Patent Publication Number: US-2003227231-A1

Title: Doubly-excited brushless alternator

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Technical Field of the Invention  
       [0002] The present invention relates generally to electrical machines, and, more particularly, to synchronous machines for generation of electric power in vehicles.  
       [0003] 2. Description of the Prior Art  
       [0004] Claw-pole, or Lundell alternators are almost the sole converters of mechanical into electrical energy in today&#39;s vehicles. Their simple construction—the rotor with only one, axially wound field coil for any number of pole pairs—along with a low manufacturing price make them the choice number one in the power range up to several kW. Beyond this power limit the rotor construction begins to show its weaknesses, the cooling problem of the field coil being the strongest of them. This is the main reason for numerous adaptations of conventional Lundell generator, most of them proposing additional permanent magnet(s) on the rotor side. Modified Lundell generators with annular permanent magnets are disclosed in U.S. Pat. No. 3,555,327, issued to Terry, in U.S. Pat. No. 4,882,515, issued to Radomski, in U.S. Pat. No. 4,959,577, issued to Radomski, U.S. Pat. No. 4,980,595, issued to Arora, and in U.S. Pat. No. 5,177,391, issued to Kusase. Several patents disclose modified Lundell alternators with tangentially oriented rotor magnets, including U.S. Pat. No. 5,693,995, issued to Syverson, U.S. Pat. No. 5,747,909, issued to Syverson and Francis, U.S. Pat. No. 5,753,989, issued to Syverson and Curtiss, and U.S. Pat. No. 6,236,134, issued to Syverson. Modified Lundell generators with radially mounted permanent magnets are disclosed among others in U.S. Pat. No. 5,130,595, issued to Arora, U.S. Pat. No. 5,552,651, issued to Radomski, U.S. Pat. No. 5,663,605, issued to Evans and Schultz, U.S. Pat. No. 5,710,471, issued to Syverson and Curtiss and in U.S. Pat. No. 6,455,978, issued to Krefta and Reiter. Probably the largest number of U.S. patents is granted to the disclosures related to modified Lundell alternators with permanent magnets between pole fingers, including U.S. Pat. No. 5,543,676, issued to York and Harris, U.S. Pat. No. 5,747,913, issued to Amlee, Matuoka, Rausch and Otipoby, U.S. Pat. No. 5,825,116, issued to Ishikawa, U.S. Pat. No. 5,892,313, issued to Harris and York, U.S. Pat. No. 5,965,967, issued to Liang, Miller and Xu, U.S. Pat. No. 5,973,435, issued to Irie and Ishikawa, U.S. Pat. No. 6,011,343, issued to Taniguchi, U.S. Pat. No. 6,013,968, issued to Lechner, Shendi and Briand, U.S. Pat. No. 6,144,138, issued to Ragaly and U.S. Pat. No. 6,448,680, issued to Akemakou. U.S. Pat. No. 5,793,143, issued to Harris and York is related to permanent magnets mounted on the rotor fan.  
       [0005] Although those designs just mentioned may have desirable features and may prove to be well manufacturable, alternative designs may provide superior power output. Thus, an alternator that is considerably robust, and that can provide significantly improved power output, particularly at low speeds, and which can do so in a simply manufacturable design, will provide advantages over the prior art.  
       SUMMARY OF THE INVENTION  
       [0006] The present invention provides an electric machine. The electrical machine includes an outer and an inner stator and a rotor rotateably mounted within a stator. The outer stator has two annular cores and the inner stator has a cylindrical core. The outer stator cores are coaxially mounted within the housing and shifted axially to each other. Each of the two outer stator cores can be laminated, and they are separated from each other by a ring of non-magnetic material. The outer stator cores are slotted, and carry a multiphase lap or wave armature winding in the slots. The inner, cylindrical stator comprises a magnetic core and an axially wound stationary coil which carries the field current. The magnetic core of the inner stator may be hollow. The rotor having an axis of rotation comprises a first magnetic flux modifying member having a plurality of axially extending pole-forming magnetic segments fixed on a ring of magnetic material and a second magnetic flux modifying member having a plurality of axially extending pole-forming magnetic segments fixed on a ring of magnetic material, the axially extending pole-forming magnetic segments of the first magnetic flux modifying member alternatively intermeshed with the axially extending pole-forming magnetic segments of the second magnetic flux modifying member. The magnetic flux modifying members in the disclosed machine change the field flux linked by the armature winding(s) as a function of the rotor shift, thus having a similar role as claw poles in a claw-pole or Lundell alternator. The pole-forming magnetic segments in the disclosed machine have a similar role as the pole fingers in a claw-pole, or Lundell alternator. The rotor is separated from the two cores of the outer stator by the outer air gap. Another, inner air gap exists between the rotor and inner stator. The second magnetic flux modifying member of the rotor is mounted on the rotor shaft and carries a non-magnetic hollow cylinder on the inner sides of the second pole-forming magnetic segments. The first magnetic flux modifying member is mounted on the non-magnetic hollow cylinder in such a manner, that the inner surfaces of the pole-forming magnetic segments of the first magnetic flux modifying member lie on the outer surface of the non-magnetic cylinder. The first magnetic flux modifying member has in addition an axial extension in a form of a hollow magnetic cylinder. Further, the rotor comprises a plurality of permanent magnets mounted on the said hollow magnetic cylinder of the axial extension of the first magnetic flux modifying member. The permanent magnets are radially magnetized. The N-pole magnets coincide tangentially with first pole fingers, and the S-pole magnets coincide tangentially with second pole fingers.  
