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Title:
MAGNETIC BEARING ARRANGEMENT AND METHOD OF OPERATING A MAGNETIC BEARING ARRANGEMENT
Document Type and Number:
WIPO Patent Application WO/2015/067309
Kind Code:
A1
Abstract:
An electromagnetic bearing arrangement comprising an electromagnet including an amplifier arrangement having a switching leg pair (32, 33), with each having two switches (34a-b, 35a-b); a coil (31) electrically connected to the junction between switches of each of the switching legs; a power supply (26); and a controller for controlling the amplifier arrangement to operate the electromagnet. Each of the switches is provided with a diode (37a-b, 38a-b) for bypassing the switch when the switch is in an off state: The controller is configured to control the amplifier arrangement to operate in a first operation mode, wherein a first pair (34a, 35b) of the switches are being switched while a second pair (34b, 35a) of the switches are kept in an off state, and in a second operation mode, wherein the first pair of the switches are switched to an off state and the second pair of the switches are being switched while the first pair of the switches are kept in the off state.

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Inventors:
LIU, Bin (Råsegelgatan 12, Västerås, S-723 48, SE)
SJÖBERG, Johan (Vasagatan 20, Västerås, S-722 15, SE)
Application Number:
EP2013/073154
Publication Date:
May 14, 2015
Filing Date:
November 06, 2013
Export Citation:
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Assignee:
ABB TECHNOLOGY LTD (Affolternstrasse 44, Zürich, CH-8050, CH)
International Classes:
F16C32/04
Foreign References:
US6297574B1
US6278251B1
EP1316738A1
US6005316A
EP1316738A1
US6005316A
Attorney, Agent or Firm:
SAVELA, Reino (Abb Ab, Intellectual PropertyIngenjör Bååths Gata 11, Västerås, S-721 83, SE)
Download PDF:
Claims:
CLAIMS

1. An electromagnetic bearing arrangement comprising

- an electromagnet (2ia-d) comprising a coil (31);

- an amplifier arrangement (23) having a switching leg pair (32, 33), each switching leg having two switches (34a-b, 35a-b), and the coil being electrically connected to the junction between the switches of each of the switching legs;

- a power supply (26) electrically connected to opposite ends of each of the switching legs; and - a controller (24) for controlling the amplifier arrangement to operate the electromagnet, characterized in that

- each of switches is provided with a diode (37a-b, 38a-b) for bypassing the switch when the switch is in an off state; and

- the controller is configured to control the amplifier arrangement to operate in a first operation mode, wherein a first pair (34a, 35b) of the switches are being switched while a second pair (34b, 35a) of the switches are kept in an off state, wherein the switches of each pair are located in different switching legs and electrically connected to different poles of the power supply; and

- the controller is configured to control the amplifier arrangement to change its operation mode to a second operation mode and to operate in the second operation mode, wherein the first pair (34a, 35b) of the switches are switched to an off state and the second pair (34b, 35a) of the switches are being switched while the first pair of the switches are kept in the off state.

2. The arrangement of claim 1 wherein the controller is configured to control the amplifier arrangement to change its operation mode to the second operation mode and to operate in the second operation mode in response to a fault in one of the switches of the first pair.

3. The arrangement of claim 2 comprising sensors configured to detect the fault in the switches of the first pair and to communicate said detection to the controller.

4. The arrangement of any of claims 1-3 wherein the controller is configured to control the amplifier arrangement to change its operation mode to the second operation mode and to operate in the second operation mode when the amplifier arrangement has operated in the first operation mode for a selected time period or for a selected number of switching operations to avoid aging and/or over-heating of the switches of the first pair. 5. The arrangement of claim 4 wherein the selected time period is a predetermined set time period or the selected number of switching

operations is a predetermined set number.

