Patent Publication Number: US-2022224186-A1

Title: Electrical machine and rotor for an electrical machine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The application is a divisional application U.S. application Ser. No. 16/303,995 filed on Nov. 21, 2018, which itself is a U.S. national stage application of PCT/EP2017/062591 filed May 24, 2017, which itself claims priority to EP 1617386.2 filed May 25, 2016, all of which are expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to the field of electrical machines, in particular to high-speed electrical machines with gas bearings. 
     Description of Related Art 
     An electric motor generally includes a rotor and a stator, the stator including a stator body supporting and housing an electrical stator and bearings. The position of the bearings relative to the stator body can be defined by bearing flanges of the stator body. Often, two journal bearings are present, typically located at opposite sides of the stator, as shown in the arrangement of  FIG. 1 a    (wherein the stator body is not shown). The precision of the alignment of the bearings in this case is mainly defined by the precision with which the bearing flange and stator body are machined. With fluid film and in particular for gas bearings, precise alignment is crucial and this arrangement in general requires special measures such as self-aligning or compliant bushing mountings, or machining, e.g. reaming, of the pair of bearings after assembly. Alternatively, the journal bearings can be arranged on the same side of the stator. This arrangement is often called overhanging motor design ( FIG. 1 b   ). With the overhanging design, the two journal bearings can be integrated into a single part, thus precise bearing alignment is easier to achieve. However, this approach generally results in longer rotors and therefore more critical dynamic behavior of the rotor. Furthermore, windage losses, caused by air resistance, are increased, with a negative impact on the overall motor efficiency. 
     U.S. Pat. No. 3,502,920 discloses a slotted electrical machine with air gap bearings, in which a bushing is located in the magnetic gap between the stator and the rotor. The bushing can be elastically suspended relative to the stator. It defines on the one hand a radial bearing and can include a centrally located thrust bearing or thrust block as an axial bearing. In order to assemble the machine, the rotor needs to be separated in the axial direction. This design is unfit for high-speed motors. 
     WO 03/019753 A2 shows a spindle motor in which the rotor rotates in the stator within a thin layer of epoxy forming a cylindrical through bore in the stator and serving to define both a radial bearing surface and an axial bearing surface. The thin layer of epoxy is directly coupled to the stator housing, and any thermally induced deformations of the housing will immediately affect the geometry of the bearing. 
     US 2006/0061222 A1 and US 2006/0186750 A1 show conventional air bearings. 
     US 2010/0019589 A1 discloses a slot configuration of an electrical machine, and, inter alia, a rotor having a multi-layer fiber-reinforced composite sleeve wrapping. Layers can be cosmetic or have functional characteristics, e.g. for achieving strength and rigidity and controlling thermal expansion in one or multiple directions. This can be done by having fibers in a particular layer oriented axially, thereby providing axial strength and limiting axial thermal expansion. In another layer, fibers can be oriented circumferentially, thereby providing circumferential strength and limiting radial thermal expansion. Thus, these layers are configured to stiffen the rotor. They would not be suited to carry an outer sleeve of a relatively stiff material required by a gas bearing, since their purpose is to control expansion, i.e. limit, thermal expansion of the rotor, rather than absorbing differences in thermal expansion between a hard rotor core and a hard rotor sleeve. 
     There is a need for an electrical machine that is suited for high-speed applications and that overcomes the abovementioned disadvantages at least in part. 
     Mainly with gas bearings, the bearing member&#39;s materials are chosen to have high rigidity and a low coefficient of thermal expansion, in order to ensure well-defined bearing clearances under the various operating and environmental conditions of the motor. The high rigidity of the materials however also causes the disadvantage of high stresses in the material at already low strain, e.g., when combining these materials with other materials having a higher coefficient of thermal expansion. 
     There is a need for a rotor that is suited for high-speed electrical machines that overcomes the abovementioned disadvantages at least in part. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to create an electrical machine of the type mentioned initially, which overcomes the disadvantages mentioned above. 
     It is therefore an object of the invention to create a rotor for a high-speed electrical machine of the type mentioned initially, which overcomes the disadvantages mentioned above. 
     According to a first aspect of the invention, the electrical machine includes a stator with a stator body supporting an electrical stator and a rotor. The rotor is supported by a bearing including a radial bearing section forming a radial gas bearing and an axial bearing section forming an axial gas bearing, the stator side parts of these bearing sections being a stator side radial bearing part and a stator side axial bearing part that are rigidly connected to one another and together form a stator bearing structure. 
     Therein, the stator bearing structure is mounted to the other parts of the stator by
         either the stator side radial bearing part being rigidly mounted to these other parts, and the stator side axial bearing part being connected to these other parts by an elastic support or not at all;   or the stator side axial bearing part being rigidly mounted to these other parts, and the stator side radial bearing part being connected to these other parts by an elastic support or not at all.       

     The abovementioned “other parts” can thus be the stator body itself or an assembly that includes the electrical stator and a carrier, with the assembly being elastically supported by the stator body. 
     Here and throughout this document, the terms “rigidly” and “fixed” are used as opposed to “elastically”. An elastic connection has a spring rate or a Young&#39;s modulus that is at least, for example, 100 or 10,000 or 1,000,000 times larger than in a rigid connection. 
     A rigid connection is a connection designed such that the connected parts do not move relative to one another during normal operation of the machine. Thus, a rigid connection can be established by screwing parts together or by pressing them against one another with a spring. In this case, the spring is not part of the rigid connection but provides a force that maintains rigidity of the connection. 
     An elastic support can be an  0 -ring, typically of a (synthetic) rubber, or a metallic spring. 
     The gas of the gas bearing can be any gas the machine operates in, such as air, a cooling agent, natural gas etc. The gas bearing can be a passive or an active gas bearing. 
