Transient Magnetic Field Research Articles

A Numerical Scheme for Fast and High Accuracy Simulation of Motion-Induced Eddy Current Testing Signals

ABSTRACT In this paper, an efficient numerical scheme and corresponding fast solver are proposed and implemented for rapid and high-accuracy numerical calculation of the motion-induced eddy current testing (MIECT) forward problems. First, inspired by the time-domain numerical formulation of the MIECT problem, a DFT-based numerical scheme in the frequency-domain was proposed through DFT of the transient excitation magnetic field signals at conductor surface caused by the moving permanent magnet of the MIECT probe. Second, a fast-forward simulator was implemented by further improving the existing Ar code for single-frequency ECT problem and adopting databases approach using the unflawed field information solved and stored a priori. As the key of the fast simulation of the MIECT signal perturbed by defects, a scheme to calculate the pickup signal perturbations was proposed and validated based on the reciprocity theorem. Finally, the validity of the numerical schemes and corresponding fast-forward solver was verified through comparing simulation results with experimental signal. Compared with the full analysis domain methods, the fast simulator can save computational burden up to 1000 times on the premise of not affecting the numerical precision, which enables its being adopted in the inverse analysis of MIECT signals for defect sizing.

Temperature Calculation, Test and Structure Improvement of Magnetic Coupling under High Slip

The temperature effect caused by high slip has an important influence on the operation performance and reliability of magnetic coupling. Taking the self-developed single disk asynchronous magnetic coupling as the research object, the heat loss equation of the magnetic coupling is established. Based on the three-dimensional transient magnetic field simulation model of the magnetic coupling, the eddy current loss, torque, and eddy current distribution law of the magnetic coupling are obtained through simulation. The space flow field and structure temperature field distribution of the magnetic coupling are analyzed by using the fluid-thermal coupling simulation method, and the heat dissipation coefficient and temperature distribution law of the structure surfaces such as copper disk, the back lining yoke iron disk, and the aluminum disk are obtained. The test platform was built to test the torque and temperature of the magnetic coupling. The results show that the error between the test and simulation is 4.8% in the torque aspect, and the maximum error between the test and simulation is only 8.1% in the temperature aspect of each component, which further verifies the effectiveness of the simulation method. On this basis, three heat dissipation improvement schemes are proposed, including installing heat dissipation blocks, setting semicircular grooves on the back lining yoke iron disk, and a hybrid design. The results show that the degree of improvement for each scheme is in the following order: hybrid design, setting semicircular grooves on the back lining yoke iron disk, and installing heat dissipation blocks. Under the hybrid design, the temperature of the back lining yoke iron plate and a copper plate of the magnetic coupling is reduced by about 8.5 °C compared with the original model, and the effect is ideal. The research results can provide an optimization reference for high-speed magnetic coupling and the temperature effect caused by an overload-locked rotor.

Research on Motor Rotor Loss of High-Speed Air Compressor in the Application of Hydrogen Fuel Cell Vehicle

As an important component of hydrogen fuel cell vehicles, the air compressor with an air foil bearing rotates at tens of thousands of revolutions per minute. The heat generation concentration problem caused by the high-speed motor loss seriously affects the safe and normal operation of the motor, so it is very important to clarify the loss distribution of the high-speed motor and adopt a targeted loss reduction design for air compressor heat dissipation. In this paper, for an air compressor with a foil bearing with a rated speed of 80,000 rpm, an empirical formula and a three-dimensional transient magnetic field finite element model are used to model and calculate the air friction loss, stator core loss, winding loss and permanent magnet eddy current loss. The accuracy of the analytical calculation method is verified by torque test experiments under different revolutions, and the average simulation accuracy can reach 91.1%. Then, the distribution of the air friction loss, stator core loss, winding loss and eddy current loss of the air compressor motor at different revolutions is obtained by using this method. The results show that the proposed method can effectively calculate the motor rotor loss of a high-speed air compressor with air foil bearing. Although the motor efficiency increases with the increase in motor speed, the absolute value of loss also increases with the increase in motor speed. Stator core loss and air friction loss are the main sources of loss, accounting for 55.64% and 29% of the total motor loss, respectively. The electromagnetic loss of winding, the eddy current and other alloys account for a relatively small proportion, which is 15% in total. The conclusions obtained in this paper can effectively guide calculations of motor loss the motor heat dissipation design of a high-speed air compressor with an air foil bearing.

