US1995894A - Circuit breaker - Google Patents

Description

March 26, 1935. J. w. McNAlRY I CIRCUIT BREAKER Filed Aug. 19, 1933 lventorz' Jacob W. Mc Nairg.

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Patented Mar. 26, 1935 UNITED STATES CIRCUIT BREAKER Jacob W. McNairy, Erie, Pa., assignor to General Electric Company, a corporation of New York Application August 19,

9 Claims.

My invention relates to circuit breakers provided with a plurality of current-carrying paths, the circuit breakers being operable to open circuit position in response to current flow, and

'5 has for an object the provision of a simple, reliable and accurate means for insuring the tripping of the circuit breaker upon the occurrence of a predetermined current flow irrespective of the division of the current between the parallel paths.

Although not limited thereto, my invention is particularly applicable to high speed circuit breakers of the type described and claimed in Tritle 81 McNairy Patent No. 1,771,935.

In the design of high speed circuit breakers arranged to carry 10,000 amperes or more, it is necessary to provide parallel paths for the load current. In each path or circuit one or more movable contacts are provided. It will be un- 20 derstood that when two or more conductors are connected in parallel to carry load current, slight differences in resistance between each path cause considerable departure from the equal division of load current between the paths. Consequently a tripping means responsive only to the load current in one branch of the circuit will not trip the circuit breaker in response to a predetermined current flowing through the circuit breaker.

In carrying out my invention in one form thereof, I provide a tripping means which will trip the circuit breaker to its open position on a predetermined load current irrespective of the current division between the parallel paths of the circuit breaker. More particularly, I provide a circuit breaker, having two parallel current paths, with a. pair of U-shaped tripping coils, each of which has its respective ends connected across a shunt provided in the respective parallel paths. The tripping coils are so arranged as to produce magnetomotive forces of the same magnitude for a given value of current.

For a more complete understanding of my invention, reference should now be had to the drawing wherein I have shown in Fig. 1 a side elevation of a circuit breaker embodying my invention, while Fig. 2 is a plan view of the circuit breaker of Fig. 1. Fig. 3 is a perspective view, partly in section, showing the mounting of the tripping coil of the circuit breaker, while Fig. 4 is a perspective view, partly in section, of a portion of a circuit breaker embodying my invention. Referring to the drawing, I have shown my 1933, Serial No. 685,859

invention in one form as applied to a circuit breaker 10 designed to carry load currents of great magnitude, for example 10,000 amperes or more. In order to carry currents of this magnitude the circuit breaker is provided with parallel paths, one path including an inductive shunt 12, flexible conductors 14 and 15, movable contacts 16 and 17, cooperating stationary contacts 18 and 19 and a conductor 20. The other parallel path of the circuit breaker includes an inductive shunt 25, flexible conductors 27 and 28, movable contacts 30 and 31, cooperating stationary contacts 32 and 33 and a conductor (not shown) similar to conductor 20. In addition there is provided an arcing contact 35 connected by a flexible conductor 36 to a stationary conductor 37 of relatively high resistance as compared with the resistance of the two parallel paths. By means of a slot (not shown) the arcing contact 35 remains in engagement with its cooperating stationary contact 39 until after the movable contacts 16, 17, 30 and 31 have been opened. The current is therefore transferred from the main contacts to the arcing contacts before the circuit is opened. Consequently, a single arc chute and blowout coil for the arcing contacts will satisfactorily take care of the arcs during circuit interruption.

It is believed that a clear understanding of the arrangement and operation of the various elements of my invention will be best understood from a complete description of the operation of the circuit breaker as a whole and therefore the construction and arrangement of certain elements only will first-be described in detail.

, Referring to Figs. 1 and 2, it will be observed that the movable contacts 16, 17, 30 and 31 are normally biased to the open circuit position by a pair of very powerful springs 45 and 46 having their upper ends connected to a stationary member 47 and their lower ends connected to the levers 48 and 49 pivotally mounted at one end on a fixed pivot pin 50, a cross member 48a. connecting the opposite ends of the levers. A pivot pin 52 forms an operating connection between the levers 48 and 49 and the levers 55 to 59, inclusive, which levers are respectively connected to the movable contact members 16, 1'7, 35, 30 and 31. As shown in Fig. 1, the circuit breaker is held in the closed circuit position by means of a holding coil 60 of a U-shaped electromagnet 61. The electromagnet is energized so as to exert an attractive effort on the cross member 48a so as to maintain the levers 48 and 49 in the position shown against their bias to the open circuit position. The holding magnet 61 is provided with an elongated rectangular slot 62 through which a pair of U-shaped tripping coils or bucking bars 65 and 66 extend. The bucking bars are so arranged as to produce a magnetomotive force in a direction to shunt the magnetic flux from the upper end of the magnet 61 across the air gap so as to release the cross members 48a and the levers 48 and 49 from the holdin magnet.

