US2172677A - Electric protective device - Google Patents

Description

Sept. 12, 1939. J. R. HATHAWAY 2,172,677

ELECTRIC PROTECTIVE DEVICE Filed Feb. 20, 1937 w zz 9/0 I I j; Ky, /7 3 a 7a 7 5/ ifl a w M WfWWZ m- 76 77 I %%W X4467 //z/% 4 w ame Sept. 12, 1939' UNITED STATES PATENT OFFICE? v smcrarc rao'rsc'rrva nsvrca John Ralph Hathaway, Freeport, m, assignor, by

assignments, to Micro |Switch,Corporation, Freeport, 111., a corporation of Illinois Application February 20, 1937, Serial No. 128,777

22 (,Iaims. (Cl. 171-97) This invention relates to methods and devices for suppressing arcs which are formed upon the opening of contacts in electric circuits, and especially to direct current circuits containing 5 load components of an inductive nature.

Itis an object of this invention to provide new and improved methods and apparatus for suppressing arcs in electric switches and to adapt switches to interrupt more severe arcs than can be extinguished by mere separation of their contacts. It is a further object to provide improved method and means for switching direct current power circuits containing inductive loads. 15 Other objects and advantages will appear as the description proceeds.

The following description of certain specific embodiments of my invention is to be read in conjunction with -the accompanying drawing in 20 which:

Fig. 1 shows my device in combination with a form of switch with which my methodand apparatus are particularly adapted to be 'used, the switch being shown in cross-sectional elevation;

25 Fig. 2 shows diagrammatically a modification 0! my invention whereby the efllciency of the construction shown "in Fig. 1 is increased somewhat for certain conditions;

Figs. 3 and 5 show diagrammatically further 30 modifications of my invention; and,

Figs. 4 and 6 show diagrammatically my invention applied to multiple load circuits.

My invention is applicable to a wide variety of switching devices, but it is especially applicable -35 to snap switches of the'type shown in U. S.

McGall Patent No. 1,960,020, issued May 22, 1934.

In a switch of this type, illustrated in Fig. 1

the movable contact member travels a small distance with a quick snap motion. Because ,of the i0 limited distance of travel, switches of this type are not adapted to interrupt heavy direct current loads and extinguish the resulting arcs unaided.

Various methods have been employed and at- 45 tempted in the past for suppressing sparhng in switches and for suppressing and extinguishing arcs. For example, a condenser and resistor have been connected in series and the resulting series combination connected across the contact points 50 of a. switch. However, this arrangement has.

proven satisfactory only when the capacity of the condenser and the value oi the resistor have been carefully adjusted to the particular switch and load. A change in' the value of the load 55 current or in the, inductive components of 'the load, or a change in the size of the switch or its speed of operation invariably called for a new adjustment of condenser and resistor values. The values of capacity and resistance have been 60 especially critical in those it was attempted to use the old condenser and resistor combination across the load contacts for suppressing an arc.

In the following description my invention is described in connection with a switch of the type illustrated in the McGall patent. The specific examples merely illustrate the invention and do not limit it. The invention is to be limited only by the scope of the appended claims.

In Fig. l the snap switch i0 is used to control 10 the current provided by generator II in a circuit operating load l2. The switch l0, comprising an insulating base l3 and cover it has a movable contact l5 mounted on the connected free ends of tension spring member i5 and compression spring members I l. The contact l5 moves between upper stationary contact i8 and lower stationary contact 19. All of the contacts are metallic, silver or silver-alloy contacts usually being used although tungsten also is suitable.

The opposite end of the tension member I6 is mounted by means of screw on metal bushing 20a in insulating base IS. The compression members ll are mounted pivotally in a V-shaped notch at 2| in member 22 which may also be 25 electrically connected to screw 20 and bushing 20s. A screw 22 provides a terminal connector atthe outer end. A screw 24 passes through insulating base I! whereby electrical connections may be made to contact or stop I at terminal 25.

A screw 2| together with a metal bushing molded in the base l3 holds the lower stationary contact I! in place and an external screw carried by this same bushing serves as a terminal connector for contact l9. Upper contact i8 is mounted on post 28 by means of screw 21, which fastens in still another metal bushing in the insulating base II which bushing carries the terminal screw 28. When movable contact I! is in contact with upper contact l8 as shown, the electric circuit is complete from generator ll through load l2, screws 22 and 20, tension and compfession members i6 and I1, contacts I! and I! to the generator.

