GB2066380A

GB2066380A - Magnet assembly for support of a rotor

Info

Publication number: GB2066380A
Application number: GB8040819A
Authority: GB
Original assignee: Ultra Centrifuge Nederland NV
Current assignee: Ultra Centrifuge Nederland NV
Priority date: 1979-12-19
Filing date: 1980-12-19
Publication date: 1981-07-08
Also published as: AU6541380A; NL189929C; DE3048071A1; SE442660B; AU544924B2; SE8008897L; JPS56130903A; GB2066380B; NL7909129A; NL189929B; DE3048071C2

Abstract

An elongated rotor 16 supported by a thrust bearing at its bottom end, is supported at the top end by coaxial tubular magnets 11 and 15. The magnet 15 is of shorter axial length and is mounted on the rotor. The inner magnet 11, poled in the same direction, is adhered or bonded to a magnet holder 12 mounted for damped radial movement by means of resilient rods 13 attached to the rotor housing. Axial relative movement of the magnets resulting from transverse forces acting on the rotor when rotating causes changes in radial stiffness and in axial forces due in each case to the magnetic interaction. The axial force can be tensile when the rotor is stationary and progressively change to a pressure against the thrust bearing as the rotor runs up to speed. Composite magnet construction, special shapes and forms of magnetization and magnetic screening can be used to control the magnetic interactions. <IMAGE>

Description

SPECIFICATION Annular magnet assembly for the support of a rotor The present invention relates to an annular magnet assembly for the support of a rotor.

U.K. Patent Specification No. 1,341,581 described and claimed an annular magnet assembly consisting of a number of tubular permanent magnets which are assembled coaxially one within the other in such a manner that similar poles of the permanent magnets all appear at the same end face of the assembly, characterised in that at least one of the tubular magnets is so arranged as to be rotatable relative to the other tubular magnet or magnets about its principal axis. As described in that specification a lubricant may be introduced between the cooperating cylindrical faces of the magnets or with a larger clearance between the faces a ball bearing may be introduced between them. The end surface of the tubular magnets may be curved or dentilculated or otherwise deviate from a flat form.

In accordance with the present invention there is provided an annular magnet assembly for the support of a rotor comprising at least two tubular magnets fitted one within the other with sufficient clearance to allow relative rotation, a first magnet being attached to the rotor and a second to a nonrotating support, each of the magnets having axially-separated poles with similar poles of the magnets pointing in the same direction and one magnet being of greater axial length than the other.

While such an assembly may be used with a bearing between the relatively-rotatable magnets, the axial magnetic forces between the magnets serving to minimize the effect of axial forces on the bearing, the assembly is especially useful for the support of long rotors. As has been explained in the introduction to the U.K. Patent Application 2,01 6,609A, long rotors tend to become shorter when rotating at high speed, in consequence of the axial contraction caused by the tangential stress in the parts of the rotor. If the magnet assembly is such that the magnet rotating with the rotor can move axially with respect to the nonrotating magnet then such relative axial displacement will result in a change in the radial stiffness and in the axial magnetic forces.The radial stiffness is measured by the change per unit of radial displacement in the radial force exerted on each other by the rotating and stationary magnets.

The magnet which is of greater axial length than the other may be constructed from a number of annular magnet elements placed axially against each other and having unlike magnetic poles resting against each other. For ease of assembly and to facilitate axial relative displacement it is advisable to make the annular magnet elements of the same diameter on the surface which faces the other tubular magnet. The diameters of the free surfaces of the annular magnet elements may be of unequal magnitudes in order to influence the characteristics of the magnet system as a function of axial displacement.

In many cases it will be preferable to fit the shorter magnet or magnets to the rotor. The stationary (non-rotating) magnet(s) can be connected with the support in a known manner which permits damped radial movement against elastic restoring forces. The longer, stationary magnet may for example be bonded or adhered to a magnet-holder which is elastically suspended from a stationary rotor housing.

