Abstract:
A stator component whose composition and processing enable the component to axially compress magnetic sheets of a stator and also inhibit joule heating of the component to the extent that the need for a separate flux shield can be eliminated. The component is formed of a ductile iron alloy containing, by weight, about 3.25 to about 3.40% carbon, about 3.70 to about 3.80% silicon, about 4.50 to about 4.70% nickel, up to about 0.20% manganese, up to about 0.06% magnesium, less than 0.02% phosphorus, less than 0.02% sulfur, with the balance being iron and incidental impurities. Following heat treatment, the component exhibits properties that inhibit joule heating of the component by eddy currents induced by alternating magnetic fields of the stator.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to dynamoelectric machines, such as generators used in the production of electrical power. More particularly, this invention relates to minimizing eddy current heating in a stator caused by magnetic fields in end-turn regions of the stator. 
         [0002]    Large turbine-driven generators used in the production of electrical power comprise a rotor that serves as a source of magnetic lines of flux produced by a wound coil carried on the rotor. The rotor rotates within a stator that comprises a number of conductors in which an alternating current is induced by the rotor as it rotates within the stator, generating a rotating magnetic field in a narrow air gap between the stator and rotor. 
         [0003]      FIG. 1  represents adjacent end portions of a stator  10  and rotor  12  illustrative of the type used in certain dynamoelectric machines, such as turbine-driven generators used to generate electrical power. The stator  10  has a generally annular shape that circumscribes the rotor  12 , which is generally a large cylindrical body from which spindles (not shown) extend for rotatably supporting the rotor  12  within the stator  10 . The rotor  12  has a series of longitudinal (axially-extending) slots  30  in its outer circumference, which result in radially-extending teeth being defined along the perimeter of the rotor  12 . Field windings  32 , each comprising multiple insulated conductor strands, are installed in the slots  30  to extend the length of the rotor  10 , longitudinally projecting from each end of the rotor  12 . The field windings  32  include end turns  34 , each of which electrically connects a winding  32  within one slot  30  to a second winding  32  in an adjacent slot  30 . As the rotor  12  spins, the end turns  34  are subjected to centrifugal forces that urge the end turns  34  radially outward. This radial movement of the end turns  34  is confined by retaining rings  36  attached to the ends of the rotor  12  to enclose the end turns  34 , as shown in  FIG. 1 . 
         [0004]    The stator  10  comprises sheets (punchings)  14  supported in a frame  16  so as to be perpendicular to the common axis of the stator  10  and rotor  12 . The sheets  14  are formed of a low loss, low magnetic reluctance material, such as a silicon steel, and compressed against each other in bundles  18 , which are axially separated by air gaps  20  maintained by nonmagnetic spacers (not shown) between the sheet bundles  18 . Armature windings  24  are positioned in slots (not shown) formed in the sheets  14 , and end turns  26  of the windings  24  extend outward from the stator  10  around the rotor retaining ring  36 . The extent to which the windings  24  extend beyond the end of the stator  10  is reduced by forming the windings  24  as involutes oriented at an angle to the longitudinal axis of the machine, as represented in  FIG. 1 . 
         [0005]    The sheets  14  of the stator  10  are axially compressed by annular-shaped flanges  22 , one of which is shown in  FIG. 1 . The flanges  22  must have adequate strength to support and maintain the positions of the sheets  14  within the stator  10 , and therefore must be formed of high strength material. A common example is ductile iron (cast nodular iron) alloys due to their strength, toughness, and machinability. As a particular example, ASTM A536 GR 60-40-18 ductile iron has been used to form stator flanges in generators produced by the General Electric Company. The alloy composition per the ASTM A536 specification is generic in nature, subordinates chemical composition to mechanical properties, and is not optimized for electrical or magnetic permeability properties. As such, components formed of ASTM A536 are mainly chosen to meet mechanical properties and obtain a spheroidal graphite microstructure with a predominantly ferritic matrix. A typical commercial grade of the ASTM A536 alloy contains, by weight, at least 3.0% carbon, at least 1.7% silicon, at least 0.03% magnesium, less than 0.1% phosphorus, less than 0.025% sulfur, the balance iron and incidental impurities. 
