Introduction

Introduction

DaWei Induction Heating Machine Co., Ltd specializes in Induction Heating machines manufacturing and marketing for more than 1 decade. The machines cover Automatic Surface Induction Hardening &Induction Tempering Machines, Flexible Induction Brazing Systems, Compact Adhesive Curing Systems Efficient Tube Welders & Thermal Straightening Systems. They are wildly used in heat treating, bonding, brazing, welding, forging, melting and heat fitting solutions.Transistor converters from 1Hz to 2000KHz frequency & power sizes from 5 to 300 KW.
Induction heating is a non-contact heating process. It uses high frequency electricity to heat materials that are electrically conductive. Since it is non-contact, the heating process does not contaminate the material being heated. It is also very efficient since the heat is actually generated inside the workpiece. This can be contrasted with other heating methods where heat is generated in a flame or heating element, which is then applied to the workpiece. For these reasons Induction Heating lends itself to some unique applications in industry.

How does Induction Heating work ?

induction heating machine



























A source of high frequency electricity is used to drive a large alternating current through a coil. This coil is known as the work coil. See the picture opposite.
The passage of current through this coil generates a very intense and rapidly changing magnetic field in the space within the work coil. The workpiece to be heated is placed within this intense alternating magnetic field.
Depending on the nature of the workpiece material, a number of things happen...
The alternating magnetic field induces a current flow in the conductive workpiece. The arrangement of the work coil and the workpiece can be thought of as an electrical transformer. The work coil is like the primary where electrical energy is fed in, and the workpiece is like a single turn secondary that is short-circuited. This causes tremendous currents to flow through the workpiece. These are known as eddy currents.

 

In addition to this, the high frequency used in induction heating applications gives rise to a phenomenon called skin effect. This skin effect forces the alternating current to flow in a thin layer towards the surface of the workpiece. The skin effect increases the effective resistance of the metal to the passage of the large current. Therefore it greatly increases the heating effect caused by the current induced in the workpiece.
(Although the heating due to eddy currents is desirable in this application, it is interesting to note that transformer manufacturers go to great lengths to avoid this phenomenon in their transformers. Laminated transformer cores, powdered iron cores and ferrites are all used to prevent eddy currents from flowing inside transformer cores. Inside a transformer the passage of eddy currents is highly undesirable because it causes heating of the magnetic core and represents power that is wasted.)

induction heating principle induction heating principle

And for Ferrous metals ?

For ferrous metals like iron and some types of steel, there is an additional heating mechanism that takes place at the same time as the eddy currents mentioned above. The intense alternating magnetic field inside the work coil repeatedly magnetises and de-magnetises the iron crystals. This rapid flipping of the magnetic domains causes considerable friction and heating inside the material. Heating due to this mechanism is known as Hysteresis loss, and is greatest for materials that have a large area inside their B-H curve. This can be a large contributing factor to the heat generated during induction heating, but only takes place inside ferrous materials. For this reason ferrous materials lend themselves more easily to heating by induction than non-ferrous materials.

It is interesting to note that steel looses its magnetic properties when heated above approximately 700°C. This temperature is known as the Curie temperature. This means that above 700°C there can be no heating of the material due to hysteresis losses. Any further heating of the material must be due to induced eddy currents alone. This makes heating steel above 700°C more of a challenge for the induction heating systems. The fact that copper and Aluminium are both non-magnetic and very good electrical conductors, can also make these materials a challenge to heat efficiently. (We will see that the best course of action for these materials is to up the frequency to exaggerate losses due to the skin effect.)

What is Induction Heating used for ?

Induction heating can be used for any application where we want to heat an electrically conductive material in a clean, efficient and controlled manner.

One of the most common applications is for sealing the anti-tamper seals that are stuck to the top of medicine and drinks bottles. A foil seal coated with "hot-melt glue" is inserted into the plastic cap and screwed onto the top of each bottle during manufacture. These foil seals are then rapidly heated as the bottles pass under an induction heater on the production line. The heat generated melts the glue and seals the foil onto the top of the bottle. When the cap is removed, the foil remains providing an airtight seal and preventing any tampering or contamination of the bottle's contents until the customer pierces the foil.

Another common application is "getter firing" to remove contamination from evacuated tubes such as TV picture tubes, vacuum tubes, and various gas discharge lamps. A ring of conductive material called a "getter" is placed inside the evacuated glass vessel. Since induction heating is a non-contact process it can be used to heat the getter that is already sealed inside a vessel. An induction work coil is located close to the getter on the outside of the vacuum tube and the AC source is turned on. Within seconds of starting the induction heater, the getter is heated white hot, and chemicals in its coating react with any gasses in the vacuum. The result is that the getter absorbs any last remaining traces of gas inside the vacuum tube and increases the purity of the vacuum.

