AC Coil Research Articles

Airplane motors employing superconducting DC field windings and conventional conductor AC windings

Many organizations are developing compact lightweight highly efficient rotating machines for airplane applications. These machines include permanent magnets for excitation and an iron-core with and without superconducting windings. Air-core (no magnetic iron) machines have the potential to be the most lightweight and efficient. Such machines can use superconductors for both DC excitation field coils and AC armature coils, which need conductors under development, like MgB2 and Bi2212. If liquid-hydrogen (LH2) is available on a plane and can be used as a coolant, it becomes feasible to develop machines with AC armature coils made from conventional conductors like copper, aluminium, and high-conductivity aluminium.This paper describes conceptual designs for a 3 MW, 4,500 RPM motor employing REBCO CORC conductor for the DC field coils and conventional conductor Litz cable for the AC armature coils cooled by LH2 available on the plane. Both rotor and stator coils are contained in separate cryostats. The DC excitation coils on the rotor are operated at 40 K to work successfully with a brushless flux pump exciter. Likewise, stator AC coils are cooled with available LH2 to take advantage of the lower resistivity of conventional conductors at cryogenic temperatures. Motor size, mass and losses are compared for stator windings employing copper, aluminium, and high-conductivity aluminium (Hyper-Al). Compared with copper and aluminium machines, the machine employing Hyper-Al has smaller size, mass and total losses.

Experimental AC loss study on REBCO coil assemblies coupled with an iron cylinder

In high-temperature superconducting (HTS) power devices, the presence of iron cores changes the magnetic field profile around the HTS coil windings, potentially affecting their AC loss characteristics. AC loss measurements for HTS coil windings coupled with an iron core using the electrical method can lead to a significant error, owing to the indirect estimation of the iron core loss through using a copper test coil. To investigate the cause of the experimental error and the influence of an iron core on coil AC losses, transport AC losses of REBCO double pancake coil (DPC) assemblies coupled with an iron cylinder were measured. A 40-turn 1DPC and an 80-turn 2DPC assembly wound with 4 mm SuperPower wire were employed in the measurements. To ensure the same iron core loss using the HTS coil assembly and the copper coil, 2D finite element method simulations were conducted iteratively to design the iron core and the copper coil to get the same local magnetic field distributions in the designed iron core for the two cases. The main cause of the error is due to the difference in local magnetic flux densities in the iron core generated by the HTS coil assembly and the copper coil even when the ampere-turns of the coils are identical. We showed that the simulation-guided measurement method can assure accurate AC loss measurement in the HTS coil assemblies coupled with iron cores. Compared with the AC losses in the 1DPC and 2DPC coil assemblies without the iron cylinder, the presence of the iron cylinder significantly increases the coil losses. Frequency dependence is observed in the coil AC losses of the 1DPC and 2DPC assemblies when coupled with the iron cylinder. This is due to the eddy current induced in the iron cylinder generating a magnetic field, which influences the coil AC loss.

A Transient-State Lumped Parameter Thermal Model for Brushless Wound Field Switched Flux Machines

In this paper, the transient-state lumped parameter thermal model (LPTM) of the wound field switched flux (WFSF) machine is characterized for the first time. The modelling methodology for the LPTM is given in details, including the process of determining the parameters of the LPTM, based on which two LPTMs are compared with the experiments, i.e. the LPTM-I and LPTM-II without and with consideration of the practical winding position in the stator slot. It is found that the practical positions of the DC and AC coils in the slots is important to improve the accuracy of the LPTM, due to the existence of slot empty space caused by overlapped DC and AC windings. By considering the practical positions of the DC and AC coils in the slots, the maximum value of the absolute error between the LPTM model predicted temperature and the measured values of the AC winding are 3.4°C, 5.23°C, 3.18°C and 5.22°C under DC test, steady state test, cycling test-I and cycling test-II, respectively, whilst they are 3.95°C, 2.00°C, 2.28°C and 4.89°C for the DC winding temperature error. The average value of the absolute error between the LPTM model predicted temperature and the measured values of the AC winding are 0.88°C, 1.25°C, 0.85°C and 1.87°C under DC test, steady state test, cycling test-I and cycling test-II, respectively, whilst they are 1.08°C, 1.47°C, 1.06°C and 2.73°C for the DC winding temperature. The temperature of the WFSF machine during a 9-hour experiment can be predicted in 27.8s by the LPTM, which has a more than 1100 times faster calculation speed.

