Ac Current Amplitude Research Articles

Combined Propulsion and Levitation Control for Maglev/Hyperloop Systems Utilizing Asymmetric Double-Sided Linear Induction Motors

This article presents a new method for combined levitation and propulsion control in maglev/Hyperloop systems by selectively applying AC and DC modes of operation to a group of asymmetric double-sided linear induction motors (ADSLIMs). Although adjusting the AC current magnitude of lower and upper primary windings in ADSLIMs allows simultaneous control of thrust and lift forces, the limitation of this current balancing technique prohibits them from producing a high lift force while operating with low thrust force. To overcome this limitation and to simultaneously control the thrust and lift forces of the ADSLIMs with high efficiency under different operating conditions, a combination of AC and DC modes of operation is proposed. AC mode of operation consists of feeding different AC current amplitudes to the upper and lower ADSLIM primary windings to produce and control the required thrust and lift forces. The DC mode of operation consists of controlling one or several ADSLIMs to operate with DC excitation to realize the desired lift force at lower thrusts which otherwise cannot be achieved by operating in AC mode alone. The concept of the new combined control strategy is studied using two-dimensional finite element (FE) electromagnetic simulations and compared with an Inductrack permanent magnet (PM) based passive magnetic levitation system.

Рекомендации по построению обобщенной модели тела человека при действии напряжения прикосновения

To improve the experimental base aimed at studying the electrophysical properties of a human body when subjected to a touch voltage in the frequency range 0-1000000 Hz, it is necessary to develop a probabilistic model of a human body as an electrical circuit element, using which it will be possible to standardize the conditions for monitoring touch voltages and currents in electrical installations with voltages of any kind and frequency. To develop a generalized human body model that would adequately represent the human body electrophysical characteristics under the effect of a touch voltage of an arbitrary frequency, it is sufficient to have experimental volt-ampere and ampere-second characteristics of the body when subjected to a constant voltage and its frequency responses in the range of 20-200000 Hz under the effect of a voltage that does not exceed the sensitivity threshold. Factors causing a gradual increase in the AC current amplitude at a constant applied voltage are revealed. The obtained computation results are in close agreement with the results of experimental studies carried out both at DC and AC voltages. The adequacy of the generalized human body model developed using the proposed approach is confirmed by good agreement between the predicted probabilistic (obtained on the model) and experimental volt-ampere characteristics at a frequency of 50 Hz and higher.

Analytical Warburg Impedance Model for EIS Analysis of the Gas Diffusion Layer with Oxygen Depletion in the Air Channel of a PEMFC

In this study, an analytical mathematical expression to simulate the low frequency impedance spectrum that represents the gas diffusion layer (GDL) and air channel of a proton-exchange membrane fuel cell (PEMFC) has been developed. The analytical expression considers the impedance response of the GDL with oxygen depletion in the air channel/GDL interface. Parameters of the finite-length Warburg impedance (ZW) reported in the literature and estimated from EIS measurements carried out in a PEMFC at different oxygen stoichiometry are considered as a base-line to simulate the GDL-channel impedance spectrum. The results demonstrate that non-stationary depletion of oxygen at the air channel/GDL interface during AC conditions yields the formation of a second loop on the GDL-channel impedance spectrum at low frequencies. In addition, it is demonstrated that the contribution of the AC current amplitude increases the second loop of the GDL-channel impedance spectrum and drifts the GDL-channel impedance spectrum away from its steady-state value which is related to the real component of the impedance response as the frequency is approaching zero. The resulting analytical expression of the GDL-channel impedance is analogous to the general mathematical expression of the finite-length Warburg component. The developed GDL-channel impedance model can simulate and separate the contribution of the impedance response of the GDL from the impedance response associated to oxygen depletion in the channel/GDL interface. This developed model could support other studies focusing on the estimation of diffusion parameters of the GDL from EIS measurements carried out in PEMFCs at different operating conditions.

