CIRCUIT BREAKER CONTACT STRUCTURE

Abstract: A circuit breaker contact configuration reduces the occurrence of contact bounce by providing a reverse force to the contacts during contact closure. This reduces the contact closing shock energy, while keeping the contact bias force constant. In one embodiment, the reverse force is obtained by means of a reversing spring (30) which engages one of the contact arms (12) at a point along the rebound path. Another embodiment comprises transferring the pivot point of one of the contacts from a first (14) to a second position (52) at a point along the rebound path.

Description: This invention relates to the configuration of electrical contacts used within electrical circuit breakers and, more particularly, to a current limiting circuit breaker such as described within EP-A-0 033 479. The above mentioned patent application discloses that current limitation is obtained by the rapid generation of a high voltage arc between the circuit breaker contacts. The rapid generation of the arc is obtained by rapidly separating the contacts to create a gap between the contacts. This gap is obtained within a very short time, in the order of milliseconds, and is created by utilizing electro-dynamic or electromagnetic forces to separate the contacts. The contact separation speed is much faster under electrodynamic or electromagnetic forces when both contacts are movable than when only one of the contacts is movable and hence the arc voltage is generated at a faster rate. However the circuit breaker ability to handle the current can be adversely influenced by having both contacts movable, because the contacts are more likely to "bounce" apart upon closing with the result that elemental arcing can occur causing the contacts to become welded together. The above pointed problem is clearly apparent from both Figures 1A-1C of the present Application, depicting the prior art as disclosed in the above mentioned EP-A-0 033 479, and DE-C-1 638 157. Looking for further information at figure 1 of the above mentioned German Patent, it is seen that a lower not controllably movable contact arm 4 is rotatable around a pivot pin 7 under the bias of a tensioning spring 9 connecting a pin 10 in the contact arm 4 to a fixed pin 11 in support means 8 supporting the arm 4 and the relating pin 7 and is prevented from counterclockwise rotating beyond a closure position by a stopping pin 12, being said pin 12 possibly responsible of abrupt stops of the contact arm 4 after an extension of the spring 9, due for example to a shock of a controllably movable upper contact arm 3 against the lower contact arm 4 at a closure, causing bounces of the abutting upper movable contact arm 3, with consequent possible contact opening, with respect to the lower contact arm 4, as it happens in figure 1C of the present Application. To overcome the above problem was made, as disclosed and claimed in FR-A-2 003 412, an attempt to reduce bounces in contact arm pairs, using many kinds of resilient, friction and/or pneumatic shock-absorbers which however affect the contact bias forces, rendering less reliable the contacs themselves, because from the contact bias force must be subtracted the resilient forces connected with the springs of the shock-absorbers and, being said resilient forces connected to the actual stresses on said shock-absorbers, it results that the contact bias forces are not constant and, consequentely, not very reliable. The purpose of this invention is to provide a contact arrangement with reduced occurrence of contact bounce without interfering with the operating mechanism or requiring increased contact bias forces. The invention concerns a contact arrangement which includes a reverse motion spring or an additional pivot to provide higher overtravel with energy dissipation to the circuit breaker contacts upon closing to limit or eliminate contact bounce without reducing the contact force or requiring higher force from the operating mechanism. The arrangement of the invention is equally applicable to two movable contacts, as well as to one movable and one stationary contact. In a first embodiment of the present invention, which includs a first contact arm and a second contact arm provided with contacts, the second contact arm being fitted with an elastic support or contact spring and a pivot around which it can rotate, the contact force operating on the second contact arm (12) is reduced by means of a reversing spring which operates at a certain point of the overtravel by opposing itself progressively to the action of the contact spring, thus decreasing as a consequence the kinetic energy of the said second contact arm and of the associated first contact arm, so that the kinetic energy reaches a null - or almost null - value during said overtravel, preventing the temporary separation of the contacts. Preferably the said reversing spring is engaged by a projection on the second contact arm and, by the deformation thereof, it develops an increasing force, opposite to the force of the contact spring, until it will be stopped by a stop integral with the carriage structure of the circuit breaker. More preferably, the said reversing spring is a leaf spring joined at one of its ends with a bracket support, that will engage the projection (22) on the second contact arm, and secured at its other end to the carriage structure of the circuit breaker. According to a modification of said first embodiment the reversing spring is a helical spring positioned between an extension of a lever member, rotatable around a fixed pin, and a fixed support, said lever member being adapted to engage said second contact arm. In a second embodiment of the present invention, which also includes a first contact arm and a second contact arm, provided with contacts, the second contact arm being fitted with elastic support or contact