Patent Publication Number: US-2006001516-A1

Title: Symmetrical phase shifting fork transformer

Description:
This application claims the benefit of provisional application Ser. No. 60/584,683 filed Jul. 1, 2004 and incorporated in its entirety herein. 
    
    
     BACKGROUND OF THE INVENTION  
      This invention relates to phase shifting fork transformers, sometimes referred to as AC/DC converters, and more particularly, to a fork transformer with minimum third harmonic and multiples thereof distortion at its outputs.  
      U.S. Pat. No. 5,455,759 to Paice, incorporated in its entirety by reference herein, discloses a symmetrical phase-shifting, fork transformer. However, this patent does not disclose or teach one of ordinary skill the details of the mechanical structure of the associated transformer used with the disclosed windings. The patent discloses what is referred to as a zig winding connected to a referred to teaser winding (which in the alternative may also be referred to as a zig winding) at a junction forming a power input terminal. A long winding has one end coupled to the junction of the zig winding and the teaser winding at the input terminal. The long winding may be tapped to form two series connected coils wherein an output terminal is connected to the tapping. The other end of the long winding is connected to like ends of other long windings of the other two phases of the three phase system at a neutral point. The zig and teaser windings are each disclosed in a further embodiment as forming a series connected auxiliary winding and an extender winding with the junction therebetween forming an output. The zig and teaser windings are wound about the core of the next adjacent so called clockwise and counter-clock wise positioned cores of the three phases. Other embodiments are also disclosed.  
      Further disclosed is a closed, but unconnected delta winding. The delta winding comprises three series connected windings with each winding wound about the core of a different corresponding leg of the three phase system and forming a closed loop independent of the other windings and not connected to the other windings. This delta winding serves an important function of siphoning off third harmonics and multiples thereof referred to as triplin harmonics by acting as a sink for these harmonics. These harmonics induce a current in the delta winding which current circulates harmlessly in the delta winding. Without the delta winding the triplin harmonics would generate undesired currents in the outputs and detrimentally affect the operation of the transformer.  
      These transformers are used to apply power to AC/DC converters forming 12, 18 and 24 pulse output signals as a result of rectification of the output AC signals. The earlier transformers typically require a minimum of five windings per leg as shown by Paice, to provide acceptable performance. One of these windings is used to form the delta winding. The delta windings provide a path in which induced currents from the third harmonics and multiples thereof flow thereby minimizing distortion at the outpputs.  
      Typical prior art transformers physically are constructed with the cores of the three phases arranged in a linear array. The cores and surrounding windings are secured to a common connection structure. These arrays are standard in the transformer industry including the construction of fork transformers having windings as disclosed by Paice.  
      The present inventors recognize that the delta winding adds cost to the transformer and also the need to minimize total harmonic distortion to minimize the need for input and output line reactors by the user of such transformers as required in the prior art transformers and which reactors also add to the cost of the system. The inventors recognize a need for an improved transformer without the need for a delta winding for the fork transformer and for a transformer with improved input and output characteristics wherein the need for input line reactors is not required providing improved lower cost operation by users of the improved transformer. The present inventors recognize that the linear array of the three phases of the transformer windings and core require the desired delta winding for minimizing the effects of the triplin harmonics. The present inventors recognize by providing the three phases in an annular array of symmetrical phase legs of cores and associated windings rather than in a linear array, the need for the delta winding can be eliminated by circulating the flux fields manifesting the triplin harmonics in annular yokes magnetically coupled to the cores rather than circulating the currents generated by such harmonics in a closed delta winding. This results in a transformer needing only a minimum of four windings per leg rather than five as in the prior art.  
      A three phase fork transformer according to an embodiment of the present invention comprises a symmetrical wye arrangement of three like transformer cores. A plurality of windings are associated with each core forming a fork transformer with the cores. Top and bottom yokes are secured to respective corresponding opposite top and bottom ends of each of the cores, the yokes being coupled to the windings and cores to receive and circulate therein third harmonic triplin generated fields generated by the windings and cores.  