     In embodiments, the stator bearing structure extends in the axial direction of the electrical machine from a first end to a second end, and the stator bearing structure is rigidly supported by the other parts of the stator near one of the two ends and near the other end is supported elastically or not at all. 
     In embodiments, the stator bearing structure is attached near one end to the stator body in a fixed manner, in particular the stator side axial bearing part is an axial bearing assembly which is rigidly mounted or attached to the stator body. 
     In embodiments, the stator side radial bearing part is a bushing that is rigidly mounted to the stator body or to a carrier carrying the electrical stator. 
     The radial bearing part extends, in the axial direction of the machine, throughout the length of the electrical stator. It can be a single part including bearing elements. 
     The drawbacks of long rotors can thus be avoided by having the bushing with the journal bearings arranged within the electric stator, therewith allowing short rotor designs. The journal bearings can be integrated into a single part, thus precise alignment can be easier to achieve than with journal bearings on separate parts. 
     The end-to-end journal bearing bushing part can increase the magnetic air gap of the motor and therefore have a negative impact on the motor efficiency. Although the end-to-end bushing concept is conceivable for a variety of machine types, slotless permanent magnet machines types are favored for this bearing concept: due to the already relatively large magnetic air gap in slotless type permanent magnet machines, the efficiency is not seriously affected. 
     The bushing can be made of a ceramic material or another material that provides sufficient mechanical stiffness and does not affect the magnetic field in the magnetic air gap. The advantage of ceramic materials is that they are suited both for gas bearings and can be placed in the magnetic air gap, where they are penetrated by the torque generating magnetic field. Generally, electrical insulators or materials with low thermal conductivity such as ceramics, glass ceramics or technical glasses, plastics, composites, mineral materials etc. can be used to avoid excessive eddy current losses caused by the alternating magnetic air gap field. 
     In embodiments, the electrical machine is of the slotless type. In other words, the electrical stator includes an air gap winding rather than slotted windings. This is advantageous since it allows for a magnetic air gap that can accommodate elements of an air bearing, such as a bushing, without substantial disadvantages. 
     In embodiments, the radial bearing section extends in the longitudinal direction of the axis of rotation and all bearing elements and cooperating rotor bearing surfaces of the radial bearing section lie outside the magnetic gap between the electrical stator an the rotor. 
     In embodiments, the radial bearing section extends in the longitudinal direction of the axis of rotation and the entirety of the radial bearing section lies in the magnetic gap between the electrical stator an the rotor. 
     In embodiments, the radial bearing section extends in the longitudinal direction of the axis of rotation and at least one bearing element and cooperating rotor bearing surface of the radial bearing section lie in the magnetic gap between the electrical stator and the rotor. 
     In embodiments, the radial bearing section extends in the longitudinal direction of the axis of rotation and at least one quarter, or at least one half, or at least three quarters of the radial bearing section lie in the magnetic gap between the electrical stator an the rotor. 
     A thrust bearing or axial bearing typically includes a rotor disc in the rotor and, at each side of the rotor disc, as seen in the axial direction, an adjacent stator disc on the stator. The two stator discs are separated by a shim or spacer element such that a well-defined gap is formed between the rotating and stationary parts of the thrust bearing. Together, stator discs, spacer and connecting elements form the axial bearing assembly. 
     In embodiments, the stator disc of the axial bearing assembly and the bushing each include an axially facing surface as an axial reference surface and the two axial reference surfaces are placed against one another. Thereby they ensure that the axis of rotation is normal to bearing surfaces of the axial bearing assembly. 
     In embodiments, a stator disc of the axial bearing assembly and the bushing are manufactured as a single piece or part. 
     In embodiments, the stator disc and the bushing that are placed against one another are pressed against one another by a resilient element. The resilient element can be as an axial compensation element, which can be a metal spring or synthetic part such as an O-ring. 
     As a result, during assembly the relative axial position and the angle between the stator disc and the bushing is well defined by the reference surfaces, whereas the relative position in the radial direction is not defined. This allows assembling the parts without mechanical constraints that might lead to a deformation of the parts. After assembly, by being pressed against each other the parts are rigidly connected. 
     For best orthogonality of journal and thrust bearings, the connecting surface on the bushing part is manufactured precisely orthogonal to the journal bearing rotational axis and the stator thrust disc faces are manufactured highly parallel. The axial bearing thrust discs together with the thrust bearing shim or spacer can be rigidly mounted to the stator body, e.g. by means of screws. Using the spring element (which can simply be an O-ring) between the stator body and the journal bearing bushing part, the bushing is orthogonally self-aligned to the axial bearing disc/shim stack or axial bearing assembly. This mounting concept is well defined, that is, not over-determined. Thus, the stator body or its parts can thermally deform in a wide range without impact on the alignment or deformation of the bearing system. 
     In embodiments, two stator discs and a spacer element arranged between the two stator discs are pressed against one another by a resilient element. This resilient element can be the same one as the one that presses a stator disc against the bushing. There also can be two resilient elements for pressing the two stator discs, the spacer element and the bushing against one another. This allows for simpler assembly and, because the system is not statically overdetermined, better precision with regard to axial alignment. 
     In embodiments, the stator bearing structure at an end at which it is not rigidly mounted is supported by means of a first elastic support. 
     Such an additional elastic support on the bushing&#39;s other end allows to compensate for a possible deformation of the stator parts and to improve vibration characteristics. 
     In embodiments, the stator bearing structure is thermally coupled to the stator body at or near the location of the first elastic support. 
     Such a thermal coupling of the bushing to the stator can be affected by using O-rings both as flexible supporting elements and for sealing off a thermally conductive paste or fluid placed in between the O-rings. The paste or fluid, depending on its viscosity, can have a dampening effect, as in a squeeze film damper. 
     In embodiments, a carrier supporting the electrical stator is mechanically decoupled from the stator body by means of elastic carrier support elements. This allows to dampen or eliminate the transmission of vibration between the electrical stator and the stator body. 
