GB2369246A

GB2369246A - Circuit breaker with magnetic coil for arc displacement

Info

Publication number: GB2369246A
Application number: GB0028207A
Authority: GB
Inventors: Christopher Aaron Nazeri
Original assignee: Whipp and Bourne Ltd; FKI PLC
Current assignee: Whipp and Bourne Ltd; Brush Holdings Ltd
Priority date: 2000-11-18
Filing date: 2000-11-18
Publication date: 2002-05-22
Anticipated expiration: 2020-11-18
Also published as: GB0028207D0; GB2369246B

Abstract

The circuit breaker includes a pair of relatively moveable contacts 11, 13, each including a respective contact surface 12, 14, adapted to move between a closed configuration in which the contact surfaces are in physical contact with one another and an open configuration in which the contact surfaces are separated by a contact gap. The first contact 11 is connected to a first current terminal 30 via a first current path 31 and the second contact 13 is connected to a second current terminal (32, Fig.1) via a second current path. A magnetic flux circuit 50 includes a coil 51, 52 wound around at least a portion of the first current path 31 and a pair of pole pieces 54, 55 defining a flux gap 56 traversing at least a portion of said contact gap. Duration of arcing between the contacts is minimised by enabling rapid extinguishing of the arc drawn between two separating contacts.

Description

CIRCUIT BREAKER WITH MAGNETIC COIL FOR ARC DISPLACEMENT The present invention relates to circuit breakers, and in particular to circuit breakers incorporating an arc displacement mechanism to drive an arc drawn between two relatively moveable contacts away from the vicinity of the contacts.

In circuit breakers capable of handling large overload currents, an important feature is the minimisation of arcing between the contacts of the circuit breaker as the contacts open during overcurrent fault situations. Various arc extinguishing techniques are known in the art, including those which aim to displace the arc from the opening contacts, thereby stretching the arc over a greater distance than that which prevails between the opening contacts and moving the arc towards an arc extinguishing structure.

It is one object of the present invention to provide a transverse magnetic field between two contacts that are moving apart in a circuit breaker so as to displace the arc from its position between the contacts.

It is another object of the invention to minimise the duration of arcing between the contacts by enabling rapid extinguishing of an arc drawn between two separating contacts.

According to one aspect, the present invention provides a circuit breaker comprising : a pair of relatively moveable contacts, each including a respective contact surface, adapted to move between a closed configuration in which the contact surfaces are in physical contact with one another and an open configuration in which the contact surfaces are separated by a contact gap;

the first contact being connected to a first current terminal via a first current path and the second contact being connected to a second current terminal via a second current path; and a magnetic flux circuit including a coil wound around at least a portion of the first current path and a pair of pole pieces defining a flux gap traversing at least a portion of said contact gap.

According to another aspect, the invention provides a method of inhibiting or minimising arc formation during current interruption comprising the steps of : using the current path to and/or from a pair of relatively moveable contact surfaces to generate magnetic flux around the current path concentrating said magnetic flux in a magnetic flux circuit that is wound around at least a portion of the current path delivering the magnetic flux to a pair of flux guides situated adjacent to a contact gap formed between the pair of relatively movable contacts as they move to an open configuration, the magnetic flux traversing a magnetic flux gap that is collocated with and transverse to the current flow between the two relatively movable contact surfaces.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a schematic cross-sectional side view of a circuit breaker according to one embodiment of the invention; Figure 2 shows a perspective schematic view of magnetic field generating components of the embodiment of figure 1; Figure 3 shows a schematic diagram of current and magnetic flux flow in the components of figure 2; and Figure 4 shows a schematic diagram of current and magnetic flux flow in the components of figure 2.

Throughout the present specification, various references to orientation may be made, such as up, down, left, right etc. It will be understood that these are for convenience in describing the specific embodiments as shown in the drawings, but are not limiting to any particular orientation in the circuit breaker as claimed or in use.

With reference to figure 1, there is shown a DC circuit breaker 10 according to a presently preferred embodiment. The circuit breaker 10 includes a fixed contact arm 11 having a contact surface 12 formed thereon and a moving contact arm 13 also having a contact surface 14 formed thereon. The moving contact arm 13 is rotatable about a pivot 15 between an open configuration (as shown in the drawing) and a closed configuration in which the contact surfaces 12 and 14 are brought together. In the open configuration, a contact gap 16 is formed between the contact surfaces 12, 14.

