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  
       [0001]     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  
       [0002]     Interior permanent magnet (IPM) rotor designs using strontium ferrite (ferrite) and neodymium-iron-boron (neo) are known in the art.  
         [0003]     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.  
         [0004]     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.  
         [0005]     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  
       [0006]     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.  
         [0007]     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.  
         [0008]     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.  
         [0009]     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.  
         [0010]     Alternatively, the invention may comprise various other methods and apparatuses.  
         [0011]     Other objects and features will be in part apparent and in part pointed out hereinafter.  
         [0012]     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  
       [0013]      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.  
         [0014]      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.  
         [0015]      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.  
         [0016]      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.  
         [0017]      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.  
         [0018]      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.  
         [0019]      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.  
         [0020]      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. 
     
    
       [0021]     Corresponding reference characters indicate corresponding parts throughout the drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     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 .  
         [0023]     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.  
         [0024]     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.  
         [0025]     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.  
         [0026]     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).  
         [0027]     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).  
         [0028]     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.  
         [0029]     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 .  
         [0030]     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.  
         [0031]     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.  
         [0032]     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.  
         [0033]     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.  
         [0034]     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 .  
         [0035]     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.  
         [0036]     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.  
         [0037]     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.  
         [0038]     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.  
         [0039]     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.  
         [0040]     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.  
         [0041]     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.  
         [0042]     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.