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Magnetics, IEEE Transactions on

Issue 5 • Date Sept. 2004

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Displaying Results 1 - 25 of 34
  • [Front cover]

    Page(s): c1 - c4
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  • IEEE Transactions on Magnetics publication information

    Page(s): c2
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  • Table of contents

    Page(s): 3217 - 3218
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  • Modeling plastic deformation effects in steel on hysteresis loops with the same maximum flux density

    Page(s): 3219 - 3226
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    Plastic deformation affects the hysteretic magnetic properties of steels because it changes the dislocation density, which affects domain-wall movement and pinning, and also because it places the specimen under residual strain. An earlier paper proposed a model for computing hysteresis loops on the basis of the effect of grain size d and dislocation density ζd. In that paper, hysteresis loops were compared that all had the same maximum flux density Bmax. The result was that coercivity Hc exhibited a linear relationship with inverse grain size (1/d) and ζd12/. The same was true of hysteresis loss WH. If one compared hysteresis loops all with the same Hmax, these linear dependences were only approximately found. Because the relationships are simpler for loops of constant Bmax, core loss experimenters compare loops that all have the same Bmax. In this paper, we modify the model to study the effect of plastic tensile deformation on hysteresis loops with the same Bmax. We found linear relationships between Hc and residual plastic strain εr and between WH and εr. With increasing residual tensile strain, Hc increases (whereas with increasing elastic tensile strain, Hc decreases). Also, with increasing residual tensile strain, the slope of the hysteresis loop decreases (whereas with increasing elastic tensile strain, the slope increases). We also consider the effect of compressive plastic deformation. View full abstract»

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  • Hysteresis parameter identification with limited experimental data

    Page(s): 3227 - 3239
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    The Preisach operator and its variants have been successfully used in the modeling of hysteresis observed in ferromagnetic, magnetostrictive, and piezoelectric materials. However, in designing with these "smart" materials, one has to determine a density function for the Preisach operator by using the input-output behavior of the material at hand. In this paper, we describe a method for numerically determining an approximation of the density function when there is not enough experimental data to uniquely solve for the actual density function by Mayergoyz's method. We present theoretical justification for our method by establishing links to regularization methods for ill-posed problems. We also present numerical results where we estimate an approximate density function from data published in the literature for a magnetostrictive actuator and two electroactive polymers. View full abstract»

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  • Computing solenoidal fields in micromagnetic simulations

    Page(s): 3240 - 3243
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    In finite-difference micromagnetic simulations, the electric field originated from time variations of magnetic induction is evaluated by means of a discrete version of Faraday's law. The electric field can be then calculated as the convolution of a tensor and the time derivative of the magnetic induction. This paper presents an analytical expression for the tensor. The paper also reports on a quantitative test of the tensor that simulates the electric field of an oscillating magnetic point dipole. View full abstract»

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  • Calculating the coupling factor in a multilayer coaxial transformer with air core

    Page(s): 3244 - 3248
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    High-voltage transformers can be built with coaxial transmission lines. This paper describes a transformer design that uses the coaxial screen as primary winding and the inner conductor as secondary. An advantage of transmission line transformers is that the insulation problem is solved and the construction can be kept simple; the coupling between the primary and secondary coils is high even though the transformer uses an air core. The air core brings another advantage: the capacity to store large quantities of magnetic energy. The combination of a high coupling factor and large energy storage capacity makes this transformer ideal for charging high-voltage capacitors fast. The winding type for the transformer is alternating Archimedean spirals. Here, we present a magnetic field analysis of the transformer's primary, secondary, and mutual inductance using a finite-element solver. We compare the measured magnetic flux density versus the calculated value. The step-up winding ratio of the transformer influences the coupling factor marginally if the construction has an even number of spiral layers for each set of windings. However, the result from the finite-element solver predicts a drop in coupling factor if the step-up transformer construction has an odd number of spiral layers. The copper conductors used in the transformer resemble isotropic copper pipes. View full abstract»

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  • Steady-state finite-element solver for rotor eddy currents in permanent-magnet machines using a shooting-Newton/GMRES approach

