Coil Losses Research Articles

Current control and tuning in a closed HTS coil

Up to today, a coil made of high temperature superconductor (HTS) usually needs a soldered joint to form a closed circuit. It is very difficult to have the joint fully superconducting. The Ohmic joint will cause Joule loss when the coil carries a quasi-persistent current. As a result, the electromagnetic energy stored in an HTS coil declines with time. We propose an approach to reduce the Joule loss of an HTS coil during the energy storing stage. The principle of the approach is to tune the current of the HTS coil smaller by introducing an iron core into the coil to increase the inductance of the coil. With this approach, the Joule loss on the HTS coil will be significantly reduced and the initially stored energy in the coil will be kept with little attenuation in a relatively long energy storing period. When a larger current is required for some functions, the current can be almost returned to the initial charged value by removing the iron core from the HTS coil. In this paper, we report our results in analytical deduction and experimental verification of this principle. Besides, the current value can be tuned to any value in a certain range by controlling the position of the iron core inside the HTS coil. This feature may be useful for some applications in which the current or magnetic field needs to be adjusted.

Clarification of the Method for Determining Distribution of Coil and Core in Small-capacity High-frequency Wireless Power Transformer of Coil Volume and Loss

Clarification of the Method for Determining Distribution of Coil and Core in Small-capacity High-frequency Wireless Power Transformer of Coil Volume and Loss

Performance evaluation of a superconducting flywheel energy storage system incorporating an AC homopolar motor layout

In this paper, a novel high-temperature superconducting flywheel energy storage system (SFESS) is proposed. The SFESS adopts both a superconducting magnetic bearing and a superconducting alternating current (AC) homopolar motor. The superconducting AC homopolar motor has structural advantages in high-speed operation, however performance of the high-temperature superconducting (HTS) field coil is easily affected by the external magnetic field generated by armature windings in the process of flywheel charge and discharge, and should be paid attention. We established an electromagnetic-thermal co-calculation model of the HTS coil to analyze the coil characteristics in this application. Simulation results demonstrate that the magnetic flux density is small near the central region of the coil and is the largest at the edges of the coil. The temperature rise caused by the change of magnetic field distribution reduces the critical current density of the tapes and increases AC loss. Furthermore, we studied the grid connection control strategy of the SFESS, which adopts double closed-loop control and direct power control, and verified the feasibility of the control strategy by multi software real-time and dynamic co-simulation. Works in this paper give some guidance to further development of a 50 kW·h SFESS prototype. All the configurations of the SFESS, the method of calculating AC loss of HTS field coil, and the control strategy of the SFESS system could be extended to large commercial units.

Magnetic Resonance Wire Coil Losses Estimation with Finite-Difference Time-Domain Method

Radiofrequency (RF) coils are used to transmit and receive signals in magnetic resonance (MR) systems. Optimized RF coil design has to take into account strategies to maximize the coil performance by choosing coil sizes and geometry for achieving the best signal-to-noise ratio (SNR). In particular, coil conductor and radiative loss contributions strongly affect the SNR value, with the first mainly playing a role in low-field MR systems especially, while the second could be the dominant coil loss mechanism for high-frequency tuned coils. This paper investigates the accuracy of the finite-difference time-domain (FDTD) method for separately estimating coil conductor and radiative loss contributions. Comparison with finite element method (FEM) analysis and workbench measurements performed on a home-built coil prototype permitted us to validate the simulation results. Moreover, this research, jointly with literature data on sample-induced losses estimation, demonstrates that an FDTD-based solver permits providing an SNR model for coils with various and complicated geometries.

AC Losses in Form-Wound Coils of Surface Mounted Permanent Magnet Vernier Machines

Surface mounted permanent magnet Vernier (SPM-V) machines, because of their high torque density and multi-pole structure, are promising candidates for low-speed direct-drive applications. To achieve high torque density, the SPM-V machines are generally designed with high gear ratios. Therefore, their operating frequencies can be much higher than those of the conventional SPM machines. This potentially increases the alternating current (AC) winding losses, especially those with form-wound coils proposed for high-power applications. This article investigates the AC losses (including skin effect, proximity effect, rotor PM induced, and circulating current losses) in form-wound stator coils of a 3 MW direct-drive SPM-V machine with different slot/pole number combinations. The study reveals that the SPM-V machines have significantly higher AC winding losses than their conventional SPM counterparts for similar operating conditions. To reduce the AC winding losses in SPM-V machines, a novel flux shunt concept is proposed along with other conventional techniques such as increasing the number of turns/coil (with terminal voltage being kept constant) and parallel strands/turn, providing extra clearance at slot opening. With the loss reduction techniques implemented, the SPM-V machines can achieve comparable efficiency but much higher torque density than their conventional counterparts. Moreover, the proposed flux shunt was also found to be very effective in reducing the potential risk of permanent magnet (PM) irreversible demagnetization, a key issue for SPM-V machines at high power ratings.

