GB2450465A

GB2450465A - Switching Flux Permanent Magnet Electrical Machine

Info

Publication number: GB2450465A
Application number: GB0704314A
Authority: GB
Inventors: Shinichiro Iwasaki; Rajesh Pranay Deodhar; Zi-Qiang Zhu
Original assignee: IMRA Europe SAS; IMRA Europe SAS UK Research Center
Current assignee: IMRA Europe SAS; IMRA Europe SAS UK Research Center
Priority date: 2007-03-06
Filing date: 2007-03-06
Publication date: 2008-12-31
Anticipated expiration: 2027-03-06
Also published as: GB2450465B; GB0704314D0

Abstract

A switching flux permanent magnet electrical machine comprises a salient pole rotor (4) rotatable about an axis within a stator 2. The stator has a plurality of elementary cells each comprising a winding 10 about a core 12, the core including a radially extending permanent magnet 8 which is polarised circumferentially. The cell has two oppositely polarised radially magnetised permanent magnets 20, 21 on the radially outer surface of the core. Alternatively the oppositely polarised magnets 30,31 may be on the axial end of the core or magnets may be present on both core surfaces - fig 6. A back iron 22 may be present as may air spaces 24 between magnets. Various configurations are disclosed. The machine may alternatively comprise a linear machine.

Description

* 2450465

I

Switching Flux Permanent Magnet Electrical Machine

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates to a switching flux permanent magnet electrical machine.

Description of the prior art

Such a machine is described in the paper "Switching Flux Permanent Magnet Polyphased Synchronous Machines" by E. Hoang, A. H. Ben Ah.rned, and J. Lucidarme, EPE'97 conference proceedings pages 3.903 to 3.908 the disclosure of which is incorporated herein by reference. The paper describes various machines which it terms type A, Al, A2 and type B. Referring to the accompanying Figure 1, a type B machine comprises a rotor 4 rotatable about an axis A within a cylindrical stator 2. The rotor comprises a plurality of salient poles 6. The stator 2 comprises a plurality of armature windings 10 each about a core 12 within which is a permanent magnet 8 and which is flanked by permanent magnets 8. The permanent magnets 8 extend radially but are magnetised circumferentially. The magnetisation of the magnets 8 alternates as shown by the arrows in the accompanying Figure 2A.

Referring to the accompanying Figure 2A, the paper defines an elementary cell of what is termed a type B machine. The cell comprises: a passive element which is a section of rotor having two rotor teeth 6; and an active element which is a section of stator comprising three circumferentially polarised permanent magnets 8 separated by a core 12 within which is a winding 10 about the core and a magnet 8 within the core.

The core is between two further magnets 8.

The other types of machine termed type A, Al and A2 have corresponding elementary cells.

It is desirable to provide a machine which allows the designer more freedom to balance performance of the machine against cost.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a switching flux permanent magnet electrical machine comprising a plurality of elementary cells, each elementary cell having an active element and a passive element which elements are relatively moveable, the passive element having at least one salient pole, the active element having a first permanent magnet extending away from the passive element, a core about the first permanent magnet, and a winding about the core and the first permanent magnet, the first permanent magnet extending wholly through the core, the first permanent magnet being polarised in the direction of relative movement or the opposite direction, the active element comprising two further permanent magnets on the core, one each side of the first permanent magnet, the one on the side of the first permanent magnet at which the first permanent magnet injects flux into the core also injecting flux into the core, the other being of opposite polarity.

The machine may be a linear machine or a rotary machine.

As shown in Figure 2A, the stator of the known machine leaks flux at the radially outer ends of the magnets 8 affecting the performance of the machine. The present invention provides the further permanent magnets which at least reduce flux leakage, and in some embodiments may enhance the flux, and provides torque which increases with the thickness of the further permanent magnets. The invention thus provides greater freedom of design for a machine designer. In addition the further permanent magnets allow the designer to choose the material and dimensions of the magnets to balance cost against performance.

