Inductotherm Group companies use electromagnetic induction for melting, heating and welding applications across multiple industries. But what exactly is induction? And how does it differ from other heating methods?
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To the typical engineer, induction is a fascinating method of heating. Watching a piece of metal in a coil turn cherry red in a matter of seconds can be surprising to those unfamiliar with induction heating. Induction heating equipment requires an understanding of physics, electromagnetism, power electronics and process control, but the basic concepts behind induction heating are simple to understand.
Discovered by Michael Faraday, Induction starts with a coil of conductive material (for example, copper). As current flows through the coil, a magnetic field in and around the coil is produced. The ability of the magnetic field to do work depends on the coil design as well as the amount of current flowing through the coil.
The direction of the magnetic field depends on the direction of current flow, so an alternating current through the coil will result in a magnetic field changing in direction at the same rate as the frequency of the alternating current. 60Hz AC current will cause the magnetic field to switch directions 60 times a second. 400kHz AC current will cause the magnetic field to switch 400,000 times a second.
When a conductive material, a work piece, is placed in a changing magnetic field (for example, a field generated with AC), voltage will be induced in the work piece (Faraday’s Law). The induced voltage will result in the flow of electrons: current! The current flowing through the work piece will go in the opposite direction as the current in the coil. This means that we can control the frequency of the current in the work piece by controlling the frequency of the current in the coil.
As current flows through a medium, there will be some resistance to the movement of the electrons. This resistance shows up as heat (The Joule Heating Effect). Materials that are more resistant to the flow of electrons will give off more heat as current flows through them, but it is certainly possible to heat highly conductive materials (for example, copper) using an induced current. This phenomenon is critical for inductive heating.
All of this tells us that we need two basic things for induction heating to occur:
There are several methods to heat an object without induction. Some of the more common industrial practices include gas furnaces, electric furnaces, and salt baths. These methods all rely on heat transfer to the product from the heat source (burner, heating element, liquid salt) through convection and radiation. Once the surface of the product is heated, the heat transfers through the product with thermal conduction.
Induction heated products are not relying on convection and radiation for the delivery of heat to the product surface. Instead, heat is generated in the surface of the product by the flow of current. The heat from the product surface is then transferred through the product with thermal conduction. The depth to which heat is generated directly using the induced current depends on something called the electrical reference depth.
The electrical reference depth depends greatly on the frequency of the alternating current flowing through the work piece. Higher frequency current will result in a shallower electrical reference depth and a lower frequency current will result in a deeper electrical reference depth. This depth also depends on the electrical and magnetic properties of the work piece.
Electrical Reference Depth of High and Low FrequencyInductotherm Group companies take advantage of these physical and electrical phenomena to customize heating solutions for specific products and applications. The careful control of power, frequency, and coil geometry allows the Inductotherm Group companies to design equipment with high levels of process control and reliability regardless of the application.
For many processes melting is the first step in producing a useful product; induction melting is fast and efficient. By changing the geometry of the induction coil, induction melting furnaces can hold charges that range in size from the volume of a coffee mug to hundreds of tons of molten metal. Further, by adjusting frequency and power, Inductotherm Group companies can process virtually all metals and materials including but not limited to: iron, steel and stainless steel alloys, copper and copper-based alloys, aluminum and silicon. Induction equipment is custom-designed for each application to ensure it is as efficient as possible.
A major advantage that is inherent with induction melting is inductive stirring. In an induction furnace, the metal charge material is melted or heated by current generated by an electromagnetic field. When the metal becomes molten, this field also causes the bath to move. This is called inductive stirring. This constant motion naturally mixes the bath producing a more homogeneous mix and assists with alloying. The amount of stirring is determined by the size of the furnace, the power put into the metal, the frequency of the electromagnetic field and the type/amount of metal in the furnace. The amount of inductive stirring in any given furnace can be manipulated for special applications if required.
