Why is pancake induction coil Better?

Process of heating an electrically conducting object by electromagnetic induction

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This article is about Induction heating. It is not to be confused with Joule heating

Component of Stirling radioisotope generator is heated by induction during testing

Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an inductor that creates an electromagnetic field within the coil to heat up and possibly melt steel, copper, brass, graphite, gold, silver, aluminum, or carbide.

An important feature of the induction heating process is that the heat is generated inside the object itself, instead of by an external heat source via heat conduction. Thus objects can be heated very rapidly. In addition, there need not be any external contact, which can be important where contamination is an issue. Induction heating is used in many industrial processes, such as heat treatment in metallurgy, Czochralski crystal growth and zone refining used in the semiconductor industry, and to melt refractory metals that require very high temperatures. It is also used in induction cooktops.

An induction heater consists of an electromagnet and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet. The rapidly alternating magnetic field penetrates the object, generating electric currents inside the conductor called eddy currents. The eddy currents flow through the resistance of the material, and heat it by Joule heating. In ferromagnetic and ferrimagnetic materials, such as iron, heat also is generated by magnetic hysteresis losses. The frequency of the electric current used for induction heating depends on the object size, material type, coupling (between the work coil and the object to be heated), and the penetration depth.

Applications

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Induction heating of 25 mm metal bar using 15 kW at 450 kHz. Keeping silicon in crucible molten at 2,650 °F (1,450 °C) for Czochralski crystal growth, 1956.

Induction heating allows the targeted heating of an applicable item for applications including surface hardening, melting, brazing and soldering, and heating to fit. Due to their ferromagnetic nature, iron and its alloys respond best to induction heating. Eddy currents can, however, be generated in any conductor, and magnetic hysteresis can occur in any magnetic material. Induction heating has been used to heat liquid conductors (such as molten metals) and also gaseous conductors (such as a gas plasma—see Induction plasma technology). Induction heating is often used to heat graphite crucibles (containing other materials) and is used extensively in the semiconductor industry for the heating of silicon and other semiconductors. Utility frequency (50/60 Hz) induction heating is used for many lower-cost industrial applications as inverters are not required.

Furnace

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An induction furnace uses induction to heat metal to its melting point. Once molten, the high-frequency magnetic field can also be used to stir the hot metal, which is useful in ensuring that alloying additions are fully mixed into the melt. Most induction furnaces consist of a tube of water-cooled copper rings surrounding a container of refractory material. Induction furnaces are used in most modern foundries as a cleaner method of melting metals than a reverberatory furnace or a cupola. Sizes range from a kilogram of capacity to a hundred tonnes. Induction furnaces often emit a high-pitched whine or hum when they are running, depending on their operating frequency. Metals melted include iron and steel, copper, aluminium, and precious metals. Because it is a clean and non-contact process, it can be used in a vacuum or inert atmosphere. Vacuum furnaces use induction heating to produce specialty steels and other alloys that would oxidize if heated in the presence of air.

Welding

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A similar, smaller-scale process is used for induction welding. Plastics may also be welded by induction, if they are either doped with ferromagnetic ceramics (where magnetic hysteresis of the particles provides the heat required) or by metallic particles.

Seams of tubes can be welded this way. Currents induced in a tube run along the open seam and heat the edges resulting in a temperature high enough for welding. At this point, the seam edges are forced together and the seam is welded. The RF current can also be conveyed to the tube by brushes, but the result is still the same—the current flows along the open seam, heating it.

Manufacturing

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In the Rapid Induction Printing metal additive printing process, a conductive wire feedstock and shielding gas is fed through a coiled nozzle, subjecting the feedstock to induction heating and ejection from the nozzle as a liquid, in order to refuse under shielding to form three-dimensional metal structures. The core benefit of the use of induction heating in this process is significantly greater energy and material efficiency as well as a higher degree of safety when compared with other additive manufacturing methods, such as selective laser sintering, which deliver heat to the material using a powerful laser or electron beam.

