Demagnetization Effects Research Articles

Damping and demagnetization analysis of a cylindrical linear electromagnetic buffer under impact load

This paper investigates the damping and demagnetization effect of an electromagnetic buffer (EMB) under impact load. The motional eddy currents are calculated first, and the Ampere circuital theorem is employed to calculate the induced magnetic field. The demagnetization effect is formulated by the magnetic Reynolds number with a correction coefficient km. Following this, the time-step finite element (FE) model of the buffering system is constructed based on the nonlinear B–H curve. The accuracy and feasibility of the FE model is preliminarily verified by small-scale impact test. The characteristics of damping displacement, velocity, damping force and damping coefficient are obtained. Then, the demagnetization effect is analysed by eddy current, magnetic field distribution, through the intensive impact load test at room temperature and time-step FE model. The value of km is obtained through the intensive impact test and the simplified expression of critical velocity. The results show that the demagnetization effect is obvious with the increase of velocity. By introducing the correction coefficient of magnetic Reynolds number, the eddy current demagnetization process is more realistic.

Modeling and Classification of Stator Inter-Turn Fault and Demagnetization Effects in BLDC Motor Using Rotor Back-EMF and Radial Magnetic Flux Analysis

This paper shall carry out unique study and investigation on the individual and combined effect of Stator Inter-turn Fault (SITF) and rotor demagnetization effects in a Surface Mounted Permanent Magnet (SPM)-type Brushless Direct Current (BLDC) motor. Comprehensive modeling of fault is accomplished using the well-established fault modeling techniques such as Electrical Equivalent Circuit (EEC), Magnetic Equivalent Circuit (MEC), Numerical Methods (NMs) and hybrid modeling-based approaches. The extensive study of SITF conditions using an Improved Winding Function Theory (IWFT) gives better analysis to winding inductance $L_{S}$ , under fault conditions, while modeling the demagnetization effects using MEC and NM based approaches gives more realistic magnetic field quantities. The outcomes obtained are further remodeled in the developed hybrid model of a BLDC motor to study the steady state performance of a machine in terms of rotor back-EMF, $E_{B}$ and radial magnetic flux density $B_{g}$ . The characteristic performance accomplished through the hybrid model system possess better accuracy with less computational time and therefore finds suitability among efficient and fast diagnosis technique feasible for industrial applications. The significant detectable changes in $E_{B}$ and $B_{g}$ under subjected fault conditions, assist in detection, identification and classification of faults in BLDC motor. By obtaining the similar outcome experimentally and under the load conditions, the proposed study on the classification of fault is validated.

Eddy current loss and its magnetization effect of electromagnetic buffer under intensive impact load

In this paper, a high-energy density electromagnetic buffer (EMB) is studied and analysed for the violent acceleration and high velocity of intensive impact loads. First, the design requirements of the EMB are proposed to select reasonable structure and magnetic circuit parameters. The equivalent current model is used to introduce the primary eddy current affected by demagnetization effect and the induced secondary eddy current. The magnetization process is studied by dividing the conductor tube into the approach end and the departure end. Considering the nonlinear damping and eddy current interaction between primary and secondary, a primary-secondary eddy current loss coupled nonlinear time-step finite element model (FEM) is established to obtain the spatiotemporal distribution characteristics of eddy current. Finally, a test experiment with weak impact, medium impact and intensive impact was carried out. The measured displacement, velocity, damping force, and time nodes responses during buffering are consistent with the established time-step FEM results. The proposed high-energy density EMB can effectively complete the impact buffering process. It is reasonable to obtain the eddy current loss and its magnetization law from the established FEM which is suitable for shock buffering with different impulse strength.

Assessment of a Novel 3T MRI Compatible Cochlear Implant Magnet: Torque, Forces, Demagnetization, and Imaging.

