Induction Heating Consultant

Induction Heating

Stanley Zinn

Stanley Zinn • Induction Consultants • Tel: 585-737-8824
15307 Strathearn Drive, Unit 11202, Delray Beach FL 33446 •
email address

Stanley Zinn
Coils Figure 1
Fig. 1: Comparative heating patterns produced by using round vs. square tubing for a solenoid induction coil (from M. G. Lozinsky, Industrial Applications of Induction Heating, Pergamon Press, London, 1969)
Figure 2
Fig. 2: Typical techniques for bracing of induction coil turns (from F.W. Curtis, High Frequency Induction Heating, McGraw-Hill, New York, 1950)
Figure 3
Fig. 3: Schematic circuit diagram indicating the inductance of the coil leads and induction coil itself: L(1), L(3)-lead inductances; L(2)-induction coil inductance; C(1)-tank capacitance; E(1) tank voltage.
Figure 4
Fig. 4: Effect of coil-lead spacing on lead inductance; closer spacing, as in (b), reduces lead inductance and thus power losses (from F.W. Curtis, High Frequency Induction Heating, McGraw-Hill, New York, 1950)
Figure 5
Fig. 5: Lead construction for multiplace inductors; lead design in (b) is preferable because of lower heat inductance (from F.W. Curtis, High Frequency Induction Heating, McGraw-Hill, New York, 1950)

Coil Design And Fabrication Tech Note

Click to view PDFby STANLEY ZINN and S. L. SEMIATIN
as featured in Heat Treating Magazine, June 1988

Part 1 • Basic Designs and Modifications

Part 2 • Speciality Coils

Part 3 • Fabrication Principles

Part 3 - Fabrication Principles

S. Zinn is an independent consultant on induction heating; (585) 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 (EPR) and distributed by ASM International. (516) 338-5151 and used with permission of EPRI

Because of its low resistivity, fully annealed, high-conductivity copper is most commonly used in the fabrication of induction heating coils. The copper is typically in a tubular form, with a minimum outer diameter of 0.125 inch (0.32 cm) to allow for water cooling. Material of this kind is available in a wide range of cross sections (round, square, and rectangular) and sizes.

Selection of Tubing
In addition to the 12R loss due to its own resistivity, the coil surrounds the load and absorbs additional heat through radiation and convection from the heated surface. Therefore, it is essential that the tubing selected for the work coil have a sufficient cooling path to remove this heat. Otherwise, the resistivity of the copper will increase due to the temperature increase, thus creating greater coil losses. In some instances, such as large coils, it may be necessary to break up the individual water paths in a coil to prevent overheating and possible coil failure.

Another factor in the selection of tubing for induction coils relates to the fact that the current in the work coils is traveling at a specific reference depth that depends on the power supply frequency and the resistivity of the copper. Accordingly, the wall thickness of the coil tubing should be selected to reference-depth limits similar to those used for induction heating of copper. Suggested wall thicknesses for various frequencies are shown in Table 1. However, copper availability must be considered, and often wall thicknesses less than twice the reference depth are used with only a nominal loss in overall coil efficiency. Square copper tubing is also commercially available and is frequently used in coil fabrication. It offers a considerable advantage in that it couples more flux to the part per turn than round tubing (Fig. 1). Moreover, it is more easily fabricated in that it will not collapse as readily on bending. It is also easily mitered to create sharp, close bends as required. If only round tubing is available, it can be flattened in a vise or other simple device to adjust the resultant thickness dimension. This flattening can be done with minimal decrease in dimension of the water-flow path.

Table 1

Table 1

Coil Forming
In fabrication of copper coils, it must be noted that the copper work hardens with increasing deformation. Thus, most fabricators anneal the tubing every few bends to relieve this condition by heating the tubing until it is bright red, then cooling it rapidly in water. These intermediate anneals prevent fracture of the tubing during fabrication.

