Patent Publication Number: US-6909352-B2

Title: Endless core for a multiphase transformer and a transformer incorporating same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation in part of U.S. application Ser. No. 09/421,897 filed Oct. 21, 1999 now abandoned, which claims the benefit of Australian Application Serial No. PQ0358 filed May 13, 1999 and Australian Application Serial No. PP7124 filed Nov. 13, 1998. 

   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to an endless core for a multiphase transformer and a transformer incorporating such a core. 
   2. Description of the Related Art 
   Multiphase transformers are well known and are used in a variety of applications including for stepping up or stepping down line voltage in power transmission systems, to provide phase shifting, modulation, star-delta converters and general power supplies. 
   A typical multiphase transformer has a planar core provided with a number of square or rectangular windows each window being bound by upper and lower branches of the core, and on opposite sides by vertical legs forming part of the core. A primary winding is wound through each window, either on a branch or leg of the window. Similarly a secondary winding is wound through each window. Irrespective of the number of phases, if the core has N windows then it will have N+1 vertical legs. This provides an inherent magnetic and therefore electrical imbalance between the phases. This arises because the magnetic flux created by current flow in the primary windings cannot circulate equally about the respective windows because of the additional vertical leg. As a result, assuming each primary phase voltage is of the same magnitude and each secondary winding has the same number of turns, then the secondary outputs cannot be the same. The transformation process is not identical between the phases due to the difference in magnetic paths surrounding each window. In order to produce equalized outputs on the secondary windings, i.e. the same magnitude output on each winding, some of the primary or secondary windings must vary the number of turns to take account of the difference in flux distribution circulating about different windows of the transformer core. Such transformers also have inherent inefficiencies due to flux leakage caused by the exposed, dead end nature of the core and the end windows having only a single oscillating flux path. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a transformer core and an associated transformer that attempts to alleviate at least the abovementioned problems in the prior art. 
   According to a first aspect of the invention there is provided a core for a multi-phase transformer. The core includes a body made of two or more rings having a common central axis, each ring consisting of a strip of magnetic permeable material wound about the central axis. The body is provided with a plurality of windows passing radially through the body, each window bound by opposed axially extending legs and opposed circumferentially extending branches, wherein said branches of each window are provided in respective axially adjacent rings. The branches and legs of each window define a closed magnetic circuit about which magnetic flux can circulate. 
   The core includes a plurality of primary windings. At least one primary winding is provided for each electrical phase, each primary winding having at least one turn wound directly about a branch or a leg of a corresponding window. This produces the magnetic flux that circulates about the corresponding window or about a periphery of an ensemble of the corresponding window and one or more sequentially adjacent windows. 
   Preferably at least one of said primary windings is wholly wound about one or both branches of said corresponding window. 
   Preferably at least one of said primary windings is wholly wound about one or both legs of said corresponding window. 
   Preferably at least one of said primary windings has a plurality of turns wherein at least one of said turns is wound about one branch of said corresponding window and at least one turn is wound about one leg of said corresponding window. 
   According to a further aspect of the invention there is provided a multi-phase transformer including at least: 
   a core according to the first aspect of this invention; 
   a plurality of primary windings, one primary winding being provided for each electrical phase of said transformer; and, 
   a plurality of secondary windings; 
   each primary winding having at least one turn wound directly about a branch or a leg of a corresponding window to produce lines of magnetic flux which circulate about said corresponding window; and, 
   at least one of said secondary windings having at least one turn wound directly about a branch or a leg of a window about which said lines of magnetic flux circulate to induce the current in said at least one secondary winding. 
   Preferably at least one primary winding is wound directly about at one or both branches of one window, and at least one secondary winding is wound directly about one or both branches of said one window. 
   Preferably at least one primary winding is wound directly about one or both branches of one window, and at least one secondary winding is wound directly about one or both legs of said one window. 
   Preferably at least one primary winding is wound directly about one or both branches of one window, and at least one secondary winding is wound directly about at least one branch and at least one leg of said one window. 
   Preferably at least one primary winding is wound directly about one or both branches of one window, and at least one secondary winding is wound directly about a branch or a leg of said one window, and directly about a branch or a leg of another window. 
