Abstract:
A method of manufacturing a stator bar in large dynamoelectric machines, with the bar insulation made of solid materials with high thermal conductivity and high dielectric strength. In the present invention, the bar insulation is provided by thermally conductive shells whose inner cavity conforms to the bar stock. The manufacturing process comprises slipping one or more thermally conductive shells at the substantially central straight portion of the bar stock, bending the ends of the bar stock to form end-arms, insulating the end-arms with tape or extrusion insulation and bonding all members to form a solid stator bar. In short, the bent end-arms are insulated by the tape or extrusion insulation while the straight portion of the bar is insulated by the thermally conductive shells. This greatly increases the effective thermal conductivity of the stator bar. The high dielectric strength of the materials significantly reduces the wall thickness of the bar insulation. The space freed by the thinner bar insulation can be filled beneficially by copper or iron thereby reducing the size of the machine. The high thermal conductivity of the bar insulation will increase the life of the stator bar, while the thinner bar insulation will reduce the size of the machine.

Description:
TECHNICAL FIELD 
     This disclosure relates to the ground wall insulation of a stator bar in large electric machines, specifically improving the thermal conductivity of the ground wall insulation. 
     BACKGROUND 
     Large dynamoelectric machines employed in the utility industry use a long insulated conductor called a stator bar  1  (see  FIG. 1-A ) placed in its core. Construction of the stator bar  1  starts with a long, straight bar stock  2  (see  FIG. 2-A ) of precise rectangular cross-section, made of plurality of transposed insulated copper strands (not shown). Both ends of the bar stock  2  are then bent at sections A and B near the end to form end-arms AC and BD. The bar stock after bending is termed a bent-bar  3  (see  FIG. 2-B ) herein. The stator bar  1  in  FIG. 1-A  is then formed by insulating the bent-bar  3  by a high voltage-resistant insulation, termed ground wall insulation or bar insulation  4 . The bar insulation  4  protects the core  10  against the large voltages of stator bar, which can range from 10 to 25 kV. The stator bar  1  hence contains a straight portion AB and bent end-arm portions AC and BD. When the stator bar  1  is inserted into the core  10 , the straight portion AB protrudes beyond the end faces PQ of the core  10 . The protruding straight portions PA and QB are called overhangs. The core  10  itself is made of lamination stack  11 . 
     The bar insulation  4  thus forms a hollow rectangular sectioned shell around the bent-bar  3 . Long ago, the bar insulation was made by wrapping a thin electrically insulative tape several times over a bent-bar  3  and curing it to form a solid bar. The thermal conductivity of the tape used to be about 0.3 w/mK, so machines used to be relatively large to keep it cool. In the past 20 years, electrically insulative tapes with thermally conductive fillers were developed, which increased the thermal conductivity to about 0.5 w/mK. A bar insulation with such higher thermal conductivity can reduce the stator temperature, beneficially increasing the life of the machine and its performance. In recent years, more innovative approaches to improving the thermal conductivity of bar insulation further are emerging, as summarized below. 
     Almost all prior-art approaches to improve the thermal conductivity of bar insulation employ thermally conductive fillers, and can be grouped into a multiphase tape  4   a  ( FIG. 1-B ), a multiphase extrusion  5  ( FIG. 1-C ) or a multiphase fabric. They use multiphased insulation, comprising a matrix or major phase of thermally insulative materials embedding minor phases of thermally conductive fillers. But admixture of multiple phases creates a large number of boundaries with innumerable discontinuities; it also suffers from potential trapped air spaces. The discontinuities and trapped air spaces obstruct heat flow, thereby reducing the net thermal conductivity. For example, the multiphase tape  4   a  may have one, two or more layers, each comprising a thermally insulative base layer with one phase of plastic tape to serve as a carrier, a second phase of fibrous weave to impart strength, a third phase of electrically insulative materials such as mica to provide electrical insulation, a fourth phase of thermally conductive fillers to provide thermal conductivity and a fifth phase of thermally insulative resin binder to bond all the phases. Several patents, e.g., U.S. Pat. No. 7,547,847 or 6,242,825 describe various layers, phases and materials used in multiphase tapes. 
