What is induction heating

Arrangement of Induction heating

Induction heating is electric heating created using electromagnetic induction principles. An electrical-conducive workpiece is placed inside a coil which is connected to an alternating current power source. The alternating magnetic field induces eddy currents in the workpiece. In principle, the system is a transformer, where the workpiece is a secondary short-circuited coil placed inside the primary induction coil, which is in induction heaters called an inductor. The eddy currents heat the inserted workpiece. The heat is transferred to the workpiece by an alternating electromagnetic field, ie not by a temperature gradient as in the case of indirect heating, and is generated directly in the workpiece. All surrounding components can be cool. This is the big benefit of induction heating.

The heat is not generated evenly across the entire cross-section of the workpiece. For example, when heating a cylindrical billet, the highest current density is on the surface, and decreases approximately exponentially to the center. This phenomenon 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

where is:

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

In practice, it is desirable to modify this relationship:

Online skin depth calculator

In the skin depth 86.5% of all the heat is generated; at two times the skin depth, 98% heat is generated, and at three times the skin depth, 99.8% heat is generated (these calculations apply to a cylindrical workpiece with a diameter greater than 8 δ).

The skin depth depends on the current frequency and on the resistivity and relative permeability of the workpiece at the operating temperature.

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

When considering operating costs, the efficiency of energy transfer from the inductor to the workpiece is interesting - the so-called electrical heating efficiency. For a long coil and a cylindrical workpiece, the electrical efficiency η_e can be approximately calculated from:

where is:

• D internal diameter of the coil
• d workpiece diameter
• δ skin depth
• ρ₁ coil resistivity
• ρ₂ workpiece resistivity
• µ_r workpiece relative permeability.

The electrical efficiency decreases as the D/d ratio increases because the coupling of the inductor's magnetic field to the workpiece is reduced. For this reason, it is not efficient to use one inductor for a large diameter range of workpieces. The efficiency also decreases with increasing skin depth of the workpiece. A low penetration depth is used, for example, for surface hardening, where a quick heating/cooling process is used on a thin surface layer.

For forming (forging) it is necessary to heat the workpiece as homogenously as possible. For this reason, a slower heating process is chosen to allow the heat to be soaked to the core. However, too slow heating worsens thermal efficiency because the time in which material heated to high temperatures loses heat by radiation is prolonged. Increasing the skin depth also helps to heat up the core faster. The frequency compromise is chosen to achieve the required in-depth heating with good electrical efficiency.

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

For heating strips and slabs, the thickness of the workpiece must be larger than 2.5 times the skin depth. In lesser thicknesses, the workpiece becomes transparent for the electromagnetic waves and the heating efficiency is reduced. This must be taken into consideration when designing the induction heating device.

Thyristor or transistor frequency converters are used to supply inductors at a frequency higher than in the distribution network (50 Hz). For supplying the power circuits of the frequency converter, an isolating transformer is mainly used, which modifies the mains voltage to the required input voltage of the converter. Its other function is the galvanic separation of the power circuits of the converter and inductor from the mains. For safety reasons, the insulation resistance of the inductor against the ground potential is measured. The delta/star connection of the transformer suppresses the disturbing penetration of odd harmonic components into the network. Compensating capacitors are connected in parallel to the inductor. Their capacitance together with the inductance of the inductor determines the operating (resonant) frequency of the converter. In addition, these capacitors compensate for the phase-shifted component of the current flowing through the inductor. As a result, several times lower current flows through the inverter output than through the inductor.

Induction heating enables good heat stabilization of the heating process. Programmable logic controllers are mostly used to control the heating process. Temperature is usually measured by non-contact pyrometers. In the case of aluminum and its alloys heating, thermocouples are also used.

One of the further benefits of induction heating is the capability of its easy mechanization and automation, which reduces the human work and is necessary in case of high production rates.

Any electrically conductive materials can be heated by induction. Induction heating of ferromagnetic materials is particularly advantageous because it achieves a very high efficiency. The workpiece can be solid, liquid or plasma. Non-conductive materials can be heated by induction in conductive crucible or pot (induction cooker).

Thanks to its advantages, induction heating has a wide application:

• Forming, forging – it is important to heat the formed part homogenously
• Melting - the crucible is often heated by induction (graphite crucible for melting various metals)
• Surface hardening - the surface is heated locally to a small depth
• Annealing - the part is heated completely and kept at a temperature for a certain time
• Soldering - solder is inserted between components that are placed in the inductor and the solder is melted
  Shrink fitting - thermal expansion of metals is used
• Welding - during longitudinal continuous welding of pipes with induction, the edges of the joint are locally melted
• plasma technology - plasma is heated by induction
• cooking - induction usually heats the bottom of a pot made of ferromagnetic material