Abstract:
A device for imposing a torque load upon rotating machinery comprises a rotary unit and plural electromagnetic units. The rotary unit includes a shaft and conductive disks discretely fastened thereto. Each electromagnetic unit includes a bracket-shaped magnetic core and one or more ferromagnetic pieces discretely fastened thereto. The device&#39;s shaft is joined end-to-end to the motor&#39;s shaft, permitting integral axial rotation of the device&#39;s shaft, the disks and the motor&#39;s shaft. Each electromagnetic unit is placed so that the core “brackets” the two extreme disks, while each piece is between two disks. During rotation, a wire (coiled around each core) conducts current of selected amperage so as to generate a magnetic field of sufficient intensity that a magnetic flux circuit is formed through the stationary core and pieces and the rotating disks, resulting in a Lorentz force associated with the magnetic field and eddy currents engendered in the disks.

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to machines (e.g., motors) that include rotational members, more particularly to methods and apparatuses for imposing torque loading upon a rotational member for purposes of conducting testing (such as involving vibration, e.g., acoustic vibration) of a machine. 
   Vibration, such as sound vibration (e.g., that which produces noise), is an important consideration in the operation of various types of machinery. The current methodologies for providing a torque load for vibration testing of machines such as motors are deficient in terms of induced load smoothness, heat generation and vibration generation. The devices conventionally used for motor testing include Prony brakes, water brakes, generator loads, and magnetic rheological devices. 
   It is desirable that a “quiet load” be used for vibration testing of a “quiet motor” so as to minimize or avoid contribution of vibration by the load to the overall vibration that is being measured with attribution to the motor. When a quiet motor is to be tested (e.g., for an endurance test), it is unsuitable to use a noisy load for testing, as such would represent an abnormal operating condition for the motor. A quiet motor is designed to operate quietly, in a quiet environment, and the introduction of a noisy test component would be incongruous. A “quiet motor,” as the term is used herein, is a motor that is designed to generate a relatively low amount of vibration during operation of the motor. A “quiet load,” as the term is used herein, is an induced load that generates as little vibration as possible at all frequencies (or all significant frequencies) during induction of the load with respect to the motor during testing (e.g., vibration testing) of the motor. It is further desirable that the load be able to operate over wide ranges of speed and torque. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide a methodology for imparting a torque load in such a way that such imparting does not compromise the accuracy of testing (e.g., vibration testing) of a machine such as a motor. 
   It is a further object of the present invention to provide such a methodology that is characterized by, or admits of, range versatility regarding speed and torque. 
   According to typical inventive practice, an axle is fastened to a rotor of a test machine so that the axle and rotor share the same axis of rotation. The axle has fastened thereto one or more electrically conductive wheels. A magnetic field and an associated magnetic circuit are electromagnetically produced, the magnetic circuit intersecting each electrically conductive wheel. When the rotor is caused to rotate, electrical eddy currents occur in each electrically conductive wheel. A Lorentz force, associated with the combination of the magnetic field and the electrical eddy currents, exerts a counter-rotational influence on the rotor. 
   The present invention is useful in the vibration testing of machines such as motors, particularly those that are designed to be characterized by relative quietness, viz., quiet motors. The present invention&#39;s methodology is practicable for acoustic testing or for various other testing purposes. The present invention provides a load without inducing any vibration that is not otherwise present. Featured by the present invention is the novel application of a non-contact, electromagnetic torque load to test electric motors. The inventive device&#39;s torque load uniquely describes an “eddy current quiet load” for a motor (e.g., quiet motor), wherein the scientific principle underlying the present invention bears some similarity to the scientific principle underlying a magnetic damper. The non-contact nature of the inventively applied load avoids (or substantially avoids) the introduction of vibration into the test. According to typical inventive practice, the applied torque load is variably controlled. 
