Coil Losses Research Articles

Optimal Number of Turns Design of the IPT Coils for Laptop Wireless Charging

The coil design of the inductive power transfer (IPT) for a transmitter (Tx) and receiver (Rx) is crucial to determine the performance of the IPT. Considering core loss and copper loss of the Tx and Rx coils, which are major loss terms of the whole IPT system, determination of the winding turns for the Tx and Rx coils becomes a key factor for maximum power transfer design of the IPT coils. In this paper, an optimal turns design of IPT coils for laptop wireless charging applications with a maximum power transfer efficiency is proposed. Under a specific design requirement of IPT, optimal turns of the Tx and Rx coils N1,op and N2,op can be determined by the proposed IPT coil design procedure with a finite-element-method (FEM) based simulation analysis. From the results of the coil design by the proposed IPT coil design methodology, N1,op and N2,op for maximum power efficiency, and input DC voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> for the target load power can be found by the proposed coil design procedure. The 40W prototypes of the two symmetry and asymmetry IPT coils for laptop wireless charging were fabricated, and verified by simulation and experiments. To separately verify the core and copper loss of the IPT coils, a resistance model and Steinmetz equation-based methods were comparatively evaluated, and it was found that they matched well at a core temperature condition of below 40oC. The results showed that the total weight and the thickness of the Tx and Rx coils as well as the maximum power efficiencies of 95.1% and 96.8% for the symmetry and asymmetry cases were obtained by the proposed IPT coil design procedure, which becomes a practical solution for the laptop wireless charging coil design.

Improved Performance of a Flat-Wire Coil With Magnetic Composite Material for Wireless Power Transfer

This letter focuses on improvements in the performance of flat-wire coils for wireless power transfer in smartphones based on the Qi interface standard. At high frequencies, alternating-current copper losses increase due to the proximity and skin effects. A low-permeability magnetic composite with low hysteresis loss was applied to a coil to reduce coil losses. A parametric analysis was used to obtain the optimum composition of the magnetic coating. Power transmission experiments on a coated coil designed for minimum loss showed that the steady-state temperature was lower than that of an uncoated coil. The coated-coil resistance was lower at high frequencies, where resistance due to skin and proximity effects are significant. When misaligned with respect to a charging pad's transmit coil, the coated coil had a smaller decrease in efficiency compared to an uncoated coil.

Influence of field-dependent critical current on harmonic AC loss analysis in HTS coils for superconducting transformers supplying non-linear loads

There are two main obstacles in front of the development of high temperature superconducting (HTS) technology for electric power network applications; tape price and cooling cost. In order to reduce cooling cost, it is vital to evaluate AC transport current loss of the tapes precisely and then reduce it by some design innovative approaches. In addition, AC transport current loss in HTS material is a critical design variable for large-scale power network applications such as HTS transformers, superconducting fault current limiters, and power cables, since they are continuously carrying the network/load current during their operating life. In existing power networks, harmonic production sources are commonly used and thus, currents are distorted. Therefore, the effect of nonsinusoidal current on the critical apparatus in the network such as transformer must be studied. In this paper, AC transport current loss of a single-turn 2G YBCO HTS coil was modeled and numerically calculated under nonsinusoidal transport current using finite element method. Furthermore, influence of dependency of critical current density to magnetic field on the AC transport current loss of HTS coil when carries distorted currents was considered. It was observed that nonsinusoidal current causes excessive losses in HTS coil. On the other hand, a case study on an HTS transformer supplying non-linear load was considered to study the loss increment as well as heat load change. It was observed that current harmonics increases the AC loss, and heat load of transformer and decreases the efficiency, consequently.

AC Losses in Noninductive SFCL Solenoidal Coils Wound by Parallel Conductors

Large scale resistive superconducting fault current limiters (SFCLs) have attracted great interest in electric power systems and aviation applications due to their compactness, lightweight, automatic fault current limiting, and fast recovery characteristics. Noninductive coil wound with stacked conductors in parallel connection is commonly used to achieve high current rating for SFCL. It is of great significance and engineering value to minimize ac losses in the noninductive coil by optimizing the configuration of the stacked conductors in order to reduce the thermal load for the cooling system. In this article, multiple possible configurations of noninductive stacks where each turn is composed of one, two, and three conductors, respectively, were proposed. Numerical models were developed to investigate the effect of noninductive stack configuration on ac loss. Eight-turn noninductive solenoidal coils wound with the noninductive stacks mentioned in the above were also simulated. It is concluded that the configuration of the noninductive stack significantly affects the perpendicular magnetic field distribution in the end edges of the stack. Single-layer configurations have limited magnetic field cancellation effect. The stack configuration with low ac loss and simple structure was suggested.

