US3896370A

US3896370A - Variable transformer

Info

Publication number: US3896370A
Application number: US421330A
Authority: US
Inventors: August Frederick Manz
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1973-12-03
Filing date: 1973-12-03
Publication date: 1975-07-22
Anticipated expiration: 1992-07-22
Also published as: FR2313754B1; DE2524325A1; FR2313754A1

Abstract

The transformer device comprises a core having a coil winding wrapped about the core making a succession of turns. A predetermined area of each turn is divided into a conductive and a nonconductive surface segment respectively. At least two electrically conductive members are arranged for sliding engagement against the predetermined area of each turn along separate paths transverse to the direction of the winding with each path including one surface segment of each of the predetermined areas. The surface segments are arranged to alternate forming a checkered geometric pattern.

Description

United States Patent Manz July 22, 1975 VARIABLE TRANSFORMER 3,376,535 4/1968 Gout 336/149 x 3,478, ll 1969 P 336 149 [75] Inventor: August Frederick Manz, Union, NJ. 29 mxmlre I [73] Assignee: Union Carbide Corporation, New Primary Examiner-A. D. Pellinen York, NY. Attorney, Agent, or Firm-E. Lieberstein [22] Filed: Dec. 3, 1973 [57] ABSTRACT PP 421,330 The transformer device comprises a core having a coil winding wrapped about the core making a succession [52] Us. Cl 323/435 R; 336/148. 336/149 of turns. A predetermined area of each turn is divided 51 Int. Cl. dost 3/04 a conductive and "Onconductive surface [58] Field of Search 323/435 R 44 336M425 ment respectively. At least two electrically conductive 336/149 members are arranged for sliding engagement against the predetermined area of each turn along separate [56] References Cited paths transverse to the direction of the winding with each path including one surface segment of each of UNITED STATES PATENTS the predetermined areas. The surface segments are ar- 2'832036 4/1958 Cutler etal' 323/435 R ranged to alternate forming a checkered geometric 3,090,906 5/[963 .lablonsky .r 323/4335 R pattern 3,293,582 l2/l966 Schmitt A t A 336/l49 3,346,804 /1967 Ryu 336/148 X 8 Claims, 7 Drawing Figures To load //i 44 V 42 i J l 32 l/ r as N c C I brush travel PATENTEDJUL 22 ms SHEET FIG.

FIG. 28

PATENTEDJUL 2 2 ms end 1 VARIABLE TRANSFORMER Tins invention relates generally to adjustable electromag *tic devices such as transformers.

BACKGROUND In general, a transformer is a structural arrangement composed of usually two coil windings wrapped a given number of turns about a ferromagnetic core. An alternating current in one winding induces an alternating emf in the other or secondary winding. The output of the transformer is taken from across all or a portion of the secondary coil. One conventional method of varying the transformer output is to couple the transformer output to an electrically conductive mechanism, such as a wiper or commutating brush, and to slide such mechanism along the surface of the secondary coil in a direction transverse to its winding turns. The surface upon which the brush mechanism slides will hereinafter be referred to as the commutator surface of the transformer.

When the sliding brush rests on one conductor turn of a winding, a current path to the load through the brush is established. When the sliding brush is not seated on one turn of winding but instead rests upon two turns simultaneously, an additional current path is established between the two turns and the brush. The current which flows in the latter path is referred to in the art as the short circuit current. With multiple brushes a separate short circuit current flows through each brush respectively. Each short circuit current flows from one of the conductor turns lying under and in contact with a brush crosswise through the brush to the adjacent turn bridged by such brush. The short circuit current creates a temperature rise in addition to that caused by the load current which cumulatively can cause an excessive rise in brush temperature ultimately leading to transformer failure. It should be clearly understood that the short circuit current is not limited or circumvented by the use of multiple parallel brushes, each of which suffers the development of its own cross current. In fact, to date the only effective control over cross current is to limit its magnitude by limiting the voltage differential between winding turns and by an appropriate selection of brush material. This, however, significantly restricts freedom of design and to a large extent determines the cost and size of the transformer. Hence the benefits that would accrue from external control over the short circuit current are obvious. For example. if the danger attributable to short circuit current were of little consequence then the brush mechanism would have few limitations especially with regard to materials of construction. At present the sliding brush mechanism is a primary and integral consideration in proper current transformer design.

SUMMARY OF THE INVENTION The transformer structure of the present invention comprises: a core; a coil winding having multiple turns of conductor successively wound around said core and being electrically insulated from one another with each turn having an exposed predetermined area consisting of at least one conductive surface segment and one nonconductive surface segment respectively; at least two electrically conductive members arranged for sliding engagement against said predetermined area of each turn along separate paths transverse to the direction of the winding with each path including one surface segment of each of said predetermined areas, and wherein such surface segments alternate so as to form a checkered geometric pattern of conductive and nonconductive surface segments respectively and circuit means for coupling said electrically conductive members in a series relationship with each other.

