US3202776A - Diverter switch for on-load tapchanger with conductive rail support means - Google Patents

Description

Aug. 24, 1965 A. J. HASELFOOT ETAL 3,202,776

DIVERTER SWITCH FOR ON-LOAD TAP-CHANGER WITH CONDUCTIVE RAIL SUPPORT MEANS 4 Sheets-Sheet 1 Filed July 51, 1963 Aug. 24, 1965 A. J. HASELFOOT ETAL 3,202,776

- DIVERTER SWITCH FOR ON-LOAD TAP-CHANGER WITH CONDUCTIVE RAIL SUPPORT MEANS Filed July 31, 1963 4 Sheets-Sheet 2 g- 1955 A. J. HASELFOOT ETAL 3,202,776

DIVERTER SWITCH FOR ON-LOAD TAP-CHANGER WITH CONDUCTIVE RAIL SUPPORT MEANS Filed July 51, 1963 4 Sheets-Sheet 3 United States Patent 3,202,776 DIVERTER SWITCH FOR 0N -LOAD TAP- CHANGER WITH CONDUCTIVE RAIL SUPPORT MEANS Arthur John Haselfoot, Keats Grove, London, and Frank Richard Scrutton, Cayhall, Ilford, England, assignors to Fuller Electric Limited, London, England, a company of Great Britain Filed July 31, 1963, Ser. No. 298,825 Claims priority, application Great Britain, Aug. 1, 1%2, 29,522/ 62 11 Claims. '(Cl. 200-18) This invention relates to an improved electric load transfer switch and in particular, but not exclusively, to an improved diverter switch for an on-load tap-changer.

According to the invention an electric load transfer switch comprises a contact movable between first and second positions, means for continuously maintaining electrical contact with the movable contact throughout its movement, a first stationary contact engageable by said movable contact in said first position and for a substantial part of the movement of the cont-act adjacent to said first position, a second stationary contact engag able by said movable contact in said second position and for a substantial part of the movement of the contact adjacent to said second position, and means for ensuring that electrical contact is maintained between said movable contact and each of said stationary contacts during respective parts of the movement of the movable contact, at least one of the stationary contacts having, for at least a part of that portion thereof which is engaged by the movable contact, a resistivity which varies in the direction of movement of the movable contact.

The movable contact may be slidably mounted for reciprocating movement between .the first and second limiting positions. Alternatively, the movable contact may be rotatably mounted, the first and second positions representing points on the arcuate path traced by a portion of the contact during its rotation. 1

The means for continuously maintaining electrical contact with the movable contact may, where possible, consist of a length of flexible, electrically conducting material attached to the movable contact. Alternatively, the means may comprise resiliently mounted electrical conducting brushes or rollers on the movable contact which brushes or rollers make continuous electrical contact with a conducting rail in all positions of the movable contact.

The first and second stationary contacts may be disposed and dimensioned so that throughout a partof the movement of the movable contact between the first and the second positions, the movable contact engages both first and second stationary contacts.

The means for ensuring that electrical contact is maint-ained between the stationary and movable contacts, during movement of the latter, may comprise electrically conducting brushes or rollers on the movable contact which, during movement of the movable contact, traverse a suitably prepared track on the respective stationary contact. Preferably there would be a set of brushes or rollers for each stationary contact.

Preferably each stationary contact has for a substantial part thereof of that portion engaged by the movable contact, a resistivity which varies in the direction of movement of the movable contact.