         [0006]    In a stator  10  having the construction described above, magnetic flux is generated by the end turns  26  and directed parallel to the longitudinal axis of the machine toward the major surfaces of the sheets  14 . This magnetic flux induces large eddy currents in the sheets  14  that cause a significant amount of joule (ohmic) heating in the sheets  14 , and consequently heating of the stator flanges  10 . The alternating magnetic fields of the stator  10  also induce eddy currents in the stator flanges  22 , resulting in further heating of the flanges  22 . In addition to energy losses that reduce the efficiency of the machine, heating of the sheets  14  and flanges  22  in the vicinity of the stator ends can be sufficient to cause local overheating that is detrimental to the operation of the machine. 
         [0007]    For this reason, the stator  10  is shown equipped with an annular-shaped flux shield  28  located adjacent the flange  22  and secured by, for example, straps (as shown), fasteners, etc. Examples of flux shields include U.S. Pat. No. 1,677,004 to Pohl and U.S. Pat. No. 4,054,809 to Jefferies. The flux shield  28  is formed of a material such as copper or a copper alloy so that magnetic flux is concentrated in the shield  28 , rather than in the flange  22 . As a result, power losses in the machine can be significantly reduced, thereby increasing the overall efficiency of the machine and reducing temperatures within the sheets  14  at the ends of the stator  10 . However, a drawback is that the flux shield  28  is heated by the eddy currents, resulting in heating of the shield  28  and heat transfer to the flange  22  by conduction and/or convection. The flux shield  28  also adds complexity and cost to the machine. Accordingly, it would be desirable if the flux shields  28  could be eliminated as separate discrete components of large dynamoelectric machines. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a stator component whose composition and processing enable the component to axially compress magnetic sheets of the stator and also inhibit joule heating to the extent that the need for a separate flux shield can be eliminated. 
         [0009]    According to a first aspect of the invention, the component is adapted for use in a stator used in combination with a rotor in a dynamoelectric machine, such that alternating magnetic fields are induced in the stator. The stator includes magnetic sheets oriented approximately perpendicular to an axis of the stator, and stator windings passing through the magnetic sheets in a direction approximately parallel to the axis of the stator. The component is configured and located on the stator to axially compress the magnetic sheets of the stator together. The component is formed of a ductile iron alloy containing, by weight, about 3.25 to about 3.40% carbon, about 3.70 to about 3.80% silicon, about 4.5 to about 4.7% nickel, up to about 0.20% manganese, up to about 0.06% magnesium, less than 0.02% phosphorus, less than 0.02% sulfur, with the balance being iron and incidental impurities. The component exhibits properties that significantly inhibit joule heating of the component by eddy currents induced by alternating magnetic fields of the stator. 
         [0010]    According to a second aspect of the invention, the component is formed by a process that includes casting a ductile iron alloy containing, by weight, about 3.25 to about 3.40% carbon, about 3.70 to about 3.80% silicon, about 4.5 to about 4.7% nickel, up to about 0.20% manganese, up to about 0.06% magnesium, less than 0.02% phosphorus, less than 0.02% sulfur, with the balance being iron and incidental impurities. The resulting casting is then subjected to a two-stage heat treatment cycle, starting with a first heat treatment at a first soak temperature of about 910±20° C. for about three hours ±30 minutes, then a second heat treatment at a second soak temperature of about 690±20° C. for about six hours ±30 minutes. Cooling can be performed by conventional methods capable of a sufficiently controlled cooling rate that avoids significant hardening and grain growth in the casting. For example, furnace cooling techniques provide such a capability, whereas cooling techniques such as conventional air cooling and liquid quenching techniques do not. 
