GB2484162A

GB2484162A - Dynamo-electric machine with rotor magnet adjustable shunt

Info

Publication number: GB2484162A
Application number: GB1106526.5A
Authority: GB
Inventors: Daiki Tanaka
Original assignee: Nissan Motor Manufacturing UK Ltd
Current assignee: Nissan Motor Manufacturing UK Ltd
Priority date: 2010-09-29
Filing date: 2011-04-18
Publication date: 2012-04-04
Anticipated expiration: 2031-04-18
Also published as: GB2484163B; GB2484163A; GB201016354D0; GB2484161A; US20130187504A1; GB2484161B; GB2484164A; GB2484162B; GB201106723D0; CN103119838A; JP2013544483A; GB2484098A; GB2484164B; GB201106613D0; GB201106338D0; GB201106526D0; WO2012042844A1; EP2622721A1

Abstract

A dynamo-electric machine includes a rotor 1 having a plurality of permanent magnets 3 and an adjustable magnetic shunt 4, for shunting the magnetic flux of at least one of the permanent magnets 3, the shunt 4 being displaceable axially between a shunting and a non-shunting position. The rotor 1 includes a magnetically anisotropic rotor core 11 and a magnetically isotropic core element 24 and extending beyond the core end to increase the flux quantity14 shunted when the shunt 4 contacts the rotor end. A further isotropic core 26 may be provided inboard of the magnets to achieve even further flux diversion 14. An end cover 28 having recesses to accommodate the ends of the core elements 24,26 may be included to define a minimum separation t4 between the shunt and the core elements. As shown in Fig 9 the cover element may have apertures adjacent the ends of core elements 24,26. The anisotropic rotor may be achieved by means of laminations and the isotropic core parts may be an electrically non-conductive material comprising a soft magnetic composite (SMC) of insulated iron powder particles. The prior art fig 2 shows an example of appropriate shunt control.

Description

Dynamo-electric machine The present invention relates to a dynamo-electric machine and in particular, but not exclusively, to a dynamo-electric machine comprising a brushless DC motor having a permanent magnet rotor mounted within an annular stator.

Dynamo-electric machines of the type described above may be used either as motors or as generators. It should be understood that although such a machine may be referred to herein as a "motor", this is not intended to preclude the possible use of the machine as a generator by driving it in reverse.

In dynamo-electric machines of the type described, the rotor carries a set of permanent magnets and the stator carries a set of stator coils. These stator coils are energised sequentially to produce a rotating magnetic field, which causes rotation of the permanent magnet rotor. When rotating, the permanent magnets of the rotor induce an electro-motive force (hereinafter abbreviated to "back EMF"), which induces a voltage in the stator coils which increases as the rotor speeds up. This induced voltage must be kept below the input voltage of the electrical supply, so as to avoid damage to the power supply devices, such as the inverter and battery. This control of induced voltage allows power to be fed into the motor to increase output. However, most of the current used to control induced voltage does not contribute directly to torque generation. It is therefore desirable to minimize current used for control purposes.

Permanent magnet motors can run faster and therefore generate more power if the magnetic flux of the permanent magnets is small, as this reduces the induced back EMF.

On the other hand, permanent magnet motors can generate more torque if the magnetic flux of the permanent magnets is large. Various systems have been proposed for modifying the flux linkage between the rotor magnets and the stator coils in order to deliver high torque at low speeds and high power at high speeds, by altering the physical or electrical layout of the stator or the rotor.

JP2007-244023A describes a permanent magnet dynamo-electric machine having a rotor that carries a set of permanent magnets and a magnetic shunt (or "short-circuit ring") that is mounted on the shaft of the rotor for axial movement towards and away from one end of the rotor. The shunt consists of an iron ring, which serves as a magnetic yoke that modifies the magnetic field on the rotor magnets. It is mounted on a drive device that transmits torque between the rotor and the shaft. A cam mechanism is provided between the rotor and the drive device, which controls axial movement of the shunt relative to the rotor according to the amount of torque generated by the rotor. When the shunt is located in a shunting position adjacent to the end of the rotor it reduces the reluctance of the rotor and effectively short-circuits the permanent magnets, thereby causing some of the magnetic flux to leak between the magnets through the shunt. This reduces the flux linkage between the rotor magnets and the stator coils. When the shunt is located in a non-shunting position in which it is displaced away from the end of the rotor, it does not significantly affect the magnetic field of the permanent magnets or the flux linkage with the stator coils.