6. The arrangement of any of claims 1-5 comprising a sensor configured to measure a temperature in the amplifier arrangement and to communicate said sensed temperature to the controller, wherein the controller is

configured to control the amplifier arrangement to change its operation mode to the second operation mode and to operate in the second operation mode in response to said measured temperature to avoid aging and/or over-heating of the switches of the first pair. 7. The arrangement of any of claims 1-6 comprising

- a plurality of electromagnets, each comprising a coil, wherein

- the amplifier arrangement has, for each of the electromagnets, a switching leg pair, each switching leg having two switches, the coil of the electromagnet being electrically connected to the junction between the switches of each of the switching legs, and each of switches being provided with a diode for bypassing the switch when the switch is in an off state;

- the power supply is electrically connected to opposite ends of each of the switching legs; and - the controller is configured to control the amplifier arrangement to operate, for each of the electromagnets, in a first operation mode, wherein, for each of the switching leg pairs, a first pair of the switches are being switched while a second pair of the switches are kept in an off state, wherein the switches of each pair are located in different switching legs and electrically connected to different poles of the power supply; and

- the controller is configured to control the amplifier arrangement to change its operation mode, for each of the electromagnets, to a second operation mode and to operate in the second operation mode, wherein, for each of the switching leg pairs, the first pair of the switches are switched to an off state and the second pair of the switches are being switched while the first pair of the switches are kept in the off state.

8. The arrangement of claim 7 wherein the plurality is four.

9. The arrangement of any of claims 1-6 comprising - at least one upper (2ia-b) and at least one lower (2ic-d) electromagnet, each comprising a coil (31, 41), wherein

- the amplifier arrangement has, for each upper electromagnet, a switching leg pair (32, 33), each switching leg having two switches (34a-b, 35a-b), the coil of the electromagnet being electrically connected to the junction between the switches of each of the switching legs, and each of switches being provided with a diode (37a-b, 38a-b) for bypassing the switch when the switch is in an off state;

- the amplifier arrangement has, for each lower electromagnet, a switching leg pair (42, 43), each switching leg having one switch (44b, 45) and one diode (47a, 48), the coil of the electromagnet being electrically connected to the junction between the switch and the diode of each of the switching legs;

- the power supply is electrically connected to opposite ends of each of the switching legs; and l8

- the controller is configured to control the amplifier arrangement to operate, for each upper electromagnet, in a first operation mode, wherein, for each switching leg pair associated with an upper electromagnet, a first pair of the switches are being switched while a second pair of the switches are kept in an off state, wherein the switches of each pair are located in different switching legs and electrically connected to different poles of the power supply; and

- the controller is configured to control the amplifier arrangement to change its operation mode, for each upper electromagnet, to a second operation mode and to operate in the second operation mode, wherein, for each switching leg pair associated with an upper electromagnet, the first pair of the switches are switched to an off state and the second pair of the switches are being switched while the first pair of the switches are kept in the off state.

10. The arrangement of claim 9 comprising two upper electromagnets and two lower electromagnets. 11. The arrangement of any of claims 1-5 wherein the switches are transistors such as e.g. FET's (Field Effect Transistors) or IGBT's (Insulated-Gate Bipolar Transistors).

12. The arrangement of any of claims 1-11 wherein the controller is configured to control the amplifier arrangement to generate two-level or three level PWM (Pulse- Width Modulated) voltages over each electromagnet.

13. A method of operating an electromagnetic bearing arrangement comprising an electromagnet (2ia-d) including a coil (31); an amplifier arrangement (23) having a switching leg pair (32, 33), each switching leg having two switches (34a-b, 35a-b), and the coil being electrically connected to the junction between the switches of each of the switching legs; and a power supply (26) electrically connected to opposite ends of each of the switching legs, wherein each of switches is provided with a diode (37a-b, 38a- b) for bypassing the switch when the switch is in an off state, said method comprising the steps of: - controlling (51) the amplifier arrangement to operate in a first operation mode, wherein a first pair (34a, 35b) of the switches are being switched (51a) while a second pair (34b, 35a) of the switches are kept (51b) in an off state, wherein the switches of each pair are located in different switching legs and electrically connected to different poles of the power supply; and

- controlling (53) the amplifier arrangement to change its operation mode to a second operation mode and operate in the second operation mode, wherein the first pair (34a, 35b) of the switches are switched (53a) to an off state and the second pair (34b, 35a) of the switches are being switched (53b) while the first pair of the switches are kept (53c) in the off state.

14. The method of claim 13 wherein the amplifier arrangement is controlled to change its operation mode to the second operation mode and to operate in the second operation mode in response to a fault in one of the switches of the first pair. 15. The method of claim 14 wherein the fault in one of the switches of the first pair is detected by a detector.

16. The method of any of claims 13-15 wherein the amplifier arrangement is controlled to change its operation mode to the second operation mode and to operate in the second operation mode when the amplifier arrangement has operated in the first operation mode for a selected time period or for a selected number of switching operations to avoid aging and/or over-heating of the switches of the first pair.

