GB1574603A - Magnetic bearing assemblies - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO MAGNETIC BEARING ASSEMBLIES (71) We, HAWKER SIDDELEY DYNAMICS LIMITED, a British Company, of Manor Road, Hatfield, HertfordThire AL10 9LL, England, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following state- ment: This invention relates to apparatus utilising a magnetic bearing or bearings as a means of supporting a rotating mass and is particularly concerned with the provision of means whereby the rotating mass and its support structure are protected from the effects of the flexibility of the component parts of the system and vibration experienced in use of the apparatus.

There are special problems of mounting introduced when a rotating mass is supported by a magnetic bearing system. These systems are fundamentally different from conventional bearings. The rotor position is not fixed relatively to the stator but is defined only by a magnetic spring. If the stator is rigidly held any radial vibration of the rotor can be controlled by incorporating sufficient radial damping. In an actual bearing the stator is not rigid but will have flexibility both within the bearing and also in the total system support structure. Both the rotor and the stator can be excited into vibration by oscillating forces. These forces can arise external to the bearing system or they can be generated by the rotor itself (e.g. out-ofbalance forces) and transmitted to the stator via the magnetic spring. The resulting vibrations of the rotor or the stator do not have to be of a very large amplitude before the necessarily small clearance between the rotor and stator will be lost and contact will occur.

This is especially likely to occur at the natural frequency of the magnetic spring mass system.

It is an object of the invention to overcome this problem.

According to the invention, there is provided an assembly comprising a rotor mounted on a stator by means of magnetic bearings, and a support structure supporting the stator, wherein the lowest natural frequency of the stator is made higher than the highest designed operating speed of the rotor, the lowest natural frequency of the support structure bearing the rotor and stator assembly is made higher than the highest designed operating speed of the rotor, and motion damping means is included such as to ensure that relative movement of the rotor and stator at right angles to the axis of rotation is less than the radial clearance between the rotor and stator, said motion damping means being inserted into the load path that runs from bearing magnets on the stator to the support structure. The damping may be inserted into the load path that runs through the following elements: (i) the stator magnets ifii) the magnet housings (iii) the stator magnet support (iv),the stator assembly base (v) the external support structure Arrangements according to the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic longitudinal section through a magnetic wheel type bearing assembly.

Figure 2 is a similar view of another construction of bearing assembly.

In Figure 1 of the drawings, a rotor wheel 7 rotates around a stator comprising a central support shaft 3 and stator magnets 1 carried in two rings on the support in magnet housings 2 opposite to two corresponding rings of inward-facing magnets 11 on the rotor. The stator support shaft 3, and oonsequently the whole wheel assembly, is mounted on a base 4 supported on supports 5.

According to the invention, the lowest natural frequency of the stator assembly 1, 2, 3, 4 is made higher than the highest designed operating speed of the wheel.

Secondly, the lowest natural frequency of the support structure 5 with the rotor and stator assembly on it is made higher than the highest designed operating speed of the wheel. Ways of raising or lowering the natural frequencies of a mechanical system are well known to engineers and need not be discussed here. Thirdly, load path motion damping 6 is inserted between the supports 5 and the assembly base 4.

Thus in the arrangement described, for proper functioning of such system employing magnetic bearing suspension and to ensure that the rotor remains free of contact with the stator for all operating speeds, the three measures detailed above are simultaneously incorporated.

In an alternative arrangement shown in Figure 2, the damping is incorporated into the stator shaft. This is accomplished by making the stator shaft 3 a composite member having a non-magnetic rigid core 8 and an outer sleeve 9 around the core of a selfdamping material, e.g. carbon-fibre reinforced material, on which the magnets 1 are mounted, the outer sleeve 9 being secured to the shaft core 8 by an intervening layer 10 of material having intrinsic damping qualities, e.g. silicon rubber. The lower end of the outer sleeve 9 is anchored in the base 4.