   Among the advantages afforded by the present invention are the following: smoothness of the applied torque load; quietness of the applied torque load; elimination (or substantial reduction) of introduction of vibration, by the torque load, into the machine being tested; precision of variable control of the torque load; directional independence (e.g., reversibility) of variable control of the torque load. As compared with conventional devices, the inventive device is much smoother with respect to the torque load that can be applied to the motor being tested. The present invention&#39;s quiet load generates only steady torques, generating no forces on the test machine rotor, and generating no torques or forces on the test machine frame. 
   In accordance with typical embodiments of the present invention, an inventive device comprises a rotating assembly and at least one stationary electromagnetic structure. More typically, at least two stationary electromagnetic structures are included. The rotating assembly includes a rigid shaft and at least two electrically conductive disks that are separated from and parallel to each other and that are perpendicular to the shaft, each electrically conductive disk being concentrically and fixedly attached to the shaft so as to be rotatable commensurately with the shaft. A housing is provided for the stationary electromagnetic structures. Each stationary electromagnetic structure includes a square-bracket-shaped (“[”-shaped or “]”-shaped) core member and at least one wedge-shaped guidance member, wherein the number of wedge-shaped guidance members is one less than the number of electrically conductive disks. Each bracket-shaped (“[”-shaped or “]”-shaped) core member is conceptually divisible into a vertical segment-shaped (“I”-shaped) main core section and two protrusive end core sections, each horizontal segment-shaped (“-”-shaped). In each stationary electromagnetic structure, each wedge-shaped guidance member is fixedly attached to the main core section of the bracket-shaped core member. 
   The stationary electromagnetic structures are positioned generally surroundingly (and generally symmetrically, according to frequent inventive practice) with respect to the rotating assembly so that, in each stationary electromagnetic structure: the main core section of the bracket-shaped core member, in longitudinal orientation, is parallel to the shaft and perpendicular to the electrically conductive disks; each of the two protrusive end core sections of the bracket-shaped core member, in longitudinal orientation, is perpendicular to the shaft and parallel to the electrically conductive disks; each of the two protrusive end core sections of the bracket-shaped core member is radially disposed with respect to the geometric rotational axis of the shaft; each wedge-shaped guidance member is radially disposed, with outwardly increasing thickness, with respect to the geometric rotational axis of the shaft; the main core section of the bracket-shaped member is situated near the rims of the electrically conductive disks; each of the two protrusive end core sections of the bracket-shaped core member is situated next to the outward faces of the two longitudinally extreme electrically conductive disks; each wedge-shaped guidance member is closely interposed between two adjacent electrically conductive disks. 
   For purposes of testing a motor, the inventive device&#39;s shaft is coaxially and fixedly attached to the motor&#39;s rigid shaft, the inventive device&#39;s shaft and the motor&#39;s shaft thereby effectively constituting a single rigid shaft having a single geometric axis of rotation. An electrical winding for conducting direct current is wound (coiled) about the main core section of the bracket-shaped core member. When the motor&#39;s shaft (and hence the inventive device&#39;s shaft) is motivatively energized, and electrical current is caused to flow through the electrical winding, a closed magnetic flux path is disposed through the bracket-shaped core, the wedge-shaped guidance block(s), and the rotating electrically conductive disks; that is, the magnetic flux flows, in a closed loop, through the stationary electromagnetic structure and through the conductive disks. As a result of the magnetic circuit established by the completely closed magnetic flux path, electrical eddy currents develop in the electrically conductive disks. The eddy currents arise due to the rotation of the electrically conductive disks in combination with the intersection of the electrically conductive disks by the magnetic flux path. The inventive device&#39;s shaft (and hence the motor&#39;s shaft) develops a resistive torque load corresponding to the generation of Lorentz forces that are associated with the physical interaction of (a) the electrical eddy currents generated in the disks and (b) the magnetic field emanating from the combination of the electrical winding and the main core section, about which the electrically winding is wound, of the electromagnetic core&#39;s bracket-shaped core member. 
   Other objects, advantages and features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein: 
       FIG. 1  is a diagrammatic perspective view of an embodiment of a torque load imposition device in accordance with the present invention, particularly illustrating the configuration and operation of the device&#39;s shaft, disks, core, winding and guide blocks. 