AC Losses Calculations for the ITER CS and PF Magnet Systems During Plasma Operation

While the ITER magnet system assembly is progressing, models are being developed to predict the performances of the coils in support of the preparation of commissioning and operation. In particular, the AC losses generated in the coils during a plasma scenario need to be estimated as input to thermo-hydraulic performance simulations. The largest AC losses during plasma operation are expected in the CS and PF coils. This paper discusses the results of simulations of AC losses in these coils during a representative ITER plasma pulse. These calculations also take into account the electromagnetic screening provided by the ITER passive structures, most importantly the vacuum vessel, discussing its impact on the conductor AC losses. The results will also be compared to earlier predictions. Finally, the CS and PF coil AC losses expected during commissioning will be briefly discussed.

Full Multiphysics Electro-Vibroacoustic Analysis of a Balanced Armature Transducer

A balanced armature transducer, also known as a receiver in some industries, is a high-performance miniature loudspeaker that is often used in hearing aids but also in other in-ear audio products like earbuds. The electrical and vibroacoustic performance of a balanced armature transducer is analyzed using a full multiphysics analysis. The model includes acoustics and associated thermoviscous losses, structural components such as diaphragm and armature (made of magnetic steel), and as the electromagnetic components including permanent magnet and coil. The vibrating armature enters a magnetic circuit with alternating magnetic flux, generated by an AC coil, which has a strong interaction with the DC magnetic field of a magnet. The electromagnetic forces are included through volumetric and surface contributions of the Maxwell stress. The analysis is performed in a commercial finite element analysis software and predicts the linear response modeled in the frequency domain and nonlinear effects modeled in the time domain. The latter include nonlinear thin-film damping and nonlinear magnetic force contributions captured using a moving mesh.

Magnetostrictive Ultrasonic Waveguide Transducer for In-Pile Thermometry

Real-time and reliable temperature measurement on nuclear fuels is crucial for the safe operation of existing pressurized-water reactors and future advanced nuclear reactors. Magnetostrictive materials deform when subjected to a magnetic field or exhibit magnetization variation when stressed. Based on these properties, this study prototyped an ultrasonic thermometer (UT) consisting of a magnetostrictive waveguide, a dc coil providing appropriate magnetic biasing, and an ac coil generating an acoustic impulse and detecting the resulting acoustic echoes. By tracking the time of flight between the excitation pulse and the echoes, the UT can potentially detect nuclear fuel cladding temperature from a long distance. In this study, magnetostrictive iron–gallium alloys, or Galfenol, were selected as the waveguide due to their large magnetostriction, superior temperature survivability, excellent radiation resilience, and high mechanical robustness. A multiphysics finite-element model considering electrical, magnetic, and mechanical dynamics in the magnetostrictive UT was then developed. The model exhibited an error of 0.24% in time-of-flight simulation and, therefore, enabled computer-aided design and guided signal processing. Between room temperature and 120 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C, the new Galfenol-based UT exhibits a linear sensitivity of 162.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\times 10{^{-6}} {^\circ }$</tex-math></inline-formula> C <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{-1}$</tex-math></inline-formula> , which is 51.7% higher than a previous magnetostrictive UT based on iron–cobalt–vanadium alloys.

Fishtail Divertor and its basis surface thermal load bearing performance

Fishtail Divertor (FTD) configuration is a dynamic magnetic field divertor configuration that uses the AC coil installed on the back of the divertor target plate to realize the swing of the strike line. The most important driving force of FTD configuration design is to improve the thermal bearing performance of divertor target plate. Under FTD configuration, the maximum temperature of divertor target plate will eventually reach a dynamic equilibrium state, and it have an obvious jump at both ends of the swing area. With the increase of the swing frequency, FTD configuration can more effectively improve the thermal load bearing performance of the divertor target plate. However, if only to alleviate the steady-state heat load, the swing frequency of FTD no need to be higher than 100 Hz. FTD configuration has engineering application prospects in future fusion reactors. Under a swing distance of 10 cm and a thermal load condition of qp=100MW/m2 and λT=5mm (steady state), Triangular wave Fishtail Divertor (TFTD) swing frequency exceeds 5 Hz or Sine wave Fishtail Divertor (SFTD) swing frequency exceeds 20 Hz, the maximum temperature of target plate can be lower than 900 K. At the same conditions expect the swing frequency is 100 Hz, the maximum temperature under TFTD and SFTD can lower than 700 K and 800 K respectively.

