ENDLESS CORE FOR A MULTIPHASE TRANSFORMER AND A TRANSFORMER INCORPORATING SAME

Abstract: Endless core (10) for a multiphase transformer (12) is formed as an endless circular or annular loop. A plurality of windows (14) are formed through and about the core (10). Adjacent windows (14) share a common portion or leg (16). Thus magnetic flux is shared between windows (14). Transformer (12) is constructed by winding respective primary windings (18) and secondary windings (20) through the windows (14). The core (10) and transformer (12) allow for an infinite possibility of phase shifting or combining because portions of the second windings (20) can be wound through more than one of the windows (14).

Description: Title ENDLESS CORE FOR A MULTIPHASE TRANSFORMER AND A TRANSFORMER INCORPORATING SAME Field of the Invention The present invention relates to an endless core for multiphase transformer and a transformer incorporating such a core. Background of the Invention 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 would 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 equalised outputs on the secondary windings, ie 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 an inherent inefficiencies due to flux leakage caused by the end windows having only a single flux return 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 one form of the present invention there is provided an endless core for a multiphase transformer said core configured as an endless loop and having a plurality of windows formed therein through which one or more primary windings and/or secondary windings can be wound, wherein adjacent windows share a common portion of the core so that the number of windows equals the number of common core portions. Preferably said core is further configured to be extendable in a manner to increase the number of windows while maintaining an equal number of common core portions. In one embodiment, the core is configured to be extendable in a plane containing said loop. However in an alternate embodiment, the core is configured to be extendable in a direction perpendicular to the plane containing the loop. Preferably the core is formed as a split core to facilitate mechanical winding of the primary and/or secondary windings through the windows. According to the present invention there is also provided a multiphase transformer comprising at least: an endless core in accordance with the first form of the present invention; one or more primary windings per electrical phase linked with selected windows to produce lines of magnetic flux that circulate at least about those windows; and, one or more secondary windings linked with one or more of said windows so that the lines of magnetic flux circulating about the window through which a secondary winding is linked induces current in that secondary winding. In one embodiment, the transformer is provided with a single turn secondary winding linked with each window where the secondary windings are in mutual electrical connection. Brief Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompany drawings in which: Figure 1 is a perspective view of a core in accordance with the present invention and a six phase transformer incorporating that core; Figure 2 is a perspective view of a second embodiment of the core and a 12 phase transformer incorporating that core; Figure 3 is a perspective view of a third embodiment of the core in accordance with the present invention; Figure 4 is a perspective view of a fourth embodiment of the core; and, Figure 5 is a cutaway perspective view of an electric motor incorporating a core in accordance with the present invention. Detailed Description of the Preferred Embodiments Referring to Figure 1, there is illustrated an endless core 10 for a multiphase (in this example, six phase) transformer 12. The core 10 is formed as an endless circular or annular loop. A plurality of windows 14,-14 6 (referred to in general as windows 14 s ) are formed through and about the core 10. Adjacent windows 14, share a common portion or leg 16, j where , and j designate the adjacent windows. For example, leg 16 1 2 is the portion of core 10 between adjacent windows 14, and 14 2 ; and leg 16 45 is the portion or leg of core 10 between adjacent windows 14 4 and 14 5 . It will be appreciated that because the core 10 is endless, 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, is bound on opposite sides by the adjacent, core portions or legs 16, j and, by upper and lower branches B H and B,. Thus, for example window 14, is bound on the left side by common core portion 16, 2 ; on the right side by common core portion 16 6 1 ; upper branch B u ; and, lower branch B,. Multiphase transformer 12 is constructed by winding respective primary and secondary windings through the windows 14,. In the embodiment shown, primary windings 18, and 18 6 (referred to in general as primary windings 18,) link with respective windows 14,. More particularly, two primary windings 18, (of the same phase) are provided for each window 14,, with one primary winding about the upper branch B u and another primary winding about a lower branch B, of each window 14,. For example, looking at window 14, a pair of primary windings 18, is provided, one of each formed about the upper branch B u and lower branch B, of the window 14,. When the primary windings 18, are coupled to respective phases of a six phase AC power supply lines of magnetic flux φ, are generated and circulate about at least the window through which the primary winding 18, is wound. Again taking for example window 14, when the primary windings 18, are connected to one phase of the six phase AC power supply, lines of magnetic flux φ, are generated that circulate about window 14,. However, it must be appreciated that the magnetic flux generated can also circulate or return about other windows 14,. Thus a part of the magnetic flux φ, can circulate about both windows 14, and 14 2 returning through legs 16 23 and 16 6 , and circulate about windows 14,, 14 2 and 14 6 returning via legs 16 23 and 16 56 . The placement of secondary windings through the windows 14, 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 s , - 20 s6 can be wound for example about the lower branch B, of each window 14, - 14 6 respectively. (Of course in a variation, the secondary windings 20 s , - 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, about each of the windows 14„ assuming that the primary voltage for each phase is of the same magnitude, the magnitude of the voltage output from the secondary windings 20 s , will be the same if each of the secondary windings 20 s , 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 s , - 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 phase of say the primary voltages supplying primary windings 18, and 18 2 then a secondary winding 20 p (shown in phantom) can be wound through both windows 14, and 14 2 ie about the common core portion 16, 2 . Now, the second winding 20 p links with the magnetic flux φ, 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, and φ 2 . This provides a 1:1 transformed combination of the phases feeding primary windings 18, 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, or common core portions 16, j of different windows. For example, in the embodiment shown in Figure 1, the primary phases are 60° apart. To obtain a secondary output having a phase 15° (ie l Λ the phase difference) in advance of the phase of the primary voltage feeding primary winding 18, a secondary winding (not shown) is provided having a 1 :4 turn ratio about branch B, of window 14, and branch B, of window 14 2 , ie the secondary winding has four turns passing through window 14 2 for every turn passing through window 14,. Figure 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, - 14, 2 and twelve common core portions 16, j , one of each between respective adjacent windows 14,. A primary winding 18, is wound about lower branch B, of each window 14,. A secondary winding 20, is wound about the upper branch B u of each window 14,. The phase of the output of any secondary winding 20, is the same as the phase of voltage driving the corresponding primary winding 18,. However, as with the previous embodiment, the secondary winding 20, can be wound partially about the upper and lower branches B u and B, or common core portions 16, j of different windows in any desired combination to produce a desired phase output in accordance with standard transformer design technics. Figure 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, - 14 6 and an upper set of windows 14 7 - 14, 2 with windows 14, and 14, +6 in vertical alignment. Primary windings 18, - 18 6 are wound about the lower branches B, of windows 14, - 14 6 respectively; and, primary windings 18 7 - 18, 2 are wound about the upper branches B, 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„ 14, +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, and upper branch B u of the windows 14„ 14, +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, - 18 6 and another providing input to primary windings 18 7 - 18, 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, - 18, 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, 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. Figure 4 further illustrates a further embodiment of the core 10"' and a corresponding 12'". In the embodiments shown in Figures 1-3 the core 10 and windows 14, are arranged so that the core 10, 10' and 10'" is endless about a first axis that is perpendicular to the axis of any particular window 14,. With the core 10"' of Figure 4, the axis of the core 10 is parallel with the axis of any window 14,. As with all previous embodiments, core 10"' is configured as an endless loop having a plurality of windows 14, where adjacent windows share a common portion of core 16, j so that they number of windows 14, equals the number of common core portions 16, j . More specifically, three windows 14, - 14 3 are formed in the core 10'" with a primary winding 18, - 18 3 respectively wound about the lower (radially outer most) branches B, of each window 14,. Respective secondary windings 20, - 20 3 are wound through the windows 14, - 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„ B u of each window 14, is the same. This assists in avoiding saturation of the core. This can be achieved by appropriate placement or configuration of the windows 14,. Figure 5 illustrates an application of the core 10 shown in Figure 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'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, - 18 ή is identical to that described in the first embodiment described in relation to Figure 1. However, instead of multi turn electrically separate secondary windings a single turn secondary winding between 20, is provided about each common core portion 16, j with each of the single turn secondary windings 20, being in mutual electrical connection. Thus, the single turn secondary windings 20, - 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, - 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, - 18 ή 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, 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 insulated strip of magnetically permeable material; winding of a strip of material then machining/cutting the windows. 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„ or loading of prewound bobbins on the common core portions 16 tJ . 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 aforegoing description.

