US2508178A - Circuit interrupter - Google Patents

Description

May 16, 1950 T. LINDsTRoM ETAL CIRCUIT INTERRUPTER Filed Feb. 1o, 1944 Patented May 18, 1950 CIRCUIT lNmRUPTEB Ture Lindstrom, E

and Bernard G.

dwmd, Tremblay, Wilkinslmrg, Pa., asslgnors to Westinghouse Electric Corporation, East Pittsburgh,

Pa., a corporation of Pennsylvania Application February 1I, 1944, Serial No. 521,788

(Cl. 20o-103) iz claim. l

This invention relates to circuit interrupters and more particularly to circuit interrupters embodying electromagnetic tripping means for tripping the interrupter in response to overloads above a predetermined value.

An object of the invention is to provide a circuit interrupter having instantaneous and time delay trip mechanism with improved means for adjust- `ing the instantaneous tripping characteristics of the interrupter.

Another object of the invention is to provide a circuit interrupter with an improved electromagnetic trip means having an armature and a fixed core wherein a portion of the magnetic core is adjustable relative to the armature to vary the tripping characteristic of the electromagnet.

Another object of the invention is to provide an improved circuit interrupter with a novel electromagnetic trip device, the electromagnet having an armature and two magnetic flux circuits and corresponding air gaps disposed to produce a differential pull on the armature, the minimum tripping current required to instantaneously trip the interrupter being determined by varying the differential pull of the magnet.

Another 'object of the invention is the provision of a circuit interrupter with a novel electromagnetic tri-p means comprising an armature and xed and movable core members for producing a differential pull on the armature, said movable core member being manually radjustable to vary the differential pull of the electromagnet.

Another object of the invention is the provision of a circuit interrupter with an improved electromagnet and trip device comprising an armature and a fixed core structure disposed to produce a differential pull on said armature, at least a part of the fixed core structure being adjustable relative to said armature to vary the differential -pull of the electromagnet to thereby adjust the tripping characteristic of the device.

Another object of the invention is the provision of a circuit interrupter with an electromagnetic trip device having a plurality of parallel magnetic flux circuits and adjustable means to vary the minimum current value for instantaneously tripping the interrupter.

The novel features that are considered characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to structure and operation, together with additional objects and advantages thereof, will be best understood from the following detailed description thereof when read in t a channel-shaped switch arm 23.

conjunction with the accompanying drawings, in which:

Figure 1 is a vertical sectional view ofA a circuit breaker embodying the invention;

Fig. 2 is a vertical sectional view on an enlarged scale of the trip device;

Fig. 3 is a bottom view of the electromagnetic trip devices for two adjacent poles of the interrupter with the trip device cover partly broken away to show the adjustment of the adjustable core member; and

Fig. 4 is a detailed sectional view taken on the line IV-IV of Fig. 3 and showing the method of mounting the fixed and adjustable core members.

Referring to Fig. 1 of the drawing, the circuit interrupter is of the three pole type and comprises a base l of molded insulating material,`

a cover 9 also o! molded insulating material secured to the base by means of bolts (not shown) operating mechanism indicated generally at Il and a trip device indicated generally at I3 having a separate electroresponsive tripping means for each of at least two of the poles of the interrupterl Each pole of the breaker is provided with a stationary contact I5 mounted on the lower end of a conductor I1 of the corresponding pole of the breaker, the conductor being suitably secured to the base 1.

The movable contact structures of the several poles are the saine, for which reason only the movable contact structure for the center pole has been shown and will be described. 'I'he movable contact I9 for the center pole is secured to a contact arm 2| which is loosely supported on The three channel-shaped switch arms of the three poles are rigidly fastened for unitary movement by means of a metal tie bar 25 which extends transversely across all of the poles of the breaker. The tie bar 25 is fastened to the switch arms 23 adjacent the pivoted ends thereof by means of metal straps 2l looped about the tie bar and clinched thereagainst. The tie bar is surrounded by an insulating tube 29 in order to insulate and prevent short circuiting of the poles of the interrupter. By means of the tie bar 25, al1 of the switch arms 23 are mechanically interconnected so that all three of the switch arms are operated by a single operating mechanism, which is hereinafter described. The channel-shaped switch arm 23 for the center pole is pivotally supported on a pin 3| mounted in a U-shaped main frame 33 secured to the base 'l by means of screws 35. The pivot pinl forms the' common pivot for the three mechanically connected switch arms 2l.