According to a further feature of the invention a diverter switch for an on-load tap-changer comprises an electrically conducting carriage reciprocatively movable between first and second limiting positions where the carriage is positively engaged, respectively, by first and second pairs of contacts, means for maintaining electrical contact between the carriage and one contact of each pair of contacts in all positions of the carriage intermediate the contacts, a first stationary contact member electrically connected to the other contact of the first pair of contacts and independently connected electrically to the carriage when the latter is in the first limiting position and throughout an initial part of movement of the carriage from the first to the second limiting positions, a second stationary contact member electrically connected to the other contact of the second pair of contacts and independently connected electrically to the carriage when the latter is in the second limiting position and throughout the final part of the aforementioned movement, said first and second stationary contact members each being constructed so that the resistivity of that part of each contact member which is electrically connected to the carriage is difierent at different points along the respective contact member in the direction of movement of the carriage, the sum of the lengths of the aforementioned initial and final parts of the movement of the carriage between limiting positions being greater than the total distance travelled by the carnage between limiting positions. Preferably each stationary contact member extends considerably more than half way across the gap between the pairs of contacts so that for a finite central portion of the movement of the carriage between the first and second pairs of contacts the carriage is making contact with both the first and second stationary contact members.

Conveniently the stationary contact members are bars extending int-o the space between the pairs of contacts, the axes of the bars being parallel. The part of the first contact member adjacent to the first pair of contacts and the part of the second contact member adjacent to the second pair of contacts are preferably of low resistivity material, the resistivity of each contact member increasing (in one example) towards the other pair of contacts.

In use the diverter switch would be associated with a tap selector which latter would connect one stationary contact member and the respective contact of the first pair of contacts to one transformer tapping and the other stationary contact member and the respective contact of the second pair of contacts to an adjacent tapping.

In a conventional diverter switch the transition resistors which are connected or disconnected across adjacent tappings or in series with the line current by the diverter switch are independent of the switch and are often mounted separately from it.

In a diverter switch in accordance with the invention the transition resistors may be incorporated into the stationary contact members, the moving contact comprising conducting brushes or rollers sliding or rolling over the stationary contact members, each of which comprises a rod or bar formed of special resistance material. By suitably dimensioning the rods or bars, initial movement of the carriage from the first contact to the second contact may cause the carriage to engage the second stationary contact member thereby inserting between adjacent tappings a resistance equal to the entire length of the rod or bar of the second stationary con-tact member. Throughout most of the remainder of the movement of the carriage the brushes or rollers slide or roll along the rods or bars maintaining a determinable resistance, due in part to each rod or bar, between the tappings until, close to the end of the movement of the carriage, full contact is made with the other tapping as the carriage is disconnected from the rod or bar of the first stationary contact member. One advantage of such an arrangement is that the current breaking duty imposed on the switch is considerably reduced if the resistance values of the rods or bars are suitably determined to effect a continual and smooth transfer of current from one rod or bar to the other.

The resistance between two points on a given rod or bar can be made to vary directly with the distance between the points, or according to some other law, e.g., as the square root of the distance. If the resistance varies in a non-linear way in this manner it can be arranged that when contact is first made and the resistance is connected bet-ween adjacenttappings, the total value of resistance can be quite high, thus reducing the current that has to be made and broken. The initial movement of the sliding or rolling contacts can then be made to cut out high resistance values, and to insert small resistance values as far as the mid-point'of the movement, and then to insert large resistance values and cut out small resistance values. By this means another advantage is obtained, that the voltage drop in the external circuit during the process of a tap-change can be considerably reduced.

The resistivity of each rod or bar can be made to vary continuously, thus following any predetermined theoretical curve, or it can be made to vary in steps, thus approximating to such a curve by segments of straight lines. The latter method is considerably easier in practice, as the rod or bar can be composed of a number of small sections connected in series, the resistivity of each section varying progressively along the length of the rod or bar.

