The disclosure relates to a controllable bearing system for supporting a rotatable element, e.g. a rotor of an electrical machine. Furthermore, the disclosure relates a machine comprising main bearings for supporting a rotatable element and at least one controllable bearing system for supporting the rotatable element in a situation where the main bearings are non-operating.

Background

In many cases there is a need to provide a rotating machine with auxiliary bear- ings in addition to main bearings. The main bearings are arranged to rotatably support the rotatable element of the machine during normal operation, and the auxiliary bearings are arranged to rotatably support the rotatable element when the main bearings are non-operating. The machine can be, for example, an electrical machine and the main bearings can be for example active magnetic bearings "AMB". When the magnetic bearings become non-operating, e.g. due to an electrical power cut, the rotatable element, i.e. the rotor of the electrical machine, is dropped to be supported by the auxiliary bearings. It is also possible that the magnetic bearings are non-operating in the sense that they are still active but their load-bearing capacity is exceeded. Also in this case, the auxiliary bearings have to support the rotor. Typically, there have to be clearances between the rotor and the auxiliary bearings when the rotor is supported by the main bearings in order that the auxiliary bearings would not disturb the normal operation of the machine.

A conventional approach is to use auxiliary bearings whose inner diameters are larger than the corresponding outer diameters of the shaft of the rotor. Therefore, in the normal operation, there are the required clearances between the auxiliary bearings and the shaft. If the active magnetic bearings shut down, the rotor drops on the auxiliary bearings and the auxiliary bearings provide a run-down of the rotor. However, depending on the rotational speed, geometrical and material properties, external forces, and/or other mechanical factors, once the rotor drops on the auxiliary bearings the rotor may get unstable and start to whirl when being sup- ported by the auxiliary bearings. This is a dangerous situation which may lead to high forces in the supporting mechanical structures and thereby may cause damages in the machine.

Summary The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

In accordance with the invention, there is provided a new controllable bearing system for supporting a rotatable element. A controllable bearing system according to the invention comprises: - a bearing,

- support elements for mechanically supporting the bearing to be movable with respect to the rotatable element in an axial direction of the bearing,

- spring equipment for generating spring force for pressing the bearing in the axial direction with respect to the rotatable element so as to make the con- trollable bearing system to support the rotatable element, and

- one or more electromagnets for generating magnetic force directed against the spring force and for keeping the controllable bearing system detached from the rotatable element in response to a situation in which electrical current exceeding a pre-determined limit is supplied to the electromagnet. The controllable bearing system is suitable for operating as an auxiliary bearing that needs to be detached from the rotatable element during a normal operation and to support the rotatable element when main bearings, e.g. active magnetic bearings, are non-operating. In a case where the main bearings are active mag- netic bearings, the one or more electromagnets of the controllable bearing system are advantageously supplied with a same electric system which is arranged to supply the active magnetic bearings. Therefore, in a case of a failure in the electric system, the one or more electromagnets of the controllable bearing system be- come inactive and the spring equipment drive the controllable bearing system to support the rotatable element. Mutually contacting surfaces of the controllable bearing system and the rotating element are advantageous conical so as to provide a centering effect when the spring equipment make the controllable bearing system to support the rotatable element. In accordance with the invention, there is provided also a new machine that comprises:

- a first element and a second element,

- a main bearing system for supporting the first element rotatably with respect to the second element, and - an auxiliary bearing system for supporting the first element rotatably with respect to the second element in a situation where the main bearing system is non-operating.

The auxiliary bearing system of the machine comprises one or more controllable bearing systems according to the invention. The machine can be for example an electrical machine where the above- mentioned first element comprises the rotor of the electrical machine and the above-mentioned second element comprises the stator of the electrical machine. The main bearing system of the machine can comprise for example active magnetic bearings "AMB". A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in connection with the accompanying drawings.

The verbs "to comprise" and "to include" are used in this document as open limita- tions that neither exclude nor require the existence of unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.

Brief description of the figures Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figures 1 a, 1 b, and 1 c illustrate a controllable bearing system according to an exemplifying and non-limiting embodiment of the invention, and figure 2 shows a schematic illustration of a machine according to an exemplifying and non-limiting embodiment of the invention.

