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?
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.
A typical induction heater system includes a power supply, impedance matching circuit, tank circuit, and applicator. The applicator which is the induction coil can be a part of the tank circuit. A tank circuit is usually a parallel set of capacitors and inductors. The capacitor and inductor in the tank circuit are reservoirs of electrostatic energy and electromagnetic energy, respectively. At the resonance frequency, the capacitor and the inductor start to swing their stored energy to each other. In the parallel configuration, this energy conversion occurs at high current. The high current through the coil helps to have a good energy transfer from the induction coil to the workpiece.
Click here to learn what induction coils are and how they work, and the different types of coils.
a) Power Supply
Power supplies are one of the most important parts of an induction heater system. They are typically rated by their operating frequency range and power. There are various types of induction power supplies which are line-frequency supplies, frequency multipliers, motor-generators, spark-gap converters, and solid-state inverters. Solid-state inverters have the most efficiency between the power supplies.
A typical solid-state inverter power supply includes two major sections; Rectifier and Inverter. Line ac currents are converted into dc in the rectifier section using diodes or thyristors. The dc current goes to the inverter, where solid-state switches, such as IGBTs or MOSFETs convert it into a current, this time at a high frequency (typically in the range of 10kHz-600kHz). According to the diagram below, IGBTs can work at a higher power level and lower frequency versus MOSFETs operating at a lower power level and higher frequencies.
b) Impedance Matching
Induction heating power supplies, like every other electronic device, have maximum voltage and current values which should not be exceeded. In order to deliver the maximum power from the power supply to the load (workpiece), the impedance of the power supply and the load must be as close as possible. In this way, the power, voltage and current values can reach their highest allowed limits simultaneously. Impedance matching circuits are used in induction heater units for this purpose. According to the application, different combinations of electrical elements (e.g. transformers, variable inductors, capacitors, etc.) can be used.
c) Resonance Tank
The resonance tank in an induction heating system is normally a parallel set of capacitor and inductor which resonates at a certain frequency. The frequency is obtained from the following formula:
where L is the inductance of the induction coil and C is the capacitance. According to the animation below, the resonance phenomenon is very similar to what happens in a swinging pendulum. In a pendulum, kinetic and potential energies convert to each other while it swings from one end to another. The motion is damped due to the friction and other mechanical losses. In the resonance tank, the energy provided by the power supply oscillates between the inductor (in the form of electromagnetic energy) and the capacitor (in the form of electrostatic energy). The energy is damped due to the losses in the capacitor, inductor, and the workpiece. The losses in the workpiece in the form of heat are desired and the goal of induction heating.
The resonance tank itself includes the capacitor and inductor. A bank of capacitors is used to provide the needed capacitance in order to reach a resonance frequency close to the capability of the power supply. At low frequencies (below 10kHz) oil-filled capacitors and at higher frequencies (more the 10kHz) ceramic or solid dielectric capacitors are used.
d) Induction Heater Inductors
The induction heating coil is specifically shaped copper tubing or other conductive material which alternating electrical current is passed through, creating a variable magnetic field. Metal parts or other conductive materials are placed within, through or close to the induction heating coil, without touching the coil and the variable magnetic field that is generated causes a friction within the metal causing it to heat.
Some conditions need to be considered when designing a coil:
1. In order to increase the induction heaters efficiency, the distance between the coil and the workpiece must be minimized. The efficiency of the coupling between the coil and the workpiece is inversely proportional to the square root of the distance between them.
2. If the part is positioned at the center of the helical coil, it will be best coupled to the magnetic field. If it is off-center, the area of the workpiece closer to the turns will receive more heat. This effect has been shown in the figure below.
3. Also, the position close to the leads-coil connection has a weaker magnetic flux density, therefore even the ID center of the helical coil is not the induction heating center.
4. The cancellation effect (figure on the left) must be avoided. This happens when the opening of the coil is very small. Putting a loop in the coil will help to provide the necessary inductance (figure on the right). The inductance of an inductor defines the capability of that inductor to store magnetic energy. Inductance is can be calculated from as:
where ε is the electromotive force and dI/dt is the rate of the current change in the coil. ε itself is equal to the rate of the magnetic flux change in the coil (- dφ/dt), where the magnetic flux φ can be calculated from NBA, with N being the number of turns, B the magnetic field and A the area of the inductor. Therefore the inductance will be equal to:
It is obvious that the value of the inductance is linearly proportional to the area of the inductor. Hence, a minimum value must be considered for the inductor loop, so that it can store magnetic energy and deliver it to the induction workpiece.
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Coil Efficiency
The coil efficiency is defined as follows:
The table below shows typical efficiencies of different coil:
In several applications, the heating object does not have a uniform profile, though needs uniform heating. In these cases, the magnetic flux field needs to be modified. There are two typical methods to accomplish this. One way is to decouple the turns where the part has a bigger cross-section (if using helical coil). A more common method is to increase the winding inter-spacing at the areas where the part cross-section is larger. Both methods are shown in the figure below.
The same situation happens when heating flat surfaces with large pancake coils. The central area will get excessive heat. To avoid this, the gap between the coil surface and the flat object will be increased by applying a conical shape to the pancake coil.
