Induction heating

Overview


Induction heating is the process of heating an electrically conducting object (usually a metal) 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 in materials that have significant relative permeability. The frequency of AC used depends on the object size, material type, coupling (between the work coil and the object to be heated) and the penetration depth.

Applications of induction heating
Induction heating allows the targeted heating of an applicable item, for applications including; surface hardening, melting, brazing and soldering, heating to fit, and many more. 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 heating has been used to heat liquid conductors (such as molten metals) and also gaseous conductors (such as a gas plasma). Induction heating is often used to heat graphite crucibles (containing other materials) and is used extensively in the semiconductor industry for the heating of silicon and other semiconductors.



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. Because it is a clean and non contact process it can be used in a vacuum or inert atmosphere. Vacuum furnaces make use of induction heating for the production of specialty steels and other alloys that would oxidise if heated in the presence of air.

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 tube welding
Induction heating is used in the manufacture of seam welded tube. 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 the seam is welded. The RF current can also be conveyed to the tube by brushes, but the result is still the same - the current flows along the open seam, heating it.

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 Brazing
Often used in higher production runs and produces uniform results and is very repeatable.

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

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

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 thermal 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:

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 lose 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.