GB2567671A

GB2567671A - Permanent magnet electrical machine

Info

Publication number: GB2567671A
Application number: GB1717232.1A
Authority: GB
Inventors: Shirokov Vladimir
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2017-10-20
Filing date: 2017-10-20
Publication date: 2019-04-24
Also published as: GB201717232D0

Abstract

A permanent magnet electrical machine 30 comprises a rotor 34 that rotates about a main rotation axis 54 and has rotor permanent magnets 48; a stator having stator windings 36; and an axially repositionable magnetic core 56 which is axially repositionable relative to the axis 54 between a first position in which a magnetic circuit extends from the rotor permanent magnet 48 through one or more stator windings 36; and a second position in which a magnetic circuit extends through the axially repositionable magnetic core 56 and returns to the rotor permanent magnet 48, bypassing the stator windings 36. The machine can be a radial flux machine with the rotor either inward or outward of the stator or an axial flux machine. Repositionable core 56 can rotate with the rotor or be static. There can be a stator core 44 and a rotor core 52 and a flux path can cross air-gaps 62, 46 between them. In the second position reluctance of the flux path through the magnet 48 and the movable magnetic core is lower than reluctance of the flux path through stator teeth.

Description

Permanent Magnet electrical Machine

The present disclosure concerns a permanent magnet electrical machine.

Electrical machines such as electrical motors and generators comprise rotors and stators. Magnetic fields generated by the stator and the rotor interact to provide torque (in the case of a motor) or electrical current through stator windings (in the case of a generator).

Various types of electrical machines exist, of which two common machine types of interest to aerospace are wound field electrical machines, and permanent magnet electrical machines. In a wound field electrical machine, the rotor comprises field windings through which current flows in operation to generate a magnetic field, whereas in a permanent magnet machine, the rotor magnetic field is provided by one or more permanent magnets. Permanent magnet electrical generators offer many advantages, such as higher power density and lower build complexity. However, since the rotor magnetic field is always present, permanent magnet electrical generators will always induce a voltage in the stator windings where the rotor is rotated even where the machine terminals are isolated. In the case of a winding fault, (such as a turn-to-turn short circuit) this can produce high uncontrollable current flow in the windings resulting in overheating and hazardous conditions. Consequently, conventional permanent magnet generators can only be switched off in the event of a turn-to-turn fault by preventing rotation of the rotor.

In view of the above, where a fault develops in the stator windings of a permanent magnet electrical generator, the rotating magnetic field from the rotor permanent magnets will cause a current to flow in the stator windings, which may result in overheating and further damage. Consequently, in safety critical applications such as aerospace, permanent magnet electrical machines are not generally used in spite of their high power density.

One alternative solution to prevent excessive electrical stator currents in the event of a fault in a permanent magnet electrical machine, is to short circuit the rotor magnetic flux in the event of a stator winding fault. US patent 7443070 proposes a permanent magnet electrical generator, in which a plurality of gates formed of magnetic material are provided between salient teeth of the stator. In normal operation, the gates are provided in an “open” position, spaced from the stator teeth. When a fault is detected, the gates are moved to a “closed” position contacting the teeth, to thereby short the magnetic flux through the teeth, before it passes through the stator windings. However, such a machine requires a gate for each stator tooth, and so requires a separate actuator for each tooth. This leads to a highly complex arrangement. The actuators are also positioned inside the stator slots, which is the source of heat inside the machine posing more complexity in keeping the system cool.

According to a first aspect there is provided a permanent magnet electrical machine comprising a rotor comprising one or more rotor permanent magnets and a stator comprising one or more stator windings, the rotor being rotatable about a main rotation axis, the machine comprising an axially repositionable magnetic core, wherein the axially repositionable magnetic core is axially repositionable relative to the main rotation axis between a first position in which a magnetic circuit extends from the rotor permanent magnet through one or more stator windings, and a second position in which a magnetic circuit extends through the axially repositionable magnetic core and returns to the rotor permanent magnet, bypassing the stator windings.

Advantageously, by having an axially repositionable magnetic core, a single actuator can be used to de-energise the stator windings in the event of a stator fault. The actuator mechanism can be accessed, repaired or replaced without disrupting the stator windings. The entire flux-bypass mechanism will be further away from the source of heat - the windings. This mechanism may allow a retrofit to existing stators, where a smaller rotor otherwise of the same design can be introduced, or a retrofit to existing rotors, where a larger stator otherwise of the same design can be introduced.