It will be remembered that the movable contacts 16 and 17 are connected in one parallel path of the circuit breaker while the movable contacts 30 and 31 are connected in the other parallel path. Assuming an equal division of load between the contacts, itwill, of course, be obvious that each contact will carry onefourth of the total load current. If the circuit breaker is rated for 10,000 amperes continuously, each contact will then carry 2500 amperes. A very slight difference in the contact pressure will greatly influence the division of current. For example, if the contact resistance of the contacts, 16 and 17 is less than the contact resistance of the contacts 30 and 31, a greater proportion of the current will flow through the contacts l6'and 1'7 thereby imposing upon them the burden of carrying more than their predetermined share of the load current. In order to minimize this tendency of unequal division of current, each parallel path is made as long as possible in the'circuit breaker itself so that the resistance of each path is relatively high with respect to the diiierence in resistance which may occur between the contacts. Consequently, a smallvariation in contact resistance has but little influence on the division of the current between the two parallel paths.

In'order to open the circuit in response to overload current, the pair of tripping coils or bucking bars 65 and 66, formed of copper, are connected respectively across the inductive shunts 12 and 25 so that each of the bars 65 and 66 produces its magnetomotive force in the same direction. The U-shaped bucking bar 65 has one'leg 67 connected by a copper conductor 70 to one side of the inductive shunt 12, while a copper conductor 72 connects the other leg '73 to the other side of the inductive shunt 12. Similarly, the copper conductors "75 and 76 on the opposite side of the holding ,coil connect the respective legs 77 and '78 of the bucking bar 66 acros's the shunt 25.

As shown, the bucking bars are identical in construction, insulation material 80 being interposed between them and between the mounting screws 81. The symmetrical tripping coil assembly is also insulated as indicated at 8a from the magnetic frame 86 of the holding coil 60. Nesting within the laterally spaced tripping bars 65 and 66 is mounted a calibrating coil 87 formed by a plurality of turns of a relatively small conductor.

The calibrating coil is fully described and claimed in my Patent No. 1,971,196 dated August 21, 1934 and assigned to the same assignee as the present invention. application the coil 87 will be described by saying that the circuit breaker may be readily calibrated in the field by the use of a storage battery, an ammeter, and a calibration chart. The calibration chart lists the related values of amperes through the calibrating coil which will produce a magnetomotive force equivalent tothe mag- Briefly, in the present netomotive forces produced by the tripping coils or bucking bars 65 and 66 for predetermined values of current. For example, 5 amperes in the calibrating coil might correspond to a current flow of 5000 amperes through each of the tripping coils. Accordingly, the circuit breaker can be calibrated at any time, the tripping springs 45 and 46 being adjustable so that the operation of the circuit breaker can be controlled as desired.

With the above understanding of the circuit breaker, the manner in which it is tripped in response to a predetermined load current irrespective of the division of current between the parallel paths will be readily understood from the description which follows.

In the operation of my invention it will be assumed that the current divides equally between the parallel paths of the circuit breaker.

The potential diiierence existing across the shunts l2 and 25 will therefore be equal, and an equal amount of current will flow through the bucking bars 65 and 66. Upon the occurrence of a predetermined steady current condition of overload, the bucking bars will produce magnetomotive forces of sufficient magnitude to shift the flux from the cross member 48a, the springs 45 and 46thereupon quickly operating the circuit breaker to the open circuit position.

If the current divides unequally between the respective parallel paths, the potential difference across one shunt will'of course be greater than the potential diiierence across the other. Consequently, a current of higher value will flow through one bucking bar than will flow through the other. The sum of the resultingmagnetomotive forces in the bars constructed in accordance with my invention however is equal to the sum of the magnetomotive forces produced by the same amount of current equally divided between the bucking bars. Accordingly, the holding flux is shifted from the cross member 48a and the levers 48 and 49 for the same current flow through'the circuit breaker irrespective of its division through the parallel paths.

If the bucking bar 65 were wound within the bucking bar 66, the sum, of the magnetomotive forces produced by them would not be proportionalto the sum of the current flowing through them. With such a construction, I have found that a predictable operation of the circuit breaker could not be obtained due to the variable division of current incident to the changing contact resistances.

Continuing with the operation of my invention, it will be remembered that the bucking bar circuits were formed of copper. In order to compensate for the increase of resistance with temperature, the resistance portions 90 and 91 of the respective shunts l2 and 25 are formed of material having a slight positive coefiicient of resistance. Consequently, the calibration of the circuit breaker does not vary with temperature because the potential difierence across the shunts increases an amount proportional to the increased resistance of the bucking bar circuits.

Under rapidly rising current conditions more current flows through the bucking coil circuits than on a slowly rising current due to the inductance of the shunts l2 and 25, the inductance of the shunt 12 for example being provided by the iron laminations 95 surrounding it. Furthermore, the division of current between the shunt l2 and the bucking bar 65 is determined by the relative inductance of the shunt circuit and the bucking bar circuit. Accordingly, iron laminations 9? can be provided for the conductors '70 and 72 forming a part of the backing bar circuit to achieve the desired tripping characteristics.