     Even though prior-art bar insulation comprises more than half-dozen phases, the thermally conductivity of filler is the only phase that contributes to increase in thermal conductivity. All other phases, being thermally insulative, tend to reduce the thermal conductivity. The geometry of a filler greatly affects its thermal resistance. The fillers geometry can be long fibers, short whiskers (e.g., E-glass with 0.99 w/mK, Dacron glass with 0.4 W/mK), particulates (e.g., boron nitride with ˜120 w/mK, aluminum oxide with 25 w/mK etc.) or flakes. Fillers with fiber or whisker geometry are mostly thermally insulative because the round shape allows thermal contact only along a line, thus obstructing heat flow; they are usually encapsulated by a resin phase that is insulative and hence obstructs heat flow. Particulate fillers are microscopic, with the largest dimension less than 15 μm (600 μinch) but microscopic nano-fillers of size less than 0.2 μm (8 μinch), are recently being introduced per U.S. Pat. Nos. 7,875,347, 7,803,457. Heat flows easily inside flake shaped particulates because of favorable aspect ratio and high thermal conductivity, but outside the flakes surface, heat transfer is obstructed by the insulative resin phase that bonds them. The problem with all fillers is that they are encapsulated by a resin binder which is mostly thermally insulative, hence overall thermal conductivity is not increased. 
     U.S. Pat. No. 7,655,868 describes an alternative embodiment that uses a tape made of thermally conductive fabric. The fabric in this invention is made by weaving a thermally conductive ceramic fiber phase in one or both directions. But a high thermal conductivity of fiber phase does not necessarily increase the thermal conductivity of bar insulation because, even though the heat flows easily through them, it encounters great resistance when trying to cross a boundary. These ceramic fibers have round cross section that contact only at a point or along a line, and this narrow path of transmission obstructs heat flow. Besides introduction of the ceramic fibers does not solve the fundamental problems of air voids and resin encapsulation that reduce the thermal conductivity. 
     Recognizing the fundamental limitations of tapes, extrusions as alternatives to tapes to increase thermal conductivity were also examined recently. The extrusion can be an in-situ or a pre-extruded member. An in-situ extruded insulation was described in U.S. Pat. Nos. 5,650,031 and 5,710,475; the method uses a moving extrusion head to deposit thermoplastic resin in-situ over a stationary bent-bar  3 . But it is very difficult for an extrusion head to follow the complex 3-D shape of bent-bar  3 . The technical difficulties of precisely depositing resin over complex 3-D shape of end-arms AC and BD are so challenging that currently stator bars are not made by this in-situ extrusion method. Alternately, pre-extruded members for bar insulation were also discussed in U.S. Pat. No. 7,832,081.  FIG. 1-C  shows one such embodiment in which the bar insulation comprises a pre-extruded insulating member  5  having a rectangular shape that defines a central cavity  9 . The extruded member has a slit  7  along the entire length of the bent-bar  3 . The slit  7  is made of pair of opposite edges  8 . Bent-bar  3  is slipped into the cavity  9  and the slit  7  is closed by plastic welding. But an extrusion insulation  5  with a slit  7  has several disadvantages. Long plastic welds that are used to close the long slit could contain electrical defects at some points over a long span. Besides, in vibratory high temperature environment, the weld expands and contracts several million times, so it may develop a crack which could propagate and cause catastrophic degradation of the insulation. Besides, manufacturing a slitted extrusion that precisely conforms to the complex 3-D shape is technically challenging. Further, extrusions rely on fillers to improve thermal conductivity, but it is well known that fillers do not greatly enhance thermal conductivity of the bar insulation. In view of these deficiencies, there still remains a need for bar insulation with enhanced thermal conductivity. 
