M. De Wulf

The effect of the guillotine clearance on the magnetic properties of electrical steels

Measurements of the bulk magnetic properties of four non-oriented electrical steels after guillotine cutting with varying clearances are reported. A heterogeneous state of deformation is observed at the sheared edge. The deformation-affected zone extends to several millimeters away from the cut edge and possibly occupies the entire sample volume. An obvious correlation was established between the clearance of the guillotine cut and the general deterioration of the magnetic properties such as the total core losses (at 50 Hz), the coercive field, the relative permeability and the remanent induction. The existence of a transition zone was demonstrated in the range of applied clearances for which the magnetic properties were severely deteriorated. This clearance transition zone corresponds with the drastic change of one shearing parameter: the knife displacement at fracture. The observed effects are discussed in terms of the strain heterogeneities that are introduced in the sample by the guillotine cutting process.

Optimizing active and passive magnetic shields in induction heating by a genetic algorithm

This paper presents a method for designing optimal passive and active shields for axisymmetric induction heaters. Such shields are needed to protect human operators and external electronic equipment from stray magnetic fields. The method uses a genetic algorithm (GA) to minimize an objective function. This function reduces the magnetic field in the target area, the power dissipation in the active and passive shields, and the influence of the shields on the heating process. The GA returns the position and height of the passive shield, the optimal current for the active shield, and the number of turns of all coils. The paper describes two optimization modes: 1) optimization of only the active shield with fixed passive shield and 2) global optimization of both active and passive shields. Several passive shields are studied: electrically conductive shields and both electrically and magnetically conductive shields. The field reduction depends on the optimization mode and the passive shield properties, but always exceeds 25 dB for combined active and passive shields. Finally, the paper compares the results of the simulations to experimental measurements.

Passive and active electromagnetic shielding of induction heaters

In induction heaters, a metallic workpiece is heated by eddy currents, induced by strong alternating magnetic fields. Using numerical models, proper active and passive shields are designed in order to mitigate the stray field of the induction heater. Two models are presented: an axisymmetric finite-element (FEM) and a hybrid finite-element-boundary-element (FEM-BEM) model. Results of both numerical methods are compared and verified with measurements on an induction heater archetype. A good correspondence is observed. Combination of passive and active shielding results in almost 20-dB field reduction.

Influence of the Electrical Steel Grade on the Performance of the Direct-Drive and Single Stage Gearbox Permanent-Magnet Machine for Wind Energy Generation, Based on an Analytical Model

The performance of a variable speed wind turbine using a direct-drive permanent-magnet synchronous generator (PMSG) as well as a PMSG with single stage planetary gearbox is compared for two grades of electric steel applied for the generator stator core lamination. A ring type, radial flux PMSG is modeled. For a fixed mechanical power input, the geometry of the generator is optimized for each turbine systems and two materials to maximize the annual efficiency of the generator. The annual efficiency is calculated based on the power curve of the generator and the probability density function of the wind speed. This function is approximated by the Weibull distribution function for a site with average wind speed of 7 m/s. For both generator systems, the annual efficiency of two optimized generators using different steel grades differs around 1%. The difference depends on a mass of active material of the generator.

Magnetic properties of high Si steel with variable ordering obtained through thermomechanical processing

Alloys with a Si content of 4.2 and 5.4 wt % Si were produced to investigate the effect of increasing the Si on the materials processing and properties and to understand the effect of the order–disorder phenomenon on its magnetic properties. Different cooling rates after hot rolling were applied: Slow cooling from 780 °C to room temperature in 26 h, air cooling and water quench, followed by cold rolling until 0.5 to 0.7 mm thickness. Magnetic properties were measured after pickling and annealing at 950 °C for 2 h. 57 Fe Mossbauer spectroscopy was used to study the effect of thermomechanical cycles on the ordering phenomena. It was noticed that the quenched samples have the highest values for the magnetic polarization, while slowly cooled samples have the lowest, for the power losses higher values are obtained for the quenched materials. The highest values for the polarization in the quenched samples were explained as a result of a higher B2 ordering.