The Influence of Electromagnetic Vibration Caused by AC Current On the Performance of Y-Ba-Cu-O Tapes

Superconducting coils may suffer from electromagnetic vibration due to changing electromagnetic fields under AC conditions. In the long term, this vibration may cause performance degradation of superconducting tapes. In order to study the influence of electromagnetic vibration between superconducting tapes on their performance, the study can be started from the simplest form of electromagnetic vibration, that is, the electromagnetic vibration between two superconducting tapes. The degree of electromagnetic vibration under different AC currents were analyzed by simulation, and the variation of critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and the n-value of Y-Ba-Cu-O (YBCO) tapes after undergoing long-term electromagnetic vibration through experiments were measured. The results show that two YBCO tapes does not show obvious performance degradation under tens of hours the electromagnetic vibration caused by the AC current amplitude of 1.2 to 1.4 times I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> .

Multiple reversals of vortex ratchet effects in a superconducting strip with inclined dynamic pinning landscape* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11702034, 11972298, and 11702218), the China Postdoctoral Science Foundation (Grant No. 2019M663812), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 300102129104, 3102018zy013, and 3102017jc01003), and the Young Talent Fund of

Using time-dependent Ginzburg–Landau formalism, we investigate the multiple reversals of ratchet effects in an unpatterned superconducting strip by the tilted dynamic pinning potential. In the case of collinear sliding potential and Lorentz force, vortices are always confined in the channels induced by sliding potential. However, due to the inclination angle of sliding pinning potential with respect to the Lorentz force, vortices could be driven out of the channels, and unexpected results with multiple reversals of vortex rectifications are observed. The mechanism of multiple reversals of vortex rectifications is explored by analyzing different vortex motion scenarios with increasing ac current amplitudes. The multiple reversals of transverse and longitudinal ratchet effects can be highly controlled by ac amplitude and dynamic pinning velocity. What’s more, at certain large current the ratchet effect reaches strongest within a wide range of pinning sliding velocity.

An inertial model of vortices to explain the extreme sensitivity of hexatic vortex fluid to low frequency ac excitation

Recently, it has been demonstrated that the Hexatic vortex fluid realized in 2-dimensional vortex state of a very weakly pinned amorphous MoGe thin film is extremely sensitive to external ac excitation. A large amplification of resistance is observed when the sample is exposed to external electromagnetic radiation or a small ac current is superimposed with the subcritical measuring dc current. The observed resistance strongly depends on both amplitude and frequency of the ac excitation. In this paper, we develop an inertial model of vortex lattice subjected to a small amplitude of ac current, by extending the flux creep theory. We show that the resistance increases exponentially with both amplitude and frequency of the ac current and explain quantitatively the variation of resistance with the amplitude and frequency of ac current using the presented model.

Inter-granular effects at high magnetic fields of cuprate and iron chalcogenide superconducting materials

The weak links effects are one of the main challenges for effective power applications of high temperature superconducting materials. Studies of these effects help for their better understanding and subsequent improvement. An overview analysis of the intergranular properties of cuprate (Y0.8Ca0.2Ba2Cu3O7-δ) and iron-based chalcogenide (FeSe0.5Te0.5) polycrystalline samples was carried out, by means of series of electro-transport experiments at different magnetic fields. The temperature evolution of the Josephson coupling and intrinsic superconductivity effects for the both systems was constructed. The FeSe0.5Te0.5 compound shows very stable and superior behavior compared to Y0.8Ca0.2BCO up to the highest magnetic fields (14T) used. We have explored FeSe0.5Te0.5 Josephson weak links influence (as a non-linear process) over the resistive transition using different AC current amplitudes and applying the sensitive AC transport third harmonics technique.

