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Induction heating is an accurate, fast, repeatable, and efficient non-contact technology for heating metals or any other conductive materials.
The induction heating system consists of an induction power supply that converts line power into alternating current and delivers it to the working head, and a working coil that generates an electromagnetic field in the coil. The workpiece is placed in the coil so that the magnetic field induces a current in the workpiece, which in turn generates heat.
The water-cooled coil is located around or adjacent to the workpiece. It is not in contact with the workpiece, and the heat is only generated by the induced current passing through the workpiece. The material used to manufacture the workpiece can be metal, such as copper, aluminum, steel, or brass. It can also be a semiconductor, such as graphite, carbon or silicon carbide.
In order to heat non-conductive materials such as plastic or glass, an induction heating conductive base, such as graphite, can be used, and then the heat is transferred to the non-conductive material.
Induction heating is suitable for processes as low as 100ºC (212°F) and as high as 3000°C (5432°F). It is also used for short heating processes that last less than half a second and heating processes that last several months.
Induction heating is used in domestic and commercial cooking for a variety of applications, such as heat treatment, welding, welding preheating, melting, heat charging in industry, sealing, brazing, curing, and research and development.
Induction generates an electromagnetic field in the coil and transfers energy to the workpiece to be heated. When current passes through the wire, a magnetic field is generated around the wire.
The advantages of induction are:
Induction heating is done using two methods:
The first method is called eddy current heating, which is caused by the I²R loss caused by the resistivity of the workpiece material. The second is called hysteresis heating, in which the alternating magnetic field generated by the coil generates energy in the component, thereby changing the magnetic polarity of the component.
When the permeability of the material decreases to 1 and the hysteresis heating decreases, hysteresis heating occurs in the component that reaches the Curie temperature. Eddy current heating constitutes the remaining induction heating effect.
When the direction of the current (AC) changes, the generated magnetic field will fail and will be generated in the opposite direction because the direction of the current is opposite. When the second wire is located in the alternating magnetic field, alternating current will be generated in the second wire.
The current transmitted through the second wire and the current transmitted through the first wire are proportional to each other, and also proportional to the reciprocal of the square of the distance between them.
When the wire in this model is replaced with a coil, the alternating current on the coil will generate an electromagnetic field. When the workpiece to be heated is in the field, the workpiece matches the second wire and generates an alternating current workpiece in it. The I²R loss of the resistivity of the workpiece material causes heat to be generated in the workpiece with the resistivity of the workpiece material. This is called eddy current heating.
With the help of an alternating electric field, energy is transmitted to the workpiece through the working coil.
The alternating current through the coil generates an electromagnetic field, which induces a current through the workpiece as a mirror image of the current through the working coil. The working coil/inductor is part of the induction heating system, which shows the effectiveness and efficiency of the workpiece during heating. There are many types of working coils, from complex to simple.
A spiral wound (or solenoid) coil is an example of a simple coil, which consists of multiple turns of copper tube wound around a mandrel. A coil that is precisely machined from solid copper and brazed together is an example of a complex coil.
The workpiece to be heated and the workpiece material determine the operating frequency of the induction heating system. It is essential to use an induction system that can provide power in the frequency range suitable for the application. The reasons for various operating frequencies can be understood through the so-called "skin effect". When the electromagnetic field induces current in the component, it mainly passes through the surface of the component.
Figure 3. (a) High-frequency induction heating has a superficial skin effect, which is more effective for small parts; (b) Low-frequency induction heating has a deeper skin effect, which is more effective for larger parts.
The higher the working frequency, the shallower the skin depth. Similarly, the lower the operating frequency, the deeper the penetration of skin depth and thermal effects. The skin depth/penetration depth depends on the temperature, operating frequency and material properties of the part.
For example (see Table 1), the stress can be relieved by heating 20 mm steel bars to 540°C (1000°F) using a 3kHz induction system. However, a 10 kHz system will be required to harden the same rebar by heating it to 870°C (1600°F).
Therefore, it can be said that a higher operating frequency (usually more than 50kHz) can be used to heat smaller parts by induction, while a lower operating frequency can be used to heat larger parts more efficiently.
In the case of an advanced solid-state induction power supply with an embedded microprocessor control system, a consistent and effective heating technology can be achieved based on the fact that all components are placed in a consistent position within the coil.
The induction heating system includes tank circuit, power supply and working coil. In industrial applications, there is enough current to flow through the coils and water cooling is required; therefore, the basic installation includes a water cooling device. The alternating current from the alternating current line is converted into alternating current in accordance with the combination of coil inductance, working head capacitance and component resistivity through the power supply.
Figure 4. Typical induction heating system
The material of the work piece determines the heating rate and power required. Iron and steel have higher electrical resistivity and are therefore easy to heat, while aluminum and copper have lower electrical resistivity and therefore require more heat to heat.
Some steels are magnetic in nature, so the resistivity and hysteresis characteristics of the metal are used in induction heating. When heated above the Curie temperature (500-600°C/1000-1150°F), the steel loses its magnetic properties; however, eddy current heating provides the heating technology required for higher temperatures.
The required power is determined by factors such as material type, workpiece size, required heating and heating time. According to the size of the workpiece to be heated, the main factor that needs to be considered is the operating frequency of the induction heating system.
Similarly, in the case of a smaller workpiece, a higher frequency (>50kHz) is required to effectively heat, while in the case of a larger workpiece, a lower frequency (>10kHz) is required and more heat penetration is generated.
When the temperature of the heated workpiece increases, heat will also be lost from the workpiece. As the temperature rises, the radiation and convection losses of the workpiece will develop into a very important factor. Insulation methods are often used at high temperatures to reduce heat loss and reduce the power required by the induction system.
Figure 5. Ambrell induction heating power supply series
This information is derived from materials provided by Ambrell Induction Heating Solutions and has been reviewed and adapted.
For more information on this source, please visit Ambrell Induction Heating Solutions.
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What happens when the induction coil melts, breaks, or touches the metal body during high-frequency operation? Assuming power 50 KW @ 420 VAC, 40 KHz
Could you please explain the ratio of current to voltage and temperature for effective induction heating
Good morning, I am interested in soldering very small wires to the PCB. I want to use solder paste. What frequency do you think should be used? Thanks amir
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