US2654813A - Overload protective relay - Google Patents

Description

/7 0: V /I/, I r /53 LEI H Oct. 6, 1953 c. HALL 2,654,813

OVERLOAD PROTECTIVE RELAY Filed Feb. 8, 1952 2 Sheets-Sheet 1 Pi Inventor":

Chester L Hall,

Mm bk! -Hs Attorney.

Oct. 6, 1953 c. 1. HALL OVERLOAD PROTECTIVE RELAY 2 Sheets-Sheet 2 Filed Feb. 8, 1952 M Ha 0 L 0 w .w w er 0 n1. A m .5 C H mm m N m o M M m n L m N r 0 7 Z w m J P W 2 w m w m M 0 0 m m. 0 M W m M 6 4 Z Patented Oct. 6, 1953 OVERLOAD PROTECTIVE RELAY Chester I. Hall, Vischers Ferry, N. Y., assignor to General Electric Company, a corporation of New York Application February 8, 1952, Serial No. 270,641

7 Claims.

This invention relates to overload protective devices for electric circuits, and it has for an object the provision of a simple, reliable, inexpensive and improved device of this character.

More specifically, the invention relates to electric circuit overload protective devices for use in locations in which high shocks may be encountered such, for example, as aboard naval vessels, and a further object of the invention is the provision of a device of this character which is capable of uniform and consistent operations under conditions of high shock.

Another object of the invention is the provision of an overload relay having an inverse squared load time of operation curve at overloads of the order of 1300% to 1500% of normal full load of the protected circuit.

A still further object of the invention is the provision of an overload relay having fail-safe operation, 1. e., a failure of the device itself will initiate the same protective operation that is produced in response to an overload.

A still further object of the invention is the provision of an overload protective relay in which substantial changes in the ambient temperature produce relatively small changes in the critical value of the tripping current.

In carrying the invention into effect in one form thereof, a pair of circuit controlling contacts have closed and open operating positions. The contacts are biased to one of these two operating positions by suitable biasing means, and a detent serves to latch them in the other operation position against the force of the biasing means. The contacts are operated by means of at hermetically sealed thermally sensitive unit which comprises a boiler and a metallic bellows in communication therewith. A volatile liquid partially fills the bellows and boiler when thebellows is in the fully compressed condition thus retaining a slight vapor space under all conditions of operation. An electric heating element is energized in response to the load of the protected circuit and supplies heat to the boiler with respect to which it is mounted in heat transfer relationship. The bellows is normally retained in a substantially, but not fully, compressed condition owing to the vacuum produced within the boiler and bellows. In response to overload on the protected circuit, the heater vaporizes a portion or the liquid in the boiler and forces the liquid out of the boiler. The resulting expansion of the bellows moves an operating member to actuate the detent to permit the biasing means to move the contacts to initiate the circuit protective operation. Failure of the device such, for example, as development of a leak in the thermally sensitive unit allows the bellows to expand and move the operating memberin the direction in which it moves when an overload occurs thereby producing fail-safe operation. Substantially shock proof operation is produced as a result of a load versus deflection characteristic of the bellows such that at normal full load of the device which is to be protected e. g. a motor, the amount of deflection is small in comparison with that which takes place in response to a small increase in load above of full load.

For a better and more complete understanding of the invention, reference should now be had to the following specification and the accompanying drawings in which Fig. 1 is a side elevation, partly in section of the protective device with the contacts in the closed position; Fig. 2 is a front elevation with the cover removed; Fig. 3 is an end section taken on the line 33 of Fig. 1 looking in the direction of the arrows; Fig. 4 is a side elevation with parts broken away, and with the contacts in the open position; Fig. 5 is an end section taken on the line 5-5 of Fig. 4; Fig. 6 is an enlarged sectional view of the boiler; Figs. 7 and 8 are plan views of heating elements having diiierent ratings; and Figs. 9 and 10 are charts of characteristic curves which facilitate an understanding of the operation of the invention and its advantages.

Referring now to the drawing, the overload protective relay device comprises a thermally sensitive unit which uses the principle of a liquidvapor system. The thermally sensitive unit is hermetically sealed and comprises a boiler l and a metallic bellows 2 which is in fluid communication with the boiler. In the illustrated embodiment the communication between the boiler and the bellows is provided by means of a capillary tube 3.