Cooking

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In induction cooking, an induction coil inside the cooktop heats the iron base of cookware by magnetic induction. Using induction cookers produces safety, efficiency (the induction cooktop is not heated itself), and speed. Non-ferrous pans such as copper-bottomed pans and aluminium pans are generally unsuitable. By thermal conduction, the heat induced in the base is transferred to the food inside.[1]

Brazing

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Induction brazing is often used in higher production runs. It produces uniform results and is very repeatable. There are many types of industrial equipment where induction brazing is used. For instance, Induction is used for brazing carbide to a shaft.

Sealing

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Induction heating is used in cap sealing of containers in the food and pharmaceutical industries. A layer of aluminum foil is placed over the bottle or jar opening and heated by induction to fuse it to the container. This provides a tamper-resistant seal since altering the contents requires breaking the foil.[2]

Heating to fit

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Induction heating is often used to heat an item causing it to expand before fitting or assembly. Bearings are routinely heated in this way using utility frequency (50/60 Hz) and a laminated steel transformer-type core passing through the centre of the bearing.

Heat treatment

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Induction heating is often used in the heat treatment of metal items. The most common applications are induction hardening of steel parts, induction soldering/brazing as a means of joining metal components, and induction annealing to selectively soften an area of a steel part.

Induction heating can produce high-power densities which allow short interaction times to reach the required temperature. This gives tight control of the heating pattern with the pattern following the applied magnetic field quite closely and allows reduced thermal distortion and damage.

This ability can be used in hardening to produce parts with varying properties. The most common hardening process is to produce a localised surface hardening of an area that needs wear resistance while retaining the toughness of the original structure as needed elsewhere. The depth of induction hardened patterns can be controlled through the choice of induction frequency, power density, and interaction time.

Limits to the flexibility of the process arise from the need to produce dedicated inductors for many applications. This is quite expensive and requires the marshalling of high-current densities in small copper inductors, which can require specialized engineering and "copper-fitting."

Plastic processing

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Induction heating is used in plastic injection molding machines. Induction heating improves energy efficiency for injection and extrusion processes. Heat is directly generated in the barrel of the machine, reducing warm-up time and energy consumption. The induction coil can be placed outside thermal insulation, so it operates at low temperatures and has a long life. The frequency used ranges from 30 kHz down to 5 kHz, decreasing for thicker barrels. The reduction in the cost of inverter equipment has made induction heating increasingly popular. Induction heating can also be applied to molds, offering more even mold temperature and improved product quality.[3]

Pyrolysis

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Induction heating is used to obtain biochar in the pyrolysis of biomass. Heat is directly generated into shaker reactor walls, enabling the pyrolysis of the biomass with good mixing and temperature control.[4]

Details

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The basic setup is an AC power supply that provides electricity with low voltage but very high current and high frequency. The workpiece to heat is placed inside an air coil driven by the power supply, usually in combination with a resonant tank capacitor to increase the reactive power. The alternating magnetic field induces eddy currents in the workpiece.

The frequency of the inductive current determines the depth that the induced eddy currents penetrate the workpiece. In the simplest case of a solid round bar, the induced current decreases exponentially from the surface. The penetration depth δ {\displaystyle \delta } in which 86% of power will be concentrated, can be derived as δ = 503 ρ μ f {\displaystyle \delta =503{\sqrt {\frac {\rho }{\mu f}}}} , where δ {\displaystyle \delta } is the depth in meters, ρ {\displaystyle \rho } is the resistivity of the workpiece in ohm-meters, μ {\displaystyle \mu } is the dimensionless relative magnetic permeability of the workpiece, and f {\displaystyle f} is the frequency of the AC field in Hz. The AC field can be calculated using the formula 1 T {\displaystyle {\frac {1}{T}}} .[5] The equivalent resistance of the workpiece and thus the efficiency is a function of the workpiece diameter a {\displaystyle a} over the reference depth d {\displaystyle d} , increasing rapidly up to about a / d = 4 {\displaystyle a/d=4} .[6] Since the workpiece diameter is fixed by the application, the value of a / d {\displaystyle a/d} is determined by the reference depth. Decreasing the reference depth requires increasing the frequency. Since the cost of induction power supplies increases with frequency, supplies are often optimized to achieve a critical frequency at which a / d = 4 {\displaystyle a/d=4} . If operated below the critical frequency, heating efficiency is reduced because eddy currents from either side of the workpiece impinge upon one another and cancel out. Increasing the frequency beyond the critical frequency creates minimal further improvement in heating efficiency, although it is used in applications that seek to heat treat only the surface of the workpiece.

