Abstract:
A transformer includes a core defining a core window, a first coil surrounding a portion of the core and including a portion located within the core window, a second coil surrounding a portion of the core and including a portion located within the core window, and a polymer barrier insulation member that is located at least partially within the core window and positioned between the first coil and the second coil.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from U.S. Provisional Application No. 60/444,968, filed Feb. 5, 2003, and titled “Polymer Sheet Core and Coil Insulation For Liquid Filled Transformers,” which is incorporated by reference. 

   TECHNICAL FIELD 
   The description below relates to polymer sheet insulation for use with a transformer core and coil assembly. 
   BACKGROUND 
   Transformers often include a core and coil assembly formed from a pair of coils interconnected by a conductive core. A three phase transformer includes three coils with multiple cores. The core and coil assembly may be positioned in a tank that is filled with a dielectric fluid. The dielectric fluid serves to cool the assembly and electrically isolate the core and coil assembly from the tank. 
   Insulation used in the transformer core and coil assembly typically is constructed using kraft or cotton-based pressboard, which is known to degrade over time as a function of temperature. This insulation normally is made from multiple individual pieces that may require cutting to the correct size. Also, the insulation needs to be thoroughly dried prior to filling the tank with dielectric fluid because moisture contributes to and increases the degradation rate, especially when combined with heat. Typical kraft-based pressboard materials absorb moisture in a range from approximately 3% to 10%, or more, based on the length of exposure to humidity. This moisture should be removed to a level of approximately 1% prior to filling the transformer with dielectric fluid. Current kraft or cotton-based pressboard material absorbs moisture and degrades rapidly at temperatures from 130° C. to 170° C. Further moisture developed during the degradation process of kraft or cotton-based pressboard, accelerates aging and degradation of the pressboard. 
   SUMMARY 
   Techniques are used to provide a polymer sheet core and coil insulation for transformers such as, for example, single-phase and multi-phase transformers. The polymer sheet core and coil insulation may be used, for example, as a barrier insulation including use as a phase-to-phase barrier insulation and as a phase-to-ground barrier insulation, and further may be used as a transformer coil support. 
   In one general aspect, a transformer includes a core defining a core window, a first coil surrounding a portion of the core and including a portion located within the core window, a second coil surrounding a portion of the core and including a portion located within the core window, and a polymer barrier insulation member that is located at least partially within the core window and positioned between the first coil and the second coil. 
   Implementations may include one or more of the following features. For example, the transformer may be a three phase transformer, a single phase transformer, or a step voltage regulator. The polymer barrier insulation member may include at least one ribbed polymer sheet, at least one flat polymer sheet, and/or two or more stacked polymer sheets. 
   The polymer barrier insulation member may be made of a high temperature polymer configured to withstand an operating temperature of approximately 130 degrees Celsius, may be configured to withstand overload conditions through approximately 200 degrees Celsius, and may be configured so as to absorb no more than approximately 1% moisture. The high temperature polymer may be, for example, syndiotactic polystyrene, a polyester thermoset molding compound, or a vinylester thermoset molding compound. The high temperature polymer may be made using a variety of techniques and, for example, may be extruded, injection molded, compression molded or thermo-formed. 
   In another general aspect, a transformer includes a core defining a core window, a first coil surrounding a portion of the core and including a portion located within the core window, and a polymer barrier insulation member that is located at least partially within the core window and positioned between the first coil and the core. 
   In another general aspect, a transformer includes a core defining a core window, a first coil surrounding a portion of the core and including a portion located within the core window, and a polymer coil support member that is located at least partially within the core window and positioned between the first coil and the core so as to support the first coil. 
   Implementations may include one or more of the following features. For example, the transformer also may include a second coil surrounding a portion of the core and including a portion located within the core window, where the coil support is positioned between the second coil and the core so as to support the second coil. In another implementation, the coil support member includes one or more standoffs. The standoffs may have several shapes such as, for example, a circular shape, a truncated circular shape, or an egg crate shape. 
   The coil support member may include one or more coolant flow channels, and may be made, for example, of a thermoset material or a high temperature thermoplastic material. In one implementation, the coil support member is made of a high temperature polymer material configured to withstand an operating temperature of approximately 130 degrees Celsius. 
