US2385983A - Electric induction furnace - Google Patents

Description

Oct. 2, 1945. M. H. HANEs ELECTRIC INDUCTION FURNACE Filed April 27, 1942 7 l Y 5 1 1 5 f f 2J H/ f//H/ ///U///V 4 2 v/ rO /n I IlhWm/hw O 1 0J 8 7 V FIG. 5

FIG. 4

FIG. 3

INVENTOR MERLE H.

HANES BY jm? FIG.

ATTORNEY Patented Oct. 2, 1945 saisiesv ELECTRIC INDUCTION FURNACE Merle H. Hanes, Wood River, Ill., assignor to Olin4 Industries, Inc., a corporation of Delaware Application April 27, 1942, Serial No. 440,641

1 Claim.

This invention pertains to electric induction furnaces, and particularly m the provision of furnaces of this type having transformer primaries provided with improved insulation.

Electric induction furnaces, particularly of the submerged channel type, have long been in extensive use in the melting of metal and these de vices have therefore been the subject of considerable study and development in various aspects. However, no significant improvement in the transformer primary insulation of such furnaces has been made for many years.

In furnaces of this type, particularly severe requirements are placed upon the insulation separating the adjacent turns of the primary coil winding due particularly to the high amperage of the current circulated and the high temperatures at which the coil is necessarily operated. Even the best material which has heretofore been available for the purpose, segments comprised of mica, has involved serious disadvantages in use v which were unavoidable, such as lack of durability, current leakage and eventual short-circuiting, high cost, and brittleness and friability of the material, causing it to flake off and drop out from between the turns of the primary coil.

It is an object of the present invention to provide improved insulation in the transformer primary of electric induction furnaces.

Another object is the provision of insulation for the primary coil of electric induction furnaces nich is free of the above-mentioned disadvanges.

A further object of the invention is to furnish electric induction furnace transformer primaries which are improved with respect to eiliciency and durability of the insulation. v

Still another object is to improve electric in duction furnace transformers by the provision of continuous insulating tape of high tensile strength and exibility between the adjacent turns of the conductor winding.

Another object of the invention is to improve electric induction furnaces lby the provision of primary coils insulated by means of continuous tape which is so applied between the adjacent turns of the conductor winding as to avoid deterioration.

Other objects will appear from the following detail description. l

In accordance with this invention, generally stated, the foregoing objects are accomplished by the provisionof a continuous length of flexible tape fabricated from inorganic insulating filaments, such as textile glass fibers, as the insulation spacing the adjacent turns of the transformer primary coil of electric induction furnaces.

The invention will be particularly described with reference to submerged channel induction furnaces such as are described in U. S. 2,076,216 to Wyatt, but is to be understood as applicable to other electric induction furnaces.

In the drawing forming part of this specification,

Figure 1 is a central vertical section of a furnace embodying the improved structure provided by the present invention.

Figure 2 is a front elevation, partly in section, of the transformer primary coil used in the apparatus of Figure l.

Figure 3 is an enlarged fragmentary view of a portion of the section shown in Figure 2.

Figure 4 is a sectional view of the transformer coil of Figure 2, taken` along line 4 4 of Figure 2, following the helical path of the insulating tape, and illustrating the manner of application.

Figure 5 is a sectional view, taken similarly to Figure 4, of a coil provided with prior art insulation.

Referring to the drawing, Figure l shows a crucible I having a depending narrow channel 2, encased in an outer metal shell 3 which is provided with linings 4 and 5 of suitable refractory. The furnace contains an` opening 6 through the loop formed by the channel, in which there is mounted an open cylinder 1 of insulating material. Cylinder 'I contains transformer core 8, consisting of laminated sheets of transformer iron, and is externally wound with primary coil 9. In operation, the passage of an alternating current through the. primary coil produces a much larger fiow of current through the metal in the single loop of channel 2, which constitutes the secondary of the transformer, thus melting and stirring the metal in the crucible.

The transformer primary comprises insulating cylinder 1, transformer core 8 internally housed therein. and conductor coil 9 helically wound on its external surface. The primary coil may, for example, consist of 40 turns of a copper rod inch wide and inch high.

The insulation l0, separating the adjacent turns of the primary coil, comprises a continuous lengthof tape, fabricated from filaments of inorganic insulating material, which is preferably held under tension throughout its length and under compression between the adjacent turns of the coil. The tape may be fabricated to the desired dimensions by any suitable method from aseaese inorganic insulating ilaments such as' textile glass fibers, andvpreferably consists of unimpregnated ltape Woven from staplebers prepared from glass containing less than 3% of alkali metal omdes. Tape having a width of 1 inch and thickness ci 0.030 inch, woven from Fiberglas staple liber, has, for example. been found to provide excellent results in use as the insulation spacing the adjacent turns of the above-described primary coil.

In assembling the transformer primary, a ring of insulating material ii, provided with a peripheral slot i2, is fastened to the cylinder l at one end; both the cylinder and ring may be formed of a cement-asbestos composition, such as that known as Transita Coil 9 is set inrplace over cylinder l, a coil lead i3 being disposed within slot i2. While the cylinder is held vertically, the

' coil 9 being compressed by its own weight, tape i@ is applied by a winding process as illustrated in Figure d, the insertion being started at the bot-l tape and metal conductor are then further compressed and are retained in this state by means of a second ring i I fastened to the other end of cyl-` inder l, the slot I2 of the ring retaining the second lead I3 of the coil. After application of a thin coating of suitable refractory or clay over the coil surface and placing of insulating sleeves I4 of wo'ven glass textile fiber, over leads i3, the assembly is ready for insertion and use in the furnace.

