AU1535100A - Endless core for a multiphase transformer and a transformer incorporating same - Google Patents

Description

WO 00/30130 PCT/AU99/01006 -1 Title ENDLESS CORE FOR A MULTIPHASE TRANSFORMER AND A TRANSFORMER INCORPORATING SAME Field of the Invention 5 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 10 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 15 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 20 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 25 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 30 having only a single flux return path.

WO 00/30130 PCT/AU99/01006 -2 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. 5 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 10 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 15 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 20 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, 25 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.

WO 00/30130 PCT/AU99/01006 -3 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 5 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 10 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 15 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,-146 (referred to in general as windows 14) 20 are formed through and about the core 10. Adjacent windows 14i share a common portion or leg 16ij where i and j designate the adjacent windows. For example, leg 161,2 is the portion of core 10 between adjacent windows 14, and 142; and leg 164,5 is the portion or leg of core 10 between adjacent windows 144 and 145. It will be appreciated that because the core 10 is endless, there are no dead ends in so far as 25 magnetic flux is concerned and therefore the core 10 facilitates the existence of WO 00/30130 PCT/AU99/01006 -4 symmetrical magnetic flux through the core 10. Each window 14 i is bound on opposite sides by the adjacent, core portions or legs 16j and, by upper and lower branches B, 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 5 core portion 166,1; upper branch Bu; 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 186 (referred to in general as primary windings 18,) link with respective windows 14i. More particularly, two primary windings 18, (of the same 10 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, and lower branch B, of the window 14,. When the primary windings 18, are coupled to respective phases of a six phase AC 15 power supply lines of magnetic flux 0, 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 0, are generated that circulate about window 141. However, it must be appreciated that the magnetic flux generated can 20 also circulate or return about other windows 14,. Thus a part of the magnetic flux 0, can circulate about both windows 14, and 142 returning through legs 162,3 and 166,1 and circulate about windows 14,, 142 and 146 returning via legs 1623 and 165,6. 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 25 windings is to be the same as the phase of the corresponding primary winding then secondary windings 20,1 - 206 can be wound for example about the lower branch B, of each window 14, - 146 respectively. (Of course in a variation, the secondary windings 20s, - 20,6 can be placed about the upper branches B u of each window or WO 00/30130 PCT/AU99/01006 - 5 even alternate between the upper and lower branches.) It will be appreciated that because of the symmetric distribution of magnetic flux 4, about each of the windows 14j, assuming that the primary voltage for each phase is of the same magnitude, the magnitude of the voltage output from the secondary windings 20 , will be the same if 5 each of the secondary windings 20, 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 20,1 - 206 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 10 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 15 182 then a secondary winding 20,p (shown in phantom) can be wound through both windows 14, and 142 ie about the common core portion 161,2. Now, the second winding 20, links with the magnetic flux , and 0 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 0, and 2 . This provides a 1:1 transformed combination of 20 the phases feeding primary windings 181 and 182. 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,B or common core portions 16j of different windows. For example, in the embodiment shown in Figure 1, the primary phases are 600 apart. To obtain a secondary output 25 having a phase 150 (ie 4 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 142, ie the secondary winding has four turns passing through window 142 for every turn passing through window 14,.

WO 00/30130 PCT/AU99/01006 - 6 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, - 141,2 and twelve common core portions 16ij, one of each between respective adjacent windows 14,. A primary winding 18i is wound about 5 lower branch B I of each window 14,. A secondary winding 20i is wound about the upper branch B u of each window 14i. The phase of the output of any secondary winding 20i is the same as the phase of voltage driving the corresponding primary winding 18i. However, as with the previous embodiment, the secondary winding 20, can be wound partially about the upper and lower branches B, and B, or common 10 core portions 16ij 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" 15 has a lower set of windows 14, - 146 and an upper set of windows 147 - 1412 with windows 14i and 1 4 ,+6 in vertical alignment. Primary windings 18, - 186 are wound about the lower branches B, of windows 14, - 146 respectively; and, primary windings 187 - 181,2 are wound about the upper branches B, of the upper set of windows 14, - 141,2. A set of secondary windings 20 are wound about the middle 20 branch Bm between vertically adjacent windows 14,, 14i+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 25 to avoid saturation it is preferred that the volume of core constituting the middle branch Bm is the sum of the volume of the core constituting the lower branch B, and upper branch B u of the windows 14,, 14i+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, 30 186 and another providing input to primary windings 187 - 181,2, the two power sources can be combined to provide a six phase output through the secondary WO 00/30130 PCT/AU99/01006 7 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 5 incorporating other windows will produce any fraction of volts required. In a different configuration (not illustrated) the primary windings 18, - 181, 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 10 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 15 perpendicular to the axis of any particular window 14i. 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 14i where adjacent windows share a common portion of core 16,j so that they number of windows 14i equals the number of common core portions 16ij. More 20 specifically, three windows 14, - 143 are formed in the core 10'" with a primary winding 18, - 183 respectively wound about the lower (radially outer most) branches B, of each window 14i. Respective secondary windings 201 - 203 are wound through the windows 141 - 143 respectively about the corresponding upper (radially inner most) branches B u . It is preferred that the core 10'" is configured so that the volume 25 of core in the upper and lower branch portions B,, B. 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 14i. 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 WO 00/30130 PCT/AU99/01006 8 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 181 - 186 is identical to that described in the first embodiment described in relation to 5 Figure 1. However, instead of multi turn electrically separate secondary windings a single turn secondary winding between 20i is provided about each common core portion 16ij with each of the single turn secondary windings 20i being in mutual electrical connection. Thus, the single turn secondary windings 20, - 206 form a wheel like structure 30 having an inner rim 32 and outer rim 34 joined by radially 10 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, - 206 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 15 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, - 186 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 20 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 25 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 30 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 WO 00/30130 PCT/AU99/01006 -9 primary and/or secondary windings about the window branches B u , BI, or loading of prewound bobbins on the common core portions 16ij. 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 5 which is to be determined from the aforegoing description.

Claims (6)

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, 5 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 I 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. 10

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 I wherein the core is formed as a split core to 15 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 20 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.