Armature reaction in a flux pump

Mawardi, Osman K.; Xu, Sen

doi:10.1109/tmag.1987.1064955

Cited by 12 publications

(5 citation statements)

References 1 publication

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“…14, where the resistive mechanism is related to the movement of remnant magnetization rather than to a reallocation of the applied emf. This secondary smaller mechanism is again similar to that described by Mawardi [54] for LTS dynamos.…”

Section: B Output Voltage Versus Current R Int Vs V Ocsupporting

confidence: 86%

Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment

et al. 2020

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High-T c superconducting (HTS) dynamos are experimentally proven devices that can produce large (more than a kiloamp) dc currents in superconducting circuits, without the thermal leak associated with copper current leads. However, these dc currents are theoretically controversial, as it is not immediately apparent why a device that is topologically identical to an ac alternator should give a dc output at all. Here, we present a finite-element model and a comparison of it with experiment that fully explain this effect. It is shown that the dc output arises naturally from Maxwell's laws when time-varying overcritical eddy currents are induced to circulate in a HTS sheet. We first show that our finite-element model replicates all of the experimental electrical behavior reported so far for these devices, including the dc output characteristics and transient electrical waveforms. Direct experimental evidence for the presence of circulating eddy currents is also obtained through measurements of the transient magnetic field profile across the HTS tape, using a linear Hall array. These results are also found to agree closely with predictions from the finite-element model. Following this experimental validation, calculated sheet current densities and the associated local electric fields are examined for a range of frequencies and net transport currents. We find that the electrical output from a HTS dynamo is governed by the competition between transport and eddy currents induced as the magnet transits across the HTS tape. The eddy currents are significantly higher (approximately 1.5 times) than the local critical current density, and hence experience a highly nonlinear local resistivity. This nonlinearity breaks the symmetry observed in a normal ohmic material, which usually requires the net transport current to vary linearly with the average electric field. The interplay between local current densities and nonlinear resistivities (which both vary in time and space) is shown to systematically give rise to the key observed parameters of experimental HTS dynamo devices: the open-circuit voltage, the internal resistance, and the short-circuit current. Finally, we identify that the spatial boundaries formed by each edge of the HTS stator tape play a vital role in determining the total dc output. This offers the potential to develop alternative designs for HTS dynamo devices, in which the internal resistance is greatly reduced and the short-circuit current is substantially increased.

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Section: B Output Voltage Versus Current R Int Vs V Ocsupporting

confidence: 86%

Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment

et al. 2020

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“…Superconducting dynamos employing low-temperature superconductors (LTS) have been reported by various previous authors. 2,[18][19][20][21][22] These devices utilise materials with a low B c2 such that a "normal-conducting spot" is considered to form in the high field region beneath the rotor magnet. 1 However, a normal spot is not formed in our HTS dynamo, as B c2 (at 77 K) of the YBCO coated conductor used in this work is much greater 23 than the applied rotor field of $0.2 T. Instead, flux vortices will penetrate in the high field region beneath the rotor magnet and move through the HTS tape.…”

mentioning

confidence: 99%

Anomalous open-circuit voltage from a high-Tc superconducting dynamo

Bumby

Jiang

Storey

et al. 2016

Applied Physics Letters

101

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We report on the behavior of a high-T c superconducting (HTS) homopolar dynamo which outputs a DC open-circuit voltage when the stator is in the superconducting state, but behaves as a conventional AC alternator when the stator is in the normal state. We observe that this time-averaged DC voltage arises from a change in the shape of the AC voltage waveform that is obtained from a normal conducting stator. The measured DC voltage is proportional to frequency, and decreases with increasing flux gap between the rotor magnet and the HTS stator wire. We observe that the DC output voltage decreases to zero at large flux gaps, although small differences between the normal-conducting and superconducting waveforms are still observed, which we attribute to screening currents in the HTS stator wire. Importantly, the normalised pulse shape is found to be a function of the rotor position angle only. Based on these observations, we suggest that the origin of this unexpected DC effect can be explained by a model first proposed by Giaever, which considers the impact of time-varying circulating eddy currents within the HTS stator wire. Such circulating currents form a superconducting shunt path which "short-circuits" the high field region directly beneath the rotor magnet, at those points in the cycle when the rotor magnet partially overlaps the superconducting stator wire. This reduces the output voltage from the device during these periods of the rotor cycle, leading to partial rectification of the output voltage waveform and hence the emergence of a time-averaged DC voltage.

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“…6-12, where the resistive mechanism is related to the movement of remnant magnetisation rather than a reallocation of the applied local-emf. This secondary smaller mechanism is again similar to that described by Mawardi [124] in the LTS dynamos.…”

Section: Output Voltage Vs Current R Int Vs V Ocsupporting

confidence: 85%

“…The only drum type dynamo in literature is an LTS dynamo constructed by Mawardi [124], see fig. 2-11.…”

Section: The Drum Dynamomentioning

confidence: 99%

The Physics of the High-Temperature Superconducting Dynamo and No-Insulation Coils

Mataira-Cole¹

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High-Tc superconducting (HTS) dynamos are a fascinating topic as practical engineering research preceded fundamental understanding, a lead then maintained for at least a decade. These devices, counter to expectation, produce a dc voltage where ‘textbook’ electromagnetism would predict a zero dc component. Simply by replacing a normal conducting stator in a standard dynamo with HTS conductor a dc — auto-rectifying — effect is created. This thesis reports my work in uncovering and codifying the underlying mechanism that gives rise to this effect — namely the broken symmetry that is usually present with Ohm’s law. An explanation of the dc voltage then leads to an explanation of the internal resistivity of such devices, which in turn allows more efficient dynamos to be designed, and modelled. The underlying logic of the HTS dynamo mechanism is also sufficiently strong to predict a complimentary electromagnetic device, a semiconducting dynamo, which remains to be experimentally verified. Ultimately, such HTS dynamos could be used to energise powerful HTS magnets. The modelling techniques developed in this thesis also provide insight into the operational behaviour of no-insulation coils (NI coils). Such coils are extremely robust to mechanical, thermal, and electrical stresses and faults. A simple model of such coils is presented that captures their essential physics with enough fidelity to predict shielding and magnetisation currents inherent with HTS conductors and turn-to-turn current flow. These two technologies represent key topics for the future of high field HTS magnet technologies and their supporting systems.

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Armature reaction in a flux pump

Cited by 12 publications

References 1 publication

Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment

Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment

Anomalous open-circuit voltage from a high-Tc superconducting dynamo

The Physics of the High-Temperature Superconducting Dynamo and No-Insulation Coils

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