A novel cochlea implant (CI) device magnet providing alignment to the static field of a magnetic resonance imaging (MRI) will lead to reduced torque, longitudinal forces, and demagnetization effects. The image void and distortion will be comparable to those obtained with standard CI magnets. MRI investigations of CI users pose several challenges such as magnet dislocation, demagnetization, and may cause pain. The presence of a CI magnet within MRI field causes image distortions and may diminish the diagnostic value of an MRI procedure. Objective of this work is to evaluate magnetic forces and imaging properties of the novel CI magnet within 1.5 and 3T MRI. Forces and torque of the novel CI magnet were measured in both 1.5 and 3T MRI and compared with the standard magnet in 1.5T. One cadaver head was implanted with the CI devices containing the novel and standard magnets in different configurations reflecting clinical scenarios and imaging properties were assessed and compared. In particular the torque has been reduced with the novel CI magnet in comparison to the standard one. Both CI magnets have not shown any signs of demagnetization. The image void and distortion was comparable between the two magnets for the main MRI clinical scanning protocols in 1.5T MRI. The novel CI magnet is safe to use for MRI investigations of CI users in 3T MRI without a need for bandaging and has acceptable level of image artefacts.

On-load demagnetization effect of high-coercive-force PMs in switched flux hybrid magnet memory machine

In the previous researches of hybrid magnet memory motors (HMMMs), the demagnetization characteristics of low-coercive-force (LCF) magnets have been already investigated extensively. Nevertheless, the possible irreversible demagnetization of high-coercive-force (HCF) magnets remains unexplored hitherto. In this paper, the demagnetization behaviour of HCF magnets in switched flux hybrid magnet memory machines (SF-HMMMs) accounting for the high-level current pulse is revealed and investigated. A simplified magnetic circuit model is built to illustrate when and how the DC current pulse poses the risk of irreversible demagnetization to the HCF magnets. Furthermore, the influences of temperature, DC current amplitude and HCF magnet thickness on the irreversible demagnetization effect of HCF magnets in the investigated SF-HMMM are analyzed based on finite-element (FE) analyses. The theoretical and FE results are experimentally verified by the tests on an SF-HMMM prototype.

Coercivity enhancement and uncoordinated deformation in PrCu-doped PrFeB/PrCo5 hybrid magnets

PrFeB/PrCo5 hybrid magnet has excellent magnetic properties and better thermal stability. In this paper, anisotropic nanocomposite PrFeB/PrCo5 hot-deformed magnets with and without doping PrCu eutectic alloy were prepared by spark plasma sintering (SPS) technique. With the addition of 6 wt% PrCu, the coercivity of the magnet (10.8 kOe) is almost double than that of undoped magnet (5.6 kOe), with an enhancement of the remanence (9.1 kG versus 8.4 kG). For the magnet with PrCu, the nonmagnetic PrCu phase is mainly distributed in the phase boundary regions, introducing a coupling effect of demagnetization on the hard-magnetic phases, which obtained an enhancement of the coercivity. The deformed Pr2Fe14B grains of the hybrid magnet have elongated platelet shapes, but the PrCo5 grains are not easy to be deformed, which affects the overall degree of deformation texture of the hybrid magnet. This implies that there is uncoordinated deformation for the PrFeB/PrCo5 hybrid magnet.

Demagnetization Analysis of Interior Permanent Magnet Machines Under Integrated Charging Operation

Integrated battery charging (IBC)—multiplexing the motor windings as line inductors for charging electric vehicle battery packs—can improve the onboard charging capability without increasing the weight and cost. However, during the charging operation, currents flowing in the stator winding definitely generate a magnetic field, which may demagnetize the permanent magnets (PMs) in the PM traction motors. In this paper, the demagnetization effects of IBC current on magnets in interior PM machines are compressively investigated. In this paper, a recursive algorithm to update the PM magnetization states by using a parallelogram hysteresis model at different demagnetization levels is applied. The PM temperature is also considered for establishing this model. By coupling this algorithm to the finite element analysis, the demagnetization transit at different demagnetization levels can be assessed. By decomposing the flux density of each node within the magnets along its magnetization direction, irreversible demagnetization can be accurately evaluated. Impacts from both the pulsating and rotating demagnetization magnetic fields during the IBC operation are assessed and compared.

Modeling of history‐dependent magnetization in the finite element method on the example of a postassembly rotor magnetizer

AbstractDemagnetization characteristics of permanent magnets play a central role for the design and optimization process of permanent magnet electrical machines and magnetic actuators. Currently, practical issues, such as loss of performance of electromagnetic energy conversion devices, due to demagnetization effects in permanent magnets or due to reduced servo‐dynamics in magnetic actuators, cannot be addressed by using conventional material models or are investigated by using rough approximations valid in restricted operation regions, if at all. This paper strives to integrate demagnetizing characteristics dependent on previous magnetizing field strength in the finite element method. The discussed examples are a magnetizer of a single permanent magnet and a spoke‐type permanent magnet rotor.