In some forming operations, it may be desirable to fill the coil with sand or salt to preclude collapse of the tubing. In addition, there are several low-temperature alloys with melting points below 212°F (100°C) that are normally used to perform this same function. When the coil is completed, it is immersed in boiling water. The alloy then flows out freely and can be reused at another time. With any of these techniques, once filled, the tubing acts as a solid rod during forming and can be simply cleared on completion.

Bracing of Coils
Because electric currents flow in both the workpiece and the coil, magnetomotive forces between the two are developed. The magnitudes of the forces depend on the magnitudes of the currents. If sufficiently large, the forces may cause the part to move in the coil. If the part has a large mass, however, the coil will tend to move relative to the workpiece. The turns may also tend to move relative to each other. It is important, therefore, that the coil turns be suitably braced to prevent movement and possible turn-to-turn shorting. Furthermore, coil motion relative to the part must be prevented to avoid undesirable changes in the heating pattern.

Much of the acoustic noise generated during low-frequency operations also occurs due to coil vibration, much as a speaker coil and magnet structure work in an audio system. Bracing and physical loading of the coil to restrict its movement will aid in reducing this condition. On very large, high-current coils, the magnetomotive force exerted L can be extremely large, and if proper bracing is not provided; the coil may gradually work harden and finally fail. Typical bracing techniques are illustrated in Fig. 2. In Fig. 2a, brass studs are brazed to every other turn. These studs are then secured to insulator posts to hold them in a fixed relation to each other. Nuts on each side of the stud at the insulator allow adjustment for characterization of the heating pattern. In Fig. 2b, the insulation has been contoured to hold the turns relative to each other after the end turns are secured with studs.

The insulation used for bracing applications must meet the criteria for the coil design. In addition to the installation being capable of withstanding the heat radiated from the workpiece, its electrical capabilities must permit it to withstand the voltage between the mounting studs or the turn-to-turn voltages of the coil. This is of particular concern when using high-voltage RF coils where up to 12,000v may be impressed across the total coil. It may be necessary in these instances to provide slots between the stud locations in the insulator boards to increase the electrical creepage path between the studs. It may also be necessary to increase the heat-resistant characteristics of the insulation by facing the area exposed to the heated surface with a sheet of high-temperature insulation.

For purposes of rigidity, cleanness, and protection, it is sometimes desirable to encapsulate work coils in a plastic or refractory material. The same kind of care with respect to voltage and temperature characteristics must be taken with these materials as with insulating boards. For low-temperature induction heating applications, epoxy encapsulation of the coil is quite common. For heating of steel billets, coils are usually cast in a refractory cement to prevent scale from the part from falling between the turns. In coating of coils with refractory materials, care must be taken to match the pH of the refractory to that of the material being heated; for example, an acidic refractory is required for the ferrous scale that drops off during high-temperature healing of steels.

Design Considerations
All coils represent an inductance to the tank circuit. However, in practice, the working portion of the coil may in fact be only a small portion of the inductance presented to the tank. Between the output terminals of the generator or heat station and the heating portion of the work coil, there may be a considerable distance of output lead. In any case, some finite distance exists between the heat-station terminations and the actual coil. Design and construction of these work-coil leads can be a major factor in determining job feasibility.

The effect of lead construction on system performance can be best understood with respect to the tank circuit of which it is a part (Fig. 3). The coil/load inductance is represented by L2. Each lead connecting the tank capacitor to the coil has its own inductance (L1, L3). If the voltage in the tank, ET, is impressed across the total of these inductances, then some voltage drop appears across each. The full voltage will thus never appear across the work coil. If the inductance of the coil (L2,) is approximately 10 times the total inductance of the leads (L1 plus L3) or greater, a maximum of 10% of the total voltage will be lost in the leads. Any loss less than this can be considered nominal.

