Abstract:
A magnet machine includes a magnet rotor. The rotor includes a sleeve and a magnet. The magnet is positioned within the sleeve. A highly electrically conductive, nonmagnetic shield surrounds the magnet. The shield reduces rotor eddy current losses and lowers rotor operating temperature, thereby improving efficiency of the machine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This patent application claims the priority of provisional patent application serial No. 60/245,697, filed Nov. 2, 2000, and serial No. 60/246,380 filed Nov. 7, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to magnetic rotary machines. More particularly, the invention relates to a rotor system for limiting eddy current losses and lowering operating temperature.  
           [0004]    2. Discussion of the Background  
           [0005]    Magnetic rotary machines include a stator with a rotatable rotor positioned therein and supported by bearings.  
           [0006]    The rotor may be subject to eddy current losses caused by magnetic field harmonics. The term eddy current losses means heat generated by eddy currents. Magnetic field harmonics are oscillations in the magnetic field at any frequency other than the fundamental frequency. The term “winding harmonics” means magnetic field harmonics caused by stator windings. The term “tooth ripple harmonics” are magnetic field harmonics which occur in a stator with teeth, and which depend upon stator tooth-slot spatial distribution.  
           [0007]    What is needed is a technique to reduce eddy current losses and lower operating temperature of the magnet.  
         SUMMARY OF THE INVENTION  
         [0008]    In one aspect of the invention, a turbogenerator system is described wherein the system includes (1) a turbine mounted for rotation on a shaft; (2) a sleeve coupled with said shaft for rotation therewith; (3) a stator surrounding said sleeve; (4) at least one permanent magnet mounted within said sleeve; and (5) a shield surrounding said at least one permanent magnet, said shield made of electrically conductive nonmagnetic material.  
           [0009]    In another aspect of the invention, a generator/motor is described wherein the generator/motor includes (1) a stator; (2) a sleeve mounted for rotation within said stator; (3) at least one permanent magnet positioned within said sleeve for rotation therewith; and (4) a shield surrounding said at least one permanent magnet to rotate therewith, said shield made of electrically conductive nonmagnetic material.  
           [0010]    In another aspect of the invention, a permanent magnet apparatus is described wherein the apparatus includes (1) a stator; and (2) a permanent magnet rotor mounted for rotation within said stator, said rotor including a permanent magnet, an electrically conductive nonmagnetic shield and a sleeve.  
           [0011]    In another aspect of the invention, a permanent magnet rotor is described wherein the apparatus includes (1) a cylindrical permanent magnet having a cylindrical permanent magnet outer surface; (2) an annular shield having an inner annular surface in contact with said cylindrical permanent magnet outer surface, said annular shield having an annular shield outer surface; (3) an annular sleeve having an annular sleeve inner surface in contact with said annular shield outer surface; and (4) wherein electrical resistivity of said shield is lower than electrical resistivity of said sleeve.  
           [0012]    In another aspect of the invention, a method for reducing eddy current losses in a permanent magnet rotor is described wherein the method includes (1) providing a permanent magnet stator; (2) providing a permanent magnet rotor designed to rotate about an axis disposed within said stator, said rotor including a permanent magnet, an electrically conductive nonmagnetic shield and a sleeve, wherein said shield has portions positioned inside said sleeve; and (3) rotating said permanent magnet rotor such that said shield reduces said eddy current losses.  
           [0013]    In another aspect of the invention, a method includes (1) providing a cylindrical permanent magnet having a cylindrical permanent magnet outer surface; (2) providing an annular shield having an inner annular surface in contact with said cylindrical permanent magnet outer surface, said annular shield having an annular shield outer surface; (3) providing an annular sleeve having an annular sleeve inner surface in contact with said annular shield outer surface, wherein resistivity of said shield is lower than resistivity of said sleeve; and (4) generating eddy currents in said shield such that said eddy current losses are reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Use of the present invention reduces rotor eddy current losses in a magnet rotary machine.  
         [0015]    Use of the present invention reduces operating temperatures in a magnet rotary machine.  
         [0016]    A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0017]    [0017]FIG. 1A is perspective view, partially in section, of an integrated turbogenerator system;  
         [0018]    [0018]FIG. 1B is a magnified perspective view, partially in section, of the motor/generator portion of the integrated turbogenerator of FIG. 1A;  
         [0019]    [0019]FIG. 1C is an end view, from the motor/generator end, of the integrated turbogenerator of FIG. 1A;  
         [0020]    [0020]FIG. 1D is a magnified perspective view, partially in section, of the combustor-turbine exhaust portion of the integrated turbogenerator of FIG. 1A;  
         [0021]    [0021]FIG. 1E is a magnified perspective view, partially in section, of the compressor-turbine portion of the integrated turbogenerator of FIG. 1A;  
         [0022]    [0022]FIG. 2 is a block diagram schematic of a turbogenerator system including a power controller having decoupled rotor speed, operating temperature, and DC bus voltage control loops;  
         [0023]    [0023]FIG. 3 a  is a cross-sectional view, taken through the permanent magnet generator portion of the turbogenerator of FIG. 1, of an alternate embodiment of permanent magnet rotor including a shield in accordance with the present invention;  
         [0024]    [0024]FIG. 3 b  is a perspective view of an alternate embodiment of permanent magnet rotor including a shield extended beyond a magnet in accordance with the present invention;  
         [0025]    [0025]FIG. 3 c  is a partial perspective view of an alternate embodiment of one end of permanent magnet rotor including a shield formed to cover ends of the permanent magnet in accordance with the present invention.  
