US3461259A - Spring contact-finger construction - Google Patents

Description

Aug. 12, 1969 M. G.LEONARD ETAL 3,46l,259

SPR ING CONTACT-FINGER CONSTRUCT ION Filed Jan. 22, 1968 FIG.

4 Sheets-Sheet 1 INVENTORS W'TNESSES: Merrill G. Leonard e. %942% Robert J.Mones Aug. 12, 1969 M. G. LEONARD ETAL 5 3 SPRING CONTACT-FINGER CONSTRUCTION 4 Sheets-Sheet 2 Filed Jan. 22, 1968 FIG. 5

Aug. 12, 1969 M LEONARD ETAL 3,461,259

SPRING CONTACT-FINGER CONSTRUCTION Filed Jan. 22, 1968 F 22 sc AND m lsa AND V 4 Sheets-Sheet :5

MOTION TO RIGHT P FIG. 8

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g- 2, 1969 M. G. LEONARD ET AL 3,46l,2591

SPRING CONTACT-FINGER CONSTRUCTION Filed Jan. 22, 1968 4 Sheets-Sheet 4 MAX. LGTH.

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i i FIG. IO TO ,Q OPEN',

MIN. LGTH. OF SPG.

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CONTACTS COME TO REST POINT o= INITIAL CONTACT BEGINNING OF FLAT SURFACE IMPACT SLIDING TRAVEL FRICTION (ZONE A) (ZONE B) United States Parent O U.S. Cl. 200-170 8 Claims ABSTRACT OF THE DISCLOSURE A switch is provided having an improved contactfinger Construction with spring characteristics, such that the contact finger does not leave the cooperable bladeshaped contact with the result that no welding occurs. By having the spring characteristics related to the contact-finger mass, there results an absence of separation between the interface contact surfaces, so that welding is substantially eliminated.

Cross references to related applications Applicant is not aware of any related pending patent application, which is pertinent to the present invention.

Background of the invention In United States patent application filed July 31, 1967, Serial No. 657,ll4, by Merrill G. Leonard, entitled Quick-Make and Quick-Break Switch, and assigned to the assignee of the instant application, there is disclosed and claimed an improved quick-make and quick-break type of switch particularly suitable for distribution transformers, where the switch contacts are immersed in transformer oil. In the aforesaid patent application, there is provided an over-center spring mechansm, which ensures a quick-make and a quick-break opening and closing movement of the rotatable moving contact structure. The rotatable moving contact structure comprised a rotatable contact carriage carrying therewith two pairs of contact fingers extending outwardly from the rotatable movable contact carriage, and making separable engagement with a pair of stationary spaced blade-shaped contacts.

It was found that for a certain type of spring-biased contact finger, welding would occur at about 3,000 amperes, when the switch closed in on a high fault current. However, by changing the spring characteristics, the contact performance would work successfully up to about 10,000 amperes. The present invention is particularly concerned with an improved type of spring contact finger Construction having unique spring characteristics.

I have discovered that there is a particular relationship between the spring characteristics of a spring contact finger and the mass of the spring contact finger, which must be met to avoid welding conditions occurring during the closing operation of the contact structure.

Summary of the invention According to the present invention, there is provided cooperable contact structure comprising a spring biased contact finger cooperable with a blade-shaped contact. The spring characteristics of the spring-biased contact finger are such that the ratio of the spring force F to the mass of the finger contact, expressed in pounds-mass, is at least equal to the constant Kf where K is equal to 216. In a certain application of the invention, a pair of such spring contact fingers move together and straddle a stationary blade shaped contact. By having the spring characteristics of such magnitude, there will occur no separation, or bouncing between the movable contact face and the contact face of the stationary blade-shaped contacts, so that welding will not occur, and instead, a constant rubbing engagement will take place between the contact nterfaces.

It is, accordingly, a general object of the present invention to provide an improved spring contact finger Construction of such characteristics that welding will be substantially eliminated.

Another object of the present invention is the provision of an improved contact finger Construction, comprising a pair of movable spring contact fingers, biased inwardly toward each other, and making separable Contacting engagement with a stationary blade-shaped contact.

Still a further object of the present invention is an improved-quick-make and quick-break type of switch in which a rotatable contact carriage, carrying two pairs of inwardly-biased spring contact fingers, makes separable Contacting engagement with a pair of stationary spaced blade-shaped Contacts, the arrangement preventing welding occurring.

