Abstract:
In a q phase-reluctance type motor including a rotor having 2n rotor protrusions with an arc angle X°, a stator having 2m stator protrusions with an arc angle Y° and m pairs of exciting coils, each of the pairs of exciting coils is disposed around one of the stator protrusions spaced apart at 180° in mechanical angle, and the following conditions are met: q≧5, X°, Y°&gt;360°/(n×q), X°+Y°=180°/n−α°, where α° is a counter torque marginal angle which is larger than 0°.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from Japanese Patent Applications Hei 11-46464 filed Feb. 24, 1999, and Hei 11-329836 filed Nov. 19, 1999, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multi-phase half-wave reluctance type electrical motor and a driving method therefor. 
     2. Description of the Related Art 
     A reluctance type motor is generally simple in structure and provides high output torque. However, it has been used in a limited field because of its high torque ripples. 
     JP-A-1-318579 proposes a method of reducing the torque ripples by increasing the number of poles. In such a method, phase current is switched on and off in an electrically tandem switching manner. However, the magnetic flux for generating output torque in the reluctance motor disclosed in the publication flows through portions of the stator that generate counter torque. This does not sufficiently reduce such torque ripples. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved reluctance type motor which can reduce such counter torque in a simple structure. 
     According to a main feature of the invention, a multi-phase reluctance type motor including a rotor having 2n rotor protrusions with an arc angle X°, a stator having 2m of stator protrusions with an arc angle Y° and m pairs of exciting coils. Each of the pairs of exciting coils is disposed around one of said stator protrusions spaced apart at 180° in mechanical angle, and the following conditions are necessary: q≧5, X°, Y°&gt;360°/(n×q), X°+Y°=180°/n−α°, where α° is a counter torque marginal angle which is larger than 0°. 
     According to another feature of the invention, at least the rotor protrusions or the stator protrusions of the above reluctance motor have a plurality of slits. As portions of the rotor protrusion and the stator protrusion overlapping with each other increase, magnetic flux flowing therethrough increases, so that a flat torque characteristic can be provided. 
     According to another feature of the invention, each of the stator protrusions has a wider base portion than other portions. As a result, magnetic saturation of the stator protrusion due to leakage flux from the sides of the stator protrusions can be reduced, thereby preventing reduction in the output torque at a latter part of the energizing period of the exciting-coils. 
     According to another feature of the invention, in a method of driving the reluctance type motor as stated above, the exciting coils are energized at a mechanical angle not larger than α° in advance of the rotor every 360°/(2n×q) mechanical angle. As a result the exciting coils can be energized in a shorter time, the torque ripples can be reduced and the output torque can be increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
     FIG. 1 is a schematic front view of a reluctance type motor according to a first embodiment of the present invention; 
     FIG. 2A is a graph showing inductance of respective phase coils, FIG. 2B is a graph showing timings of energizing the coils, and FIG. 2C is a graph showing periods of the respective phase coils to be energized; 
     FIG. 3 is a schematic diagram showing relative position between the stator protrusion and the rotor protrusion; 
     FIGS. 4A is a schematic diagram showing relative position between the stator protrusion and the rotor protrusion, and FIG. 4B is a schematic diagram showing output torque characteristic relative to the rotor position; 
     FIG. 5A is a circuit diagram of a separate-arm-type driver, and FIG. 5B is a circuit diagrams of a common arm type driver; 
     FIG. 6A is a schematic diagrams illustrating stator protrusions and rotor protrusions which have a plurality of slits thereon, FIG. 6B is a schematic diagram illustrating a distribution of magnetic flux in the stator protrusions and the rotor protrusions, and FIG. 6C is a graph showing the output torque relative to position of the rotor; 
     FIG. 7 is a chart showing inductance of an exciting coil, switching operation, and exciting current relative to the position of the rotor; 
     FIG. 8A is a graph showing inductance change of respective phase coils relative to the position of the rotor, FIG. 8B is a graph showing timings of energizing the coils, and FIG. 8C is a graph showing energizing periods of the respective phase coils; and 
     FIG. 9 is a schematic front view of a reluctance type motor according to a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     (First Embodiment) 
     A reluctance type motor according to a first embodiment of the invention is described with reference to FIGS. 1-5A, and  5 B. As shown in FIG. 1, the reluctance type motor is comprised of stator  1  having twelve stator protrusions  1   a  and rotor  2  having ten rotor protrusions  2   a . Stator  1  also has six pairs of exciting coils C 1 -C 6 . Respective exciting coils of each pair are wound around a pair of protrusions  1   a  spaced apart at 180° in mechanical angle. Stator protrusion  1   a  and rotor protrusion  2   a  have the same arc length, which corresponds to rotor protrusion arc angle X° and stator protrusion arc angle Y°, and equal to about 16°. 
     If rotor  2  rotates within the inner periphery of stator  1 , self inductance of each of coils C 1 -C 6  changes from its minimum to its maximum in mechanical angle 16° and from its maximum to its minimum in mechanical angle 16° as shown in FIG.  2 A. 