spring and with a pivot, around which it can rotate, said pivot consists in a pin integral with the second contact arm, housed into an elongate slot (45) of a support structure of the circuit breaker which permits a limited shifting of said pivot from a first position at one end of said slot to a second position at the other end of the slot in order to allow said overtravel and that, in connection with said overtravel, the moment operating on the second contact arm will be decreased as inserting a second pivot, around which rotates said stationary contact arm while the pin of the first pivot moves itself in the elongated slot, where said second pivot decreases the moment of the contact spring in respect to that related to the first pivot, so that, as a consequence of the said decreased moment operating on the second contact arm, the kinetic energy of said second contact arm and of the associated first contact arm will be decreased thereby and said kinetic energy will reach a null - or almost null - value during said overtravel, preventing the temporary separation of the contacts. Preferably, said second pivot is placed on said second contact arm into an intermediate position between a first end, carrying said first pivot, and a second end, carrying one or more contacts, to which, in case of contact closure, a force - called contact force - is applied balancing the force from the contact spring, where said intermediate pivot, by involving an arm reduced for the force of spring will decrease the moment of the same force and hence the kinetic energy associated with the first and second contact arms. Figures 1A-1C each depict a side view of the contact arrangement of the prior art; Figures 2A-2C each depict a side view of one embodiment of the contact arrangement according to the invention; Figures 3A-3C each depict a side view of a variation of the embodiment depicted in Figures 2A-2C. Figures 4A-4C each depict a side view of a further embodiment of the contact arrangement of the invention; Figure 5 is a graphic representation of the contact force as a function of a contact overtravel for the prior art arrangement of Figures 1A-1C; Figure 6 is a graphic representation of the contact force as a function of contact overtravel for the embodiment depicted in Figures 2A-2C and 3A-3C; and Figure 7 is a graphic representation of the contact force as a function of contact overtravel for the embodiment depicted in Figures 4A-4C. In order to reach a better understanding of the present invention, it is beneficial to determine what happens within a system of movable upper and lower contact arms, respectively 10 and 12, of the prior art shown in Figure 1 when the system is closed as a consequence of an external operating mechanism. In this instance, the movable contact arm 10 is lowered until its contact 10a touches the contact 12a of the lower contact arm 12. The lower contact arm 12 can be rigidly fastened to the circuit breaker structure or can be movable by pivoting it on a pin 14 abutting the contact carrying structure or carriage (not shown) and fitting it with a contact spring 16 that operates between a first pin 18, rigidly connected with contact arm 12 and a second pin 20 on the contact carriage, resulting in the biasing of contact arm 12 towards contact arm 10. Contact arm 12 is made movable to permit reciprocal motion between the contacts under the action of electrodynamic forces upon extremely high current through the contacts and contact arms as shown in Figure 1. The provision of both contact arms being movable produces a rapid separation of the contacts upon short circuit conditions so that the circuit breaker is able to achieve the desired current limiting feature. As the contacts are closed by the operating mechanism (not shown) which ensures rapid contact closing, the force upon contact arm 10 rapidly moves the contact 10a against the contact 12a of the contact arm 12, rotating contact arm 12 around pin 14 against the bias of spring 16 as shown in Figure 1A. This continued motion of contact arms 10 and 12 is termed "overtravel". From the position of maximum overtravel given in Figure 1A, contact arm 12 under the action of contact spring 16 moves to the position illustrated in Figure 1B such that a projection 22 on contact arm 12 engages a stop 24 on the contact carriage. This is the normal rest position of contact arm 12 when the contacts are closed. However, if contact arm 12 is stopped abruptly upon contact between the projection 22 and the stop 24 during return motion while contact arm 10 continues to move beyond the normal rest position shown in Figure 1B, this movement of the upper contact 10 beyond the normal rest position causes a separation between contacts 10a and 12a and temporary formation of an elemental arc 26 as shown in Figure 1C. The arc current that continues to flow through the contacts can cause erosion and welding of the contacts. Then contact 10a returns to engagement with contact 12a under action of the closing force acting on contact arm 10. A first embodiment of the invention as shown in Figures 2A-2C includes contact arm 10 and contact arm 12 with contact spring 16 and pivot point 14 around which contact arm 12 rotates. Also included is a reversing spring 30 which operates at a certain point of the contact overtravel to progressively oppose the action of the contact spring 16, thereby decreasing the kinetic energy of both contact arm 12 and contact arm 10 during overtravel so that the contacts do not become separated. Preferably, the reversing spring 30 is engaged by the projection 22 on contact arm 12 and, by becoming extended, develops an opposing force which decreases the effect of the contact spring 16 until it contacts spring stop 38 formed in the carriage structure 36 of the circuit breaker. The reversing spring 30 preferably is a leaf spring joined at one of its ends