Claims: The Claims Defining the Invention are as Follows: 1. An endless core for a multiphase transformer said core configured as an endless loop and having a plurality of windows formed therein through which one or more primary windings and/or secondary windings can be wound, wherein adjacent windows share a common portion of the core so that the number of windows equals the number of common core portions. 2. The core according to claim 1 wherein, said core is further configured to be extendable in a manner to increase the number of windows while maintaining an equal number of common core portions. 3. The core according to claim 2 wherein, the core is configured to be extendable in a plane containing said loop. 4. The core according to claim 2 wherein, the core is configured to be extendable in a direction perpendicular to the plane containing the loop. 5. The core according to claim 1 wherein the core is formed as a split core to facilitate mechanical winding of the primary and/or secondary windings through the windows. 6. A multiphase transformer comprising at least: an endless core in accordance with any one of claims 1-5; one or more primary windings per electrical phase linked with selected windows to produce lines of magnetic flux that circulate at least about those windows; and, one or more secondary windings linked with one or more of said windows so that the lines of magnetic flux circulating about the window through which a secondary winding is linked induces current in that secondary winding.

Inventors: HOBSON BARRY REGINALD, LAITHWAITE ERIC ROBERTS DI, PAOLIELLO ANGELO, REVILL CHRISTOPHER PAULL, HOBSON, BARRY, REGINALD, LAITHWAITE, ERIC, ROBERTS DI, PAOLIELLO, ANGELO, REVILL, CHRISTOPHER, PAULL
Applicants: MERLEX CORP PTY LTD, MERLEX CORPORATION PTY. LTD.
Application year: 1999
Main classification: H01F27
IPC codes: H01F27, H01F30