US3480897A

US3480897A - Adjustable sliding brush transformer and method of producing same

Info

Publication number: US3480897A
Application number: US665402A
Authority: US
Inventors: Henry J Helberg; Harry J Proxmire
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1967-09-05
Filing date: 1967-09-05
Publication date: 1969-11-25
Anticipated expiration: 1986-11-25

Description

Nov. 25, 1969 J HELBERG ET AL 3,480,897

ADJUSTABLE SLIDING BRUSH TRANSFORMER AND METHOD OF PRODUCING SAME Filed Sept. 5', 1967 2 Sheets-Sheet 1 ii/z ills g M INVENTORS.

flttorwe Nov. 25, 1969 H. J. HELBERG ET 3,430,897

ADJUSTABLE SLIDING BRUSH TRANSFORMER AND METHOD OF PRODUCING SAME Filed Sept. 5, 1967 2 Sheets-Sheet E ,-,ZZ;Z;Z?Z" Z a g 7 7 93 f w ,1:

z, 1 I I 1 1 F j /00 .98 Z, /4 v 12 '2; I lll llllllllllll mllliiiuullml lmllllllillllllllllllllillflllllllllllllllh Illlnhllllllllllllllll"llllIIllllllllll 2 1N VENTORB Henry J." He/bpr' BY lzsr'ry j/ 'vgw/i United States Patent 0 3,480,897 ADJUSTABLE SLIDING BRUSH TRANSFORMER AND METHOD OF PRODUCING SAME Henry J. Helberg and Harry J. Proxmire, Fort Wayne,

Ind., assignors to General Electric Company, a corporation of New York Filed Sept. 5, 1967, Ser. No. 665,402 Int. Cl. H01f 29/06 U.S. Cl. 336-149 6 Claims ABSTRACT OF THE DISCLOSURE An adjustable sliding brush transformer having a backing strip of interstitial or permeable insulating material sandwiched between the contact surface portion of the winding and the core insulating member. The electrical contact surface portion of the winding is bonded to the core insulating member by a cured thermosetting resin carried in the backing strip. The backing strip is disposed along the outer periphery of the core insulating member adjacent to the contact surface portion of the winding. In the process of manufacture the coil turns are wound over the backing strip and around the insulating member with the thermosetting resin impregnant being in a semicured and tacky condition. Preferably, the coil turn portions of the winding which form the contact surface portion are embedded in the backing strip and when cured, the thermosetting resin firmly bonds the contact surface portion of the winding to the core insulating member.

BACKGROUND OF THE INVENTION This invention relates generally to adjustable sliding brush transformers, and more particularly to adjustable sliding brush transformers having improved brush tracks and methods of producing same.

Variable transformers of the sliding brush variety typically have an autotransformer coil wound as a toroid upon an annular magnetic core with a surface portion of the coil forming a brush track. A brush holder, carrying one or more electrical brushes is arranged to permit the brushes to slide across and make electrical contact with the brush track. The output voltage of the transformer is varied by changing the position of the brushes with respectto the brush track, and hence it is frequently necessary to slide the brushes along the track.

One source of trouble with sliding brush transformers is in the construction of the brush track. It often happens that one or more individual turns of the coil forming the brush track is either depressed below or raised above the plane of the remainder of the brush track, resulting in a rough, uneven and/or irregular track. Under this condition, the brushes may catch at the uneven areas and either break off or be pulled from their brush holder. Further, an uneven track may cause the brushes to rotate in their brush holder and, if the brushes are then carrying load current, arcing may occur if a brush momentarily loses contact with the track. This may result in damage to the brushes, the brush holder or the conductor turns. At the very least, an uneven brush track causes hot spots, uneven and elevated temperatures and possible burning andex'cessive wearing of the brushes. It should be understood therefore that the rejection rate of the variable or sliding brush transformers due to brush track defects is quite high.

