US2535554A - Close-coupled electrical transformer - Google Patents

Description

Dec. 26, 1950 w. H. THURSTON 2,535,554

CLOSE-COUPLED ELECTRICAL TRANSFORMER Filed Jan. 24, 1949 3 Shasta-Sheet 1 Dec. 26, 1950 RSTQ 2,535,554

CLOSE-COUPLED ELECTRICAL TRANSFORMER 3 Sheets Sheet 2 Dec. 26, 1950 w, u s o 2,535,554

CLOSE-COUPLED ELECTRICAL TRANSFORMER Filed Jan. 24, 1949 5 Sheets-Sheet 3 Patented Dec. 26, 1950 CLOSE-COUPLED ELECTRICAL TRAN SFQRMER William :1. Thurston, Contra Costa, calm, as-

signor to Shell Development Company, San Francisco, Calii'., a corporation of Delaware Application January 24, 1949, Serial No. 72,380

4 Claims. (Cl. 175456) This invention relates to step-up or step-down electrical transformers having extremely high coupling between the primary and secondary instruments, such as in apparatus for determining magnetic susceptibilities of substances by subjecting them to magnetic fields of high intensities, e. g., in excess of 100,000 gausses; in video circuits wherein a very flat frequency response is desired; in various industrial appliances requiring current impulses, such as welding transformers and magnet chargers, in applications requiring voltage impulses, and for impedance matching in impulse circuits and wide frequencyrange circuits.

When the primary and secondary windings of a transformer are not closely coupled undesirable leakage flux and leakage reactances are experienced. It is recognized that in ordinary transformers having separated windings, parts of which may be appreciably remote from the core, some of the magnetic flux produced by the magnetomotive force in the primary coil will follow paths that lie wholly or partly outside of the magnetic core and that do not link with the secondary coil, and some of the flux produced by the back-magnetomotive force in the secondary coil will not link with the primary coil. It may be shown that these leakage fluxes, as they are called, are equivalent to inductances in series with the windings. Furthermore, the leakage fluxes may follow paths that lie wholly or partly within the very conductors that give rise to them, thereby generating eddy currents in the conductors, especially where the latter are of large cross-section and carry heavy currents. The eddy currents represent a parasitic load, and also, since they flow in one turn paths of finite resistance, an effective resistance is reflected into the windings. Also. the eddy currents generate heat.

The efl'ects of losses and increased impedance due to the leakage inductances and eddy currents are particularly important if a transformer is to transmit high intensity impulses of short. duraon or high frequencies in the audio and ultraaudio regions. Therefore, in the design of many impulse transformers, e. g., a 5 to l. step-down transformer, it is important that (a) the coupling, or the intimacy between the primary and secondary windings, should be exceptionally close in order to reduce the spaces available for flux leakage paths, (b) that the conductors should be thin, or else laminated, to stifle eddy currents by reducing the dimensions or continuity of the conducting metal in directions parallel to the residual leakage flux paths, and (c) that each conductor of each winding closely surround the core with no more gap than is necessary for insulation alone.

It is, therefore, an object to provide an improved transformer having improved electrical and magnetic characteristics for transmitting high intensity current or voltage impulses of short duration.

Another object is to provide a transformer that will transmit an exceptionally wide range of audio and ultra-audio frequencies with a mini mum of non-linearity.

Another object is to provide a transformer hav ing an exceptionally close coupling between the primary and secondary windings to eliminate leakage flux as far as possible.

Another object is to provide a transformer of very high coupiing and made of heavy windings for transmitting high intensity impulses wherein the windings are constructed to minimize residual leakage flux paths.

Another object is to provide an improved mechanical construction for interleaved electrical transformers in. which all conductors are made of similar or identical smooth fiat stamp ings. die-castings, sheets, or leaves without folds. edgewise bending or butt .ioints, and in the form of interrupted annuli, interconnected by bridges, which can be fabricated and assembled incxpeno sively, and which can be readily clamped together.

Another object is to provide a transformer in which leakage flux is minimized by having each conductor of each winding conform very closely to the core with only such gap as is required for insulation.

Another object is to simplify the insulation oi transformer turns by providing a construction principle that allows all insulation to be in the form of sheet stampings or dip or spray coatings.