with a bracket support 33 which engages the projection 22 or, the contact arm 12 and secured at its other end to the carriage structure 36 of the circuit breaker. A variation of the first embodiment of Figures 2A-2C is depicted by Figures 3A-3C wherein, instead of the leaf spring 30, a helical spring 30' is used as reversing spring working by compression between an extension 32' of a lever member 33', having the shape of a small metal plate pivotally mounted on a pin 34', secured to a carriage structure 36' and a support 30'a also secured to the carriage structure 36'. A second embodiment of the invention is shown in Figures 4A-4C and also includes an upper contact arm 10, a lower contact arm 12 and a contact spring 16. The pivot around which contact arm 12 rotates includes a pivot pin 14 formed on contact arm 12 and captured within an elongate slot 45 provided within the contact carriage to allow a limited movement of the pin 14 from a first position at one end of the slot 45 to a second position at the other end of the slot 45. In order to compensate for the increased overtravel, the moment of the force operating on the lower contact arm 12 is decreased by a second pivot pin 52 around which the lower contact 12 rotates while the first pin 14 moves within slot 45. The second pivot pin 52 presents a moment arm for the contact spring 16 which is decreased in respect to the first pivot pin 14 so that a decreased moment of the force operates on the lower contact arm 12. The kinetic energy of both the lower contact arm 12 and of the upper contact arm 10 is strongly decreased during the additional overtravel so that the contacts do not become separated. Ideally, the second pivot point 52 is arranged on the lower contact arm 12 at an intermediate position between a first end carrying the first pivot point 14, and a second end carrying the contact 12a. The applied contact force them balances the force from the spring 16 since the intermediate pivot point 52, presenting a reduced moment arm to the force of the contact spring 16, will decrease the resulting moment of the force and, as a result, the kinetic energy associated with both of the contact arms 10 and 12 is also decreased. A more detailed explanation as to the diminishing of the kinetic energy on the contacts with the aforementioned contact arrangements is as follows. In Figures 2A-2C, the force associated with the contact spring 16 is reduced by the reversing spring 30, since the projection 22, instead of engaging the stop 24 as with the prior art arrangement depicted in Figures 1A-1C, engages the bracket end 32 connected by means of bracket support 33 with the reversing spring 30. In the embodiment depicted in Figures 2A-2C, the leaf spring 30 is secured, for example, by means of a rivet 34 to the breaker carriage 36 and is stopped during its extension against the stop 38 formed on the circuit breaker contact carriage. Upon a contact closing operation, the upper contact arm 10 strikes the lower contact arm 12 pushing it towards the breaker support structure 36 until it reaches the position shown in Figure 2A. At this point, the contact spring 16 begins forcing the lower contact arm 12 upwards to the position shown in Figure 2B. When the lower contact arm 12 reaches the position shown, projection 22 engages the bracket top 32 which is joined through the bracket support 33 to the reversing spring 30. While the lower contact arm 12 continues its travel, the reversing spring 30 increasingly deforms and produces an increasing moment of force opposite in direction from that produced by the force of the contact spring 16 which continuosly diminishes as the contact arms 12, 10 move from the position indicated in Figure 2B to that shown in Figure 2C. The moment of force produced by the decreasing force exerted by the contact spring is effectively counteracted by the increasing moment of force provided by the reversing spring 30 in such a manner that the kinetic energy associated with the two springs in accordance with the invention is very much reduced. When the lower contact arm 12 stops upon engagement of the spring 30 against the spring stop, reversing spring 30 is able to decrease the kinetic energy associated with the upper contact arm 10 sufficiently to prevent the temporary separation of the contacts 10a and 12a. Similarly, in Figures 3A-3C, the force associated with the contact spring 16 is reduced by the reversing spring 30'. In the configuration depicted in Figure 3A the pin 18' reaches its most extended position allowing the lever member 33' to rest against a stop member 36'a of the carriage 36'. Further the pin 18', under the action of the spring 16 reaches the position depicted in Figure 3B engaging one end of the lever member 33', forcing the same to rotate clockwise around its pin 34', disengaging it from the stop member 36'a and compressing, through the extension 32' one end of the helical spring 30', the other end of which being engaged with the support 30'a pivotably mounted on the pin 30'b. While the lower contact arm 12 continues its travel, the reversing spring 30'increasingly deforms and produces an increasing moment of the force opposite in direction from that produced by the force of the contact spring 16 which continuously diminishes as the contact arms 12, 10 move from the position indicated in Figure 3B to that shown in Figure 3C. The moment of the force produced by the decreasing force exerted by the contact spring is effectively counteracted by the increasing moment of the force provided by the reversing spring 30' in such a manner that the kinetic energy associated with the two springs in accordance with the invention is very much reduced. When the lower contact arm 12 stops upon complete compression of the spring 30', said reversing spring 30' is able to decrease the kinetic energy associated with the upper contact arm 10 