     SUMMARY 
     One objective of the present invention is to increase thermal conductivity of the bar insulation significantly. Another objective is to reduce the thickness of bar insulation. Another objective is to reduce the cost of manufacturing the bar insulation. 
     Embodiments that successfully meet these objectives offer several benefits. They could increase the power rating of the machine, increase its voltage rating, increase the life of the stator bar, reduce the size of the machine, increase the efficiency of the machine, or keep the machine cool. The invention is described in two alternative embodiments—shelled bar  100  and split-shelled bar  200 . 
     Both embodiments  100  and  200  employ single phase materials—i.e., bulk or solids with high thermal conductivity and high dielectric strength. This contrasts with multi-phase approach of flexible tape insulation using low thermal conductivity materials that was pursued in the prior art. The single phase material is employed in two distinct geometries. A first embodiment  100  shown in  FIG. 2-C  employs full shell geometry, a second embodiment  200  shown in  FIG. 8-A  employs split-shell geometry. The single-phase material is applied preferably in the central straight portion of the bar stock only, while a thermally insulative material such as a tape or extrusion is used in the bent end-arm portions, both insulations being merged smoothly in a transition zone. Since heat is removed in the core span PQ only, applying a single phase material tc cover the core span PQ only will be sufficient to keep the stator bar cool. 
     Single-phase solids employed herein can be of several types, such as thermoplastics, thermosets, elastomers or ceramics. Thermoplastics may comprise a base resin (e.g., PP, ABS, PC, PA, LCP, PPS, PEEK, polyimides, bismaleimides). Thermally conductive thermoplastics can be manufactured by several firms e.g., Coolpolymers Inc, RI, Ovation Polymers, OH etc. They use thermally conductive fillers, which limits their thermal conductivity to about 3 w/mK. Thermosets and elastomers by themselves have low thermal conductivity of less than 1 w/mK. They use thermally conductive fillers to increase thermal conductivity to about 3 w/mK. So even though they are single phase solids in appearance, thermal conductivity of thermoplastics, thermosets and elastomers is defined by fillers as in the prior art. 
     Ceramics that are thermally conductive and electrically insulative and use a single phase are the preferred insulating materials in this invention. Examples include Alumina (Al2O3, 25 w/mK), Aluminum Nitride (AlN, 270 w/mK), Silicon Nitride (Si3N4, 155 w/mK), Boron Nitride (BN, 120 w/mK) They attain high thermal conductivity by using phonons instead of electrons, and also attain high electrical insulativity by molecular disposition of free electrons in the atomic structure. Ceramic shells can be made by various processes such as sintering, isostatic pressing, injection molding, slip casting, extrusion etc. Several firms can make thermally conductive ceramics such as Marketech International, WA, Ortech Advanced Ceramics, CA etc. They are made from ceramic powders of very high purity. They are literally melted and compressed to form a continuous phase. Continuous and single phase nature of these ceramics facilitates transfer of heat more easily than the two phase nature of thermoplastics, thermosets or elastomers with fillers that were used earlier in tapes and extrusions. As a result, thermal conductivity of thermally conductive ceramics is relatively high, typically greater than 25 w/mK. The following table summarizes the thermal conductivity of various materials that can be used to make bar insulation. From this table it is clear that single phase ceramics offers thermal conductivity that is significantly higher than that of multiphase materials used in the prior art. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Thermal conductivity of materials for bar insulation. 
               