The moving contacts on the carriage may be copper rollers mounted one or more on each side of each fixed rod or bar. The rollers are conveniently spring-loaded thus ensuring that there is no resultant lateral force on the rod or bar. Conveniently, just before the instant of current break with either rod or bar, the current carried by the carriage to that rod or bar is tansferred from the copper'rollers to another roller which may be mounted alongside the copper ones. This additional roller makes contact with a specially shaped portion of the rod or bar and allows the copper rollers to leave the surface of the rod or bar. These current breaking parts, consisting of the additional roller and the specially shaped contact portion, are made of some arc-resistant material, e.g., copper tungsten alloy or copper tungsten carbide alloy, and the current breaking parts are easily detachable for purposes of renewal. V

Spring-loaded brushes may bear on the copper rollers, the transfer current to or from the rollers. The contacts in which the carriage is engaged in each limiting position may be spring-loaded fixed clips of a heavy section, and ensure that the normal load current and any overload or short circuit current is not carried by the rollers.

The carriage of the diverter switch may be driven from one limiting position to the other by any convenient means known per se. For example the carriage can be moved with snap-action between end limit positions by an overcentre, stored energy mechanism.

One further advantage which may be derived from this form of diverter switch is that the relative speed of movement of the current breaking parts at the moment of current break can be kept down to quite a moderate value, as the current to be broken can be limited to quite small values by the resistance of the stationary contacts in the manner described above.

One form of switch in accordance with the invention, intended as a diverter switch for an on-load tap-changer, will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 is a side elevation of the switch,

FIGURE 2 is a plan of the switch of FIGURE lrwith the contact clips removed,

FIGURE 3 is a sectional view of the switch .taken on the line III-4H of FIGURE 1, and

FIGURE 4 is aschematic circuit diagram showing the mode of connection of the diverter switch of FIGURES 1 to 3 in a typical installation. 7

The switch, in essence, comprises a carriage (generally desi nated 1) slidably mounted on a base member 2 and adapted to be moved between first and second limiting positions.

The base member 2 is formed from a metallic base plate 3 supporting two spaced parallel upstanding electrically insulating plates 4 and .5. Also extending upwardly from the base plate 3 is a spaced pair of parallel metal guideplates 6 and 7. The median plane of the insulating plates 4 and 5 is normal to the median plane of the guide plates 6 and 7. Mounted on the base plate 3 and disposed to lie between the guide plates 6 and 7 is a lower rack 8.

The carriage 1 is formed from a main electrically conducting support plate 9 on the upper surface of which are mounted two spaced roller cradles 1t) and 11 and on the lower surface of which is mounted an upper rack 12 and spaced contact-carrying arms 13 and 14. The carriage 1 is supported on the base member 2 through the engagement of wings 15 projecting outwardly from the upper rack 12 in keyways 16 and 17 in the guide plates 6 and 7. a

Rotatably mounted between the guide plates '6 and 7 to engage the upper and lower racks 8 and 12, respectively, is a spaced pair of pinions 18, the pinions 18 being mounted on a common spindle 19 extending at each end into a slot 20 formed in each of the guide plates 6 and 7. Also mounted on the spindle 2d and disposed between the pinions 18, is an operating member 21.

From the description so far given, it will be appreciated that, referring to FIGURE 1, if the operating member Zll were to be moved a given distance to the right, the effect would be to cause the pinions 18 to roll, in a clockwise direction, over the fixed lower rack 8 and, in consequence of the interengagement between the pinions 18 and the racks 8 and 12, to cause the upper rack 12 to move to the right by an amount equal to twice the aforementioned given distance. Thus it follows that movement of the carriage between the plates 4 and 5 from a first limiting position (illustrated in the drawings) adjacent to plate 4, to a second limiting position adjacent to plate 5, can readily be effected by moving the operating member 21 by a distance equal to half the distance between the limiting positions of the carriage 1. Any suitable means (not shown) may be provided for driving the pinions 18, for example, an over-centre, stored energy mechanism.