sufficiently to prevent the temporary separation of the contacts 10a and 12a. In the embodiment depicted in Figures 4A-4C, the modification of the moment of force associated with the contact caused by spring 16 is obtained transferring the rotation of the lower contact arm 12 from a first pivot pin 14 to a second pivot pin 52 which changes the arm of the contact spring 16 force from a first length to a second length which is shorter than the first. As described earlier, the pivot pin 14 within the arrangements shown in Figures 1A-1C, 2A-2C and 3A-3C is captured within an elongate slot 45 which allows the pivot pin to move from one end to the other of the slot, such as 14 and 14' in Figures 4A-4C. Also included in the embodiment depicted in Figures 4A-4C is a second pivot pin 52 formed on the contact carriage which the lower contact arm 12 strikes upon contact closure and rotates from the position shown in Figure 4B to the position shown in Figure 4C, after which it returns to the rest position shown in Figure 4A. The displacement of pivot pin 14 within slot 45 allows the additional overtravel while the concomitant transfer of the rotation from pivot pin 14 to the second pivot pin 52 reduces the energy associated with contact spring 16 by reducing the moment of the force of the spring 16 on the lower contact arm 12 and hence the kinetic energy of upper contact arm 10, when forced upwards by the lower contact arm 12, thereby preventing the temporary separation of their contacts (10a, 12a). To better understand the operation of the present invention, reference is now made to Figures 5, 6 and 7 showing respectively the contact closing force diagrams for the structures depicted in Figures 1A-4C. Referring specifically to Figure 5, when the circuit breaker of the prior art shown in Figures 1A-1C is closed, the contact closing force in relative units takes up the values shown on the line 60 until it reaches a maximum value at A which corresponds to the maximum over-travel of the contacts with the contact spring fully extended as shown in Figure 1A. At this point, the contacts start returning towards the center point P which corresponds to the position depicted in Figure 1B. When the lower contact arm projection 22 abuts against the stop 24, the upper contact arm 10 continues moving, separating itself from the lower contact arm 12 until it reaches point C on the diagram which corresponds to the reverse overtravel position shown in Figure 1C. Figure 5 shows that the energy associated with the contacts upon closing is particularly high, as represented by the area 1 included between line 60 and the horizontal axis of the diagram and hence the reason for contact bounce when the lower contact arm 12 abuts against the stop 24 on its return travel from the maximum overtravel position at point A. Figure 6 shows a force diagram for contact arrangements depicted in Figures 2A-2C and 3A-3C. As soon as the circuit breakers closes, the lower contact arm 12 moves until it reaches the position depicted in Figures 2A end 3A and the force between the contact arms 10, 12 operating along line 70 reaches its maximum value at A corresponding to the maximum extension of spring 16. At this time, spring 16 returns contact arm 12 into the position indicated at Figure 2B and 3B where reversing spring 30 or 30' becomes engaged as indicated. The reversing spring force is illustrated by line 76 as a force operating in a direction opposite to that of spring 16, the absolute value of which increases with the distance of additional reverse overtravel, as described earlier. The additional reverse overtravel stops at point 80 corresponding in Figures 2C end 3C to the point where the reversing spring 30 or 30' abuts against the spring stop 38 or 30'a. At this point, the force operating on contact arm 12 is given by the algebraic sum of the force produced by the contact spring 16, represented by line 70, and the force generated by the receiving spring 30 or 30', represented by line 76. This is indicated as line 74. The reduction in force due to the engagement of the reversing spring 30 or 30' is indicated at step 72. From Figure 6, it follows that the energy exerted upon contact arm 12 results from the difference between the area I due to the contact spring 16 and the area II due to reversing spring 30 or 30', thus resulting in a decreased amount of energy transferred to contact arm 10. Contact arm 10 has therefore a rebound amplitude lower than that provided by the prior art structure as depicted in Figures 1A-1C sufficient to avoid contact separation. In a similar manner, Figure 7 illustrates the behavior of the force operating on the lower contact arm 12 as function of the additional overtravel in accordance with the embodiment depicted in Figures 4A-4C. As soon as the circuit breaker closes, the lower contact arm 12 overtravels until it reaches the position depicted in Figure 4A and the force between upper contact arm 10 and lower contact arm 12 operates along line 90 reaching a maximum value at A corresponding to the maximum extension of contact spring 16. At this point, the contact spring forces lower contact arm 12 to reverse overtravel to the position indicated in Figure 4B where lower contact arm 12 engages the second pivot pin 52 that provides a shorter arm of contact spring 16 than that provided by pivot 14. Step 92 represents the decrease in the force operating on the lower contact arm 12. Pin 14 now slides within the slot 45 to the position indicated at 14' which corresponds to the additional reverse overtravel stopping point 96 in Figure 7, that corresponds to the position indicated in Figure 4C. From Figure 7 it can be seen that the energy exerted upon lower contact arm 12 is decreased by the reduction of the force moment generated by the contact spring 16 in a manner similar to the energy decrease depicted earlier in Figure 6.

