Patent Publication Number: US-6335872-B1

Title: Nine-phase transformer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to transformers for converting three-phase power to nine-phase power, and more particularly to transformers for providing reduced harmonics on the AC and minimizing ripple on the DC side of an AC to DC rectifier. 
     Rectifiers are used to rectify AC voltages and generate DC voltages across DC buses. A typical rectifier includes a switch-based bridge including two switches for each AC voltage phase which are each linked to the DC buses. The switches are alternately opened and closed in a timed fashion that, as the name implies, causes rectification of the AC voltage. As well known in the energy industry the global standard for AC power distribution is three-phase and therefore three-phase rectifier bridges are relatively common. 
     When designing a rectifier configuration there are three main considerations including cost, AC line current harmonics and DC bus ripple. With respect to AC current harmonics, when an AC phase is linked to a rectifier and rectifier switches are switched, the switching action is known to cause harmonics on the AC lines. AC line harmonics caused by one rectifier distort the AC voltages provided to other commonly linked loads and therefore should generally be limited to the extent possible. In fact, specific applications may require that large rectifier equipment be restricted in the AC harmonics that the equipment produces. 
     With respect to DC link ripple, rectifier switching typically generates ripple on the DC bus. As with most hardware intensive configurations cost can be minimized by using a reduced number of system components and using relatively inexpensive components where possible. 
     It is well known in AC to DC rectification that AC current harmonics and DC ripple may be improved by increasing the number of AC phases that are rectified by the rectifier. These AC phases are phase-shifted from each other. For example, by rectifying nine-phase AC current instead of three-phase harmonics and ripple are reduced appreciably. Where AC harmonic restrictions are placed on rectifier systems such restrictions are often satisfied by employing an 18-pulse rectifier that requires a nine-phase source of AC power. As the global standard for AC power distribution is three-phase, 18-pulse rectifiers require three-to-nine phase power converters between utility supply lines and rectifier switches. 
     Isolation transformers for converting three-phase AC power to nine-phase AC power are known in the art but have several shortcomings. First isolation transformers must be rated for the full power required. Second, isolation transformers are typically relatively large as separate primary and secondary windings are required for isolation purposes. 
     Where isolation between a utility supply and a rectifier is not required, employing an autotransformer including a plurality of series and common windings may advantageously reduce the size and weight of a three-to-nine phase converter that consists of an autotransformer and a rectifier unit. Exemplary three-to-nine phase autotransformers are described in U.S. Pat. No. 4,876,634 (the “&#39;634 patent”); U.S. Pat. No. 5,124,904 (the “&#39;904 patent”); U.S. Pat. No. 5,619,407 (the “&#39;407 patent”); and U.S. Pat. No. 5,455,759 (the “&#39;759 patent”), each of which is incorporated herein for the purpose of describing the prior art. 
     The &#39;634 patent teaches the general concept of providing three-phase autotransformer coils in a plurality of series connected windings which are arranged to form a hexagon. Three-phase AC input lines are linked to three input nodes and nine output nodes provide voltages to three rectifier bridges. Phase shift between the output voltages is accomplished by providing long and short windings between the input nodes and the output nodes. Importantly, the &#39;634 patent teaches that, for each autotransformer input phase, the phase shift between three corresponding output voltages should be 20 degrees and accomplishes 20 degree phase shift by providing short windings between each two adjacent output nodes corresponding to the same input phase. Long windings are provided between adjacent output nodes corresponding to different input phases. In the &#39;634 patent the nine output voltages are provided to three separate six-pulse bridges. 
