Article: Paper Introduction
Issue 4 Applications for Induction Heating Phenomena
Induction heating analysis tends to have large calculation loads, but with improved performance in calculating machines in recent years, environments are now made so that it can be done right at design facilities. However, induction heating is used in many different fields, and involves many different issues. In this report, I will give an overview of papers on a wide variety of uses for these phenomena, organized by the issue being covered. I have tried to pull these together so that you may find hints from other fields for issues you need to solve, and I hope you will find it useful. These are taken from a survey of papers published in "IEEE Transactions on Magnetics" in recent years, with a focus on electromagnetic phenomena.
Induction Heating Phenomena
I will first give a brief introduction to induction heating phenomena. When alternating current flows in a coil and a magnetized object moves at high speed, nearby conductors produce eddy currents. If the heat generated by these eddy currents is noticeably larger than the heat given off, a temperature rise is seen, which is induction heating phenomena. In general, there are two ways of working with heating phenomena. The phenomena being treated differ greatly depending on whether you are doing heat reduction design to control the heat that is generated, or you are actively making use of the heat generated in objects. This report focuses on the latter, making active use of generated heat. Compared to traditional heating methods using hot gas, induction heating methods making active use of heat generation have many benefits, including allowing localized heating and rapid heating, and being easier to incorporate into production lines. However, they also have their problems, such as that induction heating equipment can be very expensive, and coil design and current conditions need to be controlled to ensure uniform heating. The following sections will present relevant literature and fields for various issues.
Applications by Issue
Issue: "Uniform thermal distribution"
In induction heating using heating coils, eddy currents are produced in the surface facing a heating coil, causing the temperature to rise. Because of how this is done, it is necessary to properly investigate the heating coil's shape and the current conditions if you want the temperature to rise uniformly. A look at trends in the literature shows that uniformity is used as an objective function in optimization methods.
It has been shown that a more uniform thermal distribution can be provided by induction cookers by changing the coil design from single-coil to seven-coil . With a single-coil system, the heating in the surface facing the coil becomes localized, leading to fast aging of the pan, but this can be improved with multicoils design. The influence of proximity effects among coils have also been discussed .
Coils for heating nonmagnetic strips while moving them can either come in solenoid or transverse form. Optimization calculations have been carried out for uniform heating of strips of different widths using a multi-coil system with the latter coil shape . A case has been reported of using UVW 3-phase heating coils to heat aluminum sheets uniformly while moving them. In this example, distributions of eddy current, magnetic force, and power density along the position of the strip are studied in order to evaluate the heating uniformity .
Issue: "Localized heating"
These heating methods can be seen as taking advantage of the ability of induction heating phenomena to produce heat in a limited area only. Loss produced in the work piece is concentrated in the parts facing the coil. This is because, when the amount of generated heat is increased with more current, the temperature can be rapidly raised in just the target area before the heat spreads out.
A 3D analysis example is given using an inductor for tube induction welding. The analysis of the tube uses a new strategy in displaying the movement of a metal strip .
To cause solder paste reflow in a printed circuit board (PCB), either hot air convection or infrared ovens have been relied on in the past. However, heat stress can result when the entire package is heated, and unexpected defects may occur. Localized heating using induction heating has been tried for this purpose .
An example using induction heating for shrink fits has also been reported. Shrink fits are joints in which two or more parts need to be fit together, so the dimensions of the outer metal part are increased by induction heating, another metal part is inserted there, and the whole system is consequently cooled. A complex analysis of these phenomena is performed by a fully adaptive higher-order finite element method with thermoelastic displacements in addition to coupled magnetic field and temperature field analysis .
A thermo-inductive technique for detecting defects in metallic pieces has also been written about. Eddy currents concentrate in areas with flaws, so a method has been demonstrated to measure areas of temperature rise using an infrared camera .
Issue: "Higher heating efficiency"
Use of an inductor transfers magnetic flux through the air, so energy is lost in that space. This makes it necessary to find suitable electromagnetic means of increasing efficiency. In particular, non-magnetic materials do not transfer flux as easily as magnetic materials, so problems tend to arise.
Various methods are being tried in induction cookers, including improving the coupling between the inductor and pot using a ferrite core, and placing an aluminum foil to shield the electronics from magnetic flux. In a report focusing on efficiency in planar inductors for induction cookers, comparisons are made of analyses run with variations such as materials, number of turns, current frequency, type of cable, and placement of the ferrite core .
A concept is proposed of forcing a billet to rotate at high speed in a dc magnetic field to induce heating in order to increase heating efficiency in a nonmagnetic work piece such as aluminum. This method is interesting in that it uses dc current and magnets rather than using ac current .
Issue: "Finding the circuit constant of induction coils"
When thinking about how to supply the current needed for induction heating, the focus shifts from controlling current conditions to the coil's circuit constant. Examples have been seen of obtaining circuit constants in induction cookers, etc. .
Methods of heating aluminum sheet material while moving it are being investigated. 2-D analysis is carried out to reduce solution time, but computational accuracy can be improved by accounting for the end-turn leakage inductance .
Issue: "Reducing stray field"
Devices built so that the magnetic circuit is surrounded by magnetic materials account for a minority of induction heaters. However, if it is not surrounded, magnetic flux disperses into the surrounding region. Shielding for controlling stray field is found in several different fields.
