WO2016053352A1

WO2016053352A1 - Rotor for consequent pole permanent magnet machine

Info

Publication number: WO2016053352A1
Application number: PCT/US2014/059046
Authority: WO
Inventors: Parag R. UPADHYAY
Original assignee: Abb Technology Ag
Priority date: 2014-10-03
Filing date: 2014-10-03
Publication date: 2016-04-07

Abstract

The subject matter herein includes a rotor for a consequent pole permanent magnet machine. The rotor includes a plurality of rotor laminations. Each lamination comprises a substantially disc-shaped member of magnetic material having a central aperture for surrounding the shaft. Each lamination further includes a plurality of magnet receiving apertures. Each magnet receiving aperture is located in or near an outer circumference of each lamination. Pairs of teeth extend outward from a wall of each magnet receiving aperture for holding edges of a magnet within each aperture.

Description

DESCRIPTION

ROTOR FOR CONSEQUENT POLE PERMANENT MAGNET MACHINE

TECHNICAL FIELD

The subject matter described herein relates to rotors for electric machines. More particularly, the subject matter described herein relates to a rotor for a consequent pole permanent magnet machine.

BACKGROUND

Permanent magnet machines, such as permanent magnet motors and generators, include one or more permanent magnets embedded in the rotor to produce magnetic flux that interacts with stator coils. A consequent pole permanent magnet machine includes N magnets that form 2N poles. Each permanent magnet itself forms one pole, and the opposite pole is formed as a consequence of each magnet adjacent to each magnet in the rotor material.

It is desirable in the design of consequent pole permanent magnet machines to reduce the amount of permanent magnet material required by reducing leakage flux. Leakage flux is flux that does not contribute to torque in the desired direction of rotation of the rotor. Leakage flux from the stator travels from the stator, through the air gap between the stator and the rotor, into the rotor surface, and back through the air gap to the stator in a direction that opposes torque. Because such leakage flux opposes torque, larger rotor permanent magnets are required to produce sufficient torque to perform according to specifications in the presence of the opposing torque. Because permanent magnets are typically made of rare-earth materials, which are expensive, it is desirable to reduce leakage flux and thereby reduce size or amount of permanent magnet material required.

Accordingly, there exists a need for an improved design for a rotor for a consequent pole permanent magnet machine. SUMMARY

According to another aspect of the subject matter described herein, a rotor to lamination for a consequent pole permanent magnet machine is provided. The rotor lamination comprises a body, which comprises a substantially disc shaped member. The body forms a plurality of magnet receiving apertures, where each of the magnet receiving apertures is located in or near an outer circumference of the body. The body also forms pairs of teeth extending outward from a wall of each aperture for holding edges of a magnet in each aperture.

According to yet another aspect of the subject matter described herein, a rotor for a consequent pole permanent magnet machine is provided. The rotor comprises a rotor core, which comprises a magnetic material and has a central aperture for surrounding the shaft. The rotor core includes a plurality of magnet receiving apertures, where each of the magnet receiving apertures is located in or near an outer circumference of the core. The rotor further includes pairs of teeth extending outward from a wall of each aperture for holding edges of the magnet.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter described herein will now be explained with reference to the accompanying drawings of which:

Figures 1A and 1 B are diagrams illustrating exemplary laminations for a rotor for a consequent pole permanent magnet machine according to embodiments of the subject matter described herein; Figures 2A and 2B are diagrams illustrating alternate designs for a lamination for a rotor for a consequent pole permanent magnet machine according to embodiments of the subject matter described herein;

Figure 3 is a diagram illustrating yet another design for a lamination for a rotor for a consequent pole permanent magnet machine according to an embodiment of the subject matter described herein;

Figure 4 is a perspective view of a rotor for a consequent pole permanent magnet machine and an axial view of a rotor lamination according to an embodiment of the subject matter described herein;

Figure 5 is a diagram illustrating a lamination for a rotor for a permanent magnet machine according to an embodiment of the subject matter described herein;

Figure 6 is a close-up view of a permanent magnet located in a magnet receiving aperture of the rotor of Figure 5; and

Figure 7 is a perspective view of a rotor for a consequent pole permanent magnet machine made using the laminations in Figures 5 and 6.

