Field of the disclosure

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 to a machine comprising a controllable bearing system for supporting a rotatable element of the machine. Furthermore, the disclosure relates to a method for starting a machine comprising a rotatable element.

Background Rolling bearings such as for example deep groove ball bearings and angular ball bearings are commonly used means for supporting rotatable elements such as for example a rotor of an electrical machine, a propeller, or a gear. Rotatable elements which are supported with rolling bearings are typically under-critical because the damping of rolling bearings is typically low and therefore harmful me- chanical vibrations may take place when the rotational speed achieves the first critical speed. The first critical speed is the rotational speed that excites the lowest natural frequency of the rotatable element. When the rotational speed approaches the above-mentioned natural frequency, the rotatable element begins to resonate and this, in turn, may dramatically increase mechanical vibrations of the system comprising the rotatable element. Many practical design instructions suggest that the maximum rotational speed should not exceed 70% - 85% of the above- mentioned first critical speed.

In many cases, the requirement that a rotatable element is under-critical constitutes a significant limitation in the operation of a system comprising the rotatable element. For example, the output power of the system may be smaller than it could be if the rotational speed of the rotatable element could be allowed to exceed at least the first critical speed. Thus, when designing a system comprising a rotatable element, there is often an incentive to select such design choices which lead to as high first critical speed as possible. Design choices of the kind men- tioned above may, however, require compromises with other aspects related to the mechanical design of the system comprising the rotatable element. Furthermore, in conjunction with electrical machines, the design choices that are advantageous from the viewpoint of the critical speed may require compromises with aspects related to the electromagnetic design. Thus, there is an inherent need for technical solutions for passing at least the first critical speed in a sufficiently safe way when increasing the rotational speed of a rotatable element.

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:

- rolling bearings for rotatably supporting the rotatable element, and

- an axial control device for directing, to the rotatable element, controllable axial force for changing the axial loads carried by the rolling bearings so as to control the stiffness of each of the rolling bearings.

The axial load and thereby the stiffness of each of the rolling bearings is increased in response to a change of the controllable axial force in a first direction, and the axial load and thereby the stiffness of each of the rolling bearings is decreased in response to a change of the controllable axial force in a second direction that is opposite to the first direction.

The first critical speed of the rotatable element is at least partly determined by the stiffness of the rolling bearings, and therefore the first critical speed can be controlled by controlling the above-mentioned axial force directed to the rotatable el- ement. The above-mentioned axial control device may comprise for example an electromagnet system for producing the controllable axial force directed to the rotatable element. It is, however, also possible that the axial control device comprises a hydraulic or pneumatic system that may comprise a cylinder, a piston, and an axial-thrust bearing for transferring the controllable axial force to the rotatable element.

The controllable bearing system further comprises a controller configured to control the axial control device to decrease the axial loads of the rolling bearings in response to a situation in which the rotational speed of the rotatable element achieves a limit value so as to shift the above-mentioned first critical speed from a rotational speed region above the limit value to another rotational speed region below the limit value.

In accordance with the invention, there is provided also a new machine that comprises: - a first element and a second element, and

- a controllable bearing system according to the invention for rotatably supporting the second element with respect to the first element.

The machine can be for example an electrical machine where the above- mentioned first element comprises the stator of the electrical machine and the above-mentioned second element comprises the rotor of the electrical machine.

In accordance with the invention, there is provided also a new method for starting a machine that comprises a first element and a second element that is rotatably supported with respect to the first element with rolling bearings. A method according to the invention comprises: - increasing rotational speed of the second element from zero to a limit value,

- decreasing the axial loads of the rolling bearings so as to decrease the stiffness of each of the rolling bearings and thereby to shift the critical speed of the second element from a rotational speed region above the limit value to another rotational speed region below the limit value, and subsequently

- increasing the rotational speed of the second element to exceed the limit value. 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 spe- cific 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 limitations 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: figure 1 illustrates a controllable bearing system according to an exemplifying and non-limiting embodiment of the invention, figure 2 shows a schematic illustration of a machine according to an exemplifying and non-limiting embodiment of the invention, and figure 3 shows a flowchart of a method according to an exemplifying and non- limiting embodiment of the invention for starting a machine comprising a rotatable element. Description of exemplifying and non-limiting embodiments

The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description are not exhaustive unless other- wise explicitly stated.