Control of Amount of α-Al Phase Particles in near Eutectic Al-Si Alloy by Electromagnetic Stirring

Al-Si alloy is widely used as a casting alloy. The α-Al phase in the semi-solid state has low Si content in the Al-Si alloy. Then by separation of these α-Al phases from semi-solid Al-Si alloy, refining of aluminum can be possible. But, in near eutectic Al-Si alloy, few primary α-Al phases can be crystallized. If the fraction ratio of the α-Al phase can be increased, near eutectic Al-Si alloy can refine, and this method can be used for recycling. In this study, the effect of electromagnetic stirring (EMS) on the microstructure, especially the amount of the α-Al phase particles was investigated. A rotational magnetic field was applied to JIS ADC12 alloy which has near eutectic content during slow cooling from the liquid state to the solid-state, by using a three-phase AC coil. By applying EMS at solidification, the shape of the α-Al phase became particle shape from dendrite shape, and the amount of α-Al phase particles was increased. Moreover, by applying unidirectional intermittent EMS, the volume fraction of α-Al phase particles was decreased with increasing intermittent applying time. In ADC12 alloy, the primary α-Al phases can be crystallized only 10% generally, but it could be obtained over 40% by applying EMS. This means that the semi-solid slurry of near eutectic alloy with over 40% of fraction solid can be obtained by applying EMS.

STACKED DELTA DESIGN OF THREE-PHASE PERMANENT-MAGNET FAULT CURRENT LIMITERS

This paper proposes an improvement for the dynamic performance of presaturated stacked permanent magnet biased three-phase fault current limiter (PMFCL) through COMSOL finite element simulation. The nonlinear demagnetization behavior of the permanent magnet, especially in the upper part of the B-H curve with negative magnetic field intensity, has been modelled through the Jiles-Atherton method. This enables a realistic representation of the PMFCL dynamic behavior throughout its entire operations of pre-fault, fault and fault removal, respectively. The experimental measurements have been considered to validate the trends of the simulation outcomes during the entire operation of PMFCL. Extensive finite element simulation shows that the stacked design of PMFCL can increase the capability of fault current limiting with proper selection of the number and arrangement of the AC coils around the iron core (soft magnet). Results reveal that the division of AC coils into series differential connected subcoils, with an even number, can increase the limiting capability with increasing the AC coil number of turns, without exceeding the permissible tolerances of voltage drop and power losses. Moreover, this stacked design is subjected to parametric investigation for different fault types, either symmetrical or unsymmetrical, or even when changing the fault current peak value.

The relationship between AC loss of YBCO superconducting coil and current waveform, amplitude, frequency and temperature

With the development of superconducting technology, superconducting devices have begun to be used more and more in the AC field, and AC loss is an important factor affecting the application of superconducting devices in the AC field. In this paper, the AC loss measurement experiment of the YBCO coil in the temperature of 20K and 77K is carried out. By applying sinusoidal current and triangular wave current at the frequency of 50Hz to 500Hz, the coil's AC loss is obtained. This paper compared the AC loss of the coil at different temperatures, analyzed the relationship between the AC loss and the current waveform, and gave the correlation formula between the AC loss and the current amplitude and frequency.

Real-time implementation of ring based saturated core fault current limiter to improve fault ride through capability of DFIG system

Enhancement of fault ride-through (FRT) capability in doubly-fed induction generator (DFIG) system is presented in this paper. The saturated core fault current limiter (SCFCL) works on the behaviour of magnetic field induction theory. It is composed of the ferromagnetic core with three limb construction, where both side limbs are surrounded with AC coil and middle limb with DC coil. For the application of restrictive fault, the ideal SCFCL can work for only low-power rating machines. But for the machine higher power rating, the ideal SCFCL has a drawback of developing heat across the limbs, which can damage the insulation of the coils. To overcome this glitch, the embedding of the copper ring across center limb and using forced-air cooling in SCFCL is used. This ring-based SCFCL (RSCFCL) is placed across the stator of DFIG system, which helps in minimizing fault current during any grid disturbances. Having low impedance during normal condition, it can be located permanently in the system without disturbing its stability. The feasibility of ideal SCFCL and RSCFCL is validated by studying the characteristics using thermal radiation under normal and fault conditions. The use of infrared imaging helps to monitor the thermographic patterns of the core on several conditions. The performance of RSCFCL is analyzed with the help of experimental setup and real-time simulator OPAL-RT 4510. Also, to check its effectiveness, it is used for FRT capability in the DFIG system application.