Mounted to extend between the plates 4 and 5 are spaced parallel contact members 22 and 23. Each contact member is formed from a copper end-block 24, a plurality of electrically resistive blocks 25 secured in end-to-end abutting relationship and an electrically insulating end support 26. The assemblage of blocks 25 forming each contact member is secured together by means of an insulating rod (not shown) extending axially through the contact member, on which the various resistive blocks 25 are slid, the rod being screwed into the end-block 24 at one end and into the end support 26 at the other end. The resistivities of the individual blocks 25 forming each contact member are graded, so that in moving along either contact member, the resistivity increases from block to block with the lowest resistivity block adjacent to the copper end-block 24. As can be seen from FIGURE 1, the thickness of the end supportZd is less than the thickness of the blocks 24 and 25 and intermediate the insulating end support 26 and the adjacent resistive block 25, the contact member is formed from a central tapering plate 27 and four small blocks 28 of arc-resistant material.

The contact members 22 and 23 are mounted between the plates 4 and 5 'so that the copper end-block 24- of the member 22 is adjacent to the plate 5 and the end-block 24 of the member 23 is adjacent to the plate 4-. The mode of attachment of the contact members to the plates 4 and 5 at the copperblock end of the member, is made via conducting rods 2% whichfform electrical terminals for making electrical connection to the respective contact member. V

The carriage l is electrically connected with each contact member 22 and 23 by means of the aforementioned roller cradles and 11. Each roller cradle contains a plurality of electrically-conducting roller assemblies 30, one half of the roller assemblies riding above the contact member and one half of the roller assemblies located below the contact member. Each roller assembly 30 is formed by two or more central, copper, barrel-shaped rollers 31 and two outer discs 32 of an arc-resistant material. The maximum radius of the central rollers 31 is slightly greater than the radius of the discs 32. The roller assemblies are mounted on shafts 33 supported in elongated slots 33a in the cradles and are urged together in pairs against the upper and lower surfaces of the contact member by means of tension springs 34. Good electrical contact between the individual roller assemblies 30 and the roller cradle 10 or 11, is ensured by the provision of an electrically conducting-contact blade 35 for each roller assembly 30. Each blade 35 is bolted to the respective cradle and bears against the respective roller assembly. Bolted to the top of each cradle ltl and 11 is an electrically-conducting contact blade 36.

Two pairs of electrically-conducting spring-urged clip contacts 37 are mounted on each plate 4 and 5, a first two pairs of clip contacts (i.e., pairs 37a) being positioned and dimensioned to make good electrical contact with the contact blades 36 when the carriage 1 is in one of its end positions and the other two pairs (i.e., second pairs 37b) positioned and dimensioned to make contact with the blades 36 when the carriage 1 is in the other end position. The clip contact 37b of the second pair which is mounted above the end-block 24 of contact member 22 is electrically connected to that end-block by leads 41 and similarly the clip contact 37a of the first pair which is directly above the other end-block 24 is connected to that other end block by leads 42.

Mounted on the base plate 3 are two electrically-conducting pick-up rails 38 and 39, one rail being disposed on each side of the lower rack 8. Mounted within each of the aforementioned contact-carrying arms 13 and 14, is a pair of spring-urged sliding contacts 40. The rails are disposed so that as the carriage 1 makes a stroke from one end position to the other, the contacts 40 slide along the respective pick-up rail 33 or 39 to maintain electrical contact between the carriage 1 and the rails throughout the length of the stroke.

By making the barrel-shaped rollers 31 of slightly larger maximum radius than the discs 32 of arc-resistant material, and by providing arc-resistant blocks 28 on both sides of a central tapering plate 27 at the high-resistive end of each contact member, it can readily be arranged that, as the carriage 1 moves between end positions, electrical contact is made or broken with a contact member only via discs 32 and blocks 28. This can be better appreciated from a consideration of FIGURE 2, Where itwill be seen that on contact member 23, the connection between the last resistive block 25 and the insulating end support 26 is dimensioned so that as the carriage moves to the right and the rollers reach the end of the blocks 25, the central rollers 31 on the upper surface of member 23 roll down the inclined surface of the tapered portion 2'7 and the central rollers 31 bearing against the underside of the member 23 move upwards on the inclined surface on the underside of tapered portion 27 until the discs 32 are resting on the blocks 23. As movement of the carriage 1 to the right continues, the discs 32 eventually roll oif the blocks 2% altogether, thus breaking contact between the carriage and the contact member 23, whereupon the roller assembly is supported slightly proud of the surface of the end support 26 by virtue of the shafts 33 reaching the inner ends of the slots 33a. From an examination of the carriage 10 in FIGURE 3, it will be seen that the roller assemblies 30 are not resting upon the surface of the contact member 22. Thus the make and break between the carriage 1 and either of the contact members 22 or 23 is always made via pieces of arc-re sistant material. The gradation of resistivity as a function of distance along the blocks 25 of the contact members 22 and 23, is in fact discontinuous due to each block being of substantially uniform resistivity and having a finite width, but in practice however, resistivity along the contact members can be considered as varying approximately as the nth power of the distance along the memher, it lying between 1 and t).

