The Loss Mechanism And Compensation Method Of A Flux Pumped HTS Magnet Subject To Travelling Magnetic Fields

High temperature superconducting(HTS)levitation technology has become an inevitable choice for higher speed rail transit thanks to its non-friction,low noise and high efficiency.As a significant part of HTS maglev transportation,HTS linear motor has a great application foreground because of higher thrust density,better operational reliability,smaller volume and lower noise.The core of HTS linear motor is HTS magnet.Ideally,when the HTS magnet operates in persistent current mode(PCM),it can produce a strong and stable magnetic field with quite low thermal value.However,unlike the low temperature superconducting(LTS)magnet can maintain the current for years,it is difficult for the HTS magnet to realize PCM operation due to the vibration of operating environment,the turbulence of electromagnetic circumference and the existence of joint resistance.These factors will lead to the decline of motor performance and affect the working order of linear motor.At present,the directly charging method by external driving power source is widely used to compensate the current attenuation.But the heat leakage generated by current leads can seriously fade the cooling property of cryogenic system.Moreover,it can easily induce the HTS magnet quenched.Therefore,the contactless power method is an ideal way to realize PCM operation of close-ended HTS magnet.In this paper,the HTS flux pump based on the principle of electromagnetic induction is adopted to actualize contactless power transfer for the close-ended HTS magnet.This method can not only charge the HTS magnet before it works,but also compensate the losses in time during the operation,and greatly reduce the heat leakage of cryogenic system.Via establishing the circuit model and finite element model,the theory of flux pump,which can convert AC to DC without rectifier,is clarified.In virtue of above analysis,a prototype of HTS flux pump is developed,and the impact factors of charging efficiency are investigated.Through the loss analysis of flux pump system,its advantages in reducing the heat leakage and other aspects are illustrated.Secondly,the decay characteristics of close-ended HTS magnet charged by flux pump under travelling magnetic field are studied experimentally.Taking the transport current,central magnetic field and electromagnetic forces as the core parameters,the decay curves with time are acquired by the self-designed triaxial test rig.The experimental results indicate that travelling magnetic field can enormously accelerate the current attenuation rate of HTS magnet,thereby abating the field strength produced by HTS magnet itself and diminishing the electromagnetic forces between the three-phase stator.Finally,on account of dynamic resistance,a 2D finite element model implementing the H-formulation is built,which reveals the essence of decay intensifying when the HTS magnet carrying a DC is subjected to the travelling magnetic circumference.By setting the normal resistivity,the non-linear E-J power law of superconductor is modified,and the non-convergence problem in calculating the HTS magnet quenched can be solved.The dynamic resistance variation in HTS magnet is clarified by analyzing the current density distribution of HTS magnet with different DC amplitudes,different magnetic field strengths of travelling magnetic field and different times in a period.From the calculated results,we can infer that the value of dynamic resistance is much larger than that of joint resistance.The loss caused by dynamic resistance acts on the HTS magnet,will accelerate the decay speed of internal current.As a consequence,the influence of dynamic resistance should be taken into account in the practical applications,and then the effective measures can be taken to restrain the losses.