US20070159021A1

US20070159021A1 - Composite magnet structure for rotor

Info

Publication number: US20070159021A1
Application number: US11/311,798
Authority: US
Inventors: Gary Horst
Original assignee: Emerson Electric Co
Current assignee: Emerson Electric Co
Priority date: 2005-12-19
Filing date: 2005-12-19
Publication date: 2007-07-12
Also published as: CN101026318B; MXPA06015268A; US7550889B2; US20070138892A1; CN101026318A

Abstract

An interior permanent magnet electric motor. A rotor comprising a slot radially spaced from its longitudinal axis of rotation extending parallel to the axis. First and second magnets are positioned in the slot and extend parallel to the axis. A first magnet is positioned between a second magnet and the axis.

Description

FIELD OF THE INVENTION

The present invention generally relates to an electric motor rotor design. More particularly, the present invention relates to an interior permanent magnet rotor design wherein strontium ferrite and neodymium-iron-boron are positioned in a common slot in the rotor core.

BACKGROUND OF THE INVENTION

Interior permanent magnet (IPM) rotor designs using strontium ferrite (ferrite) and neodymium-iron-boron (neo) are known in the art.
In one prior art design, the rotor has a core with long thin slots having neo in each slot. This design does not make use of ferrite. The slots are formed by using a punch press on the rotor core. In order to increase die life, decrease the core weight, and reduce flux leakage, the slots are oversized. The oversized slots allow air spaces around the neo which cause the motor to have high windage noise at high speeds. These motors can have a sinusoidal back electromagnetic flux (EMF) which is desirable.
Another option is to use ferrite in an IPM rotor design. Ferrite is less expensive and can be used to fill large slots. This results in very small air spaces which correspond to a quieter motor. The problem with ferrite is that it does not have a sufficiently high flux density to make an efficient motor.
The combination of neo and ferrite in a single rotor design has been the solution. Large slots near the center of the rotor are filled with ferrite, and smaller slots closer to the edge of the rotor have pieces of neo in them. A motor employing this design is somewhat quieter than a motor using neo alone (i.e. has less windage noise), but generally has a non-sinusoidal back EMF (i.e., it is harmonically rich). Also, the die used in manufacturing this type of rotor has a short lifespan due to the small size of the neo slot.

