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Electronics-World-1959-05

www.americanradiohistory.com Fig. 1. A conveyor carries beaters for home mixing machines through this induction heating unit which brazes the beater blades to the main stems automatically. warm. Here are some other uses. As an example of a typical automatic installation Fig. 1 shows an induction heating machine of the vacuum -tube type, coupled with an automatic conveyor and handling system for the mass production of beaters used in home mixing machines. Here the brazing of the beater blades to the main stem is done automatically. The total heating time is only 3 seconds, not long enough to discolor the stem or in any way damage the temper of the blade. Other products whose manufacture involves brazing by induction heating include ice skates, spark plugs, curtain rings, and many other consumer items as well as all types of industrial equipment. Low -frequency induction heating is usually employed to heat the entire work piece and not just a part of the surface. Low -frequency power is delivered to the work coil by one of three methods. In a few cases the work coil can be designed to connect directly to the 60 -cycle power line, but more often a transformer is used to match the work coil impedance to the line. Where higher powers are used, a rotating motor -generator set delivers the heating power, usually at a frequency higher than the 60 -cycle line. Typical are 960, 3000, and 10,000 cps. One novel application of low -frequency induction heating is as a metal melting furnace. Operating at 960, 3000, and 9600 cycles, Allis- Chalmers makes such furnaces with output ratings up to 1250 kw. Because the application of heat is quite fast and only the metal to be melted is heated, there is less oxidation, forming of scale, and other wasteful side effects which are inevitable with conventional furnaces. An unusual device is the "Frequency Transformer," made by iflductiOn Heating Corp., which generates 180 - cycle power from a 3- phase, 60 -cycle line without any rotating machinery. A transformer -like device, capacitors and resistors form an RLC network which efficiently generates the third harmonic of the power -line frequency. This equipment, in addition to metal melting, is used for heating relatively large metal pieces such as bearings or housings for shrink fitting. In the metal working industries induction heating finds wide application in the hardening and tempering of bearing portions of moving machinery. The gear tooth -hardening process in Fig. 3 is typical. Here the rack gear teeth of a business machine are hardened at the rate of one -inch-per- second. At the right, out of the picture, is a magazine containing a sizable stack of racks, which are fed into the rollers one after the other. The rack passes through the specially shaped work coil for heating. While still hot, the rack is fed into a circular quenching chamber at the left and finally the rack emerges hardened along the teeth and back edge leaving a tough area between. Other typical hardening applications include the bearing area of turbine shafts, cutting edge of blades, tool bits, drills, and practically every piece of metal which is subject to wear. Hot forming processes such as forging, bending, etc. can all be done more efficiently by using induction heating. Fig. 2. The v.h.f. energy is used here for zone-refining silicon. Double glass bells are used as vacuum chambers and shields. y In bending, for example, only the area of the bend itself need be heated and since induction heating does this so quickly, the rest of the work piece will remain rigid and retain its shape. Fig. 4 shows a moving coil fixture in which an aluminum door handle is heated to 900° F and bent at right angles in 3 seconds, the entire operation being completely automatic, accurately controlled, and without forming scales, discoloring, or distorting. Shrink fitting of bearings is another typical use of induction heating equipment. A work coil which fits inside the hole heats up the metal sufficiently to expand the hole diameter and then the shaft or other part is quickly inserted in the hole. As the metal cools it grips the insert firmly and produces a reliable, tight fit. Fig. 3. Automatic hardening of typewriter ratchet gear teeth. Fig. 4. Moving coil fixtures for heating, bending door handles.

RF GENE RATOR METAL ROD WORK COIL MAGNETIC FLUX Fig. 5. Basic relation between r.f. generator, coil, and the rod to be heated. WORK COIL RF GENERATOR RESISTANC OF METAL CURRENTS IN METAL ROO SELF- INDUCTANCE Fig. 6. Cross- section of currents in rod. In addition to the few examples outlined here, there are a host of special purpose applications where induction heating is often the only method which permits the efficient production of a particular metal part. Whenever metal must be heated, induction heating offers a rapid, efficient, and easily controllable source of heat. The basic reason for this lies in the principle of induction heating -the heat is generated electronically, directly in the work piece itself. How It Works The device that generates the power for the induction heating process is very similar to a radio transmitter. It takes low -frequency power from the power line and converts it into a high - frequency signal. In an ordinary radio transmitter the signal is sent out over the antenna and radiates through the air. In addition to the power that is radiated, a certain amount of power is lost because none of the components is "ideal." Thus we know that in a power transformer there are losses due to hysteresis and eddy currents. To keep the latter to a minimum, laminated rather than solid steel cores are used. In addition there are losses in capacitors and coils as well as in the purely resistive circuit elements. In a radio transmitter the ratio of input from the power line and the antenna output is an indication of its efficiency. In induction heating equipment, the amount of radiated energy is kept to a minimum and the losses, concentrated on the work piece, are a measure of its efficiency. Here the eddy current and hysteresis losses are utilized to heat up the work piece. The r.f. energy is concentrated in the metal by means of a work coil which is designed to fit the particular piece to be heated. Fig. 5 shows the basic relationship between the r.f. generator, the work coil, and a steel rod which is to be heated. The generator puts a current Fig. 7. Depth of penetration for metals. through the work coil and this current sets up a magnetic flux. The alternating magnetic flux, in turn, sets up a voltage to be precise, a counter -electromotive force -which causes a current to flow in the metal. This is the eddy current and, depending on the type of metal and the frequency used, Fig. 8. Photo above shows the front panel of a 7.5-kw. induction heating generator. Note simplicity of the controls required. more eddy current tends to flow on the outside of the metal than in the inner core. This characteristic. called "skin effect." is used to regulate the depth of heating by proper frequency choice and is especially useful in such applications as surface hardening. In magnetic materials there is a secondary heating effect due to hysteresis losses but these are relatively small and are not usually considered in calculations of heating efficiency. To show how the skin effect works for the smooth rod used as an example in Fig. 5, a simplified electrical presentation of the currents in the metal is shown in the cross -section drawing of Fig. 6. This shows that the current flowing in each circular path sets up a flux opposing the work coil flux, thereby acting as an electromagnetic shield for the material inside it. For this reason the flux in the inner concentric ROUND RECTANGULAR FORMED AREA TO BE - HARDENED' AREA TO BE HARDENED PANCAKE SPIRAL -HELICAL INTERNAL (A) (B) ;D `fID EXTERNAL COIL FLAT OR PANCAKE (C) INTERNAL COIL Fig. 9. This illustration shows typical work coils that are employed in induction heating equipment. (A) shows some of the common shapes used. (B) illustrates how special shaping is used to fit the metal sections that are to be heated. (C) shows placement of work coils with respect to work pieces. May. 1959 37

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