Method and apparatus for providing selectable output voltages

Abstract

A transformer is provided along with a combination of bridging tap-changers to provide a wide range of selectable output voltages in discrete, relatively small voltage steps where the highest voltage is more than double the lowest output voltage. Relatively inexpensive, off-the-shelf, bridging tap-changers are utilized in conjunction with transformer winding schemes to provide a low winding loss ratio.

1. A transformer comprising:
 a primary winding having terminals for conveying input power;  
 a plurality of secondary windings having output terminals for conveying output power, said secondary windings including a first pair of isolated center tapped windings and a second pair of isolated windings;  
 a plurality of switches electrically coupling leads of said plurality of secondary windings providing a selection of n distinct output voltages at said output terminals ranging from x voltage units to y voltage units, wherein y is greater than 2x.  
 
     
     
       2. The transformer of  claim 1 , wherein incremental steps of said n distinct output voltages are equal. 
     
     
       3. The transformer of  claim 1 , wherein said plurality of switches includes a pair of ganged five-position bridging tap changers, wired as a three-position series-parallel-series switch interconnecting said first pair of isolated center tapped windings. 
     
     
       4. The transformer of  claim 3 , wherein said plurality of switches further includes a three-position bridging tap changer interconnecting said second pair of isolated windings. 
     
     
       5. The transformer of  claim 4 , wherein n is equal to 9, x is equal to 6, and y is equal to 14. 
     
     
       6. The transformer of  claim 3 , wherein each winding of said second pair of isolated windings is center tapped and interconnected via a five-position bridging tap changer. 
     
     
       7. The transformer of  claim 6 , wherein n is equal to 15, x is equal to 10, and y is equal to 24. 
     
     
       8. The transformer of  claim 3 , wherein each winding of said second pair of isolated windings is divided into thirds by two winding taps dividing said windings and wherein said second pair of windings is interconnected via a seven-position bridging tap changer. 
     
     
       9. The transformer of  claim 8 , wherein n is equal to 21, x is equal to 14, and y is equal to 34. 
     
     
       10. The transformer of  claim 1 , wherein a fixed connection between an end lead of one winding of said first pair and an end lead of one winding of said second pair is provided via an external coil connection. 
     
     
       11. The transformer of  claim 1 , wherein a fixed connection between an end of one winding of said first pair and an end of one winding of said second pair is provided via an internal coil connection. 
     
     
       12. The transformer of  claim 1 , wherein a ratio of maximum secondary winding loss to minimum secondary winding loss is less than 1.35. 
     
     
       13. A transformer comprising:
 a primary winding having terminals for conveying input power;  
 a plurality of secondary windings having output terminals for conveying output power, said secondary windings including a first pair of isolated windings tapped at thirds and a second pair of isolated windings;  
 a plurality of switches electrically coupling leads of said plurality of secondary windings providing a selection of n distinct output voltages at said output terminals ranging from x voltage units to y voltage units, wherein y is greater than 2x.  
 
     
     
       14. The transformer of  claim 13 , wherein the incremental steps of said n distinct output voltages are equal. 
     
     
       15. The transformer of  claim 13 , wherein leads of said first pair of isolated windings are interconnected via a pair of ganged seven-position bridging tap changers connected as four-position series-parallel-series switches. 
     
     
       16. The transformer of  claim 13 , wherein leads of said second pair of isolated secondary windings are interconnected to a three-position bridging tap changer. 
     
     
       17. The transformer of  claim 13 , wherein:
 each of said second pair of isolated windings is center tapped with leads interconnected via a five-position tap changer; and  
 each of said second pair comprises k turns each divided into two groups of m turns, and each of said first pair comprises p turns each divided into three groups of q turns where k is equal to 2m, q is equal to 2k+m, and p is equal to 3q.  
 
     
     
       18. The transformer of  claim 17 , wherein n is equal to 20, x is equal to 15, and y is equal to 34. 
     
     
       19. The transformer of  claim 13 , wherein:
 each of said second pair of isolated secondary windings is tapped at thirds with leads interconnected via a seven-position bridging switch; and  
 each of said second pair comprises k turns each divided into three groups of m turns and each of said first pair comprises p turns each divided into three groups of q turns where k is equal to 3m, q is equal to 2k+m, and p is equal to 3q.  
 