Experimental Investigations of Combined DC-AC Charging Pattern for Li-ion Batteries at Low Temperatures

At low temperatures, the available capacity and power of Li-ion batteries will significantly reduce, and charging is difficult. To improve the low temperature performance of Li-ion batteries, it is essential to preheat battery and ensure the battery to maintain a certain temperature. In this work, the charging characteristics of Li-ion batteries at low temperatures are investigated through Four- Stage dropping current charging regime, and 3°C is determined as a characteristic temperature for selecting the cut-off temperature for the battery heating. In order to maintain the temperature advantage after preheating, a Combined DC-AC (Co-DC-AC) charging strategy is proposed to implement the charging and heating simultaneously. Aiming at the maximum heat generation rate with a constant polarization, the preheating AC frequency can be obtained where the maximum amplitude of AC current can be found after determining the maximum polarization voltage and impedance of battery. Six tests were designed with different combinations of preheating, Co-DC-AC, and Three- Stage dropping current (TSDC) charging. The proposed charging strategy includes preheating, Co-DC-AC and TSDC charging, which can effectively improve the charging performance at low temperature.

Переходные процессы при токовой перегрузке в цепи переменного тока с ВТСП-проводом

It has been found that the loss of superconductivity in HTSC wires under ac current overload is accompanied by the change in sign of the phase difference between current and voltage. In a stabilized second-generation HTSC wire under an overload the oscillations of ac current and voltage amplitudes have been found due to the oscillations of the normal zone size in the superconducting layer. The change in sign of the phase difference can be applied for a timely registration of a normal zone nucleation in current-carrying conductors of superconducting equipment.

Magnetic antivortex dynamics in a two nanocontact disk

Stable magnetic vortex-antivortex-vortex structures can form on a thin ferromagnetic disk at two separated nanocontacts with the vortices at the nanocontacts and the antivortex between the nanocontacts. When the vortices are pinned to the nanocontacts, the antivortex gyrotropic motion can be isolated about the stable equilibrium between the pinned vortices. The restoring force in the antivortex is from the combined exchange, magnetostatic, and Oersted fields where the exchange and the magnetostatic interactions will result in an attractive vortex-antivortex force, but the net Oersted field from the nanocontacts tends to stabilize the antivortex between both nanocontacts. A small ac driving current on one of the nanocontacts provides an efficient driving force for this antivortex oscillator that is tunable through the nanocontact current. Owing to the competing magnetostatic and Oersted forces, the oscillator can be soft along the direction between the nanocontact resulting in a high amplitude oscillation. Moreover, because of this competition, nonlinear effects are important at low ac current amplitudes. An interesting nonlinear effect is an amplitude jump that is shown to be a result of a time-dependent asymmetry of the antivortex potential.

Superconducting Power Supply for the KEDR Main Superconducting Solenoid

The power supply based on the full-wave superconducting rectifier is used for excitation of the main superconducting solenoid of the KEDR universal magnetic detector at the VEPP-4 collider. The rectifier consists of the air-core superconducting transformer and two thermally controlled superconducting switches. Several superconducting switch designs were developed, in order to achieve appropriate parameters of the rectifier operation. The present version of the switches was designed and tested in November 2013. The rectifier is cooled with a liquid helium bath. The primary winding of the superconducting transformer is excited with an ac current amplitude of 28 A at a voltage amplitude of 5 V. The heat power used for a superconducting switch control is 2.7 W. The power supply provides rising the magnetic field inside the KEDR main solenoid, long-term stabilization, and lowering the field to zero. The inductance of the solenoid is 1.2 H. The usual operating magnetic field is 0.6 T, which corresponds to the 2.4-kA current in the solenoid winding and 3.46 MJ of stored energy. The power supply pumps the field up to 0.6 T, within 110 h, and can keep it constant for several months. In this paper, the design, parameters, and performance of the superconducting rectifier are discussed.