      A fork three phase transformer according to another aspect of the present invention provides a symmetrical arrangement of three cores and associated windings coupled to top and bottom yokes which serve to receive and circulate therein the third harmonic triplin fields without the need for a delta winding. This arrangement includes electrically conductive isolation gaps between the cores and the yokes which also minimizes even harmonics reducing the need for input line reactors.  
      A three phase fork transformer according to a further aspect of the present invention comprises a symmetrical wye arrangement of three like transformer cores; a plurality of windings associated with each core forming a fork transformer with the cores; and annular top and bottom yokes secured to respective corresponding opposite top and bottom ends of each of the cores, the yokes being magnetically coupled to the windings and cores to receive and circulate therein flux formed by third harmonic triplin fields generated by the windings and cores.  
      A phase shifting three phase wye connected AC/DC fork transformer according to a further aspect comprises a plurality of like transformer magnetizable cores each having a top and a bottom. The cores each correspond to a different phase of the transformer and are arranged symmetrically about a central longitudinal axis. A plurality of windings are wound about each core, the windings of each core being substantially identical to the windings of each other core, the windings of each core being symmetrically arranged relative to the windings of each other core relative to the axis, the windings on each core comprising clockwise and counter clockwise zig windings connected to each other at a junction, and at least one central winding having one end coupled to the zig junction and its other end coupled to the same end of other central windings of each phase at a neutral point forming the fork transformer, the plurality of windings on each core and each core creating magnetic flux manifesting triplin harmonics. A yoke of magnetizable material is secured to each core at the core respective top and bottom, the cores and yokes being in juxtaposed relation with each other, the yokes being configured and located relative to the cores and windings so that the created magnetic flux corresponding to triplin harmonics is induced in and circulates within the yokes about the central axis.  
      A phase shifting three phase wye connected AC/DC fork transformer according to a further aspect of the present invention comprises three like transformer metal cores, each core having a top and a bottom, the cores each corresponding to a different phase of the transformer and arranged symmetrically about a central axis. A plurality of windings are wound about each core in a plurality of layers, a portion of each layer being spaced from the next adjacent layer forming a cooling duct with that next adjacent layer, the windings of each core being substantially identical to the windings of each other core, the windings of each core being symmetrically arranged relative to the windings of each other core about the axis, the windings on each core comprising a first clockwise and a second counterclockwise zig winding connected to each other at a junction, the first and second windings forming different layers and at least one central winding having one end coupled to the zig junction and its other end coupled to the same end of other central windings of each phase at a neutral point forming the fork transformer, the central winding forming a layer disposed between the zig winding layers, the plurality of windings on each core creating magnetic flux manifesting triplin harmonics. A yoke of magnetizable material is secured to each core at its respective top and a bottom. The cores and yokes are in juxtaposed relation with each other, the yokes being configured and located relative to the cores and windings so that the created magnetic flux of the triplin harmonics is induced in and circulates harmlessly within the yokes about the central axis; and a clamp for clamping the yokes to the cores, the cores being secured in substantial electrical conductive isolation relative to the yokes, the yokes being in magnetic field coupled relationship to the magnetic fields in said cores generated by said windings.  
      A phase shifting three phase wye connected AC/DC fork transformer according to an embodiment of the present invention comprises a plurality of like transformer magnetizable cores each having a top and a bottom, the cores each corresponding to a different phase of the transformer and arranged symmetrically about a central longitudinal axis.  
      A plurality of windings are wound about each core, the windings of each core being substantially identical to the windings of each other core, the windings of each core being symmetrically arranged relative to the windings of each other core relative to the axis, the windings on each core comprising clockwise and counterclockwise zig windings connected to each other at a junction, and at least one central winding having one end coupled to the zig junction and its other end coupled to the same end of other central windings of each phase at a neutral point forming the fork transformer, the plurality of windings on each core creating magnetic flux manifesting undesirable triplin harmonics.  
