US2336665A - Transformer apparatus - Google Patents

Description

Patented Dec. 14, 1943 UNITED STATES PATENT OFFICE TRANSFORMER APPARATUS Charles Philippe Boucher, Paterson, N. 1., assignor to Boucher Inventions, Ltd., a corporation of Delaware Original application August 21, 1940, serial No. 353,511. Divided and this application March 10, 1942, Serial No. 434,119

7 Claims. (Cl. 171-119) ticularly to electrical apparatus for supplying one or more negative loads, as, for example, in the operation of a luminescent tube system.

One of the objects of my invention is to provide simple, practical and thoroughly reliable transformer apparatus which is exceedingly compact, inexpensive and highly efilcient in operation.

Another object is to provide a transformer apparatus which is adapted to withstand the varying conditions encountered in actual use, including short-circuiting and grounding of the whole or parts of the apparatus, without damage to the apparatus and the consequent necessity for shutdown and replacement or repairs.

A further object is the provision of a transformer apparatus in which there is a minimum of flux leakage to the casing, resulting in efiicient operation and substantially no objectionable vibration of the casing.

Other objects will be obvious part pointed out hereinafter.

The invention accordingly consists in the combination of elements, features. of construction, and arrangement of parts, and in' the several operational steps, and in the relation of each of the same to one or more of the others, all as described herein, the of which is indicated in the appended claims.

. In the accompanying drawing:

' Figure 1 is a diagrammatic representation of a transformer embodying the various features of my invention, the transformer being enclosed in a casing and energizing two luminescent tubes.

in Part and in Figure 2 represents a similar transformer in which the primary coil sections are in different positions from those shown in Figure 1 and in which the load is a single luminescent tube.

As conducive to a clearer understanding of certain features of my invention, it may be noted at this point that in the'op'eration of a luminescent sign or display employing one or more luminescent gas-filled tubes of desired size and configuration, high potential electrical energy is required. The Fire Underwriters do not approve a circuit in wiiich the maximum voltage to ground substantially exceeds seven thousand five .hundred volts. Where it is desired to use a tube rescope of the application quiring a higher operating potential, for example fifteen thousand volts, the energy has in some instances been supplied by two separate transformers for each tube, the transformers being connected in series and each having an output voltage of about seven thousand five hundred volts. Such an installation is expensive'both because of the amount of equipment involved and because a separate labor charge is made for each unit installed. Moreover, a large space is required to accommodate such equipment.

In the manufacture of heretofore-known transformers having core parts of various sizes and configurations, a corresponding number of dies of different sizes have been required for the manufacture of the various core members. These dies are very expensive and add considerably to the cost of manufacture of the transformers, particularly because a separate set of dies is re quired for each transformer of different capacity. In certain heretofore-known and used trans formers, an excess of weight and space has been made necessary by core designs in which the iron has not been distributed with maximum efficiency, that is, with the proper cross-sectional area in each portion of the core to accommodate the flux coursing through that portion,'and with no greater area than that. Thus, in some transformers, the core has been shaped so that in some portions the core area is so small that the flux density has been too great and in other portions the flux density is less than that which the iron will accommodate. Such designs require the whole core to be enlarged to make large E enough the portions where the flux density is greatest, thereby increasing the weight of the core and the space it occupies. The only alternative in those designs is to have a core which is inefilcient because certain portions are too small to accommodate the flux which should normally course through them and which, therefore, seeks other paths.

Where a core type of transformer is used, the coils are mounted so that parts of them extend beyond the confines of the core. This structure has the disadvantage that a substantial portion of the magnetic flux flows into the casing instead of confining itself to the core, with a consequent loss in efliciency. Moreover, there is much waste space in the casing so that a large amount of insulating compound is required tofill the casing. This insulating compound is expensive and adds to the cost of thetransformer.

The shell type of transformer, on the other hand, has some or all of its coil sections mounted entirely within the confines of the core so that there is less waste space than in the core type. In the ordinary sh'ell type of transformer, however, there is danger that one of the coils will tion becomes charred and burned, renderingthem useless so that the transformer must be shut down for replacement or repair. While some heretofore-known and used transformers have provided magnetic shunts to limit the current in a whole circuit, these designs do not provide individual protection for each coil section. Accordingly, the coil sections must be designed to carry a heavy current and therefore must be larger, heavier and more expensive.