SUMMARY OF THE INVENTION

Embodiments of the invention include IPM rotor designs with small air spaces and large slots in order to achieve a quiet motor and improved die life. Embodiments of the invention also include IPM rotor designs that demonstrate a near sinusoidal back EMF.
In accordance with one aspect of the invention, an electric motor rotor is provided. A core has a central longitudinal axis and a slot radially spaced from the longitudinal axis extending parallel to the axis. First and second magnets are positioned in the slot and extend parallel to the longitudinal axis. The first magnet is positioned between the second magnet and the longitudinal axis.
In accordance with another aspect of the invention, a method is provided for producing an electric motor. A slot is formed in a rotor core material having a central longitudinal axis. A first magnet is inserted in the slot. A second magnet is inserted in the slot such that the first magnet is substantially between the second magnet and the central longitudinal axis. The rotor core is inserted into a stator having windings. The windings of the stator are connected to a commutation circuit.
In accordance with another aspect of the invention, an electric motor is provided. A rotor includes a core and first and second magnets. The core has a central longitudinal axis and a slot radially spaced from the longitudinal axis extending parallel to the longitudinal axis. The first and second magnets are positioned in the slot and extend parallel to the longitudinal axis. The first magnet is positioned between the second magnet and the longitudinal axis. A stator having windings is in magnetic coupling relation to the rotor. A commutation circuit is electrically connected to the windings of the stator.
Alternatively, the invention may comprise various other methods and apparatuses.
Other objects and features will be in part apparent and in part pointed out hereinafter.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross sectional view perpendicular to an axis of rotation of a motor according to one embodiment of the invention having a rectangular first magnet, a rectangular second magnet, and a trapezoidal slot.
FIG. 2 is a cross sectional view perpendicular to an axis of rotation of a rotor according to one embodiment of the invention having an arc shaped first magnet, a rectangular second magnet, and a trapezoidal slot.
FIG. 3 is a cross sectional view perpendicular to an axis of rotation of a rotor according to one embodiment of the invention having an arc shaped first magnet, two contiguous rectangular pieces of a second magnetic material, and a trapezoidal slot.
FIG. 4 is a cross sectional view perpendicular to an axis of rotation of a rotor according to one embodiment of the invention having an arc shaped first magnet, two separated rectangular pieces of a second magnetic material, and a trapezoidal slot.
FIG. 5 is a cross sectional view perpendicular to an axis of rotation of a rotor according to one embodiment of the invention having an arc shaped first magnet, a bread loaf shaped second magnet, and a precision slot.
FIG. 6 is a cross sectional view perpendicular to an axis of rotation of a rotor according to one embodiment of the invention having an arc shaped first magnet, two separated bread loaf shaped pieces of a second magnetic material, and a trapezoidal slot.
FIG. 7 is a cross sectional view perpendicular to an axis of rotation of a rotor according to one embodiment of the invention having an arc shaped first magnet and a rectangular second magnet wherein the second magnet is between the arc shaped first magnet and the axis of rotation.
FIG. 8 is a cross sectional view perpendicular to an axis of rotation of a lobed rotor according to one embodiment of the invention having a composite slot for a first and second magnet wherein the slot is trapezoidal in the area of the second magnet.
Corresponding reference characters indicate corresponding parts throughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, one embodiment of a motor 100 of the invention is illustrated in cross section including a rotor 102 having a central shaft 104 rotating about an axis of rotation A. The rotor 102 comprises a cylindrical core of steel (or other material) having a slot 110 extending parallel to the shaft. Positioned within the slot 110 are a ferrite magnet 106 and a neo magnet 108. The rotor is positioned within a stator 112 having windings 114. The windings are connected to a commutation circuit 116. Commutation circuit 116 energizes the windings 114 causing the rotor 102 to rotate about the central shaft 104. FIG. 1 illustrates one embodiment in which a single, unitary slot 110 has located therein neo and ferrite magnets each having a generally rectangular cross section perpendicular to the axis of rotation A. The magnets each have a longer rectangular dimension which is generally parallel to each other and the ferrite magnet 106 is positioned between the neo magnet 108 and the central shaft 104. In one embodiment, the slot 110 has a partial trapezoidal cross section perpendicular to the axis of rotation at the ends of the neo magnet 108. This results in generally triangular air spaces 118 bounded by the short side of the neo magnet 108, the long side of the ferrite magnet 106, and the core 102. Other rotor configurations are contemplated. For example, see the configurations illustrated in FIGS. 2-7.
Generally, motors employing the invention have a substantially sinusoidal back EMF whereas motors known in the art using ferrite and neo magnets have a harmonically rich back EMF. Motors employing the invention generally have a lower minimum inductance than motors known in the art, and the ratio of maximum inductance to minimum inductance is generally higher which improves the contribution of reluctance torque. Motors employing the invention also generate less noise at high speeds than motors known in the art because there are less air spaces in the rotor.
