Abstract:
An end band and method of forming an end band for a rotor. The end band includes a hollow cylindrical band, wherein at least a portion of the band has a grain flow in a direction parallel to the hoop stress of the band. The end band also includes at least a portion having a grain flow in a direction perpendicular to the hoop stress of the band.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to generators and, more particularly, to an end band for a generator. 
         [0002]    A generator system, such as that found in aircrafts, ships, and some terrestrial vehicles, may include three separate generators, a permanent magnet generator (PMG), an exciter, and a main generator. Rotation of the rotor of the PMG induces an alternating current in the stator of the PMG. This alternating current is commonly sent through a rectifier where it is converted to an output as a direct current. This direct current is provided to the stator of the exciter and as a result of rotation of the exciter rotor, creates and alternating current output. Rectifier circuits rectify this alternating current and the resulting direct current is provided to the rotor of the main generator. As the rotor of the main generator rotates, alternating current is induced in the main generator stator and this can be output to the system. The components of a rotor of all of these generators must remain in place during rotation. To this end and by way of example, the exciter rotor typically includes a pair of end bands that hold critical parts of the exciter rotor in a desired position while withstanding the centrifugal forces and hoop stress acting on it. 
         [0003]    Historically, end bands have been made of high strength, critical alloys well suited for use in extreme environments of pressure and heat. Presently, end bands are machined from a solid piece of bar stock having a diameter at least equal to the outside diameter of the end band. Because each end band is a hollow cylinder of a generally narrow thickness, the majority of the bar stock used to create each part is, therefore, wasted making the cost per part expensive relative to the percentage of the material used. Another method for forming an end band includes welding a rolled sheet to form a band. However, this welding method may be equally cost prohibitive because of the minimal tolerances required for the band. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    According to one embodiment of the invention, an end band is provided including a hollow cylindrical band. A portion of the band has a grain flow in a direction parallel to a hoop stress of the band. The end band also includes a portion of the band having a grain flow in a direction perpendicular to the hoop stress of the band. 
         [0005]    According to another embodiment of the invention, a rotor is provided including at least one lamination including a plurality of teeth. At least one insulation ring having a plurality of teeth is attached to the lamination. Slot insulation is disposed between the teeth of the lamination and the teeth of the insulation ring. Wire windings are wrapped around the teeth of the lamination and insulation rings. The rotor also includes at least one hollow cylindrical end band surrounding the rotor or its components. At least a portion of the end band has a grain flow in a direction parallel to a hoop stress of the end band, and at least a portion of the end band has a grain flow in a direction perpendicular to the hoop stress of the end band. 
         [0006]    According to yet another embodiment of the invention, a method for manufacturing an end band is provided including hydroforming a hollow cylinder having an open end and a closed end. The hollow cylinder is then stress-relieved. After being stress-relieved, the hollow cylinder is then aged. The closed end of the cylinder is then removed to form an end band. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a functional schematic block diagram of an exemplary generator system; 
           [0009]      FIG. 2  is an exploded perspective view of an exemplary exciter rotor assembly; 
           [0010]      FIG. 3  is an assembled perspective view of an exemplary rotor exciter assembly; 
           [0011]      FIG. 4  is an end band according to an embodiment of the invention; and 
           [0012]      FIG. 5  illustrates a process for manufacturing an end band according to an embodiment of the invention. 
       
    
    
       [0013]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Though the present invention is described in relation to a rotor of a generator, a person having ordinary skill in the art would understand that the present invention may be adapted for use with a motor as well. Referring to  FIG. 1 , a functional schematic block diagram of an exemplary generator system  100  for use with a turbine engine is illustrated. This exemplary generator system  100  may include a permanent magnet generator (PMG)  110 , an exciter  120 , a main generator  130 , a generator control unit  140 , and one or more rectifier assemblies  150 . During operation, a rotor  112  of the PMG  110 , a rotor  124  of the exciter  120 , and a rotor  132  of the main generator  130  all rotate. The rotational speed of these components may vary. As the PMG rotor  112  rotates, the PMG  110  generates and supplies alternating current (AC) power to the generator control unit  140 , which in turn supplies direct current (DC) power to a stator  122  of the exciter  120 . The exciter rotor  124  in turn supplies AC power to the rectifier assemblies  150 . The output from the rectifier assemblies  150  is DC power and is supplied to the main generator rotor  132 , which in turn outputs AC power from a main generator stator  134 . The generator control unit  140  can regulate the power output based upon monitoring signals provided to it from monitoring devices  195 . As depicted, the PMG rotor  112 , the exciter rotor  124 , and the main generator rotor  132  all rotate along a single axis  198  at the same rotational speed. In other embodiments, each of these rotor components may rotate along a different axis. The relative positioning of the PMG  110 , the exciter  120 , and the main generator  130  can be modified such that the exciter is physically between the PMG  110  and the main generator  130 . 
         [0015]    Referring now to  FIGS. 2 and 3 , the structure of an exemplary exciter rotor  124  is illustrated having a generally cylindrical hub  205  acting as the main support structure of the exciter rotor  124 . It shall be understood that while the following description is related to exciter rotors, the teachings provided herein could be applied to a rotor of any type of dynamoelectric machine. A ring-type lamination  210  is attached to the cylindrical hub  205 . In another embodiment, the ring-type lamination  210  may act as the hub for the exciter rotor. That is, in such an embodiment, the cylindrical hub  205  may be omitted. 
