Petrie Ltd

What is Induction Heating?
Induction heating is a process within a family of heating technologies known as electro-heat techniques designed to heat a range of materials. Not dissimilar to dielectric heating in that the principle is that portions of the electromagnetic spectrum are utilised as the primary energy source to heat a material, it differs distinctly in that the materials must be electrically conductive.

The application of electromagnetic radiation may be direct, where the electromagnetic energy is irradiated directly onto a material in order to heat it; or indirect, where the electromagnetic energy is used to heat an appliance which is directly applied to the material. This is fundamentally different from conventional heating techniques, based on conduction and convection, where the primary energy source to heat a material is through migration of heat flux through the material.

Fundamentals of Induction Heating
Induction heating is a procedure of heating an electrically conductive material by inducing eddy currents within the material. The induction heating process is achieved by an electromagnet, known as the workcoil, driven by a low-voltage high AC current at sub-1MHz RF frequencies. This coil creates a magnetic-field which permeates into the material, known as the workpiece. The presence of the magnetic-field in the workpiece induces eddy currents in accordance with Faraday’s Law of Electromagnetic Induction; the magnitude of the eddy currents is a function of the drive current.

The energy transfer of induction heating is a function of the distance between the coil and the workpiece. In direct heating applications the energy loss is due to Joule heating. In indirect heating applications the energy losses occur through a combination of heat conduction from workpiece to the target material, natural convection, and thermal radiation. Magnetic materials improve the induction heat process because of phenomenon known as hysteresis.

Practicalities of Induction Heating
The fundamental workcoil-workpiece configuration can be adapted to almost any design. The variability in direct and indirect designs is significant with induction heating; an example of direct heating would be when welding two steel items together, whereas an example of indirect heating would be an induction cooker where the hot-plate is heated and the food is placed on the hotplate to cook. This flexibility is one of the main reasons induction heating is so widely used across industry.

The frequency of AC used has several practical implications of how the workpiece is heated. Lower frequencies lead to deeper magnetic-field penetration depths and hence deeper heating profiles. Higher frequencies will result in greater surface heating with respect to the rest of the material. When heated, heat flux flows within the material from the hot to cold regions; hence the longer the workcoil is coupled to the workpiece the deeper the heat penetrates into the material through heat conduction. This will serve to distribute the heat evenly resulting on a more uniform heating profile across the whole workpiece. Where the workpiece is used as an appliance to heat another material, this results in greatly improved heating uniformity. For short-term heating, the greatest heating will be where the magnetic field and hence induced eddy current concentration is greatest. Since the magnetic field can be focused in a very small space through careful workcoil-workpiece design, highly-focused heating can be performed.

Applications of induction heating range from metal processing, for example induction welding, induction brazing, and induction hardening, through to forming and mouldings for the manufacture of plastic components. One of the most common applications is in the food industry, where induction heating is used to replace indirect heating by gas or electric heating elements

Advantages of Induction Heating
The advantages of induction heating for industrial processes are:

• Non-contact – There is no contact between the workcoil and workpiece; this is of importance in direct heating applications, such as welding and brazing, where there may be risk of cross-contamination.

• Rapid Heating – Induction heating can dissipate extremely high power densities within a workpiece resulting in very fast heating times. This is an advantage for direct heating applications where the need to minimise the effects of heat conduction are essential to avoid damaging the workpiece. It is also an advantage in indirect heating applications where heating times are an important parameter.

• Efficiency – Induction heating can be considerably more efficient than conventional heating; up to twice the level of efficiency.

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