Experimental and Numerical Study of AC Losses in HTS Coils of AC Electric Machines

Experimental and Numerical Study of AC Losses in HTS Coils of AC Electric Machines

A Reconfigurable Single-Stage Asymmetrical Full-Wave Step-Down Rectifier for Bidirectional Device-to-Device Wireless Fast Charging

This article proposes a reconfigurable single-stage regulating asymmetrical full-wave step-down rectifier (SDR) for fast wireless charging. The SDR is reconfigurable into a step-up power amplifier (SUPA) for device-to-device (D2D) wireless charging. The SDR and SUPA can have a direct connection with the battery without an additional voltage regulator module (VRM). With the single-stage structure, the transmitting (TX) and receiving (RX) ac voltages across the resonant coil can be boosted, which greatly reduces the coil currents and losses, improving the D2D charging power. A reconfigurable controller for the TX/RX mode selection, as well as the switching timing control, is designed. The chip is fabricated in 0.35-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS that contains 20-V devices. For the asymmetrical operation, one GaN device is used for the high current path. Also, zero-voltage switching (ZVS) is realized for all the power transistors with one small (180 nH in the 4015 package) power inductor. The bidirectional D2D WPT system achieves a maximum charging power of 5.1 W with a D2D total efficiency of 46.3&#x0025; and a charging distance of 1.6 cm. Finally, the peak total efficiency is 51.4&#x0025; with a charging power of 3.4 W and a charging distance of 1 cm.

An Efficient Method for Litz-Wire AC Loss Computation in Transient Finite Element Analysis

An efficient method is proposed to predict eddy current loss of Litz wire coils, or completely transposed multi-strand coils, in transient finite element analysis (FEA). The method is based on analytical solutions of eddy current loss in a solid wire due to skin effects and proximity effects. The bundle of Litz or solid wires is homogenized by introducing a fill factor to describe the conducting area ratio. The analytical solutions in the frequency domain for round, square, and rectangular wires are further expanded to those in the time domain. The wire length direction in each 3-D mesh is automatically detected so that the flux density vector in each mesh can be correctly decomposed to the orthogonal and tangential components for anisotropic proximity effect loss computation. The method is applied to calculating eddy current loss of a gapped inductor in both 2-D and 3-D, and the results are compared with that using real solid wires.

Improved Winding and Compensation Methods for the Multilayer Coil in IPT System

Wireless power transmission technology is used to transmit energy without contact in the field of electric vehicles, consumer electronics, and other industries. Among them, there are some applications with very low coupling coefficient where multilayer coil needs to be wound. Influenced by the parasitic capacitor between layers, the loss of the multilayer coil increases. The improved coil winding method and compensation method are proposed in this letter. Winding coils following the same sequence from outside to inside in each layer and connecting capacitors in series between layers of the coil can significantly reduce the voltage of the layers’ parasitic capacitor, which will decrease the coil loss. Compared with traditional method, the improved method increases the system efficiency at heavy load and light load by 0.74% and 2.48%, respectively. A 1-kW prototype was built with the maximum efficiency of 93.58%.

Experimental and numerical study of AC losses in HTS coils of AC electrical machines

Experimental and numerical study of AC losses in HTS coils of AC electrical machines

Temperature Distribution in the Field Coil of a 500-kW HTS AC Homopolar Motor

AC homopolar motors with high-temperature superconductors (HTSs) are considered to be a promising technology for high-power applications. With superconducting components being placed exclusively in nonrotating parts, a motor can reach high rotational speeds. However, the dynamic behavior of the system creates a nonnegligible magnetic ripple field, which generates heat and makes thermal management of the cryogenic system more challenging. In this article, we present a case study for the thermal behavior of a conduction-cooled high-temperature superconducting field coil assembly in a 500-kW asynchronous homopolar motor. The analysis focuses on the investigation of the magnetic ripple field and its impact on heat generation due to ac loss in the HTS field coil and eddy current loss in copper parts present in the assembly. Using electromagnetic and thermal finite-element models, we construct a detailed description of the thermal bus design, which considers different cooling scenarios and sizing of the conductive elements. Depending on the geometry of the thermal bus, we observe heat loads from ac losses of up to 0.4 W and eddy current losses in the copper elements of up to 1 W. These heat loads translate to temperature increases as high as 10 K. Techniques to reduce the temperature rise based on thermal bus redesign are presented, along with recommendations for the most promising cooling scenarios and optimal thermal bus geometry.

