US1971195A - Vacuum induction apparatus - Google Patents

Description

Aug. 21, 1934. i w. E. MCKIBBEN 1,971,195 V VACUUM INDUCTION APPARATUS Filed Jan. 25, 1932 Invemtorr Wayne E. MKibben.

by Hwa/w am His Attorney..

Patented Aug. 21, 1934V UNITED ySTATES 1,971,195 VACUUM INDUCTION APPARATUS Wayne E. McKibben, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application January 23, 1932, Serial No. 588,357

5 Claims.

My invention relates to electric furnaces and more particularly. to vacuum electric induction 1 furnaces of the coreless type, wherein a primary winding for generating a magnetic eld is utilized, and has for its object the provision of simple and reliable means for localizing the magnetic field in a vacuum type coreless induction furnace.

It will be understood that furnaces of the core- ?.3 less'induction type ordinarily operate at some frequency which is higher than the usual commercial frequency of 60 cycles, and are therefore often termed high frequency furnaces. For example, thefrequency of 960 cycles is much in use at the present time. The main characteristic is the absence of the core, however, since frequencies higher than 960 cycles may be used and frequencies even as low as 60 cycles may be used.

In furnaces of the vacuum type the electromagnetic fields produced in the vicinity of the furnace have a particularly detrimental effect in the inductive heating of the enclosing casing and also in the heating of=the iron, steel or other parts made of magnetic material in the vicinity of the furnace. Theresult is a very substantial energy loss and a consequent decrease in the electrical efficiency of the furnace. 'Ihe high temperatures produced in these parts furthermore constitute hazards in the operation of the furnace as well as the relatively high. voltages which may be induced in electrically independent metallic members of the furnace or of the supporting structure.

The use of a magnetic shield is disclosed and claimed in an application of N. R. Stansel, Serial No. 548,155, filed July 1, 1931, and assigned to the same assignee as the present invention. The invention of the said Stansel application was 'made prior to my invention and I, therefore, do not herein claim anything shown or described in said Stansel application which is to be regarded as prior art with respect to this present application.

In carrying out my invention in one form thereof I provide a vacuum type coreless induction furnace having an enclosing casing of highly conducting material around the primary winding, this casing being so situated and arranged that it effectively restricts the magnetic flux to the region itywhile maintaining a de- .sired electrical eiciency of the furnace.

For a more complete understanding of my invention, reference should be had to the accompanying drawing in lwhich the figure is a vertical sectional view of a vacuum coreless'induction furnace embodying my invention.

Referring to the figure of the drawing, Lhave shown my invention in'one form as applied to a vacuum coreless *induction furnace for melting metal, although it will be understood that my invention has application to vacuum furnaces of the coreless induction type as applied to various purposes, such as annealing and the like. As shown in the drawing, the furnace includes a Crucible l0 made of a suitable heat refractory material Capable of Containing a metal to be melted, which Crucible is surrounded by a primary inducing wnding 1l, shown as a helical coil coaxial with the Crucible. It is well understood that when an alternating current is supplied to the Winding 11 by means of its terminals 12 and 13 an alternating magnetic eld is set up in the interior region of the'crucible which causes eddy currents to be induced in the metal charge 14, these currents in turn causing heating of the charge. In order to prevent the outer casing 15 of the furnace from becoming heated during the operation of the furnace, this casing l5 including the cover 16 is Constructed preferably from a suitable non-ferrous, non-magnetic metal having a high electrical conductivity such as copper, aluminum, etc. The cover 16 is water cooled by means of an inlet pipe 17, the cooling channel 18 and the outlet pipe 19. To insure a leak-proof joint between the casing 15 and the cover 16, a plurality of bolts 16a are arranged so that the cover may be forced tightly against the casing 15. A gasket 15a interposed between the casing and the cover further improves the sealing of the casing. A periscope 20, provided with a mirror- 21, a lens 22, and a transparent member 23 of quartz or the like, enables the observation of the charge 14 during the operation of the furnace.v A vacuum pump 25 driven by a suitable motor 26 is connected to the casing 15 by means of the pipe 27. By means of this pump the air within the casing is exhausted and a desired vacuum maintained in the casing.

