US 3017485 A
Description (OCR text may contain errors)
Jan. 16, 1962 AGE HANSEN MEANS FOR ELECTRIC VACUUM FURNACES Filed Nov. 5, 1958 p INVENTOR.
Age Hansen United States Patent O MEANS FOR ELECTRIC VACUUM FURNACES Age Hansen, Vasteras, Sweden, assignor to Allmanna Svenska Elektriska Aktieholaget, Vasteras, Sweden, a Swedish corporation Filed Nov. 5, 1958, Ser. No. 772,133 Claims priority, application Sweden Nov. 19, 1957 2 Claims. (Cl. 219-4059) In an electric vacuum furnace, for example a high frequency furnace, corona arises on the live parts of the furnace on its coil or on the connections to the coil, due to the low pressure of the gas which is present in the space in which the furnace is accommodated. This corona may directly damage the parts on which the corona occurs, but it is also indirectly detrimental due to the generation of ozone Which damages by oxidation, both the metal parts and the insulation material.
This corona may be avoided according to the present invention, in that the induction coil and other live parts are coated with a layer of insulating material which is provided on its outer side with a conductive coating, suitably of non-magnetic material with comparatively high conduction resistance. By this measure the voltage stresses are concentrated into the insulation coating so that the low pressure gas which surrounds the live parts is entirely freed of electric stresses.
To avoid corona it is however necessary for the live parts to be entirely shielded by the conductive layer, which need not be co-herent but may consist of two or more parts which are not joined together but over-lap each other electrostatically.
The arrangement has to be such that no heavy currents can arise in the conductive coating by contact between the live parts having different potential or by electric voltages. Since, on the other hand, a comparatively good conductivity of the conductive coating is desired the shield, as a rule, has to be divided into separate parts but the interstices between these parts have to be bridged by a conductive coating which is insulated from parts of the shield. Generally speaking the live parts have to be shielded from the gas within the vacuum container by means of an electrostatically tight shield. Such a tight shielding may be obtained either by coating the live parts with an insulating lacquer layer which is coated on the outside with a thin conductive layer of a so called corona protecting lacquer suitably having a high resistance. Instead of coating the live parts with the insulating lacquer layer which is coated on the outside with a more or less conducting layer, live bodies of simple shape may be insulated by winding a strip of insulating material around them, which is provided on one side with a conducting layer, for example, metal foil; this foil should have a width which is less than the width of the strip, so that the metal foil in one turn, does not come into contact with the metal foil on the succeeding turn of the strip.
The accompanying drawing shows, in FIG. 1, a vertical view of an induction coil in an electric furnace which is assumed to be enclosed in a vacuum container not shown on the drawing. FIG. 2 shows the same induc tion coil from above. FIG. 3 is a cross-section of a conductor. FIG. 4 is a section through the winding along the line A-A in FIG. 2, while FIG. 5 illustrates an em- 3,017,485 Patented Jan. 16, 1962 "ice bodiment having a layer of insulating material on the conductor.
On the drawing 1 designates generally the turns of the coil, while 2 designates its connection leads. 3 is a part of the wall of the container surrounding the coil and 4 shows diametrically an insulating sealing gland which is attached to the wall of the vacuum container around an opening in which the leads 2 pass through the sealing gland.
In FIG. 3, a section of a conductor is shown, by way of example, which forms the furnace coil. 5 is the tubeshaped copper conductor which is surrounded by a sleeve of insulating material 6 which is in turn surrounded by a conducting shield 7. This shield 7 is connected, as can be seen from FIG. l, by conductors 8, to earth or another suitable potential. The sleeve and the surrounding shield may consist of a strip 6' of insulating material which is coated on one side with a conducting layer 7 which terminates short of both edges of the insulating strip 6'.
FIGURE 2 shows an embodiment of the invention wherein the conductive coating 7 is divided into parts along each turn of its periphery so that gaps or interstices are formed, indicated by broken lines. The gaps are covered by layers 9 of insulating material and these layers are partly covered by metal strips 10, preferably so that one rim of the metal strip is in contact with the underlying part of the conductive coating 7 while the other rim of the metal strip 10 is insulated from the underlying part of the conductive coating. See also the section 5.
I claim as my invention:
l. Means for avoiding corona of the induction coil and its associated live parts in a high-frequency induction furnace enclosed in an empty space, comprising an insulating layer on the conductor of the coil and its associated live parts, a conductive coating of comparatively high electric resistivity on said insulating layer divided into a plurality of separate parts along the turns of the coil, solid insulating layers covering the interstices between the conductive coating parts and contacting the solid conductive coating therebetween, a second layer of conductive material overlying each of the said insulating layer, said second layer being in contact with the end of one underlying conductive coating and the insulating layer and being insulated from the adjacent beginning of the other underlying conductive coating and leads connecting the said second layers of conductive material to a potential substantially lower than the potential of the coil and its associated live parts.
2. Means according to claim 1, characterised in that the insulating layer and the conductive coating are made by means of strips of insulating material which are provided on one side with a conducting coating having less Width than the strip.
References Cited in the file of this patent UNITED STATES PATENTS