US 3008027 A
Description (OCR text may contain errors)
Nov. 7, 1961 I. BECKIUS ETAL 3,008,027
PROTECTION MEANS AGAINST CORONA OF LIVE PARTS IN ELECTRIC VACUUM MELTING FURNACES Filed Nov. 24, 1959 INVENTORS Var Beckg'as and A United States Patent Ofifice 3,008,027 Patented Nov. 7, 1961 3,008,027 PROTECTION MEANS AGAINST CORONA OF LIVE PARTS IN ELECTRIC VACUUM MELT- lNG FURNACES Ivar Beckius and Arne Olin, Vasteras, Sweden, assignors to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a corporation of Sweden Filed Nov. 24, 1959, Ser. No. 855,162 2 Claims. (Cl. 21910.79)
In order to protect from corona live parts of electric vacuum melting furnaces, both the furnace coil itself and its connection leads in the vacuum container, it is usual to insulate these parts and to provide the insulation with electrically conducting earthed layers which are separated from each other. However, it is likely that crevices occur in one or more of said conducting layers so that each individual layer no longer has internal connection and therefore not even contact with the earthed lead, with the consequence that corona may occur at these crevices.
The purpose of the present invention is to improve the aforesaid corona protection and to bring about reliable and permanent earthing of every part of the corona protection layer. The feature of the invention resides in that an earthed conductor of copper or other conductive material is placed in galvanic contact on the periphery of each conductive layer of the coil turns and along the other live parts in the vacuum space.
The earthed conductor is placed suitably on the outer side and in the middle of the coil turns, which point is field-free, or where the magnetic field is weakest so that no heating of the conductor worth considering can arise. On the connection leads, the earthed conductor is so dimensioned that it is not unduly heated. The earthed conductors are fixed to their conductive shielding layers in any known manner.
The invention is illustrated by way of example on the accompanying drawing, wherein FIGURE 1 is an axial section through one turn of the coil along the line A-A in FIGURE 2, and an elevation view of one connection lead, and FIGURE 2 is a plan view of a part of the turn of the coil. The conductor, which is assumed to be hollow for cooling purpose, is provided with an insulation 2 which is coated with a shielding layer 3 of electrically conducting material. This layer is divided into a number of parts separated from each other. On the outer side of the coil and in the middle of said side an earthed conductor 4 is fixed. At this point the magnetic field is at its weakest and therefore the earthed conductors cannot be unduly heated.
By the new means the eflicacy of the corona protecting layers may be maintained even if the conductivity of the layers should be decreased or nullified by internal crevices.
Although the conductors 4 are designated in the description as earthed conductors, this term shall also apply should they be connected to potentials lying below the critical corona limit.
1. In an electric vacuum induction melting furnace comprising a coil and connection leads to the coil, insulation covering the coil and connection leads, shielding layers on said insulation separated from one another along the turns of the coil and the connection leads, a conductor in galvanic contact with each shielding layer extending along its length at the point where the magnetic field is weakest, and a conducting lead connected to said conductor and to earth.
2. Apparatus as claimed in claim 1, said conductors being located on the outer sides of the shielding layers at the middle thereof.
References Cited in the file of this patent UNITED STATES PATENTS 2,352,166 Camilli June 27, 1944 FOREIGN PATENTS 419,284 Great Britain Feb. 9, 1934 627,507 Great Britain Aug. 10, 1949