Lode Vandenbossche

Analysis of the Local Material Degradation Near Cutting Edges of Electrical Steel Sheets

Cutting leads to a certain local magnetic material degradation of the electrical steel sheet. Moreover, the material properties near the cutting edge contribute significantly to the global performance. This material degradation mostly occurs in the vicinity of critical parts of electromagnetic devices, such as stator and rotor teeth. Therefore, the need exists to characterize the local magnetic hysteresis properties due to cutting. We couple the non destructive measurements of needle signals, which are dependent on the local variations in magnetic hysteresis properties, with a numerical inverse algorithm. The inverse algorithm interprets the needle signals so that the unknown magnetic hysteresis properties can be reconstructed. The paper mainly deals with the construction of an accurate material model (numerical forward model), the correct solution of the inverse procedure and the validation of the obtained results. We reconstructed local magnetic hysteresis properties of differently cut steel sheets and we observed that it is possible to recover the material characteristics using a material model, which fully characterizes the hysteresis properties.

Comparison of Iron Loss Models for Electrical Machines With Different Frequency Domain and Time Domain Methods for Excess Loss Prediction

The goal of this paper is to investigate the accuracy of modeling the excess loss in electrical steels using a time domain model with Bertottis loss model parameters n0 and V0 fitted in the frequency domain. Three variants of iron loss models based on Bertottis theory are compared for the prediction of iron losses under sinusoidal and non-sinusoidal flux conditions. The non-sinusoidal waveforms are based on the realistic time variation of the magnetic induction in the stator core of an electrical machine, obtained from a finite element-based machine model.

Local Identification of Magnetic Hysteresis Properties Near Cutting Edges of Electrical Steel Sheets

Technological processes performed on electrical steel influence drastically their magnetic properties. The local mechanical state, changed by the introduction of cutting stresses and strains, degrades the local magnetic properties of the material. Therefore, the need exists to characterize accurately the change of the magnetic properties as a function of the distance from the cutting edge. We present a nondestructive experimental setup where spatial and time-dependent voltages are measured on the basis of needle probe sensors. We simulate the needle potentials, using a forward numerical model, starting from spatial dependent material properties, characterized by the Preisach model. Finally, we introduce a suitable algorithm which solves the inverse problem in order to recover the local magnetic properties starting from the measured potentials.

Iron loss modelling which includes the impact of punching, applied to high-efficiency induction machines

Within the market evolution towards higher efficiency machines, there is a need for more precise modelling tools, taking also higher frequency power supplies into account. This paper implements the effect of cut edge degradation, due to punching, on the local magnetization curve and local losses, aiming at improved calculation of magnetization current and machine iron losses. For an induction machine, defined by Leroy Somer and a high efficiency electrical steel, selected from ArcelorMittals range, this study combines advanced material characterization with improved modelling techniques, and is validated by selected machine testing procedures. For the sake of clear loss separation a machine with slotless rotor was characterized at two conditions regarding speed and excitation. The loss model, including already the effect of rotational losses and higher harmonics, was enhanced with the punching effect, which led to a model accuracy of 86% at synchronous speed when compared to the measured losses. This study also led to further insights in local magnetization behaviour, to be used in further design optimization.

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 Material Identification in Geometries With Non-Uniform Electromagnetic Fields Using Global and Local Magnetic Measurements

In this paper, the magnetic material characteristics are reconstructed for magnetic circuits with non-uniform electromagnetic field patterns, including excitation winding and/or air gaps, as in the case of rotating electrical machines. The identification process is done using a set of well chosen global and/or local magnetic measurements. Moreover, numerical inverse techniques are implemented in order to reconstruct the material characteristics from a limited number of global and/or local measurements.

