Electromagnetic bearing arrangement

Abstract: An electromagnetic axial shaft bearing (50) (Fig 2) reduces rotational stresses on a ferromagnetic rotor (14') by reducing its radius and displacing inwardly the radially outer, axially extending part (51) of the stator core (15) containing an axial groove (18) and electromagnet coil (19) so that the outer stator pole (22) overlies the stator electromagnet coil.

Description: This invention relates to electromagnetic bearing arrangements for supporting a rotatable shaft against thrust forces acting on that shaft along its longitudinal axis. In such an electromagnetic axial, or thrust, bearing a ferromagnetic disc fixed to, and extending radially of, the shaft comprises a rotor. Disposed adjacent at least one face of the rotor is a stator comprising one or more electromagnets which attract the rotor with an axial force that counters and balances an external thrust load on the shaft in response to detection of displacement of the shaft by the external thrust load. If such external loads occur in one direction only then only one electromagnet is necessary whereas if such external thrust loads occur in both directions an electromagnet is provided each side of the rotor. The magnitude of external axial load to be balanced determines the radial dimensions of the rotor and stator. In a typical electromagnetic axial bearing, shown in sectional elevation in Figure 1 at 10 in relation to shaft 11, each stator 12 associated with a radially extending face 13 of the rotor 14 comprises an electromagnet having an annular core 15 extending around the shaft and in the surface thereof facing the rotor intermediate its radially inner and outer ends 16, 17 respectively an annular groove 18 containing an annular electromagnet coil 19. The axially extending portions 20, 21 of the stator core radially separated from each other by the groove provide stator poles 22, 23 respectively for the electromagnet in facing relationship with the rotor face 13. A further stator shown ghosted at 12' may be disposed adjacent the other face of the rotor 14 to accommodate shaft thrust forces acting in the opposite direction. The axial force exerted on the rotor by the electromagnet in countering an axial load on the shaft is a function of the magnetic flux flowing through the magnetic circuit defined by the stator core and rotor. Such flux is maximised by having a minimal range of gaps between the stator poles and the rotor, by the areas of pole faces, materials of the core and rotor and by the electromagnet coil. It is usual for the rotor to comprise a soft ferromagnetic material but problems arise in respect of very high speed shaft rotation with respect to its mechanical strength in accommodating centrifugal stresses put upon it. Such stresses are a function of radial distance from the rotational axis of the shaft so that it has been proposed to reduce the effective radius of the rotor. In the arrangement 10 as shown, the rotor radius is limited by the radial dimensions required of the stator pole faces 22 and 23 and also by the radial dimensions of the groove 18 in order to accommodate an electromagnet coil having sufficient number of turns and current capacity. To overcome such limitations on rotor radius imposed by the stator it has been proposed to reduce the axial thickness of the rotor towards its radially outer end in steps or as a taper or both, as described in GB patent specification 2,219,357 and EP-A-0411696, which shows an electromagnetic axial shaft bearing according to the preamble of claim 1. Whereas such arrangements address the problem of rotor stresses by removing mass from its periphery the rotor itself is not reduced in radius and more complex in manufacture. It is an object of the present invention to provide an electromagnetic axial shaft bearing arrangement which permits in simple manner minimisation of centrifugal rotor stresses. According to the present invention an electromagnetic axial shaft bearing arrangement comprises a rotor of ferromagnetic material carried by, and extending radially of, the shaft for rotation therewith, and disposed adjacent at least one axial face of the rotor a stator comprising an electromagnet having an annular core extending around the shaft and in the surface thereof facing the rotor intermediate its radially inner and outer ends at least one annular groove containing an annular electromagnet coil, the axially extending portions of the stator radially separated by each groove providing stator poles for the electromagnet, the radially inner axially extending portion of the stator extending parallel to the shaft to define an electromagnet pole face adjacent the shaft and aligned with the radially inner part of the rotor face, said radially outer, axially extending portion of the stator being also directed radially inwardly to define an electromagnet pole face aligned with the radially outer part of the rotor face and which in a radial direction at least partially overlies the associated electromagnet coil, and the outer diameter of the rotor being smaller than the outer diameter of the stator. Conveniently the radially outer, axially extending portion of the stator comprises a discrete annular stator pole member secured to the radially outer, axially extending part of the stator to form an axial continuation thereof. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1, referred to hereinbefore, is a sectional elevation through a known form of electromagnetic axial shaft bearing arrangement, Figure 2 is a sectional elevation through an electromagnetic axial shaft bearing arrangement in accordance with a first embodiment of the present invention, having a rotor of uniform thickness and cranked stator pole member, Figure 3 is a sectional elevation through an arrangement similar to Figure 2 comprising a second embodiment of the present invention, having a rotor of uniform thickness and an inclined stator pole member, Figure 4 is a sectional elevation through an arrangement similar to Figure 2 comprising a third embodiment of the present invention, having a cranked stator pole member and a rotor having an outer radial part of uniform reduced thickness, Figure 5 is a sectional elevation through an arrangement similar to Figure 4 comprising a fourth embodiment of the present invention having a tapered outer radial part of the rotor, and Figure 6 is a sectional elevation through an arrangement comprising a fifth embodiment of the present invention having a tapered outer radial part of the rotor and an inclined stator pole member. Referring to Figure 2, an electromagnetic axial shaft bearing arrangement 50 in accordance with a first embodiment of the present invention is generally similar to the prior art arrangement 10 shown in Figure 1 and like parts are given like reference numerals. The arrangement differs in that the ferromagnetic rotor, 14' is radially shorter than the rotor 14 and the radially outer, axially-extending stator limb 20' of core 15 is axially shorter than limb 20. The limb 20' carries, as an axially-extending continuation, a discrete annular stator pole member 51 secured to the limb and forming part of the magnetic circuit. The member 51 is directed also radially inwardly, that is, is cranked to include a radially inwardly extending portion 52 intermediate axially extending portion 53 that forms the continuation of limb 20 and axially extending portion 54, and terminates in a radially outer stator pole face 22' that is aligned with the radially outer part of the smaller rotor face 13 and, in a radial direction, overlies the electromagnet coil 19. The arrangement thus permits of a reduction in the radius of the rotor to one that is no longer dependant upon the dimension, in a radial direction, of the electromagnet coil. Thus the coil may even be longer in the radial direction to increase the level of magnetic flux for a given current density in the coil. This may also help to compensate for any loss of efficiency due to the shape or junction with the stator pole member. It may also help to permit a reduction in axial coil dimension to accommodate the stator pole member. It will be appreciated that use of such a discrete stator pole member permits the electromagnet to be readily wound or otherwise disposed in the stator groove 18 before the member is secured to the core. However, there is nothing in principle to prevent the stator pole member being put in place prior to winding a coil in groove 18, subject to any practical difficulties in so winding a coil efficiently. In such event, it will be appreciated that the radially outer, axially extending portion of the stator may be an integral part of the core and not a separate stator pole member. Furthermore the radially outer, axially extending portion of the stator may take a form other than the cranked member described above which in integral form or as a discrete member may ease any task of subsequently winding an electromagnet coil. Referring to Figure 3, this shows in sectional elevation a further embodiment of axial shaft bearing arrangement 60, which is similar to the arrangement 50 of Figure 2 with like reference numerals for like parts. In this arrangement 60 a stator pole member 61 is secured to the end of shortened stator limb 20', after disposition of the coil 19, taking the form of a frustoconical annular member whose axial ends comprise radially extending faces 62, 63, one of which is secured to limb 20' and the other of which comprises radially outer stator pole face 22' which is disposed facing the radially outer part of the rotor and overlies the electromagnet coil 19. Such form of radially outer, axially extending portion of the stator may alternatively be integral with the stator core and more readily permits subsequent winding of the electromagnet coil than the arrangement 50, although is not as compact in an axial direction. It will be understood that the interface between the core limb 20' and a discrete member 61 may be other than in a radial plane as at 62 as long as flux is coupled efficiently; for example, the end of the limb may be frusto-conical and the end face of the member formed onthogonally to its sides as shown by broken line 64. The bearing arrangement