Racetrack Coils Research Articles

Concentric-Coil Distributed Winding Design for Large Direct-Drive Fully Superconducting Machines

Fully superconducting generators (FSCGs) use superconducting wires for both the field and armature windings to reach a high torque density for wind turbine applications. The AC armature winding of an FSCG design mostly employed racetrack tooth-coils to form fractional-slot concentrated windings. This winding type has high leakage flux and excessive harmonics, especially when used in superconducting machines. Other attempts like applying full-pitch racetrack coils can only be with one slot per pole per phase which also has significant slotting harmonics. This paper proposes an innovative concept of double-layer concentric coils to enable distributed windings with a larger integral number of slots per pole per phase. The concept successfully breaks the limitation that racetrack coils cannot be built with such integral slots. This paper then assesses the performance of the proposed winding concept within a context 20 MW FSCG design. The assessment finds out that the proposed concept has balanced three phases but reduces inductance compared to single-layer windings. The force on each concentric coil can significantly be different, thereby complicating the mechanical design of these superconducting coils.

Electro-Thermal Modelling by Novel Variational Methods: Racetrack Coil in Short-Circuit

The design of superconducting applications containing windings of superconducting wires or tapes requires electro-thermal quench modelling. In this article, we present a reliable numerical method based on a variational principle and we benchmark it to a conventional finite difference method that we implemented in C++. As benchmark problem, we consider a racetrack coil made of REBCO superconducting tape under short circuit, approximated as a DC voltage that appears at the initial time. Results show that both models agree with each other and analytical limits. Since both models take screening currents into account, they are promising for the design of magnets (especially fast-ramp magnets) and power applications, such as the stator windings of superconducting motors or generators.

Methodology for Electromagnetic Optimization of a Partially Superconducting 1.4 MW Electric Machine for Electrified Aircraft Propulsion

Large transport aircraft with electrified propulsion systems require MW-class electric machines that have higher efficiency and power-to-weight ratio than existing machines. Superconducting machines are being evaluated as potential solutions. This paper presents a method to optimize the electromagnetic design of a partially superconducting machine in terms of these metrics and the approximate fuel burn of the aircraft. The optimization is based on 2D electromagnetic finite element simulations, a prediction of the superconducting coils' critical current, and geometry relations that design the superconducting coils based on dependent variables while constraining the total cost of superconductor and ensuring adequate space for structure. The design process is completed by (1) a 3D electromagnetic finite element simulation to improve the prediction of torque and assess magnetic flux leakage, (2) refinement of the superconductor width for each individual racetrack coil, and (3) evaluating inverter-produced stator current harmonics on eddy current losses in the rotor.

Design Optimization of HTS Field Coils for High Power Density Motors Based on Continuum Sensitivity Analysis

This study proposes an optimization method for designing HTS field coils for high power density motors based on continuum sensitivity analysis. The continuum sensitivity of a superconducting system represents the gradient of the objective function with respect to the superconducting material shape, taking the critical current characteristics into account. The continuum sensitivity was coupled with the level set method through the design velocity field, and the design of the HTS field coil was performed by solving the level set equation. The applicability of the proposed method was verified by designing a 200 kW HTS motor to maximize the torque density. The design optimization provided an optimal solution through the evolution of the topology and shape of the HTS field coil. The optimized geometry was employed to design field coils consisting of racetrack coils wound with HTS tape. Finally, further designs with different cross-sectional areas of the field coils demonstrated that the proposed method can design HTS motors with minimal HTS tape for a given rated power and motor size.

