Abstract:
A procedure for centrifugally casting a shorted structure around induction motor rotors is described. The method is commonly applied to a plurality of rotors disposed and arranged for rotational balance and supported on a suitable support, or optionally, on a plurality of supports arranged in layered fashion about a common rotational axis. The method comprises forming a wax representation of the shorted structure around a lamination stack; mounting a plurality of such lamination stacks in a mounting fixture and-attaching a suitable gating and runner system; forming an investment by coating the structure with refractory followed by melting out the wax; casting molten metal into the investment while it is rotating and aligning the mold to allow the centrifugal force generated to promote mold filling; and, continuing to rotate the investment until solidification is substantially complete.

Description:
TECHNICAL FIELD 
     This application relates to processes for forming shorted structures comprising conductor bars and end rings on laminated cores of rotors for induction motors. More specifically, this disclosure relates to investment casting of shorted structures on rotor cores using rotating mold assemblies by which pairs of rotors may be produced at the same time. 
     BACKGROUND OF THE INVENTION 
     One candidate electric motor type for driving wheels of electric and hybrid vehicles is the induction motor. Induction motors, of course, may be designed in many different sizes and shapes for delivering rotational power. 
     A typical induction motor has a stationary annular wire-wound outer member of designed diameter and length called a stator. Often a three-phase alternating current is delivered to electrical leads of the stator so as to produce a magnetic field that rotates around the stator ring. A cylindrical rotor member carried on the rotating power shaft for the motor is placed closely spaced within the inner cylindrical cavity of the stator. The rotor has an inner cylindrical core of flat round steel plates, coated with electrically insulating material, and stacked as laminations with their circumferences aligned to form the cylindrical core so that it has a length complementary to that of the stator. This cylindrical core does not conduct electricity but it displays high electromagnetic permittivity. 
     Each laminated disk of the rotor core may be shaped with circumferential indentations, or the like, to carry several (e.g., 20-40) uniformly spaced, equal length, copper or aluminum electrical conductor bars extending from one end of the rotor core to the other. The spaced conductor bars may be uniformly slightly inclined to the cylindrical axis of the rotor core and the ends of each bar are connected to copper or aluminum end rings located on the rotor ends and co-axial with the rotor axis. This one-piece, cage-like structure of spaced and inclined conductor bars with end rings, carried on the laminated rotor core, is highly electrically conductive and termed a “shorted structure.” 
     Because only a small clearance is maintained between stator and rotor, the rotating magnetic field of the stator enters the rotor, inducing a current in the embedded conductors. In turn, the conductor current produces its own magnetic field which is repelled by the stator magnetic field and causes the rotor to rotate. Inclination of the conductor bars with respect to the rotational axis of the rotor cooperates with the rotation of the magnetic field produced by the stator and permits a more uniform production of torque by the induction motor. 
     The shorted structure may be fabricated by assembly and joining of its individual components, the conductor bars and end rings. An alternative approach, which promised a shorter manufacturing time, has been to overcast the conductor bars and end rings as a complete structure on the lamination stack using die casting. However, rotors manufactured using the die casting approach have exhibited problems with excessive porosity and lower than optimum shorted structure (electrical) conductivity which has reduced process yield. 
     Thus there is need for a process for rapidly fabricating induction motor rotors and particularly the shorted structure of such rotors. 
     SUMMARY OF THE INVENTION 
     This invention provides a method for casting the shorted conductor bar structure of an induction motor rotor onto a rotor lamination stack in a manner which enables consistent quality and high production rates. The shorted structure typically comprises many equal-length conductor bars and two end rings. Conductor bars, oriented to be aligned generally at an acute angle with the rotational axis of the rotor, extend the length of the rotor lamination stack and are equally spaced around the circumference of the rotor lamination stack. The conductor bars terminate in the end rings, one of which is positioned at each extremity of the lamination stack. The conductor bars are contained within and thereby mechanically restrained by the lamination stack while being generally positioned near the circumference of the rotor stack. 
