Basics of induction heating, Part 3: Implementation

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

The induction coil and workpiece are at the system’s core, but there’s much more to an induction-heating arrangement (Figure 1). In addition, there’s a power supply and resonant-tank circuit as a power amplifier which creates the primary-side AC. Note that the primary-side coil is part of this tank circuit. The amplifier resonant tank circuit must be tuned to the operating frequency and primary-side coil inductance to ensure consistent oscillation and maximum power transfer.

Fig 1: The functional block diagram of an induction-heating system is relatively simple, and usually includes a non-contact temperature sensing arrangement for closed-loop control. (Image: AZO Materials)

Due to the high power levels of tens of kilowatts in many designs, the power supply or primary coil may need water cooling. There is also a non-contact temperature sensing via a pyrometer to provide a reading which can be used in a closed loop to maintain the desired temperature at the workpiece.

The power amplifier is similar to and very different from an RF power amplifier and is often referred to as an RF amplifier. It is similar since it must produce a powerful sine-like wave-like signal. It is different since the frequencies it must produce are at the very lower end of the electromagnetic spectrum. The waveform it generates does not have to be a pure sine wave, and some reasonable amount of distortion is acceptable, so a more-efficient Class B or even Class-C amplifier could be used. But any non-sinusoidal distortion also creates considerable EMI/RFI, which can be a problem.

The frequency of the AC current in the primary coil and thus induced into the workpiece or load is critical. The depth to which heat is generated directly (not by eventual conduction from the surface) using the induced current depends on what is called the electrical reference depth or skin depth (note, this is somewhat analogous to the “skin effect” in conductors carrying RF signals but is due to a different mechanism). A brief but nonlinear equation defines the relationship among the key material and frequency parameters (Figure 2).

Fig 2: This basic equation defines the relationship among the key material and frequency parameters and associated electrical reference depth (skin depth). (Image: Bright Hub Engineering)

Higher-frequency current results in a shallower electrical reference depth, while a lower frequency current will result in a deeper electrical reference depth. This depth also depends on the electrical and physical properties of the workpiece.

Frequencies of 100-to-400 kHz produce relatively high-energy heat, best suited for quick heating of small parts or the surface/skin of larger parts (for comparison, note that the standard AM broadcast-radio band starts at 550 kHz). For deep, penetrating heat, longer heating cycles at lower frequencies of 5-to-30 kHz are used. For smaller workpieces, a higher frequency (>50kHz) is needed for efficient heating, and in cases of larger workpieces, a lower frequency (>10kHz) and more penetration of the heat are generated.

Varying the current, voltage, and frequency through an induction coil results in finely tuned and carefully engineered heating, making it suitable for precise applications like case hardening, tempering, and annealing as well other forms of heat treating. This thermal and temperature precision level is critical for many applications, including heat treatment of automotive and aerospace parts, fiber optics, ammunition bonding, wire hardening, and tempering of spring wire.

Induction heating is well suited for specialty metal applications involving titanium, precious metals, and advanced composites. It is not restricted to either ferrous or non-ferrous metals but can be used with both with the proper settings and adjustments. For example, using the same induction process to heat the same-size pieces of steel and copper will produce very different results. The reason is that steel (as well as carbon, tin, and tungsten) has high electrical resistivity and strongly resists the current flow, causing heat to build up quickly. In contrast, low-resistivity metals such as copper, brass, and aluminum take longer to heat. As an added complication, resistivity increases with temperature, so a very hot piece of steel will be more receptive to induction heating than a cold piece.

A properly designed and installed induction-heating system is a precise and fully controlled technique. All aspects of this technology have been analyzed, studied, and tested in theory and practice by physicists, metallurgists, and engineers and are well understood. The heat it induces is highly localized in the workpiece, although it will, of course, spread due to internal thermal conduction. It is relatively “clean” in that it does not generate fumes, exhaust, or residue. It is even used for highly focused spot welding of electrical contacts as well as continuous-strip welding with the system parameters set for a very shallow electrical reference depths.

The next part of this article takes induction heating to a consumer-appliance application and also provides a look at do-it-yourself advanced hobbyist systems.

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