Failure Diagnosis of Demagnetization in Interior Permanent Magnet Synchronous Motors Using Vibration Characteristics

The detection of a precursor to the demagnetization of permanent magnets is very important because a high degree of reliability is necessary in permanent magnet synchronous motors (PMSMs). This paper investigated the diagnosis of very slight PM demagnetization. A part of the permanent magnet was altered to non-magnetic material so as to mimic the effect of demagnetization. The vibration characteristics were clarified for low demagnetization in PMSMs driven under vector control by experiments and 3D finite element (FE) analysis. We found that the amplitude of some components of the vibration was approximately proportional to the demagnetization level of the PM and the load torque. Therefore, the measurement of vibration and torque is very useful for the estimation of the magnetization level of PMSMs under vector control except for under very light load.

Tuning the laser-induced ultrafast demagnetization of transition metals

The ultrafast demagnetization (UFD) dynamics of itinerant ferromagnets is theoretically investigated as a function of the characteristics of the initial laser excitation. A many-body pd-band Hamiltonian is considered which takes into account hybridizations, Coulomb interactions, spin-orbit interactions and the coupling to the laser field on the same electronic level. In this way, a fruitful connection is established between the non-adiabatic quantum dynamics and the well-known equilibrium statistical mechanics of itinerant-electron ferromagnets. The time evolution during and after the pulse absorption is determined exactly by performing numerical Lanczos propagations on a small cluster model with parameters appropriate for Ni. The most relevant laser parameters, namely, the fluence, wave length, polarization and pulse duration are varied systematically. The results show how they allow one to control the total absorbed energy, the spectral distribution of the initial excitation, and the subsequent magnetization dynamics. The calculations show that reasonable changes in these parameters do not affect the UFD dynamics qualitatively and have only a minor influence on the time scale which characterizes the initial demagnetization. In contrast, our model predicts that the degree of demagnetization correlates well with the average number of electrons excited by the laser or average number of absorbed photons. The theoretical results are discussed by comparing them with available experiments. From a fundamental perspective, the robustness of the ultrafast demagnetization effect is theoretically demonstrated, as a phenomenon reflecting the intrinsic dynamics of the metallic 3d valence electrons. A wide variety of well-focused possibilities of tailoring the efficacy of the ultrafast demagnetization process is thereby opened.

Influence of Design Parameters on On-Load Demagnetization Characteristics of Switched Flux Hybrid Magnet Memory Machine

In this paper, the influences of design parameters on the on-load demagnetization behavior of a switched flux hybrid magnet memory machine (SF-HMMM) are evaluated, which aims to provide a useful design guideline to avoid this performance degradation. The machine topology and on-load demagnetization phenomenon are described, respectively. Furthermore, some key design parameters are selected to optimize to elevate the operating point of the low coercive force (LCF) permanent magnets (PMs). Then, some geometric modifications are suggested to alleviate the on-load demagnetization effects. Finally, an SF-HMMM prototype is fabricated and tested to experimentally verify the finite-element analysis.

Demagnetization research on PMs in the Halbach magnetized pulsed alternator

Demagnetization behavior of Halbach magnetized compensated pulsed alternator (CPA) is studied by using finite element method (FEM) under the different demagnetization factors in this paper. The effect of armature reaction magnetic field and thermal rise on demagnetization of PMs is analyzed. This paper investigates the effect of different demagnetization factors on magnetic field distribution, load current, and no-load phase voltage. A series of dynamic demagnetization points in Halbach array permanent magnet (PM) are evaluated to search the worst working point. Partial demagnetization risk can be represented by the worst working point, and the global demagnetization of the PM is represented by the no-load phase voltage characteristics after discharge. The research results demonstrated that the compensation shield with a certain thickness can increase the discharge current and weaken the demagnetization influence of armature reaction. The demagnetization effect of armature reaction on PMs at high temperatures will be strengthened.