Each of the contact arms 2|, to which the movable contacts I9 are secured, is attached to its corresponding channel-shaped switch arm 23 by means oi' a headed adjusting bolt 31 which threadedly engages the switch arm and is locked in adjusted position by a lock nut 39. The head (not shown) ofthe bolt 31 loosely engages in a recess in the contact arm and is retained therein by an inwardly iianged portion of the contact arm. Contact pressure is provided by a compression spring 4I having one end seated in a cup-shaped spring seat 43 mounted on the switch arm 23 near its upper end and the other end bearing against the contact arm 2|. The spring 4I, in the open contact position of the switch arm 23, biases the contact arm 2| counterclockwise about the head of the bolt 31, the movement being limited by a tail portion of the contact arm 2| striking the channel-shaped switch arm 23.

The upper end oi' a iiexible conductor 45 is electrically and mechanically connected to the 4 portion of the operating lever is provided with a formed over portion 39. The portion 99 of the operating lever is for the purpose'o! engaging and resetting the carrier 59 to its latched position by movement of the operating handle as far as it will go in opening direction, in order to reset the mechanism following a tripping operation of the interrupter. The' carrier' 59 is normally restrained in latched position as shown in Fig. l by a latch 9I of the trip device, which engages a latching portion 93 of the carrier.

lower end of the contact arm 2| by means oi' a clamp 41 and the lower end of the flexible conductor is connected by means of a screw 49 to a conductor (Figs. 2, 3 and 4) which forms the energizing means for the corresponding pole of the trip device I3. The conductor 5| has its lower end connected by means of a bolt 53 and nuts 55 to a terminal connector 51. The upper end of the conductor I1 is similarly connected to a terminal connection (not shown) but similar to the connector 51. The circuits for the two outer poles are the same as the circuit for the center pole, consequently, only the circuit for the center pole will be traced. This circuit extends from the upper terminal (not shown) through the conductor I1, stationary contact i5, movable contact I9, contact arm 2|, flexible conductor 45, the trip device I3 energizing conductor 5I and the bolt 53 to the terminal connector 51.

The movable contact structure comprising the channel-shaped switch arms 23, the tie bar 25 and the contact arms 2| for all three poles of -the breaker, is adapted to be operated to open and closed positions to open and close the circuit of the interruptor by the single operating mechanism II. This operating mechanism comprises a carrier or support mem-ber 59 pivotally mounted by means of a pin 6| on the main-frame 33, a pair of toggle links 93 and 95 interconnecting the carrier 59 and the center pole switch arm 23, a U-shaped operating lever 91, overcenter springs 69 for connecting the operating lever 51 to the knee pivot pin 1| oi' the toggle links -63 and 65, and an insulating operating handle 13 for the operating lever.

The toggle link 63 is pivotally connected by means of a pin to the carrier 59, while the toggle link 65 is pivotally connected by means of a pin 11 to the channel-shaped switch arm 23 for -the center pole of the interrupter. The two legs of the operating lever 61 are disposed adjacent the inner sides of the main frame 33 and are pivotally supported on pins 19 projecting inwardly from the sides of the frame. The operating handle 13 projects through an opening 8| in the cover 9 and has an arcuate protective portion 83 having ,side portions 85 integral with the portion 93. The side portions 95 of the operating handle straddle the main frame 33 and are secured to the two legs of the operating lever 91 by means of screws 81 which threadedly engage the respective legs of the operating lever 61. The connecting The operation of the circuit interrupter operating mechanism is 'as follows: with the parts in the position shown in Fig. 1, in which position all of the contact means are closed and the carrier 59 is held in latched position; if the operating handle 13 is moved in a clockwise direction from the position shown to the opposite end of the slot 8| thus rotating the operating lever 61 in a clockwise direction, the overcenter spring 69 will snap overcenter below the linef`H-15 and cause the toggle links 63--65 to collapse, thereby causing simultaneous movement of all of the channelshaped switch arms 23 and the movable contacts I9 to the open-circuit position with a snap action. To reclose the interrupter, the operating handle is moved in the opposite direction back to the position shown in Fig. 1. This movement of the handle causes the overcenter springs to snap overcenter in the opposite direction and move the toggle to the in-toggle position to simultaneously close all of the contact means with a snap action.

