Toroidal Transformer Research Articles

Measurement of internal magnetic flux density distribution in air-core toroidal transformer under high-frequency excitation.

An investigation of air-core toroidal transformers has been undertaken to enable power converters to achieve megahertz operation. The characteristics of the air-core toroidal transformer govern the behaviors of the internal magnetic flux of the transformer. Understanding the behaviors at an above-megahertz level is a key issue in the design of the air-core toroidal transformer. This paper discusses the internal magnetic flux density distribution in the air-core toroidal transformer under high-frequency excitation. We propose a measurement method for the distribution and show the obtained results under a sinusoidal excitation with an amplitude of 1.5 A and a frequency of 1 MHz. The results reveal a non-uniform distribution in the tangential and normal directions. The cause of this non-uniformity was found to be the structure of the air-core toroidal transformer. In addition, the validity of the proposed measurement method was confirmed by comparing the experimentally obtained results with a numerical estimation using the Biot-Savart law. These results suggest that the proposed measurement method is capable of investigating the distributed characteristics of air-core toroidal transformers under megahertz excitation.

Design and Development of High Efficiency 150 kW Very Compact PLA Core Electromagnetic Coupler for Highly Resonant Power Transfer Technology

Highly Resonant Power Transfer (HRPT) technology is currently receiving very significant attention from the industry and the smart power grid distribution community in particular. This technology ensures electrical power transmission between two points while controlling the level of transmitted power and ensures the immediate shutdown of the transmitted power in the event of a problem. This paper reviews the inductive power transfer method and describes the design of an ultra-compact PLA core electromagnetic coupler. The proposed architecture confines the magnetic field in a toroidal PLA core transformer, and by avoiding the use of heavy and bulky shielding plates, reduces magnetic losses and avoids the Curie point. As a result, the overall unit has a weight of 5 kg and a volume of only 0.013 m3. The electromagnetic coupler is capable of transferring a peak power of 150 kW with an operating frequency of 193 kHz, giving a satisfactory efficiency of 95%. The proposed novel system was first investigated through CST 3D numerical modelling to determine the electrical parameters of the coupler’s equivalent circuit and its efficiency, to verify its compatibility with the ICNIRP 2010 standard and to evaluate its temperature rise with an air-cooling system. Afterwards, the designed coupler was built with a 3D printing device and finally tested experimentally. Simulation and experimental results are compared and show a good agreement.

MSFEM for the Eddy Current Problem in a Laminated Core Including Hysteresis

A multiscale finite-element method is extended to allow for materials with hysteresis. The method is developed for an eddy current problem, coupled with a network. As an example, a laminated toroidal transformer core is considered. Utilizing the symmetry of the domain, the problem can be rewritten as a 2-D one. The multiscale method shows good results compared to the reference solutions for both the nonlinear problem with magnetization curve and with the inclusion of hysteresis by the Preisach model while preserving its main advantages of drastically reducing the number of degrees of freedom by utilizing a coarse mesh that does not resolve each single iron sheet. Both the multiscale solution and the reference solution are compared to measurement data. It is demonstrated that the inclusion of hysteresis is necessary in order to achieve a good approximation of the measurement data, which is used to identify both the magnetization curve and the parameters for the Preisach model.

In-Rush Current Mitigation on Toroidal Transformers with Composite Cores

Abstract: Toroidal transformers are high performance transformers used in high end applications. The unique toroidal construction drives them for such performance levels, but the gap less core construction itself makes them lag in controlling high inrush currents. Transformer based solutions are always favourable in the industry over the external solutions, but most of the existing transformer based solutions weaken the toroidal transformer’s typical superior performance while mitigating the inrush current. This paper investigates an effective inrush current mitigating method with composite cores, while protecting the performance level of typical toroidal transformers and being competitive in the market. The proposed method has two cores made with different steel types which are positioned concentrically, where the inner core is uncut, but the outer core is cut with an air-gap. This paper investigates on the optimum air-gap that should be maintained to minimize the inrush current based on the transformer size and the particular two steel types. Further, this paper describes on the inrush current calculation on composite cores together with the formulae and derived characteristic curves. The results and formulae presented in this paper are verified with laboratory experiments with real transformers, built with composite cores.