      A top yoke of magnetizable material is secured to each core at the core respective top and a bottom yoke of magnetizable material is secured to each core at the core respective bottom, the cores and yokes being in juxtaposed relation with each other, the yokes being configured and located relative to the cores and windings so that the created undesirable magnetic flux is induced in and circulates within the yokes about the central axis.  
      A three phase fork transformer according to another aspect of the present invention comprises a transformer having a minimum of four windings on each phase with two windings being connected in series to provide a tapped coil in which the end of the coil electrically furthermost from the tapping is connected to form a neutral with the same coils from another phase, the end electrically closest to the tapping in each phase being arranged to be connected to one of three power source lines, the same connections on each phase being such that each of the lines of the three phase source can be connected to each of the three different phase inputs of the transformer, one winding having one end connected in a counterclockwise direction to the power input terminal of the next phase, the other winding having one end connected in a clockwise direction to the next power input terminal. Included is an assembly comprising a plurality of symmetrical positioned cores and a yoke arrangement associated with cores, the cores and yoke arrangement being magnetically coupled to the windings and arranged to cause the flux of third harmonics generated by the windings to flow in the yoke arrangement.  
      A three phase fork transformer according to a still further aspect of the present invention comprises a transformer having a minimum of four windings on each phase with one winding being connected so that the one end of the winding is connected to form a neutral with the same winding from another phase, the other end of that winding in each phase being arranged to be connected as a first power input for connection to one of three power source lines, the same input connections on each phase being such that each of the lines of the three phase source can be connected to each of the three different phase inputs of the transformer.  
      A first pair of two windings are connected in series and have a tapping therein wherein the end furthermost from the tapping is connected in a counterclockwise direction to a second power input terminal of the next phase and the end closest to the tapping is connected to the first power input.  
      A second pair of two windings are connected in series and have a tapping therein wherein the end furthermost from the tapping is connected in a clockwise direction to a third power input terminal of the next phase and the end closest to the tapping is connected to the first power input. Included is an assembly comprising a plurality of symmetrical positioned cores and a yoke arrangement associated with cores, the cores and yoke arrangement being magnetically coupled to the windings and arranged to cause the flux of third harmonics generated by the windings to flow in the yoke arrangement. 
    
    
     IN THE DRAWING  
       FIG. 1  is a circuit diagram showing a first embodiment of the present invention forming a three phase nine output transformer which may be used with a rectifier bridge to form an eighteen pulse transformer;  
       FIG. 2  is a rear elevation view of a transformer taken along lines  2 - 2  of  FIG. 3  wound with windings depicted in  FIG. 1  according to one embodiment of the present invention;  
       FIG. 3  is a top plan view of the embodiment of  FIG. 2 ;  
       FIG. 4  is a front elevation view of the transformer of  FIG. 3  taken along lines  4 - 4  with the optional terminal strip of  FIG. 2  omitted for clarity of illustration;  
       FIG. 5  is a bottom plan view of the embodiment of  FIG. 4  taken along limes  5 - 5 ;  
       FIG. 6  is an isometric view of a portion of a representative phase leg of the transformer with the windings and without the yokes attached showing the winding relationships of the different windings;  
       FIG. 7  is a sectional view of the embodiment of  FIG. 2  taken along lines  7 - 7 ;  
       FIG. 7   a  is an isometric view of a representative spacer used to space the winding layers from each other in each leg to form cooling ducts;  
       FIGS. 8 and 9  are respective top plan and side elevation views, the latter taken at lines  8 - 8  of  FIG. 8 , of the top clamp assembly of the embodiment of  FIGS. 2-5 ;  
       FIGS. 10 and 11  are respective top plan and side elevation views, the latter taken at lines  10 - 10  of  FIG. 10 , of the bottom clamp assembly of the embodiment of  FIGS. 2-5 ;  
       FIG. 12  is an isometric view of a representative yoke used in the embodiment of  FIGS. 2-5 ;  
