US3467794A

US3467794A - Tap changer

Info

Publication number: US3467794A
Application number: US715802A
Authority: US
Inventors: Donald J Ristuccia
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1968-03-25
Filing date: 1968-03-25
Publication date: 1969-09-16
Anticipated expiration: 1986-09-16

Description

Sept. 16, 1969 1:..1. IIRISTUCCIA TAP CHANGER Filed March 25, 1968 ATTORNEY INVENTOR Donald J.Ris1uccio BY jamdd/ i Z United States Patent Olfice 3,467,794 Patented Sept. 16, 1969 7 Claims ABSTRACT OF THE DISCLOSURE A tap changer adapted to be disposed within the casing of an electrical distribution transformer. The tap changer includes an insulating base member having first and second ends, and an opening which extends between its ends, an insulating shaft member disposed in the opening in the base member, bearing assemblies which secure the shaft in rotary relation with the base member, and a plurality of cylindrical electrical contact members fixed to the base member in spaced circumferential relation with one another, about the shaft. A movable contact assembly comprising a leaf spring and a contact shoe is secured to the shaft, with the leaf spring being bent to urge the contact shoe radially outward into contact with two adjacent cylindrical contact members. The contact shoe is shaped to provide a positive detent snap type wiping action at each tap changer position, as the shaft is rotated to select the desired operating position.

BACKGROUND OF THE INVENTION Field of the invention The invention relates in general to tap changers and more specifically to no-load tap changer switches of the type adapted to be disposed within the casting of a distribution transformer.

Description of the prior art Many distribution type transformers are purchased with a no-load tap changer connected to leads brought out from the high voltage winding of the transformer. The noload tap changer is disposed within the casing of the distribution transformer, with its switching mechanism usually being below the oil level, and with its operating handle and shaft extending above the oil level. The desired tap position is selected by the electrical utility at the time of installing the transformer, according to the requirements of its particular location in the electrical system. If the conditions change, necessitating a higher or lower tap position setting, the tap changer may be set to the desired new operating position. In most applications, however, once the tap changer position is selected at the time the transformer is installed, the tap changer will remain in that position throughout the service life of the transformer. Therefore, while it is imperative that the tap changer have the highest degree of reliability when set at a selected operating position, the portion of the tap changer which permits a new operating position to be selected, does not have to be constructed with the requirement of permitting a large plurality of tap changes.

In the prior art, tap changers for distribution transformers are commonly constructed using a complicated butterfly or roller shorting bar as the movable contact, and a coil spring and brass casting cemented into the handle, for providing a wiping snap action as the' movable contact is moved from operating position to operatingposition. The prior art tap changers are relatively costly to manufacture. It would be desirable to reduce the manufacturing cost of the tap changer, as well as its physical size, if these objectives may be achieved without deleteriously affecting the essential operating characteristics of the tap changer. These essential characteristics are a snap or detent action at each tap position, to assure positive positioning of the tap changer, a clean wiping action of the contacts as the tap changer is actuated to change taps, to clean any oxide formation from the contacts which could cause a high resistance joint, positive stops at the lowest and highest numbered tap positions, and a pressure contact between the movable contact and stationary contacts, to assure that the contact resistance will not increase throughout the life of the transformer. Further, since the operating mechanism of the tap changer may be disposed in oil, it is essential that the components be able to withstand such an environment without being deleteriously affected, such as swelling or warping, which may cause the tap changer to bind and prevent a subsequent change in its operating position.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved tap changer for distribution transformers which utilizes low cost components, arranged such that most components have more than one function. Thus, the number of components required in the tap changer have been reduced, simplifying its assembly and operation, as well as reducing its physical size and cost.