A coil with liner is used in applications where a wide and uniform heating area is needed, but we want to avoid using large Copper tubing. Liner is a wide sheet which is tack brazed to the coil tubing at least in two point. The rest of the joint will be soldered only to provide the maximum heat transfer connection. Also a sinusoidal profile will help to increase the cooling capability of the coil. Such a coil is shown in the figure below.
As the heating length increases, the number of turns must be increased in order to keep the heating uniformity.
Depending on the changes of the shape of the workpiece, the heating pattern vary. Magnetic flux tend to accumulate at the edges, surface cuts or indentations of the heating object, thus causing higher heating rate in these areas. Figure below shows the &#;edge effect&#; where the coil is higher than the edge of the heating element and excessive heating happens at this area. To avoid this, the coil can be brought lower, to be even or slightly lower than the edge.
Induction heating of disks can also cause excessive edge heating, as shown in the figure below. The edges will get higher heating. The height of the coil can be reduced or the ends of the coil can be made out of a bigger radius to decouple from the edge of the workpiece.
Sharp corners of the rectangular coils can cause deeper heating in the workpiece. Decoupling the corners of the coil on one hand will reduce the heating rate of the corner, but on the other hand decreases the overall efficiency of the induction process.
One of the important items to be considered while designing multiplace coils is the effect of the adjacent coils on each other. In order to keep the heating strength of each coil at maximum, the center-to-center distance between the adjacent coils must be at least 1.5 times the coil diameter.
Split inductors are used in the applications where a close coupling is needed and also the part cannot be extracted from the coil after the heating process. An important point here is that a very good electrical contact must be provided at the place where the hinged surfaces meet. Usually, a thin silver layer is used to provide the best surface electrical contact. The split parts of the coils will be cooled down using flexible water tubing. Automated pneumatic compression is often used to close/open the coil and also to provide the needed pressure at the hinged area.
In the applications like heating the tip of shafts, reaching a temperature uniformity can be difficult because of the cancellation effect at the center of the surface of the tip. A double deformed pancake coil with tilled sides, similar to the scheme below, can be used to achieve uniform heating profile. Attention must be paid to the direction of the two pancakes, in which the central windings are wound in the same direction and have adding magnetic effect.
In the applications like welding a narrow band on one side of a long cylinder where a relatively long length must be heated considerably higher than the other areas of the object, the current return path will be of importance. Using the Split-Return type of coil, the high current induced in the welding path will be divided into two which will be even wider. This way, the heating rate at the welding path is at least four times higher than the rest of the parts of the object.
Channel type of coils is used if the heating time is not very short and also fairly low power densities are needed. A number of heating parts will pass through the coil at a constant speed and reach their maximum temperature when getting out of the machine. The ends of the coil are usually bent in order to provide the path for the parts to enter and exit the coil. Where a profile heating is needed, plate concentrators can be used with multiturn channel coils.
Square Copper tubing has two main advantages compared to the round tubing: a) since it has a more flat surface &#;looking&#; at the workpiece, it provides a better electromagnetic coupling with the heating load and b) it is structurally easier to implement turns with square tubing rather than round tubing.
Lead Design: Leads are a part of the induction coil and although they are very short, they have a finite inductance. In general, the diagram below shows the circuit diagram of the heat station of an induction unit system. C is the resonance capacitor installed in the heat station, L_lead is the total inductance of the leads of the coil and L_coil is the inductance of the induction coil coupled with the heating load. V_total is the voltage applied from the induction power supply to the heat station, V_lead is the voltage drop on the lead&#;s inductance and V_coil is the voltage which will be applied to the induction coil. The total voltage is the summation of the lead&#;s voltage and the induction coil&#;s voltage:
V_lead represents the amount of the total voltage that is occupied by the leads and does not do any useful induction action. The designer&#;s goal will be to minimize this value. V_lead can be calculated as:
It is obvious from the formulae above that in order to minimize the value of V_lead, then the inductance of the leads must be several times smaller than the inductance of the induction coil (L_lead&#;L_coil).
Lead Inductance Reduction: At low frequencies, usually since high inductance coils (multiturn and/or big ID) are used, L_lead is much smaller than L_coil. However, since the number of turns and the overall size of the coil reduces for high frequency inductors, then it will become important to apply special methods to minimize the lead&#;s inductance. Below there are two examples to accomplish this.
Flux Concentrators: When a magnetic material is placed in the environment including magnetic fields, due to the low magnetic resistance (reluctance) they tend to absorb the lines of magnetic flux. The ability of absorbing the magnetic field is quantified by Relative Magnetic Permeability. This value for air, Copper and stainless steel is one, but for mild steel can go up to 400 and for Iron up to . Magnetic materials can keep their magnetic capability up to their Curie temperature, after which their magnetic permeability drops to one and they will not be magnetic anymore.
A flux concentrator is a high permeability, low electrical conductivity material that is designed to be used in construction of the induction heater coils to magnify the magnetic field applied to the heating load. The figure below shows how placing a flux concentrator at the center of a pancake coil will concentrate the magnetic field lines at the surface of the coil. Thus the materials placed on top of the pancake coil will couple better and will receive the maximum heating.
The effect of flux concentrator on the current density in the induction coil is shown in the figure below. Most of the current will be concentrated on the surface which is not covered with flux concentrator. Therefore the coil can be designed in such way the that only the side of the coil facing the heating load will be left without the concentrator materials. In electromagnetism, this is called slot effect. Slot effect will increase the efficiency of the coil significantly and the heating will need a lower power level.
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