A laboratory application for teaching the effect of harmonics on transformer core saturation

Harmonics in supply voltage or load current in a transformer have undesirable effects on the core and windings. As the harmonic content increases, higher core loss and eddy current loss in the windings results in higher operating temperatures. It causes magnetic flux density in the core to increase and shifts the operating point through the saturated region. This phenomenon is mostly taught in undergraduate education as theoretical information. However, as a difficult concept to understand, harmonics phenomenon might be taught by using experiments for active learning, visualizing the effects in a laboratory setting, and making the connection between theoretical and practical applications for deeper and meaningful learning. In this study, a teaching method for effects of harmonics is proposed and phase shift between the fundamental and harmonic component of the supply voltage on the transformer B–H characteristic in a laboratory application is introduced by an experimental work performed on toroidal transformers. Feedback from students shows that the proposed method is very efficient and useful for students to better understand the effects of harmonics on the transformer cores.

Design of a Microcontroller Based Automatic Voltage Stabilizer with Toroidal Transformer

Design of a Microcontroller Based Automatic Voltage Stabilizer with Toroidal Transformer

High Frequency Transformer’s Parasitic Capacitance Minimization for Photovoltaic (PV) High-Frequency Link-Based Medium Voltage (MV) Inverter

The high-frequency-based medium voltage (MV) inverter is used in renewable energy power sources for power transmission. However, power quality is compromised as a result of the increase in common mode noise currents by the high inter-winding parasitic capacitance in high-frequency link transformers. This fast voltage transient response leads to harmonic distortion and transformer overheating, which causes power supply failure or many other electrical hazards. This paper presents a comparative study between conventional and modified toroid transformer designs for isolated power supply. A half bridge high-frequency (10 kHz) MV DC–AC inverter was designed along with power source; a 680 W solar module renewable system was built. An FEM-simulation with Matlab-FFT analysis was used to determine the core flux distribution and to calculate the total harmonics distortion (THD). A GWInstek LCR meter and Fluke VT04A measured the inter-winding capacitance and temperature in all four transformer prototypes, respectively. The modified design of a toroid ferrite core transformer offers more resistance to temperature increase without the use of any cooling agent or external circuitry, while reducing the parasitic capacitance by 87%. Experiments were conducted along with a mathematical derivation of the inter-winding capacitance to confirm the validity of the approach.

Experimental evaluation of electric and magnetic properties of structural steel

PurposeThis paper aims to present the proposition of a new experimental method for obtaining very crucial data of the structural steel that is used in the tank of oil filled power transformers, namely, the volumetric losses and the magnetic permeability, both in function of the density of magnetic flux. Although these data are not usually available, they are fundamental for helping the transformer designer in avoiding the occurrence of hot spots in the transformer tank. The adoption of a conventional Epstein frame has restrictions because of the incompatibility between it and the samples of the steel.Design/methodology/approachThe basis of the proposition is the same as that of the Epstein frame, with significant attention paid to the additional losses in the winding that creates the magnetic flux to the samples in the core. These losses can be significant and are created by the harmonics of current along the windings and are summed to the ohmic losses. For separating these winding losses from the magnetic losses, each sample is made as being the core of a toroidal 1:1 transformer. Thus, two tests with two identic of these toroidal transformers are necessary.FindingsThe proposed methodology is simple, because it is very similar to the classical tests of transformers (no-load and short-circuit tests). The process of separation of losses requires only a numerical fitting of curves for adjusting the winding losses as a function of the current amplitude, and the obtained results are coherent with the expected behavior of the magnetic losses and the magnetic permeability of a structural steel.Research limitations/implicationsThe method gives very approximate results in comparison to those obtained using the Epstein frame. The influences of the temperature and/or of the skin effect have not been evaluated.Practical implicationsPractical, real and thus confident data of structural steel, such as the magnetic permeability and the volumetric losses (hysteresis and Foucault), become available for the transformer designer to take actions for not only reducing the tank losses but also for avoiding the occurrence of hot spots through computer simulation.Originality/valueThe proposition is very new, as it allows to test steel samples with a size that does not fit to a usual Epstein frame. It takes into account the real influence of harmonic of currents in the losses along the winding of a classical Epstein frame, which has not been so far mentioned. It allows obtaining data of structural steel that had not been considered important until now.