One cause of uneven or irregular brush tracks in sliding brush transformers is a lack of adequate backing or support behind the coil or winding turns at the track area. Without a firm and uniform backing at this area, spaces may occur behind the conductor turns, permitting a turn to fall below the track level, or conversely forcing an adjacent turn above the level of the track. It has been suggested to incorporate a mat or blanket behind the conductor turns and to dip the assembly in varnish or the like to provide backing support for the brush track. This approach has not been successful since the varnish often does not fully penetrate into the mat or blanket and hence some of the turns still are able to move away from the plane or surface of the track. Further, even when turns were pounded or depressed into the mat or blanket, the finished track surface was often irregular since the resilient coil turns would spring back away from the mat and leave spaces or gaps therebehind.

Another prior art approach involves the application of an adhesive coating on the core at the track area between the core and the coil. This technique is not satis factory since during fabrication, the assembly is messy and hence difficult to handle after the coating layer is applied. Also, when the winding is placed on the core, there is a tendency for the coating layer to extrude upwardly between adjacent turns of the winding since it is in a relatively fluid state at this time. Certain materials, such as polyester resins, cannot be used as they would contaminate the carbon brushes upon contact. Further if a polyester resin were used and exposed above the track surface, it would become soft and flow when subjected to the normal temperature of the transformer.

Accordingly, it is a general object of the present invention to provide an improved sliding brush type of variable transformer which has a stably bonded brush track and a method manufacturing such a variable transformer.

It is a more specific object of the present invention to provide an improved variable transformer and method of producing the same wherein the individual coil turns comprising the brush track are firmly held in place to form an improved and stable brush track.

It is another object of the present invention to provide an improved sliding brush transformer and method of producing same wherein arcing along the brush track due to displaced turns is effectively minimized.

SUMMARY OF THE INVENTION In carrying out the objects of this invention in one form, we provide an improved variable transformer of the sliding brush type that includes an annular core of magnetic material with a shell of insulating material disposed on the core. A carrier or backing strip of interstitial or permeable insulating material is disposed circumferentially around at least a portion of the shell, with a thermosetting resin in a semicured or B state in the carrier strip. A winding of successively disposed adjacent coil turns is disposed on the shell with portions of a majority of the coil turns being bonded thereto by the backing strip and, if desired, being partially embedded in the carrier strip. Segments of these coil turn portions are removed and provide a flat electrical contact surface or brush track. In addition, a brush holder carrying one or more electrical brushes is disposed for sliding electrical contact with the fiat electrical contact surface or brush track.

In accordance with another aspect of the present invention, in one form, we provide an improved method of fabricating or producing an adjustable sliding brush transformer that comprises assembling an annular magnetic core in a shell, wrapping a backing strip having a thermosetting resin in a semicured state impregnated therein and winding or placing a plurality of adjacent turns on the shell to form a toroidal winding thereon with portions of the turns overlying the backing strip. At this time, a compressive force is applied to the turn portions in order to insure that these turn portions are straight and in contact with the backing strip, and, if desired, to push the turn portions radially inwardly and at least partially embed them into the backing strip. The entire assembly is then cured at an elevated temperature to harden the polyester resin and thereby structurally bond the turn portions to the backing strip and the backing strip to the outer peripheral surface of the shell. Outer segments of the turn portions are then removed thereby leaving a generally flat surface on each outer turn portion, with these flat surfaces being at substantially the same distance from the axial center of the core in order to provide a flat, uniform and regular brush track for contact with the electrical brushes subsequently assembled in a brush holder and slidably mounted for contact with the 'brush track.

A primary advantage of the briefly described transformer and method of fabricating the same is that the backing provided for the coil turns at the brush track area rigidly secures the coil turns in place to provide a stable brush track. In this regard, a more solid backing is not necessary, since the coil turns, when compressed into the backing strip, will not spring back. This is due to the tacky or adhesive nature of the backing strip, with its impregnated polyester resin in a semicured state. Thus, the tacky or adhesive backing strip facilitates the fabrication of the transformer as it retains the compressed coil turns tightly against the transformer shell when they are pressed thereagainst. Furthermore, the backing strip acts as a carrier for the polyester resin, substantially preventing it from being squeezed out between the coil turns when the coil turns are compressed, and once the ployester resin is cured, the coil turns are structurally bonded in place, thereby preventing any of the turns from becoming loose and spoiling the uniformity of the brush track.

The subject matter which we regard as our invention is set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may be best understood by referring to the following description taken in connection with the accompanying drawings.

