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Fig. 10. Interior view

Fig. 10. Interior view of generator of Fig. 8, showing the r.f. oscillator. paths is reduced exponentially, which demonstrates why flux, eddy current, and the consequent heating is greatest on the surface of the work piece. The "depth of penetration" is the point at which the current density is about 37% of its value at the outside surface of the work piece. The actual values of "depth of penetration" for various metals at frequencies ranging from 10,000 cps to 100 mc. are shown in the graph of Fig. 7. By proper selection of frequency it is possible to determine accurately the depth to which the metal should be heated as well as the actual skin temperatures. The Frequency Problem The choice of frequency determines the depth of penetration as indicated in Fig. 7. In practice, frequencies ranging from those used for power transmission to high radio frequencies are employed. In addition to the effect on the work piece, selection of the right frequency must be considered along with efficiency, complexity, and cost when designing the generating equipment. Every radio ham knows that it usually costs more to put the same power on the air in the 2 -meter band than in the 80 -meter band. At the higher frequencies tubes become less efficient, losses inside the generating equipment rise, and the amount of spuriously radiated power increases. This latter type of loss is costly in two ways. First, any r.f. power that is radiated into the surrounding atmosphere is lost to the work coil; second and of more importance, radiating energy interferes with other services such as communication or radar. In most machines the exact frequency is not controlled by a crystal oscillator but depends on the tuning of the power oscillator and stable, known frequencies are difficult to maintain. This is particularly true where frequencies are adjusted for a certain work piece during one production run but must be changed during the next run. Radiation at frequencies which can interfere with aircraft beacons, communications, and similar services is possible. In addition, the harmonics generated by high - power induction heating machines may reach into the TV, FM, v.h.f., and other frequency bands. To avoid interference with other services, the FCC has issued strict limits on permissible radiation from industrial and medical equipment. For operation at any frequency except 27.55 mc., the maximum radiated field permitted is 10 microvolts -per-meter at a distance of one mile. The 27.55 mc. band is set aside as an experimental and industrial frequency allocation within which these limits need not be observed. The radiation limits apply not merely to the fundamental but also to all harmonic and subharmonic signals which might be due to induction heating machines. To insure compli- Fig. 11. Here is the basic circuit that is employed in the 7.5. kw. induction heating unit described in text. TRANS ance with FCC regulations each induction heating machine must be tested, signal strength readings must be taken, and a certificate of approval obtained from the engineering firm doing the testing. In many installations, especially at the lower frequencies, radiation does not present a problem, but where higher frequencies are used shielded work booths and similar devices must occasionally be employed to avoid interference. How serious the interference problem can be is shown in a recent case history of persistent interference with an aircraft localizer beacon. The interference was finally traced, by an FCC mobile unit, to an insufficiently shielded induction heating installation. Makers of induction heating machines are now aware of the radiation problem and usually have every new model checked for radiation before leaving their plants. Some of our readers who have experience in building transmitters may ask why crystal -controlled induction heating machines are not in wide use. Actually most manufacturers make at least one type of crystal -controlled unit, operating at 27.55 mc., which does not require extensive external shielding. For a general- purpose induction heating machine, such as the 7.5 kw. unit shown in Fig. 8, the nominal frequency is 450 kc. As the work coils are changed, as more or less power is demanded, the oscillator frequency will normally change somewhat. There is little point in keeping the frequency absolutely fixed by means of a low power crystal oscillator and a chain of power amplifiers, each of which merely wastes d.c. power. Instead, the tank circuit can be adjusted GRID LER TGNK Ti WORK COIL Fig. 12. Simplified diagram of a 20- kilowatt induction heating generator. 10KV oc PLUS 5',. RIPPLE 6000v R 500KC 10KV DC 6000V RMS S00KC. for maximum power output for each individual work piece. To minimize the external shielding required, the entire generator is housed in a grounded, electrically bonded cabinet. The Bask Circuit 7900v ,30,60^- The 7.5 kw. induction heating gen- 460V,50,60, rp á { orator just mentioned is probably typical of the medium -sized machines and its circuit, shown in Fig. 11, will be of { interest to our technically minded readers. One unusual feature of this oscillator is the fact that the plate is grounded while the grid and cathode receive a high negative voltage. Part of the r.f. signal developed across the plate tank circuit is fed back to the 38 ELECTRONICS WORLD