More specifically, the tap changer includes a molded insulating base member having first and second ends, a main opening extending between its ends, and a plurality of additional openings disposed about the main opening, in spaced circumferential relation with one another. The main opening has a first inside diameter which steps outwardly to larger second and third diameters adjacent its first and second ends, respectively. A wooden shaft member is utilized to actuate the tap changer, since it is the lowest cost material available. Binding of the shaft due to the wood swelling when disposed in oil is precluded by a new radial and thrust bearing assembly which secures the shaft member in the main opening of the insulating base member. The bearing assembly includes first and second washer members and a pin disposed through the wood shaft. The first washer member is a retainer washer sized to grip the shaft and to cooperate with the surface which defines the third inside diameter, and the step which joins the first and third inside diameters, to provide a radial and thrust bearing combination which permits the shaft to rotate while preventing axial movement of the shaft in a first axial direction. The second washer member is disposed on the shaft such that it cooperates with the surface which defines the second inside diameter, the step which joins the first and second inside diameters, and the pin. The pin is disposed through the shaft adjacent the second washer, with the pin, washer and step cooperating to provide a radial and thrust bearing which permits the shaft to rotate while preventing axial movement of the shaft in a direction opposite to the first axial direction. Thus, any swelling of the wood shaft will only cause it to be gripped tighter by the washer members. Since the relative motion is between the outside diameters of the washers and the insulating base member, there is no binding.- The pin disposed through the shaft also cooperates with a projecting portion of the insulating base member, to provide positive stops at the lowest and highest numbered tap positions.

A plurality of cylindrical, tubular, stationary contact members are disposed in fixed relation within the plurality of additional openings in the insulating base member which surrounds the shaft, with the stationary contact members extending outwardly from at least the first end of the insulating base member for a predetermined dimension. The electrical leads from the high voltage winding of the transformer are disposed within the openings in the stationary contact members, and fixed therein by crimping the contacts.

The movable contact assembly includes a leaf spring having first and second straight portions joined by an obtuse bend, with the first straight portion of the leaf spring being fixed to the shaft, and with the second straight portion, which extends outwardly from the shaft, having a contact shoe fastened to its outwardly extending end. The contact shoe has a substantially trapezoidal shape, with the small end of the trapezoidal shape extending partially between two adjacent stationary contacts, under the radially outward urging by the leaf spring. As the shaft is turned, the contact shoe will wipe across a stationary contact, and then snap into position between the next two adjacent stationary contact members. Thus, the required positive positioning of the tap changer is achieved, along with the necessary contact wiping action, without deleteriously affecting the operation of the tap changer.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIGURE 1 is an elevational view, partially in section, of a new and improved tap changer constructed according to the teachings of the invention;

FIG. 2 is a plan view of the tap changer shown in FIG. 1;

FIG. 3 is a perspective view of the tap changer shown in FIGS. 1 and 2; and

FIG. 4 is an elevational view illustrating a three-phase embodiment of the tap changer shown in FIGS. 1, 2 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and FIGS. 1, 2 and 3 in particular, there is shown elevational, plan and perspective views, respectively, of a new and improved noload tap changer constructed according to the teachings of the invention. As shown in FIG. 1, in which tap changer 10 is illustrated partially in section, it is adapted to be disposed within the casing 12 of an electrical distribution ty-pe transformer, with its operating mechanism disposed below the oil level 14 of the transformer, and connected to the high voltage winding of the transformer, such as via

electrical leads

16, 18 and 20.

More specfically, tap changer 10 includes an insulating base member 22 having first and

second ends

24 and 26, respectively. Insulating base member 22 should be constructed of a good electrical insulating material which may be accurately formed with close tolerances, and which will retain its strength and dimensions under oil. A molded glass polyester has been found to be excellent in all of these respects.

Base member 22 includes a main opening 28 which extends between its first and

second ends

24 and 26, best shown in FIG. 1. Main opening 28 has a first inside diameter 30, which steps outwardly to larger second and third inside diameters 32 and 34 adjacent its first and

second ends

24 and 26, respectively, providing a shoulder or step 36 between the first and second inside diameters 30 and 32, and a shoulder or step 38 between the first and third inside diameters 30 and 34.