     The method applies investment (or “Lost Wax”) casting process practices to a mold assembly comprising at least one mold suited for casting of a unitary shorted structure on a complementary laminated plate stack. The shorted structure comprises a first end ring attached to one end of a number of conductor bars and a second end ring attached to the other end of the conductor bars. Each mold will be constructed to permit the entry and flow of molten metal in the direction from one end ring of the conductor bars to their other end ring. The mold body is rotated about a rotation axis in a circular path with the laminated plate stack axis (the rotor axis) aligned with a radius of the circular path and molten metal is introduced at the rotation axis. Thus, the resulting centrifugal forces are suitably directed to efficiently urge the molten casting alloy into the mold along the rotation axis of the rotor to enhance feeding of any metallurgical shrinkage that may develop. Molten metal first enters the mold at a mold cavity corresponding to an end ring, then progresses along mold channels corresponding to the conductor bars and finally fills the mold cavity corresponding to the opposing end ring. Thus the mold orientation promotes metal flow in a direction substantially corresponding to the conductor bar orientation. 
     It is apparent pairs of diametrically opposing molds for the rotor structures may be rotated in combinations with the metal fed from the center of rotation of the opposing rotor mold assemblies. Thus, this casting process may be conducted to enable simultaneous casting of conductor bars and end rings for a plurality of rotors to efficiently enable higher volume production. Hence, the orientation of each of the plurality of rotors will be suitable for constructive utilization of the centrifugal force by all rotors. Thus, some number of rotor molds may be radially disposed about the rotation axis. To minimize imbalance during rotation, rotor molds may be positioned in the mold in generally symmetrical configurations, usually with pairs of molds arranged in opposition and disposed at generally equal distances from the rotation axis. Such configuration will result in an assemblage of laterally-spaced rotor molds all of which are located at a common height and thereby form a mold layer. Yet higher production volumes may be obtained by suitably stacking a plurality of such mold layers to enable casting additional rotors during a single pouring operation of the molten metal at the centers of rotation of the several molds. 
     The mold making process comprises molding a wax form or pattern corresponding to the geometry of the desired shorted structure around a rotor lamination stack or stacked individual laminations. Then a ceramic mold, an investment, is developed by application of ceramic particles to a form comprising a plurality of rotors and their associated wax pattern of the shorted structure, individually attached to a wax runner pattern and with each runner assembled to a common wax sprue pattern. The investment is heated to a temperature sufficient to melt the wax which is substantially drained from the investment. Further heating, to a much higher temperature, combusts the remaining wax and preheats the investment so that its temperature more closely matches the temperature of the casting metal. The investment is then oriented appropriately to optimize mold filling as described above, supported in compacted sand and fed with liquid metal while being rotated about an axis generally corresponding to the centerline of the common sprue. Although other configurations may be employed, it is preferred that the conductor bars be aligned generally parallel to the resultant centrifugal force and that the molten metal enters the mold at one end ring, thereafter progressing along the conductor bars and subsequently filling the end ring opposite the one by which it entered. Rotation is maintained until solidification is substantially complete. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an exemplary rotor design and construction suitable for use in an induction motor. 
         FIG. 1B  shows a partially exploded view of the exemplary induction rotor of  FIG. 1A  better illustrating the interrelationship between the lamination stack and the conductor bars and end rings, which in combination, comprise a shorted structure. Also shown are two planes of partial section, ABCD and EFGH which, when combined produce the combination section of  FIG. 3 . The two end rings shown in exploded view at the left end of  FIG. 1B  are illustrated without their conductor bars to better show their retaining slots for conductor bars. 
         FIG. 1C  shows a fragmentary plan view of a lamination as shown in  FIG. 1B  to better illustrate some of its features. 
         FIG. 2  shows, in partial cutaway, a second rotor design, suitable for the practice of this invention. 
         FIG. 3  shows, in schematic fashion, a combination sectional view of a mold suitable for application of a wax overcoat to a lamination stack. The view combines two sections, each taken along the inclined conductor bar mold cavities. Thus the combination section combines the section taken along plane ABCD of  FIG. 1B  and the section taken along plane EFGH of  FIG. 1B  thereby showing the conductor bar as continuous in both segments. For clarity the sectioning planes of  FIG. 1B  are identified. 