Brief description of the drawings FIGURE 1 is a perspective view of an adjustable sliding brush transformer embodying one form of the present invention;

FIGURE 2 is an enlarged partial sectional view taken substantially on the plane of the line 2-2 of FIG- URE 1;

FIGURE 3 is an exploded perspective view showing a first step in the process of assembling certain components of the transformer assembly in accordance with the method of fabricating the transformer illustrated in FIG- URE 1;

FIGURE 4 is a front elevational view showing another step in the process of fabricating the transformer illustrated in FIGURE 1 wherein the turn portions are pressed 7 Description of the preferred embodiment Referring now to the drawings, the improved adjustable transformer, generally denoted by reference numeral 10, comprises a core 12 of magnetic material substantially encased in an insulative shell denoted generally by reference numeral 14. The shell 14 includes upper and lower substantially

identical shell halves

16 and 18 respectively which are of a suitable insulating material such as phenolic resin or the like. Wound around the core 12 and the shell 14 of the transformer is a winding 20 of insulated wire. A portion of the surface of the winding has the insulation and some copper removed after the turns are flattened in a manner to be described hereinafter to form a generally flat annular brush track generally denoted by reference numeral 22. The core and coil assembly of the transformer 10 is bonded within a wall 25 on a base plate 27 with a resinous material 29 in the manner taught in U.S. patent application Ser. No. 582,024, filed Sept. 26, 1966 now Patent No. 3,374,452, by Winston R. Judd and assigned to the present assignee.

In the preferred embodiment, a 'brush holder assembly 28 is attached to a rotatable plate-like member 30, and a brush holder 32 urges a plurality of brush segments 34 into electrical contact with the track 22. The plate-like member 30 has a plurality of ventilation openings 36 therein, and a control knob 38 is provided for manually setting the location of the brush with respect to the brush track 22.

When the transformer is used to supply current to a load, the output current flows from the winding 20 through the brush segments 34, a brush supporting plate 40 attached to the brush holder 32, the brush holder 32 itself, and a conducting ring 42 as disclosed and claimed in U.S. Patent No. 3,160,841 owned by the assignee of the present application. A brush mounted in a housing mem ber 44 bears against the conducting ring 42 and is connected to an output terminal 46 mounted on a terminal plate 48, the terminal plate 48 being in turn mounted on the base 27. A plurality of

terminals

50, 52, 54 and 56 on the terminal plate 48 enable a variety of output voltages to be selected in order to provide a desired range of voltage for application to a load.

Referring now specifically to FIGURE 2, it will be seen that the shell halves 16 and 18 comprising the shell 14 are substantially identical, with each including an annular outer

peripheral surface

60 and 62 respectively. On this surface there is mounted a carrier or backing strip 65, the backing strip being disposed circumferentially around at least a portion of the shell 14. The backing strip 65 used in the exemplification is a glass fibre strip with polyester resin initially in a semicured or B state impregnated therein. However, it will be appreciated that other suitable interstitial insulative materials may be used as the backing strip such as for example asbestos, cotton, silk, rayon, nylon, and the like formed with interstices for containing the resin. A principal requirement for the backing strip is that it be electrically insulative and be capable of being impregnated and thereby carry the bonding material which in the exemplification as mentioned above was a polyester resin. Such backing strips are commercially available, and one particular such strip is produced under'the trademark Fusa-Fab by the General Electric Company Other suitable thermosetting resins may be used, of course, instead of the polyester resins, such as the acrylic resins, epoxy resins, ethylene propylene resins, polyurethanes, and oleoresins.

The winding 2 0;is, ofcourse, composed of a plurality of turns, each indicated by reference numeral 70, and as the turns are disposedaround the shell. 14, a portion 72 of each of the turns 70, overlies the backing strip 65 in the area of the brush track 22. Thus, as will be seen in FIGURE 2 in particular, the brush track 22 provided by the present invention. is unusually rigid and stable inasmuch as the turn portions 72 are. structurally bonded to the backing strip 65, and in the exemplification are in, fact partially embedded in the backing strip; However, it will be appreciated that it is not necessary for theturn portions 72 to be. embedded in. the strip but that all tur'n's merely must be in adhesive or. bonding contact with the backing strip 65 in order to provide the stable brush track provided by the presentinvention. Also aswillbe seen in FIGURE .2, each turn portion 72 has a segmen t removed, with the segments including at least the Wire insulation and preferably a portion of the turns themselves in order to provide a flat electrical contact surface for the brush track 22.