       FIG. 2  is a side elevation view of the inventive embodiment shown in  FIG. 1 . Additionally shown is a symbolically depicted housing/supporting structure for effecting fixed support of the core and guide blocks. 
       FIG. 3  is a top plan view of the shaft, disks (top disk shown), and core of the inventive device shown in  FIG. 1  and  FIG. 2 . Additionally shown is a symbolically depicted housing/supporting structure, such as shown in  FIG. 2 . 
       FIG. 4  is a cross-sectional plan view, similar to the view shown in  FIG. 3  and sectioned through the shaft and four guide blocks (between a pair of disks), of the shaft, disks (one disk shown), core, and four guide blocks of the inventive device shown in  FIG. 1  and  FIG. 2 . Additionally shown is the symbolically depicted housing/supporting structure shown in  FIG. 3 . 
       FIG. 5  is an elevation view, similar to the view shown in  FIG. 2 , of the shaft and disks of the inventive device shown in  FIG. 1  and  FIG. 2 . 
       FIG. 6  is a side elevation view, similar to the view shown in  FIG. 2 , of the core, winding and guide blocks of the inventive device shown in  FIG. 1  and  FIG. 2 , additionally depicting how a core portion and the winding, together, constitute an electromagnet that emanates a magnetic field when current flows through the winding. 
       FIG. 7  is a schematic view of an embodiment of a control system, implementing an inventive device such as shown in other figures herein, in accordance with the present invention. 
       FIG. 8  is a side elevation view, similar to the view shown in  FIG. 2 , of another embodiment of a torque load imposition device in accordance with the present invention, wherein the inventive device includes only one disk. Additionally shown is a symbolically depicted housing/supporting structure for effecting fixed support of the magnetic core. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1  through  FIG. 7 , inventive device  10  comprises a rotational unit  12  and at least one stationary unit  14 . Rotational unit  12  and stationary unit  14  are separately shown in  FIG. 5  and  FIG. 6 , respectively. Rotational unit  12  includes rotational shaft  16  and at least two electrically conductive disks  18 . Each stationary unit  14  includes a bracket-shaped electromagnetic core  20  and at least one wedge-shaped guide block  22 . For illustrative purposes, only one stationary unit  14  is shown in  FIG. 1  and  FIG. 2 ; however, according to more typical inventive practice, inventive device  10  includes plural (e.g., several or many) stationary units  14 , circumferentially arranged such as exemplified in  FIG. 3  and  FIG. 4 . 
   Stationary units  14  are shown in  FIG. 3  and  FIG. 4  to be arranged at approximately ninety degree angles and to be symmetrical with respect to geometric longitudinal rotational axis a of shaft  16 . Inventive practice permits utilization of one or practically any plural number of stationary units  14 . In each stationary unit  14 , the core  20  and each guide block  22  are constructed of ferromagnetic material such as iron or steel. Each stationary unit  14  extends radially outward from shaft  16 , but without being in contact with shaft  16 . As shown in  FIG. 7 , shaft  16  is rigidly connected to, and is driven by, motor shaft  160 . Thus connected, the inventive shaft  16  effectively extends, and is rotative along with, the test motor&#39;s shaft  160 . The inventive device&#39;s shaft  16  and the test motor&#39;s shaft  160  are axially connected in the direction of rotational axis a, which is shared by both shaft  16  and shaft  160 . 
   Each centrally apertured circular disk  18  is firmly mounted on shaft  16  and rotates in conjunction with shaft  16 , which extends through the center hole  19  of disk  18 . The respective geometric centers c of the plural disks  18  approximately lie in geometric axis a. Each disk  18  is made of an electrically conductive material such as aluminum or copper. Disks  18  are approximately parallel, approximately congruous, and approximately equally spaced apart from each other. To support high-speed operation, it may be preferred inventive practice to strengthen each disk  18  by means of a high strength band  24  (made, e.g., of steel or Kevlar®) situated on the outer rim of disk  18 . Shaft  16  and disks  18  do not contact any non-rotating components. As diagrammatically represented in  FIG. 2  through  FIG. 4 , frame  26  rigidly supports cores  20  using fastening means  28  such as an adhesive (e.g., a polyester or epoxy or similar material having moderate strength and electrical insulating characteristics) or one or more fasteners (e.g., bolts) that are made of plastic, aluminum or another non-magnetic material such as a non-magnetic composite material. The entire inventive device  10  is supported in a rather stiff housing such as frame  26 , which firmly maintains the positions of core  20  and guide blocks  22  (via fasteners  32 ) with respect to disks  18 . 
