Interlaminar Flux Research Articles

Influence of interlaminar flux between cores on iron loss in three‐legged wound transformer core

AbstractA three‐legged wound transformer core is composed of two inner cores and one outer core, and the interlaminar flux occurs between inner and outer cores in three‐phase excitations. Past analyses of magnetic properties of three‐legged wound transformer cores were based on the model of independent magnetic paths in the individual core, and did not factor the influence of interlaminar flux in the calculations. This paper shows experimental results of interlaminar flux between cores and iron loss increase caused by that. Results show that the interlaminar flux between the inner and outer core increases the local iron loss in the boundary region of the inner and outer core.

Influence of Interlaminar Flux between Cores on Iron Loss in Three-legged Wound Transformer Core

Influence of Interlaminar Flux between Cores on Iron Loss in Three-legged Wound Transformer Core

Interlaminar Flux Density Distribution at Joints of Overlapping Stacked Electrical Steel and Amorphous Ribbons

The design of joints in a transformer core significantly affects the transformer’s efficiency. Air gaps cause variations in the flux distribution at the joints of the laminations, which depend on the geometry. Two similar samples consisting of electrical steel strips and amorphous ribbons were made. The spatial flux distributions were determined using an array of search coils for each sample. 2D models of these samples were created and examined by finite element analysis. The magnetic flux distribution for each lamination in the samples was computed. The results show that the flux density in amorphous ribbons above and below the air gap starts to approach saturation at lower flux density levels than for electrical steel. The flux density measured using the search coil under the air gap is increased in amorphous ribbons and decreased in the electrical steel with increasing frequency.

3D Magnetic Flux Measurement in Joint Region of a Model Core Stacked with Grain-Oriented Electrical Steel

Grain-oriented electrical steels show a high degree of anisotropy that favors magnetization in the rolling direction. Magnetization also occurs in other directions in the joint regions that are characterized by interlaminar flux. For direct measurement of such flux behavior, flat sensor elements that do not create additional air gaps have been developed. The present study carried out on a model core. To determine the local flux around the joint rejoin, thin Cu film elements were vapor-deposited on both sides of individual laminates. Normal flux components were detected by frame coil arrangements, and in-plane components were determined by films with tip contacts through the coating. The obtained results showed that in the joint regions, the incoming in-plane flux is converted to interlaminar flux in the overlap region, until a high degree of global saturation is reached; at this stage the flux through the air gaps between the laminates leads to more homogeneous magnetization.

Finite-Element Modeling and Measurements of Flux and Eddy Current Distribution in Toroidal Cores Wound From Electrical Steel

Despite the apparent relative simplicity of the wound toroidal geometry, numerical modeling techniques are very difficult to implement due to the inherent large mesh size required to model a laminated core or spiral geometry. Recent advancements in computing capabilities have made the simulation and the study of such devices more feasible. Finite-element modeling of toroidal cores wound from grain-oriented electrical steel was conducted to accurately represent their magnetic circuits in order to calculate the internal magnetic flux distribution, its interaction with core geometry, and to be able to predict the associated magnetic losses. The general flux distribution trends agree with measured results from identical cores. The interlaminar flux and the associated induced planar eddy currents were also calculated. The measurements were performed using a series of search coils enclosing every five turns. In each case, the core was magnetized at 50 and 400 Hz under controlled sinusoidal flux density up to 1.5 T.

3D-measurements of magnetic flux on joint regions of SiFe steel model core

Modern types of grain-oriented SiFe show a gradually increased degree of anisotropy, which favors magnetization in rolling direction, a most recent step of development being made by the establishment of hyper-stress coatings. On the other hand, other directions of magnetization are involved in joint regions, which are characterized by interlaminar flux. For its analysis, flat sensor elements are needed which do not create additional air gaps. The present study was taken on small core models were vaporization of copper film elements could be performed on both sides of individual laminations. Normal flux components were detected by frame coil arrangements in-plane components by films with tip contacts through the coating. The results illustrates that in joints, the incoming in-plane flux is transferred to interlaminar flux in the overlap region, until a high degree of global saturation is reached, a state when flux through air gaps between laminations yields a tendency of more homogeneous magnetization.