Round Conductor With Low AC Loss Made From High-Temperature Superconducting Tapes

The second generation of high-temperature superconductor tape exhibits rapidly progressing electric transport capacity. However, its use as a conductor in magnet technology is somewhat held up by the strip-like geometry. Possible solution is the helical layout of parallel tapes on a flexible round former (recently called the Coated Conductor on Round Core or CORC). We present the study of AC properties of single-layer CORC conductors made from tapes with non-magnetic substrate. When subjected to AC magnetic field, single-layer CORC conductors exhibit negligible coupling currents because the tapes are fully transposed. On top of this the hysteresis loss can be reduced by the tape striation. Numerical simulations of magnetization loss for CORC conductors are in good agreement with experimental data. Prediction of AC loss in a coil made from CORC is compared to that calculated for a classical double pancake one made from the same amount of tape. Result indicate that the CORC-made coil produces lower losses at AC current amplitudes approaching the critical current.

Investigation of Driving Power Dependence on Magnetoimpedance Properties of Thin-Film Elements With Uniaxial Anisotropy

We investigated the dependence of driving ac current on the giant magnetoimpedance (GMI) properties of rectangular elements fabricated using thin films with uniaxial anisotropy. Two types of GMI sensor elements were fabricated. One had the typical GMI configuration, with the easy axis parallel to the width of the sensor, and the other had an inclined easy axis. The impedance changes and its change ratio deteriorated with increasing ac current amplitude in the case of the typical type of the GMI element of narrower width. The observed discontinuous impedance jump was suppressed with increasing ac current amplitude for the GMI elements having an inclined easy axis. These behaviors are explained mainly by partial magnetization saturation due to a large excitation field inside the elements. We also discuss a research direction aimed at gaining higher sensitivity in typical GMI elements and the utilization of the discontinuous jump.

Effect of soft ferromagnetic substrate on ac loss in 2G HTS power transmission cables consisting of coated conductors

As a model of second-generation high-temperature superconducting (2G HTS) power transmission cable, the effect of soft ferromagnetic (FM) substrate on hysteretic ac loss in a round cable carrying transport current or exposed to radial magnetic field is investigated theoretically. We consider the FM substrates as ideal soft magnets with an infinite permeability, neglecting the magnetic hysteresis. The analytical expressions of field and current distribution around a curved superconductor tape with FM substrate were derived applying the complex-field approach. FM components can strongly influence the ac loss behaviors in a round cable composed of coated conductors. The approximate expressions for ac loss in a round cable at small currents or low fields are obtained. It is shown that at small currents the ac loss QFM in a round cable for FM substrate depended on ac current amplitude ia and central angle θn as , while at low fields ha , the order of which differs significantly from QNM for nonmagnetic (NM) substrate. Additionally, the influence of the central angle as well as tape-width of coated conductors on the ac loss characteristic in a round cable is discussed. The ac loss QFM can be reduced by a suitable tape-width of coated conductor at higher fields as compared with QNM.

Vortex ratchet reversal in an asymmetric washboard pinning potential subject to combined dc and ac stimuli

The mixed-state resistive response of a superconductor thin film with an asymmetric washboard pinning potential subject to superimposed dc and ac currents of arbitrary amplitudes and frequency at finite temperature is theoretically investigated. The problem is considered in the single-vortex approximation, relying upon the exact solution of the Langevin equation in terms of a matrix continued fraction. The dc voltage response and the absorbed power in ac response are analyzed as functions of dc bias and ac current amplitude and frequency in a wide range of corresponding dimensionless parameters. Predictions are made of (i) a reversal of the rectified voltage at small dc biases and strong ac drives and (ii) a non-monotonic enhancement of the absorbed power in the nonlinear ac response at far sub-depinning frequencies. It is elucidated how and why both these effects appear due to the competition of the fixed internal and the tunable, dc bias-induced external asymmetry of the potential as the only reason. This is distinct from other scenarios used for explaining the vortex ratchet reversal effect so far.

