Abstract:
A permanent magnet machine (PMM) comprises: a generally cylindrical permanent magnet (PM) rotor that comprises multiple PM rotor poles arranged around a rotor axis of rotation; and a stator with two generally cylindrical and concentric yokes, an inner yoke proximate the PM rotor with associated multiple inner poles and inner armature windings suitable for multiphase alternating current operation that form a PMM magnetic flux circuit, an outer yoke with associated multiple outer poles and outer control windings suitable for connection to a direct current source, with distal ends of the outer poles in contact with the inner yoke to form an external magnetic flux circuit that diverts magnetic flux from the PMM magnetic flux circuit; wherein application of increasing direct current to the outer windings results in increased magnetic reluctance of the external magnetic flux circuit, thereby causing the external magnetic flux circuit to divert less magnetic flux from the PMM magnetic flux circuit.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to electric machines that have a permanent magnet rotor, and more particularly to permanent magnet machines that have a control winding for magnetic flux regulation. 
       BACKGROUND OF THE INVENTION 
       [0002]    There have been various proposals for multiphase electric machines of the permanent magnet type that include magnetic flux regulation by way of a control current to regulate their electromotive force (EMF) in a generating mode and developed torque in a motor mode. However, such proposals have generally involved machines of complex and costly design that have excessive weight and poor heat dissipation. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention generally comprises a permanent magnet machine (PMM) comprising: a generally cylindrical permanent magnet (PM) rotor that comprises multiple PM rotor poles arranged around a rotor axis of rotation; and a stator with two generally cylindrical and concentric yokes, an inner yoke proximate the PM rotor with associated multiple inner poles and inner armature windings suitable for multiphase alternating current operation that form a PMM magnetic flux circuit, an outer yoke with associated multiple outer poles and outer windings suitable for connection to a direct current source, with distal ends of the outer poles in contact with the inner yoke to form an external magnetic flux circuit that diverts magnetic flux from the PMM magnetic flux circuit; wherein application of increasing direct current to the outer windings results in increased magnetic reluctance of the external magnetic flux circuit, thereby causing the external magnetic flux circuit to divert less magnetic flux from the PMM magnetic flux circuit. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a cut-away end view of a PMM according to a first possible embodiment of the invention. 
           [0005]      FIG. 2  shows alternate arrangements of the rotor for the PMM shown in  FIG. 1 . 
           [0006]      FIG. 3  is a cut-away end view of the PMM shown in  FIG. 1  that shows stator magnetic flux paths for a zero or low level of control current. 
           [0007]      FIG. 4  is a cut-away end view of the PMM shown in  FIG. 1  that shows stator magnetic flux paths for a high level of control current. 
           [0008]      FIG. 5  is a graphical representation of EMF as a function of control current for the PMM in a generating mode. 
           [0009]      FIG. 6  is a graphical representation of torque as a function of speed for the PMM in a motor mode. 
           [0010]      FIG. 7  is a cut-away end view of a PMM according to a second possible embodiment of the invention. 