     Unfortunately, there are at least two problems with the 18-pulse autotransformer described in the &#39;634 patent (hereinafter the &#39;634 topology). First, there is an inherent impedance mismatch in the &#39;634 topology which results in looping currents among the three bridges and which requires additional hardware to correct. For example, when the outputs and inputs to the &#39;634 18-pulse autotransformer are linked to provide unity gain one of the three bridges is fed directly from the input power source while the other two bridges are fed through transformer windings which each are characterized by a certain amount of leakage inductance. This means that there are different impedances for each of the bridges and the different impedances cause disparate DC output voltages and hence looping currents among the bridges. A similar impedance disparity results when the &#39;634 patent 18-pulse autotransformer is linked for step-down transformation. 
     The &#39;634 topology attempts to use two inter-phase transformers to reduce the looping currents. As an initial matter Applicant believes the inter-phase transformers provided in the &#39;634 topology are erroneously specified and that six, not two, inter-phase transformers would be required to reduce the looping currents. While six inter-phase transformers can be provided, inter-phase transformers are required to carry DC bus currents. Therefore, inter-phase transformers are relatively bulky and increase system size appreciably. In addition, the six inter-phase transformers are relatively expensive and increase system costs. 
     Second, the &#39;634 topology would result in current sharing problems among the three bridges due to enclosed electrical circuits formed by the multi-phase shift bridges. The current sharing problems are exacerbated when AC line harmonics occur as different source harmonics substantially change bridge current sharing. Because AC line harmonics are often irregular and unpredictable it is impossible to balance the impedance mismatch via addition of resistance elements. While the inter-phase transformers may ease current harmonics to the power source, the inter-phase transformers are not effective as a solution for the current sharing problem. 
     Because of the current sharing problem described above all three bridges in the &#39;634 topology have to be capable of handling over-rated current conditions as high as 150% of the current level required to be handled if the bridges were able to share current equally. This is because from time to time each bridge is forced to operate close to its rated current level while the other bridges only operate at 50% of their rated level. This drastic current difference among bridges also forces the windings of the &#39;634 topology to carry appreciably disparate current magnitudes. For this reason, in addition to the bridges having high current ratings, the autotransformer also must be rated to handle high current value and therefore results in inefficient material utilization. 
     One solution to the looping and sharing current problems associated with the &#39;634 topology is to provide an autotransformer that equally spaces output voltages in phase. For example, where there are nine outputs the outputs can be phase shifted from each other by 40 degrees each. In the &#39;407 patent this is accomplished by providing an autotransformer having three coils, each coil having a plurality of serial windings and a plurality of stub windings. The serial windings form a delta and the stub windings are magnetically coupled with the serial windings from the same coil. Three terminals are provided as the apices of the delta and the three-phase AC inputs are linked to the apex terminals. A plurality of direct outputs is interposed between respective serial windings and a plurality of indirect outputs is electrically connected with the second ends of the stub windings. The windings are chosen such that the voltage magnitudes of the direct and indirect outputs are identical. Other autotransformer topologies which include stub windings are described in the &#39;904 patent and the &#39;759 patent. 
     While staggering the transformer outputs by 40 degrees essentially eliminates the looping and sharing current problems identified above, the stub winding requirement in each of the &#39;407, &#39;904 and &#39;759 patents renders those solutions wasteful of winding and core material. 
     In addition to the problems discussed above, often specific DC loads require different DC magnitudes. For example, in some cases a DC load may require a DC magnitude that is essentially identical to the AC supply magnitude and in other cases a DC load may require a stepped down DC magnitude that is less than the supply AC magnitude. One solution to this dilemma is to manufacture two different transformers, a step-down transformer and a step-up transformer. This solution, however, is relatively expensive as two designs are required and the expenses associated with manufacturing two different transformer designs can be appreciable. 