         [0011]    A significant advantage of this invention is that the component as described above is capable of exhibiting a desirable combination of strength, magnetic permeability, and electrical resistivity. This combination of properties enables the component to be less prone to joule heating, to the extent that stator end heating can be minimized without the requirement for a separate component capable of a magnetic shielding effect. The component achieves these benefits while simultaneously being capable of providing sufficient strength to support and maintain the positions of the sheets within the stator. 
         [0012]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a partial sectional view of a stator and rotor in a dynamoelectric machine, with the stator shown as being equipped with a separate flux shield in accordance with the prior art. 
           [0014]      FIG. 2  is a partial sectional view of a stator and rotor similar to that of  FIG. 1 , but with the flux shield eliminated as a result of a modified stator flange in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    A stator  50  for a dynamoelectric machine is represented in  FIG. 2 . The stator  50  and its components are merely illustrative, and their particular configurations are not to be interpreted as limiting the scope of the invention, aside from properties and characteristics necessary for use in a dynamoelectric machine. Similar to the prior art stator  10  of  FIG. 1 , the stator  50  has a plurality of thin sheets  54  of low loss, low magnetic reluctance material, such as high quality silicon steel, supported in a frame  56 . The sheets  54  are assembled in bundles  58  separated by air gaps  60  created by nonmagnetic spacers (not shown) between the bundles  58 . As part of a dynamoelectric machine, such as a generator used in the production of electrical power, a rotor  52  is rotatably supported coaxially within the stator  50 . The rotor  52  has field windings  72  that are located in longitudinal (axially-extending) slots  70  in its outer circumference and longitudinally project from the end of the rotor  52 . The field windings  72  include end turns  74  confined by retaining rings  76  attached to the core end of the rotor  52 . The stator  50  includes armature windings  64  positioned in slots (not shown) formed in the sheets  54 , with end turns  66  of the windings  64  extending outward from the stator  50  around the rotor retaining ring  76 . 
         [0016]    As in the case of the stator  10  represented in  FIG. 1 , the sheets  54  of the stator  50  are axially compressed by stator flanges  62 , one of which is shown in cross-section in  FIG. 2 . The flange  52  is depicted as being at the core end of the stator  50  with its outer perimeter abutting the stator frame  56 . The flange  52  generally has an annular shape that is coaxial with the stator  50 , and as such a cross-section of the lower portion (not shown) of the flange  52  diametrically opposed from the portion shown would have a cross-section that is substantially a mirror image of the portion shown. As before, the stator flanges  62  must have adequate strength to axially compress the sheets  54  together in order to support and maintain the positions of the sheets  54  within the stator  50 . For this reason, the flanges  62  must be formed of a high strength material. However, as evident from comparing  FIGS. 1 and 2 , the stator  50  of  FIG. 2  lacks a separate flux shield adjacent its end turns  74 . In the absence of a flux shield, such as the shield  28  shown in  FIG. 1 , the end turns  74  of the rotor  52  create magnetic flux that induces eddy currents in the sheets  54  of the stator  50 , thereby reducing the efficiency of the machine and inducing joule heating that can potentially lead to excessive temperatures in the sheets  54 , as well as in the flanges  62  located at the ends of the stator  50 . According to a preferred embodiment of the invention, the flanges  62  are formed of a material and are processed to have properties that make possible the elimination of the flux shields  28  of the prior art stator  10  of  FIG. 1 . In addition to reduced material and manufacturing costs, the elimination of the flux shields  28  also possibly allows for improved cooling flow through the stator  10 . 
         [0017]    According to a first aspect of the invention, the flanges  62  are formed of a ductile iron (cast nodular iron) alloy that exhibits desirable strength, toughness, and machinability properties, as well as desirable magnetic properties. Suitable, preferred, and nominal compositions (approximate, by weight percent) for the ductile iron alloy are summarized in Table I below. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Constituent 
                 Suitable 
                 Preferred 
                 Nominal 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Carbon 
                 3.25-3.40 
                 3.25-3.30 
                 3.25 
               