Motors employing the invention are generally less expensive to manufacture than those known in the art, but there are compromises between cost and noise. Rectangular neo magnets are less expensive than neo magnets of other shapes, but they allow some air spaces when used with an arc shaped ferrite magnet. Two small neo magnets generally conform to the arc shaped ferrite magnet better than one large neo magnet. However, using two small magnets may require a die used to form slots in a rotor core to have intricate details which means that the die will not last as long as a die that has less intricate details. Die life can be increased by not conforming to every detail of the magnets, but this will allow for air spaces which will increase acoustic noise when the motor is operating at high speeds. Because of their reduced cost, reduced acoustic noise, and reduced electrical noise, motors according to the invention may be advantageously applied in consumer appliances such as horizontal washing machines, dish washers and clothes dryers.
Referring now to FIG. 2, an embodiment of the invention using a rectangular neo magnet 208, an arc shaped ferrite magnet 206, and a trapezoidal slot is shown. A cylindrical core 202 has a central shaft 204 about which it rotates and a slot extending parallel to the shaft 204. The arc shaped ferrite magnet 206 has a convex surface 214 facing the central shaft 204 and a concave surface 216 facing away from the central shaft 204. The rectangular neo magnet 208 has a longer dimension facing the ferrite magnet 206, and the corners of the neo magnet 208 contact the concave face 216 of the ferrite magnet 206. The concave surface 216 of the ferrite magnet 206 facing the flat surface of the neo magnet 208 results in an air space 212 between the ferrite magnet 206 and the neo magnet 208. The slot is not precision cut, but is trapezoidal in the area that contains the neo magnet 208. That is, instead of fitting tightly against the outline of the combined ferrite and neo magnets, the core is cut so that it does not fit against the shorter edges of the neo magnet 208. A trapezoidal slot results in generally triangular air spaces 210 bounded by the short sides of the rectangular neo magnet 208, the concave face 216 of the ferrite magnet 206, and the core 202. This trapezoidal style slot reduces intricate details of the slot cross section which can increase the life of a die used to make the slot, making a trapezoidal slot desirable when die life is more important to the manufacturer than motor noise is to the end user. The trapezoidal slot also reduces leakage flux which contributes to a motor with a higher maximum inductance, and thus a potentially better ratio of maximum inductance to minimum inductance.
Referring now to FIG. 3, an embodiment of the invention using two rectangular neo magnets 308, an arc shaped ferrite magnet 306, and a trapezoidal slot is shown. A cylindrical core 302 has a central shaft 204 about which it rotates and a slot extending parallel to the shaft 304. The arc shaped ferrite magnet 306 has a convex surface facing the central shaft 304 and a concave surface facing away from the central shaft 304. Each rectangular neo magnet 208 has a longer dimension facing the ferrite magnet 206, and the corners of the neo magnet 308 contact the concave face of the ferrite magnet 306. The neo magnets 308 contact each other at one corner. The concave surface of the ferrite magnet 306 facing the flat surface of the neo magnets 308 results in air spaces 310 between the ferrite magnet 306 and each neo magnet 308. There is also a generally triangular air space 312 between the two neo magnets 308 bound by the concave surface of the ferrite magnet 306 and the shorter sides of each neo magnet 308. The slot is generally trapezoidal in cross section and triangular in cross section in the area that contains the neo magnets 308. That is, instead of fitting tightly against the outline of the combined ferrite and neo magnets, the core may be cut so that it does not have a precision fit with the shorter edges of the neo magnet 208. A trapezoidal slot results in generally triangular air spaces 314 bounded by the short side of the rectangular neo magnet 308, the concave face of the ferrite magnet 306, and the core 302. Air spaces 310 and 312 may be smaller than air space 212 (see FIG. 2) because two smaller neo magnets conform to the face of the ferrite magnet better than one large neo magnet. The rotor design of FIG. 3 has different acoustic characteristics than that of the design in FIG. 2 because of the difference in air spaces. The two rotors (see FIGS. 2 and 3) may be employed in different applications with different operating speeds because of their differing acoustical characteristics (i.e., reduced windage noise at certain speeds).
Referring now to FIG. 4, an embodiment of the invention using two rectangular neo magnets 408, an arc shaped ferrite magnet 406, and a trapezoid slot is shown. A cylindrical core 402 has a central shaft 404 about which it rotates and a slot extending parallel to the shaft 404. The arc shaped ferrite magnet 406 has a convex surface facing the central shaft 404 and a concave surface facing away from the central shaft 404. The two rectangular neo magnets 408 each have a longer dimension facing the ferrite magnet 406, and the corners of the neo magnets 408 contact the concave face of the ferrite magnet 406. The concave surface of the ferrite magnet 406 facing the flat surfaces of the neo magnets 408 results in air spaces 412 between the ferrite magnet 406 and the neo magnets 408. Two small neo magnets 408 conform to the concave face of the ferrite magnet 406 better than one large neo magnet thus reducing the air spaces 412 between the neo magnets 408 and the ferrite magnet 406 which tends to provide a quieter rotor design. The neo magnets 408 are spaced apart from each other by a portion of the core 414. Spacing the neo magnets 408 apart from each other allows them to be positioned in the slot more securely. The slot is trapezoidal in each area that contains each neo magnet 408. That is, instead of fitting tightly against the outline of the combined ferrite and neo magnets, the core is cut so that it does not fit against the shorter edges of the neo magnets 408. The trapezoidal slot results in generally triangular air spaces 410 bounded by the short sides of the rectangular neo magnets 408, the concave face of the ferrite magnet 406, and the core 402. This trapezoidal style slot reduces intricate details of the slot cross section which can increase the life of a die used to make the slot, making a trapezoidal slot desirable when die life is more important to the manufacturer than motor noise is to the end user. This embodiment thus allows longer die life and secure positioning of two relatively small neo magnets 408 which is cost effective regarding die life and minimizes motor noise (as compared to a design utilizing one large neo magnet).