The torque produced by the motor is usually highest at low rotational speeds and decreases as the rotational speed of the motor increases. At high torque values the shunt is displaced away from the end of the rotor by the cam mechanism, producing a gap between the rotor and the shunt. When the shunt is in this non-shunting position the flux linkage with the stator coils is at a maximum, thus allowing the machine to produce a high torque.

At low torque values the shunt is pressed against the end of the rotor by a spring. In this shunting position the shunt reduces the reluctance of the rotor and partially short-circuits the permanent magnets, effectively reducing the flux linkage with the stator coils. This reduces the induced voltage in the stator, allowing the rotor to rotate at a higher speed and increasing the maximum power of the machine.

The applicant has found that in the dynamo-electric machine described in JP2007- 244023A, although the shunt causes flux leakage and thus reduces the flux linkage between the permanent magnets and the stator, the flux linkage is only reduced by about 5%. Therefore, although the shunt increases the power of the machine at high revolution speeds, the increase is quite small.

In co-pending British patent applications No. 1016354.1 and No. 1106338.5, the present applicant describes a dynamo-electric machine including a rotor having a plurality of permanent magnets, a stator having a plurality of stator coils, and a magnetic shunt mechanism for shunting the magnetic flux of the permanent magnets, wherein the rotor includes a magnetically anisotropic rotor core and one or more magnetically isotropic core elements.

When the shunt mechanism is actuated to shunt the magnetic flux of the permanent magnets, the isotropic core elements reduce the reluctance of the rotor core in the axial direction, thus increasing flux leakage through the shunt and reducing flux linkage between the permanent magnets and the stator coils. This allows the motor to be driven faster and at a higher power level.

However, there are a number of potential problems with the mechanism described in the applicant's previous patent applications.

A first problem relates to the fact that the magnetically anisotropic rotor core in which the permanent magnets and the magnetically isotropic core elements are mounted generally has a laminated structure, comprising a plurality of thin steel sheets that extend generally perpendicular to the rotor axis. These thin steel sheets are not strongly bonded to one another. When the shunt mechanism is actuated and the shunt is located in contact with one end of the rotor, there is a strong attractive magnetic force between the shunt and the laminations nearest to that end of the rotor. The applicant has found that when the shunt is moved away from the end of the rotor, this can sometimes cause one or more of the end laminations to separate and bend away from the rest of the core, thus damaging the rotor and leading to reduced performance.

A second problem is that the magnetically isotropic core elements are also attracted magnetically to the shunt. Repeated movement of the shunt away from the end of the rotor can cause one or more of the magnetically isotropic core elements to be partially withdrawn from the rotor core. This affects the size of the air gap between the magnetically isotropic core elements and the shunt, leading to flux leakage and reduced performance. Axial displacement of the core elements may also lead to unbalanced magnetic loading on the rotor and eddy current losses. The unsupported ends of the displaced magnetically isotropic core elements may also suffer distortion caused by centrifugal forces at high rotor speeds.

Third, as well as reducing the reluctance of the rotor core in the axial direction, the magnetically isotropic core elements also slightly increase the reluctance of the rotor core in the tangential and radial directions. The flux linkage between the permanent magnets and the stator coils is thus slightly reduced, adversely affecting the performance of the machine.

It is an object of the present invention to provide a dynamo-electric machine that mitigates at least some of the aforesaid disadvantages.

According to one aspect of the present invention there is provided a dynamo-electric machine including a rotor having a plurality of permanent magnets, a magnetically anisotropic rotor core and one or more magnetically isotropic core elements, a stator having a plurality of stator coils, and a magnetic shunt for shunting the magnetic flux of at least one of the permanent magnets, said shunt being axially displaceable relative to the rotor between a shunting position and a non-shunting position, wherein each magnetically isotropic core element includes an end portion that extends axially beyond the magnetically anisotropic rotor core in the direction of the magnetic shunt.

When the shunt engages the end of the rotor, the axially-extended end portions of the magnetically isotropic core elements provide a gap between the shunt and the laminated core, which limits the magnitude of the attractive magnetic force between the shunt and the laminated core. This helps to prevent separation of the core laminations when the shunt is moved away from the end of the rotor core.

