Modeling and design of cookware for induction heating technology with balanced electromagnetic and thermal characteristics

Improving the cooking experience of induction-heating users involves, among other factors, an optimized power distribution at the bottom of the cooking vessel. Conventional ferromagnetic cookware presents high efficiency but unequal temperature distribution with flat inductors, which subsequently leads to uneven cooking results. In this work, we propose an alternative to the traditional cookware arrangement by inserting some aluminum pieces in the ferromagnetic bottom of cookware. This arrangement combines the optimal inductive performance of the ferromagnetic iron an the high thermal conductivity of aluminum. The performance of the proposed arrangement is analyzed by means of a multiphysics tool including electromagnetic and heat transfer sub-models which is applied to predict both the equivalent electrical circuit and the temperature distribution in cookware. As a result, a balanced trade-off between efficiency and temperature distribution is evidenced with the proposed solution. Experimental results also corroborates the predictions of the proposed solution. Autho

Analysis and optimization of the efficiency of induction heating applications with litz-wire planar and solenoidal coils

Optimization of the efficiency of an induction heating application is essential in order to improve both reliability and performance. For this purpose, multi-stranded cables with litz structure are often used in induction heating applications. This paper presents an analysis and optimization of the efficiency of induction heating systems focusing on the optimal copper volume of the winding with respect to different constraints. The analysis is based on the concept of a one-strand one-turn coil, which captures the dissipative effects of an induction heating system and reduces the number of variables of the analysis. An expression for the efficiency of the induction heating system is derived. It is found that, with the geometry and the other parameters of the system fixed, efficiency depends on the copper volume of the windings. In order to use this result to optimize the efficiency of an application, volume restrictions, the packing factor and the window utilization factor are also considered. The optimum frequency for an induction heating system is also studied in this work. An experimental verification for both planar and solenoidal cases is also presented

Propuestas de mejora de la información financiera externa

Una de las mayores deficiencias de la información financiera actual es su incapacidad para reflejar el valor razonable de las empresas y la evolución de su valor. Esto es debido fundamentalmente a que los estados financieros se preparan sobre la base de los principios contables legales, mientras que la valoración se realiza en función de las expectativas económicas. Las consecuencias de esta situación, el uso exclusivo de estados financieros preparados sobre la base de los principios contables tradicionales son múltiples, tanto desde el punto de vista interno porque la contabilidad como sistema de información es instrumento de gestión, de motivación y de control de las actividades, como externo, en cuanto a la asignación de los recursos de los inversores y de la sociedad en su conjunto. Diversas soluciones se han aplicado para mejorar la información externa de las empresas sobre su situación y actividades, pero las soluciones que se vienen aplicando han sido desarrolladas por las propias entidades, o por firmas de consultoría para aplicaciones específicas, por ello caras, de uso minoritario, y sin constituir un sistema normalizado que facilite su utilización generalizada y la comparación entre entidades. En este trabajo se plantea mostrar una metodología para mejorar la información de las entidades sobre su situación y actividades, preparando unos estados financieros que muestren el valor razonable de la entidad y de sus elementos, utilizando técnicas de conocimiento habitual por el profesional contable como son: análisis de negocios, valoración de empresas, comprobación de no pérdida de valor de los activos, contabilización de fusiones, consolidación de estados financieros y preparación de estados financieros previsionales

A recursive methodology for modelling multi-stranded wires with multilevel helix structure

Multi-stranded litz wires are commonly used in magnetic devices for power electronics applications at medium-high frequency range, from several kHz up to hundreds of kHz. For these applications, litz-wire structure favours the uniformity of driven current in the cross-sectional area of conductors, alleviating ac losses (skin and proximity effects) and improving the global efficiency of the application. These features are achieved by means of a special cable arrangement consisting of many isolated fine copper strands twisted together according to the manufacturing process. Often, the manufacturing process involves several twisting steps where bundles of moderate number of strands are successively twisted resulting in intricate cable structures. We present a mathematical description of the trajectories of copper strands with the purpose of obtaining the cable losses by means of Finite Element Analysis (FEA) simulation tools. Moreover, a nomenclature for this multilevel structures is also proposed. Parameters as the number of twisting steps, number of strands, strand diameter or pitch length, are included in this representation, allowing to compare the performance of different manufacturing solutions

Analysis and Modeling of the Forces Exerted on the Cookware in Induction Heating Applications

We present a semianalytical model for calculating the forces exerted on cookware in domestic induction heating applications. The developed model is based on the Maxwell''s stress tensor and is also based on the existing semianalytic expressions of the electromagnetic fields in planar induction heating systems, which are expressed in terms of Fourier-Bessel series. Taking advantage of the axial symmetry of usual domestic induction heating systems, the flux of the vertical component of the Maxwell''s stress tensor is analytically integrated and the vertical force is obtained. The proposed model captures both eddy currents and magnetization that occurs in typical ferromagnetic cookware. The model is verified by means of two-dimensional Finite Element simulations and also is tested by means of measurements of the change of the weight experimented by cookware due to the forces during the heating process

