Abstract:
A dual-input nine-phase autotransformer converts first and second three-phase AC inputs to a nine-phase AC output. The autotransformer includes input terminals for connection to a first three-phase AC input and a second three-phase AC input smaller than the first three-phase AC input. The autotransformer includes a first plurality of coils, a second plurality, and a third plurality of coils wound on respective phase legs of the autotransformer. The autotransformer includes a plurality of output terminals for providing a plurality of AC output voltages, and a plurality of internal terminals for connecting the first, second, and third plurality of coils in a configuration that provides a 40° phase shift in the AC outputs provided by the dual-input nine-phase autotransformer.

Description:
BACKGROUND 
     The present invention is related to autotransformers, and in particular to a dual-input nine-phase autotransformer. 
     An autotransformer is an electrical transformer with only one winding that acts as both the primary and secondary winding associated with a typical transformer. As a result, autotransformers can be smaller, lighter and cheaper than standard dual-winding transformers. This makes autotransformers an attractive alternative in application (such as aircraft applications) in which weight is an important factor. 
     Autotransformers are often-times employed in AC-DC power conversion systems. In theory, AC-DC power conversion may be accomplished with a plurality of diode pairs, each pair connected to a different phase of the AC input, to provide a rectified output. However, this type of rectifier leads to substantial current harmonics that pollute the electric power generation and distribution system. To reduce current harmonics, autotransformers are employed to increase the number of AC phases supplied to the rectifier unit. For example, in an eighteen-pulse converter (an AC-DC converter having an eighteen step staircase current waveform at each of the AC inputs) the autotransformer is used to transform the three-phase AC input, whose phases are spaced at 120°, into a system with nine phases spaced at 40°. This has the effect of reducing the harmonics associated with the AC-DC conversion. 
     SUMMARY 
     A dual-input nine-phase autotransformer converts first and second three-phase AC inputs to a nine-phase AC output. The autotransformer includes a first plurality of input terminals for connection to a first three-phase AC input and a second plurality of input terminals for connection to a second three-phase AC input. The autotransformer includes a first plurality of coils A 0 -A 6  wound on a first phase leg of the autotransformer, a second plurality of coils B 0 -B 6  wound on a second phase leg of the autotransformer, and a third plurality of coils C 0 -C 6  wound on a third phase leg of the autotransformer. The autotransformer includes a plurality of output terminals for providing a plurality of AC output voltages, and a plurality of internal terminals for connecting the first, second, and third plurality of coils in a configuration that provides a 40° phase shift in the AC outputs provided by the dual-input nine-phase autotransformer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a dual-input nine-phase autotransformer rectifier unit according to an embodiment of the present invention. 
         FIG. 2  is a simple cross-sectional view of the dual-input nine-phase autotransformer according to an embodiment of the present invention. 
         FIG. 3  is a vector diagram illustrating a winding configuration of the dual-input nine-phase autotransformer according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a circuit diagram of alternating current (AC) to direct current (DC) power conversion system  100  according to an embodiment of the present invention. Power conversion system  100  includes dual-input nine-phase autotransformer  102  (hereinafter, “autotransformer  102 ”), rectifier unit  104 , and DC link capacitor C DC . Autotransformer  102  includes first AC input terminals In 1 , In 2 , In 3  and second AC input terminals In 4 , In 5 , In 6 . Each of the labeled input terminals represents a terminal connection point to the windings associated with autotransformer  102 . The location of terminals associated with first AC input terminal In 1 , In 2 , In 3 , and second AC input terminal In 4 , In 5 , In 6  is described in the vector diagram shown in  FIG. 3 . First AC input terminals In 1 , In 2 , In 3  are connected to receive AC power labeled Va, Vb, Vc, respectively, while second AC input terminals In 4 , In 5 , In 6  are connected to receive AC power labeled Va′, Vb′, Vc′. For example, in an aircraft application AC power labeled Va, Vb, Vc may be 230 Volt (V) AC power provided by an on-board generator, while AC power labeled Va′, Vb′, Vc′ may be 115 V AC power delivered by a ground cart when the aircraft is on the ground. 