         FIG. 4  shows an assembly of six rotor shorted structure wax patterns, for forming the rotor design of  FIG. 2 , positioned in a circle as three sets of opposing mold pairs on one planar layer of a centrifugal casting fixture with wax runner patterns attached to a common wax sprue pattern at the center of the axis of rotation of the mold assembly. 
         FIG. 5  shows a centrifugal casting wax pattern suitable for casting a shorted structure on a plurality rotors. The pattern has two stacked layers, each layer being as shown in  FIG. 4 , and illustrating the rotational axis employed during casting. A common wax sprue pattern is used for all layers. 
         FIG. 6  shows an embodiment of a feature of the centrifugal casting fixture to better restrain relative motion of the stacked layers of  FIG. 5 . 
         FIGS. 7A , B and C show a second embodiment of a feature of the centrifugal casting fixture to better restrain relative motion of the stacked layers of  FIG. 5  and illustrate its mode of operation. 
         FIG. 8  shows a portion of the six rotor shorted structure wax patterns shown in  FIG. 4  after further processing to coat the pattern with ceramic and remove the wax to form an investment suitable for receiving molten metal. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Induction motors operate through the repulsive interaction of a rotating electrically-generated magnetic field in a stator with an induced magnetic field arising from the induced current in an arrangement of conductors positioned on the rotor. Induction motors enjoy wide application and are available in a range of configurations depending primarily on their electrical rating but influenced also by packaging constraints. Thus many variants of the motor elements exist. In particular, the rotors may exhibit pronounced differences in length, diameter etc. 
     In common with other motors, particularly large motors suitable for automotive application, the magnetic forces are substantial and require that any conductors be restrained and securely anchored. Thus the rotor conductors are typically not positioned on the surface of the rotor but are instead embedded, partially or completely, within the rotor so that they may be well supported by the rotor structure. 
     A typical rotor  10  is illustrated in  FIG. 1A  showing a lamination stack  11  (a stack of bonded disks or sheets of like shape) surrounding a supporting shaft  12  with splines  13  on one end. The lamination stack is surrounded by shorted structure  20  comprising conductor bars  14  and end rings  16  and  17 , the end rings  16  and  17  and conductor bars  14  being connected together to form an electrically-conductive cage, shorted structure  20 , around the lamination stack  11 . In this example the conductor bars  14  are shown as inclined to the axis of rotation of the rotor  10 , a relatively common configuration adopted to minimize motor speed variations or torque ripple. 
     The lamination stack  11  is fabricated as a laminated assemblage of generally annular shaped plates or disks cut or stamped from rolled sheet, usually by a blanking process using matched dies mounted in a sheet metal press. Less frequently laser cutting or electrical discharge machining may be employed. The individual disks are then suitably aligned and stacked atop one another, usually separated by an interposed electrically insulating layer or coating, and permanently attached to one another. Most often the laminations are fully formed as-separated and assembled by carefully positioning one lamination atop another in prescribed orientation. Less-commonly the desired external features are imparted by a separate machining operation conducted on the lamination stack after their assembly. 
     The laminations are magnetically ‘soft’, that is readily magnetized, and typically prepared from electrical steel with a chemistry largely comprising iron with up to 6 percent silicon by weight and less than 0.005 percent by weight carbon. A commonly-used composition is iron with 3 weight percent silicon. 
     Additional details of rotor  10  may be noted by consideration of partially-exploded view  FIG. 1B . This partially exploded view shows two individual laminations  18  separated from lamination stack  11  and illustrates the form of the conductor bar retaining slots  19 . The conductor bar retaining slots  19 , as better seen in  FIG. 1C , are generally shaped like the letter “V” but have a partially closed opening at the rotor periphery  21  to restrain the conductor bar against expulsion due to the high magnetic forces.  FIG. 1C  also shows a feature  15  to aid in angularly locating a lamination to the shaft  12  as will be discussed in greater detail later. 