In the particular switch illustrated, the blocks 25 are made of graphite based materials. It will be appreciated, however, that they could be constructed of suitable metallic alloy materials or alternatively could be made from resistance wire or strip wound into helical form and embedded or potted in a suitable electrical insulating composition (e.g., a hardened synthetic resin material). To obtain the required variation of resistivity throughout the length of the contact member the wire or strip would either have to be fashioned with a varying cross-sectional area from one end to the other, or else formed from a number of separate lengths of wire or strip, each of a differing resistivity, joined together end to end. The arcresistant material we prefer to employ, is copper-tungsten alloy or copper-tungsten-carbide alloy.

One particular application of the diverter switch illustrated is for on-load tap changing and for this application the switch would be connected with a suitable tap selector switch in the manner shown in FIGURE 4.

For convenience the tap selector has been assumed to be in the star point of a transformer winding with the diverter switch forming the neutral point of the high voltage supply (hereinafter called the H.V. neutral point), but the device could just as easily have been connected in a different part of the winding. The two rails 38 and 39 are connected together to the H.V neutral point and also to the two pairs of clip contacts 37 which are not directly connected to the contact members 22 and 23. One tapping (e.g., tapping A) of the transformer winding is connected via the tap selector, to the copper end-block 24 of contact member 23 and the pair of contact clips 37 connected thereto and the other tapping (tapping B) of the transformer winding is connected to the copper end-block 24 of contact member 22 and to the contact clip pair 37 associated therewith.

Let it be assumed that the carriage is in its illustrated position so that through the carriage 1 and the contact clips 37a, the H.V. neutral point is solidly connected to tapping A, and that a switching action commences through actuation of the operating member 21.

The carriage 1 starts to move to the right so that the blades 36 are pulled from the jaws of the two pairs of clips 37a and electrical contact between the carriage 1 and the neutral point is maintained via the sliding contacts 4t) and the rails 38 and 39. During the initial part of the movement the H.V. neutral point remains connected, substantially without any series resistance, to tapping A. With further movement of carriage 1, the discs 32 of cradle 10 meet the blocks 28 of the contact member 22 and make electrical contact therewith. The length of the copper end-block 24 of the contact member 23 is dimensioned so that the leading roller assemblies of the cradle 11 roll on to the first resistive block 25 of the contact member 23 as the leading roller assemblies in the cradle 10 make contact with the blocks 28 of the contact member 22. This means that the H.V. neutral point is connected with little or no resistance to tapping A and via the entire resistance of the contact member 22, to tapping B. As the carriage 1 continues to move away from the illustrated position towards the mid-point of its travel, the total resistance in the circuit shorting the tappings A and B becomes less (since the roller assemblies 30 in cradle 10 are rolling over higher resistance blocks 25 on the contact member 22 than the roller assemblies in cradle 11 are rolling over on the contact member 23) until it reaches a minimum value when the carriage is in its mid-position. As the carriage now continues its movement, more resistance is being added due to the carriage moving over the contact member 23 than is being removed due to the carriage moving over the contact member 22 so that the total resistance in the circuit shorting the tappings A and B rises from the minimum value until it reaches a value determined by the entire resistance of the contact member 22. Further movement of the carriage towards the right now results in the roller assemblies 30 in cradle 11 leaving the arc-resistant blocks 28 of contact member 23, so that tapping A is disconnected from the HIV. neutral point, leaving tapping B connected via little or substantially no resistance. The carriage 1 completes the movement as the blades 36 become nipped in the jaws of the two pairs of contact clips 3712, thus connecting the HV. neutral point solidly to tapping B. j