Movement of the contact arm 13 is facilitated by way of an actuating mechanism 20 according to any one of a number of actuator mechanisms well known in the art. Such actuator mechanisms 20 may include a dual mechanism 21,22 to separately provide opening and closing forces, in accordance with known practices. The actuator mechanism 20 is coupled to the movable contact arm 13 by way of drive link 23.

It will be understood that although a circuit breaker 10 using one fixed and one moving contact arm 11,13 has been described, the circuit breaker could be adapted to provide two moving contact arms. More than one contact surface on each respective contact arm could also be provided and the relative movement of contacts could generally also be provided by way of a plunger-type contact rather a pivoting contact arm 13.

The first contact 11 is coupled to a first current terminal 30 by way of a first current path 31 to be described in greater detail in connection with figures 2 and 3. The second contact 13 is coupled to a second current terminal 32 by way of a second current path 33 which includes a flexible electrical connection using or bypassing the pivot 15.

Positioned adjacent to the contact surfaces 12,14 is an arc interrupting chamber 40 which preferably includes some form of arc extinguishing mechanism as well known in the art, such as an arc chute 41. The arc chute 41 comprises a plurality of parallel, electrically conductive plates 42 each separated by a suitable dielectric medium (eg. air or other gas) which split the arc. In a preferred configuration, the arc chute 41 comprises 60 plates sustaining a 30V arc voltage between each adjacent pair of plates to give a total of 1800V. Arc chutes are well known in the art and will not be further described here.

Referring now to figure 2, a magnetic flux generating circuit is described that generates a transverse magnetic field in the vicinity of the contact gap 16 in order to accelerate movement of an arc generated in the contact gap 16 between the contact surfaces 12,14 into the interrupting chamber 40 during interruption of current between terminals 30 and 32.

The first current path 31 connecting first current terminal 30 to the fixed contact 11 is preferably split at a first electrically conductive bridging member 36 into two electrically parallel paths which will be described herein, for convenience, as a first subpath through a first, electrically conductive subpath member 34 and a second subpath through a second, electrically conductive subpath member 35. While these two subpath members 34,35 are shown in this embodiment as being both electrically

parallel and geometrically parallel, it will be understood that they need not be geometrically parallel.

The two subpath members 34,35 merge at a second bridging member 37 which also acts as a supporting member for the contact 11 and contact surface 12. The contact 11 extends upwardly from the bridging member and

thereby from the two subpath members 34, 35. The contact 11 also extends backwards from the second bridging member 37 such that the direction of current flow in the final portion of the first current path extending toward the contact surface 12 and/or the direction of current flow across the contact surfaces 12,14 is in the opposite direction to the current flow through the two subpath members 34,35. This assists in the production of a compact unit.

The movable contact 13 extends upwards between the two subpath members 34, 35 and is pivoted so that the axis of opening of the contact gap 16 between the contact surfaces 12,14 is generally parallel with the first and second subpath members.

Wrapped around each of the first and second subpath members 34,35 is a magnetic flux circuit 50 which preferably comprises a soft iron winding. The magnetic flux circuit includes a first winding section 51 which is wound around the first subpath member 34 in a first direction, and a second winding section 52 which is wound around the second subpath member 35 in the opposite direction. The first and second winding sections 51,52 are linked by a distal section 53 that links the first and second sections at the end of the subpath members 34, 35 that is distal to the contacts 11,12.

The soft iron winding extends at the ends which is proximal to the contacts to a pair of shaped pole pieces or flux guides 54,55 respectively leading

from the first winding section 51 and the second winding section 52 in a convergent manner to the vicinity of the contact gap 16. The flux guides 54, 55 are shaped so as to provide a flux gap 56 extending across the contact gap and transverse thereto. Preferably, the flux guides are shaped and positioned so that the flux gap axis extends orthogonally to the contact gap axis. Preferably, the flux guides 54,55 are also shaped to be divergent above the flux gap 56 within the interrupting chamber 40 and towards the arc chute 41.