   Preferably at least one primary winding is wound directly about one or both legs of one window, and at least one secondary winding is wound directly about one or both branches of said one window. 
   Preferably at least one primary winding is wound directly about one or both legs of one window, and at least one secondary winding is wound directly about one or both legs of said one window. 
   Preferably at least one primary winding is wound directly about one or both legs of one window, and at least one secondary winding is wound directly about at least one branch and at least one leg of said one window. 
   Preferably at least one primary winding is wound directly about one or both legs of a window, and at least one secondary winding is wound directly about at least one branch or one leg of said one window, and about at least one branch or one leg of another window. 
   Preferably at least one primary winding is wound directly about at least one branch and at least one leg of one window, and at least one secondary winding is wound directly about a branch or a leg of said one window. 
   Preferably at least one primary winding is wound directly about at least one branch and at least one leg of one window, and at least one secondary winding is wound directly about at least one branch and at least one leg of said one window. 
   The core includes a plurality of primary windings. At least one primary winding is provided for each electrical phase, each primary winding having at least one turn wound directly about a branch or leg of a corresponding window. This produces the magnetic flux that circulates about the corresponding window or about a periphery of an ensemble of the corresponding window and one more sequentially adjacent windows. 
   Preferably the core includes a plurality of primary windings, one primary winding provided for each electrical phase, each primary winding having at least one turn wound directly about a branch or a leg of a corresponding window. 
   Preferably the radially opposite branches of each window are configured to have the same volume of magnetically permeable material. 
   The invention also provides a method of constructing a core according to the first aspect of this invention, said method including the steps of stamping and winding about said central axis a strip of magnetically permeable material to form said body, said stamping arranged to produce said plurality of windows passing radially through said body. 
   Preferably the method includes the step of splitting said core through a plane passing through said windows. 
   The invention also provides a method of constructing a core according to the first aspect of this invention, said method including the steps of stamping strips of magnetically permeable material to form respective rings, aligning said rings along said common central axis, said stamping and aligning arranged to produce said plurality of windows. 
   The invention also provides a method of constructing a core according to the first aspect of this invention, said method including the steps of continuous winding about said central axis a strip of magnetically permeable material to form said body; and machining, cutting or otherwise forming said plurality of windows radially through said body. 
   Preferably the method includes the step of splitting said core through a plane passing through said windows. 
   Preferably the method includes the step of loading a prewound bobbin on one or more legs of said core. 
   The invention also provides a method of constructing a core according to the first aspect of this invention, said method including the steps of continuous winding strips of magnetically permeable material to form respective rings, and machining or forming said plurality of windows radially through respective rings of said body, aligning said rings along said common central axis to form said body, said machining and aligning arranged to produce said plurality of windows. 
   The invention also provides a method of constructing a core according to the first aspect of this invention, said method including the steps of continuous winding strips of magnetically permeable material to form respective rings, aligning said rings along said common central axis to form said body, said rings being spaced apart by an array of legs to form a plurality of windows passing radially through said body, wherein said branches of each window are provided in respective axially adjacent rings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
       FIG. 1  is a perspective view of a core in accordance with the present invention and a six phase transformer incorporating that core. 
       FIG. 2  is a perspective view of a second embodiment of the core and a 12 phase transformer incorporating that core. 
     According to another aspect of the invention, there is provided a core for a multi-phase transformer. The core includes a body made of two or more rings having a common central axis. Each ring consists of a strip of magnetic permeable material wound about a central axis for one or more turns forming radially stacked laminations co-axial with the axis. The body has a plurality of windows passing radially through the body, each window bound by opposed axially extending legs and opposed circumferentially extending branches. The branches and legs of each window define a closed magnetic circuit through which magnetic flux can circulate about the windows. 
     The core includes a plurality of primary windings. At least one primary winding is provided for each electrical phase, each primary winding having at least one turn wound directly about a branch or a leg of a corresponding window. This produces the magnetic flux that circulates about the corresponding window or about a periphery of an ensemble of the corresponding window and one or more sequentially adjacent windows. 
       FIG. 3  is a perspective view of a third embodiment of the core in accordance with the present invention. 
       FIG. 4  is a perspective view of a fourth embodiment of the core. 
