Patent Publication Number: US-2015076948-A1

Title: Power transmission apparatus

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
     The present application claims the benefit of priority of Japanese Patent Application No. 2013-194169 filed on Sep. 19, 2013, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1 Technical Field 
     This disclosure relates generally to a power transmission apparatus equipped with a first rotor, a second rotor, and a third rotor, and more particularly to a power transmission mechanism designed to minimize a leakage of magnetic flux to improve the performance thereof. 
     2 Background Art 
     Japanese Patent First Publication No. 2003-009504 teaches a power transmission mechanism equipped with a torque input shaft and a torque output shaft which is of a ring-shape and disposed around the input shaft coaxially. The power transmission mechanism also includes permanent magnets to make only, for example, the input shaft have magnetism in order to reduce manufacturing costs thereof. The power transmission mechanism also has an intermediate yoke which is installed within an air gap between an outer circumference of the input shaft and an inner circumference of the output shaft and through which magnetic flux, as produced by the input shaft, passes. The output shaft which is not magnetized and has gear teeth (which will also be referred to as pole teeth below) is formed on a circumferential surface thereof facing the intermediate yoke. Similarly, the intermediate yoke has pole teeth formed on a circumferential surface facing the teeth of the output shaft. The teeth of the intermediate yoke are arranged partially out of phase with those of the output shaft. 
     The above structure of the power transmission mechanism has the drawback in which the magnetic flux leaks from one of the pole teeth to another in the intermediate yoke. The intermediate yoke is made of an assembly of discrete blocks, but the blocks are not magnetically insulated sufficiently. This results in instability in magnetic modulation, which leads to a decrease in performance of the power transmission mechanism. 
     SUMMARY 
     It is therefore an object to provide an improved structure of a power transmission apparatus designed to minimize the leakage of magnetic flux to enhance the performance thereof. 
     According to one aspect of this disclosure, there is provided a power transmission apparatus which works to transmit power using magnetic force. The power transmission apparatus comprises: (a) a first rotor including n soft-magnetic members where n is an integer more than one; (b) a second rotor including k soft-magnetic members where k is an integer more than one; and (c) a third rotor including magnets whose number of pole pairs is m where m is an integer more than or equal to one. The number of the magnets meets a relation of 2m=|k±n|. The first rotor, the second rotor, and the third rotor are arranged in magnetic coupling with each other. The soft-magnetic members of each of the first rotor and the second rotor are arranged at intervals away from each other. 
     As viewed from the magnets of the third rotor which are disposed in a magnetic pole array, the soft-magnetic members of the first and second rotors serve as magnetic inductor arrays. The number of the magnets of the third rotor and the numbers of the soft-magnetic members of the first and second rotors, as described above, have a relation of 2m=|k±n|. In this case, the third rotor serves as a field source to create magnetic torque contributing to transmission of power or torque. 
     The soft-magnetic members of each of the first and second rotors are arranged away from each other. In other words, each of the soft-magnetic members of the first rotor is disposed to face one of the soft-magnetic members of the second rotor so as to establish magnetic coupling therebetween. This layout minimizes the leakage of magnetic flux from one of the soft-magnetic members of the first rotor to another without flux flowing to the second rotor and also minimizes the leakage of magnetic flux from one of the soft-magnetic members of the second rotor to another without flux flowing to the first rotor. 
     In the preferred mode of the disclosure, one of the first rotor and the second rotor is disposed between other two of the first, second, and third rotors. In other words, the soft-magnetic members of the one of the first and second rotors works as magnetic inductors, thereby enhancing the magnetic modulation of the power transmission apparatus to improve the ability of transmitting the power. 
     The first rotor, the second rotor, and the third rotor may be arranged radially. The third rotor is located radially most outwardly. This layout permits the magnets of the third rotor to have an increased area, thus resulting in an increase in magnetic force, which enhances the ability of transmitting the power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
       In the drawings: 
         FIG. 1  is a partial transverse sectional view which illustrates a power transmission mechanism of the first example according to the first embodiment; 
         FIG. 2  is a partial transverse sectional view which illustrates a power transmission mechanism of the second example according to the first embodiment; 
         FIG. 3  is a partial transverse sectional view which illustrates a power transmission mechanism of the third example according to the first embodiment; 
         FIG. 4  is a partial transverse sectional view which illustrates a power transmission mechanism of the fourth example according to the first embodiment; 
         FIG. 5  is a partial transverse sectional view which illustrates a power transmission mechanism of the fifth example according to the first embodiment; 
         FIG. 6  is a plane view which illustrates modifications of soft-magnetic blocks of a second rotor of a power transmission mechanism; 
         FIG. 7  is a plane view which illustrates modifications of soft-magnetic blocks of a second rotor of a power transmission mechanism; 
         FIG. 8  is a partial transverse sectional view which illustrates a power transmission mechanism of the sixth example according to the first embodiment; 
         FIG. 9(   a ) is a partial plane view which illustrates a structure of a second rotor of the power transmission mechanism of the sixth example of  FIG. 8 ; 
         FIG. 9(   b ) is a partially exploded view which illustrates a structure of a second rotor of the power transmission mechanism of the sixth example of  FIG. 8 ; 
         FIG. 9(   c ) is a partially exploded view which illustrates a structure of a second rotor of the power transmission mechanism of the sixth example of  FIG. 8 ; 
         FIG. 10(   a ) is a plane view which illustrates a modification of a second rotor of the power transmission mechanism of the sixth example of  FIG. 8 ; 
         FIG. 10(   b ) is a side view of  FIG. 10(   a ); 
         FIGS. 11(   a ),  11 ( b ), and  11 ( c ) are partial views which illustrate modifications of a second rotor of the power transmission mechanism of the sixth example of  FIG. 8 ; 
         FIG. 12  is a partial transverse sectional view which illustrates a power transmission mechanism of the seventh example according to the first embodiment; 
         FIG. 13  is a partial transverse sectional view which illustrates a power transmission mechanism of the eighth example according to the first embodiment; 
         FIG. 14  is a partial transverse sectional view which illustrates a power transmission mechanism of the ninth example according to the first embodiment; 
         FIG. 15  is a partial transverse sectional view which illustrates a power transmission mechanism of the tenth example according to the first embodiment; 
         FIG. 16(   a ) is a partial transverse sectional view which illustrates a power transmission mechanism of the eleventh example according to the first embodiment; 
         FIG. 16(   b ) is a partially perspective view which illustrates a structure of a magnet of a third rotor of the power transmission mechanism of  FIG. 16(   a ); 
         FIG. 17  is a partial transverse sectional view which illustrates a power transmission mechanism of the twelfth example according to the first embodiment; 
         FIG. 18  is a partial plane view which illustrates an axial type of power transmission mechanism of the thirteenth example according to the first embodiment; 
         FIG. 19  is a partial plane view which illustrates an axial type of power transmission mechanism of the fourteenth example according to the first embodiment; 
         FIG. 20  is a partial transverse sectional view which illustrates a modification of a power transmission mechanism according to the first embodiment; 
         FIG. 21  is a partial transverse sectional view which illustrates an electric rotating machine of the first example according to the second embodiment; 
         FIG. 22  is a partial transverse sectional view which illustrates an electric rotating machine of the second example according to the second embodiment; 
         FIG. 23  is a partial transverse sectional view which illustrates an electric rotating machine of the third example according to the second embodiment; 
         FIG. 24  is a partial transverse sectional view which illustrates an electric rotating machine of the fourth example according to the second embodiment; 
         FIG. 25  is a partial transverse sectional view which illustrates an electric rotating machine of the fifth example according to the second embodiment; 
         FIG. 26  is a partial transverse sectional view which illustrates an electric rotating machine of the sixth example according to the second embodiment; 
         FIG. 27  is a partial transverse sectional view which illustrates an electric rotating machine of the seventh example according to the second embodiment; 
         FIG. 28  is a partial plane view which illustrates an axial type of electric rotating machine of the eighth example according to the second embodiment; 
         FIG. 29  is a schematic view which illustrates an automotive power generator of the first example according to the third embodiment; 
         FIG. 30  is a schematic view which illustrates an automotive power generator of the second example according to the third embodiment; 
         FIG. 31  is a schematic view which illustrates an automotive power generator of the third example according to the third embodiment; 
         FIG. 32  is a schematic view which illustrates an automotive power generator of the fourth example according to the third embodiment; 
         FIG. 33  is a plane view which illustrates a first modification of an automotive power generator according to the third embodiment; 
         FIG. 34  is a plane view which illustrates a second modification of an automotive power generator according to the third embodiment; 
         FIG. 35  is a plane view which illustrates a third modification of an automotive power generator according to the third embodiment; and 
         FIG. 36  is a partial transverse sectional view which illustrates a modification of a third rotor of the power transmission mechanism of the tenth example according to the first embodiment, as illustrated in  FIG. 15 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention will be described below with reference to the drawings. The following disclosure will refer to a plurality of types of power transmission mechanisms. Each view illustrates only essential parts required for better understanding the embodiments of the invention, not all parts of the power transmission mechanisms. Terms of orientation, such as upper, lower, right, and left, as referred to in the following discussion, are just defined based on the drawings. The power transmission mechanisms each have a plurality of rotors arranged in non-contact with each other through an air gap so that they are rotatable. 
     First Embodiment 
       FIGS. 1 to 20  illustrate a plurality of examples of the power transmission mechanism  10  or  20  according to the first embodiment. Each of the power transmission mechanisms  10  and  20  works to transmit or output the power or torque, as inputted from an external power source, to the outside using magnetic force. The power transmission mechanisms  10 A to  10 M, as referred to below, are examples of the power transmission mechanism  10  which is of a radial type. The power transmission mechanisms  20 A and  20 B, as referred to below, are examples of the power transmission mechanism  20  which is of an axial type. Each of  FIGS. 1 to 20  is a schematic view which omits hatching except for shaded magnets for better visibility thereof and illustrates only a half of a traverse section of the power transmission mechanism  10  or  20 . 
