Abstract:
A high energy toroidal inductor addresses not only the desire for reduced weight and volume, but also the desire for minimal stray magnetic fields. Specifically, the high energy toroidal inductor includes a bucking cylinder and a predetermined number of leaves. Each leaf is of a two-ended, twisted ring configuration and includes a top portion. The top portion is the narrowest portion of the leaf, and the width of the leaf graduates outward from the top portion to each end of the ring. The bucking cylinder interfaces with each of the leaves and presents the leaves in a side-by-side, continuous toroidal configuration.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to inductors used in pulsed power networks and more particularly to a toroidal inductor that can deliver desired electromagnetic characteristics with low mass and minimal stray magnetic fields.  
       BACKGROUND OF THE INVENTION  
       [0002]     In electrical pulsed power applications, inductors are needed for pulse shaping and/or energy storage. In the past helical jelly roll inductors have been used. Helical jelly roll inductors are generally high current inductors constructed by winding long copper strips around an insulating cylinder then overlaying the windings with a layer of fiberglass to withstand internal magnetic pressure. U.S. Pat. No. 5,912,610 entitled “High Energy Inductor”, which is hereby incorporated by reference, describes this type of helical jelly roll inductor in detail.  
         [0003]     A desirable feature presented by the high energy inductor of the &#39;610 patent is that it is designed to be of sufficient strength to withstand the typically destructive magnetic field forces that result from high pulse current flow while still presenting an inductor of smaller weight and volume. In brief, high energy pulses, which involve high currents, generate such high magnetic field forces that an inductor can literally explode unless fabricated in a fashion to provide high mechanical strength. Typically, this requires that the inductor be of substantial weight and volume. However, the high energy inductor of the &#39;610 patent was able to limit the weight and volume of the inductor by imposing and maintaining a predetermined level of tensile stress upon the length of the conductor of the inductor.  
         [0004]     Thus, while the problems of weight and volume of inductors for electrical pulsed power applications have been addressed in one form, the issue of stray magnetic fields remains a problem. Specifically, helical jelly roll inductors, such as the one described in the &#39;610 patent, have been shown to emit strong magnetic fields that typically exceed MIL-SPEC acceptable limits. These fields can be very harmful to electronic equipment in an armored vehicle or on a ship. Further, large stray magnetic fields create a presence that may be detectable by enemy sensors.  
         [0005]     In one attempt to solve the stray field problem, an inductor was created by assembling several helical jelly roll inductor segments into a toroidal geometry (“segmented toroid”) by attaching the segments to a central bucking cylinder. The bucking cylinder was designed to resist the implosive forces of the inductor. This “segmented toroid” exhibited significantly lower stray fields but was quite bulky.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention comprises a high energy toroidal inductor that addresses not only the desire for reduced weight and volume, but also the desire for minimal stray magnetic fields. Specifically, the high energy toroidal inductor includes a bucking cylinder and a predetermined number of leaves. Each leaf is of a two-ended, twisted ring configuration and includes a top portion. The top portion is the narrowest portion of the leaf, and the width of the leaf graduates outward from the top portion to each end of the ring. The bucking cylinder interfaces with each of the leaves and presents the leaves in a side-by-side, continuous toroidal configuration.  
         [0007]     The bucking cylinder includes a number of slots equivalent to the number of leaves and each slot is designed to accept the top, or narrowest, portion of each leaf. Each end of each leaf is presented in a flat configuration so as to enable securement of a leaf to the next proximate leaf until a complete toroid is formed. The high energy toroidal inductor may additionally include a top and bottom support ring that is concentric to the bucking cylinder and is secured to each of the leaves. The bucking cylinder and the leaves are preferably manufactured from a light weight aluminum alloy. The high energy toroidal inductor preferably creates a stray magnetic field, at a 10 inch standoff, of less than 0.5 Tesla.  
