Abstract:
Two embodiments of rotating electrical machines wherein the cogging torque is substantially reduced by increasing the cogging number without increasing the number of pole teeth and permanent magnets. This is done by selecting the appropriate magnet angle to increase the number of coggings per revolution and this can be done using a computer analysis of the cogging torque for the individual magnets rather than by a trial and error method.

Description:
BACKGROUND OF INVENTION 
     This invention relates to a permanent magnet rotary type electrical device or machine such as an electric motor or generator and to an improved method for making such machines. 
     As is known, rotary electric machines such as a brushless DC motor or generator utilize a permanent magnet element having a plurality of permanent magnets fixed thereon. The permanent magnet element is juxtaposed to an element having a plurality of poles around which coils are wound and the elements are supported for relative rotation. In a motor, the magnetic field formed by the coil windings is controlled so as to effect rotation. In a generator, relative rotation induces voltage in coil windings. 
     In such machines, the permanent magnets are disposed at circumferentially equal intervals. The slots between the core teeth are also disposed circumferentially at equal intervals. When the machine is used as a generator, it is desirable that the torque necessary to effect rotatation when output of the generator is zero (referred to as a cogging torque) is as small as possible. Increased cogging torque will necessitates a larger driving force of an engine or the like to rotate the generator, thus resulting in a larger vibration and load. If it is used as a motor, drive torque in idling becomes larger resulting in power loss as well as increased. 
     It is generally accepted that the cogging torque is inversely proportional to the square of what is referred to as a cogging number. This cogging number is basically the number of coggings that occur each revolution of the machine. This being the number of times the magnetic cores pole passes a permanent magnet per revolution. 
     Therefore, one prior art method used to decrease the cogging torque or the variation in rotation is to increase the number of poles. For example, a motor or a generator in which the number of slots S between the poles on which the coils are wound is nine and the number of magnetic poles P is eight, that is, a system of 9-slot and 8-pole construction, is known. In this case, the cogging frequency or cogging number has a sufficiently high value of 72/revolution. Therefore, the cogging torque becomes sufficiently small. 
     With this method, however, it is necessary to increase the number of poles P and the number of slots S, and the construction becomes more complex with increasing P or S, causing an increase in the number of parts as well as making the assembly more expensive to produce. 
     Another prior art method for reducing the cogging torque employs permanent magnets or slots that are disposed oblique to the rotational axis (that is, a so-called “skew” is imparted). This method is employed to smoothen the circumferential distribution change in the magnetic field so as to decrease the cogging torque. 
     In this method, permanent magnets of special shape must be made, which increases manufacturing costs and is not suited for the automation of the production and assembly. 
     Therefore it is a principal object of this invention to provide a permanent magnet type rotary electric device and method of manufacture therefore capable of effecting decreased cogging torque and suited for automation of manufacturing without increasing cost. 
     SUMMARY OF INVENTION 
     A first feature of this invention is adapted to be embodied in a permanent magnet type rotary electric machine of reduced cogging torque. The machine has a permanent magnet element comprised of a plurality of permanent magnets of alternate polarity in a circumferential direction. These permanent magnets are of substantially the same shape and disposed circumferentially at equal intervals. An armature element is juxtaposed to the permanent magnet element and has a number of circumferentially spaced core teeth around which coils are wound. The core teeth defining a number N of slots therebetween. The permanent magnet element and the armature element are supported for relative rotation. The circumferential magnet angle made by the permanent magnets with respect to the axis of relative rotation produces an effective cogging number that is substantially greater than the least common multiple of the number of slots S and the number of poles P. 
     A second feature of the invention is adapted to be embodied in a permanent magnet type rotary electric machine of the type described in the preceding paragraph. In accordance with this feature, the magnitudes of torque exerted on the permanent magnets of N and S poles of the magnet element are selected so that both of the torque values substantially cancel each other. 
     Yet another feature of the invention is adapted to be embodied in a method of making a permanent magnet type rotary electric machine of reduced cogging torque. The method comprising the steps of constructing a permanent magnet element comprised of a plurality of permanent magnets of alternate polarity in a circumferential direction with the permanent magnets being of substantially the same shape and disposed circumferentially at equal intervals. An armature element is juxtaposed to the permanent magnet element and has a number of circumferentially spaced core teeth around which coils are wound. The core teeth define a number N of slots therebetween. The permanent magnet element and the armature element are supported for relative rotation. The circumferential magnet angle made by the permanent magnets with respect to the axis of relative rotation is selected to produce a cogging number that is substantially greater than the least common multiple of the number of slots S and the number of poles P. 