Because induction heating is accomplished using a magnetic field, the work piece (or load) can be physically isolated from the induction coil by refractory or some other non-conducting medium. The magnetic field will pass through this material to induce a voltage in the load contained within. This means that the load or work piece can be heated under vacuum or in a carefully controlled atmosphere. This enables processing of reactive metals (Ti, Al), specialty alloys, silicon, graphite, and other sensitive conductive materials.
Unlike some combustion methods, induction heating is precisely controllable regardless of batch size. Varying the current, voltage, and frequency through an induction coil results in fine-tuned engineered heating, perfect for precise applications like case hardening, hardening and tempering, annealing and other forms of heat treating. A high level of precision is essential for critical applications like automotive, aerospace, fiber-optics, ammunition bonding, wire hardening and tempering of spring wire. Induction heating is well suited for specialty metal applications involving titanium, precious metals, and advanced composites. The precise heating control available with induction is unmatched. Further, using the same heating fundamentals as vacuum crucible heating applications, induction heating can be carried under atmosphere for continuous applications. For example bright annealing of stainless steel tube and pipe.
When induction is delivered using High Frequency (HF) current, even welding is possible. In this application the very shallow electrical reference depths that can be achieved with HF current. In this case a strip of metal is formed continuously, and then passes through a set of precisely engineered rolls, whose sole purpose is to force the formed strip edges together and create the weld. Just before the formed strip reaches the set of rolls, it passes through an induction coil. In this case current flows down along the geometric “vee” created by the strip edges instead of just around the outside of the formed channel. As current flows along the strip edges, they will heat up to a suitable welding temperature (below the melting temperature of the material). When the edges are pressed together, all debris, oxides, and other impurities are forced out to result in a solid state forge weld.
With the coming age of highly engineered materials, alternative energies and the need for empowering developing countries, the unique capabilities of induction offer engineers and designers of the future a fast, efficient, and precise method of heating.
Induction heating is an accurate, fast, repeatable, efficient, non-contact technique for heating metals or any other electrically-conductive materials.
An induction heating system consists of an induction power supply for converting line power to an alternating current and delivering it to a workhead, and a work coil for generating an electromagnetic field within the coil. The work piece is positioned in the coil such that this field induces a current in the work piece, which in turn produces heat.
The water-cooled coil is positioned around or bordering the work piece. It does not contact the work piece, and the heat is only produced by the induced current transmitted through the work piece. The material used to make the work piece can be a metal such as copper, aluminum, steel, or brass. It can also be a semiconductor such as graphite, carbon or silicon carbide.
For heating non-conductive materials such as plastics or glass, induction can be used to heat an electrically-conductive susceptor e.g., graphite, which then passes the heat to the non-conducting material.
Induction heating finds applications in processes where temperatures are as low as 100ºC (212°F) and as high as 3000°C (5432°F). It is also used in short heating processes lasting for less than half a second and in heating processes that extend over several months.
Induction heating is used both domestic and commercial cooking, in several applications such as heat treating, soldering, preheating for welding, melting, shrink fitting in industry, sealing, brazing, curing, and in research and development.
Induction produces an electromagnetic field in a coil to transfer energy to a work piece to be heated. When the electrical current passes along a wire, a magnetic field is produced around that wire.
The benefits of induction are:
Induction heating is done using two methods:
The first method is referred to as eddy current heating from the I²R losses caused from the resistivity of a work piece’s material. The second is referred to as hysteretic heating, in which energy is produced within a part by the alternating magnetic field generated by the coil modifying the component’s magnetic polarity.
Hysteretic heating occurs in a component up to the Curie temperature when the material’s magnetic permeability decreases to 1 and hysteretic heating is reduced. Eddy current heating constitutes the remaining induction heating effect.
When there is a change in the direction of electrical current (AC) the magnetic field generated fails, and is produced in the reverse direction, as the direction of the current is reversed. When a second wire is positioned in that alternating magnetic field, an alternating current is produced in the second wire.
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The current transmitted through the second wire and that through the first wire are proportional to each other and also to the inverse of the square of the distance between them.
