GB2484098A

GB2484098A - Dynamo-electric machine with rotor magnet adjustable shunt

Info

Publication number: GB2484098A
Application number: GB1016354.1A
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: 2010-09-29
Publication date: 2012-04-04
Also published as: GB2484163B; GB2484163A; GB201016354D0; GB2484161A; US20130187504A1; GB2484161B; GB2484162A; GB2484164A; GB2484162B; GB201106723D0; CN103119838A; JP2013544483A; GB2484164B; GB201106613D0; GB201106338D0; GB201106526D0; WO2012042844A1; EP2622721A1

Abstract

A dynamo-electric machine includes a rotor 1 having a plurality of permanent magnets 3, a stator 5 having a plurality of stator coils 9, 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 position and a non-shunting position. The rotor 1 includes a magnetically anisotropic rotor core 11 and a magnetically isotropic core element 24 situated between the - V- of the magnets 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 (figs 11,12) to achieve even further flux diversion 14. 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 "EMF") in the stator coils, which opposes the driving current delivered to the coils. The effect of this is to limit the maximum rotational speed and the maximum output power of the motor. This force will be referred to as "back EM F" because it opposes the rotation of the motor.

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 that 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 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. Typically, high torque values will occur at low rotational speeds.

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 back EMF, 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, it 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.

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 the present invention there is provided 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 for shunting the magnetic flux of at least one of the permanent magnets, said shunt being displaceable between a shunting position and a non-shunting position, wherein the rotor includes an anisotropic rotor core and at least one 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 it 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-shaped formation helps to increase flux linkage with the stator.

In an example, the rotor includes one or more primary magnetically isotropic core elements that are 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 magnetic shunt is constructed and arranged for axial movement towards and away from one end of the rotor.

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; 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; Figures 5 and 6 are further circular axial sections of the prior art machine along line X of Figure 3, showing computer-modelled illustrations of the magnetic flux lines with the shunt in, respectively, non-shunting and shunting positions; Figure 7 is a radial cross-section showing schematically part of the rotor and stator of a dynamo-electric machine according to a first embodiment of the invention; Figure 8 is a circular axial section of the dynamo-electric machine of Figure 7 with a shunt in a shunting position; Figures 9 and 10 are circular axial sections through the dynamo-electric machine of Figure 7, showing computer-modelled illustrations of the magnetic flux lines with the shunt in, respectively, non-shunting and shunting positions; Figure 11 is a radial cross-section showing part of the rotor and stator of a dynamo-electric machine according to a second embodiment of the invention; and Figure 12 is a circular axial section through the dynamo-electric machine of Figure 11, with the shunt in a shunting position.

It should be noted that the "circular" sectional views of figures 4-6, 8-10, and 12 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 Z 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 2-6 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 on axis Z. The rotor 1 includes a plurality of permanent magnets 3 mounted in a cylindrical electromagnetic 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 5 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 5 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 roller 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 I to the shaft 10 through the roller 15, the cam plate 16 and the ball splines 17.

It will be appreciated that although the cam mechanism is shown using a roller, one or more balls may be used instead of the roller, 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 roller 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. In 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 back EMF induced in the stator coils 9 by rotation of the rotor magnets 3, allowing the rotor ito rotate at a higher speed and to deliver more power.

The arrangement of the rotor magnets 3 and the stator coils 9 in the prior art machine is illustrated in more detail in Figures 3 and 4. 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 I 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 i4a 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 ii.

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 (5) poles facing inwards. As a result, the first and second pairs of magnets produce a magnetic field having an outer part 14a 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 I and the stator 5.

The effect of the shunt 4 is shown more clearly in Figures 5 and 6. Figure 5 illustrates the magnetic flux lines of the magnetic field 14 produced by the permanent magnets 3 when the shunt 4 (not shown in this Figure) is in an inoperative or non-shunting position. This is the situation associated with low speed/high torque output, when the shunt 4 is separated from the end face of the rotor 1, and therefore does not significantly affect the strength of the magnetic field produced by the permanent magnets 3. The magnetic field lines 14 are perpendicular to the rotational axis of the rotor and are substantially evenly spaced.

Figure 6 illustrates the magnetic flux lines 14 of the permanent magnets 3 when the shunt 4 is in a shunting position. This is the situation associated with high speed and low torque output, when the shunt 4 is in a shunting condition and is pressed against the end face of the rotor 1 in order to short-circuit the permanent magnets 3. The effect on the magnetic flux lines can be seen at the left-hand end of the rotor 1, where some of the flux lines 14c pass through the shunt 4 instead of extending outwards into the stator 5. Calculations have shown that the flux linkage with the stator 5 is reduced by 4.7% when the shunt is in the shunting position, as compared to the situation in which it is in a non-shunting condition as illustrated in Figure 5. Therefore, the flux leakage through the shunt is about 4.7%.

Therefore, although the shunt 4 causes some flux leakage and a corresponding reduction in flux linkage with the stator 5, the flux leakage through the shunt is relatively small. The applicant believes that this is because the rotor I has an anisotropic laminated core 11 whose reluctance is small in the radial and tangential directions, but large in the axial direction. As a result, the shunt 4 only has a significant effect on the magnetic field in the end region of the rotor core 11 that abuts the shunt 4. The magnetic field in parts of the rotor that are separated by a greater axial distance from the shunt 4 is substantially unaffected by the shunt.