Ultrafast magnetization dynamics in an epitaxialNi54.3Mn31.9Sn13.8Heusler-alloy film close to the Curie temperature

The influence of the amplitude of an external magnetic field ($H$) and femtosecond laser pulse fluence ($F$) on ultrafast magnetization dynamics has been investigated in a ferromagnetic ${\mathrm{Ni}}_{54.3}{\mathrm{Mn}}_{31.9}{\mathrm{Sn}}_{13.8}$ Heusler-alloy film using the time-resolved magneto-optical Kerr effect. A large slowing down of the demagnetization process was observed and characteristic parameters of magnetization precession were determined for a wide range of $H$ and $F$ values. Long demagnetization times of the order of hundreds of picoseconds have been found and explained as a result of the Curie temperature (${T}_{\mathrm{C}}$) proximity in the alloy film studied. Effective magnetic anisotropy field (${H}_{\text{k}}^{\text{eff}}$) and Gilbert damping parameter dependencies were determined. A significant reduction of the precession frequency versus $F$ of the uniform Kittel mode was found. A strong decrease of ${H}_{\text{k}}^{\text{eff}}$ with $F$ was well simulated in the frame of an extended version of the microscopic three-temperature model (eM3TM), and explained by the ${T}_{\mathrm{C}}$ proximity effect. The estimated low values of the eM3TM model parameters, the demagnetization rate and electron-lattice coupling constant, appeared essential to explain the slowing down effect of demagnetization. Precession amplitude dependencies were explained by a phenomenological approach taking into account ${H}_{\text{k}}^{\text{eff}}$ and changes of the equilibrium magnetization angles induced by pump-pulse excitation.

Antiparallel exchange biased multilayers for low magnetic noise magnetic field sensors

High sensitivity magnetoelectric (ME) thin film composite sensors, which enable the detection of picotesla magnetic fields, are improved in terms of magnetic noise performance by the elimination of magnetic domain (MD) activity. Using an antiparallel (AP) exchange biasing scheme, suppression of magnetic noise is obtained. Postsetting of AP biased ferromagnetic/antiferromagnetic multilayers is accomplished by magnetic field free annealing with in-situ MD control. Overcoming the shape and demagnetization effects, stable single MD configurations in the magnetic sensing layers of magnetic multilayers are formed. Magnetic noise contributions are undetectable. The achieved single domain field stability opens the path to ultralow noise ME sensor applications. The demonstrated AP biasing scheme is applicable to other magnetic layer-based field sensing devices.

The effect of parallel pipeline parameters on the characteristics of gravity and magnetic surveys

Parallel pipeline detection is a difficult problem in underground pipeline detection. Gravity and magnetic surveys have the advantages of convenient monitoring, high sensitivity, and high accuracy. Therefore, we used gravity and magnetic surveys to detect parallel pipelines. In this paper, the effect of parallel pipeline parameters on the characteristics of gravity and magnetic surveys is studied in order to find out the quantitative relationships between them and to provide the theoretical basis for inversion interpretation. When calculating the magnetic anomaly of a pipeline, we considered the demagnetization effect of the pipeline and determined its demagnetization coefficient to be 0.12 by numerical calculation. Parallel pipeline parameters include geometric parameters (such as outer diameter ratio, pipe axis spacing, buried depth, and azimuth, etc.) and physical parameters (density and magnetic susceptibility). The characteristics of gravity and magnetic surveys include not only the amplitude of abnormal peaks, the number of abnormal peaks, and abnormal peak spacing of gravity and magnetic anomaly curves, but also the detection error of gravity and magnetic surveys in parallel pipelines. We defined the relative error of the pipe axis spacing of parallel pipelines as the detection error. We established a forward model for parallel pipeline detection and analyzed the effect of model parameters on the characteristics of gravity and magnetic surveys. Results show that gravity and magnetic surveys cannot detect pipelines parallel to the measurement line and the magnetic survey can only detect pipelines with ferromagnetism. The amplitude of abnormal peaks is affected by all parallel pipeline parameters, while the number of abnormal peaks, abnormal peak spacing, and detection error are only affected by the outer diameter ratio, pipe axis spacing and buried depth. If the upper limit of the detection error is determined to be 10%, gravity and magnetic surveys of parallel pipelines have minimum discernible pipe axis spacing. Through further research, it is found that when the outer diameter ratio is 1, the minimum discernible pipe axis spacing is minimal. The minimum discernible pipe axis spacing increases linearly with the increase of buried depth. When all parallel pipeline parameters are identical, the minimum discernible pipe axis spacing of the gravity survey is greater than that of the magnetic survey. The research results show that the magnetic survey is more suitable for detecting parallel underground pipelines than the gravity survey, and the experiment of magnetic anomaly measurement of parallel pipelines also shows that it is feasible to detect parallel underground pipelines by the magnetic survey.