     According to a second aspect of the invention, which in principle is independent from the first aspect but can be realized in combination with the first aspect, a rotor for a high-speed electrical machine is provided. The rotor includes a rotor shaft, the shaft including a rotor core and a rotor sleeve, an (essentially cylindrical) compensation element being arranged between the rotor core and the rotor sleeve to absorb differences in thermal expansion of the rotor core and the rotor sleeve. 
     In embodiments, the rotor sleeve, on which part also the rotating bearing surfaces are located, is made of a ceramic material, typically with a low coefficient of thermal expansion. For a permanent magnet machine, a permanent magnet can be mounted into the ceramic rotor sleeve. In general, permanent magnet materials have higher coefficients of thermal expansion than the targeted rotor sleeve materials, thus creating thermally induced stresses in the ceramic material when directly mounted or glued with a rigid adhesive. 
     For example, the coefficient of thermal expansion (CTE) of Samarium-Cobalt Magnets is 9 to 13 μm/m/K, that of Neodymium-Iron-Boron magnets is −1 to 8 μm/m/K. Both materials are anisotropic, but the compensation element absorbs corresponding strains as well. CTEs of typical ceramic materials for the rotor sleeve are around the range of 1.5 to 5 μm/m/K. 
     At high rotational speeds further stresses induced by centrifugal forces are superimposed over the thermally induced stresses. 
     The term “high speed electrical machine” is taken to cover machines that are suited for more than 100,000 revolutions per minute. 
     Generally, it can be the case for all embodiments that the rotor sleeve is stiffer than the compensation element. The rotor sleeve can be stiffer than the compensation element with respect to a radial pressure, and in particular wherein a radial expansion of the rotor sleeve with regard to a radial pressure is at least less than half, or less than a fifth, or less than a tenth of a radial compression of the compensation element with regard to the same radial pressure. In the assembled state, a radial pressure acting between the compensation element and the rotor sleeve acts as a compressive pressure on the compensation element and as an expansive pressure on the rotor sleeve. 
     For thermally insensitive motor designs or for permanent magnets and ceramic rotor sleeves with similar coefficients of thermal expansions, the permanent magnet can be mounted into the ceramic sleeve with a tight fit or be glued in. Thermal strain of the magnet is then directly transferred to the ceramic sleeve, thereby limiting the maximum operating temperature and maximum speed. In thermally more critical designs and/or at higher speeds, a mechanical decoupling of the two parts is needed. 
     The rotor core can include the permanent magnet fitted in a sheath in order to improve its mechanical stability. In other embodiments, the magnetic core includes a permanent magnet which that is self-supporting, in that it does not require a sheath to maintain its stability. Ideally, the sheath should be of a material with a similar CTE than the permanent magnet but with greater mechanical stability or toughness. 
     In embodiments, the compensation element includes compensation sections arranged to be deformed, in particular to yield or to be bent, when the rotor expands or contracts due to temperature changes. 
     This compensation element or compensator, can be a metallic sleeve (e.g. made from titanium), with much higher elasticity than the ceramic materials is arranged between the rotor core (permanent magnet with or without a sheath) and rotor sleeve. The compensator is designed to absorb thermally induced strain between the rotor core and the ceramic rotor sleeve and thus reduce stresses in the ceramic material. The compensator contacts the rotor core at least at the regions of their axial ends, and in other regions establishes an air gap between the rotor core and the ceramic sleeve. In these regions, the rotor core and the compensator are allowed to shrink and expand without having an impact on the ceramic sleeve. At the rotor core&#39;s axial ends, the connection to the ceramic sleeve can be made with a well-defined distance between
         contact areas joining the rotor core to the compensator, and   contact areas joining the compensator to the ceramic sleeve,
 
along which the induced strain is relieved.
       

     In an embodiment, when no preloading of the rotor core is needed, i.e. when the rotor core or just a permanent magnet constituting the rotor core is self-supporting, then the compensator can include additional points of support between the rotor core and the ceramic sleeve, or support sections of the compensation element. This can improve the dynamic behavior of the rotor. With a well-defined axial distance between these points of support, along which the induced strain can be relieved, the compensator contacts the ceramic sleeve with an air gap towards the magnet, such that the magnet can shrink and expand freely without a noticeable impact on the ceramic sleeve. 
     Thus, in embodiments, the compensation element includes first sections in contact with only the rotor core and not the rotor sleeve, and second sections in contact with only the rotor sleeve and not the rotor core, and compensating sections linking the first and second sections. 
     The second sections can include at least one flange at an end of the compensation element, at which the compensation element has an enlarged diameter. At the other end there can be another flange. In the remaining first sections the compensation element can form a tight fit with the rotor core. The compensation element can be a metal and/or have at least approximately the same CTE as the rotor core. 
     In embodiments, the second sections include one or more support sections at one or more locations between second sections that are located at ends of the compensation element. 
     In embodiments, the second sections include a plurality of separate support sections. The separate support sections are separated by hollow spaces that can include air, a gas or another substance that is more compressible than the support sections. The separate support sections can be distributed along the length and around the circumference of the rotor core. Separate support sections can be ring-like, extending around the circumference of the rotor core, or linear, extending parallel to the axis, or running along the rotor core in a helicoidal pattern. Separate support section can be point-like, with support sections spaced from one another in the axial and the circumferential direction. 
     In embodiments, for each point in a compensating section, a line in the radial direction passes through a hollow space before reaching the rotor sleeve and also passes through a hollow space before reaching the rotor core. With this, the compensating sections can act as levers (when seen in a longitudinal and/or transverse cross section), being elastically bent when absorbing differences in thermal expansion of the rotor core and the rotor sleeve. 
     In embodiments, the compensating sections extend along at least one of the axial direction of the rotor and the circumferential direction of the rotor. This allows them to bend or yield in the radial direction. 