At least the shorter magnet is preferably manufactured from a cobalt-samarium alloy.

The tubular magnets may have at least one end surface which deviates from a plane perpendicular to the axis of rotation. The end surface may be curved or dented, contain cavities or deviate in any other way from the simple planar form. These differences in form many influence the magnetic characteristics.

A further method of producing the desired magnetic characteristics is to arrange the magnetic poles of the tubular magnets, and especially those of the longer magnet, to produce particular magnetic fields inside and outside the magnets. The poles of different polarity may be arranged along two circles coaxial with the magnet. In one case these circles are at opposite ends of the magnet and on opposite surfaces with the magnetic field running obliquely across the cross-section of the magnet. In another case the circles are on the same cylindrical surface of the magnet and the path of the strongest magnetic flux across the cross-section of the magnet is curved or bowed. In this case the pole circles may be displaced inwards from the ends of the magnet while remaining near the same cylindrical surface.

For more precise control of the stray field the free cylindrical surface of each magneti, or at least of the rotating magnet, is screened with a non-magnetic material such as aluminium. Such screening also enables the characteristics of the magnet assembly to be influenced in a desired direction.

In a particular embodiment of the invention the tubular magnets which are relatively rotatable and axially displaceable are so arranged that the axial magnetic force present when the rotor is stationary is converted into an oppositely-directed force as a result of axial displacement of the magnets when the rotor is brought up to operating speed. For example the arrangement may be such, especially when the rotor is supported at the opposite end by a thrust bearing, that the rotor is subject to an axial tensile force when stationary but to an axial pressure which presses it against the thrust bearing when at operating speed.

The invention will now be described in more detail with the aid of examples illustrated in the accompanying drawings, in which: - Fig. 1 is a top plan view of an annular magnet assembly as described in U.K. Patent Specification No. 1,341,581, Fig. 2 is a vertical cross-section on the line Il-Il of the assembly of Fig. 1, Fig. 3 is a vertical cross-section of a variant of the assembly of Fig. 1, Fig. 4 is a top-plan view of an annular magnet assembly with a clearly-indicated gap between the tubular magnets, Fig. 5 is a vertical cross-section on the line V-V of the assembly of Fig. 4, Fig. 6 is a vertical cross-section of an assembly in accordance with the invention having an inner tubular magnet composed of two distinct magnet elements, Fig. 7 is a graph showing the relationship of stiffness S and axial displacement x for a rotary magnet assembly such as shown in Fig. 6, Fig. 8 is a graph of the relationship between axial force P exerted by one magnet on the other and axial displacement x for an assembly such as shown in Fig. 6, Fig. 9 is a vertical cross-section in schematic form of an annular magnet assembly in accordance with the invention, Fig. 10 is a cross-section of a cylindrical magnet, composed from a number of ringmagnets, Fig. 11 is a cross-section of a cylindrical magnet with magnetisation in the direction of the cylinder axis, Fig. 12 is the same, with magnetisation according to a cone that is coaxial with the cylinder axis, Fig. 13 is the same, with a direction of the magnetisation that in a cross-section is curved, Fig. 14 shows a variant of Figure 13, and Fig. 1 5 shows an assembled ring-magnet.

Fig. 1 represents two magnetic rings 1 and 2 fitting into each other with but little clearance. Fig.

2 shows, in a vertical cross-section of the same rings, that similar magnetic poles point in the same direction. At 3 a clearance space is provided ;nto which a lubricant may be introduced. Fig. 3 shows a set of magnetic rings 4 and 5 fitting into each other in a similar way as in Fig. 2, whilst at the same time a ball bearing 7 is incorporated in the magnetic rings.

In Fig. 4 a top-plan view of an annular magnet-assembly is given, with an appreciable clearance between the rings 1 and 2. A vertical cross-section of this is shown in Fig. 5.

Fig. 6 shows, that the inner tubular magnet 8 may be composed of two distinct cylinders 9 and 10 that are placed with the unlike poles on each other.