Simulation and Experiment of Electromagnet for High-Speed On-off Valve for Vehicle Shifting System

The high-speed on-off valve is the core component for adjusting the oil pressure of the pressure reducing valve in the clutch shifting system of engineering vehicles, and its response speed is one of the important indicators to measure its performance. In order to improve the dynamic performance of high-speed on-off valve, the electromagnet with solid magnetic isolation structure was designed and simulated by considering the eddy current loss factor. A high-speed on-off valve electromagnetic actuator with high response speed was obtained. Firstly, the theoretical calculation and verification of the theory were carried out by using the electromagnet design theory, and the theoretical model of the electromagnet was built. Secondly, the electromagnet parameter model was brought into the finite element simulation software ANSYS Electronics to simulate and optimize the static magnetic field and the transient magnetic field respectively. The different structural parameters (guide bush thickness, outer magnetic pole thickness, magnetic isolation structure, armature length) were analyzed. And the effect of the coil parameters (number of strands) on the steady-state electromagnetic force and the dynamic displacement of the armature was analyzed. The values of the structural parameters of the electromagnet were subsequently determined. Finally, the static and dynamic characteristics experiment of the magnetically separated solid electromagnet is designed and processed. The experiment and simulation results are in good agreement. The feasibility of the high-resistance electromagnet structure design and the correctness of the simulation are verified. Reasonable optimization of structural parameters and coil parameters has obvious significance for improving the dynamic characteristics of high-speed on-off valve electromagnets.

A new four‐row integral transposition method of stator windings and circulating current loss reduction in large turbo generators

Transposition of stator windings for large turbo generators is a key technology. At present, the stator windings with four-row strands for large turbo generators are usually transposed on both sides of the slot centreline and connected at the end by the joint sleeve. The different transposition methods of strands have an important effect on strand currents, circulating currents, and circulating current losses. A new four-row integral transposition method is proposed in this study. Several transposition methods based on the four-row integral transposition method are compared with the double Roebel transposition methods using 3-D time-harmonic field. The 3-D model without the end and with the end are both calculated. The results show that the four-row integral transposition method has an obvious advantage in reducing circulating current loss. Finally, a 2-D finite element model of transient magnetic field is calculated and its calculation results further support for the effectiveness of the proposed transposition method. The proposed transposition method has a great application value, which can reduce additional loss, avoid partial overheating and short circuit fault of stator windings in large turbo generators.

Nonlinear Transmission Line Performance as a Combined Pulse Forming Line and High-Power Microwave Source as a Function of Line Impedance

Nonlinear transmission lines (NLTLs) offer compact, low-cost, all solid-state high-power microwave (HPM) generation. This article experimentally investigates the RF output power for composite-based 10, 25, and 50 Ω NLTLs used as a combined pulse forming line and HPM source. We manufactured coaxial NLTLs containing 10% barium strontium titanate and 15% nickel zinc ferrite encased in polydimethylsiloxane. The output voltage and power in the time and frequency domains, respectively, showed that the 10 Ω NLTL generated the greatest RF output. The 25 Ω NLTL generated greater output power from 500–1100 MHz than the 50 Ω NLTL. This occurs because reducing the NLTL impedance induces a larger transient current for a given charging voltage. This transient current corresponds to a stronger transient magnetic field, which facilitates magnetic moment alignment to allow for coherent magnetic moment rotation to occur. This setup eliminates the separate pulse forming network and magnetic field bias that typically occurs in other NLTL systems, which provides additional flexibility in tuning the NLTL impedance and reducing device footprint.