Figure 1 illustrates a controllable bearing system according to an exemplifying and non-limiting embodiment of the invention for supporting a rotatable element 109. The rotation axis of the rotatable element 109 is parallel with the z-axis of a coordinate system 190. The controllable bearing system comprises rolling bearings 101 and 102 for rotatably supporting the rotatable element 109. In this exemplifying case, the rolling bearings 101 and 102 are angular ball bearings but as well one or both of the rolling bearings could be for example a so called deep groove ball bearing. The controllable bearing system further comprises an axial control device 103 for directing, to the rotatable element 109, controllable axial force for changing the axial loads carried by the rolling bearings 101 and 102. The radial stiffness of the rolling bearing 101 depends on the axial load of the rolling bearing 101 so that the radial stiffness increases when the axial load in the negative z- direction increases. Correspondingly, the radial stiffness of the rolling bearing 102 depends on the axial load of the rolling bearing 102 so that the radial stiffness in- creases when the axial load in the negative z-direction increases. The first critical speed of the rotatable element 109 is at least partly determined by the radial stiffness of the rolling bearing 101 and the radial stiffness of the rolling bearing 102. Therefore, the first critical speed can be controlled by controlling the axial force directed to the rotatable element 109. In the exemplifying controllable bearing system illustrated in figure 1 , the axial control device 103 comprises an electromagnet system 106 for directing, to the rotatable element 109, the controllable axial force. In this exemplifying case, the electromagnet system 106 comprises a ferromagnetic disc 1 17, electromagnets 1 13a and 1 13b, and a controller 104 for controlling the currents of the electromagnets. In this exemplifying case, the axial control device 103 further comprises a position sensor 107 for detecting the axial position of the rotatable element 109 and the controllable axial force produced by the electromagnet system 106 can be in the positive or negative z-direction. Thus, in this case, the electromagnet system 106 can be configured to operate as an axial magnetic bearing. In figure 1 , exemplifying flux-lines of the magnetic fluxes generated by the electromagnets 1 13a and 1 13b are depicted with dashed lines. In this exemplifying case, the position sensor 107 is an inductive sensor where the inductance is dependent on the distance from the position sensor 107 to a conical surface 1 12 of the rotatable element 109. The controller 104 can be configured to form a position signal indicative of the axial position of the rotatable element 109 on the basis of the inductance. In cases where it is sufficient to produce the controllable axial force only in the negative z- direction, a corresponding electromagnet system can be one-sided so that the controllable axial force can be produced only in the negative z-direction. Whether or not there is a need for being able to produce the controllable axial force both in the positive z-direction and in the negative z-direction depends on possible axial forces directed to the rotatable element 109 by a rotating object connected to the rotatable element 109. The rotating object can be for example a turbine impeller, a compressor impeller, a gear wheel, or some other object driven by or driving the rotatable element 109.

In the exemplifying controllable bearing system illustrated in figure 1 , the axial con- trol device 103 further comprises a spring system 105 for generating at least a part of the axial loads carried by the rolling bearings 101 and 102. The exemplifying spring system illustrated in figure 1 comprises springs for directing axial force to an intermediate element 1 16 that, in turn, directs the axial force to the rotatable element 109 via an axial-thrust bearing 1 15. In this exemplifying case, the spring sys- tern comprises two or more helical springs. In figure 1 , one of the helical springs is denoted with a reference number 1 14. It is, however, also possible to use a spring arrangement that is different from the one illustrated in figure 1 . For example, it is possible to use a single helical spring which surrounds the rotatable element 109. Furthermore, it is also possible to use one or more diaphragm or wave springs in- stead of the one or more helical springs.

In a controllable bearing system according to an exemplifying and non-limiting embodiment of the invention, the controller 104 is configured to control the cur- rents of the electromagnet system 106 on the basis of pre-determined data indicative of dependency between the value of the critical speed and the currents of the electromagnet system 106. The pre-determined data may comprise e.g. a look-up table and/or a mathematical formula for giving the value of the critical speed when the values of the currents of the electromagnet system 106 are inputted. The axial loads of the rolling bearings 101 and 102 can be deemed to be a single-valued mathematical function of the currents of the electromagnet system 106 in cases where the axial load caused by a rotating object driven by or driving the rotatable element 109 is known or can be estimated with a sufficient accuracy so that the axial load caused by the rotating object can be a parameter of the above- mentioned single-valued mathematical function. As a corollary, the radial stiffness of the rolling bearings 101 and 102 can be deemed to be a single-valued mathematical function of the currents of the electromagnet system 106, and thereby the value of the critical speed can be deemed to be a single-valued mathematical function of the currents of the electromagnet system 106.

In a controllable bearing system according to another exemplifying and non- limiting embodiment of the invention, the controller 104 is configured to control the currents of the electromagnet system 106 on the basis of pre-determined data indicative of dependency between the value of the critical speed and the detected axial position of the rotatable element 109. The pre-determined data may comprise e.g. a look-up table and/or a mathematical formula for giving the value of the critical speed when the value of the detected axial position is inputted. The radial stiffness of the rolling bearings 101 and 102 can be deemed to be a single-valued mathematical function of the axial position of the rotatable element 109, and thereby the value of the critical speed can be deemed to be a single-valued mathematical function of the axial position of the rotatable element 109. The control principle based on the detected axial position is suitable for cases where the axial load caused by a rotating object driven by or driving the rotatable element 109 is not known and cannot be estimated with a sufficient accuracy. The position sensor 107 has to be, however, sufficiently sensitive because even a small change in the axial position of the rotatable element 109 may strongly change the radial stiffness of the rolling bearings 101 and 102. Furthermore, the position sensor 107 is ad- vantageously axially so near to the rolling bearing 102 that possible thermal expansion of the rotatable element 109 does not cause too much error.

