Göran Engdahl

A stress-dependent magnetic Preisach hysteresis model

The authors present a generalization of the classical Preisach model which handles coupled magnetic and mechanical hysteresis. Magnetostrictive materials are known to have hysteresis with respect to both magnetic field H and mechanical stress lambda . To test the validity of the model, experiments where the two components H and lambda have been verified in many different ways have been performed on Terfenol-D and compared to results computed from the model. Some of these results are presented. This stress-dependent model is found to have an accuracy comparable to that of the classical Preisach model. >

Separation of the Energy Absorption and Overvoltage Protection in Solid-State Breakers by the Use of Parallel Varistors

Hybrid and solid-state breakers offer new possibilities in the power grid by enabling faster switching, and by simplifying dc breaking. However, they consists of expensive power electronic components that are sensitive to overvoltage transients and require energy absorbing elements mounted in parallel. At turn-off, the rapidly decreasing current in the power electronic switch and the presence of an inherent stray inductance leads to hazardous overvoltage transients across the breaker. This paper investigates the possibility to split the overvoltage protection and energy absorption into two separate components. By optimizing the voltage ratio between two varistors, one can dimension a small electronics varistor for overvoltage protection and a large power electronics varistor for energy absorption. With this setup the power electronics varistor is allowed to be in a circuit with a large stray inductance and can thus be placed further away without causing an uncontrolled overvoltage. It is shown both in circuit simulations as well as in a small-scale experiment that if the voltage ratio between the two varistors is large enough, the inner varistor only has to absorb 1-2% of the system energy.

Physics of Giant Magnetostriction

The chapter describes the physics of giant magnetostriction as the deformation of a body in response to a change in its magnetization. The change can be brought by the change in temperature or by the application of a magnetic field. All magnetic materials exhibit magnetostriction to some degree, but giant magnetostriction occurs in a small number of materials containing rare earth elements. The mechanical strength of giant magnetostrictive is important as it is used in resonance in which the amplitude of the stress can be extremely high. Magnetoelasticity is defined as the coupling between the classical properties of elasticity, strain, the intrinsically quantum mechanics, and the relativistic phenomena of magnetism. Coupling that exists at the individual electron level is known as “spin-orbit coupling.” This is one of the smallest energies used to describe the state of an atom as it derives from the relativistic aspects of the electron motion. The chapter also discusses some simplified descriptions of magnetism and magnetostriction that form a base for understanding the potential and limitations of materials exhibiting giant magnetostriction.

A magnetostrictive electric generator

An electric generator based on the magnetostrictive effect is presented. Longitudinal oscillations of a Terfenol-D rod give rise to a varying flux which induces a current in a coil wound around the rod. A small prototype of such a device is discussed, and both calculations and experiments are performed. The problem of eddy current losses is addressed. >

Comparison of Two Ultra-Fast Actuator Concepts

In this paper, two different types of ultra-fast electromechanical actuators are compared using a multi-physical finite element simulation model that has been experimentally validated. They are equipped with a single-sided Thomson coil (TC) and a double-sided drive coil (DSC), respectively. The former consists of a spirally-wound flat coil with a copper armature on top, while the latter consists of two mirrored spiral coils that are connected in series. Initially, the geometry and construction of each of the actuating schemes are discussed. Subsequently, the theory behind the two force generation principles are described. Furthermore, the current, magnetic flux densities, accelerations, and induced stresses are analyzed. Moreover, mechanical loadability simulations are performed to study the impact on the requirements of the charging unit, the sensitivity of the parameters, and evaluate the degree of influence on the performance of both drives. Finally, it is confirmed that although the DSC is mechanically more complex, it has a greater efficiency than that of the TC.

Simulation of the magnetostrictive performance of Terfenol‐D in mechanical devices

A dynamic simulation model has been developed. Registered data from static measurements of the magnetostrictive strain for different magnetizations and mechanical stresses are used as numerical input. Easy examination of differences in dynamic performance between samples of different compositions and manufacturing methods is also possible due to a computer‐aided input data handling system. The shape of the imposed magnetization can be a step, impulse, sinusoidal, or an arbitrary function. The mechanical load can be a prescribed force against the magnetostrictive element or an arbitrarily chosen mechanical impedance. The model has been verified against dynamic measurements in an experimental setup for sinusoidal and impulse magnetizations. Comparison between the model and the experimental data reveals that the model is a powerful tool for designing magnetomechanical devices based on giant magnetostrictive materials.

Nonlinear 2-D transient modeling of Terfenol-D rods

A nonlinear model for simulation of the transient behavior of magnetostrictive Terfenol-D rods has been developed. The model is based on static characterization of the magnetic and mechanical properties of the rods combined with Maxwells equations and classical mechanic laws. Registrations for different rods are stored in a database which is called upon by the model. The model is able to handle longitudinal wave propagation and eddy current influence under transient conditions. Any combination from the set of magnetic field (current), magnetic flux density (voltage), mechanical motion, mechanical stress, and mechanical impedance can be used as the two independent variables in the model. This modeling technique implies that the experimental set-up used for data acquisition of the material properties should be regarded as a part of the modeling process. The experimental set-up used for the characterization of Terfenol-D properties can produce magnetic fields and mechanical stresses independently of each other and of arbitrary shape. A specially designed sample holder provides a homogeneous magnetic field and mechanical stress field inside the rod. >

Modelling eddy currents and hysteresis in a transformer laminate

A Cauer circuit model of a transformer laminate is presented. It considers saturation, eddy currents and hysteresis. The simulation results agree to experiments with an Epstein frame in the 10-200 Hz range. The paper includes a description of a computationally fast hysteresis model with few adjustable parameters and a physical approach to derive the Cauer circuit. The model can be used under various time-transient conditions and can easily be implemented into a larger system in a circuit simulation package, such as Saber, to study a transformers interaction with switching overvoltages, for example.

Dynamic modelling of giant magnetostriction in Terfenol-D rods by the finite element method

As a contribution to the development of methods for the design and the analysis of devices based on giant magnetostrictive materials, a model for the simulation of the dynamic behaviour of the nonlinear magnetoelastic medium is presented. The coupled magnetic, magnetoelastic and mechanical equations that describe the magnetostrictive problem are solved by means of the finite element method. The thin sheets bending principle (surface splines) is used to introduce in the simulation the nonlinear properties of giant magnetostrictive materials, obtained by static characterizations. >

Experimental Testing Of An Anisotropic Vector Hysteresis Model

A vector hysteresis model is experimentally tested for two soft magnetic materials in the two-dimensional case. The model expresses net magnetization as a sum of contributions from a number of pseudoparticles, each one having a dry friction-like hysteresis mechanism. Five adjustable parameters are used to represent hysteretic properties. Comparisons between calculations and measurements on silicon-iron are made for hysteresis curves and rotational and alternating hysteresis losses.