The machine may comprise one of a radial flux electrical machine and an axial flux electrical machine.

The axially repositionable magnetic core may be configured to rotate with the rotor in use, or remain static.

The stator may comprise a plurality of generally radially extending salient stator teeth projecting from a stator core back. The stator may comprise a stator magnetic core positioned between the rotor permanent magnet and the stator windings. The stator magnetic core may comprise the axially repositionable magnetic core. Alternatively, the axially moveable magnetic core may be provided separately from the stator magnetic core. The axially moveable magnetic core may be axially moveable relative to the stator magnetic core.

A first radial air gap may be provided between the stator windings and the stator magnetic core, and a second radial air gap may be provided between the stator magnetic core and rotor permanent magnet.

The rotor may comprise a rotor magnetic core positioned on a radially or axially opposite side of the rotor permanent magnet to the stator magnetic core.

A third radial air gap may be provided between the axially repositionable magnetic core and either the rotor magnetic core or the stator magnetic core.

The second position may be defined by a position in which a magnetic flux path through the permanent magnet, stator magnetic core and rotor magnetic core defines a lower reluctance flow path than a magnetic flux path through the permanent magnet, stator magnetic core and stator teeth.

The second position may be defined by a position in which at least part of the axially repositionable magnetic core and at least one of the stator magnetic core and the rotor magnetic core are radially aligned.

The stator may be provided radially outwardly of the rotor, or the rotor may be provided radially outwardly of the stator.

The electrical machine may be one of a radial flux machine and an axial flux machine

According to a second aspect of the invention there may be provided a gas turbine engine comprising an electrical generator comprising an electrical machine in accordance with the first aspect of the invention.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

Embodiments will now be described by way of example only, with reference to the Figures, in which:

Figure 1 is a cross sectional view of a first permanent magnet electrical machine, sectioned along a radial plane ofthe machine;

Figures 2a and 2b are close ups of the view of the electrical machine shown in figure 1, showing magnetic fields lines in a first mode and the second mode respectively;

Figures 3a and 3b are axial cross-sectional views through the electrical machine of figure 1 along the line A-A in the first mode and a second mode respectively;

Figures 4a and 4b are similar view to figures 3a and 3b, but of a second permanent magnetic electrical machine;

Figures 5a and 5b are similar view to figures 3a and 3b, but of a third permanent magnetic electrical machine;

Figures 6a and 6b are similar view to figures 3a and 3b, but of a fourth permanent magnetic electrical machine;

Figures 7a and 7b are similar view to figures 3a and 3b, but of a fifth permanent magnetic electrical machine;

Figure 8a and 8b show similar views to figures 3a and 3b respectively, but of a sixth electrical machine;

Figures 9a and 9b show similar views to figures 4a and 4b respectively, but of the sixth electrical machine;

Figure 10 shows a schematic side perspective view of a seventh permanent magnet electrical machine;

Figure 11 shows an axial cross sectional view through a stator of the machine of figure 10; and

Figure 11a and 11b show similar views to figures 3a and 3b respectively, but of the machine of figure 10.

With reference to figures 1, 2a, 2b, 3a and 3b, a first electrical machine in the form of an electrical generator 30 is shown. The generator 30 comprises a static stator 32, and a rotatable rotor 34, which is mechanically driven in use to energise stator windings 36 of the stator. The rotor 34 is rotatable about a main rotation axis 54.

A circumferential segment of the electrical generator 30 is shown in further detail in figures 2a and 2b.

The stator 32 comprises a stator core back in the form of an annular ring 38 from which a plurality of salient stator teeth 40 extend radially inwardly. The ring 38 and teeth 40 are made of a material having a high magnetic permeability such as soft iron, such that the ring 38 and teeth 40 have a low magnetic reluctance.

Together, the ring 38 and teeth 40 define a plurality of slots 42 therebetween, within which the stator windings 36 are located. Both the ring 38 and the teeth 40 are generally composed of a plurality of stacked laminations to reduce eddy currents in use.

