Abstract:
A method for assembling a three-phase AC heater is disclosed. The method features swaging individual legs of a three-phase heater within a tubing, then bundling the legs together in a lineal fashion for swaging within an outer tube. The ends of the heater assembly are fitted with potting cups and potted with a compound appropriate for resisting moisture and humidity.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is directed to the field of electrical resistance heaters. Specifically, the disclosed invention is directed to methods for assembly of compact three-phase electrical heaters  
       BACKGROUND  
       [0002]     Cold temperatures are routinely encountered in high altitude operation. High altitude aircraft and satellites often require localized heating of certain components to assure proper operation when the component is in service. Electrical resistance heaters are a preferred means for delivering the localized heating because they can run off existing on-board power sources, such as 240 VAC three phase generators. Moreover, it is desirable that the local heating elements and any ancillary equipment be compact and light weight.  
       SUMMARY OF THE INVENTION  
       [0003]     A method for assembling a three-phase electrical resistance heater is disclosed. The heater assembly is of compact construction for operation in compact spaces. The heater assembly is configured for operation directly from on-board three-phase power sources, thereby negating the need for electrical transformers and rectifiers and their attendant weight.  
         [0004]     The method features individual legs of a three-phase heater combined in a lineal fashion within an outer tube. The outer tube with individual legs are swaged resulting in a compact design with a pie-shaped cross-section with the walls of each segment spaced 120 degrees. The individual legs are spaced and terminated in a potting cup providing stress relief and moisture and humidity control. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a flow chart of the method for assembly of the heater according to an embodiment of the present invention;  
         [0006]      FIG. 2  illustrates a potting cup assembly according to an embodiment of the present invention;  
         [0007]      FIG. 3  is another view of the potting cup assembly of  FIG. 2 ;  
         [0008]      FIG. 4   a  illustrates heater sub-assembly in an embodiment of the invention;  
         [0009]      FIG. 4   b  illustrates a termination of a heater subassembly according to an embodiment of the invention;  
         [0010]      FIG. 5   a  illustrates a partially assembled heater assembly according to an embodiment of the present invention; and  
         [0011]      FIG. 5   b  illustrates a heater assembly with a portion broken away according to an embodiment of the present invention; and  
         [0012]      FIG. 6  shows the detail of the locating bushing in an embodiment of the present invention.  
         [0013]      FIG. 7  is a cross-sectional view taken at line  7 - 7  of  FIG. 5   b ; and  
         [0014]      FIG. 8  is a cross-sectional view taken at line  8 - 8  of  FIG. 5   b.    
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Referring to  FIG. 1 , a flow chart  10  depicts a series of steps for a method of assembly according to an embodiment of the present invention. Numerical reference numerals for the physical configurations and components used in the various steps are identified in parentheses. A heater sub-assembly method  20  produces a heater sub-assembly  30 . A potting cup assembly method  40  produces a potting cup assembly  50 . Referring to  FIGS. 1 through 3 , the potting cup assembly  50  produced by the potting cup assembly method  40  has a strain relief  60 , a bushing  80 , an outer tube  90 , and a potting cup sub-part  100 . The outer tube  90  and the potting cup sub-part  100  may be joined to the bushing by silver brazing. The potting cup assembly  50  has two ends: a large diameter end  52 , and a small diameter end  54 .  
         [0016]     A heater assembly method  110  utilizes three heater units or sub-assemblies  30 , the potting cup assemblies  50 , and locator plugs  120  to produce a heater assembly  130 . A potting assembly method  140  is performed on the heater assembly  130 , followed by a final inspection  160  for compliance with specifications. Referring to  FIG. 4 , the heater sub-assembly  30  is a resistive wire configured as a heater coil  200 , such as nickel chromium wire, supported within compacted refractory material  220  that insulates the coil from the outer casing  210 . The refractory material may be magnesium oxide. Lead wires  210  are brazed or otherwise attached to the heater coil  200 . The outer casing can be nickel. The capped ends  230  are sealed such as by melted glass.  