Measurement of electromagnetic environment of hybrid DC circuit breaker during bipolar short circuit fault

Hybrid DC circuit breakers (DCBs) have recently been used to break short-circuit fault currents to ensure safety in flexible DC transmission projects. However, electromagnetic transients from DCB operation can interfere with the secondary equipment and cause malfunctions. In this paper, the bipolar short-circuit test, which is the first short-circuit test for DC transmission line in flexible DC project in the world, has been carried out in the 200 kV HVDC converter station equipped with two hybrid DCBs. The transient magnetic field near the secondary equipment in the DCB hall were measured, and its characteristics were discussed. Furthermore, the relationships among the space magnetic field, the short-circuit current and the action of the power electronic equipment in the DCB were also analyzed The main findings are as follows. During the bipolar short-circuit process, the maximum space magnetic field intensity near the secondary device is 109 A/m, and the maximum rising rate of the magnetic field waveform is about 350 A/m/ms. The magnetic field strength reached its peak value at the moment when the DCB was blocked, as well as the short-circuit current. It can be indicated that the DCB can cut off the short-circuit fault current accurately and reliably. The results can provide reference for the immunity evaluation and anti-interference protection of secondary equipment, and its optimal arrangement in the DCB hall.

3D modelling of the mechanical behaviour of magnetic forming systems

High-speed forming methods become attractive in manufacturing and significantly reduce the cost and energy requirements. Electromagnetic forming is a high-velocity pulse forming technique that applies electromagnetic forces to sheet or tubular workpieces using a pulsed magnetic field. In order to understand the physical behaviours of materials, numerical modeling is highly desired. Therefore, in this study, we investigate the mechanical behaviour of the electromagnetic sheet stamping and magnetic tube expansion and compression systems. For these 3D simulations, COMSOL multiphysics software is used. It provides the possibility to model the electromagnetic aspects of the problem along with the thermal and mechanical aspects in a coupled method. The developed 3D numerical fully coupled models lead to analyze the transient magnetic fields, Lorentz forces acting on workpieces, and the plastic deformations obtained in several magnetic forming systems. The effects of systems parameters are also investigated such as the coil’s form and the number of its turns.

Modelling of low-speed magnetic gear with viscose ferrofluid

PurposeIn this paper, a computational model of a coaxial magnetic gear (MG) design with viscose ferrofluid between rotors is proposed. Viscose ferrofluid is used to decrease the magnetic reluctance and therefore creates higher magnetic torque. However, viscose friction of ferrofluid is undesirable and must be minimised in this particular application. MG is supposed to operate under low rotational speeds, where the dynamic viscose friction is very low, and the effects of the viscose ferrofluid over the MG’s efficiency must be estimated. The paper aims to analyze the performance of MG with viscose ferrofluid and to estimate the MG efficiency by computational model using finite element method (FEM).Design/methodology/approachAn MG design with viscose ferrofluid between the outer low-speed rotor and modulating steel segments was modelled as a coupled transient magnetic field problem and a kinematic model with viscous friction coefficients derived from a previously computed fluid dynamics model.FindingsThe proposed computational implementation is suitable for homogeneous magnetic fluid modelling in electromagnetic actuators and rotational machines. The results regarding power and torque transmission of MG were obtained by coupled finite element modelling. The efficiency of MG significantly decreased due to ferrofluid friction.Originality/valueThe described MG design with viscose ferrofluid is a novel device with new operational characteristics, and new results for the effects of viscose ferrofluid friction in the outer magnetic field over the MG efficiency are estimated.

Enhancing Wire-Rope Damage Signals Based on a Radial Magnetic Concentrator Bridge Circuit.

Hall-effect sensors are used for the non-destructive testing of wire ropes owing to their low power consumption and high operation frequency. The high-speed operation of wire ropes causes vibration inclination at different frequencies, which makes it difficult to detect the ropes. Considering that the radial signal in the magnetic flux leakage (MFL) detection method can respond to damages to the maximum extent possible, this study proposes a radial magnetic concentrator suitable for the non-destructive testing of wire ropes based on theoretical analysis and transient magnetic field simulations. The concentrator improves the radial magnetic circuit, polymerizes the leakage of the magnetic field in the detection device, and the leakage of the magnetic field of the defect converges at the sensor position of the circumferential array to improve the signal-to-noise ratio of the Hall-effect sensor. In addition, the MFL field is homogenized through the structure of the magnetic concentrator when the wire rope is tilted, which weakens the influence of the vibration tilt of the wire rope on the test results. Finally, the experiments show that the amplitude of the wire-rope damage signal is effectively improved by using the proposed radial magnetic concentration technology, hence being convenient for defect analyses.