In a controllable bearing system according to an exemplifying and non-limiting embodiment of the invention, the controller 104 is configured to control the axial control device 103 to decrease the axial loads of the rolling bearings 101 and 102 in response to a situation in which the rotational speed of the rotatable element 109 achieves a limit value so as to shift the critical speed of the rotatable element 109 from a rotational speed region that is above the limit value to another rotational speed region that is below the limit value. Thereafter, the rotational speed can be increased to exceed the limit value. Thus, when increasing the rotational speed, the critical speed can be passed so that the critical speed is shifted to be below the prevailing rotational speed when the rotational speed achieves the above-mentioned limit value.

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 element 208, a second element 209, and a controllable bearing system for rotatably supporting the second element 209 with respect to the first element 208. The rotation axis of the rotatable element 209 is parallel with the z-axis of a coordinate system 290. In the exemplifying case illustrated in figure 2, the machine is an electrical machine where the first element 208 comprises the stator 210 of the electrical machine and the second element 209 comprises the rotor 21 1 of the electrical machine. The controllable bearing system comprises rolling bearings 201 and 202 for rotatably supporting the second element 209 with respect to the first element 208. The controllable bearing system further comprises a axial control device for directing, to the second element 209, controllable axial force for changing the axial loads carried by the rolling bearings 201 and 202 so as to control the stiffness of each of the rolling bearings 201 and 202 and thereby the critical speed of the second element 209.

In the exemplifying case illustrated in figure 2, the axial control device comprises an electromagnet system that comprises an electromagnet 206a for directing, to the second element 209, axial force in the positive z-direction and another elec- tromagnet 206b for directing, to the second element 209, axial force in the negative z-direction. The controllable bearing system further comprises a controller 204 for controlling the currents of the electromagnets 206a and 206b. The controllable bearing system further comprises a position sensor for detecting the axial position of the second element 209. In this exemplifying case, the position sensor is an inductive sensor where the inductance of a sensor element 207a is dependent on the distance from the sensor element 207a to a conical surface 212a and the inductance of a sensor element 207b is dependent on the distance from the sensor element 207b to a conical surface 212b. The controller 204 can be configured to form a position signal indicative of the axial position of the second element 209 on the basis of the difference between the inductances of the sensor elements 207a and 207b. The controller 204 can be configured to control the currents of the electromagnets 206a and 206b on the basis of for example pre-determined data that expresses the value of the critical speed as a mathematical function of the detect- ed axial position of the second element 209.

In a machine according to an exemplifying and non-limiting embodiment of the invention, the controller 204 is configured to control the electromagnets 206a and 206b to decrease the axial loads of the rolling bearings 201 and 202 in response to a situation in which the rotational speed of the second element 209 achieves a limit value so as to shift the critical speed of the second element 209 from a rotational speed region that is above the limit value to another rotational speed region that is below the limit value. Thereafter, the rotational speed can be increased to exceed the limit value. Thus, when increasing the rotational speed, the critical speed can be passed so that the critical speed is shifted to be below the prevailing rotational speed when the rotational speed achieves the above-mentioned limit value.

The exemplifying machine illustrated in figure 2 can be for example a part of a high-speed turbo-compressor, a part of a high-speed turbo-generator, a part of a lathe, a part of a milling machine, or a part of some other machine where high ro- tational speeds may be needed. Figure 3 shows a flowchart of a method according to an exemplifying and non- limiting embodiment of the invention for starting a machine that comprises a first element and a second element that is rotatably supported with respect to the first element with rolling bearings. The method comprises the following actions: - action 301 : increasing the rotational speed of the second element from zero to a limit value,

- action 302: decreasing the axial loads of the rolling bearings so as to decrease the stiffness of each of the rolling bearings and thereby to shift the critical speed of the second element from a rotational speed region above the limit value to another rotational speed region below the limit value, and subsequently

- action 303: increasing the rotational speed of the second element to exceed the limit value.

The above-mentioned rolling bearings can be for example angular ball bearings but as well one or both of the rolling bearings could be for example a so called deep groove ball bearing.

In a method according to an exemplifying and non-limiting embodiment of the invention, the axial loads of the rolling bearings are controlled with an electromagnet system that directs controllable axial force to the second element. In a method according to an exemplifying and non-limiting embodiment of the invention, one or more currents of the electromagnet system are controlled on the basis of pre-determined data indicative of dependency between the value of the critical speed and the one or more currents of the electromagnet system.

In a method according to an exemplifying and non-limiting embodiment of the in- vention, the axial position of the second element is detected with a position sensor and one or more currents of the electromagnet system are controlled on the basis of pre-determined data indicative of dependency between the value of the critical speed and the detected axial position of the second element. In a method according to an exemplifying and non-limiting embodiment of the invention, at least a part of the axial loads of the rolling bearings are generated with a spring system.

The specific examples provided in the description given above should not be con- strued 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 exhaustive unless otherwise explicitly stated.