       FIG. 5  is a cutaway perspective view of an electric motor incorporating a core in accordance with the present invention. 
       FIG. 6  illustrates one method of manufacture of a core in accordance with the present invention. 
       FIG. 7  illustrates another method of manufacture of the core. 
       FIG. 8  illustrates another method of manufacture of the core. 
       FIG. 9  illustrates another method of manufacture of the core. 
       FIG. 10  illustrates another method of manufacture of the core. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , there is illustrated a core  10  for a multiphase (in this example, six phase) transformer  12 . The core  10  is in the general form of an annulus. A plurality of windows  14   1 - 14   6  (referred to in general as windows  14   i ) are formed through and about the core  10 . Adjacent windows  14   i  share a common portion or leg  16   i,j  where  i  and  j  designate the adjacent windows. For example, leg  16   1,2  is the portion of core  10  between adjacent windows  14   1  and  14   2 ; and leg  16   4,5  is the portion or leg of core  10  between adjacent windows  14   4  and  14   5 . Due to the configuration of the core  10 , there are no dead ends in so far as magnetic flux is concerned and therefore the core  10  facilitates the existence of symmetrical magnetic flux through the core  10 . 
   Each window  14   i  is bound on opposite sides by the adjacent, core portions or legs  16   i,j  and, by upper and lower branches B u  and B l . Thus, for example window  14   1  is bound on the left side by common core portion  16   1,2 ; on the right side by common core portion  16   6,1 ; upper branch B u ; and, lower branch B l . 
   Multiphase transformer  12  is constructed by winding respective primary and secondary windings through the windows  14   i . In the embodiment shown, primary windings  18   1  and  18   6  (referred to in general as primary windings  18   i ) link with respective windows  14   i . More particularly, two primary windings  18   i  (of the same phase) are provided for each window  14   i , with one primary winding about the upper branch B u  and another primary winding about a lower branch B l  of each window  14   i .For example, looking at window  14   1  a pair of primary windings  18   1  is provided, one of each formed about the upper branch B u  and lower branch B l  of the window  14   1 . 
   When the primary windings  18   i  are coupled to respective phases of a six-phase AC power supply lines of magnetic flux φ i  are generated and circulate about at least the window through which the primary winding  18   i  is wound. Again taking for example window  14   1  when the primary windings  18   1  are connected to one phase of the six-phase AC power supply, lines of magnetic flux φ 1  are generated that circulate about window  14   1 . However, it must be appreciated that the magnetic flux generated can also circulate or return about other windows  14   i . Thus a part of the magnetic flux φ 1  can circulate about both windows  14   1  and  14   2  returning through legs  16   2,3  and  16   6,1  and circulate about windows  14   1 ,  14   2  and  14   6  returning via legs  16   2,3  and  16   5,6 . 
   The placement of secondary windings through the windows  14   i  is dependent upon the desired output. If it is desired that the phase of the output from the secondary windings is to be the same as the phase of the corresponding primary winding then secondary windings  20   s1 - 20   s6  can be wound for example about the lower branch B l  of each window  14   1 - 14   6  respectively. (Of course in a variation, the secondary windings  20   s1 - 20   s6  can be placed about the upper branches B u  of each window or even alternate between the upper and lower branches.) It will be appreciated that because of the symmetric distribution of magnetic flux φ i  about each of the windows  14   i , assuming that the primary voltage for each phase is of the same magnitude, the magnitude of the voltage output from the secondary windings  20   si  will be the same if each of the secondary windings  20   si  have the same number of turns. Thus, the core  10  and transformer  12  provide the ability to have secondary output of equal magnitude where the secondary windings  20   s1 - 20   s6  have the same number of turns. As discussed above in relation to the prior art, because of the inherent magnetic imbalance of known cores and transformers, in order to have secondary outputs of equal magnitude in a multiphase transformer one must deliberately design some of the coils to have different number of turns. 
