Showing papers in "IEEE Transactions on Magnetics in 2000"

High K/sub u/ materials approach to 100 Gbits/in/sup 2/

Abstract: High K/sub u/, uniaxial magnetocrystalline anisotropy, materials are generally attractive for ultrahigh density magnetic recording applications as they allow smaller, thermally stable media grains. Prominent candidates are rare-earth transition metals (Co/sub 5/Sm,...), and tetragonal intermetallic compounds (L1/sub 0/ phases FePt, CoPtY,...), which have 20-40 times higher K/sub u/ than today's hexagonal Co-alloy based media. This allows for about 3 times smaller grain diameters, D, and a potential 10-fold areal density increase (/spl prop/1/D/sup 2/), well beyond the currently projected 40-100 Gbits/in/sup 2/ mark, Realization of such densities will depend on a large number of factors, not all related to solving media microstructure problems, In particular it is at present not known how to record into such media, which may require write fields in the order of 10-100 kOe. Despite this unsolved problem, there is considerable interest in high Ku alternative media, both for longitudinal and perpendicular recording. Activities in this area will be reviewed and data on sputtered and evaporated thin FePt films, with coercivities exceeding 10000 Oe will be presented.

The feasibility of magnetic recording at 1 Terabit per square inch

Abstract: This paper explores the feasibility of implementing conventional magnetic recording technology at densities up to one Terabit per square inch. The key limiting physical factor is the superparamagnetic effect (thermal stability) in the recording medium. Ambient thermal energy can cause the magnetic signals to decay. The requirement for thermal stability over periods of years dictates a lower limit to the size of magnetic grains (switching units) in the recording medium. To achieve the highest areal densities, it will be necessary to use a magnetic recording configuration capable of writing and storing data on very small magnetic grains together with a signal processing system capable of recovering data reliably when each bit is recorded on very few such grains. In addition to these physical effects, there are a number of practical engineering factors that must be considered: tolerances on the head geometry, reliability of head-disk interface, track-following accuracy. In an example system, we use a perpendicular recording configuration since it appears to offer some advantage in terms of maximizing the number of stable magnetic grains per unit area. The readback signals are processed by equalization to a simple binary eye followed by soft detection of a low-rate simple parity check code. The example system approaches a density of 1 Terabit per square inch and allows 3 dB of margin against thermal decay, adjacent track interference, etc.

SNR and density limit estimates: a comparison of longitudinal and perpendicular recording

Abstract: Simplified Neel-Arrhenius analysis is utilized to determine limiting densities versus SNR for longitudinal and keepered-perpendicular media. Dominant transition noise is assumed and the effects of grain size distribution and storage above room temperature are included. Perpendicular recording potentially exhibits a higher density than longitudinal; for example, at SNR=20 dB, the limiting densities are approximately 500 and 100 Gbit/in/sup 2/, respectively.

Structural magnetic strain model for magnetostrictive transducers

Abstract: This paper addresses the modeling of strains generated by magnetostrictive transducers in response to applied magnetic fields. The measured strains depend on both the rotation of moments within the material in response to the field and the elastic properties of the material. The magnetic behavior is characterized by considering the Jiles-Atherton mean field theory for ferromagnetic hysteresis in combination with a quadratic moment rotation model for magnetostriction. Elastic properties must be incorporated to account for the dynamics of the material as it vibrates. This is modeled by force balancing, which yields a wave equation with magnetostrictive inputs. The validity of the resulting transducer model is illustrated by comparison with experimental data.

Macroscopic models of magnetization

Abstract: Four magnetization models, now considered as classical, are presented: the Stoner-Wolhfarth model, the Jiles-Atherton model, the Globus model, and the Preisach model. The paper describes the methods each model uses to simulate the magnetization mechanisms, reversible and irreversible processes, major and minor loops, and anhysteretic behavior. Improvements and changes are proposed to the Stoner-Wolhfarth and Globus models. The necessary simplifying assumptions define the limits of applicability of the models. The paper concludes with a table of the main characteristics of each model to help a potential user to make a suitable choice.

Energy-based hysteresis model for magnetostrictive transducers

Abstract: This paper addresses the modeling of hysteresis in magnetostrictive transducers in the context of control applications that require an accurate characterization of the relation between input currents and strains output by the transducer. This relation typically exhibits significant nonlinearities and hysteresis because of inherent properties of magnetostrictive materials. The characterization considered here is based on the Jiles-Atherton mean field model for ferromagnetic hysteresis in combination with a quadratic moment rotation model for magnetostriction. As demonstrated by comparison with experimental data, the magnetization model very adequately quantifies both major and minor loops under various operating conditions. The combined model can then be used to accurately characterize output strains at moderate drive levels. The advantages of this model lie in the small number (six) of required parameters and its flexibility under a variety of operating conditions.