AC loss calculation in REBCO coils or stacks by solving the equation of motion for current using an integration approach

The flux motion and its collective behaviour known as AC loss is the basic characteristic of high temperature superconducting (HTS) materials and controls the economic efficiency, stability and magnetic field uniformity of superconducting equipment. In so far, we have no method to calculate the AC loss of HTS devices in an efficient way. Normally, it will take days to simulate AC loss of HTS coils even with moderate number of turns. In this paper, we surprisingly found that an integral method is remarkably efficient in calculating AC loss of the coils in large scale. By comparing with the fastest method at present, the integral method is proved to be 4.4 times faster than the present one. We give the calculation details and discuss why this method has an advantage in calculating AC loss of the coil with a large number of turns.

AC loss measurement and simulation in a REBCO coil assembly utilising low-loss magnetic flux diverters

AC loss in high temperature superconducting (HTS) coils affects the performance of HTS devices. Using magnetic flux diverters (MFDs) is an effective way to reduce AC loss in HTS coils. In this paper, measurement and finite element method simulation of AC loss results in a REBCO coil assembly comprising four double pancake coils with two molypermalloy-powder MFDs are presented. Both experimental and numerical results show that MFDs can significantly reduce the AC loss in the REBCO coil assembly while generating negligible loss in themselves. Further, the influence of the distance between the coil assembly and the diverters on AC loss reduction is explored. Compared with the AC loss data in the coil assembly without MFDs, over 80% AC loss reduction is achieved when the distance between the coil assembly and the diverters is at its minimum value, 2 mm. The simulation results reveal that the AC loss reduction in the coil assembly is mainly due to the reduction of the radial (perpendicular) magnetic field component to the surface of REBCO wires in the end windings of the coil assembly.

Multi-parameter Analytical Method for B1 and SNR Analysis (MAMBA): An open source RF coil design tool

In Magnetic Resonance Imaging (MRI), radio frequency (RF) coils of different forms and shapes are used to maximize signal-to-noise ratio (SNR). RF coils are designed for clinical applications and have dimensions comparable with the target body part to be imaged, and they perform best when loaded by human tissue majority of which have conductivity values higher than 0.5 S/m. However, they are not properly tuned and matched for samples having low conductivity such as solid samples with low water content. Moreover, for samples with low filling factor and low conductivity, the noise in MRI is dominated by RF coil losses. In this case, RF coil design can be optimized to improve image SNR.Here, a new software tool (Multi-parameter Analytical Method for B1 and SNR Analysis) MAMBA is presented to design and compare volume coils of birdcage, solenoid, and loop-gap design for these samples. The input parameters of the tool are the sample properties, the coil design and the hardware properties, of which a relative SNR is determined. For that, a figure of merit is calculated from the coil sensitivity, applied resonant frequency and the resistive losses of sample, coil and capacitive components. The tool was tested in an ancient Egyptian mummy head which represents an extreme case of MRI with short T2*. Two optimized birdcage coils were designed using MAMBA, constructed and compared to a commercial transmit receive head coil. Calculated relative SNR values are in good agreement with the measurements.

A Hybrid Control Strategy of LCC-S Compensated WPT System for Wide Output Voltage and ZVS Range With Minimized Reactive Current

To regulate the output voltage of inductor–capacitor–capacitor-series ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -S) compensated wireless power transfer (WPT) system, a hybrid control strategy of phase shift modulation (PSM) and a switch-controlled capacitor is presented in this article. In the proposed hybrid control strategy, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -S compensated WPT system can realize wide output voltage regulation with zero voltage switching (ZVS), and the reactive current in a resonant tank can be minimized in the whole voltage range. Besides, the current in the primary coil of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -S compensated WPT system can be regulated with PSM, which helps to reduce the power loss of the primary coil at light load. Compared with conventional dual-loop hybrid control, the proposed hybrid control can regulate two control variables with one feedback loop simultaneously. To verify the theoretical analysis, a 500-W prototype with 400-V input voltage, 100−250-V output voltage is built. The experimental results show that the inverter can realize ZVS within the whole voltage range and a peak efficiency of 94.7% can be obtained.

