Coil Impedance Research Articles

Modeling and Experimental Study of Oil-Cooled Stacked Giant Magnetostrictive Actuator for Servo Valve

Giant magnetostrictive materials (GMMs) have broad application prospects in the field of servo valves, but the giant magnetostrictive actuator (GMA) has problems such as large loss and severe heat generation, which affect the output effect and accuracy. To solve these problems, this paper designs a stacked giant magnetostrictive actuator (SGMA) and analyzes the magnetic circuit and magnetic field distribution of the SGMA. Based on the magnetic field analysis and the Jiles–Atherton model, we analyze the SGMA magnetization model, simplify the traditional model, and give a solution for the simplified model using the Runge–Kutta method. We analyze the eddy current loss of the SGMA, and according to Bessel’s equation and the Kelvin function, we calculate the relationship among eddy current loss, GMM rod radius, and magnetic field frequency. By analyzing the inherent hysteresis of GMMs, a hysteresis loss model of the SGMA is established in this paper. We also calculate the coil impedance and obtain the coil loss model. Based on the loss model, the SGMA cooling system is designed. Based on the above analysis, we design a SGMA prototype, set-up the corresponding experimental platform, and conduct the necessary experiments. The experimental results show that the SGMA responds well to different signals, but as frequency increases, attenuation, deformation, and hysteresis become more pronounced, which verifies the amplitude and phase changes caused by various losses in the theoretical analysis. The experiment also observes the temperature rise of the oil-cooled SGMA at different frequencies, indicating that the cooling system can effectively control the temperature change of the SGMA, which validates the foregoing analysis.

A Simple Analytical Solution for the Designing of the Birdcage RF Coil Used in NMR Imaging Applications

A novel analytical solution for the designing of the birdcage RF coil has been demonstrated in this paper. A new concept of dominant resonance path has been introduced in this paper which is used to identify the specific closed current loop in the birdcage RF coil which is responsible for the dominant resonance frequency mode. This concept is used to determine the precise numerical values of the lumped capacitance deployed in the legs and/or end-rings of the birdcage RF coil for its proper operation at the desired resonance frequency. The analytical solution presented in this paper has been established by performing the two-port network based equivalent circuit modeling of the birdcage RF coil. The proposed analytical solution uses T-matrix theory and develops a relationship between the input impedance of the birdcage coil and the impedances of its leg and end-ring segments. The proposed analytical solution provides the information about the resonance frequency spectrum of the birdcage RF coil and solves the issue of its interfacing with external circuits without affecting its resonance characteristics. Based upon the proposed analysis and designing strategy presented in this paper, the low pass, high pass and band pass configurations of the birdcage RF coil were successfully implemented with FPCB (Flexible Printed Circuit board) technique for small volume NMR imaging applications at 1.5 T and 3.0 T MRI system. The results obtained for the implemented birdcage coils using the proposed analysis and designing technique are in closed agreement with already established methods.

Envelope dynamics of resonant inverter driven electromagnetic levitation melting system with experimental verification

The paper presents a methodology for nonlinear envelope dynamics derivation of a levitation melting type induction heating system, fed by a resonant inverter, based on recently established work piece electro-mechanical model. The work piece under consideration is formed by an aluminum sphere, floated by conical-shaped copper coil. Resonant power converter is used to drive the induction heating system due to the fact that significant impedance of the work piece coil at high operation frequency (several tens of kilohertz) limits the power transferred to the load and therefore needs to be cancelled by corresponding capacitive impedance. Since the application requires levitation of the object being heated up, low-frequency (several hertz) dynamics appears, related to floating body movement. As a result, high-frequency electrical signals become modulated by low-frequency oscillations induced by mechanical side dynamics. It is known that power transfer in sinusoidal excited electrical system is governed by magnitudes and related phases of currents and voltages. Consequently, in order to assess both the low-frequency dynamics of levitated object mechanical motion and the amount of power transferred, information regarding envelope behavior of electrical-side signals is sufficient. Once established, envelope model of the electrical subsystem combined with typical model of the mechanical subsystem describe mechanical side parameters (position, speed and acceleration) exactly as if full electrical model would have been utilized. Moreover, envelope model significantly decreases simulation time and memory requirements of simulation platform. The proposed modeling approach is validated by comparing full and envelope model simulation results, accompanied by results obtained from experimental prototype.

Improved electromagnetic coil insulation health monitoring using equivalent circuit model analysis

Impedance monitoring of electromagnetic coils is a means of monitoring insulation degradation, a major cause of failures of these coils. This paper develops an equivalent circuit model aimed at identifying diagnostic and prognostic features in the coil impedance frequency response that can be leveraged into a health-monitoring scheme. To identify impedance and frequency components that provide the greatest correlation with insulation degradation, a sensitivity analysis was performed and compared with experimental data. In contrast to previous research, the results reveal features in the impedance spectrum that are more sensitive to insulation health than the coil resonant frequency.

