Abstract:
A dual resistor for eliminating the requirement for two different value resistors. The dual resistor includes a conditioning resistor at a high resistance value and a run resistor at a low resistance value. The run resistor can travel inside the conditioning resistor. The run resistor is capable of being advanced by a drive assembly until an electrical path is completed through the run resistor thereby shorting out the conditioning resistor and allowing the lower resistance run resistor to take over as the current carrier.

Description:
The United States of America may have certain rights to this invention under Management and Operating contract No. DE-AC05-84ER40150 from the Department of Energy. 

   FIELD OF THE INVENTION 
   The present invention relates to resistors and particularly to a dual design resistor for high voltage conditioning of electrical equipment. 
   BACKGROUND OF THE INVENTION 
   Current spikes can be damaging when directing high voltage into machinery, transmission lines, or injector guns for accelerators. In the case of accelerators for example, high current spikes at high voltage can flash and damage the injector gun of the accelerator if the power supply is not properly current limited. 
   At the Thomas Jefferson National Accelerator Facility, the injector gun is designed to run at a 350 kV level. At startup, the injector gun must be conditioned for a period of time to ensure that the gun electrode is stable at high voltages and without current emissions. The voltage and current are monitored on startup and when the current fluctuations measure less than a few micro amps per 15 minutes, the gun electrode is considered stable at this voltage level. The voltage is then increased 1–2 kV and the process repeated until the voltage on the gun electrode is about 10–15% above the operating voltage. The time frame involved for the gun electrode to stabilize may be as long as two days. During the stabilization period, the current available to the gun electrode is limited by the large resistance of the conditioning resistor. After the gun electrode has been properly conditioned to a voltage 15% higher than the running voltage of 350 kV, the conditioning resistor is replaced by the running resistor, which has a much lower resistance. 
   At present, two separate resistors are used to control the current available from the high voltage power supply to the injector gun. A high value resistor is used to reduce the current available from the power supply to a low value to condition the injector gun. A low value resistor is then substituted for the high value resistor. The low value resistor is then placed on line allowing the high voltage power supply to provide higher currents when required by the injector gun for operations. 
   Unfortunately, the use of two separate resistors and the task of switching them causes a great deal of down time. High voltage power supplies for FELs may reach as high as 500 kV or higher. The separate resistors are bulky and must be secured in place between the power supply and the injector gun, within a surrounding jacket, which requires several hours of unproductive time. 
   What is needed therefore, is a dual resistor for high voltage applications that is capable of being switched from one resistance value to another without significant downtime or disassembly. The dual resistor must be capable of limiting the current available to the gun during high voltage conditioning and of delivering large current when required by the gun during operations. The dual resistor would be useful in starting up high power accelerators, high voltage transmission lines, or other high voltage equipment in which the current must be limited for conditioning or starting purposes. 
   SUMMARY OF THE INVENTION 
   The present invention is dual resistor for eliminating the requirement for two different value resistors. The dual resistor includes a conditioning resistor at a high resistance value and run resistor at a low resistance value. The run resistor can travel inside the conditioning resistor. The run resistor is capable of being advanced by a drive assembly until an electrical path is completed through the run resistor thereby shorting out the conditioning resistor and allowing the lower resistance run resistor to take over as the current carrier. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of a dual design resistor according to the present invention. 
       FIG. 2  is a side view of the dual design resistor of taken along line  1 — 1  of  FIG. 1  with the housing and the conditioning resistor cut away along line  1 — 1  to show internal details of the dual design resistor and the run resistor retracted into the housing. 
       FIG. 2A  is a detailed view of the middle portion of  FIG. 2  showing the connection of the housing to the conditioning resistor. 
       FIG. 3  is a view of the left side portion of  FIG. 2  showing the details of the run resistor portion of the dual resistor. 
       FIG. 4  is a side view of the run resistor and drive assembly portion of  FIG. 3 . 
       FIG. 4A  is an end view of the carriage taken along line  4 A— 4 A of  FIG. 4 . 
       FIG. 5  is a detailed view of the nosepiece portion of the run resistor taken along line  5 — 5  of  FIG. 4 . 
       FIG. 6  is a view of the right side portion of  FIG. 2  showing the details of the conditioning resistor portion of the dual resistor. 
       FIG. 6A  is a detailed view of a portion of the top right corner of the conditioning resistor from the area delimited by line  6 A— 6 A of  FIG. 6 . 
       FIG. 7  is a detailed view of the nosepiece portion of the conditioning resistor of  FIG. 6 . 
       FIG. 8  is a side view of the dual design resistor taken along line  1 — 1  of  FIG. 1  with the housing and the conditioning resistor cut away along line  1 — 1  to show internal details of the dual design resistor and the run resistor advanced into contact with the conditioning resistor. 
   

