Abstract:
A method for manufacturing a transfer switch includes providing a transfer switch including a first shunt contact and a second shunt contact, and operationally coupling a shunt solenoid to the first shunt contact and the second shunt contact such that when the solenoid is electrically activated the first shunt contact electrically couples a first source to a load and the second shunt contact electrically couples a second source to the load in a first pre-determined amount of time.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to electrical power transfer and, more particularly, to electrical power transfer switches. 
     Many businesses use transfer switches to switch between power sources which supply power to the business. For example, from a public utility source to a private secondary supply. Critical equipment and businesses, such as hospitals, airport radar towers, and high volume data centers are dependent upon transfer switches to provide continuous power. More specifically, in the event that power is lost from a primary source, the transfer switch shifts the load from the primary source to an alternate source in a minimal amount of time to facilitate providing continuous electrical power to such equipment and businesses. 
     At least one known transfer switch utilizes a “make-beforebreak” switch to transfer the load from the primary source to the alternate source. The make before break switch includes dual main contacts which require dual shafts and a plurality of actuators. Transfer switches including dual main contacts and dual shafts may also include dual solenoids to drive the shafts. In the event one of the solenoids fails, the main contacts may remain in an undesired position thereby preventing the transfer switch from activating to enable the business to switch to an alternate power supply. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method for manufacturing a transfer switch is provided. The method includes providing a transfer switch including a first shunt contact and a second shunt contact, and operationally coupling a shunt solenoid to the first shunt contact and the second shunt contact such that when the solenoid is electrically activated the first shunt contact electrically couples a first source to a load and the second shunt contact electrically couples a second source to the load in a first pre-determined amount of time. 
     In another aspect, an apparatus for transferring power from a first source to a second source is provided. The apparatus includes a first shunt contact, a second shunt contact, and a shunt solenoid operationally coupled to the first shunt contact and the second shunt contact such that when the shunt solenoid is electrically activated the first shunt contact electrically couples a first source to a load and the second shunt contact electrically couples a second source to the load in a first pre-determined amount of time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a power system including a transfer switch. 
     FIG. 2 is an illustration of one embodiment of a transfer switch that may be used with the power system shown in FIG.  1 . 
     FIG. 3 is a side view of the transfer switch shown in FIG.  2 . 
     FIG. 4 is a perspective view of a portion of the transfer switch in FIG. 2 in a first operating position. 
     FIG. 5 is a perspective view of a portion of the transfer switch in FIG. 2 in a second operating position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a power system  8  which includes a transfer switch  10  used to selectively switch between a plurality of power sources, e.g. between a power source  12  and a power source  14 , to supply electrical power to a load  16 . For example, in one embodiment, load  16  is a hospital, airport radar tower or other electrical power user that desires a substantially uninterrupted power supply. Load  16 , via switch  18 , draws power from source  12  under normal operating conditions. If, for example, power source  12  fails or becomes inadequate to supply load  16 , load  16  is transferred via switch  18  to draw power from source  14 . When source  12  again provides sufficient power, load  16  may be transferred via switch  18  to resume drawing power from source  12 . The foregoing description of transfer switch  10  operation is exemplary only, and additional functions may be performed by transfer switch  10 . 
     FIG. 2 is a transfer switch  18  that may be used with power system  8  (shown in FIG.  1 ). Transfer switch  18  includes a plurality of main contact phase compartments  20 . More specifically, switch  18  includes one phase compartment  20  for each phase of power entering transfer switch  18 . Compartments  20  are mechanically coupled together with a plurality of mechanical fasteners. In the exemplary embodiment, transfer switch  18  includes three phase compartments  20 . Transfer switch  18  also includes a main finger shaft  24 , a first shunt contact shaft  26 , and a second shunt contact shaft  28 . Finger shaft  24 , first shaft  26  and second shaft  28  extend from a first side  30  of transfer switch  18  to a second side  32  of transfer switch  18 . In one embodiment, finger shaft  24 , first shaft  26  and second shaft  28  are rotatably coupled to transfer switch  18  using a mechanical fastener (not shown), such as, but not limited to, a bolt and a nut, a mechanical clip or any other suitable fastener. 