Novel Microwave Resonant Technique for Accurate Testing of Magnetic Materials

A robust cavity technique is proposed to quite accurately measure the complex permeability of the magnetic materials, which gives promising results for arbitrary length samples. This technique takes into account many factors such as magnetic field correction inside the sample under test, the effect of demagnetization, and the image theory, which was not considered in past. In this paper, a numerical model of the magnetometric demagnetization factor is developed, which precisely considers the effects of the relative permeability of sample and the nonuniform magnetic field over sample volume. The sensitivity of proposed technique is better than conventional and recently modified technique assuming arbitrary length sample. The proposed scheme typically provides 98% accurate result for the sample length greater than $0.15\times $ the width of the rectangular cavity. The proposed method is quite novel and can be appreciated due to the fact that it gives higher accuracy for all the cases when compared with other standard cavity techniques.

Study Chipped Permanent Magnets of the Rotor Synchronous Machines

This article examines the effect of partial demagnetization of one of the permanent magnets of the rotor and mechanical violations of the integrity of the permanent magnets of electrical machines, which can occur both during operation and in their manufacture. The main idea of the presented work is to prove the effectiveness of using the methods of harmonic analysis of output voltage and back emf at a certain ratio of the number of grooves and poles. And also in the use of a new universal method for the diagnosis of bipolar rotors with permanent magnets. The research results will be of great practical importance for research engineers, developers and manufacturers of synchronous electrical machines with permanent magnets.

Predictive Torque Control With Online Weighting Factor Computation Technique to Improve Performance of Induction Motor Drive in Low Speed Region

This paper presents a predictive torque control (PTC) method with online weighting factor computation based on the principle of coefficient of variation (CV) while maintaining the steady state, dynamic, and low-speed performance of three-phase induction motor (IM) drive. In PTC, appropriate weighting factor values are essential for the satisfactory performance of the drive. The main challenge is the tedious tuning of weighting factors, which requires rigorous simulation to obtain the desired performance of the drive. In this paper, a method is proposed that estimates online weighting factors, which modifies the cost function consecutively based on the principle of CV. In this method, the ranking of constraint is based on its magnitude of dispersion, whereas in other tuning methods, ranks are assigned beforehand. The proposed method (CV-PTC) is compared with the conventional normalized weighting factor-based PTC method specifically to improve the demagnetization effect in the low-speed operation. In the CV-PTC algorithm, synthetic voltage vectors are implemented to improve the performance of IM in full operating speed range. The experimental analysis is carried out on the IM drive prototype to validate the results and showcase the independence imparted for the selection of weights in the cost function.

Low-Noise Fundamental-Mode Orthogonal Fluxgate Magnetometer Built With an Amorphous Ribbon Core