AC Loss and Ramp Up Related Heating Effects in Superconducting MgB2 Coils

Superconducting coils modeled by using single core MgB2 wires having copper or iron or both in the cross section as sheath materials have been examined numerically by using the magnetic field formulation and heat transfer modules of COMSOL Multiphysics software. Heating effects due to the core ac losses and skin effects in MgB2 coils are revealed for alternating currents in a liquid helium environment. Temperature changes, magnetic flux density profiles, core ac losses for each coil are obtained in one full cycle of applied alternating current. The effect of copper layer thickness on core ac loss behavior of MgB2/Cu coil and transport losses in MgB2/Cu and MgB2/Fe coils as a function of current ramp rate are investigated for an applied direct current up to 300 A.

Multi-Physics Coupling Simulation of Heat Transfer in Transformer Winding

In the electric power industry, the requirements for capacity, voltage level and compactness of transformers are getting higher and higher, which lead to significant problems, such as increased loss and more serious hot spots phenomena. Based on the three-dimensional finite element method, this paper establishes the bidirectional coupling between electromagnetic field, flow field and temperature field to calculate the temperature rise of 400 kVA oil-immersed transformer at full load and compare it with the results obtained by the heat-flow unidirectional coupling method (constant loss). The results show, due to the interaction between electromagnetic field and temperature field, the coil loss density increases, but the core loss density does not change. The hot spot temperature of coil increases and the location of coil hot spot changes. The temperature and position of the core hot spot do not change. The loss density of coil decreases as the inlet velocity increases, the hot spot temperature of coil drops faster. When the inlet velocity exceeds 0.5 m/s, increasing the inlet velocity can no longer effectively promote heat exchange. With the increase of the inlet velocity, the interaction between electromagnetic field and temperature field is weakened, but it still has a great impact on the loss density of coil. The multi-physics coupling method is of great significance to improve the accuracy of transformer temperature rise calculation.

Dynamic loss and magnetization loss of HTS coated conductors, stacks, and coils for high-speed synchronous machines

Dynamic loss is an essential parameter to consider for the design of high temperature superconducting (HTS) synchronous machine windings. For aerospace electric propulsion systems, the fundamental frequency component and harmonics in electric machines can attain kHz level because of the high rotating speed. However, for HTS coated conductors (CC), the existing definition of dynamic loss only considers the HTS layer, the validity of which at high frequencies is questionable. Besides, the variation of dynamic loss and magnetization loss under skin effect due to high frequency is still unknown. Additionally, the influence of shielding effects among distinct turns on the dynamic loss of HTS stacks and coils remains unclear. In response to the above concerns, by use of the H-formulation based numerical multilayer modelling method which considers all layers of a CC, the frequency dependence of dynamic loss and magnetization loss of HTS CCs, stacks and coils over a wide range up to 20 kHz has been investigated. Results show that the existing definition of the dynamic region is no longer valid at kHz level, which shrinks rapidly with increasing frequency and magnetization loss plays a progressively important role due to skin effect. Meanwhile, the shielding effect in HTS stacks and coils can enhance the significance of dynamic loss. This paper clarifies the characteristics of dynamic loss and magnetization loss of HTS CCs, stacks, and coils over a wide frequency band, which can serve as a useful reference for accurate loss controlling of machine windings in future aerospace HTS propulsion systems.

Reduction in AC Loss of REBCO Coils with Optimum Multi-Width Tape Induced by Part Slitting Method

The width of REBCO (REBa2Cu3O7-x, RE: rare earths)-coated conductor is an important factor associated with AC (alternating current) loss of the tape. In the same electromagnetic environment, coils winding with narrow REBCO tapes generate less AC loss than the wide one. For a single coil, the critical current density of outer turns is higher than that of inner turns, due to the inhomogeneous distribution of magnetic field. This paper presents a multi-width technique which is applicable for single REBCO coil by partly slitting single tape in place, which can effectively reduce AC loss, while maintaining current-carrying capacity in high field. The AC loss of coils with slitting in different turns is calculated by the software of finite element modeling (FEM). The results indicate that the appropriate slitting in radially outer turns of coil appears effective to reduce AC loss, especially in the case of the coils running with high current.