     Despite the relatively large size of isolation transformers, sometimes specific applications require isolated primary and secondary windings. In the isolated transformer topologies many of the same design concerns have to be considered. For example, isolation transformers should be designed so as to minimize input current harmonics, minimize DC bus voltage ripple, eliminate bus current sharing problems, reduce overall transformer size and minimize required materials thereby reducing cost. 
     Thus, it would be advantageous to have a three-to-nine phase transformer that did not cause looping and sharing current problems and that is relatively inexpensive to construct. In the case of an autotransformer it would be advantageous if the transformer could be used either as a unity gain or a step-down transformer. 
     SUMMARY OF THE INVENTION 
     The present invention includes an autotransformer for transforming three-phase AC input voltages to nine-phase AC output voltages wherein the transformer includes three coils, each coil forming a plurality of series windings, the windings arranged to form a polygon. Nodes between the windings form nine output nodes, at least one step-down input set (i.e., three step-down input nodes) and at least one unity gain input set (i.e., three unity gain input nodes). The windings are sized and configured such that the voltage magnitudes at the output nodes are identical, the voltage magnitudes of the step-down input set are identical, the voltage magnitudes of the unity gain input set are identical, the unity gain set voltage magnitudes are identical to the output node voltage magnitudes, the step-down input set voltage magnitudes are greater than the output node voltage magnitudes, adjacent output nodes are separated by 40 degree phase shifts and the nodes in each input set are 120 degrees out of phase (i.e., the second node in each input set is 120 degrees out of phase with the first and second nodes in the set and the third node is 120 degrees out of phase with the first node in the set). 
     Thus, one object is to provide an autotransformer that avoids the looping and current sharing problems discussed above. To this end, because the output voltages have identical magnitudes and are equi-phase-shifted (i.e., adjacent output node voltages are 40 degrees out of phase), looping and current sharing problems are essentially eliminated. 
     Another object is to achieve the aforementioned object relatively inexpensively. To this end the present autotransformer only includes serial windings and does not require stub windings. Thus, less winding material is required to provide the desired transformation results. 
     One other object is to provide a step-down transformer that avoids the looping and current sharing problems. To provide a step-down autotransformer the present configuration provides the step-down input set of input nodes where the magnitudes of voltages at the step-down input set are greater than the magnitudes at the output nodes. 
     Yet another object is to provide a single autotransformer that can be used either as a step-down transformer or as a unity gain transformer. This feature enables a manufacturer to provide a single transformer that can be used in two different applications and therefore reduces design and manufacturing costs as only a single autotransformer has to be designed and manufactured instead of two different autotransformers. To this end the preferred autotransformer configuration provides both the step-down input set and the unity gain input set. 
     The invention also includes an isolation transformer having three input lines linked to a primary and nine outputs linked to a secondary. The primary may be in either a delta or a Wye form including three primary windings separated by 120° phase shift. The secondary preferably has the same form as the three-phase autotransformer configuration described above with the nine outputs linked to the secondary as described. In this manner the isolation transformer achieves many of the same advantages achieved with the inventive autotransformer. In addition, as the name implies, the isolation transformer isolates the primary and secondary windings. Furthermore, the isolation transformer may be configured such that the primary windings are tap-adjustable so that the transformer turns-ratio can be adjusted “in the field” to cause step-up, step-down or unity gain to accommodate field source characteristics and facilitate optimal system operation. 