Referring now to FIGURES 3-7 inclusive, we will describe the preferred method of fabricating or producing the improved adjustable transformer shown in FIG- URES 1 and 2. Referring initially to FIGURE 3, it will be seen that the shell halves 16. and 18 are assembled on the core 12, thereby substantially completely enclosing the core 12. While We have shown, in the exemplification, the use of the shell halves 16 and 18 for insulating the core, it will be appreciated that other suitable insulating means might be used such as, for example, layer wound insulation of various types or the like. In addition, it will be appreciated that an insulation layer may be provided on the core 12 by means of a carrier strip such as the strip 65 which could be disposed on the core 12 between the core 12 and the winding 20. With the shell halves 16 and 18 assembled on the core 12, the backing or carrier strip 65 is disposed in substantial wrapping engagement about the

peripheral surfaces

60 and 62 of the shell halves, substantially encircling the shell halves. The strip 65 is retained in its position on the shell halves by a suitable retaining means such as adhesive strips or the like. With the backing strip 65 disposed on the shell 14, and With the polyester resin impregnated therein being in a semicured, tacky state, the toroidal winding 20 is placed on the shell by winding a plurality of adjacent turns 70 thereon, with portions 72 of the turns overlying the backing strip 65.

In order to produce a stable, flat brush track 22 on the core 12, as shown in FIGURE 4, the core with the shell 14 and toroidal winding 20 produced thereon is disposed on a rotatable table 80 having an upstanding mandrel or mounting post 82 thereon. The table 80 is turned by a suitable means such as the worm gear 84 which meshes with a gear 86 carried by the table 80 in order to present all the turn portions 72 to a hammering or pressing device 87 located adjacent the table 80. As the turn portions 72 are moved past the hammering or pressing device 87, the device is rapidly vibrated in order to straighten the turn portions 72 and to press them against the backing strip 65. If desired, the hammering device 87 may be driven with sufiicient force to embed portions of the turn portions 72 into the backing strip 65. The hammering device used in actual practice is a pneumatic hammer that vibrates at a relatively high rate.

In the exemplified process, when all the turn portions 72 comprising the brush track 22 have been subjected to the hammering or pressing operation, the core and coil is dipped into a varnish tank 88 as shown in FIGURE 5 in order to fill the spaces left between the adjacent turns with varnish. A varnish filler 90 is maintained in the tank 88 and the core and coil is dipped into the varnish filler 90 by means of a supporting arrangement 92, with the core and coil being left in the tank for approximately seconds. When the dipped core and coil is removed from the tank 88 .it is taken to an oven 94 and placed on a table 96 therein in order to cure the varnish as well as to cure the semicured polyester resin carried by the strip 65. It will be seen in FIGURE 6 that the oven 94 is a generally conventional industrial type oven such as the type using electric resistance heating units 98 for heating purposes. It will be understood, however, that any suitable heating means may be used to cure the varnish and the polyester resin. In actual practice, the varnish is cured by permitting the core and coil assembly to remain in the oven for approximately six hours at approximately 120 degrees centigrade. However, the polyester resin will cure in a much shorter time; for example, one type of polyester resin that we have used will cure to a tough, rigid mass in approximately one to one and a half hours at 125 degrees centigrade. It will be appreciated that polyester resins may be cured by heat alone, or by a combination of heat and pressure. Therefore, if desired, the step of applying varnish to the assembly may be omitted, and the polyester resin cured to structurally bond the turn portions 72 to the backing strip 65, and the backing strip 65 to the shell 14.