Further objects and advantages will readily become apparent upon reading the following specicaton, taken in conjunction with the drawings.

Brief description of the drawings FIGURE l is a sectional perspective View of a submersible-type transformer, designated Type SPB manufactured by the Westinghouse Electric Corporation, and developed for underground distribution systems;

FIG. 2 is a fragmentary view looking down into the underground vault showing the upper end of the submersible type transformer of FIG. 1;

FIG. 3 is a fragmentary perspective view of the loadbreak switch of the present invention mounted in the dome cover of a subway pole base transformer of the type set forth in FIGS. l and 2, and illustrating the position of a plurality of contact structures in the closed connecting position;

FIG. 4 illustrates, in Vertical section, an embodiment of the switch structure of the present invention, the contact structure being illustrated in the closed-circuit position, and the view being taken substantially along the line IV-IV of FIG. 5;

FIG. 5 is a top plan View, in section, of the switch structure of FIG. 4, the view being taken substantially along the line V-V of FIG. 4;

FIG. 6 is an inverted plan view taken substantially along the line VI-VI of FIG. 4, with a portion of the lower supporting plate broken away to more clearly illustrate the contact structure in the closed circuit position;

FIG. 7 is a considerably enlarged side elevational View of the spring-biased contact finger of the present invention, showing its Contacting engagement with the stationary blade-shaped contact;

FIG. 8 is a diagrammatic view indicating, to a considerably enlarged scale, the rightward closing movement I of the pair of cooperable contact fingers relative to the entering edge of the stationary blade-shaped contact;

FIG. 9 illustrates a plot, or graph, of the contact position as a function of time, all taken with respect to the Description of the preferred embodiment With reference to the drawings, and more particularly to FIG. 1 thereof, the reference numeral 1 generally designates a submersible-type transformer, of the type "SPB" manufactured by the Westinghouse Electric Cor- &461259 poration, and developed to provide underground distribution circuits for residential neighborhoods. Generally, the submersible type transformer 1 comprses an underground vault 2 positioned in an excavation 3 dug out of the ground. The vault 2 has a flush-rnounted grate 4 secured thereto at ground level. With reference to FIGS. 1 and 2, it will be observed that a distribution transformer 6 is located within the cylindrical housing member, or vault 2, and, at times, may be completely submerged with water. As well known by those skilled in the art, the transformer 6 comprses a high-voltage winding and a low-voltage secondary winding, which are inductively coupled.

By way of illustration only, and not by way of limitation, the transformer 6 may have a kva. rating of say, for example 25 to 167, and may be designed for systems rated 15 kv., grounded Y` and below. Two high-voltage bushings 11, 12 are secured to the upper dome cover 13, and also three bushings 15-17 are mounted on the cover 13, being connected to the secondary winding of the transformer 6. The standard-Type SPB transformer is internally connected for a loop feed primary and a 240/120 secondary. The two primary b ushings 11, 12 are connected internally by a 200 ampere bus, that completes the external feeder loop. The line side of the primary winding is connected to the internal feeder bus, and the other side of the primary winding is internally grounded to the transformer tank.

The secondary winding is internally connected at either end to the two low voltage bushings 15, 17. A neutral center tap terminates at the neutral low voltage bushing 16, and is externally grounded to the transformer tank. As shown, the neutral terminal 16 has a porcelain insulator, and is of the spade type.

Reference may be made to a typical loop distribution system, such as illustratcd in FIG. 3 of the drawings in United States patent application, filed July 31, 1967, Serial No. 657,l14, by Merrill G. Leonard, and assigned to the assignee of the instant application, to illustrate a number of transformers and their relationship to a residential neighborhood.

To provide a desired switching operation internally of the transformer tank 18, there is provided a switch 19, which may be manually operated by an externally-situated Operating handle 20, mounted on the dome cover 13 of the transformer tank 18. Such a switch 19 offers many switching positions, the advantages of which are set forth in the aforesaid patent application Serial No. 657,114.

The switching positions enable the Operating personnel to quickly isolate a line fault in a loop-feed system, while still naintaining full service. Or, a transformer 6 can be quickly disconnected from the loop for maintenance, or change-out without disrupting the service of the remaining transformers 6 on the loop. This switching exibility of the switch 19 enables utilities to minimize outage time during emergences.