         [0008]     A method of assembling a high energy toroidal inductor includes the steps of: (1) inserting a top portion of each of a predetermined number of leaves into one of a plurality of slots about a circumference of a bucking cylinder; (2) securing each of the leaves to the next proximate one of the leaves to form a continuous toroid configuration; and (3) securing a support ring, that is concentric with the bucking cylinder, to both the top and bottom of each of the leaves. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  depicts the high energy toroidal inductor of the present invention.  
         [0010]      FIG. 2  provides a front view of a leaf of the high energy toroidal inductor.  
         [0011]      FIG. 3  provides a top view of the leaf of  FIG. 2 .  
         [0012]      FIG. 4  provides a side view of the leaf of  FIG. 2 .  
         [0013]      FIG. 5  provides a side view of the bucking cylinder of the toroidal inductor.  
         [0014]      FIG. 6  provided a top view of the bucking cylinder of  FIG. 5   FIG. 7  provides a top view of the top/bottom support ring of the toroidal inductor.  
         [0015]      FIG. 8  provides a cross-section of a leaf of the inductor for force analysis.  
         [0016]      FIG. 9  provides Tables 4-9  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     A high energy toroidal inductor of the present invention is an inductor of small weight and volume that is capable of handling high electromagnetic forces, as well as an inductor having minimal stray magnetic fields, making it especially suitable to electrical pulse power applications.  
         [0018]     The high energy toroidal inductor provides the same electromagnetic characteristics as the “segmented toroid” (described in the background) but with much lower mass. In the high energy toroidal inductor, each winding is designed to be self-supporting against the internal magnetic pressure, only requiring a small lightweight support ring on the top and bottom of the toroid. These rings also reduce the radially inward forces on the central bucking cylinder. Thus, the bucking cylinder is made smaller reducing the overall weight of the high energy toroidal inductor.  
         [0019]      FIG. 1  depicts the high energy toroidal inductor (HETI)  20  of the present invention. As shown, the HETI  20  includes a plurality of leaves  22 , a central bucking cylinder  24 , a top support ring  26 , and a bottom support ring (not shown) that is identical to the top support ring.  
         [0020]     Referring to  FIGS. 2-4 , the details of the leaves  22  may be appreciated. As shown, each leaf  22  is of a ring configuration wherein the ends  30  of the ring are offset from each other so as to enable attachment of one leaf  22  to the next proximate leaf  22 . Notably, the top  32  of the leaf presents the narrowest portion of the leaf  22  with the width expanding outward from the top to the offset ends  30  of the ring. Each end  30  of the ring is provided with a flat securement member  34 , having a plurality of bores  36  therein.  
         [0021]     The bucking cylinder  24  is best seen in  FIGS. 5 and 6 . As shown, the bucking cylinder  24  is provided with a plurality of equidistantly spaced slots  40  positioned around the circumference of the cylinder. The slots  40  are sized to receive the narrow, top portion  32  of the leaf  22 . The support ring  26  is shown in  FIG. 7  and includes a plurality of bores  44  spaced equidistantly about the circumference of the ring.  
         [0022]     To assemble the HETI, each leaf  22  is positioned within a slot  40  of the bucking cylinder  24  and is secured to the both the top and bottom support rings  26  through use of a bolt  50  and bolt insulator  52 . The next leaf  22  is positioned similarly proximate the first leaf  22  and is secured to the top and bottom support rings. An insulator is placed intermediate the overlapping ends of the proximate leaves and the ends are secured with bolts. A standoff insulator  54  is provided for external connections.  
         [0023]     Design  
         [0024]     In a preferred embodiment, the HETI  20  was designed to carry at least 150 kA. The leaves  22  are of a solid metal having an offset, split ring configuration, and are thin where the forces on the leaf are small and thick where the forces on the leaf are high. The design relies on the hoop strength of the metal to carry the EM force load. In a preferred embodiment of the HETI  20 , eighteen leaf segments  22  are preferably provided with each having a nominal diameter of six (6) inches and an inner diameter of approximately three (3) inches. The basic formula for analysis is that of hoop stress, refer to the diagram of  FIG. 8 , where: 
 