     A complete understanding of the present invention will be obtained from the following description and the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a preferred embodiment of the inventive autotransformer including fifteen windings arranged to form a step-down and unity gain transformer, the transformer linked to an input three-phase supply, a nine-phase converter and a DC load; 
     FIG. 2 is a front view of a core and coils of an autotransformer according to the present invention; 
     FIG. 3 is a plan view of the core and coils of FIG. 2; 
     FIG. 4 is a waveform indicative of line current from the three-phase power source of FIG. 1 showing minimal harmonic content that results from use of the inventive autotransformer topology; 
     FIG. 5 illustrates an inventive isolation transformer including a “delta-wound” primary; 
     FIG. 6 illustrates a “Wye-wound” primary that can be substituted for the delta-wound primary in FIG. 5 to configure yet another inventive embodiment; and 
     FIG. 7 is an inventive method for configuring an autotransformer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention includes both an autotransformer configuration and also an isolation transformer configuration. 
     Step-Down and Unity Phase-Shifting Autotransformer 
     Referring now to the drawings wherein like reference numbers throughout the several views correspond to similar or like components and, specifically referring to FIG. 1, a first embodiment of the invention includes inventive autotransformer is illustrated in an exemplary environment. Transformer  10  is shown linked to a 3-phase AC source  9  via three input or supply lines  5 ,  6  and  7  and is also linked via nine output lines  30  through  38 , respectively, to a 9-phase rectifier  11  and a DC load  8 . Rectifier  11  and other rectifier designs are well known in the electrical arts and therefore rectifier  11  will not be explained here in detail. 
     Referring also to FIGS. 2 and 3 autotransformer is constructed on a laminated core  12  of electrical grade steel such as M-6 provided by Armco Incorporated of Middletown, Ohio. The laminated core  12  may have three equal paths or legs  13 ,  14 ,  15  for magnetic flux. The core  12  preferably has no other magnetic flux paths than the three traversing poles  13 ,  14 ,  15  such that the flux flowing down through one pole (e.g., pole  14  ) must return upwards through another pole (e.g., pole  13  or  15  ). 
     The poles  13 ,  14  and pass through first, second and third coils  16 ,  18  and  20 , respectively, each coil including a plurality of windings. For example, first coil  16  includes first through fifth windings  100  through  104 , the second coil  18  includes first through fifth windings  110  through  114 , and the third coil includes windings  115  through  119  as shown in FIG.  1 . 
     Each winding  100 - 104 ,  110 - 119  may be constructed using a single winding specific wire. In the alternative, several series windings may be constructed using a single wire or all of the windings may be constructed using a single wire. For example, windings  100  and  101  may be constructed using a single wire and bringing a center tap out for linking as an output node and perhaps as an input node. Preferably all of the windings have a similar construction, the distinction being primarily in the number of turns that are included in each winding. 
     Each winding  100  through  104  in coil  16  on pole  13  of the core  12  will experience the same induced volts per turn from the flux within pole  13 . The same will be true for windings  110  through  114  in coil  18  on pole  14  and for windings  115  through  119  in coil on pole  15 . Therefore, each winding  100 - 104 ,  110 - 114  and  115 - 119  on the same respective pole  13 ,  14 ,  15  will be in phase with one another. 
     Each winding  100 - 104 ,  110 - 114  and  115 - 119  has the same polarity provided that the direction of each winding  100 - 104 ,  110 - 114  and  115 - 119  is consistent. For example, assume the right end of each winding  100 - 104  shown in FIG. 1 is the start end. Then the end of winding  101  joining node  120  and the end of winding  102  joining node  125  are both start ends and so on. 
     As illustrated, the windings are linked in a series fashion to form a hexagon shape including first through sixth legs  140 ,  142 ,  144 ,  146 ,  148  and  150 , respectively. The fist coil  16  first and second windings  100  and  101  are arranged in series on leg  140 , the second coil  18  first, second and third windings  110 ,  111  and  112  are arranged in series on second leg  142 , the third coil  120  first and second windings  115  and  116  are arranged in series on third leg  144 , the first coil  16  third through fifth windings  102 ,  103  and  104 , respectively, are arranged in series on leg  146 , the second coil  18  fourth and fifth windings  113  and  114  are arranged in series on fifth leg  148  and the third coil third through fifth windings  117  through  119 , respectively, are arranged in series on sixth leg  150 . 