The circuit interrupter is opened automatically in response to overload currents occurring in the -circuit of any pole of the breaker, by operation of the trip device I3. When the latch 9| is released by the trip device in response to an overload current, it frees the carrier 59 and permits the springs 69 to rotate the carrier in a counter-clockwise direction, thus causing collapse of the toggle 53-65 and movement of the channel shaped switch arms and the movable contacts for all three poles of the breaker to the open circuit position. The breaker cannot be closed after a tripping operation until the handle is moved to the fully opened position in order to reset and relatch the carrier 59 with the latch 9|. After the carrier has been relatcheld, the operating Vhandle may then be moved to the closed circuit position to eiect closing of the contacts.

The trip' device comprises an insulating base or support member 95 (Fig. 2) secured by means of rivets 91 or other suitable means to an angle bracket 99 which is, in turn, secured to the base 1 of the interrupter by means of the screws 49. A cover |00 of insulating material encloses the trip device. The electroresponsive tripping means for the-three poles are the same, therefore only the one for the center pole will be described. The energizing conductor 5| of each pole has a loop IBI therein, the upper leg of the loop being disposed adjacent to the base 95 and secured by means of a screw |03 to the underside of the base 95. A bimetal element |05 has one end secured by means of rivets |01 or other suitable means to the lower leg'of the loop near the base of the loop. The rivets |01 also serve to rigidly secure both the lower loop of the heater element 5| andthe bimetal element |05 to a support bracket ||9 to which is also secured the electromagnetic trip means to be later described.

The bimetal element |05 extends toward the right (Fig. 2) substantially parallel to and adjacent the lower leg of the loop |0I. The free end of the bimetal is'disposed adjacent Yan adjusting screw |05 mounted in an insulating trip bar III. The trip bar is pivotally mounted by Pins Il! -supported in the trip bar engaging bearing projec tions ||5 (Fig. 3) extending downwardly from the bracket 39 through openings (not shown) in the base 95. In the normal position of the trip bar a latch member ||2 (Fig. 2) thereon engages a latch portion ||4 of the latch member 3| to restrain this latch in latching position. When the bimetal element |05 is heated a predetermined amount in response to overload currents, it deriects downwardly to engage the adjusting vscrew |09 and rocks the trip bar clockwise about magnetic means for each of at least two of the poles of the breaker operable in response to overloads above a predetermined value, or in response to short circuits to instantaneously actuate the trip bar I to trip the interrupter. Each electromagnet comprises a iixed U-shaped core member I |1 (Figs. 2 and 3) iixedly mounted on the bracket H9, a movable U-shaped core member |23, and anarmature |25. The bracket ||9 is of non-magnetic material and is secured to spaced projections |2| (Fig. 4) integral with the base 85.

The iixed core member I |1 and the bracket Il! are rigidly mounted on the projections I 2| by means of a pair of rivets |21 and |28 which extend through openings in the bracket 99, the base 35, the bracket ||3, the xed core member ||1 v and elongated slots |29 in a movable core member |23 hereinafter described. The rivets |21 and |28 have centrally located heads |3| which abut against the lower face of the bracket H9, and the upper ends of the rivets are peened over to rigidly secure the brackets 99 and I9 to the base 55 of the trip device. The lower ends of the rivets |21 and |28 are provided with spacers |33 (Fig. 4) and |34, respectively, having reduced portions abutting against the fixed core member and firmly held thereagainst. The spacers |33 and |34 are also provided with flanged portions of a larger diameter than the width of the slots |23 of the adjustable core member to retain the movable core member |23 in place but which permit adjusting movement of the core member |23. The reduced portion of the spacer |34 on the rivet (Fig. 3) is made longer than the reduced portion of the spacer |33 to accommodate a bell crank lever |35 which is pivotally mounted thereon. One arm |31 of the bell crank lever carries a pin |39 which projects into an elongated slotin the movable core member |23 whereby movement of the lever |35 is transmitted to the movable core member |23 to move it relative to the fixed core member I|1 and also relative to the armature |25. The other arm I4| of the bell crank lever extends parallel to and just inside the bottom of the cover |00 of the trip device. The free end of the arm |4| is formed at right angles to the the bell crank |35 and the movable core member |28 in the adjusted position.

I'he armature |25 is threadedly mounted on one end of a horizontally disposed trip rod |5| and is locked in place by a set screw |53. The rod I5| is slidably mounted in a formed over portion |55 of the non-magnetic bracket Il! and the armature is held in retracted position against the bracket by means of a spring |51 compressed between the portion |55 of the bracket and a nut |53 threadedly engaging an enlarged and threaded mid portion of the rod |5I. At its right-hand Aend (Figs. 2 and 3) the rod I5| projects through an open slot |6| in the trip bar and is provided .with a head |55 which, upon movement of the armature to attracted position engages and operates the trip .bar to trip the interrupter.