In a typical electric motor such as a drive motor for an electric vehicle, the rotor will have an overall length of about 10-20cm. In such a motor, the end portion of each magnetically isotropic core element typically extends axially beyond the magnetically anisotropie rotor core by a distance t1 in the range 3-10mm, usually about 4-5mm. This provides a gap that is generally sufficient to eliminate any significant risk of damage to the rotor. For larger or smaller motors, the length of the end portions may be increased or decreased accordingly.

In one preferred embodiment, each magnetically isotropic core element extends axially into the magnetically anisotropic rotor core by a distance t2. Advantageously, the distance t2 is less than the length t3 of the magnetically anisotropic rotor core. The distance t2 is preferably less than 50% oft3 and more preferably less than 20% oft3. In the end region of the rotor core, the magnetically isotropic core elements reduce the reluctance of the core in the axial direction but slightly increase the core reluctance in the tangential and radial directions. In other parts of the core that do not contain magnetically isotropic core elements, the reluctance is unaffected. The magnetically isotropic core elements therefore only increase the axial reluctance in the end region of the core, where axial flux leakage is required.

In another preferred embodiment, each magnetically isotropic core element is located on an end face of the magnetically anisotropic rotor core and does not extend significantly in the axial direction into the magnetically anisotropic rotor core. In other words, the distance t2 is zero. This ensures that the effect of the magnetically isotropic core elements on the axial reluctance of the core is confined to the very end region of the core.

Advantageously, the rotor includes a non-magnetic cover element mounted on an end of the magnetically anisotropic rotor core, so that the cover provides a predetermined minimum separation t4 between the shunt and the ends of the magnetically isotropic core elements. The cover element is preferably made of a non-magnetic material such as aluminium, stainless steel or titanium. The cover element serves a number of useful functions: it controls the separation distance t4 of the magnetically isotropic core elements from the shunt, it holds the magnetically isotropic core elements in place axially, it supports the end portions of the magnetically isotropic core elements radially, and it supports the laminated core to prevent separation of the laminations.

A cover element may also be provided in a machine where the magnetically isotropic core elements do not have axially-extended end portions. Therefore, according to another aspect of the present invention there is provided a dynamo-electric machine including a rotor having a plurality of permanent magnets, a magnetically anisotropic rotor core and one or more magnetically isotropic core elements, a stator having a plurality of stator coils, and a magnetic shunt for shunting the magnetic flux of at least one of the permanent magnets, said shunt being axially displaceable relative to the rotor between a shunting position and a non-shunting position, wherein the rotor includes a non-magnetic cover element mounted on an end of the magnetically anisotropic rotor core that provides a predetermined minimum separation t4 between the shunt and the ends of the magnetically isotropic core elements.

If the magnetically isotropic core elements have axially-extended end portions, the cover element advantageously includes a plurality of recesses for accommodating the end portions of the magnetically isotropic core elements. The cover element then supports the end portions of the magnetically isotropic core elements and protects them from centrifugal distortion at high rotor speeds.

Advantageously, the cover element includes holes that extend through an outer end face of the cover element, said holes being located adjacent the ends of the magnetically isotropic core elements. The holes prevent eddy currents being generated in the cover element adjacent the ends of the magnetically isotropic core elements.

Advantageously, the holes have a diameter D1 and the recesses have a diameter D0, where D is less than D0. This provides a shoulder where each hole meets the corresponding recess, which prevents axial displacement of the magnetically isotropic core element.

The magnetically isotropic core elements increase flux leakage through the shunt when the shunt is in the shunting position, thereby making the shunt more effective, and increasing the power of the machine at high rotational speeds.

In an example, the anisotropic rotor core comprises a plurality of laminations that extend substantially perpendicular to the rotor axis, and at least one magnetically isotropic core element that extends substantially parallel to the rotor axis. The magnetically isotropic core elements then assist the flow of magnetic flux in the axial direction of the rotor when the shunt is in the shunting position. The laminations extending substantially perpendicular to the rotor axis may be substantially circular.

In an example, the rotor includes a plurality of groups of matched permanent magnets, and at least one magnetically isotropic core element is associated with each group of permanent magnets. The magnetically isotropic core element assists the leakage of flux into the shunt for the associated group of magnets.

In an example, each group of magnets includes at least two magnets that are arranged in a V-formation with regard to a cross-section of the rotor across its axis. The V-formation helps to increase flux linkage with the stator.

In an example, the rotor includes one or more primary magnetically isotropic corc elements that arc located radially outwards of the permanent magnets, for example within the apex of the V-formation.