    Page(s): 3249 - 3253
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    This paper presents a two-dimensional steady-state finite-element solver, incorporating mechanical motion, that calculates eddy-current losses in the rotors of permanent-magnet machines. A shooting-Newton method is used to determine the periodic solution of the electromagnetics. Computation of the shooting-Newton Jacobian is avoided by using a generalized minimum residual (GMRES) linear solver. This method is more computationally efficient than performing transient analysis until convergence. The solver can be used to compare the rotor losses of different design choices for a high-speed permanent-magnet machine. Results show that rotor losses can be reduced significantly by laminating the rotor backiron, segmenting the permanent-magnet poles, increasing the number of stator slots, and closing the stator slots. View full abstract»

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  • Additional losses in induction machines under synchronous no-load conditions

    Page(s): 3254 - 3261
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    The paper examines factors and parameters that affect the additional losses produced in the rotor cage of an induction motor during a no-load test at synchronous speed. The analysis uses a numerical approach based on a finite-element step-by-step procedure, taking into account the rotor movement. Iron losses are evaluated according to the loss separation theory. Additional losses are computed on the basis of predictions of high-frequency phenomena induced in the rotor cage. The model has been validated by comparison with experiments performed on a specific laboratory setup, consisting of a stator that can be equipped with two different rotors. The numerical approach was used to investigate the role of different parameters (supply conditions, geometrical dimensions, material properties) affecting the additional losses. View full abstract»

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  • Design optimization of radially magnetized, iron-cored, tubular permanent-magnet machines and drive systems

    Page(s): 3262 - 3277
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    In this paper we deduce, from analytical field solutions, the influence of leading design parameters on the performance of a radially magnetized, iron-cored, tubular permanent-magnet machine and its drive system. We derive analytical formulas for predicting the open-circuit electromotive force, the thrust force, the iron loss, and the winding resistance and inductances, as well as the converter losses. The force density, the machine and drive system efficiencies, and the power factor and converter volt-ampere (VA) rating are established as functions of a set of machine dimensional ratios, with due account of magnetic saturation and subject to a specified thermal constraint. We validate the utility and accuracy of the analytically derived formulas by finite-element calculations. Finally, we show that the design optimization of such a linear drive system must account for the losses and VA rating of the converter as well as the design parameters of the tubular machine. View full abstract»

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  • Calculating the external magnetic field from permanent magnets in permanent-magnet motors-an alternative method

    Page(s): 3278 - 3285
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    We present an alternative method for calculating the magnetic field from a set of permanent magnets in a permanent-magnet motor. The method uses a cylindrical coordinate system to model the geometry of the structure enclosing the magnets. A Fourier series expansion yields an alternative to the more familiar multipole expansion given in spherical coordinates. The expansion is developed by using Green's function in cylindrical coordinates. A technique called charge simulation allows computation of an equivalent point charge distribution. Finally, Coulomb's law is applied to express the magnetic scalar potential in a mathematically tractable form. View full abstract»

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  • Field weakening for radial force reduction in brushless permanent-magnet DC motors

    Page(s): 3286 - 3292
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (480 KB) |  | HTML iconHTML  

    In brushless permanent-magnet dc (BLDC) machines, the attraction between the rotor permanent magnets and the stator iron causes radial stator forces that excite the stator structural response and radiate unwanted acoustic noise. In this paper, we develop an analytical model that predicts rotor torque and radial force ripple as functions of the stator currents. The model shows that field weakening of sinusoidally commutated BLDC machines can reduce radial forces but requires higher currents to maintain the desired torque. We confirmed the analytical results numerically on a BLDC motor using ANSYS finite-element analysis and found a 30% reduction in force ripple at no load. View full abstract»

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  • Analysis of eddy-current loss for design of small active magnetic bearings with solid core and rotor

