CSFT which uses stretching machine to draw the rail and make the length of it reach the standard one. This method can control the stress-free temperature of rail accurately and do not have too much influence of environment condition. But it will also cause the uneven distribution of thermal stress and the process of it is much more intricate.These problems may be eliminated by using low frequency transverse flux induction heating in which the heat (eddy current losses) is generated inside a selected volume of the rail. In comparison with other methods the induction heating (by eddy current losses) is an alternative that has numerous attractive features. Lack of any butt joints, isolation of the heating inductor from the rail and a simple way of transporting the energy to the rail are the most important advantages. This method imposes qualitatively different heat conditions than normally, (because the rail itself becomes the heat source) which influences directly the thermal processes (heating and cooling). Thus, it is possible to achieve both heat transfer and improved temperature distribution in the rail body.The main objective of the dissertation is to analyze theoretically and confirm experimentally possibility of employment of a low frequency transverse flux induction heating for CWR under track laying. The main results achieved are as follows:1. Theoretical analysis of frequency character of induction heating, using skin effect theory and magnetic saturation principle to interpret the necessity of low frequency application in the induction heating of CSFT and calculate the effective frequency domain of low frequency transverse flux induction heating (50～600Hz) to guarantee the heating efficiency and heating quality of the system.2. Theoretical three dimensional FEM analysis of magnetic field and temperature field distribution inside volume of a railway rail under low frequency heating for different structure of the heater applied. Experimental investigations for physical model of the low frequency induction heater of the railway rail to verify the theoretical approach. Analysis of heating efficiency related to heating frequency, structure of the heater and variable environmental conditions.3. Established stability model of CWR track, using inner and external moments' equilibrium theory and Hook's law calculate the critical temperature rise value of CWR track to limit and optimize the heating mode.4. Formulation of conclusions and recommendations for use of the low frequency transverse flux induction heating for CWR under track laying in practice.