         [0016]    The lamination  210  has cutouts  215  extending radially inward from the outer circumference such that the lamination  210  has teeth  220  protruding outward. Insulating sheets or rings  225  are positioned on opposite sides of the lamination  210 . Each insulating ring  225  includes cutouts  230  which form a plurality of teeth  235  disposed around the circumference. The cutouts  230  and teeth  235  of the insulating rings  225  are equivalent to and aligned with the cutouts  215  and teeth  220  of the lamination  210 . The exciter rotor  124  may include a balancing ring  240  positioned adjacent one of the insulating rings  225  to prevent movement of the laminations  220  and the insulating rings  225 . 
         [0017]    Slot insulation  245  is placed within the cutouts  215 ,  230  of both the laminations  210  and the insulating rings  225 . A first pair of insulating bands or sheets  250  is positioned around hub  205  near the insulating ring  225  and around the balancing ring  240 . Wire windings  155  are wound around the teeth  220 ,  235  of the laminations  210  and the insulating rings  225 . The slot insulation  245  insulates the wire windings  255  from the laminations  210 . The wire windings  255  include end turns  260  which cover a first and second insulating sheet or band  250  to connect the windings together. A second set of insulating bands or sheets  265  is positioned over each of the end turns  260  and first and second end bands  270  are placed over the second set of insulating bands  265 . The end bands  270  assist in holding the end turns  260  of the wire winding  255  in position despite the significant centrifugal forces experienced by the exciter rotor assembly  124  during operation. The aforementioned end band and exciter rotor are provided as a non-limiting embodiment. Numerous other configurations of an end band are considered to be within the scope of an exemplary embodiment of the invention, such as end bands used on a permanent magnet generator (PMG) rotor and a ram air turbine (RAT) main rotor. 
         [0018]    Referring now to  FIG. 4 , an exemplary end band  270  according to an embodiment of the invention is illustrated. The end band  270  is a hollow cylinder having an inner diameter large enough to fit around the outer diameter of an assembled rotor, such as exciter rotor  124  for example. The end band  270  may be made from a metal or an alloy, such as Inconel for example. The rotation of a rotor and an attached end band  270  creates a hoop stress along the circumference of the end band  270  in the direction illustrated by arrow H. Conventional end bands  270  are machined from a piece of bar stock and have a uniform grain flow in a direction. This uniform grain flow is parallel to the width of the end band and perpendicular to the direction of the hoop stress. In an exemplary embodiment, an end band  270  is formed from a sheet of material having a uniform grain flow across the sheet. The formed end band  270 , however, has a non-uniform grain flow that varies along the circumference of the end band  270 . 
         [0019]    Some portions of the end band  270 , such as section A, have a grain flow in a direction parallel to the width of the end band  270  or perpendicular to the hoop stress. Other portions of the end band  270 , exemplified by section B, have a longitudinal grain flow around the circumference of the end band  270  in a direction parallel to the hoop stress of the end band  270 . The areas of the end band  270  having grain flow perpendicular to the hoop stress are separated from the areas having grain flow parallel to the hoop stress by transitional areas where the flow is neither perpendicular nor parallel to the hoop stress. According to the teachings herein, it has been determined that the portions of the end band  270  having a transitional grain flow and the portions having a grain flow in a direction parallel to the hoop stress may have a better fatigue life than those areas having a grain flow perpendicular to the hoop stress. Therefore, because the sheet metal end band  270  has a varying grain flow around its circumference, it has a better fatigue life than a conventional end band made from bar stock. 
         [0020]    An exemplary process  300  for manufacturing an end band  270  is illustrated in  FIG. 5 . In block  302 , a sheet metal end band  270  is formed by hydroforming a hollow cylinder having an open end from a sheet of a desired material. In one embodiment, the height of the cylinder is substantially equivalent to the width of the end band  270 . In another embodiment, the height of the cylinder may be substantially equal to the width of a plurality of end bands  270 . The hydroforming process creates internal residual stresses within the formed hollow cylinder which may be reduced using a heat-treating process. Aging of the hollow cylinder directly after hydroforming, however, increases the grain size of the material. Because grain size is inversely proportional to fatigue life it is desirable to reduce the grain size of the hollow cylinder to improve its endurance and fatigue life. To control the growth of the grain size and to achieve a grain size at least comparable to the grain size of an end band machined from a piece of bar stock, the hollow cylinder is stress-relieved after being hydroformed, as illustrated in block  304 . The stress-relieving process involves heating the hollow cylinder to a temperature lower than the temperature of an aging process, and allowing the hollow cylinder to cool slowly to restore the grain size. After being stress-relieved, the hollow cylinder is then aged, as shown in block  306 , such that the sheet metal hollow cylinder has a grain size comparable or smaller than that of a conventional bar stock end band. Grain size is inversely proportional to grain size number. In one embodiment, the desired resultant grain size number of the hollow cylinder and resultant end band  270  is approximately 9. Because deformation of the hollow cylinder may occur during the aging processes, the bottom of the hollow cylinder is removed afterwards, shown in block  308 , such as by machining for example, to form an end band  270  having a desired wall thickness and inner diameter. 
         [0021]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.