    Page(s): 3293 - 3301
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    As the size of rotors levitated by active magnetic bearings (AMBs) gets smaller, it becomes increasingly difficult to make laminated cores and rotors that have low eddy-current loss, and solid cores and rotors have to be substituted. Thus, accurate modeling of eddy-current loss is important for small-size AMB systems with solid cores and rotor. In this paper, we propose a new eddy-current loss model for AMB systems, based on the eddy-current brake concept. We show that the eddy-current loss in AMBs strongly depends on the arrangement and size of poles. We compare test results for hetero- and homopolar AMBs having nonlaminated cores and rotor to analytical findings based on the eddy-current loss model. The experimental results confirm that the eddy-current loss in small homopolar AMBs with nonlaminated cores and rotor can be greatly reduced by optimizing the arrangement and size of poles. View full abstract»

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  • Effect of magnetic hysteresis on rotational losses in heteropolar magnetic bearings

    Page(s): 3302 - 3307
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    This paper extends a method for predicting rotational losses for laminated rotors of heteropolar magnetic bearings by using an eddy-current model to include the effect of magnetic hysteresis in the rotor material. It compares the modeling results to the experimental data that were used earlier to assess the loss model neglecting hysteresis. The correction to the total electromagnetic loss in the rotor due to the hysteresis is significant at rotational speeds below 6000 revolutions per minute (RPM), where the model including hysteresis effects provides much better agreement with existing experimental data. View full abstract»

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  • Fault-tolerant homopolar magnetic bearings

    Page(s): 3308 - 3318
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    This paper summarizes the development of a novel magnetic suspension that improves reliability via fault-tolerant operation. The suspension is suitable for flywheels used in satellites and space stations for attitude control and energy storage. Specifically, we show that flux coupling between poles of a homopolar magnetic bearing can deliver desired forces even after termination of coil currents to a subset of "failed poles". Linear, coordinate-decoupled force-voltage relations are also maintained before and after failure by bias linearization. We determined current distribution matrices that adjust the currents and fluxes following a pole set failure for many faulted pole combinations. We used one-dimensional magnetic circuit models with fringe and leakage factors derived from detailed, three-dimensional finite-element field models to obtain the current distribution matrices and the system response. Reliability is based on the success criterion that catcher bearing-shaft contact does not occur after pole failures. The magnetic bearing reliability is improved by increasing the number of the radial poles. An advantage of our method over other redundant approaches is a significantly reduced requirement for backup hardware such as additional actuators or power amplifiers. View full abstract»

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  • Electromagnetic forming by distributed forces in magnetic and nonmagnetic materials

    Page(s): 3319 - 3330
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    In this paper, we discuss the electromechanical force densities associated with pulsed electromagnetic fields in inhomogeneous, linear media with conductive losses, in the context of a process of shaping metal objects. We show that the conductivity and the gradients in permittivity and in permeability lead to volume forces, while jump discontinuities in permittivity and permeability lead to surface forces. These electromagnetic forces are assumed to act as volume (body) source densities in the elastodynamic equations and as surface source densities in the corresponding boundary conditions that govern the elastic motion of deformable matter. As an example, we apply the theory to the calculation of the elastic field in a hollow cylindrical object made of a conducting magnetic or nonmagnetic material. We compare the numerical results with those for the classical theory of elasticity with concentrated forces on the boundaries of the material as the source of the elastodynamic field. View full abstract»

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  • Design of a bias magnetic system of a magnetostrictive sensor for flexural wave measurement

    Page(s): 3331 - 3338
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    We report on an investigation of the voltage output from a magnetostrictive sensor for the measurement of elastic flexural waves in a cylindrical steel waveguide. Since the sensor performance is strongly influenced by the bias magnetic field, the bias field optimization is one of the most critical issues in the design of magnetostrictive sensors. For a magnetic system consisting of a yoke and an electromagnet, we formulate a method for optimizing yoke topology in order to maximize the sensor output. Both linear and nonlinear magnetization relations are considered in our analysis. For the verification of the performance of the proposed sensors, we conducted several experiments involving flexural waves to assess the performance of the optimized sensors, and we analyze their results here. View full abstract»

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  • Reducing dielectric losses in MnZn ferrites by adding TiO2 and MoO3

    Page(s): 3339 - 3345
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    We investigated the complementary effects of TiO2 and MoO3 additives on the magnetic properties, the core loss, and the dielectric properties of MnZn ferrites. We analyzed the composition factors influencing the permeability of MnZn ferrites. A high initial permeability base ZnO-MnO-Fe2O3 composition ratio was selected. Electrical and dielectric analyses indicated that the introduction of TiO2 can improve the high-frequency loss properties of MnZn ferrite cores, but such introduction will also have a negative effect on the magnetic properties of the sintered samples. Introduction of MoO3 can alleviate the negative influence of TiO2 and also reduce the internal polarization intensity. As a result, the dielectric constant and corresponding dielectric loss may be reduced. We give experimental results in the paper. View full abstract»

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  • Plasma-sprayed MnZn ferrites with insulated fine grains and increased resistivity for high-frequency applications

    Page(s): 3346 - 3351
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    Plasma-sprayed MnZn ferrite thick films are built up by splats, which consist of columnar grains with diameter ∼200 nm and height ∼1 μm. The existence of the conductive wustite FeO in the as-sprayed films greatly reduces the dc resistivity. However, a useful structure can be developed in these ferrite films with fine equal-axis ferrite grains insulated by the high-resistivity hematite Fe2O3 because of the polygonization of the columnar grains and the oxidation of the wustite during an annealing process. The dc resistivity increases significantly after the annealing process, an effect ascribed to the growth of hematite Fe2O3 on the basis of impedance analysis. The magnetic properties of these ferrite films improve concurrently. The high-frequency response of the annealed plasma-sprayed MnZn ferrites shows a permeability of ∼700 stabilized to above 10 MHz. The maximum Q factor at about 10 MHz increases from 5 to 20 as a result of the increase of the dc resistivity. View full abstract»

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  • Core-current-enhanced domain-wall pinning in nanocrystalline Fe73.5Cu1Nb3Si15.5B7 ribbon

    Page(s): 3352 - 3357
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    We have studied the influence of surface fields Hp (generated by either direct or alternating core current) on soft magnetic properties of amorphous and nanocrystalline Fe73.5Cu1Nb3Si15.5B7 ribbon. While in an amorphous ribbon the coercive field Hc decreases with Hp, in the same optimally annealed ribbon (Hc=1.3 A/m, Mm≈Ms) Hc increases with Hp for all the explored types of Hp (static and dynamic with different phases with respect to that of the magnetizing field H). The unexpected increase of Hc in nanocrystalline ribbon is associated with the influence of Hp on the surface and main (inner) domain structure. Here, we develop a model that takes into account this influence and explains the experimental results. View full abstract»

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  • Off-diagonal magnetoimpedance in NiFe-Au-NiFe layered film and its application to linear magnetic sensors

    Page(s): 3358 - 3367
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    We have measured the field dependence of the off-diagonal impedance in the megahertz frequency range for a NiFe-Au-NiFe layered film using a helical microcoil. The film and the coil were deposited by means of radio-frequency sputtering, and a transverse anisotropy in magnetic layers was established by applying a dc magnetic field during the deposition and by postproduction annealing. The film had 5 mm length, 50 μm width, and 1.5 μm total thickness. The helical microcoil had 23 turns with a 50 μm turn width. We applied high-frequency excitation by means of the coil current and measured the induced voltage across the film stripe. This voltage response is directly proportional to the off-diagonal component of the total impedance tensor. We found that the plots of the real and imaginary parts of the off-diagonal impedance, as functions of the applied dc magnetic field, are antisymmetrical with respect to the field direction. The dc bias current through the film plays an important role: without the bias current, the measured signal is very small and irregular. The field antisymmetry demonstrated by the off-diagonal impedance can be utilized in highly sensitive and linear magnetic sensors, and we discuss the principles of operation of such sensors here. View full abstract»

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  • Influence of AC magnetic field amplitude on the surface magnetoimpedance tensor in amorphous wire with helical magnetic anisotropy

