The induction coil, also known as an ‘inductor’, is essential to the induction heating process. Many factors contribute to a coil’s effectiveness: the care taken to make it, the quality of the materials used, its shape, its maintenance, its correct matching with the power source, etc. That’s why it’s so important to insist on professionally made and maintained coils—preferably from the same people who made your induction heating system.
Inductin coil design is therefore one of the most important aspects of the overall induction heating machine. A well-designed inductor provides the proper heating pattern for your part and maximizes the efficiency of the induction heating power supply, while still allowing easy insertion and removal of the part.
The induction coil does not have to be shaped in a helix. With the right design, it is possible to heat conductive materials of any size and form, and also possible to heat only the portion of material required. It is even possible to heat different zones of the part at the same or different temperatures by means of a proper design of the inductor geometry. Temperature uniformity within your part is achieved through correct inductordesign. The most effective uniformity can be achieved in round parts. Due to the nature of electrical current path flow, parts with sharp edges could preferentially heat in those areas if the proper inductor design is not used.
The inductor is similar to a transformer primary, and the workpiece is equivalent to the transformer secondary (Fig.1). Therefore, several of the characteristics of transformers are useful in the development of guidelines for coil design. One of the most important features of transformers is the fact that the efficiency of coupling between the windings is inversely proportional to the square of the distance between them.In addition, the current in the primary of the transformer, multiplied by the number of primary turns, is equal to the current in the secondary, multiplied by the number of secondary turns. Because of these relationships, there are several conditions that should be kept in mind when designing any coil for induction heating:
1) The induction coil should be coupled to the part as closely as feasible for maximum energy transfer. It is desirable that the largest possible number of magnetic flux lines intersect the workpiece at the area to be heated. The denser the flux at this point, the higher will be the current generated in the part.
2) The greatest number of flux lines in a solenoid coil are toward the center of the coil. The flux lines are concentrated
inside the coil, providing the maximum heating rate there.
3) Because the flux is most concentrated close to the coil turns themselves and decreases farther from them, the geometric center of the coil is a weak flux path. Thus, if a part were to be placed off center in a coil, the area closer to the coil turns would intersect a greater number of flux lines and would therefore be heated at a higher rate, whereas the area of the
part with less coupling would be heated at a lower rate; the resulting pattern is shown schematically in Fig. 2. This effect is more pronounced in high-frequency induction heating.