Modelling and Impact of HTS Tapes Soldering on the Electromagnetic Characteristics of HTS Cage Rotor Induction Machines

Integrating high-temperature superconductors (HTS) into the rotor of induction machines promises significant advancements in the development of high-specific-power AC electric motors. However, solder joints between rotor HTS tapes pose a critical performance factor. This paper presents a methodology to model and analyse the impact of soldering in the HTS rotor cages of induction machines, avoiding the need for 3D computational-expensive simulations. First, the solder resistance is estimated based on experimental tests conducted on 1st and 2nd generation HTS tapes, and its impact on AC losses is determined and validated both experimentally and through 3D Finite Element models (FEM). Then, to avoid the need for 3D simulations, two 2D-FEM approaches for representing solder connections are implemented: a FEM coupled to an Equivalent Electric Circuit and a Fully FEM based on modified Electric Field approach. Both methods are adapted to operate with an applied rotor induced voltage instead of an applied current due to the nature of the induction machine. The proposed modified formulations are validated experimentally using a single-phase transformer with a secondary short-circuited single-turn HTS coil is used to mimic the conditions at which the HTS coils will operate in an induction motor. Finally, the formulations are used to evaluate the locked rotor and synchronous conditions of a 90W induction machine with HTS rotor tapes. Results show that the modified formulations accurately account for the presence of solder in the short-circuited HTS tapes and that the solder resistance has a significant impact on the near-synchronous operation of the HTS induction machine.