A stator magnetic core 44 is provided radially inwardly of the teeth 40. The magnetic core again comprises a high magnetic permeability material to provide a low magnetic reluctance. The stator magnetic core 44 is in the form of an annular ring, laminated circumferentially or interrupted at the gaps between stator teeth, which remains static in use (i.e. does not rotate with the rotor 34). The stator magnetic core 44 is separated from the stator teeth 40 by a first air gap 46, shown more clearly in figures 3a and 3b. The first air gap 46 extends around the whole annulus of a radially outer face of the stator magnetic core 44. In this embodiment, the first air gap 46 may be very small, or may be omitted.

A plurality of rotor permanent magnets 48 are provided radially inwardly of the stator magnetic core 44. A first rotor soft (i.e. formed of a material having a high magnetic permeability) magnetic core in the form of a ring 50 is provided radially inwardly of the permanent magnets 48, and is separated from the stator soft magnetic core 44 by a second air gap 58. A second rotor magnetic core in the form of a second ring 52, either laminated circumferentially or not laminated at all, is provided radially inwardly of the first ring 50, and is again formed of a high magnetic permeability material.

A cross sectional view along the line A-A from figure 1 is shown in figures 3a and 3b in first and second positions respectively, as will be explained in further detail below. As can be seen, an axially repositionable magnetic core 56. The core 56 is located at a radial position radially inward of the stator magnetic core 44, and radially outward of the second rotor magnetic core 52. The axially repositionable magnetic core 56 radially is axially spaced from and overlaps with the rotor permanent magnet 48 and the first rotor magnetic core 50.

The axially translatable magnetic core 56 is spaced radially outwardly from the second rotor magnetic core 52 to define a third air gap 62. Notably, the third air gap 62 has a lesser radial extent than the first air gap 46, for reasons that will be explained below. In this embodiment, the axially repositionable magnetic core 56 is rotationally static (i.e. does not rotate with the rotor 34), though rotatable axially repositionable magnetic cores 56 are envisaged. The axially repositionable magnetic core 56 is in the form of a ring of high magnetic permeability material. As will be appreciated, a single axially repositionable magnetic core 56 can be provided, which can be actuated with a single actuator to bypass magnetic flux from all of the rotor permanent magnets 48 to all stator windings 36 at once. Consequently, the present arrangement provides significant advantages over the prior art.

The axially repositionable magnetic core 56 is repositionable in an axial direction from a first position (as shown in figure 3a) to a second position (as shown in figure 3b), i.e. in a direction parallel to the rotor rotational axis. In the first position, the axially repositionable magnetic core 56 is axially spaced from the stator magnetic core 44, permanent magnet 48, and second rotor magnetic core

52. Consequently, and as more clearly shown in figures 2a and 3a, a magnetic flux path B is provided which extends generally radially from the rotor permanent magnets 48, generally radially and circumferentially through the first rotor magnetic core 50 to loop back to the permanent magnet 48, then generally radially and circumferentially through the second air gap 58, the stator magnetic core 44, first air gap 46 and stator teeth 40, and generally circumferentially though the stator ring 38, before flowing around the stator windings 36 and radially back to the rotor permanent magnets 48 to form a magnetic circuit which extends around the stator windings 36 in a plane which extends generally normal to the main rotor axis 54. In view of this arrangement of rotor magnets and stator windings, in which the magnet flux extends between the rotor magnet and the stator windings in a generally radial direction, such an arrangement is known as a “radial flux” electrical machine. In the figures, solid flux lines represent flux lines in the illustrated plane, whereas dotted lines represent flux lines out of the illustrated plane. Consequently, with the axially repositionable magnetic core 56 in the first position, the magnetic field generated by the rotor permanent magnets 48 interacts with the stator windings 36 to generate an electrical current in the windings 36 as the rotor 34 rotates.

