US3022401A - Electrical switching device and method employing liquid conductor - Google Patents

Description

Feb. 20, 1962 Filed NOV. 26, 1958 R. B. WILKERSON ELECTRICAL SWITCHING DEVICE AND METHOD EMPLOYING LIQUID CONDUCTOR 2 Sheets-Sheet 1 Inventor:

R0 1:? B. Wi I kerson H i s torn e g.

Feb. 20, 1962 R. B. WILKERSON 3,022,401

ELECTRICAL SWITCHING DEVICE AND METHOD EMPLOYING LIQUID CONDUCTOR 2 Sheets-Sheet 2 Filed NOV. 26, 1958 Inventor Robert B. Wilkerson by W m His" borneg.

trite This invention relates to electrical switches and more particularly to switches employing liquid metal for making and breaking electrical continuity between terminals.

The life of a contactor is limited primarily by the mechanical and electrical wear of the contact surfaces. Existing liquid metal contactors, i.e., mercury switches, partially overcome the problem of contact wear by the use of one or more liquid contact surfaces. Failure of these contactors is most often due to contamination of the mercury, wear of the electrodes, or breakdown of the insulator separating the electrodes. Contamination of the mercury can result from oxidation of the mercury in the presence of air or from the formation of sludge or scum on the mercury surface, which sludge is formed of particles separated from the electrodes or from the insulator by arcs. Insulation breakdown due to arcing can also cause a permanently conducting path to be established between the electrodes.

It is one of the objects of the present invention to provide an electrical contactor in which electrical contact is both made and broken between liquid metal contacting surfaces, thereby eliminating arcs across or between solid portions of the contactor.

Another object of this invention is to provide a completely sealed liquid metal switch having no exposed moving parts and which is suitable for use in explosive or contaminating atmospheres.

A further object of this invention is to provide a mercury switch in which there are few or no movable elements in the switch except the liquid mercury itself.

A still further object of the invention is to provide an electrical switch which has as an inherent function the automatic limiting of current to predetermined maximum values both on establishment and interruption of current therethrough.

In accordance with these teachings there is provided an electrical contactor which utilizes the physical separation of a solid stream or jet of falling mercury into discontinuous droplets to accomplish the actual opening of the circuit at points remote from the solid parts of the contactor. A mercury stream flowing through an orifice in an upper reservoir bearin one of the terminals and falling into a lower pool of mercury in contact with another terminal is converted at the surface of the pool from a solid conducting jet to separate nonconducting droplets or vice verse by varying the rate of flow of the mercury through the orifice.

While the scope of the present disclosure should not be limited except by a fair interpretation of the appended claims, the manner in which the above objects and advantages are achieved, as well as additional significant features of the invention, will more readily be understood in connection with the accompanying illustrations wherein:

FIG. 1 is a perspective view of an electrical contactor embodying the present teachings;

FIG. 2 is an enlarged cross-sectioned perspective view of a portion of the contactor shown in PEG. 1;

FIGS. 3 through 8 represent pictorially the various stages of operation of the electrically contacting portions of the switch shown in FIGS. 1 and 2; and

FIGS. 9-12 are cross-sectional views of still another embodiment of these teachings showing the condition of the contactor at different stages of operation.

atent O ice The contactor shown in FIGS. 1 and 2 comprises a solid insulating casing 1 completely enclosing a continuous path within which mercury is circulated. The volume Within the casing not occupied by mercury may desirably be filled with an inert gas. Into the interior of the casing extend two terminals 2 and 3 located one above the other. The upper terminal in the embodiment shown contains a hollow chamber 4 having an orifice 5 in the bottom thereof. Spaced below the orifice 5 is a pool 6 of mercury collected in a cup-like receptacle 7 and in electrical communication with the lower electrode. An overflow passage 8 permits excess mercury from pool 6 to spill over in a broken, non-conducting stream into a reservoir 9 from which the mercury may be pumped through a return path 10 back to the upper chamber 4. To accomplish this pumping function the contactor shown makes use of an electromagnetic pump generally shown at 11, the details of which will be more fully set forth below. Mercury pumped into the upper chamber may flow through the orifice 5 into the pool of mercury below. When the mercury stream between the orifice and the pool below is unbroken, electrical continuity is established between electrodes 2 and 3.

