How does the induction heater work?

Process of heating an electrically conducting object by electromagnetic induction

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This article is about Induction heating. It should not be confused with Joule heating

Component of Stirling radioisotope generator is heated by induction during testing

Induction heating is the process of heating electrically conductive materials, such as metals or semiconductors, through electromagnetic induction. This involves heat transfer through an inductor that creates an electromagnetic field within a coil, which in turn heats or melts materials like steel, copper, brass, graphite, gold, silver, aluminum, or carbide.

An important feature of induction heating is that the heat is generated within the object being heated, rather than from an external heat source. This allows objects to be heated very quickly. Additionally, there is no need for external contact, which is important when contamination is a concern. Induction heating is used in various industrial processes such as heat treatment in metallurgy, Czochralski crystal growth, and zone refining for the semiconductor industry. It is also used to melt refractory metals that require very high temperatures, and in induction cooktops.

An induction heater consists of an electromagnet and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet. The rapidly alternating magnetic field penetrates the object, generating electric currents inside the conductor called eddy currents. These eddy currents flow through the material's resistance, heating it by Joule heating. In ferromagnetic and ferrimagnetic materials, such as iron, additional heat is generated by magnetic hysteresis losses. The frequency of the electric current used for induction heating depends on the object's size, material type, coupling (between the work coil and the object), and the penetration depth.

Applications

Induction heating of 25mm metal bar using 15kW at 450kHz

Induction heating allows the targeted heating of an applicable item for applications including surface hardening, melting, brazing, soldering, and heating to fit. Due to their ferromagnetic nature, iron and its alloys respond best to induction heating. Eddy currents can also be generated in any conductor, and magnetic hysteresis can occur in any magnetic material. Induction heating is used to heat liquid conductors, such as molten metals, and gaseous conductors, such as a gas plasma – see Induction plasma technology. Induction heating is also used extensively in the semiconductor industry for heating silicon and other semiconductors. Utility frequency (50/60Hz) induction heating is used for many lower-cost industrial applications as inverters are not required.

Furnace

An induction furnace uses induction to heat metal to its melting point. Once molten, the high-frequency magnetic field can also be used to stir the hot metal, ensuring alloying additions are fully mixed into the melt. Most induction furnaces consist of a tube of water-cooled copper rings surrounding a container of refractory material. Induction furnaces are used in most modern foundries as a cleaner method of melting metals than reverberatory furnaces or cupolas. Sizes range from a kilogram of capacity to a hundred tonnes. Induction furnaces often emit a high-pitched whine or hum when they are running, depending on their operating frequency. Metals melted include iron and steel, copper, aluminum, and precious metals. Because it is a clean and non-contact process, it can be used in a vacuum or inert atmosphere. Vacuum furnaces use induction heating to produce specialty steels and other alloys that would oxidize if heated in the presence of air.

Welding

A similar, smaller-scale process is used for induction welding. Plastics may also be welded by induction if they are either doped with ferromagnetic ceramics, where magnetic hysteresis of the particles provides the heat required, or by metallic particles.

Seams of tubes can be welded this way. The currents induced in a tube run along the open seam and heat the edges, resulting in a temperature high enough for welding. At this point, the seam edges are forced together and welded. The RF current can also be conveyed to the tube by brushes, but the result is the same – the current flows along the open seam, heating it.

Manufacturing

In the Rapid Induction Printing metal additive printing process, a conductive wire feedstock and shielding gas are fed through a coiled nozzle, subjecting the feedstock to induction heating and ejection from the nozzle as a liquid, to fuse under shielding to form three-dimensional metal structures. The core benefit of induction heating in this process is significantly greater energy and material efficiency, as well as a higher degree of safety compared to other additive manufacturing methods, such as selective laser sintering, which uses a powerful laser or electron beam to deliver heat to the material.

Cooking

In induction cooking, an induction coil inside the cooktop heats the iron base of cookware by magnetic induction. Induction cookers provide safety, efficiency (the cooktop itself is not heated), and speed. Non-ferrous pans, such as copper-bottomed and aluminum pans, are generally unsuitable. The heat induced in the base is transferred to the food inside by thermal conduction.

Brazing

Induction brazing is often used in higher production runs. It produces uniform results and is very repeatable. There are many types of industrial equipment where induction brazing is used. For instance, induction is used for brazing carbide to a shaft.