Referring now to FIG. 5, an embodiment of the invention using a bread-loaf shaped neo magnet 508, an arc shaped ferrite magnet 506, and a precision cut slot is shown. A cylindrical core 502 has a central shaft 504 about which it rotates and a slot extending parallel to the shaft 504. The arc shaped ferrite magnet 506 has a convex surface 510 facing the central shaft 504 and a concave surface 512 facing away from the central shaft 504. A bread-loaf shaped neo magnet 508 is generally rectangular, however, one of the longer sides is generally complementary to the concave face 512 of the ferrite magnet 506. The curved side of the neo magnet 508 is substantially in contact with the concave face 512 of the ferrite magnet 506. The precision cut slot is an alterative to a slot that is trapezoidal or triangular in the area of the neo magnet. The slot is precision cut to accept the ferrite magnet 506 and neo magnet 508 while maintaining a minimum air space between the ferrite and neo magnets and between each magnet and the rotor core. This means that the core 502 fits tightly against the outline of the combined neo and ferrite magnets. This embodiment has essentially no air spaces either between the two magnets or between the magnets and the core and thus is quiet when operating at high speeds. However, the large bread-loaf shaped neo magnet 508 and precision slot mean that this embodiment may be one of the more expensive to manufacture due to shortened die life and increased neo magnet expense. Also, embodiments utilizing a precision slot generally have a lower maximum inductance than embodiments utilizing a trapezoidal slot which means that such embodiments may not be as efficient as other embodiments.
Referring now to FIG. 6, an embodiment of the invention using two bread-loaf shaped neo magnets 608, an arc shaped ferrite magnet 606, and a trapezoidal slot is shown. A cylindrical core 602 has a central shaft 604 about which it rotates and a slot extending parallel to the shaft 604. The arc shaped ferrite magnet 606 has a convex surface facing the central shaft 604 and a concave surface facing away from the central shaft 604. Bread-loaf shaped neo magnets 608 are generally rectangular, however, one of their longer sides is complementary to the concave face of the ferrite magnet 606. The curved side of each neo magnet 508 is substantially in contact with the concave face of the ferrite magnet 606. The neo magnets 608 are spaced apart from each other. The slot is not precision cut, but is trapezoidal in the area that contains the neo magnets 608. That is, instead of fitting tightly against the outline of the combined ferrite and neo magnets, the core is cut so that it does not fit tightly against the shorter edges of the neo magnets 608. The trapezoidal slot results in generally triangular air spaces 610 bounded by the short sides of the rectangular neo magnets 608, the concave face of the ferrite magnet 606, and the core 602. This trapezoidal style slot reduces intricate details of the slot cross section which can increase the life of a die used to make the slot, making a trapezoidal slot desirable when die life is more important to the manufacturer than motor noise is to the end user. This embodiment allows for longer die life, secure positioning of two relatively small neo magnets, and reduced air spaces as compared to the embodiment illustrated in FIG. 4.
Referring now to FIG. 7, an embodiment of the invention using a rectangular neo magnet 708, an arc shaped ferrite magnet 706, and a precision slot is shown. A cylindrical core 702 has a central shaft 704 about which it rotates and a slot extending parallel to the shaft 704. The generally arc shaped ferrite magnet 706 has a convex surface facing the central shaft 704 and a concave surface facing away from the central shaft 704. The rectangular neo magnet 708 is positioned with a longer edge in contact with the convex surface of the ferrite magnet 706. The neo magnet 708 is offset from the center of the convex face of the ferrite magnet 706. The slot is precision cut to fit against the outline of the combined neo and ferrite magnets. However, generally triangular air spaces 710 exist bound by the convex surface of the ferrite magnet 706, a portion of the long side of the neo magnet 708, and the core 702. This embodiment allows for a long die life and relatively small air spaces as compared to certain other embodiments. However, locating the neo magnet 708 closer to the shaft 704 than the ferrite magnet 706 reduces the maximum inductance of the rotor.