The present invention is particularly concerned with a non-welding type of contact finger Construction 22 for a quick-make and quick-break load-break switch 19, which insures positive and reliable contact separation and nonwelding contact closing. The overcenter spring mechanism 24, which is associated with the rotatable contact carriage 26, is set forth and claimed in the aforesaid patent application. Briefly, the overcenter spring mechanism 24 comprses an externally-disposed manually-operable handle, which effects rotative opening and closing movement of a driver crank-arm 27. Seated at the end of the driver crank-arm 27 is an overcenter tension spring 28, the other end of which is secured to an upwardly-extending spring seat pin 29, which is secured to and rotatable with, a driven crank-arm 30, the latter being fixedly secured to the rotatable contact carriage 26. Reference may be made to FIGS. 4 and in this connection.

It will be observed that the rotatable contact carriage 26 comprses a guide portion 26a extending through a lower guide opening 31 provided in an insulating generally U-shaped supporting member 32, secured by brackets 33 to the upper support 34 of the switch structure. Extending laterally through openings 35 provided in the side walls of the insulating support 32 is a pair of spaced stationary contact blades 36, with which the two pairs of springbiased contact fingers 37, secured to the rotatable carriage 26, make separable opening and closing Contacting engagement. FIG. 4 shows the position of the contact structure in the closed-circuit position thereof, wherein it will be obvious that the circuit extends from the right-hand bladeshaped contact 36 through the first pair of cooperable spring-biased contact fingers 37, through the conducting body portion of the movable contact carriage 26, through the left-hand second pair of spring-biased contact fingers 37, and to the left-hand stationary blade-shaped contact 35. As well known by those skilled in the art, suitable conducting leads connect to the contacts for the switching Operations, which have been mentioned hereinbefore.

With reference to FIG. 5, it will be apparent that clockwise rotative driving motion of the crank-arm 27, as eectcd through the manual rotation of the extemallydisposed handle 20, will eifect tensioning of the tension spring 28 and will effect quick-opening motion of the driven crank-arm 30, and consequent opening motion of the rotatable movable contact carriage 26. Quick-break opening motion will consequently ensue. During the closing operation, the externalIy-provided manuallyoperable handle 20 is rotated in a counter-clockwise direction to consequently bring the line of the action of the overcenter spring 28 through the axis 25 to effect quick closing of the separable contact structure.

To insure the positive opening and closing motion of the separable contact structure, there is provided a pair of pivotally-mounted dogs 39, which are forced by a lug 40, movable with the driving crank-arrn 27, so as to forceably engage the spring seat pin 29 to forceably cause opening and closing movement of the movable contact carriage 26. This action is more fully set forth in detail in the aforesaid patent application Serial No. 657,114, and generally constitutes no part of the present invention. A detailed description thereof is, therefore, deemed unnecessary.

FIG. 7 illustrates, to an enlarged scale, one of the spring-biased contact fingers 22. It will be noted that, generally, there is provided a semi-spherical contact 41 having a rivet portion 42, which extends through a plurality of copper laminations 44, and through an upper leaf spring 46, so that the head proton 47 may be riveted, as shown in FIG. 7. Addtionally, to provide added spring pressure, there is provided a separate leaf spring 49, which engages the riveted portion 47 of the contact 22, and, in turn, is riveted by the rivet pin 51 to the movable contact carriage 26. Reference may be had to FIG. 4 in this connection.

In the design of a high-voltage switch 19, it is important that the switch 19 will close-in on very high fault current. It has been found that a particular Construction of contacts such as shown in the above-patent application will fail by welding at about 3,000 amps, but another construction of rather specific characteristics will work successfully up to about 10,000 amps. This inven'tion describes the latter improved contact Construction 22.

The new Construction is shown in FIG. 7 of the drawings. This shows a particular shape of contact, and also it shows a fiat or leaf-type spring 49, which serves to force the pair of contacts 22 against the blade 36. The approach condition is shown in an enlarged view in FIG. 8. It has been found that the force of the spring, and its relationship to the mass of the contact 22, as well as the path of contact travel, are important factors in the construction that will prevent welding.