       FIG. 13  is an isometric view of an optional representative core and yoke alignment bracket for each phase of the embodiment of  FIGS. 2-5 ;  
       FIG. 14  is a side elevation view of the top yoke of  FIG. 2  without the rest of the transformer structure illustrating the optional terminals used with the transformer of  FIGS. 2-5 ;  
       FIG. 15  is a circuit diagram showing a second embodiment of the present invention forming a three phase nine output transformer which may be used with a rectifier bridge to form a twelve pulse transformer;  
       FIG. 16  is a circuit diagram showing a rectifier bridge to form the DC outputs of the twelve pulse transformer of  FIG. 15 ; and  
       FIGS. 17 and 18  are respective circuit diagrams showing a further embodiment of the present invention forming a three phase twelve output transformer and a corresponding rectifier bridge to form a twenty-four pulse DC output; and  
       FIGS. 19 and 20  are respective circuit diagrams showing the embodiment of the present invention forming a three phase nine output transformer as shown in  FIG. 1  and a corresponding rectifier bridge to form an eighteen pulse DC output. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      In  FIG. 1 , a phasor diagram  8  shows representative windings of a three phase fork AC/DC transformer. This diagram is identical to a diagram in the Paice patent mentioned in the introductory portion except this diagram has no delta winding as in the Paice diagram. In this diagram, the length of the rectangles representing windings represent the approximate relative length of the windings. The windings are arranged into three phases A, B and C at corresponding three legs  10 ,  12  and  14  of the physical construction of the transformer. The windings of each phase is identical to the windings of each of the other phases and a description of phase A is representative of the other phases. Phase A has a power input terminal  16  at the junction of clockwise zig winding w 4  and counterclockwise zig winding w 3  (referred to as a teaser winding in Paice). The terms “clockwise” and “counterclockwise” refer to the relative clock positions of the windings about the center of the diagram. A central winding comprises a relatively small winding w 1  connected in series with a longer phase winding w 2 . w 1  is referred to herein as a teaser winding and w 2  is referred to herein as a phase winding. The nomenclature of the windings is a matter of choice as there are no set rules as to such nomenclature. The functions of the windings in  FIG. 1  are the same as the functions of the windings in the Paice diagrams that are in the same relative diagram location, regardless the nomenclature assigned to the windings. The windings w 1  and w 2  are referred to by Paice as a long winding. The zig winding w 4  is wound about the core of the phase B leg  12  in the clockwise direction and has one end connected to the power input of phase A and the other unterminated end forms an output terminal z 1 . The zig winding w 3  is wound about the core of phase C in the counterclockwise direction and has one end connected to the power input of phase A and the other unterminated end forming an output terminal x 1 . The zig windings of each of the phases is wound in the identical manner. All of the zig windings of the different phases are identical to each other. All windings of each of the phases wound about the corresponding cores to be described below are matched in impedance to the windings of each other phase in a known way in this art.  FIG. 1  corresponds generally to the phasor diagram of  FIG. 7A  of Paice and reference to this patent should be made for more complete description of these windings. The major and significant difference is that there is no delta winding in  FIG. 1  as compared to Paice,  FIG. 7A , which includes a delta winding. The other winding  
      embodiments of Paice may also be utilized in the alternative as desired for a given transformer implementation without the delta closed loop windings.  
      The windings w 1  and w 2  are connected in series with their interconnection forming a tapped point having an output y 1 . The other end of smaller winding w 1  is connected to the power input terminal of phase A at the junction of zig windings w 3  and w 4 . The other end of winding w 2  is connected to a like end of the windings w 2  of each of the phases A, B and C to form a neutral point.  
      In  FIGS. 2-7 , transformer  18  comprises three identical rectangular metal cores  20 ,  20 ′ and  20 ″ and which may be other geometrical shapes as desired, e.g., round or cylindrical, square and so on. A round cylindrical core is desirable because it requires less winding lengths and thus is lower in cost than planar surface cores. In  FIG. 13 , representative core  20  may preferably comprise a stack of sheet metal laminations as known in this art. The core  20  is rectangular in plan cross section shape. The core  20  has a longitudinal axis  22 .  