The movable core member |23 is of the same` general U-shape as the fixed core member ||1 and lies iiat against the lower face of the fixed core member. When the movable core member |23 is in its position closest to the armature |25, the pole faces oi the two core members are substantially in line. The movable core member provides a path for a portion of the magnetic flux parallel to the magnetic iiux path through the xed core member. The magnetic flux through the armature |25 and both the fixed and movable core members produces a pull on the .armature tending to move it in tripping direction. The initial air gap between the armature |25 and the core member |1 is fixed but the air gap between the movable core member |23 and the armature may be varied by adjusting the position of the movable core member in the previously described manner. This varies the over-all pull on the armature produced by both core members.

The ends of the movable core member |23 are formed over at right angles and lie adjacent the ends of the fixed core member. Each formedover portion of the movable or adjustable core member |23 is provided with an arm |63 which arms extend to the right (Fig. 3) and are formed inwardly toward each other. The purpose of the arms |63 is to provide a limited path for at least .a portion of the magnetic flux and thereby provide a differential pull on the armature |25 which may be varied according to the adjust` ment of the core member I 23. The ends of the armature |25 and of the core means IIT-|23 are slanted inwardly along av line extending toward the center of U of the core means. This not only increases the pole face area and thereby provides for maximum pull on the low trip setting where the current is low but also provides a corner on the armature of relatively small area facing the parallel extensions of the core member |23 so that the lateral iiux leakage to the parallel extensions is reduced and there is a strong back pull between the inturned arms |63 on the extensions and the back of the armature |25,

Referring to the lower portion of Fig. 3, it may be seen that the instantaneous magnetic tripping means -for the right-hand pole of the interrupter is adjusted to trip in response to overload currents of high value. In this adjusted position the adjustable core member |23 has been moved to the left away from the armature and way from the pole faces of the fixed core member ||1, thus differentially increasing theA air gap of the magnet and decreasing the pull-of theA magnet for a given amount of current.- Movement of the adjustable core member |23l also moves the bent arms |53 thereof closer' to the outer side of the armature. the movement oi' the member |23 in this direction being limited by the arms |33 ensains the portion |55 of the non-magnetic bracket ||3 (Fig. 2). In this position there is a slight air gap between the arms |83 and the right-hand face of the amature |25. In the adjustment oi the trip device just described, the inward pull of the magnetic core members and |23 on the amature in tripping direction is differentially decreased and the back pull of the arms |63 is increased for a given value of overload current.. When the bell crank lever |25 is rotated clockwise from the high" position toward the low" position, the adjustable core member |23 ismoved toward the armature |25 and the arms |63 are moved away from the armature. This has the eiIect oi' decreasing the magnetic air gap and increasing the pull of'the magnet on the amature in tripping direction. Also, as the arms |83 move away from the armature, the back pull on the armature opposing tripping movement thereof decreases so that when the device is set at a position below the highest position which it is capable of adjustment, the trip device will be actuated in response to overloads of lesser magnitude to trip the interrupter.

By way oi' example, assuming that the breaker is rated to carry a normal load indenitely without actuating the trip device, the bimetal may be calibrated to trip the breaker with an inverse time delay in response to overloads of say between 100% and 300% of the normal rated current. The electromagnetic trip device may be calibrated and adjusted to instantaneously trip the breaker on overloads of, for instance, between 300% and 1100% of rated current depending on the setting of the lever |35 and the corresponding adjustment of the movable core member |23. Assuming that the trip device is calibrated to the above arbitrary values, then with the device set to the low" position, as shown in the upper part oi' Fig. 3, the electromagnet will operate to instantaneously trip the breaker upon the occurrence of an overload current of 300% or more of normal. If the lever |35 is moved to the high position to correspondingly adjust the movable core member |23, the armature will not 'be attracted until the occurrence of an overload current of 1100% of normal. By adjusting the lever |35 to any point between the maximum and minimum settings, the magnet will function to instantaneously trip the breaker in response to an overload current of a minimum value corresponding to the particular setting ofthe device. Thus the device may be set to cause instantaneous tripping of the breaker in response to a minimum overload of, for instance, 300%,A

500% or 700%, etc., up to 1100% of normal rated current. It will be understood that if the device is adjusted to instantaneously trip out at a minimum overload of, for instance, 700% of normal, the bimetal element |05 will function to trip the breaker after a time delay upon the occurrence of overload currents of any value between 100% and 700%, or between 100% and the particular minimum current value between 300% and 1100% to which the magnet is adiusted.