17. The method of claim 16 wherein the selected time period is a

predetermined set time period or the selected number of switching

operations is a predetermined set number.

18. The method of any of claims 13-15 comprising the step of measuring a temperature in the amplifier arrangement, wherein the amplifier

arrangement is controlled to change its operation mode to the second operation mode and to operate in the second operation mode in response to said measured temperature to avoid aging and/or over-heating of the switches of the first pair.

Description:
MAGNETIC BEARING ARRANGEMENT AND METHOD OF OPERATING A MAGNETIC BEARING ARRANGEMENT

TECHNICAL FIELD

The invention relates generally to magnetic bearings and specifically to amplifier arrangements for magnetic bearings and methods for operating such arrangements.

BACKGROUND

Active magnetic bearings are used to suspend shafts of rotating equipment subject to load spectra which vary with respect to time. Control of the position of the rotating assemblies of such equipment is effected by electromechanical systems which combine the use of feedback control and switching amplifiers. For the practical application of such systems in industrial rotating machinery, the output of these amplifiers must vary in the order of thousands of cycles per second in order to maintain or adjust the desired position.

Fig. 1 is diagrammatic illustration of a known amplifier arrangement, which has two switching legs 12, 13, each having one switch 14, 15 and one diode 17, 18, which is used to excite an electromagnet. The coil 11 of the electromagnet is electrically connected to the junction between the switch 14, 15 and the diode 17, 18 of each of the switching legs 12, 13. A power supply 26 is connected to opposite ends of each of the switching legs and a controller is provided for controlling the amplifier arrangement to operate the

electromagnet. Typically, a controller generates PWM (Pulse- Width

Modulated) signals at terminals G2 and G3 to achieve a desired current in the coil of each of the electromagnets used in the magnetic bearing in response to sensed position signals. It is of outmost importance that the switching operates satisfactorily in order to avoid faults that may lead to a touch-down. EP 1 316 738 Ai discloses an example of such an amplifier arrangement.

US 6,005,316 discloses another known amplifier arrangement of power switches. The amplifier arrangement comprises two switching legs and with a load, such as a bearing coil, connected between the two switch legs. Each leg is provided with two switches. The legs are connected in parallel with a power supply and a capacitor. Each power switch is typically in the form of a FET (Field Effect Transistor) and is responsive to a control signal. Two schemes are used to control the switches to allow the current to track the set point: bi- state switching and tri-state switching.

SUMMARY

An aim of the present invention is to provide an electromagnetic bearing arrangement and a method of operating an electromagnetic bearing arrangement, which are more reliable and reduce the risk of a touch-down or other fault operation.

A further aim of the invention is to provide such arrangement and method, by which the components of an amplifier arrangement of the electromagnetic bearing arrangement will have a longer lifetime. According to one aspect of the invention, there is provided an

electromagnetic bearing arrangement comprising an electromagnet including a coil; an amplifier arrangement having a switching leg pair, each switching leg having two switches, and the coil being electrically connected to the junction between the switches of each of the switching legs; a power supply electrically connected to opposite ends of each of the switching legs; and a controller for controlling the amplifier arrangement to operate the

electromagnet. Each of the switches is provided with a diode bypassing the switch when the switch is in an off state. The switches may be transistors such as e.g. FET's (Field Effect Transistors) or IGBT's (Insulated-Gate Bipolar Transistors).

The amplifier arrangement provides for a switching redundancy since only one switch per switching leg is actually required for operation using two-level or three level PWM (Pulse- Width Modulated) voltages. The controller is configured to control the amplifier arrangement to operate in a first operation mode, wherein a first pair of the switches are being switched while a second pair of the switches are kept in an off state, wherein the switches of each pair are located in different switching legs and

electrically connected to different poles of the power supply. By the term "operate in a first operation mode, wherein a first pair of the switches are being switched" is meant that the controller is configured to control the first pair of the switches of the amplifier arrangement to generate suitable voltage over the electromagnet for its operation. For instance, the controller may be configured to control the amplifier arrangement to generate two-level or three level PWM (Pulse- Width Modulated) voltages over the electromagnet. By the term "a second pair of the switches are kept in an off state" is meant that the controller is configured to keep the second pair of the switches of the amplifier arrangement off and they are thus not used in the generation of the electromagnet voltages. These switches are simply by-passed by the diodes connected over them.

