Abstract:
A high speed aircraft generator may utilize a prefabricated crossover subassembly to interconnect field coils. The crossover may include two attachment leads interconnected with a section of braided wire. The braided wire may remain free of brazing filler metal after the crossover subassembly is brazed into position between field coils of the generator. Consequently, the crossover may remain flexible and may have reduced susceptibility to fatigue failure that may otherwise result from circumferential relative displacement of the field coils from one another during changes of rotational speed of the field coils.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is in the field of electrical generators and motors and, more particularly, electrical starter generators operating at very high rotational speeds. 
         [0002]    In certain applications of generators such as those employed in aircraft, there is a requirement to produce a high power density with a generator that is small in size and light in weight. In these applications, a desired high power density may be achieved with relatively small generators which operate at very high rotational speeds. A typical aircraft generator may operate at rotational speeds of 12,000 to 24,000 rpm. 
         [0003]    When a generator is operated at such high rotational speeds, rotatable components of the generator may be subjected to correspondingly high centrifugal forces. Some rotatable components may be particularly vulnerable to damage from centrifugal forces and fatigue. Examples of these vulnerable components are interconnections between field coils of the generators. 
         [0004]    In a typical aircraft high-speed generator, field coils may be interconnected to one another with crossovers. Fatigue inducing stresses may arise in crossovers because high rotational speeds and temperatures of the aircraft generators produce a radial displacement of the field coils relative to an axis of rotation. This radial displacement may cause variation of circumferential spacing between the field coils. While such spacing variation may be relatively small, it is nevertheless large enough to produce bending of the crossover during each change of rotational speed of the generator. Repeated bending of the flat strip crossovers may produce stresses which may lead to fatigue failure of the crossovers. 
         [0005]    As can be seen, it would be desirable to construct high-speed generators which do not incorporate crossovers that are vulnerable to fatigue failure. Additionally, it would be desirable to provide a method for constructing such generators without producing work hardening of the crossover or wicking of brazing filler metal into the crossover. 
       SUMMARY OF THE INVENTION 
       [0006]    In one aspect of the present invention a generator comprises at least a first and at least a second field coil adapted to rotate around an axis. The first and second field coils each comprise wound flat conductors. The first and second field coils are interconnected electrically in series by a crossover attached to the flat conductors of both of the field coils. The crossover comprises a flexible interconnecting member. The flexible interconnecting member has at least a first and at least a second attachment member attached thereto. The flat conductor of the first field coil is attached to the first attachment member and the flat conductor of the second field coil is attached to the second attachment member. 
         [0007]    In another aspect of the present invention a crossover for electrically interconnecting field coils in a generator comprises a flat braided wire member having two ends and flat metal strips attached to the ends of the braided wire member. 
         [0008]    In still another aspect of the present invention a method for producing electrical current with a high-speed electrical generator comprises the steps of, passing electrical current through at least a first and at least a second field coil, passing electrical current between the first and the second field coil on an electrically conductive crossover, rotating the field coils about an axis while producing centrifugal force on the field coils which results in a variation of circumferential spacing between the field coils and allowing the crossover to flex during the variation in circumferential displacement so that fatigue inducing stress is not produced in the crossover. 
         [0009]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is partial cross-sectional perspective view of a generator constructed in accordance with the invention; 
           [0011]      FIG. 2  is a perspective view of a field coil of the generator of  FIG. 1  in accordance with the invention; 
           [0012]      FIG. 3  is a schematic view of interconnections between field coils of the generator of  FIG. 1  in accordance with the invention; 
           [0013]      FIG. 4  is a perspective view of a portion of the generator of  FIG. 1  showing a crossover in accordance with the invention; 
           [0014]      FIG. 5  is a partial perspective view of the generator of  FIG. 1  showing interconnected field coils in accordance with the invention; 
           [0015]      FIG. 5A  is a detailed view of the portion of the generator of  FIG. 5  in accordance with the present invention; and 
           [0016]      FIG. 6  is a detailed view of a portion of the generator of  FIG. 1  showing an inner crossover in accordance with the present invention; 
           [0017]      FIG. 7  is a perspective view of a crossover in accordance with the invention; 
           [0018]      FIG. 7A  is a detailed view of a portion of the crossover of  FIG. 7  in accordance with the invention; 
           [0019]      FIG. 8  is an illustration of a technique for brazing the crossover of  FIG. 7  in accordance with the invention; 
           [0020]      FIG. 9  is a schematic view of a crossover attachment to field coils in accordance with the invention; 
           [0021]      FIG. 9A  is a detailed view of a portion of the crossover attachment of  FIG. 9  in accordance with the invention; and 
           [0022]      FIG. 10  is a flow chart of a method of generating electric power in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0024]    Various inventive features are described below that can each be used independently of one another or in combination with other features. However, any single inventive feature may not address any of the problems discussed above or may only address one of the problems discussed above. Further, one or more of the problems discussed above may not be fully addressed by any of the features described below. 
         [0025]    Broadly, embodiments of the present invention may be useful in improving high-speed electrical generators. More particularly, embodiments of the present invention may provide a simple expedient to reduce damage from radial displacement of field coils resulting from centrifugal forces. Embodiments of the present invention may be particularly useful in aircraft generators which operate at high rotational speeds of up to about 24,000 rpm. 