The axially repositionable magnetic core 56 is moveable from the first position to the second position by axially sliding the axially repositionable magnetic core 56 toward the rotor permanent magnet 48 such that a radially outer face of the axially repositionable magnetic core 56 abuts a radially inner face of the stator magnetic core 44, though a very small air gap may alternatively be maintained therebetween. In the second position, the axially repositionable magnetic core 56 is moved axially toward the stator magnetic core 44, permanent magnet 48, and second rotor magnetic core 52. Consequently, and as more clearly shown in figure 3b, a “shorted” magnetic flux path is provided which extends from the rotor permanent magnets 48, generally radially through the first rotor magnetic core 50, generally axially through the second magnetic core, generally radially through the third air gap 62, the axially repositionable magnetic core 56, third air gap 60, generally axially through the stator magnetic core 44, before flowing back to the rotor permanent magnets 48 via the third air gap 60 to form a magnetic circuit which bypasses the stator windings 36 and flows in a plane generally parallel to the rotor rotational axis 54. This magnetic flux path has a lower magnetic reluctance when the axially repositionable magnetic core 56 is in the second position than the magnetic flux path that flows through around the stator windings, in part as a consequence of the smaller third air gap 62 compared to the first air gap 46. This lower reluctance could also be provided by careful selection of low reluctance materials in the shorted flux path. Consequently, with the axially repositionable magnetic core 56 in the second position, the magnetic field generated by the rotor permanent magnets 48 does not substantially interact with the stator windings 36 to generate an electrical current in the windings 36 as the rotor 34 rotates, and so the electrical generator is effectively “turned off”, i.e. generated substantially no electrical power in the stator coils 36, irrespective of whether the rotor 34 is rotating as no voltage is induced in the stator windings 36 due to absence of magnetic flux, which is now bypassed.

Consequently, the electrical generator 30 can be de-activated by axially repositioning the axially repositionable magnetic core 56 from the first position to the second position, and activated by repositioning the axially repositionable magnetic core 56 from the second position to the first position. De-activation may take place in the event of the detection of a winding fault in the stator electrical windings 56.

For example, in the present invention, a motor damage prevention controller 64 is provided (only shown in figure 3a for brevity). The controller 64 is configured to detect a fault in the windings, such as a short circuit or open circuit fault. The controller 64 is also in signal communication with an actuator 66 which is configured to axially reposition the axially repositionable magnetic core 56. In normal operation, the controller 64 controls the actuator 64 to maintain the axially repositionable magnetic core 56 in the first position. However, in the event of a fault being detected, the controller 64 controls the actuator 64 to maintain the axially repositionable magnetic core 56 in the second position, to thereby de-activate the generator 30. The controller 64 and actuator 66 could be “fail safe”, such that the actuator 66 drives the axially repositionable magnetic core 56 to the second position in the event of no signal being received from the controller 64. For example, the actuator could comprise an electromagnetic actuator that is spring biased toward the second position.

Consequently, there is provided a permanent magnet electrical machine in which the rotor permanent magnet flux can be decoupled from the stator windings, to thereby reduce or substantially eliminate electrical power generator in the stator windings, whilst still permitting rotation of the rotor. Such a machine could be useful for example as an aircraft gas turbine engine electrical generator, since it is not desirable to shut-down an engine in the event of a generator electrical fault, and any added weight and mechanical complexity from a mechanical clutch is highly undesirable. This solution providing a decoupling mechanism which does not experience mechanical wear under normal operating conditions.

Referring to figures 4a and 4b, a second electrical machine in the form of a generator 130 is shown. The generator 130 is similar to the generator 30, and so only differences are described in detail.

The generator 130 comprises a stator 132 similar to the stator 32, and having a core back 138, teeth 140 and stator magnetic core 144 similar to the core 44. The generator 130 also comprises a rotor 136 similar to the rotor 36, having a permanent magnet 148 similar to the permanent magnet 48 and a rotor magnetic core 152 similar to the core 52.

The generator 130 also comprises an axially repositionable magnetic core 156. However, the core 156 differs from that of the core 56, in that, when in the second position, an axial end face of the core 156 abuts an axial end face of the core 144, as shown in figure 4b. Again, a low reluctance magnetic flux path is defined by the cores 156, 144, 152 and permanent magnet 148 when the core 156 is in the second position, and the permanent magnet, core 144, teeth 140 and core back 138 when the core 156 is in the first position.

Figures 5a and 5b show a third electrical machine in the form of a generator 230. Again, the generator 230 is similar to the generator 30, and so only differences are described in detail.

The generator 230 comprises a stator 232 similar to the stator 32, and having a core back 238, teeth 240 and stator magnetic core 244. The generator 230 also comprises a rotor 236 similar to the rotor 36, having a permanent magnet 248 similar to the permanent magnet 48 and a rotor magnetic core 250 similar to the core 50.

