Abstract:
There is disclosed a rotor structure of an inner rotor type motor comprising a stator including annular stator yokes each having a plurality of pole teeth along an inner circumference thereof and coils arranged inside the stator yokes, each coil constructed by winding a magnet wire and a rotor rotatably disposed with a small gap from the pole teeth of the annular yokes and having a permanent magnet arranged opposite to the pole teeth. The permanent magnet comprises a plurality of discrete segment magnets, which are arranged apart from each other by resin molding. Thus, the inexpensive inner rotor type motor rotor can be structured by using the magnet having excellent magnetic characteristics.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotor structure of an inner rotor type motor, and more particularly of an inner rotor type motor using a rare earth magnet. 
     2. Description of the Prior Art 
     With higher performance achieved for various OA equipments in recent years, there have been strong demands for a higher output, a higher speed and lower power consumption in various motors including spindle motors, PM (permanent magnet) type stepping motors and others. 
     To meet such demands, use has been made of a rare earth magnet having a high energy product for a rotor magnet material. However, because of a low mechanical strength of this magnet, if a highly productive resin forming method based on a high polymer material is used as a rotor assembling method, cracks may occur in the magnet due to a forming pressure, making it impossible to maintain the function of the rotor. Under the circumstances, therefore, the only way available is a low-productive method such as adhesion or the like. 
     FIG. 1A shows an example of a conventional rotor structure using a magnet of the foregoing kind. A sleeve  12  is fitted in around a shaft  3  and, on its periphery, a cylindrical rare earth bond magnet  25  which is reputed to be more inexpensive than a sintered rare earth magnet is arranged in a thinned form to minimize the quantity of its use to a necessary limit whereby the magnet cannot be formed in a larger dimension, and hence is divided into two portions. If this rotor is manufactured by insert molding, two cylindrical rare earth bond magnets  25  and a sleeve  12  are set in a metallic mold and then a resin is injected. However, because of a low mechanical strength of the cylindrical rare earth bond magnet  25 , cracks may occur due to an injection pressure, or even breaks may occur depending on molding conditions. 
     Therefore, an improved rotor structure has been proposed as shown in FIG.  1 B. According to the improved structure, a rotor is manufactured in such a way that a cylindrical rare earth bond magnet  25  is beforehand fixed by adhesive K onto the periphery of a metal sleeve  26  serving as a reinforcing material, then is set in a metallic molding die together with a sleeve  12  holding a shaft  3 , and the sleeves  12  and  26  are integrated by a resin  27 . 
     The improved rotor structure has enhanced characteristics, but necessity of the adhesion step and the reinforcing sleeve inevitably leads to cost increase, which may limit application of the rotor structure manufactured as such. 
     SUMMARY OF THE INVENTION 
     The present invention was made with the foregoing problems in mind, and it is an object of the invention to provide a rotor structure of an inner rotor type motor, which uses a high-performance rare earth magnet but is low in costs. 
     In order to achieve the foregoing object, in accordance with the invention, there is disclosed a rotor structure of an inner rotor type motor comprising a stator including annular stator yokes each having a plurality of pole teeth along an inner circumference thereof and coils arranged inside the stator yokes, each coil being constructed by winding a magnet wire and a rotor rotatably disposed with a small gap from the pole teeth and having a permanent magnet arranged opposite to the pole teeth, wherein the permanent magnet comprises a plurality of discrete segment magnets which are integrally molded by a resin of a high polymer material and arranged apart from each other. 
     The permanent magnet used for the rotor comprises a plurality of discrete segment magnets, which are arranged apart from each other like a ring on the outer circumference of the rotor to keep the reduction of a surface area at a minimum limit. The segment magnets are made integral and held in place by filling a space between the segment, magnets with a resin by molding. Thus, portions holding segment magnets serve as a relief for a resin pressure caused by molding, and any cracks or breaks which may be caused in the segment magnet by a molding pressure can be prevented. In addition, reduction of the surface area for entire segment magnets is kept at the minimum limit, so there is no substantial deterioration of a torque as one of characteristics of a motor. Moreover, as the usage of magnets which are of relatively high price is minimized, reduction of cost can also be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are sectional views showing two different examples of conventional rotor structures. 
     FIG. 2 is a partial cutaway view showing an entire constitution of a permanent magnet (PM) type stepping motor using a rotor structure of the present invention. 
