Abstract:
Permanent magnets are mounted on a rotor of an electrical motor by an apparatus that keeps an adhesive bonding layer between the magnet and rotor in compression over wide variations in temperature and speed of rotation. The mechanical stresses on the magnets are controlled using compliant layers and spacers. The apparatus allows the removal and replacement of individual magnets.

Description:
This application claims the benefit of the filing date of U.S. Provisional Application; Ser. No. 60/666,539; filed Mar. 30, 2005; entitled “Method and Apparatus for Retention of Magnets on a Permanent Magnet Motor Rotor,” the entirety of which provisional application is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to permanent magnet motors. More particularly, the present invention relates to retaining magnets on rotors useful in permanent magnet motors. 
   BACKGROUND OF THE INVENTION 
   A permanent magnet motor consists of a wound stator within which a rotor rotates. Permanent magnets are attached to the rotor to produce alternating north and south magnetic fields that interact with electrical current through the stator to produce torque. The permanent magnets are attracted to the steel core of the rotor, but the centrifugal force created by the rotation of the rotor tends to pull the magnets off of the rotor. 
   Magnet retention is difficult, involving several factors. First, the magnets are brittle ceramics and structurally weak. Second, the centrifugal forces are high, especially with very high-speed rotors. Third, radial space (i.e., the space between the rotor and the stator) is at a premium because the magnetic field weakens as the radial separation between the rotor and stator increases. Fourth, permanent magnet motors frequently are required to operate in environments spanning a wide range of temperatures and the rates of thermal expansion of the components of the rotor may differ substantially over the temperature range. 
   Many methods have been proposed to retain magnets on rotors. Magnets can be bonded to the surface of the rotor, and then held in place by an outer wrap of high-strength material such as glass or carbon fiber, typically with an encapsulant filling the spaces between magnets. These methods have a drawback in that the thickness of the wrap reduces the mechanical clearance (i.e., the radial space) between the stator and rotor. Also, the expansion rate of the wrap under tension and temperature makes it difficult to keep the adhesive bond in compression at high rotational speed. In the absence of compression the adhesive bond can peel, which then allows the magnets to move axially. Finally, since these approaches depend on the integrity of the outer wrap, it is not feasible to repair or replace a magnet after the rotor has been built. The prior art teaches a similar method without the outer wrap. This approach eliminates the radial thickness penalty of the approaches described above, but relies totally on the encapsulant and bond for retention. 
   Another prior art approach teaches a detachable magnet carrier to hold the magnets. Essentially, the magnets are packaged in a stainless steel box that provides structural strength. This is an expensive approach, and the thickness of the box subtracts from the radial clearance between the rotor and stator. 
   Magnets can also be contained inside of the rotor, such that the rotor structure retains the magnets. Interior magnet constructions require compromises in the magnetic circuit that reduce performance in some applications. 
   Accordingly, there still exists a need in industry for a magnet mounting method and structure that places the magnets on the surface of the rotor, using a minimal radial thickness of structural material, so that the performance of the magnetic circuit is maximized. Further, the mounting should maintain compression on the magnets under a wide range of rotational speed and temperature, avoiding excessive mechanical stress on the brittle ceramic magnets. Finally, the mounting should allow the replacement of individual magnets after the rotor has been built. 
   SUMMARY OF THE INVENTION 
   In one aspect the present invention embodies a retainer for holding a magnet to a rotor. Magnet retainers of the present invention generally include a retainer body having an opening to accept a fastening device to secure the retainer body to the rotor and an angled surface adapted for engaging an angled surface of a magnet when the retainer body is secured to the rotor; a fastening device disposed in the opening to secure the retainer body to the rotor; and a spring mechanism disposed in the opening between the fastening device and the retainer body to provide a force reactive to a centrifugal force during rotation of the rotor. 
   In a second aspect, the present invention embodies a rotor useful in a permanent magnet motor. Rotors of the present invention generally include a rotor body having a cylindrical surface having a circumference; a plurality of magnets disposed on the substantially cylindrical surface at equal intervals around the circumference, each magnet having an angled surface; an adhesive layer disposed between each magnet and the cylindrical surface; and a plurality of magnet retainers, each magnet retainer disposed between a neighboring pair of the magnets, each retainer including a retainer body having an opening to accept a fastening device to secure the retainer body to the rotor body and having an angled surface engaging the angled surface of one of the magnets; a fastening device disposed in the opening, the fastening device securing the retainer body to the rotor body; and a spring mechanism disposed between the fastening device and the retainer body to provide a force reactive to a centrifugal force during rotation of the rotor body. 
