Abstract:
A method for coupling a permanent magnet to a rotor body of an electric motor includes providing a rotor body having an outer surface. A mounting hole is formed in the rotor body. A permanent magnet in the form of an annular section is provided. The concave surface of the permanent magnet has a diameter generally equal to the outer diameter of the rotor body. The permanent magnet has a hole formed therein to receive the threaded connector, the hole having a countersink at the convex surface of the permanent magnet. The permanent magnet is positioned on the outer surface of the rotor body such that the hole lines up with the mounting hole, an elastomeric body is positioned in the countersink, and the threaded connector is positioned through the elastomeric body and the hole of the permanent magnet to couple to the rotor body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application which claims priority from U.S. provisional application No. 61/935,185, filed Feb. 3, 2014. 
    
    
     TECHNICAL FIELD/FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to permanent magnet electric motors, and specifically to the bonding of permanent magnets to a rotor or stator of a permanent magnet electric motor. 
     BACKGROUND OF THE DISCLOSURE 
     In general, electric motors operate by rotating a rotor relative to a fixed stator by varying the orientation of a magnetic field induced by one or more coils. In some electric motors, both the rotor and stator include coils. In such an induction motor, the magnetic field induced by the stator coils induces current within the rotor coils which, due to Lenz&#39;s law, causes a resultant torque on the rotor, thus causing rotation. 
     In a permanent magnet motor, on the other hand, the rotor includes one or more permanent magnets. The permanent magnets, in attempting to align with the magnetic field induced by the coils in the stator, cause a resultant torque on the rotor. By varying the orientation of the magnetic field, the rotor may thus be caused to rotate. In high-torque permanent magnet motors, multiple permanent magnets may be positioned on the exterior of the rotor (for an internal rotor permanent magnet motor). 
     While in operation, the components of the permanent magnet motor may heat up in response to, for example, electrical resistance in the stator coils, losses in iron core of stator, induced currents in rotor caused by harmonics, mechanical friction, etc. Because of this increase in heat, the permanent magnets must be bonded to the rotor in such a way that any thermal expansion of the rotor or permanent magnets will not cause the permanent magnets to fracture or separate from the rotor. Additionally, in cases where the permanent magnets are formed by, for example, sintering, the permanent magnets themselves may be relatively brittle. Furthermore, where the permanent magnets are constructed of a material with a different thermal expansion coefficient than the rotor, as is often the case, the thermal expansion of the rotor may cause the permanent magnets to crack. 
     SUMMARY 
     The present disclosure provides for a method for coupling permanent magnets to a rotor. The method may include providing a rotor body, the rotor body being generally cylindrical in shape, the rotor body having an outer surface; forming a mounting hole in the rotor body, the mounting hole positioned to couple to a threaded connector; providing a permanent magnet, the permanent magnet being generally in the form of an annular section, the concave surface of the permanent magnet having a diameter generally equal to the outer diameter of the rotor body, the permanent magnet having a hole formed therein positioned to receive the threaded connector, the hole having a countersink formed therein at the convex surface of the permanent magnet; positioning the permanent magnet on the outer surface of the rotor body so that the hole of the permanent magnet is in alignment with the mounting hole; positioning an elastomeric body within the countersink; positioning the threaded connector through the elastomeric body and the hole of the permanent magnet; coupling the threaded connector to the rotor body. 
     The present disclosure also provides for a rotor for a permanent magnet electric motor. The rotor may include a rotor body, the rotor body being generally cylindrical in shape and having an outer surface. The rotor body may include a mounting hole positioned to couple to a threaded connector. The rotor may also include a permanent magnet. The permanent magnet may be generally in the form of an annular section. The concave surface of the permanent magnet may have a diameter generally equal to the outer diameter of the rotor body. The permanent magnet may have a hole formed therein positioned to receive the threaded connector. The hole may have a countersink formed therein at the convex surface of the permanent magnet. The rotor may also include an elastomeric body positioned within the countersink between the threaded connector and the permanent magnet. 
     The present disclosure also provides for a method. The method may include providing a rotor body. The rotor body may be generally cylindrical in shape. The rotor body may have an outer surface. The outer surface of the rotor body may have at least one dovetail channel. The method may also include providing a permanent magnet. The permanent magnet may be generally in the form of an annular section. The concave surface of the permanent magnet may have a diameter generally equal to the outer diameter of the rotor body. The permanent magnet may include at least one dovetail adapted to fit into the dovetail channel. The method may further include sliding the permanent magnet on the outer surface of the rotor body so that the dovetail couples to the dovetail channel. 