   The core  10  and transformer  12  also allow for an infinite possibility of phase shifting or combining. If one wanted to obtain a secondary output of a phase halfway between the phases of say the primary voltages supplying primary windings  18   1  and  18   2  then a secondary winding  20   p  (shown in phantom) can be wound through both windows  14   1  and  14   2  i.e. about the common core portion  16   1,2 . Now, the second winding  20   p  links with the magnetic flux φ 1  and φ 2  and thus the secondary output is of a magnitude and phase corresponding to the vector or phasor addition of the voltage induced by fluxes φ 1  and φ 2 . This provides a 1:1 transformed combination of the phases feeding primary windings  18   1  and  18   2 . However combinations of other ratios and thus different amounts of phase shifting can be achieved at will by simply winding the secondary winding  20   p  about the upper or lower branches B u , B l  or common core portions  16   i,j  of different windows. For example, in the embodiment shown in  FIG. 1 , the primary phases are 60° apart. To obtain a secondary output having a phase 15° (i.e. ¼ the phase difference) in advance of the phase of the primary voltage feeding primary winding  18   1  a secondary winding (not shown) is provided having a 1:4 turn ratio about branch B l  of window  14   1  and branch B l  of window  14   2 , i.e. the secondary winding has four turns passing through window  14   2  for every turn passing through window  14   1 . 
     FIG. 2  illustrates a core  10 ′ suitable for constructing a twelve phase transformer  12 ′. Here, the core  10 ′ is again in shape of a ring or annulus but this time provided with twelve windows  14   1 - 14   1,2  and twelve common core portions  16   i,j , one of each between respective adjacent windows  14   i . A primary winding  18   i  is wound about lower branch B l  of each window  14   i . A secondary winding  20   i  is wound about the upper branch B u  of each window  14   i . The phase of the output of any secondary winding  20   i  is the same as the phase of voltage driving the corresponding primary winding  18   i . However, as with the previous embodiment, the secondary winding  20   i  can be wound partially about the upper and lower branches B u  and B l  or common core portions  16   i,j  of different windows in any desired combination to produce a desired phase output in accordance with standard transformer design techniques. 
     FIG. 3  illustrates an extending (vertically stacked) core  10 ″ and a multiphase transformer  12 ″ constructed using the core  10 ″. The core  10 ″ can be considered as being two six window cores vertically stacked upon each other. Thus the core  10 ″ has a lower set of windows  14   1 - 14   6  and an upper set of windows  14   7 - 14   12  with windows  14   i  and  14   i+6  in vertical alignment. Primary windings  18   1 - 18   6  are wound about the lower branches B l  of windows  14   1 - 14   6  respectively; and, primary windings  18   7 - 18   1,2  are wound about the upper branches B l  of the upper set of windows  14   7 - 14   1,2 . A set of secondary windings  20  are wound about the middle branch B m  between vertically adjacent windows  14   i ,  14   i+6 . Therefore, in this particular illustrated embodiment, there are only six secondary coils  20 . The output of any particular secondary winding  20  would be the transformed phasor or vector addition of voltages induced by the magnetic flux generated by the primary windings linked with the windows common to that particular secondary winding  20 . In order to avoid saturation it is preferred that the volume of core constituting the middle branch B m  is the sum of the volume of the core constituting the lower branch B l  and upper branch B u  of the windows  14   i ,  14   i+6 . This embodiment then allows the combination of two six phase supplies that are out of phase with each other. For example, if there are two six phase power supplies, one providing input to coils  18   1 - 18   6  and another providing input to primary windings  18   7 - 18   1,2 , the two power sources can be combined to provide a six phase output through the secondary windings  20 . This could be particularly useful in for example coupling two multiple phase power supplies to a common power transmission grid. The core configuration will also allow for the ability to have 6 primary and 12 secondary windings. Also a turns ratio of 1/0.5 primary to secondary, or secondary to primary, as well as incorporating other windows will produce any fraction of volts required. 
   In a different configuration (not illustrated) the primary windings  18   1 - 18   1,2  of transformer  12 ″ can be connected to a different phase of a twelve phase power supply and primary windings  20  round through various windows  14   i  to provide a transformed twelve phase output. Again, the phasing of the output from the secondary windings can be arranged as required in accordance with known transformer design techniques to provide the desired secondary phase output. 
