Induction heating is a process where electrically conductive material is heated when it's placed within a dynamic magnetic field without touching the inductor. It is a simple and cost-effective heating process that delivers fast and consistent heat when compared to other conventional heating processes used for preheating and stress-relieving welds. Heat is generated by circulating electric current as it is placed on the magnetic field(electromagnetic induction). To develop heat, the resistance of the material must be low(metals) and the voltage must be high. For example, metals with high resistance like iron will heat up much faster than low-resistance metals like copper.
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Heat is generated through resistance losses and hysteresis losses when the induced electrical current flows. Hysteresis losses mainly occur in ferromagnetic materials when they are magnetized and demagnetized.
Induction heating is mostly used in industrial processes when manufacturers want to change the physical properties of metals(bonding, hardening, and softening). During the induction heating process, there are no residual combustion emissions because metals are not heated with fire and smoke. Also, the rate of heat transfer is regulated and steady during the process with minimum heat loss. Unlike traditional heating processes(flame, resistance heating...), induction heating is easy to set up, fast time to temperature, safer, and also it is more efficient, accurate, and uniform.
How does Induction Heating work
We discussed in the paragraph above what&#;s induction heating, now let's talk about how induction heating works. When electrical current flows through a copper conductor, it produces a magnetic field around the conductor. The direction of the electric field depends on the direction of the electrical current using the right-hand thumb rule.
The more current that passes through the conductor, the bigger and stronger the magnetic field will be around the conductor. When the electrical current flow in the conductor is changed to the opposite direction, also the magnetic field changes. Passing an alternating magnetic field through a conductive material generates localized electrical currents within the metal. The generated electrical currents are called Eddy currents. The stronger the magnetic field, the more Eddy currents are generated.
Metals have a certain amount of electrical resistance, and the Eddy currents circulate against the resistance of the metal which causes the metals to heat up. This process is called Joule heating, and it is responsible for the generation of most of the heat.
The electrical resistance of the conductive material that is being heated, plays a major role in the heat that is generated. For example, metals with a low resistance value require more Eddy currents to heat than metals with a high resistance value. While heating ferrous materials, hysteresis losses should be taken into consideration. This occurs due to the material's resistance to a changing magnetic field. Hysteresis losses generate less heat than Joule heating but still significantly contribute to the total heat within the material.
Also, the magnetic properties of the conductive material that's being heated play a big role in the amount of heat that is generated. For example, magnetic materials such as iron generate more heat due to hysteresis losses, while non-magnetic materials like copper or aluminum won't generate any heat due to hysteresis.
Eddy currents produce heat at the surface of the part which is directly next to the heating coil. The heating depth is determined by how fast the alternating field switches back and forth through the material. The remainder of the part's thickness is heated from conduction from the part.
Components of Induction Heaters
An induction heater consists of 4 main parts:
Induction heating coil
Workpiece
Power supply
Component circuit
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HISTORY OF INDUCTION HEATERS
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Michael Faraday was the first to discover induction heating in using a battery and two copper wires wrapped around an iron core. However, the first time it was implemented with success was around 100 years later, in the year in England, where the first induction melting system had been installed by EFCO. With the need for a reliable and fast process to manufacture metals for engine parts during the second world war, the technology of induction heating advanced swiftly. As the focus shifted toward lean manufacturing and enhanced quality control, the technology of induction heating was rediscovered with the development of controlled induction power supplies.
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Induction heating is a process for heating metals and other electrically-conductive materials that is precise, repeatable and a safe non-contact method. It involves a complex combination of electromagnetic energy and heat transfer that passes through an induction coil, creating an electromagnetic field within the coil to metal down materials. Materials such as Steel, Copper, Brass, Graphite, Gold, Silver, Aluminum, and Carbide can be heated for a range of applications, which include various heat treating applications such as hardening, annealing, tempering, brazing, soldering, shrink fitting, heat staking, bonding, curing, melting and many more.
Two key phenomena must be learned to understand the fundamentals of induction heating; Faraday&#;s Law of Induction and Skin Effect.
Faraday&#;s Law of Induction
When an electrically conducting material (such as a metal) is placed within a time-varying magnetic field, an electric current (called an &#;eddy current&#;) is induced in the part producing a second magnetic field which opposes the applied field (figure below). The reason behind this phenomenon is that a time-varying magnetic field disturbs the relaxed environmental condition of the electrically conducting material. In return, the material tries to oppose this change by producing another magnetic field to cancel the imposed field.
The induction phenomenon has two important consequences:
i. Induced force. An example is shown in the figure below, where a permanent magnet is dropped into a copper tube. The induced force according to the Faraday&#;s law tries to stop the magnet&#;s motion inside the tube.
ii. Induced heat. When an electrically conductive material is exposed to an alternating magnetic field, depending on the material, heat is induced by two mechanisms; Joule Heating and Magnetic Hysteresis. The latter occurs in the magnetic metals (such as Carbon Steel below Curie temperature) in which the rotation of the adjacent magnetic dipoles due to the direction change of the imposed magnetic field will lead into friction and heat. This effect increases by increasing the frequency of the imposed magnetic field.
Joule Heating is the main heating effect caused by induction phenomenon. Any current I, ac or dc, passing through an electrically conducting material causes voltage drop V resulting in energy conversion to heat. Heat power is defined by V.I=R.I^2, where R is the electrical resistance of the current path. The resistance of the current path is inversely proportional to the cross-section area in which the current is flowing.
If an electrically conducting material is exposed to a magnetic field, eddy currents are induced in the material. Special characteristics of such currents result in a phenomenon which we call &#;Induction Heating&#;. The eddy currents are concentrated at the surface of the material. The reason is that at high frequency, the imposed magnetic field changes its direction very fast. Therefore, the induced currents in one direction do not have enough time to penetrate into the depth of the metal before their time is up. The thickness of the current penetration in the material is called &#;Skin Depth&#;. Skin depth depends on the electromagnetic properties of the material and also is inversely proportional to frequency. Figure below shows the dependence of the skin depth to frequency. Here, δ is the skin depth, ρ is the electrical resistivity, ω is the angular frequency and μ is the magnetic permeability.
Using high frequencies in induction heating industry (Mainly 10kHz to 700kHz) implies very thin penetration depths in metals (typically less than 1mm). Passing high current density (big I) through that shallow depth (big R) results in high R.I^2. Consequently, high energy conversion from electrical to heat occurs.
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Video credits: https://www.youtube.com/watch?v=5BeFoz3Ypo4
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