According to these teachings the size of the orifice 5 and the vertical distance through which the mercury stream must fall to the pool below are carefully selected so that the continuity of the mercury stream may be established or interrupted by varying the pressure of the mercury at the orifice. Variations in the pressure at the orifice may be controlled by varying the height of mercury above the orifice, by varying the force exerted by the pump, or by a combination of these methods.

By the use of the electromagnetic pump 11 shown in the preferred embodiment of FIG. 1 the force of pumping may be very readily controlled. This pump includes a flattened portion 12 of the return path and a magnetic core structure arranged with pole pieces 13 on opposite sides of the flattened portion. The core structure is excited by alternating current to establish an alternating flux through the narrow dimensions of flat portion 12. Fixed within the flattened portion of return path at opposite ends thereof in space quadrature to the pole pieces are a pair of electrodes 14 and 15 in electrical contact with the mercury therein. When these electrodes are supplied with alternating potentials, electrical currents flow through the mercury in directions orthogonal to the magnetic fluxes and, assuming the phasing between the electrical currents and the alternating field is correct, a pumping force is exerted on the mercury in the direction shown by the arrows. By using a three-legged magnetic structure 16 with a second ary winding 17 on one of the legs to supply current to the electrodes 14 and 15, a primary winding 18 on another of the legs, and with the pole pieces 13 constituting the third leg, a substantially constant phase relationship requiring no adjustments is maintained between the magnetic flux and the secondary current. Supplying the primary winding 18 with operating potentials therefore causes a pumping action which builds up the pressure on the upstream side of orifice 5.

For a better understanding of what takes place before, during, and after the pumping action reference may be had to the diagrammatic illustrations shown in FIGS. 3 to 8 illustrating the nature of the mercury stream at different times and its course from the upper chamber 4' through the orifice 5' and into the lower pool 6. As a purely graphic illustration of the mercury pressure in the upper chamber, different heights of a mercury column 20 are shown, although in actual practice the height of the mercury above the orifice might vary but little with the pressures being supplied principally by the force of the pump. In FIG. 3 which represents the instant when the pumping action has just begun as represented by the flow of mercury into the upper chamber, the pressure is insufiicient to cause any passage of mercury through the orifice and no electrical continuity is established bctwee electrodes 2 and 3. As the mercury pressure in the upper chamber builds up, as shown in FIG. 4, mercury begins to fiow through the orifice into the pool below. At this stage the mercury stream 21 is divided into two portions: an upper continuous portion, and below that a discontinuous flow of droplets. It will be shown in greater detail below that the length of the continuous portion of the mercury stream for a given orifice is directly proportional to the velocity of mercury at the orifice. This in turn depends upon the pressure above the orifice. Therefore, as the pressure in the upper chamber increases the length of the continuous portion of the mercury stream also increases until, as shown in FIG. 5, it establishes a continuous current-conducting connection between terminals 2' and 3' through the mercury stream. it should be noted that any arcing which takes place at the moment when electrical continuity is established occurs at the sur-- face of the mercury pool betwen boundaries of mercury, and that no arcing is possible either between solid surfaces or between mercury and a solid surface.

FIG. 6 illustrates the condition of the mercury stream contactor at the instant when the pumping action has ceased. It should be noted that electrical continuity between electrodes 2 and 3' is not broken the instant the pumping action stops, but that the pressure or rate of flow at the orifice must be reduced before the circuit can be broken. The small time delay which occurs between the initiation of the pumping action and closing of the contactor or between cessation of the pumping action and opening of the contactor may, if desired in specific applications, be extended by providing an enlarged upper chamber. With an appreciable time delay the contactor is effectively insensitive to transient encrgizations or interruptions in energization.