A PMNN‐PZT Piezoceramic Based Magneto‐Mechano‐Electric Coupled Energy Harvester

AbstractIt is desired to obtain a piezoceramic with a high piezoelectric coefficient and low dielectric loss simultaneously for energy harvester application. Herein, it is reported that the 0.025Pb(Mn1/3Nb2/3)O3‐0.525Pb(Ni1/3Nb2/3)O3‐0.135PbZrO3‐0.315PbTiO3 (PMNN‐PZT) ceramic exhibits superior piezoelectric charge parameter e33 of 37.74 pC m−2 and low dielectric loss tanδ of 0.45%. Furthermore, a PMNN‐PZT ceramic‐based magneto‐mechano‐electric coupled energy harvester (MMEC‐EH) with a varying‐stiffness cantilever is designed and fabricated, which shows the strong self‐resonance effect in response to a random, transient impulse vibration or magnetic field stimulus. The investigations show that under a 0.6 s pulse vibration stimulus, the MMEC‐EH can tune itself into self‐resonance damping oscillation lasting for six seconds at its resonance frequency of 16 Hz, and the produced maximum power is 1.96 mWRMS. Under both weak magnetic field and force‐field dual‐stimulus (Hac = 0.5 Oe and a = 0.05 g), the generated power density is about 60 mWRMS Oe−2g−2cm−3, which is one to two orders of magnitude higher than those previously reported MMEC‐EHs. Finally, the MMEC‐EH is successfully demonstrated to power temperature/humidity sensors, indicating its potential for harvesting both weak vibration and magnetic field energy from environments for self‐powered sensor application.

Patient-Specific Magnetic Catheters for Atraumatic Autonomous Endoscopy.

Despite increasing interest in minimally invasive surgical techniques and related developments in flexible endoscopes and catheters, follow-the-leader motion remains elusive. Following the path of least resistance through a tortuous and potentially delicate environment without relying on interaction with the surrounding anatomy requires the control of many degrees of freedom. This typically results in large-diameter instruments. One viable solution to obtain dexterity without increasing size is via multiple-point magnetic actuation over the length of the catheter. The main challenge of this approach is planning magnetic interaction to allow the catheter to adapt to the surrounding anatomy during navigation. We design and manufacture a fully shape-forming, soft magnetic catheter of 80 mm length and 2 mm diameter, capable of navigating a human anatomy in a follow-the-leader fashion. Although this system could be exploited for a range of endoscopic or intravascular applications, here we demonstrate its efficacy for navigational bronchoscopy. From a patient-specific preoperative scan, we optimize the catheters' magnetization profiles and the shape-forming actuating field. To generate the required transient magnetic fields, a dual-robot arm system is employed. We fabricate three separate prototypes to demonstrate minimal contact navigation through a three-dimensional bronchial tree phantom under precomputed robotic control. We also compare a further four separate optimally designed catheters against mechanically equivalent designs with axial magnetization profiles along their length and only at the tip. Using our follow-the-leader approach, we demonstrate up to 50% more accurate tracking, 50% reduction in obstacle contact time during navigation over the state of the art, and an improvement in targeting error of 90%.

Design and Starting Performance Study of a Soft-Start Brushless Doubly Fed Machine

Traditional brushless doubly fed machine(BDFM) are difficult to take into account both high starting characteristics and high operating efficiency, which limits the range of industrial application. In order to solve this problem, with the composite coil structure adopted by the rotor winding of BDFM and based on the magnetomotive force(MMF) harmonic theory of stator winding, this paper presents a soft starting method. This method enables the motor to automatically increase the starting resistance when starting, which reduces the starting current and the impact on the grid. And the mechanical characteristics become hard. When running, the starting resistance returns to normal, which makes the running efficiency high. First, this paper introduces the working principle of soft-start BDFM, and gives several designs of stator winding and rotor winding(RW) for comparative analysis. Then, the two-dimensional transient magnetic field analysis method is used to analyze the magnetic field changes. Finally, a test prototype was developed and an experimental platform was built. Under soft starting and asynchronous starting, the no-load and load starting performance of the soft-start BDFM is studied through simulation and experiment. The results confirmed the feasibility of the design.