     Those second sections where the compensation element is attached to the rotor sleeve are preferably concentrated in one region as seen in the axial direction. In this manner rotor sleeve and compensation element at the remaining second sections are free to slide relative to one another in the axial direction. For example, if there are two flanges, one each at one of the two ends of the rotor, the sleeve is attached to one flange and is free to slide in the axial direction on the other flange and on any optional remaining second sections. 
     The same kind of distribution of attached and not attached sections can be realized for the first sections where the compensation element and the rotor core are in contact. 
     In embodiments, the compensation element is configured to be elongated in the axial direction, thereby reducing its outer diameter from being larger than an inner diameter of the rotor sleeve to being smaller than the inner diameter of the rotor sleeve. This allows the rotor sleeve to be assembled around the compensation element. This allows assembling the shaft by a method including the following steps:
         sliding the compensation element over the rotor core;   applying a force for elongating the compensation element in the axial direction, thereby reducing its outer diameter until the outer diameter is smaller than the inner diameter of the rotor sleeve;   sliding the rotor sleeve over the compensation element;   reducing the force elongating the compensation element, thereby increasing the outer diameter of the compensation element such that it pushes against the inner surface of the rotor sleeve, thereby centering the rotor sleeve on the compensation element.       

     The last step also establishes a force fit between the compensation element and the rotor sleeve. 
     The force for elongating the compensation element can be applied by means that are not part of the rotor or shaft and are removed after assembly. Alternatively, the force can be applied and optionally also controlled by elements that are part of the rotor or shaft when it is in operation. 
     In embodiments, the compensation element has the shape of a corrugated cylinder. 
     In embodiments, hollow spaces are formed between the compensation element and the rotor core, and/or between the compensation element and the rotor sleeve, and optionally the hollow spaces are ventilated by ventilation openings, the ventilation openings being, for example, holes in the compensation element. 
     This ensures that, when the motor is to be used in an environment with an explosive gas, air (containing oxygen and posing a risk when mixed with the explosive gas during operation of the motor) can be flushed out before the motor is taken into operation. 
     For high rotational speeds it is also possible to implement the contact between the rotor core and the compensator as a shrink fit. This allows to preload the rotor core material and compensate for stresses in the rotor core caused by centrifugal forces. 
     Instead of using a compensator in the form of a compensation sleeve to relieve the induced strains, a strain tolerant adhesive or molding material can be used. The clearance between rotor core and ceramic sleeve is filled with a material that compensates for the different strains of magnet and ceramic sleeve. The elasticity of the adhesive or molding material is chosen to be low enough to yield to strain differences of rotor core and ceramic sleeve, but still high enough in order to prevent mechanical resonance between the rotor core and the ceramic sleeve which could be excited by the rotational frequency. The compensation material can be silicone. 
     Regarding Young&#39;s modulus, achievable operating temperature and durability, filled silicone molds are suitable materials for the compensation material serving as a buffer between the magnet and the ceramic sleeve. 
     Thus, in embodiments, the compensation element includes a synthetic elastic compensation material, and in particular the compensation material can include a filler material for adjusting its Young&#39;s Modulus. 
     However, silicones have the drawback of being nearly incompressible (i.e. their Poisson ratio is close to 0.5). Therefore, high pressure is generated in the silicone between the magnet and the ceramic sleeve when the silicone expands at higher temperatures. Different measures can be taken to avoid this problem. 
     One approach is to include gas bubbles in the silicone which yields the compound a more compressible behavior. The resulting (filled) silicone foam may contain less than 1% or less than 10% or less than 30% of gas bubbles. To high gas content however will lower the overall elastic modulus of the compound such that rotor dynamics is impaired. A preferred method to fabricate such a rotor is to mold the magnet into the ceramic sleeve using a filled silicone mold in combination with a foaming additive to control the gas content in the resulting compound. Another method would be to coat the magnet first with the silicone and then shrink or press it into the ceramic sleeves. 
     Instead of tiny gas bubbles, the problem of low compressibility and high stresses in the ceramic rotor sleeve can be solved by a patterned or structured silicon layer containing 1% to 50% of empty space. As an example, the silicon layer can contain axial, ring or spiral shaped grooves, circular cutouts or be coated in lines, circles or arbitrary shaped spots. When the magnet and silicon layer expands with higher temperature, the silicone is allowed to expand into the nearby grooves rather than build up pressure against the ceramic sleeve and produce tensile stresses. The ratio of pattern width to layer thickness is in the range of 5:1 to 20:1. Thickness is typically 1/10 mm to 5/10 mm. Preferably, the silicone layer/structure is brought on the permanent magnet first and then the silicone coated magnet is shrunk or pressed into the ceramic sleeve. 
     Another option is to first apply a (filled) silicone coating on the permanent magnet and then roughen the coating&#39;s surface to bring in some empty space where the material is allowed to expand under increased temperature. 
     Thus, in embodiments, between the rotor core and the rotor sleeve, and adjacent to or enclosed by the compensation material, pockets of gas are present, increasing the compressibility of the compensation material. 
     Such pockets of gas can be formed by one or more of the following measures:
         by gas bubbles within the compensation material, or   by grooves in the inner and/or outer surface of the compensation material, or   by roughening of the inner and/or outer surface of the compensation material, or   by arranging the compensation material at disjoint locations between the rotor core and the rotor sleeve.       

     In embodiments, the gas bubbles are formed by expandable hollow microspheres, in particular thermoplastic expandable hollow microspheres. These can be thermally expandable. 
     In embodiments, a nominal diameter of the microspheres, when expanded, is larger than a nominal distance between the rotor core and the rotor sleeve. In other words, this nominal diameter is larger than the difference between the outer radius of the rotor core and the inner radius of the rotor sleeve. The rotor sleeve can be assembled on the rotor core by the steps of
         arranging the rotor core inside the rotor sleeve;   filling the gap between them with the compensation material including the microspheres; and   causing the microspheres to expand (e.g., by thermal activation) to their nominal diameter.       