In an example, the rotor includes one or more secondary magnetically isotropic core elements that are located radially inwards of the permanent magnets. The secondary magnetically isotropic core elements increase flux leakage through the shunt by encouraging the magnetic flux to flow radially through the shunt. This supplements the tangential flux path through the shunt that is encouraged by the primary magnetically isotropic core elements.

In an example, the magnetically isotropic core elements are made of a material that is electrically non-conductive, so as to prevent the creation of eddy currents.

In an example, the magnetically isotropic core elements are made of a soft magnetic compound material.

In an example, the dynamo-electric machine includes a drive mechanism for controlling axial movement of the magnetic shunt.

In an example, the drive mechanism for controlling axial movement of the magnetic shunt comprises a roller and cam drive mechanism.

In an alternative example, the drive mechanism for controlling axial movement of the magnetic shunt comprises a ball and cam drive mechanism.

In an example, the drive mechanism for controlling axial movement of the magnetic shunt is automatically operated and is driven by motor torque output.

Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic isometric view of a rotor of a dynamo-electric machine, illustrating the axial (A), radial (R), and tangential (T) directions thereof Figure 2 is an axial section showing part of a prior art dynamo-electric machine as described in JP2007-244023A; S Figure 3 is a schematic radial cross-section showing part of the rotor and stator of the prior art machine, showing the magnetic flux lines of the rotor magnets; Figure 4 is a circular axial section of the prior art machine along dashed line X of Figure 3, with the shunt 4 in a shunting position; Figure 5 is a radial cross-section showing schematically part of the rotor and stator of a dynamo-electric machine of the type described in the applicant's co-pending British patent applications No. 1016354.1 andNo. 1106338.5; Figure 6 is a circular axial section of the dynamo-electric machine of Figure 5 with a shunt in a shunting position; Figure 7 is an axial section of the dynamo-electric machine of Figure 2, illustrating potential problems with that machine; Figure 8 is an axial section of a dynamo-electric machine according to a first embodiment of the present invention; Figure 9 is an axial section of a dynamo-electric machine according to a second embodiment of the present invention; and Figure 10 is an axial section of a dynamo-electric machine according to a third embodiment of the present invention.

It should be noted that the "circular" sectional views of figures 4 and 6 could also be called "developed" sections. These views present a view of the rotor and stator as if it were cut through along the dashed line X in Figure 3, and then flattened out. Hence, the rotor shaft 10 of Figures 1 and 2 cannot be seen. These views are not easy to visualize in terms of looking at a motor and its components, but are invaluable in terms of understanding the flow of magnetic fields. The top and bottom of each of these Figures are adjacent (rather than opposed) on the actual components.

Figures 2-4 illustrate a prior art dynamo-electric machine of the type described in JP2007- 244023A. The machine includes a rotor 1 mounted by means of an angular bearing 2 and a needle bearing 6 on a shaft 10, which rotates on axis Z. The rotor 1 includes a plurality of permanent magnets 3 mounted in a cylindrical magnetic rotor core 11, which is supported by an inner rotor body member 12. The rotor core 11 is made of laminated steel sheets that extend substantially perpendicular to the rotor axis Z and serve to reduce energy losses by hysteresis and eddy currents. As the core 11 is laminated only in the axial direction (as if it were a stack of compact discs), it has anisotropic magnetic properties and encourages the magnetic field 14 of the permanent magnets 3 to flow in the tangential direction (T, Fig. 1) and the radial direction (R), but not in the axial direction (A) of the rotor.

An annular stator S surrounds the rotor 1, a small radial air gap being provided between the outer surface of the rotor 1 and the inner surface of the stator 5. The stator S comprises a plurality of stator coils 9 wound onto cores 8. The stator cores 8 are mounted in a case 7 that forms a housing of the dynamo-electric machine. By supplying electrical current sequentially to the coils 9, a rotating magnetic field can be generated within the stator, which causes the rotor 1 to rotate by sequentially attracting and repelling the rotor magnets 3.

A magnetic shunt assembly 13 is mounted on the shaft 10 adjacent to one end of the rotor 1. The shunt assembly 13 comprises a shunt 4 in the form of an annular iron ring or yoke, and a cam plate 16 that is mounted via ball splines 17 on the shaft 10 for axial movement towards or away from the rotor 1. The cam plate 16 is urged towards the adjacent face of the rotor body 12 by a disc spring 21 that is compressed between the cam plate 16 and a nut 18 on the shaft 10. Cam plate 16 is rigidly connected to the shunt 4 so that they move together, both rotationally and longitudinally. Alternatively, the cam plate and shunt may comprise a single, integrated component.