Fabrication of 3D air-core MEMS inductors for very-high-frequency power conversions

We report a fabrication technology for 3D air-core inductors for small footprint and very-high-frequency power conversions. Our process is scalable and highly generic for fabricating inductors with a wide range of geometries and core shapes. We demonstrate spiral, solenoid, and toroidal inductors, a toroidal transformer and inductor with advanced geometries that cannot be produced by wire winding technology. The inductors are embedded in a silicon substrate and consist of through-silicon vias and suspended windings. The inductors fabricated with 20 and 25 turns and 280-350 μm heights on 4-16 mm2 footprints have an inductance from 34.2 to 44.6 nH and a quality factor from 10 to 13 at frequencies ranging from 30 to 72 MHz. The air-core inductors show threefold lower parasitic capacitance and up to a 140% higher-quality factor and a 230% higher-operation frequency than silicon-core inductors. A 33 MHz boost converter mounted with an air-core toroidal inductor achieves an efficiency of 68.2%, which is better than converters mounted with a Si-core inductor (64.1%). Our inductors show good thermal cycling stability, and they are mechanically stable after vibration and 2-m-drop tests.

Influence of the chemically synthesis conditions on the microstructure and magnetic properties of the Co-Fe-B nanoparticles

Co-Fe-B particles present a high potential for applications in microwave domain (electromagnetic shielding, toroidal transformer, etc.) due to their special soft magnetic properties like high saturation magnetization, low coercivity, large anisotropy and high magnetic permeability. However, their microwave applications are limited to about few gigahertzes due to the eddy current losses if the size of the particles is larger than few hundred of nanometers. Chemical synthesis method gives the possibility to obtain nanoparticles with diameters from few nanometers to tens of nanometers by varying the parameters of the chemical synthesis. One way to avoids the agglomeration of the particles in the utilization of the polyvinyl-pyrrolidone (PVP) which is acting as dispersant and dimensions controlling agent for nanoparticles.The aim of this paper is to study the influence of the synthesis conditions on the magnetic properties and microstructure of Co-Fe-B nanoparticles prepared by chemical reduction method in order to obtains nanoparticles with magnetic properties suitable for high frequency applications in the 0.1 ÷ 12 GHz frequency range.Co-Fe-B nanoparticles were prepared by chemical reduction of CoCl2·6H2O and FeSO4·7H2O salts in aqueous solution of sodium borohydride (NaBH4) in presence of the polyvinyl-pirrolydone (PVP). The experimental results indicate that the amount of PVP, Fe/Co ratio and the temperature of the chemical synthesis are important parameters which have to be controlled in order to obtain nanoparticles with desired dimensions, nanostructure and soft magnetic properties with suitable properties for high frequency applications.

Measurement uncertainty of the instantaneous characteristics of non‐linear coil obtained by Dommel's method

The characteristics of coil are obtained through the following measurement: the root mean square (RMS) coil current, as a function of RMS coil voltage, and the coil loss, as a function of RMS coil voltage. These values were then transformed, using Dommel's method, into instantaneous characteristics of non-linear coil, in a form of a non-linear inductance, representing saturation effect; in parallel with the non-linear resistance representing coil loss. The instantaneous characteristics are composed of peak values of resistance current and voltage, and inductance current and flux. The uncertainties of the characteristics are defined through the uncertainty of the peak values. The uncertainty of instantaneous characteristics is calculated on an example of a coil realised as unloaded iron-cored toroid transformer with a strip-wounded core made of oriented transformer sheets (M5-type) using the adaptive Monte-Carlo method (MCM) and uncertainty framework of ‘Guide to the expression of uncertainty in measurement’ (GUM). Results reveal the peculiarities of both methods and the impact of the uncertainty of measured values on the uncertainty of instantaneous characteristics.