     Fifteen separate nodes are formed at the joining points between adjacent windings including nodes  120  through  134 . Referring still to FIG. 1, the transformer representation can be thought of as a voltage plane wherein distance between any of nodes  120  through  134  and a neutral or origin  160  represents the voltage magnitude at the corresponding node. For example, a vector  162  is formed between origin  160  and node  120  that represents voltage magnitude at node  120 . A slightly shorter vector  164  is formed between origin  160  and node  134 . Thus, comparing vectors  162  and  164  the voltage magnitude at node  120  is clearly greater than the voltage magnitude at node  134 . Similar voltage magnitude vectors can be drawn for each node in FIG.  1 . 
     The angle between two vectors (e.g.,  162  and  164 ) represents a phase shift angle between two node voltages. Nine of the fifteen nodes in FIG. 1 are output nodes. The nine output nodes include first through ninth output nodes  121 ,  122 ,  124 ,  126 ,  127 ,  129 ,  131 ,  132  and  134 . Importantly, the windings are arranged and sized such that the phase shift between each two adjacent output node voltages is essentially 40 degrees. For example, the phase angle between voltages at nodes  134  and  121  is 40 degrees. Similarly, the phase shifts between voltages at nodes  121  and  122 ,  122  and  124 ,  124  and  126 ,  126  and  127 ,  127  and  129 ,  129  and  131 ,  131  and  132  and  132  and  134  are each 40 degrees. 
     Similarly, the windings are sized and arranged such that the voltage magnitude vectors at each output node are identical. For example, each of vectors  164 ,  166  and  168  are identical (see still FIG.  1 ). 
     Autotransformer can be used as either a step-down transformer or as a unity gain transformer. To use transformer  10  as a step-down transformer, either one of two node sets can be selected as input nodes to be linked to the supply lines  5 ,  6  and  7  (see supply lines in FIG.  1 ). Each of the two input node sets that can be used for step-down transformation is separately referred to herein as a step-down input set. Referring still to FIG. 1, a first step-down input set includes nodes  123 ,  128  and  133 , each of those nodes separated by 120 degrees of phase shift. It should be appreciated that voltage magnitude vector  170  corresponding to input node  123  is longer than the output node voltage magnitude vectors (e.g.,  168 ,  164 ,  166 , etc.). Similarly, although not illustrated, the voltage magnitude vectors corresponding to each of input nodes  128  and  133  are identical to vector  170  and are each longer than the output voltage magnitude vectors. Hence, the resulting output voltage is stepped down when lines  5 ,  6  and  7  are linked to input nodes  123 ,  128  and  133 . 
     The step-down magnitude between primary and secondary voltages is proportional to the ratio of the lengths of supply and output voltage magnitude vectors in FIG.  1 . For example, the step-down magnitude will be proportional to the ratio of the length of vector  164  (i.e., the length of the output vector) to the length of vector  162  (i.e., the length of a supply vector). The following equations can be formulated from the trigonometric relationship in FIG.  1 :              x   =         tan     -   1            (       cos                 20      °                   (     1   -     cos                 40      °                  )           sin                 20      °     +     cos                 20      °                 sin                 40      °         )       ≈     13.08      °               Eq   .              1                                         V   sec       V   pri       =         164   _         162   _     _       =       cos        (       40      °     -   x     )       ≈   0.8916               Eq   .              2                         
     Thus the voltage step-down magnitude is 10.84%. FIG. 1 can also be used to identify the lengths of each of windings  100  through  104  with respect to the length of the supply voltage magnitude vector  162 . The relationships among the lengths of windings  110  through  114  and  115  through  119  are the same as among the lengths of windings  100 - 104 . Using FIG. 1 the following relationships can be developed: 
     