of the present invention enables a bearing rotor of smaller diameter than hitherto to be used. However this arrangement may be used in combination with the proposals of the above-mentioned patent specifications, that is, in combination with a rotor having a radial outer portion of reduced axial thickness. Referring to Figure 4, which is similar to the arrangement of Figure 3 and uses like reference numerals for corresponding parts, a third embodiment 70 has the radially outer portion of the reduced radial rotor 14'' also of reduced thickness at 14 1 ''. The cranked stator pole member 71 has one axially extending limb 72 extending axially further than the radially inner stator pole 23 and provides radially outer stator pole 22" adjacent the reduced thickness rotor portion 14 1 '', the reduction in rotor dimensions at its radial extremity providing maximal reduction in inertia and centrifugal stresses. Referring to Figure 5, in an arrangement 80 alternative to the form shown in Figure 4, the cranked stator pole member 81 has a pole face portion 82 that is inclined to the radial direction and aligned with a conically tapered peripheral edge 14 1 ''' of the reduced radius rotor 14'''. Figure 6 shows a fifth embodiment 90 which combines the tapered rotor structure of Figure 5 with the inclined radially outer, axially extending portion 61 of the core of Figure 3. In any of the above, it will be appreciated that the radially outer, axially extending portion of the core may be provided by a discrete stator pole member or be formed integrally with the core. It will also be appreciated that other configurations may be defined within the scope of the claims. The axial shaft bearing arrangement described hereinbefore have all comprised stator cores of 'C'-shaped section containing a single annular electromagnet coil. It will be appreciated that the invention may be applied with no change with stator cores of 'E'-shaped section employing two electromagnet cores intermediate three stator poles faces. Although the axial shaft bearings described in accordance with the present invention have comprised a single stator arrangement to counter axial shaft thrust forces in one direction it will be appreciated that to accommodate forces in both axial directions a second stator arrangement may be disposed adjacent the other radially extending face of the rotor.

Claims: An electromagnetic axial shaft bearing arrangement (50,60,70,80,90) comprising a rotor (14',14'',14''') ferromagnetic material carried by, and extending radially of, the shaft (11) for rotation therewith, and disposed adjacent at least one axial face (13) of the rotor a stator (12) comprising an electromagnet having an annular core (15) extending around the shaft and in the surface thereof facing the rotor intermediate its radially inner and outer ends (16,17) at least one annular groove (18) containing an annular electromagnet coil (19), the axially extending portions (20',21) of the stator radially separated by each groove providing stator poles (22',23,22'',82) for the electromagnet and the radially inner axially extending portion of the stator (21) extending parallel to the shaft to define an electromagnet pole face (23) adjacent the shaft and aligned with the radially inner part of the rotor face, characterised by said radially outer, axially extending portion of the stator (51,61,71,81) being also directed radially inwardly to define an electromagnet pole face (22',22'',82) aligned with the radially outer part of the rotor face and which in a radial direction at least partially overlies the associated electromagnet coil, and by the outer diameter of the rotor being smaller than the outer diameter of the stator. An electromagnetic axial shaft bearing arrangement as claimed in claim 1 in which said radially outer, axially extending portion of the stator comprises a discrete annular stator pole member (51,61,71,81) secured to the radially outer, axially extending part of the stator (20') to form an axial continuation thereof, and characterised in that said stator pole member is adapted to be secured to the radially outer, axially extending part of the stator subsequently to disposition of the electromagnet coil in the associated groove. An electromagnetic axial shaft bearing arrangement as claimed in claims 1 or claim 2 in which the rotor (14",14"') is axially thinner at the radially outer end (14 1 ",14 1 "') associated with said radially outer stator pole (22'',82) than radially inwardly and associated with any other stator pole and said radially outer, axially extending portion is adapted to provide said radially outer stator pole face axially displaced with respect to each other stator pole face (23), and characterised in that the stator pole face (22',22'') provided by said radially outer, axially extending portion extends substantially radially and parallel to each other stator pole face radially inwardly thereof.

Inventors: NEW NIGEL HENRY, NEW, NIGEL HENRY
Applicants: GLACIER METAL CO LTD, THE GLACIER METAL COMPANY LIMITED
Application year: 1993
Main classification: F16C39
IPC codes: F16C39, F16C32