As shown in Fig. 6, the boiler is flat in general appearance, i. e., its height is small in comparison with its other dimensions. It is illustrated as being cylindrical in form. It is preferably made in two portions comprising the cylindrical portion having an upper closed end and a lower open end, and a supporting plate la, to which the wall of the open end is fastened by a suitable method such as brazing with silver solder.

The cylindrical portion of the boiler is made of any suitable gOOd heat conducting material such as copper or Phosphor bronze, and its end surface, which after assembly is in heat transfer relationship but not in actual physical contact with a heating element, is blackened to provide good heat absorption qualities. The supporting plate la is preferably made of a metal having high strength and rigidity and relatively poor thermal conductivity such, for example, as stainless steel. An annular internal supporting member t prevents collapse of the boiler when operating under conditions of partial vacuum.

The bellows 2 is made of a suitable strong flexconductivity. At its rear edge, i eg," the edge,-

which is remote from the cover, the plate 2a is provided with an upturnedlug Z b'provided; with holes through which machine screws exte'nd into threaded engagement with the wall of the enclosing casing or housing. Similarly the fi'ontedig'e of the plate is provided with an upturned lug Ale having a threaded opening for receiving a; screw passing through the cover of the enclosing casing. The capillary tube 3 enters openings in the supporting plates to and 2a and is fastened thereto bysilver' solderi'ngto provide tightly sealed joints.

-' The chemical composition ofthe boiler fluid 1 sheets the'critical temperature of operation of the" relay.' When this temperature is high with respect to the ambient temperature, the effect of the ambient temperature upon relay operation is small. How'ever'the power requirements are high. Preferably theboil'er fluid should have a low freezing point'so that low ambient temperatures do not afiect it. It must be chemically stable and mustnot combine chemically with the boiler or the bellows. Although several liquids produce satisfactory operating results, it is preferred to use" para-cymene GH3CGH4CH(CH3)2. Paracymene-has a boiling'point of 177 C. Other liquids which may be; used and which provide successful operation a re aniline, o-dichlorobenzene zindbhlord-inethirlbsnzene. These liquids have boiling points of"l 84'.4'"C.', and 180.6 C. and" 1626" C. respectively? z order that the operating characteristics shall be consistent in successiv'e operations; it is necessary'that the boiler fluid shall be in contact with its own vapor. Thisjconditionis achieved byfilling the boiler and' belows with the bellows fully compressed a i'idjthenj sealing" the unit at 100 C; When reIeased after filling and sealing, the external atmospheric pressuremaintains the? bellows substantially compressed. However; the spring'restorin force of the compressed'belw r uce gi l eht xban ion hic is 'sufii cient' to' cause the bellows to? produce a small negative pressure" in the boiler with the result ant formation of a vapor pocket. Withoutjthis val mi'pbckety the operating characteristics are ,r I. .sr. J

"To the movable closed end of the bellows is securely fastened a rodj-like 'operatingmember .5 which is preferably made of "a suitable plastic insulating memberwhichj doesnot deteriorate at relatively high temperatures, e, g; temperatures of the'order'of l1 5':C .""Atfit's opposite or lower end the operating member 5 provided with a. trip detent: l havingfa" circumferential slotted portion 'idwhi'clr'provides'for movement 'of a contact bridging member} out of contact-mahing'; relationship with; stationary control contacts Qand l0 responseto the force of a biasin'g spring" 'I l as the (let'er'it is moved downward in; response toj expanjsion of the bellows. As shown, the contactbri'dging member is provided with a circular opening'flq inwhich' the'enlarged' j portion of the'detent rests when the bellows is fully contracted; and this" circular opening is further extended toward the contact and the bridging member'to pio'vide a slot" 81) which co acts with. the slotted portion ofthe detent The contact bridging member is mounted on a rod {2 made of insulating material. Ifhis rod s Provided w th a pin is gainst which the biasirig'sprin'g I l presses to bias'th'e movable contact bridging member toward the contact opening position.

