BASICS OF INDUCTION HEATING
In induction heating, an alternating current (AC) source is used to supply current to an induction heating coil. As a result, the coil generates an alternating magnetic field. When an object is placed in this field, two heating effects occur:
- hysteresis losses – these occur only in magnetic materials such as iron, nickel, cobalt, etc. due to the friction between the molecules when the material is being continuously magnetized in different directions. Higher magnetic field oscillation frequency results in faster particle movement, which causes more friction and thus more heat.
- eddy-current losses – these occur as a Joule heating effect in any conductive material because of the electric currents induced by the fluctuating magnetic field.
Both effects result in heating of the treated object but the second one is most commonly the main heat source in IH processes. Moreover, hysteresis is not observed in non-magnetic materials, and magnetic materials lose their magnetic specificities if heated above a specific temperature (the so-called Curie point).
Eddy currents also depend on the magnetic field frequency due to the skin effect – at high frequencies, the currents flow close to the conductor surface. This specificity is used to control the penetration depth of the induction heating process. As a result, either the whole object or only a specific part of it (only the surface, for example) can be heated. Thus, induction heating can be used for different applications – from metal melting to brazing and surface hardening.
Skin effect is also observed inside the induction coil conductor. Therefore, pipes can be used instead of solid wires. When the current flows through the inductor, similar resistive losses are observed due to the Joule effect. In order to prevent the coil from melting and damage, water cooling is often applied.
ADVANTAGES OF INDUCTION HEATING
Compared to some of the classic heating techniques (resistance heating, flame heating, furnaces, etc.), induction heating has the following advantages:
- reduced time – via induction heating, the target is heated directly resulting in a reduction of both heating time and wasted heat. This method provides high power density and low or no thermal inertia.
- high efficiency – efficiency values higher than 90% are obtained due to the proper design of the power converter and the coil. In addition, high temperatures can be reached quickly and easily as the ambient heat loss is significantly reduced.
- improved control – precise regulation of the heating power can be achieved via appropriate coil design and control of the power converter. As a result, additional features such as local heating, and pre-heating, predefined temperature profiles may be implemented.
- industrial automation option – induction heating allows improvement of both the productivity and the quality of the processes. Quality is additionally guaranteed as the heating is contactless (no interference by the heating tool).
- safety and cleanliness – there is no thermal or air pollution as the target is heated directly and no fuel substances are used.
INNOVATIONS AND FUTURE DEVELOPMENT
Although induction heating systems have already reached maturity as a technology, the development of modern technologies continuously provides options for new research trends and industrial interest. In the coming years, the following topics are expected to be of significant interest:
- efficiency improvement – induction heating systems with even higher efficiency are expected with the improvement of semiconductor technology. Moreover, special coil shapes and designs are provide increased efficiency. The aim of these efforts is to improve not only the performance but also the reliability of the induction heating systems.
- induction heaters with multiple coils – better heat distribution, higher performance and flexibility can be achieved using several simultaneously-operating coils. These systems represent a major technological breakthrough and are more and more commonly implemented not only in industrial but also in domestic applications. Efforts should be made to optimize multiple-output power converter designs and advanced control algorithms. Another issue to be considered is the coupling effect between the individual coils.
- advanced control – robust control algorithms are required to provide proper power converter operation for different induction heating loads and operating points. The control of multi-coil systems is another challenge. Improved performance and optimization of the transient processes is expected by the implementation of real-time identification control units with adaptive algorithms.
- special applications –the range of induction heating applications is expected to increase even more with increased technological development. Heating of low-resistivity materials, as well as heating of biological tissues for medical purposes, are topics of particular interest. There are still other applications that need further research to optimize the process parameters.
HISTORY OF INDUCTION HEATING
Induction heating was first discovered by Michael Faraday as he studied the induction of currents in wires by a magnet. The fundamental principles of induction heating were later established and developed by James C. Maxwell in his unified theory of electromagnetism. James P. Joule was the first to describe the heating effect of a current flowing through a conductive material.
In 1887, Sebastian Z. de Ferranti proposed induction heating for metal melting and filed the first patent on the industrial applications of induction heating. The first fully-functional induction furnace was presented in 1891 by F. A. Kjellin, and the first high-frequency furnace application of induction heating was implemented by Edwin F. Northrup in 1916.
During the Second World War and afterwards, the use of the induction heating technology was boosted by the aircraft and the automotive industries. Induction heating was not only used for metal melting but also for advanced material treatment, which significantly increased the range of induction heating applications.
The development of solid-state generators using new power semiconductor technologies provided the potential for IH beyond the industrial environment. Since the late 1980s, different domestic applications have appeared. In recent years, a special interest in induction heating for medical treatments has appeared, as this method provides precise and targeted local heating.
Today, induction heating technology provides highly efficient and reliable systems for a wide variety of applications.
- Lucia, O., P. Maussion, E. J. Dede, J. Burdio, Induction Heating Technology and Its Applications: Past Developments, Current Technology, and Future Challenges, (2013) IEEE Transactions on Industrial Electronics, vol. 61 ( 5), pp. 2509-2520.
- Tudbury, C. A., Basics of Induction Heating, vol. 1, J. F. Rider, May 1960, New York, US.
- Magnet, Wikipedia article https://en.wikipedia.org/wiki/Magnet#Magnetic_metallic_elements
- Joule heating, Wikipedia article https://en.wikipedia.org/wiki/Joule_heating
- Curie temperature, Wikipedia article https://en.wikipedia.org/wiki/Curie_temperature