grid through the tickler coil. R, and C, make up the grid -leak network. R, limits grid current during the positive portion of the cycle. The working ri. energy is coupled by transformer T, to the work coil and its load. If the plate were at a high d.c. voltage, a coupling capacitor would be required or the transformer would have to have high - voltage d.c. insulation. The simplicity of the basic circuit of Fig. 11 is shown by the physical appearance of the r.f. power panel of the Thermonic model 750 generator, Fig. 8. Fig. 12 is a simplified diagram of a 20 kw. G -E induction heating generator. Here a coupling capacitor isolates the work coil from the d.c. power and the work coil forms a part of the oscillator tank circuit. In addition to the r.f. oscillator circuit there is a d.c. supply to furnish the necessary power. The control circuits to regulate the "on" time and to protect the equipment in case of failure of the cooling system, are quite a plant in themselves. The 7.5 kw. generator shown in Fig. 8 requires 300 cubic feet of forced -air cooling per minute and 8 gallons of water at a pressure of 40 -45 psi. The water is used to cool the transmitting tube and the various power coils and then the heated water gives off its heat to the cooling air. This means that an internal pump circulates water to the hot points in the system and then, just like in an automobile, the water is cooled down again by passing through a radiator while a fan blows cool air through it. This cooling system removes the d.c. and filament power which is not turned into working r.f. power. The Work Coil Once an induction heating generator is installed, the design of a suitable work coil for the particular application is the most important problem. Since the magnetic field generated by the work coil decreases rapidly with distance, the coil is placed as close as possible to the area to be heated. Therefore, work coils are designed to fit each application. Many work coils are cooled by the main water cooling system and are made of copper tubing or hollow copper fixtures. Some typical shapes for various jobs of hardening, brazing, and soldering are shown in Fig. 9. Note how the shape of the coil determines the shape of the area which is heated. It is possible to heat inside surfaces as long as the work coil can be made small enough to fit into the opening. In industrial practice, once the basic work coil for a particular generator has been designed, special variations are often built by plant technicians who are familiar with induction heating methods. Larger automatic and semiautomatic installations serving continuous production runs usually employ carefully designed and tested work coils, especially if an additional operation such as quenching or bending is part of the heating set -up. Typical of this is an area hardening process where a certain spot is rapidly heated and May. 1959 COVER STORY Melting Silicon For Semiconductors THE cover photograph shows D. R. Ginter. electronic -grade silicon has a total impurity a technicien in the Semiconductor Lab- content of one atom or less in one billion oratory of the Chemical and Metallurgical atoms of silicon. Division of Sylvania Electric Products Inc., The molten silicon will be cast into rods at Towanda, Pennsylvania. operating equip- to permit further processing in one of the ment for the melting and casting of silicon. many and exacting steps in the manufac- The silicon is being melted under a protec- ture of diodes, transistors, and rectifiers. tive cover by power supplied from a radio - Silicon in rod form is necessary for frequency oscillator through a fourteen - "floating zone" purification required for turn coil. The resultant eddy currents cause special types of transistors. In this process intense heat to be produced. a long uniform rod of silicon has a trans- Molten silicon at 2600 degrees Fahren- verse zone melted through it by induction heit is an extremely reactive material and heating. This "floating" molten zone rewill attack and dissolve nearly all sub- ceives its only support from silicon above stances. The silicon is being melted, prior and below, and comes in contact with no fo casting, in a quartz crucible in an argon other substance. The zone is made to travel atmosphere. The argon prevents oxidation the length of the rod a number of times. of the silicon while quartz is the only re- The impurities, which remain in the liquid fractory material with which the molten state, move to one end. They are subsematerial does not react appreciably. The quently removed simply by cutting off the radio -frequency field of the coil causes a end of the rod. If the operation is carried graphite susceptor, into which the crucible on in a vacuum, some impurities will also fits, fo become hot enough fo melt the be lost by sublimation. In addition, silicon silicon. rods are made in various diameters which The power for the operation is supplied are then cut into ingots for crucible meltby a I0- kilowatt radio- frequency unit with ing and drawing of doped single crystals. an output frequency of 450 kilocycles. The Sylvania is a leading producer of silicon unit is manufactured by the Lindberg En- that is used in the fabrication of single gineering Co., Chicago. Ill. Silicon, which crystals which are cut into wafers, shaped, has a high melting point and is an ex- and processed into transistors, diodes, and tremely reactive material in the molten other semiconductor devices. The performstate, must be kept out of contact with all ance of all semiconductor devices is demetals and with almost all nonmetals. The pendent ultimately on fhe purify of the use of radio- frequency power, rather than basic material used. Extremely minute quansome other method of heating, allows the tities of any contaminant can seriously melting chamber to be kept free of resist- hamper the performance of a semiconance units and other heater elements which ductor device. Therefore, every possible would contaminate the silicon. This is ex- precaution is exercised during production tremely important because much of todav's to maintain the highest purify. then sprayed with a cooling fluid or else dropped into a coolant bath. Here the hardening cycle would he automatically controlled by a timer and at the end of the hardening cycle the spray would be turned on for a short period. Repair and Maintenance Many of our readers in the servicing field are wondering if the repair and maintenance of induction heating equipment does not offer a new field for the electronic service technician. A survey of the major manufacturers shows that service is usually handled by their own personnel. Induction heating generators are installed and tested at the customer's plant by the manufacturer's own engineers. After a short instruction period plant maintenance men or plant electricians are usually capable of replacing tubes, fuses, and similar parts by referring to the service manual and possible phone consul- (Cover photo by John Miller) tation. According to one of the leading manufacturers, service calls by field engineers are quite rare because of the extremely rugged design and ultra - reliable components used in this type of equipment. Much of the hardware inside a typical high- or medium -power generator consists of the cooling system and most maintenance people are capable of repairing leaky plumbing, worn out blower motors, water pumps. There is a place for people with electronics training in the induction heating field, but it is usually as an employee with the manufacturer. Here knowledge of electronics need not exceed amateur radio experience or a general theoretical understanding of radio equipment, but there should be a strong background of metal working, machine shop, and production processes. Jobs in the induction heating field are not too hard to find and most firms are on the lookout for capable electronic technicians. -- 39

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