   Inventive practice usually provides for the implementation of housing/support structure for the stationary electromagnetic elements. In the light of the instant disclosure, multifarious configurations and techniques for housing or supporting the stationary electromagnetic elements will be evident to the ordinarily skilled artisan. For instance, according to many inventive embodiments, a common frame joins all of the electromagnetic elements. As illustrated in  FIG. 3  and  FIG. 4 , rotational unit  12  is characterized by circular symmetry. Stationary units  14  are radially disposed around shaft  16 . The circularity of frame  26  affords some compatibility with the circular symmetry of rotational unit  12 . It is emphasized that frames  26  and fasteners  28  shown in  FIG. 2  through  FIG. 4  and  FIG. 8  are merely representative, in highly diagrammatic fashion, of the large variety of housing/support systems that the ordinarily skilled artisan who reads the instant disclosure can bring to bear in practicing the present invention. 
   Notable is the absence of any bearing means for supporting the rotating assembly. The implementation of bearings (such as bearings on shaft  16  that are associated with frame  26 ) would defeat an important feature of the present invention, namely, the complete lack of physical coupling between the rotor component (which includes main shaft  160  and rotational unit  12 ) and the stator component (which includes stationary units  14 , windings  42  and frame  26 ). An important principle of the present invention is the application of a torque via the magnetic field in the aether (action at a distance) without the need for any contact forces. The inclusion of any bearing apparatus would create contact forces between the rotor and the stator, thus transferring forces between the rotor and the stator via contact, and possibly also generating forces due to irregular surface shapes. As the present invention is usually practiced, the present invention&#39;s intent is that the rotor be supported on the rotating test object, while the stator be placed in the proper proximity to the rotor and carefully aligned but without any physical contact with the rotor. 
   Each stationary unit  14  includes one ferromagnetic bracket-shaped core  20  and plural (two shown in  FIG. 1 ,  FIG. 2  and  FIG. 6 ), discrete, ferromagnetic, wedge-shaped guide blocks  22 . In each stationary unit  14 , the combination of a core  20  and plural guide blocks  22  describes a largely solid, substantially rectilinear geometric shape, wherein core  20  mechanically supports guide blocks  22  by non-magnetic means, such as through utilization with respect to each guide block  22  of fastening means  32  comprising either an adhesive material (e.g., a polyester or epoxy material or a similar material having moderate strength and electrical insulating characteristics) or one or more one or more fasteners  32  (e.g., one or more brackets attached to core  20 ) that are made of plastic, aluminum or another non-magnetic material such as a non-magnetic composite material. The term “fastening means” is broadly used herein to refer to any means that can be used for attaching, joining, affixing, fastening, connecting or holding together two or more objects, including but not limited to any one of or any combination of devices such as nail, screw, bolt, nut, washer, clamp, clasp, clip, bracket, peg, pin, staple, rivet, hook, tie, weld, adhesive, etc. As illustrated in  FIG. 6 , bracket-shaped core  20  is conceptually divisible into three generally straight sections, viz., a back (longitudinal) core section  34  (which defines an axis of symmetry that is parallel to axis a), a top end core section  36  (which defines an axis of symmetry that is perpendicular to axis a), and a bottom end core section  38  (which defines an axis of symmetry that is perpendicular to axis a). 