OBJECTS It is therefore the principal object of the present invention to provide a variable output voltage transformer.

This and other objects and advantages of the present invention will become apparent from the following description and accompanying drawings of which:

FIG. 1 is a simplified illustration of a typical transformer construction showing one coil winding having a plurality of conductor turns wrapped around the core. one side of which is shown broken away to permit a view in section of a conventional commutating surface and sliding brush assembly;

FIG. 2(a) and 2(b) are simplified electrical equivalent circuits of the transformer device of FIG. 1 showing in FIG. 2(a) the brush resting entirely upon a single turn whereas in FIG. 2(b) the brush is shown shorting two turns;

FIG. 3 is an electrical diagrammatic illustration of the transformer coil winding and an enlarged plan view of the current commutating surface arrangement of the present invention;

FIG. 4 is a view in perspective illustrating the commutator and brush arrangement of the present invention;

FIG. 5 is an electrical resistive equivalent circuit of FIG. 4; and

FIG. 6 is a view in perspective of a coil preform which will facilitate the formation of the commutating surface of the present invention.

DETAILED DESCRIPTION OF INVENTION Referring now to FIG. 1 in which is shown a section of a conventional adjustable transformer assembly I0, comprising; an iron core 12, and a coil winding 14 which is circumferentially wound about the core 12 to provide a plurality of conductor turns. For simplicity, only one coil winding 14, representing, for example, the transformer secondary, is shown. In fact, the invention is equally applicable to an autotransformer arrangement in which only a single coil winding is used. The coil winding I4 is preferably in the form of a rectangular strip of conductive material such as copper with a coated insulating jacket so that adjacent turns of winding do not make electrical contact. To prepare a commutating surface it is conventional to machine down one face of the coil winding 14 to expose a conductive section of each turn along such face. In FIG. 1 the letter S is intended to designate the commutator surface formed on the coil winding 14. A commutating brush 18 is held in sliding engagement against the commutating surface S by a drive screw assembly (not shown). The brush 18 transfers current to a transformer load (not shown) from the coil winding 14 with the current magnitude depending upon the physical location of the brush along the commutator surface S.

The electrical equivalent of FIG. 1, with the transformer 10 connected to a load L, is shown in FIGS.

2(0) and 2(1)). In FIG. 2(a) the brush l8 completes an electrical circuit when it is resting upon a conductive section such as section 20 of the first turn. In this case, the load current I, flows through all of the turns of winding 14, the brush I8 and the load L. In FIG. 2(b), the brush is shown bridging the two

sections

20 and 22 respectively thus shorting out one turn. In such instance a short circuit current l, is caused to flow in the closed loop formed about such turn which is in addition to the flow of load current I,

In accordance with the commutator arrangement of the present invention the closed loop through which the short circuit current l flows is completed through a circuit path external of the coil winding 14 as will be explained in detail hereafter.

FIG. 3 shows the commutating coil winding surface arrangement of the present invention. For simplicity, each turn of coil winding 14 is shown represented as a line except for the surface segments of each turn which are identified by the capital letters C" and N" respectively. The letter C connotes a conductive sur face segment whereas the letter "N connotes a nonconductive surface segment. The aggregate of all such surface segments C and N respectively comprise the commutating surface S. The preferred method for forming the segments C and N respectively upon one side of the coil winding 14 will be discussed hereafter. However, it is significant to note that each nonconduc tive segment N is limited only to the surface of the turn upon which it is formed, i.e., it does not extend down through its full cross section. Thus, the coil winding 14 remains electrically continuous notwithstanding the discontinuity along the surface of each turn of winding due to the nonconducting segments N.

The segments alternate from conductive C to nonconductive N along each turn of winding 14 as well as between turns forming thereby rows of alternating segments. The resulting geometry of the commutating sur face S is a checkerboard pattern of electrically con ducting and electrically nonconducting surface segments C and N respectively.

Each row of segments in the direction transverse to the coil winding 14 represents a path of travel for a conductive mechanism such as a conventional commutating brush. At least two rows of such segments and two corresponding commutating brush mechanisms are necessary to form the adjustable output transformer structure of the present invention. Although only one brush per path of travel is necessary any number of additional brushes may be coupled in parallel to such brush to decrease the current density for such path. Electrically, such parallel connected brushes represent the equivalent of a single brush. Hence, if more than one brush per path is used, the brushes should be coupled to move in unison and must physically be con tained within the bounds of such path of travel. Any conventional type of sliding brush may be used for transferring current from the commutating surface S to the transformer load. In addition, the assembly (not shown} for maintaining and slidably engaging the con ductive surfaces of the brushes against the commutating surface S is also conventional.