     The shorted structure may be fabricated as an assembly. However a more promising approach is to cast the shorted structure as a single piece over the lamination stack  11 . Such an approach is challenged by the thermal mass of the lamination stack which will tend to rapidly extract heat from the inflowing molten metal and may choke off the flow of molten liquid prematurely causing flow passages to freeze before the mold fills completely. Die casting, which may employ a water cooled mold and uses mechanical assistance to rapidly charge the molten liquid to the mold, has been used but has generally failed to consistently generate the desired quality or to deliver the expected productivity enhancement required by hybrid traction motors. 
     The subject invention employs a one piece ceramic mold or investment formed using the lost wax process. The mold is then rotated before being charged with molten metal. Rotation is maintained during pouring and continues until solidification occurs. Rotation induces and generates a centrifugal force which, in combination with appropriate mold positioning will be effective in urging the molten metal into the mold and promoting mold fill before the conductor bars, sprue and/or runner structure freezes and prohibits further metal addition. It is preferred that the direction of rotation be such as to generate a centrifugal force which acts in a direction parallel to the conductor bars. 
     As is well known, because of shrinkage and contraction, the volume of a casting is usually less than the volume of the mold into which it is cast. Thus, suitable adjustment to the mold dimensions, usually described as a pattern-maker&#39;s allowance, is made to ensure the finished casting dimensions. These considerations apply to the process under discussion. Thus, where reference is made to a wax pattern it will be appreciated that the general geometry of the cast feature and the pattern will be substantially identical but that the dimensions of the wax and cast features will differ. 
       FIG. 2  shows a rotor  10 ′ of a second design with a lamination stack  11 ′ comprising laminations  18 ′ and shorted structure  20 ′ comprising conductor bars  14 ′ and end rings  16 ′,  17 ′. Rotor  10 ′ is likewise representative of those suitable for practice of this invention. It will be appreciated that the details of rotor  10 ′ differ from those of rotor  10  with respect to at least length, external diameter, internal diameter and conductor bar placement. Such design variances are commonly encountered and are not prejudicial to the practice of this invention which is intended for broad application to induction motor rotor variants in common use. 
     A point of difference between the rotor design of  FIGS. 1 and 2  is that in  FIG. 2  the conductor bars  14 ′ are fully surrounded by the lamination stack  11 ′. Inasmuch as the conductor bar opening will generally be formed during a single press stroke such a feature may be readily accommodated. This design ensures that if the rotor is subjected any machining or grinding processes for balance or concentricity or to achieve rotor-stator clearance tolerances, the current-carrying capability of the conductor bar will not be compromised. 
       FIG. 3  shows, a composite section, obtained by combining sections like those shown at ABCD and EFGH in  FIG. 1B , of a lamination stack  11  positioned in a split mold  50  suitable for casting wax in locations corresponding to the desired locations of conductor bars  14  and end rings  16  and  17  shown in  FIG. 1B . The planes of section ABCD and EFGH as shown in  FIG. 1B  are chosen to enhance the clarity of the figure and specifically to convey that the conductor bar openings  114 , are continuous. It will be appreciated that conductor bar openings  114  of  FIG. 3  are intended to receive cast, electrically conducting material and thereby form conductor bars  14  of  FIGS. 1A and 1B  with a section as best shown at  19  in  FIG. 1C . Similarly end ring openings  116  and  117  of  FIG. 3  when filled with cast electrically conducting material will form end rings  16  and  17  respectively of  FIGS. 1A and 1B . Thus the planes of section are inclined to the rotational axis of the rotor and sectioning planes ABCD and EFGH are oppositely inclined to the rotational axis. The individual sections corresponding to sectioning planes ABCD and EFGH have been combined in the composite section of  FIG. 3 . It will be appreciated that  FIG. 3  will show a true section for a rotor geometry where conductor bars are not inclined to the axis of rotation but instead are parallel to the rotation axis. 