             
          
           
               
                 Nr. 
                 Material 
                 w/mK range 
               
               
                   
               
               
                 1 
                 multiphase tape with fillers 
                 0.3 to 0.6 
               
               
                 2 
                 multiphase thermoplastics with fillers 
                 1 to 3 
               
               
                 3 
                 Single phase ceramics without fillers  
                  25 to 280 
               
               
                   
               
             
          
         
       
     
     In the shell embodiment  100  shown in  FIG. 2-C , the shell  99  is shaped to have hollow rectangular section with a central cavity which fits the stator bar. The cross section of the shell  99  is closed in the sense that it does not have a slit; this is in contrast to an extrusion 5 with a slit 7 investigated in U.S. Pat. No. 7,832,081. Besides, since alumina&#39;s dielectric strength is more than twice that of tape insulation, the thickness of the shell  99  in the present invention will be nearly half the thickness of traditional taped bar insulation. The manufacturing process for making embodiment  100  involves four major steps—a method to slip thermally conductive shells over central region of the bar stock  2 , a means to bend the bar stock to form end-arms, a method to apply electrical insulation over the end arms, and a method to bond all parts with each other to form a solid body. In the first step the thermally conductive shell member is slipped over the bar stock. The shell insulation comprises at least one long member or multiple shorter shell members which are jointed to each other. Fewest possible joints should be used to join the shell members, and joints are designed to prevent voltage breakdown. In the second step, stator bar is bent to form end-arms. In the third step, thermally insulative tape or extrusion is applied over the end-arms. Both thermally conductive insulation in the straight portion and thermally insulative insulation in the bent portion overlap each other around a transition zone where the straight and bent portions meet. Corona protection layers may be added to the insulation at appropriate places if needed. In the fourth step, the thermally conductive shell, thermally insulative end-insulation and the bent-bar  3  are bonded together to form the stator bar  1 . 
     The present invention has several innovative features which result in significant benefits. Higher thermal conductivity of shells helps increase power or efficiency of the machine, or reduce temperature rise in the stator bar  1 . Reducing temperature rise keeps the stator bar cooler, hence increases its life, as it is well known that every 10° C. reduction in temperature doubles the life. Alternatively, one can pump more current and hence increase power of the machine. Or, one can use a smaller conductor to carry same current and hence reduce the size of the machine or reduce its cost. Further, the higher dielectric strength of shells reduces the thickness of bar insulation, freeing insulation space that can be used to conduct flux, thereby reducing the size of the machine. For example, dielectric strength of typical tape insulation is about 60 to 90 volt/mil. In contrast, the dielectric strength of alumina ceramic is about 300 to 750 volt per mil per Harper (1974). Thus even when an alumina ceramic insulation is designed at a conservative 200 volt per mil design point, the thickness of the bar insulation that uses alumina shells will be half that of bar insulation made of conventional tape. The freed bar insulation space can be filled innovatively with either iron or copper and used to reduce size of the machine. In addition, the shells in the present invention do not have any slit in the cross section. Elimination of the slit reduces the risk of insulation failure. Further, thermally conductive shells can be mass produced inexpensively as extrusion, reducing the cost. In addition, the method needs less labor for assembly compared to winding a tape, thereby further reducing the overall cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a stator bar and its location within the stator of an electric machine. 
         FIG. 2  shows major steps in making thermally conductive ground wall insulation in accordance with the present invention, with  FIG. 2-C  showing the shell embodiment  100  of the present invention. 
         FIG. 3  shows a sample butt-joint means to mate two or more shells in the straight portion of the bar stock. 
         FIG. 4  shows an alternative lip-joint means to mate two or more shells in the straight portion of the bar stock. 
         FIG. 5  shows an alternative tongue and groove joint means to mate two or more shells in the straight portion of the bar stock. 
         FIG. 6  shows an alternative dual shell means to mate two or more shells in the straight portion of the bar stock. 
         FIG. 7  shows a thermal model of thermally conductive bar insulation. 