     The last step causes the rotor sleeve to be centered on the rotor core, since all microspheres expand to the same nominal diameter, at least on average. 
     In other embodiments, the expandable hollow microspheres have a nominal diameter, when expanded, that is smaller than the above nominal distance. Then a particular microsphere does not touch both the rotor core and sleeve. 
     In embodiments, the compensation element includes compensation spheres of an elastic material, arranged and thereby elastically deformed between the rotor core and the rotor sleeve. The rotor sleeve can be assembled on the rotor core by the step of:
         sliding the rotor core inside the rotor sleeve while inserting the compensation spheres in the gap between them; and   thereby elastically compressing the compensation spheres.       

     This causes the rotor sleeve to be centered on the rotor core, assuming that all compensation spheres have the same diameter, at least on average. 
     In embodiments, the compensation element includes a plurality of elastic elements attached to the rotor core and deformable by sliding the rotor sleeve over the rotor core with the elastic elements, thereby aligning and centering the rotor sleeve with respect to the rotor core 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings, which show: 
         FIG. 1 a - b    longitudinal section views of prior art machines with gas bearings; 
         FIGS. 2 a - c    a first embodiment of an electrical machine, with variations; 
         FIG. 3  an alternative elastic suspension of one end of a bushing of the first embodiment; 
         FIG. 4  a second embodiment of an electrical machine; 
         FIG. 5  a partial longitudinal section of a rotor with a layer of compensation material; 
         FIGS. 6 a -7 g    partial longitudinal sections of rotors of a first group of embodiments; 
         FIG. 7 h    a transverse section of a rotor of the first group; 
         FIG. 8  a partial longitudinal section of a rotor with a layer of compensation material; 
         FIGS. 9-13  rotors with structured stress relieving adhesives or filling materials; 
         FIGS. 14 a - b    a rotor with a controllable compensation element. 
     
    
    
     In principle, identical parts or parts with an analogue function are provided with the same reference symbols in the figures. 
     DETAILED DESCRIPTION 
       FIG. 1 a    schematically shows a prior art electrical machine with air or gas bearings. Therein, a stator body (not shown) carries an electrical stator  3  with a coil  31  and a core  32  and further carries an axial bearing section  12  and a radial bearing section  17  in which a rotor  5  with a shaft  51  and a permanent magnet  52  is arranged to rotate. Individual bearing elements  19  constituting the radial bearing section  17  are arranged at opposite ends of the machine, with the electrical stator  3  in between.  FIG. 1 a    shows the same elements in a prior art overhanging arrangement, with the individual bearing elements  19  arranged at the same end of the machine, both at the same side of the electrical stator  3 . 
     Here and in the other arrangements, a fan  6  driven by the electrical machine operating as a motor is shown as an example for an application of the machine. Naturally, any other end device, in particular one requiring a high speed drive can arranged to be driven by the electrical machine. 
       FIG. 2 a    schematically shows a first embodiment, with the elements already mentioned, but in a different arrangement. In addition, the stator  1  is shown, including a stator body  25  supporting a carrier  4  elastically by means of elastic carrier supports  41  such as O-rings. The elastic carrier supports  41  serve to decouple the stator body  25  and the carrier  4  with respect to thermal distortion. Alternatively, the carrier  4  can be rigidly attached to the stator body  25 , or the two can be one and the same part. The carrier  4  carries the electrical stator  3 . The stator body  25  can be the housing of the machine or just part of it. 
     The parts of the axial bearing section  12  and the radial bearing section  17  that are attached to the stator as opposed to the rotor form a stator bearing structure. This stator bearing structure includes the stator-side bearing surfaces of the axial bearing section  12  and the radial bearing section  17  and defines the relative position of these surfaces. The stator bearing structure is designed to be rigid in itself and to be easily and reliably machined and assembled to high precision. 
     The axial bearing section  12  or thrust bearing section on the part of the rotor includes a generally disk-like thrust plate or rotor disc  54  extending outwardly from an outer peripheral surface of the shaft  51  near one end of the rotor  5 . The rotor disc  54  has two oppositely facing axially facing surfaces, which in cooperation with two stator discs  14 , between which the rotor disc  54  is arranged to rotate, form the axial bearing. 
     The axial bearing section  12  on the part of the stator includes these stator discs  14 , which are part of an axial bearing assembly  11 , which further includes a spacer element  15 , typically a washer, which defines a distance between axially facing surfaces of the of the stator discs  14  facing each other, and fasteners  16  that clamp the stator discs  14  and spacer element  15  together. The same fasteners  16  can also attach the axial bearing assembly  11  to the stator body  25 . 
     The radial bearing section  17  or journal bearing section on the part of the rotor includes at least part of the outer peripheral surface of the shaft  51 . This part functions as a rotor bearing surface  53 , which in cooperation with bearing elements  19  of a bushing  18  form the radial bearing. 
     The radial bearing section  17  on the part of the stator includes this bushing  18 . 
     In order to align the axis of rotation, which is defined by the bearing surfaces of the bushing&#39;s bearing elements  19  of the radial bearing section  17 , to be orthogonal to the bearing surfaces of the axial bearing section  12 , only a single pair of surfaces needs to be machined with high precision and mounted against one another: These are axial reference surfaces  13  located on an inner one of the stator discs  14  and on an axially facing surface of the bushing  18 . These axial reference surfaces  13  are machined to define a right angle between the axially facing bearing surfaces of the axial bearing section  12  and the axis of rotation. 
     The axial reference surface  13  of one (inner) stator disc  14  and the bushing  18  can be attached to each other by various means of fastening, including further fastening elements or welding, gluing etc. Alternatively, as shown in  FIG. 2 , they can be placed against one another without being directly attached, but rather being pressed against one another by an axial compensation element  20 , such as a plate spring (conical spring washer) or spring washer or an  0 -ring, arranged to press the bushing  18 , in particular a shoulder or flange of the bushing, in the axial direction away from the stator body  25  and against the stator disc  14 . The stator disc  14  in turn is held against the stator body  25  by the fastener  16  in a fixed manner. 