     The wax-casting mold  50  is intended to be reusable and will generally be fabricated of metal for durability. Since the low melting point of wax does not mandate use of more heat resistant materials, aluminum alloy is a suitable mold material and offers easy machining. It will be appreciated that operation of the mold will require that it be mounted in a press or similar device and require additional features such as a guide pins, mounting plates etc. which have been omitted for simplicity. 
     The mold comprises a first mold section  58  including a core feature  59  and a second mold section  60  separated along a parting line XX. The mold incorporates provision for injection of molten wax through runner  52  and has vents  56 . The cylindrical periphery  21  of lamination stack  11  is fitted tightly to the cylindrical walls  66  of second mold section  60  to effectively bar deposit of wax on the outer periphery of lamination stack  11 . Further, the close fit between the laminations and the mold section facilitates aligning the laminations. A similarly close fit is desired between the inner bore of the laminations and the outer surfaces of core  59 . Introduction of complementary features on the inner bore of the laminations and the outer surfaces of core  59  may also be used to facilitate alignment of stacked laminations. For example the inclination of conductor rods  14  as shown in  FIG. 1A  would be readily achieved by inclining a protuberance (not shown) on core  59  complementary to a slot or recess on the bore of the lamination such as is shown at  15  in  FIG. 1C , or vice versa. It may also be noted that the cast conductor rods will act to mechanically secure the laminations to form the lamination stack so that the laminations may be loaded into the mold, or more preferably, onto the core individually, potentially facilitating stack assembly. 
     The outwardly-facing end lamination  70  of lamination stack  11  is sealingly spaced apart, such as by stops  72 , from second mold surface  68  to create annular opening  116 . Similarly the outwardly-facing end lamination  71  of lamination stack  11  is sealingly spaced apart, such as by stops  74 , from first mold surface  76  to create annular opening  117 . Thus molten or flowable injection molding wax formulated from hydrocarbon wax, natural ester wax, synthetic wax, natural and synthetic resins, organic filler materials and water to achieve suitable characteristics as is well known to those skilled in the art, may be introduced through runner  52 . As depicted in  FIG. 3 , the wax on entering the mold will first fill the annular region  116  corresponding to a first end ring, then flow along channels  114  corresponding to the conductor bars before filling regions  117  on the opposing end surface of the rotor to form the second end ring. Vents  56  will enable venting of air initially present in the mold. Alternatively the mold may be evacuated prior to introduction of wax and the vents eliminated. 
     When the wax has solidified and hardened, mold segments  58  and  60  may be separated along split line XX by motion in a direction indicated by double arrow YY. As depicted, the wax over-molded lamination stack including the wax runner pattern (designated  52 ′ in  FIGS. 4 and 5 ) may now be readily removed from the mold, if necessary with the aid of an ejector pin (not shown), again along direction YY and the wax sections corresponding to vents  56 , if present, removed. 
     It will be appreciated that  FIG. 3  is exemplary and not restrictive and that alternate mold designs incorporating different wax fill geometries and mold segment geometries may be employed. Such variants are fully comprehended by the invention. Further, although not preferred, the wax features corresponding to the conductor bars, end rings and runner, may also be built up by hand, for example, by laying up shaped wax forms and attaching them together by co-melting the contacting forms. 
       FIG. 4  shows a plurality of wax-overmolded lamination stacks  110 ′ after being overmolded with wax in a mold such as shown in  FIG. 3 . These overmolded lamination stacks are positioned on a casting fixture  100  which includes a wax sprue pattern  80 , attached to all of the wax runner patterns  52 ′ associated with each of the wax-overmolded lamination stacks  110 ′. Attachment of wax runner pattern  52 ′ to wax sprue pattern  80  is accomplished by co-melting and adjoining the abutting portions of each individual pattern. Preferably a common wax is used for all patterns so that when the wax cools and solidifies it will constitute a joint with the same characteristics as the pattern features. Wax-overmolded lamination stacks  110 ′ are derivative of the rotor  10 ′ shown in  FIG. 2  and comprise lamination stack  11 ′, wax features  114 ′ corresponding to conductor bars  14 ′, wax feature  116 ′ corresponding to end ring  16 ′ and wax feature  117 ′ corresponding to end ring  17 ′. These features are shown and indicated on only one of the overmolded lamination stacks of the figure but are common to all overmolded lamination stacks. 