     The present disclosure also provides for a method. The method may include providing a rotor body. The rotor body may be generally cylindrical in shape. The rotor body may have an outer surface. The method may further include providing a retaining ring. The method may further include providing a permanent magnet. The permanent magnet may generally be in the form of an annular section. The concave surface of the permanent magnet may have a diameter generally equal to the outer diameter of the rotor body. The permanent magnet may have at least one flange extending from an end of the permanent magnet. The flange may be adapted to allow the retaining ring to hold the permanent magnet to the rotor body by compressing the flange to the rotor body. The method may further include positioning the permanent magnet on the outer surface of the rotor body. The method may further include positioning the retaining ring about the rotor body and permanent magnet such that the retaining ring is generally aligned with the flange. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  depicts a rotor having permanent magnets affixed thereto consistent with embodiments of the present disclosure. 
         FIG. 2  depicts a partial cross section of the rotor of  FIG. 1 . 
         FIG. 3  depicts a partial cross section of a rotor having permanent magnets affixed thereto consistent with embodiments of the present disclosure. 
         FIGS. 4 a , 4 b    depict a rotor having permanent magnets affixed thereto consistent with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     As depicted in  FIG. 1 , rotor  101  for use in a permanent magnet motor may include rotor body  103 . Rotor body  103  may be generally cylindrical in shape. In some embodiments, rotor body  103  may be coupled to output shaft  105 . As rotor  101  is rotated within the permanent magnet motor, output shaft  105  serves to transfer the rotational power generated by rotor  101  to other equipment (not shown). 
     Rotor  101  may, in some embodiments, include one or more permanent magnets  107  positioned about the exterior surface of rotor body  103 . In some embodiments, as depicted in  FIG. 1 , permanent magnets  107  may be annular in shape. The concave surface of each permanent magnet  107  may have generally the same diameter as the exterior surface of rotor body  103 . Permanent magnets  107  may be configured such that the magnetic axis of each permanent magnet is substantially aligned to be normal to the surface of rotor body  103 . In some embodiments, the magnetic field of adjacent permanent magnets  107  are in opposition, so that the magnetic pole of permanent magnets  107  alternate between North and South. In some embodiments, permanent magnets  107  may be formed by sintering of permanent magnet material such as, for example and without limitation, a rare-earth magnet such as neodymium. In other embodiments, permanent magnets  107  may be formed by a rapid solidification process as understood in the art. 
     As depicted in  FIG. 2 , permanent magnet  107  may be coupled to rotor body  103 . In some embodiments, permanent magnet  107  may have one or more holes  109  formed therein. Hole  109  is aligned so that when permanent magnet  107  is placed on the outer surface of rotor body  103 , hole  109  extends in a direction normal to the surface of rotor body  103 . In some embodiments, hole  109  may include countersink  111 . Rotor body  103  may include one or more mounting holes  113  positioned to align with holes  109  of permanent magnets  107 . In some embodiments, mounting holes  113  may be tapped to accept the thread of threaded fastener  115 . In some embodiments, threaded fastener  115  may be, for example and without limitation, a screw, bolt, or other threaded fastener. Countersink  111  may allow threaded fastener  115  to, when installed, remain below the outer surface of permanent magnet  107  which may, for example, avoid interference between threaded fastener  115  and other parts of the permanent magnet motor. 
     In some embodiments, a thread-locking compound may be applied to threaded fastener  115  to, for example, prevent threaded fastener  115  from unintentionally unthreading from rotor body  103 . In some embodiments, a potting material or adhesive may be applied between, for example, rotor body  103  and permanent magnet  107 . 
     In the embodiment depicted in  FIG. 2 , threaded fastener  115  is a flathead screw with a matching tapered profile to that of countersink  111 . One having ordinary skill in the art with the benefit of this disclosure will understand that threaded fastener  115  may be replaced by a threaded connector having a different profile without deviating from the scope of this disclosure. Likewise, countersink  111  may have a different profile such as, for example and without limitation, a counterbore without deviating from the scope of this disclosure. For the purposes of this disclosure, the term “countersink” is intended to include both countersinks and counterbores unless specifically differentiated. 
