Coil Impedance Research Articles

Impedance-Based Condition Monitoring for Insulation Systems Used in Low-Voltage Electromagnetic Coils

Electromagnetic induction coils are widely used in a variety of applications, many of them safety-critical. The insulation around the wire in an electromagnetic coil accounts for a significant portion of the failures in solenoid-operated valves and in electric motors. This paper presents a method of detecting the degradation of insulation used in low-voltage applications by assessing changes in impedance responses. The results indicate that coil impedance, resolved into resistance and reactance, evolves differently when the coil is subjected to different loading conditions, which reflects insulation degradation signatures due to different failure mechanisms. This method can be used to assess the insulation life of an electromagnetic coil, allowing replacement prior to the formation of harmful shorts or critical coil opens.

A Thickness Measurement System for Metal Films Based on Eddy-Current Method With Phase Detection

This paper proposes a high-speed measurement system for the thickness of metal films based on the eddy-current method. Theoretical derivation shows the effects of film thickness on the impedance of the sensor coil. In a voltage division circuit, the thickness information is converted to the phase difference of two measured signals. The tangent value of the phase difference is proportional to thickness h , which is a result that has been verified theoretically and experimentally. The prototype used in this paper can cover a measurement range of 10 μ m–0.5 mm for copper films, and can achieve a high measurement speed (up to excitation frequency) without a complex demodulation circuit. The measurement error caused by the temperature variation is roughly –0.2%/°C. The influence of external magnetic field on the prototype is also tested. The entire sensor size can be easily controlled in 50 × 50 × 30 mm3 as a handheld device. The instrument can meet many industrial requirements for both online measurement and postinspection.

Semi-Analytical Approach for Finite-Element Analysis of Multi-Turn Coil Considering Skin and Proximity Effects

Native application of finite-element method (FEM) to the analysis of skin and proximity effects in multi-turn coils results in large equation systems, whose solution needs long computational time. This paper proposes a semi-analytical approach to overcome this problem. For the analysis of the proximity effect, the complex permeability of a round conducting wire immersed in uniform time-harmonic magnetic fields is represented in a closed form. Then, the homogenized complex permeability over the cross section of the multi-turn coil is analytically evaluated using the Ollendorff formula. The magnetoquasistatic problem is thus replaced by the magnetostatic one, in which the multi-turn coil is treated as a uniform material with the homogenized complex permeability. The skin effect is taken into consideration by introducing the corresponding impedance in the circuit equation. The proposed method is shown to give the impedance of multi-turn coils, which is in good agreement with that obtained by the conventional FEM as well as experiments.

Strategy for Predictive Modeling of the Common-Mode Impedance of the Stator Coils in AC Machines

This paper presents a methodology for the predictive modeling of common-mode (CM) impedance of the stator windings of AC three-phase machines taking into consideration all the parasitic inductive and capacitive phenomena. An electrical model with lumped parameters ( RLC ) is generated, based on a physical approach of the machine, which is implantable in a SPICE-type software. The prediction results are in satisfactory agreement with the measurements ones on a large frequency bandwidth [10 kHz to 100 MHz]: The low-frequency capacitance is predicted with better than 5% accuracy, while the peaks and dips in the CM impedance plot are very well predicted beyond 5 MHz.

Free-Positioning Wireless Power Transfer to Multiple Devices Using a Planar Transmitting Coil and Switchable Impedance Matching Networks

In this paper, an efficient free-positioning wireless power transfer (WPT) system for charging multiple devices is proposed. For the system, a planar transmitting (Tx) coil that consists of a spiral coil and concentric multiple parallel loops and switchable impedance matching networks (IMNs) at a transmitter are developed. Using the Tx coil, mutual inductance between the Tx coil and the receiving (Rx) spiral coils is made uniform in the effective charging area. The parallel loops are placed in the interior of the Tx coil, and the magnetic field is enhanced and controlled by adjusting the space in each parallel loop. For optimal design of the coil, an analysis method using an equivalent circuit is explained in detail. It is found that the current of one loop of a parallel loop is more than the input current and negative current flows on the other loop, while the total current of each parallel loop is constant. An array of parallel capacitors with a switch as the switchable IMN is used for the maximum power transfer efficiency corresponding to the number of devices. The reflection coefficients of the fabricated WPT system are under −10 dB at 6.78 MHz, regardless of the number of devices and positions. The WPT system has the maximum power transfer efficiencies of 93% for two devices and simultaneously transfers uniform power to up to four devices with high efficiency.