   In yet another general aspect, a transformer includes a core defining a core window, a first coil surrounding a portion of the core and including a portion located within the core window, a second coil surrounding a portion of the core and including a portion located within the core window, a first polymer barrier insulation member that is located at least partially within the core window and positioned between the first coil and the second coil, a second polymer barrier insulation member that is located at least partially within the core window and positioned between the first coil and the core, and a polymer coil support member that is located at least partially within the core window and positioned between the first coil and the core so as to support the first coil. 
   Other features will be apparent from the description and drawings, and from the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view of a three phase transformer core and coil assembly. 
       FIG. 2  is a cut-away top view of the three phase transformer core and coil assembly of FIG.  1 . 
       FIG. 3  is a cut-away side view of the three phase transformer core and coil assembly of  FIGS. 1 and 2 . 
       FIG. 4  is a cut-away end view of the three phase transformer core and coil assembly of  FIGS. 1-3 . 
       FIGS. 5 ,  7  and  8  are side views of corrugated barrier insulation that may be used in the three phase transformer core and coil assemblies of  FIGS. 1-4 . 
       FIG. 6  is a side view of exemplary stacking configurations of the corrugated barrier insulation of  FIG. 5   
       FIG. 9  is an expanded view of a section of the corrugated barrier insulation of FIG.  8 . 
       FIG. 10  is a side view of flat barrier insulation that may be used in the three phase transformer core and coil assemblies of  FIGS. 1-4 . 
       FIGS. 11 and 14  are top views of coil supports that may be used in the three phase transformer core and coil assemblies of  FIGS. 1-4 . 
       FIG. 12  is an expanded top view of a section of the coil support of FIG.  11 . 
       FIG. 13  is a cross-sectional view of the coil support of  FIG. 12  taken along section A—A of FIG.  12 . 
       FIG. 15  is an expanded top view of a section of the coil support of FIG.  14 . 
       FIG. 16  is a cross-sectional view of the coil support of  FIG. 15  taken along section A—A of FIG.  15 . 
       FIG. 17  is a perspective view of a coil support that may be used in the three phase transformer core and coil assemblies of  FIGS. 1-4 . 
       FIG. 18  is an expanded perspective view of a section of the coil support of FIG.  17 . 
       FIG. 19  is a perspective cross-sectional view of the coil support of FIG.  18 . 
       FIG. 20  is a cross-sectional view of the coil support of FIG.  18 . 
     Like reference symbols in the various drawings indicate like elements. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a transformer core and coil assembly  100  includes coils  105 ,  110  and  115  and cores  120 ,  125 ,  130  and  135 . A frame  140  surrounds the cores and coils, and includes a top  145 , a bottom  150 , and sides  155  and  160 . As shown, the transformer core and coil assembly  100  is for a three-phase transformer. Each of coils  105 ,  110  and  115  includes a primary winding and one or more secondary windings for an individual phase of a three-phase system. The transformer core and coil assembly  100  typically is placed in a tank and immersed in a dielectric fluid (not shown). The transformer core and coil assembly  100  may be used in applications such as, for example, distribution and power transformers, and in one example, may include core and coil assemblies having operating ratings through 10,000 kVA. 
     FIG. 2  shows a cut-away top view of the three phase transformer core and coil assembly  100  of FIG.  1 . As shown in  FIG. 2 , core  120  defines a core window  205  through which at least a part of coil  105  extends. Core  125  defines a core window  210  through which at least a part of coil  105  and at least a part of coil  110  extend. Core  130  defines a core window  215  through which at least a part of coil  110  and at least a part of coil  115  extend. Core  135  defines a core window  220  through which at least a part of coil  115  extends. Each of coils  105 ,  110  and  115  is associated with a different phase of a three-phase system. 
     FIG. 3  illustrates a cut-away side view of the three-phase transformer core and coil assembly  100  of  FIGS. 1 and 2 . As shown, core window  205  of core  120  contains at least a portion of coil  105 , barrier insulation  305 , a coil support  310 , and barrier insulation  315 . 
   Barrier insulation  305  is located adjacent coil  105  and between the coil  105  and the core  120  in the core window  205 . Barrier insulation  305  serves as a coil-to-ground (core) barrier and is made of a polymer material. Barrier insulation  305  also provides dielectric and mechanical strength between the coil and the core. Typically, barrier insulation  305  is a flat or corrugated sheet. Cooling duct configurations may be included in the barrier insulation  305 . Multiple pieces of barrier insulation may be used to fill the space to an appropriate thickness given the dielectric strength required. In certain implementations, the barrier insulation is made from interlocking pieces, such as, for example, corrugated pieces. In other implementations, the barrier insulation is made from flat pieces. 
