Patent Publication Number: US-2003224142-A1

Title: Methods for making slot cell insulation and slot cell insulation produced thereby

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to the field of electrical power generators and motors, and, more particularly, to insulation for such devices and associated methods.  
       BACKGROUND OF THE INVENTION  
       [0002] A commercial electrical power generator typically includes a rotor and a stator which surrounds the rotor. The rotor is mounted on a shaft which is driven by a steam turbine or combustion turbine, for example. The rotor typically includes a forged body including slots therein, and windings are positioned in the slots. An exciter delivers electrical power to the windings so that the rotor generates a rotating magnetic flux which cuts through corresponding windings in the stator to thereby produce electrical power.  
       [0003] The rotor windings are typically insulated from the adjacent portions of the rotor body by an insulating body typically formed of a number of layers laminated together and shaped to be received in the generally U-shaped rotor slot. U.S. Pat. No. 5,164,142 to Simmonds, for example, discloses a process and apparatus for step molding elongated pieces, such as rotor slot cell insulation.  
       [0004] The slot cell insulation typically includes openings therein to receive a radial flow of cooling air or hydrogen to thereby cool the windings. For example, U.S. Pat. No. 4,560,896 to Vogt et al. discloses a composite insulation comprising aramid paper slot armor and an epoxy-glass sub-slot cover.  
       [0005] A static excitation system may be used to power the rotor, and this may place additional stress on the slot cell insulation. The static excitation system typically includes a static exciter which is an electronic device including semiconductor switches to generate the DC current for the rotor, but also including an AC component. The static exciter has an input connected to an exciter transformer, and its output is coupled to the rotor windings via brush rigging and a coupling ring assembly. Static excitation is advantageous because it is relatively simple, has no rotating parts, has a fast response time, and has a relatively compact size. The disadvantage is that the slot cell insulation and other rotor components are subject to repetitive voltage pulses of substantial amplitude.  
       [0006] The assignee of the present invention has provided slot cell insulation formed of five stacked layers including an inner TEFLON®-glass layer (0.005″), an epoxy-glass prepreg layer (0.010″), a NOMEX® 410 layer (0.010″), an epoxy-glass prepreg layer (0.010″), and an outer NOMEX® 410 layer (0.010″).  
       [0007] The slot cell insulation is typically made by laying up the layers on a flat surface and then positioning the layers in a female mold having the shape of the rotor slot. Slip between the layers is relatively.limited so that wrinkles may be formed at the corners or bends. A male mold is pressed into the layers so that the layers take the desired U-shape of the slot. Unfortunately, the layers may also be stretched at the corners by this molding technique also producing defects in the slot cell insulation.  
       SUMMARY OF THE INVENTION  
       [0008] In view of the foregoing background, it is therefore an object of the invention to provide high quality slot cell insulation and a method for making the slot cell insulation that is better able to resist heat and/or electrical deterioration.  
       [0009] This and other objects, features and advantages in accordance with the invention are provided by a method for making slot cell insulation comprising providing a male mold having an outer shape corresponding to the rotor slot; stacking a plurality of layers adjacent the male mold; and laminating the stacked layers together on the male mold to thereby form the slot cell insulation. The slot cell insulation can then be removed from the male mold. The male mold may have a generally U-shaped outer surface. Accordingly, corner portions of the slot cell insulation can be made more uniform and with a greatly reduced likelihood of defects.  
       [0010] The laminating may include heating the stacked layers. In addition, the stacked layers may comprise at least one heat curable layer having a heat curing temperature, and heating may comprise heating to at least the heat curing temperature. For example, the at least one heat curable layer may be an epoxyglass prepreg layer.  
       [0011] Laminating may also comprise covering the stacked layers with an evacuable membrane, and evacuating the evacuable membrane to thereby press the stacked layers together and remove volatile materials therefrom. This vacuum pressing may be performed prior to heating and/or during heating, for example. The laminating may further include subjecting the evacuable membrane to an elevated pressure to further press the stacked layers together.  