A shunt drive mechanism is provided for controlling axial movement of the shunt assembly 13. In this case the shunt drive mechanism comprises a cam mechanism that includes at least one ball 15 located in ramped grooves 19, 20 in opposed end faces of the rotor body member 12 and the cam plate 16. It should be noted that the rotor 1 is rotatably mounted on the shaft 10 via the angular bearing 2 and the needle bearing 6. Torque is transmitted from the rotor 1 to the shaft 10 through the ball 15, the cam plate 16 and the ball splines 17.

It will be appreciated that although the cam mechanism is shown using one or more balls, one or more rollers may be used instead of the balls, as may be suitable to the application.

At high torque values the rotor 1 rotates relative to the shunt assembly 13 and the movement of the ball 15 within the ramped grooves 19, 20 drives the shunt assembly 13 axially away from the rotor, so that there is a gap between the shunt 4 and the rotor magnets 3. In this non-shunting position, the shunt 4 does not significantly affect the

magnetic field generated by the rotor magnets 3.

At low torque values, the shunt assembly 13 is pressed by the spring 21 against the end face of the rotor 1, as shown in Figure 2. Tn this shunting position, the shunt 4 partially short-circuits the rotor magnets 3, so that the magnetic flux 14 flows partially through the shunt 4. This reduces the magnetic flux linkage between the rotor 1 and the stator 5, and thus reduces the voltage induced in the stator coils 9 by rotation of the rotor magnets 3, allowing the rotor 1 to rotate at a higher speed and deliver more power.

The rotor 1 includes a plurality of planar permanent magnets 3, the poles of the magnets being located on their radially outer and inner faces. The permanent magnets 3 extend axially along the length of the rotor 1 and are arranged in matched pairs, both magnets of each pair 3a, 3b having the same polarity and each pair of magnets having an opposite polarity to the adjacent pairs. The two magnets of each pair are inclined towards each other in a V-shaped formation. As shown in Figure 3, this has the effect of shaping the magnetic field 14 so that the outer part l4a of the field extends radially outwards to increase flux linkage with the stator 5, while the inner part 14b of the magnetic field passes directly between the magnets 3 through the rotor core 11.

In the example shown in Figure 3, the first pair of magnets 3a have their South (5) poles facing outwards and their North (N) poles facing inwards relative to the axis Z of the rotor, whereas the second pair of magnets 3b have their North (N) poles facing outwards and their South (S) poles facing inwards. As a result, the first and second pairs of magnets produce a magnetic field having an outer part 1 4a that extends radially outwards beyond the cylindrical surface of the rotor 1, and an inner part 14b that extends inwards to a far lesser radial extent.

The stator 5 includes a large number of coils 9 that are arranged around the internal face of the stator. These coils are energised consecutively to produce a rotating magnetic field within the stator, which causes rotation of the rotor 1.

In Figure 4 the shunt 4 is shown in a shunting position, in which it abuts the end of the rotor. The shunt has a low reluctance and therefore when it is located in this position it effectively short-circuits the permanent magnets 3, causing flux leakage through the shunt 4, and thus reducing the flux linkage between the rotor 1 and the stator 5.

A dynamo-electric machine of the type described in the applicant's co-pending British patent applications No. 1016354.1 and No. 1106338.5 is illustrated in Figures 5 and 6.

Except as described below, the machine is similar to the prior art machine shown in Figures 2 to 4, which is further described in JP2007-244023A. The description of the prior art machine therefore applies equally to the machine shown in Figures 5 and 6, except where indicated otherwise.

In this machine, rotor 1 includes -in addition to the permanent magnets 3 and the laminated rotor core 11 -a plurality of elongate magnetic core elements 24 that extend through the rotor core 11, substantially parallel to the rotor axis Z. One core element 24 is associated with each pair of magnets 3, the core element 24 being located within the "I-shaped gap between the outer faces of the magnets 3 and the outer cylindrical surface of the rotor core 11. Hence, the core elements 24 may be surrounded at least partly on at least two sides by permanent magnets, which may be arranged in a V-formation. Thus, as illustrated in Figure 5, a first core element 24a is associated with the first pair of magnets 3a and a second core element 24b is associated with the second pair of magnets 3b.