As the pressure in the upper chamber decreases, a point represented in FIG. 7 is reached when the length of the continuous portion of the mercury stream is less than the distance to the surface of the mercury pool. When this occurs the circuit continuity is interrupted at the surface of the pool and, as formerly, any arcing which eventuates will take place solely between mercury surfaces. Thereafter the rest of the mercury flows out of the upper chamber in a non-continuous stream until the upper chamber is essentially empty and the condition of the switch is as shown in FIG. 8, ready once more for actuation.

Certain definite relationships exist between the length of the continuous portion of an electric stream flowing through an orifice of a given diameter and the velocity of the stream at the orifice. For a given orifice and given liquid properties the length of the continuous portion of the stream, that is, the distance betwen the orifice and the point where the stream separates into droplets, is directly proportional to the orifice velocity. The equation theoretically relating continuous stream-length with fluid and orifice characteristics is:

22 it KV( where L==continuous stream length, V=velocity at the orifice, D diameter at the orifice, =density of the liquid, r=surface tension and K=a constant dependent upon vis cosity and orifice configuration. Beyond a certain point turbulence, surface tension, air resistance and other effects cause the continuous stream to disintegrate to a non-continuous shower of droplets. This invention, as can be seen, makes use of these characteristics of fluid fiow to ensure that circuit completion or interruption takes place at the terminus of the continuous portion of the conducting liquid stream and entirely betwen liquid boundaries. There is actually a critical orifice velocity below which the direct relationship between velocity and continuous stream-length formulated above ceases to hold. This critical orifice velocity depends not only upon the characteristics of the liquid but also upon the diameter of the orifice. For example, with mercury as the liquid the critical velocity for a 40 mil circular orifice is about 25 inches per second created by a pressure of 1.1 inch of mercury above the orifice. For a 30 mil orifice the critical velocity is approximately 39 inches per second established by a column of mercury 2.3 inches above the orifice. Below the critical velocity, although the direct relationship between orifice velocity and continuous stream length no longer exists, an increase in pressure above the orifice is still accompanied by an increased length of the continuous portion of the stream except at the very lowest orifice pressures. At very low orifice pressures the droplets form on and break away at the end of the orifice and the stream has, therefore, no steady or continuous length.

From the foregoing it can be seen that an important feature of this invention resides in spacing the surface of the mercury pool a suflicient distance so that single droplets may not bridge the gap from orifice to pool, but a smaller distance than the maximum continuous stream length. Only thus will it be possible to control the making and breaking of circuit continuity by varying the orifice pressure and, hence, the orifice velocity. The distance of the surface of the mercury pool below the orifice will depend upon orifice size for, as can be seen from the equation above, other things being equal, the larger the orifice the greater the continuous stream length. The selection of orifice size on the other hand will depend upon the desired current-carrying capability of the contactor, and the capacity of the pump.

One of the interesting features of the mercury stream contactor described is its ability to limit the average current through its terminals by the so-called pinch effect, the compression of a conductor by its own concentric magnetic field. For example, a mercury stream 0.25 inch long and 0.035 inch in diameter will absorb or dissipate 35 watts in carrying 50 amps. With no current a stream of this length and diameter can be maintained with an orifice pressure as little as 0.82 inch. However, any current flow at all through the stream has a tendency to pinch 0d the stream and therefore to shorten its etiective length. For the stream to carry a full 50 amps. without pinching the orifice pressure should be at least 1.44 inches. With this higher orifice pressure, if the available current is then increased to 6% amps, the stream alternately pinches and remakes, resulting in an average current of about 30 amps. Pinching would then continue unless the orifice pressure were raised still further to 1.76 inches. The mercury stream contactor thus has the ability to reduce currents greater than that for which it may be designed. It cannot, however, be considered primarily a protective device required to interrupt system faults where currents on the order of thousands of amperes are available.

Although the maximum current capacity of a single stream is not known with certainty, it is probably near the 250 ampere limit of conventional mercury switches. Current-carrying capacity can however be increased by operating multiple streams in parallel. I have found, for example, that a pair of streams each capable of carrying only 50 amperes can sucessfully control amperes when run in parallel.