Sealing

Induction heating is used in cap sealing of containers in the food and pharmaceutical industries. A layer of aluminum foil is placed over the bottle or jar opening and heated by induction to fuse it to the container. This provides a tamper-resistant seal since altering the contents requires breaking the foil.

Heating to Fit

Induction heating is often used to heat an item, causing it to expand before fitting or assembly. Bearings are routinely heated in this way using utility frequency (50/60Hz) and a laminated steel transformer-type core passing through the center of the bearing.

Heat Treatment

Induction heating is commonly used for the heat treatment of metal items. The most typical applications include induction hardening of steel parts, induction soldering/brazing to join metal components, and induction annealing to selectively soften an area of a steel part.

Induction heating can produce high-power densities, allowing short interaction times to achieve the required temperature. This ensures tight control of the heating pattern, which follows the applied magnetic field closely, reducing thermal distortion and damage.

The process can be used in hardening to produce parts with varying properties. The most common process is to produce localized surface hardening for wear resistance while retaining the toughness of the original structure where needed. The depth of induction-hardened patterns can be controlled through the choice of induction frequency, power density, and interaction time.

The flexibility of the process is limited by the need to create dedicated inductors for many applications. This can be expensive and requires handling high-current densities in small copper inductors, which can require specialized engineering.

Plastic Processing

Induction heating is used in plastic injection molding machines to improve energy efficiency for injection and extrusion processes. Heat is directly generated in the barrel of the machine, reducing warm-up time and energy consumption. The induction coil can be placed outside thermal insulation, so it operates at low temperatures and has a long life. The frequency used ranges from 30kHz down to 5kHz, decreasing for thicker barrels. The reduction in the cost of inverter equipment has made induction heating increasingly popular. Induction heating can also be applied to molds, offering a more even mold temperature and improved product quality.

Pyrolysis

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Induction heating is used to obtain biochar in the pyrolysis of biomass. Heat is directly generated into shaker reactor walls, enabling the pyrolysis of the biomass with good mixing and temperature control.

Details

The basic setup involves an AC power supply that provides low voltage but very high current and high frequency. The workpiece to be heated is placed inside an air coil driven by the power supply, usually in combination with a resonant tank capacitor to increase the reactive power. The alternating magnetic field induces eddy currents in the workpiece.

The frequency of the inductive current determines the depth at which the induced eddy currents penetrate the workpiece. For a solid round bar, the induced current decreases exponentially from the surface. The penetration depth, where 86% of power is concentrated, can be derived as:

    δ = 503 * sqrt(ρ / (μ * f))
  

Where δ is the depth in meters, ρ is the resistivity of the workpiece in ohm-meters, μ is the dimensionless relative magnetic permeability of the workpiece, and f is the frequency of the AC field in Hz. The efficiency is a function of the workpiece diameter over the reference depth, increasing rapidly up to a point. Decreasing the reference depth requires increasing the frequency. Since the cost of induction power supplies increases with frequency, supplies are often optimized to achieve a critical frequency. Below this frequency, heating efficiency is reduced.

Relative depth varies with temperature because resistivities and permeability vary with temperature. For steel, the relative permeability drops to 1 above the Curie temperature. This variation can be significant for nonmagnetic conductors and even more so for magnetic steels.

Frequency (kHz)	Workpiece type
5–30	Thick materials (e.g., steel at 815°C with diameter 50mm or greater).
100–400	Small workpieces or shallow penetration (e.g., steel at 815°C with diameter of 5–10mm or steel at 25°C with a diameter around 0.1mm).
480	Microscopic pieces

Magnetic materials improve induction heating efficiency due to hysteresis. High permeability materials are easier to heat using induction. Hysteresis heating occurs below the Curie temperature, where materials retain their magnetic properties. Temperature difference, mass, and specific heat influence the workpiece heating.

Energy transfer in induction heating is affected by the distance between the coil and the workpiece. Energy losses occur through heat conduction, natural convection, and thermal radiation. The induction coil is usually made of copper tubing and fluid coolant. The diameter, shape, and number of turns influence the efficiency and field pattern.

Core Type Furnace

The furnace comprises a circular hearth that contains the charge to be melted in the form of a ring. The metal ring is large and is magnetically interlinked with an electrical winding energized by an AC source. Essentially, it is a transformer where the charge to be heated forms a single-turn short circuit secondary, magnetically coupled to the primary by an iron core.