DETAILED DESCRIPTION

The subject matter described herein includes a rotor for a consequent pole permanent magnet machine. Figure 1A is a diagram illustrating a lamination for a rotor for a consequent pole permanent magnet machine according to an embodiment of the subject matter described herein. Referring to Figure 1A, rotor lamination 100 compress a substantially disc shaped member with a central aperture 102 for receiving a shaft. Lamination 100 further includes a plurality of magnet receiving apertures 104 for receiving permanent magnets 106. Magnet receiving apertures 104 are circumferentially spaced from each other about the outer circumference of lamination 100. In the illustrated example, four apertures 104 are spaced equidistantly from each other in the circumferential direction, making the apertures ninety degrees apart from each other. However, more or fewer apertures may be included to accommodate more or fewer magnets, depending on the torque and speed requirements of the electric machine. Each magnet receiving aperture 104 includes a central portion 108 that is defined by pair of teeth 110 that engage edges of magnet 106. Teeth 110 extend outward from the wall of aperture 104 that is closest to the center of the lamination 100. In the example illustrated in Figure 1A, teeth 110 in central portion 108 define a triangular shaped wall of each aperture where the apex of the triangle is replaced by a slot or trench for holding the lateral edges of a magnet 106.

Each magnet receiving aperture 104 further includes radially and circumferentially extending portions 112 that extend circumferentially from central portion 108 and radially inwardly from the outer circumference of lamination 100.

During assembly, teeth 110 facilitate insertion of the magnets into the rotor. For example, teeth 110 may be spaced from each other with a spacing corresponding to a width of permanent magnets 106 so that permanent magnets 106 can be slid axially into the rotor from the end of the rotor. Thus, teeth 1 0 of each rotor lamination 100 form an axial guide for permanent magnet insertion, which reduces assembly time and cost. Teeth 110 also facilitate the maintaining of axial alignment of magnets 106 when the rotor is spinning.

In the illustrated example, each magnet receiving aperture 104 is closed by a bridge 114 that is part of the outer circumference of rotor lamination 100. Each bridge 114 mechanically holds each magnet 106 in place while the rotor is rotating.

The magnetic pole configuration for one of magnets 106 is also illustrated in Figure 1A. In particular, Figure 1A illustrates the magnetic flux lines for the magnet 106 at the 12 o'clock position. It is understood that the remaining magnets 106 illustrated in Figure A produce similar flux lines at their respective circumferential positions around lamination 100. The result of the magnetic flux is a north pole near the center of each magnet 106 and a south pole on the surface of lamination 100 equidistant between adjacent magnets 106. The south pole is referred to as a consequent pole because it occurs as a consequence of the flux emanating from each magnet 106. Thus, in the example illustrated in Figure 1A, four magnets produce eight poles. In general, N magnets in a consequent pole permanent magnet machine will produce 2N poles. It should also be noted that the body of lamination 100 can be made of any suitable magnetic material, such as 400 material.

In Figure 1A, each magnet receiving aperture 104 has an inverted-U shape. The U-shape of each aperture is inverted in that the open end of each U faces radially inward and downward with the respective aperture is in the twelve o'clock position. It should be noted that during operation of the rotor, the U shape of each aperture will be inverted when the aperture is in the twelve o'clock position, uninverted when the aperture is in the 6 o'clock position, open to the left when the aperture is in the 3 o'clock position, and open to the right when the aperture is in the 9 o'clock position. The term "inverted-U shape" as used herein is intended to refer to the orientation of a given aperture when the aperture is at the twelve o'clock position. In addition, the term "inverted-U shape" is not intended to be limited to an shape that is exactly like the English letter "U" and is intended to include other designs where the shape of the aperture when viewed in profile is open on one end and closed on the other end.

Figure 1 B illustrates an alternate embodiment of a rotor lamination where the radially and circumferentially extending portions 112 of each magnet receiving aperture 104 extend further inwardly towards the center of lamination 100. In particular, each magnet receiving aperture 104 further includes an extension 116 that extends inwardly from its respective radially and circumferentially extending portion 112. Each extension 116 comprises a triangular shaped portion that extends from its respective radially and circumferentially extending portion 112 towards the center of lamination 100. Each extension 116 further reduces leakage flux from the stator by providing a larger air gap in the rotor to block the leakage flux induced by the stator. For example, flux from the stator (not shown in Figure 1 B) evidenced by flux line 117 would encounter extension 116, which forms a radial air gap. The radial air gap provides a region of low magnetic permeability, which would block or reduce flux lines 117 emanating from the stator. In one embodiment, the permeability of extension 116 may be similar to that of free space.