However, in this embodiment, the stator magnetic core 244 is axially moveable between first and second positions. The stator magnetic core 244 defines first and second air gaps 246, 258 at radially outer and inner faces respectively.

A further rotor magnetic core 256 is provided, which radially overlaps with and contacts the rotor magnetic core 250. The further rotor magnetic core 256 rotates with the rotor 236, and defines a third radial air gap 262 between a radially outer face of the further rotor magnetic core 256 and the stator magnetic core 244.

In use, where the stator magnetic core 244 is provided in the first position (as shown in figure 5a), magnetic flux extends through the rotor magnetic core 250, permanent magnet 248, stator magnetic core 244, teeth 420 and core back 238, and consequently extends around the stator windings. On the other hand, where the stator magnetic core 244 is provided in the second position (as shown in figure 5b), the third radial air gap 262 between the radially outer face of the further rotor magnetic core 256 and the stator magnetic core 244 is closed, so that the core 256 and core 244 contact one another. Consequently, the magnetic flux extends through the rotor magnetic core 250, permanent magnet 248, magnetic core 244 and further rotor magnetic core 256, thereby bypassing the stator windings.

Figures 6a and 6b show a fourth electrical machine in the form of a generator 330. Again, the generator 330 is similar to the generator 30, and so only differences are described in detail.

The generator 330 comprises a stator 332 similar to the stator 32, and having a core back 338 and teeth 340. The generator 330 also comprises a rotor 336 similar to the rotor 36, having a permanent magnet 348 similar to the permanent magnet 48 and a rotor magnetic core 350 similar to the core 50.

The rotor further comprises a further rotor magnetic core 344 which rotates with the rotor 336 in use. The further rotor magnetic core 344 is located radially between the permanent magnet 348 and the stator teeth 340, and defines a first air gap 346 between a radially outer surface of the core 344 and the radially inner surface of the teeth 340.

An axially repositionable rotor magnetic core 356 is provided, which is axially repositionable relative to the main rotation axis to define first and second positions. The axially repositionable rotor magnetic core 356 is provided radially inwardly of the further rotor magnetic core 344 and radially overlaps with the rotor core 350 and permanent magnet 348. In the first position, as shown in figure 6a, magnetic flux extends through the rotor magnetic core 350, permanent magnet 348, stator magnetic core 344, across the air gap 346, through the teeth

320 and core back 338, and consequently extends around the stator windings. On the other hand, where the rotor magnetic core 344 is provided in the second position (as shown in figure 5b), the radially inner face of the further rotor magnetic core 344 abuts a radially outer face of the axially repositionable rotor core 256, so that the core 356 and core 344 contact one another. An axial face of the core 356 also abuts axial faces of the magnetic core 350 and permanent magnet 348. Consequently, the magnetic flux extends through the rotor magnetic core 350, permanent magnet 348, core 344 and core 356, thereby bypassing the stator windings.

Figures 7a and 7b show a fifth electrical machine in the form of a generator 430. Again, the generator 430 is similar to the generator 30, and so only differences are described in detail.

The generator 430 comprises a stator 432 similar to the stator 32, and having a core back 438 and teeth 440. The generator 430 also comprises a rotor 436 similar to the rotor 36, having a permanent magnet 448 similar to the permanent magnet 48 and a rotor magnetic core 450 similar to the core 50.

The generator further comprises an axially repositionable magnetic core 456, which may be static, or may rotate with the rotor 436. The core 456 comprises axially extending radially inner 452 and radially outer 444 fingers connected by a generally radially extending connecting portion 445. The radially inner finger 452 is provided radially inward of the rotor core 50, while the radially outer finger 444 is provided radially between the permanent magnet 448 and stator teeth 440. An air gap 446 is defined between the rotor 446 and stator 432.

The core 456 is axially repositionable between a first position shown in figure 7a and a second position shown in figure 7b. In the first position, the fingers 444, 452 are axially spaced from the core 450, magnet 448 and teeth 440, such that magnetic flux extends through the rotor magnetic core 450, permanent magnet 448, across the air gap 446, through the teeth 320 and core back 338, and consequently extends around the stator windings. On the other hand, where the core 456 is in the second position, the fingers 444, 452 axially overlap with the core 450, magnet 448 and teeth 440, such that the finger 444 is located in the air gap 446. Consequently, the magnetic flux extends through the finger 452, rotor magnetic core 450, permanent magnet 448 and finger 444, and thereby bypasses the stator windings.