Relative depth varies with temperature because resistivities and permeability vary with temperature. For steel, the relative permeability drops to 1 above the Curie temperature. Thus the reference depth can vary with temperature by a factor of 2–3 for nonmagnetic conductors and by as much as 20 for magnetic steels.[7]

Applications of frequency ranges Frequency (kHz) Workpiece type 5–30 Thick materials (e.g. steel at 815 °C with diameter 50 mm or greater). 100–400 Small workpieces or shallow penetration (e.g. steel at 815 °C with diameter of 5–10 mm or steel at 25 °C with a diameter around 0.1 mm). 480 Microscopic pieces

Magnetic materials improve the induction heat process because of hysteresis. Materials with high permeability (100–500) are easier to heat with induction heating. Hysteresis heating occurs below the Curie temperature, where materials retain their magnetic properties. High permeability below the Curie temperature in the workpiece is useful. Temperature difference, mass, and specific heat influence the workpiece heating.

The energy transfer of induction heating is affected by the distance between the coil and the workpiece. Energy losses occur through heat conduction from workpiece to fixture, natural convection, and thermal radiation.

The induction coil is usually made of copper tubing and fluid coolant. Diameter, shape, and number of turns influence the efficiency and field pattern.

Core type furnace

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The furnace consists of a circular hearth that contains the charge to be melted in the form of a ring. The metal ring is large in diameter and is magnetically interlinked with an electrical winding energized by an AC source. It is essentially a transformer where the charge to be heated forms a single-turn short circuit secondary and is magnetically coupled to the primary by an iron core.

References

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• Brown, George Harold, Cyril N. Hoyler, and Rudolph A. Bierwirth, Theory and application of radio-frequency heating. New York, D. Van Nostrand Company, Inc., 1947. LCCN 47003544
• Hartshorn, Leslie, Radio-frequency heating. London, G. Allen & Unwin, 1949. LCCN 50002705
• Langton, L. L., Radio-frequency heating equipment, with particular reference to the theory and design of self-excited power oscillators. London, Pitman, 1949. LCCN 50001900
• Shields, John Potter, Abc's of radio-frequency heating. 1st ed., Indianapolis, H. W. Sams, 1969. LCCN 76098943
• Sovie, Ronald J., and George R. Seikel, Radio-frequency induction heating of low-pressure plasmas. Washington, D.C. : National Aeronautics and Space Administration; Springfield, Va.: Clearinghouse for Federal Scientific and Technical Information, October 1967. NASA technical note. D-4206; Prepared at Lewis Research Center.

How efficiently and effectively a workpiece is heated is determined by the design and type of the induction coil. Work coils range in complexity from a simple solenoid- or helical-wound coil (made up of a number of turns of copper tube wound around a mandrel) to a coil which is precision-machined from solid copper and brazed.

The most ubiquitous induction coil design is the helical solenoid coil. Since the heating area or part is found within the coil, in the area of greatest magnetic flux, this coil supplies a large scope of heating behaviors. Flux lines in a solenoid coil are concentrated inside the coil, supplying the maximum heating rate in that place.

6 Common Types of Solenoid Coils for Induction Heating Applications

1. Multi-Turn Helical Coil

The most common and the most efficient coil by a long way is the helical (solenoid). As the amount of turns determines the length of the heating pattern, the design is relatively simple.

