US2586326A

US2586326A - Circuit interrupter trip device

Info

Publication number: US2586326A
Application number: US579976A
Authority: US
Inventors: George G Grissinger; Lindstrom Ture; Sandin Jerome
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1945-02-27
Filing date: 1945-02-27
Publication date: 1952-02-19
Anticipated expiration: 1969-02-19

Description

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Feb- 19, 1952 G. G. GRlsslNGER ITAL CIRCUIT INTERRUPTER TRIP DEVICE Filed Feb. 27, 1945 I lum WITNESSES:

Patented Feb. 19, 1952 UNITED STATES PATENT OFFICE CIRCUIT INTERRUPTER TRIP DEVICE of Pennsylvania Application February 27, 1945, Serial No. 579,976

11 Claims. l

This invention relates to circuit interrupters and more particularly to circuit interrupters embodying electromagnetic tripping means for tripping the interrupter in response to overload currents 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 adjusting the instantaneous tripping characteristics of the interrupter.

Another object of the invention is to provide a circuit interrupter having instantaneous and time delay trip mechanism with improved adjusting means for adjusting, over a wide range, the instantaneous tripping characteristics of the interrupter.

Another object of the invention is to provide a circuit interrupter with an improved electromagnetic trip means with a Xed core and a movable armature wherein the armature is adjustable relative to the core to vary the minimum tripping current required to instantaneously trip the interrupter over a wide range of current values.

Another object of the invention is to provide a circuit interrupter with an electromagnetic trip device, a plurality of parallel magnetic ux circuits wherein one element of said electromagnetic trip device is adjustable to vary the minimum overload current value for instantaneously tripping the interrupter.

Another object of the invention is to provide a circuit interrupter with an improved electromagnetic trip device comprising a movable armature and a iixed core structure disposed t produce a differential pull on said armature, wherein the armature is adjustable relative to said core structure to Vary the differential pull of the electromagnet, the minimum tripping current required to instantaneously trip the interrupter being selectively determined by varying the diiierential pull of the electromagnet.

Another object of the invention is to provide a circuit interrupter with an improved electromagnetic trip device comprising yan armature and a fixed core structure disposed to produce a differential pull on said armature wherein means is provided for adjusting the armature relative to the fixed core to adjust the instantaneous tripping characteristics of the interrupter and the adjusting means is adjustable for Calibrating said trip device.

The novel features that are considered characteristic of the invention are set forth in particular in the appended claims. The invention 2 e itself, however, both as to structure and operation, together with additional advantages thereof. will be best understood from the following detailed description thereof when read in conjunction with the accompanying drawing, in which:

Figure l is a vertical sectional view of a circuit interrupter embodying the principles of the invention;

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

Fig. 8 is a horizontal sectional view of the electromagnetic trip devices for two adjacent poles of the interrupter taken on line III- III of Fig. 2, showing the adjustment of the movable armature and the adjustment of the armature adjusting means for one of the poles. The adjusting device for the pole shown in the upper part of the gure has been omitted to more clearly illustrate the magnet structure.

Referring to Fig. 1 of the drawing, the circuit interrupter is of the three pole type and comprises a base 1 of molded insulating material, a cover 9 also of 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 ofthe interrupter. Each pole oi the breaker'is provided with a stationary contact l5 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 same, for which reason only the movable contact structure for the center pole has been shown and will be described. The movable contact i9 for the center pole is secured to a contact arm 2| which is loosely supported on a channel-shaped switch arm 23. 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 21 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, all 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 1 by means of screws 35. The pivot pin 3| forms the common pivot for the three mechanically connected switch arms 23.