Electric induction furnace transformer coils insulated in accordance with this invention as described above have displayed a number of outstanding and unexpected advantages in use. The durability, that is length of useful service life, far exceeds that of the best prior insulating material. Signicantly improved electrical insulation has been obtained by the use of the glass fiber tape, resulting in increase in the power factor, and therefore of the thermal elciency of the furences involved in the handling of the assembled transformer.

The flexibility of the glass :fiber tape is another property which is advantageously utilized particularly when the tape is held under lateral compression, as described above. This results in the highly desirable feature that the tape is held snugly against the external surface of insulating member "l, even though the latter is not truly cylindrical, either as a result of faulty manufacture or of warpage produced through use in the furnace at an elevated temperature of 700 F. to over 1000 F., followed by cooling to ordinary temperatures, and reheating. Provision is thus made against the occurrence of gaps between the coil insulation and the surface of the insulating cylinder which might otherwise be the site of short circuits between adjacent turns of the transformer winding. I

Short circuits of the type above referred to, which result in destroying the primary coil andl requiring its replacement, have been unavoidable. with even the best prior coil insulation. Such insulation, as illustrated in Figure 5, has consisted of arc-shaped segments i5 of pressed mica-refractory binder compositions, the size being generally such as to require two or three for completing a full turn. Such insulation has the disadvantage of high cost for the material and for the labor of assembly, which involves the tedious procedure of inserting the segments successively one by one throughout the coil, due precaution being required to maintain previously inserted segments in place. Since the segments are of fixed circular shape, no means are provided for preventing the occurrence of gaps between the same and the external surface of sleeves l when the two curvatures do not conform exactly.

' turns of the primary coil.

naces; higher metal production capacity per furnace has thus been secured. Further economy results from the lower cost of material and labor in insulating-a coil in accordance with this invention. l

The foregoing and other advantages resulting from the present extended research in the-application of unimpregnated inorganic filament tape in the insulation of high temperature electric transformers have been secured despite the known fact, that filaments of inorganic material such as glass deteriorate by abrasion, friction be- -tween the individual bers tending to cause scratching and breakage, indicating that such tape would not successfully withstand the vibra- A further serious deficiency of the prior art insulation material has been its friability and tendency to flake and drop out, particularly after having been subjected to the elevated operating teml perature, thus tending to shorten the life of the coil because of short circuits. Furthermore, the ilaked mica tended to collect inthe free space of opening B, thus impairing the flow of air which is 'tion and handling. to which the transformer coil .adjacent turns of the primary coil.

provided during operation for cooling the transformer primary.l It was generally necessary to reinsulate the transformer primary whenever a furnace was shut down and disassembled, as for the repair or renewal of lining 4 or 5, because the mechanical handling incident to the removal of the primary from its mounting in the furnace caused the dropping out of insulating material from be'- tween the adjacent windings of the primary. At times, short circuits have also occurred during furnace operation due to the dropping of entire mica segments from proper position in the coils, necessitating shut-downs and replacement.

'I'he foregoing disadvantages have been overcome in transformer coils insulated, in accordance with lthis invention, by means of continuous flexible tape of insulating filament spacing the i Such transformer assemblies have not only served in the melting of millions of pounds of metal without showing any signs of deterioration, but have also at the same time withstood the mechanical handiing, incident to the removal from and reinsertion inthe furnace, without any failure.

As indicative of the increased efficiency which is made possible by this novel insulation for high temperature electric transformer coils, a power factor of about 95% has been obtained by its use ina furnace using about 150 kw. at 440 volts, provided withA a forty-turn primary carrying a cur- `rent of about 350 amperes. The increased production which is made available by this invention is illustrated by the fact that in` an extended comparative test, an average of five heats of a given alloy were melted, ready for casting, in a furnace provided with a transformer coil insulated with glass-uber tape, as contrasted with an average of three heats produced in the same period of time by a similar furnace, differing only in the provisionl of a transformer oeil insulated with mica segments.

As known in the art. the size and number of turns of the primary conductor coil will vary in gether.

ness of the tape should be adequate to provide the necessary insulation and may be substantially greater than the minimum required, if excess space is available, adequate provision having been made for the number of turns required in the coil.

It will be further understood that the principles of this invention are generally applicable to the insulation of transformer primary coils intended for use in electric induction furnaces.

Accordingly, the invention is to be understood as not limited to the specific details described herein, but to include modifications within the spirit of the advance rnade in thetart which will be realized by those skilled therein from study of `the above description, such changes being contemplated as within the scope of the appended claim.

Having now described the invention, what is claimed as new and is desired to be secured by Letters Patent is:

In an electric induction furnace, a transformer primary comprising an insulating cylinder, a

single layer of helical turns of a nat conductor thereonl and a continuous glass-fiber tape disposed edgewise to the said cylinder between the `turns and spacing said turns apart, whereby said tape is held in place by compression at least partly due to said turns tending to spring to-

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