Abstract:
An electric machine including an outside rotor having a first member providing a magnetic path between adjacent ones of a circumferential array of magnets, and a second member surrounding and inducing a compression load in the first member.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to electric machines such as generators and motors and, more particularly, to an improved outside rotor electric machine. 
       BACKGROUND OF THE ART 
       [0002]    Some inside rotor electric machines, i.e. machines having a rotor received inside a stator, have a rotor with magnets retained around a shaft and surrounded by a non-magnetic ring. Generally, the ring compresses the magnets and the rotor shaft to pre-stress the rotor such as to prevent the magnets from being separated from the shaft at high rotation speeds. However, such a ring defines a layer of non-magnetic material between the magnets and the stator, thus interfering with the magnetic flux by effectively increasing the air gap. As such, thicker magnets must generally be used to obtain a given magnetic flux, which results in a generally heavier rotor, thus a lower maximum rotation speed and/or a lower power density. 
         [0003]    Some outside rotor electric machines, i.e. machines having a rotor surrounding the stator, have a rotor with magnets retained within a magnetic ring which acts both as part of the magnetic circuit (also known as the back iron) and provides the necessary strength to resist the loads produced during use. Generally, the retaining ring is sized according to the necessary load carrying capability or hoop strength, and is significantly thicker that would otherwise be required to obtain the necessary magnetic capability, i.e. the hoop strength requirements substantially exceed the magnetic capability requirements. The use of a thicker ring for a given magnetic capability generally results in a heavier rotor, with consequent adverse effect on the rotor dynamics of the assembly thus a lower rotation speed and/or a lower power density. 
         [0004]    Accordingly, improvements are desirable. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of this invention to provide an improved outside rotor for an electric machine. 
         [0006]    In one aspect, the present invention provides an electric machine comprising an inner stator and a rotor surrounding the stator, the rotor including an array of circumferentially spaced apart magnets forming alternating poles and defining at least part of a circumferential inner surface adapted to extend adjacent the stator, a retaining ring surrounding and retaining the magnets, the retaining ring including a magnetic material and defining a magnetic path between adjacent ones of the magnets, and a containment ring surrounding the retaining ring, the containment ring being disposed outside of the magnetic path and having an interference fit with the retaining ring such as to produce a hoop compression stress in the retaining ring. 
         [0007]    In another aspect, the present invention provides an electric machine comprising an inner stator and a rotor surrounding the stator, the rotor including an array of circumferentially spaced apart magnets forming alternating poles and defining at least part of a circumferential rotor surface adapted to extend adjacent the stator, first means for providing a magnetic path between adjacent ones of the magnets, and second means for surrounding and inducing a compression load in the first means to increase a load resistance of the first means during rotation of the rotor, the second means being disposed outside of the magnetic path. 
         [0008]    In a further aspect, the present invention provides an outside rotor for an electric machine, the outside rotor comprising a circumferential array of spaced apart magnets forming alternating poles, the magnets defining at least part of an inner surface adapted to extend adjacent a stator, a retaining ring surrounding and retaining the magnets, the retaining ring including a magnetic material and defining a magnetic path between adjacent ones of the magnets, and a containment ring surrounding the retaining ring and producing a hoop compression stress in the retaining ring, the containment ring being remote from the magnetic path. 
         [0009]    Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
     
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    Reference is now made to the accompanying figures depicting aspects of the present invention, in which: 
           [0011]      FIG. 1  is a transverse cross-sectional view of an outside rotor electric machine according to a particular aspect of the present invention; 
           [0012]      FIG. 2  is an exploded perspective view of the machine of  FIG. 1 ; and 
           [0013]      FIG. 3  is a partial longitudinal cross-sectional view of the machine of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Referring to the Figures, an electric machine according to a particular aspect of the present invention is generally shown at  10 . The machine  10  has an “outside rotor” configuration, i.e. the machine comprises a rotor  12  which surrounds a stator  14 . 
         [0015]    The stator  14  is schematically shown in dotted lines, and may be any appropriate type of stator, including, but not limited to, a stator such as described in U.S. Pat. No. 6,965,183, issued Nov. 15, 2005 to Dooley, and which is incorporated herein by reference. A rotor air gap  24 , radially defined between a circumferential inner surface  26  of the rotor  12  and an outer surface  28  of the stator  14 , separates the rotor  12  and stator  14 . 
         [0016]    The rotor  12  generally comprises an array of circumferentially spaced apart magnets  30 , which in the embodiment shown are permanent magnets, forming alternating poles. The magnets  30  are retained by a non-magnetic yoke  32  (in the present specification and claims, “non-magnetic” is also intended to include elements that have some, but negligible, magnetic capability relative to the magnets  30 ) and a retaining ring  34 . The yoke  32  is generally crown-shaped, and includes an array of circumferentially spaced apart spacers  36  (not shown in  FIG. 3 ) extending from a ring  38  (best seen in  FIG. 2 ). Each magnet  30  is received between adjacent spacers  36 , and abuts the adjacent spacers  36  and the ring  38 , such that the magnets  30  and yoke  32  together define the cylindrical inner surface  26  of the rotor  12 . In an alternate embodiment, the magnets  30  fill the inner circumference of the retaining ring  34  and as such the yoke  32  is omitted. 
         [0017]    Referring to  FIGS. 1 and 3 , the retaining ring  34  surrounds the magnets  30  and yoke  32 . The retaining ring  34  includes an attachment flange  40  extending radially inwardly and abutting the yoke ring  38  opposite of the magnets  30  to engage driving and/or drivable means such as a rotating shaft, which is represented in  FIG. 3  by an axis of rotation  42 . The attachment flange  40  is shown as a substantially conical flange, although other alternate flange geometries are also possible. 