A heating element for the boiler is illustrated as comprising a pair of heater supports I4 and I5 andfa strip of" resistance material [6 which is bent upat each, end to provide a channel-shaped cross sectioril The ends of the resistance strip arev silver soldered tov the heater supports so that the supports are held in spaced apart relationship; The heater supports may be made of any suitable material having relatively high thermal resistivity and relatively low electrical resistivity. For heater currents'of less than 25 ampere's, the heater supports are preferably made of monel metal which contains approximately 67% nickel, 28% copper and 5% other elements such as iron and. manganese. The heater element strip I6 may be made of 'a suitable material such as a Nichrome V alloy"containing nickel, 20% chromium. As shown, the protruding ends 'of the heatersupportsare bent down at right angles to engage with the terminalplates Ma and I551. with which they aremaihtainedin contact by suitable means such as machine screws;

The heaterelementand the thermally sensitive unit are mountedinasuitable molded housing member m'ade of an insulating compound which is illustrated as a. generally rectangular box-shaped member I -T' which is provided with a cover l8. Apair of ledges- F91 andZll formed on the inside rear surface of the casing'm'ember provide support for maintaining the'heating' element in the upperehd spac'ef above such ledges.

Beneath thesefledges, the opposite walls Ila and ll'b are provided withslots 2] and 22 for receiving the boiler'supporting plate Ia. A U- shaped curved leaf" spring-1a between the lower surface of the plate andth lower surfaces of the slots maintains thetop 'surfaceof the boiler against the'hea'tin'g element. The boiler is electrically insulated from the heating element by means of a sheetfof'mica (not shown) approximately 10 mils inlthicliness which i's'inserted between the upper surface" of the boiler and the lower surface'ofthe} heating element. A second pair of slots 14; and 25; the, side, walls receive the supporting plate 2?]; v of the bellows and serve tosupport the bellows in anintermediate'posi tion within the; housing Near'th'ebot tom end of the housingifapartition member 26 serves to define the upper wallbrachamb r'w tmn which the stationary 'control' embeds. 9 and; 10 are mountedfandprovidejs a guidefor. operating members Infoperatio theheater terminals are connected to th'circuiti'which; is to bepr'otected and which forthefpurposes of illustration may be assumed to} heal motorcircuit. The connection is madewit fthe' heating elementconnected directly. in se" .relationship with the motor. The bellews, is normally. partiallyv decompressedfas a .resiilt o'flits spring tension which expands the'bellow's' 'a, slight. amount fromthe fully: compre 'edflco dit'on,untilalcondition of equilibriumis producj gbe'tweehthe spring tension ofthe bellowsandthe compressive force of, the atmos pl 1ere In this nor mal condition; the detent l onthe operatingmember 5* maintains the movable contac't'8 iir-"br dging'position with respect to the stationarycontrol contactsi 9 -and I0 which are preferably connected inthecontrol circuit of a protective device for the motor such, for example, as a circuit breaker.

As long as the motor current is less than the critical full load value at which the relay is set to trip, the liquid in the boiler is below the boiling point, and the relay remains in the normal partially decompressed condition. If the motor current increases to the critical full load value, a, small portion of the liquid in the boiler is vaporized. The vapor being trapped at the upper surface of the boiler forces the unvaporized liquid out of the boiler through the capillary tube into the bellows and causes expansion of the bellows.

As a result of the expansion of the bellows, the operating rod 5 moves downward until the edge of the detent 1 passes below the bottom surface of the movable contact bridging member 8. This permits the biasing spring I I to move the rod 12 toward the left so that the restricted portion 8b of the opening 8a is moved into the slot in the detent member. The contact 8 is thus moved out of bridging engagement with the stationary contacts 9 and H). The opening of the control contacts 9 and [0 results in opening the main circuit to the motor and in deenergizing the heating element 16 thus permitting the thermally responsive system to cool.

When the vapor in the boiler was condensed,

the liquid which was transferred to the bellows is returned to the boiler. The bellows therefore is constrained by external atmospheric pressure to collapse but is prevented from doing so by the contact carrying rod 8 which prevents upward movement of the detent 1. Depression of the manually resettable contact carrying arm 12 permits the detent I to move upward and thus permits the bellows to collapse to its initial partially decompressed position.

An important feature of this invention is the relationship between load current in the protected circuit and the expansion of the bellows and deflection of the detent member 1. This relationship is illustrated in Fig. by the curve 21 of which ordinates represent deflection of the detent and abscissae represent heater current or motor circuit load current. It will be noted from this curve that at normal full load on the motor, i. e., 100% full load, the detent is deflected approximately mils, and that at 118% full load the detent is deflected more than 60 mils which represents a deflection more than 3 times the deflection at 100% load. In other words, at normal full load of the motor, the amount of deflection of the detent is small in comparison with the ultimate deflection for a very small increase above full load. This assures full locking condition of the detent for all normal loads on the motor which in turn enables the relay to withstand high shock without producing false operation.