     FIG. 4  further illustrates a further embodiment of the core  10 ′″ and a corresponding  12 ′″. In the embodiments shown in  FIGS. 1-3  the core  10  and windows  14   i  extend perpendicular to the axis of the core  10 . That is the windows  14   i  are formed through the radial direction of the core. With the core  10 ′″ of  FIG. 4 , the axis of the core  10  is parallel with the axis of any window  14   i . As with all previous embodiments, core  10 ′″ is configured in the general form of an annulus or ring having a plurality of windows  14   i  where adjacent windows share a common portion of core  16   i,j  so that they number of windows  14   i  equals the number of common core portions  16   i,j . More specifically, three windows  14   1 - 14   3  are formed in the core  10 ′″ with a primary winding  18   1 - 18   3  respectively wound about the lower (radially outer most) branches B l  of each window  14   i . Respective secondary windings  20   1 - 20   3  are wound through the windows  14   1 - 14   3  respectively about the corresponding upper (radially inner most) branches B u . It is preferred that the core  10 ′″ is configured so that the volume of core in the upper and lower branch portions B l , B u  of each window  14   i  is the same. This assists in avoiding saturation of the core. This can be achieved by appropriate placement or configuration of the windows  14   i . The core  10 ′″ is depicted as a disc having a relatively small axial length in comparison to its radius. However it may of course be formed with an axial length exceeding the length of its radius. 
     FIG. 5  illustrates an application of the core  10  shown in FIG.  1 . The core  10  is used in this application in a transverse flux motor  26 . Full operation and constructional details of the transverse flux motor are described in the Applicant&#39;s Australian Application No PP 7124 the contents of which is incorporated herein by way of reference. The structure of core  10  and the placement of primary windings  18   1 - 18   6  is identical to that described in the first embodiment described in relation to FIG.  1 . However, instead of multi turn electrically separate secondary windings a single turn secondary winding between  20   i  is provided about each common core portion  16   i,j  with each of the single turn secondary windings  20   i  being in mutual electrical connection. Thus, the single turn secondary windings  20   1 - 20   6  form a wheel like structure  30  having an inner rim  32  and outer rim  34  joined by radially extending spokes  36 . The outer rim  34  is depicted as residing in the air gap  38  of a cockcroft ring  40 . Without going into the detail of operation of the motor  26 , currents are induced through the single turn secondary windings  20   1 - 20   6  that interact with magnetic flux passing through the air gap  38  of the cockcroft ring  40  thereby generating transverse forces on the outer rim  34  of the wheel  30  causing it to move. The path of motion of the wheel  30  can be controlled at will by variation of the magnitude and frequency of the primary voltages supplied to the primary coil  18   1 - 18   6  and the phase relationship therebetween. 
   Now that embodiments of the present invention have been described in detail it will be apparent to those skilled in the relevant arts and numerous modifications and variations may be made without departing from the basic inventive concepts. For example, in each of the embodiments shown, the core  10  is depicted essentially as being in a ring, annulus or circular type form. However it can assume other shapes provided that it is continuous or endless and is provided with equal numbers of windows and common core portions. Also, the exact number of windows provided is simply dependent upon the application and in particular the number of primary phases. Also, the position and placement of the secondary windings  20   i  is dictated solely by the desired magnitude and phase of the secondary outputs. 
   The core  10 ,  10 ′,  10 ″,  10 ′″ can be made by casting; continuous stamping and winding of an strip of magnetically permeable material; winding of a strip of material then machining/cutting the windows. Naturally, the strip is wound so that its width extends in the direction of the axis of the core. The manufacture of the core by winding of a strip of material is depicted in  FIGS. 6-9 . With particular reference to  FIG. 6 , a strip of magnetically permeable material  40 , stamped with rectangular cutouts  42  is wound about an axis  44  to produce a core  10 . The cutouts  42  are disposed within the periphery of the strip  40  and align circumferentially to produce windows  14 . 