In order to show the circuit of Fig. 1 as clearly as possible, the lines'of the diagram of the circuit connections are run directly to contact pieces l8 and" rather than to their terminal screws 2| and 28.

The snap switch is operated by means of plunger 29 pressing upon tension member I6. When the tension center line of tension member IS passes through the pivotal point 2| of com: pression members I'I, contact ll snaps from the upper-contact to the lower contact. If the load is sufllciently heavy and the current is undirectional an are forms between separated 'II and I.

when switches such as the switch II were used without making special provision "forqarc." ot

pressing or extinction, the arc would persist even after the movable contact reached lower contact IS, with the result that the contacts aii'ected by the are quickly became fused and valueless. For example, when silver or silver-alloy contacts were used, destructive arcing occurred with a load greater than 0.5 ampere direct current at volts. The area of each contact was 0.1335 square inch and the distance between the separated contacts was 0.015 inch. The time required for the moving contact to travel from one stationary contact to the next was approximately .006 second. Where the line voltage was increased to 600 volts the safe operating current fell to approximately 0.015 ampere.

In the system of Fig. 1 I suppress and extinguish the arc formed by the interruption of the direct current load by first opening the load carrying contacts to form the arc and then connecting a condenser across them to extinguish the arc; This is accomplished by the stationary contact I9 and condenser 30, condenser 30 being connected across the two stationary contact pieces l8 and I9 as shown diagrammatically in Fig. 1.. I have found that the arc which is formed between stationary contact and movable contact l5 as it travels to lower contact 19 is immediately extinguished by this arrangement so that the silver is not sufficiently heated to cause fusion or-serious erosion.

Tests show that with a switch of the dimensions referred to above in the system of Fig. 1, up to 20,000 breaks may be made with a direct current load of 6.5 amperes at 220 volts with only slightly perceptible wear of the contacts. In addition the condenser capacity is not critical. However, within certain limits, the size of the condenser approximately determines the size of load that may be interrupted. In the particular test referred to, the condenser capacity was approxi-- mately 1 micro-farad. With the above switch in the system of Fig. 1, twenty amperes at 220 volts direct current was repeatedly interrupted Without excessive erosion oi the silver contact surfaces.

A switch of the type shown in Fig. 1 having the dimensions given above was able to interrupt only 10 amperes, 230 volts, direct current on noninductive load when using the known device of connecting a 2 micro-farad condenser across the load contacts. A. 13 ampere load produced a sustained arc. A series resistor would not have improved the arc interrupting performance of the switch but would only have reduced sparking or welding of contacts upon closing. On the other hand, a similar switch using the circuit of Fig. l with a condenser with any value from 1. to 3 micro-farads interrupts a current of 20 amperes, 220 volts direct current on non-inductive load.

The system of my Fig. 1 is not critical as to the value of the condenser required for suppressing the arc, whereas the values of condenser and resistor connected across the load contacts in prior devices are critical. Furthermore, my im proved circuit and apparatus are-effective for suppressing more severe arcs than are the prior art devices.

I have no satisfactory and complete explanation or theory of operation for my improved apparatus. The action of the electric arc is so complex and my; invention operates under so many seemingly different conditions that it is diilicult to formulate a theory that completely explains all the phenomena that I have observed. However, I do believe that I take advantage o the fact that the arc is most diflicult to maintain, and therefore, most easily extinguished when it is drawn out to its maximum length. I do this by first drawing the arc to its maximum length and then connecting the condenser across the contact gap to shunt the arc.