     FIG. 3 is a side view of transfer switch  18 . Transfer switch  18  includes a single coil shunt contact solenoid  40  mechanically coupled to transfer switch  18 , a first shunt linkage  42 , a second shunt linkage  44 , and a third shunt linkage  46 . First shunt linkage  42  includes a first end  50  rotatably coupled to first shunt shaft  26 , and a second end  52  rotatably coupled to third linkage  46 . Second shunt linkage  44  includes an opening  60 , a first end  62  rotatably coupled to second shunt shaft  28  and a second end  64  rotatably coupled to third linkage  46 . Solenoid  40  is controlled by a controller (not shown). 
     Solenoid  40  includes a plunger  70  and a spring  72 . Plunger  70  also includes a connector  74  which extends through opening  60  to couple connector  74  in slidable contact with second shunt shaft  28 . Transfer switch  18  also includes a first limit switch  80  and a second limit switch  82  mechanically coupled to transfer switch  18 . 
     FIG. 4 is a perspective view of a portion of transfer switch  18  in a first operating position  90 . FIG. 5 is a perspective view of a portion of transfer switch  18  in a second operating position  92 . Phase compartment  20  includes a first source bus  100 , a second source bus  102 , and a load bus  104  mounted in a housing  106 . In the exemplary embodiment, housing  106  is fabricated from a non-conductive material and electrically isolates each first source bus  100 , second source bus  102 , and load bus  104 . First source bus  100  includes a first contact  110 , a second contact  112 , and a third contact  114  that are each electrically coupled to a power source such as source  14  (shown in FIG.  1 ). Second source bus  102  includes a first contact  120 , a second contact  122  and a third contact  124  each electrically coupled to a source such as source  12  (shown in FIG.  1 ). Load bus  104  includes a first contact  130 , a second contact  132 , and a third contact  134  each electrically coupled to a load, such as load  16  (shown in FIG.  1 ). 
     Phase compartment  20  also includes a finger assembly  140  mechanically coupled to finger shaft  24 . Finger assembly  140  includes a movable finger  142  and two contact pads  144  mounted on finger  142 . Finger assembly  140  also includes an electrical conductor  146 , such as a braid assembly attached to finger  142  to electrically couple finger  142  to load bus  104  as finger  142  is re-positioned. In the exemplary embodiment, finger  142  is symmetrical about a centerline  148 . 
     Phase compartment  20  also includes a first shunt contact  150  and a second shunt contact  152  that are each positioned within phase compartment  20 . First shunt  150  is mechanically coupled to first shunt shaft  26 , and second shunt contact  152  is mechanically coupled to second shunt shaft  28 . First shunt contact  150  includes a first contact  160  and a second contact  162 . First contact  160  and second contact  162  are electrically isolated from shunt shaft  26  by an electrically non-conductive device (not shown). Second shunt contact  152  includes a first contact  170  and a second contact  172  which are electrically isolated from shunt shaft  28 . In an alternative embodiment, first shunt contact  150  and second shunt contact  152  are fabricated such that electrical current is transferred through first shunt contact  150  and second shunt contact  152  without the use of contacts  160 ,  162 ,  170 , and  172 , respectively. 
     During use, when the controller senses the available power from either source  14  or source  16 . More specifically, the controller monitors a phase differential between source  12  and source  14  to determine when source  12  and source  14  are in approximate synchronism and when the available power is below a pre-set value. When source  12  and source  14  are approximately in synchronism, the controller causes solenoid  40  (shown in FIG. 2) to actuate thereby retracting plunger  70  (shown in FIG. 2) into solenoid  40  to move first shunt contact  150  and second shunt contact  152  from first operating position  90  (shown in FIG. 4) to second operating position  92  (shown in FIG.  5 ). In first operating position  90 , first shunt contact  150  and second shunt contact  152  are “open” such that contacts  150  and  152  do not conduct electricity. In second operating position  92 , first shunt contact  150  rotates such that first shunt contacts  160  and  162  electrically couple with contacts  112  and  130 , respectively, thereby allowing electricity to flow from first power source  12  to load  16 . Additionally, second shunt contact  152  is rotated such that second shunt contacts  170  and  172  electrically couple with contacts  122  and  132 , respectively, thereby allowing electricity to flow from second power source  14  to load  16 . For example, retracting plunger  70  causes second shunt linkage  44  (shown in FIG. 2) to translate to move third linkage  46  (shown in FIG.  2 ). As third linkage  46  is re-positioned, first shunt linkage  42  (shown in FIG. 2) is rotated at approximately the same rate as second shunt linkage  44 . Because first shunt contact  150  is coupled to first shunt shaft  26 , and second shunt contact  152  is coupled to second shunt shaft  26 , actuating solenoid  40  causes contacts  150  and  152  to translate from a non-conducting state to a conducting state to enable electricity to flow from first power source  12  to load  16  and from second power source  14  to load  16 . 
