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What is important when designing induction coils?

What is important when designing induction coils?

Correctly designed and built induction coils are absolutely critical for successful, cost-effective induction heating. In fact, designing and testing coils is often the process with the longest lead time when devising an induction heating solution.

A key reason for this is the fact that coils are task specific. They must be designed to achieve specific results on specific materials under specific conditions. There are no—or at least there shouldn’t be—‘off-the-shelf’ coil designs.

Rigorous testing of a coil’s design and construction is essential. Too few people realize that coils are often the part most exposed to harsh operating conditions. Testing and computer-aided simulation is therefore sometimes needed to arrive at a design that is both safe and fatigue resistant. And of course, it takes repeated testing to achieve optimal part-heating patterns.

Nothing can be taken for granted when designing induction coils. With very high power density coils, for example, one even needs to determine the correct speed at which cooling water should flow through the coil. Too low a speed will result in insufficient thermal transference. But even when the  correct speed has been found, the coil designer must decide whether a booster pump is necessary in order to achieve and maintain the desired water through-flow rate. The competent coil designer will also specify a purity level for the cooling water, in order to  minimize corrosion on the inside of the coil. So something as apparently straightforward as the coil’s water, is in fact a complex matter demanding technical competence and specialist equipment.

Magnetic flux concentrators are another area of an overall induction solution that at first glance seems relatively straightforward. As the name suggests, the main function of such concentrators is to concentrate the coil’s current in the area of the coil facing the workpiece. Without a concentrator, much of the magnetic flux is free to propagate around the coil. This uncontrolled flux will then ‘engulf’ adjacent conductive components. But when channeled by a concentrator, the magnetic flux can be restricted to precisely defined areas of the workpiece, resulting in the localized heating zones characteristic of induction heating. Many variables must be considered when making flux concentrators. The workpiece’s material, the coil’s shape, the application—each influences the concentrator’s final design. Even deciding what material to use for the concentrator can be a complicated task. Basically, concentrators are made from  laminations, or from pure ferrites and  ferrite- or iron-based powders.

Each concentrator material has its own drawbacks and advantages. Laminations have the highest flux densities and magnetic permeability; they are also less expensive as parts than iron- and ferrite-based powders. Laminations must however be stamped to a few standardized sizes and are therefore less flexible. They are also labor intensive to mount. Pure ferrites can also offer outstanding magnetic permeability. However, they suffer from low saturation flux density, and their brittleness makes them difficult to machine (diamond-tipped cutters must be used). Iron-powders are easy to shape, offer high flux densities, and are easy to shape. But great care must be taken to provide against over-heating, as internal losses or heat transfer from the heated part means such powders have a relatively low working temperature.

Of course, many other factors need to be considered when designing induction coils. Correct impedance matching between the coil and the power source, for instance, is crucial in order to use the full power from the power source. Plus the fact that coils need five to ten times as much  reactive as active power. Then there is the science of choosing the appropriate electrical insulation: should the coil be dipped in an epoxy coating, or should it be molded with high-temperature concrete? Again, these are complicated decisions influenced by several variables. As we have seen, a professionally designed and fabricated induction coil is an advanced, complex  component. Unfortunately, too many induction users persist in viewing coils as low-tech copper tubes. The results of this misconception are incorrect—even dangerous—coil designs, amateurish repairs, insufficient or incorrect maintenance, and ultimately, process and equipment failures.