Referring to FIG. 8, an embodiment of the invention shows a lobed core using either a rectangular neo magnet or bread-loaf neo magnet, an arc shaped ferrite magnet, and a trapezoidal slot is shown. This embodiment is shown without the magnets to better depict the cross section of a composite slot 808. A cylindrical core 802 has a central shaft 806 about which it rotates and the composite slot 808 extends parallel to the shaft 806. An arc shaped ferrite magnet for use with this embodiment has a convex surface facing the central shaft 806 and a concave surface facing away from the central shaft 806. A neo magnet for use with this embodiment has a longer dimension facing the ferrite magnet, and either has the corners of the neo magnet contacting the concave face of the ferrite magnet 206 (if the neo magnet is rectangular), or has one of the longer sides generally complementary to the concave face of the ferrite magnet and substantially in contact with the concave face of the ferrite magnet (if the neo magnet is bread-loaf shaped). The composite slot 808 is trapezoidal in cross section perpendicular to the axis of rotation forming generally triangular air spaces with the shorter edges of a neo magnet used in this embodiment. This trapezoidal slot reduces intricate details of the slot cross section which can increase the life of a die used to make the slot, making a trapezoidal slot desirable when die life is important to the manufacturer.
In the embodiment of FIG. 8, the core 802 is lobed. A rotor with lobes generally has reduced cogging torque and a more sinusoidal back EMF. The cross section of the core 802 is shown surrounded by a perfect circle 804. The outer edge 812 of the core 802 varies in distance from the perfect circle 804. The distance 810 from the outer edge 812 of the core 802 to the perfect circle 804 is generally less than the distance 814 from the outer edge 812 of the core 802 to the perfect circle 804 over a slot 808. In one embodiment, the distance 810 over a slot is 0.020″ and the distance 814 not over a slot is 0.040″. Embodiments of the invention may have lobes over each slot in the rotor, or lobes over selected slots in the rotor.
In yet another embodiment, the present invention is a method of manufacturing an IPM motor having a rotor wherein a ferrite magnet and a neo magnet are both located in the same slot. One or more slots are formed in a cylindrical rotor core having a central longitudinal axis about which the core rotates. The neo magnet is inserted in the slot. The ferrite magnet is placed in the slot between the neo magnet and the central longitudinal axis of the cylindrical core. The ferrite magnet is arc shaped when viewed in cross section relative to the central longitudinal axis. The neo magnet is rectangular when viewed in cross section relative to the central longitudinal axis. The slot may be precisely complementary to the outline of the combined ferrite and neo magnets so as to minimize air spaces, or it may have a trapezoidal area around the rectangular neo magnet. The rotor core is secured within a stator having windings, and a commutation circuit energizes the windings. A magnetic field of the stator interacts with the magnets in the rotor causing the rotor to turn.
It is contemplated that aspects of the embodiments described above may be combined in numerous ways without deviating from the invention. For example, the embodiment shown in FIG. 6 may use a precision slot instead of a trapezoidal slot, or the embodiment shown in FIG. 5 may use a trapezoidal slot. FIGS. 1-7 show 4 slots having magnets in them, but the rotor may have any number of slots, some of which may be empty. Also, the same rotor may contain more than one configuration of neo and ferrite magnets. The central shaft shown in the above embodiments may be cast, forged, or machined as part of the core or engage the core by some other means such as splining. Additionally, any of the rotor configurations may have lobed cores as shown in FIG. 2.
Some embodiments of the invention have advantages over other embodiments. For example, using two rectangular (i.e., viewed in cross section) pieces of neo magnet allows small air spaces than one larger piece of neo magnet because they better conform to the curvature of the ferrite magnet. Embodiments of the invention utilizing a trapezoidal slot will generally have a higher maximum inductance than embodiments utilizing a precision slot because a precision slot tends to increase leakage flux. Embodiments using lobed rotor cores generally have a lower cogging torque and more sinusoidal back EMF than embodiments using cylindrical rotor cores. Also, embodiments with a neo magnet further from the center of the rotor than the ferrite magnet tend to develop a higher maximum inductance than embodiments with neo magnets closer to the center than the ferrite magnet.
The above description is also applicable to other motor configurations such as inside out motors and/or motors having windings in the rotor and permanent magnets in the stator, and visa versa. For example, embodiments of the invention in an inside out motor include neo and ferrite magnets located in a single slot. Magnet configurations and air space considerations are similar to those of the above described rotor designs.
This description refers to ferrite and neo throughout, but one skilled in the art will recognize that magnetic materials other than neo and ferrite may be used without deviating from the invention and more than one piece of neo and/or ferrite may be used in each slot. One skilled in the art will also notice that different shapes of neo magnets, ferrite magnets, and slots are possible without deviating from the invention. The cylindrical rotor core may be made with steel or some other material. The description refers to an IPM motor rotor throughout, but one skilled in the art knows that an electric motor may be configured as a generator.
Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.
The order of execution or performance of the methods illustrated and described herein is not essential, unless otherwise specified. That is, it is contemplated by the inventors that elements of the methods may be performed in any order, unless otherwise specified, and that the methods may include more or less elements than those disclosed herein. For example, it is contemplated that executing or performing a particular element before, contemporaneously with, or after another element is within the scope of the various embodiments of the invention.
When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.
As various changes could be made in the above products and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