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 or" 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) of the bolt 31 loosely engages in a recess in the contact arm and is retained therein by an inwardly flanged portion of the contact arm. Contact pressure is provided by a compression spring 4| having one end seated in a cupshaped 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 4|, inthe 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 of a flexible conductor 45 is electrically and mechanically connected to the lower end of the contact arm 2| by means of a clamp 41 and the lower end of the flexible conductor is connected by means of a screw 49 to a conductor 5I 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 5l. The upper end of the conductor is similarly connected to a terminal connector. 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. 'I'his circuit extends from the upper terminal 51 through the bolt 53, the conductor |1, stationary contact l5, movable contact I9, contact arm 2 flexible conductor |15, the trip device I3 energizing conductor 5| and the lower bolt 53 to the lower 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 interrupter by the single operating mechanism This operating mechanism comprises a carrier or support member'59 pivotally mounted by means of a pin 6| on the main frame 33, a pair of toggle links 63 and 65 interconnecting the carrier 53 and the center pole switch arm 23, a U-shaped operating lever 61, overcenter springs 63 for connecting the operating lever 61 to the knee pivot pin 1| of 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 15 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 Aframe. 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 83. The side portions 85 of the Aoperating handle straddle the main frame 33 and are secured to the two legs of the operating lever 01 by means of screws 81 which threadedly engage the respective legs of the operating 'lever 61. rEhe connecting portion of the operating lever is provided with a formed over portion 83. The portion 89 ofthe operating lever is for the purpose of 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 53 is normally restrained in latched position as shown in Fig. 1 by a latch 9| of the trip device, which engages a latching portion 93 of the carrier.

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 53 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 3| thus rotating the operating lever 61 in a clockwise direction, the overcenter spring 63 will snap overcenter below the line 1|--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 intoggle position to simultaneously close all of the contact means with a snap action.

rEhe circuit interrupter is opened automatically in response to overload currents occuring in the circuit of any pole of the breaker, by operation of the trip device I3. Whenthe latch 3| is released by the trip device in Vresponse to an overload current, it frees the carrier 53 and permits the springs 69 to rotate the carrier in a counterclockwise direction, thus causing collapse of the toggleA 63--65' and `movement of the channelshaped 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 relatoh the carrier 59 with the latch 9 I.

lAfter the carrier has been relatched, the operating handle may then be moved to the closed circuit position to effect 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 ||l| therein, the upper leg of the loop being disposed adjacent to the base 95 and secured by suitable means 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| and the bimetal element |95 to a support bracket ||9 to which is also secured the electromagnetic trip'meansto be later described.

The bimetal element |05 extends toward the right (Fig. 2) substantially parallel to and adjacent the lower leg of the loop l The free end When the bimetal element is heated a predetermined amount in response to overload currents. it deects downwardly to engage the adjusting screw |09 and rocks the trip bar clockwise about its pivot H3. This movement of the trip bar disengages the latch ||2 from the latch portion H4 of the latch H6 thus freeing the latch member I6 which, in turn, frees the latch 9| causing an opening operation of the breaker mechanism in the previously described manner.

The latch member 9| is pivotally and slidably supported on a pin 92 mounted in the side members of the bracket 99. The latch H6 is pivotally supported on a pin H9 mounted in one side member of the bracket 99 and a portion |20 of the bracket formed parallel to the side members from the connecting portion of the bracket. The latch members 9| and H5 are biased to latching position by means of a spring |23 coiled about the pin 92. The bight |24 of the spring |23 engages and biases the latch 9| in a counterclockwise direction to .latching position. One end |29 of the spring |23 engages an opening in an ear |3I on the latch member HS and biases this latch to latching position. The other end |39 of the spring engages the connecting portion of the bracket 99 to provide tension for the spring. A roller latch |35 carried by the latch member H5 maintains the latch 9| in latching position as long as the latch I2 on the trip bar engages latch portion H4 and maintains the latch H6 in the position shown in Fig. 2.

Referring to Fig. 1, it will be seen that the overcenter springs 69, in the closed position of the interrupter, acting through the toggle link 63 biases the releasable member 59 in a counterclockwise direction. Movement of the member 59 is normally prevented by engagement of the latch end 93 thereof with the latch member 9|.

When the trip bar |H (Fig. 2) is actuated by the bimetal element |05 as previously described, or by the electromagnetic tripping means to be hereinafter described, the latch element ||2 is disengaged from the latch portion H4 of the latch member H6. The force of the overcenter springs 99 (Fig. 1) overcomes the spring |23 and moves the latch member H9 clockwise, which movement disengages the roller latch from the latch 9| permitting the latter to rotate in a clockwise direction to release the carrier 59 thereby effecting opening operation of the interrupter. As soon as the latch portion 93 of the carrier 59 is free of the latch 9|, the spring |23 acts to restore the latch 9| counterclockwise to its latching position and also to restore the latch H6 counterclockwise to its latching position. The counterclockwise movement of the latch members 9| and H6 is limited by portions of each latch member engaging a stop |31 bent inwardly from the bracket 99. When the latch I6 is biased against the stop |31, the latch por- ,tion l I4 thereof is slightly beyond latching position to permit movement of the latch H2 to latching position.