   Blocks  22  are styled herein “guide” blocks because they serve as magnetic flux guides. That is, blocks  22  afford guidance with respect to the path of magnetic flux Φ in the context of magnetic circuit M, which represents the completely closed (circuitous) magnetic flux Φ path. According to frequent inventive practice, each guide block  22  is characterized by a wedge shape, having a trapezoidal (nearly triangular) cross-sectional profile, to spatially facilitate the radial distribution of guide blocks  22  about axis a. When two or more guide blocks  22  are associated with a core  20 , it is typical inventive practice that the guide blocks  22  represent congruous segments that are aligned end-to-end so as to describe a combined geometric form having the same trapezoidal cross-sectional profile. Each core  20  has associated therewith an electrically conductive wire  40  through which flows a current i. Wire  40  includes a winding portion  42  in which wire  40  is wound or coiled, circumferentially, on back section  34  of core  20 . 
   When a current flow i is applied to core  20  via wire  40  in the manner and direction shown in  FIG. 1 ,  FIG. 2  and  FIG. 7 , an upward magnetic flux Φ is generated in back section  34  of such core  20 . As shown in  FIG. 6 , the back core section  34  and the coiled wire portion  42  together represent an electromagnet that generates a magnetic flux Φ. Magnetic flux Φ is then guided (leftward as shown) through top section  36  of core  20 , toward shaft  16 . Magnetic flux Φ then leaves top section  36  of core  20  in a downward direction, passing through the first disk  18 , viz., top disk  18   a.  Magnetic flux Φ is then guided down further through the upper guide block  22   a  until it reaches the next (second) disk  18 , viz., middle disk  18   b.  Magnetic flux Φ then passes downward through middle disk  18   b,  and then continues downward to and through the next guide block  22 , viz., lower guide block  22   b.  Magnetic flux Φ then reaches and passes through the next (third) disk  18 , viz., bottom disk  18   c.  Magnetic flux Φ is then guided (rightward as shown) through bottom section  38  of core  20  and toward back section  34  of core  16 , whereupon magnetic flux Φ reaches and proceeds upward through back section  34  of core  16 , thereby closing the magnetic flux Φ path so as to form a magnetic circuit M. Application of current i to winding  42  causes magnetic flux Φ to return to core  20  from bottom disk  14   c  and turn rightward and then upward, thereby effecting closure of the path of magnetic flux Φ. Current i is applied continuously during the time that inventive device  10  is in operation. Steady magnetic flux Φ exists as long as steady current i is flowing in winding  42 . The description in this paragraph disregards all “fringing” magnetic flux Φ as being negligible. 
   Magnetic flux Φ passes through each disk  18  in a corresponding region  44 . The region  44  where magnetic flux Φ passes through each disk  18  is indicated in  FIG. 1  via a solid outline. The total surface area of regions  44  (which is the sum of the individual surfaces areas of regions  44 ) increases in accordance with the number of cores  20  that the inventive device  10  includes. That is, the more cores  20  that are utilized, the greater is the overall surface area of each disk  18  through which magnetic flux Φ passes. In each disk  18 , eddy currents e are created at the corresponding region  44 , the location of through-passage of magnetic flux Φ. Eddy currents e occur in each disk  18  as a consequence of the rotation of disk  18  in synchronism with the intersection thereof of magnetic flux Φ. The interaction of the eddy currents e with the magnetic field causes a Lorentz force F. Because of the geometrical configuration of the inventive system, this Lorentz force F is manifested as a torque t developed on the disk  18  in a direction such as to oppose the rotation r of shaft  16 . Torque t will oppose (be counter-rotational with respect to) shaft rotation r regardless of whether shaft rotation r is clockwise or counterclockwise. With some approximation, torque t will be directly proportional to the rotational speed (i.e., the speed of rotation r) of shaft  16 . No torque t whatsoever will be produced when shaft  16  is at a standstill, or in other words when the rotational speed equals zero. For this reason, inventive practice will generally not be useful for determining the ability of the motor  100  to start under load. 