Another object of my invention,therefore, is

to provide a transformer apparatus which is inexpensive in construction, employing windings of minimum necessary current-carrying capacity and of minimum cost, and yet which is of such construction as to reliably withstand the many varying conditions encountered in actual practical use by virtue of the individual protection provided for each secondary winding coil section.

Referring now more particularly to the prac tice of my invention, attention is invited to Figure 1 of the drawing in which I have shown a transformer, generally indicated by the numeral I0, enclosed in a casing II, a source of single phase alternating-current electrical energy I2, and two luminescent tubes TI and T2 energized by the transformer. The transformer I comprises a core, a primary winding and a secondary winding. The core is preferably made of laminated magnetic steel and comprises two outer longitudinal bars I3 and M and an inner longitudinal member I5. The member I3 has lateral extensions I3a. and I3!) at its ends and an extension I3c at its center which extend into abutting they are connected in series. The connections are made in such a manner that both coils generate magnetic flux flowing from the bar I5 into the members I3 and M, or in the reverse direc-'- tion, at the same instant. The primary winding is energized through the leads I! and I8 from the source I2, which may be the standard singlephase, GO-cycle commercial mains or any other suitable source of alternating current energy.

One secondary winding coil section SI is mounted on the member I5 near its left end in the space between the members I3 and M and between the extensions I31; and Ma, and the shunt arms Kid and Md, respectively. Similarly, another secondary coil section S2 is mounted on the member I5 near its right end in the space between the members I3 and I4 and between'the extensions I31) and Mb and the shunt arms Be and Me, respectively.

After the primary and secondary Winding coil sections have been mounted in their proper positions on the core, steel core bands I9, 20 and H are put into the positions indicated for holding the core parts in close contact. The core memhers I3, M and I5 are then forced together under heavy pressure, as by an arbor press, and wedges 22 are inserted under the bands to hold them taut, the wedges 22 being insulated from the core members by pads 23.

The coil SI is grounded to the core by a metal strip 24 connected to one terminal of the coil and held in contact with the core under the band I9. The other terminal of the coil SI is connected by a lead 25 to one terminal of the tube TI, whose other terminal is grounded at 26 to complete the circuit. The core itself is grounded as at 21. Similarly, the coil S2 is grounded to the core by a metal strip 28 held under the band 2| and is connected from its relationship with opposite end and center portions of the member I5. Likewise the member M has lateral extensions Ma, Mb and Me which abut the ends and center of member I5. Member I3 also has lateral arms I3d and I3e which extend closely toward but do not abut the bar I5 at points intermediate its ends, forming therewith air-gaps GI and G2 of high magnetic reluctance. Similarly, member M has lateral arms Md and Me which form air-gaps G3 and G4 with portions of the bar I5 intermediate it's ends. The arms I3d, I3e, Md and Me are all preferably of the same cross-sectional area and therefore of the same reluctance. Also, the air-gaps GI,

G2, G3, and G4 are preferably of the same width and-reluctance. The-member t5-and theextensions I30 and Me of the members I3 and M preferably have the same cross-sectional area, which is double that of the main portions of the memhere I3 and M and of their extensions I3a, I3b,

Ma and Mb. The cross-sectional areaof the "shunt'arms I3d, I3e, Md and Me may be less than that of the extensions I3a, I3b, Ma and Mb in' order that the reluctances of the shunt magnetic paths may be considerably higher than the reluctances of the main magnetic paths.

One primary winding coil section PI is mounted on the extension I3c and another coil P2 is mounted on the extension Mc. These coil sections are connected in series by the lead It or in parallel. high resistance they "are connected in parallel. Where they are of heavy wire and low resistance Where the coils are of fine wire and other terminal to one terminal of the tube T2 by a lead 29. The tube T2 is also grounded at 26. Because of the ground connections, two complete circuits are formed. One circuit includes the coil SI and tube TI. The other circuit includes coil S2 and tube T2. The coils SI and S2 can therefore be connected in their circuits either in a series 01: an opposition arrangement. In other words, it is feasible to have the ungrounded terminals of the coils SI and S2, to which the leads 25 and 23, respectively, are connected, either at opposite potentials or at the same potential at any given instant. The arrangement in which the two terminals are always at approximately the same potential is preferable where the tubes TI and T2 are mounted in parallel relationship with their sides close together. Adjacent portions of the respective tubes with suchta connection will-:-always-be at substantially the same potential and therefore there will be no tube-puncturing dielectric discharge.