The controller is configured, at some point, to control the amplifier arrangement to change its operation mode to a second operation mode and to operate in the second operation mode, wherein the first pair of the switches are switched to an off state and the second pair of the switches are being switched while the first pair of the switches are kept in the off state. That is, the electromagnet voltages are generated only by the second pair of the switches.

There may be various triggering mechanisms for triggering the controller to change the operation mode from the first operation mode to the second operation mode in order to obtain different advantages.

In one embodiment, the controller may be configured to control the amplifier arrangement to change its operation mode from one to another one of the operation modes and to operate in the second operation mode in response to a fault in one of the switches of the first pair. Such fault may be detected by the controller directly by analyzing the currents through the coil or by means of signaling from the switches.

Alternatively, the electromagnetic bearing arrangement may comprise sensors configured to detect the fault in the switches of the first pair and to communicate this to the controller. These sensors may be dedicated sensors for this purpose, or existing sensors used in a feedback control of the operation of the amplifier arrangement may be used.

In another embodiment, the controller may be configured to control the amplifier arrangement to change its operation mode to the second operation mode and to operate in the second operation mode when the amplifier arrangement has operated in the first operation mode for a selected time period or for a selected number of switching operations e.g. to avoid aging and/or over-heating of the switches of the first pair. The selected time period and the selected number of switching operations may be predetermined based on estimated lifetimes.

When another selected time period has passed or another number of switching operations has been performed, the controller may be configured to control the amplifier arrangement to change its operation mode back to the first operation mode and to operate in the first operation mode for the next selected time period or number of switching operations.

In yet another embodiment, a sensor may be configured to measure a temperature in the amplifier arrangement and to communicate the sensed temperature to the controller, wherein the controller may be configured to control the amplifier arrangement to change its operation mode to the second operation mode and to operate in the second operation mode in response to the measured temperature e.g. to avoid aging and/or over-heating of the switches of the first pair. To this end, the controller may be triggered to change the operation mode from the first operation mode to the second operation mode if the sensed temperature exceeds a threshold temperature. In still another embodiment, the electromagnetic bearing arrangement may comprise a plurality of electromagnets, each including a coil, wherein the amplifier arrangement may have, for each of the electromagnets, a switching leg pair, wherein each switching leg comprises two switches, the coil of the electromagnet is electrically connected to the junction between the switches of each of the switching legs, and each of switches is provided with a diode for bypassing the switch when the switch is in an off state.

The power may be electrically connected to opposite ends of each of the switching legs, and the controller may be configured to control the amplifier arrangement to operate in a first operation mode, wherein, for each of the switching leg pairs, a first pair of the switches are being switched while a second pair of the switches are kept in an off state, and the switches of each pair are located in different switching legs and electrically connected to different poles of the power supply. The controller may be configured, at some point, to control the amplifier arrangement to change its operation mode, for each of the electromagnets, to a second operation mode and to operate in the second operation mode, wherein, for each of the switching leg pairs, the first pair of the switches are switched to an off state and the second pair of the switches are being switched while the first pair of the switches are kept in the off state. In one version, the electromagnetic bearing arrangement may comprise four electromagnets located equiangularly around a rotating equipment having a horizontal rotation axis. The electromagnets may be located 45 degrees off an orientation wherein two electromagnets are located along a horizontal axis and two electromagnets are located along a vertical axis. In another version, the electromagnetic bearing arrangement may comprise four electromagnets located equiangularly around a rotating equipment having a vertical rotation axis.

It shall be appreciated that the changing of operation mode for the

electromagnets may be performed independently of one another. If the operation mode is changed due to a fault in one of the switches, only the operation mode for the electromagnet, for which the fault occurs, is changed. On the other hand, if the operation mode for the electromagnets is changed when a selected period of time has lapsed or when the same switches have been used for a selected number of switching operations, the operation mode for all the electromagnets may be changed concurrently with one another.

In yet another embodiment, the electromagnetic bearing may comprise a plurality of electromagnets located around a rotating equipment having a horizontal rotation axis. The electromagnetic bearing arrangement may comprise at least one upper and at least one lower electromagnet, each including a coil, wherein the amplifier arrangement may have, for each upper electromagnet, a switching leg pair, each switching leg having two switches, the coil of the electromagnet being electrically connected to the junction between the switches of each of the switching legs, and each of switches being provided with a diode for bypassing the switch when the switch is in an off state. Further, the amplifier arrangement may have, for each lower electromagnet, a switching leg pair, each switching leg having one switch and one diode, the coil of the electromagnet being electrically connected to the junction between the switch and the diode of each of the switching legs.