A system for high-temperature levitation melting induction heating has been reported. Heating coils are positioned so that magnetic flux will not leak into the surrounding region .
Issue: "Finding and controlling electromagnetic force distributions"
Lorentz force from generated eddy currents cannot be ignored when a large amount of electric power is applied in order to heat a target object to a high temperature, because the magnetic flux density also increases. Just as there is a field of study for restraining and regulating the Lorentz force distribution, there is a field for actively making use of Lorentz force to convey electricity-conducting liquids.
A study was reported relating to a pump for high-temperature molten metal. This uses a mechanism to move metal by applying a twisted rotating magnetic field to liquid metal from outside. Reverse flow can occur if the Lorentz force produced in a cross-section is nonuniform, so the study pays particular attention to uniformity of electromagnetic force .
In relation to the forces generated when 3-phase alternating current is applied to thin steel plate being conveyed, another report gives a comparison of the attractive force caused by magnetization and the repelling force caused by Lorentz force. The frequency determines the relationship between both of their magnitudes .
Issue: "Conveying electrically conductive liquids"
If a liquid is electrically conductive, it can be adjusted to flow in the desired direction by producing Lorentz force.
One paper presents a system to move molten metal by generating a rotating magnetic field in it . Another gives a report of a triply coupled magnetic field/temperature field/field of flow analysis of inductively heated incompressible flow of electrically conductive liquid in ceramic pipe . There is also a report of contactless purification of metal by levitation melting using the Lorentz force generated in it .
Issue: "Material modeling"
For induction heating analysis, accounting for temperature dependency in the material being heated is important from the standpoint of accuracy. A particular problem is that it is difficult to obtain magnetic properties taking account of temperature dependence in magnetic materials. Simplified methods are also needed for analyzing composite materials with complex structure.
It has been reported that it is important in terms of accuracy to take account of the temperature dependence of the B-H curve when doing analyses of billet heaters that are used, for example, for the shaft of a car. I think this can serve as a useful reference for the temperature dependence of carbon steel .
Induction heating is being used for heat treatment of carbon fiber reinforced polymer (CFRP) composites, which have high mechanical and chemical resistance per unit weight. Issues regarding how to model multilayer CFRP composites and ways to handle anisotropy in electrical properties for analysis have been published .
Issue: "Accurate control"
Compared to gas heating methods, induction heating allows simple adjustment of the amount of heat by simply controlling the current. I found an example of control of position with attention to regulation. Traditionally, position has been controlled by pneumatic elements, but a possibility of producing thermoelastic dilatation using induction heating with the goal of more precise regulation has been suggested .
Issue: "Contactless application"
Compared to electric heating and gas heating, induction heating allows higher temperatures to be reached in the target object without any contact. Active applications are being investigated in fields where as little mixture with other substances as possible is necessary.
A case has been reported of refining metal to a high degree of purity without contact through levitation melting . There is also a method of heating magnetic fluids with induction heating as a form of magnetic induction hyperthermia treatments to cancers. The report points out that the induction heating depends on the relation between the magnetic core size and the coating layer thickness . When tension is applied to thin steel plate that is being conveyed by the contacting pinch rollers used to prevent it from slackening by its own weight, surface cracks and exfoliation of coating can occur. The use of a linear induction motor to apply electromagnetic force to steel sheet without contact has been tried as a way to avoid this problem .
These examples are not all necessarily based in industry, but they help to reaffirm the wide range of uses and studies that can be achieved with induction heating phenomena. While each of them deals with its own issues, common points can be seen when looking at the phenomena involved.
 L.C.Meng, K.W.E.Cheng, W.M.Wang, "Thermal Impacts of Electromagnetic Proximity Effects in Induction Cooking System With Distributed Planar Multicoils", IEEE Trans. Magn., vol.47, no.10, pp.3212-3215, Oct. 2011.
 J.Wang, Y.Wang, S.L.Ho, X.Yang, W.N.Fu, G.Xu, "Design and FEM Analysis of a New Distributed Vernier Traveling Wave Induction Heater for Heating Moving Thin Strips", IEEE Trans. Magn., vol. 47, no. 10, pp.2612-2615, Oct. 2011.
 F.Dughiero, M.Forzan, C.Pozza, E.Sieni, "A Translational Coupled Electromagnetic and Thermal Innovative Model for Induction Welding of Tubes", IEEE Trans. Magn., vol. 48, no. 2, pp.483-486, Feb. 2012.
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 H.Kagimoto, D.Miyagi, N.Takahashi, N.Uchida, K.Kawanaka, "Effect of Temperature Dependence of Magnetic Properties on Heating Characteristics of Induction Heater", IEEE Trans. Magn., vol. 46, no. 8, pp.3018-3021, Aug. 2010.
 G. Wasselynck, D. Trichet, J. Fouladgar, "Determination of the Electrical Conductivity Tensor of a CFRP Composite Using a 3-D Percolation Model", IEEE Trans. Magn., vol. 49, no.5, pp.1825-1828, May. 2013.
 A.H.Habib, M.G.Ondeck, K.J.Miller, R.Swaminathan, Michael E.McHenry, "Novel Solder-Magnetic Particle Composites and Their Reflow Using AC Magnetic Fields", IEEE Trans. Magn., vol.46, no.6, pp.2187-2190, Jun. 2010.