      A first elongated yoke alignment bracket  24  on each leg is wedged to one outer surface  26 ,  FIG. 13 , of the corresponding leg core and the respective inner most winding  40 ′,  42 ′ and  44 ′ (zig winding w 3 ) of respective windings  40 ,  42  and  44  of each leg,  FIG. 6 . Bracket  24  is made of glastic, a combination fiberglass, epoxy and plastic electrically insulating and magnetic field transparent material. This bracket is used to align the respective top and bottom yokes  34  and  34 ′ to the respective cores  20 ,  20 ′ and  20 ″ of the three legs. A second elongated glastic bracket  28  is at the opposite outer surface  30  of the core, is also wedged between the core and the innermost winding w 3  of each leg and also used to align the yokes to the cores. The bracket  24  is narrower than bracket  28 . The bracket  24  has a through hole  32  at each end beyond the plane of the core upper and lower respective surfaces  34 ,  36 . The bracket  28  has two through holes  32  at each end. These holes are used to manipulate the brackets  24  and  28 , either to move them or remove them as desired, after the yokes described below are assembled thereto, the brackets being optional. The brackets are held in place by the wedging action of the later wrapped windings as best seen in  FIGS. 6 and 7 . Each core  20 ,  20 ′ and  20 ″ preferably has an identical set of brackets  24  and  28 ,  FIG. 7 .  
      The cores  20 ,  20 ′ and  20 ″ are disposed in a symmetrical annular array 120° apart forming a Y format in plan view as seen in  FIG. 7 . The brackets  24  are radially outward of the cores and the brackets  28  are radially inward of the cores and extend beyond the cores in alignment with a portion of the top and bottom yokes. These extended portions receive the yokes in the space therebetween aligning the upper and lower yokes  34  and  34 ′ to the cores.  
      The brackets  24  and  28  are first assembled abutting the core  20  upright as shown,  FIG. 13 , with a portion protruding above and below the core  20 . The space between the brackets  24  and  28  is just sufficient to receive the yoke  34  therein ( FIGS. 2 and 4 ). This spacing serves to align the yokes with the cores. The yoke dimension d,  FIGS. 3 and 5 , is substantially the same as the dimension d′ of the yokes  34  and  34 ′, which yokes are identical. The dimension d′ of each of the cores  20 ,  20 ′ and  20 ″ of the array of cores defines an annular ring which receives and aligns the yokes  34  and  34 ′ thereto.  
      In  FIGS. 2-5  and  12 , top yoke  34  (and bottom yoke  34 ′ identical to top yoke  34 ) is a metal cylinder having a central bore  36 . The yoke is toroidal. The yoke preferably is made of laminated magnetizable sheet metal such as steel and wound about a cylindrical form to form the toroid ring shape with overlapping layers. The yokes preferably have an ID formed by bore  36 ,  FIG. 12 , that is coextensive with the diameter of a circle that is tangent to the inner surfaces  26  ( FIG. 13 ) of the cores  20 ,  20 ′ and  20 ″,  FIG. 7 , to which the brackets  24  are abutting. The yokes have an outer diameter (OD) that is coextensive with a circle that is tangent with the outer surface  30  ( FIG. 13 ) of the cores to which the bracket  28  is abutting. This tangency of the yoke OD and ID is best seen in  FIG. 7 . Thus the yoke inner diameter (ID) minus its OD, dimension d,  FIG. 5 , is the same as dimension d′ of each of the cores. Dimension d′ is the radial dimension of the cores from surface  26  to surface  30 ,  FIG. 13 . The brackets  24  and  28  serve to align the yokes with the cores as explained. The dimension d of the yokes times the height of the yoke  34  (and yoke  34 ′), dimension h,  FIGS. 4 and 12 , is equal to the transverse cross sectional area of each of the cores  20 ,  20 ′ and  20 ″ (in a plane normal to the drawing sheet in  FIGS. 2 and 4 ).  