       [0012] Other important aspects of the invention relate to the layers used in making the slot cell insulation. The stacked layers may comprise a pair of aromatic polyamide layers and a polyimide layer therebetween. These layers may be prelaminated, that is, laminated together prior to the overall lamination of the layers. Each aromatic polyamide layer may comprise NOMEX® paper, for example. In addition, the polyimide layer may be a corona-resistant polyimide layer, such as a KAPTON® CR layer. The NOMEX® layers provide a protective sandwich for the relatively mechanically fragile KAPTON® CR layer.  
       [0013] The stacked layers may also comprise one or more epoxy-glass prepreg layers. A polytetrafluoroethylene (PTFE)-glass layer may be included in the stacked layers and be positioned so that the PTFE portion engages the rotor windings when installed. The stacked layers may also include an additional aromatic polyamide layer positioned to engage the rotor.  
       [0014] The invention is also directed to a method for making slot cell insulation including providing a mold having a shape corresponding to the rotor slot; and laminating a plurality of stacked layers together adjacent the mold to thereby form the slot cell insulation, and wherein the stacked layers comprise a pair of aromatic polyamide layers and a polyimide layer therebetween. In words, in accordance with this aspect of the invention the unique material combination may be used for any molding/laminating approach.  
       [0015] Yet another aspect of the invention is directed to the slot cell insulation. More particularly, the slot cell insulation may include a laminated structure for positioning in the rotor slot and comprising a plurality of layers laminated together. The plurality of layers, in turn, may comprise a pair of aromatic polyamide layers and a polyimide layer therebetween. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is a fragmentary perspective view of a portion of a dynamoelectric machine including the slot cell insulation in accordance with the present invention.  
     [0017]FIG. 2 is a greatly enlarged cross-sectional view of a portion of the slot cell insulation as shown in FIG. 1.  
     [0018]FIG. 3 is a schematic cross-sectional view during manufacturing of the slot cell insulation as shown in FIG. 1.  
     [0019]FIG. 4 is a flow chart for a method of making the slot cell insulation as shown in FIG. 1.  
     [0020]FIG. 5 is a flow chart for an alternate method of making the slot cell insulation as shown in FIG. 1.  
     [0021]FIG. 6 is graph of comparative test results for the slot cell insulation as shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0022] The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.  
     [0023] Referring now initially to FIGS. 1 and 2 the slot cell insulation  20  in accordance with the invention is first described. The slot cell insulation  20  is positioned in a rotor slot between rotor windings  17  and adjacent rotor portions  16  of a dynamoelectric machine  15 . The dynamoelectric machine may be either a motor or generator as will be readily appreciated by those skilled in the art.  
     [0024] In the illustrated embodiment, the dynamoelectric machine  15  also includes a cooling gas channel member  21  positioned beneath a stacked arrangement of rotor coil copper layers  22  and rotor turn insulation  23 . A filler insulation block  24  is positioned over the uppermost copper layer, and a wedge  25  secures the windings  17  within the rotor slot. The gas channel member  21  is connected in fluid communication with radial slots  26  in the windings  17  to pass cooling fluid therethrough. Other winding configurations are also contemplated by the present invention.  
     [0025] The illustrated slot cell insulation  20  includes a plurality of layers stacked and laminated together. Of particular advantage is the inclusion of a prelaminated layer  31  which, in turn, includes a pair of aromatic polyamide layers  32   a ,  32   b  and a polyimide layer  33  therebetween. These layers may preferably be prelaminated, that is, laminated together prior to the overall lamination of the remaining layers of the insulation  20 . The prelaminated layer  31  is advantageous in that the outer aromatic polyamide layers  32   a ,  32   b  provide mechanical strength and protection for the inner polyimide layer  33 . The polyimide layer  33  has very desirable electrical characteristics, but is typically relatively expensive and easily damaged if handled by itself.  
     [0026] Each aromatic polyamide layer  32   a ,  32   b  may comprise NOMEX® 410 paper, for example. In addition, the polyimide layer  33  may be a KAPTON® layer. More preferably, the polymide layer  33  may be a corona-resistant polyimide layer, such as a KAPTON® CR layer. The external surfaces of the prelaminated arrangement advantageously provide the same bonding surfaces (NOMEX® 410 paper) as the prior art. Both the NOMEX® and KAPTON® CR materials are available from DuPont. Other similar materials may also be used in the slot cell insulation  20  as will be appreciated by those skilled in the art.  