     Throughout the drawings, like reference numbers refer to like parts. The explanation of the second and following examples will omit the same parts as those in the first example for the brevity of disclosure. 
     First Example 
     The power transmission mechanism  10 A is, as shown in  FIG. 1 , equipped with a first rotor  11 A, a second rotor  12 A, and a third rotor  13 A. The first rotor  11 A, the second rotor  12 A, and the third rotor  13 A are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 A. The first rotor  11 A is an example of a first rotor  11  of the power transmission mechanism  10 . The second rotor  12 A is an example of a second rotor  12  of the power transmission mechanism  10 . Similarly, the third rotor  13 A is an example of a third rotor  13  of the power transmission mechanism  10 . 
     The first rotor  11 A includes n (=integer more than one, in other words, more than or equal to two) soft-magnetic blocks  11   a  which are arrayed at regular intervals away from each other in a circumferential direction thereof. Each of the soft-magnetic blocks  11   a  is of a trapezoidal shape and oriented with the long side thereof facing the second rotor  12 A in a radially outward direction. The second rotor  12 A includes k (=integer more than one) soft-magnetic blocks  12   a  which are arrayed at regular intervals away from each other in a circumferential direction thereof. Each of the soft-magnetic blocks  12   a  is of a rectangular or square shape, but may alternatively be formed to have another shape. 
     The third rotor  13 A includes a soft-magnetic cylinder  13   a  and magnets  13   b  whose number of pole pairs is m (=integer more than or equal to one). Each of the magnets  13   b  is implemented by a permanent magnet made of material showing an electrical resistivity of 3 μΩm or more. The magnetization direction that is a direction in which each of the magnets  13   b  is magnetized is expressed by an arrow in the drawing. The magnets  13   b  are located inside the soft-magnetic cylinder  13   a , in other words, arranged to face the second rotor  12 A in order to facilitate the ease of flow of magnetic flux, as produced thereby, to the second rotor  12 A. The soft-magnetic cylinder  13   a  is disposed outside the magnets  13   b  in order to make the magnetic flux, as produced by the magnets  13   b , flow through the soft-magnetic cylinder  13   a . The soft-magnetic cylinder  13   a  of this embodiment in  FIG. 1  is not needed in a structure where an armature is disposed to face the third rotor  13 A (see the second embodiment). 
     The n soft-magnetic blocks  11   a  of the first rotor  11 A may be made up of at least two discrete soft-magnetic segments each of which serves as a pole segment. Similarly, the k soft-magnetic blocks  12   a  of the second rotor  12 A may be made up of at least two discrete soft-magnetic segments each of which serves as a pole segment. Each of the pole segments is made of, for example, a stack of thin magnetic steel plates. The soft-magnetic blocks  12   a  of the second rotor  12 A interposed between the first rotor  11 A and the third rotor  13 A work as magnetic inductors. Each of the soft-magnetic blocks  11   a  of the first rotor  11 A is, as can be seen in  FIG. 1 , disposed to face at least one of the soft-magnetic blocks  12   a  of the second rotor  12 A in the radial direction of the first and second rotors  11 A and  12 A in order to establish magnetic coupling therebetween. In other words, each of the soft-magnetic blocks  11   a  of the first rotor  11 A functions as one of discrete gear teeth of a typical magnetic gear which is magnetically coupled with one of the soft-magnetic blocks  12   a  of the second rotor  12 A. Similarly, each of the soft-magnetic blocks  12   a  of the second rotor  12 A functions as one of discrete gear teeth of a typical magnetic gear which is magnetically coupled with one of the soft-magnetic blocks  11   a  of the first rotor  11 A. This layout minimizes the leakage of magnetic flux from one of the soft-magnetic blocks  11   a  to another without allowing it to flow to the second rotor  12 A and also minimizes the leakage of magnetic flux from one of the soft-magnetic blocks  12   a  to another without it flowing to the first rotor  11 A. 
     The n soft-magnetic blocks  11   a , the k soft-magnetic blocks  12   a , and the magnets  13   b  whose number of pole pairs is m are selected to meet a relation of 2m=|k±n|. In the structure of  FIG. 1 , n=20, k=32, and m=6 (i.e., 2m=k−n). These numbers may be determined depending upon the type or rating of the power transmission mechanism  10 A. It is advisable that the number of pole pairs of the soft-magnetic blocks  12   a  of the second rotor  12 A be greater than that of the soft-magnetic blocks  11   a  of the first rotor  11 A. 
     Second Example 
       FIG. 2  illustrates the power transmission mechanism  10 B which is, like the power transmission mechanism  10 A, equipped with the first rotor  11 A, the second rotor  12 A, and the third rotor  13 A. The power transmission mechanism  10 B is different from the power transmission mechanism  10 A in layout of the first rotor  11 A, the second rotor  12 A, and the third rotor  13 A. Specifically, the power transmission mechanism  10 B has the first rotor  11 A, the second rotor  12 A, and the third rotor  13 A arranged radially from the outside to the inside thereof. Other arrangements are identical with those in the first example. The structure of the second example is also substantially identical in operation and beneficial effects with the first example. 
     Third Example 
       FIG. 3  illustrates the power transmission mechanism  10 C which is, like the power transmission mechanism  10 A, equipped with the first rotor  11 A, the second rotor  12 A, and the third rotor  13 A. The power transmission mechanism  10 C is different from the power transmission mechanism  10 A in layout of the first rotor  11 A and the second rotor  12 A. Specifically, the power transmission mechanism  10 C has the second rotor  12 A disposed inside the first rotor  11 A in the radial direction thereof. 
     The power transmission mechanism  10 C, although not illustrated, may be designed to have the second rotor  12 A, the first rotor  11 A, and the third rotor  13 A arranged in this order in the radial direction from outside to inside thereof. Other arrangements are identical with those in the first example. The structure of the third example is also substantially identical in operation and beneficial effects with the first example. 
     Fourth Example 
       FIG. 4  illustrates the power transmission mechanism  10 D which is equipped with the first rotor  11 B, the second rotor  12 A, and the third rotor  13 A. The first rotor  11 B, the second rotor  12 A, and the third rotor  13 A are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 D. The first rotor  11 B is an example of the first rotor  11  and includes n soft-magnetic blocks  11   b  which are arrayed at regular intervals away from each other in the circumferential direction of the power transmission mechanism  10 D. Each of the soft-magnetic blocks  11   b  is of a square or rectangular shape. Other arrangements are identical with those in the first example. The structure of the fourth example is also substantially identical in operation and beneficial effects with the first example. 
     The power transmission mechanism  10 D, although not illustrated, may be designed to have the first rotor  11 B, the second rotor  12 A, and the third rotor  13 A arranged in this order in the radial direction from the outside to the inside thereof. The power transmission mechanism  10 D may also be engineered to have the second rotor  12 A, the first rotor  11 B, and the third rotor  13 A in this order radially from the inside to the outside or the outside to the inside thereof. Other arrangements are identical with those in the first example. The structure of the fourth example is also substantially identical in operation and beneficial effects with the first example. 
     Fifth Example 
       FIG. 5  illustrates the power transmission mechanism  10 E which is equipped with the first rotor  11 A, the second rotor  12 B, and the third rotor  13 A. The first rotor  11 A, the second rotor  12 B, and the third rotor  13 A are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 E. The second rotor  12 B is an example of the second rotor  12  and includes k soft-magnetic blocks  12   b  which are arrayed at regular intervals away from each other in the circumferential direction of the power transmission mechanism  10 E. Each of the soft-magnetic blocks  12   a  in  FIG. 1  is, as described above, rectangular or square with all flat surfaces, while each of the soft-magnetic blocks  12   b  in  FIG. 5  is shaped to have non-planar side surfaces. The side surfaces of the soft-magnetic blocks  12   b , as referred to herein, are surfaces thereof facing each other. In the illustrated case where the soft-magnetic blocks  12   b  are arrayed in the circumferential direction of the second rotor  12 B, the side surfaces of the soft-magnetic blocks  12   b  are the surfaces thereof facing each other in the circumferential direction. Other arrangements are identical with those in the first example. The structure of the fifth example is also substantially identical in operation and beneficial effects with the first example. 
     It is advisable that a radially intermediate one of the three rotors  11 ,  12 , and  13  (e.g., the second rotor  12 B in the fifth example of  FIG. 5 ) be engineered to have soft-magnetic blocks with non-planar side surfaces. The non-planar side surfaces are shaped to have irregularities, one or more concavities, one or more convex portions, and/or curved surfaces. The non-planar side surfaces of each of the soft-magnetic blocks  12   b  in  FIG. 5  are V-shaped in cross section, but may be formed in another shape. For instance, the second rotor  12 B may be shaped, as illustrated in  FIG. 6 , to have any of types of soft-magnetic blocks  12   c ,  12   d ,  12   e ,  12   f , and  12   g . For facilitating comparison of the shape among them,  FIG. 6  shows the soft-magnetic blocks  12   b  at the upper left hand corner thereof. The soft-magnetic block  12   c  has side surfaces with a chevron protrusion. The soft-magnetic block  12   d  has side surfaces with a U-shaped or arc-shaped recess. The soft-magnetic block  12   e  has side surfaces with an arc-shaped or domed protrusion. The soft-magnetic block  12   f  has side surfaces with a combination of flat and curved areas. The soft-magnetic block  12   g  has side surfaces: one having a V-shaped concave portion and the other having a V-shaped concave portion. Of course, the second rotor  12 B may have soft-magnetic blocks with non-planar side surfaces of another shape. The non-planar side surfaces of the soft-magnetic blocks  12   b  work to minimize a leakage of magnetic flux from one of them to another, which facilitates flow of the magnetic flux from the surfaces facing the first and third rotors  11 A and  13 A. The first to fifteenth examples may have any of the soft-magnetic blocks  12   b  to  12   g.    