The power supply may be electrically connected to opposite ends of each of the switching legs, and the controller may be configured to control the amplifier arrangement to operate in a first operation mode, wherein, for each switching leg pair associated with an upper electromagnet, a first pair of the switches are being switched while a second pair of the switches are kept in an off state, wherein the switches of each pair are located in different switching legs and electrically connected to different poles of the power supply. The controller may be configured, at some point, to control the amplifier arrangement to change its operation mode of each upper electromagnet to a second operation mode and to operate in the second operation mode, wherein, for each switching leg pair associated with an upper electromagnet, the first pair of the switches are switched to an off state and the second pair of the switches are being switched while the first pair of the switches are kept in the off state. In one version, the electromagnetic bearing arrangement comprises two upper electromagnets and two lower electromagnets arranged equiangularly around a rotating equipment such as in the above disclosed 45 degrees orientation.

According to another aspect of the invention there is provided a method of operating an electromagnetic bearing arrangement comprising an

electromagnet including a coil; an amplifier arrangement having a switching leg pair, each switching leg having two switches, the coil being electrically connected to the junction between the switches of each of the switching legs; and a power supply electrically connected to opposite ends of each of the switching legs, wherein each of switches is provided with a diode for bypassing the switch when the switch is in an off state. According to the method, the amplifier arrangement is controlled to operate in a first operation mode, wherein a first pair of the switches are being switched while a second pair of the switches are kept in an off state, wherein the switches of each pair are located in different switching legs and electrically connected to different poles of the power supply., The amplifier arrangement is, at some point, controlled to change its operation mode to a second operation mode and operate in the second operation mode, wherein the first pair of the switches are switched to an off state and the second pair of the switches are being switched while the first pair of the switches are kept in the off state. The operation of the electromagnetic bearing arrangement according to the second aspect may be modified to comprise method steps for performing any of the functions, actions, or operations disclosed with respect to

electromagnetic bearing arrangement according to the first aspect.

The present invention provides for electromagnetic bearing arrangements and methods of operating an electromagnetic bearing arrangement, which are more reliable and which reduce the risk of a touch-down or other fault operation. Further, the lifetimes of the components of the electromagnetic bearing arrangements can be prolonged. Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig l is a diagrammatic illustration of a known amplifier arrangement,

Fig 2 illustrates schematically an electromagnetic bearing arrangement according to an embodiment of the invention.

Fig 3 is a diagrammatic illustration of an amplifier arrangement of an electromagnetic bearing arrangement according to an embodiment of the invention.

Fig 4 is a diagrammatic illustration of an amplifier arrangement of an electromagnetic bearing arrangement according to an embodiment of the invention.

Fig 5 illustrates schematically in a flow chart a method of operating an electromagnetic bearing arrangement according to an embodiment of the invention.

DETAILED DESCRIPTION

Fig 2 illustrates schematically an electromagnetic bearing arrangement according to an embodiment of the invention. The electromagnetic bearing arrangement comprises four electromagnets 2ia-d arranged e.g.

equiangularly around a rotating equipment 22 such as a rotor of an electric machine. The rotating equipment 22 may be oriented such that its rotation axis is horizontal and the four electromagnets 2ia-d may be arranged 45 degrees off the vertical and horizontal axis such that not a sole electromagnet will be responsible for counteracting the gravitational forces of the rotating equipment. Thus, the two electromagnets 2ia-b may be referred to as upper electromagnets and the two electromagnets 2ic-d may be referred to as lower electromagnets. It shall be appreciated that the electromagnetic bearing arrangement of the invention may comprise any number of electromagnets arranged in any suitable configuration.

The electromagnetic bearing arrangement further comprises an amplifier arrangement 23 for operating the electromagnets 2ia-d, a power supply 26 for supplying the amplifier arrangement 23 with power, and a controller 24 for controlling the amplifier arrangement 23 to operate the electromagnets 2ia-d.