      An electrically insulating sheet layer  38 ,  FIG. 2 , is between the yoke  34  and cores. The insulating layer  38  is washer-like in shape the same as the yoke  34  and is juxtaposed with the yoke  34 . Layer  38  is preferably Nomex, a trademark of the Dupont corporation, for a callendered sheet of paper about 3 mils in thickness. In the alternative, an air gap may be used instead of layer  38 . The layer  38  is electrically insulating and transparent to magnetic fields. The cores  20 ,  20 ′ and  20 ″ top surfaces  29  ( FIG. 13 ) and yoke  38  facing bottom surface abut the layer  38 . An insulating layer  38 ′ identical to layer  38  is between and abutting the cores  20 ,  20 ′ and  20 ″ bottom surface  31  ( FIG. 13 ) and the facing surface of the bottom yoke  34 ′.  
      In  FIGS. 3, 6  and  7 , the windings  40  of the phase A leg  10  are wrapped about the core  20 . Similarly, the windings  42  and  44  of the respective phase B and C legs are identical to each other and are wrapped in an identical manner about the corresponding cores  20 ″ and  20 ′ of each leg. Representative winding  40  has three layers of windings. The outer layer  46  comprises the wires of the clockwise zig winding w 4  of phase C associated with the core  20  of leg  10 . The innermost layer  48  comprises the wires of the counterclockwise zig winding w 3  of phase B. The middle layer  50  comprises the wires of windings w 1  and w 2  forming the tapped (output y 1 ,  FIG. 1 ) phase A winding (the tapping not being shown in these figures). The windings  46 ,  48  and  50  are all wound in an identical direction about the respective core  20 . The corresponding windings of each other legs are identical to and wound identically about each of the other respective cores  20 ′ and  20 ″. All windings of each of the legs are identical in gage and are sized to carry the expected currents in a manner calculated in a known manner to those of ordinary skill in this art. The windings of each leg are arranged to provide matched impedances in each leg as also known to those of ordinary skill in this art.  
      The innermost windings of layer  48  are wrapped directly about the core  20  and the brackets  24  and  28  wedging the brackets in place by the force of the windings. The terminations of the various windings are not shown in these figures. The middle winding layer  50  is wound about the innermost winding layer  48 . The layer  50  is wound spaced from the winding layer  46  by winding about spacers  52  and  54  are made of glastic material. In  FIG. 7   a , representative spacer  52  is rectangular and elongated having a length L corresponding to the length of the core  20  (top to bottom of the drawing figure). The spacers  52  are identical and the spacers  54  are identical and all have the length L. The spacers may be about ⅜×½ inch, ¾×½ inch or ½×½ inch by way of example or may have other dimensions depending upon a given implementation. These spacers form cooling ducts between the innermost layer  48  and middle layer  50 . The windings of layers  46  and  50  abut each other and layer  50  abuts the layer  48  windings on the radially innermost side of the core  20  adjacent to and facing bracket  24  I this embodiment due to the physical size of the cores and windings. In other larger embodiments, the windings may be spaced from each other to form cooling ducts at all sides of the cores. The windings  48  and  50  adjacent to the core  20  bracket  28  have the greatest spacing therebetween.  
      The outermost winding layer  46  is spaced from the middle layer  50  by spacers  56  and  58 . Spacers  56  may be dimensioned the same as spacers  54  and spacers  58  may be dimensioned the same as spacers  52 . However, the spacers may also differ in relative sizes from that shown according to a given implementation. The drawings of the various figures are not to scale. Thus cooling ducts are provided between the outer layer  46  of windings and the middle layer  50 . All of the spacers are made of the same material and have the same general shape as spacer  52 ,  FIG. 7   a . All of the legs are wound in identical fashion as described with respect to leg  10 . The windings of each layer are all wound in the same direction in each leg. The legs are thus symmetrical as much as possible in all ways. Any change of configuration of one leg needs to be identical to each other leg to provide matched impedences for all of the legs.  