           [0011]      FIG. 8  is a detailed cut-away end view of an adaptor for a PMM according to a third possible embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  is a cut-away end view of a PMM  2  according to a first possible embodiment of the invention. It comprises a PM rotor  4  that revolves about a rotor axis of rotation  6  and a stator  8 . The rotor  4  typically rotates with a coupled drive shaft  10  that has an axis of rotation coincident with the rotor axis  6 . By way of illustration only, the rotor  4  has four PM rotor poles  12  arranged about the rotor axis that comprise surface mounted PMs. The rotor  4  may have a different number of rotor poles  12 , and the rotor poles  12  may have a different configuration. For instance,  FIG. 2  shows the rotor  4  with two alternate configurations for the rotor poles  12 , one with the rotor poles  12  comprising PMs embedded in the rotor  4  and another with the rotor poles  12  comprising PMs mounted with associated ferromagnetic pole faces  14 . 
         [0013]    The stator  8  has two generally cylindrical yokes, an inner yoke  16  and an outer yoke  18 . The inner yoke  16  is proximate the PM rotor  4  and it has associated multiple inner poles  20  and inner armature windings  22  that are suitable for multiphase alternating current (AC) operation to form a PMM magnetic flux circuit. The outer yoke  18  envelopes the inner yoke  16  and it has associated multiple outer poles  24  and outer control windings  26  suitable for connection to a direct current control source, with distal ends  28  of the outer poles  24  in contact with an outer surface  30  of the inner yoke  16  to form an external magnetic flux circuit that diverts magnetic flux from the PMM magnetic flux circuit. 
         [0014]    The PMM  2  according to the first embodiment shown in  FIG. 1  has inner poles  20  and outer poles  24  of the salient type. By way of example only, the inner armature windings  22  and the outer control windings  26  are concentrated coils of one slot coil pitch. The external magnetic circuit, comprising the outer yoke  18  and the outer poles  24 , should have a very low level of magnetic reluctance so that a large portion of the magnetic flux in the PMM magnetic flux circuit will divert through it. Since the magnetic flux in the inner yoke  16  varies with time, the outer yoke  18  and the outer poles  24  should comprise ferromagnetic laminations or sintered magnetic powder. 
         [0015]    If the stator  8  comprises a homogeneous material and construction of magnetic permeability μ 1 , the outer yoke  18  should have a radial thickness h out  that is substantially greater than the radial thickness h in  of the inner yoke  16  to insure that the level of magnetic reluctance in the external magnetic circuit is suitably less than that of the PMM magnetic circuit. To satisfy this condition, it is preferable that the radial thickness h out  of the outer yoke  18  is greater than or equal to four times the radial thickness h in  of the inner yoke  16 . Alternatively, if the stator  8  does not comprise a homogeneous material and construction but the inner yoke has a magnetic permeability μ 1 , the outer yoke  18  may have approximately the same radial thickness as the inner yoke  16  if the outer yoke  18  comprises a material and construction such that it has a magnetic permeability μ 2  that is substantially greater than magnetic permeability μ 1 . In this case, it is preferable that the magnetic permeability μ 2  is greater than or equal to four time the magnetic permeability μ 1 . Of course, the outer yoke  18  may similarly have a combination of greater radial thickness and magnetic permeability than the inner yoke  16  to achieve the desired level of magnetic reluctance. The outer control windings  26  generally couple to each other in a series-connected or parallel-connected configuration so that a direct current control signal I C  applied to them will lower the magnetic permeability of the external magnetic circuit and therefore increase its magnetic reluctance in proportion to the level of I C . 
         [0016]    The number of outer poles  24  should be at least the number of PM rotor poles  12 . The PMM  2  in  FIG. 1  shows six of the outer poles  24  by way of example only. For this embodiment, with its inner armature windings  20  comprising concentrated coils of one slot coil pitch, the number of inner poles  24  should satisfy the condition 
         [0000]    
       