Some coils have many turns, a large cross-sectional area, and thus fairly high inductance. Hence, the comparative lead inductance is small. As the frequency increases, coils often become smaller in size, and their inductance and inductive reactance decrease. As the distance between the heat station and coil increases, therefore, these lead inductances can become critical.

Several coil designs that illustrate the effect of lead design are shown in Figs. 4 and 5. In Fig. 4a, a coil with leads far apart is depicted. The space between the leads presents an inductance almost equal to that of the coil. Thus, a major portion of the voltage will not appear in the working area. A better design (Fig. 4b) minimizes this gap and thus improves heating efficiency. Fig. 5b also shows single-turn, multiplace coils with an extremely poor and an improved lead design.

Another factor to consider is the interaction of the leads with nearby metal structures. Because all leads have some inductance, they can act as work coils. Thus, a conductor placed within their field will be heated. Leads placed adjacent to metal structures will tend to heat them. In addition to unwanted heat, this loss reduces the power available to the load. It is important that lead-to-lead separation be minimized and proximity to metallic structural members be considered. Whenever possible, duct housings, trays, or conduits must be of low-resistivity or insulating materials, such as aluminum or plastic.

Typical Lead Design
Induction heating lead designs typically make use of water-cooled copper plates or tubes. When coil voltages are low (less than or equal to 800v), a low-inductance structure known as a fishtail is often utilized. A fishtail is a pair of parallel copper plates that are water cooled to maintain low resistivity. They are placed with their wide bus faces parallel, and are either separated physically with air as an insulator or held together by nylon bolts and nuts with teflon or a similar material acting as a spacer. Extending from the heat station to a point as close as possible to the operating area of the work coil, they present minimum inductance and provide maximum power at the coil. Depending on conditions and construction, efficient runs of up to 15 feet are practical. The thickness of the copper plates should be consistent with the frequency, as noted in Table I, and cooling-water paths and sizes must be consistent with the power being transmitted as well. The copper plates should increase in width with generator power and the distance of the run. Moreover, they should be spaced as close together as possible with only enough space for proper insulation to prevent arcing.

As the coil inductance increases (e.g., as the number of turns or the coil diameter increases), lead length becomes less critical, and plain copper tubing leads then become more practical. However, larger coils also require higher terminal voltages. These leads must also be kept as close as possible to each other while maintaining sufficient spacing to prevent arcing. However, good practice still dictates that coil leads be kept to a minimum length and that copper tubing sizes be used that are consistent with frequency, current, and cooling requirements.

Rigid leads, whether tubing or bus, built to the above guidelines are inherently more effective than flexible, water-cooled cable. In some cases, however, it is absolutely necessary to use flexible connections. There are several variations in flexible leads, but it must be kept in mind that the inductive lead losses in flexible cables are usually much greater than those for rigid connections. The most common flexible lead is generally used in applications similar to tilt-type induction melting furnaces and consists of a water-cooled, spiral-wound inner conductor (similar to BX cable, but made of copper) with an outer insulating covering. These leads are used in pairs with one for each lead connection. Not only must they be sized for current and frequency, but the insulation must be capable of handling the voltage rating of the system. Flexible leads should be tied together with insulating straps.

Coaxial leads are also available and may be rigid or flexible. They consist of an inner conductor and an outer sheath or housing that is also used as the return conductor. This outer sheath is generally at ground potential. In addition to providing an extremely low-inductance lead, the outer ground acts to eliminate possible strong radiation or inductive coupling to adjacent structures.

Rigid coaxial lead is generally quite expensive and is usually limited to those applications where it is imperative to transmit high power at high frequency over some distance.

Another type of coaxial cable is the water-cooled type generally used at radio frequencies. It consists of a low-inductance, braided inner conductor that runs through a water-cooled tube, and an outer return braid that is also water cooled. This construction is generally utilized with medium-to-high-inductance coils because its construction does not greatly minimize lead inductance but does provide flexibility. This last type of lead is most common when the operator must physically move the coil from part to part as in bottle sealing.