     In the exemplary embodiment, shunt contact  150  and shunt contact  152  are rotated together such that buses  100  and  102  respectively, are electrically coupled to load bus  104  when source  12  and source  14  are approximately synchronized. Shunt contacts  150  and  152  remain in a closed position for a first pre-determined amount of time. In one embodiment, shunt contacts  150  and  152  are closed between approximately seventy five milliseconds and approximately one hundred and twenty five milliseconds. In another embodiment, shunt contacts  150  and  152  are closed approximately one hundred milliseconds. 
     When first shunt contact  150  and second shunt contact  152  are in the closed position, i.e. conducting electricity, a single coil solenoid (not shown) coupled to finger  142  is activated to cause finger  142  to traverse from a first finger position  200  to a second finger position  202 . For example, if finger  142  is in position  200 , contacts  120  and  144  are electrically coupled such that electricity flows from power source  14  to load  16 . Furthermore, activating the finger solenoid causes finger  142  to traverse to position  202  such that contacts  110  and  144  are coupled to enable power to flow from source  12  to load  16 . 
     In one embodiment, main finger  142  traverses from first finger position  200  to second finger position  202  in a second pre-determined amount of time. In one embodiment, main finger  142  traverses from first position  200  to second position  202  between approximately sixty milliseconds and approximately seventy milliseconds. In the exemplary embodiment, the first pre-determined amount of time, i.e. amount of time shunt contacts  150  and  152  remain closed, is greater than the second pre-determined amount of time, i.e. time required for main finger  142  to traverse from first position  200  to second position  202 . When a main finger shaft of main finger  142  reaches second position  202 , another limit switch, such as limit switch  80  or limit switch  82  signals solenoid  40  to cut off. Spring  72  then extends plunger  70  causing shunt contacts  150  and  152  to return to an “open” position. For example, if finger  142  is in position  200 , when finger  142  traverses to second position  202 , finger  142  depresses first limit switch  80  and power to solenoid  40  is terminated, allowing energy stored within spring  72  to force plunger  70  out of solenoid  40 , thereby moving linkage  46  causing first shunt contact  150  and second shunt contact  152  to return to an open position. In another embodiment, if finger  142  is in second position  202 , when finger  142  traverses to first position  200 , finger  142  depresses first limit switch  82  and power to solenoid  40  is terminated, allowing energy stored within spring  72  to force plunger  70  out of solenoid  40 , thereby moving linkage  46  causing first shunt contact  150  and second shunt contact  152  to return to an open position. 
     In the exemplary embodiment, transfer switch  18  facilitates transferring load  16  from source  12  to source  14 , in phase, and without a loss of power to load  16 . Furthermore, transfer switch  18  includes only the main finger shaft coil and coil of solenoid  40  which can both operate as an open or a closed transition switch. In the event of a shunt contact failure, switch  18  can continue to operate as an open transition switch. Additionally, shunt contacts  150  and  152  are not required to be the same ampacity as the main contacts, nor have a load breaking or an arc quench capability. 
     Transfer switch  18  is adaptable for a two-pole, a three-pole, and a four-pole modular configuration with minimal additional hardware. Symmetrical and one-piece design of parts such as compartments  20  facilitate reducing a number of components, thus facilitating cost reductions. 
     Additionally, transfer switch  18  facilitates reducing the inertia of a shunt contact train. For example, during a closed transition operation without direct frequency control of the alternate source, the alternate source, typically a generator set, is usually set to a tenth of a hertz higher than the alternate source (a utility). An electronic controller monitoring the phase differential between the primary source and the alternate source will determine when the two are in synchronism. At that point a shunt signal will be issued causing the load to be connected to both the primary source and the alternate source and the main finger to transfer. Since shunt contacts  150  and  152  are physically smaller than bus  100  and bus  102 , shunt contacts  150  and  152  facilitate shunting the circuit more rapidly. Therefore, a length of time before shunting with a subsequent shifting to a more out of phase condition is reduced. This is beneficial to the load not seeing an electrical anomaly. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.