   Coil support  310  is located between the coil  105  and the core  120  beneath the coil  105 . Coil support  310  serves to support the weight of coil  105 , and also serves as a dielectric barrier between the coil and the core. A barrier  315  may be located above coil  105  between coil  105  and core  120  in the core window  205 . 
   Core window  210  of core  125  contains at least portions of coil  105 , coil  110 , barrier insulation  320 , a coil support  325 , and barrier insulation  330 . Barrier insulation  320  is located between coil  105  and coil  110 , and acts as a coil-to-coil insulation barrier. Barrier insulation  320  also provides dielectric and mechanical strength between the coils. In a similar manner to that described with respect to coil support  310 , coil support  325  is located beneath at least a portion of coil  105  and at least a portion of coil  110 , and serves to separate the coils  105  and  110  from the core  125 . Barrier insulation  330  may be located on top of at least portions of coils  105  and  110 , and serves to separate the coils  105  and  110  from the core  125  in a manner similar to that described with respect to barrier insulation  315 . 
   Core window  215  of core  130  contains at least portions of coil  110 , coil  115 , barrier insulation  335 , a coil support  340 , and barrier insulation  345 . Barrier insulation  335  may be placed between coil  110  and coil  115  in a manner similar to that described with respect to barrier insulation  320 . Coil support  340  may be placed underneath a portion of coil  110  and a portion of coil  115  in a manner similar to that described with respect to coil support  325 . Barrier insulation  345  may be placed on top of coil  110  and coil  115  in a manner similar to that described with respect to barrier insulation  330 . 
   Core window  220  of core  135  contains at least portions of coil  115 , barrier insulation  350 , a coil support  355 , and barrier insulation  360 . Barrier insulation  350  is placed between coil  115  and core  135  in a manner similar to that described above with respect to barrier insulation  305 . Coil support  355  is placed underneath coil  115  in a manner similar to that described above with respect to coil support  310 . Barrier insulation  360  is placed on top of coil  115  in a manner similar to that described above with respect to barrier insulation  315 . 
   Each of barrier insulation  305 ,  315 ,  320 ,  330 ,  335 ,  345 ,  350 , and  360  is made of a polymer material. The polymer material may be, for example, a high temperature polymer with low water absorption properties. The polymer material used for insulation should have less than 1% moisture and should not absorb moisture when exposed to humid air. For example, it is possible to use a polymer material with less than 0.5% moisture. Temperatures of approximately 130° C. to approximately 200° C. may exist in the areas of the barrier material, and it is beneficial to use a barrier polymer material that does not absorb moisture beyond the 0.5% level, and that operates at elevated temperatures from 130° C. through 200° C. with minimal degradation. Polymer materials that operate in a transformer for extended periods of time, during overload conditions, at temperatures from 130° C. through 170° C. and excursions through 200° C. are very desirable. The material should also be cost effective. 
   The polymer material reduces the need to dry the insulation prior to filling the transformer with dielectric fluid and thereby reduces the transformer manufacturing cycle time. The barrier insulation may be molded or extruded as a single part, and molding or extrusion can be used to add functionality, such as, for example, coolant flow channels, locating features, interlocking features, and stacking features. 
   In particular implementations, the polymer material is of a heat resistant type, such as, for example, Questra (syndiotactic polystyrene), high temperature nylon, PPS (polyphenylene sulfide), RADAL-R (polyphenylsulfone), or another appropriate heat resistant or thermoset material. The polymer material typically has a superior dielectric strength to allow for a reduction in thickness of the electrical insulation, and consequently to allow a size and weight reduction in the transformer. The barrier insulation may be used in multi-phase transformers, shell-type single phase transformers, and step voltage regulators, among other applications. The barrier insulation may be used for coil-to-coil insulation barriers and coil-to-ground (core) barriers. The barriers typically serve as barriers for dielectric and mechanical strength. Within the transformer coils  105 ,  110 , and  115 , polymer sheet wire spacers may be used to space sections of the windings apart from one another. 