               
                   
                 Silicon 
                 3.70-3.80 
                 3.72-3.78 
                 3.75 
               
               
                   
                 Nickel 
                 4.50-4.70 
                 4.55-4.65 
                 4.60 
               
               
                   
                 Manganese 
                 up to 0.20 
                 0.17-0.20 
                 0.185 
               
               
                   
                 Magnesium 
                 up to 0.06 
                 0.035-0.06  
                 0.0375 
               
               
                   
                 Phosphorous 
                 &lt;0.02 
                 &lt;0.02 
                 &lt;0.02 
               
               
                   
                 Sulfur 
                 &lt;0.02 
                 &lt;0.02 
                 &lt;0.02 
               
               
                   
                 Iron 
                 balance 
                 balance 
                 balance 
               
               
                   
                   
               
             
          
         
       
     
         [0018]    The alloy may also contain incidental impurities, for example, preferably less than 0.002% lead, less than 0.001% antimony, less than 0.01% tin, less than 0.02% arsenic, less than 0.05% aluminum, and less than 0.02% tellurium. Castings of the alloy preferably have a metallurgical microstructure containing spheroidal graphite iron nodularity of greater than 90% and a desirable nodule count. Desired metallurgical microstructures in the alloy castings can be confirmed through the use of cast-on test coupons per ASTM standard A536, on the basis that such test coupons contain spheroidal graphite iron nodularity of greater than 90% and a nodule count of greater than 100 per square millimeter. 
         [0019]    In an investigation reported below, it was determined that, regardless of silicon content, increasing nickel content in a range of about 0.2 to 5.0 weight percent coincided with increasing resistivity (specific resistance), and regardless of nickel content, increasing silicon content in a range of about 2.5 to 4.2 weight percent coincided with increasing resistivity in the alloy. It was also determined that permeability in as-cast alloys decreased with increasing nickel content regardless of silicon content. Magnetic permeability influences eddy current response and has a significant effect over conductivity, while eddy current losses due to joule heating can be reduced by increasing resistivity. As such, high permeability and resistivity are believed to be desirable properties for minimizing joule heating of the flanges  62 , yet neither was attained by simply increasing the silicon and nickel contents of alloy specimens prepared for investigations leading to this invention. 
         [0020]    Alloys prepared for the investigations are summarized in Table II below, as are certain mechanical and magnetic properties of the alloys. In Table II, “AC” identifies alloys in the as-cast condition, and “HT” identifies alloys whose compositions are similar to the numerically corresponding AC alloys (e.g., HT1 to AC1) but further underwent heat treatment in an attempt to influence the permeability and resistivity of the alloys. Heat treatment of ferritic ductile irons tends to improve electrical resistivity properties with ferritization. In the investigation, two-stage heat treatments were devised in an attempt to improve electrical and magnetic permeability properties of the ductile iron alloys being evaluated. All heat treatments entailed heat from room temperature to a soak temperature of about 910±20° C. at a rate of about 2° C./minute, holding at the soak temperature for about three hours ±30 minutes, cooling from the soak temperature to a second soak temperature of about 690±20° C. at a rate of about 1° C./minute, holding at the second soak temperature for about six hours ±30 minutes, cooling from the second soak temperature to a temperature of about 200±20° C. at a rate of about 1.5° C./minute, and then air cooling to room temperature. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                   
                   
                   
                   
                 Max. 
               
               
                 Alloy 
                 Composition (weight %) 
                 UTS 
                 Resistivity 
                 Permeability 
               
             
          
           
               
                 No. 
                 C 
                 Si 
                 Ni 
                 Fe 
                 (ksi) 
                 (μ-ohm · cm) 
                 (H/m) 
               
               
                   
               
             
          
           
               
                 AC1 
                 3.54 
                 2.58 
                 0.32 
                 bal. 
                 44.9 
                 53.2 
                 1557 
               
               
                 AC2 
                 3.57 
                 2.48 
                 0.24 
                 bal. 
                 43.3 
                 52.8 
                 1619 
               
               
                 AC3 
                 3.24 
                 3.08 
                 1.44 
                 bal. 
                 62.3 
                 60.0 
                 856 
               
               
                 AC4 
                 3.37 
                 3.74 
                 1.43 
                 bal. 
                 71.9 
                 67.7 
                 1443 
               
               
                 AC5 
                 3.31 
                 3.10 
                 4.86 
                 bal. 
                 85.5 
                 63.6 
                 490 
               
               
                 AC6 
                 3.28 
                 3.75 
                 4.60 
                 bal. 
                 99.5 
                 73.7 
                 755 
               