Claims: A contact arrangement for electrical circuit breakers comprising an operating mechanism, a first contact arm (10) and a first contact (10a), a second contact arm (12) and a second contact (12a), a contact spring (16) acting on the second contact arm for biasing said second contact arm (12) around a pivot (14) towards said first contact arm (10) to limit overtravel by said second contact arm (12) after said first and second contacts (10a, 12a) are closed by said operating mechanism; a contact stop (22, 18') associated with said second contact arm (12) for limiting the extent of return travel of said second contact arm (12) in the direction of said first contact arm (10) after said two contacts (10a,12a) are closed, characterized by a reverse spring (30, 30') acting on said second contact arm (12) against the return travel of said second contact arm (12) in the direction of said first contact arm (10) to decrease the action of the contact spring (16) without causing said first and second contacts (10a, 12a) to become separated. The contact arrangement of claim 1 characterized in that said contact stop (22, 18') associated with said second contact arm (12), during said second contact arm (12) return travel in the direction of said first contact arm (10), engages said reverse spring (30, 30') providing an opposing force to return said second contact arm (12) to a rest position. The contact arrangement of claim 1 characterized in that said contact stop (22, 18') comprises a projection on said second contact arm (12) and said reverse spring (30, 30') is attached to a contact carriage (36,36') which supports said second contact arm (12). The contact arrangement of claim 3 characterized in that said reverse spring comprises a leaf spring (30) having a bracket end (32, 33) for engaging said contact stop (22) and an opposite end secured to said contact carriage (36). The contact arrangement of claim 3 characterized in that the stop (18') of said second contact arm (12) engages a lever member (33'), rotatably pivoted on a carriage structure (36') of said contact arrangement, said lever member (33') engaging through an extension (32') one end of a helical reversing spring (30') the other end of which engages a support (30'a) pivotally mounted on said contact carriage (36'). A contact arrangement for electrical circuit breakers comprising an operating mechanism, a first contact arm (10) and a first contact (10a), a second contact arm (12) and a second contact (12a), a contact spring (16) acting on the second contact arm (12) for biasing said second contact arm around a pivot (14) towards said first contact (10) arm to limit overtravel by said second contact arm (12) after said first and second contacts (10a, 12a) are closed by said operating mechanism characterized by a contact carriage (56) supporting said second contact arm (12) and having an elongate slot (45) supporting the pivot (14) of the second contact arm (12) and a contact stop pin (52) on said contact carriage (56) working as a second pivot pin for said second contact arm (12), the contact stop pin (52) being contacted by said second contact arm (12) during return travel of said second contact arm (12) in the direction of said first contact arm (10) to decrease the moment of the force of the spring (16) acting on the second contact arm (12). The contact arrangement of claim 6 characterized in that said pivot pin (14) moves from a first position within said slot (45) before contact between said second contact arm (12) and said contact stop (52) to a second position within said slot (45) after contact between said second contact arm and said contact stop (52). The contact arrangement of claim 7 characterized in that said return spring (16) provides a first return moment on said second contact arm (12) when said pivot pin (14) is in said first position and said return spring (16) provides a second return moment on said second contact arm (12) when said pivot pin (14) is in said second position. The contact arrangement of claim 8 characterized in that said first moment is greater than said second moment. The contact arrangement of claim 6 characterized in that said contact stop is located intermediate the second contact (12a) at one end of said second contact arm (12) and said pivot pin (14) at the opposite end of said second contact arm (12).

Inventors: BANFI, CLAUDIO, PARDINI, FRANCO
Applicants: CGE- COMPAGNIA GENERALE ELETTROMECCANICA S.P.A.
Application year: 1984
Main classification: H01H1
IPC codes: H01H1