The time delay tripping may be adjusted by rotating the screw |09 in the trip b ar to vary the distance of theend of the screw from the bi'- metal element. Y

The high instantaneous trip setting is determined by the small air gap between the amature |25 and the arms |63 of the movable core mem-'- 8 ber.andbytheairgapbctweenthemovablecorc member |23 and the armature. The low in- .stantaneous trip setting is determined by the initial force of the spring |51. The initial tension ot the spring |51 may be varied by turning the nut |53 on the threaded portion of the rod |5| to increase or decrease the tension oi' the spring. This varies the low instantaneous tripping current required to trip the breaker when the adjustable magnet core is set to the low position.

From the foregoing description, it will be seen that the invention provides a circuit breaker having an improved trip device wherein the electromagnetic trip means is provided with two parallel magnetic flux circuits and corresponding air gaps disposed to produce a differential pull on the armature so that a wide range of adjustment is obtained, the minimum current value required to trip the breaker being selectively determined by varying the air gaps to thereby vary the differential magnetic pull on the armature.

While the invention has been disclosed in accordance with theprovisions of the patent statutes, it is to be understood that various changes in the structural details and arrangement of parts thereof may be made without departing from some oi the essential features of the invention. It is desired, therefore, that the language of the appended claims be given as reasonably broad interpretation as the prior art permits.

We claim as our invention:

1. In a circuit interrupter, tripping means including electromagnetic means comprising an armature and ilxedly mounted magnetic core means, a unitary movable magnetic core means movable relative to said amature and having portions disposed on each side of said amature to cause said electromagnet to produce a differential pull on said armature. and means tor adjusting said movable core means relative to said armature to vary the din'erential pull o! said electromagnet to thereby selectively determine the minimum value of overload current required to operate said armature.

2. In a circuit interrupter the combination of an armature operable to effect automatic operation of said interrupter, electromagnetic means comprising a fixed main pole structure cooperating with said armatm-e to denne a main air. gap an auxiliary pole structure disposed adjacent said fixed pole structure and movable relative to said nxed pole structure and said armature to vary the main magnetic air gap between said armature and said pole structure, means integral with said auxiliary pole structure providing a back drag on said armature, said integral means being movable with said auxiliary pole structure to vary the back drag on said armature, and means for adjusting said auxiliary pole structure and said integral means to selectively determine the tripping characteristic oi' said electromagnetic means.

3. In a circuit interruptor, the combination of a tripping armature, an energizing winding traversed by the current of the circuit of the interrupter, a main pole structure comprising a tlxedly mounted main pole member and a unitary auxiliary pole member cooperating with said main pole member to produce a forward pull on said armature and movable relative to said main pole member and said armature, said auxiliary pole member having a part disposed to cause said pole structure to produce a back pull on said armature, and manual means for moving said auxiliary :,soaive pole member relative to said main pole member and said armature to vary the magnetic air gap between the armature and said main pole struc. ture and to simultaneously vary the back pull of said pole structure'.

4. In a circuit interrupter, an armature, electroresponsive means comprising a pole structure including a main pole member fixedly mounted in front of said armature defining a main air gap, a unitary auxiliary pole member slidably mounted in front of said armature adjacent to said main pole member` and` movable relative thereto to vary the main air gap of said electromagnet, said auxiliary pole member having portions integral therewith disposed to cause said pole structure to produce a back pull on said armature, said pole structure being effective when energized in response to overload currents above a predetermined magnitude to operate said armature, and means for adjustably positioning said auxiliary pole member and said integral portions relative to said main pole member and said armature to selectively determine the magnitude of the current required to instantaneously operate said armature.

5. In a circuit interrupter, an armature, electromagnetic means energized in response to the current in the circuit of the interrupter for operating said armature, said electromagnetic means comprising a xedly mounted main pole member disposed in front of said armature delining a main air gap, an auxiliary pole member disposed in front of said armature and movable relative to said main pole member to vary the main air gap of the electromagnetic means, means integral with said auxiliary pole member disposed back of said armature to produce a back. drag on said armature, said integral means being movable with said auxiliary pole member to vary the back drag on said armature.

