Michael Ertl

Physical modeling and numerical computation of magnetostriction

Purpose – Magnetostrictive alloys are widely used in actuator and sensor applications. The purpose of this paper is to developed a realistic physical model and a numerical computational scheme for their precise computation.Design/methodology/approach – The main step in the physical modeling is the decomposition of the mechanical strain and the magnetic induction into a reversible and an irreversible part. For the efficient solution of the arising coupled nonlinear partial differential equations the authors apply the finite element method.Findings – It can be demonstrated, that the hysteresis operators can be fitted by appropriate measurements. Therewith, the developed physical model and numerical simulation scheme is applicable for the design of magnetostrictive actuators and sensors.Originality/value – The decomposition of the mechanical strain and the magnetic induction into a reversible and an irreversible part. The reversible part is described by the linear magnetostrictive constitutive equations, whe...

Investigation of load noise generation of large power transformer by means of coupled 3D FEM analysis

Purpose – This paper aims to present a 3D numerical analysis of the load noise generation associated with large, oil immersed three‐phase power transformers.Design/methodology/approach – After studying the mechanical behavior of the winding structures of transformers, the results of coupled magneto‐mechanical simulations are presented.Findings – An appropriate modeling strategy of the vibratory winding structures of transformers is necessary to reduce complexity and computational resources.Originality/value – The presented model setup describes a fully transient, 3D coupled magneto‐mechanical simulation of the vibratory winding structure of large power transformers.

Accurate magnetostatic simulation of step-lap joints in transformer cores using anisotropic higher order FEM

Purpose – The purpose of this paper is to propose a solution strategy for both accurate and efficient simulation of nonlinear magnetostatic problems in thin structures using higher order finite element methods. Special interest is put in the investigation of the step-lap joints of transformer cores, with a focus on the spatial resolution of the field quantities. Design/methodology/approach – The usage of hierarchical finite elements of higher order makes it possible to adapt the local accuracy in different spatial directions in thin steel sheets. Due to explicit representation of gradients in the basis functions, a simple Schwarz-type block preconditioner with a conjugate gradient solver can efficiently solve the arising algebraic system. By adapting the block size automatically according to the aspect ratio, deterioration of convergence in case of thin elements can be prevented. The resulting Newton scheme is accelerated utilizing the hierarchical splitting in a two-level scheme, where an initial guess i...

Anisotropic model for the numerical computation of magnetostriction in grain-oriented electrical steel sheets

Purpose – The modeling of magnetostrictive effects is a topic of intensive research. The authors’ goal is the precise modeling and numerical simulation of the magnetic field and resulting mechanical vibrations caused by magnetostriction along the joint regions of electric transformers. Design/methodology/approach – The authors apply the finite element (FE) method to efficiently solve the arising coupled system of partial differential equations describing magnetostriction. Hereby, they fully take the anisotropic behavior of the material into account, both in the computation of the nonlinear electromagnetic field as well as the induced magnetostrictive strains. To support their material models, the authors measure the magnetic as well as the mechanical hysteresis curves of the grain-oriented electrical steel sheets with different orientations (w.r.t the rolling direction). From these curves they then extract for each orientation the corresponding commutation curve, so that the hysteretic behavior is simplified to a nonlinear one. Findings – The numerical simulations show strong differences both in the magnetic field as well as mechanical vibrations when comparing this newly developed anisotropic model to an isotropic one, which just uses measured curves in rolling direction of the steel sheets. Therefore, a realistic modeling of the magnetostrictive behavior, especially for grain-oriented electrical steel as used in transformers, needs to take into account the anisotropic material behavior. Originality/value – The authors have developed an enhanced material model for describing magnetostrictive effects along the joint regions of electric transformers, which fully considers the anisotropic material behavior. This model has been integrated into a FE scheme to numerically simulate the mechanical vibrations in transformer cores caused by magnetostriction.

Investigation of the dynamics of electromagnetic valves by a coupled magneto‐mechanical algorithm including contact mechanics

Purpose – The fast and flexible development of fast switching electromagnetic valves as used in modern gasoline engine demands the availability of efficient and accurate simulation tools. The purpose of this paper is to provide an enhanced computational scheme of these actuators including all relevant physical effects of magneto‐mechanical systems and including contact mechanics.Design/methodology/approach – The finite element (FE) method is applied to efficiently solve the arising coupled system of partial differential equations describing magneto‐mechanical systems. The algorithm for contact mechanics is based on the cross‐constraint method using an energy‐ and momentum‐conserving time‐discretisation scheme. Although solving separately for the electromagnetic and mechanical system, a strong coupling is ensured within each time step by an iterative process with stopping criterion.Findings – The numerical simulations of the full switching cycle of an electromagnetic direct injection valve, including the b...

Sound Power Measurements in the Near Field of Transformers

The international standard for the determination of the sound power level of transformers allows both the sound pressure and the sound intensity measurement method. Since the sound measurements take place in the reactive near-field next to the vibrating transformer tank walls, local disturbances influence the sound field characteristics at the measurement positions. As a result, the measured mean sound power level differs commonly up to 6dB at comparative measurements with both methods.Beyond these near field effects, the influence of an industrial measurement environment (background sound sources, hard-reflecting floor, semi-reverberant walls, and standing waves) to the sound pressure and sound intensity field characteristics is investigated. Hereby, numerical analyses based on 3D-FEM with consideration of the fluid-structure-coupling are used. The measured sound level differences can be re-produced and clarified in numerical analyses.Copyright

Investigation of the origin of load-controlled vibration of large power transformers by coupled 3D-FEM analysis

This paper presents a three-dimensional numerical analysis of the load noise generation associated with large, oil immersed three-phase power transformers. Based on investigations of an appropriate modeling of the vibratory winding structures of transformers, the mechanical behavior of these devices are studied. By using an efficient coupled simulation scheme based on magneto-mechanical finite element calculation, the solution of magnetic and mechanical systems can be achieved simultaneously by taking account of their strong mutual coupling.