     FIG. 3 is a sectional view showing a stator assembly of the motor shown in FIG.  2 . 
     FIG. 4 is a sectional view of the motor shown in FIG.  2 . 
     FIGS. 5A and 5B are views showing two different examples of end surface shapes of segment magnets used for a rotor of the invention. 
     FIGS. 6A to  6 F are views showing different examples of measures to prevent axial pulling-out of the segment magnets. 
     FIG. 7 is a perspective view showing an example of an end surface shape of one segment magnet. 
     FIGS. 8A and 8B are views showing two different examples of rotor magnetization positioning means of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 illustrates, in partial cutaway fashion, the entire constitution of a permanent magnet (PM) type stepping motor using a rotor structure of the invention. Reference numerals  1  and  15  denote flanges manufactured by punching stainless steel plates;  2  a bearing made of an oil containing alloy; and  5  a stator assembly including stator yokes  9  and  10  which are made of a soft magnetic material steel plate stereoscopically bent in a doughnut shape and have a plurality of pole teeth  13  formed on its inner circumference, and a coil  8  made by winding a magnet wire on a bobbin  6  which is arranged between the respective stator yokes  9  and  10 . 
     On the other hand, a rotor  19  is located in the center of the stator assembly  5 , and constructed in such a manner that a sleeve  12  made of an aluminum material (not shown in FIG. 2, see FIG. 4) is fitted in by pressure almost to the center of a shaft  3  as a rotational center, a holder  17  made of a high polymer material is formed around the sleeve  12  to be united therewith, and a plurality of segment magnets  4  having plural magnetic poles are disposed on the outer circumference of this holder  17  apart from each other at a specified interval to face the pole teeth  13  of the stator yokes  9  and  10 . 
     The holder  17  may be manufactured by integrally insert-molding the sleeve  12  and the plurality of segment magnets  4 . Alternatively, the holder  17  may also be manufactured by providing a holding dovetail groove in the fixing position of the segment magnet  4 , insert-molding only the sleeve  12 , and then inserting and fixing the plurality of segment magnets  4  onto the outer peripheral surface of the holder  17 . On the end surface portion of the holder  17 , a positioning groove  20  is provided for positioning magnetic poles during magnetization. Needless to mention, such a magnetization positioning means may be configured as a hole or a projection. 
     FIG. 3 illustrates in section the constitution of the stator assembly  5 . 
     The doughnut-shaped stator yoke  10  is fitted on its outer periphery with the stator yoke  9  in such a manner that the pole teeth  13  of the former engage with the pole teeth  13  of the latter at equal intervals, and contains therein the coil  8  made by winding the magnet wire on the bobbin  6 , thereby constituting the stator assembly  5 . Two of such upper and lower stator assemblies  5  are disposed back to back one on another to constitute a stator of the motor. 
     FIG. 4 illustrates in vertical section a stepping motor of an embodiment of the invention. 
     The rotor  19  is constructed in such a manner that the shaft  3  is pressed into the sleeve  12  and fixed thereto, and the holder  17  made of a resin is located on the outer circumference of the sleeve  12 . On the outer peripheral surface of the holder  17 , a plurality of segment magnets  4  are arranged coaxial to and at equi-distance from each other. Regarding the fixing method of the segment magnets  4 , as described above, the segment magnets  4  and the sleeve  12  may be subjected to insert molding. Alternatively, the holder  17  and the sleeve  12  may be insert-molded, and the segment magnets  4  may be inserted and fixed in the holding dovetail groove provided for positioning magnet in the outer periphery of the holder  17 . 
     The rotor  19  thus constructed is subjected to magnetization by using the magnetization positioning groove  20  provided in the end surface of the holder  17  as a reference, so that a magnetic pole is formed on each of the plurality of segment magnets  4  by using a magnetizer (not shown). Then the rotor  19  is rotatably held by the bearings  2  and  16  made of oil containing alloys which are arranged in the centers of the flanges  1  and  15  by caulking such that the magnetic pole thus provided can face the pole teeth  13  of the stator assembly  5  a very small distance away from the pole teeth  13  and coaxial to the same. 
     The flanges  1  and  15  are fixed by a method such as plasma welding or the like so as to set the pole teeth  13  of the stator assembly  5  coaxial to the bearings  2  and  16 . 
     It is necessary to take measures to prevent the segment magnets  4  from pulling out radially or axially of the rotor  19  during high-speed rotation. Some of such measures are shown in FIGS. 5A and 5B and FIGS. 6A to  6 F. 