         [0026]    [0026]FIG. 3 d  is a sectional view of an alternate embodiment of permanent magnet rotor including shield located outside the sleeve.  
         [0027]    [0027]FIG. 3 e  is a sectional view of an alternate embodiment of permanent magnet rotor including shield disposed at the center of the rotor.  
         [0028]    [0028]FIG. 3 f  is a sectional view of an alternate embodiment of permanent magnet rotor including sleeve made of electrically conductive nonmagnetic material.  
         [0029]    [0029]FIG. 4 a  is a sectional view of an alternate embodiment of permanent magnet rotor including a central shaft, magnet, sleeve and a shield disposed between magnet and shaft.  
         [0030]    [0030]FIG. 4 b  is a sectional view of an alternate embodiment of permanent magnet rotor including a central shaft, magnet, sleeve and a shield disposed between magnet and sleeve.  
         [0031]    [0031]FIG. 4 c  is a sectional view of an alternate embodiment of permanent magnet rotor including a central shaft, magnet and a shield disposed between magnet and shaft. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]    Referring now to the drawings, like reference numerals designate identical or corresponding parts throughout the several views.  
         [0033]    Mechanical Structural Embodiment of a Turbogenerator  
         [0034]    With reference to FIG. 1A, an integrated turbogenerator  1  according to the present invention generally includes motor/generator section  10  and compressor-combustor section  30 . Compressor-combustor section  30  includes exterior can  32 , compressor  40 , combustor  50  and turbine  70 . A recuperator  90  may be optionally included.  
         [0035]    Referring now to FIG. 1B and FIG. 1C, in an embodiment of the present disclosure, motor/generator section  10  may be a permanent magnet motor generator having a permanent magnet rotor or sleeve  12 . Throughout the present disclosure rotor or sleeve  12  is referred to as a permanent magnet rotor, any suitable rotor technology may be used including wound rotors. Permanent magnet rotor or sleeve  12  may contain a permanent magnet  12 M. Permanent magnet rotor or sleeve  12  and the permanent magnet disposed therein are rotatably supported within permanent magnet motor/generator stator  14 . Preferably, one or more compliant foil, fluid film, radial, or journal bearings  15 A and  15 B rotatably support permanent magnet rotor or sleeve  12  and the permanent magnet disposed therein. All bearings, thrust, radial or journal bearings, in turbogenerator  1  may be fluid film bearings or compliant foil bearings. Motor/generator housing  16  encloses stator heat exchanger  17  having a plurality of radially extending stator cooling fins  18 . Stator cooling fins  18  connect to or form part of stator  14  and extend into annular space  10 A between motor/generator housing  16  and stator  14 . Wire windings  14 W exist on permanent magnet motor/generator stator  14 .  
         [0036]    Referring now to FIG. 1D, combustor  50  may include cylindrical inner wall  52  and cylindrical outer wall  54 . Cylindrical outer wall  54  may also include air inlets  55 . Cylindrical walls  52  and  54  define an annular interior space  50 S in combustor  50  defining an axis  51 . Combustor  50  includes a generally annular wall  56  further defining one axial end of the annular interior space of combustor  50 . Associated with combustor  50  may be one or more fuel injector inlets  58  to accommodate fuel injectors which receive fuel from fuel control element  50 P as shown in FIG. 2, and inject fuel or a fuel air mixture to interior of  50 S combustor  50 . Inner cylindrical surface  53  is interior to cylindrical inner wall  52  and forms exhaust duct  59  for turbine  70 .  
         [0037]    Turbine  70  may include turbine wheel  72 . An end of combustor  50  opposite annular wall  56  further defines an aperture  71  in turbine  70  exposed to turbine wheel  72 . Bearing rotor  74  may include a radially extending thrust bearing portion, bearing rotor thrust disk  78 , constrained by bilateral thrust bearings  78 A and  78 B. Bearing rotor  74  may be rotatably supported by one or more journal bearings  75  within center bearing housing  79 . Bearing rotor thrust disk  78  at the compressor end of bearing rotor  74  is rotatably supported preferably by a bilateral thrust bearing  78 A and  78 B. Journal or radial bearing  75  and thrust bearings  78 A and  78 B may be fluid film or foil bearings.  