     The wax-overmolded lamination stacks are positioned in opposition to facilitate balance and are individually supported on a supporting feature  84  dimensioned to slidably engage the inner diameter of wax-overmolded lamination stacks  110 ′ with minimal clearance. Supporting features  84  are themselves attached to a supporting structure comprising a stacked array of annuli  82  supported and attached by a plurality of ribs  86 . All wax runner patterns  52 ′ are attached to a common wax sprue pattern  80 . The eventual axis of rotation  81 , corresponding to the centerline of wax sprue pattern  80  is also shown. 
     The structure depicted for the fixture is illustrative only and various modifications to the structure shown are comprehended in this invention. Without limitation these may include: variations in rotor support features  84 ; or variations in the number or distribution of rotors accommodated provided the resulting assembly is substantially balanced; or of the nature of the supporting structure  82 ; or of its support members  86 . For example: the rotor and shaft assembly of  FIG. 1  might be supported using an internally splined hollow cylinder sized to slidably engage splines  13  on shaft  12  of  FIG. 1 ; the supporting structure might comprise more or fewer annular features like that shown at  82  in  FIG. 4  and the features might be of greater or lesser diameter and/or of alternate cross-section; and finally the support members shown as  86  in  FIG. 4  might be modified in number, cross-section or incorporate additional features for improved performance as illustrated in  FIGS. 6 and 7 . 
       FIG. 5  illustrates a partial build-up of a casting fixture  300  comprised of two of the layers shown in  FIG. 4 , depicted as a first layer  100  and a second layer  200 . Ribs  86  are aligned and serve to releasably join first layer  100  to second layer  200 . For convenience the rotors in each of the layers are depicted as aligned but it may be advantageous to stagger the rotor positioning in the different layers if such a configuration improves rotational balance. As will become clearer from the discussion of the methods of attaching the layers staggering the rotor orientation may require that additional ribs  86  be provided. The additional ribs may be positioned symmetrically in the substantially 120° sectors between the ribs depicted in  FIGS. 4 and 5 . 
     Any convenient attachment procedure may be followed. For example as represented in  FIG. 4 , ribs  86  are hollow. Thus layer alignment may be enabled by sliding a tight-fitting rod of complementary shape through ribs  86  of each layer and tying layers  100  and  200  together with wire or other suitable material. 
     Alternatively, in a second embodiment, best illustrated at  FIG. 6 , ribs  86 ′ and  86 ″, respectively mounted on annular features  82 ′ and  82 ″, are shown. Ribs  86 ′,  86 ″ have been formed by the incorporation of shaped plugs  87 ′ and  87 ″ permanently attached to one end of each of ribs  86 ′ and  86 ″, for example by riveting or other mechanical fastener or by welding or by interference fit or other suitable means. Plug  87 ′ extends beyond surface  92  of rib  86 ′ and is adapted for easy insertion into the open end of rib  86 ″, for example by adoption of a tapered cross-section as shown. Thus, as shown in  FIG. 6 , layer  200  may, provided ribs  86 ′ and  86 ″ are approximately aligned, be positioned atop layer  100 , enabling plug  87 ′ to guide and engage the opening of rib  86 ″ so that end surface  92  of rib  86 ′ is brought into contact with surface  94  of rib  86 ″. Thus layers  100  and  200  are locked together against rotation but again would require tying together to restrain them from being pulled apart. 