6. An electroresponsive device comprising an energizing winding, an armature cooperating therewith, a xedly mounted pole member cooperating with said armature to define a main air gap, a movably mounted auxiliary pole member cooperating with said xedly mounted pole member to produce a forward pull on said armature, means on said auxiliary pole member disposed to produce a back drag on said armature, said auxiliary pole member and said means together being movable to vary the main air gap of the electroresponsive device and to vary the back drag on said armature.

7. The combination of an energizing winding. an armature cooperating therewith, a pole structure comprising a main ixedly mounted pole structure and an auxiliary pole structure movable relative to said main pole structure, said main pole structure and said auxiliary pole structure cooperating to produce a forward pull on said armature, means mounted on and movable with said auxiliary pole structure for causing said pole structure to produce a diiferential pull on said armature, and means for selectively adjusting said auxiliary pole structure.

8. An electromagnet comprising an energizing winding, a movable armature, a fixedly mounted main pole member disposed adjacent one side of said armature, a movable pole member mounted adjacent said xedly mounted pole member and movable relative thereto, said pole members producing a forward pull on said armature, means on said movable pole member and disposed adjacent the opposite side of said armature for producing a back drag on said armature, said pole members overcoming said back drag and operating said armature when energized in response to current of a predeterminedl magnitude traversing said winding, and manual means for adjusting said movable pole member to selectively determine the magnitude of current required to operate said armature.

9. An electroresponsive device comprising an energizing winding, an armature cooperating therewith, a U-shaped pole member mounted on a fixed support 'and cooperating with said arma@ ture to define a main air gap, a slidablymounted U-shaped auxiliary pole member disposed` adjacent to said ilxedly mounted pole member and cooperating therewith to produce a forward pull on said armature, projections on said auxiliary pole member cooperating with said armature to produce a back drag thereon, said 'auxiliary pole member and said projections being movable relative to said fixedly mounted pole member and said armature to vary the main air gap of said electroresponsive device and to vary the back drag on said armature.

l0. In a circuit interrupter, an electromagnet comprising an armature, a U-shaped magnetic core operable in response to overload currents to effect operation of said armature, said core having a portion disposed in front of said armature, said U-shaped core having pole faces at the ends thereof slanting inwardly toward the center of the U and having substantially parallel extensions at the outer edges of the U extending beyond said slanting pole faces, said extensions having at the outer ends thereof inwardly directed portions extending in back of the end portions of the armature, said armature having slanting faces adjacent to and cooperating with the slanting pole faces on the core, and said slanting faces of the armature terminating in a corner at each end of the armature of small area for reducing the lateral ux leakage to said extensions and enhancing the back pull and a portion disposed back of said armature to produce a differential pull on said armature, and manual means for effecting relative movement between said armature and both portions of said core to vary the differential pull of said electromagnet.

11. In a circuit interrupter comprising operating means for causing automatic operation of said interrupter, an electromagnet comprising an armature, a core member having portions arranged in relation to said armature to provide a main air gap on one side of said armature and a secondary air gap on the opposite side of said armature, a core having an extension beyond the end of the armature and having pole portions positioned oppositely on each side of the armature to provide a main air gap on one side of the armature and a secondary air gap on the other side of the armature producing a differential pull on the armature, the pole portion for one of said air gaps being slanted at an angle relative to the i polel portion of the other air gap, said armature having a slanting end portion facing said slanting pole portion and providing an end of reduced section for reducing the lateral flux leakage to said extension of the core, producing a differential `pull on said armature, and manual means for effecting relative movement between said armature and said core member to vary both of said .air gaps to thereby vary the differential pull on u electromagnetic means operable in response to 11 overload currents to effect automatic opening of said contact means. said electromagnetic means including an amature, a xed core member, a

.movable core member cooperative with said xed core member to produce a forward pull on said armature to operatesaid amature, a portion integral with said movable core member disposed and arranged to cause said electromagnetic means to produce a differential pull on said armature, and adjusting means for selectively positioning said movable core member and said integral portion relative to said armature to vary the dinerential pull to thereby selectively determine the minimum overload current value required to operate said armature.

EURE LINDBTROM.

BmYARD Qi.

REFERENCES crrlm The foiiovving references are oi record in the ie of. this patent:

UNITED STATES PATENTS Number Name Date Re. 18,683 Y Scott Dec. 6. 1933 130.795 Edison Aug. 27, 1872 685,431 Pearson Oct. 29. 1901- 1,227,341 Stratton May 22, 1917' 2,044,133 Stolz et ai. June .18. 1936 2.047,73?l y Liugai July 14. 193#I

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