Figure 2A illustrates yet another alternate embodiment of rotor lamination 100. Referring to Figure 2A, rotor lamination 100 includes radially extending slots 120 located circumferentially equidistantly between magnet receiving apertures 104 yet close to (e.g., within 1-3 mm) the outer circumference of lamination 100. Each radially extending slot 120 comprises an elongate gap structured to reduce leakage flux caused by the stator, as evidenced by flux line 122 illustrated in Figure 2A. Each radial extending slot may form an air gap that provides a region of low magnetic permeability. As such, each radially extending slot 120 blocks or reduces stator leakage flux that would otherwise pass through the corresponding region of rotor lamination 100 if slots 120 were not present. In addition, adjacent radially extending slots 120 may block pole to pole leakage flux indicated by dashed line 123. This flux is also called harmonic flux. Radially extending slots 120 can be distinguished from radially extending slots in some rotor designs that are designed to isolate flux from adjacent rotor magnets. Because stator flux leakage reduction slots 120 are located close to the outer circumference of lamination 100, e.g., within 1-3 mm of the outer circumference of lamination 100, a large portion of the leakage flux generated by the stator in operation can be reduced. It should also be noted that although the magnet receiving apertures 104 in Figure 2A are the same as those in Figure 1A, the magnet receiving slots in Figure 2A could also include extensions 116, as illustrated in Figure 1 B.

Figure 2B illustrates yet another embodiment of a lamination and associated components for a rotor for a consequent pole permanent magnet machine accordingly to an embodiment of the subject matter described herein. Referring to Figure 2B, each magnet 106 is surrounded by a pair of axially extending conductors 124. Each conductor 124 extends axially through the stack of laminations 100 that form the rotor core adjacent to an edge of each magnet 106. End caps (not shown) may be included to connect conductors 124 electrically to each other on each end of the rotor core. The presence of conductors 124 creates an opposition to the flux variation in the rotor poles. Under transient and short circuit conditions, current induced in conductors 124 by the stator will short circuit, which protects each magnet 106 from demagnetization at high temperatures. Conductors 124 may be a solid metal or stranded to limit the current and adjust the resistance. In addition, conductors 124 can be supplied an excitation voltage via a slip ring and brushes to control the current and further protect magnets 106. The percentage of excitation voltage supplied to conductors 124 can be varied to produce varying degrees of magnetomotive force relative to that produced by magnets 106 (e.g., from 20% electromagnetic + 80% magnet to 80% electromagnetic + 20% magnet) to make the design suitable for control as a motor or as a generator.

Figure 3 illustrates an alternate embodiment of lamination 100 and associated components. In Figure 3, axially extending conductors 126 may be positioned between adjacent magnet receiving apertures 104 near the surface of lamination 100. Axially extending conductors 126 may be solid or a stranded conductive material and can be used for self-starting of the consequent pole machine and to provide damping during transient operation. Conductors 126 may be shorted through end rings or end caps (not shown) or supplied with current through slip ring rotor excitation to support the magnets. Although conductors 126 are only shown between two of magnet receiving apertures 104, it is understood that conductors 126 would extend between each magnet receiving aperture 104.

In addition, in the embodiments illustrated in Figures 1A - Figure 3, four circumferentially spaced magnets are included. However, the number of magnets and magnet receiving apertures depends on the desired speed and torque of the machine. More magnets can be included to increase the speed and torque. For example, in the examples illustrated in Figures 1A - Figure 3, the number of magnets may support a low speed (less than 1000 RPM) high torque machine. If the number of magnets is increased to twelve, resulting in twenty-four poles, the speed of the machine may be further reduced to less than 100 rpm. Similarly, with number of magnets decreased to three, resulting in six poles, the speed of the machine may be greater than 1500 RPM. Figure 4 is a perspective view of a rotor 400 and an axial view of a rotor lamination 100. In Figure 4, rotor 400 is formed by bolting a stack of laminations 100 together and adding an end cap 401 on each end. Using rotor laminations versus a solid core decreases eddy currents in the rotor. However, a solid core rotor is intended to be within the scope of the subject matter described herein. For example, a solid core rotor may have the same radial profile as any of the laminations described herein. Thus, a solid core rotor, in one embodiment, may comprise a substantially cylindrically shaped member of magnetic material having a central aperture for surrounding the shaft. The substantially cylindrical core of such a solid rotor may include a plurality of magnet receiving apertures. Each of the magnet receiving apertures may be located in or near an outer circumference of the core. Each of the magnet receiving apertures may define pairs of teeth that extend outward from a wall of each aperture for holding edges of a magnet in each aperture. In one embodiment, the magnet receiving apertures may have an inverted-U shape (when in the twelve o'clock position) and may be closed by the outer circumference of the rotor or rotor lamination, as illustrated in Figures 1A-3. In an alternate embodiment, the magnet receiving apertures may open along the outer circumference of the rotor or rotor lamination, as illustrated in Figure 7.