The core elements 24 are made of a magnetically isotropic material that conducts the magnetic flux equally in all directions, but which is preferably electrically non-conductive since an electrically conductive material would encourage eddy current losses. For example, the core elements 24 may be made of a soft magnetic composite (SMC) material comprising insulated iron powder particles. The isotropic core elements 24 therefore serve to reduce the overall magnetic reluctance of the rotor core in the axial direction without significantly increasing eddy current losses.

The effect of the isotropic core elements 24 is to modify the flux linkage with the stator when the shunt 4 is in the shunting position. When the shunt 4 is removed from the end of the rotor core 11 to a non-shunting position, the isotropic core elements 24 do not significantly affect the magnetic flux of the permanent magnets 3, as in the absence of the shunt 4, there is virtually no magnetic flux flowing in the axial direction of the rotor.

When the shunt 4 is located against the end of the rotor 1, as shown in Figure 6, the isotropic core elements 24 help to create a magnetic flux circuit that passes through the shunt 4 and extends through the core elements 24 further into the length of the rotor core 11 in the axial direction than in the prior art machine. Within the rotor 1, the magnetic flux 14 flows in the axial direction within the isotropic core elements 24 and in the tangential direction within the laminated core 11. In the shunt ring 4 the magnetic flux I 4c flows mainly in the tangential direction between adjacent magnets 3. The isotropic core elements 24 thus help to short-circuit the magnetic flux between adjacent permanent magnets, and thus to reduce the flux linkage with the stator coils 9. In this configuration, the applicant has calculated that the flux linkage with the stator coils is reduced by 6.7% as compared to the situation when the shunt 4 is in a non-shunting condition. Therefore, the flux leakage is about 6.7%. This represents a 44% increase in flux leakage as compared to the value of 4.7% achieved with the prior art machine illustrated in Figures 2 to 4.

A potential problem with the previously described machines is illustrated in Figure 7.

When the shunt 4 is located against the end of the rotor 1, a strong attractive magnetic force AMF is created between the shunt 4 and the laminated rotor core 11, and also between the shunt 4 and the magnetically isotropic core elements 24. When the shunt 4 is moved away from the end of the rotor 1, this large magnetic force can potentially cause damage to the rotor 1 in two different ways, as described below. It may be noted incidentally that in the machine shown in Figure 7, two core elements 24 are associated with each permanent magnet 3.

The first way in which the rotor 1 can be damaged is through damage to the laminations of the rotor core 11. When the shunt element 4 is moved away from the end of the rotor 1, the large attractive magnetic force between the shunt element 4 and the laminated core 11 can cause one or more laminations at the end of the rotor core 11 to be separated from the other laminations and bent towards the shunt element, as shown at lid. This reduces the magnetic efficiency of the rotor, and can lead to reduced performance and/or mechanical failure.

A second way in which the rotor 1 can be damaged is that one or more of the core elements 24 may be pulled partially out of the rotor core 11 by the attractive magnetic force AMF when the shunt element 4 moves away from the end of the rotor core 11, as shown in Figure 7. Movement of a core element 24 will affect the size of the air gap AG between the core element 24 and the shunt element 4, leading to flux leakage and reduced performance. Axial displacement of the core elements 24 may also lead to unbalanced magnetic loading to the stator coils 9 which may create unwanted loop currents within different coils of the same phase.

The first of the problems mentioned above may be avoided by using the modified rotor configuration shown in Figure 8, in which the magnetically isotropic core elements 24 have end portions 26 that extend beyond the end of the laminated rotor core ii by a distance ti. This prevents the shunt 4 from coming into contact with the end of the rotor core 11, and thus avoids a large attractive magnetic force being developed between the shunt and the end laminations of the core 11. Therefore, when the shunt 4 is moved away from the end of the rotor 1, there is a reduced risk of the end laminations of the rotor core being separated and bent away from the other laminations, as shown in Figure 7.

The distance ti by which the end portions 26 extend beyond the end of the laminated rotor core 11 is generally quite small. For example, in a motor having a rotor length t3 of about 10-20cm, such as a drive motor for an electric vehicle, distance ti may typically be in the range 3 to 10mm, usually about 4 to 5mm. The distance ti depends on the size of the motor and the strength of the permanent magnets 3, and must be sufficient to ensure that the attractive magnetic force between the shunt 4 and the rotor core 11 is reduced to a level at which damage to the rotor core 11 cannot result when the shunt 4 is moved away from the end of the rotor 1.

