Abstract:
Difficulties in self-starting a permanent magnet motor are eliminated in a structure including a stator with a rotor journalled within the stator for rotation about an axis. The rotor includes a body of ferromagnetic material having a nominally cylindrical peripheral surface concentric with the axis. Permanent magnets are located on the peripheral surface to define equally angularly spaced magnetic poles with alternating ones of the poles being of opposite polarity. A thin, hollow cylinder formed of a good electrically conducting material is disposed on the body to sandwich the magnets against the peripheral surface of the body and provides a situs for the generation of localized induced electrical current which generates magnetic fields that react with rotating magnetic fields in the stator to start the motor from a dead stop without the need for position sensors or controlled electronics.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to electric motors, and more particularly, to a permanent magnet motor that is capable of self starting when operated directly on line.  
         BACKGROUND OF THE INVENTION  
         [0002]    Permanent magnet motors are typically unable to operate without elaborate controls because they cannot be started when connected directly to the line. Thus, they typically employ rotor position transducers and control electronics in order to start. These components quite clearly add to both the cost and the complexity of the motor system.  
           [0003]    As one means of avoiding position transducers and control electronics, while providing for a direct on-line starting performance, squirrel cage rotor bars and magnets have been employed and located on the surface of the rotor of the motor. The magnets are located in spaces between the conducting bars on the rotor. This produces two disadvantages. Firstly, the rotor bars effectively create slots in the surface of the magnets, thereby reducing the effectiveness of the operation of the permanent magnet motor. Secondly, the magnets that are located on the surface of the rotor reduces motor inductance, thereby reducing the effectiveness of the motor operation as driven by induction. As a consequence, poor motor efficiency results.  
           [0004]    Another conventional alternative includes a motor having a rotor with squirrel cage rotor bars located on or near the surface of the rotor and magnets buried within the rotor. Although this construction provides reasonable performance as regards efficiency of both induction and permanent magnet induced operation, the construction is relatively difficult to manufacture. This is due to the requirement of properly locating the magnets in slots within the rotor itself.  
           [0005]    The present invention is directed to overcoming one or more of the above problems.  
         SUMMARY OF THE INVENTION  
         [0006]    It is the principal object of the invention to provide a new and improved permanent magnet motor. More specifically, it is an object of the invention to provide such a motor that is capable of starting when connected directly on-line and without the need for the use of rotor position transducers and/or control electronics.  
           [0007]    An exemplary embodiment of the invention achieves the foregoing objects in a self-starting permanent magnet motor that includes a stator with a rotor journalled within the stator for rotation about an axis. The rotor includes a body of ferromagnetic material having an approximately cylindrical peripheral surface concentric with the axis. Permanent magnets are located on the peripheral surface so as to define “n” equally angularly spaced magnetic poles with alternating polarity. “n” is an even integer of at least two. A thin, hollow cylinder is disposed on the body to sandwich the magnets against the peripheral surface. The hollow cylinder is formed of a good electrically conducting material.  
           [0008]    In one embodiment, the structure further includes corrosion resistant sealing end pieces at opposite ends of the body and a corrosion resistant hollow cylinder disposed on the body to sandwich the conducting cylinder against the magnets. The hollow corrosion resistant cylinder is sealed to both of the end pieces.  
           [0009]    An embodiment of the invention contemplates that the sides of the poles be circumferentially spaced from one another and that the spaces thus formed are filled with a rotor forming material. In one embodiment, the rotor forming material is part of the ferromagnetic body while in another embodiment, the rotor forming material is a potting compound.  
           [0010]    In a highly preferred embodiment of the invention, each of the magnets is made of plural pieces and each in turn has a flat surface. The peripheral surface of the body has a plurality of flats against which respective ones of the plurality of magnet pieces are abutted.  
       
    
    
       [0011]    Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.  
       DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a sectional view of a self-starting permanent magnet motor made according to the invention;  
         [0013]    [0013]FIG. 2 is a partially schematic, sectional view taken approximately the line  2 - 2  of FIG. 1;  
         [0014]    [0014]FIG. 3 is an enlarged, fragmentary, sectional view of part of the periphery of the rotor of the motor;  
         [0015]    [0015]FIG. 4 is a sectional view of a modified embodiment of a rotor made according to the invention;  
         [0016]    [0016]FIG. 5 is a sectional view of the modified embodiment taken approximately along the line  5 - 5  of FIG. 4; and  
         [0017]    [0017]FIG. 6 is an enlarged, fragmentary view of the rotor periphery of the embodiment of FIGS. 4 and 5.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    Referring to FIG. 1, an exemplary embodiment of a permanent magnet motor is illustrated and is seen to include a stator, generally designated  10 , having a central opening  12  and windings, only the end turns  14  of which are shown. The stator  10  may be of conventional construction and will be energized by placing an alternating current voltage across the windings, including the end turns  14 . A rotor, generally designated  16 , includes a body of ferromagnetic material  18 . As seen in FIG. 2, the body  18  has a nominally cylindrical outer surface  20 . The body  18  is mounted on a shaft  22  of any desired configuration and in turn is journalled as by bearings schematically shown at  24  for rotation about an axis  26 . The peripheral surface  20  of the body  18  is concentric with the axis  26 .  
         [0019]    Turning again to FIG. 2, the peripheral surface  20  of the body  18  includes a series of flats  28  that extend longitudinally along the length of the body  18 , that is, in parallel relationship to the axis  26 . A series of permanent magnet segments or pieces  30  and  32  have flat sides  34  which are abutted against the flats  28  and held in place by any suitable means. If desired, a thin layer of adhesive (not shown) may be used for the purpose. It will be observed that the magnets  30  extend about a circumferential extent of 180° of the rotor  16  as do the magnets  32 , although lesser circumferential extents can be used. The magnets  32  have their north poles located radially inwardly while the magnets  30  have their south poles located radially inwardly.  
         [0020]    Those skilled in the art will thus recognize that the rotor illustrated in the drawings is a two pole rotor having one north pole and one south pole, the north pole being defined by the magnets  30  and the south pole being defined by the magnets  32 . Of course, a greater number of poles could be employed as desired so long as the poles are equally angularly spaced about the periphery of the rotor  16 . In general, the number of poles “n” will be an even integer, i.e., two, four, six, etc.  
         [0021]    Sandwiching the magnets  30 ,  32  against the body  18  is a thin can or sleeve  36  in the configuration of a hollow cylinder made of a good electrical conductor. Copper is preferred because of its relatively low cost when compared to other good conducting materials. However, other good conductors, including aluminum, silver, etc. could be used where their particular characteristics provide a useful function in the apparatus.  
         [0022]    As usual, a small air gap  38  exists between the stator  10  and the rotor  16 .  
         [0023]    The rotor may also include a pair of end pieces  40  and  42  which abut opposite ends of the body  18  and which are axially spaced along the axis  26 . The hollow cylinder  36  is supported by the periphery of the end pieces  40  and  42 . The end pieces  40  and  42  will be formed of a relatively poor magnetic conductor such as stainless steel.  
         [0024]    In an alternate embodiment, the end pieces  40  and  42  are formed of a corrosion resistant material and a second hollow cylinder  43  (FIG. 6) is fitted over the outside of the first hollow conducting cylinder  36 . This second hollow cylinder  43  is also formed of a corrosion resistant material and is sealed to the end pieces  40  and  42  hence creating a sealed environment for the permanent magnets  30 ,  32  and the hollow conducting cylinder  36 .  
         [0025]    An alternate embodiment is illustrated in FIGS.  4 - 6 . Like components are given like reference numerals. In this embodiment, it will be seen that not only are the magnets  30 ,  32  forming each of the two poles in separate pieces spaced circumferentially about the axis  26 , they may also be formed in separate pieces extending along the length of the axis  26  as well. Again, flats  28  are employed on the rotor body  18  as well as flat surfaces  34  on the magnets  30 ,  32 . The rotor may also be provided with a corrosion resistant inner sleeve  44  sealed and welded to the end pieces  40 ,  42  to completely encapsulate the rotor body.  