Impedance-based Health Monitoring of Electromagnetic Coil Insulation Subjected to Corrosive Deterioration

Electromagnetic coils are widely used components in a variety of industries and systems, and their failure can lead to catastrophic failure of the system in which they are placed. Past studies have revealed the electromagnetic coil insulation to be a weakness, and there are presently no available methods to detect degradation in the coil insulation in-situ. Prior work in the AC motor community on twisted pairs of magnet wire has shown that insulation capacitance measurements can reveal useful diagnostic information. Yet, no studies have tracked the coil impedance spectrum over an aging/degradation period, no methods are available to identify frequencies in the impedance spectrum to be used, nor is there discussion concerning methods to employ coil impedance to determine a degradation mechanism. This paper develops an approach of detection of the aging of insulation in low-voltage electromagnetic coils when subjected to corrosive environmental conditions by assessing changes in impedance responses. The complex impedance was resolved into its real and imaginary parts and the Spearman correlation coefficient was used to find regions of interest within the impedance spectrum. The results indicate that real and imaginary impedance provide information that can assist in condition-based maintenance procedures for electromagnetic coils. Furthermore, by incorporating frequency-correlation analysis, the individual frequencies which provide diagnostic and prognostic information can be identified.

Thickness Measurement of an Iron Plate Using Low-Frequency Eddy Current Testing With an HTS Coil

We applied low-frequency eddy current testing (ECT) using a high-temperature superconducting (HTS) coil in order to measure the thickness of an iron plate. Using this method, we measured changes in coil impedance when the iron plate was placed below the coil. Although low-frequency measurements were necessary to avoid the skin effect, changes in coil impedance became very small at low frequencies. For this reason, an HTS coil was used in order to sensitively measure these small changes. First, changes in the inductance L and the resistance R of the coil were measured when the iron plate was positioned 18 mm below the HTS coil, and the thickness of the plate was changed in increments from 6 to 22 mm. The results show that we were able to estimate the thickness of the plate up to 22 mm based on the changes in R when an excitation frequency of 4 Hz was used. Next, the effect of the liftoff between the iron plate and the HTS coil on the changes in Land R was studied while the liftoff was changed from 18 to 28 mm. The results show that the liftoff could be estimated from the changes in L. We were therefore able to determine the liftoff and the thickness of the iron plate by measuring changes in L and R, respectively. Results obtained from numerical simulation using the finite element method were in agreement with the experimental results.

An experimental system for extraction of pectin from orange peel waste based on the o-core transformer structure

An experimental system based on the o-core transformer structure was established and used to extract pectin from orange peel waste. Unlike other electric-field-assisted techniques, it avoids the use of powered electrodes. The acidic solvent acts as the secondary coil connected to a purpose-made glass chamber which forms a closed loop. Then the induced electric field in the system appears to be under the influence of alternating magnetic flux based on Faraday's law of induction. We found that an increase in the excitation voltage causes enhancement of the pectin yield. Increasing frequency from 20 to 200 kHz had a negative impact on pectin yield partially due to the increased impedance of the primary coil. The ionic conduction was enhanced at the same excitation voltage since there are more free ions in the mixture at lower pH. This means that a lower impedance in the mixture is conducive to extraction. This multidisciplinary technique combines the transformer concept with the electric-field-assisted process, thus providing a reference for the application into agricultural by-products treatment.

Design of Magnetic Cloak for Experiments in AC Regime

The magnetic shielding device causing only negligible distortion of external magnetic field is called the magnetic cloak. We have designed and tested cloaks with a coaxial bilayer structure where a superconducting cylindrical layer is placed inside the tube from a ferromagnetic material. Such object when inserted in the bore of a coil generating ac magnetic field should not change the coil's impedance. This experiment is suitable for checking the cloak performance in the ac regime by measuring the voltage on the coil terminations, particularly the change in its amplitude and phase. The impact on signal amplitude comes from the nonzero magnetic moments of the cloak components. Then, the opposite character of ferromagnetic and diamagnetic material can be used to suppress the response, e.g., by modifying the thickness of the ferromagnetic layer. On the other hand, the shift in the phase comes from the hysteresis behavior of both constituents, and their impacts are additive. We achieved decent results using the ferrite powder/epoxy composite for the outer tube and REBCO superconducting tapes for the inner cylinder. Numerical simulations and experimental verification were used to investigate the electromagnetic response at various geometrical arrangements of the superconducting part of the cloak.