Adapting of Non-Metallic Cookware for Induction Heating Technology via Thin-Layer Non-Magnetic Conductive Coatings

We analyze the feasibility of heating non-metallic cookware, unappropriate for heating by means of induced currents, with the purpose of extending the applicability range of the current induction heating cooktops. In order to turn materials as glass, ceramic, wood or plastic into suitable for the induction heating technology, we propose the use of thin layers of a metal (not necessarily a ferromagnetic material) which can be deposited on a surface by means of a thin or thick layer technology. For this purpose, the inductive performance of these layers is investigated by means of an analytical electromagnetic model, finite element simulations and experimental measurements. Calculations point out that for a specific induction arrangement working at a fixed frequency, it exists a thickness which maximizes the induction efficiency for each layer material. The suitability of this result is tested by means of a set of samples with copper thin layers whose thicknesses range from one hundred of nanometers to tens of micrometers, which are implemented using a phase vapor deposition (PVD) technology. The obtained induction efficiency and equivalent resistance are compared with those obtained with conventional ferromagnetic materials. As a proof of concept, the inner and outer bottoms of two glass pots are covered with a copper layer of 2µm, and 1.5µm , respectively, and 1 kW is inductively supplied by means of a series resonant inverter, reaching the boiling water conditions

Domestic induction heating system with standard primary inductor for reduced-size and high distance cookware

In this work, a hybrid wireless power transfer system which combines induction heating (IH) and inductive power transfer (IPT) functionalities is proposed to improve the performance of a domestic induction heating application with small loads weakly coupled to distant inductors. Considering the basic single-inductor domestic IH application, the addition of a secondary inductor with series compensation capacitor directly attached to the small ferromagnetic cookware. This inductor allows to adapt the primary inductor to the load size, extending load distance while avoiding increased power losses and stress in electronic components. The extended distance can be used to implement the glassless induction concept, where the ceramic glass of typical cooktops is substituted by the kitchen surface itself. The design of the secondary coil is carried out by means of a combination of Finite Element simulations and electrical simulations. A design process including the housing of the resonant capacitors and the selection of the secondary winding number of turns and cabling is presented. As a result, a prototype is implemented and tested under working conditions up to 1500 W at several distances. Experimental results validate the electrical modelling and simulation. Moreover, thermal results confirm the feasibility of the proposal and validate the adopted strategies for the capacitor housing. IEE

Design and Implementation of a Test-Bench for Efficiency Measurement of Domestic Induction Heating Appliances

The operation of a domestic induction cooktop is based on the wireless energy transfer from the inductor to the pot. In such systems, the induction efficiency is defined as the ratio between the power delivered to the pot and the consumed power from the supplying converter. The non-transferred power is dissipated in the inductor, raising its temperature. Most efficiency-measuring methods are based on measuring the effective power (pot) and the total power (converter output). While the converter output power is directly measurable, the measurement of the power dissipation in the pot is usually a cause of inaccuracy. In this work, an alternative method to measure the system’s efficiency is proposed and implemented. The method is based on a pot with a reversible base to which the inductor is attached. In the standard configuration, the inductor is placed below the pot in such a way that the delivered power is used to boil water, and the power losses are dissipated to the air. When the pot base is flipped, the inductor is immersed into the water. In this case the losses in the inductor also contribute to heating up and boiling the water. The induction efficiency is calculated from the boiling rates in both configurations. A commercial inductor was tested under real working conditions with consistent results

Induction Heating of Two Magnetically Independent Loads With a Single Transmitter

This article introduces the design of a system capable of heating two magnetically independent ferromagnetic loads placed on different horizontal planes, which uses a combination of induction heating and inductive coupling, called inductively coupled heating. The system uses a single primary inductor acting as a transmitter to transfer power to a secondary inductor attached to the bottom load, which is connected electrically with a third inductor that heats the top load. Since power of the whole system is supplied by a simple half-bridge inverter, the ratio of the delivered power to each of the loads, which is critical for cooking results, is entirely dependent on the system's geometry, coil's number of turns, and compensation capacitors. A finite-element model is used to simulate the magnetic fields generated by inductor currents and calculate the impedance matrix. With the impedance, capacitor values and inductors’ number of turns are selected with the objective of achieving a high power ratio between the top and bottom zones, as well as minimizing stress in the electronics. First, a prototype was built to validate the impedance results in the small-signal regime, and then, the full power regime was used to verify power and current simulation