US3190992A - Vacuum switch having channel in contacts to prevent migration of roots of an arc - Google Patents

Description

June 22, 1965 B. T. DIGBY 3,190,992

' VACUUM SWITCH HAVING CHANNEL IN CONTACTS TO PREVENT MIGRATION 0F ROOTS OF AN ARC Filed May 1, 1962 INVENTOR:

Basil Ti/xfor-d Dfyby United States Patent 3,l9tl,92 VACUUM SWlTCH HAVHNG CHANNEL IN USN- TACTS Ti) PREVENT MEGRATEUN 0F RCUTS OF AN ARC Basil Tuxford Digby, London, England, assignor to Asso= ciated Electrical Industries Limited, London, England, a British company Filed May 1, M62, Ser. No. 192,035 Claims priority, application Great Britain, May 2, 1961, 15,881/61 2 Claims. (Cl. 2tltl-l44l) The present invention relates to vacuum switches and is particularly applicable to the control of an are discharge occuring between the electrodes, of such switches.

It is known that an arc is formed when two electrodes, carrying either a substantially steady or a varying or alternating current and which are initially in contact with one another within an evacuated enclosure, are separated. Subsequent to separation, a metal bridge is formed between the electrode which ruptures on further separation thereof, the root of that bridge remaining on the electrode which is negative with respect to the other, namely the cathode becomes incandescent and causes gradual evaporation and erosion of the cathode surface by virtue of the incandescent area or areas produced thereupon. These incandescent spots, from which electron emission occurs, are known as cathode spots, and in general a number of such spots will exist and move at random over the cathode surface. The electrons emitted from the cathode spots will establish and maintain a plasma between the positive electrode or anode and the cathode and thus permit a discharge current to continue to flow from the anode to the cathode after mechanical contact thercbetween has been broken.

Each of the electrodes when operating in connection with an alternating current source and used therefore to interrupt an alternating current flowing therethrough, are capable of existing at a negative potential with respect to the other, at the instant of interruption, and are each capable therefore of acting as the cathode from which the are formed on interruption roots.

Erosion of both electrode surfaces is thus likely to occur during the intervals between the initiation of the arc, formed on separation thereof, and the subsequent extinction of the are which occurs when the polarity of the source changes and the arc current falls to Zero.

When operated in connection with, and used therefore to interrupt, a direct current flowing from a substantially direct current source, only that electrode connected permanently to the negative terminal of the supply is capable of acting as the cathode on interruption of the direct current flowing therethrough. The arc formed on interruption will thus root only on that electrode which is negative; only that electrode is therefore subject to erosion.

Moreover the roots of the arcs formed as hereinbefore described are liable to wander not only over the contacting surfaces of the cathode, where they are initially formed, but are also liable to migrate from the contact surface to the vulnerable surfaces of the vacuum switch, for example the region of the vacuum seal between the cathode and the enclosing evacuated envelope. Such vacuum seals may, for example, be thin flexible metallic bellows if the cathode is arranged to be movable with respect to the envelope. Furthermore the roots of the arcs are liable to migrate on to those regions of the envelope located beyond the vacuum seal and cause erosion and perforation thereof, and consequent leakage of gas therethrough.

It is an'object of the present invention therefore to devise an electrode assembly suitable for use in a vacuum amass: Patented June 22, 1965 switch and which confines the random movement of the arc roots and prevents the migration of the arc roots to the vulnerable parts of the switch.

The electrode assembly of the kind in which two electrodes arranged to come into contact with one another within an evacuated enclosure and connectible to an electrical supply source, is formed, according to the present invention, such that each electrode capable of acting as a cathode is provided with at least one continuous channel extending into that region of the peripheral side wall adjacent to and separated from the contacting face thereof, said channel having a narrow width and having a depth substantially greater than its width.

Conveniently, the or each channel extends circumferentially about the axis of separation of the electrode and is located in a plane lying substantially perpendicularly to said axis. In an electrode having dimensions substantially as shown in the accompanying drawing (approximately to scale) the channel preferably has a width of not less than 1.5 mm. and a depth not less than 3.0 mm. in order efficiently to intercept the movement of the arc roots. A suitable relation of the depth of the channel to the electrode is that the depth be approximately /5 the diameter.