Smart load management of distribution‐class toroidal transformers using a dynamic thermal model

Thermal behaviour is a prime factor in the accurate performance assessment of power transformers as well as in the prediction of their life expectancy. This study presents a computer modelling tool based on an electro-thermal equivalent circuit of transformers that is able to predict the hot-spot temperature and average surface temperatures for all internal layers of distribution-class toroidal transformers. Temperature is the limiting factor that prevents running transformers for hours or days in overload conditions. The modelling tool presented in this study is capable to identify the safe maximum overload current and duration that a transformer can handle without introducing damage or loss of life. The model is helpful to predict the short-term (few hours) and long-term (few days) overload capabilities of transformers. The electro-thermal model can also be used as a tool to optimise the design and evaluate the performance of transformers. This study is specifically focused on the implementation of the proposed method on dry-type distribution-grade toroidal transformers. The model is built using circuit components (lumped R and C ) obtained from the thermal-electrical analogy. The model is validated with numerous finite-element method simulations and laboratory tests with transformers of various power ratings.

Transformers, the unsung technology [Numbers Don't Lie

Ihave always disliked exaggerated claims of imminent scientific and technical breakthroughs, like inexpensive fusion, cheap supersonic travel, and the terraforming of other planets. But I am fond of the simple devices that do so much of the fundamental work of modern civilization, particularly those that do so modestly, even invisibly. . No device fits this description better than a transformer. Nonengineers may be vaguely aware that such devices exist, but they have no idea how they work and how utterly indispensable they are for everyday life. . The theoretical foundation was laid in the early 1830s, with the discovery of electromagnetic induction by Michael Faraday and Joseph Henry. They showed that a changing magnetic field can induce a current of a higher voltage (known as stepping up) or a lower one (stepping down). But it took another half century before Lucien Gaulard, John Dixon Gibbs, Charles Brush, and Sebastian Ziani de Ferranti could design the first useful transformer prototypes. Next, a trio of Hungarian engineers-Otto Blathy, Miksa Deri, and Karoly Zipernowsky-improved the design by building a toroidal (doughnut-shaped) transformer, which they exhibited in 1885. Though it worked well, its winding was difficult to make.

A new bidirectional hybrid multilevel inverter with 49-level output voltage using a single dc voltage source and reduced number of on components

This paper proposes an isolated bidirectional asymmetrical multilevel inverter topology composed by an H-bridge (HB) inverter connected to the primary side of transformer. The transformer secondary side is composed by two windings, and each one is connected to a new cell introduced in this work. The proposed new cell is based on the conventional HB inverter, where two bidirectional switches are added to each HB leg, thus resulting in the structure referred to as CHB-2bs. The transformer can also be composed by multiple secondary winding in order to provide the necessary voltages to supply as many as CHB-2bs cells are. A 600W laboratory prototype with dc input voltage of 48V and an ac output voltage of 220V, 60Hz using a grain-oriented silicon–steel toroidal transformer operating at 300Hz is implemented to validate the theoretical assumptions and the advantages addressed to the CHB-2bs cell.