         FIG. 8  illustrates the split-shell embodiment  200  of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2-C  shows the shell embodiment  100  of present invention. The embodiment  100  employs one or more of thermally conductive and electrically insulative shells  99 , abbreviated herein as “shells”. The shells  99  have hollow rectangular cross section that defines a central cavity  51  whose axis synchronizes with that of the bar stock  2 . The straight bar stock  2  is fitted in this cavity  51 . Span MN of shells  99  is greater than the span PQ of the core, but less than the straight span AB of the bar stock  2 . The end faces M and N of shells  99  has a beveled portion MF that is less than the overhang MP. At the edge M and N, the thickness  98  of shells  99  equals the thickness of one layer of an insulative tape. The end-arms AC, BD are insulated by end-insulation  4   a ,  4   b  made of a tape winding or extruded insulation. End-insulation  4   a ,  4   b  cover not only end-arms AC, BD but also cover the overlap zone MR. As shown in  FIG. 2-C , the overlap zone MR is defined by the end face M of the shells  99  and end face R of the end-insulation  4   a ,  4   b . In the overlap zone MR both shell-insulation  99  as well as end-insulation  4   a ,  4   b  protect the core against voltage breakdown. The tape type end-insulation comprises multiple layers of insulative tape  4   a ,  4   b  wrapped tightly. Alternatively, the extruded type end-insulation can be used over the ends. Additional corona protection layers may also be applied if needed in any part of the bar. The process of making the shell embodiment  6  comprises four steps, and starts with a pre-assembled bar stock  2 . 
       FIG. 2-A  shows a first step of sliding the shells  99  over a stationary bar stock  2 , with the longitudinal axis of shells  99  aligned with that of bar stock  2 . In this step, shells  99  cover loosely over the bar stock  2  and are not bonded to it. The clearance between the shells  99  and the bar stock  2  is made as small as practical (to minimize thermal resistance), and just enough for the shells  99  to slide over the bar stock  2  without damaging it. To protect the bar during sliding, the outer surface of the bar stock  2  may optionally be provided with a powder or a thin fabric.  FIG. 2-A  shows the bar stock  2  covered loosely by the shells  99  in mid-region of the straight portion of bar stock  2  at the end of first step. 
       FIG. 2-B  shows a second step of bending, in which the bar stock  2 , covered loosely by shells  99 , is bent in a bending fixture (not shown) to form bent end-arms AC, BD. Normally the bar stock  2  can be clamped at sections M, N to facilitate this bending. But in some designs, it may be necessary to clamp the bar stock  2  at its mid-section before bending at sections A, B. In such cases, one clamps the bar stock  2  only, and not the shells  99  so that the copper conductors within bar stock  2  will take up the bending stresses. One arrangement that transfers bending stresses to the copper conductors is to employ shells  99  that are along the axis into two or more smaller shells; the smaller shells are then moved outwards along axis of the bar stock  2  until they reach the beginning of bending, viz., the end faces M, N of the shells  99  synchronize with end sections A, B of straight portion of the bar stock  2 . By moving the shells outwards, one creates a space mid-section that is devoid of shells, enabling clamping of the bar stock  2  at its mid-section, before bending it. Alternatively, the split shells can be separated by a small gap, and clamps can be used in these small gaps to clamp the stator bar. After bending the bar stock  2 , the clamping means (not shown) is removed and the loose shells  99  are slid inwards along axis of bar stock  2 . In an alternative arrangement, one uses shells  99  that are shorter than the core span PQ. These shorter shells  99  are then split into two halves along axis and the split shorter shells are moved outward along the axis of the bar stock  2  allowing clamping of the bar conductors at its mid-section. After bending, the shorter shells  99  are moved back inwards. A person skilled in the art can conceive of other means to bend the bar stock  2  when covered by loose shells  99  without altering the invention. 
       FIG. 2-C  shows a third step of end-arm insulating means, as well as the finished stator bar. In this step, a thermally insulative material (termed “end-insulation”)  4   a ,  4   b  is applied over the end-arms. As already mentioned, at the edge M and N, the thickness  98  of shells  99  equals the thickness of an insulative tape. Winding the tape is detailed in  FIG. 2-D . Winding the tape starts with an edge of a tape  97  in the first layer  91  in flush with the thin edge  98  of shells  99 . First layer  91  of the tape  97  thus smoothly transitions the end-insulation from thermally conductive shell to a thermally insulative tape with near-zero air-gap. Tape  96  in the second layer  92  is wound with half-lap over the tape  97 , i.e., half of its width in contact with tape  97  while other half is in contact with shell&#39;s beveled edge. This half-lapping “caps” any air-gap in the first layer, thereby preventing voltage breakdown. Multiple layers of tape are wound in this half-lap fashion over the face M normal to axis, over the beveled surface MF and over the outer surface FR of shells  99 . A person skilled in the art can conceive of alternative arrangements to create similar robust tape/shell transition without altering the invention. Alternatively, the end-insulation can be made of an extrusion with a hollow rectangular section with a central cavity shaped to fit with the end arms  4   a ,  4   b . The extrusion can be made of a thermoplastic or an elastomeric material. They can be formed as an elastomeric shell without slit or a thermoplastic shell with slit. An elastomeric shell without slit is stretched at inner diameter to fit over end-arm. With a thermoplastic shell containing a slit, after jacketing over end-arm, one closes the slit using techniques such as thermoforming, plastic welding or chemical bonding. 