The utility of shock coil and left ventricular tip–right ventricular coil impedance as a predictor of pneumothorax in cardiac resynchronization therapy implantation

A high defibrillation threshold (DFT) infrequently occurs during device implantation. The major cause of high DFT is pneumothorax. However, there are few data about shock coil and left ventricular tip–right ventricular coil (LV tip–RV coil) impedance as a marker of a high DFT caused by pneumothorax. A 65-year-old man, diagnosed with dilated cardiomyopathy, was referred for cardiac resynchronization therapy (CRT) defibrillator implantation with single shock coil lead. The shock coil impedance was high and LV tip–RV coil impedance was normal just before DFT test. Then, high DFT was detected. Afterwards, pneumothorax was found. After the treatment of the pneumothorax, the shock coil impedance decreased. However, the LV tip–RV coil impedance did not change. This discrepancy can be a useful predictor of the high DFT caused by pneumothorax. With these markers, we can predict high DFT and consequently can treat pneumothorax earlier without the unnecessary shock delivery.<Learning objective: Cardiac resynchronization therapy (CRT) implantation is widely performed. Pneumothorax is one of the complications of CRT implantation. High defibrillation threshold (DFT) is known as a predictor of the pneumothorax. However, the predictor of this complication has not fully been evaluated. The high shock coil impedance combined with normal left ventricular tip–right ventricular coil impedance can be a useful predictor of the high DFT caused by pneumothorax.>

Wireless Metal Detection and Surface Coverage Sensing for All-Surface Induction Heating.

All-surface induction heating systems, typically comprising small-area coils, face a major challenge in detecting the presence of a metallic vessel and identifying its partial surface coverage over the coils to determine which of the coils to power up. The difficulty arises due to the fact that the user can heat vessels made of a wide variety of metals (and their alloys). To address this problem, we propose and demonstrate a new wireless detection methodology that allows for detecting the presence of metallic vessels together with uniquely sensing their surface coverages while also identifying their effective material type in all-surface induction heating systems. The proposed method is based on telemetrically measuring simultaneously inductance and resistance of the induction coil coupled with the vessel in the heating system. Here, variations in the inductance and resistance values for an all-surface heating coil loaded by vessels (made of stainless steel and aluminum) at different positions were systematically investigated at different frequencies. Results show that, independent of the metal material type, unique identification of the surface coverage is possible at all freqeuncies. Additionally, using the magnitude and phase information extracted from the coupled coil impedance, unique identification of the vessel effective material is also achievable, this time independent of its surface coverage.

A novel concept for tissue-metal detection and differentiation using an inductive proximity sensor

In this paper a novel application of inductive proximity sensors for detection of living tissue by means of measurements of the coil impedance changes at different frequencies is described. The mathematical analyses utilizing Bessel function estimation include detected object size and its distance from a sensor. The main aim of this study is to prove the possibility of distinguishing between metal objects and living tissues. The movement of an object can be also detected, assuming it is in a distance comparable with a size of the induction sensor.

Analytical approaches to eddy current nondestructive evaluation for stratified conductive structures

The excitation and propagation of an electromagnetic field is a very complex problem in Stratified conductive structures (SCS) because of their complicated physical properties. In single air-cored coil above multilayered conductive structures, the impedance of an eddy-current coil is calculated based on the electromagnetic field. This paper provides insight into eddy current models for stratified conductive structures using three types of analytical approaches, including Cheng’s matrix method, Truncated region eigenfunction expansion (TREE) method, and the innovative method based on the reflection and transmission theory of the electromagnetic waves. The analytical expressions of the three models are discussed and compared. Numerical simulation of an FEM model is performed, and the experimental results verify the developed models.

E-Cored Coil With a Circular Air Gap Inside the Core Column Used in Eddy Current Testing

The problem of an axially symmetric E-cored coil with a circular air gap inside the core column located above a two-layered conductive half-space is presented. The truncated region eigenfunction expansion method is used to derive expressions describing the magnetic vector potential of the filamentary coil. The final expressions for the impedance of the rectangular cross-sectional coil are obtained, and calculations for various frequency values are carried out. The results are compared with those from the COMSOL package, which shows a very good agreement.

Equivalency Assessment for an Eddy Current System Used for Steam Generator Tubing Inspection

Eddy current testing is widely used for inspecting steam generator tubing in nuclear power plants (NPPs). The inspection technique for steam generator tubing in NPPs should be qualified in accordance with examination guidelines. When the components of a qualified system such as eddy current tester, probe, and data analysis program, are changed, the equivalency of the modified system to the originally qualified system must be verified. The eddy current tester is the most important part of an eddy current testing system because it excites and transmits alternating currents to the probe, receives coil impedance of the probe and generates signals for anomalies. The Korea Hydro & Nuclear Power Co., Ltd. (KHNP) developed an eddy current testing system with an eddy current tester and data acquisition-analysis program for inspecting the steam generator tubing in NPPs; this system can be used for an array probe and as a bobbin and rotating probes. The equivalency assessment for the currently developed system was carried out, and we describe the results in this paper.