      In  FIGS. 2-5  and  8 - 11 , the yokes, insulating layers and legs with the windings on the cores are clamped together by clamp assembly  60  at the top of the transformer  18  and clamp assembly  62  at the bottom of the transformer  18 . Clamp assembly  60  includes a Y-shaped clamp  61  having three identical legs  64 ,  66  and  68  each identically arcuately spaced apart 120° to match the spacing and orientation of the transformer  18  legs  10 ,  12  and  14 . The clamp legs  64 ,  66  and  68  are each juxtaposed with and aligned centrally in a circumferential direction with a corresponding transformer leg  10 ,  12  and  14 . The clamp  61  is preferably formed from three identical U-shaped steel channels forming each of the three legs of the clamp. The channels each have planar sides and bottom walls welded or otherwise joined together at the central junction of the three legs  64 ,  66  and  68 . In the alternative, the legs  64 ,  66  and  68  of the clamp  61  may be formed of hollow square or rectangular steel tubing. The legs may also be formed of sheets of steel welded together to form the channels or tubing. The configuration of the clamp  61  may differ according to a given implementation.  
      The bottom clamp assembly  62  has a clamp  70  that is substantially the same as clamp  61  except the clamp  70 , in addition, has transformer support feet  72  welded to each of the clamp  70  legs,  FIGS. 2 and 4 . Feet  72  are L-shaped brackets wherein one leg of the L forms the support foot and the other normal leg is welded or bolted to a corresponding leg of the clamp  70 . The upper clamp assembly  60  also includes two sheet steel planar washers  74 ,  74 ′, the former on top of and the latter underneath the Y-shaped clamp  61 . The washers may be any shape and are shown as hex shaped by way of example. They could be round, square, rectangular and so on. The washers  74  and the clamp  61  have a central bore  76 ,  FIG. 9 . The lowermost washer  74 ′,  FIG. 9 , is located in the bore of the upper yoke  34 ,  FIGS. 2 and 4 . Identical washers  74  and  74 ′ are associated with the lower yoke  34 ′. The upper washer  74  of the lower clamp assembly  62 ′ is located in the bore of the lower yoke  34 ′. The washers serve as support and reinforcement gussets for the legs of the Y-shaped clamps  61  and  70 . A long bolt  78 ,  FIGS. 2 and 4 , is located in the central bore  76  of the washers and clamps. The bolt  78  clamps the clamp assemblies  61  and  70  to the yokes and the yokes to the cores via nuts at the threaded ends of the bolt  78 . While a single bolt is used in this embodiment, more bolts attached to each leg of the clamps at their outer edges may also be used for large transformers.  
      In  FIG. 14 , an optional termination strip assembly  80 ,  80 ′ and  80 ″ is attached to the outer surface of the top yoke  34  at each phase leg  20 ,  20 ′ and  20 ″. Assembly  80  is representative of the identical assemblies and includes five copper rectangular strips  84  attached to a U-shaped glastic insulation support  82 . The strips  84  are configured to receive and be connected to wire conductors to which the outputs of the transformer are to be connected. The supports  82  are attached to the outer peripheral surface of the top yoke  34  by a steel band  86 .  
       FIG. 15  is a phasor diagram representing a twelve pulse transformer having twelve outputs  1 - 12 . This diagram corresponds to the diagram  FIG. 3A  of the Paice patent noted in the introductory portion. Reference to that patent should be made for further description of this diagram. However, like  FIG. 1  herein, there is no delta winding as is in the Paice patent  FIG. 3A .  FIG. 16  is a rectifier bridge circuit also shown in  FIG. 3A  of the Paice patent and used to generate the twelve pulse output at the DC load.  