               
                 AC7 
                 3.27 
                 3.95 
                 11.3 
                 bal. 
                 108.8 
                 77.2 
                 61 
               
               
                 HT1 
                 3.50 
                 2.53 
                 0.22 
                 bal. 
                 41.8 
                 54.2 
                 2085 
               
               
                 HT2 
                 3.53 
                 2.56 
                 0.24 
                 bal. 
                 43.8 
                 54.2 
                 1865 
               
               
                 HT3 
                 3.25 
                 3.15 
                 1.51 
                 bal. 
                 57.1 
                 60.1 
                 1609 
               
               
                 HT4 
                 3.26 
                 3.93 
                 1.54 
                 bal. 
                 70.3 
                 70.3 
                 1894 
               
               
                 HT5 
                 3.25 
                 3.14 
                 4.92 
                 bal. 
                 76.4 
                 62.3 
                 1203 
               
               
                 HT6 
                 3.26 
                 4.12 
                 5.58 
                 bal. 
                 91.5 
                 73.4 
                 1174 
               
               
                 HT7 
                 3.38 
                 4.16 
                 11.2 
                 bal. 
                 111.0 
                 77.5 
                 66 
               
               
                   
               
             
          
         
       
     
         [0021]    From the results in Table II it can be seen that strength increased with increasing silicon and nickel levels, but that permeability decreased with increasing nickel content. Furthermore, for alloys with nickel levels of about 1.4 to 1.5% (AC3, AC4, HT3, and HT4) and about 4.6 to 5.6% (AC5, AC6, HT5, and HT6), increasing silicon contents resulted in improved permeability and slightly higher resistivities. Still further, by comparing the alloys with silicon levels of about roughly 4% (AC6, AC7, HT6, and HT7), it can be seen that increasing nickel contents were responsible for slightly higher resistivities but drastically lower permeabilities. For alloys having the lowest nickel contents (AC1, AC2, HT1, and HT2), low resistivities (below 55 μ-ohm·cm) were obtained, and improved permeability could be achieved through heat treatment (comparing HT1 and HT2 to AC1 and AC2). Finally, by comparing the heat treated alloys (HT1-HT7) to the untreated alloys with similar compositions (AC1-AC7, respectively), it can be seen that all heat treated alloys significantly outperformed their corresponding untreated alloys in terms of permeability, and six of the seven heat treated alloys exhibited higher resistivities relative to their corresponding untreated alloys, the exception being the approximately equal resistivities exhibited by alloys AC6 and HT6. As such, the investigation showed that the heat treatment could increase permeabilities and resistivities over those obtained in the as-cast condition for the alloys evaluated. 
         [0022]    Based on the above results, the alloy compositions approximately corresponding to AC6 and HT6 were identified as exhibiting a desirable balance of properties, such as a resistivity of at least 70 μ-ohm·cm and a maximum permeability of at least 500 H/m, that would render a stator flange  62  formed of these materials capable of eliminating the requirement for a separate component (e.g., shield  28 ) having a magnetic flux shielding capability. In view of the effect that high nickel levels had on permeability, it was concluded that a heat treated alloy having a composition closer to that of AC6 than HT6, corresponding to the nominal composition of Table I, would more nearly exhibit optimal properties. Finally, limited additions of manganese as set forth in Table I are capable of improving mechanical properties, while limited additions of magnesium as set forth in Table I are desirable to obtain the desired nodular graphite shape and offset deleterious effects of impurities. 
         [0023]    In terms of power loss characteristics that might be expected for a generator whose stator  50  utilizes a flange  62  formed of an alloy of this invention, it should be noted that a slender, small-area hysteresis (BH) loop corresponds to reduced power losses. In particular, a slender BH loop indicates low retentivity, low residual field, and easier magnetization with low reluctance. Furthermore, a higher saturation induction (Bs) with a small hysteresis loop is desirable to minimize the size of the stator. The relatively high permeabilities and resistivities of the alloy set forth in Table I is believed to provide such benefits. 
         [0024]    While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the flange  62  and the dynamoelectric machine (including the stator  50  and rotor  52 ) in which it is used could differ from that shown. Therefore, the scope of the invention is to be limited only by the following claims.