     It is found necessary that the stator protrusion and the rotor protrusion have to overlap with each other at a ratio more than 50% when they conduct magnetic flux, and the number q of the electric phases of the reluctance type motor according to this embodiment should be calculated as follows: 
     q≧2m/(the greatest common divisor between 2n and 2m)=5, 
     where 2m is the number of stator protrusions that is 12, 2n is the number of rotor protrusions that is 10, and the greatest common divisor is 2. 
     In order to switch the exciting current from one to another, the smaller one of the protrusion arc angles X° and Y° should be double the switching interval or more. That is: X°, Y°≧360°/(n×q) 
     Exciting coils C 1 -C 6  are sequentially energized in tandem at the following driving intervals: 
     360°/(2n×q)=6° (in mechanical angle). 
     Output torque of the motor can be generated continuously if each of exciting coils C 1 -C 6  is energized by a driver while rotor  2  rotates through 12° in mechanical angle, which is twice as long as the above energizing interval, as shown in FIG.  2 B. Such a driver D 1  or D 2  is shown in FIG.  5 A and FIG.  5 B. Driver D 1  is a separate arm type driver, and driver D 2  is a common arm type driver. 
     The distance between adjacent rotor protrusions  2   a  is calculated as follows: 
     360°/2n−X°=36°−16°=20° (in mechanical angle) 
     FIG. 3 illustrates stator  1  and rotor  2  when coils C 6  are switched off and coils C 2  are switched on while coils C 1  are already being energized as shown in FIG.  2 B. In this instance, the rotor protrusion  2   a  is positioned in advance of the same stator protrusion  1   a  in the rotation direction. That is, the rear edge of rotor protrusion  2   a  is positioned in front of the front edge of stator protrusion  1   a  at a mechanical angle α°, while the rear edge of the same stator protrusion  1   a  and the front edge of the next rotor protrusion meet. 
     That is, α=(the arc angle between adjacent rotor protrusions  2   a : 360°/2n−X°)−(the arc angle of the stator protrusion  1   a : Y°), or 
     X°+Y°=180°/n−α° 
     In this Embodiment, angle α°=20°−16°=4°. This angle is provided, as a counter torque marginal angle, to prevent leakage magnetic flux Φl, which is a portion of magnetic flux Φm generated by coil C 1 , from leaking to coil C 2  through rotor protrusion  2   a , so that counter torque can be suppressed as shown in FIG.  4 B. 
     If the marginal angle α° is zero, as illustrated in FIG. 4A, the leakage magnetic flux Φl causes counter torque that reduces the output torque. 
     (Second Embodiment) 
     A reluctance type motor according to a second embodiment is described with reference to FIGS. 6A,  6 B and  6 C. 
     A plurality of radially extending grooves or slits  3  are formed in each of the stator and rotor protrusions  1   a ,  1   b  as shown in FIG.  6 A. The grooves or slits  3  (hereinafter referred to slits) are effective to orient the magnetic flux Φm in the radial direction, so that the motor can provide a more flat output torque characteristic as indicated by a broken line in FIG. 6C than a motor without slits as indicated by a dotted line in FIG.  6 C. The slits  3  can be formed only at ones of the stator protrusions  1   a  and rotor protrusions  2   a.    
     Each of stator protrusions  1   a  may have the base portion that is wider than the inner edge (radially inner end). In other words, each of the stator protrusions  1   a  has a trapezoidal front shape as illustrated in FIG.  6 B. This shape allows a portion of the magnetic flux to flow from the side surface thereof, thereby, to moderate the magnetic saturation. Accordingly, the output torque of those of coils C 1 -C 6  is prevented from lowering during the latter part of the period in which they are energized. As a result, the motor having the slits can provide a more flat output torque characteristic than others, as indicated by a solid line in FIG.  6 C. 
     (Third Embodiment) 
     A reluctance type motor according to a third embodiment is described with reference to FIGS. 7,  8 A,  8 B and  8 C. 
     The exciting current can not be fully supplied to each of exciting coils C 1 -C 6  until a certain delay time passes as indicated by a thick broken line in FIG.  7 . This causes torque ripples and reduces the output torque. 
     This delay can be reduced if the timing of the energizing is advanced a certain angle such as the marginal angle α° (e.g. 4°) because the advanced angle brings the exciting coil to the position where the inductance thereof is reduced when it is energized. The result is shown in FIGS. 8A,  8 B and  8 C. 
     Because an optimum advanced angle varies with the rotation speed, the advanced angle should be controlled according to the rotation speed. 
     (Fourth Embodiment) 
     A reluctance type motor according to a fourth embodiment of the invention is described with reference to FIG.  9 . Stator  1  has twelve protrusions  1   a  and rotor  2  has fourteen protrusions  2   a . The protrusion arc angle of both stator protrusions and rotor protrusions is 10.3° in mechanical angle. The motor according to the fourth embodiment is operated in substantially the same manner as the previous embodiments. 
     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention in this document is to be regarded in an illustrative, rather than restrictive, sense.