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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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Induction heating is the process of heating conductors, (usually metals), by inducing an electric current to flow in the object to be heated. Current is induced into the object in the same manner that current is induced into the secondary of a transformer.
In induction heating, a coil of copper is wound around an object to be heated. The coil of copper can be compared to the transformer primary, and the object to be heated can be compared to the secondary of the transformer.
The object to be heated acts like a single turn secondary in a transformer. Additionally, the object acts as if the single turn secondary were short circuited.
An alternating current is applied to the primary of a transformer, which creates an alternating magnetic field. The secondary of the transformer is located within the magnetic field. Faraday's Law shows that an electric current will be induced into the secondary of the transformer.
Thus, applying an alternating current to the induction coil induces a current into the object to be heated. Imagine how a short circuit secondary on a transformer would heat up if you connected power to the primary!
Thus, applying an alternating current to the induction coil induces a current into the object to be heated. Imagine how a short circuit secondary on a transformer would heat up if you connected power to the primary!So, you can heat metals without flames and without touching the object to be heated. You can even heat the metal underwater.. Growth in the induction heating industry expanded very rapidly duringSurface hardening, or case hardening, was one of the main growth areas during WWII.and weaponry using case hardening on axles and engine components could outlast those without case
After the war, the technology improvements moved rapidly into the civilian sector as the demand for reliable automobiles increased.
Induction heating equipment must create alternating currents at frequencies from 60 Hz to over 1 MHz. In the beginning, spark gap oscillators, motor driven generators and vacuum tubes were used to create the alternating current. Technology advanced and soon SCR, (Silicon Controlled Rectifier), based power supplies were used to replace older generators. Very large and powerful transistors are now used in power supplies for induction heating.
must create alternating currents at frequencies from 60 Hz to over 1 MHz. In the beginning, spark gap oscillators, motor driven generators and vacuum tubes were used to create the alternating current. Technology advanced and soon SCR, (Silicon Controlled Rectifier), based power supplies were used to replace older generators. Very large and powerful transistors are now used in power supplies for induction heating.Anis comprised of several major components.. Power Supply ( generates the high frequency current). Load Matching Station ( matches the impedance of the coil to the power supply). Induction Coil (copper coil wrapped around object to be heated. Water Cooling ( high power systems are water cooled to remove waste heat)
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