An HTS Module for Magnetic Combustion Control in a Lab-Scale Hybrid Rocket Engine

Magnetohydrodynamic control of diffusive flames can provide a contactless method for altering combustion in hybrid rocket engines. The presence of a magnetic field modifies conditions within the thermal boundary layer and provides an opportunity for active control of fuel release. However, the realization of the process requires the application of substantial magnetic fields, beyond the capability of copper electromagnets. This work presents a feasibility study of an HTS magnet designed for a lab-scale technology demonstration. The presented magnetohydrodynamic combustion model shows that a magnetic field in the range of 1 T is required to enhance the conditions in the combustion chamber. The realization of cooling to sustain the superconducting state is assumed to be achieved through the utilization of the existing cryogenic environment needed for the liquid oxygen in combustion. The numerical analysis shows that the required magnetic field can be delivered with a mid-sized racetrack coil magnet made of two double pancake coils operating at 65 K. The coil arrangement is shown to be able to withstand the heat generated by the combustion of a 10 s burn. For the lab-scale application, a ferromagnetic core will be used to reduce the amount of superconducting tape required, while retaining a suitable magnetic field. The analysis is concluded with the scaling studies for an HTS coil without an iron core, which shows that a magnetic field of up to 2 T can be achieved.

Calculations of Self- and Mutual Inductances for Racetrack Coils Using Equivalent Models

This paper presents the calculations of self and mutual inductances for thin racetrack coils using a novel approach. Approximate rectangular and diamond shape models for racetrack coils are used for the analysis with the same cross-sectional area and circumference as the racetrack coils. A 3D analytical method using separation of variables method and Fourier series is developed for the magnetic fields analysis and calculation of self and mutual inductances for thin racetrack coils. The analytical method utilizes magnetic vector potential with two components to solve Maxwell differential equations. Different racetrack coils with different dimensions are considered for the validation of analytical modeling for the inductance calculations. The inductance calculations of circular coils are also presented as special cases for racetrack coils. The approaches for the mutual inductance calculation of racetrack and circular coils could be utilized for wireless power transfer and charging applications.

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

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 capability 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.

Effect of a DC transport current on the AC loss in no-insulation ReBCO racetrack coils exposed to AC parallel magnetic field at 77 K and 4.2 K

ReBCO coils are developed as DC field coils in linear motor systems to increase the force density, in favor of permanent magnets. Such coils have to sustain a relatively large heat load stemming from the AC magnetic field environment in which they operate. The use of no or partial turn-to-turn insulation can make them more stable against the effects of local heating. Conversely, the radial electrical connections in no-insulation (NI) coils allow for large coupling currents, causing additional AC loss on top of the already significant heat load. Here we report on the AC loss in sub-scale NI, 4 mm wide single-tape, ReBCO racetrack coils exposed to parallel-to-the-tape magnetic field in the frequency range of 10−4 to 1 Hz at 77 K and 4.2 K, while carrying a DC transport current. AC loss is measured magnetically and electrically. The main goal of these experiments is to validate our 2D numerical model, which provides more insight into the origin of the AC loss. At low frequencies, inter-turn coupling currents are spread more or less homogeneously throughout the winding pack. Whereas at high frequencies, the skin effect causes shielding of the interior of the coil and large induced currents only occupy the coil’s outer surface.

Fabrication of racetrack-type double-pancake coils using 2G tapes and online mechanical shock test

Racetrack-type double-pancake (DP) coil was wound successfully with YBCO high-temperature superconducting tapes. The characteristics and impact resistance of the coils were investigated. The characteristics of the dependent DP coil did not change after applying different acceleration values in the X and Y directions. With the increasing value of acceleration (i.e., up to 30 g in X direction and 15 g in Y direction), the voltage of the racetrack coil remained unaffected. Based on the tests, three racetrack-type coils were wound and assembled in a certain sequence to form a magnet. The optimum temperature range for impregnating the magnet was confirmed by investigating the viscosity and shear stress of paraffin at different temperatures. After the assembly of the coils, the magnet was vacuum impregnated with liquid paraffin. Subsequently, the magnet was fixed in a Dewar that was filled with liquid nitrogen to cool the coils. The magnet was energized after cooling down. Then, the mechanical shock resistance of the coils in Dewar was tested dynamically. Results showed that the coil characteristics did not change after paraffin impregnation. The voltage and magnetic field were almost unaffected by the dynamic increase in the acceleration value to up to 30 g every 5 g interval in the X direction. Thus, these methods of construction and paraffin impregnation and fixtures can be used in the racetrack-type magnet.