In Figure 4, end cap 401 includes slots 402 to allow insertion of magnets into magnet receiving apertures 104. Slots 402 need not have the same configuration as magnet receiving apertures 104 as long as the magnets will slide axially through magnet receiving slots 402. End cap 401 further includes apertures 406, which may be used for insertion of conductive bars, rotor windings, or for cooling. It should also be noted that another end cap 401 with the same or different configuration from end cap 401 can be included on the opposite end of rotor 400.

Rotor 400 surrounds a shaft 404 and rotates with shaft 404. If the electric machine that houses rotor 400 is a motor, shaft 404 may be mechanically coupled to a load to perform work on the load. For example, in the case of a direct drive for a paper mill, shaft 404 may drive a mixer that mixes wood pulp. If the electric machine is a generator, then the spinning of shaft 404 causes rotor 400 to produce a rotating magnetic field, which induces current in stator coils, and the current can be used to electrically drive a load.

Figure 5 is a diagram of a rotor lamination 100 according to an alternate embodiment of the subject matter described herein. Referring to Figure 5, magnet receiving apertures 104 of rotor lamination 100 are open along the outer circumference of lamination 100. As illustrated in the close- up view in Figure 6, each magnet receiving aperture 104 includes teeth 110 extending from the wall of each aperture closest to the center of lamination 100 to hold each magnet 106 in place circumferentially and to provide an axial guide for sliding each magnet 106 in place during assembly. In addition, circumferentially extending teeth 128 extend from opposite sidewalls of each aperture 104 to provide mechanical force that opposes outward movement of magnet 106 when aperture 104 is filled with epoxy 130. In the illustrated example, the regions of magnet receiving aperture 104 on opposing sides of magnet 106 may be filled with epoxy 130. Epoxy 130 also covers the outward facing surface of each magnet 106. Epoxy 130 may be a low magnetic permeability material to reduce leakage flux.

In Figure 6, a portion of stator 134 is also visible. The topology illustrated in Figures 5 and 6 may be suitable for large pole machines where the speed of the machine is low (less than 100 RPM) and the centrifugal force is on the magnets are lower. In such low speed machines, magnet retention is not as severe an issue as with a high-speed (greater than 1500 RPM) machines. These machines have larger numbers of poles (greater than 24) and therefore the leakage flux is prominent, which necessitates a larger magnet size or one of the rotor designs described herein.

Creating the topology illustrated in Figures 5 and 6 without surface bridges facilitates magnet insertion during assembly of the rotor. Teeth 110 facilitate axial alignment of the magnets, as with the embodiments illustrated in Figures 1A-3. The magnetic force between the magnets and the rotor also helps retain the magnets radially in place. To further increase the retention of magnets 106, trapezoidal wedges 132 may be inserted on opposing sides of magnet 106 in magnet receiving aperture 130. Each trapezoidal wedge 132 may be solid or hollow in the center. Making the wedges hollow may allow wedges 132 to function as circumferential springs to hold each magnet 106 in its respective aperture 104. Trapezoidal wedges 132 may extend axially for the length of permanent magnets 106. For high- torque machines, torsional forces are severe and can be overcome by teeth 110, wedges 132 and epoxy 130.