Additional objects are: To provide a transformer in which the current densities in both primary and secondary turns are automatically the same; and to provide a transformer having a minimum distributed capacitance in each winding because the turns of one winding act as shields between the turns of the other winding.

Other objects will become apparent from the following description. taken in connection with the drawingforming a part of this specification and illustrating. by way of example, a preferred embodiment of the invention. In the drawing:

Fig. 1 is a side elevation view of a. transformer according to the invention;

Fig. 2 is a fragmentary perspective view showing the relative arrangement of some of the winding leaves and the insulating sheets (both shown thicker than in actual construction), the core being omitted;

' Fig. 3 is a plan view of an insulating sheet;

Fig. 4 is a perspective view of a bridge;

5 is a fragmentary enlarged sectional view take ,on line 5-5 of Fig. 1, showing the arrangement of the leaves, insulating layers and the bridges; and

Fig. 6 is a schematic wiring diagram indicating the circuit arrangement.

' Briefly, according to the present invention there is provided a core of magnetic material having a winding leg about which there are stacked a plurality of thin, flat, preferably smooth winding leaves of conductive metal in the form of open or interrupted annuli, separated from each other and from the core by intervening layers of insulating material. The winding leaves are parallel to each other, i. e., their surfaces are perpendicular to the axis of the winding leg of the core, and each winding leaf forms a single turn of one of the windings. Leaves forming turns of the secondary winding are interleaved between those forming turns of the primary winding and evenly distributed along the length of the primary winding, regardless of the relative numbers of leaves in the two windings. Adjacent turns or winding leaves of each winding are interconnected by bridges to form separate primary and secondary windings. In the preferred embodiment, which is illustrated, the two windings contain the same number of leaves regardless of the turns ratio of the transformer, and the desired turns ratio is attained by omitting bridges on one (or both) of the windings at equally spaced points along the length to subdivide such winding (or windings) into a plurality of fractional windings having winding leaves whose number is an aliquot fraction of the total number of leaves in the composite winding; these fractional windings are' then connected in shunt by suitable electrical conductors.

The leaves may be clamped together by tie rods or frameworkshaving engaging means with thi at H on' it are a plurality of winding leaves it made of thin sheet material of conductive metal, preferably copper, separated bylayers M of insulating material. These layers are quite thin and their arrangement is shown more clearly in Fig. 2. While any insulating material may be used, such as material which is coated to one or both sides of the leaves, it is preferred to use thin sheet material of high dielectric strength. it was found that empire cloth is suitable for this purpose when the leaves are carefully finished to remove all burrs and are perfectly flat; however, laminated sheet material, reinforced for greater mechanical durability, such as the product available commercially under the trade name Armatite, consisting essentially ofa layerof empire cloth bonded to heavy fish paper, is more general- 137 suitable.

The winding leaves for the primary and secondary windings may be of identical construction; as seen in Fig. 2, they are thin sheet copper stampings forming interrupted annuli, i. e., providing an insulating gap ill, the central opening being of sufilcient size to accommodate the core I I. A pair of contact lugs 15 and 16 having holes ll extend to one side on opposite sides of the a parallel lugs on the same side of each leaf. The

winding; the tie rods are, in this case, insulated to avoid electrical contact with the lugs and bridges.

insulating sheets may be continuous or may have slits 20 to facilitate assemblyabout the winding leg of the core. The tabs l8 and I9 have holes 2| located for juxtaposition with the holes ll. For the sake of clarity both the winding leaves and insulating sheets are shown in Fig. 2 greatly thickened in relation to length and breadth; typical dimensions appear in the last paragraph hereof. The winding leaves are stacked about the winding leg in alternate orientations, whereby alternate leaves, which form the primary winding and are indicated by the letter 12 in Fig. 2, extend in one direction, and the alternate leaves, which form the secondary winding, and are indicated by the letter s, extend in the opposite direction. In Fig. 1, the lugs of the leaves forming the primary winding are indicated at 22 and those of leaves forming the secondary winding are indicated at 23. The insulating sheets 13 are also stacked in alternate orientations, causing the tab 18 of one sheet to lie over the contact lug of each primary leaf and the tab iii of the next sheet to be over the contact lug E6 of the primary leaf. Sheets l3 whose tabs i8 are over lugs E5 of the primary winding have their tab l9 fitting over lugs i6 of the secondary leaves, while the sheets having tabs l8 covering lugs 16 on the primary leaves have tabs I9 covering lugs 15 on the secondary leaves.