     In some embodiments, elastomeric body  117  may be positioned between the head of threaded fastener  115  and permanent magnet  107  when permanent magnet  107  is installed to rotor body  103 . Elastomeric body  117  may be formed of an elastomeric material, allowing elastomeric body  117  to be installed under elastic compression between threaded fastener  115  and permanent magnet  107 . Because threaded fastener  115  may have a thermal expansion coefficient and/or thermal conductivity different from that of permanent magnet  107 , threaded fastener  115  may thermally expand and increase in length more rapidly than permanent magnet  107  as permanent magnet  107 , threaded fastener  115 , and rotor body  103  increase in temperature during normal use. In such a case, the compressive stress on elastomeric body  117  between threaded fastener  115  and permanent magnet  107  may decrease. Elastomeric body  117 , being elastically deformed, increases in size as the stress thereon decreases, which may maintain the compressive force between threaded fastener  115  and permanent magnet  107 . Elastomeric body  117  may thus, for example, prevent any loosening of the attachment between permanent magnet  107  and rotor body  103 . 
     Although depicted as a single O-ring, elastomeric body  117  may, in some embodiments, be, for example and without limitation, a single O-ring, multiple O-rings, an elastomeric washer, or a combination thereof. 
     Likewise, as threaded fastener  115  and permanent magnet  107  decrease in temperature during normal operation of the permanent magnet motor, for example when the permanent magnet motor is shut off, threaded fastener  115  may thermally contract more rapidly than permanent magnet  107 . In this case, the compressive stress on elastomeric body  117  between threaded fastener  115  and permanent magnet  107  may increase. Elastomeric body  117  may elastically deform to, for example, prevent excess force from being exerted on permanent magnet  107  by threaded fastener  115 . Elastomeric body  117  may thus, for example, prevent threaded fastener  115  from crushing permanent magnet  107 . 
     In order to assemble rotor  101 , a rotor body  103  may be provided. One or more mounting holes  113  may be formed in the exterior surface of rotor body  103 . In some embodiments, mounting holes  113  may be tapped to receive a threaded fastener. One or more permanent magnets  107 , having at least one hole  109  formed therein, each hole  109  positioned to align with a corresponding mounting hole  113 , each hole  109  having countersink  111 , is then positioned onto the outer surface of rotor body  103 . Elastomeric body  117  is then placed within countersink  111 . A threaded fastener, such as threaded fastener  115 , is then threaded into hole  109  and mounting hole  113 , such that the head of threaded fastener  115  mechanically couples permanent magnet  107  to rotor body  103 . 
     Although  FIG. 1  depicts a permanent magnet  107  being coupled to rotor body  103  by only one threaded fastener  115 , one having ordinary skill in the art with the benefit of this disclosure will understand that multiple screws  115  may be utilized for each permanent magnet  107 . Additionally, although depicted as being used for an internal rotor permanent magnet motor, one having ordinary skill in the art with the benefit of this disclosure will understand that permanent magnets  107  may be installed to the interior surface of a tubular rotor of an external rotor permanent magnet motor without deviating from the scope of this disclosure. Likewise, although described with permanent magnets  107  coupled to the rotor of a permanent magnet motor, permanent magnets  107  may be coupled to the stator of a permanent magnet motor in which the coils are positioned on the rotor without deviating from the scope of this disclosure. 
     In some embodiments, rotor  201  may include rotor body  203  as depicted in  FIG. 3 . Rotor body  203  may include one or more dovetail channels  205  adapted to interface with permanent magnets  207 . In such embodiments, permanent magnets  207  may include magnet dovetail  209  adapted to fit into dovetails  205  and thus retain permanent magnet  207  to rotor body  203 . In such an embodiment, permanent magnet  207  may be slid into dovetail channels  205  during assembly. In some embodiments, dovetail channels  205  may be formed by removing material from rotor body  203 . In some embodiments, dovetail channels  205  may be formed as a separate piece from rotor body  203  and affixed thereto by, for example and without limitation, threaded couplers. In some embodiments, dovetail channels  205  may be coupled to rotor body  203  by threaded couplers as described above. 
     In some embodiments, rotor  301  may include rotor body  303  as depicted in  FIGS. 4 a , 4 b   . Permanent magnets  305  may include one or more flanges  307  as depicted in  FIG. 4 a   . Flanges  307  may, for example and without limitation, be adapted to receive retaining ring  309  when installed as depicted in  FIG. 4 b   . Retaining ring  309  may be adapted to encircle rotor body  303  and flanges  307  of permanent magnets  305  in order to retain permanent magnets  305  to rotor body  303 . In some embodiments, retaining ring  309  may be a split ring, the ends of which being coupled to one or more of rotor body  303  or the other end of retaining ring  309 . 
     The foregoing outlines features of several embodiments so that a person of ordinary skill in the art may better understand the aspects of the present disclosure. Such features may be replaced by any one of numerous equivalent alternatives, only some of which are disclosed herein. One of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. One of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.