Due to the distribution of the magnetic field produced by the primary inducing winding 11, the length of the casing 15 is preferably at least 50% greater than the length of the inducing winding 11. In performing its shielding function the casing 15 acts to absorb by eddy'current effect that portion of the magnetic flux exterior to the primary winding which would normally be in the space occupied by the casing and beyond the casing. By making `the thickness of' the casing great enough, no appreciable flux will be present in the space exterior to, it.v

There is a certain minimum thickness of the wall which is determined with referenceV to the depth of penetration. The depth of penetration may be expressed by the equation equals a constant, the value of which is dependent upon the physical relationship between the electrical and magnetic circuit; ,f equals the frequency of the supply source; u equals the permeability of the material of the shield; and c equals the conductivity of the material of the shield.

The value of the Wall thickness given by this equation is the theoretical depth of magnetic penetration of the Wall, as described in the book entitled oscillations chapter VI, 1920 edition, page 371, C. P. Steinmetz. Ordinarily, however, I prefer to have this thickness somewhat greater than the minimum thickness.

It may be observed from this equation that the eifectiveness of the shield for a given material varies inversely with the square root of the permeability and conductivity of the material in'the shield. Also for a given proportion of parts an increase in the frequency of the current supplied to the primary winding increases the effectiveness of the wall asa shield.

Another consideration relative to the shield thickness is that it must be great enough to provide sufficient strength to withstand the air pressure of the atmosphere incident to the evacuation of the furnace. As a general proposition, the thickness, of the materials at present available with the requisite properties, Will be great enough due tothe considerations of strength that they will greatly exceed the minimum depth of penetration iigure which is obtained for a particular furnace from the equation described above.

At present the effectiveness of my high conductivity casing is believed to be due to an opposing magnetic field produced by the eddy currents in the enclosing casing. The resulting flux opposes that produced by the inducing winding 11 thereby shielding the parts exterior to the furnace as well as diminishing to a considerable degree the magnetic iiux which would otherwise cause considerable heating of the enclosing casing.

Inasmuch as the energy consumed in the wall is proportional to the square root of the resistivity of the material of the Wall, the use of a material having a very low resistivity, such as aluminum or copper is particularly advantageous.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not Wish to be limited thereto, since Transient Electric Phenomena and` .said casing.

many modifications may be made, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1. A vacuum coreless induction furnace comprising an inducing winding surrounding a charge to be heated, and a sealed enclosing casing made of non-magnetic material havingl a resistivity within the range of copper and aluminum at ordinary temperatures surrounding said charge and said Winding.

2. A vacuum coreless induction melting furnace including a Crucible, a primary Winding surrounding said crucible for producing a magnetic field in an interior region of said Crucible, an enclosing casing Within which a vacuum may be maintained formed of non-magnetic elec trically conducting material having an electrical resistivity Within the range of copper and aluminum at ordinary temperatures and entirely enclosing said Winding and Crucible, the walls of said casing being spaced a substantial distance from said Winding to localize said field within said casing and to increase the operating efficiency of said furnace.

3. A vacuum coreless induction furnace ccmprising an inducing Winding surrounding the charge to be heated and a sealed non-magnetic enclosing casing made of solid copper surrounding said charge and said winding.

4. A vacuum coreless induction furnace cornprising an inducing winding surrounding the charge to be heated and a sealed non-magnetic 11g enclosing casing made of solid aluminum surrounding said charge and said winding.

5. A vacuum coreless induction furnace comprising an inducing winding arranged to produce a magnetic field in a charge to be heated, a 15 sealed enclosing casing made of non-magnetic material having a resistivity Within the range of copper and aluminum at ordinary temperatures surrounding said charge and said Winding, said casing having its thickness greater than the depth of magnetic penetration whereby all of the stray flux passing into it is absorbed by eddy current effect, and means for exhausting the air from WAYNE E. MCKIBBEN. ,915

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