Experimental Parameter Determination and Laboratory Verification of the Inverse Hysteresis Model for Single-Phase Toroidal Transformers

This paper presents a laboratory test procedure to determine the parameters of the inverse hysteresis model (IHM) applicable to single-phase toroidal transformer low-frequency modeling. The model parameters are obtained through a fitting procedure of the history-independent IHM of a series of open-circuit experiments and waveform recordings. These experiments can be easily executed during the transformer acceptance testing at the factory laboratory. The performance of the model is verified under various operational conditions, such as sinusoidal excitation, varied frequencies and voltages, and non-sinusoidal excitations. The results show that the model performs very well under all tested conditions giving great confidence that the proposed method can be used for parameter determination.

Investigation of Transformer-Based Solutions for the Reduction of Inrush and Phase-Hop Currents

A comprehensive literature review shows that transformer-based solutions are superior for the mitigation of inrush currents than external (to the transformer) solutions. The use of air gaps and low-permeability (iron) materials are known techniques for this purpose. This paper investigates the effectiveness of these approaches for reducing inrush and phase-hop currents. Studies are carried out on toroidal transformers, due to their broad application in power electronics devices. Contrary to common belief, this paper demonstrates that air gaps do not reduce the inrush currents when a transformer is fully demagnetized. However, inrush currents can be mitigated by the use of low-permeability iron materials. It is also demonstrated that air-gaps significantly reduce inrush currents when transformers have residual flux, e.g., for phase-hop conditions. Analytical expressions are derived to compute the mitigation factor for a specific gap length. The results and formulae presented in this paper are verified with laboratory experiments, transient simulations with validated circuit models, and 2-D finite element simulations.

Reduction of Inrush Currents in Toroidal Transformers by Sector Winding Design

In this letter, the principles for controlling the saturation inductance of toroidal transformers by leaving unwound sectors are presented. It is shown that inrush currents can be reduced substantially with this technique. This concept is supported by finite-element simulations, transient analyses, and laboratory experiments.

Influence of losses on the output voltage of ferrite transformer in case of strong magnetic field in the core

Taking into account the nonlinear properties of ferrites the output voltage of toroidal transformer with magnetically soft ferrite core is calculated. Calculations are based on methods analogous to those used in nonlinear optics. Initially demagnetized ferrite is assumed to be isotropic. At this condition apart from the harmonic on fundamental frequency the output voltage of transformer contains also higher harmonics of odd order. Nonlinear susceptibilities of third and fifth order are used in calculations Magnetic losses are taken into account considering linear and nonlinear susceptibilities as being complex. These losses affect amplitudes of harmonics in the output voltage of transformer and create phase shifts between components of magnetization on different frequencies in the core and the intensity of exciting harmonic magnetic field. Results obtained in calculations are confirmed by experimental measurement of output voltage of toroidal transformer with MnZn ferrite core. In order to calculate amplitudes of harmonics in the output voltage of transformer numerical values of susceptibilities in question should be known.

Harvested Power Wireless Sensor Network Solution for Disaggregated Current Estimation in Large Buildings

In modern economies, large service buildings are responsible for an important part of the global electrical energy consumption. The implementation of energy saving strategies can benefit from disaggregated consumption monitoring. To tackle this problem, a number of technological solutions exist, being however, expensive in equipment, installation and maintenance. Because of its decentralized operation principle, wireless sensor networks (WSNs) are an important tool to implement disaggregated electrical energy monitoring. The development of a self-powered, battery-free current sensor node for large WSNs may contribute to the implementation of monitoring solutions for large buildings. In this paper, a solution to monitor disaggregated consumption is presented, based on a contact-less power source for Zigbee nodes using a split-core toroidal coil current transformer (SCCT). The proposed device is able to power a battery-free wireless node estimating also the current drawn by the electrical load with a single SCCT. The SCCT is successfully applied to power a battery-free wireless device running a complex communication software stack. The proposed system is described through simulation as well as the experimental results.

Nonlinear Properties of Soft Ferrites

In framework of methods used in nonlinear optics the output voltage of toroid transformer with soft ferrite core is calculated. It shown that apart from the fundamental frequency, it contains also harmonics of third and fifth order. An experimental verification on real transformer with MnZn ferrite core confirms the statements developed.