US 2806926 A
Description (OCR text may contain errors)
M. RIGERT FAL ELECTRIC CONTACT Sept 17 1957 Filed June 29, 1955 Y l @H ,mm ...n Wm .my ,d Rm Y l. W .mv Y M B '2,806,926 Patented Sept. 17, 1957 ELECTRiC CoNrAC'r Max Rigert, West Allis, and William J. Weinfurt, Wau- Watosa, Wis., assignors to McGraw-Edison Company, a corporation of Delaware Application June 29, 1955, Serial No. 518,744
Claims. (Cl. 269-166) This invention relates to electric circuit breakers and more particularly to contacts adaptable for installation in breakers having high current interrupting capacity.
Closing the movable contact of la high capacity circuit breaker into or against the stationary contact is an operation which normally takes place with extreme rapidity. Upon this event, the movable contact not only electrically engages the stationary Contact but the kinetic energy of the former is transferred to the latter with a sharp impact. Since the collision with the stationary contact is somewhat elastic, it follows that the stationary contact will be deflected from its normal position of engagement with the movable contact and that subsequently, the contact pressure creating springs will estore the stationary contact to engagement with lthe movable contact.
ln reality, the stationary contact is caused to vibrate periodically in a natural frequency or vibration somewhat indeterminate but governed largely by the mass of the `contacts and the constant of the pressure creating springs. ln high capacity breakers, this vibration, which is in fact a minute opening and closing of the circuit through short amplitude swings, is especially deleterious because it causes arcing which rapidly ercdes the contacts and shortens their lives.
it is a primary object of the instant invention to overcome the aforementioned vibration problem, together with its consequent arcing, by providing the relatively stationary electric contact with vibration damping means.
It is a further object of this invention to provide simple and compact vibration damping means which are easily incorporable in existing contact structures without extensive modification thereof.
A general Object is to provide a novel and more en cient electric contact: Other more specific objects will appear from time to time throughout the specification.
Referring to the single sheet drawing in conjunction with which the invention will be described in detail:
Fig. l is a sectional elevational View of a contact assembly embodying the invention, the contact being in closed circuit position;
Fig. 2 is a fragmentary View of the contact assembly shown in 1Eig. 1 as the same appears in open circuit position;
Fig. 3 is a fragmentary vievv of the stationary contact illustrated in Fig. 1 but with a modied form of vibration damping means;
Fig. 4 represents the conta-ci assembly of Fig. 3 in closed circuit position;
Fig. 5 is a fragmentary View of the stationary Contact showing an alternative form of the vibration damping means; and
Fig. 6 shows the contact assembly of Fig. 5 in closed circuit position.
For illustration of a particular circuit interrupter to which the anti-vibration contact forming the subject of the forthcoming description may be applied, reference may be had to the copending application of W. R. Harry entitled Circuit Interrupter, Serial No. 141,336, tiled January 30, 1950, and assigned to the same assignee as the instant invention.
For the sake of brevity, only the improved contact struc- -ture per se will be described herein. Referring to Fig. 1, the Contact assembly comprises a stationary contact 1 arranged to coact with a coaxially movable contact designated generally by the reference numeral 2. Movable contact 2 is pinned to a bridging cross head 3 which may carry a like movable contact on its opposite end (not shown). Cross head 3 is supported by an insulating rod 4 which is adapted to execute a reciprocatory movement, thus causing contact 2 to telescope into and out of electrical engagement with stationary Contact 1.
An insulating tube 7 houses the contact assembly which is restrained therein by any suitable means such as snap rings 8 and 9. Customarily, housing 7 is immersed in a dielectric liquid such as oil filling the circuit interrupter tank (not shown). Thus,` when movable contact 2 is rapidly disengaged from stationary contact 1, some of the dielectric liquid is vaporized due to heat associated with the electric arc thus established, and the resulting gases are relieved through a radially extending aperture 10.
The path 0f current ow through the contact assembly is evident from Fig'. 1, it being from wire connecting stud 13, contact supporting terminal 14, stationary contact 1, movable contact 2 and to a simil-'ar adjacent contact structure through conductive cross head 3.