Wireless Power Transfer under Wide Distance Variation Using Dual Impedance Frequency

A dual-impedance operation, where coil impedance is controlled by operating frequency selection, is proposed to maintain optimum reflected impedance across coupling variation. More specifically, this work focuses on how high coupling between coils presents excessively high reflected resistance to transmitter (Tx) inverters, degrading the efficiency and output power of the inverter. To overcome this problem, the proposed system is equipped with dual-impedance coil and selects high- or low-impedance coil based on the ability to operate both at 200 kHz and 6.78 MHz frequencies. The reactive impedances of 6.78 MHz coils are designed to be higher than that of 200 kHz coils. Since the reflected resistance is proportional to the coil impedances and coupling squared, at close distance with high coupling coefficient, 200 kHz coils with low coil impedances are activated to prevent an excessive rise in reflected resistance. On the other hand, at large distance spacing with low coupling coefficient, 6.78 MHz coils with high coil impedances are activated so that sufficient reflected resistance is obtained even under the small coupling. The proposed system’s advantages are the high efficiency and the elimination of bulky mechanical relay switches. Measured efficiencies are 88.6–50% across 10 coupling variations.

Electromagnetic Sensor for Detecting Wear Debris in Lubricating Oil

The detection technology on wear debris in lubricating oil is critical to ensure the safe operation of the engines in the aerospace industry. In this article, a triple-coil reverse double-excitation solenoid sensor is discussed. Inside the sensor, electromagnetic field generated by two excitation coils is sensitive to the occurrence of wear debris, and its changes can be captured by the induction coil. Thus, the impedance and inductive voltage of the induction coil can be used to detect the wear debris. Assuming the debris is spherical, an electromagnetic model is established. Via time-harmonic electromagnetic field analysis method, the expressions of the induction coil’s impedance increment and inductive voltage are derived. The relationships between the impedance increment and the parameters of the debris and the coils are investigated. The theoretical results are validated by Maxwell from Ansys, Canonsburg, PA, USA, simulations and experiments. In order to get an accurate voltage output signal, a passband filter amplifier is designed to connect to the output of the induction coil and empirical mode decomposition is presented to process the digital voltage signal. The derivation method on the coil impedance increment and inductive voltage and the signal processing method can be extended to design sensors measuring more complicated wear debris.

Maximizing Transfer Distance for WPT via Coupled Magnetic Resonances by Coupling Coils Design and Optimization

The technology of wireless power transfer via coupled magnetic resonances (CMR-WPT) has a good transfer performance in the “middle range”, but it is far from satisfying the application requirements in practice. To achieve a longer transfer distance, coils are usually complicatedly designed, making the system unwieldy and costly. Based on the intuitive and easily adjustable coil parameters, a design and optimization method was proposed in this paper, which can realize the minimum size coil that meets the requirements at 6.78 MHz. First, an equivalent model of the CMR-WPT system was developed, and the influences of coil parameters on the effective transfer distance and power transfer efficiency (PTE) were analyzed, indicating that the effective transfer distance and PTE were not always proportional to the coil radius, the number of turns or the pitch. Then, the coil impedance and the quality factor of different coils were calculated. In addition, the influence degree of coil parameters on the transfer performance was also compared using variable control. Finally, an experimental platform was built, and experimental results agreed well with the simulation results.

Power Stability Control Strategy of the WPT System Based on Load Detection

The output power fluctuates when offset occurs between the receiving coil and the transmitting coil or load equivalent impedance changes in a wireless power transfer system (WPT). These two conditions will affect the safety of the system and the life of the battery load. Therefore, effective control methods are needed to ensure the stability of the load power. This paper analyzes the WPT system with LCC/S compensation topology and the secondary side containing DC/DC circuit. The relationship between mutual inductance and load voltage and current, mutual inductance and the system transmission efficiency, DC/DC circuit duty ratio and load voltage and current are deduced. The output power stability control strategy through load voltage and load current detection and control of DC/DC circuit is proposed. Finally, the WPT system with 1 kW output power is built through simulation and experiment, which verifies the effectiveness and correctness of theoretical analysis and control strategy in this paper.

