Behaviour prediction of closed-loop HTS coils in non-uniform AC fields

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Dynamic resistance leads to the demagnetization of high-T c superconducting (HTS) permanent magnets (such as closed-loop coils and tape stacks) when exposed to AC fields. This letter reports on the systematic study of an unexpected dynamic resistance occurring in first cycle AC field, which can be induced by any weak AC field-even below the threshold value 'conventionally' defined for generating dynamic resistance. This resistance is predicted by a critical-state analytical calculation, and its physical origin is shown to be from the asymmetrical flux penetration. Numerically, this resistance is verified, and simulated for a closed-loop HTS coil, showing a non-negligible demagnetization effect. Experimentally, the current decay (i.e. demagnetization) characteristics of a closed-loop HTS coil in a sufficient range of AC fields are measured. This resistance manifests itself in the closed-loop coil as a relatively sharp current decay in the first cycle of the applied field; this observation is in qualitative agreement with the simulation. A comparative experiment is performed to eliminate the contribution from the index loss or 'conventional' dynamic resistance that also possibly results in a greater demagnetization in the first cycle owing to the larger coil current. As the result, this unexpected dynamic resistance is verified. The unexpected, sharp demagnetization caused by this resistance might need to be considered when designing HTS closed-loop coil applications.

Section: Closed-loop Hts Coilmentioning

confidence: 99%

Sharp demagnetization of closed-loop HTS coil in first cycle of external AC fields induced by unexpected dynamic resistance

Zhong

Jin

2021

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“…Each H model is built on a 2D axisymmetric geometry. The azimuthal current distribution from the ECG model is input into these H models to calculate the dynamic resistance distribution, because the dynamic resistance is dependent on the transport current of HTS tapes [17,43]. Meanwhile, the dynamic resistance distribution is fed back from the H model to the ECG model to update the I ϕ and I r distributions.…”

Section: Equivalent-circuit Grid Modelmentioning

confidence: 99%

Time-variant magnetic field, voltage, and loss of no-insulation (NI) HTS magnet induced by dynamic resistance generation from external AC fields

Zhong

et al. 2023

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High-temperature superconducting (HTS) coils serving as DC magnets can be operated under non-negligible AC fields, like in synchronous machines of maglev trains and wind turbines. In these conditions, dynamic resistance generates in HTS tapes, causing redistribution/bypassing of the transport current inside the no-insulation (NI) coil and its unique operational features. This issue was studied by experiments on an NI coil with DC current supply put into external AC fields. Due to the current redistribution induced by dynamic resistance, the central magnetic field and voltage of the NI magnet initially undergo various transient processes, and eventually exhibit a stable central magnetic field reduction and a DC voltage. These time evolutions have implications for a time-varying torque and loss of an HTS machine. These time evolutions are strongly affected by the contact resistivity distribution, and whether it is the first time of the NI magnet being exposed to the AC field, showing several qualitatively different waveforms (e.g., some are even non-monotonic with time). The magnitudes of the stable central field reductions, and their observed linear correlation with the DC voltages are found to be decided by the local contact resistivity of the innermost and outermost several turns. It is also noted that the non-insulated turn-to-turn contact help lessening the loss induced by the dynamic resistance. A numerical model is established to analyse/explain those experimental results by observing the microscopic current distribution. Two risks of quench are noticed: (i) azimuthal current of the middle part turns increases as the AC field is applied; (ii) a concentration of radial current is observed near the terminals of the NI coil.

“…higher I c . However, at a lower applied current, the tape with a high I c is not effectively penetrated by the small applied magnetic flux density at the working air gap [40][41][42].…”

Section: Effect Of Superconducting Tape Icmentioning

confidence: 99%

“…One of the application scenarios of the flux pump is to use the device to compensate for the magnetic field decay. Such applications include HTS magnets for magnetic levitation which suffers from the field decay due to the harmonics of ground windings [42], and the magnetic resonance imaging (MRI) magnets where 0.1 ppm h −1 level of field stability is required [43] and no HTS joint is good enough. In such applications, to run the magnet under driven mode will introduce huge heat load due to heat conduction along the current leads.…”

Section: Introductionmentioning

confidence: 99%

Miniaturized HTS linear flux pump with a charging capability of 120 A

Chen

et al. 2023

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Owing to the presence of joint resistance and flux creep, high-temperature superconducting (HTS) coils without a power supply inevitably suffer from current decay. A flux pump is a voltage supply that requires connections with smaller footprints and a lower heat load than traditional current leads. In this study, we explain the principle of the upper limit for the output current of the traveling wave flux pumps. Based on this principle, a miniaturized linear flux pump device was developed. With narrow and misaligned iron teeth, elaborate 3D geometry of the iron pieces, and optimized driving current waveform, the miniaturized flux pump can support more than 120 A output current with only a 10 mm wide HTS tape and a compact size of 4.6 × 4.6 × 3.4 cm. Our experimental results show that the critical current of the HTS tape has a significant effect on the flux pump output. An HTS tape with a larger critical current supports a higher maximum transport current, whereas an HTS tape with a smaller critical current requires less applied current for positive output. Finally, excitation tests on HTS coils were performed. Charge/active discharge and field supplement experiments were done on a maglev HTS racetrack coil of 0.4 H, where charging/field supplement capbility of the miniaturized flux pump were demonstrated up to 46.8 A (close to the critical current of the coil). It has also been proved that the flux pump can work together with an external power supply with persistent current switch. The miniaturized flux pump can also independently charge an HTS coil of 60 μH to 91.6 A, which is the critical current of the coil at a low voltage criterion.