A further advantage of this invention is that the relay fails safe. For example, if a leak should develop in the thermally sensitive unit while the relay is in the contact closed position, the bellows would expand and move the detent in thesame direction as that in which it is moved in response to an overload condition.

Heaters of different construction may be used to provide different ratings for the relay. A spirally wound heater 29 of the type illustrated in Fig. 8 may be used with a protected circuit in which 100% full load is within the range of 1 to 8 amperes. The heater 30 illustrated in Fig. 7 may be used in the 8 to 30 ampere range and the strip resistor illustrated in heating relationship with boiler I of Fig. 1 may be used in the 31 to 50 ampere range.

A still further advantage of the invention is that the timing of operation of the relay in response to overloads of the order of 1300% to 1500% of normal full load varies in inverse relationship to the square of the load as illustrated in Fig. 9 by the curve 28 in which ordinates represent percentages of full load and abscissae represent operating time. The curve is plotted on semilogarithmic coordinates. The explanation of this phenomenon is that at low loads the slow rate of heat transfer from the heating element to the boiler, owing to the heat transfer through the liquid and the metal of the boiler, elevates the entire thermally sensitive system including the liquid to the boiling point of the liquid before vaporization occurs. On the other hand, at high loads vaporization is produced at the top surface of the liquid in the boiler before the temperature of the main body of the liquid has increased appreciably and forces the liquid into the bellows to effect expansion and releases the detent.

Although in accordance with the provisions of the patent statutes, this invention is described as embodied in concrete form and the principle thereof has been explained together with the best mode in which it is now contemplated applying that principle, it will be understood that the elements shown and described are merely illustrative and that the invention is not limited thereto since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or from the scope of the annexed claims.

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

1. An electric circuit overload protective device comprising in combination, a thermally sensitive unit comprising a boiler, a bellows having communication with said boiler, a vaporizable liquid filling said unit in the fully compressed condition of said bellows and at a predetermined temperature of said liquid thereby to provide a negative pressure within said unit in a partially decompressed condition of said bellows and at a temperature of said liquid substantially lower than said predetermined temperature, contacts having open and closed operating positions and operable from one of said positions to the other in response to expansion of said bellows from said partially decompressed condition, and a heating element mounted in heat transfer relationship with said boiler and having input terminals for connection with a circuit to be protected.

2. An electric circuit overload protective device comprising in combination, a housing member, a thermally sensitive unit mounted within said housing member comprising a shallow boiler, a bellows, means providing communication from the space immediately above the bottom of said boiler to the space within said bellows, a vaporizable liquid filling said unit in the full compressed condition of said bellows and at a predetermined temperature of said liquid substantially above room temperature thereby to provide a negative pressure within said unit at room tem perature and in a partially decompressed condition of said bellows, contacts having open and closed operating positions and operable from one of said positions to the other in response to expansion of said bellows from said partially de- 7. compressed condition. and a heating element mounted above said boiler and having its. lower surface in heat transfer relationship with the top. surtace of said boiler and: having input. terminals for connection with a circuit to beprotected;

3., electric circuit. overloads protective device comprising; in combination, a pair of contacts having closed and open. operating positions; a detentter latching said contacts. in a; first olfi'said positions; meansz biasing. Sfifid contacts to a sec- (and (it; said positions, a thermally sensitive unit comprising a boiler, a bellows havingcommunication with said boiler: and. a uapcrizablaliquid izable liquid filling said; unit. in. tliev fully" commg: said u'nitfat. a predeoermined temperanirethe min. compressed condition or said bellowsto 4*. A-nelectric circuit: overloadprot'eotivedevice comprising in. con'rloina-tiim,-w pair of contacts. having closeda and? open. operating positions; a latch. fonretaining said. contactsin afirst: of said. positions, means. biasing said contacts to-.the-sec'- nd. of said. positions, a;- thermaliy. sensitive unit comprising aboilen, a bellows=rnember,- a tubular. member providing communication between said boiler and said bellows, and a volatile-liqui'dcompletely filling said boiler, said tubular. member and said bellows in" the GOmPlBSSEdiCOHGlt'lOILOf L said: bellows: at: a. predetermined temperature. to provide within said boiler in. the released conditionof; said: bellows. a: liquid: pressure less. than atmospheric at; a. temp eraturez substantially lQWe er. than. said: predetermined; temperature; an: op.-

erating member of? insulating mateziali connected to said. bellows for releasing; said latch-.130; effect movement on said contacts :togs'aid second position in. response. to expansion, of said. bellows, and a; heating element. mountedLi'n heat; transfer. relationship. with said: boiler and; havinginput termina'ls; for: connection with'. an circuit. to, be pro tecteda.