In the operation of this transformer, as the voltage impressed across the primary winding coil sections by the source I2 rises in one direction from a zero value, -a current begins to flow through the primary winding. This current geneiates a magnetomotive force which causes a magnetic flux to flow thorugh the core as indicated by the arrows for one-half cycle of the applied electromotive force. The path for the flux interlinking the secondary winding coil SI and the primarywinding is in two parts. One part is from the extension I3c through the left half of the member I5, extension I31: and the left half of the member I3, then back to the extension I30. This part includes and interlinks the pri- Tbeoth assaees section Pl with the secondary coil 8|. partis from the extension llc through.

maryco the left half of member ll, extension Ila, and

the left half of member ll back into extension llc, including interlinking the primary coil P2 with the co SI. Similarly. the flux linking thecoils2isintwoparts. Onepartpasses from extension l lc into the right half of member It, then through extension lib and the right half of member it back into extension I30. This part interllnks the coil 52 with the primary coil PI. The other part courses from extension Ilc into the right half of member I I, then through extension Ilb and the right half member I back into extension llc, interlinking the coil 82 with-the primary coil P2. It will be understood that during the other half cycle the flux will course in a direction opposite to that outlined above and shown by the arrows.

T e ma netic flux links the secondary winding coil sections SI and S2 and as it changes in magnitude induces an electromotive force in those coils. When the potential across the terminals. of one coil, say the coil SI, and also across the terminals of the luminescent tube Tl through the lead 25 and the ground conhections, reaches a value sufliciently high to ionize and render conductive the gas in the tube TI, a current flows in the circuit comprising that tube and its coil section. This current renders the tube Tl luminous. When the gas in the tube strikes or becomes ionized the resistance of thetube becomes very low. The current flowing through the circuit would therefore reach a high value were it not for the peculiar magnetic shunt construction provided. The-current flowing through the coil.S| creates a back magnetomotive force which op poses the coursing of the main flux through that part of the core on which the coil is mounted.

When the back magnetomotive force reaches such a value that the reluctance of, the ath through the left-hand end of the member 5 is gaps and shunt arms. One part is from the extension I la into the member l5, then across airgap GI into shunt arm lid and through the member l2 back to extension lie. The other part of the shunt path is from member He into member I! and across air-gap G2 into shunt arm lld, then through member it back into extension c. It will, of course, be understood that the major portion of the main flux will traverse the same path as that outlined above but in the opposite direction when the electromotive force applied across the terminals of the primary winding is inthe other half cycle. By virtue of the magnetic shunt construction, the current in the coil SI is limited to a safe value, so that there is no danger of the 0011's becoming overheated and its insulation. becoming charred- Enough current continues to flow through the coil SI and its load, tube TI, to maintain the tube in a luminous condition until the electromotive force applied across the primary winding decreases almost to zero in following through the cycle of alternation. The tube Tl then becomes extinguished and remains so until the electromotive force across the primary winding reaches a value in the opposite direction such that a voltage is the bar liwhen the tube II has not yet become greater than that of the right-hand end of themember I5 or of the magnetic shunt paths, the

gi-eater portion of the main flux seeks a path of less reluctance through one of those paths. Thus if the tube'Tl has struck before the tube T2, the

ordinarily pass through the left-hand end of .the member I! is through the right-hand end of that member, augmenting the flux normally coursing path of least reluctance for the flux which would therethrough and linking the coil S2. Since the iiux linking the coil 52 is greater in amount at this time, the potential induced in coil 82 will be increased and therefore the tube T2 will in normal operation become lighted .very soon after the tube T2. A similar condition of operation ismei when it is the tube .132 which strikes first. In

that case, however, the main portion. of the-flux normally linking the coil 82 will be diverted to link the coil SI. until tube Tl strikes. Where operativeor, when it has, through the shunt paths which include the air-gap G2 and shuntv arm-lie, on the one hand, and air-gap G2 and shuntarm Me, on the other hand. Enough flux continues to link the coil S2 to maintain a flow of current through the tube T2 until the potential applied across the terminals of the primary winding approaches a zero value in following through the cycle of alternation. Where a cycle sourceof electromotive force is used, the

tubes become luminous 120 times per second.

Due to persistence of vision, they appear to emit a steady glow.