The mercury stream contactor shown in FIGS. 1 and 2 and described in the foregoing materials possesses several distinct advantages. It prevents erosion due to arcing between solid contacts or across the insulators. It has an inherent continuous current limiting action. Since it is completely enclosed it can be used in explosive or contaminating atmospheres. Because there are no moving parts except the conducting liquid itself, it possesses an exceptionally long useful life. And because of the continuous flow of the liquid through the enclosed path of circulation, the contactor has a naturally efiicient heat distribution which tends further to prolong its life and extend its utility.

Many of these advantages are also present in the alternate embodiment of a mercury stream contactor shown in FIGS. 9 through 12 which embodiment also possesses certain distinct advantages of its own. This contactor includes a three-legged, completely enclosed path within an insulating enclosure 31 with a reservoir of mercury 32 at the bottom. The rest of the interior is, as in the previous example, preferably filled with an inert gas under pressure. Extending into the enclosure are a pair of electrical terminals 33 and 34 between which electrical contacts to be controlled. The upper terminal 33 communicates with a metallic funnel at the bottom of upper chamber 35 having an orifice 36 in the bottom thereof. The lower terminal extends inward to define a cup-like receptacle for pool 37 of mercury. Directly below these elements is a check valve constituting a restriction 38 in the sidewalls of the chamber and a ball 39 formed of steel or some other material which floats in mercury. The function of this valve is to permit mercury to flow only downward therethrough.

To direct the mercury into the upper chamber, there is provided a pump in the form of a solenoid 41 and a cooperating magnetic plunger 42 which fits closely within its portion of the enclosure. Before initiation of the switch action the condition of the switch is as shown in FIGURE 9. When the solenoid is energized, plunger 42 is pulled downward as shown in FIG. forcing mercury up through the center well 43 and over into the upper chamber 35 into which it flows because of the sloping spillway 44. Because of the unidirectional effect of the check valve no mercury is forced upward therethrough upon downward movement of plunger 42. When a sufiicient height of mercury exists above orifice 36 electrical contact is established between terminals 33 and 34 in the manner shown and heretofore described in connection with FIGS. 3, 4 and 5. However, as mercury continues to flow through orifice 36 it is prevented from passing through the check valve because of the back pressure on it. Therefore the area below orifice 36 fills up with mercury as shown in FIG. 11, the gas in the space below the upper chamber escaping through vent 45 into the space 46 above the plunger.

This arrangement also provides current limiting when initial contact is made between the mercury stream and the surface of the pool below. However, after the space below the upper chamber fills with mercury, the crosssectional area of mercury between the electrodes is greatly increased, resulting in substantially no current limiting but decreasing the total resistance of the mercury path through the contactor.

When the solenoid is deenergized, the plunger floats back upward as shown in FIG. 12 relieving the back pressure on the check valve and permitting the mercury above this valve to seek a lower level. When this happens the mercury drains from the vicinity about the mercury pool and the upper chamber is free to empty. As it empties electrical continuity between the electrodes will be interrupted in the manner shown in connection with FIGS. 6, 7 and 8. It should be noted that the mercury should be free to drain through the check valve faster than it does through orifice 36. If the check valve impedes the flow of mercury in the downward direction too much, the space below the orifice will not drain until the upper chamber is empty. Consequently no mercury stream, as such, will form and electrical continuity through the contactor will be interrupted, not at the sur faces between the falling stream and pool below, but betwen the metal parts surrounding the orifice 36 and a gradually sinking mercury surface.

While certain embodiments of these teachings have been shown and described in detail it should be obvious that these examples have been selected as illustrative in nature and not necessarily limiting on the scope of the present contributions. Many variations in form and detail will naturally occur to those skilled in the art to which this disclosure pertains within the scope of these teachings in their broader aspects.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical switch comprising: an insulating enclosure containing a reservoir of an electrically conducting liquid; an upper chamber Within said enclosure having a restricted orifice to permit liquid in said chamber to flow out of said chamber in a free-falling stream, a container having therein a pool of said liquid, the surface of which is spaced below said orifice in a position to intercept said stream; means for introducing electrically conducting liquid from said reservoir into said chamber at a controlled rate to vary the liquid pressure at said orifice thereby to control the length of the uninterrupted or continuous portion of the liquid stream issuing from said orifice into said pool, the surface of said pool being spaced below said orifice a sufficient distance that single drops of said liquid do not span the distance from the orifice to said pool but a lesser distance than the maximum length of the continuous portion of said stream; a first terminal in electrical contact with liquids issuing from said orifice; and a second terminal in electrical contact with said pool.

