TECHNICAL FIELD

The present invention relates to a simplified radial magnetic bearing.

BACKGROUND ART

Rotary electric machines such as motors, generators and magnetic bearings include a stationary component known as the stator and a rotating component known as the rotor. The stator and rotor are separated by an air gap across which a magnetic field traverses. In the case of magnetic bearings, the magnetic field between the stator and rotor is used to support a shaft so as to allow rotation of the shaft in the bearing without the need for further mechanical support.

Such magnetic bearings are well known and are described in further detail in, for example United States Patent No. 4,500,142.

Normally, the stator of a magnetic bearing is formed from a number of pole pieces having an electrical coil wound about them so as to generate a magnetic field between the stator and rotor. The magnetic field established between the rotor and stator attracts the rotor towards the stator. Thus, the position of the rotor can be maintained at the desired datum by regulating the current supplied to each coil in response to a signal indicative of the position of the rotor within the stator.

Magnetic bearings have highly desirable operational characteristics compared with other types of bearings. However, one drawback of magnetic bearings is their relatively high cost. Magnetic bearings typically contain several electromagnets. Each electromagnet requires a separate coil and connections and hence requires numerous manufacturing steps. This results in a costly bearing assembly. Moreover, there are often four or more electromagnets within a bearing, with each electromagnet requiring an expensive current switch capable of modulating the current delivered to each electromagnet coil. This further increases the cost of the magnetic bearing system and restricts the number of applications in which magnetic bearings can be used.

Accordingly, an object of this invention is to provide a simplified magnetic bearing.

DISCLOSURE OF INVENTION

The invention provides:

(a) a rotor assembly;

(b) a stator assembly, wherein said stator assembly comprises a plurality of pole pieces circumferentially spaced about the axis of rotation of said rotor assembly and terminates at a gap separating said rotor assembly and said stator assembly, and wherein each of said pole pieces is associated with at least one adjacent pole piece to define a sector of said stator assembly, such that said stator assembly contains at least one steady state sector and at least two controllable sectors;

(c) means to produce a steady state magnetic field between said at least one steady state sector and said rotor assembly; and

(d) at least two individually controllable means to generate a controllable magnetic field between said at least two controllable sectors and said rotor assembly.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a sectional view of a magnetic bearing according to this invention;

Figure 2 is a sectional view of an alternate embodiment of a magnetic bearing according to this invention; and

Figure 3 is a block diagram of a typical control system for a controllable sector of a magnetic bearing.

Referring to Figure 1, a magnetic bearing 1, consists of a stator assembly 34 and rotor assembly 36. The rotor assembly 36 is fitted to shaft 35 via rotor sleeve 37. The stator assembly housing 2 is preferably formed from non-magnetic material to provide a barrier to magnetic flux (i.e. to mitigate the flow of magnetic flux from the magnetic bearing into adjacent machine components). Contained within stator housing 2 is the stator core 3. The stator core 3 is preferably formed from a series of laminations of magnetic material stacked along the longitudinal axis of the bearing assembly. The use of laminations to form the stator core reduces the generation of

eddy currents during variations in the magnetic field within the core and is a conventional construction technique for magnetic bearings. Likewise, rotor assembly 36 is comprised of a series of laminations assembled along the longitudinal axis of the bearing assembly. The laminations are fitted on rotor sleeve 37 which is then mounted on shaft 35.

Slots 5 are formed in the stator core 3 to define poles 38, 39, 40, 41, 42 and 43. Each pole extends radially inwards and terminates at gap 6 which lies between the stator assembly 34 and the rotor assembly 36. As shown in Figure 1, a pair of adjacent poles define each sector of the bearing and accordingly three pairs of poles define sectors 8, 26 and 27. For clarity, the boundary of each sector is shown on Figure 1 by a dotted line. Multiple turns of electrically conductive wire encompass each of the poles in the stator to form coils 9, 10, 11, 12, 13 and 14 which are associated with each pole. The coils of adjacent poles within a sector are connected in series.

Coils 9 and 10, of sector 8, are connected to a source of steady state current (not shown) which, for example, may be a direct current power supply. The current supplied to coils 9 and 10 generates a steady state magnetic field, which pulls the rotor towards sector 8 of the stator assembly. The steady state magnetic field is not provided with any active control means capable of varying the attractive strength of the magnetic field in response to nutations of the rotor. However, although no active control system is associated with the magnetic field, a means of selecting one of a number of steady state field strengths (not shown) is not outside the scope of this invention. For example, if the magnetic bearing is installed in a machine which is capable of accepting a variety of shafts of different weights, it would be beneficial to minimize the current supplied to the coil while at the same time providing sufficient magnetic force to support the various shaft weights. Through a switch means the magnitude of the steady state current supplied to the coils could be set in accordance with the weight of the shaft. In other words, the switch means would simply provide the capability to select from more than one steady state currents.