       
         {overscore ( 100 )}={overscore ( 101 )}=sin(40°− x ){overscore ( 162 )}≈0.4527*{overscore ( 162 )}  Eq. 3 
       
     
     
       
         {overscore ( 103 )}=2 sin(20°)cos(40 °−x ){overscore ( 162 )}≈0.6099*{overscore ( 162 )}  Eq. 4 
       
     
     
       
         {overscore ( 102 )}={overscore ( 104 )}=sin(20° +x ){overscore ( 162 )}−0.5*{overscore ( 103 )}≈0.2409*{overscore ( 162 )}  Eq. 5 
       
     
     The lengths expressed in Equations 3 through 5 are proportional to the turn ratios of windings  100  through  104 . Thus, for windings  100  through  104  the turn ratios are: 
     
       
         {overscore ( 100 )}:{overscore ( 101 )}:{overscore ( 102 )}:{overscore ( 103 )}:{overscore ( 104 )}:=1:1:0.5321:1.3472:0.5321  Eq. 6 
       
     
     Although non-integer numbers of winding turns can be achieved, integer numbers of turns are preferred for ease of manufacturing. The following table lists possible winding turn combinations available to achieve the required turn ratios indicated above. In addition, a maximum error introduced because of integral winding turn numbers is also indicated. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Combination # 
                 {overscore (100)} 
                 {overscore (101)} 
                 {overscore (102)} 
                 {overscore (103)} 
                 {overscore (104)} 
                 Max Error 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1 
                 15 
                 15 
                 8 
                 20 
                 8 
                 0.77% 
               
               
                 2 
                 28 
                 28 
                 15 
                 38 
                 15 
                 0.31% 
               
               
                 3 
                 32 
                 32 
                 17 
                 43 
                 17 
                 0.07% 
               
               
                 4 
                 43 
                 43 
                 23 
                 58 
                 23 
                 0.25% 
               
               
                   
               
            
           
         
       
     