     An outermost or innermost one of the first to third rotors  11  to  13  (e.g., the first rotor  11 A in  FIG. 5 ) is preferably shaped to have soft-magnetic blocks with long and short sides. Each of the soft-magnetic blocks  11   a  of the first rotor  11 A is, as described above, of a trapezoidal shape, but may be made to have another shape.  FIG. 7  illustrates examples of the shape of each of the soft-magnetic blocks  11   a . For facilitating comparison of the shape among them,  FIG. 7  shows the soft-magnetic blocks  11   a  on the left hand side thereof. The soft-magnetic block  11   c  is substantially of a trapezoidal shape with stepwise side surfaces The soft-magnetic block  11   d  is of a fan or sectorial shape with arc-shaped concave surfaces facing the adjacent first and third rotors  11 A and  13 A. The soft-magnetic blocks  12   a  to  12   g  in  FIGS. 1 and 6  may be designed to have the fan-shape, like the soft-magnetic block  11   d . Each of the soft-magnetic blocks  11   a  may alternatively be made to be rectangular or non-rectangular. The soft-magnetic blocks  11   c  or  11   d  may be employed in any of the first to fourth or sixth to fifteenth examples. Although not illustrated, the power transmission mechanism  10 E of the fifth example may be modified in the same way as described in the second to fourth examples to achieve the same effects. 
     Sixth Example 
       FIG. 8  illustrates the power transmission mechanism  10 F which is equipped with the first rotor  11 A, the second rotor  12 C, and the third rotor  13 A. The first rotor  11 A, the second rotor  12 C, and the third rotor  13 A are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 F. The second rotor  12 C is an example of the second rotor  12  and, as illustrated in  FIGS. 9(   a ) to  9 ( c ), includes k soft-magnetic blocks  12   c  and bridges  12   h . The k soft-magnetic blocks  12   c  are arrayed at regular intervals away from each other in the circumferential direction of the power transmission mechanism  10 F. The bridges  12   h  work as fasteners to retain some or all of the soft-magnetic blocks  12   c . Specifically, the k soft-magnetic blocks  12   c  are, as illustrated in  FIG. 9(   b ), arrayed at regular intervals away from each other. The bridges  12   h  are, as illustrated in  FIG. 9(   c ), arranged at a given interval away from each other in the radial direction of the power transmission mechanism  10 F to hold some or all of the k soft-magnetic blocks  12   c  firmly in a given manner. The holding of some or all of the k soft-magnetic blocks  12   c  through the bridges  12   h  may be achieved by bolts, screws, soldering, arc-welding, or glueing (or bonding). The bridges  12   h  may be made of soft-magnetic material. In this case, some or all of the k soft-magnetic blocks  12   c  and the bridges  12   h  may be formed integrally with each other. 
     The some or all of the k soft-magnetic blocks  12   c  may be retained or joined together in another way without use of the bridges  12   h .  FIGS. 10(   a ),  10 ( b ),  11 ( a ),  11 ( b ), and  11 ( c ) illustrate second rotors  12 D and  12 E that are modifications of the second rotor  12 C. The second rotor  12 D of  FIG. 10  includes the soft-magnetic blocks  12   c , fasteners  12   i , and a plate  12   j .  FIG. 10(   a ) is a plane view of the second rotor  12 D.  FIG. 10(   b ) is a side view of the second rotor  12 D. The plate  12   j  is used as a bridge and has an annular or hollow cylindrical shape. The soft-magnetic blocks  12   c  are secured to the plate  12   j  through the fasteners  12   i . The fasteners  12   i  are implemented by, for example, screws or bolts. The plate  12   j  may be made of material other than non-magnetic material, but preferably made of it. 
     The second rotor  12 E, as illustrated in  FIGS. 11(   a ) to  11 ( c ), includes the soft-magnetic blocks  12   c  and a fastener  12   k . The fastener  12   k  is of an annular or hollow cylindrical shape.  FIG. 11(   a ) is a plane view of the second rotor  12 E.  FIGS. 11(   b ) and  11 ( c ) are side views which show first and second modifications of the fastener  12   k , respectively. The fastener  12   k  of  FIG. 11(   b ) has formed therein holes extending through a thickness thereof. The soft-magnetic blocks  12   c  are fit through the holes. The fastener  12   k  of  FIG. 11(   c ) is made of, for example, a plate and used as a bridge. The fastener  12   k  has formed therein non-through holes such as recesses or concavities in which the soft-magnetic blocks  12   c  are embedded or fit. The fasteners  12   k  may be made of material other than non-magnetic material, but preferably made of it. 
     The fastening mechanisms, as illustrated in  FIGS. 9(   a ) to  11 ( c ), may be used to retain the soft-magnetic blocks  12   a ,  12   b ,  12   d  to  12   g  of the second rotor  12  or the soft-magnetic blocks  11   a  to  11   d  of the first rotor  11 . Other arrangements of the power transmission mechanism  10 F are identical with those in the first example. The power transmission mechanism  10 F is also substantially identical in operation and beneficial effects with the first example. The power transmission mechanism  10 F may be modified in the same way as described in the second to fifth examples to achieve the same effects. 
     Seventh Example 
       FIG. 12  illustrates the power transmission mechanism  10 G which is equipped with the first rotor  11 A, the second rotor  12 F, and the third rotor  13   x . The first rotor  11 A, the second rotor  12 F, and the third rotor  13   x  are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 G. The third rotor  13   x  is an example of the third rotor  13  and includes magnets  13   y  whose number of pole pairs is m and m′ soft-magnetic blocks  13   x  where m′=2m. The magnets  13   y  and the soft-magnetic blocks  13   x  are arranged alternately in the circumferential direction of the power transmission mechanism  100 . In the illustrated example, the soft-magnetic blocks  13   x  are continuously joined together by an annular body of the third rotor  13 B, but may alternatively be formed to be discrete. Specifically, the third rotor  13   x , as illustrated in  FIG. 12 , may be made of a single annular soft-magnetic body. The magnets  13   y  are embedded in the annular soft-magnetic body at given intervals away from each other. In other words, a portion of the annular soft-magnetic body is interposed between every adjacent two of the magnets  13   y  as one of the soft-magnetic blocks  13   x . The second rotor  12 F is an example of the second rotor  12  and includes k soft-magnetic blocks  12   a  and magnets  12   m  whose number of pole pairs is k′ where 2k′=k. The soft-magnetic blocks  12   a  and the magnets  12   m  are arranged alternately in the circumferential direction of the power transmission mechanism  10 G. In other words, the k soft-magnetic blocks  12   a  are disposed at intervals away from each other. Similarly, the k magnets  12   m  are disposed at intervals away from each other. Note that the third rotor  13   x  may include at least two of the magnets  13   y  (i.e., permanent magnets) which are magnetized in given directions and have of the soft-magnetic blocks  13   x  and the at least two of the magnets  13   y  arranged alternately in the circumferential direction of the third rotor  13   x.    
     As viewed from the magnets  13   y  of the third rotor  13   x  which are disposed in a magnetic pole array, the soft-magnetic blocks  11   a  and  12   a  of the first and second rotors  11 A and  12 F serve as magnetic inductor arrays. The number of the magnets  13   y , the number of the soft-magnetic blocks  11   a , and the number of the soft-magnetic blocks  12   a  meet a relation of 2m=k−n. In this case, the third rotor  13   x  serves as a field source to create first magnetic transmission torque. Additionally, as viewed from the magnets  12   m  of the second rotor  12 F which are arranged in a magnetic pole array, the soft-magnetic blocks  11   a  and  13   x  of the first and third rotors  11 A and  13   x  serve as magnetic inductor arrays. The number of the magnets  12   m , the number of the soft-magnetic blocks  13   x , the number of the soft-magnetic blocks  11   a  meet a relation of 2k′=m′+n. In this case, the second rotor  12 F works as a field source to create second magnetic transmission torque. The power transmission mechanism  10 G is capable of outputting the sum of the first and second magnetic transmission torques, thereby enhancing the ability of transmitting the power. Other arrangements of the power transmission mechanism  10 G are identical with those in the first example. The power transmission mechanism  10 G is also substantially identical in operation and beneficial effects with the first example. The power transmission mechanism  10 G may be modified in the same way as described in the second to sixth examples to achieve the same effects. 
     Eighth Example 
       FIG. 13  illustrates the power transmission mechanism  10 H which is equipped with the first rotor  11 A, the second rotor  12 A, and the third rotor  13 B. The first rotor  11 A, the second rotor  12 A, and the third rotor  13 B are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 H. The third rotor  13 B includes soft-magnetic blocks  13   a  and magnets  13   b  whose number of pole pairs is m. The magnets  13   b  are made up of first magnets  13   b   1  whose number of pole pairs is m and second magnets  13   b   2  whose number of pole pairs is m. The first magnets  13   b   1  and the second magnets  13   b   2  are arranged alternately in the circumferential direction of the power transmission mechanism  10 H (i.e., the circumferential direction of the third rotor  13 X). In other words, each of the second magnets  13   b   2  is disposed between adjacent two of the first magnets  13   b   1 . The boundary or interface between each of the first magnets  13   b   1  and adjacent one of the second magnets  13   b   2  is preferably aligned with the radial direction of the power transmission mechanism  10 H. Adjacent two of the first magnets  13   b   1  are, as indicated by arrows in  FIG. 13 , magnetized in opposite circumferential directions of the power transmission mechanism  10 H. Each adjacent two of the second magnets  13   b   2  are magnetized in opposite radial directions of the power transmission mechanism  10 H. The layout of the magnets  13   b  in  FIG. 13  is generally referred to as a Halbach array. The magnets  13   b  have increased areas, thus resulting in an increase in magnetic flux, which enhances the ability of transmitting the power. The third rotor  13 B may alternatively be made up of a single annular soft-magnetic block  13   a  and magnets  13   b  embedded in the soft-magnetic block  13   a . In this case, a portion of the soft-magnetic block  13   a  is interposed between every adjacent two of the magnets  13   b   1 . Other arrangements of the power transmission mechanism  10 H are identical with those in the first example. The power transmission mechanism  10 H is also substantially identical in operation and beneficial effects with the first example. The power transmission mechanism  10 H may be modified in the same way as described in the second to seventh examples to achieve the same effects. 