Preliminary Design of CFETR TF Prototype Coil

China fusion engineering test reactor (CFETR), based on ITER technology and bridged between ITER and DEMO, has been supported by China government to start technologies R&D and engineering design. The field of CFETR at plasma core is 6.5 T, maximum field of TF coil is about 14.8 T. TF coil is wound by Nb3Sn and NbTi CIC conductors. Coil weight is about 650 tons with height about 21.7 m and width about 12.3 m. Prototype TF coil manufacture design consists of mechanical & electro-magnetic design and analysis, conductor design and analysis, coil AC loss analysis, thermal–hydraulic analysis and coil cooling, quench detection and coil protection, coil winding, case manufacture, and coil assembling. The TF prototype coil is one sub-task of the CRAFT project, which will last for 5 year and 8 months and is constructed by ASIPP and SWIP cooperatively. The preliminary design of the TF prototype coil has been finished at the present stage. In this paper, the basic design requirements of TF magnet are introduced firstly. Next, the preliminary design and analysis of the TF coil is described, which includes the winding package (WP) design, the coil case (CC), the conductor design and analysis, the electromagnetic, mechanical and AC losses analysis. Then the process and tooling design for the WP and CC manufacturing and assembling is presented. Finally, a summary and a plan are given.

Analysis of key factors affecting the non-limiting state reactance of superconducting fault current limiter

With the rapid development of the power grid, the voltage level is getting higher, and the degree of interconnection is becoming stronger. As a result, the level of the short-circuit fault current of the power grid keeps increasing, which seriously threatens the security of the power grid. The saturated-core superconducting fault current limiter (SCSFCL) is a new type of current limiting device, which can effectively limit the short-circuit current without influencing the flexibility of the power grid. The non-limiting state reactance is very important to the normal operation of the power line protected by the SCSFCL, but there is little detailed analysis of the key factors affecting its non-limiting state reactance so far. In this paper, the effects of the height of AC coil, the turns of AC coil, the cross-section area of the iron core and the excitation of the DC coil on the non-limiting state reactance are analysed and verified by simulation with ANSYS Maxwell. Finally, the design suggestions are put forward, including the method of designing the DC excitation value of the superconducting coil and how the key factors affect the non-limiting state reactance.

Acoustic resonance for contactless ultrasonic cavitation in alloy melts.

Contactless ultrasound is a novel, easily implemented, technique for the Ultrasonic Treatment (UST) of liquid metals. Instead of using a vibrating sonotrode probe inside the melt, which leads to contamination, we consider a high AC frequency electromagnetic coil placed close to the metal free surface. The coil induces a rapidly changing Lorentz force, which in turn excites sound waves. To reach the necessary pressure amplitude for cavitation with the minimum electrical energy use, it was found necessary to achieve acoustic resonance in the liquid volume, by finely tuning the coil AC supply frequency. The appearance of cavitation was then detected experimentally with an externally placed ultrasonic microphone and confirmed by the reduction in grain size of the solidified metal. To predict the appearance of various resonant modes numerically, the exact dimensions of the melt volume, the holding crucible, surrounding structures and their sound properties are required. As cavitation progresses the speed of sound in the melt changes, which in practice means resonance becomes intermittent. Given the complexity of the situation, two competing numerical models are used to compute the soundfield. A high order time-domain method focusing on a particular forcing frequency and a Helmholtz frequency domain method scanning the full frequency range of the power supply. A good agreement is achieved between the two methods and experiments which means the optimal setup for the process can be predicted with some accuracy.

A Novel Multi-Function Saturated-Core Fault Current Limiter

A novel multi-function saturated-core fault current limiter (MFCL) with two operating modes is proposed in this paper. MFCL can achieve fault current limitation and power flow management by switching operating mode: current-limiting mode and power-flow-controlling mode, which realizes technology convergence. The MFCL consists of an iron core with rotatable magnetic valve, ac coils, dc coils, and a fault-limiting reactor. The operating principle of the MFCL and the design of the rotatable magnetic valve are analyzed in this paper. The finite-element analysis (FEA) model based on Maxwell is used to analyze the MFCL. Simulation results depict that the MFCL can limit fault current effectively and provide variable impedance for power flow controlling.