When any disturbing force aots upon the rotor its resulting radial movement exerts a force on the stator magnets 1 and the carbon fibre support shaft sleeve 9 then acts as though cantilevered from a point at the bottom end of the shaft, thereby squashing the intervening layer 10 of the composite shaft with consequent damping of the movement.

WHAT WE CLAIM IS:- 1. An assembly comprising a rotor mounted on a stator by means of magnetic bearings, and a support structure supporting the stator, wherein the lowest natural frequency of the stator is made higher than the highest designed operating speed of the rotor, the lowest natural frequency of the support structure bearing the rotor and stator assembly is made higher than the highest designed operating speed of the rotor, and motion damping means is included such as to ensure that relative movement of the rotor and stator at right angles to the axis of rotation is less than the radial clearance between the rotor and stator, said motion damping means being inserted into the load path that runs from bearing magnets on the stator to the support structure.

2. An assembly according to claim 1, wherein the stator comprises a central support shaft carrying bearing magnets, and a base carrying the support shaft, and the damping means is inserted between said base and the said support structure or between the stator bearing magnets and the central support shaft.

3. An assembly according to claim 1, wherein the stator comprises a central support shaft carrying bearing magnets, and the damping means is incorporated in the support shaft.

4. An assembly according to claim 3, wherein the support shaft comprises a rigid central core, a surrounding sleeve of a selfdamping material on which the stator magnets are mounted, and an interveniing layer of a resilient material with damping properties.

5. An assembly comprising a rotor mounted on a stator by means of magnetic bearings, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. designed operating speed of the wheel. Secondly, the lowest natural frequency of the support structure 5 with the rotor and stator assembly on it is made higher than the highest designed operating speed of the wheel. Ways of raising or lowering the natural frequencies of a mechanical system are well known to engineers and need not be discussed here. Thirdly, load path motion damping 6 is inserted between the supports 5 and the assembly base 4. Thus in the arrangement described, for proper functioning of such system employing magnetic bearing suspension and to ensure that the rotor remains free of contact with the stator for all operating speeds, the three measures detailed above are simultaneously incorporated. In an alternative arrangement shown in Figure 2, the damping is incorporated into the stator shaft. This is accomplished by making the stator shaft 3 a composite member having a non-magnetic rigid core 8 and an outer sleeve 9 around the core of a selfdamping material, e.g. carbon-fibre reinforced material, on which the magnets 1 are mounted, the outer sleeve 9 being secured to the shaft core 8 by an intervening layer 10 of material having intrinsic damping qualities, e.g. silicon rubber. The lower end of the outer sleeve 9 is anchored in the base 4. When any disturbing force aots upon the rotor its resulting radial movement exerts a force on the stator magnets 1 and the carbon fibre support shaft sleeve 9 then acts as though cantilevered from a point at the bottom end of the shaft, thereby squashing the intervening layer 10 of the composite shaft with consequent damping of the movement. WHAT WE CLAIM IS:-

1. An assembly comprising a rotor mounted on a stator by means of magnetic bearings, and a support structure supporting the stator, wherein the lowest natural frequency of the stator is made higher than the highest designed operating speed of the rotor, the lowest natural frequency of the support structure bearing the rotor and stator assembly is made higher than the highest designed operating speed of the rotor, and motion damping means is included such as to ensure that relative movement of the rotor and stator at right angles to the axis of rotation is less than the radial clearance between the rotor and stator, said motion damping means being inserted into the load path that runs from bearing magnets on the stator to the support structure.

2. An assembly according to claim 1, wherein the stator comprises a central support shaft carrying bearing magnets, and a base carrying the support shaft, and the damping means is inserted between said base and the said support structure or between the stator bearing magnets and the central support shaft.

3. An assembly according to claim 1, wherein the stator comprises a central support shaft carrying bearing magnets, and the damping means is incorporated in the support shaft.

4. An assembly according to claim 3, wherein the support shaft comprises a rigid central core, a surrounding sleeve of a selfdamping material on which the stator magnets are mounted, and an interveniing layer of a resilient material with damping properties.

5. An assembly comprising a rotor mounted on a stator by means of magnetic bearings, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.