Adjacent leaves of the same winding are interconnected by bridges 24 formed of stampings of conductive sheet material such as copper. They are bent to provide parallel contact portions 25 and 26 which are offset from each other, i. e., lie in different planes, by a distance to permit them to lie in fiat engagement with the con- Referring to the drawings in detail, a magnetic core, which may be of any suitable magnetic material, construction and shape, is shown at Ill. The transformer core shown may be formed of numerous laminations and to form a closed magnetic path, although a core having gap may be used. The winding leg of the core is indicated tact lugs and positioned by the tabs [8 and I! as shown in Fig. 5. As shown, one contact portion 25 is in flat engagement with the bottom of a (first) contact lug IS on a winding leaf, and the other contact portion 26 is in flat engagement with the upper face of other (second) contact lug l6 of the adjacent leaf of the same winding. Because of the alternate orientation of the insulating sheets 13 there is a tab l8 or 19 on the face of each contact lug l5 or l6 that is not engaged by a bridge. The contact portions and 26 are provided with holes 21 which may be aligned with the holes ll of the contact lugs. End plates or caps 28 and 29 of rigid material are in electrically insulated engagement with the outmost leaves and are clamped together by means of insulated tie rods 30 whichextend through the holes l1, 2| and 21, thereby conveniently retaining the leaves, insulating sheets and bridges in alignment.

To insure contact between the bridges and contact lugs either or both should be thickened slightly, or the tabs l8 and 19 should be twice the thickness of the body of the insulator. In the embodiment shown the bridges have their contact portions 25 and 26 of thickness equal to the sum of the thickness of one leaf and one insulating sheet.

The contacting faces of the lugs I5 and I6 and of the contact portions 25 and 2B of the bridges may optionally be tinned before assembly and sweated together after assembly to insure good electrical contact.

In the embodiment illustrated the transformer has a 3:1 turns ratio. Hence, all adjacent pairs of leaves of the primary winding are interconnected by bridges. Two bridges are om'tted from the secondary winding at the third points, indicated at 3| and 32, thereby subdividing the secondary winding into three fractional windings, each having four turns, i. e., one-third of the number of leaves of the primary winding. These three fractional windings are connected in shunt by the circuit indicated in Fig. 6, thereby attaining an overall turns ratio of 3:1. By this arrangement the turns of the primary winding are connected in series, and the turns of the secondary winding are connected in a series-parallel combination. Other arrangements, involving series-parallel connections in both windings are, of course, also possible. It will be understood that actual transformers may have many more than twelve leaves for each winding. 1

The conductors for effecting the series-parallel arrangement are indicated by heavy flat bars 33 and 34 in Fig. 1. These are connected to the contact lugs of the leaves where the bridges are omitted and have holes so as to be secured by the tie rods 2?. Thus, bars 33 engage the upper faces of contact lugs 16, and bars 34 engage the lower faces of contact lugs IS. The free contact lugs at the ends of both windings are, similarly, connected by heavy fiat bars 35, 36, 31 and 38, having similar holes for receiving the tie rods, and adapted for connection with external circuits. not shown. The connectors 33, 35 and 37 are connected to the tabs N5 of their respective leaves, and the connectors 34, 36 and 38 are connected to the tabs l5 of their respective leaves.

It will be seen that the construction described affords excellent coupling in that the primary and secondary leaves can lie fiat against one another, spaced apart only by the thickness of the insulating layer, such as the sheets l3 or other painted or sprayed insulating material. All local double thickness of the leaves, such as would occur when strips of metal are folded to build up the windings and would result in increased axial spacing, is avoided thereby. This results in a greatly improved space factor and better coupling.

A further advantage of the use of entirely fiat Ill] leaves or stampings is that the current does not go around folds or sharp bends, which would provide inductive paths.