Should the tube Tl become grounded in operation, as by formation of a conductive film of moisture, dirt and the like between its electrodes or between the high potential electrode and the ground, the flow of current in the 'circuit will not substantially exceed that which flows during normal operation of the tube. This is due to the peculiar magnetic shunt construction which provides protection from the flow of excessive current to eachindivid'ual coil in the same manner as the the tubes become ionized and conductive at substantially the same instant, as is the casein this condition of operation, there is little potential difference between adjacent portions of the respective tubeswhen they are mounted closely together. take place between the tubes.

When both tubes have become operative, the coil S2 as well as the coil SI generates a countermagnetomotive force which opposes the coursing of the main flux. That flux which links the coil SI until it becomes ionized will thereafter seek a path of less reluctance. Thus the major portion of the flux courses in two parts through the air- As a result, no dielectric discharge will 0011 isprotected during normal operation. The

same protection will be aflorded where it is the coil SI itself, instead of the tube Tl, which becomes grounded. As current begins to flow in the coil-SI, a countermagnetomotive force is developed by that coil which causes the major portion of the main flux to seek a path of less reluctance. That path will be through the right-hand end of bar l5 linking the coil S2 when that coil has no current flowing through it. When current is flowing through coil S2,'as for example, when tube T2 is operating, the flux follows two shunt paths. One shunt path is from extension I30 throirh member l5, air-gap GI, shunt arm I 3:!

comes ground'ed,- the current flowing through I coil S2 will be limited to a safe value by virtue of the diversion of the major portion of the magneticfiux from the coil S2 through the part of the core linking the. coil SI or through two magnetic shunt paths. The shunt paths in this instance are those through the air-gap G2 and shunt arm I3e and through air-gap G4 and shunt arm Me. Where such individual protection is not provided for the secondary coils, as in the ordinary transformer, those coils must be made of large diameter wire to withstand the heavy short circuit current. Due to the magnetic shunt construction in my transformer, the coils can be made of fine wire, which has the advantages of being cheaper and lighter, in

weight.

Where one coil, say the coil SI, becomes opencircuited, as by breaking of the tube TI or of one of the leads, no current will flow in that coil. Hence no countermagnetomotive force will be generated by coil SI and the core path linking it will be of very low reluctance. Therefore, the flux linking coil S2 will be diminished in amount since it willfollow the path of least reluctance through the left-hand end of bar I5. The volt-,

' age generated in coil S2 will not be sufiicient to ionize the gas in tube T2 and no current will flow in the coil S2. It will thus be apparent that both coils are fully protected. Similarly, when the coil S2 is open-circuited, neither it nor the coil SI will be damaged.

It will be understood that, where desired, the tubes TI and T2 may be of different resistances.

For example, the tubeTI may be of higher re-- sistance than tube T2. In that event the shunt paths through air-gap GI' and shunt arm I3d and through air-gap G3 and shunt arm I4d must be clesigned'so as to have comparatively high reluctances. At the same time, the shunt paths through air-gap G2 and shunt arm 13c and through air-gap G4 and shunt arm l4e should be of somewhat lower reluctances.

Considering now another embodiment of my invention, attention is invited to Fig. 2, in which I have shown a transformer Il0 energizing a single tube Tand comprising a core, a primary winding and a secondary winding. The transformer may be enclosed in a casing similar to the one shown in Fig. 1. The core comprises three members H3, H4 and H5, which are preferably made of laminated magnetic'steel. The members H3 and 4 have lateral extensions II3a, II3b, H411 and II4b at their ends and extensions II3c and H40 at their centers, all of which extensions are turned toward the inner memberI l5 against which they are forced under pressure and held in firmly abutting relationship by core bandsI I9, I20, and I2I which are slipped over the parts. Wedges I22, insulated from the core by pads I23, are placed under the core bands to hold them tight. The member "-3 has shunt arms H317. and -II3e which form air-gaps G5 and G6 with the member H5. Member II4 also has shunt arms HM and II4e which form airgaps G1 and G8 with the member II5. gSince the amount of flux normally coursing through member I I5 and extensions H30 and I I4c is twice as great as that normally coursing through the main portions of members H3 and H4 andthe extensions II3a, I I4a, H31) and H41), the former parts have cross-sectional areas twice as great as the areas of the latter parts. The shunt arms I I3d, I I3e, I Md and I I4e preferably have smaller cross-sectional areas than any other parts.