     
     
       20. The transformer of  claim 19 , wherein n is equal to 28, x is equal to 21, and y is equal to 48. 
     
     
       21. The transformer of  claim 13 , wherein a fixed connection between an end lead of one winding of said first pair and an end lead of one winding of said second pair is provided via an external coil connection. 
     
     
       22. The transformer of  claim 13 , wherein a fixed connection between an end of one winding of said first pair and an end of one winding of said second pair is provided via an internal coil connection. 
     
     
       23. The transformer of  claim 13 , wherein a ratio of maximum secondary winding loss to minimum secondary winding loss is less than 1.29. 
     
     
       24. A method for providing incrementally selectable electrical power outputs from a transformer, said method comprising:
 providing a primary winding and a plurality of secondary windings including a first set of secondary windings and a second set of secondary windings;  
 providing at least a first switch and a second switch for interconnecting leads of said plurality of secondary windings, said first switch being independently operable and comprising a ganged pair of bridging tap-changers and said second switch comprising a single bridging tap-changer;  
 creating a plurality of sub-windings by electrically connecting taps to selected points on said secondary windings to stationary terminals of said first and second switch; and  
 connecting end leads of said secondary windings and tap leads of said sub-windings with said first and second switch such that said secondary windings and associated sub-windings are configurable in parallel, series, and series-parallel combinations that provide n distinct output voltages ranging from x voltage units to y voltage units, wherein y is greater than 2x.  
 
     
     
       25. The method of  claim 24 , wherein said connecting step further comprises:
 interconnecting said first set of secondary windings utilizing said first switch, wherein one switch position of said first switch connects said first set of windings in parallel; another switch position connects said first set of windings in series and all other positions of said first switch connects only portions of said first set of windings in parallel and connects a remainder of said first set of windings in series.  
 
     
     
       26. The method of  claim 24 , wherein said connecting step further comprises:
 interconnecting said second set of secondary windings utilizing said second switch, wherein one switch position of said second switch fully bypasses said second set of windings; another switch position fully series connects said second set of windings, and all other switch positions of said second switch connects only a portion of said second set of windings in series and bypasses a remainder of said second set of windings.  
 
     
     
       27. The method of  claim 24 , further comprising providing a three-position bridging tap changer as said second switch. 
     
     
       28. The method of  claim 24 , wherein said creating a plurality of sub-windings is accomplished by center-tapping each of said second set of secondary windings, said method further comprising providing a five position bridging tap changer as said second switch. 
     
     
       29. The method of  claim 24 , wherein said creating a plurality of sub-windings is accomplished by tapping each of said second set of secondary windings at thirds, said method further comprising providing a seven position bridging tap changer as said second switch. 
     
     
       30. A transformer comprising:
 a primary winding;  
 a plurality of secondary windings having a plurality of sub-windings created by taps at selected points on selected ones of said plurality of secondary windings;  
 a first switch and a second switch having terminals that connect end leads of said secondary windings and tap leads of said sub-windings, said first switch being independently operable and comprising a ganged pair of bridging tap-changers, and said second switch comprising a single bridging tap-changer, wherein said secondary windings and associated sub-windings are configurable in parallel, series and series-parallel-series combinations to yield n distinct voltage outputs ranging from x voltage units to y voltage units, wherein y is greater than 2x, and wherein a substantially low maximum to minimum winding loss is achieved.  
 
     
     
       31. The transformer of  claim 30 , wherein said first switch is interconnected between corresponding windings of a first set of secondary windings, wherein at least one switch position of said first switch connects said corresponding windings in parallel, at least one other switch position connects said corresponding windings in series, and other positions of said first switch connect only a portion of said corresponding windings in parallel and connect a remainder of said corresponding windings in series. 
     
     
       32. The transformer of  claim 30 , wherein said second switch is interconnected between corresponding windings of a second set of secondary windings, wherein one switch position of said second switch fully bypasses said corresponding windings; another switch position fully connects in series said corresponding windings, and all other switch positions of said second switch connects in series only a portion of said corresponding windings and bypasses the remainder of said corresponding windings. 
     
     
       33. The transformer of  claim 30 , wherein said second switch comprises a three position bridging tap changer. 
     
     
       34. The transformer of  claim 30 , wherein said second switch comprises a five position bridging tap changer. 
     
     
       35. The transformer of  claim 30 , wherein said second switch comprises a seven position bridging tap changer.