     Depending on the application, autotransformer  102  is configured to step up or step down the voltage provided at first input terminals In 1 , In 2 , In 3  and second input terminals In 4 , In 5 , In 6 . For example, in one embodiment the voltage provided at the first input terminals is stepped down within a range defined by the ratio between the output voltage of the autotransformer (e.g., voltage Vout provided at output terminal Out 1 ) and the input voltage Va provided at one of the first input terminals (e.g., Vout/Va=γ, where 0.5≦γ≦1). Likewise, in another embodiment the voltage provided at second input terminals is stepped up within a range defined by the ratio between the output voltage of the autotransformer (e.g., voltage Vout provided at output terminal Out 1 ) and the input voltage Va′ provided at one of the second input terminals (e.g., Vout/Va′=γ, where 1≦2γ≦2). In this way, two input sources may be employed to generate the desired DC output voltage for provision to attached loads. Likewise, autotransformer  102  includes nine output terminals Out 1 , Out 2 , Out 3 , Out 4 , Out 5 , Out 6 , Out 7 , Out 8 , Out 9  that are connected to rectifier unit  104  for rectification to the desired DC output. 
     Rectifier unit  104  includes a plurality of diode pairs (labeled D 1  and D 1 ′, D 2  and D 2 ′, D 3  and D 3 ′, D 4  and D 4 ′, D 5  and D 5 ′, D 6  and D 6 ′, D 7  and D 7 ′, D 8  and D 8 ′, and D 9  and D 9 ′), each pair connected to one of the plurality of output phases provided by autotransformer  12 . Diodes D 1 -D 9  are connected to output terminals Out 1 -Out 9 , respectively, to provide a positive rectified output voltage to DC output voltage Vdc+. Likewise, diodes D 1 ′-D 9 ′ are connected to output terminals Out 1 -Out 9 , respectively, to provide a negative rectified output voltage to DC output voltage Vdc−. In the embodiment shown in  FIG. 1 , rectifier unit  104  includes 18 diodes, making AC-DC power conversion system an eighteen-pulse converter. 
       FIG. 2  is a simple cross-sectional diagram of dual-input nine-phase autotransformer  102  according to an embodiment of the present invention. In the embodiment shown in  FIG. 2 , autotransformer  102  includes three phase-legs labeled  110   a ,  110   b , and  110   c . Each phase leg  110   a ,  110   b ,  110   c  is associated with one phase of the three-phase AC input provided to autotransformer  102 . For example, AC input voltage Va provided to autotransformer  102  at input terminal In 1  is provided to coils wound around phase leg  110   a . Likewise, AC input voltage Vb provided to autotransformer  102  at input terminal In 2  is provided to coils wound around phase leg  110   b , and AC input voltage Vc provided at input terminal In 3  is provided to coils wound around phase leg  110   c . As a dual-input autotransformer, each phase leg also includes a second input terminal for connection to a second AC input. For example, AC input voltage Va′ provided to autotransformer  102  at input terminal In 4  is provided to coils wound around phase leg  110   a . Likewise, AC input voltage Vb′ provided to autotransformer  102  at input terminal In 5  is provided to coils wound around phase leg  110   b , and AC input voltage Vc′ provided to autotransformer  102  at input terminal In 6  is provided to coils wound around phase leg  110   c.    
     The plurality of output terminals Out 1 -Out 9  are connected to one of the three phase legs  110   a ,  110   b , and  110   c . For example, AC output terminals Out 6 , Out 7 , Out 8  are associated with phase leg  110   a . Likewise, AC output terminals Out 1 , Out 2 , Out 9  are associated with phase leg  110   b , and AC output terminals Out 3 , Out 4 , and Out 5  are associated with phase leg  110   c.    
