Basics of induction heating, Part 1: Introduction

Induction heating is widely used in industry and even consumer appliances as a contact-free heating technique with many distinct advantages.

A very common design challenge for many engineers is to manage and dissipate excess heat produced within or impinging on a system so the temperature of that system or some of its components does not exceed some specified limit. However, there are designs for which the engineering objective is to create heat – often, lots of it – and deliver that heat to a load.

There are several ways to generate heat and convey it from that source to the load via some varying combination of the classic heat-energy transfer modes of convection, conduction, and radiation (Figure 1).

Fig 1: There are three ways to transfer heat energy from a source to a load, but induction heating uses an approach in which the load is induced to self-heat via magnetic energy rather than heat transfer. (Image: Amazon).

The historic and obvious way is to use an open flame (fired by wood, coal, oil, gas, or other fuel), but this has low efficiency and obvious physical-arrangement safety concerns. Another way is entirely electrical, by employing electrical resistive loss – usually considered a bad thing – into a heat source. With either of these approaches, the heat created must be directed to flow to the load.

An indirect approach is to use microwave energy to stimulate the vibration of molecules in the load and create heat but are not suitable for use with many types of materials, especially those that are electrically conductive. Also, the microwave energy must be confined in an RF-shielded enclosure for user safety, as microwaves can have adverse effects on living cells and tissues, as well as on inanimate materials.

There is still another approach that seems to be almost “magical” at first: induction (or inductive) heating. This is based on electromagnetic principles and the classic AC transformer first investigated by Michael Faraday in the 1830s, before electronics and even basic electricity as we know them were even established ideas.

Induction heating uses loss induced by the flow of magnetic energy rather than resistive loss due to the flow of electrical current to create heat in a targeted load. Unlike resistive-based heating, where the heat is created at the source and then conveyed to the load by some varying combination of conduction, convection, and radiation, the induction-created heat is created at the load itself.

This is done via the interaction of an alternating current (AC), which develops a magnetic field that, in turn, interacts with the vessel in which the load (material) to be heated is placed or the material itself. Induction heating is yet another case of clever engineering where a normally negative attribute – in this case, resistance and loss – is turned to positive use.

This article will look at the principles and applications of induction heating. Due to its many favorable characteristics such as non-contact approach, cleanliness, relative safety, and controllability, it is often used in commercial and industrial settings for basic heating but also melting large quantities of metallic substances.

It is also used in material test systems to heat a metal sample (Figure 2). I have seen induction heating in action, taking a solid steel test-sample rod about 5 cm long and 1 cm in diameter to glowing cherry red in a few seconds – that’s a dramatic demonstration you don’t forget.

Fig 2: Induction heating is often used in materials test systems as a quick and effective way to locally heat a test sample even to extreme temperatures, yet without the need for a large furnace. (Image: ZwickRoell LP)

Induction heating is not a new development. As noted earlier, it began with Michael Faraday and his study of the induction of currents in wires by a battery being switched on and off – a crude way of creating a non-DC current flow (Figure 3). The fundamental principles of induction heating were later established and developed by James C. Maxwell in his unified theory of electromagnetism, while James P. Joule was the first to describe the heating effect of a current flowing through a conductive material.

Fig 3: Faraday’s discovery of the principle of magnetic induction and transformers is at the heart of induction heating/ (Image source: Engineering Timelines)

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. Edwin F. Northrup implemented Kjellin and the first high-frequency furnace application of induction heating in 1916.

Some consumers are familiar with induction heating, if not the principles, as it is used in some home stove-top ranges. Induction heating finds applications in processes where temperatures are as low as 100⁰C (212°F) and as high as 3000⁰C (>5400°F). It is used in short “flash” heating processes lasting for less than a second as well as in long-term heating processes that extend over several months.

The next part of this article will look at the principles of induction heating.

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