H. Pfützner

Relevance of magnetostriction and forces for the generation of audible noise of transformer cores

Recently, the two most significant characteristics of power transformer cores, i.e., loss and noise, have been reduced considerably because of improved material and improved core design. An undisputed base value of noise results from magnetostriction (MS) due to the global magnetization of limbs and yokes, whereas excess values are attributed to different mechanisms, such as excess MS and magnetic forces (MF), in a controversial way. However, a multistep lap (MSL; number of overlap steps N>1) yields considerably lower noise than a single-step lap (SSL; N=1), indicating a specific role of joint design. The present work was aimed at a deeper understanding as a tool for further improvements. For a large number of differently built-up single-phase model cores, local distributions of surface displacements d/sub x/, d/sub y/, d/sub z/ and velocities v/sub x/, v/sub y/, v/sub z/ as well as the corresponding noise n were determined. As a major finding, MSL yielded weak reductions of in-plane vibrations v/sub x/ and V/sub y/, considerable reductions of v/sub z/, and strong reductions of n. Interpretations of results were based on numerical Spice modeling of linearized joint vibrations, which were also established experimentally. The results indicate high values of off-plane flux density B/sub z/ and the corresponding Maxwell stress arising at the outermost sheet ends for SSL, and considerably lower values for MSL, provided that B is below its critical value B/sub c/. MSL yielded reductions of d/sub z/, v/sub z/, and n in 80% of all cases. Three additional core parameters-small air gap lengths, high overlap lengths, and high lamination factors A-all lead to reduced n; the use of oil yielded strong reductions. It is concluded that joint designs actually play a predominant role in noise generation. Apart from some contribution from attractive in-plane forces due to B/sub z/, in air gaps, interlaminar flux B/sub z/ yields primary sources of vibrations that propagate to the entire core surface. In core regions of reduced /spl Lambda/, B/sub z/ yields vibrations of sheet ends due to MF according to an experimentally determined effective Youngs modulus that considers practical conditions like waviness, burrs, and thickness variations of laminations. For high h, oscillations of the whole core thickness due to B/sub z/-caused MS will arise close to joints. For MSL, all of these mechanisms become less relevant: Gap regions show much lower B/sub x/ and, provided B<B/sub c/, also lower B/sub z/, apart from being regionally distributed. Further, imperfections of joint assembling are less critical. As a consequence, the noise level of modern MSL power transformer cores, as commercially assembled with homogeneous compression and high N, can be well predicted by mere consideration of MS resulting from global magnetization.

Method for continuous nondisturbing monitoring of blood pressure by magnetoelastic skin curvature sensor and ECG

This paper concerns continuous nondisturbing estimation of blood pressure using mechanical plethysmography in connection with standard electrocardiography (ECG). The plethysmography is given by a novel magnetoelastic skin curvature sensor (SC-sensor) applied on the neck over the carotid artery. The sensor consists of a magnetoelastic bilayer partly enclosed by a coil. Bending the bilayer causes large changes of magnetic permeability which can be measured by the coil. The SC-sensor signal and the ECG signal are adaptively processed in order to estimate blood pressure according to a specifically established theoretical model. The model uses estimated vessel radius changes and pulse transit time as parameters. The results show cross correlation coefficients in the range 0.8 up to 0.9 between reference and estimated values of systolic blood pressure, diastolic blood pressure, and systolic/diastolic blood pressure change, whereas the estimation error was below 4 +/- 7 mmHg at rest and increased with the stress level. Limitations of the model applicability are given by a hysteretic behavior of both model parameters due to inert changes in artery stiffness. The SC-sensor and the ECG electrodes cause minimal inconvenience to the patient and offer an approach for a continuous nondisturbing monitoring of blood pressure changes, as being relevant for sleep monitoring or biomechanic feedback

Problems of loss separation for crystalline and consolidated amorphous soft magnetic materials

Losses were measured as a function of magnetization frequency f/sub d/ for two advanced soft magnetic alloys: laser-scribed 0.23-mm HI-B steel ZDKH (designated HB) and consolidated amorphous Powercore (designated PC). Although domain observations on HB yielded distinct domain refinements due to the laser treatment, anomalous losses W/sub a/ still proved to be higher than the classical ones. A further refinement resulted from increasing f/sub d/. Thus, anomalous eddy current losses were reduced while hysteresis losses were increased. As a consequence, it is suggested to separate W/sub a/ into an eddy current component and a hysteresis component, which arises in addition to the classical hysteresis loss. With increasing f/sub d/, especially wide domains were refined. Results of loss measurements performed at high f after demagnetization with low f/sub d/ indicated similar bar-type domain structures in HB and PC, the domain widths of the latter exceeding those of HB, however, by one order of magnitude. A Kerr-effect scanning technique confirmed the existence of an inner bar-type domain structure with domain widths of some millimeters. >

Three-dimensional flux distributions in transformer cores as a function of package design

In spite of extensive optimizations of transformer core designs, investigations of full sized cores showed distinct inhomogeneities of flux density B. Limbs showed discontinuous variations of B in peripheral packages and minima of B in thick central ones. The latter are not caused by global eddy currents but rather by localized flux components /spl Phi//sub z/ normal to the sheet plane. Attempts to determine the respective effective ac-permeability /spl mu//sub z/ yielded values below 100, i.e., almost three orders below /spl mu//sub x/ of the rolling direction. For given B, (planar) eddy current losses P/sub z/ proved to exceed the respective values P/sub x/ by two orders, a ratio which increases with increasing length L of the magnetized sheet region. The low ratio /spl mu//sub z///spl mu//sub x/ yields a tendency of constant package flux throughout the whole core. A key criterium for /spl Phi//sub z/-components between packages proved to be the overlap regions which were studied in a comparative way for several step-lap configurations. Distinct differences of respective values of lap-region excitation V/sub L/ were observed as a function of air gap lengths and the step number N, respectively. Variations of V/sub l/-and especially overlaps of high V/sub L/ in connection with shifted overlap regions-proved to yield /spl Phi//sub z/-components including flux transfer between packages. In a complex way, shifts yielded decreasing excitation power, but increased core losses due to planar eddy currents. In addition, package shifts cause both the discontinuities of B of thin peripheral packages and the minima of local B in thick central ones. With respect to core design, it can be assumed that small shifts favor take over of flux without causing significant planar eddy current losses (due to small L), while large shifts increase total losses in a disadvantageous way. With increasing N, these effects become less significant. >

Novel 3-phase excited single sheet tester for rotational magnetization

Abstract This paper describes a novel apparatus based on a 3-phase magnetic excitation which allows arbitrary multi-directional magnetization including dynamic rotational magnetization. It is based on a triple-yoke technique, preferably used with a sample of hexagonal shape. Using a 3-phase power source, the most important magnetization patterns can be produced even without dynamic control of the induction vector B due to the fact that also practical 2-D patterns result from 3-phase excitation.

The needle method for induction tests: sources of error

The regional flux density in laminations of soft magnetic materials is usually determined by means of a single-winding coil inserted through drilled holes. An alternative method is to use needle contacts arranged at lamination surface points through the surface coating. This method is simple and nondestructive. However, concern exists whether the results of measurements are fully equivalent with results from the coil method. We show here that full equivalence can be expected in most cases. The test results prove to be fully equivalent if the investigated region exhibits quasi-homogeneous induction or linear induction variations. If the region encloses a local induction extremum, a systematic error arises, but it can be neglected in practice. On the other hand, strong errors may arise if the in-plane flux is combined with asymmetric off-plane flux. This is because the method takes advantage of the surface eddy-current configuration, which reflects both components of flux.

Magnetostrictive amorphous bimetal sensors

The paper describes the application of a magnetostrictive amorphous ribbon (AR) for the detection of bending. In order to increase sensitivity, a bimetal structure is used which consists of AR and a nonmagnetic carrier ribbon. Several methods for the preparation of the bimetal are discussed. Results of the bending sensitivities are given for various combinations of the material types indicating crucial problems of bimetal preparation.

Acoustical signal properties for cardiac/respiratory activity and apneas

Traditionally, auscultation is applied to the diagnosis of either respiratory disturbances by respiratory sounds or cardiac disturbances by cardiac sounds. In addition, for sleep apnea syndrome diagnosis, snoring sounds are also monitored. The present study was aimed at synchronous detection of all three sound components (cardiac, respiratory, and snoring) from a single spot. The sounds were analyzed with respect to the cardiorespiratory activity, and to the detection and classification of apneas. Sound signals from 30 subjects including 10 apnea patients were detected by means of a microphone connected to a chestpiece which was applied to the heart region. The complex nature of the signal was investigated using time, spectral, and statistical approaches, in connection with self-defined time-based and event-based characteristics. The results show that the obstruction is accompanied by an increase of statistically relevant spectral components in the range of 300 to 2000 Hz, however, not within the range up to 300 Hz. Signal properties are discussed with respect to different breathing types, as well as to the presence and the type of apneas. Principal component analysis of the event-based characteristics shows significant properties of the sound signal with respect to different types of apneas and different patient groups, respectively. The analysis reflects apneas with an obstructive segment and those with a central segment. In addition, aiming for an optimum detection of all three sound components, alternative regions on the thorax and on the neck were investigated on two subjects. The results suggest that the right thorax region in the seventh intercostal space and the neck are optimal regions. It is concluded that for patient assessment, extensive acoustic analysis offers a reduction in the number of required sensor components, especially with respect to compact home monitoring of apneas.

Rotational Magnetization in Transformer Cores—A Review

Usually, rotational magnetization (RM) is associated with rotating machine cores. However, in more restricted ways, it also arises in three-phase transformer cores. Modern designs of T-joint yield detours of flux, as a source of RM in the T-joint, the middle limb ends, as well as in the yokes. Simulation of RM is possible by means of so-called rotational single sheet testers which should consider the large grains of highly grain oriented materials. Their high effective anisotropy yields induction patterns of rhombic or lancet-like type with maximum values of axis ratio a up to 0.5, and very high angular velocity round the materials hard directions. Compared to elliptic RM-as arising in non-oriented materials-the corresponding losses are lower due to restricted induction in the hard direction. But they show strong increase with (i) rising a and (ii) rising angular velocity of the induction vector. The magnetostrictive strain shows a pronounced (negative) maximum in the rolling direction with values up to 10 ppm, the transverse direction and normal direction exhibiting positive maxima of lower extent. With respect to industrial relevance, significant RM effects are restricted to the vicinities of T-joints. They represent the location of maximum core loss and also of maximum magnetostrictive strain as a source of audible core noise.

Mechanisms of noise generation of model transformer cores

Copyright (c) 1996 Elsevier Science B.V. All rights reserved. The audible noise of transformer cores is generated by vibrations due to both magnetostriction and magnetic forces which gain relevance in the case of highly oriented SiFe. The Maxwell stress vector depends strongly on the flux distribution in the core corners which show maximum dislocations d z normal to the sheet planes. Multi-step-lap assembling increases d z but reduces harmonic distortions which can be assumed to be one reason for the decrease in A-weighted noise.