Figure 8 also illustrates another optional modification of the previous arrangement as illustrated in Figure 7. In this modification the magnetically isotropic core elements 24 extend into the laminated rotor core 11 by a distance t2 that is less than the overall stack length t3 of the laminated core 11, and is also less than the length of the permanent magnets 3. In some circumstances, it may be unnecessary for the core elements 24 to extend through the entire length of the laminated core 11, as significant flux leakage in the axial direction takes place only in the end region of the rotor adjacent the shunt 4. Away from that end of the core, the flux lines are substantially radial and the magnetically isotropic core elements 24 therefore provide no benefit; and may actually slightly increase the reluctance of the core in the radial and tangential directions. Thus, the efficiency of the machine may be increased slightly by decreasing the length of the magnetically isotropic core elements 24, so that they are provided only in the end region of the rotor adjacent the shunt 4.

The distance t2 may be substantially less than the stack length t3 of the rotor, for example less than 50% of the stack length t3, and is preferably a small fraction of t3, for example approximately 10-20% oft3. In some circumstances, it may be desirable for the distance t2 to be equal to zero, in which case the magnetically isotropic core elements 24 will be surface-mounted on the end face of the rotor core 11.

A second embodiment of the invention is shown in Figure 9. This is similar to the embodiment shown in Figure 8, except that it includes a cover plate 28 made of a non-magnetic material such as aluminium or stainless steel, which is mounted on the end of the rotor 1 adjacent the shunt 4. Recesses 30 are provided on the axially inner face (adjacent to rotor 1) of the cover plate 28 to accommodate the outwardly extending end portions 26 of the magnetically isotropic core elements 24. The thickness of the cover plate 28 is slightly greater than the distance ti (Figure 8) by which the end portions 26 extend beyond the end of the laminated rotor core 11. Thus, the cover plate 28 provides a designed separation t4 between the ends 26 of the magnetically isotropic core elements 24 and the shunt 4 when the shunt is pressed against the axially outer face of the cover plate 28, this separation distance t4 being equal to the difference between the thickness of the cover plate 28 and the distance ti (Figure 8) by which the end portions 26 extend beyond the end of the laminated rotor core 11.

By controlling the separation distance t4, the cover plate 28 ensures that each of the magnetically isotropic core elements 24 short-circuit the same amount of magnetic flux into the shunt 4, thus avoiding unwanted loop currents being generated in the stator coils 9.

The cover plate 28 also holds the magnetically isotropic core elements 24 in place, preventing axial displacement of the core elements by the attractive magnetic force between the core elements 24 and the shunt 4. Furthermore, the cover plate 28 supports the end portions 26 of the magnetically isotropic core elements 24, to prevent radial distortion of the core elements by the centrifugal forces experienced at high rotor speeds.

Finally, the cover plate 28 supports the laminated core 11 and prevents axial distortion of the core laminations, which can be caused either by vibration of the rotor or by the attractive magnetic forces developed between the shunt 4 and the core 11.

A third embodiment of the invention is shown in Figure 10. This is similar to the second version shown in Figure 9 except that holes 32 are provided in the cover plate 28, which extend from the ends of the recesses 30 to the outer face of the cover plate 28. Each hole 32 has a diameter D that is less than the diameter D0 of the recess 30, and also less than the diameter of the magnetically isotropic core element 24. The shoulder 34 formed at the point where the hole 32 meets the recess 30 engages the end of the magnetically isotropic core element 24 to prevent axial movement of the core element.

The cover plate 28 of the third embodiment therefore serves a number of useful functions: it controls the separation distance t4 of the magnetically isotropic core elements 24 from shunt 4; it holds the magnetically isotropic core elements 24 in place axially; it supports the end portions 26 of the magnetically isotropic core elements 24; and it supports the laminated core 11 to prevent separation of the core laminations. Tn addition, the holes 32 prevent eddy currents from forming within the cover plate in the regions between the ends of magnetically isotropic core elements 24 and the shunt 4 (compare with the eddy currents EC shown in Figure 9). This reduces parasitic losses, and improves the efficiency of the machine.