     Ninth Example 
       FIG. 14  illustrates the power transmission mechanism  10 I which is equipped with the first rotor  11 A, the second rotor  12 A, and the third rotor  13 C. The first rotor  11 A, the second rotor  12 A, and the third rotor  13 C are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 I. The third rotor  13 C includes a soft-magnetic cylinder  13   a  and magnets  13   b  whose number of pole pairs is m. The magnets  13   b  are broken down into magnetic pairs which are embedded in the soft-magnetic cylinder  13   a . Each of the magnetic pairs works as one pole. The magnets  13   b  of each magnetic pair are arranged away from each other through a small gap (which will also be referred to as a first interval), in other words, located close to each other. The magnets  13   b  of each magnetic pair are oriented asymmetrically with respect to the radial direction of the power transmission mechanism  10 I, so that long sides of each of the magnets  13   b  intersect with the radial direction of the third rotor  13 C at an angle other than 90 degrees. The magnetic pairs of the magnets  13   b  are arrayed at a second interval away from each other. The second interval is longer than the first interval at which the magnets  13   b  of each magnetic pair are disposed away from each other. The magnets  13   b  of each magnetic pair may alternatively be oriented symmetrically with respect to the radial direction of the power transmission mechanism  10 I, in other words, mirror-image symmetrical about the radial direction of the third rotor  13   x.    
     The magnets  13   b  are embedded in the soft-magnetic cylinder  13   a , thus minimizing the probability that they detach accidentally from the soft-magnetic cylinder  13   a  when subjected to centrifugal force during rotation of the third rotor  13 C. The structure of the power transmission mechanism  10 I is, therefore, high in safety. Other arrangements of the power transmission mechanism  10 I are identical with those in the first example. The power transmission mechanism  10 I is also substantially identical in operation and beneficial effects with the first example. The power transmission mechanism  10 I may be modified in the same way as described in the second to eighth examples to achieve the same effects. 
     Tenth Example 
       FIG. 15  illustrates the power transmission mechanism  10 J which is equipped with the first rotor  11 A, the second rotor  12 A, and the third rotor  13 D. The first rotor  11 A, the second rotor  12 A, and the third rotor  13 D are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 J. The third rotor  13 A in  FIG. 1 , as described above, has the magnets  13   b  whose number of pole pairs is m and which are mounted on the inner circumference of the soft-magnetic cylinder  13   a , in other words, arranged to face the second rotor  12 A, while the third rotor  13 D of this example includes the soft-magnetic cylinder  13   a  and the magnets  13   b  which are disposed in the soft-magnetic cylinder  13   a  at intervals away from each other in the circumferential direction of the third rotor  13 D. In other words, a portion of the soft-magnetic cylinder  13   a  is disposed between every adjacent two of the magnets  13   b . All the magnets  13   b  are magnetized in the same direction. The magnets  13   b  arranged in the layout of  FIG. 15  are generally referred to as being of a consequent-pole type. 
     The magnets  13   b  are, as indicated by arrows in  FIG. 15 , all magnetized inwardly toward the center of the third rotor  13 D, but however, may alternatively be magnetized in a radially outward direction. Other arrangements of the power transmission mechanism  10 J are identical with those in the first example. The power transmission mechanism  10 J is also substantially identical in operation and beneficial effects with the first example. The power transmission mechanism  10 J may be modified in the same way as described in the second to ninth examples to achieve the same effects. 
     Eleventh Example 
       FIGS. 16(   a ) and  16 ( b ) illustrate the power transmission mechanism  10 K which is equipped with the first rotor  11 A, the second rotor  12 A, and the third rotor  13 E. The first rotor  11 A, the second rotor  12 A, and the third rotor  13 E are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 K. The third rotor  13 E includes the soft-magnetic cylinder  13   a  and magnets  13   c  whose number of pole pairs is m. The magnets  13   c  are magnetized in directions, as indicated by arrows. 
     Each of the magnets  13   c  works as one pole and is, as illustrated in  FIG. 16(   b ), made up of a plurality of magnetic segments  13   d . In the illustrated example, the fifteen magnetic segments  13   d  are arranged continuously in a 3×5 matrix to form one pole. The magnetic segments  13   d  of each of the magnets  13   c  are magnetized in the same direction. The number of the magnetic segments  13   d  of each of the magnets  13   c  is not limited to fifteen, but may be changed as needed. Additionally, at least one of the magnets  13  may be made up of the plurality of magnetic segments  13   d.    
     Each of the magnetic segments  13   d  is electrically insulated from the adjacent ones. Specifically, the magnetic segments  13   d  are isolated from each other through an electrically insulating film or an electrically insulating material. For instance, only mutually facing side surfaces or whole surfaces of every adjacent two of the magnets  13   d  may be isolated from each other. The electric insulation among the magnetic segments  13   d  avoids generation of an eddy current, as indicated by an arrow D 11  expressed by a two-dot chain line, but creates eddy currents, as indicate by arrows D 12  expressed by solid lines, one in each of the magnetic segments  13   d . This eliminates a loss of energy arising from the eddy current, as indicated by the arrow D 11 . 
     Other arrangements of the power transmission mechanism  10 K are identical with those in the first example. The power transmission mechanism  10 K is also substantially identical in operation and beneficial effects with the first example. The power transmission mechanism  10 K may be modified in the same way as described in the second to tenth examples to achieve the same effects. 
     Twelfth Example 
       FIG. 17  illustrates the power transmission mechanism  10 L which is equipped with the first rotor  11 , the second rotor  12 , and the third rotor  13 . The second rotor  12 , the third rotor  13 , and the first rotor  11  are arranged in this order radially from the inside to the outside of the power transmission mechanism  10 L. The first rotor  11 , the third rotor  13 , and the second rotor  12  may alternatively be, as indicated by parentheses, arrayed in this order radially from the inside to the outside of the power transmission mechanism  10 L. The power transmission mechanism  10 L is designed to have the third rotor  13  interposed between the first rotor  11  and the second rotor  12 . 
     The first rotor  11  may be implemented by either of the first rotor  11 A or the first rotor  11 B, as described in the first to eleventh examples. The first rotor  11  may also be made to have any of the structures, as referred to in the sixth example of  FIGS. 9(   a ) to  11 ( c ). Similarly, the second rotor  12  may be implemented by any of the second rotor  12 A to  12 F, as described in the first to eleventh examples. The third rotor  13  may be implemented by any of the third rotor  13 A to  13 E, as described in the first to eleventh examples. Other arrangements of the power transmission mechanism  10 L are identical with those in the first example. The power transmission mechanism  10 L is also substantially identical in operation and beneficial effects with the first example. The power transmission mechanism  10 L may be modified in the same way as described in the second to eleventh examples to achieve the same effects. 
     Thirteenth Example 
       FIG. 18  illustrates the power transmission mechanism  20 A which is of an axial type. Specifically, the power transmission mechanism  20 A has the first rotor  21 , the second rotor  22 , and the third rotor  23  disposed in this order coaxially with each other. In other words, the first rotor  21 , the second rotor  22 , and the third rotor  23  are shaped to be arranged coaxially and adjacent each other in a multi-layer form. The first rotor  21  structurally corresponds the first rotor  11  of the radial type of power transmission mechanism, as described above. Similarly, the second rotor  22  structurally corresponds the second rotor  12  of the radial type of power transmission mechanism. The third rotor  23  structurally corresponds the third rotor  13  of the radial type of power transmission mechanism. Specifically, the first rotor  21  may be implemented by either of the first rotor  11 A or the first rotor  11 B, as described in the first to twelfth examples, which are modified to be arranged coaxially with the second rotor  22  and the third rotor  23 . The second rotor  22  may be implemented by any of the second rotor  12 A to  12 F, as described in the first to twelfth examples, which are modified to be arranged coaxially with the first rotor  21  and the third rotor  23 . The third rotor  23  may be implemented by any of the third rotor  13 A to  13 E, as described in the first to twelfth examples, which are modified to be arranged coaxially with the first rotor  21  and the second rotor  22 . Other arrangements of the power transmission mechanism  20 A are identical with those in the first example. The power transmission mechanism  20 A is also substantially identical in operation and beneficial effects with the first example. 
     Fourteenth Example 
       FIG. 19  illustrates the power transmission mechanism  20 B which is of an axial type. Specifically, the power transmission mechanism  20 B has the second rotor  22 , the first rotor  21 , and the third rotor  23  disposed in this order coaxially with each other. In other words, the second rotor  22 , the first rotor  21 , and the third rotor  23  are shaped to be arranged coaxially and adjacent each other in a multi-layer form. The power transmission mechanism  20 A is different from the thirteenth example only in that the first rotor  21  is disposed between the second rotor  22  and the third rotor  23 . Other arrangements of the power transmission mechanism  20 B are identical with those in the first example. The power transmission mechanism  20 B is also substantially identical in operation and beneficial effects with the first example. 
     Although not illustrated, the power transmission mechanism  20 B may be designed, like the twelfth example, to have the third rotor  32  arranged between the first rotor  21  and the second rotor  22 . In other words, the first rotor  21 , the third rotor  23 , and the second rotor  22  may be arranged coaxially in this order in a multi-layer form. This structure is also substantially identical in operation and beneficial effects with the first to eleventh examples. 