Circular magnetization and energy loss in Fe-based soft magnetic wires

The circular permeability, μ=μ′−jμ″, of two Fe-based soft magnetic wires with longitudinal (S1) and transverse (S2) domain structure, respectively, were determined from the measurements of impedance, Z=R+jωL, as functions of frequency (f=102–104Hz), AC current amplitude (I=0.1–100mA) and DC axially applied field (H=0–5.4kA/m). So the influences of f and H upon the circular magnetization have been studied experimentally. Although the two samples exhibit similar changes of μ=μ′−jμ″ with the circular-field, Hφ, different technique magnetization processes, referred to different domain structures of samples, has been discussed by analyzing the energy loss according to a current theoretical model. Consequently, we have concluded that the circular magnetization is dominated by domain magnetization rotations and domain wall displacements in samples S1 and S2, respectively.

Nonadiabatic ratchet effect in superconducting films with a tilted cosine pinning potential

The influence of an ac current of arbitrary amplitude and frequency on the mixed-state dc-voltage-ac-drive ratchet response of a superconducting film with a dc current-tilted uniaxial cosine pinning potential at finite temperature is theoretically investigated. The results are obtained in the single-vortex approximation, i.e., for non-interacting vortices, within the frame of an exact solution of the appropriate Langevin equation in terms of a matrix continued fraction. The dc voltage ratchet response is discussed as a function of ac current amplitude and frequency for various dc biases, in a wide range of corresponding dimensionless parameters. Our theoretical results are discussed in comparison with recent experimental work on the high-frequency ratchet response in nanostructured superconducting films.

Dynamic Desaturation Process in Saturated Cores Fault Current Limiters

The process of driving the magnetic core of saturated-cores fault-current-limiters (SCFCL) out of saturation has been studied numerically using finite elements calculations. The de-saturated core section was found to grow dynamically starting from the central region below the AC coils and gradually expanding towards the coil edges with increasing AC current amplitude. During the transition, the core volume underneath the coils is non-homogeneously magnetized and the system is highly non-linear. An effective core volume that contributes to the SFCL impedance during fault can be defined for different current amplitudes. Two “open core” SCFCL models have been studied and compared for their dynamic de-saturation process. The results demonstrate the importance of the dynamic effective volume size to the current limiting process and suggest that prior art approximations based on full core desaturation are not sufficient for a detailed SFCL design.

Frequency-dependent ratchet effect in superconducting films with a tilted washboard pinning potential

The influence of an ac current of arbitrary amplitude and frequency on the mixed-state dc-voltage-ac-drive ratchet response of a superconducting film with a dc current-tilted uniaxial cosine pinning potential at finite temperature is theoretically investigated. The results are obtained in the single-vortex approximation, i.e., for noninteracting vortices, within the frame of an exact solution of the appropriate Langevin equation in terms of a matrix continued fraction. Formulas for the dc voltage ratchet response and absorbed power in ac response are discussed as functions of ac current amplitude and frequency as well as dc current induced tilt in a wide range of corresponding dimensionless parameters. Special attention is paid to the physical interpretation of the obtained results in adiabatic and high-frequency ratchet responses taking into account both running and localized states of the (ac+dc)-driven vortex motion in a washboard pinning potential. Our theoretical results are discussed in comparison with recent experimental work on the high-frequency ratchet response in nanostructured superconducting films [B. B. Jin et al., Phys. Rev. B 81, 174505 (2010)].

Influences of axial field upon circular magnetization and eddy‐current anomaly factor in Fe‐based nanocrystalline wire

AbstractFe‐based soft magnetic wire with transverse magnetic anisotropy was obtained by stress current annealing. Its circular permeability, µ = µ′ – jµ″, has been determined from the measurements of impedance, Z = R + jωL, as a function of frequency (f = 10 – 105 Hz), AC current amplitude (I = 0.1 – 100 mA) and DC bias field (H = 0 – 5.4 kA/m). An experimental study of the influence of H upon the circular magnetization and the f‐dependent µ = µ′ – jµ″ has been performed. It is concluded that the axial field, H, evolution of the circular permeability, µ = µ′ – jµ″, the relaxation frequency, fτ, and the eddy‐current anomaly factor, η, can be explained in terms of the modification of domain structure by the applied axial field H.