Sigrid Jacobs

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.

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.

Improved iron loss modelling approach for advanced electrical steels operating at high frequencies and high inductions in automotive machines

ArcelorMittal recently developed a specific new range of electrical steel grades for automotive applications, as an answer to the specific requirements for electrical traction machines (requirements such as excellent efficiency over a wide speed range, high power density, high reliability and robustness). These electrical steel grades are optimised over a wide frequency range to achieve the specific material property compromise of magnetic, electric, mechanical and thermal performance. Moreover, to maximise the potential of these advanced electrical steel grades, careful machine design is needed: when optimising the machines power density for instance, its crucial not to over-dimension the machine, neither in magnetic nor in mechanical aspects of the electrical steel laminations. Therefore more precise modelling tools are becoming more essential: ArcelorMittal developed an improved approach for the iron loss modelling of machines based on a better model description and appropriate material characterisation of the relevant phenomena influencing the machines iron losses (such as lamination punching and pressing of the stacks). Furthermore, ArcelorMittal has also in-house advanced mechanical characterisation tools such as fatigue testing on electrical steels, for tighter mechanical machine design. When such advanced magnetic and mechanical design tools are used, on the optimised electrical steel grades of the ArcelorMittal iCAReTM offer for automotive applications, the maximum potential of these electrical steels can be exploited.

Metallurgical solutions for new top performance non-oriented electrical steel for cores

The link is made between magnetic properties, metallurgical parameters & process control, by using a material modeling approach. Within the iron loss formulation, the coefficients of the hysteresis, the eddy current and the excess losses are linked to chemical, structural and textural elements of the steel metallurgy. A particular focus is put on the surface and subsurface precipitation state. In particular for high frequency applications, such as automotive traction, thin electrical steels are used to reduce the eddy current losses, so the surface related effects become relatively more important and their proper control can bring a relevant performance enhancement. Such production parameter optimization is implemented in ArcelorMittals iCARe™ automotive product range.

Influence of initial heating during final high temperature annealing on the offset of primary and secondary recrystallization in Cu-bearing grain oriented electrical steels

The industrial production route of Grain Oriented Electrical Steels (GOES) is complex and fine-tuned for each grade. Its metallurgical process requires in all cases the abnormal grain growth (AGG) of the Goss orientation during the final high temperature annealing (HTA). The exact mechanism of AGG is not yet fully understood, but is controlled by the different inhibition systems, namely MnS, AlN and CuxS, their size and distribution, and the initial primary recrystallized grain size. Therefore, among other parameters, the initial heating stage during the HTA is crucial for the proper development of primary and secondary recrystallized microstructures. Cold rolled 0.3 mm Cu-bearing Grain Oriented Electrical Steel has been submitted to interrupted annealing experiments in a lab tubular furnace. Two different annealing cycles were applied:• Constant heating at 30°C/h up to 1000°C. Two step cycle with initial heating at 100°C/h up to 600°C, followed by 18 h soaking at 600°C and then heating at 30°C/h up to 10...

Numerical and experimental evaluation of magnetostriction and magnetic forces on transformer stacks and joints for the assessment of core vibrations

This paper deals with the prediction of mechanical vibrations that exist on transformer cores due to magnetostriction and magnetic forces. Several single-phase transformer topologies are investigated, demonstrating the consequences of the configuration of the joints, with the aim to reveal the relative contributions of magnetostrictive and magnetic forces to the final vibration level. The results are obtained through advanced 3D electromagnetic and mechanical FE simulations, taking into account non-linear and anisotropic magnetic and mechanical material properties as well as parasitic airgaps. The results are compared with measurements to allow an assessment of the methodology. The research is based on a non-oriented, low-loss electrical steel from ArcelorMittal, that is typically used in small auxiliary transformers.

Improved calculation of iron losses in large salient-pole synchronous hydro-generators

To bridge the existing gap between the calculated and the actually measured iron losses in low loss electrical steel parts of electrical machines, one of the possible improvements is to incorporate the effect of punching into the loss modelling. Therefore, within the finite element analysis the electrical steel parts geometry is divided into different sub-regions, and affected local magnetisation curves and magnetic loss parameters are attributed to each of these regions. Both these local magnetic properties are deduced from measurements on industrially punched samples of different widths. In this paper such improved iron loss calculation methodology is carried out on two large synchronous hydro-generators. The additional losses due to punching occur mainly at the punched edge and are of both hysteretic and excess loss nature, but also in the unaffected bulk of the geometry all iron loss components increase due to a magnetic flux re-distribution from the edge to the bulk of the stator tooth geometry. Another finding is that the higher harmonic dynamical losses in the stator teeth increase due to the punching. As a conclusion, at least part of the unexplained measured total losses of large hydro-generators can be attributed to the punching effect on the iron losses, which enables more accurate design of such generators.