A programmable and self-adjusting class E amplifier for efficient wireless powering of biomedical implants.

In this paper, an enhanced approach of a class E amplifier being insensitive to coil impedance variations is presented. While state of the art class E amplifiers widely being used to supply implanted systems show a strong degradation of efficiency when powering distance, coil orientation or the implant current consumption deviate from the nominal design, the presented concept is able to detect these deviations on-line and to reconfigure the amplifier automatically. The concept is facilitated by a new approach of sensing the load impedance without interruption of the power supply to the implant, while the main components of the class E amplifier are programmable by software. Therefore, the device is able to perform dynamic impedance matching. Besides presenting the operational principle and the design equations, we show an adaptive prototype reader system which achieves a drain efficiency of up to 92% for a wide range of reflected coil impedances from 1 to 40 Ω. The integrated communication concept allows downlink data rates of up to 500 kBit/s, while the load modulation based uplink from implant to reader was verified of providing up to 1.35 MBit/s.

Solution and Extension of a New Benchmark Problem for Eddy-Current Nondestructive Testing

We report on further measurements and a numerical solution for a proposed benchmark problem involving an eddy-current nondestructive evaluation configuration. This is a simplified version of the inspection of fastener holes in aircraft structures, comprising combinations of plates with through holes and a crack. Previously, only line (B-scan) data were provided for the coil impedance, while in this paper, we provide precision surface (C-scan) data as well as details on the evaluation of the critical parameters of the problem. Furthermore, the experimental data are cross validated with numerical ones using a model that combines an integral formulation and the FEM approach.

Simulation research of eddy current separation technology in scrap copper based on ANSYS finite element method

Copper mineral resources are seriously insufficient in China. Scrap copper recycling is an inevitable choice to solve the shortage problem of the copper resources. To achieve rational utilization of the scrap copper in high quality, it's greatly significant to separate the scrap copper finely. In this paper, modeling and simulation analysis for the separation of the scrap copper with different materials by eddy current sensor based on ANSYS finite element method are made. The detection model for three situations such as the surface of the scrap copper is smooth and clean, coating and concavo-convex about four kinds of scrap copper materials (red copper, brass, manganin and bronze) are involved respectively in magnetic field lines, contour of magnetic flux intensity and equivalent impedance of sensor coil. Intercomparison and analysis of the simulation results testify that eddy current separation technology of the scrap copper is feasible. The simulation results are validated by the experiments.

Improving the Performance of Saturated Cores Fault Current Limiters by Varying Winding Density in the AC Coils

Effect of the winding density in the AC coils on the performance of a saturated cores fault current limiter (SCFCL) has been studied, exploiting Finite Element Analysis. For a given design, a fixed number of turns was concentrated at the center of the AC coils limb, resulting in high windings density. The coil length was then increased gradually along the limb, decreasing the winding density and therefore decreasing the coil impedance. However, we found that the ratio between the fault to nominal state impedances increases with decreasing winding density. The results are discussed and explained as originating from the change in the flux linkage of windings in the coil for various core states. In the nominal-state of the SCFCL, the core is saturated and the coupling between the windings is lower, given the lower winding density. However, when the core is desaturated during a fault, the magnetic interaction of the windings with the core strengthens, the coupling between the windings increases and contributes to higher fault-state impedance. Thus, reducing the winding density may serve in increasing the impedance ratio of the device and improving the performance of SCFCLs. The results suggest that the windings density in SCFCLs should be used as a significant design parameter.

An &lt;italic&gt;LC&lt;/italic&gt;-Type Passive Wireless Humidity Sensor System With Portable Telemetry Unit

This paper presents a high-sensitivity passive wireless humidity sensor system with a portable telemetry unit for applications in sealed environments. A complementary metal oxide semiconductor (CMOS) interdigital capacitive humidity sensor die was attached to an organic substrate (FR-4) on which a fixed planar spiral copper inductor was fabricated. The variable capacitor and the fixed inductor were wire bonded to form an inductor-capacitor (LC) tank circuit. The resonant frequency of the sensor tank is dependent on the sensor capacitance, which changes in response to the humidity. The sensitivity of the capacitive sensor was improved significantly using graphene oxide as a sensing material. The package-level integration was achieved by employing the embedded inductor on an organic packaging substrate. The LC-type sensor is interrogated wirelessly using our homemade portable telemetry unit, which is based on a standing wave ratio bridge to measure the real part of the readout coil impedance. Measurements show a sensitivity of -18.75 kHz/%RH over a range of 15%-95% RH. The implemented telemetry unit addresses the need for a low-cost, portable, and universal reader of the LC-type passive wireless sensors.