Electric heating – Inductive heating – Metal working
Reexamination Certificate
2001-04-18
2002-11-05
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
Metal working
C219S634000, C219S635000, C219S651000, C219S655000, C219S676000
Reexamination Certificate
active
06476366
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for degassing and brazing preassembled vacuum interrupters in one brazing operation.
RELATED TECHNOLOGY
Vacuum interrupters for vacuum circuit breakers, vacuum contactors, vacuum load interrupters in the medium-voltage range and also in the low-voltage range, for example, as motor protection switch, have a variety of uses.
The structure of a typical vacuum interrupter is depicted in the drawing in FIG.
2
. Vacuum interrupter
1
includes movable conductor
12
and stationary conductor
13
which are equipped with a contact piece at the front-side ends facing each other. The actual vacuum tube is formed by metallic cover parts
15
including an insulator
14
located therebetween. Movable conductor
12
is sealed with respect to the cover by a bellows
16
. The outer parts forming the vacuum tube are interconnected via brazing points L, just as contact pieces
18
,
19
with conductors
12
,
13
. The contact pieces are laterally surrounded by cylindrical screen
17
which is also fixedly connected to cover
15
via a brazing point.
The oldest method for manufacturing the vacuum interrupter, namely the degassing, welding, and brazing of the parts, is the so-called “pinch-off method”. In this context, the parts made of high-grade steel are initially pre-degassed at approximately 1,000° C., then the stationary contact group and the movable contact group are each pre-brazed individually, and the complete vacuum interrupter is assembled of these parts and coupled to an ultra-high vacuum pump system via a copper pipe, and subsequently heated to a temperature of 400° to 500° C. during at least 24 hours, cooled down, and the copper pipe is squeezed together, i.e., pinched off via a hydraulic pressing device. Then, the vacuum interrupter is separated from the ultra-high vacuum pump system.
Another known method for manufacturing vacuum interrupter is described as direct brazing sealing technique. In this case, the high-grade steel parts are equally pre-degassed at approximately 1,000° C., and the stationary contact subassembly and the movable contact subassembly are pre-brazed between 700° C. and 960° C. Subsequently, the brazed subassemblies, together with the appropriate ceramic bodies and corresponding brazing foils, are preassembled to form the vacuum interrupter, pumped empty at temperatures between 700° C. to 860° C., degassed, the getter is activated, and the vacuum interrupter is sealed ultra-high vacuum tight inside a high-vacuum brazing furnace. The aforesaid manufacturing techniques are multi-step methods.
Moreover, the “one-shot brazing” technique is known, in which all brazing and degassing operations for manufacturing the vacuum interrupter are carried out in an integrated manner in only one brazing cycle.
For carrying out the direct brazing sealing technique and the one-shot brazing technique, special high-vacuum brazing furnaces are required which can reach a final pressure between 5×10
−7
and 5×10
−8
mbar in the cold condition. These high-vacuum brazing furnaces have the feature of being composed of a double-walled cylinder which is closed with a double walled dished head on both ends. One of the dished heads is designed as a door so that the furnace can be loaded and unloaded. The furnace contains heating elements made of graphite, molybdenum or tungsten. If molybdenum and tungsten heating elements are used, the furnace is provided with a metallic lining made of molybdenum and/or high-grade steel. Depending on the size of the useful space of the furnace and on the geometry of the vacuum interrupters to be manufactured, it is possible for 50 to 500 vacuum interrupters to be brazed and degassed in one batch. Such a brazing cycle is composed of:
1. Evacuating furnace and batch to 5×10
−4
to 5×10
−5
mbar in 45 to 80 minutes.
2. Switching on the heating and carrying out a predetermined temperature program for furnace and charge until the brazing temperature is reached:
letting furnace and charge cool off
switching on the rapid cooling and waiting until approximately 40 to 70° C. are reached.
Depending on the batch, a complete brazing cycle takes approximately 10 to 12 hours and is always a discontinuous process, involving the three working steps of loading the furnace, brazing the batch, and unloading the furnace.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a faster and more inexpensive method for degassing and brazing vacuum interrupters based on the one-shot brazing technique, in particular, to reduce the outlay of energy and time.
The present invention provides a device for degassing and brazing preassembled vacuum interrupters in only one brazing operation, including a base plate (
21
) having at least one brazing site (
2
) which features an opening (
27
) of the base plate for connecting a suction pump and a bell (
20
) which covers the opening (
27
) and can be put on the base plate and lifted off, and including an excitation coil (
3
) which surrounds the bell (
20
) on the outside and to which medium- or high-frequency energy can be applied, including a medium- or high-frequency generator for the excitation coil, and a susceptor (
4
) which is arranged inside the bell (
20
) and designed as a cylindrical tubular piece for receiving at least one preassembled vacuum interrupter.
As a holding device for the vacuum interrupters, a grid-like locating plate for inserting the preassembled vacuum interrupters is preferably provided in the opening of the base plate or on the base plate above the opening. Depending on the size of the vacuum interrupter and of the grid, possibly, several smaller vacuum interrupters can be arranged in lieu of a large vacuum interrupter. The grid-like structure of the locating plate brings about a high flow conductance during the pumping out of the gases.
To attain a high vacuum, moreover, it is proposed for an annular groove to be formed on the upper side of the base plate, the annular groove surrounding the opening, and a suitable sealing ring being arranged therein in a manner that it is joined to the base plate in a high vacuum-tight manner.
For exact heating to attain the brazing temperature and for cooling, provision is made for thermoelements which are arranged in the head region of the bell via a high vacuum-tight gland. Also provided are pressure-measuring elements for the inside space and for the suction line to the suction pump, i.e., an ultra-high vacuum pump system for achieving the appropriate degassing and the corresponding high vacuum with certainty.
For this, preferably, suction connecting pieces which are fixedly connected or which can be connected in a vacuum-tight manner are provided on the bottom side of the base plate at the opening.
The fast heating via the excitation coil and the allocated susceptor which is made of a material which very easily absorbs magnetic field lines and is able to very quickly absorb magnetic energy is essential to the invention. The susceptor is preferably composed of soft magnetic materials, such as iron and iron alloys, for example, Fe, FeNi, FeNiCo, etc. This applies especially in the lower frequency spectrum. In the higher range of the used frequency spectrum, it is also possible to use higher-melting metals and alloys, such as Mo (molybdenum), W (tungsten), Ta (tantalum), high-grade steels and superalloys. Using the medium- or high-frequency energy according to the present invention, the object to be brazed, i.e., the preassembled vacuum interrupter, can be very quickly heated to the desired brazing temperature.
The arrangement may be such that the excitation coil has an annular design and surrounds the bell, namely transversely to the vertical switching axis of the vacuum interrupter which is appropriately arranged inside the bell, the excitation coil being arranged approximately centrically with respect to the susceptor surrounding the vacuum interrupter. For an optimum heating, provision is made for the susceptor to be arranged on insulating
Davidson Davidson & Kappel LLC
Leung Philip H.
Moeller GmbH
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