Stationary contact 1 comprises a plurality of fingers 16 spaced from each other by slots 1'7 and arranged to det'lne a substantially cylindrical resilient enclosure. One end of each finger 16 is provided with arc resistant tips 18, preferably of copper tungsten alloy. Axially opposed ends of each tinger are conformed into a hook-like arcuate guide edge 19 which bears in an annular channel 20 in the contact supporting terminal 14. A pair of contact pressure creating closed coil springs 21 surround the pivotal fingers 16 intermediate arcing tips 18 and guide edges 19. Obviously, the tension in springs 21 causes arcing tips 18 to bear in compressive relation upon the contact head 22 of the movable Contact 2. When the movable contact 2 is withdrawn from arcing tips 1S by reason of cross head 3 being vertically reciprocated (-see Figs. 2, 3, and 5) fingers 16 are collapsed radially inwardly toward each other an amount dependent upon the Width of slot 17 until adjacent ends of arcing tips 18 abut each other. An annular axial protuberance 15 bearing on supporting terminal 14 constitutes la fulcrum on which lingers 16 ex or pivot radially in order to establish an entrance for receiving movable contact head 22 against the force of springs 21.
From the structure and operation described thus far it should be evident that execution of a rapid vertical reciprocatory motion by insulating rod 4 would cause contact head 22 to strike arcing tips 18 with considerable impact. Because it is necessary for movable contacts 1 to be inherently flexible in order that movable contact 2 be admissible therebetween, it is further evident that the impact of the collision will result in a vibratory motion being set up in movable contact fingers 16. The nature of this vibration is governed by well known physical principles and depends primarily upon the mass of movable contact 2, its velocity, the mass of stationary Contact 1, and the constants and characteristics of the springs 21. ln reality, this vibratory movement amounts to a making and breaking of the electr-ic circuit which it is desired to establish between contact head 22 and arcing tips 18 when the circuit breaker is closing in. Of course, after a suitable time lapse following closure of the breaker the energy of the `impact. incidental to closing will decline to zero, being dissipated by the vibratory action of stationary contact ngers 16. In heavy current circuits, however, the
heat developed by such vibration, even though it is of corporation in the contact structure of a knovel damper or vibrational energy absorbing element preferablyein the form of a ring of dielectric impervious rubber or other ref silient material. Y
In Figs. l and 2 the resilient damping ring is designated by the reference numeral and is shown surrounding stationary :Contact fingers 16 concentrically thereof iin tension and radially spaced from the interior wall 11 of insulating tube 'l'. The ring is supported on a shoulder 26 on arciug tip-s 18 and a fixed shoulder 27 of insulating material. Inthe respective figures, rings 25, 30 land 35 are each displaced axially from the free ends of arcing tipsl 18 so that the heat generated by arcing incident to separation of contact head 22 will not effect them adversely.
The effect of incorporating the resilient ring 25 is to daimpen the natural period of vibration of Ithe contact assembly when compared with the natural frequency of the same'assenrbly Yif-rubber ring 25 were omitted. It is evident that the effect of the stiffness of the Vspring 21 -is augmented when the springs are moved axially closer to ring 25. Correspondingly, the amplitude of vibration of the movable contact fingers 16 is greatly reduced by rubber ring 25 because ofthe greater effectiveness of the latter resulting from its being placed more remote from the pivot point 15 on the individual Yfingers 16. Thus, it
is seen that the metall-ic springs 21 and resilient ring 25 constitutes the combination of a vibratory spring having an energy absorbing spring damper in parallel therewith both acting on a single mass so that the amplitude and frequency of vibration of the mass is damped appreciably although the total force required to close the contacts is increased' only slightly.
While it is not here intended to go into a complete discussion of the properties of all resilient materials which could be used to accomplish ythe purpose of ring 25, it may be noted that rubber or synthetic rubber like materials which are unaffected by Vthe dielectric fluid performed the damping function particularly elicaciously. This is so in part because of the inherent energy absorbing capabilities of rubber like materials. Reference to the hysteresis curvesv of such materials will 'demonstrate that the area included in the hysteresis loop of rubber is relatively large as compared with that of other resilient metallic materials,
and consequently, this area being a representation of' the work performed, the internal work is relatively large. Not only is the hysteresis loop of rubber like materials large, but within a certain practical range the area and Vthe energy absorbed increases according to rapidity of applicatio-n of the load. Thus, in circuit breakers where a movable contact strikes and comes to rest in a stationary contact during a small fraction of a cycle, the shock of closure is particularly enormous and it is advantageous to have as part of the contact assembly a high energy obsorbing material such as rubber which increases its capacity to 'absorb energy when it is needed most and exerts a lesser effect during slow closures. Moreover, an important characteristic of the rubber like material is that it absorbs much of the energy it receives under impact and manifests it in the form of heat; whereas, a metallic spring returns almost all the energy which it receives in the form of an opposite mechanical force. Hence, the advantage accompanying use of a rubber like mass as a damping means is apparent.