2. An electrical switch comprising: an insulating enclosure containing a reservoir of an electrically conducting liquid; an upper chamber within said enclosure having a restricted orifice to permit liquid in said chamber to flow out of said chamber in a free-falling stream, a container having therein a pool of said liquid the surface of which is spaced a predetermined distance below said orifice in a position to intercept said stream; a first terminal in electrical contact with liquid issuing from said orifice; a second terminal in electrical contact with said pool; means for introducing said electrical-1y conducting liquid from said reservoir into said upper chamber and for controllably varying the rates of flow through said orifice to establish in said stream a continuous or uninterrupted portion whose length is controllably greater than and less than the distance between said orifice and the surface of said pool, whereby initiation and interruption of electrical continuity between said terminals takes place solely between said liquid stream and said pool surface.

3. An electrical switch comprising: an insulating enclosure containing a reservoir of an electrically conducting liquid; an upper chamber within said enclosure having a restricted orifice to permit liquid in said chamber to flow out of said chamber in a free-falling stream; a container having therein a pool of said liquid, the surface of which is spaced a predetermined distance below said orifice in a position to intercept said stream; a first terminal in electrical contact with liquid issuing from said orifice; a second terminal in electrical contact with said pool; means for introducing said electrically conducting liquid into said upper chamber, for increasing the liquid pressure at said orifice to occasion the liquid stream issuing from said orifice to have a continuous length at least equal to the distance from said orifice to the surface of said pool and for decreasing said liquid pressure at said orifice to reduce the continuous length of said stream to less than said distance, whereby the initiation and interruption of electrical continuity between said terminals takes place solely between said liquid stream and said pool surface.

4. An electrical switch comprising: an insulating enclosure containing a reservoir of an electrically conducting liquid; an upper chamber within said enclosure having an orifice to permit liquid therein to flow out of said chamber in a free-falling stream; a container having therein a pool of said liquid the surface of which is spaced below said orifice in a position to intercept said stream, and arranged such that excess liquid from said pool spills into said reservoir; means for returning said electrically conducting liquid from said reservoir to said upper chamher and for creating higher and lower liquid pressures at said orifice; the relationship between the size of said orifice and the distance between it and the pool being such that only said higer pressures result in a continuous electrically conducting stream of fiow between said orifice and said pool; a first terminal in electrical contact with liquids issuing from said orifice; and a second terminal in electrical contact with said pool.

5. An electrical switch comprising: an insulating enclosure containing a reservoir of mercury; an upper chamber within said enclosure having a restricted orifice to permit mercury in said chamber to flow out of said chamber in a free-falling stream; a container within said enclosure having therein a pool of mercury the surface of which is spaced a predetermined distance below said orifice in a position to intercept said stream of mercury; means permitting excess mercury to spill from said pool into said reservoir in an electrically discontinuous state to maintain the surface of said pool at a substantially constant level; a first terminal in electrical contact with it e stream of mercury issuing from said orifice; a second terminal in electrical contact with said pool of mercury; means defining a return path between said reservoir and said upper chamber; and an electromagnetic pump for introducing mercury from said reservoir into said upper chamber at controlled pressures including a pair of elec trodes arranged opposite each other in a portion of said return path, and a pair of magnetic pole pieces positioned on opposite sides of said portion of said return path in space quadrature to said electrodes.

6. An electrical switch comprising: an insulating enclosure containing a reservoir of an electrically conducting liquid; an upper chamber within said enclosure having a restricted orifice to permit liquid in said chamber to fiow out of said chamber in a free-falling stream; a container having therein a pool of said liquid, the surface of which is spaced a predetermined distance below said orifice in a position to intercept said stream; a first termi nal in electrical contact with liquid issuing from said orifice; a second terminal in electrical contact with said pool; means for controllably forcing said electrically conducting liquid through said orifice at rates of flow up to a maximum at least sufiicient to cause said stream to have a continuous or uninterrupted portion long enough to bridge the distance between said orifice and the surface of said pool.