       [0007] The embodiment of this invention is an alternator that provides a significantly higher power output, particularly at low speeds, over other claw pole alternator designs. The disclosed alternator is simply manufacturable, an advantage over many other designs. It does not have slip rings and brushes for the field coil, an advantageous feature over many other designs that increases robustness and helps much better utilize the limited packaging volume of the alternator. For these reasons, the present invention provides advantages over the prior art. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008] These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where:  
     [0009]FIG. 1 is a perspective view of an electric machine according to the embodiment of the present invention.  
     [0010]FIG. 2 is an exploded view of the rotor of the disclosed machine showing separately the rotor assembly and the inner stator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0011] Referring now to FIG. 1, an electrical machine according to the embodiment of the present invention will be described. The cores  1  and  1   a  of the outer stator have axially extending slots  11  and  12 , respectively, in which the armature multiphase winding (not shown) is disposed. Those skilled in the art will recognize that the axially extending slots  11  and  12  can carry any type of electric machine winding, such as concentric, lap, wave etc. Teeth  13  separate slots in the outer stator core  1 , and teeth  14  in the outer stator core la from each other. The outer stator cores  1  and  1   a  are preferably constructed of iron laminations, each punched with the cross-sectional features shown in FIG. 1. Although the outer stator cores  1  and  1   a  are preferably constructed of laminations, the present invention is not so limited. By way of example, the outer stator cores  1  and  1   a  can also be made solid, as opposed to laminated. Each stator winding conductor goes through a slot  11  in the outer stator core  1 , continues axially through the space between the outer stator cores  1  and  1   a  and then comes into a corresponding slot  12  in the outer stator core  1   a,  said corresponding slot  12  being coaxial with said slot  11  in the stator core  1 . The magnetically conducting rotor cylinder  8  carries a plurality of permanent magnets on its outer surface, said permanent magnets having alternating magnetic polarity. Permanent magnets  9  have all the N-polarity, and permanent magnets  10  have all the S-polarity. Magnetic flux of permanent magnets  9  goes radially through the outer air gap into the teeth  13  of the outer stator core  1 , following through the stator core  1  yoke, back to the stator core  1  teeth, outer air gap, permanent magnets  10  and then to the rotor cylinder  8 .  
     [0012] The body of the inner stator  2  may have a coaxial hole  2   a  in order to allow for the rotor shaft to go along the whole machine. The inner stator has an axially wound coil  3  which carries the field current of the generator.  
     [0013] Referring now to FIG. 2, further details of the inner stator part and the rotor will be described. Magnetic flux generated by field current in coil  3  leaves the iron core of the inner stator  2  through the core lateral surface  2   c,  crosses the inner air gap, and enters radially into the ring  5  of the first magnetic flux modifying member, where it changes its direction. From the ring  5  the flux goes axially into the pole forming magnetic segments  7  of the first magnetic flux modifying member, said pole forming magnetic segments  7  all having the same magnetic polarity. In the pole forming magnetic segments  7  the flux changes its direction into radial, crosses the outer air gap and enters radially into teeth  14  of the outer stator core  1   a.  On its way through the outer stator core  1   a  the flux of the field coil goes further through the yoke, and than through the stator teeth  14  into the pole forming magnetic segments  6  of the second magnetic flux modifying member. The pole forming magnetic segments  6  have an opposite magnetic polarity to the polarity of the pole forming magnetic segments  7 . From the second pole forming magnetic segments  6  the magnetic flux goes into to the ring  4  and then radially through the inner air gap into the inner stator surface  2   b.    
     [0014] The field current flowing through the coil  3  can be either positive, or negative. The reason for such bi-directional current will become apparent during the upcoming discussion.  
     [0015] Referring to FIGS. 1 and 2 the operation of disclosed alternator will now be discussed. With current flow in one direction (say positive) in field coil  3 , all pole forming magnetic segments  7  will have magnetic N- polarity, and all pole forming magnetic segments  6  will have magnetic S-polarity. Since the magnets  9  all have N-polarity, and magnets  10  all have S-polarity, the flux in the outer stator core  1   a  induces at certain rotor speed the voltages in the stator winding conductors which have the same polarity as the voltages induced by the flux of permanent magnets  9  and  10 . As a result, the total induced voltage in each stator conductor increases with positive field current. If the field current becomes negative, the rotor fingers  7  become magnetically S-poles, and rotor fingers  6  magnetically N-poles. The induced voltages in stator conductor axial sections in stator cores  1  and  1   a  oppose each other, thus decreasing the total induced voltage in the armature winding.  
     [0016] It will be now appreciated that there has been presented a synchronous alternator with field coil and permanent magnets. The rotor of the presented machine is built out of magnetic flux modifying members along with a magnetic hollow cylinder. A plurality of permanent magnets is placed on the rotor magnetic hollow cylinder. The presented machine has two outer stator cores, said outer stator cores carrying a common armature winding, and an inner stator, said inner stator carrying an axially wound field coil.  
     [0017] While the invention has been described with particular reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements of the preferred embodiment without departing from invention. As is evident from the foregoing description, certain aspects of the invention are not limited to the particular details of the examples illustrated, and it is therefore contemplated that other modifications and applications will occur to those skilled in the art. It is accordingly intended that the claims shall cover all such modifications and applications as do not depart from the true spirit and scope of the invention.