         [0017]     Referring to  FIG. 4   b , a metal ceramic hermetic termination  280  is brazed to the casing. The lead wire  190  is brazed to the cap  240  at the ceramic metal termination as is the insulated exterior lead wire.  
         [0018]     Referring to  FIG. 5 , a heater assembly  235  is illustrated. The heater assembly  235  is constructed using the heater assembly method  110  as described below: 
        1. Obtain three (3) heater sub-assemblies  30  and straighten them.     2. Obtain a length of outer tubing  240 . The tubing may be made of nickel.     3. Group the three heater sub-assemblies  30  in a lineal bundle  35  and align them on one end. Feed the bundled sub-assemblies  30  into one end  242  of the outer tubing  240  and push the sub-assemblies  30  through the outer tubing  240  until the leads  190  emerge out the other end  244  of the outer tubing  240 . Align each 4-in. lead mark on the emerging lead  190  so that it is even with the end  244  of the outer tubing  240 .     4. Insert end  242  inside a swaging tool (not shown) and swage the outer tubing  240  to a specified diameter.     5. Swage the outer tubing  240  over the length of the lineal bundle  35 .     6. Trim the outer tubing  240  on each end to assure a cold section  250  is on each end, and that the cold sections  250  are of approximately equal length.     7. Measure the end-to-end length of outer tubing  240 . This length should be between 51-in. and 53-in.     8. Anneal the outer tubing  240  containing the heater sub-assemblies  30 .     9. Oven bake the outer tubing  240  containing the heater sub-assemblies  30 .     10. Test electrical isolation between the heater coil assembly  200  and the outer tubing  240  with a high potential electrical tester (“Hypot”) at 800 volts. Confirm that leakage current between the heater coil assembly  200  and the outer tubing  240  is less than 0.05-mA.     11. For each end of the heater assembly  110 , slide potting cup assembly  50  over the outer tubing  240  so that the lead  190  projects through the large diameter end  52 . That is, when the potting cup assembly has been fed over the end of the outer tubing  240 , the large diameter end faces the nearest lead end of the heater assembly  110 .     12. For each end of the heater assembly  110 , slide a terminal locating bushing  270  over the over the internal leads  190  so the internal leads  190  are in the grooves  272  of the bushing  270 . Insert terminal locating plug  274  into center of bushing  270  to retain leads  190  in grooves  272 .     13. Braze a ceramic metal termination  280  onto the end of each lead  190  with silver braze or by TIG welding.     14. Mark smaller diameter elements at 0.85-in. and at 1.0-in. from end of outer tubing  240 . Position the ceramic metal termination  280  so that a 0.40-in. length of the leads  190  extend into the ceramic metal termination  280 . This space allows for re-alignment of ceramic metal termination pieces without damage of the tubes  210 .     11. Insert the locating plug  120  inside the terminal locating bushing  270 .     12. Silver braze terminal locating bushing  280 , leads  190  and locator plugs  120  at each end of heater assembly  110 .     13. Braze on a specified length of external lead wires  290 . The lead wires  290  may be of 18 gage MGT with silicone exterior (Duraflex 550S 5400), rated for 450° C. service.     14. Check resistance of the heater assembly  30  for compliance within specified parameters.     15. Fill the potting cups with suitable potting material  296 .        
 
         [0038]     Note the swaging of the three individual heater sub assemblies, creates the pie shaped cross-section shown in  FIG. 8 . The ends of the heater sub assemblies are terminated in the potting cup and embedded in suitable potting material  296 . This provides a highly efficient reduced diameter heater that is extremely robust and suitable for aerospace or outer space applications.  
         [0039]     It is noted that the same methods disclosed are not limited to the creation of the specific embodiments described herein. For example, the physical dimensions of the resultant assembly, (e.g. hot section length, cold section length, wire diameter, and the diameters of the various tubing) may vary according to specific requirements and still be within the scope of the present invention. Likewise, the heating elements may be of a straight or a coiled configuration, and may be comprised of any suitable resistive element. Also, the annealing temperatures may differ, depending on the materials being utilized, and still be within the scope of the claimed method. Other embodiments will also be obvious to skilled artisans who utilize the methods disclosed herein.