Further, it is feasible with this construction to form the leaves to conform closely to cores of standard or readily manufactured cross-sectional shapes, whereby the air gap between the core and winding can be made extremely small. Also, the leaves can be readily constructed to surround the core closely on all sides, leaving only a minor space opposite the insulating gap l4. Finally, by avoiding the use or need for wrapped or discontinuous insulation labor costs during manufacture are reduced and a thinner layer of insulation can be used because the insulation sheets 13 or other equivalent fiat, continuous insulating layers can be pretested and, hence, do not require the use of space-consuming excess insulating material. The use of such thin insulating material is facilitated because the leaves are flat and present no local areas at which insulating material would be subjected to higher than average mechanical stress or wear.

The transformer described above results in extremely close coupling and low residual leakage flux, and is suitable for producing high intensity impulses. Thus, by using a transformer with a 5:1 step-down turns ratio, having copper leaves 0.056 inch thick, 1 inch wide, with insulation 0.013 in thickness, 30 leaves in each winding, and a core cross-section 1 /4 by 3 inches. magnetic field intensities of approximately one million gausses have been obtained when a microfarad condenser charged to 1500 volts was discharged through the primary and the secondary was connected to an air core work coil having twenty turns of by /2 inch copper edgewise wound into a coil measuring inch I. D., 1% inches O. D., and 3 inches long.

I claim as my invention:

1. An interleaved, closely coupled transformer including a magnetic core having a winding leg,-

, outwardly from the winding at a different peripheral location from that of the lugs on the secondary winding all the lugs of the primary winding being aligned in register with each other and all the lugs of the secondary winding being aligned in register with each other, and a plu rality of bridges of conductive sheet metal for each winding having engaging portions shaped for flat engagement with alternate lugs of adj acent leaves of the same winding for connecting said leaves in series.

2. An interleaved, closely coupled transformer including a magnetic core having a winding leg, a primary winding and a secondary winding, each of said windings having: a plurality of substantially flat, thin winding leaves of conductive metal closely stacked parallel to one another and surrounding said winding leg, each winding leaf forming a single turn of one of said windings and being shaped as an annulus fitting closely about the winding leg and having an insulating gap,

aesasss the leaves of the primary winding being alternated with an equal in number to the leaves of the secondary winding, each leaf havin a first and second contact lug extendin to one side thereof on opposite sides of said insulating gap, the contact lugs on the leaves forming the primary winding having a difierent peripheral location on the side of the winding from the location of contact lugs on leaves forming the secondary winding, and all corresponding lugs on adjacent leaves of the same respective windings being aligned in register with each other; a plurality of bridge for each winding interconnect= ing pairs of adjacent winding leaves of the same winding in series and made of conductive sheet metal bent for flat engagement with a first contact lug of one leaf and for flat en agement with the second contact lug of the adjacent winding leaf of the same winding, one or more bridges being omitted from at least one of said windings at evenly spaced intermediate points thereof to subdivide said winding into a plurality of fractional windings whose turns are an aliquot fraction of the total number of leaves therein; and means for interconnecting said fractional windings of the same composite winding in parallel.

3. In an interleaved, closely coupled transformer having a plurality of windin leaves shaped as flat broken annuli forming single turns of the primary and secondary windings stacked.

about a magnetic core and separated by sheets of insulating material, the improved construction wherein all winding leaves are constructed of stampings of thin conductive sheet metal of the same outlines, each leaf having an insulating gap and a pair of contact lugs extending outwardly on one side and a central cut-out portion for receiving the core, the leaves of the primary having like orientation and the leaves of the secondary winding being interleaved with the primary winding leaves and having like orienta tion which is diiierent from that of the primary windin leaves; all of. the insulating sheets are of the same outlines corresponding to the winding leaves but having two tabs, one matching the location of one of the contact lugs of the primary winding leaves and the other matching one of the contact lugs of the secondary winding leaves in assembled positions, alternate insulating sheets having alternate orientations to cause the tabs to fit over difierent pairs of contact lugs; and bridges are provided in each winding be tween contact lugs of adjacent leaves of the same winding, said bridges being disposed to connect the leaves of each winding in series.

4. The transformer accordin to claim 3 wherein said contact lugs, bridges and tabs are provided with holes in alignment, and the said parts are maintained in position and clamped together by means of electrically insulated tie rods passing through said holes.

WILLIAM H. THURSTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 316,354 Gaulard et al Apr. 21, 1885 2,474,395 Early et a1. June 28, 1949 FOREIGN PATENTS Number Country Date 171,836 Great Britain Dec. 1, 1921 887,228 France Nov. 8, 1943

Images