In this type of coil the workpiece can be stationary in order to supply a defined heating band in “single shot heating.” Alternatively, the workpiece can be moved through the coil to heat a longer part using a highly uniform heating pattern called “scan heating.”

2. Pancake Induction Coil

Pancake induction coils are employed when it is necessary to heat the workpiece from just one side, or when surrounding the part is not possible. Pancake coils can also be utilized to heat a small narrow band in the center of the part. As the flux from only one surface intersects with the workpiece, the pancake coil supplies a huge scope of heating behaviors.

3. Multi-Position Helical Coil

Multi-position coils are frequently utilized to generate more parts in a certain time while enabling the entire heating process. Any amount of positions are possible but usually up to eight positions are practical. Depending on the heating process needed, parts can be heated at the same time, or be indexed in and out of different positions.

4. Split Helical Coil

When it is not possible to access the target heating area using a helical coil, single- or multiple-turn split helical coils are employed.

Conducting Through Split Coils

The faces of the fixed and hinged portions of the coil need good surface-to-surface contact. These are usually faced with silver, or special alloys which are matched to supply good contact. To ensure closure during heating, clamps are utilized. High currents pass through this interface at high frequency, which generally leads to the life of the contact being limited.

Securing Split Coils

To maintain proper coupling distance, split coils usually need a means of finding the part in the coil. Typically, ceramic buttons or pins are secured to the face of the inductor. During heating/quenching they are subject to thermal shock and must be designed for simple replacement.

Quenching Through a Split Coil

Some applications need the split-coil design to have the ability to quench through the face of the inductor. So that excessive heating does not happen in the movable section during the cycle, coolant for the coil chamber of the split inductor is carried by flexible hoses that bypass the hinge.

The quench chamber is fed using a separate hose arrangement. During heating, the face of the quench chamber is nearest to the work, and carries most of the current. It needs to be thick enough to avoid distortion or melting during the heating cycle.

5. Master Work Coils and Inserts

Generally, master coils and inserts are utilized for small batches where a single-turn coil can be employed. Master work coils supply a fast, simple means of altering coil diameters or shapes in order to match a variety of parts.

Usually, master work coils are made up of copper tubing which supply both an electrical connection to the power supply and a water-cooled contact surface for connection to a coil insert. The copper tube is bent into the form of a single-turn coil and soldered to a copper band which is adapted to the slope of the coil insert and is recessed.

6. Butterfly Coils

Butterfly coils are perfect for producing an even heating pattern at the end of a shaft or bar and they have two specially formed pancake coils. So that the current paths are additive, center turns must be wound in the same direction. To create the desired pattern, only these center turns should couple directly with the part. The butterfly “wings” can either be bent up to decouple their fields from the shaft, or can be coupled with the shaft itself.

Solenoid Coil Design Calculations

Coupling Distance

Usually, distance grows with the diameter of the part, normal values being 0.75, 1.25, and 1.75 inches (19, 32 and 44 mm) or billet-stock diameters of around 1.5, 4 and 6 inches (38, 102, and 152 mm), respectively. Uniform heating at high power densities becomes challenging when the length of the coil surpasses 4-8 times its diameter. Single-turn or multi-turn coils that scan the length of the workpiece are usually preferable in these cases.

Coupling Efficiency

Efficiency of coupling between the windings is inversely proportional to the square of distance between them.

Voltage Loss

A maximum of 10 % of the total voltage will be lost in the leads if the inductance of the coil (L2) is approximately ten times the total inductance of the leads (L1 plus L3) or more. Any loss which is lower than this can be considered nominal.

Current

(current in the secondary * # of secondary turns) = (Current in the primary of the transformer * # of primary turns).

Minimum Tubing OD

To allow for water cooling the minimum outer diameter of copper tubing is 0.125 in (0.32 cm).

Rotational Speed

Rotational speeds generating a minimum of 10 revolutions during the heating cycle should be employed for the majority of hardening operations, which are of short duration.

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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