         
           
             
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         [0000]    wherein N is the number of inner poles  24 , 2p is the number of PM rotor poles  12 , m 1  is the number of phases of the multiphase AC of the inner armature windings  22 , GCD is the greatest common divisor of N and 2p, and k=1,2,3, . . . . For example, if the PMM  2  as shown in  FIG. 1  has N=6, m 1 =3 and 2p=4, then 
         [0000]    
       
         
           
             
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         [0000]    where GDC(6,4)=2 and k=1. 
         [0017]    In a generating mode, a prime mover (not shown) coupled to the drive shaft  10  rotates the PM rotor  4 . The magnetic flux Φ PM  that the rotating PM rotor generates in the stator  8  primarily flows through the external magnetic circuit. Thus, although the total magnetic flux Φ PM  equals the magnetic flux Φ of the PMM inner magnetic circuit plus the magnetic flux Φ C  of the external magnetic circuit, with little or no control current I C , Φ C  is approximately equal to Φ PM . In other words, the magnetic flux that the PM rotor  4  produces represented by 
         [0000]    
       
         
           
             
               
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         [0000]    generates a level of EMF in the stator  8  represented by EMF≈k E Φ PM n, where N C  is the total number of turns of the outer windings  24 , I C  is the control current, R is the magnetic reluctance of the outer yoke  18 , k E  is the EMF constant and n is the rotational speed of the PM rotor  4 . 
         [0018]      FIG. 3  is a cut-away end view of the PMM  2  shown in  FIG. 1  that shows stator magnetic flux paths for a zero or low level of control current I C  when operating in a generating mode within a three-phase alternating current system. PMM magnetic flux paths  32  pass through the PMM magnetic circuit in the inner yoke  16  and represent the magnetic flux Φ of the PMM magnetic circuit. External magnetic flux paths  34  pass through the external magnetic circuit in the outer yoke  18  and represent the magnetic flux Φ C  of the external magnetic circuit. Since the total magnetic flux Φ PM  equals the magnetic flux Φ of the PMM magnetic circuit plus the magnetic flux Φ C  of the external magnetic circuit, with little or no control current I C , Φ C  is approximately equal to Φ PM  because most of the magnetic flux travels through the external magnetic flux paths  34 . The low reluctance path of Φ C  through the external magnetic circuit allows the PM rotor  4  to induce a high level of EMF into the inner armature windings  22 . 
         [0019]      FIG. 4  is a cut-away end view of the PMM  2  shown in  FIG. 1  that shows stator magnetic flux paths for a high level of control current I C  in a generating mode. With a high level of control current I c , the magnetic reluctance of the external magnetic circuit represented by the external magnetic flux paths  34  is high, forcing most of the total magnetic flux Φ PM  through the PMM magnetic circuit represented by the PMM magnetic flux paths  32 . However, the high reluctance path of Φ through the PMM magnetic circuit forces the PM rotor  4  to induce a lower level of EMF into the inner windings  22 . 
         [0020]      FIG. 5  is a graphical representation of EMF as a function of control current I C  for the PMM  2  in a generating mode for two different rotational speeds of the PM rotor  4 . Constant speed line  36  represents a rotational speed n 1  and constant speed line  38  represents a rotational speed n 2  that is less than speed n 1 . By adjusting the level of control current I C  between level I C2 , represented by point  40 , and level I C1 , represented by point  42 , along constant EMF line  44 , it is possible to maintain constant EMF output from the PMM  2  between speeds n 2  and n 1 . 
         [0021]    In a motor mode, increasing the level of the control current I C  serves to reduce the torque that the PMM  2  develops in the PM rotor  4 , thereby allowing a high degree of developed torque at low rotational speeds of the PM rotor  4  and high speed operation at lower levels of developed torque.  FIG. 6  is a graphical representation of torque as a function of speed for the PMM  2  with three different levels of control current I C . Constant current line  46  represents torque as a function of speed for I C  equal to zero, constant current line  48  represents torque as a function of speed for an intermediate level of control current I C1 , and constant current line  50  represents torque as a function of speed for a high level of control current I C2  that is greater than I C1 . It is evident that with I C  equal to zero, the PMM  2  develops high torque, but its maximum speed is limited. With control current I C1 , the PMM  2  develops a lower level of torque but may reach higher speed. With control current I C2 , the PMM  2  develops still lower torque but may reach a still higher speed. 
         [0022]      FIG. 7  is a cut-away end view of a PMM  52  according to a second possible embodiment of the invention. It is much the same as the PMM  2  shown in  FIG. 1 , but multiple slots  54  in the inner yoke  16  form non-salient inner poles  56 . This embodiment allows the use of distributed multiple inner armature windings  58 , in which case the number of alternating current phases, selection of slots  54  and inner armature windings  58  determine the effective number of poles coupled to the inner yoke  16 . 
         [0023]      FIG. 8  is a detailed cut-away end view of an adaptor  60  for a PMM (not shown) according to a third possible embodiment of the invention. It is simply the outer yoke  18  with associated multiple outer poles  24  and outer control windings  26  suitable for connection to a direct current control source, with distal ends  28  of the outer poles  24  in contact with an outer surface of the PMM to form an external magnetic flux circuit that diverts magnetic flux from the PMM magnetic flux circuit of the PMM. The adaptor  60  may be useful for converting a standard PMM to a regulated one for use in either a generating mode or motor mode. 
         [0024]    The described embodiments of the invention are only some illustrative implementations of the invention wherein changes and substitutions of the various parts and arrangement thereof are within the scope of the invention as set forth in the attached claims.