Simiiarly, the shunt 25 is provided with iron laminations 98 while the conductors '75 and '76 ior'ming-a part of the circuit for the bucking bar 66 is provided with iron laminations 79. Preferably, the inductances are so selected that the inductance vontage predominates over the resistance voltage down to relatively low rates of rise of current. In all cases however either with steady or unsteady current conditions, the bucking bars and 66 cooperate to produce magnetomotive forces proportional to the total current flowing through the circuit breaker irrespective of the relative magnitudes of current in the respective tripping bars at a given instant.

While I have shown a particular embodiment of my invention, it will be understood of course that I do not wish to be limited thereto since many modifications may be made, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

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

1. The combination with a circuit breaker provided with a plurality of contacts and parallel current paths, means for biasing said contacts to the open circuit position, of a holding magnet provided with a holding winding for maintaining said contacts in a closed circuit position against the bias of said biasing means, a series coil for each of said parallel paths, each of said series coils being arranged in such relation with said magnet as to produce a magnetomotive force in a direction to shift the magnetic flux away from the armature to release said circuit breaker for operation to its open circuit position.

2. The combination with a circuit breaker normally biased to its open position, parallel paths for current flow through said circuit breaker, holding means for maintaining said circuit breaker in its closed position, tripping means responsive to the sum of the load current through said parallel paths for rendering said holding means ineilective so as to permit high speed operation of said breaker to its open position.

3. The combination with a circuit breaker normally biased to its open position, parallel paths for current flow through said circuit breaker, holding means for maintaining said circuit breaker in its closed position, means responsive to a predetermined load current through said circuit breaker for rendering said holding means ineflective irrespective of the division of current between said parallel paths comprising a bucking bar for each current path energized in accord ance with the current through its current path, and means mounting said bucking bars in laterally-spaced relation with each other and in operative relation with said holding means.

4. A circuit breaker provided with parallel current paths comprising a plurality of stationary contacts, a plurality of movable contacts, connections for connecting said contacts in circuit with said parallel paths, an inductive shunt connected in each of said paths, biasing means for biasing said movable contacts from a closed circuit position to an open circuit position, holding means for maintaining said contacts in said closed position against said bias, tripping means responsive to the sum of the load current through said respective paths for rendering said holding means inefiective so as to permit high speed operation of said breaker to its open position, comprising apair of U-shaped bucking bars of identical construction, connections for connecting saidrespective bucking bars in parallel rela- I tion with said inductive shunts whereby said bucking bars produce magnetoniotive forces in response to the load current flowing through said inductive shunts.

5. A circuit breaker provided with parallel current paths comprising a plurality of stationary and movable contacts, connections for connecting said contacts in said parallel paths, an inductive shunt connected in each of said paths, biasing means for biasing said movable contacts from a closed circuit position to an open circuit position, holding means for maintaining said contacts in said closed position against said bias, magnetic means for maintaining said breaker in said closed position, tripping means responsive to the sum of the load current through said respective paths for rendering said holding means ineflective thereby to obtain high speed operation of said breaker to its open position, comprising a pair of U-shaped bucking bars mounted opposite one another, connections for connecting said bucking bars to said inductive shunts whereby said bucking bars produce magnetomotive forces in response to the load current flowing through said inductive shunts.

6. A circuit breaker comprising a plurality of stationary and movable contacts, conductors for connecting said contacts in parallel paths, biasing means for biasing said contacts from a closed circuit position to an open circuit position, a. U-shaped holding magnet for maintaining said contacts in their closed position, an inductive shunt connected in each of said parallel paths, said holding magnet being provided with an elongated rectangular slot, a U-shaped bucking bar having its adjoining legs extending from one side of said slot, a second bucking bar having its adjoining legs extending from the other side of said slot, means for electrically insulating and laterally spacing said bucking bars from each other and from said magnet, and connections for connecting said bucking bars to said inductive shunts.

7. The combination with a circuit breaker normally biased to its open position, parallel paths for current flow through said circuit breaker, holding means for holding said circuit breaker in its closed position, a U-shaped bucking bar for each of said paths energized by the current through its current path, means mounting said bucking bars in operative relation with said holding means and in laterally spaced relation with each other, and a calibrating coil for said circuit breaker nesting within said bucking bars.

8. The combination with a circuit breaker provided with parallel current paths, a U-shaped bucking bar for each current path, means mounting said bucking bars in laterally spaced relation with each other, the open ends of said bars being alternately disposed with respect to each other, and a calibrating coil nesting within said bucking bars.

9. A circuit breaker comprising a plurality of stationary and movable contacts, conductors for connecting said contacts in parallel paths, biasing means for biasing said contacts from a closed circuit position to an open circuit position, a U-shaped holding magnet for maintaining said contacts in their closed position, an inductive shunt connected in each of said parallel paths,

said holding magnet being provided with an elongated rectangular slot, a U-shaped bucking bar having its adjoining legs extending from one side of said slot, a second bucking bar having its adjoining legs extending from the other side of said slot, said bucking bars being electrically insulated and laterally spaced from each other and from said magnet, connections for connecting said bucking bars to said inductive shunts, and a calinetomotive force equal to and in the same direction as the magnetomotive force produced by said bucking bars for a given predetermined-load current.

- JACOB W. McNAIRY.

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