The Influence of Substrate Size Changes on the Coil Resistance of the Wireless Power Transfer System

Wireless power transfer (WPT) technology has been widely used in many fields. Nevertheless, in the field of high power transmission, such as the WPT system of electric vehicles, the power transmission efficiency of WPT system lags behind that of wired charging due to losses brought by substrate shielding materials. In this regard, the conduction resistance of Litz-wire coils without substrate is analyzed first in this paper. Secondly, the induction resistance of the coil with single-layer and double-layer substrate materials is modeled. Then, through the establishment of a coil simulation and experimental platform with a single-layer substrate, a contrastive analysis of the variation trend of coil equivalent series resistance (coil ESR) at changing thickness and area and constant volume of the substrate is carried out in combination with the theory. The variation law of coil ESR at changing thickness and area and constant volume of double-layer substrate is also explored at the end of this paper. This is expected to contribute to the reduction of coil losses in the WPT system through a systematic study of the influence of substrate size changes on the coil resistance of the WPT system.

Loss characteristics of superconducting pancake, solenoid and spiral coils for wireless power transfer

Wireless power transfer (WPT) is an emerging technology with widespread applications, such as wireless charging for electric vehicles (EVs), which has become a major point of interest. Conventionally, it is used for stationary charging, but also dynamic systems emerge. Key drawbacks of standard WPT systems are the limited transfer distance between the copper coils and the transfer efficiency. By employing high-temperature superconductors (HTS) as coil material these limitations can be alleviated. However, HTS coils have highly nonlinear ac loss characteristics, which will be studied. This paper investigates the transport current loss and the magnetisation loss of HTS coils individually and when combined in the high frequency range relevant to WPT for EVs. A multilayer 2D axisymmetric coil model based on H-formulation is proposed and validated by experimental results as the HTS film layer is inapplicable at such frequencies. Three of the most commonly employed coil configurations, namely: double pancake, solenoid and circular spiral are examined. While spiral coils experience the highest transport current loss, solenoid coils are subject to the highest magnetisation loss due to the overall distribution of the turns. Furthermore, a transition frequency is defined for each coil when losses in the copper layer exceed the HTS losses. It is much lower for coils due to the interactions between the different turns compared to single HTS tapes. At higher frequencies, the range of magnetic field densities, causing a shift where the highest losses occur, decreases until losses in the copper stabilisers always dominate. In addition, case studies investigating the suitability of HTS-WPT are proposed.

A Comparative study of transport current loss in HTS coils for superconducting wireless power transfer

This paper focuses on the transport current loss modelling for HTS coated conductor coils under high-frequency AC current, which is essential when applying superconducting technology in wireless power transfer system (WPT). The pancake coil, spiral coil and solenoid coil designs were proposed, whose study were carried out based on the Finite Element Method in 2D using H-formulation. The transport current loss in each layer for each coil when applying high-frequency AC current were simulated in the FEM platform. The variation of the transport current loss distribution from different layers of the HTS coated conductor according to changing current input was simulated and compared for different operating frequencies. Finally, the frequency and current dependence of the transport current loss in different layers was shown and compared. The simulation demonstrates that the transport current loss in copper layer could be much higher than the transport current loss in the HTS layer at high current and frequency level due to the skin effect. The results also present that the pancake coil design and spiral coil design could be applied to relatively lower frequency WPT application while solenoid coil design can achieve lowest total loss in higher frequency application. These results are essential when designing HTS coils for WPT system.

Loss analysis of superconducting wireless charging for electric vehicles

Wireless power transfer (WPT) is an emerging technology with widespread applications. It can be used for charging in multiple applications, such as wireless charging for electric vehicles (EVs), which has become a major point of interest. Conventionally, it is used for stationary charging, but also dynamic systems emerge. Key drawbacks of standard WPT systems are the limited transfer distance between the copper coils and the transfer efficiency. By employing high-temperature superconductors (HTS) as coil material these limitations can be alleviated. However, HTS coils have an interesting ac loss characteristic which will be studied. This study investigates the transport current and magnetisation loss of HTS coils in the frequency relevant to WPT for EVs. In addition, a comparison between the full anisotropic dependency of the critical current and only considering perpendicular field components is conducted. A homogenous 2D axisymmetric coil model is used to examine three of the most employed coil configurations and their loss characteristics depending on turn numbers.