A dynamo-electric machine according to a first embodiment of the present invention is illustrated in Figures 7-10. Except as described below, the machine is similar to the prior art machine shown in Figure 2, which is further described in JP2007-244023A. The description of the prior art machine therefore applies equally to the embodiment shown in Figures 7-10, except where indicated otherwise.

In this embodiment, rotor I 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 (Figures I and 2). One core element 24 is associated with each pair of magnets 3, the core element 24 being located within the V-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 vee-formation. Thus, as illustrated in Figure 7, 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, as 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 illustrated in Figures 9 to 10. When the shunt 4 is removed from the end of the rotor core 11 to a non-shunting position, as shown in Figure 9, 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 1 0, 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 whose shunted magnetic flux circuit is shown in Figure 6. 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 14c 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, as illustrated in Figure 9. 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 5 and 6.

A dynamo-electric machine according to a second embodiment of the invention is illustrated in Figures 11 and 12. The machine is similar to the first embodiment shown in Figures 7 to 10, and the previous description applies equally to the second embodiment, except where indicated otherwise.

The rotor I includes, in addition to the permanent magnets 3, the laminated rotor core 11 and the set of primary elongate magnetic core elements 24, a set of secondary elongate magnetic core elements 26 that extend through the rotor core 11 substantially parallel to the rotor axis. One secondary core element 26 is associated with each pair of magnets 3, each secondary core element 26 being located between the inner faces of the magnets 3 and the inner cylindrical surface of the rotor core 11. Thus, as illustrated in Figure 11, a first secondary core element 26a is associated with the first pair of permanent magnets 3a and a second secondary core element 26b is associated with the second pair of permanent magnets 3b.

The primary and secondary core elements 24, 26 are both made of a magnetically isotropic material that conducts the magnetic flux equally in all directions, but which is preferably electrically non-conductive. For example, the core elements 24, 26 may be made of a soft magnetic composite (SMC) material comprising insulated iron powder particles. The isotropic core elements 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 magnetic core elements 24, 26 is illustrated by the magnetic flux lines 14 shown in Figures 11 and 12. When the shunt 4 is located against the end of the rotor I, as shown in Figure 12, the core elements 24, 26 create a magnetic flux circuit that passes through the primary and secondary core elements 24, 26 and the shunt 4 and extends even further into the length of the rotor core in the axial direction than in the first embodiment shown in Figures 7 to 10. In particular, as illustrated in Figure 11, the magnetic flux within the shunt 4 includes a first component 14d that passes tangentially between adjacent primary core elements 24a, 24b and a second component 14e that passes radially between the paired primary and secondary core elements 24a, 26a. The core elements 24, 26 thus help further to short-circuit the magnetic flux between adjacent permanent magnets and thus further to reduce the flux linkage with the stator coils. They also help each permanent magnet to short-circuit flux within itself, from one pole to the other, in addition to assisting flux leakage between adjacent magnets.

When the shunt 4 is removed from the end of the rotor core 11, the primary and secondary core elements 24, 26 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.

Certain modifications to the various forms of the dynamo-electric machine described above are of course possible. For example, although in each of the drawings the isotropic core elements 24, 26 are shown extending through the entire axial length of the rotor 1, they may be of a shorter length. For example, they may be provided only at or adjacent one or both ends of the rotor. The isotropic core elements 24, 26 may also extend beyond the rotor core at one or both ends of the rotor.

Claims

CLAIMS1. 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 for shunting the magnetic flux of at least one of the permanent magnets, said shunt being displaceable between a shunting position and a non-shunting position, wherein the rotor includes a magnetically anisotropic rotor core and at least one magnetically isotropic core element.
2. A dynamo-electric machine according to claim 1, wherein the magnetically anisotropic rotor core comprises a plurality of laminations that extend substantially perpendicular to the rotor axis, and at least one magnetically isotropic core element extends substantially parallel to the rotor axis.
3. A dynamo-electric machine according to claim I or claim 2, wherein 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.
4. A dynamo-electric machine according to claim 3, wherein each group of permanent 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.
5. A dynamo-electric machine according to claim 4, wherein each magnetically isotropic core element is located within the V-formation of a pair of permanent magnets.
6. A dynamo-electric machine according to any one of the preceding claims, wherein the rotor includes one or more primary magnetically isotropic core elements that are located radially outwards of the permanent magnets.
7. A dynamo-electric machine according to claim 6, wherein the rotor includes one or more secondary magnetically isotropic core elements that are located radially inwards of the permanent magnets.
8. 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.
9. A dynamo-electric machine according to claim 8, wherein the magnetically isotropic core elements are made of a soft magnetic compound material.
10. A dynamo-electric machine according to any one of the preceding claims, wherein the magnetic shunt is constructed and arranged for axial movement towards and away from one end of the rotor.
11. A dynamo-electric machine according to claim 10, including a drive mechanism for controlling axial movement of the magnetic shunt.
12. A dynamo-electric machine according to claim 11, wherein the drive mechanism for controlling axial movement of the magnetic shunt comprises a roller and cam drive mechanism.
13. A dynamo-electric machine according to claim 11, wherein the drive mechanism for controlling axial movement of the magnetic shunt comprises a ball and cam drive mechanism.
14. A dynamo-electric machine according to any one of claims 11 to 13, wherein the drive mechanism for controlling axial movement of the magnetic shunt is automatically operated and is driven by motor torque output.
15. A dynamo-electric machine constructed and arranged according to any one or more of the accompanying Figures 7 to 12.