When the polyester resin has been cured and the turn portions 72 structurally bonded to the backing strip 65, the core and coil is placed on a rotatable mandrel 100 at a grinding station 101 and the core and coil rotated by shaft 102. A grinding head 104 carried on rotatable shaft 106 is actuated to grind off or remove outer segments of all of the turn portions 72 in the area of the brush track 22 to produce a flattened surface. At this time, the core and coil is assembled with the base plate 27, and the remaining components of the transformer 10 are assembled thereon to produce the final transformer unit 10. However, if desired, the core and coil may be subjected to additional processing steps, e.g., (1) an additional varnish dip and cure after the first cure (to insure a water seal, as certain types of wire enamels are sensitive to attack by water); and/or (2) plating the coil turns at the brush track 22 with gold or other suitable electrical contacting surface platings to provide good electrical contact w 1th the brush segments 34 and to prevent excessive wearlng of the turns 70. I

In the actual reduction to practice of our invention, 1n certain units all of the above described steps are carried out, while on other units some of the steps are omitted. In practice, we use a backing strip that is approximately 0.03 of an inch in thickness, but we may, of course, use thicker or thinner strips so long as they carry a resin composition.

From the aforegoing description, it will be appreciated that we have provided an improved variable transformer of the adjustable brush type having firmly bonded turns that are not readily displaced during manufacture or operation. While we have illustrated and described a particular exemplification of the invention, it will be apparent that many modifications will occur to those skilled in the art. Thus, this invention is not intended to be limited to the particular embodiment shown and it is intended by the appended claims to cover all modifications that come within the true spirit and scope of the invention.

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

1. An adjustable sliding brush transformer comprising: an annular core of magnetic material; a shell of insulating material disposed on said annular core; a backing strip of interstitial insulating material disposed circumferentially around at least a portion of said shell, said backing strip having a cured thermosetting resin impregnated therein; a winding of successively disposed adjacent wire turns wound on said shell and overlying said backing strip, .said winding having a flat electrical contact surface portion, and said cured thermosetting resin in said backing strip bonding the electrical contact surface portion to said shell; and electrical brush means for sliding electrical contact with said flat electrical contact surface.

2. The adjustable sliding brush transformer of claim 1 wherein said backing strip comprises a glass fibre strip wrappingly engaging an outer surface of said shell and wherein said thermosetting resin is a polyester that is initially in a semicured state prior to Winding of said wire turns on said shell.

3. In an adjustable sliding brush transformer having an annular core of magnetic material, an insulating means covering substantially said magnetic core, a toroidal winding including Winding turns wound on said insulating means and magnetic core, and a movable brush means for making electrical contact with said toroidal winding, the improvement comprising: an electrical brush track formed by a contact surface portion of said winding, a permeable backing strip disposed between the insulating means and said winding adjacent to said contact surface portion, said permeable backing strip having cured thermosetting resin impregnated therein, said contact surface portion being 7 a partially embedded in said backing strip, and said backing strip bonding said contact surface portion to said insulating means.

4. The adjustable sliding brush transformer of claim 3 wherein said thermosetting resin impregnated in said backing strip is in a semicured state when said toroidal winding is placed over said insulating means on said magnetic core.

5. A method of producing a variable transformer having an annular core of magnetic material, a toroidal winding carried on the core and electrical brush means disposed for sliding contact with the toroidal winding, the methodcomprising the steps of: assembling on the annular core at least one insulating member having an outer annular surface, applying a backing strip impregnated with a thermosetting resin on the outer annular surface of said insulating member in substantial wrapping engagement therewith, said thermosetting resin being in a semicured state; placing the toroidal winding on said insulating member and annular core by winding a plurality of adjacent turns thereon with portions of the turns overlying the backing strip; curing the thermosetting resin;

and forming a contact surface on said toroidal winding to provide a brush track.

6. The method of claim 5 including the step of embedding the turn portions at least partially into said backing strip by applying a compressive force during winding thereby to force the turn portions radially inwardly into the backing strip.

References Cited UNITED STATES PATENTS 1,402,587 1/1922 Evershed 338-474- XR 2,147,824 2/1939 Webb 336205 XR 2,246,159 6/ 1941 Work et al. r 336205 XR 2,949,592 8/1960 Smiley 336-448 3,316,517 4/1967 Ellin 33148 XR 3,339,162 9/1967 Burnsteel et al 336205' 3,396,356 8/1968 Whipple 336-205 XR LEWIS H. MYERS, Primary Examiner 20 T. .T. KOZMA, Assistant Examiner US. Cl. X.R.