Reduction of AC Loss in HTS Coils of Superferric Magnets for Rapid-Cycling Synchrotrons by Changing Cross-Section of Coils and Iron Yoke Geometry

We have been studying the potential of energy consumption reduction in rapid-cycling synchrotrons (RCSs) by using superferric magnets with coils wound with high temperature superconductor (HTS). By conducting three-dimensional electromagnetic field analyses on superferric magnets with different coil cross-sections and iron yoke geometries, ac loss of each magnet design was estimated. Rough estimations of the iron loss were also conducted to obtain overall energy consumption of each magnet design. By the comparison, influence of coil cross-section and iron yoke geometry on electricity consumption of the magnet were evaluated.

Experimental AC loss analysis of braid type non-inductive coil for superconducting fault current limiter

Resistive superconducting fault current limiter (SFCL) plays a key role to realise the stability and safety of power grid and the future electric aircraft. This paper presents the design and test of a braid type non-inductive coil for SFCL. One inner coil and one outer coil were wound around a G10 tube using 4 mm-wide SuperPower wires. Two configurations of the braid type non-inductive coil were achieved by connecting the inner coil and outer coil in series and in parallel, respectively. AC loss results of these two non-inductive coils were measured and compared to investigate the AC loss dependency on coil configuration and current distribution. It has been concluded that braid type non-inductive coil has low AC loss due to magnetic field cancellation. Serial connection of the inner and outer coils leads to a good degree of field cancellation, which results in higher critical current and lower AC loss of the non-inductive coil, compared to parallel connection.

Numerical Study on AC Loss of an HTS Coil Placed on Laminated Silicon Steel Sheets With Distorted AC Transport Currents

AC loss is an intractable and inevitable issue on high temperature superconducting (HTS) coils. In some HTS devices with the iron core, AC loss in HTS coils have been investigated. However, practically, partial distorted currents can flow in HTS coils, and the AC loss in this situation has rarely been conducted. Here, a normal and distorted current were respectively applied into a double-layer racetrack coil (DRC) to induce AC losses. The numerical and experimental results were obtained with and without laminated silicon steel sheets (LSSS), which showed that the AC loss of the DRC significantly increased when put on the LSSS. Furthermore, compared with the loss generated by the fundamental waveform, currents with 3rd harmonic in phase and 5th out of phase, both with 10% total harmonic distortion (THD) could increase AC loss by 9.3% and 7.8%, respectively. To reduce AC loss of the DRC on the LSSS, two methods are proposed, and it is found that increasing the 5 mm distance vertically and horizontally between the DRC and LSSS could significantly decrease the AC loss by 20.7% and 18.0%, respectively. This study could offer an indispensable reference for designing and operating HTS coils with the iron core for AC applications.

Minimum power loss based design of SMES as influenced by coil material

• Optimizing the dimensions of the SMES coil to achieve minimum coil loss • The Genetic Algorithm (GA) is utilized to minimize the coil loss as a multi-objective function • Optimal AC loss, refrigeration loss and total loss of SMES coil • Different coil materials are considered to select the best material with minimum coil loss. • The effect of the twist pitch and circumferential stress on the total losses in the SMES coil is studied. Superconducting magnetic energy storage (SMES) system is one of the commonly used techniques by the end-users to mitigate the voltage sag at their premises from the distribution system. The SMES is a superconductor coil wound on a nonmagnetic core. The paper is aimed at optimizing the dimensions of the coil to achieve minimum coil loss. The Genetic Algorithm (GA) is utilized to minimize the coil loss as a multi-objective function with indices including AC loss and refrigeration loss. The results show that the optimal total loss of SMES coil at capacity energy of 4 MJ is 85.04 W against 114.5 W in a previous work. The corresponding optimal dimensions of the coil which include the inner diameter, outer diameter and coil length are 0.19, 0.2 and 2.17 m, respectively. Different low-temperature superconductivity coil materials at operating temperature of 4.2 o K are considered to select the best material with minimum coil loss. The results conclude that Nb–Ti material achieves minimum coil loss when compared with MgB 2 and YBCO. The effect of the twist pitch and circumferential stress on the total losses in the SMES coil is studied.