Sensors 27 such as position sensors may be provided for measuring the positions of the electromagnets 2ia-d and for forwarding the measured positions to the controller 24, which controls the operation of the amplifier arrangement 23 and thus the electromagnets 2ia-d in response thereto, thus providing feedback control.

For exciting at least one of the electromagnets 2ia-d, the amplifier

arrangement 23 may comprise a switching leg pair 32, 33 as being illustrated in Fig. 3. Each switching leg 32, 33 has two switches 34a-b, 35a-b, and the coil 11 of the electromagnet is electrically connected to the junction between the switches 34a-b, 35a-b of each of the switching legs 32, 33. Each of switches 34a-b, 35a-b is provided with a diode 37a-b, 38a-b for bypassing the switch when the switch is in an off state. The power supply 26 is electrically connected to opposite ends of each of the switching legs 32, 33. The switches 34a-b, 35a-b are controlled by the controller 24 via the control terminals Gi- G 4 .

The switches 34a-b, 35a-b may be transistors such as e.g. FET's (Field Effect Transistors) or IGBT's (Insulated-Gate Bipolar Transistors).

It shall be appreciated that the controller 24 may be configured to control the amplifier arrangement of Fig. 3 to generate two-level or three level PWM (Pulse- Width Modulated) voltages over the electromagnet by using only two switches at a time: either switches 34a and 35b or switches 34b and 35a. This switching redundancy is used in the present invention. Thus, the controller 24 (Fig. 1) is configured to control the amplifier arrangement 23 to operate in a first operation mode, wherein a first pair of the switches 34a-b, 35a-b, say switches 34a and 35b, are being switched while a second pair of the switches 34a-b, 35a-b, say switches 34b and 35a, are kept in an off state. The switches of each pair are located in different switching legs 32, 33 and electrically connected to different poles of the power supply 26.

The controller 24 is further configured to control the amplifier arrangement 23, at some point, to change its operation mode to a second operation mode and to operate in the second operation mode, wherein the first pair 34a, 35b of the switches are switched to an off state and the second pair 34b, 35a of the switches are being switched while the first pair 34a, 35b of the switches are kept in the off state.

The controller 24 is capable of controlling the amplifier arrangement 23 to change the operation mode between the first and second operation modes at any point, and to alternately change between the first and second operation modes.

Several triggering mechanisms for triggering the controller to change the operation mode in order to obtain different advantages can be contemplated. The controller 24 may be configured to control the amplifier arrangement 23 to change the operation mode to the second operation mode and to operate in the second operation mode in response to a fault in one of the switches 34a, 35b of the first pair. The sensors 27 may be configured to detect the fault in the switches 34a, 35b of the first pair and to communicate this to the controller 24. Otherwise, dedicated sensors may be provided for this purpose. Yet alternatively, the default is detected by the controller 24 by analyzing the currents through the coil 11 or by means of signaling from the switches 34a-b, 35a-b.

In another embodiment, the controller 24 may be configured to control the amplifier arrangement 23 to change its operation mode to the second operation mode and to operate in the second operation mode when the amplifier arrangement 23 has operated in the first operation mode for a selected time period or the switches of the first pair has operated for a selected number of switching operations e.g. to avoid aging and/or over- heating of the switches of the first pair. The selected time period may be a predetermined set time period and the selected number of switching operations may be a predetermined set number of switching operations as performed by the switches of the first pair. In this embodiment, the operation mode may be changed back and fourth between the first and second modes at selected intervals.

Yet alternatively, the electromagnetic bearing arrangement may comprise a sensor configured to measure a temperature in the amplifier arrangement 23 and to communicate the sensed temperature to the controller 24, wherein the controller 24 may be configured to control the amplifier arrangement 23 to change its operation mode to the second operation mode and to operate in the second operation mode in response to the measured temperature e.g. to avoid aging and/or over-heating of the switches of the first pair.

Further, it shall be appreciated that any of the amplifier arrangement structures and operation mode changing capabilities described above may be implemented for more than one of the electromagnets, such as for half of them or for each of them. For the electromagnetic bearing arrangement of Fig. 2 any of the amplifier arrangement structures and operation mode changing capabilities described above may be implemented for each of the electromagnets 2ia-d or for only the upper electromagnets 2ia-b. Fig 4 is a diagrammatic illustration of an amplifier arrangement of an electromagnetic bearing arrangement for exciting two electromagnets according to an embodiment of the invention.