   In a third aspect, the present invention embodies a rotor useful in a permanent magnet motor that includes a rotor body having a cylindrical surface having a circumference; a plurality of magnets disposed on the cylindrical surface at equal intervals on the circumference, each magnet having an angled surface; an adhesive layer disposed between each magnet and the rotor body; an open-mesh screen disposed between each magnet and the cylindrical surface; a plurality of magnet retainers, each magnet retainer disposed between a neighboring pair of the magnets, each magnet retainer including a retainer body having an opening to accept a fastening device to secure the retainer body to the rotor body and having an angled surface engaging the angled surface of one of the magnets; a fastening device at least partially disposed in the opening, the fastening device securing the retainer body to the rotor body; and a spring mechanism disposed between the fastening device and the retainer body to provide a force reactive to a centrifugal force during rotation of the rotor body; and a plurality of axial retainers, each axial retainer disposed at an end of a magnet retainer, the axial retainers adapted to prevent axial movement of a magnet. 
   In a fourth aspect, the present invention embodies a method for retaining magnets on a rotor. Methods of the present invention generally include attaching magnets, having an angled surface, to a cylindrical surface of the rotor body using an adhesive layer; positioning retainer bodies, having angled surfaces, each retainer body positioned between a neighboring pair of the magnets on the cylindrical surface such that the angled surfaces of the retainer bodies engage the angled surfaces of the magnets; and securing the retainer bodies to the rotor body. Preferred embodiments include one or more of the following: positioning a spacer between each magnet and the cylindrical surface, such that the spacer shares the same volume as the adhesive layer; positioning a compliant layer between the angled surfaces of the retainer bodies and the angled surfaces of the magnets; and positioning a spring mechanism between the retainer bodies and fastening devices used to secure the retainer bodies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in the various figures. The drawings are not meant to limit the scope of the invention. For clarity, not every element may be labeled in every figure. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  illustrates a retainer body of a magnet retainer in accordance with an embodiment of the present invention. 
       FIG. 2  illustrates the retainer body of  FIG. 1  with an axial retainer secured to an end of the retainer body. 
       FIG. 3  shows an isometric view of a portion of a rotor in accordance with an embodiment of the present invention. 
       FIG. 4  shows a close-up view of a magnet retainer on the cylindrical surface of the rotor of  FIG. 3 . 
       FIG. 5  shows a cross-sectional view of the rotor of  FIG. 3 . 
       FIG. 6  shows another cross-sectional view of the rotor of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides for improved retention of magnets on rotors useful in permanent magnet motors. In accordance with the present invention, magnet retainers are advantageously utilized to aid in retaining magnets onto a rotor body that has a cylindrical surface. The magnets are disposed on the cylindrical surface at equal intervals around the circumference of the cylindrical surface. An adhesive layer is disposed between the magnet and the cylindrical surface. Magnet retainers are disposed between each pair of magnets and secured to the rotor body. The magnet retainers are secured to the rotor body in a manner that applies a centripetal force on the magnets, pulling the magnets down onto the cylindrical surface of the rotor body in a manner that exerts a compressive force on the adhesive layer even at the highest rotational speeds at which the rotor is used. Spring mechanisms are used to counter centrifugal force on the magnets during use (i.e., rotation of the rotor) and maintain this compressive force. Axial retainers can be used to prevent axial movement of the magnets upon failure of the adhesive layer. 
   Rotor bodies useful in permanent magnet motors are generally well known in the art. They have a cylindrical surface and rotate about an axis within a stator. The rotor body is typically made of steel, although the present invention may be advantageously used with rotor bodies made of other materials. In a preferred embodiment, features are created in the cylindrical surface at equal intervals around the circumference of the cylindrical surface. Thus, the phrase cylindrical surface as used herein refers to a surface that may have features as herein described and does not necessarily refer to, but may include, a surface that is cylindrical in a mathematically precise meaning. The intervals are preferably as precisely equal as can feasibly be made. The features are shaped such that placing magnets into the features helps properly locate and align the magnets. The manner of creating the features is not particularly critical to the present invention. A preferred method for creating the features is to cut the features into the surface. 