   Polymer coil supports  310 ,  325 ,  340 , and  355  each insulate a coil end from a core ground plane. The coil supports may be formed to have a minimally obstructed oil flow horizontally above and below the coil in the core window, which provides for consistent cooling with lower thermal gradients. The coil supports provide mechanical support for the coils in the core windows. Sufficient support area is provided to prevent the crushing of the coil margins. The coil supports are sufficiently rigid to withstand telescoping forces during short circuit. When multiple layers are stacked together, the coil supports are designed to minimize the probability of allowing individual layers to move axially in relation to each other during shipping or short circuit. The use of the polymer coil supports reduces stacking time through, among other things, part count reduction. Use of the polymer coil supports also provides more consistent core coolant (oil) flow, improved thermal performance and consistency, and improved mechanical performance because among other properties, it does not compress. 
   With respect to the polymer coil supports  310 ,  325 ,  340 , and  355 , various production methods may be used for providing dielectric coolant flow to the side ducts of the coils  105 ,  110 , and  115 . For example, the polymer coil support  310 , which insulates the ends of coil  105  from the core  120  ground plane, provides minimal obstruction of coolant flow horizontally above and below the coil  105  in the core window  205  and provides for consistent coolant flow. The coil support  310  also provides mechanical support for the coil  105  in the core window  205 . This support serves to prevent crushing of the coil margins. In addition, the coil support  310  is sufficiently rigid to withstand telescoping forces during a short circuit. A single sheet or multiple stacked sheets may be used. Where stacked sheets are used, the coil support  310  is designed to minimize the probability of individual layers moving axially in relation to each other during shipping or short circuit. The coil support  310  has sufficient dielectric strength in the core window  205 . The coil support  310  may be made from a high temperature thermoplastic or a thermoset material. 
     FIG. 4  shows a cut-away end view of the three phase transformer core and coil assembly of FIG.  1 .  FIG. 4  illustrates coil  115 , core  135 , barrier insulation  350 , coil support  355 , barrier insulation  360 , and frame  140 . 
     FIG. 5  illustrates a side view of an exemplary corrugated barrier insulation that may be used in the three phase transformer core and coil assembly of  FIGS. 1-4 . For example, with respect to  FIG. 3 , the corrugated barrier insulation  500  may be used as barrier insulation  305 ,  320 ,  335 , or  350  and/or barrier insulation  315 ,  330 ,  345 , or  360 . Corrugated barrier insulation  500  has upper faces  505  and lower faces  510  that are connected by transverse members  515 . Protrusions  545  are provided at the lower face  510  in order to allow for stacking of multiple layers of corrugated barrier insulation  500 . The upper face  505  has a length  520  and a thickness  525  that may be uniform or may vary. Transverse member  515  has a length  530  and a thickness  535  that may be uniform or may vary. The lower face  510  has a length  540  and a thickness  550  that may be uniform or may vary, and typically includes protrusions  545 . Though not shown, top face  505  may also include protrusions. Corrugated barrier insulation  500  may be made from polymer materials as discussed above with respect to barrier insulation  305 ,  315 ,  320 ,  330 ,  335 ,  345 ,  350 , and  360 . 
     FIG. 6  shows a side view of a stacked configuration of multiple sheets of the corrugated barrier insulation of FIG.  5 . The stack  600  of corrugated barrier insulation  500  sheets includes an aligned stack  605  of sheets  500 A,  500 B,  500 C, and  500 D, and also includes a staggered stack  610  of sheets  500 E,  500 F, and  500 G. In the aligned stack  605 , the upper faces of sheets  500 A,  500 B,  500 C, and  500 D are aligned, the lower faces of  500 A,  500 B,  500 C, and  500 D are aligned, and the transverse members of  500 A,  500 B,  500 C, and  500 D are aligned. The protrusions  545  assist in maintaining the stacked configuration of the corrugated barrier insulation sheets  500 A,  500 B,  500 C, and  500 D. 
   The staggered stack  610  includes sheets  500 E,  500 F, and  500 G aligned in a staggered configuration. For example, corrugated barrier insulation sheet  500 E is aligned to be offset with respect to the adjacent corrugated barrier insulation sheet  500 F. A lower face  510 F of sheet  500 F is staggered with respect to a upper face  505 E of sheet  500 E, and an upper face  505 F of sheet  500 F is staggered with respect to a lower face  510 G of sheet  500 G The staggered pattern may continue as more layers are added. The number of layers in the stacked configuration will depend on, among other things, the dielectric strength required and the physical dimensions of the core and the coil. 