     A fourth step involves bonding all the members, viz., the thermally conductive shells  99 , thermally insulative end-insulation  4   a ,  4   b  and bent-bar  3 , to form a rigid mass of an insulated stator bar. The bonding is a very complex process that requires several steps, e.g., defining the bond line requirements, designing a bondline, selecting a bonding resin, selecting a bonding method, defining the bonding procedure including surface preparation, designing a bonding fixture and actual bonding. Some of these steps such as selecting a bonding resin, bonding method etc. are well known and a person skilled in the art can make the correct choice. A key step is selecting the bonding method. Thermoforming may be used to bond thermoplastic shells  99 , while chemical bonding may be used to bond elastomeric shells. Vacuum assisted press-curing may be one option to bond tape winding. Vacuum assisted resin transfer molding or vacuum pressure impregnation are preferred methods to bond all parts. Basic features of vacuum assisted bonding a joint  39  joining shell  50  and  59  is shown in  FIG. 3-D . The stator bar assembly is warmed up to a specified temperature to remove trapped air. A peelable strip  391  is applied first over the joint  39  to seal it. A vacuum pump  395  is attached to the bond line  39  via a nozzle  396 , and a resin injection pump  392  is attached to bond line  39  via a nozzle  393  using an aperture  394  in the peelable strip  391 . Resin is pumped into interspaces spaces between all members using the resin injector  392  while vacuum pump  395  is used to vacuum out the trapped air. The resin is injected into the air gaps at prescribed vacuum, pressure and temperature conditions and the resin is then cured at specified temperatures for specified time. Strip  391  is then peeled out and the finished stator bar  1  is tested for voltage and temperature endurance. 
     Designing the bond line and jointing of two or more shells requires special attention to achieve high voltage protection. The number of joints must be minimized to reduce any chance of voltage failure.  FIGS. 3-6  illustrates several examples of various jointing means. This involves defining the jointing surfaces and bondline geometry to prevent relative movement between shells in all directions other than longitudinal direction. Joinable shells may have an optional locking means that prevents their relative movement in the longitudinal direction also. Generally a first end  52  of a shell has a male plug feature while a second end  53  has a female socket feature. Male plug in one shell, when mated with a female socket in another shell forms a jointed shell. The zone at which they mate and join together is called a joint. Joints can be made in various styles with distinctive features as discussed below. 
       FIG. 3  illustrates a shell  30  with the plug and socket features for joining by a butt-joint. The cross-section  56  of a shell  30  is rectangular and hollow, without any slit, and defines a central cavity  51  which conforms to bar stock  2 . Shell  30  has a first end  52 , and a second end  53  which are perpendicular to its axis. Both ends  52 ,  53  have flat faces  36   a ,  36   b  which are used for making a butt joint.  FIG. 3-B  shows an isometric view of how the shells  44 ,  50 ,  59  are joined at  38 ,  39  by butting and bonding one face  36   a  of one shell with second face  36   b  of adjacent shell, thereby forming a butt-joint.  FIG. 3-C  shows cross sectional view of butt joints  38 ,  39  joining the shells  44 ,  50 ,  59 . Butt joints  38 ,  39  are mechanically weakest as they rely on the bonding resin to attach shells together. It is also electrically weakest as length of the bond line is small so it forms an easy path for voltage breakdown. Because a butt-joint is weak, they are not a preferred choice. It is presented herein only to illustrate the geometrical features of thermally conductive shells. 
       FIG. 4  shows lip-joint features in another shell  44 .  FIG. 4-A  shows a front view of the shell  44  when a first end  52  faces the viewer. End  52  has an outer lip  46   a , i.e., a protrusion bounded by outer face  48   a  and inner face  47   a .  FIG. 4-B  shows another shell  30  when a second end  53  faces the viewer. The second end  53  has inner lip  46   b  which is bounded by inner face  47   b  and outer face  48   b .  FIG. 4-C  shows how these two shells  44  and  30  are joined by lap-joint. To join, the inner face  47   a  of the outer lip  46   a  mates with the inner face  47   b  of inner lip  46   b . In addition, the outer face  48   a  of outer lip  46   a  mates with the outer face  48   b  of inner lip  47   b .  FIG. 4-D  illustrates the cross-section of shells  44 ,  30 ,  49  that are joined by lap-joints  47 ,  48 . 