   Magnets are attached to the rotor body at equal intervals around the circumference of the cylindrical surface. Several magnets may be attached at different axial positions at the same radial position, in effect forming a longer magnet. In preferred embodiments, the magnets are placed into features in the cylindrical surface. The magnets are attached to the rotor body by a layer of adhesive. The adhesive layer is disposed between the cylindrical surface (in the feature, if present) and the magnet. Preferably, a spacer is also positioned between the cylindrical surface and the magnet, sharing the same volume as the magnet. The spacer ensures that the compressive force on the adhesive layer does not reduce the thickness of the adhesive layer to below a minimum thickness needed for the adhesive layer to perform acceptably. In a preferred embodiment, the spacer comprises an open-mesh screen. The optimum thickness of the adhesive layer may vary from one embodiment to another, depending on the size of the magnets and the particular adhesive being used. However, the needed thickness of the adhesive layer can be determined without undue experimentation by one of ordinary skill in the art of adhesive technology. Adhesives generally have good strength in shear, to resist axial movement, but have much less strength in peel, to resist radial movement away from the rotor body. 
   Magnet retainers are inserted between each pair of magnets separated by an angular interval. The magnets on each side of a magnet retainer are collectively referred to herein as a neighboring pair of magnets. The magnet retainers comprise a retainer body having an angled surface adapted for engaging an angled surface of a magnet when the retainer body is secured to the rotor body. In preferred embodiments, the retainer bodies are made of stainless steel. By engage, it is herein meant that the angled surfaces of the magnet retainers press down on the angled surfaces on the magnets, exerting a centripetal force on the magnets that pulls the magnets down onto the rotor, which exerts a compressive force on the adhesive layer. In preferred embodiments, the body of each magnet retainer comprises two angled surfaces with each angled surface engaging an angled surface on one of the neighboring pair of magnets. The length and angle of the engaged angled surfaces are selected to keep the mechanical stress within the magnets to an acceptably low level. In preferred embodiments, a compliant layer is positioned between the magnet&#39;s angled surface and the angled surface of the retainer body. The compliant layer can assist in distributing the centripetal force more uniformly across the angle surfaces, limiting the contact stress concentrations. The compliant layer in preferred embodiments comprises an epoxy coating. In one preferred embodiment, the compliant layer consists essentially of an epoxy coating on the angled surface(s) of each magnet. 
   The retainer body of a magnet retainer has an opening to accept a fastening device to secure the retainer body to the rotor. Retainer bodies typically have a plurality of such openings. The fastening device is disposed in the opening and secures the retainer body to the rotor body. In a preferred embodiment, the fastening device is a threaded bolt. The bolt is disposed in the opening of the retainer body and screwed into the rotor body, securing the retainer body, and therefore the magnets, to the rotor body. The bolts may be referred to as radial bolts as they exert a centripetal force on the retainer body, which exerts a centripetal force on at least one magnet of a neighboring pair of magnets. The bolts can be advantageously secured by safety lock wiring at a radially inward end. 
   An individual magnet can be removed by removing the two magnet retainers on either side of the magnet, and then peeling the magnet away from the rotor, breaking the adhesive bond. A new magnet can be installed by the same process used in initial construction. 
     FIG. 1  illustrates a retainer body  100  of a magnet retainer. The retainer body  100  has three openings  102  for accepting fastening devices. The shape of the openings  102  in the retainer body  100  are cylindrical because the retainer body  100  is designed for use with bolts as fastening devices. The retainer body also has two angled surfaces  104 , one on each side  106  of the retainer body  102  for engaging angled surfaces of neighboring magnets (not shown).  FIG. 1  also shows one end  108  of the retainer body  100 . 
   The effect of the centripetal force (provided by the fastening device) holding the magnet retainers and magnets in place and producing the compressive force on the adhesive layer may be lessened during operation of the rotor. For example, the centrifugal force exerted on the retainers and magnets at high rotation speeds can be extreme. The centrifugal force is exerted in the opposite direction of the centripetal force, and therefore diminishes the effect of the centripetal force. Additionally, the rotors are typically designed to work in different environments that can result in significantly different operating temperatures. Thermal expansion and contraction caused by different operating temperatures can reduce the centripetal force. Changes in both the centripetal and centrifugal forces during different operating conditions can induce peel stresses on the adhesive layer and may cause some radial deflection of the magnets. 