     Alternatively, as shown in  FIG. 2 b   , the stator discs  14  and the spacer element  15  can be clamped together by means of a mounting disc  14   a  and a plate spring  14   b  (or O-ring which acts as spring element). The plate spring  14   b  clamps the stator discs  14  against the stator body  25  and also must compensate the clamping force of the axial compensation element  20 . Thus, the plate spring  14   b  must be preloaded more than the axial compensation element  20  and is therefore generally larger and stiffer. 
     Another alternative is shown in  FIG. 2 c   . The stator discs  14 , the spacer element  15  and the shoulder of the bushing  18  are clamped against the stator body  25 . For robustness against possible tilting of the stator discs  14  relative to the bushing  18 , the diameter where the plate spring  14   b  is in contact with the (outer) stator disc  14  can be smaller than an outer diameter of the bushing  18 , in particular of a shoulder of the bushing  18  that abuts the other (inner) stator disc  14 . 
     At its other end, with respect to the axial bearing section  12 , the bushing  18  is supported by the stator body  25  by first elastic supports  21 , for example, O-rings. This dampens mechanical oscillations that might arise at the otherwise free end of the bushing  18 . A gap between the bushing  18  and the stator body  25  can be filled with thermally conducting filler  23 . This allows dissipating heat from the bushing  18  to the stator body  25 . 
     In the embodiment of  FIG. 2 , the first elastic supports  21  and the conducting filler  23  are arranged at the outside of the peripheral surface of the bushing  18 .  FIG. 3  schematically shows an alternative arrangement of these elements: here the stator body  25  includes or carries an end piece  24 . The end piece  24  projects into the inside of the bushing  18 . At least one first elastic support  21  and optionally the conducting filler  23  are arranged on the inside of the bushing  18  between the bushing  18  and the end piece  24 . 
       FIG. 4  schematically shows a second embodiment, with the elements already mentioned, but in a different arrangement. Again the axial bearing assembly  11  and the bushing  18  are in contact at the axial reference surfaces  13 . However, in this case it is not the axial bearing assembly  11  but rather the bushing  18  that is attached to the stator body  25  in a fixed manner. The axial bearing assembly  11  can be linked to the stator body  25  by means of optional second elastic supports  22 , for example, O-rings. Optionally, thermally conducting filler  23  is arranged in a gap between the second elastic supports  22 . If the bushing  18  and axial bearing assembly  11  are not joined by other means, the axial compensation element  20 , such as a plate spring, can be arranged to press the bushing  18  in the axial direction away from a projection  11  a of the axial bearing assembly  11  against the stator disc  14 . Alternatively, the arrangement of  FIG. 2 c    can be implemented in combination with the remaining elements of  FIG. 4 . 
     The embodiment of  FIG. 4  can be implemented in combination with a carrier  4  for the electrical stator  3 , supported by elastic carrier supports  41 , as in  FIG. 2 . In this case, the bushing  18  can be rigidly attached to the stator body  25 , with the carrier  4  and electrical stator  3  remaining movable relative to the bushing  18 , or the bushing  18  can be rigidly attached to the carrier  4  and thereby be in a fixed position relative to the electrical stator  3 . 
     In both cases, i.e. the bushing  18  being attached to the stator body  25  or the electrical stator  3 , the bushing  18  can be mounted by means embedding it, in particular with a thermally conductive molding material to obtain improved thermal coupling to the stator. 
     For the embodiments of both  FIG. 2  and  FIG. 4 , the following holds: 
     The bushing  18  lies within the magnetic (air) gap that separates the electrical stator  3  and the rotor  5 . Furthermore, the bearing elements  19  and cooperating rotor bearing surfaces  53  of the radial bearing section  17  lie completely or mostly within the volume through which the magnetic flux driving the motor passes. 
     The bearing air gap  7  lies between the bushing  18  and the rotor  5 . It is narrowest at the location of the bearing elements  19  and cooperating rotor bearing surfaces  53 , and can be wider at other locations in the axial direction in order to reduce friction losses. 
     The position of the bushing  18  relative to the axial bearing assembly  11  is rigidly constrained by only one mechanical link. This link is defined by the axial reference surfaces  13  on the bushing  18  and one of the stator discs  14 . During assembly, these surfaces can slide on one another. Afterwards, they are pressed together by the axial compensation element axial compensation element  20  and are in essence rigidly connected. The only other mechanical links between the bushing  18  and the axial bearing assembly  11 —via the stator body  25 —are elastic or resilient since they run
         via the axial compensation element  20  and the optional first elastic supports  21  ( FIGS. 2 and 3 ).   via the optional second elastic supports  22  ( FIG. 4 )       

     In this way, the relative position of these parts and in particular of the bushing  18  with respect to the stator discs  14  is not overdetermined. Thus, the precision of the alignment of the axial and radial bearing sections is easy to achieve, by precise machining of the axial reference surfaces  13 , and can be maintained under thermal and mechanical stress. 
     In other words, the axial bearing assembly  11  and bushing  18 —together forming the stator bearing structure—and the rotor  5  can part of one or more kinematic loops, where each loop includes at least one resilient element. Conversely, the axial bearing assembly  11  and bushing  18  are not part of an overconstrained loop or arrangement. 
     Furthermore, the location of the bushing  18 , and thus the axis of rotation, is constrained by the location of the stator body  25  in a fixed manner by not more than one mechanical link, that is
         via the attachment of the axial bearing assembly  11  to the stator body  25  ( FIGS. 2 and 3 ).   via the attachment of the bushing  18  to the stator body  25  ( FIG. 4 ).       