     Combination 3 has the lowest maximum error and therefore is preferred. 
     Referring now to FIG.  4  and also still to FIG. 1, a line current waveform to power source  9  which results when using the inventive autotransformer configuration  10  is illustrated. The system used to generate the waveform of FIG. 4 was a 100 KVA AC to DC rectifier system. Fourier spectrum analysis of the current waveform in FIG. 4 yields a total harmonic distortion (THD) of 5% which is well below the typical 32% associated with a conventional three-phase AC to DC rectifier system. The KVA rating with respect to the DC output using the configuration of FIG. 1 is 0.84 which, when compared to the KVA rating of 2.3 associated which a conventional isolated transformer, is appreciably reduced. 
     A second stepdown input set that causes stepping down transformation includes nodes  120 ,  125  and  130 . The comments above with respect to nodes  123 ,  128  and  133  are applicable to this second set of input nodes. 
     Although not illustrated transformer can also be linked to supply lines  5 ,  6  and  7  to provide unity gain. Herein, each of the unity gain input node sets is referred to separately as a unity gain input set to differentiate the unity gain input sets from the step-down input sets. In addition, while the step-down input nodes were referred to as first, second and third nodes, the unity gain nodes in each set will be referred to as fourth, fifth and sixth nodes to differentiate between the step-down and unity gain nodes. 
     A first unity gain input set includes fourth, fifth and sixth nodes  121 ,  126  and  131 , each of the fourth, fifth and sixth nodes separated by 120 degrees of phase shift. It should be appreciated that when transformer  10  is connected for unity gain, voltage magnitude vector  164  corresponding to fourth input node  121  has the same length as the output node voltage magnitude vectors (e.g.,  164 ,  166 , etc.). Similarly, although not illustrated, the voltage magnitude vectors corresponding to each of the fifth and sixth input nodes  126  and  131 , respectively, are identical to vector  164 . Hence, as the nomenclature indicates, the voltage gain is unity when transformer  10  is linked in this manner. 
     It should be appreciated that voltage magnitude vector  164  corresponding to input node  134  has the same length as the output node voltage magnitude vectors (e.g.,  168 ,  164 ,  166 , etc.). Similarly, although not illustrated, the voltage magnitude vectors corresponding to each of input nodes  124  and  129  are identical to vector  164 . Hence, the voltage gain is unity when transformer  10  is linked in this manner. 
     Other unity gain input sets that yield unity gain include node set  122 ,  127  and  132  as well as node set  124 ,  129  and  134 . Operation of transformer  10  when linked to each of these other unity gain sets is essentially as described above. 
     Isolation Phase-Shifting Transformer 
     Referring now to FIG. 5, an exemplary inventive isolation transformer configuration  310  is illustrated. Configuration  310  includes a primary  312  and a secondary  314 . Primary  312  includes three delta wound coils including coils  316 ,  318  and  320 . The coils are arranged such that they are phase shifted by 120° with respect to each other. Coils  316  and  318  are linked at apice  324 , coils  318  and  320  are linked at apice  326  and coils  320  and  316  are linked at apice  322 . Although not illustrated in FIG. 5, referring also to FIG. 1, in this isolation transformer  10  embodiment, three-phase AC source  9  is linked via three supply lines  5 ,  6  and  7  to apices  322 ,  324  and  326 , respectively. 
     Referring still to FIG.  5  and also to FIG. 1, secondary  314  has a configuration which is essentially identical to the configuration of autotransformer  10  in FIG. 1 except that the input lines  5 ,  6  and  7  are linked to primary  312  instead of to secondary  314 . Because of the similar construction between secondary  314  and autotransformer  10 , similar components in autotransformer  314  have number is identical to the numbers in FIG.  1  and the description above should suffice to indicate the configuration of secondary  314 . As with the autotransformer  10 , in FIG. 5, secondary nodes are formed by the linking of adjacent windings. The secondary nodes include nine output nodes. Although not illustrated, output lines  30 - 38  in FIG. 1, are linkable to output nodes  134 ,  121 ,  122 ,  124 ,  126 ,  127 ,  129 ,  131  and  132 , respectively. As illustrated, secondary windings  110 ,  111 ,  112 ,  113  and  114  are in phase with primary winding  316 , secondary windings  115 ,  116 ,  117 ,  118 , and  119  are in phase with primary winding  320  and secondary windings  100 ,  101 ,  102 ,  103  and  104  are in phase with primary winding  318 . Moreover, in this embodiment, at least a sub-set of secondary node voltage magnitudes (e.g. magnitudes at nodes  120 ,  123 ,  125 ,  128 ,  130  and  133  ) are greater than the output node voltage magnitudes (e.g., at nodes  121 ,  122 , etc.). 
     When the inputs and outputs are linked to primary  312  and secondary  314  as indicated above, the output voltages and current are phase shifted by 40° as desired and all of the advantages associated with the autotransformer as described above are associated with the isolation transformer illustrated. It should be noted that an additional advantage with the isolation transformer is that adjustable tappings can be added to the primary to change the tums-ratio between the primary and secondary windings in the field thereby enabling a transformer user to modify the transformer action to either result in step-up transformation, step-down transformation or unity gain. 
     Referring now to FIGS. 5 and 6, instead of providing a delta wound primary  312  in conjunction with secondary  314 , a “Wye-wound” primary  350  may be substituted for delta-wound primary  312  to provide a slightly different isolation transformer  10  configuration. To this end, Wye-wound primary  350  includes windings  336 ,  338  and  340 . One end of each winding  336 ,  338  and  340  is linked at a central point  334  and the other ends of windings  336 ,  338  and  340  form primary input nodes  328 ,  330  and  332 , respectively. Referring also to FIG. 1, although not illustrated in FIG. 6, input lines  5 ,  6  and  7  from source  9  are linkable to input nodes  328 ,  330  and  332 , respectively. Referring also to FIG. 5, secondary  314  is linked to nine output lines as indicated above. Operation of this Wye-wound primary configuration is essentially identical to operation indicated above and therefore will not be explained here in detail. 
     Referring now to FIG. 7, therein is illustrated an inventive method of configuring a transformer. To this end, at process block  600 , the first step is to provide an autotransformer with nine 40° phase-shifted identical magnitude output voltages. Next, at step  602 , at least a unity gain input node set and a step-down input node set are provided. At step  604  three-phase AC input lines are linked to the nodes in one of the unity gain or step-down input sets wherein the interset nodes are separated by 120° phase shift. At step  606  the nine output nodes are linked via lines to nine inputs of a rectifier and at step  608  three-phase power is provided to the nodes. When three-phase power is provided to the input nodes, the three-to-nine phase transformation occurs. 
     It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. 
     To apprise the public of the scope of this invention, the following claims are made: