G. Krismanic

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.

Relevance of multidirectional magnetostriction for the noise generation of transformer cores

As well known, T-joint regions of three-phase transformer cores tend to show rotational magnetisation (RM) patterns which yield distinct increases of magnetostriction. The present work was focused on the question whether these local phenomena exhibit practical relevance for the cores global magnetostriction-caused strain. A main effect was found for the yokes horizontal direction, RM causing about 80% increase of total strain in the T-joint region and 40% for the total yoke.

A hand-held sensor for analyses of local distributions of magnetic fields and losses

Abstract The paper describes a novel sensor for non-destructive analyses of local field and loss distributions in laminated soft magnetic cores, such as transformer cores. It was designed for rapid information on comparative local degrees of inhomogeneity, e.g., for the estimation of local building factors. Similar to a magnifying glass with handle, the compact hand-held sensor contains extremely sharp needle electrodes for the detection of the induction vector B as well as double-field coils for the vector H . Losses P are derived from the Poynting law. Applied to inner — or also outer — core regions, the sensor yields instantaneous computer displays of local H , B , and P .

Automatic scanning system for the determination of local loss distributions in magnetic cores

The paper describes an automatic scanning system for the determination of local loss distributions in magnetic cores based on the rise-of-temperature method. The drawback of long measurement times is accepted opposite to the performance of unsupervised overnight operation. The reported measurements on model transformer cores indicate a good accuracy close to 5%.

On the practical relevance of rotational magnetisation for power losses of transformer cores

Abstract The paper discusses the relevance of rotational magnetisation (RM) for power losses in laminated transformer cores. Experimental results are given for the local distribution of the loci of the induction vector B and the magnetic field vector H . Further, typical local amplitude ratios e for magnetisation in transverse direction and regional building factors are shown for defined regions of the core. An approximate estimation indicates that about 13% of the total building factor can be attributed to RM phenomena, including effects of flux distortion.

Practical relevance of the “hard directions” of h.g.o. Si–Fe

Abstract Crystallographically, the magnetically “hard direction” (h.d.) of goss-textured Si–Fe arises in an angle Ψ h.d. close to 55° from the rolling direction. This paper reports the “effective h.d.” for h.g.o. Si–Fe subject to rotational magnetisation (RM). While circular RM yielded two values exceeding 55°, elliptical RM showed much lower values, which in addition proved to be affected by the intensity of magnetisation as well as by mechanical stress. Generally, the h.d. showed minimum values of magnetostrictive strain.

Stress effects on the multidirectional magnetic behaviour of grain-oriented silicon iron sheets

The paper discusses effects of tensile and compressive stress on highly grain oriented SiFe sheets subject to rotational magnetisation which was simulated according to elliptical B-patterns of model transformer cores. Experimental results are given for all three H-patterns, losses and multidirectional magnetostriction. Interpretations are based on domain theory, and conclusions are presented with respect to transformer cores.

Application of neural networks for the prediction of multidirectional magnetostriction

This paper describes attempts to use artificial neural networks (ANNs) for the prediction of magnetostriction (MS) characteristics of transformer core materials. In this first approach, the ANNs were trained with data from a rotational single-sheet tester to predict MS in rolling direction (r.d.) as a function of material grade, amplitude and shape of multidirectional magnetisation as well as the level of additional mechanical stress. It is shown that ANNs are able to forecast the corresponding relative MS changes in an approximate way.