I have found that it is immaterial whether the condenser 30 acts to shunt a part of the current away from the are or whether the condenser actually discharges itself through the are so as to add temporarily to the current in the are. In Fig. 1 the switch is shown in the load closed position. In this position the condenser 30 is disconnected from other parts of the circuit so that the condenser may maintain itself either charged or discharged during the time that the load is energized. Assume then, that the switch is in the position shown in Fig. 1 so as to energize the load, the condenser 30 being disconnected thereby from other parts of the circuit. Assume also that precaution is taken to discharge the condenser 30. Then when the switch is operated to open the load circuit an arc is drawn between the contacts l5 and i8, and when the contact l5 comes into engagement with the contact ill to connect the condenser 30 across the arc, the condenser extinguishes the arc. At the moment that the contacts l5 and i9 come into engagement the arc current is temporarily reduced, at part of the current being shunted through the condenser 30. Now, with the load contacts open and the arc extinguished the condenser 30 becomes charged to the full voltage of the generator ll. Operation of the switch ID to close the load circuit does not discharge this condenser 30. Upon subsequent opening of the load contacts l5 and i8 the are again is drawn between them and this are is extinguished when the moving contact l5 again engages the contact ill to connect the condenser across the arc. However, this time the condenser has a charge and instead of shunting a part of the current from the arc it actually discharges itself through the arc ther by temporarily increasing the current of the arc. The condenser appears to be just as efiective for extinguishing the are between the contacts 55 and it when it is charged as when it is discharged by the engagements of the moving contact l5 with the stationary contact id.

My present invention isparticularly adapted to reduce arcing in switches as distinguished from sparking. Thus the system of Fig. 1 does not appear to have any effect upon mere sparking between switch contacts and does not appear to materially improve the operation of switches when those switches are used with loads which can be interrupted easily by merely separating the sfitch contacts. My invention apparently is most effective when the switch to which it is applied is unable to interrupt the arc unaided. However, my invention does not interfere in any way with the operation of a switch in handling its load circuit and, therefore, it may be applied to a switch which ordinarily is capable of inter rupting its own load circuit unaided in order to insure that the switch shall not fall under an abnormal load. While my invention might not greatly improve the ordinary operation of such a switch, it would protect the switch by standing ready to extinguish the arc whenever conditions should become so severe that the switch would not be able to interrupt the arc unaided.

While the system of Fig. 1 is extremely effective in suppressing arcs so as to greatly increase the arc interrupting ability of a switch, under avast? certain conditions, especially when handling inductive loads, there may be some tendency for the arc suppressing contacts to stick, that is, there may be some tendency for the moving contact II to weld to the stationary contact II which connects the condenser 30 across the are alter the arc has been drawn out by the separation of the load contacts. I have found that this tendency of the suppressor contacts to weld may be overcome by modifying the system 01 Fig. l in accordance with one of the arrangements shown inFlgs. 2, 3 and 5.

In the system of Fig. 2 the electrical circuit contains in series the generator 35, load It and snap switch 31. The snap switch I'I comprises upper contact 38, lower contact 39 and movable -contact 40. The condenser 4i is placed across contacts 38 and 39. When the movable contact 40 snaps from upper contact 38 to lower contact 39, condenser 4i suppresses the are formed by the breaking of the circuit. In this modification a glowvalve 42 is inserted between the lower contact 39 and the movable contact 40.

In one test, I found that a certain load of 15 amperes at 220 volts direct current handled by the system of Fig. 1 produced some sticking because of slight welding between the suppressor contacts, but that when a glow valve was added in the position shown by the glow valve 42 in Fig. 2, the sticking was entirely eliminated. In that particular test the glow valve 42 consisted oi a 110 volt neon lamp of 1 watt rating. Such a lamp consists of a glow valve with a resistor of approximately 5000 ohms in series with it. Other types of glow valves may be employed.

Fig. 3 shows a further modification, the switch and circuit being shown in diagrammatic form. The circuit comprises generator 45, load 45 and switch 41 in series. The load circuit is complete when movable contact 40 abuts upper contact 49. Condenser 50 is placed between upper contact 40 and lower contact 5| with which movable contact 4! establishes electrical contact upon movement from upper contact 49. In this particular modification a high resistance 52 is placed in parallel with condenser 50, that is, across the stationary contacts 49 and ii. In a switch of the McGall type already described in connection with Fig. 1, improved results are obtained by using a resistance of approximately 20,000 ohms across the condenser. The value of this resistance is not critical. It is made high to keep to a minimum the current which flows when contacts 48 and ii are closed.

In Fig. 4 my improved arc suppressing device is shown in connection with a circuit in which two loads are controlled by a single switch. The circuit consists of generator 55, switch 56. having a movable contact 51, stationary contacts 58 and 59 with loads SI and 6| connected to stationary contacts 58 and 59 respectively. The benefits of my invention may be obtained by inserting condenser 62 across stationary contacts BI and 59 as shown.