Normal Flux Distribution in a Three-Phase Transformer Core Under Sinusoidal and PWM Excitation

Transformer no-load loss and noise have been reduced considerably because of improved core material and improved core design. The variation of normal or interlaminar flux distribution (z direction) in a three-phase transformer core has been measured under pulse width modulation (PWM) excitation (no-load) with switching frequencies in the range of 1-3 kHz. The planar eddy current loss and attractive interlaminar force were analyzed for selected values of modulation index and magnetization condition (frequency 50 Hz; induction 1.5 T). Results showed significant reduction of the planar eddy current component and attractive interlaminar force for low modulation index with increasing switching frequency

2D finite-element analysis of interlaminar flux density distribution at joints of zip-type unicore

Normal flux distribution along the rolling directions at joints of overlapped stacked electrical laminations is investigated to set up a flux distribution similar to that in the zip-type unicore. A 2D model of a C-core and three layers of overlapped laminations with joints with similar characterisitics to a zip-type unicore were formed and an FE analysis was carried out to solve the problem using a static magnetic analysis and unidirectional vector potential formulation. The results show that when the reluctance changes in the closed magnetic paths causes the flux deviates towards shorter magnetic paths moving to adjacent layers increasing the magnetic flux density. This redistribution causes the flux to transfer easier at joints in the unicore compared to other jointed cores.

Prediction of core losses of three phase transformers from estimation of the components contributing to the building factor

An algorithm is produced to estimate the iron loss of power transformers from quantification of the contributions of the effect of joints, rotational and harmonic flux, stress, interlaminar flux and core geometry. Better agreement is achieved between predicated and actual performance of 20 full size cores than when the normal empirical method is used particularly between 1.5 and 1.7 T (50 Hz).

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 Young's 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.

Comparison of transformer loss prediction from computed and measured flux density distribution

Losses in power transformer cores can be predicted from knowledge of localised flux distribution within the assembled laminations. This together with a knowledge of the stress sensitivity of the losses of grain-oriented, 3% silicon iron can ideally be used to estimate the total core loss. The localised flux density is computed given the magnetic material B-H data and the core geometry. This paper reports results in which localised flux density and losses have been measured experimentally in model cores and compared with data obtained from a FEM analysis of the same core geometry. Effects of interlaminar flux, rotational flux and stress are taken into account. Agreement within 5% is found between the measured core loss and that calculated from the localised experimental data but predictions using the FEM results were only within 10% of the measured value. This greater difference is believed to be due to neglect of interlaminar flux and rotational magnetisation in the computation of flux distribution. However, these are shown to be accountable by using corrective factors whose magnitudes depend on core geometry.

Relevance of step-lap joints for magnetic characteristics of transformer cores

It has been stated that multistep-lap (MSL) jointed cores show distinct improvements regarding power losses P in comparison to single-step-lap (SSL). On the other hand, model core experiments tend to yield contradictory results. In the paper, special emphasis was put on the underlying physical mechanisms, taking into consideration that model cores usually show &apos;perfect&apos; stacking, while full-size cores exhibit considerably high air gap lengths g as well as shifts s between the overlap regions of adjoining packages. It is shown that an increase of the number N of steps reduces the sensitivity of the core in respect to variations of g and s. This is due to increasing &apos;critical induction&apos; BC as of which the excitation VL of the overlap region steeply increases. Low VL corresponds to lower balancing interlaminar flux in the overlap/gap regions and thus to a generally more homogeneous flux distribution linked with lower P. The quantity BC can be assumed to be of high practical relevance for the design of transformer cores.

Flux distribution analysis in a mitred transformer core corner

A detailed knowledge of magnetic flux distributions and interlaminar flux takeover mechanisms is a prerequisite of transformer core design using modern extremely anisotropic materials. This paper presents the results of a corner analysis using extremely thin search coils. All relevant flux components within a mitred corner were measured, and the distribution is discussed in terms of an equivalent circuit. A local loss analysis by means of thermistor scanning showed that the losses are much larger close to the inner corner edge.