Fig. 7 shows, how the stiffness S varies with axial displacement of the shorter tubular magnet 1 in Fig. 6 relative to the longer tubular magnet 8.

From a mid-position indicated with the number 0, both a movement in the one direction and one in the other direction shows an augmentation of the stiffness. This means that, for example, the north pole of the shorter magnet approaches more closely the north pole of the longer magnet, causing this to undergo a greater repulsive force, with a result that the stiffness, being the reaction of the magnet-system on transverse loads, becomes greater. The same occurs if the shorter magnet 1 should displace itself downwards in Fig. 6, so that also there the similar south poles would be placed opposite or nearer each other, with likewise an augmentation of the transverse stiffness. During the stoppage of the rotor the mid cross-section of the shorter magnet is located at the point a, to be shifted gradually to the right as the rotor is speeding up.At the normal running speed the mid cross-section will have reached b, with the result that at the lower running speeds as well as at the normal speed the highest transverse stiffness may be developed.

In Figure 8 it is shown, that by means of an analogous displacement X along the axis of rotation of the shorter magnet cylinder 1 of Fig. 6, an axial force P likewise undergoes a change. If the shorter magnet is located in the midst of the longer magnet, causing the two outermost south poles to be just as far from each other as the two outermost north poles, there is no resultant axial force. As soon, however, as the shorter magnet cylinder 1 moves somewhat higher, the repelling force between the north poles becomes predominant and a downward directed resulting force develops. The same happens in an analogous way if the shorter magnet cylinder 1 is moved downwards, because in tilis case a resulting upwards directed force manifests itself, becoming steadily greater, as Fig. 8 shows, as the displacement from the mid position increases.

With stoppage of the rotor the mid cross-section of the shorter magnet is located at C, to be shifted gradually to the right. At the normal running speed point d is reached. The direction of the axial force is therefore by degrees reversed.

Figure 9 shows how the longer magnet 11 which is to act as the stationary magnet, is fixed, for example by means of a sticky substance or a glue, against a magnetholder 12 which is suspended with a number of thin rods 13 from a part 14 of the housing of the installation. The rotating magnet 1 5 is firmly attached to the schematically indicated rotor 1 6. This rotor may be sustained by a bearing, for example in the manner that is shown in Fig. 1 of U.K. patent specification No. 1,544,489.

The rods 13 may be executed as blade or plate springs. These rods or springs will have to take also a vertically directed force.

If so required at the other end of the magnetholder also thin rods (indicated by the dotted lines 17) may be attached.

With 1 8 is indicated a magnetic screening, for example of aluminium. Ho!der 12 may also be manufactured of a non-magnetic material.

Figure 10 shows how the magnetic cylinder 1 5 of Figure 9 may also be executed differently, namely as a number of magnet rings that are stacked together with the dissimilar poles adjacent, the rings having unequal inner diameters.

Figure 11 shows a cross-section of a magnet cylinder that has been magnetized in vertical direction, whereby the strongest poles are located at the middle of the end surfaces.

In Figure 12 the strongest poles are located at the corners 24 and 25 of the cross-section.

Figure 1 3 shows a magnet with the strongest poles at 26 and 27.

Figure 14 shows a magnet with the strongest magnetpoles at 28 and 29.

Figure 15 shows an embodiment of Figure 14, wherein the magnet is composed of two cylinders 30 and 31 and two end-rings 32 and 33. The parts 34 and 35 are made of material with magnetic screening properties.

A remarkable embodiment is obtained, if the magnets used in the configuration shown in Figure 6 are made so strong, that after the assembling of the outer and inner rings, the rings are magnetically locked and the inner ring is unable to fall through the held outer ring, because the effect of gravity on the inner ring is completely countered by the magnetic forces. In this way a floating magnetic bearing results, that is extremely useful for minimizing bearing losses and therefor is capable of saving a great deal of energy, now lost in all kinds of fluid lubricated bearings, or bearings of the ball bearing type.

