A typical induction heater system includes a power supply, an impedance matching circuit, an energy storage circuit, and an applicator. The applicator, which is an induction coil, may be part of the energy storage circuit. The energy storage circuit is typically a set of capacitors and inductors connected in parallel. The capacitors and inductors in the energy storage circuit are reservoirs of electrostatic energy and electromagnetic energy, respectively. At a resonant frequency, the capacitors and inductors begin to oscillate their stored energy with each other. In a parallel configuration, this energy conversion occurs at high currents. The high current through the coil helps to transfer the energy well from the induction coil to the workpiece.
The induction coil determines how effectively and efficiently the workpiece is heated. Induction coils are water-cooled conductors made of copper tubing that can be easily formed into coil shapes for the induction heating process. The induction heating coil itself does not become hot when water flows through it.
Working coils vary in complexity from simple spiral or solenoid wound coils, consisting of multiple turns of copper tubing wrapped around a mandrel, to coils precision machined and brazed from solid copper.
The coil generates an alternating electromagnetic field (EMF) through the alternating current flowing in it, which transfers energy from the power source to the workpiece. The alternating electromagnetic field (EMF) of the coil produces induced currents (eddy currents) in the workpiece, which generate heat due to I Squared R losses (core losses).
The current in the workpiece is proportional to the strength of the electric potential of the coil. This energy transfer is known as the transformer effect or eddy current effect.
Success in the use of induction heating processes depends greatly on the proper design of the induction device. Inductors used for high-frequency induction heating are often referred to as heating coils and can be made in a variety of types and styles, depending on the shape of the metal surface to be heated. Their design must follow certain principles in order to obtain maximum efficiency from the high frequency generator.
In general, localized induction heating is limited only by the ability to construct the coil to accommodate the surface to be treated and by the capacity or power output of the generator. In other words, the generator should have enough power to heat the surface quickly. The job is then suitable for induction heating if the shape of the part fits the surrounding coil.
Eddy currents induced in the workpiece are often a mirror image of the coil current. It is useful to keep this in mind when designing coils for heating strange shapes. The current establishes a magnetic field perpendicular to the current. The voltage induced by this field is greatest in the direction at right angles to the field and parallel to the original current. The result is that the eddy currents are parallel to the coil current within the limits imposed by the shape of the workpiece.
The eddy currents and the coil current are also attracted to each other. They are concentrated near the surface due to the skin effect. When the frequency is low, there is less tendency for the currents to attract each other than when the frequency is high. In general, if the frequency is high enough for effective induction heating, the coil currents and eddy currents will try to get as close to each other as possible.
Almost any continuous shape on a flat workpiece surface can be heated. The current induced in the workpiece follows the shape of the coil. The air gap between the coil and the workpiece plays an important role in this type of heating. As the space between the coil and the workpiece increases, the magnetic flux connected to the workpiece decreases rapidly. The clarity of the heating pattern in the workpiece reflecting the shape of the coil increases with frequency and the proximity of the coil to the surface of the workpiece.
Induction coil design can have a significant impact on process efficiency and final part quality, and the optimal coil design for your product depends greatly on your application. Certain coil designs tend to be best suited for specific applications, while less than ideal coil application pairings can result in slow or irregular heating, higher defect rates and lower quality products.
Designing an Induction Coil for Your Application
Start by understanding where heat needs to be generated in the part to perform the process, then design the coil to achieve the heating effect. Again, the choice of frequency will depend on the induction heating application you will be using.
Before designing an induction coil, consider the following three factors as well as your induction application:
Movement of the part relative to the coil - Some applications rely on the movement of the part with the aid of a conveyor, turntable or robot. A properly designed induction coil can accommodate these individual handling requirements without loss of heating efficiency.
Frequency - Higher frequencies are used for applications such as brazing, soldering, annealing or heat treating that require surface heating. Lower frequencies are preferred for applications where parts need to be heated to the core, such as forging and mold heating.
Power Density Requirements - Short cycle heating applications requiring high temperatures require higher power densities. Higher power densities may also be required to confine the hot zone to a smaller area, thereby reducing the heat affected area.
Coupling is the transfer of energy that occurs in the space between the heated portion of the coil and the workpiece. Therefore, the coupling distance is how much space is required to balance efficiency and manufacturing requirements.
Generally, the distance increases with part diameter, with typical values of 0.75, 1.25, and 1.75 inches (19, 32, and 44 millimeters), or billet diameters of approximately 1.5, 4, and 6 inches (38, 102, and 152 millimeters) ), respectively.
Magnetic flux tends to concentrate in the center of the length of the solenoid's working coil. This means that the heating rate generated in this region is usually greater than that generated at the ends. In addition, if the part being heated is long, conduction and radiation will
carry heat away from the ends at a greater rate. Coils can be modified to provide better heating uniformity along the length of the part. The technique of adjusting the number of turns, spacing, or coupling of the coil to the part to achieve a uniform heating pattern is sometimes called "characterizing" the coil.
Coil Characterization
Six other common methods for improving heating uniformity are
Heating two separate zones on a part
Heating a tapered part
Heating parts with secondary operations
Heating many different parts with one coil
The type and design of the induction coil determines how effectively and efficiently the workpiece is heated. Working coils vary in complexity from simple helical or solenoid wound coils, consisting of multiple turns of copper tubing wound around a mandrel, to coils precision machined and brazed from solid copper.
The spiral solenoid coil is the most common induction coil design. It offers a wide range of heating behaviors because the part or heated area is located in the area of maximum magnetic flux within the coil. The magnetic flux lines in a solenoid coil are concentrated inside the coil, thus providing the maximum heating rate at that location.
Double deformation pancake coils
In applications such as heating the tip of a shaft, achieving temperature uniformity can be difficult due to the counteracting effect at the center of the tip surface. Similar to the scheme below, double deformed pancake coils with inclined sides can be used to achieve uniform heating distribution. Attention must be paid to the orientation of the two pancakes, where the center winding is wound in the same direction and has a magnetizing effect.
Separate loop coils
In applications such as welding a narrow strip on one side of a long cylinder, where the relatively long length must be heated much higher than the rest of the object, the current return path will be very important. With a shunt type coil, the high current induced in the weld path will be split into two parts with wider currents. This results in a heating rate at the weld path that is at least four times higher than the rest of the object.
Channel coils
Channel type coils are used if the heating time is not very short and a fairly low power density is required. Multiple heated parts pass through the coil at a constant rate and reach maximum temperature as they leave the machine. The ends of the coils are usually curved to provide a path for parts to enter and exit the coil. Plate concentrators can be used with multi-turn channel coils where contour heating is required.
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