     Modification 
     The radial type of power transmission mechanism  10  may be engineered to have one of all possible combinations of the first rotors  11 A and  11 B, the second rotors  12 A to  12 F, and the third rotors  13 A to  13 E in the first to twelfth examples. One such example is illustrated in  FIG. 20 . The power transmission mechanism  10 M of  FIG. 20  is engineered to have the first rotor  11 A (i.e., the first rotor  11 ), the second rotor  12 B (i.e., the second rotor  12 ), and the third rotor  13 B (i.e., the third rotor  13 ). Any of all possible combinations of the first rotors  11 A and  11 B, the second rotors  12 A to  12 F, and the third rotors  13 A to  13 E is identical in operation with and offers substantially same beneficial effects as the first to twelfth examples. 
     Second Embodiment 
       FIGS. 21 to 28  illustrate a plurality of examples of an electric rotating machine  100  or  200  according to the second embodiment. The electrical rotating machines  100  and  200  are constructed as, for example, a motor-generator. The electric rotating machines  100 A to  100 G, as referred to below, are examples of the electric rotating machine  100  which is of a radial type. The electric rotating machine  200 A, as referred to below, is an example of the electric rotating machine  200  which is of an axial type. Each of  FIGS. 21 to 28  is a schematic view which omits hatching except for shaded magnets for better visibility thereof and illustrates only a half of a traverse section of the electric rotating machine  100  or  200 .  FIGS. 21 to 28  also omit a winding of an armature. Throughout the drawings, like reference numbers refer to like parts. The explanation of the second and following examples of the second embodiment will omit the same parts as those in the first example for the brevity of disclosure. 
     First Example 
     The electric rotating machine  100 A is, as shown in  FIG. 21 , of an inner rotor type and includes the first rotor  11 A, the second rotor  12 A, the third rotor  13 F, and the armature  101 . The first rotor  11 A, the second rotor  12 A, and the third rotor  13 F, and the armature  101  are arranged in this order radially from the inside to the outside of the electric rotating machine  100 A. The third rotor  13 F includes the magnets  13   b  whose number of pole pairs is m and which are arrayed in a circumferential direction of the third rotor  13 F. The electric rotating machine  100 A is a modification of the power transmission mechanism  10 A of  FIG. 1 . Specifically, the electric rotating machine  100 A omits the soft-magnetic cylinder  13   a  from the third rotor  13 A to have the armature  101  in order to ensure a required flow of magnetic flux. 
     The magnets  13   b  of the third rotor  13 F establish magnetic couplings between the armature  101  and the third rotor  13 F and between the third rotor  13 F and the second rotor  12 A. How to create magnetic torque acting on the first rotor  11 A, the second rotor  12 A, and the third rotor  13 F is the same as in the first example of the first embodiment except that the third rotor  13 F is used instead of the third rotor  13 A. Between the first rotor  11 A which is located most inwardly and the second rotor  12 A disposed intermediate between the first rotor  11 A and the third rotor  13 F, U-shaped flows of magnetic flux, as indicated by arrows D 21 , are created. This achieves good magnetic modulation, thus enhancing the ability of torque transmission in the electric rotating machine  100 A. 
     Second Example 
       FIG. 22  illustrates the electric rotating machine  100 B which is of an outer rotor type. The electric rotating machine  100 B is, like the electric rotating machine  100 A of  FIG. 21 , equipped with the first rotor  11 A, the second rotor  12 A, the third rotor  13 F, and the armature  101 , but different therefrom in that the first rotor  11 A, the second rotor  12 A, and the third rotor  13 F, and the armature  101  are arranged in this order radially from the outside to the inside of the electric rotating machine  100 B. Other arrangements of the electric rotating machine  100 B are identical with those in the first example of  FIG. 21 . The electric rotating machine  100 B is also substantially identical in operation and beneficial effects with the first example. 
     Third Example 
       FIG. 23  illustrates the electric rotating machine  100 C which is of an inner rotor type. The electric rotating machine  100 C is equipped with the first rotor  11 A, the second rotor  12 A, the third rotor  13 G, and the armature  101 . The first rotor  11 A, the second rotor  12 A, the third rotor  13 G, and the armature  101  are arranged radially in this order from the inside to the outside of the electric rotating machine  100 C. 
     The third rotor  13 G has a plurality of magnets  13   e  embedded in an outer circumferential portion of the soft-magnetic cylinder  13   a . The layout of the magnets  13   e  is the same as that of the magnets  13   b  in  FIG. 14 . The magnets  13   e  establish a magnetic connection between the third rotor  13 G and the armature  101  to transmit the power or torque therebetween. The third rotor  13 G is, as can be seen in the drawing, made by a combination of the third rotor  13 A and the magnets  13   e . The third rotor  13 A, as described above, has the magnets  13   b . The magnets  13   b  work to establish a magnetic connection between the third rotor  13 G and the second rotor  12 A to transmit the torque therebetween. Although not illustrated, the same flows of magnetic flux as those in  FIG. 21  are created, thus providing the same operation and beneficial effects as those in the first example. 
     Although not illustrated, the electric rotating machine  100 C, like the second example of  FIG. 22 , may be of an outer rotor type which is designed to have the first rotor  11 A, the second rotor  12 A, the third rotor  13 G, and the armature  101  arranged in this order radially from the outside to the inside thereof. Other arrangements are identical with those in the first example of  FIG. 21 . The structure of the third example is also substantially identical in operation and beneficial effects with the first example. 
     Fourth Example 
       FIG. 24  illustrates the electric rotating machine  100 D which is of an inner rotor type. The electric rotating machine  100 D is equipped with the first rotor  11 A, the second rotor  12 A, the third rotor  13 H, and the armature  101 . The first rotor  11 A, the second rotor  12 A, the third rotor  13 H, and the armature  101  are arranged in this order radially from the inside to the outside of the electric rotating machine  100 D. 
     The third rotor  13 H is formed by a combination of the third rotor  13 A of  FIG. 1  and the third rotor  13 F of  FIG. 21 . The third rotor  13 A in this example is disposed on the inner circumferential side of the third rotor  13 H and faces the second rotor  12 A. The third rotor  13 F is disposed on the outer circumferential side of the third rotor  13 H so that it faces the armature  101 . The magnets  13   b  of the third rotor  13 F establish a magnetic connection between the third rotor  13 H and the armature  101  to transmit the power or torque therebetween. The magnets  13   b  of the third rotor  13 A work to establish a magnetic connection between the third rotor  13 H and the second rotor  12 A to transmit the torque therebetween. Although not illustrated, the same flows of magnetic flux as those in  FIG. 21  are created, thus providing the same operation and beneficial effects as those in the first example of  FIG. 21 . 
     Although not illustrated, the electric rotating machine  100 D, like the second example of  FIG. 22 , may be of an outer rotor type which is designed to have the first rotor  11 A, the second rotor  12 A, the third rotor  13 H, and the armature  101  arranged in this order radially from the outside to the inside thereof. Other arrangements are identical with those in the first example of  FIG. 21 . The structure of the third example is also substantially identical in operation and beneficial effects with the first example. 
     Fifth Example 
       FIG. 25  illustrates the electric rotating machine  100 E which is of an inner rotor type and equipped with the first rotor  11 B, the second rotor  12 A, the third rotor  13 F, and the armature  101 . The first rotor  11 B, the second rotor  12 A, the third rotor  13 F, and the armature  101  are arranged in this order radially from the inside to the outside of the electric rotating machine  100 E. This structure is substantially identical with that of the power transmission mechanism  10 D of the fourth example of the first embodiment. Although not illustrated, the same flows of magnetic flux as those in  FIG. 21  are created, thus providing the same operation and beneficial effects as those in the first example of  FIG. 21 . 
     Although not illustrated, the electric rotating machine  100 E, like the second example of  FIG. 22 , may be of an outer rotor type which is designed to have the first rotor  11 B, the second rotor  12 A, the third rotor  13 F, and the armature  101  arranged in this order radially from the outside to the inside thereof. Other arrangements are identical with those in the first example of  FIG. 21 . The structure of the fifth example is also substantially identical in operation and beneficial effects with the first example. 
     Sixth Example 
       FIG. 26  illustrates the electric rotating machine  100 F which is of an inner rotor type and equipped with the first rotor  11 A, the second rotor  12 A, the third rotor  13 I, and the armature  101 . The first rotor  11 A, the second rotor  12 A, the third rotor  13 I, and the armature  101  are arranged in this order radially from the inside to the outside of the electric rotating machine  100 F. 
     The third rotor  13 I is formed by a combination of the third rotor  13 A of  FIG. 1 , the third rotor  13 C of  FIG. 14 , and the third rotor  13 F of  FIG. 21 . The third rotor  13 C in this example is disposed on the inner circumferential side of the third rotor  13 I and faces the second rotor  12 A. The third rotor  13 F in this example is disposed on the outer circumferential side of the third rotor  13 I and faces the armature  101 . The magnets  13   b  of the third rotor  13 F establish a magnetic connection between the third rotor  13 I and the armature  101  to transmit the power or torque therebetween. The magnets  13   e  of the third rotor  13 C work to establish a magnetic connection between the third rotor  13 I and the second rotor  12 A to transmit the torque therebetween. Although not illustrated, the same flows of magnetic flux as those in  FIG. 21  are created, thus providing the same operation and beneficial effects as those in the first example of  FIG. 21 . 
     Although not illustrated, the electric rotating machine  100 F, like the second example of  FIG. 22 , may alternatively be of an outer rotor type which is designed to have the first rotor  11 A, the second rotor  12 A, the third rotor  13 I, and the armature  101  arranged in this order radially from the outside to the inside thereof. Other arrangements are identical with those in the first example of  FIG. 21 . The structure of the sixth example is also substantially identical in operation and beneficial effects with the first example. 