Magnetic nanowires in hexagonally ordered pores of alumina

Abstract: Acid-anodized aluminum forms amorphous alumina with long and columnar nanopores with approximately hexagonal ordering ("alumite"). Excellent hexagonal ordering of these nanopores has been achieved by 24 hours of anodization, but with restricted domain size (2-4 /spl mu/m/sup 2/), which can be increased to 100 /spl mu/m/sup 2/ with longer anodization. We have deposited Fe in disordered pores and Co in ordered pores; we can control the average length and diameter of these nanowires, but there is still a distribution of nanowire lengths. Previously, we described Fe nanowires with diameters down to 11 nm in disordered pores. Here we focus on longer (770 nm) and shorter (64 nm) Co nanowires with diameters of 25 nm in ordered pores with 100 nm pore-to-pore separation. The longer wires have an easy axis out-of-plane, with squareness >0.9, coercivity=1900 Oe, and a fluctuation field of 5.3 Oe. The shorter wires are more isotropic, with lower coercivities (/spl ap/1300 Oe) and larger fluctuation fields /spl ap/8.4 Oe.

Effect of substitution of divalent ions on the electrical and magnetic properties of Ni-Zn-Me ferrites

Abstract: The effect of Cu/sup 2+/, Cd/sup 2+/, Co/sup 2+/, Ca/sup 2+/, Mn/sup 2+/, and Mg/sup 2+/ ions on the physical properties of stoichiometric Ni/sub 0.5/Zn/sub 0.5/Fe/sub 2/O/sub 4/ ferrite is investigated. The specimen is prepared by the conventional manufacturing method without atmosphere control. The divalent ions replace one-half the Ni/sup 2+/ ion molar content. We found that the magnetic and electrical properties of Ni-Zn ferrite changes considerably with the substituent species. Copper ions play an important role in lowering the sintering temperature and increasing density. Manganese and cobalt ions increase electrical resistivity by about two orders of magnitude, while the Ca and Mn ions improve thermal stability of the initial permeability. We also discuss our investigation of the physical properties of the Ni-Zn-Me ferrites on the basis of site occupation of the cation species in the spinel structure.

Design and finite-element analysis of an outer-rotor permanent-magnet generator for directly coupled wind turbines

Abstract: This paper presents the design and finite-element analysis of a permanent-magnet generator using neodymium-iron-boron magnets for directly coupled wind turbines. For the sake of small size and light weight with extra low speed for direct coupling, the outer rotor structure is used. The simple magnetic equivalent circuit approach is used for initial design iteration, and the finite-element method is applied to analyze the detailed characteristics. Comprehensive experimental tests were conducted to verify the theoretical predictions. Agreement between the theoretical work and testing results was good.

Relevance of magnetostriction and forces for the generation of audible noise of transformer cores

Abstract: Recently, the two most significant characteristics of power transformer cores, i.e., loss and noise, have been reduced considerably because of improved material and improved core design. An undisputed base value of noise results from magnetostriction (MS) due to the global magnetization of limbs and yokes, whereas excess values are attributed to different mechanisms, such as excess MS and magnetic forces (MF), in a controversial way. However, a multistep lap (MSL; number of overlap steps N>1) yields considerably lower noise than a single-step lap (SSL; N=1), indicating a specific role of joint design. The present work was aimed at a deeper understanding as a tool for further improvements. For a large number of differently built-up single-phase model cores, local distributions of surface displacements d/sub x/, d/sub y/, d/sub z/ and velocities v/sub x/, v/sub y/, v/sub z/ as well as the corresponding noise n were determined. As a major finding, MSL yielded weak reductions of in-plane vibrations v/sub x/ and V/sub y/, considerable reductions of v/sub z/, and strong reductions of n. Interpretations of results were based on numerical Spice modeling of linearized joint vibrations, which were also established experimentally. The results indicate high values of off-plane flux density B/sub z/ and the corresponding Maxwell stress arising at the outermost sheet ends for SSL, and considerably lower values for MSL, provided that B is below its critical value B/sub c/. MSL yielded reductions of d/sub z/, v/sub z/, and n in 80% of all cases. Three additional core parameters-small air gap lengths, high overlap lengths, and high lamination factors A-all lead to reduced n; the use of oil yielded strong reductions. It is concluded that joint designs actually play a predominant role in noise generation. Apart from some contribution from attractive in-plane forces due to B/sub z/, in air gaps, interlaminar flux B/sub z/ yields primary sources of vibrations that propagate to the entire core surface. In core regions of reduced /spl Lambda/, B/sub z/ yields vibrations of sheet ends due to MF according to an experimentally determined effective Young's modulus that considers practical conditions like waviness, burrs, and thickness variations of laminations. For high h, oscillations of the whole core thickness due to B/sub z/-caused MS will arise close to joints. For MSL, all of these mechanisms become less relevant: Gap regions show much lower B/sub x/ and, provided B