The tap change is complete and the tap selector can (if a further tap-change in the same direction is desired), move the contact member 23 from its connection with tapping A and place it into connection with tapping C ready for a fresh tap change on the return movement of the carriage.

By arranging for the resistance in the shorting circuit of the tappings to be reduced between the point where contact with the contact members is made or broken and the mid-point of the switch travel, the resistive voltage drop in series with the external circuit which occurs 1 during a tap-change operation can be considerably reactual moment of make or break.

Many modifications can clearly be envisaged to the particular switch described, for example the disposition and construction of the contact members could easily be changed and the sliding contacts 453 and rails 38 and 39 could be completely dispensed with, the carriage 1 being connected directly to the requisite two sets of contact clips by a bundle of flexible conducting braid. Sliding contacts may be employed between the carriage and the contact members in place of the roller assemblies and alternative- -ly the sliding contacts 4d may be replaced by roller assemblies.

What is claimed is:

1. An electric load transfer switch, comprising housing means;

a movable contact positioned within said housing means and movable between spaced first and second positions;

moving means coupled to said movable contact for moving said movable contact between first and second positions within said housing means and along intermediate positions between said first and second positions;

a first stationary electrical Contact supported by said housing means at a position in said housing means in which said first stationary contact is engaged by said movable contact in said first position and for a po tion of said intermediate positions adjacent said first position;

a second stationary electrical contact supported by said housing means at a position in said housing means in which said second contact is engaged by said movable contact in said second position and for a portion of said intermediate positions adjacent said second position, said second stationary contact being spaced from said first stationary contact, said first and second stationary contacts and said intermediate positions and said movable cont-act being so positioned that said movable contact is disengaged from said first stationary contact and its adjacent intermediate positions when engaged with said second stationary contact and its adjacent intermediate positions and is disengaged from said second stationary contact and its adjacent intermediate positions when engaged with said first stationary contact and its adjacent intermediate positions; p means supported in said housingmeans continuously i3 maintaining electrical contact with said movable contact throughout the movement of said movable contact; and means cooperating with said movable contact and said first and second stationary contacts and said intermediate positions for maintaining electrical contact between said movable contact and each of said first and second stationary contacts at said first and second position's'and for maintaining electrical contact between said movable contact and both said first and second stationary contacts at a portion of said intermediate positions, at least one of said first and second stationary contacts having an electrical resistance varying in the direction extending between said first and second positions for at least a part of the portion thereof and of the adjacent intermediate positions engaged by saidmova-ble contact. 2. An electric load transfer switch as claimed in claim ll, further comprising a carriage slidably mounted in said housing means for movement between said first and second positions, said movable contact being supported by said carriage.

3. An electric load transfer switch as claimed in claim 1, wherein said means continuously maintaining electrical contact with said movable contact comprises an electrically conductive rail, electrically conductive means on said movable contact and resilient means urging the electrically conductive means on said movable contact into continuous electrical contact with said electrically conductive rail. 4. An electric load transfer switch as claimed in claim 1, wherein said first and second stationary electrical contacts are positioned in determined relation to each other by said housing means and have determined dimensions to provide electrical contact between said movable contact and both said first and second stationary contacts at a major portion of said intermediate positions.

5. An electric load transfer switch as claimed in claim 1, wherein said means for maintaining electrical contact between said movable contact and each of said first and second stationary contacts comprises track means on said first and second stationary contacts and electrically conductive roller means mounted on said movable contact and contacting the track means of said first stationary contact and the track means of said second stationary contact at determined times.