Magnetic evaluation of the hardening and softening of thermally aged iron–copper alloys

We evaluated the variations of the magnetic hysteretic behaviour of Fe–1wt%Cu model alloy samples due to thermal aging and over-aging. In these alloys, the formation and growth of Cu-precipitates during the thermal aging process results in mechanical hardening, whereas for aging times higher than a certain critical value, mechanical softening occurs. Magnetic hysteretic properties such as permeability, remanence, peak value and width of the local interaction field distribution, which is related to the Preisach model, are measured as a function of various Cu-precipitation stages obtained by time dependent heat treatments at 773 K (500 °C). We found that all magnetic parameters exhibit an extremum value for the peak hardening sample. Furthermore, the variation of permeability, remanence, and the peak value of the local interaction field distribution mimics the behaviour of the reciprocal value of the yield stress as a function of aging time. These results suggest that the magnetic domain wall movement is hindered mainly by Cu-precipitates upon thermal aging. The peak hardening values of the investigated magnetic parameters change by approximately 50% when compared with the initial values. This pronounced sensitivity indicates the potential of magnetic non-destructive evaluation for the assessment of the hardening and softening phenomena induced by Cu-precipitation.

Energy considerations in a micromagnetic hysteresis model and the Preisach model

Finding the relations between the microstructural material parameters and the macroscopic hysteresis behavior is indispensable in the design of ferromagnetic materials with minimal (hysteresis) losses. Micromagnetic hysteresis simulations enable a rigorous and structured investigation of these relations since in the numerical model each material parameter can be altered independently. This paper describes a procedure to extract the Preisach distribution function, quantifying the macroscopic hysteresis properties, from micromagnetic simulations incorporating the materials’ microstructure. Furthermore, the instantaneously added, stored and dissipated energy while running through the hysteresis loop as described in the macroscopic Preisach model and in the micromagnetic hysteresis model are compared, evidencing a very good agreement. Moreover, using the micromagnetic model, the energy rearrangements between the different micromagnetic interaction terms is studied at each time point of the hysteresis loop. It...

Drag force measurement: A means for determining hysteresis loss

A method for determining hysteresis losses in thin strips of soft magnetic materials is described. It is based on the measurement of a drag force which arises with the movement of the sample through the strong field existing in the space near a permanent magnet. Not associated with macro eddy currents, the force is shown to originate from the magnetic hysteresis of the material, having, in fact, an amplitude equal to the product of hysteresis loss and the area of the sample cross section. Correlation within 18% with the measurements made by conventional methods is shown for a wide range of experimental materials.

Magneto-optical and field-metric evaluation of the punching effect on magnetic properties of electrical steels with varying alloying content and grain size

The punching of electrical steel sheets to manufacture iron cores for electrical machines can influence drastically the final magnetic properties of stator and rotor cores. In this paper, the influence of punching on the magnetic properties is studied on 5 mm wide ring core samples. Six different fully processed non-oriented electrical steels with varying alloy content and varying grain size were investigated. Both global (integral) as well as local magnetic characterisation is carried out. The global magnetic properties are obtained by field-metric magnetic measurements. Punching leads to an increase in the global ring core losses: for the specific case of 5mm wide punched rings, this increase is in the range of 10% to 30% (for losses at 1 T and 400 Hz) depending on the investigated material (this range corresponds to a loss increase of 2.5 to 5.5 W/kg). The local magnetic degradation due to punching becomes more important when approaching the cutting edge. For local magnetic characterisation the magneto-optical Kerr-effect is used. The method allows the observation of the magnetic domain behaviour near the cut edge. Moreover this magneto-optical method allows for a quantitative analysis of the effect of punching on the local magnetic contrast as a function of distance from the cut edge. In addition also micro hardness measurements are performed near the cut edges to determine the mechanically affected zone. Furthermore the influence of punching on the investigated local and global magnetic properties is discussed as a function of the differences in alloying content and grain size of the investigated materials. For the high alloyed electrical steels, the material with medium grain size deteriorates less due to punching than the material with high grain size. This can be observed both from the dc magnetization curves which exhibit higher permeability and from the iron losses at medium frequency (400Hz) after punching. At 50Hz the losses remain lowest for the high grain sized material, even after punching.