To prevent unrestricted translation of rotor assembly 37 towards sector 8, two controllable sectors 26 and 27 are provided. Sectors 26 and 27 are termed

controllable sectors because it is possible to independently modulate the magnitude of the magnetic force between the rotor and each of the sectors 26 and 27. Thus, in summary, the magnetic bearing shown in Figure 1 contains a single steady state stator section 8 having a steady state magnetic field and only two controllable stator sectors 26 and 27 having controllable magnetic fields. Preferred bearings according to this invention contain only one steady state sector (although more than one such sector may be employed) and only two controllable sectors (although more than two such sectors may be employed). A particular advantage of using only two controllable sectors is reduced cost, as such a bearing requires only the use of two sector control circuits.

The major components of a conventional sector control circuit used to modulate the magnetic force of a controllable sector of a rotor are shown in Figure 3. Coils 11 and 12 of Figure 3 are representative of coils 11 and 12 (or alternatively, coils 13 and 14) of Figure 1. Transducer 19 is responsive to the motion of the rotor relative to the stator. The output signal from the transducer is supplied to processor 20 through conductor 22. The response characteristics for the bearing system are preset in processor 20. The output signal from the processor supplied by conductor 23 to amplifier 21, represents the desired response to the motion detected by the transducer. Amplifier 21 modulates the current being supplied to coils 11 and 12 through conductors 24 and 25 in response to the input received through conductor 23. For a specific magnetic bearing, there is a relationship between the magnitude of the current delivered to the coils of the electromagnets, and the attractive force between the stator and rotor of the bearing. It is, therefore, possible to modulate the force of the magnetic bearing in response to the motion of the rotor. Further details regarding conventional sector control circuits are known to those skilled in the art and are described, for example in: "Magnetic Bearings for Vibration Reduction and Failure Prevention" (Allaire, P.E. et al, presented at the 40th Meeting of the Mechanical Failures Prevention Group, Gaithersburg, Md. April 16-18, 1985).

It will be readily understood by those knowledgeable in the art of magnetic bearings, that a multitude of variations in stator assembly and rotor assembly designs are possible. An example of one such variations would be where a sector contains

two poles but, a coil is wound around only one pole in the sector. These variations are in general a function of the requirements and constraints of specific applications. These variations generally arise from common engineering design practices readily understood by those skilled in the design of magnetic bearings.

An alternative embodiment of the invention is shown Figure 2. In this embodiment, discrete magnets 28, 29 and 30 each represent a distinct pair of pole pieces (and each pair of pole pieces represents a sector) and are positioned at circumferentially spaced locations about rotor 31. By way of explanation, magnet 28 has two radially extending limbs 28(a) and 28(b) which constitute a pair of pole pieces. Coil 44 of electromagnet 28 is supplied with a steady state current. Electromagnets 29 and 30 (which both have a pair of radially extending limbs that constitute a pair of pole pieces), and associated coils 32 and 33 form the controllable sectors of the magnetic bearing of Figure 2. Coils 32 and 33 are connected to separate sector control circuits which supply current to the coils in response to the motion of rotor 31.

Thus, the bearing shown in Figure 2 contains only one steady state sector (i.e. represented by magnet 28) and only two controllable sectors (i.e. represented by magnets 29 and 30). Moreover, the controllable sectors are each controllable by simply adjusting the current supplied to only one coil (i.e. coil 33 controls magnet 30 and coil 32 controls magnet 29).

One further advantage of employing discrete magnets is the ease with which commercially available oriented grain structure magnetic materials can be used to form the magnets. The oriented magnetic material is readily available in tape form and can be wound around a mandrel to form a magnet of the desired shape. Further details about the use of oriented material is contained in co-pending United States Patent No. 5,095,237.

In certain situations where further simplification of the bearing is desired, the electromagnet associated with the steady sector of the bearing may be replaced in part or completely by a permanent magnet. The permanent magnet would provide the necessary steady state force on the rotor while eliminating the need for a coil and power supply associated with the uncontrolled sector of the magnetic bearing.

INDUSTRIAL APPLICABILITY

The invention relates to a simplified radial magnetic bearing which is suitable in rotating machinery (especially compressors, turbines and the like).