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    Ia sense, coil design for induction heating is built upon a large store of empirical data whose development springs from several simple inductor geometries such as the solenoid coil. Because of this, coil design is generally based on experience. This series of articles reviews the fundamental electrical consider ations in the design of inductors and describes some of the most common coils in use.

    Basic design considerations 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 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. 4) At the point where the leads and coil join, the magnetic field is weaker; therefore, the magnetic center of the inductor is not necessarily the geometric center. This effect is most apparent in single-turn coils. As the number of coil turns increases and the flux from each turn is added to that from the previous turns, this condition becomes less important. Due to the impracticability of always centering the part in the work coil, the part should be offset slightly toward this area. In addition, the part should be rotated, if practical, to provide uniform exposure. 5) The coil must be designed to prevent cancellation of the magnetic field. The coil on the left in Fig. 3 has no inductance because the opposite sides of the inductor are too close to each other. Putting a loop in the inductor (coil at center) will provide some inductance. The coil will then heat a conducting material inserted in the opening. The design at the right provides added inductance and is more representative of good coil design. Because of the above principles, some coils can transfer power more readily to a load because of their ability to concentrate magnetic flux in the area to be heated.

    For example, three coils that provide a range of heating behaviors are: ? a helical solenoid, with the part or area to be heated located within the coil and, thus, in the area of greatest magnetic flux; Coil design and fabrication: basic design and modifications Induction by STANLEY ZINN and S. L. SEMIATIN S. Zinn is executive vice president, Ameritherm, Inc., Rochester , N.Y.; (716) 427-7840.S.L. Semiatin is a project manager in the Center for Materials Fabrication at Battelle Columbus Division; (614) 424-7742. This article is excerpted from the book “Elements of Induction Heating,” published by Electric Power Research Institute (EPRI) and distributed by ASM International, (516) 338-5151 and used with permission of EPRI 32 HEAT TREATING/JUNE 1988 Ep = primary voltage (V); Ip = primary current (A); Np = number of primary turns; Is = secondary current (A); Ns = number of secondary turns; Es = secondary voltage (V); Rl = load resistance(?) Fig. 1: Electrical circuit illustrating the analogy between induction heating and the transformer principle. Fig. 2: Induction heating pattern produced in a round bar placed off center in a round induction coil. Fig. 3: Effect of coil design on Inductance (from F. W. Curtis, High Frequency Induction Heating, McGraw-Hill, New York, 1950) ? a pancake coil, with which the flux from only one surface intersects the workpiece; and ? an internal coil for bore heating, in which case only the flux on the outside of the coil is utilized. In general, helical coils used to heat round workpieces have the highest values of coil efficiency and internal coils have the lowest values (Table I).

    Coil efficiency is that part of the energy delivered to the coil that is transferred to the workpiece. This should not be confused with overall system efficiency. Besides coil efficiency, heating pattern, part motion relative to the coil, and production rate are also important. Because the heating pattern reflects the coil geometry, inductor shape is probably the most important of these factors. Quite often, the method by which the part is moved into or out of the coil can necessitate large modifications of the optimum design. The type of power supply and the production rate must also be kept in mind. If one part is needed every 30 seconds but a 50-second heating time is required, it will be necessary to heat parts in multiples to meet the desired production rate. Keeping these needs in mind, it is important to look at a wide range of coil techniques to find the most appropriate one. Medium-to-high-frequency Simple solenoid coils are often relied on in medium-to-high-frequency applications such as heat treatment. These include single- and multiple-turn types. Fig. 4 illustrates a few of the more common types based on the solenoid design. Fig. 4a is a multiturn, single-place coil, so called because it is generally used for heating a single part at a time. A single-turn, singleplace coil is also illustrated (Fig. 4b). Fig. 4c shows a single-turn, multiplace coil. In this design, a single turn interacts with the workpiece at each partheating location. Fig. 4(d) shows a multiturn, multiplace coil. More often than not, medium-tohigh-frequency applications require specially configured or contoured coils with the coupling adjusted for heat uniformity.

    In the simplest cases, coils are bent or formed to the contours of the part (Fig. 5). They may be round (Fig. 5a), rectangular (Fig. 5b), or formed to meet a specific shape such as the cam coil (Fig. 5c). Pancake coils (Fig. 5d) are generally utilized when it is necessary to heat from one side only or when it is not possible to surround the part. Spiral coils (Fig. 5e) are generally used for heating bevel gears or tapered punches. Internal bores can be heated in some cases with multiturn inductors (Fig. 5f). It is important to note that, with the exception of the pancake and internal coils, the heated part is always in the center of the flux field. Regardless of the part contour, the most efficient coils are essentially modifications of the standard, round coil. A conveyor or channel coil, for example, can be looked at as a rectangular coil whose ends are bent to form “bridges” in order to permit parts to pass through on a continuous basis. 

   The parts, however, always remain “inside” the channels where the flux is concentrated. Fig. 6 illustrates similar situations in which the areas to be hardened are beside the center of the coil turns, and thus are kept in the area of heaviest flux.