The amplifier arrangement may comprise a switching leg pair 32, 33 as the one of Fig. 3 and a further switching leg pair 42, 43. The further switching leg 42 has two switches 44a-b whereas the further switching leg 43 has one switch 45 and one diode 48. A coil 41 of a further electromagnet is electrically connected to the junctions between the switches 44a-b of the further switching leg 42 and between the switch 45 and the diode 48 of the further switching leg 43. Each of switches 44a-b of the further switching leg 42 is provided with a diode 47a-b for bypassing the switch when the switch is in an off state. The power supply 26 is electrically connected to opposite ends of each of the further switching legs 42, 43. Drives ACS Ml and ACS 850 commercially available from ABB comprise the topology of the amplifier arrangement of Fig. 4 and may be used in this invention. It shall be appreciated that the amplifier arrangement of Fig. 4 comprises a first part with switching redundancy (as disclosed with reference to Fig. 3) and one part without switching redundancy. Therefore, the switch 44a and the diode 47b of the further switch leg 44a may be dispensed with. If the switch 44a is present, it should be kept in an off state during operation. The switches 44a (if present), 44b, and 45 are controlled by the controller 24 via the control terminals G6, G5, and GB.

A controller may be configured to control the amplifier arrangement of Fig. 4 to operate one electromagnet (i.e. electromagnet having the coil 31) in a first operation mode, wherein, for each switching leg pair 32, 33 associated with this electromagnet, a first pair 34a, 35b of the switches are being switched while a second pair 34b, 35a of the switches are kept in an off state. The switches of each pair are located in different switching legs and electrically connected to different poles of the power supply 26.

The controller is then configured to, at some point, control the amplifier arrangement to change its operation mode for the electromagnet having the coil 33 to a second operation mode and to operate in the second operation mode, wherein, for the switching leg pair 32, 33 associated with the electromagnet having the coil 11, the first pair of the switches are switched to an off state and the second pair of the switches are being switched while the first pair of the switches are kept in the off state. Since the further switching leg pair 42, 43 has no switching redundancy, it cannot change operation mode.

If the amplifier arrangement of Fig. 4 is used in the electromagnetic bearing arrangement of Fig. 2, the switching leg pair 32, 33 with redundancy is used for exciting one of the upper electromagnets 2ia-b, whereas the switching leg pair 42, 43 without redundancy is used for exciting one of the lower electromagnets 2ia-b since an upper electromagnet 2ia-b is normally exposed to higher currents than a lower electromagnet due to the

gravitational forces. Obviously, two of the amplifier arrangement of Fig. 4 are required for exciting all electromagnets 2ia-d of the electromagnetic bearing arrangement of Fig. 2. In such arrangement, the operation mode can be changed for only two of the electromagnets 2ia-d, e.g. the upper electromagnets 2ia-b.

Fig 5 illustrates, schematically, in a flow chart, a method of operating an electromagnetic bearing arrangement according to an embodiment of the invention. The method is implemented in an electromagnetic bearing arrangement comprising an electromagnet including a coil; an amplifier arrangement having a switching leg pair, each switching leg having two switches, and the coil being electrically connected to the junction between the switches of each of the switching legs; and a power supply electrically connected to opposite ends of each of the switching legs, wherein each of switches is provided with a diode for bypassing the switch when the switch is in an off state.

According to the method, the amplifier arrangement is, in a step 51, controlled to operate the electromagnet in a first operation mode. The controlling comprises switching, in a substep 51a, a first pair of the switches and keeping, in a substep 51b, a second pair of the switches in an off state during the switching of the first pair of the switches. The switches of each pair are located in different switching legs and electrically connected to different poles of the power supply. Next, the amplifier arrangement is, in a step 53, controlled at some point to change its operation mode to a second operation mode and operate in the second operation mode. This controlling comprises switching, in a substep 53a, the first pair of the switches to an off state, switching, in a substep 53b, the second pair of the switches, and keeping, in a substep 53c, the first pair of the switches in the off state during the switching of the second pair of the switches.

The described control method may be modified to include method steps using any of the arrangements, devices and/or apparatuses as described above with respect to Figs. 2-4 for performing any of the disclosed functions. In particular, the method may comprise any of the conditions disclosed above to be met in order to perform the substeps 53a-c.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. In particular, it shall be appreciated that the different embodiments, features, and alternatives disclosed above may be combined to reach yet further embodiments of the invention.