FAST METHOD OF CALCULATING THE AIR-CORED COIL IMPEDANCE USING THE FILAMENTARY COIL MODEL

FAST METHOD OF CALCULATING THE AIR-CORED COIL IMPEDANCE USING THE FILAMENTARY COIL MODEL

Fast measurement of the coating thickness and conductivity using eddy currents and plane wave approximation

An analytical formula for the electromagnetic field generated by a current-carrying cylindrical coil above a half-infinite conductive plate has been studied. The theoretical analysis showed that when the radius of the coil was much larger than the penetration depth, the electromagnetic wave at the plate surface could be approximated as a plane wave, and the real part of the plane wave impedance would not be affected by lift-off changes. Using normalized plane wave impedances as theoretical values and normalized coil impedances measured by swept-frequency eddy current technique as experimental values, a novel approach is proposed to rapidly and simultaneously solve the coating thickness, coating conductivity and substrate conductivity of a conductive plate with ultra-thin metallic coatings by Levenberg−Marquardt algorithm. The experimental results show that the maximum relative measurement errors of the proposed method for single- and double-coated plates are less than 8% and 18%, respectively, and the measurement time is less than 1 s, which is suitable for fast parameter detection.

Homogenization of multi-turn coil with elliptic cross-section using complex permeability

Purpose The tensor complex permeability of a multi-turn coil with elliptic cross-section is analytically expressed. In field analysis, a multi-turn coil can be modeled by the uniform material that has the present tensor complex permeability. It is shown that the frequency characteristic of the present tensor complex permeability is in good agreement with that evaluated by finite element method applied to a unit cell of the multi-turn coil region. Design/methodology/approach The authors introduce a new method to evaluate the complex permeability of a multi-turn rectangular coil. To obtain the complex permeability of a rectangular coil in a closed form, it is approximated as an elliptic coil. Because the rectangular coil has different complex permeabilities in the vertical and horizontal directions, the complex permeability have to be defined in a tensor form. It suffices to discretize the coil region into rather coarse finite elements without considering the skin depth in contrast to the conventional finite element method. Findings The proposed method is shown to give the impedance of multi-turn coils which is in good agreement with results obtained by the conventional finite element (FE) analysis. By extending the proposed approach, the authors can easily perform 3D FE analysis without difficulty in discretization of the coil region with fairly fine finite elements. Moreover, they found that the approximation of rectangular coils as the elliptic coils is valid for analysis of quasi-static fields using this homogenization method. Originality/value The novelty of this study is in the approximation of the rectangular coils with elliptic coils, and the complex permeability for them is formulated here in a closed form. The proposed formula includes that for the round coils. Using the present method, the authors analyze the rectangular coils without fine discretization.

Implementing an Impedance Compression Network to Compensate for Misalignments in a Wireless Power Transfer System

This paper presents an impedance compression network (ICN) design in a wireless power transfer (WPT) system to compensate for distance or alignment variations between coils. In midrange WPT applications, magnetic resonant coupling coils are mainly implemented to achieve high efficiency for charging batteries. However, a distance or horizontal alignment variation between coils changes their coupling coefficient, and it decreases the overall performance of WPT systems because the zero voltage switching in a resonant inverter is lost. In order to reduce coil impedance variation, we propose an ICN that compresses variations in coil impedance. The ICN consists of a resistance compression network and a phase compression network to suppress magnitude and phase variations, respectively, in the coil impedance. Only passive components, such as inductors and capacitors, are used to implement an ICN.

Influence of transmission distance on magnetically coupled resonant radio energy transmission system

Transmission distance is a key factor in designing a wireless energy transmission system. The change in coil spacing will have a huge impact on the output characteristics of the system.Therefore, this paper theoretically analyzes the mutual inductance model of two coils from the perspective of circuit theory.Studies have shown that coil spacing, impedance, self-inductance, operating frequency, load resistance and many other factors can affect the output characteristics of the system.So, this thesis uses the control variable method to study the relationship between transmission distance and output characteristics, and proposes the optimal output power point.Finally, an experimental prototype was made to verify the feasibility of the method.

Smartcoil—design, assembly and test of a 10 MVA superconducting air coil fault current limiter

Within a German government funded project the partners Siemens AG and KIT have developed an iron-free inductive current limiter. The project aimed at a device for the medium voltage range, i.e. U = 10 kV with a nominal power of 10 MVA. One phase of this limiter was designed, manufactured and successfully tested. The limiter is based on a stack of closed superconducting ReBCO rings inserted into a conventional reactor coil. Under normal operation conditions, the rings shield the inner part of this coil. With the occurrence of a short circuit the superconducting rings quench and the impedance of the reactor coil becomes effective. The paper reports the principles of the design and the development of the superconducting ReBCO rings. It describes in detail the test of such rings, in particular the contactless determination of contact resistances and critical currents. Of particular importance was the development of a cryostat which minimizes the distance between the primary reactor coil and the superconducting insert. Finally, the device was tested in the Siemens test field in Berlin under the most relevant operation conditions. The limiting behavior complied with the predicted properties.