The primary winding comprises two coil sections P3 and P4 which are mounted on the member II5 on opposite sides of the extensions H30 and H40 in the spaces between those extensions and the shunt arms H311 and II4d for the coil P3 and the shunt arms I I3e and 4e for the coil P4. The coils P3 and P4 areconnected in series by the lead I I6 or they may be connected in parallel. They are energized through the leads II! and H8 from the source II2 of alternating current electrical energy. it is not necessary in this transformer-that the primary coils be connected in any particular manner. The trans-' former will operate satisfactorily whether the coils P3 and P4 generate flux which courses in the same direction through the member H5 at any given instant or which courses in opposite directions.

The secondary winding comprises coil sections S3 and S4 which are mounted on the central member H5 and occupy respectively the spaces between shunt arms NM, 411 and extensions II3a, H411 and between shunt arms 3e, II4e and extensions I I3b, II4b. The coil S3 is grounded to the core'at I24 and its other terminal is connected by lead I25 to one terminal of the tube T. Coil S4 is likewise grounded to the core at I28 and-its other terminal is connected to the other terminal of tube T by a lead I29. The grounding connections must be such as to place the coils S3 and S4 in series. of the lead I25 is at a maximum value in a positive direction, the lead I29 must be at a maximum negative potential. The voltage across the tube T then is the sum of the voltages of the two coils and yet the maximum voltage to ground at no point exceeds thevoltage across any one coil section. The core may be grounded as at I 21.

In the operation of this transformer, as the electromotive force impressed by the source 2 across the terminals of the primary winding rises from a zero value a current flows through the primary winding and generates a magnetomotive force which causes flux to flow in the core. The flux generated by each primary coil follows two parallel paths. With the connections shown,v the flux generated by coil P3 during one-half cycleof the applied electromotive force courses to the right in the left-hand end of member I I5, then in two paths through extensions H30 and H40 into the left-hand ends of members H3 and H4, returning to member II5 through extensions H311 and H411. Similarly, the flux generated by coil P4 during the same half-cycle courses to the left in the right-hand end of member II5, then in two paths through extensions II3c and 40, the

right-hand ends of members I I3 and H4 and through extensions I I3b and I I4b back into member H5. These paths are indicated in the figure by arrows. The fiux links the secondary winding coils S3 and S4 and induces in them an electromotive force. When the electromotive force across the outer terminals of the secondary coils and across the terminals of the tube T reaches a value high enough to render the gas in the tube That is, when the potential V ionized and conductive, a current flows through the tube and through the secondary winding coil sections. As in the transformer of Fig. 1, this current does not each an unsafe value be cause of the magnetic shunt construction provided. The current flowing through the secondary coils creates a back magnetomotive force which opposes the coursing of the main flux and causesit to seek a path of less reluctance through the magnetic shunt paths provided, thereby reducing the amount of flux which interlinks the primary and secondary windings and thus reducing the voltage and current in the secondary circuit. Thus the major portion of the flux normally linking the coil S3 is diverted around that coil, following two paths from member H into extensions H and H40, members H3 and H4, shunt arms H3d and H ld, across air-gaps G5 and G1 and back to member H5. Similarly the major portion of the flux nor ally linking the coil S4 is diverted around that coil, following two paths from member I I5 into e tensions H30 and H40, members H3 and H4, unt arms Hits and 4e, across air-gaps G6 and G8,

and back to member I l 5. Enough flux continues voltage across that coil is insu'fiicient to ionize the gas in the tube, no current flows in that coil.

- It will be understood that for the other halfcycle of electromotive force applied across the terminals of the primary winding the fiux will follow the paths outlined above, but will be coursing in the opposite direction.

It will be seen that I have provided a transformer which is economical in construction and installation and which is highly eflicient and safe in operation. In both the embodiments, I have shown there are only two different shapes of core members. The outer core members have exactly the same configuration. Therefore, only two dies are needed for the manufacture of the core parts. This results in a saving in manufacturing cost. \Moreover, the cores are efilciently designed so that the flux is distributed symmetrically and with more or less uniform density. Accordingly, there is no waste iron in the core and it is of minimum weight. Moreover, the heat developed in the transformer is distributed evenly and dissipated emciently.