Insulating base member 22 includes a plurality of additional openings which extend between its first and second 4

ends

24 and 26, such as

openings

40 and 42 shown in FIG. 1, for receiving the stationary contact members of the tap changer. The plurality of additional openings are disposed about the main opening 28, spaced circumferentially about an imaginary circle whose center coincides with the axis of the main opening 28. The number of these additional openings depends upon the number of operating positions the tap changer 10 is to have. As illustrated in FIGS. 1, 2 and 3, tap changer 10 has five operating positions, which requires a total of six

stationary contacts

44, 46, 48, 50, 52 and 54, and thus six openings through the base member 22.

The

stationary contacts

44, 46, 48, 50, 52 and 54 each have first and second ends, such as first and

second ends

58 and 60, respectively, on stationary contact 44, with their second ends being disposed in one of the openings which encircle the main opening 28. The stationary contacts are fixed in the openings such that their first ends extend perpendicularly outward from at least the first end 24 of the base member 22 for a predetermined direction. As will hereinafter be described, the loading on the stationary contacts will be radial, and not axial. Therefore, they may be constructed of cylindrical, tubular conductive material, such as copper tubing, having predetermined outside and inside diameters. The predetermined outside diameter is selected to provide a clearance fit with the diameter of the plurality of openings in the base member 22. The contacts are cut to length from the tubing, and they are each flared outwardly, with a suitable tool, to provide a circumferential bulge or protrusion therein a predetermined dimension from their second ends, such as the circumferential bulge 56 shown on contact 48. The stationary contact members may have their second ends placed into the plurality of openings starting at the first end 24 of the insulating base member 22, until their circumferential protrusions reach the start of its associated opening in the base 22. The dimension of the circumferential protrusion of the second end of each contact is selected such that its second end extends outwardly from its associated opening for a distance sufiicient to allow it to be flared radially outward and provide a lip, such as lip 62 on the second end 60 of contact 44, adjacent the opening, to firmly secure the stationary contacts within their associated openings.

The opening in each of the tubular stationary

electrical contacts

44, 46, 48, 50, 52 and 54 facilitates the connection of the electrical leads from the transformer winding thereto. For example, as shown in FIG. 1, the insulation may be stripped from the ends of the tap leads, they may be inserted into the opening in one of the stationary contacts starting at its second end, such as lead 16 shown entering the second end 60 of contact 44, and the wall of the tubular contact adjacent the bared end of the lead may be crimped or otherwise pressed inwardly, as shown at 64 on contact 44, to firmly grip the lead and provide an excellent electrical contact between the stationary contact member and its associated electrical lead.

An operating shaft member 70 having first and

second ends

72 and 74, respectively, is disposed in the main opening 28. Shaft member 70 is preferably constructed of wood, such as maple or birch, and it has an outside diameter selected such that its maximum diameter due to any swelling after being disposed below the oil level for a period of time, Will be less than the first inside diameter 30 of opening 28.

Shaft member 70 is fixed within opening 28, in rotary relation with base member 22, by first and

second washer members

76 and 78, respectively, and a pin member 80. The first washer member 76 is a metallic retainer washer formed of a material such as steel, which has an inside diameter sized to allow it to be pressed over the second end 74 of shaft member 70 to the desired location, and to bite into the shaft and resist relative movement between the shaft and washer when an effort is made to push it back towards the second end 74 of the shaft 70.

The first washer member 76 has an outside diameter sized to cooperate with the surface which defines the third inside diameter 34 of opening 28, to provide a radial bearing which allows the shaft 70 and washer member 76 to rotate when the shaft is turned. In other words, the outside diameter of washer member 76 is slightly less than the inside diameter 34 of the opening 28. Washer member 76 is disposed adjacent the step 38, which is the surface joining inside diameters 34 and 30, with step 38 and washer member 76 cooperating to provide a thrust bearing when an attempt is made to move shaft 70 axially in the direction from end 26 to end 24 of insulating base member 22.