As previously set forth, the interruptor cannot be closed following an automatic opening operation until the carrier 59 is reset and relatched. This is accomplished by movement of the handle to the fully open position. During the clockwise or resetting movement of the carrier 59, the latch end thereof engages the latch 9| and, since this latch cannot rotate due to the engagement of the latch roller |35 therewith, it is slid downwardly between the latch roller and the limit stop |31. When the carrier 59 has moved to the left (Fig. 2) of the latch 9|, the spring |23 acts to slide the latch upwardly to latching position. The interrupter may now be closed by counterclockwise movement of the handle to the closed position.

It will be noted that the latch HB restrains the latch 9| with a large moment arm and that the latch H2 restrains the latch H6 with a large moment arm thereby effecting a large mechanical advantage and an extremely light latch load.

The trip device is also provided with electromagnetic means or at least two poles of the breaker operable in response to overload currents above a predetermined value, or in response to short circuits to instantaneously actuate the trip bar Hi to cause automatic opening of the interrupter. Each electromagnet comprises a fixed U-shaped core member H1 xedly mounted on the bracket H9 and a movable armature |25. The bracket H9 is of nonmagnetic material and is suitably secured by means of rivets |21 to a projection (not shown) molded integral with the base 95. The rivets |21 extend through openings in the base 95, the bracket H9 and the core member I |1 thereby rigidly securing the bracket and the core member H1 to the base.

The armature |25 is threadedly mounted on one end of a horizontally disposed rod l5! and is locked in place by a set screw |53. rhe rod |5| is slidably mounted in an opening in a formed over portion |55 of the nonmagnetic bracket H9 and the armature is biased to retracted position against the bracket by means of a spring |51 compressed between the portion |55 of the bracket and a collar |59 secured on the rod |51. At its right hand end (Figs. 2 and 3) the rod projects through an open slot |5| 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 interrup-ter.

The core member I1 is of laminated construction and the ends of the outer lamination |2| (Figs. 2 and 3) are bent at righi-J angles and lie adjacent the ends of the other laminations of the 'lxed core member. Projections |52 of the outer lamination |2| extend to the right (Fig. 3) and are bent inwardly toward each other to form small auxiliary pole pieces |63 which, when the magnet is energized, exert a back p-ull on the armature |25. rlhe purpose of the projection |92 and the auxiliary pole pieces |63 is to provide a limited path for at least a portion of the magnetic flux to thereby provide a differential pull on the armature |25 which may be varied according to the adjustment of the armature relative to the core member H1 and the auxiliary pole pieces The armature |25 is adjusted by means of an eccentric cam |91 (Fig. 3) which engages the back face of the armature. The cam Ii'iA .is

Referring to Fig. 3, it will be seen th-at the instantaneous trip magnet shown at the bottom of the gure is adjusted to trip the interrupter in response to overload currents of high value. In this adjusted position, the armature |25, biased by the spring |51 against the cam |01, occupies the position of greatest retraction permissible by adjustment of the lever |09. This position of the armature |25 provides the greatest air gap between the armature and the core member Il and the smallest air gaps between the armature |25 and the auxiliary pole pieces |53. llhis decreases the pull of the magnet for a given value of over- -load current and increases the value of the current required to operate the trip magnet and trip the interrupter. Movement of the adjusting lever |69 in a clockwise direction from the Fig. 3 position through the action of the cam Il moves the armature |25 toward the left thereby decreasing the main air gap. This has the effect of increasing the pull of the magnet on the armature in tripping direction and of decreasing the back drag opposing movement of the armature in tripping direction. When the device is adjusted to a position below the highest position to which it is capable of adjustment, the trip device will be actuated in response to overload currents of lesser magnitude to trip the interrupter. The adjusting lever |69 is retained in adjusted position by engagement with a series of holes |83 in the plate |13 which is spring biased toward the adjusting lever by means of a spring washer |85.