7. An electrical switch comprising: a reservoir containing mercury; a container containing a pool of mercury normally insulated from the mercury in said reseryoir; means defining a restricted orifice spaced directly above the surface of said pool of mercury; switch operating means for directing mercury from said reservoir through said orifice in a free-falling stream at rates of flow up to a maximum rate at least sufficient to establish in said stream a continuous or uninterrupted portion long enough to bridge the distance between said orifice and the surface of said pool; a first electrode in electrical contact with the mercury passing through said orifice; and a second electrode in electrical contact with the mercury in said pool.

8. The method of making and breaking electrical continuity between two electrodes which includes: forming a freely falling stream of conductive liquid in electrical contact at its upper end with one of said electrodes; intercepting said freely falling stream solely within a pool of conductive liquid in electrical contact with the other of said electrodes; and selectively controlling the length of that portion of said freely falling stream which is uninterrupted by the formation of droplets, the uninterrupted steam length being made at least great enough to reach the surface of said pool to establish electrical continuity between said electrodes and being made inmind set

S sufficient to reach the surface of said pool to interrupt continuity between said electrodes, whereby electrical contin ity between said electrodes is made and broken entirely between the terminus of said uninterrupted stream and the surface of said pool.

9. The method of making and breaking electrical contlnuity between two electrodes which includes: forcing a body of electrically conductive liquid in electrical contact ith one of said electrodes through an orifice to form a freely falling stream thereof; intercepting said freely falling stream solely within a pool of conductive liquid in electrical contact with the other of said electrodes; and selectively varying the length of that portion of the freely falling stream which is uninterrupted by the formation of droplets by increasing the rate of the flow of said stream from a lower rate to a higher rate at which the uninterrupted portion of said stream becomes suflicient to bridge the distance between said orifice and said pool and by decreasing the rate of fiow of said stream from a higher rate to a lower rate at which the uninterrupted portion of said stream becomes insutficient to bridge the distance between said orifice and said pool, whereby the initiation and interruption of electrical continuity between said electrodes takes place solely at the point of contact between the surface of said pool and the lower terminus of said stream.

10. The method of making and breaking electrical continuity between two electrodes which includes: forming a freely falling stream of electrically conductive liquid in electrical contact at its upper end with one of said electrodes by forcing said liquid through an orifice; intercepting said freely falling stream solely within a pool of conductive liquid in electrical contact with the other of said electrodes; and selectively varying the length of that portion of the freely falling stream which is uninterrupted by the formation of droplets by selectively varying the rate of flow of said liquid through said orifice between a lower rate at which the uninterrupted portion of the stream is insulficient to bridge the distance between said orifice and said pool and a higher rate at which the uninterrupted portion of the stream is sufficient to bridge the distance between said orifice and said pool, whereby the initiation and interruption of electrical continuity between said electrodes takes place solely at the point of contact between the lower terminus of said stream and the surface of said pool.

11. An electrical switch comprising: A container containing a pool of electrically conducting liquid, means defining a restricted orifice spaced above the surface of said pool by a predetermined distance, means for directing electrically conducting liquid through said orifice in a freefalling stream at variable rates of flow between a miximum rate at least sufiicient to establish in said stream a continuous portion long enough to bridge the distance between said orifice and the surface of said pool, and a minimum rate at which the length of said continuous portion is less than the distance between said orifice and the surface of said pool, a first electrode in electrical contact with the liquid passing through said orifice, and a second electrode in electrical contact with the liquid in said pool and located beneath the surface of said pool whereby establishment and interruption of electrical continuity between said terminals takes place solely between said liquid stream and said pool surface.

References Cited in the file of this patent UNITED STATES PATENTS 330,451 Weston Nov. 17, 1885 ,0 4 Bainbridge Feb. 11, 1930 2,490,785 De Vany Dec. 13, 1949

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