Induction heating (continued.....)

Induction heating is a method of providing fast, consistent heat for manufacturing applications which involve bonding or changing the properties of metals or other electrically-conductive materials. The process relies on induced electrical currents within the material to produce heat. Although the basic principles of induction are well known, modern advances in solid state technology have made induction heating a remarkably simple, cost-effective heating method for applications which involve joining, treating, heating and materials testing.

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.The basic components of an induction heating system are an AC power supply, induction coil, and workpiece (material to be heated or treated). The power supply sends alternating current through the coil, generating a magnetic field. When the workpiece is placed in the coil, the magnetic field induces eddy currents in the workpiece, generating precise amounts of clean, localized heat without any physical contact between the coil and the workpiece.

There is a relationship between the frequency of the RF field and the depth to which it penetrates your workpiece; low frequencies (up to 30kHz) are effective for thicker materials requiring deep heat penetration, while higher frequencies (100 to 400kHz) are effective for smaller parts or shallow penetration. The higher the frequency, the higher the heat rate.

Due to the effects of hysteresis , magnetic materials are easier to heat than non-magnetics; these materials naturally resist the rapidly changing magnetic fields within the induction coil. The resulting friction produces hysteresis heating in addition to eddy current heating. A metal which offers high resistance is said to have high magnetic permeability which can vary from 100 to 500 for magnetic materials; non-magnetics have a permeability of 1. Hysteresis heating occurs at temperatures below the "Curie" point - the temperature at which a magnetic material loses its magnetic properties.The induced current flow within the part is most intense on the surface, and decays rapidly below the surface. So the outside will heat more quickly than the inside; 80% of the heat produced in the part is produced in the outer "skin". This is described as the "skin depth" of the part. The skin depth decreases when resistivity decreases, permeability increases or frequency increases.

Coupling refers to the proportional relationship between the amount of current flow in the workpiece and the distance between the workpiece and the coil. Close coupling generally increases the flow of current and therefore increases the amount of heat produced in the workpiece.

The induction coil, made from copper tubing, is water-cooled. The size and shape of the coil (single or multiple turn; helical, round or square; internal or external) follows the shape of your workpiece and variables of your process so that the proper heat pattern is achieved and the efficiency of the induction system is maximized. The system generates an RF field in the induction coil,producing a magnetic field around your workpiece. System output determines the relative speed at which the workpiece is heated: a brazing process accomplished with a 3 kW system could be completed more quickly with a 5 kW system. However, additional power capability may increase the system's, size and weight and utility requirements; larger ones require 3-phase electrical connections and facilities for water cooling.

For your application, you must consider: the degree of temperature change required, the mass, specific heat and electrical properties of the workpiece, the coupling efficiency of the coil design and thermal losses due to conduction of heat into workpiece fixturing, convection and radiation.

Electromotive force (emf, ) is a term used to characterize electrical devices, such as voltaic cells,thermoelectric devices, electrical generators and transformers, and even resistors. For a given device, if an electric charge Q passes through that device, and gains an energy W, the net emf for that device is the energy gained per unit charge, or W/Q. This has units of volts, or joules per coulomb, and hence can be thought of as a voltage induced by the device in question. Since force has the unit of the newton, emf is a misnomer, but one that over time has resisted change.

Induction Heating

To understand Induction Heating, firstly we need to understand what is eletromagnetic Induction and later we switch to Induction Heating, in upcoming posts of this blog .

Electromagnetic Induction
In electronics, the production of an Electromotive force (emf) in a circuit by a change of magnetic flux through the circuit or by relative motion of the circuit and the magnetic flux. As a magnet is moved in and out of a coil of wire in a closed circuit an induced current will be produced. All dynamos and generators produce electricity using this effect. When magnetic tape is driven past the playback head (a small coil) of a tape recorder, the moving magnetic field induces an emf in the head, which is then amplified to reproduce the recorded sounds.

Electromagnetic induction takes place when the magnetic field around a conductor changes. If the magnetic field is made to change quickly, the size of the current induced is larger. A galvanometer can be used to measure the direction of the current. As a magnet is pushed into a coil, the needle on the galvanometer moves in one direction. As the magnet is removed from the coil, the needle moves in the opposite direction.

If the change of magnetic flux is due to a variation in the current flowing in the same circuit, the phenomenon is known as self-induction; if it is due to a change of current flowing in another circuit it is known as mutual induction.

Recent Trends in Induction Heating

The recent trends in induction heating have thrown open new vistas...