How Induction Heating Works

What is Induction Heating?

Induction heating is a highly efficient and fast method that uses a magnetic field to heat conductive materials, such as metals and semiconductors, without contact. This method has become increasingly popular for industrial, medical, and domestic applications due to its many advantages over traditional heating techniques such as resistance, flame, and ovens/furnaces. Induction heating is beneficial for highly precise or repetitive operations, where consistent heating and temperature control are critical for the quality and repeatability of the end product.

Basics of Induction Heating

In induction heating, an alternating current (AC) source supplies current to an induction heating coil. As a result, the coil generates an alternating magnetic field. When an object is placed in this field, two heating effects occur:

• Hysteresis losses – These occur only in magnetic materials, such as iron, nickel, cobalt, due to the friction between the molecules when continuously magnetized in different directions. Higher magnetic field oscillation frequency results in faster particle movement, causing more friction and, thus, more heat.
• Eddy-current losses – These occur as a Joule heating effect in any conductive material because of the electric currents induced by the fluctuating magnetic field.

Both effects result in the heating of the treated object, but the latter is commonly the main heat source in IH processes. Hysteresis is not observed in non-magnetic materials, and magnetic materials lose their magnetic properties if heated above a specific temperature (the Curie point).

Eddy currents also depend on the magnetic field frequency due to the skin effect – at high frequencies, the currents flow close to the conductor surface. This specificity controls the penetration depth of the induction heating process. As a result, either the whole object or only a specific part of it can be heated. Therefore, induction heating can be used for different applications – from metal melting to brazing and surface hardening.

Skin effect is also observed inside the induction coil conductor. Therefore, pipes can be used instead of solid wires. When the current flows through the inductor, similar resistive losses are observed due to the Joule effect. Water cooling is often applied to prevent the coil from melting and damage.

Advantages of Induction Heating

• Reduced time – Induction heating heats the target directly, reducing heating time and wasted heat. This method provides high power density and low or no thermal inertia.
• High efficiency – Efficiency values higher than 90% are obtained with proper power converter and coil design. High temperatures can be reached quickly and easily, and ambient heat loss is significantly reduced.
• Improved control – Precise regulation of the heating power can be achieved via appropriate coil design and power converter control. This allows for features such as local heating, pre-heating, and predefined temperature profiles.
• Industrial automation option – Induction heating improves both productivity and process quality. The contactless heating ensures no interference by the heating tool.
• Safety and cleanliness – There is no thermal or air pollution as the target is heated directly, and no fuel substances are used.

Innovations and Future Development

Induction heating systems, while mature, continuously evolve with modern technology advancements. The following areas are expected to see significant interest in the coming years:

• Efficiency improvement – Induction heating systems with higher efficiency are anticipated due to semiconductor technology improvements. Special coil shapes and designs will also enhance efficiency.
• Induction heaters with multiple coils – Better heat distribution, higher performance, and flexibility can be achieved using several simultaneously-operating coils. These systems represent a major technological breakthrough and are increasingly implemented in both industrial and domestic applications.
• Advanced control – Robust control algorithms are needed to ensure proper power converter operation for different induction heating loads and operating points. Improved performance is expected through real-time identification control units with adaptive algorithms.
• Special applications – The range of induction heating applications is expected to grow with technological development. This includes heating low-resistivity materials and biological tissues for medical purposes. Many other applications require further research to optimize process parameters.

History of Induction Heating

Induction heating was first discovered by Michael Faraday during his studies of electromagnetic induction. The principles were further developed by James C. Maxwell in his unified theory of electromagnetism. James P. Joule first described the heating effect of a current flowing through a material.

In 1887, Sebastian Z. de Ferranti proposed induction heating for metal melting and filed the first patent in this field. The first fully-functional induction furnace was presented in 1891 by F. A. Kjellin, with high-frequency furnace applications by Edwin F. Northrup in 1916.

During World War II and afterward, induction heating technology grew, driven by the aircraft and automotive industries. The technology's range expanded to advanced material treatment, significantly increasing its applications.

The development of solid-state generators using new power semiconductor technologies pushed IH beyond industrial environments. Since the late 1980s, domestic applications have emerged. Recently, there has been a significant interest in induction heating for medical treatments due to its precise and targeted heating.

Today, induction heating technology provides highly efficient and reliable systems for a wide variety of applications.

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