To explain this, we must consider the complex motion of the contact 22 as it starts from rest, accelerates under the force of the toggle spring 28, and snaps into place in its closed position, as shown in FIG. 4. This whole sequence of events will typically be complete in a time of about second, or 50 milliseconds. The motion cannot be precisely calculated, for it depends, in part, on such uncertain values as oil viscosity, hearing friction, amount of damping action of brush-copper Shunt 52, contact and blade deformation on impact, and friction forces at contact-to-blade interface. However, we can make such a ssumptions that will simplify the problem, and with this, a calculation of approximate contact movement can be made.

We will assume, then, that the contact assembly 37 accelerates uniformly and completes its free travel in 20 milliseconds (slightly over l-cycle of a 60 cycle wave). The lnear travel in a 90 arc is about 2", hence the average velocity V,=distance/time=2"/.02 sec.=l in./sec. The final velocity (V would then be twice this, or 200 in./sec.

However, a large part of this will be dissipated when the contacts 22 encounter the wedge-shape blade 36, as shown in FIGURE 8. By a calculation, we estimate that 70% of the energy is dissipated on impact; this cuts the velocity to (.70) =.49 or about /2 of the maximum. Hence, the velocity after entry is .49 200=100 in./sec. In more detail, the total energy E imparted to the contacts is equal to the energy given up by the driving spring (neglecting hearing friction and viscous drag). Therefore, total energy E =Av. force of Spg and change of length of spring.

:ww (See 5) :20 in. lb.

This is used up in (Zone A) and (Zone B).

In Zone B, the energy used up is in sliding friction.

Assume Coef. of friction .20. Then with 30 lb. Contact Load, Fricton drag, F =30 .20=6 And, energy used up per contact 6 -=l /2 in. lb. But, Total energy used up for 4 contacts E =4 1 /2= 6 The remainder of the energy must be used up in Zone A; this energy E =20-6+l4 in. lb.

From this point, the contacts 22 will slow down rapidly; hence, this is the :maximum velocity (V under which the contacts will have a tendency to jump or lift oh the surface as it leaves the point O of FIGURE 8. (Prior to this, the contacts are being wedged apart as they travel the surface P-O.)

At the point O, we are conoerned with any movement toward or away from the blade 36. This, we will call displacement velocity V The spring force F tries to hold the contact 22 firmly on the blade 36; it fails to do so only if this irregular surface falls away so fast that the spring 49 cannot accelerate the contact 22 as much as needed. The question then is, how fast can the spring 49 accelerate the contact; that is, what would be the path of free movement if the contact 22 were instantly released while it is a-dvancing at the speed V =100 ft.sec.?

The net effective weight of the contact 22 is estmated at .07 lb., and the force driving it toward the blade 36 is assumed to be at the maximum test level of 30 lb. It is well known that 1 lb. force will accelerate l lb. mass at 32.2 ft./sec. therefore, 30 lb. force will accelerate .07 lb. mass at a value :14,000 ftJsee. or 170,000 in. /sec.

Now we can determine the path of the contact under the -assumed conditions.

Let t= /2 millisecond, then the "spin-travel" (closng movement) of the contact is S V t= l00 .0005 =.05 in. Meantime, the fall-travel" (displacement movement) S at z X l70,000 (0005 .021 in.

6 Similarly we calculate: t=1 millisecond S =.10 in. S .084 in. t=2 millisecond S=.20 in. S =.34 in.

Now let us consider the case in which the switch 19 did not work properly. Here the spring force was approximately 6 lb. and this actng on the contact mass, will produce an acceleration.

a: 6 l/.07 32.2=2,800 ft./sec. or 34,000 in./sec. From this, we co'mpute S as before:

t= /2 millisecond S =.05 S =.004 t=l millisecond S .10 S =.'017 t=2 millisecond S =.20 S =.070

These points are plotted on FIGURE 9.

Now to determine whether the contacts 22, 36 will separate, we must take account of the fact that the contact itself is not a sharp point, but has a crown shape, so we must observe the movement of the entire contact surface with respect to the entire blade surface. The simplest way to do this is to draw an Outline of the contact to the same scale on a template, and observe its action as its center is made to follow this curve of free fall".