The second washer member 78 may be a conventional metallic washer formed of steel, which has an inside diameter slightly larger than the outside diameter of shaft member 70, to allow it to be easily slipped over the shaft, and an outside diameter sized to cooperate with the surface which defines the second inside diameter 32 of opening 28, to provide a radial bearing which allows shaft 70 to rotate when the shaft is turned. The radial bearing at end 24 of base member 22 may initially be between the shaft 70 and the inside diameter of washer member 78, since the shaft outside diameter is slightly smaller than the inside diameter of washer member 78. However, if the outside diameter of the wood shaft 70 swells to the point where it causes the washer member 78 to be fixed to the shaft, the shaft and washer member 78 will turn together and the radial hearing will then be between the outside diameter of washer member 78 and the surface which defines inside diameter 32.

The pin member 80 is disposed through a transverse opening in shaft member 70 immediately adjacent washer member 78, such that the pin member 80 will retain washer member 78 within the second inside diameter 32. When an attempt is made to move shaft 70 axially in the direction from the first end 24 to the second end 26 of base member 22, the pin 80 functions as a thrust bearing by contacting the surface of base member 22 surrounding the opening 28, and the surface of washer member 78. Any axial force placed on washer member 78 will be resisted by step 36 between the first and second inside diameters 30 and 32 of opening 28. Therefore, the washer 78 and step 36 also cooperate to provide a thrust bearing.

The pin 80 extends outwardly from one side of the shaft for a predetermined dimension, which also allows it to cooperate with a projection 82 which projects outwardly from end 24 of base member 22, to provide a positive stop at both the lowest and highest numbered tap positions. The arcuate dimensions of projection 82, best shown in FIG. 3, and the orientation of pin member 80, are selected according to the number of tap positions tap changer is to have. As illustrated in FIG. 2, projection 82 is sized to allow five tap positions, with the first tap position starting between

contacts

46 and 48, and the fifth tap position being between

contacts

54 and 44. The projection 82 and pin 80 will not allow a tap changer position between

stationary contacts

44 and 46.

A movable contact assembly 84 is fastened to shaft 70. Movable contact assembly 84 includes a leaf spring 86 and the contact shoe 88. Leaf spring 86, which may be formed of a good spring material such as 188 stainless steel, has an obtuse bend 90 therein, forming a first straight portion 92 and a second straight portion 94. The first straight portion 92 of the leaf spring 86 is fastened to shaft 70, such as by screws 96, such that the second straight portion 94 extends outwardly from shaft 70 towards end 74 of the shaft. Leaf spring 86 is fixed to shaft 70 at a location which will cause the end of the second straight portion 94 to be adjacent the plurality of stationary contacts.

Contact shoe 88 may be formed of any suitable good electrical conductor. For example, sintered powdered copper or brass, or combinations thereof, have been found to be excellent, since the contact can be molded to its required shape. Contact shoe 88 is fixed to the outwardly extending end of the second straight portion 94 of leaf spring 86, and it is sized such that it will extend between and contact two adjacent stationary contact members, such as

contact members

48 and 50 as illustrated in FIG. 2, under a positive radial force provided by leaf spring 86. A suitable method of fastening the contact shoe 88 to the leaf spring 86, is to provide notches in the end of the leaf spring 86 and then swage the contact shoe into the notches. To provide the positive detent or snap action necessary when the tap changer reaches an operating position, as well as to wipe the contacts clean as the tap changer is actuated to change tap positions, the contact shoe 88 should have a substantially trapezoidal shape or configuration, in a plane which is substantially perpendicular to the axis of shaft 70. This trapezoidal configuration of the contact shoe 88 is best shown in FIG. 2, with the small end 98 of the trapezoidal configuration extending between the two adjacent spaced

stationary contacts

48 and 50. The large end 100 of the trapezoidal configuration should be longer than the distance between two stationary contacts, to cause the contact shoe 88 to contact the stationary contacts with its tWo surfaces which slope inwardly from the large end 100 to the small end 98.

Since the force applied to the stationary contacts is radial, the movable contact assembly 84 should be positioned axially with respect to shaft 70 such that contact shoe 88 will contact the stationary contacts as close as practical to the points at which they are secured to the base member 22, in order to insure that the stationary contacts will remain parallel with one another.