Mutual Impedance of Eddy-Current Coils Above a Second-Layer Crack of Finite Length

A model is presented to determine the change in mutual impedance between a driver and pickup coil due to eddy-current interaction with a crack in a conductive two-layer plate system where the plates are separated by a non-conducting layer of constant thickness. Here, the impedance change of interest arises from a through-thickness crack of finite length located in the hidden lower plate. The model assumes that both conducting plates are electromagnetically thin and is therefore valid for large electromagnetic skin-depth. The model predictions are compared with experimental measurements using a pair of cylindrical air-cored coils arranged to simulate an eddy-current sliding probe. Agreement between the calculated and measured impedance change is obtained over the frequency range 10 Hz–3 kHz for which the plates can be treated as electromagnetically thin, and reasonable qualitative agreement was obtained at higher frequencies. A method for computing the induced current streamlines in the presence of the crack is also described and visualisation of the computed streamlines is used to assist in the interpretation of the impedance change for different probe orientations. These results provide a valuable initial insight into the interaction between a buried crack in a multilayer structure and the electromagnetic fields induced and detected by eddy-current sliding probes.

Study of the effect of sugar concentration on the characteristic impedance of a magnetic coil

The work is devoted to the development of technology for non-invasive measurement of glucose in human blood. Modern blood glucose meters are discrete devices that do not fully provide a picture of the level of glucose in the blood. Some cases of hyperglycemia or hypoglycemia may not be recorded between measurements. In addition, finger piercing causes pain and subsequently leads to tissue damage. In this paper, the effect of glucose concentration in the human body on the complex resistance of an inductance coil, with the finger of a test person placed in it, is investigated.

Calculation of the Impedance of an E-cored Coil Placed Above a Conductive Material with a Surface Hole

Abstract In eddy current testing, probe-coils with the E-type pot core are commonly used for detecting defects in test objects. In this paper, an analytical mathematical model of such a probe placed above a two-layered conductive material with a surface hole has been presented. The final formulas in the closed form that make it possible to calculate the coil impedance were worked out using the truncated region eigenfunction expansion (TREE) method and implemented in Matlab. Changes in resistance and reactance were determined for both material without a hole and for space containing no conductor. The results were compared with those obtained for the air-cored coil and the I-cored coil. The correctness of the calculations was confirmed through experimental measurements and with the finite element method (FEM) in the COMSOL Multiphysics package

Influences of Coil Radius on Effective Transfer Distance in WPT System

The improvement of effective transfer distance is significant for wireless power transfer (WPT) via coupled magnetic resonances, with coil radius being the most important influencing factor. To study the relationship between effective transfer distance and coil radius, a model of WPT is established at first. Based on analyses of circuit and magnetic field, the influencing factors for effective transfer distance are obtained. Second, the models of WPT are constructed using software COMSOL, and the influences of Tx and Rx coil radius on the effective transfer distance are studied in detail. In addition, they are not proportional and the coil impedance is the most important factor. On this basis, a coil design method is proposed. Finally, an experiment device was built, and experimental results were well consistent with the theoretical analysis, showing the rationality and effectiveness of the proposed method.

An EIGA driven coupled of electromagnetic-thermal field modeling in the induction melting process

A crucible-free, an Electrode Induction melting Gas Atomization (EIGA) technique for induction melting of the reactive and refractory metals/alloys is developed. The impedance matching of the superalloy Rene95 in a conical induction has been investigated. An evaluation of the complex electromagnetic and thermal fields on the free surface of the superalloy Rene95 is carried out by both numerical and experimental methods under alternating currents with the output power of 120 kW and operational effective current 2 kA. The two types of coupling induction coils have been devised, tested and compared with the experimental results, revealing the impedance of four-turn induction coil with superalloy Rene95 releasing the electrical energy that causes the most high-temperature distribution in the surface of superalloy Rene95. Induction coil is also known as an “inductor” which is the core component in the melting process of nickel-based superalloy, leading to design a good induction coil to reduce the power consumption and increase the efficiency of the process. The achievements are the melting rate enhancement and a steady-state of continuous liquid metal flow producing the liquid metal streams with diameter of about 5 mm.

Finite element analysis of multi‐turn magnetoplated coils using homogenization method

AbstractMagnetoplated wires (MPW) are effective to reduce eddy current losses in power devices such as converters and contactless power transfer systems. In the conventional finite element analysis (FEA) of MPW, elements must be sufficiently smaller than the skin depth. This results in large number of unknowns. This article proposes a homogenization method using complex permeability for FEA of magnetoplated coils so as to circumvent this problem. The coil impedances obtained by the proposed method with coarse elements are shown to agree well with those computed in the conventional FEA with fine elements.