     FIG. 7  shows a side view of another exemplary corrugated barrier insulation  700  that may be used in the three phase transformer core and coil assembly of  FIGS. 1-4 . Corrugated barrier insulation  700  includes upper faces  705  and lower faces  710  that are connected by transverse members  715 . Protrusions  745  are provided at the lower face  710  in order to allow for stacking of multiple layers of corrugated barrier insulation  700 . Though not shown, the upper face  705  may also include protrusions. The transverse member may be of uniform or non-uniform thickness. For example, an indentation  750  is formed in a transverse member  715  so that the transverse member has a non-uniform thickness. Another transverse member  718  has a flat wall  755  so that the transverse member has a uniform thickness. 
     FIG. 8  illustrates a side view of yet another exemplary corrugated barrier insulation  800  that may be used in the three phase transformer core and coil assembly of  FIGS. 1-4 . Corrugated barrier insulation  800  includes flat regions  805  and ribs  810  located on an upper surface  802  of insulation  800 . A rib  810  includes an indentation  815  located on a lower surface  803  of the insulation  800 . Indentation  815  and rib  810  facilitate the stacking of multiple sheets of corrugated barrier insulation  800  and enable the stacked sheets to resist transverse motion with respect to each other. Corrugated barrier insulation  800  also includes a rounded end  835  and a raised section  845 . 
   The corrugated barrier insulation  800  has an overall length  825  that may be divided into a set of one or more distances  820  between consecutive ribs  810 , a distance  830  from a rib  810  to an end  835 , and a distance  840  from a rib  810  to the other end  835 . 
     FIG. 9  shows an expanded view of a section of the corrugated barrier insulation of  FIG. 8  near a rib  810 . The section  900  includes a rib  810  located on an upper surface  802 , and an indentation  815  located on a lower surface  803 . The indentation  815  has a shape that is configured to engage rib  810 . The rib  810  has a height  905  above surface  802 . The flat area  805  transitions to a rib  810  through a bottom curve  925  to a side  920  to a top curve  915  and to a rib top  910 . The rib top  910  has a length  912 . Although rounded corners are illustrated, other shapes, such as square corners or other angled corners, may be used. Also, the sides may have different shapes and slopes from those illustrated in FIG.  9 . 
   The barrier insulation has a thickness  960  and a total height from the lower surface  803  to the rib top  910 . 
   The indentation  815  has corners  930  to form an indentation of depth  955 , and corners  935  to form a total depth  950 . A recess  945  is optional. Although rounded corners are shown, various other shapes may be used for the indentation. Also, other shapes may be used for the rib  810  and the indentation  815 . 
   As discussed, rib  810  and indentation  815  are configured to engage and enable stacking of multiple sheets  800  in such a manner that the sheets do not slide transversely with respect to one another when subjected to compressive loading forces. The combination of rib  810  and indentation  815 , or other corrugated shapes, enables, among other benefits, easier stacking of multiple sheets, and prevents sliding of the various sheets in a stack with respect to each other. 
     FIG. 10  shows a side view of flat barrier insulation that may be used in the three phase transformer core and coil assembly of  FIGS. 1-4 . Flat barrier insulation  1000  has no corrugation (i.e., no ribs or indentations). Flat barrier insulation  1000  has a thickness  1010  and an overall length  1005 . Flat barrier insulation  1000  also includes optional rounded corners  1015 . Although flat and corrugated shapes have been shown, other shapes of barrier insulation sheets may be used. 
     FIG. 11  shows a top view of an exemplary coil support that may be used in the three phase transformer core and coil assembly of  FIGS. 1-4 . For example, referring to  FIG. 3 , coil support  1100  may be used as any or all of coil supports  310 ,  325 ,  340 , and  355 . Coil support  1100  has a length  1105  and a width  1110 . Although coil support  1100  is shown as a rectangular shape, other shapes may be used. Coil support  1100  has standoffs  1115  and channels  1120  between the standoffs that provide for coolant flow above and/or below the coil support  1100 . 