FIG. 4 is a perspective showing of the commutating surface S and brush arrangement of the present invention. The commutating surface includes two

rows

30 and 32 of alternating conductive and nonconductive segments C and N respectively. A pair of brushes 34 and 36 are electrically coupled in parallel and are physically connected to slide in unison over row 30. Likewise, brushes 38 and 40 are electrically connected in parallel and are physically coupled to slide in unison over row 32. ln view of the fact that each pair of brushes operate as the electrical equivalent of a single brush each of such pairs will hereafter be referred to as a brush set. It should be understood that in fact only one brush per path of travel is necessary as was earlier mentioned. Alternatively each brush set could represent more than two brushes.

The brush set 34, 36 is coupled electrically to the brush set 38, 40 through the circuit network 42 which in turn is coupled to the transformer load L.

The existence of a circuit network 42 between the brush sets 34, 36 and 38, 40 respectively provides not only control over the short circuit current but also permits the transformer characteristics to be readily modified. Before touching upon such variations however, the operation of the transformer structure embodying the commutator surface and brush arrangement of the present invention as illustrated in FIG. 4 will be ex plained.

In accordance with the preferred arrangement as shown in FIG. 4, the circuit network 42 consists of two cross current resistors RI and R2 joined at terminal 44 for connection to the transformer load. When the brush set 34, 36 rests upon a conductive segment C a current path to the load is established from the winding through brush set 34, 36 and resistor R1. Likewise, a current to the load will flow through brush set 38, 40 and resistor R2 when the brush set 38, 40 rests upon a conductive segment C. By aligning each brush set realtive to the other such that when one brush set rests entirely upon a conductive segment C the other is resting entirely upon a nonconductive segment N, as shown in FIG. 4, the load current will flow alternately from each brush set to the load as the brushes are moved along the commutating surface S. Thus, the load current flows either only through one brush set or proportionally through both. The latter case occurs when each brush set 34, 36 and 38, 40 respectively is located intermediate a conducting and nonconducting segment. It is during such latter case when a short circuit path between the turns is established. For the configuration illustrated in FIG. 4 however, the magnitude of such short circuit current is limited and controlled by the circuit network 42. In the case where RI and R2 are both zero the network 42 reduces to only a common junction for electrically connecting the brush sets together. The commutating operation would then be the equivalent of a conventional transformer.

The electrical equivalent of FIG. 4 for the case when both brush sets simultaneously conduct current reduces to the simplified circuit schematic shown in FIG. 5 provided one assumes that the brush impedance and winding impedance is negligible compared to the resistances R1 and R2 of circuit network 42. The load resistance is designated as R, and the load current whereas V represents the voltage across one turn of winding and V the voltage across n-l turns of winding where n total turns. An analysis of the circuit of FIG. 5 follows from which the advantages of the present transformer structure will become apparent.

It is desirable, although not essential, to have identical cross current resistors R1 and R2 to more evenly balance the transformer currents from alternate turns. Determine: Therefore, as a first condition let the parameters listed in Table l 1 R1 R Calculation:

(1) 5 I. From equations l and 3. determine the correct cross current resistor size,

As a second desirable condition, let the maximum circuit current under normal circumstances be no greater than the rated load current For this to oc- R R2 cur. the cross current (I, in this case) must be zero at l rated load current, i.e. at l rated, 1, 0.

With these two conditions, the resistance ratio equals the voltage ratio 0.0055 ohms 2. Determine the cross current resistor watts at zero l load from Table 1 R2 1L W W (1.8) (320)/4 I44 watts;

total watts 288 3. Determine the cross current resistor watts at rated Then the load current is the same as the current in load from Table 1 cross current resistor, R W2 (1.8) (320) 576 watts V, V, Thus from the above example, the cross current resis- (3),: R1. tors must be able to dissipate 576 watts each, even though only one carries the total power at a time. Shift- When the load Circuit s pen t ing the brushes, by one turn will switch the flow of cur- 2 1 the Circuit CI'OSS current '5'! rent to the opposite resistor. In no case does any com- Also, from the circuit of FIG. 5 and equat o mutator resistor even carry more than ratedload curabove, rent.