     FIGS. 5A and 5B show examples of measures to prevent the pulling-out of the segment magnets  4  in a radial. direction of the rotor  19 . In case the segment magnets  4  and the sleeve  12  are insert-molded using a high polymer material to be united with each other, an end surface of each segment magnet  4  which is viewed in an axial direction of the rotor  19  is shaped in such a manner that a length W 2  of one side edge i.e., an outer circumferential edge of each segment magnet  4  which is located at the outer part of the rotor  19 , is shorter than a length W 1  of the other side edge i.e., an inner circumferential edge of each segment magnet  4  which is located at the inner part of the rotor  19 . In this way, a high polymer material is filled between the adjacent segment magnets during molding, and the radial pulling-out of the segment magnets can be prevented. 
     Next, FIGS. 6A to  6 F show some examples of measures to prevent the pulling-out of the segment magnets  4  in an axial direction of the rotor  19 . A recessed portion  4   a  is formed substantially in the center of both end surfaces of each segment magnet  4  as shown in FIG.  7 . Accordingly, when the segment magnets  4  are fixed to the holder  17  by insert-molding as shown in FIG. 6A, a high polymer material fills the recessed portion  4   a  of each segment magnet  4  to press both end surfaces thereof. Thus, the axial pulling-out of the segment magnets can be effectively prevented. 
     As another measure, as shown in FIG. 6B, rotary flanges  22  may be attached to entire surfaces of both ends of the rotor  19  in the axial direction. Alternatively, as shown by “A” in FIG. 6C, the inner peripheral edge of both end surfaces of each segment magnet  4  may be chamfered. Otherwise, as shown in FIG. 6D, the inner peripheral edge of both end surfaces of each segment magnet  4  may be provided with a stepped portion B. In this way, if insert molding is carried out, a high polymer resin enters the chamfered portion A or the stepped portion B to prevent the axial pulling-out of the segment magnets  4 . The chamfered portion A and the stepped portion B may take other shapes. 
     As yet another measure, as shown in FIG. 6E, a concaved portion C may be provided almost in the center between both end surfaces of the segment magnets  4  where a rotor magnet has less magnetic effect with the result that the rotor magnet is of a narrower width at the concaved portion C. In an example shown in FIG. 6F, a serrated portion D is provided on both lateral sides of the segment magnet  4 . Various structures and shapes other than the above may be employed for prevention of the axial pulling-out and it is important to install any of these measures in a position where no or least possible effects are caused on motor characteristics. 
     With the rotor structured in the foregoing manner, no cracks or breaks occur in the magnet with a low tensile strength even if the metallic molding die is subjected to a molding pressure during rotor manufacturing, because no tensile pressure is applied to the magnet. This means that a thickness of the magnet can be ½, or less, of a pitch of a magnetic pole, which is a minimum limit in view of magnetic flux distribution between rotor magnetic poles effective for a motor operation. Therefore, the amount of magnets to be used can be reduced to a minimum limit. 
     Since the rotor of the present invention includes a plurality of segment magnets disposed apart from each other, a nonmagnetic portion of the resin-made holder is present between the segment magnets. Thus, it is necessary to appropriately position rotor magnets in a magnetizer for magnetizing rotor magnetic poles. 
     FIG. 8A shows an example of a rotor with a magnetization positioning hole  20   a  necessary for magnetization provided in the end surface of the holder  17 , and FIG. 8B shows an example of a rotor with a magnetization positioning notch  20   b  provided in the end surface of the holder  17 . 
     The invention has been described with reference to the rotor structure of the inner rotor type motor using the rare earth magnet. However, the invention is applicable to a rotor of an inner rotor type motor using any other cylindrical magnet irrespectively of kinds of magnets and similar effects can be obtained. 
     According to the present invention, no inconveniences which may grow into serious drawbacks such as magnet cracks or the like occur even if a rotor is manufactured with a magnet having a relatively low mechanical strength, with the result that a high-performance rotor using a rare earth magnet which is particularly excellent in magnetic characteristics can be provided. At the same time, since the amount of magnets to be used, which is small, can be further reduced by thinning of the magnet and adjustment of a width thereof, a low-cost rotor can be provided. Moreover, as it is possible to employ the insert molding in the manufacturing process, a high-performance rotor can be manufactured in a short time.