For this type of floating magnetic bearing the magnet flux distribution used is preferably the flux distribution shown in Figure 13 for the longer magnet and the flux distribution shown in Figure 14 for the shorter magnet, the cylindrical surfaces of the magnetic rings that contain the magnetic poles being situated on opposite sides of the air gap. Nevertheless it is possible to use for example the flux pattern shown in Figure 12 for two annular rings, fitted together like the rings 9 and 10 in Figure 6, the upper ring 9 having the same flux orientation as Figure 1 2 and ring 10 having the flux directed from the left bottom corner to the right upper corner, resulting in an approximation of the flux pattern of Figure 13.

Other floating bearing flux configurations will be possible, without departing from the invention idea. In all these cases it is necessary to have a radially symmetric flux distribution in the annular rings that is as accurate as possible, to ensure a perfect centric alignment of the floating part.

Claims

1. An annular magnet assembly for the support of a rotor comprising at least two tubular magnets fitted one within the other with sufficient clearance to allow relative rotation a first magnet being attached to the rotor and a second to a nonrotating support, each of the magnets having axially-separated poles with similar poles of the magnets pointing in the same direction and one magnet being of greater axial length than the other.

2. A magnet assembly as claimed in claim 1 in which the magnet of greater axial length is constructed from a number of annular magnet elements placed axially against each other and having unlike magnetic poles resting against each other.

3. A magnet assembly as claimed in claim 2 in which the annular magnet elements are of the same diameter on the surface facing the other tubular magnet.

4. A magnet assembly as claimed in claim 4 in which the annular magnet elements are of different diameters on the surface remote from the other tubular magnet to produce a desired magnetic field system.

5. A magnet assembly as claimed in claim 2, 3 or 4 in which the end surface of at least one of the magnet elements deviates from a plane perpendicular to the axis of rotation.

6. A magnet assembly as claimed in any of the preceding claims in which the shorter magnet is attached to the rotor.

7. A magnet assembly as claimed in claim 6 in which the longer magnet is connected to a stationary support in such a manner that it can execute damped radial movements against an elastic restoring force.

8. A magnet assembly as claimed in claim 4 in which the longer magnet is adhered or bonded to an elastically-suspended magnet holder.

9. A magnet assembly as claimed in any of the preceding claims in which one of the tubular magnets has its magnetic poles of opposite polarity arranged along two circles coaxial with the magnet.

10. A magnet assembly as claimed in claim 9 in which the said circles are on the same cylindrical surface of the magnet.

1 A magnet assembly as claimed in claim 10 in which the circles are displaced inwards from the opposite ends of the magnet.

12. A magnet assembly as claimed in claim 9, 10 or 11 in which it is the longer magnet which has its magnetic poles arranged as defined.

13. A magnet assembly as claimed in any of the preceding claims in which the free cylindrical surface of at least the rotating magnet is screened with non-magnetic material.

14. A magnet assembly as claimed in any of the preceding claims in which the magnetic forces between the two tubular magnets are sufficient to form a free-floating magnetic bearing for the rotor.

15. A magnet assembly as claimed in any of claims 1 to 13 in which the two tubular magnets are so positioned that when the rotor is not rotating the axial magnetic force is in one direction but as a result of axial displacement as the rotor speed is increased the axial force is reversed at normal running speeds.

16. A magnetic assembly as claimed in claim 1 5 in which the rotor is supported at its other end by a thrust bearing and the magnets are arranged to exert an axial tensile force when the rotor is stationary and an axial pressure to force the rotor against the thrust bearing at normal running speeds.

17. A magnet assembly as claimed in any of the preceding claims in which the rotor is of elongated form.

1 8. A magnet assembly as claimed in claim 1 7 in which the rotor has a length of at least 10 times the rotor diameter.

19. A magnet assembly as claimed in any of the preceding claims in which at least the shorter tubular magnet is made of a cobalt-samarium alloy.