The circuit network 42 thus clearly provides a means for controlling the short circuit current. Desirable design guidelines are apparent from the equations set forth above under the assumed conditions using a symmetrical and resistive circuit network 42 and limiting the maximum current to the ratedload current. These conditions would be preferred for most welding transformer applications. Other design guidelines can be 20411, 2 readily determined for an asymmetrical circuit network 42 and/or for a more complex circuit. It should also be The short circuit cross current equals half the load understood that the circuit network 42 is not limited to Substitution from (3) yields current. resistive or passive circuitry. In this regard, a source of These simple equations show that the short circuit potential, as one example, may be introduced to supply cross current varies between the limits of one-half the a bias or vary in a predetermined manner the regulation rated load current and zero, as the load current varies (volt/ampere-slope) of the transformer. from zero to rated. Table 1 summarizes the circuit 45 The description of the invention to this point related characteristics just discussed. to the structural arrangement between the commutat- TABLE 1 Load Current Cross Currents Cross Current Resistor Watts I I, l, R Watts=W R, Watts=W Total Warw 0 1,12 2 v 4 v,l, 4 V,I,j2 t 1 1 g t L IL 0 [IL 0 IIL l, =rated load current V,=transformcr volts per turn The variations shown in Table l were verified experiing Surface S and the brushes. Although any procedure mentally. Treatment of the circuit as purely resistive may be used to fabricate the commutating Surface S on did not introduce any significant errors. The following the coil winding 14, the preferred technique involves example will illustrate a simple design application takthe use of a coil form 50 as illustrated in FIG. 6. The ing advantage of the characteristics set forth in Table coil form 50 represents a mold mounted upon a surface 1 of a conventional ferromagnetic core, which surface is to become the commutating Surface S. When a coil EXAMPLE winding 14 is wound about a core having this coil form Given: 50 mounted thereon, each turn of winding will auto- VI lts (volts/turn selected by the transformer matically rise and fall in conformity to the surface undesigner) dulations upon the form. It should be recalled that the l 320 amps (required rated load current) coil winding 14 is a conductor preferably in the form of a rectangular strip with an insulated covering. A turn of such winding laid in the receding section 52 of the coil form 50 falls below the plane of the commutator surface and then rises to above the plane of the commutator surface by following the level portion 54 of coil form 50. On the very next loop around the transformer core, the coil winding will first pass over another level portion 54 and then follow down the receding section 52. After the coil is fully wound a coating of insulating material such as epoxy is applied over the turns of winding formed upon the coil form. Thereafter, and in a conventional manner, such surface is machined down flat leaving alternate segments of exposed conductor in the checkerboard pattern of FIG. 4 to form the commutator surface S.

Although the invention has been described with particular reference to only one circuit containing two resistors for controlling the short circuit current it is apparent that the circuit may be modified depending upon the application desired. Moreover, although the advantages of an external adjustable circuit to a transformer designer has been only briefly discussed it is evident that through the use of such circuit the designer is free to consider using high conductivity brushes of, for example, material such as copper, silver, aluminum and/or alloys as well as conventional low conductivity brushes.

I claim:

1. A variable output voltage transformer comprising: a core; a coil winding having multiple turns of conductor successively wound around said core and being electrically insulated from one another with each turn having an exposed predetermined area consisting of at least one conductive surface segment and one nonconductive surface segment respectively; at least two electrically conductive members arranged for sliding engagement against said predetermined area of each turn along separate paths transverse to the direction of the winding with each path including one surface segment of each of said predetermined areas, and wherein such surface segments alternate so as to form a checkered geometric pattern of conductive and nonconductive surface segments respectively and circuit means for coupling said electrically conductive members in a series relationship with each other.

2. A variable output voltage transformer as defined in claim 1 wherein said means for electrically coupling said conductive members. comprises; a passive electrical circuit including impedance means.

3. A variable output voltage transformer as defined in claim 2 wherein said impedance means comprises a first and second resistor connected in series circuit relationship and wherein said passive circuit is adapted to be connected to a load at a point between said first and second resistor.

4. A variable output voltage transformer as defined in claim 3 wherein each nonconducting surface segment consists of an insulating material such as an epoxy resin.

5. A variable output voltage transformer as defined in claim 3 wherein said coil winding is the secondary winding of such transformer and further comprising another coil winding representing the primary winding.

6. A variable transformer as defined in claim 3 wherein each electrically conductive member is of a low conductivity material such as carbon.

7. A variable output voltage transformer as defined in claim 3 wherein each electrically conductive member is of a high conductivity material selected from the class consisting of copper. silver, aluminum and alloys thereof.

8. A variable transformer as defined in claim 1 wherein said means for electrically coupling said conductive members comprises an electrical circuit network including a source of potential energy.

Claims

2. A variable output voltage transformer as defined in claim 1 wherein said means for electrically coupling said conductive members, comprises; a passive electrical circuit including impedance means.

7. A variable output voltage transformer as defined in claim 3 wherein each electrically conductive member is of a high conductivity material selected from the class consisting of copper, silver, aluminum and alloys thereof.