Induction heating is the process of heating an electrically produced heat by electromagnetic induction, through heat generated in the object by eddy currents. 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 flowing through the resistance of the material heat it by Joule heating. 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.

 

CHAPTER ONE
1.1                                                        INTRODUCTION
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, through heat generated in the object by eddy currents (also called Foucault currents). 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 flowing through the resistance of the material heat it by Joule heating. In ferromagnetic (and ferrimagnetic) materials like iron, heat may also be generated by magnetic hysteresis losses. The frequency of current used depends on the object size, material type, coupling (between the work coil and the object to be heated) and the penetration depth.
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 which require very high temperatures. It is also used in induction cooktops for heating containers of food; this is called induction cooking.
An induction heater is a key piece of equipment used in all forms of induction heating. Typically an induction heater operates at either medium frequency (MF) or radio frequency (RF) ranges.

1.2                             OBJECTIVE OF THE PROJECT
The objective of this work is design a device that allows one to rapidly heat a metal object. With enough power, one can even melt metal. The induction heater works without the need for fossil fuels, and can anneal and heat objects of various shapes.

1.3                          SIGNIFICANCE OF THE PROJECT
Why choose induction heating over convection, radiant, open flame or another heating method? For many modern manufacturing processes, induction heating offers an attractive combination of speed, consistency and control. Here's a short summary of the major advantages that modern solid state induction heating offers for lean manufacturing.

• When you use a cooktop that works with induction, you get instant heat. Now you might be thinking you already get that with gas but it will surprise you to know that induction cooktops can heat a pan of water far quicker than gas simply because of the technology they use.
• Staying with heat for the moment, you will also find with induction technology that there is no waste. You see an induction cooktop will only heat the cookware and not the whole surface. In fact, this type of cooking technique is up to 70% more efficient than either conventional electric or gas.
• Safety is an important consideration when you’re cooking especially with children around, and this happens to be one of my favourite features with induction. When you take the pan off the burner, the unit will automatically switch off, and in most cases you can touch the burner seconds afterwards without burning yourself.
• This safety feature is also great when you are cooking because as anyone with kids will know, they can get a little curious when you’re putting together a tasty dinner. If little fingers do happen to touch the cooktop, they won’t get burnt.
• Cleaning a cook top can be one of the biggest headaches, and I speak from experience with this one. With traditional electric or gas hobs it’s impossible to avoid food being baked on to the surface. Not only that, traditional gas cooktops have lots of places where food can hide. With an induction cooktop this isn’t the case. Food cannot become baked on because the surface is cool, and cleaning can be as easy as wiping it down with a damp cloth.
• Looks for some are important, and although last on my list of pros this deserves a mention. Induction cooktops have gone one step further in terms of design, and many of them blend into the décor of your kitchen seamlessly. It’s not the most important point to mention, but if you spend a lot of time in the kitchen having a nice environment to work in really helps. Well, I think so anyway.

1.4                       LIMITATION OF THE PROJECT

• Induction cooktops will cost you more to buy than traditional electric or gas. However, you have to think about the technology that goes into building them. You also have to consider how much quicker you can cook any type of dish, no matter how complicated. Add to that the savings you can make on your energy bills, and the chances are your initial outlay will pay for itself.
• Cookware is an issue I come across all the time, and there are many people who think they will have to buy a completely new set of pots and pans. The trouble is not everyone understands that all you have to do is make sure a magnet will stick to the base of your cookware. If it does, it will work with induction. However, there are cases when you will have to replace some items.
• Power can be a bit of an issue. If you have an outage for some reason, you won’t be able to cook (unlike with gas). Also, you must make sure you’re careful when looking at the wattage any induction cooktop offers you, especially if it has more than two burners.

1.5                           APPLICATION OF THE PROJECT
Induction heating allows the targeted heating of an applicable item for applications including surface hardening, melting, brazing and soldering and heating to fit. Iron and its alloys respond best to induction heating, due to their ferromagnetic nature. 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. 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.
Induction furnace
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 capacity. 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 make use of induction heating for the production of specialty steels and other alloys that would oxidize if heated in the presence of air.
Induction welding
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.
Induction cooking
In induction cooking, an induction coil in the cook-top heats the iron base of cookware by cyclic magnetic induction. Copper-bottomed pans, aluminium pans and other non-ferrous pans are generally unsuitable. The heat induced in the base is transferred to the food via (metal surface) conduction. Benefits of induction cookers include efficiency, safety (the induction cook-top is not heated itself) and speed. Both permanently installed and portable induction cookers are available.
Induction brazing
Induction brazing is often used in higher production runs. It produces uniform results and is very repeatable.
Induction sealing
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 heating by induction to fuse it to the container. This provides a tamper-resistant seal, since altering the contents requires breaking the foil.
Heating to fit
Induction heating is often used to heat an item causing it to expand prior to 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
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 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
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 temperature and has a long life. The frequency used ranges from 30 kHz down to 5 kHz, decreasing for thicker barrels. The reduction in 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.