Transient dynamic analyses of presaturated core fault current limiters through flux and inductance versus current modelling

Here, the outcomes of experimental measurements and finite-element simulations are used to develop MATLAB/Simulink models for evaluating the transient dynamic behaviour of the well-known dual-core presaturated core fault current limiter (PCFCL), either for single- or three-phase configurations. These models are based on calculating the total flux linkage–current characteristics of the AC coils at different levels of DC biasing current, taking into consideration the induced voltage across the DC coil terminals due to significant flux variation during the fault condition. On the other hand, the time-varying self-inductance and the self-inductance–current characteristics of the PCFCL are directly developed through Simulink modelling. It is worth mentioning that the self-inductance–current characteristic enables an important and quick design optimisation tool for PCFCL, where the dynamic inductance term significantly contributes to the total voltage drop across PCFCL in comparison to the static inductance. These total flux linkage and inductance versus current models can be directly used for modelling the dynamic transient behaviour of PCFCL when attached to any electrical transient programme used for analysing complex electrical power systems, with remarkable accuracy, quicker, and easier for various network scenarios and fault conditions.

Design Optimization of a Permanent-Magnet Saturated-Core Fault-Current Limiter

Designs of saturated-cores fault current limiters (FCLs) usually implement conducting or superconducting DC coils serving to saturate the magnetic cores during nominal grid performance. The use of coils adds significantly to the operational cost of the system, consuming energy, and requiring maintenance. A derivative of the saturated-cores FCL is a design implementing permanent magnets as an alternative to the DC coils, eliminating practically all maintenance due to its entirely passive components. There are, however, various challenges such as the need to reach deep saturation with the currently available permanent magnets as well as the complications involved in the assembly process due to very powerful magnetic forces between the magnets and the cores. This paper presents several concepts, achieved by extensive magnetic simulations and verified experimentally, that help in maximizing the core saturation of the PMFCL (Permanent Magnet FCL), including optimization of the permanent magnet to core surface ratios and asymmetrical placement of the permanent magnets, both creating an increase in the cores’ magnetic flux at crucial points. In addition, we point to the importance of splitting the AC coils to leave the center core point exposed to best utilize their variable inductance parameters. This paper also describes the stages of design and assembly of a laboratory-scale single phase prototype model with the proposed PMFCL design recommendations, as well as an analysis of real-time results obtained while connecting this prototype to a 220 V grid during nominal and fault states.

Unidirectional focusing of horizontally polarized shear elastic waves electromagnetic acoustic transducers for plate inspection

The ultrasonic guided wave testing method is very effective in monitoring the safety of complex structures. The fundamental shear horizontal (SH) wave mode has the advantage that the propagation velocity does not change with the frequency. However, accurately recognizing the ultrasonic signal is difficult due to the SH guided wave’s bidirectional propagation characteristics generated by the Lorentz force mechanism. Therefore, it is necessary to achieve unidirectional focusing of Lorentz force-generated SH guided waves. A newly designed SH guided wave electromagnetic acoustic transducer (EMAT) is proposed with an interlaced periodic permanent magnet (PPM) and AC coils of different phases. A 3D model based on the electromagnetic mode and the solid mechanics mode using COMSOL software is calculated and compared with the bidirectional focusing EMATs. The results show that there are many influencing factors in the structural design of the transducers, and the focal radius and the angle of the individual PPMs are discussed in this work. The results indicate that the new unidirectional EMAT shows superiority in many cases, and the decrease of the focal radius with a suitable aperture angle will increase the focusing intensity of the signal.

The Influence of Flux Diverter Structures on the AC Loss of HTS Transformer Windings

Compared to conventional transformers, high temperature superconducting (HTS) transformers have higher efficiency and fewer hazards, whose main loss comes from the ac loss of high temperature superconducting windings carrying ac current. Their radial leakage magnetic fields are perpendicular of HTS winding tapes, which is the main parameter affecting the ac loss. In the study of superconducting magnets, the flux diverter has been used to reduce the radial leakage magnetic field. The effect of the flux diverter installed at the top of the transformer windings on the coil ac loss was explored in this paper. The influence of different structure, slotting, and the number of slots on the transformer was also discussed, which laid a theoretical foundation for the optimization of flux diverter on the 6.6 MVA HTS transformers. Besides, in this paper, we choose the homogenization treatment method to reduce the computing time and memory when modeling.

Eddy Current Linear Speed Sensor

Novel eddy current speed sensor with axisymmetric coils and solid iron rod as moving part is presented. The analysis is performed for both dc and ac coil currents and for variable iron rod translational speed. The coil inductance and induced voltage results using the analytical method and finite-element method calculation are compared with the measured values. Two different coil configurations are used for simulations and measurement.