     As described in more detail with respect to the vector diagram shown in  FIG. 3 , a plurality of coils is wound around each phase leg. For example, in one embodiment three groups of seven coils (labeled in  FIG. 3  as coils A 0 -A 6 , B 0 -B 6 , and C 0 -C 6 ) are wound around phase legs  110   a ,  110   b , and  110   c , respectively. The number of turns (i.e., length) of each coil is varied, and a plurality of interconnections internal to autotransformer  102  allow connections to be made between various coils on each of the three phase legs  110   a ,  110   b ,  110   c . The number of coils, the turns of each coil, and the interconnection between various coils affects the performance of autotransformer  102 . The simple cross-sectional view shown in  FIG. 2  does not illustrate the plurality of coils associated with each phase leg, or the turns or various interconnections of the coils with one another. A particular configuration of the plurality of coils associated with each phase leg according to an embodiment of the present invention is illustrated in the vector diagram shown in  FIG. 3 . 
       FIG. 3  is a vector diagram illustrating a winding configuration of dual-input nine-phase autotransformer  102  according to an embodiment of the present invention. In the embodiment shown in  FIG. 3 , autotransformer  102  is a symmetrical system, such that the number of coils, and winding turns associated with each of the coils is symmetrical between each of the phase legs  110   a ,  110   b , and  110   c . The phase shift between respective output terminals is illustrated by the angle measured between two output terminals based on point n (located in the middle of the triangular shape). For example, the phase shift between output terminal Out 1  and output terminal Out 9  is 40°. Similarly, the phase shift between output terminal Out 9  and output terminal Out 8  is 40°. It is a goal of autotransformer  102  to provide a nine-phase output in which each of the output phases is shifted 40° relative to one another. 
     The vector diagram shown in  FIG. 3  illustrates schematically the electrical configuration of coils in autotransformer  102 . In particular, all straight line arrows in the vector diagram represent coils, with the length of the straight line arrow being proportional to the number of winding turns of the coil. The polarity of the coil is defined by the direction of the arrow. All lines of the same orientation represent a same phase of the three-phase input provided to autotransformer  102 . Output terminals for connection to rectifier unit  104  are denoted with black dots and are labeled Out 1 -Out 9 , as denoted in  FIG. 1 . Internal connections within autotransformer  102  are denoted with circles and are labeled internal terminals T 1 -T 9 . Each winding connected between either output terminals Out 1 -Out 9  or internal terminals T 1 -T 9  is denoted with a coil number. For example, coils associated with phase leg  110   a  includes coils A 0 -A 6 , while coils associated with phase leg  110   b  include coils B 0 -B 6  and coils associated with phase leg  110   c  includes coils C 0 -C 6 . The direction of the arrows representing each of the windings is dictated by the phase of the winding. For example, all coils associated with phase leg  110   a  (e.g., coils A 0 -A 6 ) point the same direction, with the same holding true for all coils associated with phase legs  110   b  and  110   c , respectively. The phase difference or angle between the AC inputs Va, Vb, Vc provided to first AC input terminals In 1 , In 2 , In 3  is 120°, respectively. Similarly, the phase difference between the AC inputs Va′, Vb′, and Vc′ provided via second AC input terminals In 4 , In 5 , In 6  is also 120°. 
     In the embodiment shown in  FIG. 2 , first AC input terminals In 1 , In 2 , In 3  form the corners of a triangle. Likewise, second AC input terminals In 4 , In 5 , In 6  are connected at the midpoint of coils A 2 , B 2 , and C 2 , respectively. Coils A 0 -A 3  are connected in series with one another via the plurality of internal terminals T 1 , T 2 , and T 3 . Likewise, coils B 0 -B 3  are connected in series via the plurality of internal terminals T 4 , T 5 , T 6 , and coils C 0 -C 3  are connected in series via the plurality of internal terminals T 7 , T 8 , and T 9 . Coils A 0  and C 3  are connected together at input terminal In 1 , which is connected to AC input voltage Va. Likewise, coils B 0  and A 3  are connected together at input terminal In 2 , which is connected to AC input voltage Vb, and coils C 0  and B 3  are connected together at input terminal In 3 , which is connected to AC input voltage Vc. 