    Page(s): 3368 - 3377
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    We have performed experimental and theoretical studies on the influence of ac magnetic field amplitude on the magnetoimpedance tensor in an amorphous wire with helical magnetic anisotropy. For the experimental measurements, we used an amorphous wire of composition (Co0.94Fe0.06)72.5Si12.5B15 with negative, nearly zero magnetostriction constant, excited either by an ac circular hφ or by an axial hz magnetic field created by an ac electric current. We changed the ac current amplitude from 7.5 to 40 mA and the current frequency f from 1.5 to 20 MHz. The values of the asymmetric giant magnetoimpedance ratio associated with the sweeping direction of the dc field Hex and the corresponding sensitivity were 211% and 0.64 V/Oe, respectively, for an ac current of 37.5 mA at 3 MHz. For the theoretical study based on the magnetization rotation, we obtained the second-order harmonic of the ac magnetization m&oarr;(2) induced by the relatively high ac magnetic field by solving the Landau-Lifshitz-Gilbert (LLG) equation. We also considered a second-order surface impedance tensor ςˆ(2), which allowed us to analyze quantitatively the influence of the ac magnetic field amplitude on the impedance tensor of the wire. We obtained the domain model of the wire with helical magnetic anisotropy having multidomains and the magnetization vector ±M0 directed in the easy direction, and the corresponding static magnetic configurations, by solving the static LLG equation. For the given magnetic configurations, we calculated the second-order impedance tensor ςˆ(2). The results can well explain the irregular field characteristics of the voltage responses at low dc field value, when the wire was excited at high frequency and at large ac magnetic field. View full abstract»

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  • Designing an efficient resistive magnet for magnetic resonance imaging

    Page(s): 3378 - 3381
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    We present an alternative procedure to design a 0.1 T resistive magnet for magnetic resonance imaging. The procedure considers the conductor to be uniformly located over the cylindrical surface and treats it as coil elements. It applies the linear programming method with upper and lower bounds to constrain the current density to a fixed value in order to produce a desired magnetic field over a region of interest. The approach minimizes the power and preserves the predefined homogeneity, resulting in spatial clusters that define the coil's magnet. We demonstrate the method in a practical design situation. View full abstract»

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  • Permanent conical magnet for interventional magnetic resonance imaging

    Page(s): 3382 - 3386
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    We describe an open structure of permanently magnetized material for interventional magnetic resonance imaging of the brain. We transformed the ideal magnet, contained between two coaxial cones, into a practical device by a series of geometrical steps that minimize field perturbations. By taking advantage of the quasi-linear demagnetization characteristics of rare-earth materials, we could analyze the structure with an exact mathematical model. The magnet, built of material of remanence 1.38 T, generates a field of 0.45 T within its central gap 30 cm wide. Our numerical computations show a remarkable 0.25% field uniformity in the imaging region. The large opening makes the conical magnet suitable for interventional and surgical imaging. View full abstract»

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  • A new electromagnetic hearing aid using lightweight coils to vibrate the ossicles

    Page(s): 3387 - 3393
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    As the first stage in the development of a noninvasive electromagnetic hearing aid, we made a new transducer that generates a high-excitation force to vibrate ossicles via the tympanic membrane. This transducer consists of a core, driving and induction coils, a rare-earth magnet, and a vibrator coil. We designed the core, the driving and induction coils, and the magnet so as to generate the greatest excitation force possible when installed in the external ear canal of humans. With regard to the vibrator coil, which was attached to the center of the tympanic membrane to vibrate the ossicles, we determined its optimal mass, position, and shape both by finite-element method (FEM) analysis and by experiments using an artificial middle ear. A prototype of the optimally designed transducer can generate an excitation force of more than 95 dB sound pressure level (SPL) in terms of sound pressure at frequencies between 0.1 and 10 kHz. This result indicates that the transducer developed in this study can be used to treat patients with a hearing loss up to 70 dB hearing level (HL). View full abstract»

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Aims & Scope

IEEE Transactions on Magnetics publishes research in science and technology related to the basic physics and engineering of magnetism, magnetic materials, applied magnetics, magnetic devices, and magnetic data storage.

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Meet Our Editors

Editor-in-Chief
Pavel Kabos
National Institute of Standards and Technology