An Electromagnetic Design of a Fully Superconducting Generator for Wind Application

A fully superconducting wind generator employs superconductors in stator and rotor to enable high torque density and low weight, that is, enable an ultra-light electric machine for wind application. However, the level of the AC loss of the stator armature coils is a critical issue, which lacks investigations in the design of the fully superconducting generators. In this paper, an in-house model was developed to analyze the potential of a fully superconducting generator by integrating the electromagnetic design with the AC loss estimation. The electromagnetic model was made through analytical equations, which take into consideration the geometry, the magnetic properties of iron, and the nonlinear E–J constitutive law of superconductors. Since the permeability of iron materials and the critical current of the superconductors depend on the magnetic field, an iteration process was proposed to find their operating points for every electromagnetic design. The AC loss estimation was carried out through finite element software based on the T–A formulation of Maxwell’s equations instead of analytical equations, due to the complexity of magnetic fields, currents and rotation. The results demonstrate that the design approach is viable and efficient, and is therefore useful for the preliminary design of the generator. In addition, it is found that smaller tape width, larger distance between the superconducting coils in the same slot, smaller coil number in one slot and lower working temperature can reduce the AC loss of the stator coils, but the reduction of the AC loss needs careful design to achieve an optimum solution.

A method to predict AC loss on HTS Coils of a 30-kW generator using the T-A formulation

High-temperature superconducting (HTS) conductors are gradually being used in electrical applications and have promising market prospects in the near future. However, HTS conductors inevitably generate AC loss under time-varying currents and magnetic fields. Loss estimation should be of great importance because it influences the generator's structure design and cryocooler's option for HTS applications. In this work, rotating machinery, magnetic (RMM); partial differential equation (PDE); and electrical circuit (CE) modules are fully coupled to calculate the heat dissipation on 30 kW high-temperature superconducting-stator (HTS-stator) coils with the help of Maxwell equations and the T-A formulation. In the whole model, macroscopic results, such as output the voltages and current, are calculated by the RMM and CE modules, while microscopic results, such as the current density distribution and magnetic field penetration, are estimated from the coupling of RMM and PDE modules. It should be noted that an extra deviation value for the current density needs to be introduced into the RMM module for superimposition on the original current density in the HTS coil derived from previous results. By using our model, the AC loss of an HTS coil in this generator is successfully estimated to be 37.3 W. This value is consistent with the result obtained using the magnetic field boundary extraction method. In addition, the AC loss estimation of an HTS coil in the generator is completed under different rotation speeds/frequencies and is positively correlated with them.

A new vision of short-time and long-time AC loss measurement and modelling: A superconducting power electronic circuit

This paper provides a new vision of loss measurement and modelling for superconducting coils, with both the short-time test and long-time test, as the behaviour of the AC loss in superconductors is actually a dynamic process with the temperature and time dependence, particularly for the medium-to-high frequency AC operation. A single-pancake High temperature superconducting (HTS) coil was tested in a power electronic circuit (DC-AC inverter). The operating current was from 15 A to 50 A, and the operating frequency was from 1 kHz to 5 kHz. The AC loss measurements were carried out using the function of fully decoupling power by a commercial power analyzer. This method was able to measure the real-time AC loss, and the loss measuring process is much easier than the conventional electrical and magnetic method. As expected, for short-time loss tests, the experiment, the FEM H-formulation model, and the conventional analytical model agree with each other. However, the AC losses in long-time tests are found having power-law relations with the operating time (above kHz level). As the accumulated heat in the HTS coil as well as the interrelation between the critical current and temperature, the real-time AC losses are strongly time-dependent. A new time-dependent analytical model is developed, for the purpose of accurately modelling the real-time loss in the HTS coil. The new analytical model well matches the AC loss tests over both the long and short testing time. The experimental method and new analytical model offer a new sight at the AC loss of superconducting coil for both the short-time and long-time operation in a power electronic circuit, which is able to explore more reasonable guidelines of safe operating area (SOA) during the practical AC operation of superconducting applications.

AC Loss Evaluation of a Superconducting Pancake Coil with Coated Conductors using an Extended A-V Formulation

In this study, an extended A-V formulation is used to determine the alternating current (AC) losses of a coil made of high temperature superconducting (HTS) tape. The current is the same at each turn by balancing the voltages of each turn. By this modification, this method can be applied in a wide application area. This method also provides the chance to determine the calculations at currents higher than the critical current values in a more stable way. The AC loss characteristics of the pancake coil are examined in detail by changing the current value and frequency. Also, the losses at individual turns (1st, 5th, and 10th) are presented. It was seen that the most outer and inner turns have higher losses than that of the other turns. The AC loss results obtained from the extended A-V formulation used in this study are compared with experimental and H-formulation-based results and are in good agreement. The frequency dependence of the pancake coil's loss is revealed at 5 frequency values. The results show that the loss is frequency and current amplitude dependent as expected. Similarly, the calculated losses for each frequency are in good agreement with that obtained from the H- formulation. The current distributions of a single tape and the entire coil are also presented, and all results are discussed in detail.