     FIG. 12  shows an expanded top view of a section of the coil support of FIG.  11 . Section  1200  of the coil support shows a topographical view of standoffs  1115  and channels  1120 . There is a spacing  1215  between standoffs in the vertical direction and a spacing  1220  between standoffs in the horizontal direction. The spacings  1215  and  1220  may be uniform or may vary. The standoffs  1115  may include an inner radius  1205  and an outer radius  1210 , with the inner radius being at the highest elevation in the topographical view and the outer radius being at the lowest elevation in the topographical view. 
     FIG. 13  is a cross-sectional view of the coil support of  FIG. 12  taken along section A—A of FIG.  12 .  FIG. 13  shows three standoffs  1115  and two channels  1120 . Each standoff  1115  has a height  1305  and each channel  1120  has a depth  1315 . In particular implementations, the height  1305  and depth the  1315  may be uniform or may vary. The polymer sheets forming standoffs  1115  and channels  1120  have a thickness  1310 , which may be uniform or variable. 
     FIG. 14  shows a top view of another exemplary coil support that may be used in the three phase transformer core and coil assembly of  FIGS. 1-4 . Coil support  1400  has a length  1405  and width  1410 . Although shown in a rectangular shape, other shapes may be used. Coil support  1400  has standoffs  1415  of a first type and standoffs  1417  of a second type. Coil support  1400  also has channels  1420  that run between the standoffs  1415  and  1417 . 
     FIG. 15  is an expanded top view of a section of the coil support of FIG.  14 . The section  1500  shows standoffs  1415  of a first type, which in this case is a circular type, and standoffs  1417  of a second type, which in this case is a truncated circular type. Channels  1420  are formed between the standoffs  1415  and  1417 . 
   Although  FIG. 15  shows a pattern of alternating standoffs  1415  and  1417 , other implementations may use other patterns. For example, two or more standoffs of a first type  1415  may be placed adjacent to each other, rather than alternating between standoffs of a first type and a second type.  FIG. 15  shows a horizontal standoff spacing  1505  between the same type of standoff  1417  and a horizontal standoff spacing  1515  between different types of standoffs. Similarly, there is a vertical spacing  1510  between the same type of standoff and a vertical spacing  1520  between alternating types of standoffs. In particular implementations, the horizontal and vertical spacings may be uniform or variable. 
     FIG. 16  shows a cross-sectional view of a section of coil support of  FIG. 15  taken along section A—A of FIG.  15 .  FIG. 16  shows standoffs of a first type  1415  and channels  1420 . Due to the cross-section shown, a standoff of the second type  1417  is not shown. Standoff  1415  has a height  1605  and channel  1420  has a depth  1615 . In particular implementations, height  1605  and depth  1615  may be uniform or variable. The polymer sheet in which standoff  1415  and channel  1420  are formed has a thickness  1610  that may be uniform or may vary. 
     FIG. 17  shows a perspective view of yet another exemplary coil support that may be used in the three phase transformer core and coil assembly of  FIGS. 1-4 . Coil support  1700  has a length  1705  and a width  1710 . Although shown in a rectangular configuration, other shapes may be used. Coil support  1700  has standoffs  1715  and dimples  1717 . Channels  1720  are formed between the standoffs  1715  and the dimples  1717  to enable coolant flow above and/or below the coil support  1700 . 
   The shape of the coil support  1700  is symmetric in the x and the y directions, and has the same shape on the top and the bottom. This allows for installation without regard to orientation. The coil support  1700  is of uniform wall thickness, improving material flow characteristics during the molding process. The repetitive design assists with tooling construction. This shape provides a high degree of flow area for the dielectric coolant and tends to minimize the potential for continuous blockage of any single coolant flow channel in the coil support  1700 . 
     FIG. 18  shows an expanded perspective view of a section of the coil support of FIG.  17 . Section  1800  shows one or more standoffs  1715 , dimples  1717 , and channels  1720 . 
     FIG. 19  shows a perspective cross-sectional view of the coil support of FIG.  18 . Cross-section  1905  shows standoffs  1715  and dimples  1717  in a profile view. 
     FIG. 20  shows a cross-sectional view of the coil support of FIG.  18 .  FIG. 20  shows one or more standoffs  1715  and one or more dimples  1717 . Each standoff  1715  has a height  2005  and each dimple  1717  has a depth  2010 . 
   Other implementations are within the scope of the following claims.