In Fig. 5 the circuit consists of generator 65, load 66, a switch 61 with movable contact 68 and stationary contacts 69 and I0, condenser H across the two stationary contacts oi the switch and resistor 12 between the lower contact Iii and the movable member ll of the switch. when using a switch of the McGall type having the dimensions and characteristics described in connection with Fig. 1 and a load oi 20 amperes at of 20 ohms and'condenser li a capacity oi two micro-iarads. Resistor l2 eliminates the tendency oi contacts Cl and II to weld to each other, especially when heavier values of current are interrupted and condenser ii is of V micro-farad or more in capacity. The exact action of resistor 12 is not understood but experience shows that its use improves the action of the arc suppressive arrangement of Fig. l in that it reduces the tendency of the switch to weld its suppressor contacts on inductive loads.

The elimination of welding 01' the suppressor contacts in the systems of Figs. 2, 3 and 5 is believed to result simply from the elimination of high condenser currents incidental to the closing of the suppressor contacts to connect the condenser across the arc. The glow valve 42 of Fig. 2 and the resistors 52 and i2 Figs. 3 and respectively apparently control the condenser voltage previous to the closing of the suppressor contacts to bring the potential difference between them to a low value. It is believed that the resistor 52 of Fig. 3 simply discharges the condenser iii and that this eliminates welding because the arc voltage is low. The are voltage necessarily is low in this system because, if the arc voltage were high the switch would be better able to interrupt the are without the aid of the condenser 50. It is permissible to control the condenser voltage in this manner because the actual state oi charge of the condenser at the time that it is connected to the are apparently is immaterial in the operation of my system.

In the system of Fig. 5, the condenser Ii apparently comes to such a condition oi charge at the time when the moving contact 8! comes into'engagement with the stationary suppressing contact II, that the condenser voltageis substantially equal to the arc voltage. Consequently, the condenser ii is unable either to detract irom, or add to, the current of the are which is drawn between the moving contact 88 and the stationary load contact 6!. Assuming that the condenser l'i has a capacity of 2 micro-farads and that the resistor 12 has a resistance of 20 ohms, the time constant oi the combination of the condenser ii and resistor 12 is 0.00004 second. The time required for-the operation of the switch 62, that is .the time required for contact 68 to move from the stationary contact 69 to the staionary contact 10, is approximately 0.0033 second. This time is approximately eightytimes as long as the time constant of the combination of the condenser Ii and resistor 12. As the load contacts open, that is, as the moving contact 68 moves away from the stationary contact 69 and draws an are between them, a potential appears between these two contacts, which potential is the value required to maintain the arc; Inasmuch as the time constant of the combination oi the condenser ii and resistor 12 is much less than the time required for the operation of the switch, it is believed that the condenser ii, at any given instant, has a potential substantially equal to the potential across the two contacts 68 and 89, because the time required for charging the condenser ii through the resistor 12 is small compared to the time required for the operation of the switch. Consequently, when the moving contact 8| does finally come into engagement with the stationary contact II to short circuit the resistor 12, there is little or no voltage between those contacts, and therefore, little or no current will be shunted from the arc'through the condenser li.

Actual tests have demonstrated that the system of Fig. 5 is substantially equivalent to the system of Fig. 1 insofar as its arc interrupting and suppressing ability is concerned. However, the sys-.

tem of Fig. 5 is superior to the system of Fig. 1 in that the addition of the resistor 12 in Fig. 5 reduces the sparking which occurs between the moving contact and the stationary suppressing contact of the switch to induce welding of the surfaces of those contacts to each other.