   If the current i in the portion  42  windings of wire  40  is held constant, the load torque t applied to shaft  16  will rise with the rotational speed of shaft  16 . In some applications, the inventive practitioner may desire that torque t be maintained at a constant value while passing from a first non-zero rotational speed of shaft  16  to a second, higher rotational speed of shaft  16 . Holding torque t constant requires that the current i in the portion  42  wire windings be appropriately decreased as the rotational speed of shaft  16  increases. If the inventive system is operating at one load level, and it is desired to increase the load to a higher level at the same rotational speed, the current i in the portion  42  wire windings must be increased. The limiting loading rate is largely controlled by the voltage rating of power supply  52  (shown in  FIG. 7 ) and by the inductance (and resistance) of the portion  42  wire windings. 
   The operation of inventive device  10  is somewhat similar in principle to that of the magnetic motion damper (also referred to simply as a “magnetic damper”) that is found in some scientific balances and similar apparatus. In accordance with previously known applications of the “magnetic damper” type, the magnetic field is constant and, usually, is supplied by a permanent magnet. In contrast, in accordance with typical embodiments of the present invention, the magnetic field B is specifically supplied by an electromagnet, and the current i in the electromagnet is controlled in order to achieve control over the developed torque t. 
   As schematically illustrated in  FIG. 7 , test motor shaft  160  is included in or associated with test motor  100 . Controller  50  is electrically connected to power supply  52  (e.g., a battery or other device providing direct current) and rotational speed sensor  54 . The rotational speed of motor shaft  160  (and hence of inventive shaft  16 ) can be controlled either through existing mechanism included in or associated with test motor  100 , or through controller  50  so as to be dedicated to control of the entire load testing procedure. Inventive shaft  16  and motor shaft  160  are fixedly structurally connected in the direction of rotational axis a, and rotate together as an integral unitary shaft. Inventive practice will normally necessitate utilization of a rotational speed sensor  54  for sensing the rotational speed of motor shaft  160  (and hence of inventive shaft  16 ). Rotational speed sensor  54 , electrically connected to inventive shaft  16  or motor shaft  160 , communicates with controller  50  in order that controller  50  can set a current i to produce a given torque t. As the rotational speed changes, the current i will, in most cases, need to be continuously adjusted to produce the required torque-time characteristics. 
   As shown in  FIG. 2  and  FIG. 6 , each space between two guide blocks  22 , or between a guide block  22  and a core end section  36  or  38 , represents a slot  48  that accommodates a disk  18  so as to leave two air-gaps  46 . In order to minimize the required magnetomotive force (mmf) , it is preferable to minimize the air-gaps  46 . By “air-gap” is meant herein a space between a guide block  22  and the non-contactingly abutting surface of an adjacent disk  18 , or a space between a core end section  36  or  38  and the non-contactingly abutting surface of an adjacent disk  18 . In order to minimize the reluctance of the magnetic circuit M, and therefore the associated magnetomotive force (mmf) and hence current i, inventive practice frequently prefers a shortening of the magnetic circuit M insofar as is reasonably possible. Such reduction or minimization of the length of magnetic circuit M can be achieved by doing one or more of the following to the extent that it is suitably practicable: spacing disks  18  closer together on shaft  16 ; decreasing the length (in the direction of axis a) of guide blocks  22 ; decreasing the clearance (width) of air-gaps  46 ; increasing the cross-sectional area of main core section  34  of core  16 ; increasing the cross-sectional area of end core sections  36  and  38  of core  16 ; increasing the cross-sectional area (i.e., the mathematical area of region  44 ) of guide blocks  22 ; increasing the number of turns of wire  40  in winding portion  42  (provided that there is room to do so). 
   It may be useful in inventive practice—particularly in situations in which an inventive device is being custom designed to be built for a specific test—to perform lateral vibration analyses on the rotor and on the complete system after the inventive design is considered completed. In the absence of a forced vibration analysis, all that can be done in this regard is to extract the natural frequencies and assure that there are none in the intended operating range. If there is significant lateral vibration, this can tilt disks  18  out of plane, causing closure of air-gaps  46  and concomitant metal-to-metal contact between disks  18  and guidance blocks  22  (and/or between disks  18  and upper and lower core end sections  36  and  38 ). This type of failure is unacceptable and generally demands that the inventive system be designed anew. The present invention can thus be practiced in the manner of designing an inventive device to suit a specific test machine or purpose. The present invention can also be practiced in the manner of providing an inventive device to suit various test machines and purposes. For instance, many inventive device embodiments can be associated with whatever machine needs to be tested; generally, in such cases, the inventive practitioner accepts the rotor dynamics that exist for any such combination of inventive device and test machine. 