   Reference Numerals Used in the Specification and Drawings 
   
       
       
         
             10 —dual design resistor 
             12 —housing 
             14 —first end of housing 
             16 —second end of housing 
             18 —conditioning resistor 
             20 —input end of dual design resistor 
             22 —input electrical contact 
             24 —output end of dual design resistor 
             26 —output electrical contact 
             28 —upstream corona ring 
             30 —downstream corona ring 
             32 —first end of conditioning resistor 
             34 —second end of conditioning resistor 
             36 —run resistor 
             38 —first end of run resistor 
             39 —bayonet mount 
             40 —second end of run resistor 
             41 —first mounting ring 
             42 —tubular cavity 
             43 —second mounting ring 
             44 —drive assembly 
             45 —screws for bayonet mount 
             46 —motor 
             47 —corona ring mounting screws 
             48 —ball screw 
             50 —shaft 
             52 —drive gear 
             54 —driven gear 
             56 —first end of ball screw 
             58 —carriage 
             60 —top end of carriage 
             62 —bottom end of carriage 
             64 —ball nut 
             66 —limit switch striker 
             68 —front bracket 
             70 —rear bracket 
             72 —front limit switch 
             74 —rear limit switch 
             76 —tie rod 
             78 —nosepiece of run resistor 
             80 —elongated ceramic cylinder 
             82 —nosepiece of conditioning resistor 
             83 —downstream mounting ring 
             84 —silver layer 
             86 —resistive layer 
             88 —protective layer 
             89 —silver-filled epoxy 
             90 —faceplate 
             92 —aperture 
             94 —contact head 
             96 —spring 
             98 —electrical receptacle 
             100 —electrical wiring 
         
       
     