The performance of the system of Fig. 5 provides a vivid comparison between my present invention and prior art spark suppressing circuits. It is to be observed that were the stationary suppressing contact 10 of the switch 61 omitted, the circuit connections of the system of Fig. 5 would become similar to those of the old arrangement of a series combination of a condenser and resistor shunted across the load contacts of the switch, although the optimum values of resistance and capacity might differ between two arrangements. It also is to be observed that no arc is ever drawn between the contact 68 and the stationary suppressing contact 10, during the motion of the moving contact 58 away from the stationary load contact 69 toward the stationary suppressing contact Ill, and that therefore, the contact 10 performs no operation during the interval that contact 68 is in motion. Consequently, during the short interval that the contact 68 is in motion, the system of Fig. 5, except for the difference in the values of the resistances employed, can act no differently than does the old system already mentioned. The system of Fig. 5 differs from the prior system only in that it provides the. suppressing contact 10 which shunts the condenser Ii directly-across the are after the arc has been drawn out to its maximum length. However, as I have already pointed out, the prior system requires precise adjustment of the values of its condenser and resistor and then will interrupt a current of only 10 amperes at 220 volts with a switch of the type already described, whereas the system of Fig. 5 does not require any such careful adjustment and will interrupt a current of 20 amperes at 220 volts with the same switch.

The system of Fig. 5 is especially useful in that it may be employed for efliciently suppressing both sparking and arcing in a switch. A spark as distinguished from an arc is transitory and expires before the load contacts complete their separation. An arc persists and, as I have discovered, is most easily extinguished after the load contacts have reached their maximum separation. The system of Fig. 5, during the time that the contact 68 is in motion, functions in the same manner as does the prior art condenser and resistor to reduce sparking. But if the load is so severe as to produce an are instead of a spark, the arc is extinguished by the condenser Ii when the contact 68 stops against contact it. Thus the system of Fig. 5 offers both the benefits of the old condenser and resistor shunt and also the benefits of connecting the condenser across the are after drawing the arc to its full length.v I

It appears to be important in practicing my invention that the condenser be connected directly across the are. I have found that connecting resistance in series with the condenser 1i and shunting the resulting combination across the two stationary contacts of the switch is far less eifective'than shunting only the condenser across the contacts of the switch. For example,

as little as one-half ohm connected in series with the condenser 1i in Fig. 5, or in series with the condenser 30 in Fig. 1, seriously reduces the arc suppressing ability of the system.

In Fig. 6 the circuit consists of generator I5 5 and two loads 16 and 1'! connected respectively to the two stationary contacts 18 and 18 of switch 80. Instead of using one condenser as in Fig. 4, two condensers 8i and 82 are placed across the stationary contacts with a resistor 83 between the common terminal of the condensers and the movable member 84 of the switch. For example, with the McGall type of switch, condensers 8i and 82 may have one or four microfarads capacity each and resistance 83 may be approximately thirty ohms. This arrangement results in improved operation over that of Fig. 4 insofar as sticking of the switch contacts is concerned.

It is thus seen that my invention provides a combination of a condenser and a switch which may be marketed for use on power circuits generally without precise restriction as to size and character of the load, for inasmuch as the con denser value is not critical for any load, a wide range of different load conditions may be accommodated by any one condenser value.

To secure the maximum benefits of my invention, it is obvious thatthe movable contact member should move as rapidly as possible from the contact with which it arcs to the second contact whereby the condenser'or other means is put in operation to suppress the are. If the movable contact member moves slowly, the temperature of the contact pieces may be raised sufliciently that the contacts may be damaged before the arc is suppressed.

Although my invention may in some cases be used. with some beneficial effect if an alternating current circuit is controlled by the switch, the principal use of the invention is in connection with the control of direct current circuits.

With the illustration of the principles of this invention by the above described examples, those skilled in the art will be able to apply these principles to various types of snap and other switches and by means of modified constructions and circuits other than those given by way of illustration. Accordingly, I do not wish to be limited except by the scope of the appended claims.

I claim:

1. The method for preventing the erosion of electrical contacts by the arcing between them when an electrical circuit is broken by their separation, which comprises separating said contacts rapidly and immediately thereafter connecting a condenser across said contacts before the arc heats the contacts appreciably.

2. In combination in a switching apparatus, load controlling contacts, a condenser and resistor connected in series with each other across said load controlling contacts, and means for first separating said load controlling contacts and for then connecting a shunt acrosssaid resistor.

3. In an electrical switching device in which the contacts are separated to break the electric current passing therethrough, a condenser, one side of said condenser being connected to the circuit on one side of said switch and the other side of said condenser being connected to the opposite side of said switch when said contacts are separated, and a glow discharge valve connected between the second side of said condenser and said opposite side of said switch.