A handle 102 having a pointed end 112 may be disposed through a transverse opening near end 72 of shaft 70, and secured thereto with a pin 104. The transverse openings in shaft member 70 for the pin and the handle 102, and the location of the leaf spring 86 on shaft 70, are preselected such that handle 102 may also be used as a pointer to indicate the operating position of tap changer 10 on a suitably marked position indicating board 106. The position indicator 106 may formed of heavy cardboard, or other suitable insulating material. The position indicator 106 may be bent into a substantially Z configuration, with the end marked with the tap changer operating positions having an opening 108 which allows it to be slipped over the first end 72 of shaft 70,

and the handle 102, and with the other end having two spaced openings therein which allow it to be secured into insulating base member 22. As shown in FIGS. 1 and 2, base member 22 may have a pin 108 molded therein which fits in one of the openings in the position indicator 106, and an opening therein which coincides with the other opening in the position indicator 106 for receiving a nut and bolt combination shown generally at 110.

The construction of tap changer 10 facilitates assembly of its component parts. The

stationary contacts

44, 46, 48, 50, 52 and 54 may be inserted into the openings in the insulating base member 22, and then their second ends may be flared outwardly with a suitable tool, to secure them in their desired operating positions. The shaft 70 may have pin 80 disposed through its transverse opening, movable contact assembly 84 may be fastened thereto, and handle 102 may be inserted through its transverse opening and secured by pin 104. Washer member 78 may be dropped into place in the base member 22, or it may be disposed over the second end of shaft 70, The second end of shaft 70 may then be inserted through opening 28, starting at the first end of base member 22. Retainer washer 76 may then be forced over end 74 of shaft 70, which secures the shaft 70 in the desired rotary relation with the insulating base member 22. Position indicator 106 may then have its marked end slipped over handle 102 and shaft 70, and its other end secured to base member 22. This completes the assembly of the tap changer.

The operation of tap changer 10 requires only that the handle and shaft be turned until the pointed end 112 of the handle 102 is in the desired tap position. Turning the handle counterclockwise, looking at the tap changer in the direction shown in FIG. 2, until pin 80 hits the positive stop provided by projection 82, will cause the tap changer to be in operating position No. 1. Turning the handle and shaft clockwise until pin 80 hits the positive stop provided by the other side of the projection 82 will cause the tap changer to be in operating position No. 5. As the handle and shaft are turned, one side of the contact shoe will wipe on one of the stationary contacts until reaching the small end of its trapezoidal configuration. The small end will continue to wipe across the contact, with the outwardly extending end of the leaf spring being forced inwardly toward the shaft 70. As soon as the last portion of the small end of the contact shoe has moved over the stationary contact, the contact shoe Will snap into the next operating position between two adjacent stationary contacts, under the influence of the bent leaf spring. Once in the new operating position, the leaf spring will continue to maintain a radial urging of the contact shoe against the stationary contacts, to assure a good electrical contact.

While tap changer has been described in FIGS. 1, 2 and 3 as being single-phase, the construction of tap changer 10 allows it to be readily adapted to polyphase operation. This embodiment of the invention is shown in FIG. 4, which illustrates a three-phase tap changer 120 having

switching mechanisms

122, 124 and 126. Each of the switching

mechanisms

122, 124 and 126 are similar to the single-phase tap changer 10 hereinbefore described and shown in FIGS. 1, 2 and 3, except they have a common operating shaft 130. The three switching

mechanisms

122, 124 and 126 are mounted on a suitable mounting base 132, which is adapted for mounting within the casing of a three-phase transformer.

In summary, there has been disclosed a new and improved tap changer of the type adapted to be disposed within the casing of a distribution transformer. The tap changer has a less complicated structure than tap changers for providing the same function in the prior art, it requires less space, and its manufacturing cost is substantially less than tap changers of the prior art. Further, the disclosed tap changer construction provides all of the essential functions required of such a tap changer, including a positive detent and snap action when the tap changer reaches a new operating position, contact wiping action, and positive stops at the lowest and highest numbered operating positions.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made Without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative, and not in a limiting sense.