  
   DETAILED DESCRIPTION 
   With reference to  FIG. 1 , a top view is shown of a preferred embodiment of a dual design resistor  10  according to the present invention. The dual design resistor  10  includes an electrically conductive tubular housing  12  having a first  14  and second end  16  and a tubular conditioning resistor  18  in electrical contact with and extending from the second end  16  of the housing  12 . The dual resistor  10  includes an input end  20  with an input electrical contact  22  and an output end  24  with an output electrical contact  26 . An upstream corona ring  28  with a bayonet mount  39  secures the housing  12  to the conditioning resistor  18  and a downstream corona ring  30  secures the conditioning resistor  18  to the output electrical contact  26 . The conditioning resistor  18  includes a first  32  and a second  34  end. 
   Referring to  FIG. 2 , a side view of the dual design resistor  10  with the housing  12  and the conditioning resistor  18  cut away along line  1 — 1  of  FIG. 1 , a run resistor  36  having a first- 38  and second  40  end is disposed within the electrically conductive tubular housing  12 . The first end  38  of the run resistor  36  is in electrical contact with the first end  14  of the housing  12 . 
   As shown in  FIG. 2A , the housing  12  and the conditioning resistor  18  are secured together by a bayonet mount  39  including a first mounting ring  41  and a second mounting ring  43 . The first mounting ring  41  is secured to the housing  12  and includes keyhole slots (not shown) for accepting screws  45  secured to the second mounting ring  43 . The upstream corona ring  28 , having a smooth outer surface with no burrs or sharp edges to cause electrical arcing or flashing, covers the bayonet mount  39 . Corona ring mounting screws  47  secure the upstream corona ring  41  to the second mounting ring  43 . The housing  12  and the mounting rings  41 ,  43  are preferably constructed of an electrically conductive metal such as aluminum. The first mounting ring  41  is typically secured to the housing  12  by welding. The second mounting ring  43  is typically secured to the ceramic conditioning resistor  18  by a conductive adhesive, such as silver-filled epoxy type 761A11 or 761A12, available from McMaster-Carr Supply Company of Los Angeles, Calif. 
   The housing  12  and the conditioning resistor  18  define a tubular cavity  42  extending the length of the dual design resistor  10  therein. An insulating gas is disposed to completely surround all components both inside and outside of the dual resistor  10  to suppress high voltage arcing. The insulating gas is preferably sulfur hexafluoride. The housing  12  further includes a drive assembly  44  for advancing the run resistor  36  linearly within the tubular cavity  42 . The conditioning resistor  18  is of a higher resistance value than the run resistor  36 . The drive assembly  44  is capable of advancing the run resistor  36  until the second end  40  of the run resistor  36  establishes electrical contact with the contact head  94  on the second end  34  of the conditioning resistor  18  thereby shorting out the higher resistance conditioning resistor  18  with the lower resistance run resistor  36  and allowing the run resistor  36  to take over as the current carrier. The run resistor  36  preferably has a resistance of between 450 and 500 ohms. A particularly preferred run resistor  36  is a Type 1044AS non-inductive tubular ceramic resistor, which can be obtained from Kanthal Globar of 3425 Hyde Park Boulevard, Niagara Falls, N.Y. 
   Referring to  FIG. 3 , the drive assembly  44  of the dual design resistor is secured within the portion of the tubular cavity  42  that is within the housing  12 . The drive assembly  44  includes a motor  46  and a ball screw  48 . 
   Details of the drive assembly  44  are shown in  FIG. 4 . The motor  46  of the drive assembly  44  includes a shaft  50  and a drive gear  52  secured to the shaft  50 . A driven gear  54  is included on a first end  56  of the ball screw  48 . The drive assembly  44  includes a carriage  58  that has a top  60  and a bottom  62  end. A ball nut  64  is secured to the bottom end  62  and a limit switch striker  66  secured the top end  60  of the carriage  58 . A front  68  and rear  70  bracket include respectively a front  72  and rear  74  limit switch. As shown the end view of the carriage  58  in  FIG. 4A , two tie rods  76  are included outboard of the ball screw  48 . The second end  40  of the run resistor  36  includes a nosepiece  78  secured thereto. 
   With reference to  FIG. 6 , the conditioning resistor  18  is constructed of an elongated ceramic cylinder  80 . A nosepiece  82  is attached with screws to a downstream mounting ring  83  on the second end  34  of the conditioning resistor  18 . 
   Referring to  FIGS. 6 and 6A , a downstream mounting ring  83  is secured to the ceramic cylinder  80  with silver-filled epoxy  89  and the downstream corona ring  30  is secured to the downstream mounting ring  83  by screws. A silver layer  84  covers the end of the ceramic cylinder  80 . A resistive layer  86  overlapping a portion of the silver layer  84  and a protective layer  88  is applied over the resistive layer  86 . The area between the downstream mounting ring  83  and the silver layer  84  is filled with silver-filled epoxy  89 . The silver-filled epoxy  89  provides good electrical contact between the downstream mounting ring  83  and the silver layer  84  of the conditioning resistor  18 . The resistance value of the conditioning resistor  18  is set by the thickness of the resistive layer  86  and the time and temperature to which it is fired. The silver layer  84  provides a method of connecting to the resistive layer  86 . The protective layer  88  protects the outer surface of the conditioning resistor  18  against abrasion. The elongated ceramic cylinder  80  that forms the conditioning resistor  18  preferably has a wall thickness of between ⅛-inch and ¼-inch thick. Preferably, the conditioning resistor  18  has a resistance of between 100 and 200 Mohms. 
   A detailed view of the nosepiece  82  portion of the conditioning resistor is shown in  FIG. 7 . The nosepiece  82  includes a faceplate  90  having an aperture  92  with the output electrical contact  26  fitted therein. A contact head  94  is affixed to the output electrical contact  26  by a spring pin. The spring  96  is used to maintain a good electrical contact between the nosepiece  78  of the run resistor and the contact head  94  of the conditioning resistor&#39;s nosepiece  82 . The contact head  94  is constructed of electrically conductive material and will carry an electrical current to the output electrical contact  26  when it is contacted by the run resistor (not shown). 
   Referring to  FIG. 3 , the housing  12  further includes an electrical receptacle  98  and electrical wiring  100  (dashed lines) connecting the electrical receptacle  98  with the motor  46  via the front  72  and rear  74  limit switches. DC power and control for the motor  46  is provided by an external power supply and switch (not shown). The motor  46  and the ball screw  48  are disposed within the tubular cavity  42 . 