       FIG. 17  is a phasor diagram similar to that of  FIG. 15 , but coupled to a rectifier bridge  88 ,  FIG. 18 , as a twenty four pulse transformer. The rectifier bridge  88  is coupled to the twelve outputs of the circuit of  FIG. 17  as shown. The outputs of  FIG. 17  are labeled  1 - 12 . The bridge comprises  12  pairs of diodes  90 ,  92  connected in series with each other and in parallel to each of the other pairs. Each transformer output is connected to the anode-cathode junction between each of the series connected diodes  90 ,  92 . The output comprises twenty four pulses of DC current applied to the DC load.  
       FIG. 19  is a phasor diagram which is the same as that of  FIG. 1 . Its outputs are labeled  1 - 9 . These outputs are connected to the rectifier bridge  94 ,  FIG. 20 , to produce an eighteen pulse output current to the load. Presently, eighteen pulse prior art transformers are in wide use. Twenty four pulse transformers have tradeoffs including more components and thus are more costly to implement.  
      Thus there has been disclosed preferred and alternative embodiments of a transformer construction of the fork type wherein a minimum of four windings can be used per phase in comparison to a minimum of five windings as used in the prior art which require a closed loop unconnected delta winding not required in the transformers of the present invention. The delta winding provided a path in which third harmonic currents and multiples thereof could flow.  
      The transformer of the present invention has improved performance with regard to reduced total harmonic distortion on the input lines and requires only four windings per leg or phase. The flux of the third harmonics and multiples thereof is circulated through the yokes by use of symmetrically oriented legs of the three phases so that magnetic flux can flow through each individual core leg and circulate in the yokes. A round or toroidal yoke is preferably used because less stress is placed on the sheet steel forming the yoke when winding the sheets of steel about a round form compared to tighter smaller bends that may result in triangular or other shapes at the corners of such shapes. Yokes of such other shapes however may also be used according to a given implementation. What is required is that the yoke be able to circulate the flux in an annular path and any shape yoke which permits this flux flow is acceptable for this purpose. However, the round yoke yield of lower stress on the steel forming the yoke results in lower yoke core losses, eliminates the need to anneal the steel after winding the yoke, and effectively produces a more efficient yoke core.  
      In a nine phase transformer, the windings of the autotransformer are interconnected between phases in a manner that results in nine phase voltages that are symmetrically displaced from each other as shown in  FIG. 19  for example. The primary purpose, however, is to provide power to 12, 18, or 24 pulse AC/DC converters while mitigating or minimizing total harmonic distortion present on the input power lines. The transformer of the present invention accomplishes this result by eliminating the even harmonics via the insulating layers  38 ,  38 ′ and the third harmonics and multiples thereof, i.e., the triplin harmonics, via the transformer configuration as described herein. Thus acceptable total harmonic distortion levels are provided without input line reactors and output line reactors typically required for prior art transformers.  
      The insulating layers  38 ,  38 ′ provide a high reluctance path, inherent to the construction of the transformer disclosed above due to the gap between the cores of each leg and the top and bottom yokes. Additionally, the grain orientation of the steel in the cores of the legs is perpendicular to the yokes. The combination of this grain orientation and the gap between the cores and yokes results in a reluctance higher than that of three phase, shell type cores, which historically have been built for third harmonic circulation. An added benefit of the gaps between the cores of each leg and the yokes is the elimination of the even harmonics that are common to the rectifier circuits of the prior art. The provision of a path for the flux of the third harmonics and multiples thereof permits the flux to circulate harmlessly and prevents these harmonics from being transmitted to the incoming lines, and out to the power distribution system. These harmonics can have detrimental effects on equipment being used with the power system and can result in lower efficiencies and higher operating costs. The transformer of the present invention accomplishes these advantages without the prior art closed unconnected delta winding which also adds to the cost of transformers.  
      It will occur to those of ordinary skill that the disclosed embodiments may be altered to provide still further embodiments, the invention being not limited to the disclosed embodiments. It is intended that the invention be defined by the appended claims.