Description of exemplifying and non-limiting embodiments

Figures 1 a, 1 b, and 1 c illustrate a controllable bearing system 101 according to an exemplifying and non-limiting embodiment of the invention for supporting a rotata- ble element 109. Figure 1 a shows a view of a section taken along a line B-B shown in figure 1 c. Figure 1 c shows a view of a section taken along a line A-A shown in figure 1 a. Figure 1 b shows a view of a section corresponding to that shown in figure 1 a. Figures 1 a and 1 b illustrate the controllable bearing system 101 in different operational situations. In figures 1 a and 1 b, the section plane is parallel with the yz-plane of a coordinate system 190, and in figure 1 c the section plane is parallel with the xy-plane of the coordinate system 190. The controllable bearing system comprises a bearing 102 and first and second support elements 103 and 104 for mechanically supporting the bearing 102 so that the bearing is movable with respect to the rotatable element 109 in the axial direction of the bearing. The axial direction is parallel with the z-axis of the coordinate system 190. In this exemplifying case, the bearing is a ball bearing that is advantageously a so called deep groove ball bearing so as to provide axial load-bearing capacity. The bearing could as well be a combination of two different bearings where one of the bearing provides most of the axial load-bearing capacity and the other of the bearings provides most of the radial load-bearing capacity.

The controllable bearing system 101 comprises spring equipment for generating spring force for pressing the bearing 102 in the axial direction with respect to the rotatable element 109 so as to make the bearing 102 to support the rotatable ele- ment. In the exemplifying and non-limiting controllable bearing system illustrated in figures 1 a-1 c, the bearing 102 is attached to the first support element 103 and the spring equipment comprises four compression springs between the first and second support elements 103 and 104. In this exemplifying case, each of the compression springs is a helical spring. In figure 1 a, one of the helical springs is de- noted with a reference number 105. It is, however, to be noted that many kinds of spring equipment are possible. For example, it is also possible to use a single helical spring which surrounds the rotatable element 109. Furthermore, it is also possible to use one or more diaphragm springs instead of the one or more helical springs. In the exemplifying case illustrated in figures 1 a-1 c, the spring equipment is arranged to generate the above-mentioned spring force by pushing the support element 103 in the positive z-direction. It is also possible to use such spring equipment which is arranged to generate the above-mentioned spring force by pulling the support element 103 in the positive z-direction. In this case, the spring equipment comprises one or more extension springs pulling the support element 103 in the positive z-direction.

The controllable bearing system 101 comprises an electromagnet 106 for generating magnetic force directed against the above-mentioned spring force and for keeping the controllable bearing system detached from the rotatable element 109 when electrical current exceeding a pre-determined limit is supplied to the elec- tromagnet. Figure 1 a illustrates a situation where there is no electrical current in the electromagnet 106 or where the electrical current is so small that the springs are capable of pressing the bearing 102 against the rotatable element 109. Figure 1 b illustrates a situation where the electromagnet 106 generates magnetic force acting against the spring force and capable of keeping the bearing 102 detached from the rotatable element 109. In figure 1 b, exemplifying flux-lines of the magnetic flux generated by the electromagnet 106 is depicted with dashed lines. The exemplifying and non-limiting controllable bearing system illustrated in figures 1 a-1 c comprises a conical surface 107 for supporting the rotatable element 109 so that the conical surface surrounds the rotatable element. The rotatable element comprises advantageously a corresponding conical surface 1 16 that matches the conical surface 107 of the controllable bearing system. In this exemplifying and non-limiting case, the conical surface of the controllable bearing system is the inward surface of the inner ring of the bearing 102 but it is also possible that the inner ring of the bearing has a cylindrical inward surface and the controllable bearing system comprises a sleeve element whose outward surface is attached to the inward surface of the inner ring and whose inward surface is conical. In the exemplifying and non-limiting controllable bearing system illustrated in figures 1 a-1 c, a winding 108 of the electromagnet 106 is attached to the second support element 104 with respect to which the bearing 102 is movable in the axial direction. It is, however, also possible that the winding of the electromagnet is attached to the first support element 103, i.e. to the same support element as the bearing. In this exemplifying and non-limiting case, the electromagnet 106 is rota- tionally symmetric with respect to the rotational axis of the bearing 102. It is, however, to be noted that many kinds of electromagnet arrangements are possible. For example, it is also possible to have many electromagnets which are circumfer- entially distributed around the rotational axis of the bearing 102 in the same way as the four helical springs are circumferentially distributed around the rotational axis as illustrated in figure 1 c. Furthermore, it is also possible that there are windings on both the first and second support elements 103 and 104.