Magnetic circuit model for the mutually coupled switched-reluctance machine

Abstract: The mutually coupled switched-reluctance motor (SRM) appears to have several performance advantages over other motor technologies. The existence of strong coupling between phases, however, makes the analysis of this machine quite complicated. Preliminary design of this machine can be greatly accelerated by the ability to evaluate potential motor geometries quickly. This paper introduces a general magnetic circuit model of the mutually coupled SRM that adapts to any geometry, unlike existing geometry-dependent approaches (such as finite elements), which are numerically intensive and require excessive computation time. The model uniquely implements the magneto-motive force (mmf) sources necessary to accommodate complex flux paths through the machine and includes the effects of magnetic saturation. The results are compared to those of a finite element solver to demonstrate the performance of this method as a first-step to evaluating candidate designs.

Nonlinear varying-network magnetic circuit analysis for doubly salient permanent-magnet motors

Abstract: In this paper, a nonlinear varying-network magnetic circuit (VNMC) modeling method is developed for doubly salient permanent-magnet (DSPM) motors. The method is used to analyze the static characteristics of DSPM motors by specific performance calculations, in which the interaction between the permanent-magnet field and the armature current field, as well as magnetic saturation, are taken into account. Two newly proposed DSPM motors (12/8-pole and 6/4-pole) are used as examples. There is good agreement between the results of the nonlinear VMMC modeling method and finite-element analysis. Moreover, the nonlinear VNMC method offers the advantage of calculating, effectively and efficiently, the static characteristics of DSPM motors having different dimensions, parameters, and conditions. Finally, the proposed method is verified by experimental testing of a 6/4-pole prototype.

Yee-like schemes on staggered cellular grids: a synthesis between FIT and FEM approaches

Abstract: We propose an analysis (discretization techniques, convergence) of numerical schemes for Maxwell equations which use two meshes (not necessarily tetrahedral), dual to each other. Schemes of this class generalize Yee's "finite difference in time domain" method (FDTD). We distinguish network equations (the discrete equivalents of Faraday's law and Ampere's relation) which can be set up without any recourse to finite elements, and network constitutive laws, whose validity cannot be assessed without them. This establishes a complementarity between "finite integration techniques" (FIT) and the finite element method (FEM). As an example, a Yee-like method on a simplicial mesh and its so-called "orthogonal" dual, is described, and its convergence is proved.

Influence of machine symmetry on reduction of cogging torque in permanent-magnet brushless motors

Abstract: We study two methods to reduce cogging torque, both based on the analysis of the rotational symmetry in a brushless permanent-magnet motor. The first one seeks to lower the symmetry in the flux of a conventional motor with asymmetrical magnet distribution. The second one uses auxiliary slots to increase the frequency of the cogging. Analytical calculations have been made to predict the harmonic spectra of the models. Numerical calculations by the finite-element method were carried out. The results support the analytical conclusions. The frequency of the torque pulsation was increased, as predicted. We obtain a peak-to-peak cogging torque reduction of about 85% for each of the two methods. Dynamical calculations show no changes in the mean torque produced. The asymmetrical motor could introduce some ripple due to the EMF shifting.

An approach to a suitable stator length for minimizing the detent force of permanent magnet linear synchronous motors

Abstract: This paper describes a technique for minimizing the detent force using the finite element method. The detent force of the whole stator core is the total of two magnetic forces arising at both side edges of a stator core. Computed results show the phase difference between the two magnetic forces. Therefore, we can cancel out the two forces by adjusting a stator length to minimize the detent force. The stator with the smooth formed edge shape is also contrived to reduce the detent force in the practical use. The detent force of this model is successfully minimized by proposed method.