In a first embodiment, the said two further permanent magnets are on the core on the side of the cell remote from the passive element one being polarised in the direction of extent of the first permanent magnet through the core towards the passive element and the other in the opposite direction. This first embodiment may further comprising another two permanent magnets wherein the said another two permanent magnets are on the face of the core facing in the direction perpendicular to the said direction of relative movement and to the direction of extent of the first permanent magnet, the another two permanent magnets on the core being one each side of the first permanent magnet, the one on the side of the first permanent magnet at which the first permanent magnet injects flux into the core also injecting flux into the core, the other being of opposite polarity.

In a second embodiment, the said two further permanent magnets are on the face of the core facing in the direction perpendicular to the said direction of relative movement and to the direction of extent of the first permanent magnet, one being polarised in the direction faced by the face of the core and the other in the opposite direction.

According to another embodiment of the present invention the switching flux permanent magnet electrical machine comprises a salient pole rotor rotatable about an axis within a stator, the stator having a plurality of windings about respective cores, each core including a radially extending permanent magnet which is polarised circumferentially, each active element of the machine having the two further permanent magnets which are radially magnetised permanent magnets on the radially outer surface of the core.

The radially magnetised magnets reduce leakage flux from the radial ends of the circumferentially magnetised magnets and increase the flux coupled to the rotor.

In an embodiment, the cores further comprise axially magnetised permanent magnets arranged on the axially facing surfaces thereof. Each core has two of said axially magnetised permanent magnets, one on each circumferentially facing side of the circumferentially magnetised magnet of the core, the axially magnetised magnets being of opposite polarity.

Another embodiment of the invention provides a switching flux permanent magnet electrical machine comprising a salient pole rotor rotatable about an axis within a stator, the stator having a plurality of windings about respective cores, each active element including a radially extending permanent magnet which is polarised circumferentially, and the said two further permanent magnets which are axially magnetised permanent magnets on the axial face of the core.

The axially magnetised magnets reduce leakage flux from the axial ends of the circumferentially magnetised magnets and increase the flux coupled to the rotor.

A machine of the invention may be a motor. A machine of the invention may be a generator. -5

Bnef description of the drawings

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings in which: Figure I is a schematic end view of a known switching flux permanent magnet electrical machine of type B; Figure 2A is a schematic diagram of an elementary cell of the type B machine ofFigiirel; Figure 2 B is a schematic diagram of an elementary cell of a type B machine modified in accordance with one embodiment of the invention by the provision of radially polarised magnets; Figures 3A to D show end views of examples of switching flux permanent magnet electrical machines according to the invention, elementary cells of those machines and graphs of torque versus thickness of radial permanent magnets of those machines together with a reference torque value indicated by the dashed line and corresponding to the absence of radially polarised magnets; Figure 4 shows a modification of the first example of Figure 3; Figure 5 is a schematic, partially exploded, perspective view of another example of a switching flux permanent magnet electrical machine according to the invention; Figure 6 is an axial cross sectional view of the machine of Figure 5; Figures 7, 8 and 9 show variants of arrangements of magnets used in an elementary cell of the machine of Figure 6, in which A shows the arrangement of the magnets when viewing the cylindrical surface of the stator, B shows the arrangement of the magnets when viewing the axial end of the stator and C shows the arrangement of the magnets when viewing an axial cross section of the stator; and Figure 10 is a schematic diagram of an elementary cell of a type Al machine.

Referring to Figures 2B and 3A, denoted Embodiment 1, the type B machine of Figure 1 is modified by the addition to the elementary cell of two radially polarised permanent magnets 20 and 21 of opposite polarity and a stator back iron 22. The magnets 20 and 21 which are polarised radially are placed between the core 12 and the stator back iron 22. The magnets 20 and 21 space the ends of the magnets 8 away from the back iron 22. The radially polarised magnets 20 and 21 extend circumferentially between successive radially extending circumferentially polarised magnets 8 of the unit cell. The polarity of the radially polarised magnets 20 and 21 relative to the circumferentially polarised magnets is shown in embodiment I. The direction of polarisation of the radially polarised magnets 20, 21 relative to the magnets 8 is defined by the following rule; when the two adjacent circumferentially magnetised magnets 8 together inject flux into the the stator core, the radially polarised magnet 20 or 21 is also polarised to inject flux into the stator core.