     Seventh Example 
       FIG. 27  illustrates the electric rotating machine  100 G which is of an inner rotor type and equipped with the first rotor  11 A, the second rotor  12 A, the third rotor  13 B, and the armature  101 . The first rotor  11 A, the second rotor  12 A, the third rotor  13 B, and the armature  101  are arranged in this order radially from the inside to the outside of the electric rotating machine  100 G. The third rotor  13 B, like the one in  FIG. 13 , has the magnets  13   b  disposed in a Halbach array. 
     Although not illustrated, the same flows of magnetic flux as those in  FIG. 21  are created, thus providing the same operation and beneficial effects as those in the first example of  FIG. 21 . 
     Although not illustrated, the electric rotating machine  100 G, like the second example of  FIG. 22 , may alternatively be of an outer rotor type which is designed to have the first rotor  11 A, the second rotor  12 A, the third rotor  13 B, and the armature  101  arranged in this order radially from the outside to the inside thereof. Other arrangements are identical with those in the first example of  FIG. 21 . The structure of the seventh example is also substantially identical in operation and beneficial effects with the first example. 
     Eighth Example 
       FIG. 28  illustrates the electric rotating machine  200 A which is of an axial type. Specifically, the electric rotating machine  200 A has the first rotor  21 , the second rotor  22 , the third rotor  23 , and the armature  201  disposed in this order coaxially with each other. In other words, the first rotor  21 , the second rotor  22 , the third rotor  23 , and the armature  201  are shaped to be arranged coaxially and adjacent each other in a multi-layer form. 
     The first rotor  21  structurally corresponds the first rotor  11  of the radial type of electric rotating machine, as described above. Similarly, the second rotor  22  structurally corresponds the second rotor  12  of the radial type of electric rotating machine. The third rotor  23  structurally corresponds the third rotor  13  of the radial type of electric rotating machine. In other words, the electric rotating machine  200 A may be engineered to have one of all possible combinations of the first rotors  11 A and  11 B, the second rotors  12 A to  12 F, and the third rotors  13 A to  13 E, as used in the first and second embodiments, which are modified to be arranged coaxially in the multi-layer form. The structure of the eighth example is substantially identical in operation and beneficial effects with the first to seventh examples. 
     Although not illustrated, the electric rotating machine  200 A, like the thirteenth example in the first embodiment, may alternatively be designed to have the second rotor  22  disposed outside the first rotor  21 . This structure is also substantially identical in operation and beneficial effects with the first example. 
     Modification 
     The radial type of electric rotating machine  100  may be engineered to have one of all possible combinations of the rotors, as described above, and the armature  101  or  201  mounted adjacent the third rotor  13 . Specifically, the electric rotating machine  100  may include one of all possible combinations of the first rotors  11 A and  11 B, the second rotors  12 A to  12 F, and the third rotors  13 A to  13 E in the first to twelfth examples of the first embodiment. Some such combinations have been discussed in the first to seventh examples of the second embodiment. The axial type of electric rotating machine  200  may also be designed to include a combination of the first rotor  21 , the second rotor  22 , and the third rotor  23  and have the armature  201  disposed to face the third rotor  23 . One such combination has been discussed in the eighth example of the second embodiment. Any and all possible combinations of the above described rotors is identical in operation with and offers substantially the same beneficial effects as the first to twelfth examples of the first embodiment or the first to seventh examples of the second embodiment. 
     Third Embodiment 
       FIGS. 29 to 32  illustrate a plurality of examples of a power generator  500  engineered as a power unit for vehicles such as automobiles according to the third embodiment. The power generators  500 A to  500 D, as referred to below, are examples of the power generator  500  which are equipped with the radial type of electric rotating machine  100 , as described above. Each of  FIGS. 29  to  32  is a schematic view which is simplified in the same way, as referred to in the first and second embodiments, for better visibility thereof. Throughout the drawings, like reference numbers refer to like parts. The power transmitting members  501  to  503  and  506  to  513 , as discussed below, may be made of any material as long as they are connectable with rotors of the power generator  500 . For instance, the power transmitting members  501  to  503  and  506  to  513  may be implemented by one or a combination of a rotary shaft, a cam, a ring, a crank, a belt, a gear, a rack-and-pinion, and a torque converter. 
     First Example 
     The power generator  500 A is, as illustrated in  FIG. 29 , equipped with the electric rotating machine  100 A of  FIG. 21  and the power transmitting members  501  and  502 . The power transmitting member  501  works as a first power transmitting member joined to the second rotor  12 A to transmit the power only from or to the second rotor  12 A or bi-directionally between itself and the second rotor  12 A. The power transmitting member  502  works as a second power transmitting member joined to the first rotor  11 A to transmit the power only from or to the first rotor  11 A or bi-directionally between itself and the first rotor  11 A. One of the power transmitting members  501  and  502  is mechanically connected to the engine Eg such as an internal combustion engine illustrated in  FIG. 32 . The other of the power transmitting members  501  and  502  is mechanically connected to the axle  515  to which road wheels Wh are attached. The armature  101  is energized in response to a control signal Sig, as outputted from the rotation controller  520 , to control rotation of the rotors (mainly the speed of the third rotor  13 F). Even when the armature  101  is not operating or is deenergized, the first rotor  11 A and the second rotor  12 A are magnetically coupled together, thus enabling the power or torque to be transmitted therebetween. 
     The power transmitting mechanism  10  (i.e., the power transmitting mechanisms  10 A to  10 M), as referred to in the first embodiment, may be engineered to have either or both of the power transmitting members  501  and  502  coupled to the second rotor  12  and the first rotor  11 . Similarly, the electric rotating machine  100  (i.e., the electric rotating machines  100 A to  100 G), as referred to in the second embodiment, may be designed to have the same structure, as illustrated in  FIG. 29 , except for the rotation controller  520 . The same applies to the second to the fourth examples, as will be described below. 
     Although not illustrated, the power transmitting member  501  may alternatively be joined to the first rotor  11 A or the third rotor  13 F. Similarly, the power transmitting member  502  may alternatively be joined to the second rotor  12 A or the third rotor  13 F. In either case, the power is enabled to be transmitted between magnetically coupled two of the rotors  11 A,  12 A, and  13 F. 
     Second Example 
     The power generator  500 B is, as illustrated in  FIG. 30 , equipped with the electric rotating machine  300 A and the power transmitting members  503  and  506 . The electric rotating machine  300 A is an example of the electric rotating machine  300  and includes the power transmitting mechanism  10 A of  FIG. 1 , the rotor  102 , and the armature  101 . The rotor  102  is basically identical in structure with the third rotor  13 A except that the soft-magnetic cylinder  13   a  is disposed radially inside the magnets  13   b . The third rotor  13 A and the rotor  102  are disposed adjacent each other in an axial direction of the power generator  500 B (i.e., a lateral direction in  FIG. 30 ) and coupled together by the connecting member  504 . The first rotor  11 A, the second rotor  12 A, and the third rotor  13 A are arranged radially (i.e., the vertical direction in  FIG. 30 ). The power transmission mechanism  10 A and the rotor  102  are arranged axially (i.e., the lateral direction in  FIG. 30 ). 
     The power transmitting member  503  works as the first power transmitting member joined to the second rotor  12 A. The power transmitting member  506  works as the second power transmitting member joined to the first rotor  11 A. One of the power transmitting members  503  and  506  is mechanically connected to the engine Eg in  FIG. 32 . The other of the power transmitting members  503  and  506  is mechanically connected to the axle  515  to which road wheels Wh are attached. The connecting member  504 , as described above, connects between the third rotor  13 A and the rotor  102 . The connecting member  505  supports the rotor  102  to be rotatable relative to the power transmitting member  506 . The armature  101  is disposed so as to face the rotor  102 . The armature  101  is energized in response to the control signal Sig, as outputted from the rotation controller  520 , to control rotation of the rotors (mainly the rotor  102 ). Even when the armature  101  is not operating or is deenergized, the first rotor  11 A and the second rotor  12 A are magnetically coupled together, thus enabling the power or torque to be transmitted therebetween. 
     Although not illustrated, the power transmitting member  503  may alternatively be joined to the first rotor  11 A or the third rotor  13 A (or the rotor  102 ). Similarly, the power transmitting member  502  may alternatively be joined to the second rotor  12 A or the third rotor  13 A (or the rotor  102 ). A soft-magnetic material may also be disposed between the third rotor  13 A and the rotor  102  to unite them together. This eliminates the need for the connecting member  504 . In either case, the power is enabled to be transmitted between magnetically coupled two of the rotors  11 A,  12 A, and  13 A. 
     Third Example 
     The power generator  500 C is, as illustrated in  FIG. 31 , equipped with the electric rotating machine  300 B and the power transmitting members  507 ,  508 , and  509 . The electric rotating machine  300 B is an example of the electric rotating machine  300  and includes the first rotor  11 A, the second rotor  12 A, the third rotor  13 A, and the armature  101 . The first rotor  11 A, the second rotor  12 A, the third rotor  13 A have substantially the same structures as those of the power transmission mechanism  10 B in  FIG. 2 , respectively, and joined in the same way as in the power transmission mechanism  10 B. The third rotor  13 A is, however, shaped to be longer than the one in  FIG. 2  in the axial direction (i.e., the lateral direction in  FIG. 31 ) of the electric rotating machine  300 B. Additionally, the armature  101  is axially disposed adjacent the first rotor  11 A and the second rotor  12 A. The first rotor  11 A is joined to the power transmitting member  508 . The second rotor  12 A is joined to the power transmitting member  507 . The power transmitting members  507  and  508  are arranged coaxially with each other. The third rotor  13 A is joined to the power transmitting member  509 . 