Finite-element computation of the deformation of ferromagnetic material taking into account magnetic forces and magnetostriction

This paper presents a method for computing the deformation of ferromagnetic material due to magnetic forces and magnetostriction by using the finite-element method. In particular, the two-dimensional case, viz. the analysis of stresses and deformation in electrical steel sheet, is discussed. The experimental determination of the magnetoelastic properties is presented as well.

Magnetic properties of electrical steel with Si and Al concentration gradients

Electrical steel with a Si content up to 6.5 wt% is a good soft magnetic material, because the power losses are reduced through the increased electrical resistivity of the bulk material by the presence of alloying elements like Si and Al. Nevertheless, with the increasing of the alloying elements, the concentration of Fe atoms in the bulk is reduced and a lower magnetic saturation is reached. Samples are produced by hot dipping in a Si-Al hypereutectic bath, followed by annealing treatments for diffusion. Series of different concentration profiles have been obtained and the magnetic properties measured. A short immersion is sufficient to form a Si-Al enriched layer with D0/sub 3/ ordered structure (25at% Si-Al) in the steel surface: 10 /spl mu/m of D0/sub 3/ or more can be obtained after 20 s immersion (dependent on substrate and experimental conditions). A short diffusion annealing lowers and broadens this level, e.g., to a classical level of 6.5 wt% of Si, while the center of the steel remains at the original level (e.g., 3 wt% Si). Similar gradients are obtained with the simultaneous presence of Si and Al. Magnetic measurements were performed at 50 and 400 Hz after different thermal cycles, leading to different gradients of Si and Al. The power losses are reduced more than 50% in the final material compared with the original. It appears that a short annealing after the dipping already gives low power losses, even before a homogeneous concentration profile is obtained over the sample thickness. A smooth concentration gradient is preferable, with a lower Si content in the center, in order to maintain a good magnetic saturation. This can also be shown through modeling of the magnetic behavior with the concentration gradient. The reduction of power losses is more pronounced at 400 Hz, proving the importance of the skin effect. A further advantage of this procedure is the shorter annealing time after hot dipping.

Evolution of magnetic properties and microstructure of high-silicon steel during hot dipping and diffusion annealing

Hot dipping and diffusion annealing is an alternative production process to obtain steel with high silicon contents while avoiding rolling problems. Surface alloying with Si and Al is achieved on a low-Si steel substrate (/spl ap/ 3%Si) by hot dipping in a hypereutectic Al-27%Si molten bath. To obtain a sufficient amount of Al and Si in solid solution over the thickness, a diffusion annealing treatment is performed after hot dipping. A diffusion annealing at 1250/spl deg/C allows the homogenization of the composition obtaining a concentration of 6.3% Si and 4.5% Al over the thickness. The evolution of microstructure, crystallographic texture, and magnetic properties in each step of the process is reported. After dipping and annealing, the losses decreased proportionally with the total Si-Al concentration achieved during the process. Values as low as 0.6 W/kg at 50 Hz (1T) and 10 W/kg at 400 Hz (1T) have been obtained in the experimental materials with high Si + Al content and 0.35-mm thickness.

Computation of the preisach distribution function based on a measured everett map

Modeling the hysteresis behavior of soft magnetic materials based on Preisach theory, requires a Preisach distribution function characterising the material. Although it is possible to obtain all the data for a classical Preisach model from experimental first-order curves, obtaining the Preisach distribution function by differentiating this data twice leads to large errors and even to negative values in the distribution function, This work reports on how to avoid these errors in the Preisach function when it is computed starting from measured data. Measured BH-loops are arranged into an Everett function, which is then corrected in order to serve as a basis for the Preisach distribution function. The computation method is tried out on a nonoriented, a grain oriented and an iron-powder material.

Modelling of electromagnetic losses in asynchronous machines

This paper deals with the numerical modelling of electromagnetic losses in electrical machines, using electromagnetic field computations, combined with advanced material characterisations. The paper gradually proceeds to the actual reasons why the building factor, defined as the ratio of the measured iron losses in the machine and the losses obtained under standard conditions, exceeds the value of 1.