When the wire in this model is substituted with a coil, the alternating current on the coil generates an electromagnetic field and while the work piece to be heated is in the field, the work piece matches to the second wire and an alternating current is produced in the work piece. The I²R losses of the material resistivity of the work piece causes heat to be created in the work piece of the work piece’s material resistivity. This is called eddy current heating.
Figure 1.
With the help of an alternating electric field, energy is transmitted to the work piece with a work coil.
The alternating current passing via the coil produces the electromagnetic field which induces a current passing in the work piece as a mirror image to the current passing in the work coil. The work coil/inductor is a part of the induction heating system that displays the effectiveness and efficiency of the work piece when it is heated. Work coils are of numerous types ranging from complex to simple.
The helical wound (or solenoid) coil is an example of simple coil, which consists of many turns of copper tube wound around a mandrel. A coil precision-machined from solid copper and brazed together is an example of complex coil.
Figure 2.
The work piece that needs to be heated and the work piece material decide the operating frequency of the induction heating system. It is vital to use an induction system that provides power over the range of frequencies suitable for the application. The reasons for various operating frequencies can be understood by what is referred to as the “skin effect”. When the electromagnetic field induces a current in a component, it passes primarily at the component surface.
Figure 3. (a) High frequency induction heating has a shallow skin effect which is more efficient for small parts; (b) Low frequency induction heating has a deeper skin effect which is more efficient for larger parts.
When the operating frequency is higher, the skin depth is shallower. Similarly when the operating frequency is lower, the skin depth and the penetration of the heating effect are deeper. Skin depth/penetrating depth is based on the temperature, operating frequency, and material properties of the part.
For example (see Table 1), a 20mm steel bar can be stress-relieved by heating it to 540°C (1000°F) using a 3kHz induction system. However, a 10 kHz system will be needed to harden the same bar by heating it to 870°C (1600°F).
Approximate smallest diameter for efficient heating at different induction frequencies Material Temperature 1 kHz 3 kHz 10 kHz 30 kHz Steel below curie 540 °CHence it can be said that higher operating frequencies, mostly more than 50kHz, can be used to heat smaller parts with induction and lower operating frequencies can be used to heat larger parts more efficiently.
In the case of advanced solid-state induction power supplies with embedded microprocessor control systems, consistent and effective heating techniques are achievable based on the fact that all parts are placed at a consistent location within the coil.
An induction heating system comprises a tank circuit, a power supply, and a work coil. In industrial applications, there is sufficient current passing via the coil to need water cooling; therefore a basic installation contains a water cooling unit. The alternating current from the AC line is converted through a power supply to an alternating current that is in line with the combination of coil inductance, workhead capacitance, and component resistivity.
Figure 4. Typical Induction Heating System
The work piece material dictates the heating rate and power needed. Iron and steel heat easily as they have higher resistivity while aluminum and copper need more power to heat due to their lower resistivity.
Certain steels are magnetic in nature hence the resistivity and hysteretic properties of the metal are used when heated with induction. The steel loses its magnetic properties when heated above the Curie temperature (500-600°C/1000-1150°F); however eddy current heating provides the required heating technique for higher temperatures.
The power required is determined by factors such as the type of material, size of the work piece, required temperature increase, and the time to temperature. According to the size of the work piece to be heated, the essential factor to be considered is the operating frequency of the induction heating system.
Likewise, in the case of smaller work pieces a higher frequency (>50kHz) is needed for efficient heating, and in cases of larger work pieces a lower frequency (>10kHz) and more penetration of the heat is generated.
When the temperature of the heated work piece increases, heat is also lost from the work piece. Radiation and convection losses from the work piece develop into a very essential factor with higher temperatures. Insulation methods are frequently used at high temperatures to reduce heat losses and to decrease the power required from the induction system.
Figure 5. Family of Ambrell Induction Heating Power Supplies
This information has been sourced, reviewed and adapted from materials provided by Ambrell Induction Heating Solutions.
For more information on this source, please visit Ambrell Induction Heating Solutions.
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