         [0018]    The retaining ring  34  also includes a magnetic material in order to complete a magnetic path between the magnets  30  with a minimum path length and as such maximize the magnetic flux density in the rotor air gap  24 . The materials for the rotor  12  may be any deemed suitable by the designer, and may include in a particular embodiment samarium cobalt for the permanent magnets  30 , maraging steel for the retaining ring  34 , and aluminium, titanium or another appropriate lightweight non-magnetic material for the yoke  32 . 
         [0019]    The rotor  12  further comprises a containment ring  44  surrounding the retaining ring  34 . In the embodiment shown, the retaining ring  34  includes a cylindrical shoulder  46  (see  FIG. 3 ) against which the containment ring  44  is abutted. The containment ring  44  has an interference fit with the retaining ring  34 , such as to produce a hoop compression load in the retaining ring  34  at initial assembly. In a particular embodiment, this is achieved by having the containment ring  44  shrink fitted around the retaining ring  34 . Having this initial compressive stress in the retaining ring  34  has the effect of lowering the hoop stress in the rotor  12  during operation, through superposition of the compression pre-stressing load and the tensile operating load. 
         [0020]    In a particular embodiment, the containment ring  44  is a sleeve comprising a lightweight, high stiffness, non-magnetic filament material wound around the retaining ring  34 . Alternately, the containment ring  44  could also comprise an appropriate lightweight, high stiffness, magnetic material. Suitable material for the containment ring  44  include high strength composite materials, e.g. a fiber reinforced composite including carbon fiber such as reinforced carbon-carbon, carbon fibers in a matrix of polyimide, and other lightweight materials providing high strength and stiffness. 
         [0021]    As is the case for conventional permanent magnet machines, the machine  10  may operate in a generator mode or a motor mode. When operated in a generator mode, an external torque source forces rotation of the flange  40  (and thus the rotor  12  and the magnets  30 ), and the interaction of the magnets  30  and the stator  14  causes a magnetic flux to loop therein. As the rotor  12  rotates, the magnetic flux in the stator  14  changes, and this changing flux results in an output current that can be used to power electrical devices, or be stored for later use. When operated in a motor mode, a voltage from an external source is applied to the stator  14  which causes current flow therein and results in a magnetic flux to be set up in a magnetic circuit therein. When current is supplied in an appropriate manner to stator  14 , the rotor  12  can be made to rotate and thus produce usable torque. 
         [0022]    The retaining ring  34  of the machine  10  is advantageously thinner and lighter when compared to the retaining ring of a similar rotor without a containment ring  44 . As the pre-stressing of the retaining ring  34  by the containment ring  44  effectively decreases the hoop stress in the rotor  12  during use, a thinner and lighter retaining ring  34  can be used. For example, the retaining ring  34  can be sized according to the desired magnetic capability, while the containment ring  44  provides the necessary hoop strength to the rotor  12 . 
         [0023]    The containment ring  44  is made of a material having a stiffness at least equal to, and preferably greater than, the stiffness of the material of the retaining ring  34 , for example a material having a Young&#39;s modulus approximately 2 times that of the material of the retaining ring  34 . In a particular embodiment, the containment ring  44  is made of a material having a stiffness per unit of mass greater than that of the material of the retaining ring  34 , such as to further reduce the overall weight of the rotor  12 . A lighter rotor advantageously allows for rotation at higher maximum speed and/or acceptable dynamic characteristics, for an increase in power density of the machine  10 . 
         [0024]    In another particular embodiment, the containment ring  44  is made of a material having a stiffness per unit of volume greater than that of the material of the retaining ring  34 , such as to reduce the overall size of the machine  10 , thus allowing for a higher power density for a given machine size. Advantageously, the containment ring  44  can be made of a material having both a stiffness per unit of mass and a stiffness per unit of volume greater than those of the material of the retaining ring  34 , such as is the case with a containment ring  44  made of reinforced carbon-carbon and a retaining ring  34  made of maraging steel, or any other suitable “soft” magnetic alloy material. 
         [0025]    Since the containment ring  44  adds strength directly to the retaining ring  34 , as opposed to a ring surrounding the magnets of an inside rotor for example, the rotor  12  is reinforced without introducing a material thickness between the magnets  30  and the stator  14 . The absence of material between the magnets  30  and the stator  14  allows for a grater magnetic flux for a given magnet thickness, or, in other words, the use of thinner, lighter magnets for a given magnetic flux. Thinner magnets reduce the load created on the retaining ring  34  for a given speed of rotation, thus allowing for a greater speed of operation and thus a greater power capacity for a given size/weight of the machine  10 . Accordingly, a higher power machine is obtained for a given machine weight. 
         [0026]    Also, since the containment ring  44  provides the necessary strength to the rotor  12 , the retaining ring  34  can be made a better, “soft” magnetic material, which may have lower strength, the containment ring  44  compensating for the lack of strength of the retaining ring  34 . Such “soft” magnetic material include, for example, cobalt-iron alloys, silicon-iron alloys, and nickel-iron alloys. 
         [0027]    In addition, the interference fit of the containment ring  44  around the retaining ring  34  results in friction which may advantageously act as a damper to free ring vibrations of the rotor  12 . 
         [0028]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. For example, the containment ring can be used with outside rotors having a different geometries than the rotor described herein, including rotors having different types of magnets, e.g. electromagnets. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.