Demagnetisation of grain-oriented electrical steels (GOES)

The magnetisation processes in grainoriented electrical steels, which are the basic component of transformers cores, are very complex and depend on the amplitude and frequency of excitation. Nevertheless, demagnetisation frequency is of less significance because good quality procedure can result in the same demagnetised state. Demagnetisation is important for transformer testing, especially when using Sweep Frequency Response Analysis, as well as for saturation testing of current transformers

Rotational magnetisation

Rotational magnetisation is a phenomenon that occurs locally in the magnetic cores of three-limb transformers. The power loss dissipated under rotational magnetisation can be several times higher than under usual magnetisation conditions as defined by manufacturers of the electrical steels. The phenomena were discovered over 100 years ago and have been studied since, but it is difficult to model it theoretically and apply the knowledge practically in transformer design. This article focuses on the definition and discussion of the phenomenon in order to introduce it to a wider audience of the transformer community

FEM modelling and experimental validation of proximity loss

Skin depth and proximity effects in transformer windings are important phenomena influencing the design even at power frequencies (50-60 Hz). They become critically important at elevated frequencies, especially for high-frequency transformers. This article presents results of computer simulations and experimental validation on physical prototypes. The investigated frequency range was from 1 Hz to 10 kHz

Qualitative FEM study of proximity loss reduction by various winding configurations – Part II

Skin depth and proximity effects in transformer windings are important phenomena influencing the design even at power frequencies (50-60 Hz). However, they become critically important at elevated frequencies, especially for high-frequency transformers, operating in switched-mode power supplies for example, at any power level. This article presents a numerical study of optimum winding configurations which can drastically reduce the proximity effects. It is possible to make transformers operating even at 1 MHz without the use of very expensive Litz wire

Qualitative FEM study of proximity loss reduction by various winding configurations – Part I

Skin depth and proximity effects in transformer windings are important phenomena influencing the design even at power frequencies (50-60 Hz). However, they become critically important at elevated frequencies, especially for high-frequency transformers, operating in switched-mode power supplies for example, at any power level. This article presents a numerical study of optimum winding configurations which can drastically reduce the proximity effects. It is possible to make transformers operating even at 1 MHz without the use of very expensive Litz wire

Spinor Bose-Einstein condensates

An overview on the physics of spinor and dipolar Bose-Einstein condensates (BECs) is given. Mean-field ground states, Bogoliubov spectra, and many-body ground and excited states of spinor BECs are discussed. Properties of spin-polarized dipolar BECs and those of spinor-dipolar BECs are reviewed. Some of the unique features of the vortices in spinor BECs such as fractional vortices and non-Abelian vortices are delineated. The symmetry of the order parameter is classified using group theory, and various topological excitations are investigated based on homotopy theory. Some of the more recent developments in a spinor BEC are discussed.Comment: To appear in Physics Reports. The PDF file with high resolution figures is available from the following website: http://cat.phys.s.u-tokyo.ac.jp/publication/review_of_spinorBEC.pd

Effect of off-axis H-coil sensitivity on clockwise-anticlockwise differences of rotational power loss in isotropic samples

Wire wound H-coils used for rotational measurements are not completely immune to off-axis components of magnetic field strength H. As a result, such components are capable of introducing the significant difference between the rotational power loss measured in the clockwise (CW) or anticlockwise (ACW) direction of rotation. These measurement errors do not influence the average value, and they are distinctly different from the errors caused by the angular misalignment of the sensors in the field metric method. As demonstrated in this study even ideal angular positioning of the H-coils could still produce CW-ACW differences at the level of ±15% at lower flux density and ±30% close to saturation

Experimental verification of 2.4 kVAr and 12 kVAr prototype variable inductors controlled by virtual air gaps with magnetic orthogonality

This paper describes experimental verification of single-phase variable inductor prototypes rated at 2.4 kVAr (75 mH, 10 A, 230 V, 24 kg) and 12 kVAr (43 mH, 30 A, 400 V, 78 kg). Results for much smaller devices rated at 1-10 Var are also shown. The variability of inductance of the main AC winding was achieved by adding a DC winding, wound through the apertures placed symmetrically in the core such that both windings were magnetically orthogonal. Local saturation caused by the magnetic field due to DC winding creates virtual air gaps. This method is capable of lowering the main inductance by over 90% if no real air gap is present, but for industrial applications the range of variability is likely to be closer to 20-30% range, depending on the size of the real air gap in the magnetic circuit. The amount of variation/control can be improved by using more than one control winding. Very good repeatability of performance was observed, with relatively small total harmonic distortion introduced due to control. The experimental findings agree with a generalised analytical model