Improvement in calculation of the self- and mutual inductance of thin-wall solenoids and disk coils

Abstract: The self-inductance expressions given by Yu and Han (1987) for air-core circular coils with rectangular cross sections, thin solenoids, and disk coils can be solved only by the numerical integration methods. We propose as an alternative a combined analytic and numeric approach. The approach brings some improvement in the calculations of self-inductance of thin-wall solenoids and disk coils that can be encountered in superconducting magnetic energy storage (SMES) problems. We also give a method for the calculation of mutual inductance of disk coils and of thin-wall solenoids. The results are obtained in an analytical form over the complete elliptic integrals of the first and second kind and Heuman's Lambda function. It is important to mention that the kernels of these integrals are always continuous functions on intervals of integration, so singularities are avoided. The results enable one to calculate the self-inductance and the mutual inductance of any thin air-core coil precisely and fast. For practical applications, the results are so simple to use that we recommend them to avoid the problems of solving the singular cases.

Analytically computing the flux linked by a switched reluctance motor phase when the stator and rotor poles overlap

Abstract: This paper develops an analytical model for the flux linked by a phase of a switched reluctance motor (SRM) starting with the basic laws for magnetic fields. Thus this model is not empirical nor does it require any data from the machine being modeled or any finite-element analysis results. The model is applicable to rotor positions where the stator and rotor poles overlap, and it includes iron saturation. This paper gives a detailed development of the analytical model including the approximations that are inherent to it and the justification for them. The analytical model for the flux linked by a phase is used to find the analytical equations of motion for the SRM. It is also used to obtain an analytical expression for the SRM's coenergy. Differentiating the coenergy with respect to rotor position, holding the current constant gives an analytical expression for the torque. Finally, this paper compares the analytical model's predictions to measured data from experimental machines.

Improved parameter modeling of interior permanent magnet synchronous motor based on finite element analysis

Abstract: This paper presents an investigation of the parameter modeling on the basis of an improved finite element (FE) analysis in which the variable frequency characteristic in field weakening is considered in interior permanent magnet synchronous motors (IPMSM). The parameters of IPMSM have a nonlinear characteristic under variable load conditions and due to the current phase angle in a system fed inverter. From the analysis of FE, a new vector control algorithm is proposed by a neural network using a variable inductance estimator. The performances of variable frequency are simulated by using the proposed algorithm, and the validity of the proposed FE analysis is compared with experimental results.

Room-temperature magnetic refrigerator using permanent magnets

Abstract: A magnetic refrigerator device based on adiabatic magnetic refrigeration is described. The magnetic material is cyclically magnetized and demagnetized by permanent magnets in an adiabatic process. A temperature difference of 1.6 K between the hot and cold regions was obtained under a low magnetic field (0.3 T). Gadolinium was the magnetic material used in experiments at room temperature. The range of working temperatures is between 70 and 300 K for a variety of active magnetic materials. The optimized experimental setup increased the device efficiency by achieving a temperature difference between hot and cold sources up to 5 K.

Polarization method for static fields

Abstract: An overview of the polarization method is presented. The method can by applied for different regimes of the electromagnetic field as well as for electric circuits. Criteria for the choice of the permeability are proposed, so that the iterative scheme leads to a Picard-Banach fixed point procedure. The errors are evaluated. An efficient overrelaxation method is presented. The modality of using FEM numerical method is analyzed in order to ensure the convergence of the method.

Magnetic ground state of a thin-film element

Abstract: By means of three-dimensional numerical calculations we studied possible micromagnetic configurations in a rectangular Permalloy-like thin-film element. The parameters were chosen to be compatible with the so-called micromagnetic standard problem 1. We demonstrate that for these parameters a diamond domain pattern is the lowest energy state that replaces cross-tie patterns favorable in larger elements. Only at smaller sizes does the originally envisaged Landau pattern form the ground state. The transition to high-remanence structures (or what would be comparable to a "single-domain" state) is found for lateral sizes that are an order of magnitude smaller than the benchmark parameters. The transitions among the different domain patterns become plausible in view of the energy of symmetric Neel walls in extended thin films. The features of the high-remanence structures can be derived from the principle of uniform charge distribution.

Analysis of iron loss in interior permanent magnet synchronous motor over a wide-speed range of constant output power operation

Abstract: The paper presents the analysis of iron loss in interior permanent magnet synchronous motor over a wide-speed range of constant output power operation. The analysis is based on finite element method and uses the information on local flux density variation in the motor components. The total iron loss is calculated by summing up separately the hysteresis, classical eddy current and excess losses for each element of discretization over one electrical cycle of magnetic field variation in all parts of motor iron. The results of iron loss calculation were compared with the results obtained from experiments on the actual machine.