     The power transmitting members  507  and  508  serve as the first power transmitting member. The power transmitting member  509  serves as the second power transmitting member. At least one of the power transmitting members  507 ,  508 , and  509  is mechanically connected to the engine Eg in  FIG. 32 . The others of the power transmitting members  507 ,  508 , and  509  are mechanically connected to the axle  515  to which road wheels Wh are attached. The armature  101  is energized in response to the control signal Sig, as outputted from the rotation controller  520 , to control rotation of the rotors (mainly the third rotor  13 A). Even when the armature  101  is not operating or is deenergized, the first rotor  11 A and the second rotor  12 A are magnetically coupled together, thus enabling the power or torque to be transmitted therebetween. 
     Although not illustrated, the power transmitting member  507  may alternatively be joined to the first rotor  11 A or the third rotor  13 A. Similarly, the power transmitting member  508  may alternatively be joined to the second rotor  12 A or the third rotor  13 A. The power transmitting member  509  may alternatively be joined to the first rotor  11 A or the second rotor  12 A. In either case, the power is enabled to be transmitted between magnetically coupled two of the rotors  11 A,  12 A, and  13 A. 
     Fourth Example 
     The power generator  500 D is, as illustrated in  FIG. 32 , equipped with the electric rotating machine  100 A of  FIG. 21 , the electric rotating machine  300 C, and the power transmitting members  510 ,  511 ,  512 , and  513 . The electric rotating machines  100 A and the  300 C are driven independently from each other in response to the control signals Sig transmitted from the rotation controller  520 . The layout of the electric rotating machines  100 A and  300 C is not limited to the illustrated one. The power generator  500 D may also be equipped with an additional electric rotating machine(s). 
     The second rotor  12 A of the electric rotating machine  100 A is joined to the engine Eg through the power transmitting member  510 . The first rotor  11 A is connectable to the electric rotating machine  300 C through the power transmitting members  511  and  513  and also to the axle  515  through the power transmitting members  511  and  512 . The gear  514  is mounted between the power transmitting member  512  and the axle  515 . The axle  515  has the wheels Wh affixed thereto. The electric rotating machine  300 C is equipped with the rotor  102  and the armature  101 , as illustrated in  FIG. 30 . The power transmitting member  510  serves as the first power transmitting member. The power transmitting member  511  serves as both the second power transmitting member and the third power transmitting member. The power transmitting member  512  serves as the third power transmitting member. 
     The transmission of power when the engine Eg and/or the electric rotating machine  100 A is driven will be described below. 
     When the engine Eg is run, the power, as generated thereby, is transmitted to the second rotor  12 A, so that it rotates. This causes the power to be transmitted from the second rotor  12 A to the first rotor  11 A. When the electric rotating machine  100 A is driven, the power, as produced by the armature  101 , works to rotate the third rotor  13 F, so that the power is transmitted to the first rotor  11 A. The power, as inputted to the first rotor  11 A, is enabled to be transferred to the electric rotating machine  300 C or the wheels Wh through one of lines, as indicated by arrows D 100 . Specifically, when the first rotor  11 A is mechanically connected to the power transmitting members  511  and  513 , the power, as outputted from the first rotor  11 A, works to rotate the rotor  102 , so that the electric rotating machine  300 C operates in an electric power generation mode. The electric power may be then stored in a battery. When the first rotor  11 A is mechanically connected to the power transmitting members  511  and  512 , the power, as outputted from the first rotor  11 A, works to rotate the wheels Wh through the axle  515 . 
     The transmission of power when the engine Eg and the electric rotating machine  300 A are driven will be described below. 
     The power generated by the engine Eg is, as described above, transmitted to the power transmitting member  511 . The power, as produced by the armature  101  of the electric rotating machine  300 C, works to rotate the rotor  102 , so that the power is transmitted to the power transmitting member  513 . The power, as inputted to the power transmitting member  511 , and the power, as inputted to the power transmitting member  513 , are combined together. Such resultant power is transmitted to the wheels Wh through the power transmitting member  512  and the axle  515 . The electric rotating machine  100 A may be energized simultaneously in the motor mode. In this case, the power, as outputted from the electric rotating machine  100 A, is added to the above resultant power. When being placed in the deenergized state, the electric rotating machine  100 A may be used in the electric power generation mode. 
     The transmission of power when the electric rotating machine  100 A and/or the electric rotating machine  300 A is driven will be described below. 
     When it is required to start the engine Eg, the electric rotating machine  100 A is energized. The power, as generated by the armature  101  of the electric rotating machine  100 A, works to rotate the third rotor  13 F. The power of the third rotor  13 F is then transmitted to the power transmitting member  510  through the second rotor  12 A and to the engine Eg, so that the engine Eg is started. The electric rotating machine  100 A, therefore, works as an engine starter. When it is required to actuate the electric rotating machine  300 C, the armature  101  of the electric rotating machine  300 C is energized. The power, as produced by the armature  101  of the electric rotating machine  300 C, is transmitted to the power transmitting member  513  through the rotor  102 . The electric rotating machine  300 C, therefore, works in the motor mode to drive the vehicle. As apparent from the above discussion, the system equipped with the rotation controller  520 , as illustrated in the  FIG. 32 , works to actuate the engine Eg, the electric rotating machine  100 A, and/or the electric rotating machine  300 C in the way, as described above, to establish the transmission of power or torque through the power transmitting members  511  to  513  to start the engine Eg, generate the electric power, and/or run the wheels Wh as required. 
     Modification 
     The radial type of electric rotating machine  100  or  300  may be engineered to have one of all possible combinations of the rotors, as described above, and the armature  101  mounted adjacent the third rotor  13 . Specifically, the electric rotating machine  100  may include one of all possible combinations of the first rotors  11 A and  11 B, the second rotors  12 A to  12 F, and the third rotors  13 A to  13 E in the first to twelfth examples of the first embodiment. Some such combinations have been discussed in the first to seventh examples of the second embodiment. The axial type of electric rotating machine  200  may be employed in addition to or instead of the radial type of electric rotating machines  100  and  300 . The electric rotating machine  200  may be designed to include a combination of the first rotor  21 , the second rotor  22 , and the third rotor  23  and have the armature  201  disposed to face the third rotor  23 . In other words, the power generator  500  for vehicles may be engineered to include one of all possible combinations of the electric rotating machines, as described above. In either modification, the power generator  500  offers substantially same beneficial effects as the first to twelfth examples of the first embodiment or the first to seventh examples of the second embodiment. 
     Other Embodiments 
     While the present invention has been disclosed in terms of the first to third embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims. The invention may be embodied as described below. 
     a) In the eighth example of the second embodiment, the electric rotating machine  200 A is, as illustrated in  FIG. 28 , equipped with only the armature  201 , but however, may alternatively be designed, like in  FIG. 33 , to have two armatures. The electric rotating machine  200 B in  FIG. 33  is an example of the axial type of electric rotating machine  200  and includes a single first rotor  21 , two second rotors  22 , two third rotors  23 , and two armatures  201  and  202 . The first rotor  21  is disposed at the middle of the electric rotating machine  200 B. The second rotors  22 , the third rotors  23 , and the armatures  201  and  202  are disposed symmetrically with respect to the first rotor  21  in the axial direction (i.e., the vertical direction in  FIG. 33 ) of the electrical rotating machine  200 B. The second rotors  22  may be mechanically joined together or not. Similarly, the third rotors  23  may be mechanically joined together or not. When the second rotors  22  are mechanically separate from each other, it enables the power to be transmitted between the second rotors  22 . The same is true for the third rotors  23 . Other arrangements are identical with those in the eighth example of the second embodiment. The structure of this modification is substantially identical in operation and beneficial effects with the eighth example of the second embodiment. 
     In the absence of the armatures  201  and  202 , the electrical rotating machine  200 B may be employed as the power transmission mechanism  20  identical in operation with the power transmission mechanism  20 A of  FIG. 18 . The second rotors  22  may be disposed at the middle of the electrical rotating machine  200 B. In this case, the electrical rotating machine  200 B may be employed as the power transmission mechanism  20  identical in operation with the power transmission mechanism  20 B of  FIG. 19 . The electrical rotating machine  100 A,  300 A, or  300 B in the third embodiment, as shown in  FIGS. 29 to 32 , may be replaced with or in addition to the electric rotating machine  200 B. 
     b) The first example of the third embodiment (i.e., the power generator  500 A), as described above in  FIG. 29 , has the power transmitting member  501  joined to the second rotor  12 A and the power transmitting member  502  joined to the first rotor  11 A, but however, may be engineered, as illustrated in  FIG. 34 , to have a switch or selector  530  disposed between the rotors  11 A and  12 A and power transmitting member  501  and a selector  531  disposed between the rotors  11 A and  12 A and power transmitting member  502 . The power generator  500 A may alternatively be equipped with only one of the switches  530  and  531 . The selector  530  works as a first selecting mechanism to switch a mechanical connection of the power transmitting member  501  among the first rotor  11 A, the second rotor  12 A, and the third rotor  13 F. Similarly, the selector  531  works as a second selecting mechanism to switch a mechanical connection of the power transmitting member  502  among the first rotor  11 A, the second rotor  12 A, and the third rotor  13 F. 
     Specifically, the selector  530  is responsive to the control signal Sig, as outputted from the rotation controller  520  of  FIG. 32 , to establish the mechanical connection of the power transmitting member  501  to one of the first rotor  11 A, the second rotor  12 A, and the third rotor  13 F. Similarly, the selector  531  is responsive to the control signal Sig, as outputted from the rotation controller  520 , to establish the mechanical connection of the power transmitting member  502  to one of the first rotor  11 A, the second rotor  12 A, and the third rotor  13 F. The rotation controller  520  works to control the operations of the selectors  530  and  531  so as not to simultaneously connect the same one of the first rotor  11 A, the second rotor  12 A, and the third rotor  13 F to both the power transmitting members  501  and  502 . For instance, when it is required for the selector  530  to make the connection between the first rotor  11 A and the power transmitting member  501 , the rotation controller  520  establishes the mechanical connection of the second rotor  12 A or the third rotor  13 F to the power transmitting member  502  through the selector  531  or disconnects the power transmitting member  502  from the rotors  11 A,  12 A, and  13 F. 
