What is induction heating

Induction heating is electric heating created using electromagnetic induction principles. An electrical-conducive metal is placed inside a coil which is connected to an alternating current power source. The alternating magnetic poles in the core of the coil are induced into the metal and this metal subsequently flows with eddy currents. In principle, the system is a transformer, in which a metal billet(in principle, a short-circuited coil)is the secondary coil with is placed into the primary coil, which is referred to in this industry as the inductor. The eddy currents heat the metal piece placed in the winding core. The heat is transferred to the billet using alternating magnetic poles, with some normal heat loss (as is the case with most indirect heating systems), and this heat builds inside the billet. All other pieces in the surrounding area can be cool. This is the big benefit of induction heating.

The heat in the billet is not distributed evenly across the entire cross-section of the billet. For example, while heating a cylinder-shaped billet, the highest current density is on the surface, and the closer to the center, this density decreases almost exponentially. This is called the "skin effect."

j_x = J_o e^-kx

The depth at which the current density value drops to J_o/e, that is, to 0,368 density on the surface, is called the skin depth δ

kde je:

• ω = 2πf angular frequency, f is frequency
• ρ resistivity of the conductive material
• µ_o absolute magnetic permeability (4π x 10-7Hm-1)
• µ_r average permeability of the billet material.

In practice, it is desirable to modify these relationships:

In the surface-most layer of the skin depth 86.5% of all the heat is created; at two times the skin depth, 98% heat is created, and at three times the skin depth, 99.8% heat is created (these calculations apply to a cylinder-shaped billet with a diameter larger than 8 δ).

Of course, the skin depth is dependant on the inductor current frequency and the specific resistance and the proportional permeability of the material during operating temperatures.

For illustration, we present skin depths for copper and carbonaceous steel(mm):

When considering operating costs, the heating efficiency is interesting. Approximately, the efficiency η can be assessed from the association

kde je:

• D internal diameter of the inductor core
• d material diameter
• δ skin depth
• ρ₁ average resistance of the inductor material
• ρ₂ average resistance of the billet material
• µ_r proportional permeability of the billet material.

The efficiency of D/d is proportionately lessened because the bonds between the magnetic poles of the inductor and material are reduced. For this reason, it is not efficient to use one inductor for a large diameter range of materials. The efficiency is reduced even when the the proportions δ/d are increased. A low value of δ/d is used, for example, for surface hardening, where a quick heating/cooling process is used on a thin surface layer.

For forming (forging) is is necessary that the material be heated as evenly as possible. For this reason, a slower heating process is chosen to allow the heat to be carried to the center of the material. Evenly heating also helps increase the skin depth. A frequency compromise is chosen for achieving the necessary warm-through while attaining good energy transfer from the inductor to the material.

Experience has shown the following operating parameters for various sizes of carbonaceous steel heated to 1200°C:

For heating strip-shaped material(height different than width), the thickness of the strip must be larger than 2.5 times the skin depth. In lesser thicknesses, radiotransperancy takes place and the heating efficiency is reduced, and this is necessary to take into consideration during the design phase of the heating equipment.

For powering an inductor with frequencies higher than that supplied by a standard power grid (50Hz), a static thyristor or transistor frequency converter is used.

ROBOTERM spol. s r.o. in Chotěboř manufacturers thyristor-based frequency converters from 25 to 1200 kW with frequencies to 8 kHz, and transistor-based models to 200 kW with frequencies of up to 25 kHz .

Induction heating enables good heat stabilization of the heated material. The main component in the control system is a programmable logic controller, while heating is typically measured using non-contact pyrometers. In the case of aluminum alloy heating, thermocouples are also used.

One of the further benefits of induction heating is the capability of its mechanization, and in many cases automatization. This reduces the human work load, as is typically desired when working with most high-power equipment.

Induction heating can be pratically applied in industries such as:

• for forming (forging) – likely the most common industry using this technology, where it is important to heat the entire piece evenly
• for melting iron and nonferrous metals with low and mid-frequencies
• for surface hardening – ROBOTERM spol. s r.o. Chotěboř also partners with external technology firms for the manufacture of hardening equipment
• for soldering – between the the soldered metal parts, solder is placed, the part set is placed into an inductor, and the solder is liquified
• for pressing while hot – uses the temperature expansivity of metal
• for special technology – welding, plasma, vacuum melting, maintaining the temperature of melted glass. Presently, ROBOTERM spol. s r.o. Chotěboř has not yet encountered these types of installations.