R&D of on-board metal-insulation REBCO superconducting magnet for electrodynamic suspension system

Metal-insulation (MI) REBCO high-temperature superconducting (HTS) magnet has the advantages of short charging delay, low contact losses, and self-protection abilities, which shows important application prospects in many fields. Recently, Institute of Electrical Engineering, Chinese Academy of Sciences developed a prototype on-board MI HTS coil used for electrodynamic suspension (EDS) system working with an acceleration of beyond 8 g. To design a reliable engineering on-board magnet, specific Ic angle dependence data of short sample under different fields were fully considered to estimate the operating margin and losses accurately of the on-board magnet. A semi-analytical method, combination of the dynamic circuit method and finite element method, was used to analyze the dynamic electromagnetic force distribution inside the on-board magnet under motion state, in which the effect of the propulsion coils and the figure-eight-shaped coils were taken into considerations. Finally, a full-size MI double pancake racetrack coil was fabricated and tested in a liquid nitrogen bath. The test results meet the design requirements and verify the feasibility of the design, which provides an important reference for the experiment of on-board EDS with high acceleration in the future.

Development of a phased array flexible Rayleigh-wave electromagnetic acoustic transducer for pipe inspection

In this paper, a novel Phased Array fully Flexible Rayleigh-wave EMAT (PAFR-EMAT) suitable for inspection of curved surface based on flexible coil bias field magnet EMAT is proposed. The PAFR-EMAT consists of a flexible racetrack coil generating long pulsed bias magnetic field instead of the conventional permanent magnets, and flexible meander coils triggered in sequence to induce transient eddy current for exciting enhanced Rayleigh wave of MHz frequency. Based on the Lorentz force mechanism, pulsed electromagnetic fields and the generated ultrasonic waves of the proposed new EMAT are studied through numerical simulation at first. Experiments are conducted then to check the practical performance of the PAFR-EMAT for pipe inspection. Results show that the PAFR-EMAT has good performance in Rayleigh wave generation for inspection of the curved surface of metallic pipe and is capable to detect surface crack of small depth.

Design and test of a setup for calorimetric measurements of AC transport losses in HTS racetrack coils

The estimation and measurement of AC losses in coils based on high-temperature superconductors (HTS) are relevant aspects of HTS applications. They influence the cooling power requirement, operating temperature and efficiency, which can be decisive in the construction and implementation of superconducting equipment, such as superconducting electrical machines. The measurement of losses due to AC transport current (without external magnetic field) is not an easy task. Several efforts have been made to measure these AC transport losses in superconducting coils with a calorimetric approach by trying to minimize the influence of the environment, improve the accuracy and ensure the reproducibility of the results. This work presents the design and construction of a setup to measure transport AC losses in high-temperature superconducting coils based on a calorimetric approach (boil-off method). The evaporated cryogen (nitrogen) related to the dissipation of energy is collected by using a 3D printed bubble collector that guides the gas into a flow sensor. A box-inside-a-box approach is used to surround the measurement chamber with a cryogenic environment. This approach allows re-directing the heat transfer from the surroundings into an intermediate zone (space between external and internal box). Since this intermediate zone operates under cryogenic temperatures, the noise and the heat transfer in the internal part of the setup are reduced. A statistical analysis of the results based on a standard load cycle, average value, and standard deviation calculations allows assessing the variability in the measurements and expressing the results as average value and uncertainty range. The calibration and reproducibility of the measurements are verified with a set of resistors under different conditions and during different weeks. Finally, the AC transport losses in a racetrack coil for an electrical machine application are measured and compared with 3D simulation results based on the homogenization of the T-A formulation.

Calculation, Design, and Winding Preliminary Tests of 90-kW HTS Machine for Small-Scale Demonstrator of Generating System for Future Aircraft With Hybrid Propulsion System

Achieving high values of specific power for electric machines needs application of high temperature superconductors (HTS). Production of HTS machines requires development of calculation methods, technology and experimental verification of applied methods. This article represents the results of calculation and modeling of a 90-kW 6000-r/min synchronous machine with HTS armature winding. The considered machine consists of permanent magnet rotor and three-phase stator HTS winding which contains nine double pancake racetrack coils. All coils have been produced and critical current for each coil and n-value were obtained. Machine design was developed. Obtained results allow us to finally assemble the machine and provide experimental research.