Fig. 3 shows a fragmentary section of the contact structureV illustrated in Fig. l modified by inclusion of an altei-native form of 4the resilient vibration damping ring designated by the reference Vnumeral 30. Note ini this gure that when contact head 22 is withdrawnV from stationary contact 1 that ring 30 assumes a-square cross sec-y tion and that it fully bridges the space betweeniingers 16 and the interior-Wall 11 of insulating tube 7. Fig. 4 demonstrates the deformation of rubber ring resulting from' entryV of contact head 22 between arcing tips 18.
By comparison with Fig. l, where the rubber damping ring is placed in tension by closing of the contact, it be noted in Fig. 4 that the loading of rubber ring 30 is purely compressive in nature because of the ring being closely coniined by the wall 11 and outer periphery of fingers 16. Moreover, it is to be noted that even though lingers 16 have swung radially 'outward compressing ring 30, that the ring has substantially the Vsame area as represented in Fig. 3. The fact that the total cross sectional area and the total volume ofthe rubber like materialremain substantially constant when itis subjected to compressive loading accounts to a large extent for the rubber j high energy absorbing characteristics. In fact, it is knownV that rubber is capable of absorbing approximately four times more energy per oubic'inch than metal such as steel, when energy is expressed in inch pounds. Thus, from purely volumetiic considerations, it is seen that there is considerable advantage in using the rubber like material as a damping means in preference to anothermetallicV spring.
Figs. 5 and 6 illustrate an alternativeformy of the re-Y silient damping ring designated by the reference numeral 3S. Here it is to be noted that ring 35 isi roundv in cross section when: not loaded compressively as iii'Fig. 5'A and that itis elliptical in cross section when compressive#V of the inventionV are merely illustrative and that greater specificity regarding the contiguration of the damping. ring is difficult in View of the large number of variables affecting. a vibrating mechanical system'. As implied above, these factors include the `constants of the associated coil springs, the mass of the stationary Contact,V
and theY velocity of the impact occurring when the movable contact engages the arcing tips. Accordingly, it'A should be a relatively easy matter, using the basic ideas disclosed hereinabove, to attainA exactly thefri'ghtamount of damping; desired for anyrparticular*circuitbreakerl c`ro`n4 tact of the type analogous to that describedherein.
While preferred forms of thel invention have been illustrated, it should be understood that the invention isV not limitedto the specific details of constructionlandarrangement thereof herein illustrated 'but that' the general principles disclosed herein are subject to modification and adaptation by those versed in the arti to various" other contacts, for example, butt type or knife type', without departing from the scoper of the instant invention'.A
l. An electric contact assembly including a stationary contact element and an axially movable contact' elementr adapted to electrically engage each other with sharpY impact, one of said contact elements comprisingl aY plurality of lingers juxtaposed to each other in circular arrangement and having pivot ends and free ends for receiving the other contact element therebetween, a metallic coil spring surrounding'said lingers intermediate said'ends and biasing them radially inwardly toward each other, a resilient rubber-like ring surrounding said lingers near their free ends around the zone of interengagement between contact elements and axially remote from said metallic spring, whereby the periodic vibration of said fingers and metallic spring resulting from said impact are damped by said rubber-like ring.
2. An electiic contact assembly including a stationary contact element and an axially movable contact element adapted to electrically engage eachl other with sharp' impact, an insulating tube housing said Contact elements, said stationary contact comprising a plurality of lingers juxtaposed to each other in circular arrangement"'andl each having a pivot end and free end disposed to receive said movable contact element therebetween, a contact pressure metallic coil spring surrounding said ngers intermediate their ends wherein vibrations are established by engagement of said elements, and a resilient rubberlike ring surrounding said fingers near their free ends around the zone of Contact nterengagement and in compressive relation with the interior of said tube axially remote from said metallic spring, whereby the periodic vibrational energy of said metallic spring and fingers is absorbed by said reslient ring.
3. The invention according to claim 2 wherein the resilient rubber-like ring is rectangular in cross section.
References Cited in the le of this patent UNITED STATES PATENTS Hunt July 24, 1923 Scott Oct. 10, 1944 Cotter Nov. 21, 1944 Caldwell Sept. 20, 1955