If we draw the line O-A, FIGURE 8, at 30 angle, and then make the contact center follow the path of free fall O-B, we can see the contact separates from the blade 36 for a time of about 2 /2 milliseconds, which is more than enough to cause welding. This explains the failure on the test with the 6-lb. spring.

Now if we try the 30-1b. spring, making the contact follow the path O-C (FIG. 9), we see that the contact 22 overlaps the blade 36 all the way, which means that in the actual case it would be exerting pressure, so no welding should occur. This was confirmed by test results.

Againg, if we move the contact 22 so that it just follows the surface of the blade 36, then we trace a path O-D, (FIG. 9) which corresponds approximately to a spring force of about 15 lb., curve O-E.

From this, we conclude that a minimum force of 15 lb. is necessary on the contact 22 a actually used in this switch mechanism. Higher levels may be used, up to the point where other restrctions intervene, such as the friction forces becoming so high that the contacts 37 do not close completely.

In actual design, the 30 angle which the wedge makes with the blade surface 36 does not have to intersect at a point; this can be smoother-contoured by a radius, which will improve the contact action slightly, and thus add a factor of safety to the marginal condition described above.

From the foregoing description, it will be apparent that there has been provided an improved non-welding type of spring-contact finger construction 22 in which, by the application of spring force of particular values, non bouncing or separation of the contact surfaces, is eliminated. By such elimination, non-welding is achieved.

As mentoned hereinbefore, the improved spring contact finger construction 22 worked successfully up to about 10,000 amperes, where other spring contact finger construction, not observing the critical characteristics, as set forth herein, failed by welding, at about 3,000 amperes. The foregoing will clearly show the considerably improved results and the necessity for observing the spring characteristics, as set forth herein. As mentoned, the spring characteristics of the spring-biased contact finger are such that the ratio of the spring force F to the mass of the finger contact, expressed in pounds-mass, is at least equal to the constant K," where K is equal to 216.

Although there has been illustrated and described a specific structure, it is to be clearly understood that the same was merely for the purpose of illustraton, and that changes and modifications may readily be made therein, by those skilled in the art, without departing from the spirit and scope of the invention.

&461259 We claim:

1. A switch comprising, in combination:

(a) a blade-shaped contact,

(b) a spring-biased finger contact engaged with the blade-shaped contact to complete a circuit,

(c) one of said contacts being relatively movable with respect to the other,

(d) the ratio of the spring force F to the mass of the finger contact expressed in pounds-mass being at least equal to K, where K=216.

2. The switch of claim 1, wherein two contact spring fingers move together and on opposite sides of the stationary blade contact, and the ratio of the spring force F to the mass of each of the two finger contact, expressed in pounds-mass being at least equal to "K, where K=216.

3. The switch of claim 1, wherein the spring finger contact comprises a semispherical contact welded to a plurality of conducting larninations for conduction of current thereto, and a separate leaf spring Supplies at least a portion of the spring face F 4. The combination of claim 2, wherein each spring finger contact comprises a semispherical contact welded to a plurality of conductng laminations for conduction of current thereto, and a separate leaf spring Supplies at least a portion of the spring face F 5. A snap-opening and snap-closing switch including, in combination:

(a) a pair of spaced stationary blade-shaped contacts,

(b) a rotatable contact carriage carrying therewith two oppositely-extending pairs of spring-biased movable contact fingers,

(c) each pair of sprng-biased movable contact fingers straddling a stationary blade-shaped contact, and,

(d) the ratio of the spring force F of each springbiased contact finger to the mass of the respective contact finger, expressed in pounds-mass being at least equal to Kf where K=216.

6. The combination of claim 5, wherein each spring finger contact comprises a semispherical contact welded to a plurality of conducting laminations for conduction of current thereto, and a separate leaf spring supplies at least a portion of the spring force F 7. The combination of claim 1, wherein the slope angle 6 of the entering edge of the blade-shaped contact is approximately 30 to the horizontal.

8. The combinaton according to claim 1, wherein the spring-biased finger contact has a semi-spherical contacting portion.

References Cited UNITED STATES PATENTS 2,962,590 11/ 1960 Kussmaul et al. 3,l54,662 10/1964 Heupel et al. 3,197,599 7/ 1965 Lingenfelter. 3,192,328 6/1965 Wilson.

H. O. J ONES, Primary Examner U.S. Cl. X.R. 200-67

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