 

1.6 DEFINITION OF TERMS

As you investigate an induction heating solution for your manufacturing process, the following definitions may be useful to you:
Annealing: A process which softens a metal by first heating and then slowly cooling it. Annealing removes stresses and improves machineability.
Bell Jar System: A relatively small protective atmosphere heating system designed for low volume production or lab use. Parts are heated in a sealed quartz "bell jar" to eliminate oxidation, scaling and carbon build-up.
Brazing: A method of joining two pieces of heated metal together with a third, molten filler metal. Brazed joints have great tensile strength – they are often stronger than the two metals being bonded together.
Capacitor: An electronic component that stores energy. Capacitors are used to smooth out or "decouple" the output of an induction heating power supply.
Conductive: An adjective describing a material that transfers heat. Induction heating coils are usually made of copper, a highly-conductive metal.
Conduction Losses occur when heat is transferred between materials in direct physical contact. Heat loss to the fixture holding the part being heated in an induction coil is a good example of conduction loss. Conduction losses also occur in the part if only a small section is being heated.
Convection Losses are losses due to the flow of air/gasses across the surface of a heated part. Convection losses are typically smaller in magnitude and do not generally have a major impact in most applications.
Curie temperature: The reversible point in a heating process in which a magnetic material loses its magnetic properties. For Iron (Fe) the Curie point is 770°C; for Nickel (Ni), it is 358°C; for Iron Oxide, (FE2O3), it is 622°C.
Capillary action: In brazing, capillary action is the pulling force that draws the melted alloy into the gap between the parts being joined.
Carburization: A process by which carbon is added t to a metal. Excess carbon can make metals brittle.
Continuous Flow Manufacturing: A process that results in a balanced, just-in-time production system that minimizes excess inventory and maximizes throughpout.
Controlled Atmosphere: A regulated atmosphere in which a heating process can be completed; usually used to ensure quality results. Brazing in a controlled atmosphere of nitrogen or argon instead of open air produces cleaner joints.
Coupling Efficiency: The proportional relationship between the amount of current flow and the distance between coil and part. Placing the part close to the coil increases the flow of current and the amount of heat induced in the part.
Curing: The toughening or hardening of a polymer material such as paint, epoxy or other adhesive, either by heat or the addition of chemical additives.
Eddy Current: Circulating movement of electrical current within an electrical conductor caused by the intersection of the conductor with a moving magnetic field.
Faraday's Law explains how a change in a magnetic field can create voltage. The law states that the amount of voltage created is equal to the change in magnetic flux divided by the change in time. The greater the change in the magnetic field, the greater amount of voltage.
Fusing: A manufacturing process which utilizes heat to melt or bond to materials together.
Glove Box: An enclosed workspace equipped with gloved openings which enable operator to allow manipulation materials inside the box. Glove boxes provide an excellent environment for controlled atmosphere brazing.
Heat Station: Part of the induction heating system that incorporates the heating coil. For improved flexibility, many induction heating power supplies are equipped with a remote heat station, which is connected by cabling to the main unit.
Heat Treating: A combination of controlled heating and cooling cycles applied to a metal for hardening or softening. Hardening, annealing, and stress relieving are examples of heat treating processes.
Heat Staking: The process of inserting a piece of metal into plastic. The metal is first heated, so that the plastic around the metal melts at first contact, and then cools around the metal, forming a firm bond [also known as shrink fitting].
Hysteresis: Heat produced by internal friction that is created when magnetic materials pass through the induction coil. Magnetic materials naturally offer electrical resistance to the rapidly changing magnetic fields within the coil. This resistance produces internal friction – from the process of repositioning the magnetic dipoles in the material – which in turn produces heat.
Inductor: Specially-shaped copper tubing or other conductive material through which alternating electrical current is passed, creating a varying magnetic field. Depending on the application, the metal parts to be heated are either positioned close to the inductor or passed through it. But the parts are heated without actually touching the inductor.
Induction Heating: A process which is used to bond, harden or soften metals or other conductive materials, by placing the part to be heated in a copper coil. For many modern manufacturing processes, induction heating offers an attractive combination of speed, consistency and control.
Joule Effect: A scientific formula – also known as Joule's first law – expressing the relationship between heat produced by electrical current passed through a conductor. It is expressed as "Q = I2 x R x t" where Q is the amount of heat produced, I is the current flowing through the part (conductor), R is the electrical resistance of the part, and t = time.
Pre-tinning: The process of pre-applying solder to metallic parts to improve the soldering process during the actual induction heating cycle.
Quenching: Cooling a part with water after it has been heated.
Radiation: Energy that comes from a source and travels through some material or through space in the form of rays or waves or particles. Light, heat, and sound are all examples of radiation.
Radio frequency (RF): a frequency of electromagnetic radiation in the range at which radio signals are transmitted, anywhere from 3 Hz to 300 GHz. Radio frequencies are higher than audio frequencies and lower than infrared frequencies.
Resistivity: The ability of a material to resist passage of electric current through itself or on its surface.
Shrink Fitting: The process of inserting a piece of metal into plastic. The metal is first heated, so that the plastic around the metal melts at first contact, and then cools around the metal, forming a firm bond [also known as heat staking].
Silver brazing: Joining two pieces of heated metal with a third, molten silver alloy. Silver brazing produces a very strong bond but requires very precise machining tolerances.
Soldering: the process of joining two pieces of metal by applying a third molten metal which has a lower melting point. Soldering is similar to brazing but it is done at lower temperatures and produces a less-strong bond.
Susceptor: A metal plate which is heated by the induction process and then used to transfer heat to a non-metallic material.
Tempering: A heat treatment applied to a metal after it has been hardened. The metal is first heated and then rapidly cooled to decrease hardness and increase toughness.
Vacuum Furnace: An industrial furnace which heats parts to very high temperatures in a controlled atmosphere or vacuum; frequently used for brazing to improve joint quality.

 

 

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