 A   interior   =d×L   (Eq. 1) 
 
 F   interior   =P×d×L   (Eq. 2) 
 
 F   wall =σ×2 t×L   (Eq. 3) 
 
         [0025]     where: L=length into page; P=internal pressure; d=mean diameter of a toroid leaf; sigma=material stress; and t=thickness.  
         [0026]     Knowing that F wall =F interior , then: 
 
 d×L×P=σ 2 t×L   (Eq. 4) 
 
         [0027]     then solving for sigma,  
             σ   =           P   ×   d       2   ⁢   t       ⁢           ⁢   or   ⁢           ⁢     t   min       =       P   ×   d       2   ⁢           ⁢     σ   max                   (     Eq   .           ⁢   5     )             
 
         [0028]     It is assumed that the outward forces on an inductor leaf act like forces on a sealed pressure vessel, i.e., the internal forces are the magnetic forces rather than the forces of a compressed gas.  
         [0029]     The forces, then, may be calculated as follows with reference to  FIG. 8 :  
                     F   AA     =     F   EE                 =       1   2     ⁡     [               ⁢         F   8     ⁢   Sin   ⁢           ⁢     (   9   )       +       F   27     ⁢   Sin   ⁢           ⁢     (   27   )       +       F   26     ⁢   Sin   ⁢           ⁢     (   45   )       +                     ⁢         F   25     ⁢   Sin   ⁢           ⁢     (   63   )       +       F   24     ⁢   Sin   ⁢           ⁢     (   81   )       +       F   23     ⁢   Sin   ⁢           ⁢     (   81   )       +                     ⁢         F   22     ⁢   Sin   ⁢           ⁢     (   63   )       +       F   21     ⁢   Sin   ⁢           ⁢     (   45   )       +                     ⁢         F   20     ⁢   Sin   ⁢           ⁢     (   27   )       +       F   19     ⁢   Sin   ⁢           ⁢     (   9   )                 ]                     (     Eq   .           ⁢   6     )                 F   AA     =       1   2     ⁡     [           2434   +   6492   +   6939   +   6089   +   4974   +               3889   +   2900   +   2008   +   1178   +   389           ]               (     Eq   .           ⁢   7     )                 F   AA     =     18646   ⁢           ⁢   N             (     Eq   .           ⁢   8     )                       F   BB     =     F   DD                 =       1   2     ⁡     [               ⁢         F   26     ⁢   Sin   ⁢           ⁢     (   9   )       +       F   25     ⁢   Sin   ⁢           ⁢     (   27   )       +       F   24     ⁢   Sin   ⁢           ⁢     (   45   )       +                     ⁢         F   23     ⁢   Sin   ⁢           ⁢     (   63   )       +       F   22     ⁢   Sin   ⁢           ⁢     (   81   )       +       F   21     ⁢   Sin   ⁢           ⁢     (   81   )       +                           ⁢         F   20     ⁢   Sin   ⁢           ⁢     (   63   )       +       F   19     ⁢   Sin   ⁢           ⁢     (   45   )       +                     ⁢         F   19     ⁢   Sin   ⁢           ⁢     (   27   )       +       F   20     ⁢   Sin   ⁢           ⁢     (   9   )                       ]                     (     Eq   .           ⁢   9     )                 F   BB     =       1   2     ⁡     [           1535   +   3103   +   3561   +   3508   +   3215   +               2805   +   2312   +   1761   +   1130   +   406           ]               (     Eq   .           ⁢   10     )                 F   BB     =     1168   ⁢           ⁢   N             (     Eq   .           ⁢   11     )                       F   CC     =       ⁢         F   23     ⁢   Sin   ⁢           ⁢     (   9   )       +       F   22     ⁢   Sin   ⁢           ⁢     (   27   )       +       F   21     ⁡     (   45   )       +                     ⁢         F   20     ⁢   Sin   ⁢           ⁢     (   63   )       +       F   19     ⁢   Sin   ⁢           ⁢     (   81   )                       (     Eq   .           ⁢   12     )                 F   CC     =     8873.3   ⁢           ⁢   N             (     Eq   .           ⁢   13     )             
 
         [0030]     The forces, F AA , F BB , F CC , F DD , and F EE , are the forces that tend to pull the leaf  22  apart in tension, like the force in a rubber band. These forces are derived from finite element computer calculations of the magnetic fields in and around the toroid. The sections are always aligned with the axis of the main leaf circle at radial planes. Section AA is 180 degrees away from the bucking cylinder side of the leaf, section BB is 135 degrees away, section CC is 90 degrees away, section DD is 45 degrees away, and section EE is 0 degrees away. A summary of the force calculations is provided below in Table 1.  
                                                     TABLE 1                       Section   Force, N   Design Force, N   Length, m   Width, m                                AA   18646   55938   0.02284   0.01       BB   11668   35004   0.02284   0.01       CC   8873   26619   0.02284   0.027       DD   11668   35004   0.02284   0.044       EE   18646   55938   0.02284   0.048                  
 