       FIG. 5  schematically shows a partial longitudinal section of a rotor  5  according to the prior art. The rotor  5  includes a rotor core  55  inside a rotor sleeve  56 . 
       FIGS. 6 a  through 6 c    schematically show embodiments with a mechanically resilient decoupling of the rotor core  55  from the rotor sleeve  56  by means of a compensation element  57 . The compensation element  57  absorbs differences in thermal expansion and allows to combine a rotor sleeve  56  with a relatively low coefficient of thermal expansion (CTE) with a rotor core  55  with a relatively high CTE. 
     The compensation element  57  can be made of a metal such as titanium or a titanium alloy, steel, a nickel alloy. Alternatively, it can be made of a synthetic material such as PEEK (Polyetheretherketone), PAI (Polyamide-imide, e.g. trademarked as Torlon), etc. 
     The compensation element  57  includes first sections  61  in contact with only the rotor core  55  and not the rotor sleeve  56 , and second sections  62  in contact with only the rotor sleeve  56  and not the rotor core  55 , and compensating sections  63  linking the first and second sections. Typically, there is a tight fit or pressure fit at the first sections  61  and/or at the second sections  62 . Alternatively or in addition, they may be glued. A hollow space  64  lies between the rotor sleeve  56  and the compensation element  57 . The hollow space  64  is ventilated by ventilation openings  60 . 
     In the embodiments of  FIGS. 6 a  and 6 b   , the second sections  62  are flanges at the two ends of the compensation element  57 , where the compensation element  57  has an enlarged diameter relative to the first section  61 . The embodiment of  FIG. 6 a    can be manufactured by a forming process. The embodiment of  FIG. 6 b    can be manufactured by a machining or cutting process. In the embodiment of  FIG. 6 c   , the compensation element  57  including the second sections  62  is of a substantially cylindrical shape, without flanges having an enlarged diameter. Instead, the rotor sleeve  56  at the two ends of the compensation element  57  has inwardly protruding elements that are in contact with the second sections  62 . At locations without such protruding elements, the hollow space  64  is arranged between the rotor core  55  and the rotor sleeve  56 . 
       FIGS. 7 a  through 7 g    schematically show embodiments with second sections that form a support section for the rotor sleeve  56 . Such support sections can be provided at one or more locations in one of the arrangement of  FIGS. 6 a  to 6 c   , or embodiments in which the rotor sleeve  56  is not supported at its ends.
         The support sections can be realized as a plurality of separate projections or bumps shaped in the compensation element  57 , or as one or more projecting ribs extending along at least part of the rotor core  55  ( FIG. 7 a   ).   The support sections can establish a distance corresponding to a hollow space  64  between the rotor sleeve  56  and the compensation element  57  by an outwardly projecting element of the compensation element  57  ( FIG. 7 b   ) and/or an inwardly projecting element of the rotor sleeve  56  ( FIGS. 7 c  and 7 d   ).   The support sections can establish a distance corresponding to a hollow space  64  between the compensation element  57  and the rotor core  55  by a cavity in the rotor core  55  ( FIG. 7 c   ) and/or a cavity in the compensation element  57  ( FIG. 7 d   ).       

     Whereas  FIGS. 7 b  through 7 d    show support sections with projections between the rotor sleeve  56  and the compensation element  57  and cavities between the compensation element  57  and the rotor core  55 , other embodiments have cavities between the rotor sleeve  56  and the compensation element  57  and projections between the compensation element  57  and the rotor core  55 . 
       FIGS. 7 e  through 7 h    schematically show arrangements in which the compensation element  57  includes several separate parts or compensation parts  57   a  arranged between the rotor core  55  and the rotor sleeve  56 . Each of these separate compensation parts  57   a  can correspond to one support section. The compensation parts  57   a  of  FIGS. 7 e  through 7 g    can be manufactured by molding, in particular injection molding. They can be manufactured from a synthetic material or from a metal material. The compensation parts  57   a  can be ring shaped, i.e. extend in a circular fashion around the rotor core  55 . They can be molded separately and then slid onto the rotor core  55 , or they can be molded in place on the rotor core  55 . This can result in the compensation parts  57   a  being stressed. Such stress can be mitigated by incorporating reinforcement rings  65  made, for example, of a metal, in particular titanium or steel, on the inside of the compensation parts  57   a  where they contact the rotor core  55 . This is shown in  FIG. 7   f.    
       FIG. 7 e    shows ring-shaped compensation parts  57   a  that can be manufactured with a simple two-part mold without undercuts, with the molds moving in the axial direction for removing the part after molding. As seen in the longitudinal cross section, the ring shape extends in the axial direction with an outer diameter that increases monotonously from the first section  61  to the second section  62 , and the outer diameter also increases monotonously from the first section  61  to the second section  62 . 
       FIG. 7 g    shows compensation parts  57   a  with a Y-shaped cross section. X-shaped compensation parts  57   a  are also possible.  FIG. 7 h    shows separate compensation parts  57   a  seen in the axial direction. The compensation parts  57   a  abut one another in the circumferential direction. Thereby they can provide good centering of the rotor sleeve  56  on the rotor core  55 . The compensation parts  57   a  can be manufactured by extrusion or by (injection) molding. The compensation parts  57   a  can extend in the axial direction, i.e. with the cross section of  FIG. 7 h    remaining unchanged at different points along the axis. Alternatively, the compensation parts  57   a  can be arranged in a helix configuration. In other embodiments, not shown, the compensation parts  57   a  are not separate but are manufactured as a single piece. 
     In further embodiments, not shown, projections and cavities are arranged on the rotor sleeve  56  and/or the rotor core  55  and/or the compensation element  57  and running in the axial direction, in analogy to the embodiments of  FIGS. 7 b  through 7 d    where they run in the circumferential direction. 
