Induction Coil design and fabrication: basic design and modifications by STANLEY ZINN and S. L. SEMIATIN n a sense, coil design for induc- 2) The greatest number of flux lines in a them, the geometric center of the coil is tion heating is built upon a large solenoid coil are toward the center of a weak flux path. Thus, if a part were store of empirical data whose the coil. The flux lines are concentrated to be placed off center in a coil, the development springs from sev- inside the coil, providing the maximum area closer to the coil turns would in- Ieral simple inductor geometries such as heating rate there. tersect a greater number of flux lines the solenoid coil. Because of this, coil 3) Because the flux is most concen- and would therefore be heated at a design is generally based on experi- trated close to the coil turns them- higher rate, whereas the area of the ence. This series of articles reviews selves and decreases farther from part with less coupling would be heated the fundamental electrical consider- at a lower rate; the resulting pattern ations in the design of inductors and is shown schematically in Fig. 2. This describes some of the most common effect is more pronounced in high-fre- coils in use. quency induction heating. 4) At the point where the leads and Basic design considerations coil join, the magnetic field is weaker; The inductor is similar to a transformer therefore, the magnetic center of the primary, and the workpiece is equiva- inductor is not necessarily the geomet- lent to the transformer secondary (Fig. ric center. This effect is most appar- Ep = primary voltage (V); Ip = primary current (A); Np = number of primary turns; I = secondary current (A); N = 1). Therefore, several of the charac- s s ent in single-turn coils. As the number number of secondary turns; Es = secondary voltage (V); Rl teristics of transformers are useful in = load resistance(Ω) of coil turns increases and the flux the development of guidelines for coil from each turn is added to that from design. Fig. 1: Electrical circuit illustrating the the previous turns, this condition be- analogy between induction heating and the One of the most important features comes less important. Due to the im- transformer principle. of transformers is the fact that the ef- practicability of always centering the ficiency of coupling between the wind- part in the work coil, the part should ings is inversely proportional to the be offset slightly toward this area. In square of the distance between them. addition, the part should be rotated, if In addition, the current in the primary practical, to provide uniform exposure. of the transformer, multiplied by the number of primary turns, is equal to 5) The coil must be designed to pre- the current in the secondary, multiplied vent cancellation of the magnetic field. by the number of secondary turns. Be- The coil on the left in Fig. 3 has no cause of these relationships, there are inductance because the opposite sides several conditions that should be kept of the inductor are too close to each in mind when designing any coil for Fig. 2: Induction heating pattern produced other. Putting a loop in the inductor induction heating: in a round bar placed off center in a round (coil at center) will provide some 1) The coil should be coupled to the induction coil. inductance. The coil will then heat a part as closely as feasible for maxi- conducting material inserted in the mum energy transfer. It is desirable opening. The design at the right pro- that the largest possible number of vides added inductance and is more magnetic flux lines intersect the work- representative of good coil design. piece at the area to be heated. The Because of the above principles, denser the flux at this point, the higher some coils can transfer power more will be the current generated in the part. readily to a load because of their abil- ity to concentrate magnetic flux in the S. Zinn is executive vice president, Ameritherm, Inc., Rochester , N.Y.; (716) 427-7840.S.L. area to be heated. For example, three Semiatin is a project manager in the Center for coils that provide a range of heating Materials Fabrication at Battelle Columbus Divi- behaviors are: sion; (614) 424-7742. This article is excerpted from the book “Ele- • a helical solenoid, with the part ments of Induction Heating,” published by Elec- Fig. 3: Effect of coil design on Inductance or area to be heated located within the tric Power Research Institute (EPRI) and dis- tributed by ASM International, (516) 338-5151 (from F. W. Curtis, High Frequency Induc- coil and, thus, in the area of greatest and used with permission