     In a yet further variant shown in  FIG. 7A , tapered plug  89  attached to one end of rib  86 ′ incorporates a recess  96  and rib  86 ″ incorporates a pin  99 , complementary in shape to recess  96 , extending through sidewall  102  of rib  86 ″ and supported on spring strip  98  attached to sidewall  102  by rivet  97 . Thus as rib  86 ′ is lowered in the direction indicated by arrow  110  after being brought into general alignment with rib  86 ″ the end of plug  89  guides and engages the open end of rib  86 ″. As rib  86 ′ descends, the taper of plug  89  displaces pin  99  and bends and tensions spring strip  98  as shown in  FIG. 7B . With continued motion of rib  86 ′ surface  92 ′ of rib  86 ′ is brought into contact with surface  94 ′ of rib  86 ″ and recess  96  aligns with pin  99 , which under the urging of tensioned spring strip  98  is displaced into recess  96  as indicated in  FIG. 7C . When configured as shown in  FIG. 7C , layers  100  and  200  ( FIG. 5 ) are fully restrained. 
     It will be appreciated that the specific locking mechanisms and devices described above are intended to be illustrative and not limiting and that other designs and configurations may be employed without departing from the scope of the invention. 
     Returning to  FIG. 5  it will be noted that layers  100  and  200  share a common wax sprue pattern  80 ′. Additional layers  100  ( FIG. 4 ) may be incorporated and it is anticipated that a casting fixture may include up to four layers. Additional layers would continue to share a common wax sprue pattern developed by extension of sprue pattern  80 ′ at end  130  or end  132 . Such a configuration would, based on the configuration shown as  100  in  FIG. 4  enable up to 24 rotors to be cast in a single pour. During casting and solidification the casting fixture will be rotated about axis  81  coincident with the centerline of wax sprue pattern  80 ′. A suitable direction of rotation is indicated by arrow  120 , but rotation opposite that shown by arrow  120  would also be effective. 
     The casting fixture is then used to create an investment, a ceramic mold suitable for containing molten metal. Typically the investment is produced by a series of sequential steps. First the casting fixture is dipped into a slurry of fine refractory material which will deposit as a thin layer on the fixture surfaces and then letting any excess drain off, so that a uniform surface is produced. The slurry may incorporate a variety of ceramics in varying proportions ranging in size from about 45 to 75 micrometers (200-325 mesh) and suitable to enable any fine details of the finished casting to be accurately reproduced. Next, the casting fixture is stuccoed, or overcoated with coarser ceramic particles, including mullite, ranging in size from about 300 to 1000 micrometers (18-50 mesh), by dipping it into a fluidized bed, placing it in a rain sander, or by applying by hand. Finally, the coating is allowed to harden. These steps may be repeated to build up the ceramic coating to the desired thickness, which is usually 5 to 15 mm (0.2 to 0.6 in). 
     Common refractory materials are used to create the investments. These include: silica, zirconia, various aluminium silicates, and alumina. The silica may be quartz or fused silica. Aluminium silicates, mixture of alumina and silica, typically have an alumina content ranging from 42 to 72% and include mullite at 72% alumina. Particularly during the initial slurry-based coat the choice of refractory will be informed by the need to suppress reaction between refractory and molten metal and may promote the use of zirconia-based ceramics. The binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica or silica sol, set by drying, sodium silicate, and a hybrid of these controlled for pH and viscosity. Alcohol-based binders may be preferred in practice of this invention to minimize corrosion of the ferrous lamination materials. Where aqueous binders are used the laminations may be protected by a thin barrier coating, for example of shellac, applied by spraying or by dipping in a dilute solution with a fast-evaporating and non-corrosive solvent. 
     Once the refractory has been applied in required thickness and dried, the entire structure of  FIG. 5  is enclosed in a substantially-continuous layer of ceramic with all locations into which molten metal is to cast being occupied with wax. 
     The wax is initially removed by gently heating the casting fixture, for example in a steam autoclave, so that the wax will melt and run out for collection and recycling. The casting fixture is then ‘burned out’, that is heated to a temperature of about 1800-2200° F. in an oxidizing atmosphere to combust and remove all remnant wax and render the investment suitable for receipt of the molten metal. 
