Structure Optimization And Charging Performance Investigation Of A Dynamo-type High Temperature Superconducting Flux Pump

Compared with low temperature superconducting(LTS)magnets,the high temperature superconducting(HTS)magnets have many advantages such higher critical temperature and stronger current capacities,which have great potentials in electric power,superconducting maglev,medical,aerospace,national defense and military fields.However,it is too hard to make a superconducting joint on the HTS magnet,leading to a considerable joint resistance for it.The joint resistance as well as the flux creep make the HTS magnet unable to work in the persistent current mode(PCM).Therefore,to ensure that HTS magnet can generate a stable enough magnetic field,it generally working in the open circuit mode charged by an external power supply,which need a pair of copper current leads to bridge the room temperature(300K)and low temperature(77K).The current leads are an unignorably heat source for the cooling system,which will generate a lot of heat loss leading to a higher cooling cost and the instabilities of the HTS magnets.Flux pump is a kind of power source which is an ideal and promising method for the charging of HTS magnets,being able to charge the HTS magnets with a large saturated current.Among all kinds of flux pumps,the dynamo-type flux pump can realize a contactless charging for a closed HTS coil with simple structure as well as low cost,which is valuable for the application of engineering by eliminate the current leads,complicated external power source and control system.Hence,in this paper,we study the principle of a dynamo-type flux pump,and optimize the structure as well as the charging performance.First of all,we analyzed the characteristics of the high temperature superconductors under alternating magnetic field.Taking the E–J power law and Kim model into account,we built a finite-element model(FEM)with H-formulation.The results show that according to the E–J power law,the open circuit voltage along with the HTS stator increased when the frequency increased and the flux gap reduced,which meant it was the changing ratio of the magnetic field that would affect the charging performance.Secondly,we studied the influence brought by the magnetic field,changing ratio of the magnetic field and the magnetic field distribution.In order to promote the charging performance,we added a ferromagnetic(FM)slice into the system,and optimized the geometry parameters such as the width of permanent magnet and the HTS stator.The simulation results depicted that the FM slice below the HTS stator would enhance the magnetic field around the HTS stator which would promote the charging performance.On the contrary,the FM slice above the HTS stator would weaken the magnetic field and the charging performance.Besides,the open circuit voltage would increase with the increase of the stator width.Thirdly,we optimized the structure of dynamo-type flux pump by removing the rotating shaft that bridge the room temperature and low temperature,applying a pair of magnetic coupling to realize the contactless charging.Also,we studied the transmission performance with FEM,which showed that the torque reached about 10 N·m when the gap was 14 mm.Ultimately,we built a prototype of the dynamo-type flux pump with a pair of magnetic coupling and conducted relevant experiments on it.The experimental results denoted that,the saturated current and the charging speed would increase when the frequency grew and the flux pump decreased.More significantly,when a FM slice was put under the HTS stator,the charging speed was promoted dramatically,and the saturated current was enhanced for about 30%.