5,. Anielectriczcircuit.dverloadzprotective device comprising: in: combination; a; pair; of contacts having; closed. and openoperating: positions; means: biasing; said; contacts; to; a: first. of said operating positions, a: detent: for: latchingv said contacts ain.thesecondzof.saidpperating: positions against-itheiorcezof; saidzbiasins means :azsealfiti thermally: sensitive. unit: comprising a: boiler; a. hellowshavihe; communication with: saicb bioilen and: a vaporizable 1 liquidcompletelyfilling; said: unit. in; thesnbstantially fully; compressed icondie tion: of said bellows; at a; temperature of I said: liquid of; 100% C';. toiprov-ide a negativepressure; within; said; unit in. a; partially decompressed condition of: said-1bellowsnataroomtemperature. substantially; lower -than; 1 .0? C a; heatingele:-- ment having-intuit terminals; for; connectionwith arcirouiti t r. benrotected and. mo t d in h attransfer;relationshinwitnlsaidlboiler toefiect .ex-.

pansionoisaid bellows from said partially del compressed condition, and an operating member of insulating material mechanically connected to-one end. of said: bellows for retaining said de tent the latching position. said partially decompressed condition of said bellows and: mov able in; response to: expansion of said bellows from said) partially deconipress'ed condition to actuane said; detent: to effect movement of' said contacts; to said: second position:

6av An; electric. circuit overload= protective tie-- vicecomprising in; combinatiom a pair of con tacts. having.- closed and open operating" post-- tions, a detent for latching said contact-'5 in a first. or said positions; means biasing said} contactsto the second of said: positions, a thermally sensitive unit comprising a; boiler, a; bellows member having communication withsaid boiler" and at vaporizable. liquid filling: said unit at a predetermined temperature in a fully com pressed condition of said bellows to provide a negative pressure within said unitin a partially decompressedt condition of said. bellows at: atemperature substantially less than said piecete'rmined? temperature, a heating element having input terminals forconnection-wi'tha circuit to" be protected and mounted in heat transfer re lationship with said boiler to effect further d compressionv of' said bellows from" said partially decompressedcondition, andan operating member connected to said bellowsfor retaining said? detent in it's latchingpo'sition in saidpa tisan decompressed condition" ofs'a'id bellows and movg able in. response to said further decompression of said bellows to actuate said detent to efi'ect' movement of said contacts to said second position and to provide fail safe operation of said device in: response to a leak in said thermal sensitive unit.

7. An' electric circuit o ie'rload protectlve devi'cecomp'risi'ng in combination, a:- thermally s'e'r' 'si tive unit com rising aboiler 'havi'n'g' spaced apart top and bottom end walls andan ncl01singwau oinin s'aid end walls to provide an enclosed space,- a bellows member disposed in" spaced apart relationship with said boiler, a tubular member providing communication be tween said ellows andsaid tuner through said bottoni' end wall, a vaporizable liquidfilli'ng said unit in' the' full'y' compressed conditiono'f 'said'i' bellows and at a predetermind temperature ofsai'd liquid tl'l'ereby 'to providef a negati'iie pres surewi'tltin saidumt in apartiauy decompressed condition of said bellows and at a temperature or said liquid substantiallylower than saidpredetermined temperature; contacts having open and-closed operating =positionsand-operable from one ofseiidpositibns'td-th otherin' response to' expansion of said bellows fromsaid'lp'artialfy dcompre'ssed condition; and a heating el'er nnt;

mounted in heat transfer relationship with said top end? wall" of said: boiler and having-- input terminals forconnection with a secure to be protected: 5 r

CHESTER I. Rofrenc es file of" this patent UNITED sTATEsrATENT's" Name; Date Longeetlal. May, 291.1951 Stroup Apr. 15; 1952

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