Each of figures 1 to 7 show electrical machines in which the rotor is provided radially inwardly of the stator. However, the disclosure is also applicable to electrical machines in which the rotor is provided radially outwardly of the stator.

Figures 8a, 8b, 9a and 9b show a sixth electrical machine 530 having a stator 532 and rotor 534. However, unlike the previous embodiments, the rotor 534 is provided radially outward of the stator 532. Consequently, the radial order of the components is generally reversed.

Starting from radially inwardly, the sixth electrical machine 530 comprises the stator 532 comprising a stator core back 538 and teeth 540 defining slots 542 therebetween. These components are similar to corresponding components of the first embodiment. Further radially outwardly, a first air gap 546 is provided, which is defined between radially opposite faces of the stator 532 and rotor 534.

Again, starting radially inwardly from the first air gap 546, the rotor 534 comprises a first rotor magnetic core 544, rotor permanent magnets 548 and second and third rotor magnetic cores 50, 52.

Referring to figures 9a and 9b, the machine 530 further comprises an axially moveable magnetic core 556, which operates in a similar manner to the previous described axially repositionable magnetic cores. As shown in figures 9a and 9b, the axially repositionable magnetic core 556 is axially moveable to define an “on” position (as shown in figures 8a and 9a), magnetic flux flows through the stator windings, and an “off” position, (as shown in figures 8b and 9b), in which in which the core 556 traverses the radially inner and outer cores 544, 552 of the rotor, such that the core 556 provides a return path for the magnetic flux B, such that it bypasses the stator windings.

Figures 10 to 13 show a seventh electrical machine 630 comprising a stator 632 and rotor 634. The rotor 634 is configured to rotate about an axis A, which defines an axial direction of the machine 630. However, unlike the previously described electrical machines, this machine is an axial flux machine rather than a radial flux machine.

Figure 10 shows a schematic side view of the seventh electrical machine 630. As can be seen, the stator 632 and rotor 634 are axially separated, such that magnetic flux flows at least partly in an axial direction between the stator and rotor 632, 634 across an air gap therebetween.

The stator 632 comprises a soft iron magnetic core in the form of a stator core back 638. The stator core back 638 is in the form of a disk formed of a soft iron. A plurality of soft iron teeth 640 axially extend from a front face of the disk of the core back 638. Stator windings 636 are wound around the teeth 640.

A stator magnetic core is provided axially forward of the teeth 640. The stator magnetic core comprises a plurality of radially extending, circumferentially spaced fingers 644. The fingers 644 extend from a radial position of a radially inner end of the teeth 640, to a position radially outward of a radially outer end of the stator core back 638, such that the fingers 644 extend radially outward to a greater extent than the core back 638.

An air gap is provided axially forward of the fingers 644, and the rotor 634 is provided axially forward of the air gap. The rotor 634 similarly comprises soft iron, and also comprises a plurality of permanent magnets 648.

The machine 630 further comprises a plurality of axially moveable magnetic cores 656, each comprising a material such as soft iron. A moveable core 656 is provided between each pair of adjacent fingers 644, though only two cores 656a, 656b are shown in figure 10 for clarity.

As can be seen from figure 10, the axially moveable magnetic cores are translatable between an “off” position (i.e. the position shown for core 656b) and an “on” position (i.e. the position shown for the core 656a). in some cases, the core 656 would all be in the same position, so that only a single actuator is required to move all cores 656 to an on or an off state. However, individual cores may be moveable to allow a sub-set of electrical windings 636 to be isolated in the event of a fault.

When in the “on” position (as shown by core 656a in figure 10, and in figure 12a), the cores 656b are located axially spaced from the radial plane of the fingers 644, either adjacent the stator core back 638, or further axially spaced in the backward axial direction. Consequently, in this position, a magnetic circuit Bi is defined. The magnetic circuit Bi flows through the rotor 634, the fingers 644, the stator windings around the stator teeth 640, and through the stator core back 638. Consequently, when operated as a generator, the magnetic field generated by the rotating rotor 634 interacts with the magnetic field of the stator windings to generate an electrical current in the field windings, such that the generator generates electricity.