The operation of the circuit breaker will now be described with reference to figures 3 and 4.

In a first mode of operation, current flows from the first current terminal 30 through first current path 31 and second current path 33 to the second current terminal 32. Current therefore flows leftwards as shown through first and second subpath members 34,35 as I, and 12 merging to form 13 (=I, +I2)./3 flows rightwards as shown in the current path leading into contact surface 12 and, when the contacts are closed, into contact surface 14

from where it forms current 14 flowing through contact 13 downwards towards current terminal 32.

Magnetic fields (DI and C2 are generated in the direction shown, by the currents I, and 12 flowing in subpath members 34, 35. These fields are concentrated in the magnetic flux circuit and guided to the magnetic flux gap 56. By the direction of the first winding section 51, the flux cl) is guided generally in the direction shown by (D3 towards the end of subpath member 34 which is proximal to the contacts, ie in the same direction as current flow

Il. Conversely, by the direction of the second winding section 52, the flux C2 is guided generally in the direction shown by #4 towards the end of subpath member 35 that is distal from the contacts, ie. opposite to the direction of current flow I2.

The flux (D3 and04 is guided by flux guides 54, 55 to generate a flux across the flux gap 56 as indicated by Cc which is transverse to the current 13 across the contacts.

In the event of an overcurrent occurring in the circuit being protected by the circuit breaker 10, the actuator mechanism 20 initiates current interruption by driving the movable contact 13 from a closed position to an open position. During the initial separation of the contact surfaces 12,14, an arc may be struck therebetween if the current and voltage are sufficient. An arc is disadvantageous for two reasons: it (a) delays current interruption and (b) causes damage to the contact surfaces. In the configuration of the present

invention, the magnetic flux Cc traversing the contact gap 16 serves to blow the arc upwards into the interrupting chamber 40 and arc chute 41 where the arc is rapidly suppressed.

The strength of magnetic flux generated across the flux gap 56 is proportional to the current flowing in the first current path 31 and thus increases the magnetic blow out force according to the magnitude of the overcurrent.

It will be noted that the magnetic force on the arc will be in the upward direction towards the interrupting chamber 40 regardless of the direction of current flow through the first current path. If the current flow in first and second subpath members 34,35 is reversed, it will be observed that the flux flow in magnetic flux circuit 50 is also reversed. Thus, the reversed current direction across the contact gap 16 is counteracted by the reversed flux direction in the magnetic flux circuit 50.

Thus, although a DC circuit breaker has been described in an exemplary embodiment, it will be apparent that the circuit breaker will work in the event of current reversal and can also be readily adapted for use as an AC circuit breaker.

The magnetic flux traversing the flux gap 56 also serves to expel ionised debris that may be generated in the arc or at the contacts, away from the contact surfaces.

In the preferred embodiment, the magnetic flux circuit is formed of soft iron and can be adapted to reach saturation at a predetermined level so as to limit the magnetic forces generated under exceptionally high currents, thus helping to avoid damage to the circuit breaker structure.

In a preferred embodiment, the magnetic permeability of the flux circuit material is particularly chosen to provide a required degree of transverse magnetic flux (DG across the flux gap 56 for a given level of current flowing across the contacts 12,14. Important in such calculations is also the crosssectional area of the magnetic flux circuit 50. In a presently preferred embodiment of the invention, the flux circuit is comprised of a square section soft iron member 51,52, 53 that is approximately 20 mm x 20 mm in section. This provides an appropriate degree of flux density and saturation at a desired current level.

For optimum efficiency and effectiveness, the magnetic flux circuit 50 is preferably formed from a low remanence material so that magnetic flux (DG across the flux gap 56 is reduced to zero or near-zero rapidly after current flow ceases. For example, in a preferred arrangement, remanent flux density after saturation should be as low as possible and preferably less than 20mT,

or more preferably less than 10mT. Ideally, it should be possible to reduce this to zero or near-zero with a reverse current of less than 200 A. Substantial decreases in the arc duration upon separation of the contact surfaces 12,14 are achieved with the present invention, even at low currents, thus significantly reducing contact erosion. In a preferred embodiment, for a

current flow (13) of 200 amps, the five windings of the first and second winding sections 51, 52 generate a field (DG across the flux gap 56 of lOmT orthogonal to the current flow, resulting in a reduction of the current arcing time from 150 ms to 50 ms.