Calculation and measurement of coupling loss in a no-insulation ReBCO racetrack coil exposed to AC magnetic field

No-insulation coils are self-protecting and can therefore generally be operated at higher current densities. However, the electrical turn-to-turn connections may cause additional AC loss when charging the coil or when it is exposed to a time-dependent magnetic field. In this work, we study the case of a no-insulation ReBCO tape racetrack coil exposed to a uniform AC field applied parallel to the tape surface. We show that an anisotropic continuum model allows to formulate analytical approximations for coupling loss in the low- and high-frequency limits. For intermediate frequencies, the continuum model needs to be evaluated numerically. The model was validated with representative measurements of AC loss in the coils, measured calorimetrically as well as magnetically using pick-up coils. The validation experiment confirms the predicted frequency dependence of the coupling loss, which is at low frequencies and at high frequencies, due to the skin effect. The transition between low- and high-frequency regimes occurs around a characteristic frequency that is directly related to the characteristic time constant associated with the current decay in (dis)charge experiments.

Dynamic resistance and total loss in small REBCO pancake and racetrack coils carrying DC currents under an AC magnetic field

In many high-temperature superconducting applications, REBCO (Rare-earth barium copper oxide) coils carry DC currents under AC magnetic fields, such as the field winding of rotating machines, linear synchronous motors and the electro-dynamic suspension system of maglev. In such operating conditions, REBCO coils generate AC loss—total loss which includes the magnetization loss due to the shielding currents, and the dynamic loss arising from dynamic resistance caused by the interaction of DC currents and AC magnetic fields. In this work, dynamic resistance and total loss in a small double pancake coil (DPC) and a small double racetrack coil (DRC) are investigated via experiments in the temperature range between 77 K and 65 K. The DC currents are varied from zero to 70% of the self-field critical currents of the REBCO coils, with AC magnetic fields up to 100 mT. The experimental results in the DPC are well supported by the finite element simulation results using 3D T-A formulation. Our results show that the critical current of the DRC is approximately 2%–5% higher than that of the DPC in the temperature range. For given experimental conditions, the magnetization loss in both coils is much greater than the dynamic loss. The dynamic loss and magnetization loss in the DRC are greater than those in the DPC, which we attribute to the large perpendicular magnetic field component in the straight sections of the DRC.

Status and powering test results of HTS undulator coils at 77 K for compact FEL designs

The production of low emittance positron beams for future linear and circular lepton colliders, like CLIC or FCC-ee, requires high-field damping wigglers. Just as compact free-electron lasers (FELs) require high-field but as well short-period undulators to emit high energetic, coherent photons. Using high-temperature superconductors (HTS) in the form of coated ReBCO tape superconductors allows higher magnetic field amplitudes at 4 K and larger operating margins as compared to low-temperature superconductors, like Nb-Ti.This contribution discusses the development work on superconducting vertical racetrack (VR) undulator coils, wound from coated ReBCO tape superconductors. The presented VR coils were modularly designed with a period length of 13 mm. Powering tests in liquid nitrogen of multiple vertical racetrack coils were performed at CERN. The results from the measurements are presented for three VR coils and compared with electromagnetic simulations.

A Lorentz Force EMAT Design with Racetrack Coil and Periodic Permanent Magnets for Selective Enhancement of Ultrasonic Lamb Wave Generation