     A still further feature of the invention is adapted to be embodied in a method as set forth in the preceding paragraph, however the magnitudes of torque exerted on the permanent magnets of N and S poles of the magnet element are selected so that both of the torque values substantially cancel each other. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an end elevational view of a rotating electrical machine constructed in accordance with a first embodiment of the invention with the coil windings removed but the phases thereof indicated by reference characters. 
     FIGS. 2A,  2 B and  2 C are graphical views showing three different magnet angles and the cogging torque resulting therefrom. 
     FIG. 3 is a cross sectional view taken along a plane containing the axis of rotation of a rotating electrical machine constructed in accordance with a second embodiment of the invention. 
     FIG. 4 is a developed view showing the coil windings of this embodiment. 
    
    
     DETAILED DESCRIPTION 
     Before going directly to a description of the preferred embodiments, some general discussion of the theory of the invention will assist those skilled in the art to understand the invention. It has been generally assumed that under certain conditions, the cogging number per revolution of is the least common multiple of the number of slots S and the number of poles P. However the inventors have determined that by selecting the circumferential angle (magnet angle) of the permanent magnet with respect to the axis of rotation the cogging number can be significantly increased from this calculated cogging number. This invention therefore selects the magnet angle of the permanent magnets to increase the actual cogging number and also to decrease the cogging torque by canceling out the cogging torques caused by the alternate poles. This could have been achieved by testing various physical relationships, but has been expedited by computer analysis, as will be described shortly. 
     Referring now in detail to the embodiment of FIG. 1, a rotating electrical machine constructed in accordance with this embodiment is identified generally by the reference numeral  11 . Since the invention deals primarily with the number of permanent magnets and their circumferential extent and spacing and the number of cooperating pole teeth, the general overall construction is not illustrated and only the permanent magnet element, indicated generally by the reference numeral  12  and the cooperating magnetic pole tooth element, indicated generally by the reference numeral  13  are shown. 
     In addition, the winding of the pole teeth of the element  13  are not illustrated but it is to be understood that the individual teeth are wound in a three phase pattern with the phases indicated by the reference characters U, V, W. The pole teeth are wound in opposite directions with those with the overhead bar being oppositely wound with respect to those lacking the overhead bar. 
     In this illustrated embodiment, the permanent magnet element  12  rotates and is comprised of an outer shell  14  on which four permanent magnets, indicated by the reference numeral  15  are affixed in circumferentially spaced positions. The permanent magnets  15  have an arcuate configuration and each has substantially the same physical characteristics with the magnet angle θ of each being equal and of an extent which will be described shortly. 
     The coil winding element  13  is comprised of a fixed central part  16  from which eighteen (18) core teeth  17  extend in a radial direction with their tips being spaced closely proximate to the permanent magnets  15 . The winding of the coils thereon is as afore described. Although the system is described in conjunction with a rotating permanent magnet and a fixed pole core element which is disposed circumferentially inwardly thereof, the opposite relation can be employed and the rotational elements and affixed elements being reversed. The angle between the centers of the permanent magnets  15  is 90° in this embodiment. 
     In accordance with the invention, the magnet angle θ is chosen in such a way that even though the normally assumed cogging number for the machine  11  would be 36, this being the lowest integral equally divisible by the number (4) of permanent magnets  15  and number (18) of magnetic pole teeth  17 , the magnet angle is chosen so that the cogging torques of the individual magnets will cancel each other out substantially and to provide a cogging number that is twice that would normally be expected i.e. 72. Although this effect could be achieved by running a number of tests at varying magnet angles, the inventors have discovered that the computation can be simulated through readily available software. 
     This software is one of electromagnetic field analyzing tools for electromechanical design and is capable of calculating torque exerted on a selected portion. For example software sold under the name “Maxwell 2D Field Simulator” supplied by “Ansoft Japan Co., Ltd.” (head office: 3-18-20 Shin-Yokohama, Kohoku-ku, Yokohama-shi, Kanagawa Japan) can be employed. 
     Thus, with this software, if various conditions such as a magnet angle θ of a permanent magnet, dimensions and the disposition of the permanent magnet, and the like are inputted, torque T N  exerted on the permanent magnets  15 N of N poles and torque T S  exerted on the permanent magnets  15 S of S poles can be individually determined. Also, the total cogging torque T( N + S ) can be determined by summing the torques T N  and torque T S  exerted on the permanent magnets  15 N and  15 S of N and S poles are added together to determine cogging torque generated in the rotor  12 . 
