GB2233498A

GB2233498A - Vacuum switch arc control

Info

Publication number: GB2233498A
Application number: GB8914230A
Authority: GB
Inventors: John Ernest Jenkins
Original assignee: General Electric Co PLC; GEC Alsthom Ltd
Current assignee: General Electric Co PLC; Alstom UK Ltd
Priority date: 1989-06-21
Filing date: 1989-06-21
Publication date: 1991-01-09
Also published as: GB8914230D0

Abstract

An arc formed in a vacuum switch is dispersed by imposing an axial magnetic field which is provided by a helical coil 19 mounted externally of the vacuum envelope 5 coaxial with the contacts 13,15. The coil 19 is connected between the fixed contact 15 and a terminal 31. The diameter of the coil may thus be kept within that of the vacuum envelope. <IMAGE>

Description

Vacuum Switch This invention relates to vacuum switches and the control of an arc in such a switch.

It is well known that at currents of about lOkA and above, a vacuum arc changes from a mode in which the arc is diffuse to one in which it becomes constricted. In this constricted mode the energy input to the arcing anode can be such that gross melting of a region of the anode can take place. The relatively slow cooling of this molten area results in metal vapour being produced after the switch has been opened, and more particularly after current zero, and the vapour pressure can be such that the dielectric strength of the gap is reduced causing reignition of the arc.

Two methods, both based on the application of magnetic fields to the arc, have been developed to overcome this problem.

In the first method, the geometry of the conductors within the switch is such that the current produces a radial magnetic field within the gap. This field causes the constricted arc to travel in a circular path with the result that the energy input to the arcing anode is evenly distributed thus preventing gross melting.

In the second method an axial field is applied to the switch by means of an external single turn coil centred on the contact gap.

This field helps to prevent the arc constricting. The coil is connected in series with the switch so that coil current is the same as that for the switch.

Both these methods have a number of advantages and disadvantages. The first method has the advantage that no extra space is required outside the switch, but has the disadvantage that the conductors are expensive to machine and are a permanent feature of the switch. The second method has the advantage of flexibility in that the same switch can be used with or without the coil for different current ratings. The disadvantage is that the effective diameter of the switch is increased. Also if a higher field is required to prevent constriction it is not easy to increase the number of turns on the coil.

An object of the invention is therefore to provide a vacuum switch having arc control by means of an externally applied magnetic field but which does not involve an increase in the switch diameter.

According to one aspect of the invention, in a vacuum switch having a vacuum envelope enclosing a pair of contacts mounted on respective conductive members extending through and sealed to the envelope, the contacts and the conductive members lying on a common axis, a helical coil is fixed at one end to one of the conductive members and at its other end to a terminal connection for the switch, the axis of the helical coil being substantially aligned with the common axis so as to provide, in operation, a magnetic field substantially aligned with the common axis, passing through the arc gap, and effective to reduce constriction of an arc struck between the contacts.

The helical coil is preferably fixedly connected to the fixed contact.

The helical coil may be fixed to said one conductive member by means of a ferromagnetic cylindrical member extending axially through the helical coil, the cylindrical member preferably being a steel bolt which is screwed into the conductive member so increasing the permeability of the flux path of the axial magnetic field.

The turns of the helical coil may be spaced apart by an insulating layer and the coil compressed by the bolt between the one conductive member and a threaded terminal member.

According to another aspect of the invention in a method of controlling an arc in a vacuum switch, an axial magnetic field is imposed upon the arc gap by means of a helical coil mounted externally of the vacuum envelope, coaxially with the arc gap and adjacent one terminal of the switch.

An a.c. vacuum switch in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is an outside view of a vacuum switch embodying the invention; and Figure 2 is a diagrammatic sectional view of part of the switch.

The switch comprises a pair of axially aligned contacts enclosed within an evacuated vacuum tight envelope. The lower contact is sealed and fixed to the envelope and the upper contact is sealed to the envelope by a flexible bellows in known manner. The upper contact is then driven by a solenoid to close the contact gap against the force of a release spring. The movable contact is connected by a flexible lead to the upper switch terminal.

Figure 1 shows the upper terminal 1, upper and lower metal pressings 3 and 5 and the ceramic cylinder 7 which completes the vacuum envelope. The upper and lower metal pressings 3 and 5 are fixed to conductor members which are threaded (9, 11) and normally provide terminal fixings.

Figure 2 shows the contacts and lower fixing in more detail, the upper, moving contact 13 being shown closed against the fixed, lower contact 15. The fixed contact 15 is mounted on a conductor member 17 mentioned above, which is threaded (11) to receive a normal terminal connection. An assembly comprising a helical copper coil 19, a brass washer 25, and a threaded brass terminal sleeve 31 are fixed against the flat end face 21 of the conductor 17 by means of a steel bolt 23. The turns of the coil are separated by an insulating layer 27 and the bolt 23 is separated from the coil and the terminal sleeve 31 by an insulating sleeve 29. The end faces of the coil 19 are shaped to produce a large surface area abutment with the conductor face 21 and the brass washer 25.The result of this construction is that current carried by the contacts 13 and 15 can only reach the terminal sleeve 31 by way of the helical coil 19. Thus the full contact current provides an axial magnetic flux within the coil which extends through the contact gap region and tends to disperse an arc between the contacts over a wider contact area.

Melting and vapourisation of the contacts is thus counteracted and re-striking is very largely inhibited. It should be noted that this axial field is achieved without any increase in the switch diameter such as would result from a coil (or rather single turn) in the plane of the constant gap. The helical coil is well within the diameter of the vacuum envelope.

The use of a ferromagnetic bolt 23, eg, conveniently, steel, increases the permeability of the flux path close to the contact gap and tends to confine the axial field at this position, so enhancing and making maximum use of the available field.

The construction has the further advantage that the number of turns of the helical coil can be increased significantly with a corresponding increase in the magnetic field. Thus a range of different coils may be adapted to fit according to the current rating.

The coil may instead be added or not to a standard switch depending on the rated current. Below a certain current rating the arc control coil may not be necessary.

While an alternating current switch has been described, it will be clear that the same principle could be applied to a d.c.

switch.

Claims

1. A vacuum switch having a vacuum envelope enclosing a pair of contacts mounted on respective conductive members extending through and sealed to the envelope, the contacts and the conductive members lying on a common axis, wherein a helical coil is fixed at one end to one of said conductive members and at its other end to a terminal connection for the switch, the axis of the helical coil being substantially aligned with said common axis so as to provide, in operation, a magnetic field substantially aligned with said common axis, passing through the arc gap, and effective to reduce constriction of an arc struck between said contacts.

2. A vacuum switch according to Claim 1, having a fixed contact and a movable contact wherein said helical coil is fixedly connected to the fixed contact.

3. A vacuum switch according to Claim 1 or Claim 2, wherein said helical coil is fixed to said one conductive member by means of a ferromagnetic cylindrical member extending axially through the helical coil.

4. A vacuum switch according to Claim 3, wherein said cylindrical member is a steel bolt which is screwed into said conductive member so increasing the permeability of the flux path of the axial magnetic field.

5. A vacuum switch according to Claim 4, wherein the turns of the helical coil are spaced apart by an insulating layer and the coil is compressed by said bolt between said one conductive member and a threaded terminal member.

6. A vacuum switch substantially as hereinbefore described with reference to the accompanying drawings.

7. A method of controlling an arc in a vacuum switch, in which an axial magnetic field is imposed upon the arc gap by means of a helical coil mounted externally of the vacuum envelope, coaxially with the arc gap and adjacent one terminal of the switch.