   With reference to  FIGS. 5 and 7 , the run resistor nosepiece  78  is secured to the run resistor  36 . The nosepiece  78  is constructed of aluminum and, upon traveling to the second end  34  of the conditioning resistor  18  (see  FIG. 8 ), transfers electrical current from the run resistor  36  to the contact head  94  of the nosepiece  82 . The end of the run resistor nosepiece  78  forms the second end  40  of the run resistor  36 . 
   For operation of the dual design resistor  10  of the present invention, the reader is referred to  FIGS. 3 and 4 . The run resistor  36  is supported within the internal cavity  42  of the housing  12 . In an initial state, the first end  38  of the run resistor  36  is cantilevered from the first end  14  of the housing  12 . The input electrical contact  22  at the first end  14  of the housing  12  is typically at a voltage of 350,000 volts. The housing  12  is typically constructed of aluminum. The housing  12  therefore conducts electrical current from its first end  14  to its second end  16  with minimal current loss. 
   With reference to  FIG. 2 , since the insulating gas fills the tubular cavity  42  within the dual design resistor  10 , and the run resistor  36  is cantilevered from the first end  14  of the housing  12 , the run resistor  36  is therefore electrically insulated from the second end  34  of the conditioning resistor  18 . Therefore, with a high voltage applied to the first end  14  of the housing  12 , all the current flow is through the conditioning resistor  18 . In a preferred embodiment, the conditioning resistor  18  has a resistance of 150 Mohms and the run resistor  36  has a resistance of 500 ohms. Applying Ohm&#39;s Law, with 350 kilovolts applied to the input electrical contact  22 , and all current flow through the conditioning resistor  18 , the maximum current output available at the output electrical contact  26  equals 0.0023 amps. Thus, with the run resistor  36  retracted until the limit switch striker  66  contacts the rear limit switch  74 , as shown in  FIG. 2 , all current flow is through the conditioning resistor  18  and the maximum output current available at the output electrical contact  26  equals 0.0023 amps. 
   To switch the current flow of the dual design resistor  10  to the run resistor  36 , the 350 kV is turned off and DC power is sent through wiring  100  and front limit switch  72  to operate motor  46 , which drives carriage  58  and ball nut  64  along ball screw  48 . Carriage  58  is thereby carried along ball screw  48  and carries with it run resistor  36 . The run resistor  36  is therefore driven from the left to right in the  FIG. 2 , advancing the second end  40  of the run resistor  36  toward the contact head  94  at the second end  34  of the conditioning resistor  18 . 
   Referring to  FIG. 8 , the second end  40  of the run resistor  36  eventually contacts and depresses the contact head  94  and the limit switch striker  66  contacts the front limit switch  72  stopping the motor  46  and the linear advancement of the run resistor  36  into the conditioning resistor  18 . When the second end  40  of the run resistor  36  contacts the contact head  94 , the run resistor  36 , being of a lower resistance value than the conditioning resistor  18 , shorts out the conditioning resistor  18  and nearly all current will now flow through the run resistor  36  when high voltage is reapplied. In the preferred embodiment, with the run resistor  36  having a resistance of 500 ohms and 350 kilovolts applied to the input electrical contact  22 , the maximum current available through the run resistor  36  is 700 amps. 
   The dual design resistor  10  of the present invention therefore includes a first position, as shown in  FIG. 2 , when the second end  40  of the run resistor  36  is insulated from the second end  34  of the conditioning resistor  18  and a second position, as shown in  FIG. 8 , when the run resistor  36  establishes electrical contact with the contact head  94  at the second end  34  of the conditioning resistor  18 . The maximum current available from the dual design resistor  10  is therefore determined by the position of the run resistor  36 . The maximum current available from the dual resistor  10  is 0.0023 amps with the run resistor  36  retracted, or in the first position shown in  FIG. 2 , and the maximum current available is 700 amps with the run resistor  36  extended, or in the second position shown in  FIG. 8 . 
   The dual design resistor  10  of the present invention is especially useful for introducing a high voltage to downstream electrical components that are susceptible to damage by current spikes or fluctuations. Typically, when very high voltage is first applied to electrical equipment the voltage must be brought up gradually and the maximum current limited in order to prevent any major damage to the equipment. A corona discharge can emanate from any sharp or rough surfaces and they must be “high voltage processed” smooth by controlling the power (current and voltage) of the discharge. On startup of high voltage equipment, it may take one or two days to establish a steady electric field on the equipment without corona or other discharges. Power supplies to photocathode injector guns, such as those used to create electrons for accelerators that produce photons for FELs, may supply between 300 and 500 kV DC. With such high voltages involved, it is very critical to not introduce a high current immediately on startup to the injector gun, as slight fluctuations in the current can cause electrical arcing, flashing, or other damaging results. It is therefore desirable to first introduce the downstream components to a relatively low voltage with the maximum current available limited to a small value and gradually raise the voltage when there is no or very minimal current activity. Both the power supply voltage and current are monitored during high voltage processing and startup of the equipment. When the downstream equipment is able to hold a voltage that is higher than the desired operating voltage with only very low current drain, then the equipment is finished with the high voltage processing. The power supply is turned off and the dual design resistor  10  is switched from the conditioning setup, with all current through the conditioning resistor  18 , to the run setup with all power through the run resistor  36 . 
   To ensure that there is no arcing or flashing within the dual design resistor, the tubular cavity  42  and in fact the entire dual design resistor  10  is engulfed with an insulating gas, such as sulfur hexafluoride. In an especially preferred embodiment in which the run resistor  36  has a length of 24-inches and a diameter of 1.5-inches, the conditioning resistor has a length of 25.5-inches and a diameter of 5.0-inches with a wall thickness of 0.188-inch. 
   As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.