     These calculated torques are shown in FIGS. 2A,  2 B and  2 C which show changes in the torque when the magnet angle θ of each permanent magnet  18  is varied between 60°, 67.5° and 70°, respectively, with other physical conditions being maintained the same. In these figures, torque T N , T S , and T( N + S ) are represented by N, S and N+S. The horizontal axis represents the mechanical angle, in degrees and a mechanical angle of 20° corresponds to an electrical angle of 40°. 
     From this analysis, in the case of the magnet angle θ=60° (FIG.  2 A), the positions of the positive and negative peaks of torque T N  and torque T S  are offset by 5° in mechanical angle (10° in electrical angle). Therefore the cogging torque T( N + S ) of the sum of these torque values increases greatly in the positive and negative directions in a range of the mechanical angle of 10°. Also the peaks of the cogging torque T( N + S ) appear 36 times in one revolution of the rotor  12 , the same as the normally calculated cogging number. In this example, the cogging torque T 
     ( N + S ) required to pass over the peaks is about 0.054 N.m. 
     Similarly, in the case of the magnet angle θ=70° (FIG.  2 C), while peaks of T N  and T S  moves to the left on FIG. 2C, the peaks appear 36 times for one revolution of the rotor  12  as in the case of θ=60°. The cogging torque T( N + S ) required to get over the peaks is about 0.022 N.m. 
     On the other hand, in the case of the magnet angle θ=67.5° (FIG.  2 B), the changes in the torque become greater. That is, mechanical angles at which peaks of the torque T N  exerted on the permanent magnets  15 N of N poles and the torque T S  exerted on the permanent magnets  15 S of S poles are produced come to very close to each other. As a result, peaks of the sum T( N + S ) of these torque values appears at half the period of that of the cases ( 2 A), ( 2 C), and 72 times for one revolution of the rotor  12 . A much smaller cogging torque T N + S ) required to get over these peaks is 0.008 N.m. As a result of the foregoing analysis, it can be seen that in the case of the magnet angle θ=67.5°, the cogging number per revolution of the rotor  12  is 72, which is two times the least common multiple 36 of the number S (=18) of slots and the number P (=4) of poles. 
     Although in this embodiment the magnet angle θ is set at an angle near θ=67.5°, a magnet angle θ at which the cogging number increases, can also be found in a range below θ=60°. For example, it is expected that there is a desirable magnet angle θ at an angle near θ=57°. Also, as already noted, this invention can be applied not only to the outer rotor type motor or generator, but also to the inner rotor type. 
     FIGS. 3 and 4 show a second embodiment of the invention with FIG. 3 being a cross sectional view taken through the axis of rotation of the rotating electrical machine constructed in accordance with this embodiment and which is indicated generally by the reference numeral  51 . In this embodiment, the machine  51  is a motor, but again it will be apparent to those skilled in the art how the invention can be employed in conjunction with generators. 
     The motor  51  has an outer housing, indicated generally by the reference numeral  52  which is comprised of a circumferentially extending stator shell  53  which has a plurality of circumferentially spaced cylindrical permanent magnet segments  54  affixed thereto with alternating polarities. As with the previously described embodiment, there are four permanent magnets  54  and these magnets have a magnet angle of about 67.5°. 
     The motor housing  52  is completed by end closures  55  and  56  that are detachably affixed to the stator shell  53  by fasteners  57 . Bearings  58  and  59  are fixed in the end closures  55  and  56  and journal the end of a rotor shaft  61 . The rotor shaft  61  has affixed to it a laminated core assembly, indicated generally by the reference numeral  62  which has a cylindrical inner portion that is suitably fixed to the shaft  61  and from which eighteen (18) pole teeth  63  extend. The tip ends  64  of the pole teeth are in confronting relationship to the permanent magnets  54 . 
     Windings  65  encircle these pole teeth  63  and are wound in a pattern as shown in the developed view of FIG.  4 . These windings  65  are connected at their ends to respective commutator strips  66  as shown in FIGS. 3 and 4. These commutator strips  66  are suitably affixed to the rotor shaft  61  and are engaged by brushes  67  held by respective brush holders  68  so as to transfer electrical current to the windings  65 , in a manner well known in this art. Thus, this embodiment like the previously described embodiment, results in a cogging number of 72 which is at least twice the common multiple 36 of the number of magnets  54  and pole teeth  63 . 
     Therefore, from the foregoing description it should be readily apparent that it is possible to reduce the cogging torque by increasing the cogging number without necessarily increasing the number of pole teeth or permanent magnets as previously thought. Of course, this advantage applies to electrical motors or electrical generators and, in fact, can be applied to linear electrical machines. Of course, the foregoing description is that of preferred embodiments of the invention and various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.