       FIG. 5  shows tongue-and-lip joint features in another shell  60 .  FIG. 5-A  shows shell  60  when a first end  52  faces the viewer. The first end  52  has tongue  66   a , i.e., a protrusion bounded by an outer face  67   a  and faces  68   a ,  69   a .  FIG. 5-B  shows shell  60  when a second end  53  faces the viewer. The second end  53  has a groove  66   b  that conforms to the tongue  66   a  and is bounded by middle face  67   b  and faces  68   b ,  69   b .  FIG. 5-C  illustrates cross-section shells  64 ,  60 ,  69  joined together by tongue-and-groove joints  67   a ,  67   b . Comparing  FIG. 4-D  and  FIG. 5-C , it is clear that the bond line for the tongue- and groove-joint is longer than that of lap-joint. Hence the tongue-and-joint has a greater ability to prevent voltage breakdown. 
       FIG. 6  shows dual butt joint features of dual shells. The outer shell  75  in  FIG. 6-A  is made of half the thickness of the shell insulation, has a hollow rectangular cross-section  74   b  and its end faces  72   b ,  73   b  define a cavity  76   b .  FIG. 6-B  illustrates an inner shell  73  which is also half thickness of insulation and having a hollow rectangular cross-section  74   a  and with end faces  72   a ,  73   a , its outer periphery fitting into cavity  76   b  of outer shell  75 . Inner periphery of cross section  74   a  defines a cavity  76   a . This cavity  76   a  conforms to and fits the bar stock  2 . The outer shell  75  is slipped over inner shell  73  until end face  72   b  is at midline MM of inner shell  73 .  FIG. 6-C  shows cross-section of the shell  99  assembled over bar stock  2  using outer shells  75 ,  76  and inner shells  71 ,  73 ,  77 . The shells form but joints  78   a ,  78   b , and  78   c  as shown in  FIG. 6-C . but alternate joints such as lip, tongue and groove etc can also be used to increase voltage endurance. 
     Jointing surfaces in ends  52 ,  53  may be optionally provided with a bonding coat for bonding. Outer surface of shell  50  may be optionally provided with a resistive layer, termed slot armor, to provide for small relative motion and to even the electrical stresses. One flat face of shell  50  may be optionally provided with through holes for venting or injecting impregnating resin that bonds shell with shell, and shell with bar stock. Even though only few styles of joining shells, viz., lip joint, tongue and groove joint, dual shell joint are illustrated here in, a person skilled in art can conceive of alternative styles of joints that can resist high voltages. The scope of this invention encompasses any means which are used to join two or more shells. 
       FIG. 8-A  illustrate the split-shell embodiment  200  of the present invention. The split-shell embodiment  200  is identical to that shown in  FIG. 2-C  except that the shells  99  in  FIG. 2-C  are split along longitudinal axis into two equal halves  209 ,  210  which are then bonded by a longitudinal joint  47 . The end arms  3   a ,  3   b  are then insulated by tape winding or extrusions  4   a ,  4   b  and contain transition zones  36  as in the shell embodiment shown in  FIG. 2-C .  FIG. 8-B  shows the cross section of split shells  209 ,  210 . the cross-section of each split shell having a the shape of U.  FIG. 8-C  shows a dove tail joint  47  that joins end faces of split shells  209 ,  210 . The split shells  209 ,  210  are made of single phase material as in shells  99  and occupy the same geometrical space. The advantage of embodiment  200  over embodiment  100  is that in the embodiment  200 , the bar stock  2  can be bent using existing bending fixtures to form bent-bar  3 , and the split-shells  209 ,  210  can then be inserted in the central straight portion of the bent bar  3 . Hence embodiment  200  eliminates the step  3  required in formulating the embodiment  100 . But since the bar insulation  200  is made of brittle ceramic and hence is unbroken, the resulting stator bar  1  is somewhat brittle. So it may not be flexible enough to meet the bending requirements, whereas in embodiment  100  the bond lines are designed to be flexible, so the bending stresses are taken up more by the flexible joints and less by the ceramic shells  99 . 
       FIGS. 7-A  and  7 -B further detail how the embodiments  100  or  200  can produce a stator bar with superior thermal conductivity.  FIG. 7-A  shows the key components in the heat path. These are first, the shell  99  and second, the bond line  43  of the resin that bonds the bar stock  2  and shell  99 .  FIG. 7-B  shows a simplified thermal resistance model of the bar stock. The model comprises two resistances—resistance of thermally conductive shell R 1 , and resistance of the bond line R 2 . Since both resistances are in series, the net thermal resistance R is given by 
                   R   =         R   1     +     R   2       =         l   1         k   1     ⁢   A       +       l   2         k   2     ⁢   A                   (   1   )               
where l 1 , k 1  denote thickness and thermal conductivity of shell  99  while l 2 , k 2  denote thickness and thermal conductivity of bond line  43  respectively. “A” denotes the heat transfer area perpendicular to the heat flow path. Net resistance can then be written as R=l 1 /(k eff  A) where effective thermal conductivity of stator bar k eff  is
 