In particular, the invention has been found to be particularly beneficial in smaller circuit breakers adapted to trip at levels in the region of 100 amps where conventional circuit breakers have been unable to generate significant electromagnetic arc displacement forces and where arc chutes (typically used for much higher interrupt currents) have previously been relatively ineffective.

While the invention has been described in connection with a few exemplary embodiments, it will be understood that variations and modifications to the embodiments described may be made within the scope of the accompanying claims.

In particular, while the arrangement of current path division to enable two series-connected windings to be used in the magnetic flux circuit is preferred to provide a compact unit with good cooling characteristics, a single, undivided current path could be used.

In another embodiment, the magnetic flux circuit could be wound around both the first and second current paths (eg. in the current paths leading both

to and from the circuit breaker contacts) or separate flux circuits could be provided around each of the first and second current paths, each circuit having flux guides that contribute to the transverse flux across the contact gap 16.

In another embodiment, the magnetic flux circuit could be wound around the second current path only, eg. the current path that leads to a moving contact rather than to a fixed contact.

Claims

CLAIMS 1. A circuit breaker comprising: a pair of relatively moveable contacts, each including a respective contact surface, adapted to move between a closed configuration in which the contact surfaces are in physical contact with one another and an open configuration in which the contact surfaces are separated by a contact gap; the first contact being connected to a first current terminal via a first current path and the second contact being connected to a second current terminal via a second current path; and a magnetic flux circuit including a coil wound around at least a portion of the first current path and a pair of pole pieces defining a flux gap traversing at least a portion of said contact gap.
2. The circuit breaker of claim 1 in which the first contact comprises a fixed contact non-movable with respect to the magnetic flux circuit and the second contact comprises a moveable contact.
3. The circuit breaker of claim 1 or claim 2 in which the first current path is divided into a first and a second current subpath electrically parallel to one another, and in which the flux circuit comprises a pair of seriesconnected windings in which a first one of the series connected pair is wound in a first direction around the first current subpath and the second one of the series connected pair is wound in a second direction around the second current subpath.
4. The circuit breaker of any preceding claim in which said flux circuit coil is formed from a relatively high permeability material, with a low remanance.
5. The circuit breaker of claim 4 in which the flux circuit coil is formed from soft iron.
6. The circuit breaker of claim 3 in which the second one of said contacts lies between said first and second current subpaths, the first and second contacts relatively movable in a direction that is substantially parallel to the flux circuit coils so as to create a contact gap substantially parallel to the current flow in said first and second current subpaths and in which the first current subpaths having said flux circuit series-connected windings extend in a first direction, and a portion of the first current path extending to the first contact is in a second direction opposite to the first direction.
7. The circuit breaker of any preceding claim in which the flux circuit is configured to define a flux gap that is adapted to drive an arc away from the contact gap and towards an arc interrupting chamber.
8. The circuit breaker of claim 7 wherein the arc interrupting chamber further includes arc suppression means.
9. The circuit breaker according to any preceding claim further including an actuator for driving at least one of the relatively moveable contacts between said open and closed configurations.
10. A method of inhibiting or minimising arc formation during current interruption comprising the steps of : using the current path to and/or from a pair of relatively moveable contact surfaces to generate magnetic flux around the current path concentrating said magnetic flux in a magnetic flux circuit that is wound around at least a portion of the current path delivering the magnetic flux to a pair of flux guides situated adjacent to a contact gap formed between the pair of relatively movable contacts as they move to an open configuration, the magnetic flux traversing a magnetic flux gap that is collocated with and transverse to the current flow between the two relatively movable contact surfaces.
11. The method of claim 10 further including the steps of : dividing the current path to and/or from the pair of relatively movable contact surfaces into to electrically parallel subpaths; concentrating magnetic flux in a magnetic flux circuit wound around at least a portion of both subpaths; and combining the magnetic flux from both subpaths by series connection of said flux circuit windings.
12. A circuit breaker substantially as described herein with reference to the accompanying drawings.
13. A method of inhibiting or minimising arc formation during current interruption substantially as described herein with reference to the accompanying drawings.