6. An electric load transfer switch as claimed in claim 5, wherein at least a portion of the track means of each of said first and second stationary contacts comprises arcresistant material.

7. An electric load transfer switch, comprising housing means;

an electrically conductive carriage positioned within said housing means and movable between spaced first and second positions, said carriage having electrically conductive rollers mounted thereon;

moving means coupled to said carriage for moving said carriage between first and second positions within said housing means and along intermediate positions between said first and second positions;

a first pair of substantially resilient contacts supported by said housing means at a position in said housing means in which said first pair of resilient contacts is engaged by said carriage in said first position and at said intermediate positions;

a second pair of substantially resilient contacts supported by said housing means at a position in said housing means in which said second pair of resilient contacts is'engaged by said carriage in said second position and at said intermediate positions, said second pair of resilient contacts being spaced from said first pair of resilient contacts;

means supported in said housing means continuously maintaining electrical contact with said carriage throughout the movement of said carriage;

means cooperating with said carriage and said first and second pairs of resilient contacts and said intermediate positions for maintaining electrical contact between said carriage and one of the pair of contacts of each of said first and second pairs of resilient contacts at all of said intermediate positions;

a first stationary electrical contact supported by said housing means, said first stationary electrical contact being electrically connected to the other of the pair of contacts of said first pair of resilient contacts and supported by said housing means at a position in said housing means in which said first stationary electrical contact is electrically connected via selected ones of the rollers of said carriage to said carriage in said first position and for a portion of said intermediate positions adjacent said first position; and

a second stationary electrical contact supported by said housing means, said second stationary electrical contact being electrically connected to the other of the pair of contacts of said second pair of resilient contacts and supported by said housing means at a position in said housing means in which said second stationary electrical contact is electrically connected via selected others of the rollers of said carriage to said carriage in said second position and for a portion of said intermediate positions adjacent said second position, said second stationary contact being spaced from said first stationary contact, said first and second stationary contacts and said intermediate positions and said carriage being so positioned that said carriage is disengaged from said first stationary contact and its adjacent intermediate positions when engaged with said second stationary contact and its adjacent intermediate positions and is disengaged from said second stationary contact and its adjacent intermediate positions when engaged with said first stationary contact and its adjacent intermediate positions.

8. An electric load transfer switch as claimed in claim 7 wherein each of said first and second stationary electrical contacts comprises a plurality of interconnected segments of different electrical resistance, the electrical resistance" 10 of said segments electrically connected to said carriage varying in magnitude at different ones of said intermediate positions.

9. An electric load transfer switch as claimed in claim 7, wherein each of said first and second stationary electrical contacts extends in a direction substantially toward the other for a distance greater than half of the space between said first and second pairs of resilient contacts so that at selected ones of said intermediate positions said carriage is electrically connected with both said first and second stationary electrical contacts.

10. An electric load transfer switch as claimed in claim 9, wherein a portion of said first stationary electrical contact adjacent said first pair of resilient contacts and a portion of said second stationary electrical contact adjacent said second pair of resilient contacts each comprise material of low electrical resistance, each of said first and second stationary electrical contacts having an electrical resistance which varies as each of said stationary contacts extends toward the other.

11. An electric load transfer switch as claimed in claim 10, wherein the electrical resistance of each of said first and second stationary electrical contacts increases in magnitude as it extends toward the other of said stationary electrical contacts.

References Cited by the Examiner UNITED STATES PATENTS 1,433,740 10/22 Nagel 338-97 X 1,867,147 7/32 Haller.

2,134,870 11/38 Fruth 338142 2,704,799 3/55 Kwasniewski 338126 3,030,599 4/62 Bourns et a1 338-l83 OTHER REFERENCES German application, 1,149,776, June 6, 1963.