     In the embodiment shown in  FIG. 3 , connection to each of the plurality of output terminals is as follows. Coil B 6  is connected between output terminal Out 1  and internal terminal T 1 , located between coils A 0  and A 1 . Coil B 5  is connected between output terminal Out 2  and internal terminal T 2 , located between coils A 1  and A 2 . Coil C 4  is connected between output terminal Out 3  and internal terminal T 3 , located between coils A 2  and A 3 . Coil C 6  is connected between output terminal Out 4  and internal terminal T 4  located between coils B 0  and B 1 . Coil C 5  is connected between output terminal Out 5  and internal terminal T 5  located between coils B 1  and B 2 . Coil A 4  is connected between output terminal Out 6  and internal terminal T 6  located between coils B 2  and B 3 . Coil A 6  is connected between output terminal Out 7  and internal terminal T 7  located between coils C 0  and C 1 . Coil A 5  is connected between output terminals Out 8  and internal terminal T 8  located between coils C 1  and C 2 . Coil B 4  is connected between output terminal Out 9  and internal terminal T 9  located between coils C 2  and C 3 . 
     The configuration of windings illustrated in  FIG. 3  generates nine phase-shifted outputs (via output terminals Out 1 -Out 9 ) that are provided to rectifier unit  104 , which includes a pair of diodes associated with each input to provide an 18-pulse rectifier unit. The AC outputs (Out 1 -Out 9 ) provided by autotransformer  102  are phase-shifted relative to one another by the desired amount (e.g., 40°). In addition, the size of autotransformer  102  is determined, in part, by the number of windings employed and the number of turns or length of each coil. For example, first output terminal Out 1  is provided at a phase equal to that of first AC input terminal In 1 . Coil A 0  (located on phase leg  110   a ) is connected to input terminal In 1  on one end, and to internal terminal T 1  at the other end. Coil B 6  (located on phase leg  110   b ) is connected to internal terminal T 1 , and terminates at AC output terminal Out 1 . As illustrated by the physical location of AC output terminal Out 1  in the vector diagram shown in  FIG. 3 , AC output terminal Out 1  is in-phase with the AC input Va provided at input terminal In 1 . Coil A 1  is connected to internal terminal T 1 , and terminates at internal terminal T 2 . Coil B 5  is connected to internal terminal T 2 , and terminates at AC output terminal Out 2 . The phase difference between the AC output provided at output terminal Out 1  and the AC output provided at output terminal Out 2  is 40°. 
     The length or number of turns associated with each coil is a function of the desired step up/step down voltage associated with autotransformer  102 . For example, for a step down ratio of γ=0.875, the following coil configurations are employed: 
     
       
         
               
               
               
             
           
               
                   
               
               
                   
                 Coil 
                 Number of turns 
               
               
                   
               
             
             
               
                   
                 A0, B0, C0 
                 n 0   
               
               
                   
                 A1, B1, C1 
                 n 1  = 1.638 * n 0   
               
               
                   
                 A2, B2, C2 
                 n 2  = 6.725 * n 0   
               
               
                   
                 A3, B3, C3 
                 n 3  = 2.638 * n 0   
               
               
                   
                 A4, B4, C4 
                 n 4  = 2.578 * n 0   
               
               
                   
                 A5, B5, C5 
                 n 5  = 2.578 * n 0   
               
               
                   
                 A6, B6, C6 
                 n6 = 0.5 * n 0   
               
               
                   
               
             
          
         
       
     
     In other embodiments, depending on the step-up/step-down ratio, the number of turns associated with each coil is varied to provide the desired output. A benefit of the configuration illustrated in  FIG. 3 , is the ability to include both step-up/step-down functionality in a single, symmetrical autotransformer. In addition, the configuration of coils minimizes the apparent power kVA rating of the autotransformer. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.