I claim as my invention:

1. A tap changer adapted to be disposed within the casing of an electrical distribution transformer, comprismg:

an insulating base member having first and second ends and an opening extending between its ends,

an insulating shaft member disposed in the opening in said insulating base member, and extending outwardly from at least the first end thereof,

bearing means securing said shaft member in rotary relation with said insulating base member,

a plurality of stationary cylindrical electrical contact members fixed to said insulating base member in spaced, circumferential relation with one another, about said shaft member, and extending outwardly from at least the first end of said insulating base member,

a movable contact assembly, said movable contact assembly including a bent leaf spring member having first and second straight portions, and a contact shoe fixed to the end of its second straight portion, the

first straight portion of said leaf spring member being fixed to said shaft member, with the bend in the leaf spring member being disposed to urge said contact shoe outwardly to contact two of said spaced cylindrical electrical contact members,

and handle means disposed on said shaft member for rotating said shaft member to selectively short two cylindrical electrical contact members with said contact shoe.

2. The tap changer of claim -1 wherein said shaft member=is formed of wood, and said bearing means includes first and second washer members disposed in spaced relation on said shaft member, and a pin member disposed through said shaft and extending outwardly from one side thereof, said first washer member being a retainer washer which is sized to grip said wood shaft and cooperate with said insulating base member to provide a first radial and thrust bearing combination which allows the shaft to rotate while preventing axial movement of the shaft in a first axial direction, said second washer member cooperating with said insulating base member and said pin :member to provide a second radial and thrust bearing combination which allows the shaft member to rotate while preventing axial movement of the shaft in a direction opposite to the first axial direction.

3. The tap changer of claim 2 wherein the opening which extends between the first and second ends of said insulating base member has a first inside diameter which steps outwardly to larger second and third inside diameters adjacent said first and second ends, respectively, providing a first shoulder adjacent the first end between the first and second inside diameters, and a second shoulder adjacent the second end between the first and third inside diameters, said first washer member being disposed on said shaft member immediately adjacent the second shoulder and sized to cooperate with the surface which defines the third inside diameter to provide a radial bearing, said second washer member being disposed on said shaft member immediately adjacent the first shoulder and sized to cooperate with the surface which defines the second inside diameter to provide a radial bearing, said pin member being disposed through said shaft member immediately adjacent said second washer member.

4. The tap changer of claim 2 wherein said insulating base member includes a portion which extends outwardly from its end adjacent said pin member, which cooperates with said pin member to restrict the arcuate travel of said movable contact assembly and provide positive stops at the lowest and highest numbered tap positions.

5. The tap changer of claim 1 wherein the contact shoe fixed to the second end of said leaf spring member has a substantially trapezoidal shape perpendicular to the plane of the leaf spring, with the smaller end of the trapezoidal shape being sized to extend between two adjacent cylindrical contact members for a predetermined dimension, under the urging influence of said leaf spring member, to provide a movable contact assembly having a wiping snap action as it is actuated to change tap connections.

6. The tap changer of claim 1 wherein said plurality of cylindrical electrical contact members are tubular, being adapted to receive electrical leads from the transformer winding, and to secure them by crimping the wall of the tubular contact members.

7. The tap changer of claim 1 wherein it is for polyphase operation, including one of said insulating base members, with its plurality of stationary contacts, said bearing means, and said movable contact assemblies, for each of the phases, said insulating base members being disposed in spaced relation on said insulating shaft mem ber, each fixed in rotary relation with said shaft member by one of said bearing means, one of said movable contact assemblies being fixed to said shaft member adjacent each of said insulating base members, to contact its associated stationary contact members, said handle means 9 1O actuating all of the movable contact assemblies simul- 2,680,174 6/ 1954 Foley et al. taneously to change tap positions in each phase in unison. 2,903,530 9/ 1959 Wilson ZOO-66 References Cited ROBERT K. SCHAEFER, Primary Examiner UNITED TA E ATE 5 J. R. SCOTT, Assistant Examiner 2,255,501 9/1941 Blume. 2,470,625 5/1949 Kuhn 200-4 2,547,658 4/1951 Palmeetal.