   Further the core can be split through a plane passing through the windows  14  to facilitate mechanical/automatic winding of the primary and/or secondary windings about the window branches B u , B 1 , or loading of prewound bobbins on the common core portions  16   i,j . The splitting can be effected after winding of the strip, or alternately the core can be initially formed as a split core, i.e. from two separate strips which are wound to form respective rings or loops which can be aligned along a common axis to form the core. This is depicted in  FIGS. 7 and 8 . In  FIG. 8 , one (of two) rings or loops  46  is shown being formed by winding of a strip  40 ′ about an axis  44 . The strip  40 ′ is provided with cutouts  42 ′ which open onto one edge  48  of the strip  40 ′. By axially aligning two rings or loops  46  with respective edges  48  and cutouts  46 ′ facing each other, a core is produced. The facing cutouts  46 ′ form windows  14  and the formed core is effectively split in a plane passing centrally through the windows  14 . 
   The stamping is for the purpose of producing the windows. If desired when producing the core from separate axially aligned rings or loops, the stamping could be performed on the strips used to form one of the rings or loops only with the full length of the legs provided on that one ring or loop. Thus the one ring or loop provides one branch and two legs of each window  14 . Then a second non-stamp ring or loop can be axially aligned and abutted with the previous ring or loop to provide a second branch for each window. This arrangement is depicted in  FIG. 8  where two loops  46 ″ and  46 ′″ are wound from respective strips  40 ″ and  40 ′″. The strip  40 ″ is provided with cutouts  42 ″ which open onto edge  48  and include the full length of the legs  16  of the windows  14  of the final assembled core  10 . The strip  40 ′″ forming the ring  46 ′″ has no cutouts as is simply axially aligned with and abutted against edge  48  of ring  46 ″ to form the core  10 . 
   The strip can of course be wound for more than 360° as shown most clearly in FIG.  6 . In this event it would be preferable to form the windows  14  after winding of the strip by appropriate machining techniques such as laser, wire or water cutting, spark erosion, grinding or milling. Stamping could still be used although the stamping would need to be incremental or indexed to take into account the change in diameter to ensure correct circumferential alignment of the voids left by the stamping to create the windows. When wound for more than 360° it is preferable for the strip to be insulated to reduce the effects of eddy currents in adjacent windings of the strip. This can be achieved with known techniques such as applying a layer of varnish to the strip. 
   In a further method of construction separate rings or loops can be formed by continuous winding of strips of magnetically permeable material with the rings or loops forming the branches only of the windows and forming the legs separately which are disposed between axially aligned rings or loops. The legs can be formed from the same material as the rings or loops as separate stacked short lengths which are bound or otherwise held together. Thus the axial ends of the separate lengths abut individual turns of the strips forming the rings or loops. In this way a closed magnetic circuit is maintained about each window and each turn or layer constituting each window. This arrangement is shown in  FIG. 9  where the core  10  is formed by two rings  46 ′″ separated by separate intervening legs  16 ′. Each ring is formed by winding a plain strip of material having no cutouts. The legs  16 ′ are formed from individual curved plates  50  of material of the same thickness of the strips used for winding the rings  46 ′″. The number of plates  50  used to form each leg is equal to the number of turns of the strip in each ring  46 ′″. 
   In yet a further variation in the method of manufacture, instead of winding a single strip as described in relation to  FIGS. 6-9  a plurality of individual strips could be wound into individual rings with adjacent/abutting ends and axially stacked one inside the other to produce a core. This is depicted in  FIG. 10  which illustrates a core  10  made form four strips  52   a - 52   d  of material each of which is wound for approximately 360° only about an axis to form corresponding rings  54   a - 54   d . The successively outer rings have greater diameter. Each strip  52   a - 52   d  is prior stamped to produce rectangular cutouts  42  which radially align to form windows  14 . Due to the change in diameter of successive rings  54  the spacing of the cutouts needs to be indexed or incremented from ring to ring. Clamps  56  may be applied about the core  10  to hold the rings  54  together and ensure that the opposite ends A and B of each ring are closely adjacent each other or abutting. Preferably the ends A and B of the ring are staggered or offset about the core. All of the manufacturing methods described in relation to  FIGS. 6-9  can be replicated with individual rings of the type described above in relation to  FIG. 10 , e.g. the rings  46 ,  46 ′,  46 ″ and  46 ′″ of  FIGS. 6-9  can be formed of concentric separate rings stacked inside each other with closely adjacent/abutting ends A, B. 
   All such variations and modifications together with others that would be obvious to a person of ordinary skill in the art are deemed to be within the scope of the present invention the nature of which is to be determined from the a foregoing description.