KATHLEEN H. CLAFFY, Primary Examiner. BERNARD A. GILHEANY, Examiner.

Claims (1)

1. AN ELECTRIC LOAD TRANSFER SWITCH, COMPRISING HOUSING MEANS; A MOVABLE CONTACT POSITIONED WITHIN SAID HOUSING MEANS AND MOVABLE BETWEEN SPACED FIRST AND SECOND POSITIONS; MOVING MEANS COUPLED TO SAID MOVABLE CONTACT FOR MOVING SAID MOVABLE CONTACT BETWEEN FIRST AND SECOND POSITIONS WITHIN SAID HOUSING MEANS AND ALONG INTERMEDIATE POSITIONS BETWEEN SAID FIRST AND SECOND POSITIONS; A FIRST STATIONARY ELECTRICAL CONTACT SUPPORTED BY SAID HOUSING MEANS AT A POSITION IN SAID HOUSING MEANS IN WHICH SAID FIRST STATIONARY CONTACT IS ENGAGED BY SAID MOVABLE CONTACT IN SAID FIRST POSITION AND FOR A PORTION OF SAID INTERMEDIATE POSITIONS ADJACENT SAID FIRST POSITION; A SECOND STATIONARY ELECTRICAL CONTACT SUPPORTED BY SAID HOUSING MEANS AT A POSITION IN SAID HOUSING MEANS IN WHICH SAID SECOND CONTACT IS ENGAGED BY SAID MOVABLE CONTACT IN SAID SECOND POSITION AND FOR A PORTION OF SAID INTERMEDIATE POSITIONS ADJACENT SAID SECOND POSITION, SAID SECOND STATIONARY CONTACT BEING SPACED FROM SAID FIRST STATIONARY CONTACT, SAID FIRST AND SECOND STATIONARY CONTACTS AND SAID INTERMEDIATE POSITIONS AND SAID MOVABLE CONTACT BEING SO POSITIONED THAT SAID MOVABLE CONTACT IS DISENGAGED FROM SAID FIRST STATIONARY CONTACT IS DISENGAGED INTERMEDIATE POSITIONS WHEN ENGAGED WITH SAID SECOND STATIONARY CONTACT AND ITS ADJACENT INTERMEDIATE POSITIONS AND IS DISENGAGED FROM SAID SECOND STATIONARY CONTACT AND ITS ADJACENT INTERMEDIATE POSITIONS WHEN ENGAGED WITH SAID FIRST STATIONARY CONTACT AND ITS ADJACENT INTERMEDIATE POSITIONS; MEANS SUPPORTED IN SAID HOUSING MEANS CONTINUOUSLY MAINTAINING ELECTRICAL CONTACT WITH SAID MOVABLE CONTACT THROUGHOUT THE MOVEMENT OF SID MOVABLE CONTACT; AND MEANS COOPERATING WITH SAID MOVABLE CONTACT AND SAID FIRST AND SECOND STATIONARY CONTACTS AND SAID INTERMEDIATE POSITIONS FOR MAINTAINING ELECTRICAL CONTACT BETWEEN SAID MOVABLE CONTACT AWND EACH OF SAID FIRST AND SECOND STATIONARY CONTACTS AT SAID FIRST AND SECOND POSITIONS AND FOR MAINTAINING ELECTRICAL CONTACT BETWEEN SAID MOVABLE CONTACT AND BOTH SAID FIRST AND SECOND STATIONARY CONTACTS AT A PORTION OF SAID INTERMEDIATE POSITIONS, AT LEAST ONE OF SAID FIRST AND SECOND STATIONARY CONTACTS HAVING AN ELECTRICAL RESISTANCE VARYING IN THE DIRECTION EXTENDING BETWEEN SAID FIRST AND SECOND POSITIONS FOR AT LEAST A PART OF THE PORTION THEREOF AND OF THE ADJACENT INTERMEDIATE POSITIONS ENGAGED BY SAID MOVABLE CONTACT.

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