See for example magnet 21 on the left side of Figure 2B. The polarity alternates for successive ones of the radially polansed magnets 20, 21 around the stator.

The radially polarised magnets 20 and 21 are spaced by an air gap 24, which is coincident with the end of the circumferentially polarised permanent magnet 8 between the magnets 20 and 21. Thus there are air gaps 24 between the ends of the circumferentially polansed magnets 8 and the back iron 22. The radially polarised magnets 20, 21 are spaced from those of adjacent cells by air gaps 24 coincident with the ends of the outer radially polarised magnets 8 of the unit cell. In embodiment 1 the back iron has a thickness of 1mm but could have another value of thickness at the choice of the designer.

The entire machine comprises a cylindrical stator surrounded by radially polarised magnets 20, 21 spaced by air gaps 24 coincident with the ends of the circumferentially polarised magnets 8. The magnets 20 and 21 are between the cylindrical back iron 22 and the stator. The magnets 20 and 21 alternate in polarity.

As shown by the torque graph of embodiment 1, torque varies with the thickness of the radially polarised magnets 20 and 21. The radially polarised magnets 20 and 21 reduce flux leakage from the ends of the circumferentially polarised magnets 8 and add to the flux of the machine.

Embodiment 2 of Figure 3 differs from embodiment I only in that the radially polarised magnets 20 and 21 abut above the circumferentially magnetised magnets 8: i.e. there are no air gaps between the magnets 20 and 21. In the example of embodiment 2, the back iron has a thickness of 2mm but could have another value of thickness at the choice of the designer.

The torque graph of embodiment 2 has been calculated for only one point but that shows increased torque relative to the prior art machine of Figure 1 consistent with the graph of Embodiment 1.

The entire machine has no air gaps between successive radially polarised magnets20and2l.

Embodiment 3 of Figure 3 differs from embodiment 1 only in that the circumferentially polarised magnets 8 extend to the back iron 22 between the radially polarised magnets 20, 21 the radial ends of which abut the sides of the magnets 8.

As shown by the torque graph of embodiment 1, torque varies with the thickness of the radially polarised magnets 20 and 21. The radially polarised magnets and 21 reduce flux leakage from the ends of the circumferentially polarised magnets 8 and add to the flux of the machine.

Embodiment 4 of Figure 3 is the same as Embodiment 3 but omits the back iron 22. Any of the embodiments ito 3 may omit the back iron.

As shown by the torque graph of embodiment 1, torque varies with the thickness of the radially polarised magnets 20 and 21. The radially polarised magnets and 21 reduce flux leakage from the ends of the circumferentially polarised magnets 8 and add to the flux of the machine even without the back iron. A machine may have a non-magnetic casing of, for example plastic, instead of a back iron.

Figure 4 shows an embodiment 1' structurally identical to Embodiment 1. The graph of embodiment 1' compares flux linkage per coil of embodiment I or 1' with flux linkage of the prior art machine of Figurel for a range of variation of magnet remanent flux density Br (Tesla or Wb/m"2). The graph shows that a machine in accordance with embodiment 1 achieves greater flux linkage than the equivalent prior art machine shown in Figure 1. For example, for a machine having a stator outside radius Rso= 45mm, a rotor bore radius Rsi = 33.5 mm and a gap between the rotor and stator is g=0,Smm, the machine of embodiment 1' with Br0.6 T achieves the same performance as the prior art machine with Br=1.OT. Thus either performance is improved for the same magnet remanent flux density, or the same performance can be achieved, with cheaper magnets which produce less flux. Although the embodiment of Figure 4 requires more magnets than the prior art of Figure 1, the overall cost using cheaper magnets may be reduced. The same may apply to the other embodiments.