     
       
         
           
             
               
                 
                   
                     k 
                     eff 
                   
                   = 
                   
                     
                       k 
                       1 
                     
                     
                       1 
                       + 
                       
                         
                           
                             k 
                             1 
                           
                           
                             k 
                             2 
                           
                         
                         ⁢ 
                         
                           
                             l 
                             2 
                           
                           
                             l 
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Using this equation, we show below that a stator bar  2  employing thermally conductive shell  99  has superior thermal conductivity. 
     Example 
     Consider a stator bar  2  comprising 0.125″ thick Alumina shell  99  (k 1 =25 w/mK, l 1 =0.125″) with a 0.005″ thick bond line (k 2 =0.3 w/mK, l 2 =0.005″). Plugging these values into equation (2) indicates that the effective thermal conductivity k eff  of Alumina shelled stator bar will be 5.8 w/mK. This is significantly higher than the 0.6 to 1 w/mK thermal conductivity that has been achieved so far by the prior art employing tape insulation. Since temperature rise is directly proportional to the thermal resistance, a stator bar using thermally conductive shell insulation will be significantly cooler as shown below. 
     Consider a prior-art stator bar with taped bar insulation, having an effective thermal conductivity of 0.6 w/mK. Assume that the stator core is at 100° C. while the stator bar is at 150° C., so that temperature rise is 50° C., and assume that this rise is mostly due to the bar insulation. Replacing the prior-art taped bar insulation with thermally conductive shell insulation in accordance with the present invention leads to an effective thermal conductivity of 5.8 w/mK as shown above. This higher thermal conductivity reduces the temperature rise from 50° C. to (50) (0.6/5.8)=5.2° C. Thermally conductive shell insulation hence greatly reduces the stator bar temperature from 150° C. to 105° C. This is a significant reduction in the stator bar temperature. Thus the thermally conductive shell insulation presented in this invention keeps the stator core and stator bar significantly cooler.