         [0031]     Next, the minimum thickness for the toroid leaf may be calculated according to Eq. 5 above. Table 2 provides a summary of the minimum thickness, as measured in meters, of each section for various materials including Hard Copper, 2014 Aluminum, 6061 Aluminum, and 7075 Aluminum.  
                               TABLE 2                       Sec-   Hard CU   2014 Al   6061 Al   7075 Al       tion   Min. Thickness   Min. Thickness   Min. Thickness   Min. Thickness                   AA   1.70E−02   1.36E−02   2.07E−02   1.17E−02       BB   1.06E−02   8.54E−03   1.30E−02   7.29E−03       CC   2.99E−03   2.40E−03   3.65E−03   2.05E−03       DD   2.41E−03   1.94E−03   2.95E−03   1.66E−03       EE   3.53E−03   2.84E−03   4.32E−03   2.43E−03                  
 
         [0032]     A summary of the yield strength, resistivity, and density of each of these materials is provided in Table 3.  
                               TABLE 3                       Property   Hard Cu   2014 T6 Al   6061 T6 Al   7075 T6 Al                   Yield Strength,   3.30E+08   4.10E+08   2.70E+08   4.80E+08       Pa       Resistivity,   1.70E−08   4.30E−08   4.00E−08   5.20E−08       ohm*m       Density,   8900   2800   2700   2800       Kg/m{circumflex over ( )}3                  
 
         [0033]     Finally, a voltage drop for each of the materials may be calculated, with reference to  FIG. 8  and the numbered sections. The section numbers are obtained from a computer Lorentz force calculation and they designate 18 degree sections of the toroid leaf. The numbers start at the bucking cylinder (9 degrees at the center plane of the section), with number 19, go up to number 27 (153 degrees at the center plane of the section), and end with number 8 (171 degrees at the center plane of the section). As can be seen, section 8 is out of sequence. The calculated voltage drop through the sections is just the DC voltage drop through the sections due to the required thickness. So, when the thickness is higher, the voltage drop is less because there is a thicker “wire”. Because, there is so much current going through the toroid, even small resistances lead to significant energy losses.  
         [0034]     Tables summarizing the voltage drops of the various materials may be found in  FIG. 9 , with Table 4 providing the voltage drop for hard drawn copper (varying leaf thickness), Table 5 providing the voltage drop for hard drawn copper (constant leaf thickness), Table 6 providing the voltage drop for 6061 Aluminum (varying leaf thickness). Table 7 providing the voltage drop for 6061 Aluminum (constant leaf thickness), Table 8 providing the voltage drop for 2014 Aluminum (varying leaf thickness), and Table 9 providing the voltage drop for 2014 Aluminum (constant leaf thickness). In view of the above, the preferred material for the high energy toroidal inductor is the aluminum alloy 6061-T6 due to its light weight and high strength.  
         [0035]     Table 10 provides a comparison between the high energy toroidal inductor of the present invention, the segmented toroid (described in the background) and the helical jelly roll toroid (described in the background). As can be seen, toroidal inductors have small external magnetic fields relative to comparable helical inductors and the high energy toroidal inductor of the present invention provides the additional benefits of significantly reduced weight and reduced stray magnetic fields.  
         [0036]     As indicated by Table 10, a preferred embodiment of the high energy toroidal inductor of the present invention is designed to be 8 μH with 18 turns. Each turn is constructed by machining a block of 6061-T6 aluminum into the requisite shape; to minimize the electrical resistance there are no welded joints between turns. Notably, each turn is made thick enough to support the electromagnetic forces which act radially outward on each turn. In this way, the toroidal inductor can be made with a similar weight to an equivalent helical inductor since no bracing is needed and can additionally be made with similar reduced magnetic fields to an equivalent segmented toroidal inductor.  
                                                             TABLE 10                                           Stray Field               Max       Space   @ 10 in       Inductor   Inductance   Current,   Weight,   Claim,   standoff       Design   μHenries   kAmp   kg   m 2     Tesla                                Helical   5.5   150   18   0.0083   1-5       Jelly Roll       Segmented   60   100   122   0.14   0.2       Toroid       High Energy   8   150   17   0.0033   0.3       Toroidal                  
 
         [0037]     The present invention may be embodied in other specific forms without departing from the essential attributes thereof; therefore the illustrated embodiments should be considered in all respects as illustrative and not restrictive.