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Induction heating

Induction heating is the process of heating a metal object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. An induction heater (for any process) consists of an electromagnet, through which a high-frequency Alternating current (AC) is passed. Heat may also be generated by magnetic hysteresis losses.

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Applications of induction heating

Induction heating allows the precision heating of an applicable item, for applications from surface hardening to melting. Often, iron and its alloys respond best to induction heating, due to their ferromagnetic nature. Eddy currents can, however, be generated in any conductor, and magnetic hysteresis can occur in any magnetic material.

Induction furnace

An induction furnace uses induction to heat a metal to its melting point. Once molten, the high-frequency magnetic field can also be used to stir the hot metal, which is useful in ensuring that 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 a reverberatory furnace or a cupola. Sizes range from a kilogram of capacity, to a hundred tonnes capacity. 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, aluminium, and precious metals.

Induction 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.

Induction cooking

In induction cooking, an induction coil in the cook-top heats the iron base of cookware. Copper bottomed pans, aluminium pans and most stainless steel pans are not suitable.

The heat induced in the base is transferred to the food via conduction. Benefits of induction cookers include efficiency, safety (the induction cook-top is not heated itself) and speed. Drawbacks include the fact that non-ferrous cookware such as copper, aluminium and glass cannot be used on an induction cook-top. Both installed and portable induction cookers are available.

Induction sealing

Induction heating is often used in Induction sealing or "cap sealing".

Heat treatment

Induction heating is often used in the heat treatment of metal items. The most common applications are induction hardening of steel parts and induction soldering/brazing as a means of joining metal components. Induction heating can produce high power densities which allow short interaction times to reach the required temperature. This gives tight control of the heating 'pattern' with the pattern following the applied magnetic field quite closely and allows reduced themal distortion and damage. This ability can be used in hardening to produce parts with varying properties. The most common hardening process is to produce a localised surface hardening of an area that needs wear-resistance, while retaining the toughness of the original structure as needed elsewhere. The depth of induction hardened patterns can be controlled through choice of induction-frequency, power-density and interaction time. There are limits to the flexibility of the process - mainly arising from the need to produce dedicated inductors for many applications. This is quite expensive and requires the marshalling of high current-densities in small copper inductors, which can require specialized engineering and 'copper-fitting'.

Details

The basic setup is an AC power supply that output electricity with low voltage but very high current and high frequency. The workpiece to heat is placed inside an air coil driven by the power supply. The alternating magnetic field induces eddy currents in the workpiece.

Suitable frequency:

Frequency [kHz]	Workpiece type
5 - 30	Thick materials
100 - 400	Small workpieces or shallow penetration
480	Microscopic pieces

Magnetic materials improve the induction heat process because of hysteresis. In essence materials with high permeability (100-500) are easier to heat with induction heating. Hysteresis heating occurs below the curie temperature where materials loose their magnetic properties.

So high permability and temperatures below curie temperature in the workpiece is useful. Also temperature difference, mass, and specific heat influence the workpiece heating.

The energy transfer of induction heating is coupled to the distance between the coil and the workpiece. Energy losses occur through heat conduction from workpiece to fixture, natural convection, and thermal radiation.

The induction coil is usually made of 3.175 mm - 4.7625 mm diameter copper tubing and fluid cooled. Diameter, shape, and number of turns influence the efficiency and field pattern. ^[1]

Notes

^ Induction Heating Fundamentals. 070710 ameritherm.com