     [0027] Using selected high Tg thermoplastic polymer films can increase the ultimate withstand voltage stress and voltage endurance of the slot cell insulation  20 . Important thermal conductivity improvement can also be realized. These high performance polymer films, like corona resistant KAPTON® CR are relatively expensive and difficult handle in large sheet form. These polymer films are typically very thin and fragile where dents, wrinkles, pinholes, or contamination can readily occur during slot cell stack-up prior to molding. The preferred polymer film application method is a prelaminate of the polymer film between opposed layers of NOMEX®. This prelaminated material made before slot cell stack-up for molding will prevent film damage and provide easy to handle sheet insulation. Also, the polymer film can be pretreated with corona discharges or heat treated to promote chemical adhesion in the lamination line prior to laminating. The laminating adhesive in the prelaminate is for high temperature applications including the polymer film-NOMEX® interface bonding.  
     [0028] The slot cell insulation  20  illustratively includes first and second epoxy-glass prepreg layers  34   a ,  34   b  adjacent respective opposite sides of the laminated layer  31 . A polytetrafluoroethylene (PTFE)-glass layer  35  is included in the stacked layers and is positioned so that the PTFE portion  35   a  engages the rotor windings  17  when installed. The PTFE portion  35   a  facilitates desired slippage between the windings  17  and slot cell insulation  20 . The slot cell insulation  20  also includes an additional aromatic polyamide layer  32   c , such as NOMEX® 410 paper, positioned to engage the rotor  16 . As will be readily appreciated by those skilled in the art, the epoxy-glass prepreg layers  34   a ,  34   b  are heat curable layers that serve to bind the adjacent layers together.  
     [0029] The starting thickness for the various layers may be as follows: PTFE-glass layer  35  about 0.005 inches, epoxy-glass prepreg layers  34   a ,  34   b  about 0.010 inches, aromatic polyamide layers  32   a - 32   c  about 0.005 inches, and the inner polyimide layer  33  about 0.0015 inches. Of course, for better thermal conductance these thicknesses may be reduced, while for better electrical insulation the thicknesses may be greater. Use of the prelaminated layer  31  allows the overall thickness to be reduced while maintaining the mechanical strength and voltage endurance. Those of skill in the art will appreciate that the desired thicknesses are somewhat application specific. It is noted, however, that the listed materials and thicknesses should meet the new Class H requirements for operation up to 180° C. During laminating, the stacked layers may experience a decrease in total thickness from the starting 0.055 inches down to about 0.045 inches, for example.  
     [0030] With thinner starting layers and/or additional pressure the resulting thickness may also be reduced down to 0.030 to 0.035 inches as will be appreciated by those skilled in the art. Thinner slot cell insulation  20  may permit more copper and, hence, more current carrying capacity for the rotor windings. Thicker slot cell insulation  20  may be readily produced by including additional epoxy-glass prepreg and NOMEX® layers on either or both sides of the prelaminated central layer  31 , for example, as will also be appreciated by those skilled in the art.  
     [0031] Turning now additionally to the apparatus  50  of FIG. 3 and the flow chart  60  of FIG. 4, methods of making the slot cell insulation  20  are now described. From the start (Block  62 ), the method illustratively includes at Block  64  providing a male mold  51  having an outer shape corresponding to the rotor slot. At Block  66  the layers for the slot cell insulation  20  are stacked onto the male mold  51 . A release layer, not shown, may be provided to facilitate later release from the mold  51  as will be appreciated by those skilled in the art. At Block  68  the stacked layers are laminated together on the male mold  51  to thereby form the slot cell insulation  20 . The slot cell insulation  20  can then be removed from the male mold  51  (Block  70 ) before stopping at Block  72 .  
     [0032] As shown in the illustrated apparatus  50 , the male mold  51  may have a generally U-shaped outer surface (shown in inverted position in FIG. 3) so that corner portions of the slot cell  20  insulation can be made more uniform and with a greatly reduced likelihood of defects. In other words, unlike the prior art which assembles the layers first on a flat surface and then bends the layers into a female mold, the layers are laid up on the male mold  51 . Also according to the prior art, positioning of the male mold within the female mold would tend to cause non-uniformities at the corners. The present invention also addresses this shortcoming as will be appreciated by those skilled in the art. Another advantage of using the male mold  51  in this embodiment of manufacturing is that the cost and lead time for a female mold can be avoided.  