         [0026]    In this embodiment the circumferential extent of each of the two poles is but 125° for the FIG. 4 embodiment, rather than 180°. Again, the circumferential extent of the poles may be at other angles. Thus, spaces  46 ,  47  exist between the edges of the two poles and the spaces  46  are filled with rotor forming material. As seen in the lower part of FIG. 5, the rotor forming material filling the space  46  is nothing more than a continuation of the ferromagnetic material of the body  18 , that is, formed by a ridge on the cylindrical peripheral surface of the body  18 . The purpose is to enhance the structural strength of the rotor but where such additional strength is not required, the material can be omitted. At the top of FIG. 5, the space  47  may alternatively be filled with a potting compound such as an appropriate epoxy resin. In general, both of the spaces  46 ,  47  will be filled with the same material, i.e., ferromagnetic material or potting compound.  
         [0027]    In one alternative embodiment, axial grooves or slots are formed in the material filling the spaces  46 ,  47 . Electrically conducting bars or rods  48  are located in the grooves thus formed. The bars  48  are joined to electrically conducting rings  49  at opposite ends of the rotor body  16 .  
         [0028]    This construction can be employed as an alternative to the use of the hollow conducting cylinder  36  or in addition to it.  
         [0029]    In operation, these embodiments operate in essentially the same way. When an alternating current voltage is applied to the stator  10 , a rotating magnetic field is induced by the stator  10 . This rotating magnetic field, in turn, induces localized electrical currents within the hollow cylinder  36  and/or the conducting bars  48  which react with the rotating magnetic field in the stator  10  to initiate rotation of the rotor  16  within the stator  10 . In other words, magnetic fields generated by induced current within the hollow cylinder  36  and/or bars  48  reacting with the rotating magnetic field in the stator  10  generates a torque to initiate rotation of the rotor  16 . This torque accelerates the rotor  16  toward synchronous speed.  
         [0030]    As the rotor  16  gains speed, the interaction between the rotating magnetic field in the stator  10  and the magnets  30 ,  32  increases to further accelerate the rotor  16 . When synchronous speed is achieved, there will be no induced current flowing in the hollow cylinder  36  and/or bars  48  because of a lack of relative rotation and any losses that might otherwise be experienced due to such induced current cease as long as synchronous speed is maintained. At this point, the rotor  16  is driven solely as a result of the interaction of the rotating magnetic field in the stator  10  with the magnets  30 ,  32 .  
         [0031]    From the foregoing, it will be appreciated that a motor made according to the invention solves the problems listed previously. Losses are minimized because conductors on the rotor, namely the hollow cylinder  36  and/or bars  48 , are not effective to form notches in the magnetic poles which impede efficiency. At the same time, it is not necessary to locate the magnets  30 ,  32  well within the body  18  so that construction problems, and the costs thereof are eliminated. Similarly, because the start up torque is supplied solely by induced current within the hollow cylinder  36  and/or bars  48 , there is no need for position sensors or electronic controllers to be used during start up.  
         [0032]    A motor made according to the invention is ideally suited for any of a variety of uses requiring self-starting permanent magnet motors. In applications where corrosion resistance is desired, as, for example, in pumps or the like, the use of non-corroding material in forming the end pieces  40 ,  42  and the addition of a second hollow cylinder of non-corroding material located outside the conducting cylinder and sealed to the end pieces  40 ,  42  readily prevent damage to the rotor that might otherwise be caused by contact with corrosive materials.  
         [0033]    The hollow cylinder  36  can be made sufficiently thin so as to not effectively increase the air gap  38  between the rotor  16  and the stator  10  which would cause the loss of efficiency so that a high efficiency, self-starting alternating current permanent magnet motor is provided. A wall thickness of 1.2 mm for the hollow cylinder  36  has proved to be effective.  
         [0034]    To avoid losses due to an enlargement of the air gap, the hollow corrosion resistant cylinder should also have a thin wall, for example, 0.56 mm when 316L stainless steel is employed.