         [0026]    An embodiment of the present invention may provide a generator that is constructed with crossovers that may be produced as sub-assemblies and then inserted into the generator. The embodiment of the present invention may use a sub-assembly that incorporates a flexible braided wire member as a crossover. The embodiment of the present invention may comprise a unique construction technique which may preclude wicking of brazing filler metal into the braided wire member thus keeping the crossover flexible so that a resultant crossover may be provided with a reduced susceptibility to fatigue failure. These desirable improvements of a high-speed generator may be achieved by constructing a generator in an inventive configuration illustrated in  FIG. 1 . 
         [0027]    Referring now to  FIGS. 1 and 2 , a generator, shown symbolically and designated generally by the numeral  10 , may be comprised of field coils  12  which may be assembled in a rotor  14  adapted for rotation about an axis  16 . The generator  10  may also comprise an exciter assembly  17 . The generator may be constructed with a plurality of field coils electrically interconnected in series. The field coils  12  may be comprised of a tightly wound conductor  18  which may have a generally flat configuration such as that illustrated in  FIG. 2 . 
         [0028]    Referring now to  FIGS. 3  through  FIG. 6 , an illustrative embodiment of the present inventive generator  10  may comprise four of the field coils  12  interconnected with one another. Interconnection between the field coils  12  may be provided with outer crossovers  20  and inner crossovers  22 . The outer crossovers  20  may be attached to outer sides  12 - 1  of the field coils  12 . The inner crossovers  22  may be attached to inner sides  12 - 2  of the field coils  12 .  FIGS. 4 ,  5  and  5 A illustrate locations of the outer crossovers  20 .  FIG. 6  illustrates a location of one of the inner crossovers  22 . 
         [0029]    Referring now to  FIGS. 7 and 7A , one of the outer crossovers  20  is shown in detail. While  FIG. 7  shows construction of one of the outer crossovers  20 , it should be noted that the inner crossovers  22  may be constructed in a similar manner. The outer crossover  20  may comprise a sub-assembly of attachment leads  20 - 1  and an interconnection member  20 - 2 . The attachment leads  20 - 1  may be constructed from flat meta; strip such as annealed copper. The interconnection member  20 - 2  may comprise braided wire. The attachment leads  20 - 1  may be connected to the interconnection member  20 - 2  with a brazed connection  20 - 3  that employs a brazing filler metal  20 - 4 . A flexible portion  20 - 5  of the crossover  20  may be located between the brazed connections  20 - 3 . The brazed connection  20 - 3  may be referred to as crossover-subassembly brazed connection because it may be produced while the crossover  20  is being constructed as a subassembly which may be subsequently assembled into the generator  10 . 
         [0030]    Referring now to  FIG. 8  it may be seen that the crossover-subassembly brazed connection  20 - 3  may be produced without allowing any wicking of brazing filler metal  20 - 4  into the flexible portion  20 - 5  of the outer crossover  20 . This desirable prevention of wicking may be achieved by immersing the flexible portion  20 - 5  in a cooling fluid  30  while heat may be applied to the attachment lead  20 - 1  during brazing. The cooling fluid  30  may keep the flexible portion  20 - 5  at a temperature below a melting temperature of the brazing filler metal  20 - 4 . Thus, the brazing filler metal  20 - 4  may not wick into the flexible portion  20 - 5  of the crossover  20 . Consequently, a resultant one of the crossovers  20  may be produced with desirable flexibility. 
         [0031]    Referring now to  FIGS. 9 and 9A , it may be seen that the generator  10  of  FIG. 1  may be produced by brazing the crossovers  20  to the field coils  12 . In  FIGS. 9 and 9A , a simplified example of construction of the generator of  FIG. 1  is illustrated. Only one of the crossovers  20  is shown interconnecting only two of the field coils  12 . It may be seen that crossover-attaching brazed connections  40  may be produced with crossover-attaching brazing filler metal  40 - 1 . It may also be seen that application of heat to melt the filler metal  40 - 1  may not result in melting of the filler metal  20 - 4  of  FIG. 7 . Thus the coils  12  may be interconnected without a resultant wicking of either the filler metal  40 - 1  or the filler metal  20 - 4  into the flexible portion  20 - 5  of the crossover  20 . In other words, brazing metal wicking may not produce undesirable rigidity in the flexible portion  20 - 5  of the crossover  20 . Consequently, the coils  12  may be interconnected with a flexible interconnection that may be resistant to fatigue inducing stress. In other words fatigue inducing stress that might otherwise result from multiple changes of rotational speed of the rotor  14  (see  FIG. 1 ) may be avoided. 
         [0032]    In one embodiment of the present invention, a method is provided for producing electrical current with a high speed generator (e.g. the generator  1   0 ). In that regard the method may be understood by referring to  FIG. 11 . In  FIG. 11 , a flow chart portrays various aspects of an inventive method  100 . In a step  102 , current may be passed through a first field coil (e.g., one of the field coils  12 ). In a step  104 , current may be passed from a first field coil to a second field coil through a flexible crossover (e.g., current may pass through the crossover  20  from one of the field coils  12  to another one of the field coils  12 ). In a step  106 , current may be passed through a second field coil (e.g., one of the field coils  12 ). In a step  108 , a magnetic field may be produced (e.g. by passage of current through the field coils  12 ). In a step  110 , the field coils may be rotated about an axis (e.g., the field coils  12  may be rotated about the axis  16  by rotation of the rotor  14 ). In a step  112 , the crossover may be allowed to flex to compensate for relative displacement of the field coils as a result of centrifugal force produced by rotation in the step  110  (e.g., the flexible portion  20 - 5  of the crossover  20  may flex). In a resultant step  114  electrical power may be produced. Thus when the steps of the method  100  are practiced, the generator may operate with multiple variations of rotational speed without producing fatigue inducing stress in the crossover. 
         [0033]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.