Yet another common application for induction heating is a process called Zone purification used in the semiconductor manufacturing industry. This is a process in which silicon is purified by means of a moving zone of molten material. An Internet Search is sure to turn up more details on this process that I know little about.

Other applications include induction melting, induction welding,induction forging and induction  brazing or metals. Induction cooking hobs and rice cookers. Metal hardening of ammunition, gear teeth, saw blades and drive shafts, etc are also common applications because the induction process heats the surface of the metal very rapidly. Therefore it can be used for surface hardening, and hardening of localised areas of metallic parts by "outrunning" the thermal conduction of heat deeper into the part or to surrounding areas. The non contact nature of induction heating also means that it can be used to heat materials in analytical applications without risk of contaminating the specimen. Similiarly, metal medical instruments may be sterilised by heating them to high temperatures whilst they are still sealed inside a known sterile environment, in order to kill germs.

What is required for Induction Heating ?

In theory only 3 things are essential to implement induction heating:

A source of High Frequency electrical power, A work coil to generate the alternating magnetic field, An electrically conductive workpiece to be heated,Having said this, practical induction heating systems are usually a little more complex. For example, an impedance matching network is often required between the High Frequency source and the work coil in order to ensure good power transfer. Water cooling systems are also common in high power induction heaters to remove waste heat from the work coil, its matching network and the power electronics. Finally some control electronics is usually employed to control the intensity of the heating action, and time the heating cycle to ensure consistent results. The control electronics also protects the system from being damaged by a number of adverse operating conditions. However, the basic principle of operation of any induction heater remains the same as described earlier.

The Importance of Induction Coil Design

induction coilThe induction coil, typically made from copper tubing with a 1/8" to 3/16" diameter, is normally cooled with water. The size and shape of the coil - single or multiple turn; helical, round or square; internal or external - should reflect the shape of your workpiece and variables of your process.

With good coil design, the proper heat pattern is achieved and the efficiency of the induction heating power supply is maximized without making it difficult to insert and retrieve your part. You can read more about this important aspect of induction heating in our free tech note.

Typical applicationsTypical applications of induction are the melting of metals, the heating of metals for design, the brazing and welding and all sorts of surface treatments. However, by using electric conductive recipients (e.g.graphite) also other materials like glass can be heated

1.Induction Melting of metals by means of induct ion crucible furnaces

An induction crucible furnace essentially consists of a crucible with refractory lining, that contains the material to be melted and that is surrounded by the induction coil. The coil is water-cooled and is surrounded by an iron core, in order to improve magnetic coupling.
There are applications at 50Hz as well as mid-frequency applications. The power range (up to 10MW and more) and the specific powers (up to 1200 kW/ton) are extremely high. The melting can therefore occur very quickly.
Low-frequency induction crucible furnaces (50Hz) are usually applied for big applications (large power and large capacity). Mid-frequency furnaces are rather used in smaller applications. They offer more flexibility and are more compact. In general there is a trend towards using mid-frequency furnaces at the expense of low-frequency furnaces.

2.Induction Brazing

Induction Brazing is an assembly technique where two pieces are joined together by means of a third material that is brought to its melting temperature. In the connection zone both pieces are heated up to a temperature higher than the melting temperature of the third material. Induction is frequently applied because of the precise localisation of the heating. Moreover the heating happens very quickly which makes that the oxidation or structural or compositional changes can be controlled. Brazing under inert atmosphere is possible. Induction heating is suited for high production speeds in automatized production lines.

3.Inductive hardening of steel

Steel with a carbon percentage of at least 0.3% is qualified for surface hardening. For this the workpiece is heated up to approximately 900°C and after that it is chilled. The technique is used for the hardening of gear wheels, crankshafts, valve stems, saw blades, spades, rails, and many other things.The inductive process has the advantage that the treatment can be localised very accurately. Moreover, the chemical composition of the surface layer doesn’t change, which is the case for other surface hardening techniques. Because of the selective heating less energy is required than for a complete heating of the product and distortion can be avoided. Typical for inductive hardening are the very high energy densities (1.5 to 5kW/cm²) and the short treatment times (2 seconds). Figure 4 shows some realisations of inductors. Some inductors are equipped with a spraying system that allows chilling of the workpiece right after the heating.

4.Inductors for hardening

Induction hardening is especially applied in automated production processes with sufficient production volume. With induction heating a constant, high production quality can be reached. The energy consumption and the production losses are lower than for conventional techniques. dielectric heating Polycondensation glues and polyvinyl acetate glues are especially suited for high frequency bonding. It has already been used in woodwork and wood processing, drying and bonding……