     In each embodiment corresponding to  FIGS. 6 a  through 6 c  and 7 a  through 7 h    the ribs or compensation parts  57   a  or, in general, the support sections can be separate from one another, and/or extend in one direction following a linear or circular or spiral trajectory. In each case, a hollow space  64  lies between the compensation element  57  and the rotor core  55  and/or between the compensation element  57  and the rotor sleeve  56 . In each case, ventilation openings ventilation opening  60  (not drawn in each case) can be present as well. 
     For all embodiments including features of  FIGS. 6 a  through 6 c  and 7 a  through 7 h    it is the case that the rotor core  55  and rotor sleeve  56  are radially decoupled. In other words, each line in the radial direction which passes through the rotor core  55  and the rotor sleeve  56  passes, in between the rotor core  55  and the rotor sleeve  56 , at least once through a hollow space  64 . 
       FIG. 7 a    also schematically shows an optional variant in which the rotor core  55  includes not solely the permanent magnet but the permanent magnet  52  arranged in a sheath  59  in order to maintain mechanical stability at high speeds. This variant can be combined with the embodiments of the other figures. 
       FIG. 8  schematically shows a decoupling by means of a layer of compensation material  58 . This can be an adhesive or filling material which accommodates the different CTE&#39;s of the rotor core  55  and rotor sleeve  56 , and thereby relieves corresponding stress. 
     The compensation material  58  can be silicone, to which filling materials can be added in order to determine its Young&#39;s modulus to a desired value. Such values can be 5 to 50 MPa. Filling materials can be ceramic particles with sizes of less than 50 micrometers. Unfilled silicone can have values around 1 MPa or 2 MPa to 4 MPa. 
       FIG. 9  shows a rotor construction with a structured stress relieving adhesive and/or filling as a compensation material  58 . Grooves in the compensation material  58  provide pockets of air and increase the compressibility of the body of the compensation material  58  as a whole, as opposed to the situation where the entire volume between the rotor core  55  and the rotor sleeve  56  is filled with the compensation material  58 . The grooves are shown to run in the axial direction. With grooves running circumferentially, and with the grooves extending all the way from the rotor core  55  to the rotor sleeve  57 , the embodiment corresponds to that of  FIG. 13 . 
       FIG. 10  shows rotor a construction with an elastic compensation material  58  including gas bubbles. Such gas bubbles can be formed, for example, by means of a foaming additive or by means of (thermally) expandable hollow microspheres  64   a.  The wall thickness of such microspheres typically is so small that their mechanical behavior is like that of gas bubbles. Here the gas bubbles or microspheres have diameters that are smaller than the distance between the rotor core  55  and the compensation element  57 . 
       FIG. 11  shows a rotor construction with an elastic compensation material  58  including expandable hollow microspheres  64   a  with a nominal diameter after expansion that is larger than the distance between the rotor core  55  and the compensation element  57 . This causes the rotor core  55  and the compensation element  57  to be automatically aligned and centered when the microspheres  64   a  expand. 
       FIG. 12  shows a rotor construction with compensation spheres  58   a  of an elastic material, arranged and thereby elastically deformed between the rotor core  55  and the rotor sleeve  56 . The compensation spheres can be inserted in the gap between the rotor core  55  and rotor sleeve  56  during or after assembly. The ability of the spheres to roll can be blocked by at least one of mechanically enclosing the spheres in the gap, heating them, coating the spheres with an adhesive before or during assembly and hardening or curing the adhesive after assembly. The hardening or curing can be effected by at least one of heat, irradiation, ultrasound, chemical activation, waiting a certain time, etc. 
       FIG. 13  illustrates an embodiment in which the compensation element  57  includes a plurality of elastic elements  58   b  attached to the rotor core  55 . An outer diameter of the elastic elements  58   b  is larger than the inner diameter of the rotor sleeve  56 . Sliding the rotor sleeve  56  over the rotor core  55  therefore deforms the elastic elements  58   b,  aligning and centering the rotor core  55 . 
       FIGS. 14 a - b    show a rotor with a controllable compensation element  57 . The compensation element  57  has the shape of a corrugated cylinder. Applying a force to the axial ends of the cylinder, pulling them apart, reduces the amplitude of the corrugation. In particular, the outer diameter is reduced, allowing to slide the rotor sleeve  56  over the compensation element  57 . In a relaxed state, the outer diameter of the compensation element  57  is larger than the inner diameter of the rotor sleeve  56 . 
     Pulling the ends apart can be done with means that are part of the shaft  51 , as shown in  FIG. 14 a   : one end of the compensation element  57  has a hook-like extension  57   c  that abuts a first end the rotor core  55  and limits movement of the compensation element  57  in one direction along the axis. The other end has elements for pulling the compensation element  57  in that direction by pushing against the other, second end of the rotor core  55 . These elements can be a screw  57   a  engaging threads in the compensation element  57  and pushing against the rotor core  55  via an axially resilient element  57   b  such as a spring washer or conical spring washer. By turning the screw  57   a,  the tension force pulling at the compensation element  57  can be adjusted, and thereby the outer diameter of the compensation element  57  as well. After reducing this outer diameter and sliding the rotor sleeve  56  over the compensation element  57 , the tension on the compensation element  57  can be reduced and adjusted by unscrewing the screw  57   a.  The screw  57   a  can be left at a certain position for setting a radial force between the compensation element  57  and the rotor sleeve  56 , or can be removed completely for maximum force. 
     Thus, pulling the ends apart can be done during assembly only. In the embodiment of  FIGS. 14 b    this is done with an attachment element  57   d , such as a hole, to which an element (not shown) for pulling at the compensation element  57  can be attached for the purpose of assembly. After assembly, the force acting between the compensation element  57  and the rotor sleeve  56  is a function of the dimensions and material properties of these elements. 
     The compensation element  57  can be made of a metal, in particular titanium or a titanium alloy, nonmagnetic steel, a nickel alloy, etc. Alternatively, it can be made of a synthetic material or plastic.