     A fragmentary view of such an investment  100 ′ is shown in  FIG. 8 , which focuses on only a portion of the pattern shown in  FIG. 4 , with the pattern shown in ghost for reference. Supporting structure elements  82 ,  84  and  86 , though not shown for clarity, remain to provide support. The investment is covered by a continuous layer of ceramic material  120 . Removal of the wax has created a sprue  80 ″ with a centerline  81  and runners  52 ″, shown generally in ghost and in cut-away at location ‘A’. Thus molten metal entering the sprue  80 ″, may be transported to ceramic-encased lamination stack  11 ″ through runners  52 ″. Absent the wax, ceramic-encased lamination stack will contain cavities  114 ″,  116 ″ and  117 ″ (commonly present in all of the ceramic-encased lamination stacks but shown and identified only at cutaway section ‘B’) suitable for transporting and accepting the molten metal to form the conductor bars  14 ′ and end rings  16 ′,  17 ′ shown in  FIG. 2 . 
     The ‘burn out’ step is also effective in preheating the investment and thereby reducing the temperature difference between the molten metal and investment during the casting process. The preheated investment will be effective in increasing the fluidity of the cast metal and act to prevent or minimize opportunity for misruns during the casting process. The investment is then inserted and positioned in a chamber or container which is agitated or vibrated while sand of prescribed composition, typically mullite although silica may also be used, and of minimal moisture content with a distribution of particle sizes ranging from 150 to 840 micrometers (100-20 mesh) is added at a controlled rate. This procedure will compact the sand around the investment, providing support and rendering it capable of sustaining the, possibly at least partially unbalanced, centrifugal forces generated during casting. The assemblage of the container and its sand-supported investment comprise the mold. 
     Because the lamination stacks comprising the rotors are ferrous, they may function as chills during the casting process, efficiently extracting heat from the inflowing molten metal, lowering its temperature and causing it to freeze before the mold is filled and producing misruns. To forestall this it is generally desirable to at least preheat the investment, including the lamination stacks, to a temperature at least close to the melting point of the casting alloy. The preheating which occurs on burnout may be adequate if sand fill, mold preparation and pouring occur promptly, before the investment loses appreciable heat to the poorly heat-conducting sand. However, although less preferred, additional heat may be provided, for example by heating the mold in an oven, prior to pouring if necessary. 
     The mold with its preheated investment is then rotated about axis  81  (see  FIGS. 5 and 8 ) with a rotational speed of between 1 and 300 rpm and the molten metal is introduced to the mold. For example the molten metal may be top fed, by being poured into a pouring cup, not shown, attached directly to end  130 ′ of sprue  80 ″ or, more preferably with the addition of further melt distribution channels (not shown), bottom fed, so that the molten metal enters the mold at end  132 ′ of sprue  80 ″. The molten metal will typically be high purity electrical grade copper or aluminum to assure minimal electrical resistance in the finished casting but it may be preferred to use aluminum or copper alloys which may impart additional strength. The use of such higher strength alloys will be more preferred in higher performance motors which will subject the rotor shorted structure to higher operating loads. The melt will be maintained at some temperature greater than its melting temperature, the excess being superheat, with the degree of superheat and the investment temperature being cooperatively selected to assure mold filling. The rotation imparted to the mold will induce centrifugal forces directed outward to the periphery of the investment and will promote radial flow outward along the conductor bars where the individual metal flows will combine to form the end ring. 
     Although the gating geometry is depicted as comprising a common sprue and a single runner in  FIG. 8 , those skilled in the art will recognize that alternative or supplementary gating or venting may be beneficial in achieving consistent mold fill. Similarly it will be appreciated that the casting process may be conducted under at least partial vacuum and that the rotor forms shown with the conductor bars aligned with the direction of the applied centrifugal force may be inclined or otherwise oriented in an alternative manner without departing from the gist of the invention. 
     After the mold is filled, rotation is continued until solidification is substantially complete. After solidification concludes, the sand will be discharged from the mold, the investment broken open and the gating removed to recover the rotor with its cast shorted structure in conventional fashion. 
     The practice of the invention has been illustrated with some exemplary designs and configurations which are not intended to limit the scope of the invention.