By way of 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 |69 and the corresponding adjustment of the movable armature |25. Assuming that the trip device is calibrated to the above arbitrary values, then with the device set to the low position, 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 |69 is moved to the high position to correspondingly adjust the armature |25, the armature will not be attracted until the occurrence of an overload current of 1l00% of normal. By adjusting the lever |69 to any point between the maximum and minimum settings,

the magnet will function toinstantaneouslytrip the breaker in response to an overload'current of a minimum value corresponding to the .particular setting of the device. Thus the device may be set te cause instantaneous tripping of the breaker in response to a minimum overload of, for instance, 300%, 500% or 700%, etc., up to l% 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 tripI 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 ll00% to which the magnet is adjusted.

The time delay tripping may be adjusted by rotating the screw |09 in the trip bar to vary the distance of the end of the screw from the bimetal element.

The high instantaneous trip setting is determined by the small air gap between the armature |25 and the auxiliary pole pieces |53 of the core member, and by the air gap between the core member ||1 and the armature. The low instantaneous trip setting is determined by the initial force oi' the spring |51. The initial tension of the spring 551 may be varied by backing out the set screw |53 and turning the rod |5| into or out of the armature to increase or decrease the tension of the spring. This varies the low instantaneous tripping current required to trip the breaker when the adjustable armature 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 electromagnet 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 the provisions 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 of 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:

l.. A trip device lor a circuit interrupter comprising a tripping armature, a pole structure comprising a main iixed pole member disposed in iront of said armature and a plurality of hired auxiliary pole members disposed back of said armature to cause said pole structure to produce a diierential pull on said tripping armature, an energizing winding traversed by the current of the circuit of the interrupter for energizing said pole structure, manual means for moving said tripping armature relative to said pole structure to vary the differential pull of said pole structure on said armature, a pivoted plate supporting said manual means, and means for adjustably positioning said plate to vary the initial setting of said manual means.

2. The combination of an energizing winding normally venergized by current of predetermined 2,5se,32e

value, an armature cooperating therewith, a unitary pole structure comprising a fixed main pole member and a plurality of fixed auxiliary pole members integral with said main pole member and energized in response to the abnormal currents in said winding for operating said armature, said auxiliary pole members being disposed to produce a back drag on said armature, manual means for moving said armature relative to said main pole member and said auxiliary pole members to vary the magnetic pull on said armature for operating said armature and for varying the back drag on said armature to thereby selectively determine the magnitude of overload current required to instantaneously operate said armature, a support member for said manual means, and adjusting means for adjustably positioning said support member to thereby vary the initial .setting of said manual means.

3. The combination of an energizing winding,

an armature cooperating therewith, a fixedlyl mounted pole structure comprising a main pole member4 disposed on oneside of said armature armature, manual means for selectively adjusting said armature relative to said pole structure to vary the differential pull thereof on said armature, support means for said adjusting means, and means for adjusting said support means for said adjusting means to vary the initial setting of said manual means.

4. The combination of an energizing winding normally energized by current of predetermined Value, an armature cooperating therewith, a xedly mounted pole structure comprising a main pole member disposed on one side of said armature providing a main magnetic flux circuit and a pair of spaced auxiliary pole members integral with said main pole member and disposed on the opposite side of said armature, said auxiliary pole members providing a secondary magnetic flux circuit, said pole structure producing a differential pull on said armature, manual means for adjusting said armature relative to said pole structure to thereby vary the minimum overload current value required to operate said armature, a movable support on which said manual means is mounted, and adjusting means for varying the position of said support to vary the initial setting of said manual means.

5. A circuit interrupting device comprising a winding, a movable armature attracted by the field thereof for movement to circuit interrupting position, a fixed pole structure magnetized by said winding, a part of said fixed pole structure being disposed to produce a magnetic force for restraining said armature against movement by said attraction except for field strengths above a predetermined value, manual means for adjusting said armature relative to said pole structure to selectively determine the eld strength required to effect circuit interrupting movement of said armature against said restraining force, pivot means for said manual means, and adjusting means for moving said pivot relative to said armature to calibrate said device.