AC Copper Losses in Edge-Wound Coils in a YASA Motor

AC Copper Losses in Edge-Wound Coils in a YASA Motor

Magnetization loss of no-insulation coil for an electrodynamic suspension system

A high-temperature superconducting no-insulation (NI) coil, with its self-protection property, high engineering current density and unique demagnetization property, becomes a potential candidate for an electrodynamic suspension (EDS) system. Compared with the applications in high field magnets, the NI coil used in the EDS system is considered as working in a dynamic state, the magnetization loss generated in the NI coil is essential for the design of a cryogenic system. This paper presents the study on AC magnetization loss of NI coils by both numerical and experimental methods. Firstly, a 3D finite element numerical model representing the full geometry of the NI coil is built to analyse the effect of field frequency, field magnitude, as well as the radial characteristic resistance. Then, systematic discussions are conducted to figure out the working mechanism of NI coils. Finally, a calibration-free method testing platform is installed to validate the numerical model, and a modified model is proposed to represent the non-uniform radial characteristic resistance caused by stress distribution. The conclusions of this paper will be used in the future optimization of NI coils and the cryogenic design of the EDS system.

Electromagnetic Fault Analysis of Superconducting Wind Generator With Different Topologies

Superconducting (SC) direct-drive wind generators are one of the best candidates for the offshore wind energy applications. The gearbox that exists in the traditional wind generators can be removed. Using superconductors in the armature and excitation windings will largely increase the machine torque density. However, the synchronous reactance in a traditional superconducting synchronous machine is usually low due to its large air gap. Consequently, operating faults will largely threat the machine itself. Also, the superconductor needs to work in the low temperature, usually around 30 K. The magnetic field around it and the current flows in it are also limited. Thus, to improve the machine reliability, the superconducting system needs to be taken care. In this paper, a double stator superconducting flux modulation machine (DSSFMM) is studied and its fault performances are analyzed. From the results, both fault current and fault torque in this kind of machine are low. It is also indicated that the AC loss of the SC coils during fault condition is low.

Thermal Characteristics of REBCO Coil in a Wireless Power Transmission System for the Railway Vehicle in Liquid Nitrogen

A wireless power transmission (WPT) system using the high temperature superconducting (HTS) coil for a railway vehicle is required to transmit the electric power of several hundred kW in a short time (60 s). In order to design the WPT system using HTS coil with the thermal stability, and it is necessary to clarify the thermal characteristics of the HTS coil at the operating current in kHz band and the operating conditions considering thermal stability in the WPT system. We analyzed the frequency characteristics of the critical current in the HTS coil above 1 kHz and investigated the operating conditions considering thermal stability in the WPT system for the railway vehicle using HTS coil in liquid nitrogen. As a result, the critical current of the HTS coil in the kHz band decreased with the frequency. This is because the AC loss in the HTS coil increased with the frequency, and the critical current density of the HTS coil decreased with increasing the surface temperature. In order to design the HTS coil with thermal stability in the kHz band, and it is necessary to operate the HTS coil on alternating current less than about 50% of the critical current measured by direct current. We found that the thermally stable WPT system capable of fast charging for 60 seconds can be realized by operating current at the frequency of about 2 kHz.

AC Loss Reduction Through Flux Diverters for Superconducting Wireless Charging Coils at High Frequencies

Wireless power transfer (WPT) for electric vehicles (EVs), also known as wireless charging, is a fast-developing technology with many advantages over conventional cable charging. Novel materials such as high temperature superconductors (HTS) can be used to increase system performance. However, HTS coils have high ac losses at wireless charging frequencies (10-100 kHz) and this is crucial for WPT systems. Flux diverters have been used in the past for dc and low frequency (~50 Hz) superconducting applications but their effectiveness at high frequencies is unexplored. This article investigates the impact of flux diverters on HTS double pancake coils operating at high frequencies up to 85 kHz, commonly used for wireless charging of EVs. Various geometric characteristics of flux diverters are investigated such as air gap between diverter and coil, thickness, and width. A 2-D axisymmetric multilayer model using finite element analysis was used to examine the coil and diverter losses at such frequencies and different load factors (LFs) between 0.1 and 0.8. It is demonstrated that flux diverters are a viable option to reduce the coil losses even at high frequencies and the width of the flux diverter has the biggest impact on the loss reduction. In general, flux diverters are more suitable for applications using high LFs. Lastly, the impact of the diverter in terms of magnetic field distribution above the coil and overall loss distribution in the HTS coil was examined.