1. An electric motor rotor comprising:

a core having a central longitudinal axis and a slot radially spaced from the longitudinal axis and extending parallel to the axis; and

first and second magnets positioned in the slot and extending parallel to the longitudinal axis, wherein the first magnet is positioned between the second magnet and the longitudinal axis.

2. The rotor of claim 1 wherein said core has an outer surface parallel to said longitudinal axis, said outer surface having a lobe.

3. The rotor of claim 2 further comprising an air space adjacent to the first or second magnet.

4. The rotor of claim 1 wherein, when viewed in cross section, the first magnet is arch-shaped, having a convex surface facing the central longitudinal axis and a concave surface facing the second magnet.

5. The rotor of claim 4 wherein at least one of the following:

(1) when viewed in cross section, the second magnet has a convex surface generally complementary to and in contact with the concave surface of the first magnet; and

(2) when viewed in cross section, at least a portion of the first magnet contacts the concave surface of the second magnet.

6. The rotor of claim 1 further comprising a third magnet of the same material as the second magnet sized and shaped substantially the same as the second magnet.

7. The rotor of claim 6 wherein, when viewed in cross section, the second and third magnets are substantially rectangular and at least a portion of each is in contact with the first magnet.

8. The rotor of claim 1 wherein one of the first or second magnets is strontium ferrite, the other of the first and second magnets is neodymium-iron-boron, and the core is steel.

9. A method of producing an electric motor comprising:

forming a slot in a rotor core having a central longitudinal axis;

inserting a first magnet in the slot;

inserting a second magnet in the slot, wherein said first magnet is substantially between the second magnet and the central longitudinal axis;

inserting the rotor core into a stator having windings; and

connecting the windings of the stator to a commutation circuit.

10. The method of claim 9 wherein the first magnet is strontium ferrite, the second magnet is neodymium-iron-boron, and the core is steel.

11. An electric motor comprising:

a rotor including:

first and second magnets positioned in the slot and extending parallel to the longitudinal axis, wherein the first magnet is positioned between the second magnet and the longitudinal axis;

a stator in magnetic coupling relation to the rotor having windings; and

a commutation circuit electrically connected to the windings of the stator.

12. The rotor of claim 11 wherein said core has an outer surface parallel to said longitudinal axis, said outer surface having a lobe.

13. The motor of claim 12 further comprising an air space adjacent to the first or second magnet.

14. The motor of claim 11 wherein, when viewed in cross section, the first magnet is arch-shaped, having a convex surface facing the central longitudinal axis and a concave surface facing the second magnet.

15. The motor of claim 14 wherein at least one of the following:

16. The motor of claim 11 further comprising a third magnet of the same material as the second magnet sized and shaped substantially the same as the second magnet.

17. The motor of claim 16 wherein, when viewed in cross section, the second and third magnets are substantially rectangular and at least a portion of each is in contact with the first magnet.

18. The motor of claim 17 wherein, when viewed in cross section, the second and third magnets are spaced apart.

19. The motor of claim 11 wherein the rotor has at least two slots equally spaced radially and circumferentially about the longitudinal axis.

20. The motor of claim 11 wherein one of the first or second magnets is strontium ferrite, the other of the first and second magnets is neodymium-iron-boron, and the core is steel.