Bdot probe and Rogowski coil cross-calibration and sensor fusion in pulsed direct current capacitor discharges.

Bdot probes and Rogowski coils are used in the measurement of transient magnetic fields and currents, respectively. They both share the mechanism of creating an induced electromotive force response via Faraday's law, which scales linearly with the pulsed magnetic field. High power capacitor direct current (DC) discharge systems release a single pulse of current that is both very high and very fast (≲1 ms). To capture these transient data and characterize these systems, high current tolerant and fast response time sensors are required. While these measuring devices have been well studied and utilized for almost 100 years, a comprehensive and detailed description of the custom design, calibration, and sensor fusion application of these tools for use in various pulsed DC capacitor value discharges is largely missing in the literature. Using robust analytical calculations, finite element analyses, and empirical methods, we have developed a sensor fusion protocol for current and magnetic field probes (with relative errors of ±13% and ±15%, respectively) for use in any geometry of high speed pulsed DC current calibrated capacitor discharge systems. This paper comprehensively outlines the design and sensor fusion methodologies that allow for the deployment of in-house built Bdot probes and Rogowski coils to a wide range of pulsed DC systems and demonstrates their use in a characteristic plasma environment.

Waveform conversion and validation of transient magnetic field due to ESD using equivalent circuit of magnetic near‐field probe

AbstractWith a 6 GHz band high resolution magnetic near‐field probe (XF‐R 3‐1) produced by Langer, the transient magnetic fields due to collision ESDs (electrostatic discharges) between metal balls at a charging voltage of 600 V were measured near the spark point to investigate a dipole radiation mechanism. In this study, as an object of considering the measured magnetic field waveforms, two different equivalent circuits of the magnetic near‐field probe are derived based on the probe response waveform observed by a TDR (time domain reflectometer) with a 10 ps rise‐time pulse and the probe reflection coefficient measured with a 26 GHz network analyzer. Waveform conversion formulae from the probe output voltage to the magnetic field are given. The validity of the conversion methods is verified by comparing the measured conversion waveforms and their frequency spectra in addition to the converted spectra by the Langer field correction curve with the calculated waveforms of the transient magnetic far‐fields from a dipole model consisting of image charge pairs and the Rompe‐Weizel spark resistance formula. The probe conversion formulae presented here are valid within the frequency range of the 6 GHz probe band, however, beyond the band the resonance peaks at multiple frequencies over 8 GHz appear on the spectra, which causes damping oscillations peculiar to the probe with multiple frequencies to the time domain waveform.

The Design and Magnetic Field Analysis of a Double Rotor Permanent Magnet Braking Device

During long-term continuous braking, high-intensity braking, or frequent braking, the temperature of the brake disc or brake drum will increase significantly, resulting in a decrease in the friction coefficient, the aggravation of the wear degree, and the dangerous heat recession of partial or even total loss of braking efficiency. This paper focuses on an innovative double rotor permanent magnet braking device, which is located on the inner side of the wheel hub, to improve the heat decay resistance of the friction braking device. The structure and principle of the double rotor permanent magnet braking device were given, and its main structural parameters were designed, calculated, and optimized. The geometric model and finite element model of the double rotor permanent magnet braking device were established. The static and transient magnetic field analysis and the braking torque characteristic analysis of the double rotor permanent magnet braking device were carried out by using Maxwell electromagnetic analysis software. The results show that the magnetic flux density in the working area of the double rotor permanent magnet braking device increases with the increase in rotation speed, the braking torque changes with the change of rotation speed, and the maximum braking torque occurs in the low-speed area, which is consistent with the theoretical calculation results. This provides a theoretical basis for the follow-up prototype test of the double rotor permanent magnet braking device.

