Basics of induction heating, Part 5: Do-It-Yourself versions

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

We conclude this article by looking at how advanced amateurs can build their own induction-heating system. Induction heaters are among the many dramatic projects constructed by experimenters and hobbyists. After all, it’s impressive to heat up that metal sample in a few seconds.

Just as with Tesla coils, there are websites that go into detail describing how to build your own induction-heating system. Unlike Tesla coils, the electronic parts needed are fairly standard, as is the copper or other tubing needed for the primary coil. Also, unlike Tesla coils, the voltages used are much lower, so there is far less danger of shock, while insulation requirements and clearance dimensions are greatly reduced.

Still, building your own system is an ambitious project. Whenever you have large currents – as is inherent in induction heating – the component values are critical, and seemingly trivial changes can affect power amplifier performance and operation. Also, the high currents mean that ohmic losses at contact and connections are an issue, and these can actually overheat themselves through I²R self-heating with large current flows.

As with the Tesla coil, the schematic of an induction heater is fairly simple (Figure 1). The oscillator is based on the classic Royer design (devised in 1954 by George H. Royer). This relaxation oscillator uses a saturable-core transformer and creates square waves with just a few basic components. (This oscillator topology is also used in some unrelated DC/AC inverter circuits.) In most implementations, including the induction heater, the coarse-square wave output is often modified to be a more sine-like wave using a resonant output-tank circuit (C1/L1).

Fig 1: The basic schematic of an induction heater circuit is relatively simple and is usually based on the classic Royer oscillator. (Image: RM Cybernetics)

The objective of the PA is to deliver large amounts of current at a specified frequency to the low-impedance primary coil (substitute “antenna” for “coil” here, and the similarities between a low-frequency broadcast transmitter and an induction PA are clear). In contrast, the Tesla-coil power supply needs to deliver low current but at very high voltage.

The typical power supply for induction-heating projects starts with a modest-level standard AC/DC supply rated for 15 VDC/20 A, which is then used for the PA/oscillator circuit. Suppose a commercial line-operated AC/DC supply is used. In that case, the rest of the circuit is at relatively low voltage (typically, under 100 V), so it is dangerous – but not as dangerous as a Tesla coil which is at thousands of volts and prone to all sorts of voltage-induced material breakdowns.

A big challenge is in fabricating the special electromechanical components such as the primary coil and their assembly and connection. Small changes and seemingly insignificant issues can have a major impact given the current levels and affect the ability of the circuit to function well or at all. Large-gauge wires, connections, and fasteners, along with solid construction, are critical. You can even buy kits with all or most electronic parts and a circuit board, but there are still many parts you have to fabricate yourself.

In many industrial and some hobbyist designs, the self-heating of the primary coil due to unavoidable I²R loss (large currents through the coil’s DC resistance, combined with heat radiating from the workpiece back to the primary coil, can overheat the primary coil material itself. Water cooling is often used with water pumped through the primary coil tubing to prevent this occurrence. This obviously complicates the physical design and assembly (Figure 2).

Fig 2: Even a small induction heater may need cooling of its primary coil, which complicates the physical arrangement; note the fish-tank pump on the left with the clear plastic tubing going to the coiled pipe, which is also electrically connected to the amplifier. (Image: RM Cybernetics)

Conclusion

Induction heaters play an important role in industry and research as a non-contact, efficient, electrically driven, and very controllable technique for heating and even melting various metals. It is widely used and analyzed by scientists and metallurgists who have investigated both the underlying theory and the critical performance details, along with the efforts of electrical-power and mechanical engineers who have built the needed circuits and systems. It is also amenable to DIY projects by serious experimenters, although it should be approached with caution due to the associated currents, voltages, and temperatures.

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