My transformer is compact in construction. All the coil sections are contained wholly within the core so that a minimum of flux is lost by fringing to the casing. Moreover, the shunt paths are all within the outer confines-of the core. The casing in which the transformer is enclosed, therefore, does not aflect the reluctances of these shunt paths and is made to fit my transformer closely. Thus, there is no waste space in the casing. .A minimum of insulating compound is required to fill the interstices in the transformer efiects further savings in first cost and in operation. Less space is required for installation of this transformer than is required for the installation of two transformers to do the same work. Negative loads of either equal or unequal resistances may be energized from that transformer. Due to the fact that one side of the circuit is through the ground, expensive high potential cables and bushings need be used only on th other side. Where the connections are such that the high potential sides are always at the same potential, a single multi-conductor cable can be used to accommodate both high potential leads and thus effect a further economy.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiments hereinbefore set forth, it will be understood that all matter described herein, or shown in the accompanying drawing, is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. In electrical transformer apparatus of the character described, in combination, a central linear core member, two parallel core members disposed on opposite sides of said central member, said parallel members having long transverse arms at their centers and ends extending-into abutting relationship with said central member and also having short transverse arms inter mediate said long arms separated from said cen tral member by air-gaps and forming shunt paths; 2. primary winding comprising two coil sections mounted on said central member between said long central arms and said short arms of said parallel members and connected in bucking relationship; and a secondary winding comprising two coil sections mounted on said central member between said long end arms and short arms of said parallel members;

2. In electrical transformer apparatus of the character described, in combination, a central linear core member, two parallel core members disposed on opposite sides of said central mem- 'ber, each of said parallel members having one central and two end extensions abutting said central and end extensions forming air-gaps with said central member; a primary winding comprising twocoil sections mounted on said central memberon opposite sides of said central extensions and connected in parallel and in bucking relationship; and a secondary winding comprisingtwo coil sections each mounted on said central member near one of its ends between said shunt arms and said end extensions.

3. In electrical transformer apparatus of the character described, in combination, a central linear core member, two parallel core members .disposed on opposite sides of said central member,

each of said parallel members having one central and two end extensions abutting said central 1 does the work of two transformers and therefore 76 member and also having two shunt arms intervmediate said central and end extension forming air-gaps with said central member; a primary winding comprising two coil sections mounted on said central member on opposite sides of said central extensions and connected in series and in bucking relationship; and a secondary winding comprising two coil sections eachanoun'ted on said central member near one of its ends between said shunt arms. arid said end extensions.

4. In electrical transformer apparatus ofthe character described, in combination, a transformer core including a central linear core member and two parallel core members disposed on opposite sides of said central member, said parallel members having long transverse arms at their centers and ends joining said central member and also having short transverse arms intermediate said long arms separated from said central member by air-gaps and forming shunt paths; a primary winding comprising two coil sections mounted on said central member between said long central arms and said short arms of said parallel members and connected in parallel and in bucking relationship; and two secondary winding coil sections mounted on said central member each of the same being positioned between said long end arms and said short arms of said parallel members.

5. In electrical transformer apparatus of the character described, in combination, a transformer core including a central linear core member and two parallel core members disposed on opposite sides of said central member, said parallel members having long transverse arms at their centers and ends joining said central member and also long arms separated from said central member by air-gaps and forming shunt paths; a primary winding comprising two coil sections mounted on said central member between said long central arms and said short arms of said parallel members and connected in series and in bucking relationship; and two secondary winding'coil sections mounted on said central member between said long end arms and said short arms of said parallel members.

6. In electrical transformer apparatus of the character described, in combination, a central linear core member, two parallel core member: disposed on opposite sides of said central member,

having short transverse arms intermediate said said parallel members having long transverse arms at their centers and ends extending into abutting relationship with said central member and also having short transverse arms intermediate said long arms separated from said central member by air-gaps and forming shunt paths; a primary winding comprising two coil sections mounted on said central member between said long central arms and said short arms of said parallel members and connected in bucking relationship; and a secondary winding comprising two coil sections mounted on said central member between said long end arms and said short arms of said parallel members, and having a terminal of each said coil sections grounded to the core.

7. In electrical transformer apparatus of the character described, in combination, a casing; a core snugly fitting in said casing and including, a central linear core member, two parallel core members, disposed on opposite sides of said central member, said parallel members having long transverse arms at their centers and ends joining said central member and forming parallel magnetic paths and also having short transverse arms intermediate said long arms separated from said central member by air-gaps and forming shunt paths; a primary winding comprising two coil sections mounted on said central member between said long central arms and said short arms of said parallel members and connected in bucking relationship; and a secondary winding comprising two coil sections mounted on said central core member between said long end arms and said short arms of the parallel core members.

CHARLES PHILIPPE BOUCHER.

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