As an additional measure in confining the arc roots, each channel is preferably so positioned within the peripheral wall, but in a direction along the common axis of the electrode system, that it lies adjacent to a tin which forms part of each electrode and extends therearound. One of the plane surfaces of the tin therefore also forms a wall of the channel, and the outer edge of the fin may conveniently lie in the same plane as, and form part of, the peripheral wall of the electrode structure.

in order that the invention may be more readily understood, an embodiment thereof and its method of operation will now be described with reference to the accompanying drawing, which illustrates in axial crosssection a form of electrode structure in accordance with the invention incorporated in a vacuum switch arranged to be used in connection with an alternating current source and arranged therefore to interrupt an alternating current flowing therefrom.

Referring now to the drawing, we have shown one example of a vacuum switch suitable for use with and used to interrupt a current flowing from an alternating current source which incorporates the electrode assembly disclosed by the invention. In this witch, the two electrodes l and 2 have their contacting faces 3, 4, enclosed within the evacuated envelope Ztl, shown as being of glass, the electrodes being capable of disengagement by virtue of the movement of the electrode 1 along the common axis of the two electrodes. Electrode 2 is supported by a metal thimble l4 sealed to a re-entrant stem 15 forming part of the envelope 20. Movement of the electrode l with respect to the electrode 2 to cause disengagement therebetween, is permitted by the flexible thin metallic bellows 13, forming glass-to-metal and metal-to-metal seals, respectively, between the envelope 2d and the electrode 1, respectively, and is produced by the mechanical assembly of the members 16, 1'7, 18 and 19. In this assembly, movement of the plunger 17 toward and away from the switch will produce engage- .rent and disengagement of the contacting faces of the electrodes respectively. Formation of an are between the electrodes when carrying an alternating current will thus occur on disengagement of the contacting faces thereof.

If, during the first half-cycle of operation, the electrode 1 is the negative electrode, the cathode spot of the are will play upon the contacting surface thereof. The electrodes, preferably cylindrical in form, are mounted on support members and 6, which are connected to the current-carrying conductors 7 connectible in turn to the external circuit. In accordance with the present invention, the random motion of the arc roots over the electrode surfaces is limited by the provision of the channels 8, conveniently annular in form if the electrode structures are cylindrical, extending around the peripheral side walls 9 thereof. It is in addition desirable as an additional measure in preventing the arc wandering beyond the channel and on to other adjacent parts of the electrode 3 that the channels should be so positioned axially along the electrode system such that they lie adjacent to the fins 10, also annular in form if the electrode structure is cylindrical, extending therearound, and which are conveniently formed integrally therewith. In this way, one of the plane surfaces of the fin forms a wall of the channel and the outer edges of the fins may lie in the same plane as, and form part of, the peripheral walls 9 of the electrode structure. Both electrodes are constructed in this manner, as that electrode which is negative during one half cycle of AC. in one operation at the instant of switching or interruption may not necessarily be the negative electrode in another later operation of the switch.

When used in connection with a direct current source however, only that electrode permanently connected to the negative terminal of the supply need be provided with a channel in the peripheral side wall thereof since at the instant of the interruption of the direct current flowing therethrough, only the negative electrode is liable to be eroded by virtue of the arc roots being formed thereupon. However, the channel which is provided on the electrode and which restricts the movement of the arc roots does not as hereinafter described extinguish the whole of the arc. Accordingly since only a part of the arc is extinguished it is usual to employ in conjunction with a DC. vacuum switch, having at least one electrode constructed according to the invention, means for reducing the current in the circuit, in which the switch is included, to zero, within a short interval after initiation of the arc has occurred. Such a means is of course unnecessary in the case of an AC. switch in which the arc is self-extinguishing by virtue of the current automatically falling to zero when the polarity of the supply is reversed.