     [0033] The laminating may include heating the stacked layers, such as by using the schematically illustrated heater  52  of the apparatus  50 . Of course, the stacked layers may comprise at least one heat curable layer having a heat curing temperature, and heating may comprise heating to at least the heat curing temperature. For example, the at least one heat curable layer may be an epoxy-glass prepreg layer. The heating may include an initial upward ramping of about 5° C./minute until reaching a temperature of about 175-185° C. This heating may be maintained for about 1 to 4 hours, prior to a cool down cycle.  
     [0034] Laminating may also comprise covering the stacked layers with an evacuable membrane  53 , and evacuating the evacuable membrane via the schematically illustrated vacuum source  54  to thereby press the stacked layers together and remove volatile materials therefrom. A felt breathing layer, not shown, may be provided between the stacked layers and the evacuable membrane  53  as will also be appreciated by those skilled in the art. This vacuum pressing may be performed prior to heating and/or during heating, for example.  
     [0035] The laminating may further include subjecting the evacuable membrane  53  to an elevated externally applied pressure to further press the stacked layers together. The pressure may be provided by an inert gas or a layer of shrink tape (not shown) and range from about 50 to 300 psi, and, more preferably about 85 to 100 psi. Other pressure and temperature ranges may be used depending upon the materials for the slot cell insulation  20  as will be appreciated by those skilled in the art. The vacuum and pressure may cause the resin of the epoxy-glass prepreg layer to saturate the adjacent NOMEX® paper layers, for example.  
     [0036] Turning now additionally to the flow chart  80  of FIG. 5, and as noted above, another important aspect of the method relates to the material layers used to make the slot cell insulation  20 . In particular, the stacked layers may preferably comprise a pair of aromatic polyamide layers and a polyimide layer therebetween. More particularly, from the start (Block  82 ), the stacked layers may include a prelaminated NOMEX®-KAPTON® CR-NOMEX® layer  31  (Block  84 ). This prelaminated arrangement  31  may then be positioned adjacent a mold (male or female, although a male mold  51  may be preferred) and laminated together at Block  86 . The slot cell insulation  20  may then be removed from the mold at Block  88  before stopping at Block  90 .  
     [0037] The slot cell insulation  20  and associated manufacturing methods provide a number of advantages. For example, improved voltage endurance, thermal conductivity, and thermal conductance are achieved. Warpage and moisture sensitivity may also be reduced. The mechanical properties are equal to or better than existing slot cell insulation. The slot cell insulation may also meet the more stringent Class H requirements, and/or may be more desirable in applications using static excitation. The molding time may also be reduced to provide additional cost savings.  
     [0038] In other embodiments, a central laminate structure including the following may be used: polymer film-mica paper-polymer film; NOMEX®-polymer film-NOMEX®; and Dacron-PET-Dacron. A number of thermoplastic polymer films may be used including: polyetherimide (PEI), polyether ether keytone (PEEK), polyimide (PI), high performance polyethylene terephtalate (PET), and DACRON®-MYLAR®-DACRON® (DMD) polyester felt-polyester film laminate. In other embodiments a mica-epoxy/glass structure may be used. For some applications, the mica paper may provide reduced mechanical qualities from those typically desired for slot cell insulation. The mica paper embodiments do, however, enjoy excellent electrical properties.  
     [0039] The plots of FIG. 6 illustrate the comparative aging time versus dielectric stress for various slot cell configurations. The plot labeled  91  is for the seven-layer construction described above and including the NOMEX®-KAPTON® CR-NOMEX® central portion, and the plot labeled  90  is for a mica-epoxy/glass central portion. The plot labeled  92  is for a conventional NOMEX®-epoxy/glass construction.  
     [0040] Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Accordingly, it is understood that the invention is not to be limited to the illustrated embodiments disclosed, and that other modifications and embodiments are intended to be included within the spirit and scope of the appended claims.