Figure 7 is a perspective view illustrating a portion of rotor 400 with the open slot design illustrated in Figures 5 and 6. Rotor 400 is formed by bolting a plurality of rotor laminations 100 together in an axial stack. A magnet 106 extends axially in magnet receiving aperture 104. It can be seen from the design in Figure 7 that assembly of rotor 400 can be achieved easily by placing magnet 106 in magnet receiving aperture 104 from a radial direction. The magnetic attraction between magnet 106 and rotor laminations 100 will serve to partially hold magnet 106 in place. As stated above, epoxy placed on opposing sides of magnet 106 coupled with the radially extending teeth and trapezoidal wedges may increase the force that retains magnet 106 within magnet receiving aperture 104.

It should be noted that the rotor and rotor lamination design illustrated in Figure 5 and 7 may be structured to hold conductors 124 that extend axially adjacent to each magnet 106. In one embodiment, wedge shape members 132 may be formed of a conductive material for protecting each magnet 106 from demagnetization. It should also be noted that rotor lamination 100 may include apertures between at least some of the magnet receiving apertures for holding axially extending conductors 126 for self- starting and for reducing demagnetization of permanent magnets 106. Further, as stated above, rotor 400 may be formed of a stack of laminations or may have a solid core. The subject matter described herein may at least partially overcome the deficiency of using a higher volume of permanent magnet material to meet the high performance requirements of a direct drive machine. For example, a rotor for a consequent pole permanent magnet machine as described herein may reduce leakage flux and thereby reduce the amount of permanent magnet material required for a given application over rotor designs without the leakage flux reduction mechanisms described herein.

In some embodiments, for example, as illustrated in Figure 2B, providing a conductive shield over or around a rectangular shaped magnet may make a machine that incorporates such a shield around the magnets more robust, similar to cage induction motors, and the optimized magnet volume and shape improves performance, like surface permanent magnet machines. The thermal stability of the magnets when surrounded by conductive shields may allow the machine to operate under high temperature (e.g., up to 180°C) environments.

In some embodiments, for example, as illustrated in Figure 3, the optional rotor bars or conductors 126 located between adjacent magnets 106 may improve the transient behavior of the machine under short circuit conditions and provide for self-starting of the machine.

The simple and robust construction of the rotor and rotor lamination embodiments described herein in terms of assembly and the compatibility with a rectangular permanent magnet shape may reduce product costs.

In some embodiments, the possibility of accommodating a large number of poles (e.g., 24 or more poles) in the rotor may give better drive performance because the drive fundamental frequency may be higher for large numbers of poles at very low speed.

As stated above, the design of the rotor laminations may allow rectangular magnets to be used, which may save in the production costs of the magnets versus using arc-shaped magnets.

At least some of the rotor designs described herein may be capable of achieving high magnetic flux density (e.g., on the order of 0.9 Teslas) in the air gap between the rotor and the stator, even with a 25% reduction in permanent magnet material used over prior designs and with a rectangular magnet shape.

Some embodiments of the rotor design described herein may be suitable for low speed applications, such as in motors for paper mills, hoists, elevators, wind generators, and cooling tower motors. For such applications, especially where a high number of poles are used, pole to pole leakage flux can be significant. However, some embodiments of the subject matter described herein (Figures 2A and 2B) may reduce such leakage flux using magnet receiving apertures and the radial slots located near the outer circumference of each rotor lamination.

As stated above, different numbers of rotor poles than those illustrated above are intended to be within the scope of the subject matter described herein. In addition, the poles may be moved circumferentially forward or backward (clockwise or counterclockwise) from the positions of the poles illustrated in Figures 1A-7 to minimize armature reaction effects. In addition, different magnetic materials and magnet shapes other than those described herein can be used.

It will be understood that various details of the presently disclosed subject matter may be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.

Claims

What is claimed is:

1 . A rotor for a consequent pole permanent magnet machine, the rotor comprising:

a plurality of rotor laminations, each comprising a substantially disc-shaped member of magnetic material having a central aperture for surrounding a shaft, each lamination further including:

a plurality of magnet receiving apertures, each of the magnet receiving apertures being located in or near an outer circumference of each lamination; and

pairs of teeth extending outward from a wall of each aperture for holding edges of a magnet in each aperture.

2. The rotor of claim 1 wherein each of the magnet receiving apertures has an inverted-U shape when the respective aperture is in a twelve o'clock position.

3. The rotor of claim 2 wherein the inverted-U shape includes a central portion defined by the teeth for receiving a magnet and outer portions extending circumferentialiy and radially inwardly from the central portion.