This article proposes an electromagnetic acoustic transducer (EMAT) for selectively improving the purity and amplitude of ultrasonic Lamb waves in non-ferromagnetic plates. The developed EMAT consists of a racetrack coil and a group of periodic permanent magnets (PPMs). Two-dimensional finite element simulations and experiments are implemented to analyze the working mechanism and performance of the PPM EMAT. Thanks to the specific design, the eddy currents increase with increasing wire density and the directions of the magnetic fields and Lorentz forces alternate according to the polarities of the magnet units. Wires laid uniformly beneath the magnets, and the gaps between adjacent magnets generate tangential and normal Lorentz forces, resulting in-plane (IP) and out-of-plane (OP) displacements, respectively. The constructive interference occurs when the wavelength of the generated Lamb wave is twice the spacing of the magnets, leading to large amplitudes of the targeted ultrasonic Lamb waves. Therefore, the PPM EMAT is capable of generating pure symmetric or antisymmetric mode Lamb waves at respective frequencies. The results prove that the developed PPM EMAT can generate pure either S0 or A0 mode Lamb waves at respective frequencies. The increase in wire width and wire density further increases the signal amplitudes. Compared with the case of conventional meander-line-coil (MLC) EMAT, the amplitudes of the A0 and S0 mode Lamb waves of our PPM EMAT are increased to 880% and 328%, respectively.

Magnetization loss and transport current loss in ReBCO racetrack coils carrying stationary current in time-varying magnetic field at 4.2 K

ReBCO racetrack coils may be used in high-dynamic superconducting linear motor systems, typically replacing either permanent- or electromagnets in the DC stator. Even so, in order to achieve a significant increase in force density, the superconductor needs to carry a high transport current while simultaneously experiencing the time-varying magnetic field from the copper mover coils. To aid with the design of such devices, a 2D numerical model has been developed that predicts the AC loss under motor-relevant conditions, i.e. under the combined influence of a stationary transport current and an alternating external magnetic field. The main aim of the experiments described in this paper is to validate this model with dedicated AC loss measurements. To this end, we constructed a set-up that simultaneously measures magnetization-, transport current- and overall AC loss. Two identical insulated sub-scale ReBCO racetrack coils were tested at 4.2 K while carrying a stationary transport current of up to 700 A in a sinusoidal, alternating magnetic field up to 1.5 T, applied perpendicular to the broad face of the windings. Just like with metallic superconductors, the transport current significantly increases the AC loss level and lowers the penetration field. The inductive, electric and calorimetric data were found to be consistent with each other, validating the experimental calibration methods involved. Furthermore, the numerical model accurately predicted all AC loss components in the coils without any fitting to the data and can thus reliably be used in the design of superconducting machines.

Development and performance evaluation of a conduction-cooled warm bore HTS steering magnet

A conduction-cooled warm bore high-temperature superconducting magnet has been designed, built and tested to serve high rigidity beam steering from K500 cyclotron at the Variable Energy Cyclotron center. The objective of the program is the design and fabrication of HTS magnet system for future accelerator magnet. The magnet is specified for beam steering of ± 3° in horizontal plane and ± 1.5° in vertical plane for maximum beam rigidity of 3.3 T-m. An optimized design has been carried out to achieve the specified field quality by suitably positioning the racetrack coils while satisfying several other design constraints. The window-type magnet is composed of double pancake coils wound with high-temperature superconducting tape mounted inside iron yoke and integrated with two-stage cryocoolers. The magnet was cooled down to operating temperature of approximately 20 K with cryocoolers and reached the designed field of 0.70 T (By-direction) and 0.35 T (Bx-direction). Test results demonstrate the technology feasibility of developing cryogen-free warm bore superconducting magnet for accelerator use. This paper describes the design, manufacture and test results of the HTS magnet.

Characteristic Analysis of Coreless-Type Linear Synchronous Motor With the Racetrack Coils as Secondary for EDS Maglev Train

The characteristics of coreless-type linear synchronous motor, which is applied in the propulsion system of electrodynamic suspension maglev train are calculated and analyzed by the more accurate analytical model. First, the magnetomotive force distribution model of the vehicle coils is proposed according to their racetrack shape. Then, on the basis of the space harmonic method, the characteristic analysis formula of coreless-type linear synchronous motor with single-layer and double-layer armature winding are presented. Second, the air-gap flux density, back electrodynamic force, and thrust of the coreless-type linear synchronous motor are analyzed by the three-dimensional finite element simulation method and the analytical methods. Then, the relationship of efficiency, power factor, and power angle with the number of feeder units is obtained. Finally, the accuracy of the analytical model and numerical result are experimentally validated by comparing with the transient operation data on Yamanashi and Miyazaki test line.