Design sensitivities for scattering-matrix calculation with tetrahedral edge elements

Abstract: The rates of change of the scattering matrix of a microwave device with respect to geometric or material parameters are of interest to designers, and useful in automatic optimization. They are most efficiently calculated by the adjoint variable method, but this usually entails the additional cost of funding the adjoint solutions. In the finite-element formulation presented here, no new solutions are needed beyond those required to calculate the complete scattering matrix. Explicit expressions are given for the calculation of the required matrix for a tetrahedral edge element. Results are presented for three rectangular waveguide problems: a uniform length terminated in a short-circuit; an H-plane miter bend; and a two-step waveguide impedance transformer.

Homogeneous magnet design using linear programming

Abstract: We introduce a technique for designing homogeneous magnets using linear programming, We first show that minimum-power homogeneous magnet design can be cast as a linear programming problem. We also show that the method is applicable to minimum conductor mass superconducting magnet design. The method has several advantages over existing techniques including: it allows complete flexibility in arbitrary geometric constraints on both the coil locations and the shape of the homogeneous volume; it guarantees a globally optimal solution; and it offers rapid computation speed (about 30 s). Three resistive magnet design examples and one shielded superconducting magnet design are presented to illustrate the flexibility of the method.

A new surface-inset, permanent-magnet, brushless DC motor drive for electric vehicles

Abstract: A new five-phase, surface-inset, permanent-magnet (PM), brushless DC motor drive is proposed in this paper. The motor drive has advantages of both the PM brushless DC motor drive and the DC series motor drive. The originality is that the air-gap flux of the motor is generated by both the PM excitation and the specially controlled stator currents (two particular phases) under the same PM pole. The motor configuration and principle of operation are so unusual that the magnetic field distribution and steady-state performance are analyzed by the finite-element method. Experimental results for a prototype verify that the proposed motor drive is promising for modern electric vehicle applications.

Unbalanced force calculation in switched-reluctance machines

Abstract: This paper presents a detailed analytical model for computing the radial magnetic forces that arise in switched reluctance machines (SRM's). The model is general and includes iron saturation, displacements of the rotor from its center location, and arbitrary angular rotor positions. The force between an individual stator pole and its corresponding rotor pole is calculated. The model is used to calculate the unbalanced magnetic forces on the SRM rotor, due to the rotor being displaced from its center location, by calculating the difference in the radial magnetic forces on opposite stator poles. The calculation of the unbalanced magnetic rotor forces requires an especially accurate model for the radial magnetic forces since the unbalanced forces are the difference between the two large radial forces on opposite sides of the rotor. The side pull created by the unbalanced forces will stress the bearings of the motor. The detailed analytical model presented here will simplify the bearing system design and will be especially useful if less stiff magnetic bearings are being employed. Finite-element analysis is used to validate the detailed analytical model. This same model for calculating the radial magnetic forces can be used as the input to a calculation of stator yoke ovulation due to radial magnetic forces and of the resulting acoustical noise production.

Overview of latest work on perpendicular recording media

Abstract: Recent works on perpendicular recording media were reviewed, Co-Cr alloy films are still under improvment and show a recording ability of 20 Gbit/in/sup 2/ and beyond. New perpendicular recording media that have high perpendicular anisotropy have been developed. These would be candidate media for ultrahigh density recording beyond 100 Gbit/in/sup 2/.

Optimal core shape design for cogging torque reduction of brushless DC motor using genetic algorithm

Abstract: The cogging torque in the small brushless DC (BLDC) motors used in the digital versatile disk (DVD) driving system or hard disk drive (HDD) system can cause some serious vibration problems. In this paper, some core shapes that reduce cogging torque are found by using the reluctance network method (RNM) for magnetic field analysis and genetic algorithms (GAs) for optimization. The outer rotor type BLDC motor for the DVD ROM driving system is optimized as a sample model.

Structural optimization in magnetic devices by the homogenization design method

Abstract: The homogenization design method (HDM) is extended to obtain an optimal topology of magnetic fields. This is accomplished by maximizing the magnetic mean compliance in a given region of the device. HDM is applied to a three-dimensional case, taking into account the saturation effect of the material. Results show that HDM is valid to maximize the vector potential and the magnetic energy. This method can also be applied to increase the performance of motors and antennas.

Solution strategies for transient, field-circuit coupled systems

Abstract: Transient simulation time for field-circuit coupled models of realistic electromagnetic devices becomes unacceptably high. A magnetodynamic formulation is coupled to an electric circuit analysis, yielding a sparse, symmetric and indefinite matrix. The unknown circuit currents correspond to negative eigenvalues in the matrix spectrum. The Quasi-Minimal Residual method performs better than the Minimal Residual approach that is restricted to positive definite preconditioners. The positive definite variant is solved by the Conjugate Gradient method without explicitly building the dense coupled matrix. As an example, both approaches are applied to an induction motor.