We have found from practical experiments that only that portion of the arc originating from those roots which migrate and enter the channel positioned in the peripheral side wall of an electrode formed in accordance with the invention, becomes extinguished. A possible explanation of this effect is that when the cathode spot enters the narrow channel 8 formed in the peripheral wall of the cathode, and is for example positioned on the channel wall at the region it, the electric field within the'channel at a point in the arc plasma and immediately in front of the cathode spot at it, is reduced as a result of the electrical shielding effect provided by the channel walls. Hence the arc fails to be self-maintaining since ions in the plasma are not accelerated suficiently by the electric field to cause in turn the required heating of the cathode spot (by their kinetic energy). Consequently, the cathode emits an insufiicient number of electrons All which can be accelerated by the electric field to collide with vapour molecules to form ions. Thus, a reduction to the electric field in the channel is thought to have two efiects; firstly, the acceleration of the ions in the arc plasma is reduced, with the consequent lowering of the cathode spot temperature, and the emission of fewer electrons from the cathode; secondly, electrons so produced will be accelerated less rapidly and hence are less likely to produce ions in the vapour of the metal of the electrode structure. An additional effect is that some of the electrons and metal vapour are rapidly removed from the arc as they are collected or condensed on the opposite wall 12 of the channel.

During the manufacture of the switch, it is necessary to process the completed assembly at a high a temperature as permissible. This requires that the metal-tometal and metal-to-glass joints withstand the degree of heating involved. The current-carrying conductors 7 have to be made of a high conductivity metal, e.g. copper, or copper alloy, while the bellows 13 may be of Monel, Phosphor-bronze or stainless steel, and the metal thimble lid of Milo K. The metal bellows may also lose its elasticity if heated to too high a temperature. It is, therefore, preferred to form the joints between the thimble l and the electrode 2, and between the bellows l3 and electrode It by means of arc-Welding in an inert atmosphere, such as argon.

What I claim is:

l. A direct current vacuum switch comprising:

(a) an evacuated envelope,

(b) two relatively movable electrodes extending respectively into opposite ends of the envelope,

(0) a cylindrical contact member at the inner end of each electrode so that electrical contact can be established at the opposed faces of the contact members,

(d) a circumterentially continuous channel provided in the peripheral side wall of at least one contact member, the channel having substantially parallel opposed walls and extending into the contact member substantially radially inwardly of the axis of the electrodes, so that upon disengagement of the contact members to interrupt a current, the roots of an are formed on the contact face of the one contact member are intercepted by and extinguished within, the channel and are thereby prevented from migrating to vulnerable parts of the switch.

2. A direct current vacuum switch according to claim ll, wherein the depth of the channel is twice the width of the channel.

References Cited by the Examiner UNITED STATES PATENTS BERNARD A. GILHEANY, Primary Examiner.

Claims (1)

1. A DIRECT CURRENT VACUUM SWITCH COMPRISING: (A) AN EVACUATED ENVELOPE, (B) TWO RELATIVELY MOVABLE ELECTRODES EXTENDING RESPECTIVELY INTO OPPOSITE ENDS OF THE ENVELOPE, (C) A CYLINDRICAL CONTACT MEMBER AT THE INNER END OF EACH ELECTRODE SO THAT ELECTRICAL CONTACT CAN BE ESTABLISHED AT THE OPPOSED FACES OF THE CONTACT MEMBERS, (D) A CIRCUMFERENTIALLY CONTINUOUS CHANNEL PROVIDED IN THE PRIPHERAL SIDE WALL OF SAID LEAST ONE CONTACT MEMBER; THE CHANNEL HAVING SUBSTANTIALLY PARALLEL OPPOSED WALLS AND EXTENDING INTO THE CONTACT MEMBER SUBSTANTIALLY RADIALLY INWARDLY OF THE AXIS OF THE ELECTRODES, SO THAT UPON DISENGAGEMENT OF THE CONTACT MEMBERS TO INTERRUPT A CURRENT, THE ROOTS OF AN ARC FORMED ON THE CONTACT FACE OF THE ONE CONTACT MEMBER ARE INTERCEPTED BY AND EXTINGUISHED WITHIN CHANNEL AND ARE THEREBY PREVENTED FROM MIGRATING TO VULNERABLE PARTS OF THE SWITCH.

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