Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2003-08-15
2004-07-13
Le, Dang (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C505S166000
Reexamination Certificate
active
06762522
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a magnetic bearing where a shaft which can rotate is borne magnetically within a stator. The magnetic bearing is intended to have the following features:
a first bearing part is rigidly connected to the shaft and is surrounded by a second bearing part, which is associated with the stator, forming a bearing gap between these bearing parts,
the first bearing part contains a magnet arrangement with permanently magnetic elements,
the second bearing contains a superconducting arrangement with a high-T
c
superconductor material, with magnetic bearing forces being produced between the superconducting arrangement and the permanently magnetic elements of the magnet arrangement, and
a cooling apparatus is provided for cooling the superconductor material of the superconducting arrangement to an operating temperature below the critical temperature of the superconductor material.
A magnetic bearing such as this is disclosed in DE 44 36 831 C2.
Magnetic bearings allow moving parts to be provided with bearings which make no contact and are therefore free of wear. They require no lubricants and can be constructed to have low friction. In this case, a body which can rotate (rotating body) can be hermetically sealed, that is to say in a vacuum-tight manner, from the outer area surrounding it.
Known magnetic bearings use magnetic forces between stationary electromagnets on a stator and ferromagnetic elements, which are on a rotor body and rotate with it. With this type of bearing, the magnetic forces are always attractive. In consequence, it is in principle impossible to achieve a bearing which is inherently stable in all three spatial directions (see “Eamshaw's Theorem” in “Trans. Cambridge Phil. Soc.”, Vol. 7, 1842, pages 97 to 120). Magnetic bearings such as these therefore require active bearing control, which uses position sensors and a control loop to control the currents and the supporting magnets and to counteract any discrepancies of the rotor body from a nominal position. The control process, which needs to have a plurality of channels for this purpose, requires complex power electronics. In addition, a mechanical emergency bearing must be provided as a precaution against sudden failure of the control loop. Corresponding magnetic bearings are used, for example, in turbo-molecular pumps, ultra-centrifuges, high-speed spindles for machine tools, and X-ray tubes with rotating anodes; use for motors, generators, turbines and compressors is likewise known.
In principle, superconductors allow a new type of magnetic bearing: one of the bearing parts is in this case formed with permanently magnetic elements which induce shielding currents in the event of a position change, as a consequence of field changes in the superconductor material of a further, second bearing part which surrounds the first bearing part with a gap. The forces which result from this may be repulsive or attractive, but are directed such that they counteract the deflection from a nominal position. In contrast to known magnetic bearings, it is possible in this case to achieve an inherently stable bearing (see, for example, “Appl. Phys. Lett.”, Vol. 53, No. 16, 1988, pages 1554 to 1556). In contrast to known magnetic bearings, there is no need here for any complex control system that is susceptible to defects; however, a cooling apparatus must be provided in order to cool the superconductor material to an operating temperature below the critical temperature of the superconductor material.
Appropriate superconducting bearing parts for magnetic bearings such as these may be one of the first fields of use for the metal-oxide high-T
c
superconductor materials which have been known since 1987, such as those based on the Y-Ba-Cu-O material system, which can be cooled to an operating temperature of about 77 K using liquid nitrogen.
Use of appropriate high-T
c
superconductor material is envisaged for the magnetic bearing which is disclosed in the DE-C2 document cited initially. The magnetic bearing contains a large number of permanently magnetic elements which are in the form of annular discs and are located one behind the other in the axial direction on a rotor shaft. These elements are polarized such that the polarization alternates when seen in the axial direction of the shaft. Comparatively thin ferromagnetic intermediate elements are arranged in each case between adjacent elements. These intermediate elements primarily have the task of magnetically concentrating the magnetic lines of force of adjacent permanently magnetic elements, so that a particularly high magnetic field strength is produced on the side of each intermediate element which faces the bearing gap. This bearing part of the rotor body, together with its magnet arrangement composed of permanently magnetic elements, is surrounded by a fixed-position bearing part of a stator. This bearing part contains a superconducting arrangement with a high-T
c
superconductor material such as Yba
2
Cu
3
O
x
, with the abovementioned magnetic bearing forces being produced between the superconducting arrangement and the permanently magnetic elements of the magnetic arrangement. The superconductor material of the conductor arrangement is kept at about 77 K by liquid nitrogen (LN
2
). For this purpose, cooling channels through which this coolant is passed are provided on the outside of the superconducting arrangement.
In the case of magnetic bearings in which parts which need to be cryogenically cooled are adjacent to the bearing gap, one problem that can occur is that environmental air can reach the cold components through the bearing gap, with the moisture in the air freezing there. Corresponding icing can lead to functional restrictions or damage to the bearing. In the case of the magnetic bearing which is disclosed in the abovementioned DE-C2 document, such icing of the bearing gap can be avoided by emitting vaporizing nitrogen. The necessary cooling power for the bearing is in this case from a few watts up to the order of magnitude of 10 W at 50 to 80 K. However, if other cooling techniques than those used for the known magnetic bearings are envisaged, especially using so-called cryogenic coolers with only indirect cooling, there is no corresponding capability to avoid the risk of icing in the bearing gap, since no vaporizing coolant gas is then available.
SUMMARY OF THE INVENTION
One possible object of the present invention is therefore to refine the magnetic bearing having the features mentioned initially, such that such risk of bearing icing is minimized irrespective of the chosen cooling technique, and such that the sealing complexity can be kept low.
This object may be achieved in that, in the case of the magnetic bearing having the features mentioned initially, the superconducting arrangement and the magnetic arrangement are also jointly surrounded by at least one isolation area, and in that an additional area is provided, which is separated from the at least one isolation area and comprises the bearing gap and subareas which extend on side end faces of the superconducting arrangement and of the magnet arrangement radially as far as the shaft and are sealed there with respect to the shaft.
The advantages which are associated with this embodiment of the magnetic bearing are, in particular, that the complexity for sealing the additional area from the parts which can rotate can be kept low. This is because the seal uses the smallest possible diameter, so that the circumferential speed of the parts of the seal which also rotate is minimized. This makes it simpler for the seal to operate, and correspondingly lengthens its life. The simplified sealing, which may thus also be designed to be effective, of the additional area also results in the risk of ingress of gases which can freeze at least largely being avoided.
The additional area of the magnetic bearing can thus be evacuated in a simple manner. This advantageously allows friction losses to be reduced. In the event of any leakage of the sealant on the shaft, a small
Le Dang
Siemens Aktiengesellschaft
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