On the other hand, when in the “off” position (as shown by core 656b in figure 10, and in figure 12b), the cores 656b are located in the radial plane of the fingers 644, circumferential between adjacent pairs of fingers, radially outwardly of the stator windings. Consequently, in this position, a magnetic circuit B₂ is defined. The The magnetic circuit B₂ flows through the rotor 634, the one of the fingers 644, the axially moveable magnetic core 656b, back through an adjacent one of the fingers 644, and back to the rotor 634. Consequently, the magnetic circuit bypasses the stator windings, such that the magnetic field generated by the rotor does not interact with the stator windings, such that no electrical current is generated in the windings.

It will be understood that the invention is not limited to the embodiment abovedescribed and various modifications and improvements can be made without departing from the concepts described herein.

For example, the present invention could be applied to an electrical machine in the form of a motor/generator, which can be configured as either a motor or a generator..

In a further example, the electrical machine could be arranged with the stator radially inward of the rotor.

Claims

1. A permanent magnet electrical machine (30) comprising a rotor (34) comprising one or more rotor permanent magnets (48) and a stator (32) comprising one or more stator windings (36), the rotor (34) being rotatable about a main rotation axis (A), the machine (30) comprising an axially repositionable magnetic core (56), wherein the axially repositionable magnetic core (56) is axially repositionable relative to the main rotation axis (A) between a first position in which a magnetic circuit extends from the rotor permanent magnet (48) through one or more stator windings (36), and a second position in which a magnetic circuit extends through the axially repositionable magnetic core (56) and returns to the rotor permanent magnet (48), bypassing the stator windings (36).

2. An electrical machine according to claim 1, wherein the machine comprises one of a radial flux electrical machine and an axial flux electrical machine.

3. An electrical machine according to claim 1 or 2, wherein the axially repositionable magnetic core (56) is configured to rotate with the rotor (34) in use, or remain static.

4. An electrical machine according to any of the preceding claims, wherein the stator (32) comprises a plurality of generally radially extending salient stator teeth (40) projecting from a stator core back (38).

5. An electrical machine according to any of the preceding claims, wherein the stator (32) comprises a stator magnetic core (44) positioned between the rotor permanent magnet (48) and the stator windings (36).

6. An electrical machine according to any of the preceding claims, wherein the stator magnetic core (244) comprises the axially repositionable magnetic core (244).

7. An electrical machine according to any of claims 1 to 5, wherein the axially moveable magnetic core (56) is provided separately from the stator magnetic core (44), and wherein the axially moveable magnetic core (56) is axially moveable relative to the stator magnetic core (44).

8. An electrical machine according to any of the preceding claims, wherein a first radial air gap (46) is provided between the stator windings (36) and the stator magnetic core (44).

9. An electrical machine according to any of the preceding claims, wherein a second radial air gap (58) is provided between the stator magnetic core (44) and rotor permanent magnet (48).

10. An electrical machine according to any of the preceding claims, wherein the rotor (34) comprises a rotor magnetic core (52) positioned on a radially or axially opposite side of the rotor permanent magnet (48) to the stator magnetic core (44).

11. An electrical machine according to claim 5 or claim 10, wherein a third radial air gap (62) is provided between the axially repositionable magnetic core (56) and either the rotor magnetic core (52) or the stator magnetic core (44).

12. An electrical machine according to claims 4, 5 and 10 or any claim dependent thereon, wherein the second position is defined by a position in which a magnetic flux path through the permanent magnet (48), stator magnetic core (44) and rotor magnetic core (56) defines a lower reluctance flow path than a magnetic flux path through the permanent magnet (48), stator magnetic core (44) and stator teeth (40).

13. An electrical machine according to claim 5 or claim 10 or any claim dependent thereon, wherein the second position is defined by a position in which at least part of the axially repositionable magnetic core (56) and at least one of the stator magnetic core (44) and the rotor magnetic core (52) are radially aligned.

14. An electrical machine according to any of the preceding claims, wherein

5 the stator (32) is provided radially outwardly of the rotor (34).

15. An electrical machine according to any of claims 1 to 13, wherein rotor is provided radially outwardly ofthe stator.

10

16.A gas turbine engine comprising an electrical generator comprising an electrical machine in accordance with any ofthe preceding claims.