Room temperature operating magnetometers able to detect a few pT magnetic field would play a crucial role in such areas as bio-magnetic field detection and very tiny ferromagnetic contaminats detection in battery production lines. We have already reported fundamental mode orthogonal fluxgate having a noise density $\sim 3$ pT$/ \surd $ Hz at 1 Hz using a Co-based amorphous wire (made by Unitika) core sensor head [1]. However, the supply of the amorphous wire is limited. In this paper, we present that the narrow amorphous ribbon core is an alternative to the wire core if the dc bias current for the fundamental mode operation [2] is properly adjusted. In a particular case of as-cast Metglas 2714A ribbon of 1 mm width, the dc bias current elevated to 200 mA, which is to be 40 mA for the wire core sensor head, reduces the noise down to a few pT$/ \surd $ Hz. In the experiment, we made several sensor heads listed in Table I using the above mentioned ribbon. A 1 mm width ribbon which is made by slitting a wider sample is bent in a U-shape or more likely in a hair-pin shape and inserted into a cylindrical pickup coil of length 30 mm, bore diameter of 2 mm and having 500 turn copper winding. The driving and signal processing electronics used is the one developed for the wire core sensor head. The only difference is that an adopter circuit is inserted between the ribbon core sensor head and the driving electronics to supply a large dc bias current from the battery and to moderately $( \approx 2\mathrm {x})$ amplify the ac excitation current; the ac excitation current for ribbon core sensor heads is 24 mArms while that for wire core one is 12 mArms. The driving frequency is 100 kHz and the magnetometer is configured to operate in a closed loop (feedback) using $\mathrm {a}10 \mathrm {k}\Omega $ feedback resistor. The sensitivity increases slightly with increase in length of the core, because the demagnetization effect becomes smaller for longer cores. The sensitivity of the magnetometer with the wire core sensor head is about one half of that of the magnetometer with the ribbon core sensor head because the number of turns of the pickup coil used for the wire core is 1000. It should be noted that the sensitivity of the closed loop system is $R/ n[\mathrm {V} /\text{A}$/m] where $R$ is the feedback resistance and $n$ is the winding density per meter of the pickup coil. The noise spectral density of the magnetometer was measured by placing the sensor head in a five shell cylindrical shield and by using an FFT analyzer (Stanford Research Systems SR780). Results are listed in Table I from which we can conclude that the noise level obtained with a ribbon core sensor head is comparable to that obtained with a wire core sensor head. Fig. 1 shows the noise spectral density of the ribbon core sensor head R19 for three different dc bias currents. It is interesting to see that the noise level is dramatically reduced by increasing the dc bias current. This is probably related to the nature of the as-cast Metglas 2714A ribbon such that weak magnetic anisotropies are distributed randomly in the ribbon plane. It is also interesting to compare the magnetic field produced by the dc bias current at surfaces of the ribbon core and of the wire core. For the wire core, the radius is about $60 \mu \mathrm {m}$ and the bias current is 40 mA, hence the magnetic field is about 106 A/m. For the ribbon core, because the thickness $( < 20 \mu \mathrm {m})$ is negligible compared to the width (1 mm), the magnetic field at the surface of the ribbon is quite uniform along the width direction, hence the magnetic field can be calculated by dividing the current (220 mA) by the peripheral length which is twice the width, yielding 110 A/m which is similar to the one for the wire. One may expect that necessary dc bias current can be reduced by making the width of the ribbon narrower. A very weak magnetic field of rectangular waveform of 15 pT in peak-to-peak is well detected with a short averaging. Details on the adapter circuit will be explained.

A Novel Dual-Sided PM Variable Flux Memory Machine

Memory machine (MM) was regarded as a promising candidate for wide-speed-range applications [1]–[6]. Since the low coercive force (LCF) PM is employed in MM, the air-gap flux can be flexibly adjusted by changing the magnetization state (MS) of PM with a temporary current pulses. The speed range can be subsequently extended with high efficiency maintained. According to the magnetizing current pattern, MMs can be generally categorized into AC- [1]–[3] and DC-magnetized [4]–[6] types. The former MM normally employs vector-control algorithm to apply d-axis current pulse in stator windings to vary the magnetization level of LCF PMs, while the latter one adopts auxiliary DC magnetizing coils to facilitate the magnetization control. Besides, the rotor-PM topologies of MMs have structures similar to conventional interior PM machines, which shows comparable torque density with those conventional surface-mounted PM machines . Nonetheless, LCF magnets on the rotor generally suffer from to armature reaction demagnetization effect, and the integration of armature and magnetizing functions in the stator winding results in complicated online FW control. On the other hand, the stator-PM MMs have the advantages of easy online magnetization control, and a simple and robust salient rotor as well as easy thermal management can be obtained. Nevertheless, the fact that two kinds of PMs, i.e., NdFeB and LCF PMs, as well as two sets of windings are located on stator leads to excessively crowded stator space and low torque density. Therefore, this paper attempts to propose a novel dual-sided PM memory machine (DSPM-MM) by combing the distinct advantages of “high torque density” of rotor-PM MM and “simple online PM flux control” of stator-PM MM. In the proposed design, the consequent-pole NdFeB PMs are placed in the rotor, while LCF PMs are mounted between the adjacent stator teeth to enable flexible air-gap flux adjustment. The machine topology and operating principle are introduced and addressed, respectively. Then, the electromagnetic performance is analyzed, which confirms the feasibility of the proposed design.