B-dot probe measurements on plasma plume expansion across the magnetic field

Time-varying diamagnetism in laser-produced plasma moving across a transverse magnetic field for different field strengths has been studied using fast imaging and magnetic probe. The emphasis of the present work is on the development of suitable B-dot probe, quantitative analysis of induced diamagnetic field in an expanding plasma plume and its effect on the applied magnetic field profiles. A Helmholtz coil with pulsed power system is used to produce uniform magnetic field varying from 0.13 T to 0.57 T. Helmholtz coil allows the plume imaging along the magnetic field lines, which gives the direct structural information of the induced diamagnetic cavity. A high frequency three-axis B-dot probe has been developed to measure the transient magnetic field. Different experimental approaches have been used to test the response, sensitivity and calibration of the developed probe. It has been observed that induced magnetic field displaced the external magnetic field that is plasma plume shows the diamagnetic behaviour for considered magnetic field range. The probe signals in directions orthogonal to the applied/induced magnetic lines are correlated with the distortion in applied magnetic field in three dimensional space, which is consistent with recently simulated topology of external magnetic field in similar experimental conditions [Patel et al, 2021 Plasma Phys. Control. Fusion 63 115020 ].

Measurements and analysis of response function of cold atoms in optical molasses

We report our experimental measurements and theoretical analysis of the position response function of cold atoms in a magneto-optical trap (MOT) by applying a transient homogeneous magnetic field as a perturbing force. We observe a transition from a damped oscillatory motion to an over-damped relaxation, stemming from a competition between the viscous drag provided by the optical molasses and the restoring force of the MOT. Our observations are in agreement with the predictions of our model based on the Langevin equation. We also study the diffusion of the atomic cloud in the optical molasses and find the measured value of diffusion coefficient matching with the prediction of our theoretical model.

Novel ESD device design for STT-MRAM memory chip

An investigation of the influence of the magnetic field induced by electrostatic discharge (ESD) current on magnetic tunnel junction (MTJ) device is presented in the paper. The transient magnetic field generated by the ESD current is not considered in the traditional CMOS ESD protection strategy, which may be a reason of the degradation of the performance and the robustness of STT-MRAM. An ESD protection circuit of STT-MRAM is proposed in the paper, including the diode-based ESD devices, the filter circuit and the power clamp. Based on the waveform analysis of the three ESD standard test models, including HBM, MM and CDM, a filter circuit is added in the ESD device to decrease the influence of the high frequency components on the magnetic device. The diode-based ESD devices are integrated in the ESD structure, which is used for the ESD window of STT-MRAM for its low operating voltage. Based on the operation mechanism of the STT-MRAM, its ESD window allows a low operating voltage, thus the diode-based devices are chosen to be integrated in the ESD device. The distribution and magnitude of the magnetic field are studied in the paper. The results show that the ESD protection circuit of STT-MRAM can effectively suppress the magnetic field generated by the ESD current, hence increasing the stability of STT-MRAM. Under the influence of the HBM, MM and CDM events, the ESD disturbance probability of the MTJ device are 2.0 × 10−12, 5.8 × 10−9 and 1.1 × 10−12 respectively. The influence of the memory density and the distance between pads and memory array are also investigated in this paper. The ESD protection strategy proposed in the paper can be adopted for the future high performance STT-MRAM.

3D-FOX—A 3D Transient Electromagnetic Code for Eddy Currents Computation in Superconducting Magnet Structures: DTT TF Fast Current Discharge Analysis

Bulky metallic structures are needed in the toroidal field (TF) superconducting magnets for fusion applications to withstand the large Lorentz forces acting on the winding. The pulsed coil operation during a plasma scenario and the fast current discharge, or a plasma disruption event in off-normal operating conditions, cause transient magnetic fields, inducing eddy currents in the TF structures. The eddy currents generate heat in the structures heating in turn the winding pack, eroding the temperature margin: such power deposition is a key input for thermal-hydraulic (TH) analyses. However, the computation of eddy currents in fusion magnets is a challenging topic since a transient, fully 3D electromagnetic (EM) model is required. The EM problem is solved here by means of the finite element (FE) open source code FreeFEM++. First, the correct implementation of the EM problem is verified by means of suitable benchmarks against both simple analytical cases and the results obtained with state-of-the-art FE commercial codes on the DTT TF coil, used as a reference geometry. Then the EM code is applied to the evaluation of the magnetic fields and eddy currents induced in the same reference coil during the normal (static) and off-normal (transient) operation; the output of the EM analysis is used as input to the TH analysis carried out with the 4C code, aimed at computing the temperature margin evolution during the transient.