The second and third embodiments shown in Figures 9 and 10 may be modified by omitting the end portions 26 of the magnetically isotropic core elements 24 that extend beyond the end face of the laminated rotor core 11. In that case, the recesses 30 can also be omitted and the thickness of the cover plate 28 can be reduced. In this modified version, the cover plate 28 controls the separation distance t4 of the magnetically isotropic core elements 24 from the shunt 4, it holds the magnetically isotropic core elements 24 in place axially and it supports the laminated core 11 to prevent separation of the core laminations. Preferably, holes 32 are provided in the cover plate adjacent the ends of the magnetically isotropic core elements 24 to prevent the formation of eddy currents.

Claims

CLAIMSA dynamo-electric machine including a rotor having a plurality of permanent magnets, a magnetically anisotropic rotor core and one or more magnetically isotropic core elements, a stator having a plurality of stator coils, and a magnetic shunt for shunting the magnetic flux of at least one of the permanent magnets, said shunt being axially displaceable relative to the rotor between a shunting position and a non-shunting position, wherein each magnetically isotropic core element includes an end portion that extends axially beyond the magnetically anisotropic rotor core in the direction of the magnetic shunt.
2. A dynamo-electric machine according to claim 1, wherein the end portion of each magnetically isotropic core element extends axially beyond the magnetically anisotropic rotor core by a distance tj in the range 3-10mm.
3. A dynamo-electric machine according to claim 1 or claim 2, wherein each magnetically isotropic core element extends axially into the magnetically anisotropic rotor core by a distance t2.
4. A dynamo-electric machine according to claim 3, wherein the distance t2 is less than the length t3 of the magnetically anisotropic rotor core.
5. A dynamo-electric machine according to claim 1 or claim 2, wherein each magnetically isotropic core element is located on an end face of the magnetically anisotropic rotor core and does not extend significantly in the axial direction into the magnetically anisotropic rotor core.
6. A dynamo-electric machine according to any one of claims 1-5, wherein the rotor includes a non-magnetic cover element mounted on an end of the magnetically anisotropic rotor core, so that the cover provides a predetermined minimum separation t4 between the shunt and the ends of the magnetically isotropic core elements.
7. A dynamo-electric machine including a rotor having a plurality of permanent magnets, a magnetically anisotropic rotor core and one or more magnetically isotropic core elements, a stator having a plurality of stator coils, and a magnetic shunt for shunting the magnetic flux of at least one of the permanent magnets, said shunt being axially displaceable relative to the rotor between a shunting position and a non-shunting position, wherein the rotor includes a non-magnetic cover element mounted on an end of the magnetically anisotropic rotor core that provides a predetermined minimum separation t4 between the shunt and the ends of the magnetically isotropic core elements.
8. A dynamo-electric machine according to claim 6 or claim 7, wherein the cover element includes a plurality of recesses for accommodating end portions of the magnetically isotropic core elements.
9. A dynamo-electric machine according to any one of claims 6 to 8, wherein the cover element includes holes that extend through an outer end face of the cover element, said holes being located adjacent the ends of the magnetically isotropic core elements.
10. A dynamo-electric machine according to claim 9 when dependent on claim 8, in which the holes have a diameter D and the recesses have a diameter D0, where D1 is less than D0.
11. A dynamo-electric machine according to any one of the preceding claims, wherein the magnetically anisotropic rotor core comprises a plurality of laminations that extend substantially perpendicular to the rotor axis, and at least one of the magnetically isotropic core elements extends substantially parallel to the rotor axis.
12. A dynamo-electric machine according to any one of the preceding claims, wherein the magnetically isotropic core elements are made of a material that is electrically non-conductive.
13. A dynamo-electric machine according to claim 12, wherein the magnetically isotropic core elements are made of a soft magnetic compound material.
14. A dynamo-electric machine according to any one of the preceding claims, including a drive mechanism for controlling axial movement of the magnetic shunt.
15. A dynamo-electric machine according to claim 14, wherein the drive mechanism for controlling axial movement of the magnetic shunt comprises a roller and cam drive mechanism.
16. A dynamo-electric machine according to claim 14, wherein the drive mechanism for controlling axial movement of the magnetic shunt comprises a ball and cam drive mechanism.
17. A dynamo-electric machine according to any one of claims 14 to 16, wherein the drive mechanism for controlling axial movement of the magnetic shunt is automatically operated and is driven by motor torque output.
18. A dynamo-electric machine constructed and arranged according to any one or more of the accompanying Figures 7 to 10.