of EPRI tion Heating, McGraw-Hill, New York, 1950) magnetic flux; 32 HEAT TREATING/JUNE 1988 • a pancake coil, with which the flux 5c). Pancake coils (Fig. 5d) are gener- from only one surface intersects the ally utilized when it is necessary to heat workpiece; and from one side only or when it is not pos- • an internal coil for bore heating, in sible to surround the part. Spiral coils which case only the flux on the outside (Fig. 5e) are generally used for heating of the coil is utilized. bevel gears or tapered punches. Inter- nal bores can be heated in some cases In general, helical coils used to heat round with multiturn inductors (Fig. 5f). It is workpieces have the highest values of important to note that, with the excep- coil efficiency and internal coils have the Fig. 4: Typical configurations for induc- tion of the pancake and internal coils, lowest values (Table I). Coil efficiency tion coils: (a) multiturn, single place; (b) the heated part is always in the center is that part of the energy delivered to single-turn, single place; (c) single-turn, of the flux field. the coil that is transferred to the multiplace (from F. W. Curtis, High Fre- Regardless of the part contour, the workpiece. This should not be confused quency Induction Heating, McGraw-Hill, most efficient coils are essentially modi- with overall system efficiency. New York, 1950) fications of the standard, round coil. A Besides coil efficiency, heating pat- conveyor or channel coil, for example, tern, part motion relative to the coil, can be looked at as a rectangular coil and production rate are also important. whose ends are bent to form “bridges” Because the heating pattern reflects in order to permit parts to pass through the coil geometry, inductor shape is on a continuous basis. The parts, how- probably the most important of these (a) Round (b) Rectangular (c) Formed ever, always remain “inside” the chan- factors. Quite often, the method by nels where the flux is concentrated. Fig. which the part is moved into or out of 6 illustrates similar situations in which the coil can necessitate large modifi- the areas to be hardened are beside the cations of the optimum design. The center of the coil turns, and thus are kept type of power supply and the produc- (d) Pancake (e) Spherical-helical (e) Internal in the area of heaviest flux. tion rate must also be kept in mind. If one part is needed every 30 seconds Fig. 5: Multiturn coils designed for heat- Internal coils but a 50-second heating time is re- ing parts of various shapes: (a) round; Heating of internal bores, whether for quired, it will be necessary to heat (b) rectangular; (c) formed; (d) pancake; hardening, tempering, or shrink fitting, is parts in multiples to meet the desired (e) spiral-helical; (f) internal (from F. W. one of the major problems most com- production rate. Keeping these needs Curtis, High Frequency Induction Heat- monly confronted. For all practical pur- ing, McGraw-Hill, New York, 1950) in mind, it is important to look at a wide poses, a bore with a 0.44-inch (1.1-cm) range of coil techniques to find the internal diameter is the smallest that can most appropriate one. be heated with a 450-kHz power sup- ply. At 10 kHz, the practical minimum Medium-to-high-frequency ID is 1.0 inch (2.5-cm). Area to be Simple solenoid coils are often relied hardened Tubing for internal coils should be on in medium-to-high-frequency ap- made as thin as possible, and the bore plications such as heat treatment. should be located as close to the sur- These include single- and multiple-turn face of the coil as is feasible. Because Area to be hardened types. Fig. 4 illustrates a few of the Coil (c) the current in the coil travels on the in- more common types based on the sole- in position side of the inductor, the true coupling of noid design. Fig. 4a is a multiturn, the maximum flux is from the ID of the single-place coil, so called because it coil to the bore of the part. Thus, the is generally used for heating a single Fig. 6: Coil modifications for localized conductor cross section should be mini- part at a time. A single-turn, single- heating (from F. W. Curtis, High Fre- mal, and the distance from the coil OD place coil is also illustrated (Fig. 4b). quency Induction Heating, McGraw-Hill, New York, 1950) to the part (at 450 kHz) should approach Fig. 4c shows a single-turn, multiplace 0.062-inch (0.16-cm). In Fig.7a, for coil. In this design, a single turn inter- example, the coupling distance is too Too deep Minimum acts with the workpiece at each part- Flat tubing great; coil modification improves the de- Keep close heating location.