The embodiments of Figures 3 and 4 use radially polarised magnets around the circumference of the machine to reduce radial flux leakage. However, flux also leaks from the axial ends of the magnets 8 of the elementary cells. The embodiments of Figures 5 to 9 use techniques similar to those of Figures 3 and 4 on the axial ends of the machine.

Figure 5 is a perspective view of any one of the machines of Figure 3, showing the axial end of the machine. Magnets 30 and 31 are fixed to the axial end of an elementary cell each side of the circumferentially polarised (as indicated by Rc) magnet 8 within the core 12 of the cell. The magnets extend circumferentially, radially outside the winding 10 of the unit cell. The magnets 30 and 31 are polarised axially as indicated by Ra, one 30 of the magnets being polarised oppositely to the other magnet 31. In Figure 5, magnets like 30 and 31 are provided each side of every magnet 8. As shown in Figures 7, 8 and 9, the magnets 30, 31 and 8 may be arranged in various configurations similar to those of Figure 3. The rule for the polarisation of the axially polarised magnets 30 and 31 is the same as for the radially polarised magnets 20 and 21 as described above.

Figures 7, 8 and 9 show the relationship of axially polansed magnets 30 and 31 to one circumferentially polarised magnet 8. All magnets 8 in one machine have the same physical arrangement of magnets 30 and 31; the polarisation of the magnets may vary from magnet 8 to magnet 8.

Referring to Figure 7, each of the radially extending circumferentially polarised magnets 8" of the machine has an axial projection 8lwhich extends between the axially polarised magnets 30 and 31, spacing the magnets 30 and 31 apart.

Referring to Figure 8, the axially polarised magnets 30 and 31 abut at the axial end of the circumferentially polarised magnet 8.

Referring to Figure 9 the axially polarised magnets 30 and 31 are spaced apart by an air gap 34 at the axial end of the circumferentially polarised magnet 8.

The machines described above may be motors or may be generators. The rotor and stator may be of any suitable construction, for example of laminations, as known in the art.

Machines in accordance with the invention may have: the axial magnets 30 and 31 shown in Figures 5 to 9; together with the radially polarised magnets 20 and 21 of Figure 3; or the magnets 20 and 21 without the axial magnets 30 and 31; or the axial magnets 30 and 31 without the radially polarised magnets 20 and 21 Whilst the invention has been described by way of example with reference to a type B machine, the invention could be applied to a type A, Al and A2 machines as described in the paper by Hoang et al. Referring to Figure 10, an elementary cell of a type Al machine comprises a radial magnet 8 which is circumferentially polarised.

The magnet 8 extends radially through a core 12 containing a winding 10. Adjacent cells are spaced by an air gap 40. To reduce flux leakage from the radial end of the magnet 8, two circumferentially extending but radially polarised magnets 20 and 21 are placed one each side of the circumferentially polarised magnet 8. In Figure 10 the two magnets 20 and 21 abut over the magnet 8. The rule for the polarisation of the magnets 20 and 21 is the magnet 20, on the side of magnet 8 at which magnet 8 injects flux into the core, also injects flux into the core. Adjacent magnets 20 and 21 have opposite polarity. Any of the arrangements shown in Figure 3, 4 and 5 to 9 may be applied suitably modified to type A, Al or A2 machines.

The invention may be applied to a linear machine having a plurality of linear elementary cells as described above.

Claims

1. A switching flux permanent magnet electrical machine comprising a plurality of elementary cells, each elementary cell having an active element and a passive element which elements are relatively moveable, the passive element having at least one salient pole, the active element having a first permanent magnet extending away from the passive element, a core about the first permanent magnet, and a winding about the core and the first permanent magnet, the first permanent magnet extending through the core, the first permanent magnet being polarised in the direction of relative movement or the opposite direction, thc active element comprising two further permanent magnets on the core, one each side of the first permanent magnet, the one on the side of the first permanent magnet at which the first permanent magnet injects flux into the core also injecting flux into the core, the other being of opposite polarity.

2. A machine according to claim 1, wherein the said two permanent magnets are on the core on the side of the cell remote from the passive element one being polarised in the direction of extent of the first permanent magnet through the core towards the passive element and the other in the opposite direction.