   Reference is now made to  FIG. 8 , which is illustrative of inventive embodiments that involve implementation of a single electrically conductive disk  18 , and that hence obviate implementation of any guide blocks  22 . Although three disks  18  are shown in  FIG. 1 ,  FIG. 2  and  FIG. 5 , it is to be understood that an inventive device  10  can be embodied to include a single disk  18  or any plural number of disks  18 . Inventive device  10 ′ shown in  FIG. 8  includes a single disk  18 ; nevertheless, the inclusion of at least two disks  18  represents generally much preferred inventive practice. The inventive single-disk configuration shown in  FIG. 8 , while possible in accordance with inventive principles, will usually be highly impractical insofar as it will require enormous current in the windings  40  to develop sufficient torque t, unless the rotor (motor shaft  160 , and hence inventive shaft  16 ) is operating at a very high speed. Note that similar inventive principles obtain for inventive device  10 ′ in that a magnetic field B (oriented generally parallel to the length of back core section  34 ) is associated with the combination of circumferential winding portion  42  and back core section  34 , and in that magnetic circuit of flux Φ is created through the combination of core  20  and one disk  18  (as distinguished from typical inventive embodiments involving implementation of plural disks  18 , wherein a magnetic circuit of flux Φ is created through the combination of core  20 , at least two disks  18 , and at least one guide block  22 ). 
   In the light of the instant disclosure, the ordinarily skilled artisan will be capable of practicing the present invention with desired effect by selectively varying among parameters including the following: (i) diameter of shaft  16 ; (ii) length of shaft  16 ; (iii) number of disks  18 ; (iv) material of disk  18 ; (v) diameter of disk  18 ; thickness of disk  18 ; (vi) geometrical shape of core  20 ; (vii) number of turns of wire  40  on each winding portion  42 ; number of cores  20  (with a winding portion  42  on each core  20 ); operating current i; geometrical shape of each guide block  22 ; “air-gap” widths or clearances within contiguous pairs of guide blocks  22  and disks  18 ; “air-gap” widths or clearances within contiguous pairs of core end sections ( 36  or  38 ) and disks  18 . These and other factors are available to the inventive practitioner for adjustment, as needed, to satisfy particular design requirements. Another consideration is that it may become necessary in inventive practice to water-cool disks  18 ; however, water-cooling of disks  18  is to be avoided if at all possible, as it would tend to complicate inventive practice. The desirability of avoiding such water-cooling would tend to militate in favor of a greater number of disks  20  sharing a reduced magnetic field that requires less winding current and therefore has less Ohmic losses . 
   According to one approach to fabricating an inventive device&#39;s stationary unit  14 , a coil  42  is wound around a core member  20 , and the combination thereof is vacuum impregnated in epoxy. Next, the guide members  22  are positioned relative to the wound core member  20 , and epoxy is applied to hold the guide members  22  in place. Then, the entire inventive stationary unit  14  assembly is dipped in epoxy. Subsequently, epoxy can be used for adhering or affixing the inventive stationary unit  14  assembly with respect to a housing/support structure  26 . In the light of the instant disclosure, the ordinarily skilled artisan will appreciate the various methods and techniques for making a test device  10  in accordance with the present invention. 
   The present invention, which is disclosed herein, is not to be limited by the embodiments described or illustrated herein, which are given by way of example and not of limitation. Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of the instant disclosure or from practice of the present invention. Various omissions, modifications and changes to the principles disclosed herein may be made by one skilled in the art without departing from the true scope and spirit of the present invention, which is indicated by the following claims.