Presentation Information
[APP2-07]Relationship between Induced Voltage and Velocity of Magnetic Material in a Superconducting Coil Magnet for a Reciprocating-type Magnetic Refrigeration System
*Shoki Fujii1, Daiki Kobayashi2, Shinnosuke Matsunaga1,3, Kyohei Natsume3, Koji Kamiya3 (1. National Institute of Technology, Gifu College (Japan), 2. University of Tsukuba (Japan), 3. National Institute for Material Science (Japan))
Keywords:
Superconducting Magnet,Magnetic Refrigeration System
[Purpose]
Reciprocating-type magnetic refrigeration systems have emerged as promising environmentally friendly alternatives to conventional cooling, since they eliminate greenhouse gases as refrigerants. These systems utilize the magnetocaloric effect, in which the temperature of a magnetic material changes under varying magnetic fields. For liquid hydrogen production, superconducting coil magnets are indispensable to generate strong fields. However, their operation faces the risk of quench, a collapse of the superconducting state that results in a rapid increase in electrical resistance. Quenches may cause localized heating and thermal stress, leading to severe damage. Conventional detection relies on monitoring voltage drops across magnet terminals. In reciprocating systems, however, motion of magnetic materials induces periodic voltages from magnetic flux changes, which overlap with quench signals and reduce reliability. This study aims to clarify the characteristics of such induced voltages to provide foundational data for improving quench detection systems.
[Method]
Finite element method (FEM) simulations were conducted using Femtet®, a commercial software developed by Murata Software. A three-dimensional transient magnetic field analysis in Cartesian coordinates was applied, incorporating motion of magnetic materials and coupling with an external circuit. The model replicated an actual superconducting coil magnet and included two magnetic materials and two shield magnets arranged symmetrically. The pair of magnetic materials was moved together at constant velocities of 22, 50, 100, 150, and 220 mm/s. For each velocity, time steps were adjusted to ensure a travel distance of about 200 mm. A uniform mesh size of 6 mm was applied. In the external circuit, a constant current of 58 A was supplied, and a 130 Ω protection resistor was connected in parallel with the magnet.
[Results]
The simulations showed that induced voltage increased with the velocity of the magnetic material. Waveform amplitudes grew larger as velocity rose, and peak induced voltage demonstrated an approximately linear dependence on velocity. However, slight deviations from strict linearity were observed, indicating that induced voltage may approach an asymptotic value at high speeds.
[Consideration]
These findings confirm earlier reports of velocity dependence while adding new evidence of non-linear tendencies. The results suggest that motion-induced voltages cannot be regarded as strictly proportional to velocity, especially in the high-speed regime. This complicates quench detection strategies that rely solely on voltage monitoring, since motion-induced voltages overlap with true quench signals. Therefore, new approaches that incorporate additional sensing or advanced signal processing are necessary.
[Conclusion]
This study clarified the relationship between induced voltage and magnetic material velocity in superconducting coil magnets for reciprocating-type magnetic refrigeration systems. The results confirmed the general increasing trend but also suggested a saturation tendency at higher velocities. By providing essential data on motion-induced voltages, this work contributes to the development of more reliable quench detection systems. Future research will expand the simulations under wider conditions and conduct experimental validation at the National Institute for Materials Science (NIMS).
Reciprocating-type magnetic refrigeration systems have emerged as promising environmentally friendly alternatives to conventional cooling, since they eliminate greenhouse gases as refrigerants. These systems utilize the magnetocaloric effect, in which the temperature of a magnetic material changes under varying magnetic fields. For liquid hydrogen production, superconducting coil magnets are indispensable to generate strong fields. However, their operation faces the risk of quench, a collapse of the superconducting state that results in a rapid increase in electrical resistance. Quenches may cause localized heating and thermal stress, leading to severe damage. Conventional detection relies on monitoring voltage drops across magnet terminals. In reciprocating systems, however, motion of magnetic materials induces periodic voltages from magnetic flux changes, which overlap with quench signals and reduce reliability. This study aims to clarify the characteristics of such induced voltages to provide foundational data for improving quench detection systems.
[Method]
Finite element method (FEM) simulations were conducted using Femtet®, a commercial software developed by Murata Software. A three-dimensional transient magnetic field analysis in Cartesian coordinates was applied, incorporating motion of magnetic materials and coupling with an external circuit. The model replicated an actual superconducting coil magnet and included two magnetic materials and two shield magnets arranged symmetrically. The pair of magnetic materials was moved together at constant velocities of 22, 50, 100, 150, and 220 mm/s. For each velocity, time steps were adjusted to ensure a travel distance of about 200 mm. A uniform mesh size of 6 mm was applied. In the external circuit, a constant current of 58 A was supplied, and a 130 Ω protection resistor was connected in parallel with the magnet.
[Results]
The simulations showed that induced voltage increased with the velocity of the magnetic material. Waveform amplitudes grew larger as velocity rose, and peak induced voltage demonstrated an approximately linear dependence on velocity. However, slight deviations from strict linearity were observed, indicating that induced voltage may approach an asymptotic value at high speeds.
[Consideration]
These findings confirm earlier reports of velocity dependence while adding new evidence of non-linear tendencies. The results suggest that motion-induced voltages cannot be regarded as strictly proportional to velocity, especially in the high-speed regime. This complicates quench detection strategies that rely solely on voltage monitoring, since motion-induced voltages overlap with true quench signals. Therefore, new approaches that incorporate additional sensing or advanced signal processing are necessary.
[Conclusion]
This study clarified the relationship between induced voltage and magnetic material velocity in superconducting coil magnets for reciprocating-type magnetic refrigeration systems. The results confirmed the general increasing trend but also suggested a saturation tendency at higher velocities. By providing essential data on motion-induced voltages, this work contributes to the development of more reliable quench detection systems. Future research will expand the simulations under wider conditions and conduct experimental validation at the National Institute for Materials Science (NIMS).