4. The rotor of claim 2 wherein the outer circumference of each lamination closes each magnet receiving aperture.

5. The rotor of claim 1 wherein the magnet receiving apertures comprise slots formed in the outer circumference of each lamination.

6. The rotor of claim 5 comprising circumferentialiy extending teeth located on opposing sides of each slot.

7. The rotor of claim 6 comprising first and second wedge shaped members located on circumferentially opposite sides of each magnet for holding the magnet in its respective slot. 8. The rotor of claim 1 wherein each lamination includes radially extending slots located near the outer circumference of each lamination for reducing leakage flux induced by a stator.

9. The rotor of claim 1 comprising a plurality of magnets located between the teeth in the magnet receiving apertures.

10. The rotor of claim 9 comprising conductors extending axially adjacent to each of the magnets for reducing demagnetization of the magnets. 1 . The rotor of claim 1 comprising axially extending conductors extending through the laminations between the magnet receiving apertures to form a cage for reducing demagnetization of the magnets and leakage flux. 12. A rotor lamination for a consequent pole permanent magnet machine, the rotor lamination comprising:

a body comprising a substantially disc shaped member, the body including:

a plurality of magnet receiving apertures, each of the magnet receiving apertures being located in or near an outer circumference of the body; and

pairs of teeth extending outward from a wall of each aperture for holding edges of a magnet in each aperture. 13. The rotor lamination of claim 12 wherein each of the magnet receiving apertures has an inverted-U shape when the respective aperture is in a twelve o'clock position.

14. The rotor lamination of claim 13 wherein the inverted-U shape includes a central portion defined by the teeth for receiving a magnet and outer portions extending circumferentially and radially inwardly from the central portion.

15. The rotor lamination of claim 13 wherein the outer circumference of each lamination closes each magnet receiving aperture.

The rotor lamination of claim 12 wherein the magnet receiving apertures comprise slots formed in the outer circumference of each lamination.

17. The rotor lamination of claim 16 comprising circumferentially extending teeth located on opposing sides of each slot.

18. The rotor lamination of claim 17 wherein the slots are configured to receive first and second wedge shaped members located on circumferentially opposite sides of each magnet for holding the magnet in its respective slot.

19. The rotor lamination of claim 12 wherein each lamination includes radially extending slots located near the outer circumference of each lamination for reducing leakage flux induced by a stator.

20. The rotor lamination of claim 12 wherein the magnet receiving apertures are structured to hold conductors extending axially adjacent to each of the magnets for reducing demagnetization of the magnets.

21 . The rotor lamination of claim 12 wherein the body is configured to hold axially extending conductors between the magnet receiving apertures to form a cage for reducing demagnetization and leakage flux.

22. A rotor for a consequent pole permanent magnet machine, the rotor comprising:

a rotor core comprising a magnetic material and having a central aperture for surrounding the shaft, the rotor core including:

a plurality of magnet receiving apertures, each of the magnet receiving apertures being located in or near an outer circumference of the core;

pairs of teeth extending outward from a wall of each aperture for holding edges of the magnet in each aperture.

23. The rotor of claim 22 wherein each of the magnet receiving apertures has an inverted-U shape when the respective aperture is in a twelve o'clock position.

24. The rotor of claim 23 wherein the inverted-U shape includes a central portion defined by the teeth for receiving a magnet and an outer portion extending circumferentially and radially inwardly from the central portion.

25. The rotor of claim 23 wherein the outer circumference of the rotor closes each magnet receiving aperture.

26. The rotor of claim 22 wherein the magnet receiving apertures comprise slots formed in the outer circumference of the core.

27. The rotor of claim 26 comprising circumferentially extending teeth located on opposing sides of each slot. 28. The rotor of claim 27 comprising the first and second wedge shaped members located on the circumferentially opposite sides of each magnet for holding the magnet in its respective slot.

29. The rotor of claim 22 wherein the core includes radially extending slots located near the outer circumference of the core for reducing leakage flux induced by a stator. 30. The rotor of claim 22 comprising a plurality of magnets located between the teeth in the magnet receiving apertures.

31 . The rotor of claim 30 comprising conductors extending axially adjacent to each of the magnets for reducing demagnetization of the magnets.

32. The rotor of claim 22 comprising a plurality of axially extending conductors extending through the core between the magnet receiving apertures to form a cage for reducing demagnetization of the magnets and leakage flux.