6. A circuit interrupter comprising a magnet winding, a tripping armature cooperating therewith, means for producing a magnetic back drag normally restraining tripping movement of said armature by the field of said winding, manual means for adjusting said armature to selectively vary the effective magnitude of said magnetic back drag and to vary the magnitude of the attraction of said field required to operate said armature, a support member for said manual adjusting means and said support member being adjustable to vary the initial setting of said manual means.

7. A trip device for a circuit interrupter comprising an electromagnet having a movable armature operable to effect opening of said interrupter, a fixed U-shaped core structure disposed on one side of said armature to operate said ar.- mature upon energization of said electromagnet in response to overload currents of predetermined value, spaced auxiliary core members integral with said fixed core structure and disposed en the other side of said armature to restrain said armature against operation in response to overload currents below said predetermined value, manual means comprising a pivoted lever having a cam surface thereon for engaging and adjusting said larmature relative to said fixed ``.core member and said auxiliary core members vary the minimum overload` current required4 tooperate said armature, and apivoted support n 1e m b ={1` on which said lever is pivotallymounted, said pivoted support member being adjustable to calibrate said trip device. A

8. A trip device for a circuit interrupter comprising an electromagnet having an armature operable to effect opening of said interrupter, a U-shaped main core member having a pair of pole pieces disposed on one side of said armature, said main core members being operable upon energization of said electromagnet in response to overload currents above a predetermined value to operate said armature, auxiliary core members extending from the pole pieces of said main core member and disposed to restrain said armature against operation in response to overload currents below said predetermined Valuey adjusting means for said armature comprising a cam member operable to move said armature relative to said main core member and said auxiliary core member to selectively determine the minimum overload current required to operate said armature, an adjustable support member for pivotally supporting said cam member, and. means for adjusting said support member to vary the initial setting of said cam member.

9. A trio device for a circuit interrupter operable to effect opening of said interrupter comprising an armature, a U-shaped magnetic core energizable in response to overload currents to operate said armature, said magnetic core having a portion disposed in front of said armature to produce a forward pull on said armature, said U-shaped magnetic core having pole faces at the ends thereof slanting inwardly toward the center of the U and havingsubstantially parallel eX- tensions at the outer edges of the U extending beyond said slanting pole faces, the outer ends of said extensions being formed inwardly and disposed back of the end portions of said armature for producing a back pull on said armature, said armature having slanting pole faces cooperating with the slanting pole faces on said magnetic core, said slanting pole faces of the armature terminating in a corner at each end of the armature of small area for reducing the lateral flux leakage to said extensions and enhancing the back pull on said armature, manual means operable to move said armature relative to said front portion and said back portion of said magnetic core to vary both the forward pull and the back pull on said armature, a support member for said manualmeans, and said support member being adjustable to vary the initial setting of said manual means.

1'0. A trip device for a circuit interrupter operable to effect opening of said interrupter comprising an electromagnet comprising an armature, a xed core member having an extension beyond the end of said 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 said armature producing a dif- Aferential pull on said armature, the pole portion for one of said air gaps being slanted at an angle relative to the pole 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 said core, man- 'ualm'eans for adjusting said armature relative tosaid core member to vary said main and secondary 'air gaps to thereby vary the differential 'pull on sai'dfarmature, a member supporting said "manual "adjusting means, and .means for adjusting said supporting `member for' said 'manual 'adjusting' means to vary the initial setting' of 'said 'tiireproducingfa differential pull on said armature, the pole portion of one of said airgaps being slanted at an angle relative to the pole portion of the other air gap, said armature having a sianting end portion facing said slanting pole portion providing an end of reduced section for reducing the lateral iiux leakage to said extension of said core, manual means for adjusting the position of rest of said armature relative to said xed core member to simultaneously vary both said main and secondary air gaps to thereby vary the differential pull on said armature, and a member adjustably supporting said manual means.

GEORGE G. GRISSINGER.

TURE LINDSTROM.

JEROME SANDIN.

REFERENCES CITED The following references are of record in the file of this patent:

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