4. In an electrical switching device in which I the contacts on the opening of said device during the flow of electric current therethrough, separate a distance insuflicient to break the are formed thereby, a condenser one side of which is connected to the circuit on one side of said switch, a high resistance connected across said condenser, and means whereby the opposite side of said condenser is connected to the circuit on the opposite side of said switch when said circuit is broken by the separation of said contacts.

5. In an electrical switching device in which the contacts on the opening of said device during the flow of electric current therethrough, separate a distance insuflicient to break the arc formed thereby, a condenser one side of which is connected to the circuit on one side of said switch, a resistance connected between the opposite side of said condenser and the opposite side of said switch, and means whereby said opposite side of condenser is connected directly to said circuit on said opposite'side of saidswitch when said circuit is broken by the separation of said contacts. i

6. In combination, a direct current power supply, a load device therefor, a snap switch having load contacts for completing and interrupting a circuit through said load device and power supply, said load contacts consisting of a first stationary contact and a moving contact which separate too small a distance to interrupt the are which forms between said load contacts as they separate, a second stationary contact engaged by said moving contact when said load contacts are open, and a condenser connected between said first and second stationary contacts.

7. The system of claim 6 wherein there is included means connected between the second stationary contact of said switch and one of the other contacts thereof for draining a charge from said condenser.

8. As an article of manufacture a switch for electric power circuits including in combination, double throw switch means having two contact gaps in series, a snap mechanism for operating said switch means for opening the first of said contact gaps and then closing the second gap with a continuous snap motion, and a condenser connected across said series connected gaps.

9. The combination of claim 8 wherein a re sistor shunts one of said contact gaps.

10. In combination in a switching apparatus, load contacts, a condenser, andmeans for first separating said load contacts and then shunting said condenser across them.

vice, means for operating said switching means.

to open one gap before closing the othenwhereby to prevent said gaps from being in closed condition simultaneously, and meansfor draining a charge from said condenser.

13. The combination of claim 12 wherein the means for draining the charge from the condenser includes a glow valve.

14. The combination of claim 12 wherein the means for draining the charge from the condenser shunts said condenser.

15. As an article of manufacture, the combination of claim 8, wherein there is included means connectedacross at least one of said contact gaps for draining from said condenser a charge left there by said switch means when interrupting a power circuit.

16. In combination in a switching apparatus, load controlling contacts, are controlling contacts, and condenser means, all connected in series, means for operating said contacts first to open said load controlling contacts and then to close said are controlling contacts, and an impedance device for draining a condenser charge, said impedance being connected between (1) a point on said condenser means removed from the connection of said condenser means to said load controlling contacts, and (2) the point of connection between said load controlling contacts and are controlling contacts.

17. In combination in a switching apparatus, load controlling contacts, a condenser and resistor in series together shunting said circuit path through' said load controlling contacts, and means for first separating said load controlling contacts and for then effectively removing said resistor from the shunt path and leaving only said condenser shunting said load controlling contacts.

18. The method of interrupting an electric current which comprises first drawing an are between eelctrodes and directing said current through said arc, and second, shunting said are with a condenser, and timing those operations to set a time interval between the introduction of the current to the arc and the shunting of said are by the condenser.

19. The method of interrupting an electric current which comprises drawing an are between electrodes, directing said current through said arc, and thereafter shunting said arc with a condenser.

20. The method of interrupting an electric current in an electric circuit which comprises-drawing an arc and directing said current of said circuit through said arc, and thereafter changing said circuit to extinguish said arc.

'21. The method of interrupting an electric current in an electric circuit which comprises drawing an arc and directing said current through said arc, and thereafter switching said circuit to change the electrical characteristics thereof for extinguishing said arc.

22. In combination in a switching apparatus, a power supply, a load device and load controlling contacts all connected in series, a resistor and condenser each having one terminal connected to the other, the other terminal of said resistor being connected to the circuit path that extends from said power supply through said load to said load controlling contacts, the other terminal of the condenser being connected to the other circuit path between said power supply and load controlling contacts, and means for first opening said load contacts and for then connecting the joined terminals of said condenser and resistor to said circuit path extending from said power supply through said load device to said load controlling contacts.

' JOHN RALPH HATHAWAY.

Images