   Magnet retainers of the present invention utilize a spring mechanism to maintain the centripetal force, and therefore the compressive force, at high rotation speeds and at varying operating temperatures. The spring mechanism is disposed between the fastening device and the retainer body and provides a force reactive to a centrifugal force during rotation of the rotor body. If centrifugal force causes radial deflection of a magnet retainer, the spring mechanism will tend to compress and exert a centripetal force on the magnet retainer, countering the centrifugal force. In a preferred embodiment, the spring mechanism comprises at least one disc spring (also referred to as Belleville Washers), such as the disc springs available from Belleville Springs, Ltd of the United Kingdom. The disc springs can be used in parallel or series combinations to obtain a desired spring constant. 
   In preferred embodiments, axial retainers are disposed at an end of a magnet retainer to prevent axial movement of the magnets if the adhesive bond fails. In one preferred embodiment, the axial retainers are flat plates welded to the ends of magnet retainers. The plates extend beyond the edge of the magnet retainer and block the magnet from axial movement.  FIG. 2  illustrates an embodiment  200  of the present invention comprising the retainer body  100  of  FIG. 100  with an axial retainer  202  secured to the end  108  of the retainer body  100 . The axial retainer  202  covers the end  108  of the retainer body  100  such that it can not be seen in  FIG. 2 . 
     FIG. 3  shows an isometric view of a rotor  300  in accordance with the present invention. The rotor  300  comprises a rotor body  302  having a cylindrical surface  304 . Magnets  306  are attached to the rotor body  302  at equal intervals around the circumference of the cylindrical surface  304 . In  FIG. 3 , two magnets  306  are attached at different axial positions at the same radial position, in effect forming a longer magnet. Each pair of magnets  306  has a magnet retainer  308  disposed between the magnets  306 . Two magnet retainers  308  are secured at different axial positions at the same radial position, in effect forming a longer retainer. Each magnet retainer  308 , in  FIG. 3 , is secured to the rotor body  302  with three bolts  310 . 
     FIG. 4  shows a close-up view  400  of a magnet retainer  308  on the cylindrical surface  304  of the rotor body  302  of  FIG. 3 . The magnets  306  are disposed into features in the cylindrical surface  304 . An axial retainer  412  is secured to an end of the retainer body  108 , extending beyond the edges of the magnets  306 , and preventing movement of the magnets  306  in an axial direction.  FIG. 4  also shows a gap  416  between the retainer body and the cylindrical surface  304 . In preferred embodiments, the magnet retainers do not exert any centripetal force directly onto the cylindrical surface. Designing the magnet retainer such that a gap exists between the retainer body and the cylindrical surface ensures that no centripetal force can be exerted by the retainer body directly onto the cylindrical surface. In this manner, all centripetal force on a retainer body is exerted onto the magnets via the engaged angled surfaces. 
     FIG. 5  shows a cross-sectional view  500  of the rotor  300  of  FIG. 3 . The view  500  shows two retainer bodies  100 , at two different radial positions, on each side of a magnet  306 . The retainer bodies  100  are secured to the rotor body  302  using bolts  514  that are disposed in openings  102  of the retainer body  100 . Spring mechanisms  520  are disposed between the bolts  514  and the retainer body  100 . Angled surfaces  104  on the retainer body  100  engage angled surfaces  522  on the magnet  306 . The bolts  514  have openings  524  at the radially inward ends  526  of the bolts  514  for use with safety lock wiring (not shown), which can advantageously aid in securing the bolt  514  to the rotor body  302 . 
     FIG. 6  shows another cross-sectional view  600  of the rotor  300  of  FIG. 3 . This view  600  shows two magnet retainers  308  at different axial positions of the same radial position of the rotor  300 . The view  600  is a cross-section view through the length of the magnet retainers  308  so the magnets  306  are not shown in  FIG. 6 . Each retainer body  108  has an axial retainer  412  at one end. 
   In accordance with the present invention, novel means and methods for retaining magnets on rotors have been provided. While the present invention has been shown and described herein with reference to specific embodiments thereof, it should be understood by those skilled in the art that variations, alterations, changes in form and detail, and equivalents may be made or conceived of without departing from the spirit and scope of the invention. Accordingly, the scope of the present invention should be assessed as that of the appended claims and by equivalents thereto.