Figure 2 shows a schematic illustration of a machine according to an exemplifying and non-limiting embodiment of the invention. The machine comprises a first ele- ment 209, a second element 210, and a main bearing system for supporting the first element rotatably with respect to the second element. Furthermore, the ma- chine comprises an auxiliary bearing system for supporting the first element rotat- ably with respect to the second element in a situation where the main bearing system is non-operating or the load-bearing capacity of the main bearing system is exceeded. In the exemplifying case illustrated in figure 2, the machine is an elec- trical machine where the first element 209 comprises a rotor 217 of the electrical machine and the second element 210 comprises a stator 218 of the electrical machine. The main bearing system comprises active magnetic bearings 21 1 and 212 each comprising a radial bearing section and an axial bearing section. The auxiliary bearing system comprises controllable bearing systems 201 and 221 according to an embodiment of the invention. Each of the controllable bearing systems 201 and 221 can be for example such as the controllable bearing system 101 illustrated in figures 1 a-1 c. A device that comprises the machine comprises a control system 213 for supplying electrical currents to the active magnetic bearings 21 1 and 212 to the electromagnets of the controllable bearing systems 201 and 221 . The device can be, for example but not necessarily, a high-speed turbo-compressor.

In the normal operation of the machine, the above-mentioned electromagnets produce magnetic forces which act against the mechanical springs of the controllable bearing systems 201 and 221 in a way that there are clearances between the controllable bearing systems and the rotatable element 209. In figure 2, exemplifying flux-lines of the magnetic fluxes generated by the electromagnets are depicted with dashed lines. If electricity is lost, in which case the active magnetic bearings 21 1 and 212 lose their bearing power, the above-mentioned electromagnets become inactive and the springs of the controllable bearing systems 201 and 221 drive the bearings of the controllable bearing systems into contact with the rotata- ble element 209, and therefore the controllable bearing systems 201 and 221 start to support the rotatable element 209.

In the exemplifying machine illustrated in figure 2, the rotatable element 209 comprises a first conical contact surface 214 for contacting with the controllable bearing system 201 and a second conical contact surface 215 for contacting with the controllable bearing system 221 . The contact surfaces 214 and 215 taper towards mutually opposite ends of the rotatable element 209 as shown in figure 2. The controllable bearing system 201 comprises a first conical surface matching the contact surface 214 and, correspondingly, the controllable bearing system 221 comprises a second conical surface matching the contact surface 215. In a case where the electricity is lost, the conical surfaces of the controllable bearing systems 201 and 221 engage the conical contact surfaces 214 and 215 of the rotata- ble element 209 and thus the rotatable element 209 stays centric during the rundown of the machine. The above-described auxiliary bearing system provides centric positioning of the rotatable element 209 also when the machine is at rest so that the electricity is switched off. Therefore, it is possible to have smaller tolerances in an actuator, e.g. a turbine impeller and a turbine chamber, attached to the machine than in a case where radial movements of the rotatable element 209 are limited with auxiliary bearings that are fixed, i.e. non-movable, with respect to the rotatable element 209. Furthermore, the non-alternating axial loads caused by the springs of the controllable bearing systems 201 and 221 are beneficial for the operation of the mechanical bearings of the controllable bearing systems. Support elements 203 and 223 of the controllable bearing systems 201 and 221 are arranged to slide axially so that sufficient radial stiffness is provided and tilting is within an acceptable tolerance area. Furthermore, the support elements 203 and 223 are advantageously mechanically constrained not to rotate because friction forces in the mechanical bearings of the controllable bearing systems try to rotate the support elements 203 and 223 during run-down.

In a machine according to an exemplifying and non-limiting embodiment of the invention, the windings of the electromagnets of the controllable bearing systems 201 and 221 are connected in series with suitable windings of the active magnetic bearings 21 1 and 212 in order to ensure a fast response of the controllable bear- ing systems 201 and 221 in a case of a power loss in the active magnetic bearings 21 1 and 212.

The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given above are not exhaus- tive unless otherwise explicitly stated.