3. A machine according to claim 1, wherein the said two permanent magnets are on the face of the core facing in the direction perpendicular to the said direction of relative movement and to the direction of extent of the first permanent magnet, one being polarised in the direction faced by the face of the core and the other in the opposite direction.

4. A machine according to claim 2, further comprising another two permanent magnets wherein the said another two permanent magnets are on the face of the core facing in the direction perpendicular to the said direction of relative movement arid to the direction of extent of the first permanent magnet, the another two permanent magnets on the core being one each side of the first permanent magnet, the one on the side of the first permanent magnet at which the first permanent magnet injects flux into the core also injecting flux into the core, the other being of opposite poJ arity.

5. A machine according to claim 1, 2, 3 or 4, which is a linear machine.

6. A machine according to claim 1, 2, 3 or 4, which is a rotary machine.

7. A switching flux permanent magnet electrical machine according to claim 2 or 4, comprising a salient pole rotor rotatable about an axis within a stator, the rotor comprising a plurality of the said passive elements, the stator having a plurality of said active elements including the said windings about respective cores, each active element comprising the said first permanent magnet which is a radially extending permanent magnet which is polarised circumferentially, wherein the said further permanent magnets are radially magnetised permanent magnets on the radially outer surface of the core.

8. A machine according to claim 7, wherein, in each active element, the two radially magnetised magnets of opposite polarity are spaced by an air gap.

9. A machine according to claim 7 wherein, in each active element, the two radially magnetised magnets of opposite polarity abut each other above the circumferentially polarised magnet.

10. A machine according to claim 7, wherein, in each active element, the two radially magnetised magnets of opposite polarity abut respective sides of the circumferentially polari sed magnet.

11. A machine according to any one of claims 7 to 10, further comprising a back iron, the radially magnetised magnets being between the back iron and the radially outer surface of the cores. *1

12. A machine according to any one of claims 7 to 10, comprising a non-magnetic casing the radially magnetised magnets being between the non-magnetic casing and the radially outer surface of the cores.

13. A machine according to any one of claims 7 to 12 when dependent on claim 4, wherein each active element comprises the said another two permanent magnets which are axially magnetised permanent magnets arranged on an axially facing surface of the core of the active element.

14. A machine according to claim 13, wherein the two axially magnetised magnets of opposite polarity are spaced by an air gap.

15. A machine according to claim 13, wherein the two axially magnetised magnets of opposite polarity abut each other on the axial face of the circumferentially polarised magnet.

16. A machine according to claim 13, wherein the two axially magnetised magnets of opposite polarity abut respective sides of the circumferentially polarised magnet.

1 7. A machine according to any one of claims 7 to 16, wherein the stator comprises further radially extending, circumferentially magneti sed, permanent magnets between the cores.

18. A switching flux permanent magnet electrical machine according to claim 3, comprising a salient pole rotor rotatable about an axis within a stator, the rotor comprising a plurality of the said passive elements, the stator having a plurality of said active elements including the said windings about respective cores, each active element comprising the said first permanent magnet which is a radially extending permanent magnet which is polarised circumferentially, wherein the said further permanent magnets are axially magnetised permanent magnets on the axially facing surface of the core of the active element.

19. A machine according to claim 18, wherein the two axially magnetised magnets of opposite polarity are spaced by an air gap.

20. A machine according to claim 18, wherein the two axially magnetised magnets of opposite polarity abut each other on the axial face of the circumferentially polarised magnet.

21. A machine according to claim 18, wherein the two axially magnetised magnets of opposite polarity abut respective sides of the circumferentially polansed magnet.

22. A machine according to any one of claims 18 to 21, wherein the stator comprises further radially extending, circumferentially magnetised, permanent magnets between the cores.

23. A switching flux permanent magnet electrical machine substantially as hereinbefore described with reference to: Figure 3, or Figure 4, andlor Figures 5 and 6 as modified by Figure 7, 8 or 9; or to Figure 10; of the accompanying drawings.