     The selectors  530  and  531  may be employed in any of the second to fourth examples of the third embodiment. One such example is illustrated in  FIG. 35 .  FIG. 32  shows the power generator  500 F that is a modification of the power generator  500 D of  FIG. 32 . Specifically, the power generator  500 F has the selector  532  disposed between the power transmitting member  510  and the rotors  11 A,  12 A, and  13 F and the selector  533  disposed between the power transmitting member  511  and the rotors  11 A,  12 A, and  13 F. The power generator  500 F may alternatively be equipped with only either of the selectors  532  and  533 . The selector  532  works as the first selecting mechanism. The selector  533  works as a second selecting mechanism. The power generator  500 F works to transmit the power only from the power transmitting member  510  to any of the rotors  11 A,  12 A, and  13 F or vice versa or bi-directionally between the power transmitting member  510  and any of the rotors  11 A,  12 A, and  13 F. The power generator  500 F also works to transmit the power only from the power transmitting member  511  to any of the rotors  11 A,  12 A, and  13 F or vice versa or bi-directionally between the power transmitting member  511  and any of the rotors  11 A,  12 A, and  13 F. 
     c) The magnets  13   b  of the third rotor  13 D in the tenth example of the first embodiment in  FIG. 15  are all magnetized radially toward the center of the power transmission mechanism  10 J, but may alternatively be magnetized radially outwardly. The number of pole pairs of the magnets  13   b  is m.  FIG. 36  shows the power transmission mechanism  10 P that is a modification of the power transmission mechanism  10 J. The power transmission mechanism  10 J includes the third rotor  13 L equipped with the magnets  13   b  whose number of pole pairs is m and which are magnetized in a circumferential direction of the third rotor  13 L. The orientations of every adjacent two of the magnets  13   b  are in opposite directions. Other arrangements are identical with those in the tenth example of the first embodiment. The structure of this modification may be altered in the same way as in the second to ninth examples of the first embodiment. Such modifications offer substantially the same beneficial effects. 
     Beneficial Effects 
     The above described first to third embodiments provide the following advantages. 
     1) The power transmission mechanism  10 , as described above, has the first rotor  11 , the second rotor  12 , and the third rotor  13  which are arranged to make a magnetic coupling among them. Similarly, the power transmission mechanism  20  has the first rotor  21 , the second rotor  22 , and the third rotor  23  which are arranged to make a magnetic coupling among them. The numbers of respective sets of the soft-magnetic blocks  11   a  to  11   d , the numbers of respective sets of the soft-magnetic blocks  12   a  to  12   g , and the numbers of respective sets of the magnets  13   b ,  13   c , and  13   e  are, as described above, selected to meet a relation of 2m=|k±n| (see  FIGS. 1 to 5 ,  8 ,  12  to  20 , and  36 ). The magnets  13   b ,  13   c , or  13   e  of each of the third rotors  13  and  23  are permanent magnets and arranged as a magnetic pole array functioning a field source to create magnetically transmitting torque. The soft-magnetic blocks of the other rotors, therefore, serve as magnetic inductor arrays. This arrangement makes each of the power transmission mechanisms  10  and  20  function as, for example, a magnetic gear, to achieve the transmission of power or torque. The soft-magnetic blocks  11   a  to  11   d  and  12   a  to  12   g  are formed as discrete pole segments which are separate from each other through air gaps or magnetically insulating material, thus reducing the leakage of magnetic flux from one of the pole segments to another to ensure good magnetic modulation.
 
2) One of the first rotor  11  or  21  and the second rotor  12  or  22  is disposed between other two of the first rotor  11  or  21 , the second rotor  12  or  22  and the third rotor  13  or  23  (see  FIGS. 1 to 5 ,  8 ,  12  to  20 , and  36 ). In this layout, the soft-magnetic blocks  11   a  to  11   d  and  12   a  to  12   g  work as magnetic inductors, thereby enhancing the magnetic modulation and improving the ability of transmission of power.
 
3) In the structure, as illustrated in  FIGS. 1 ,  3  to  5 ,  8 ,  12  to  16 ,  20 , and  36 , in which the first rotor  11  or  21 , the second rotor  12  or  22 , and the third rotor  13  or  23  are arranged to overlap radially, the third rotor  13  or  23  is disposed most outwardly. This layout permits the size or area of the magnets  13   b ,  13   c , or  13   e  of the third rotor  13  or  23  to be increased, which results in increased ability of the field system, which will enhance the transmission of power.
 
4) The structure in which one of the first rotor  11  or  21  and the second rotor  12  or  22  is disposed between other two of the first rotor  11  or  21 , the second rotor  12  or  22 , and the third rotor  13  or  23 , as illustrated in, for example,  FIGS. 5 and 6 , has the one have non-planar or uneven side surfaces. This reduces the leakage of magnetic flux in each of sets of the soft-magnetic blocks  11   a  to  11   d  and  12   a  to  12   g , thus enhancing the ability of transmission of power.
 
5) In the structure in which the first rotor  11  or  21 , the second rotor  12  or  22 , and the third rotor  13  or  23  are disposed in three circular arrangements, and one of the first rotor  11  or  21  and the second rotor  12  or  22  is disposed on either of an outer or inner side of the three circular arrangements, the one has soft-magnetic members (i.e., one of sets of the soft-magnetic blocks  11   a  to  11   d  or  12   a  to  12   g ) which are non-rectangular in shape, such as trapezoid or fan-shape, as illustrated in, for example,  FIGS. 1 to 8 ,  12  to  20 , and  36  (especially see  FIG. 7 ), with the longest side surfaces thereof facing the middle of the three circular arrangements. This layout results in a decrease in degree of magnetic resistance between each of the longest side surfaces and a corresponding adjacent one of side surfaces of an adjacent one of the first rotor  11  or  21 , the second rotor  12  or  22 , and the third rotor  13  or  23 , thus enhancing the magnetic modulation and improving the ability of transmission of power.
 
6) Either or both of the first rotor  11  or  21  and the second rotor  12  or  22  have a plurality of soft-magnetic members (i.e., the soft-magnetic blocks  11   a  to  11   d  or  12   a  to  12   g ), illustrated in, for example,  FIGS. 8 to 10(   c )), which are jointed or assembled together in a circle by fasteners (i.e., the bridges  12   h  or  12   j ), thereby enhancing the stiffness or mechanical strength of the circular assembly of the soft-magnetic members.
 
7) Either or both of the first rotor  11  or  21  and the second rotor  12  or  22  have a plurality of soft-magnetic members (i.e., the soft-magnetic blocks  11   a  to  11   d  or  12   a  to  12   g ), illustrated in, for example,  FIGS. 11(   a ) to  11 ( c )), which are jointed or assembled together in a circle by a fastener (e.g., the fastener  12   k ). Specifically, the fastener is made of non-magnetic material and has the soft-magnetic members embedded therein. The soft-magnetic members (i.e., the soft-magnetic blocks  11   a  to  11   d  or  12   a  to  12   g ) serve as magnetic inductors, while the fastener serves as a non-magnetic member. This enables a magnetic gear (i.e., the power transmission mechanism) to be produced which has a high mechanical strength and an enhanced ability of transmitting torque.
 
8) In the structure in which one of the first rotor  11  or  21 , the second rotor  12  or  22 , and the third rotors  13  or  23  is disposed between other two of them, as illustrated in, for example,  FIGS. 1 to 5 ,  8 ,  12  to  20 , and  36 , the one has the number of pole pairs which is greater than those of the other two. In other words, one of the first rotor  11  or  21 , the second rotor  12  or  22 , and the third rotors  13  or  23  which is the greatest in number of pole pairs is disposed at the middle of three arrangements of the first rotor  11  or  21 , the second rotor  12  or  22 , and the third rotors  13  or  23 , thus achieving good magnetic modulation and enhancing the ability of torque transmission.
 
9) The third rotor  13  or  23  is, as illustrated in, for example,  FIGS. 15 ,  20 , and  36 , designed to have a soft-magnetic material (i.e., the soft-magnetic block  13   a ) disposed between adjacent two of magnetics (i.e., the magnets  13   b ,  13   c , or  13   e ) which are magnetized in a circumferential direction or a radial direction of the third rotor  13  or  23 . For instance, the third rotor  13  or  23  is designed to have at least two soft-magnetic members and at least two magnets which are arranged alternately. This permits the size or area of the magnets of the third rotor  13  or  23  to be increased, thus resulting in increased ability of the field system, which will enhance the transmission of power.
 
10) The magnets  13   b ,  13   c , or  13   e  of the third rotor  13  or  23  are made of material showing an electrical resistivity of 3 μΩm or more. This results in a decrease in eddy current generated in the magnets  13   b ,  13   c , or  13   e , thus decreasing the amount of heat arising from the eddy current to ensure the stability in operation of the magnets  13   b ,  13   c , or  13   e.  
 
11) The magnets  13   c  of the third rotor  13  or  23 , as illustrated in, for example,  FIG. 16 , are each made up of a plurality of magnetic segments  13   d  which are arranged continuously, for example, in a matrix, and all of which are magnetized in the same direction. Every adjacent two of the magnetic segments  13   d  are electrically isolated from each other. This structure of the third rotor  13  or  23  has each of assemblies of the magnetic segments  13   d  function as one of the magnets  13   c  and serves to create a flow of eddy current only within each of the magnetic segments  13   d , thereby improving the ability of transmission of torque. At least one of the magnets  13   c  may be made up of the plurality of magnetic segments  13   d.