         [0038]    Turbine wheel  72 , Bearing rotor  74  and Compressor impeller  42  may be mechanically constrained by tie bolt  74 B, or other suitable technique, to rotate when turbine wheel  72  rotates. Mechanical link  76  mechanically constrains compressor impeller  42  to permanent magnet rotor or sleeve  12  and the permanent magnet disposed therein causing permanent magnet rotor or sleeve  12  and the permanent magnet disposed therein to rotate when compressor impeller  42  rotates.  
         [0039]    Referring now to FIG. 1E, compressor  40  may include compressor impeller  42  and compressor impeller housing  44 . Recuperator  90  may have an annular shape defined by cylindrical recuperator inner wall  92  and cylindrical recuperator outer wall  94 . Recuperator  90  contains internal passages for gas flow, one set of passages, passages  33  connecting from compressor  40  to combustor  50 , and one set of passages, passages  97 , connecting from turbine exhaust  80  to turbogenerator exhaust output  2 .  
         [0040]    Referring again to FIG. 1B and FIG. 1C, in operation, air flows into primary inlet  20  and divides into compressor air  22  and motor/generator cooling air  24 . Motor/generator cooling air  24  flows into annular space  10 A between motor/generator housing  16  and permanent magnet motor/generator stator  14  along flow path  24 A. Heat is exchanged from stator cooling fins  18  to generator cooling air  24  in flow path  24 A, thereby cooling stator cooling fins  18  and stator  14  and forming heated air  24 B. Warm stator cooling air  24 B exits stator heat exchanger  17  into stator cavity  25  where it further divides into stator return cooling air  27  and rotor cooling air  28 . Rotor cooling air  28  passes around stator end  13 A and travels along rotor or sleeve  12 . Stator return cooling air  27  enters one or more cooling ducts  14 D and is conducted through stator  14  to provide further cooling. Stator return cooling air  27  and rotor cooling air  28  rejoin in stator cavity  29  and are drawn out of the motor/generator  10  by exhaust fan  11  which is connected to rotor or sleeve  12  and rotates with rotor or sleeve  12 . Exhaust air  27 B is conducted away from primary air inlet  20  by duct  10 D.  
         [0041]    Referring again to FIG. 1E, compressor  40  receives compressor air  22 . Compressor impeller  42  compresses compressor air  22  and forces compressed gas  22 C to flow into a set of passages  33  in recuperator  90  connecting compressor  40  to combustor  50 . In passages  33  in recuperator  90 , heat is exchanged from walls  98  of recuperator  90  to compressed gas  22 C. As shown in FIG. 1E, heated compressed gas  22 H flows out of recuperator  90  to space  35  between cylindrical inner surface  82  of turbine exhaust  80  and cylindrical outer wall  54  of combustor  50 . Heated compressed gas  22 H may flow into combustor  54  through sidewall ports  55  or main inlet  57 . Fuel (not shown) may be reacted in combustor  50 , converting chemically stored energy to heat. Hot compressed gas  51  in combustor  50  flows through turbine  70  forcing turbine wheel  72  to rotate. Movement of surfaces of turbine wheel  72  away from gas molecules partially cools and decompresses gas  51 D moving through turbine  70 . Turbine  70  is designed so that exhaust gas  107  flowing from combustor  50  through turbine  70  enters cylindrical passage  59 . Partially cooled and decompressed gas in cylindrical passage  59  flows axially in a direction away from permanent magnet motor/generator section  10 , and then radially outward, and then axially in a direction toward permanent magnet motor/generator section  10  to passages  98  of recuperator  90 , as indicated by gas flow arrows  108  and  109  respectively.  
         [0042]    In an alternate embodiment, low pressure catalytic reactor  80 A may be included between fuel injector inlets  58  and recuperator  90 . Low pressure catalytic reactor  80 A may include internal surfaces (not shown) having catalytic material (e.g., Pd or Pt, not shown) disposed on them. Low pressure catalytic reactor  80 A may have a generally annular shape defined by cylindrical inner surface  82  and cylindrical low pressure outer surface  84 . Unreacted and incompletely reacted hydrocarbons in gas in low pressure catalytic reactor  80 A react to convert chemically stored energy into additional heat, and to lower concentrations of partial reaction products, such as harmful emissions including nitrous oxides (NOx).  
         [0043]    Gas  110  flows through passages  97  in recuperator  90  connecting from turbine exhaust  80  or catalytic reactor  80 A to turbogenerator exhaust output  2 , as indicated by gas flow arrow  112 , and then exhausts from turbogenerator  1 , as indicated by gas flow arrow  113 . Gas flowing through passages  97  in recuperator  90  connecting from turbine exhaust  80  to outside of turbogenerator  1  exchanges heat to walls  98  of recuperator  90 . Walls  98  of recuperator  90  heated by gas flowing from turbine exhaust  80  exchange heat to gas  22 C flowing in recuperator  90  from compressor  40  to combustor  50 .  
         [0044]    Turbogenerator  1  may also include various electrical sensor and control lines for providing feedback to power controller  201  and for receiving and implementing control signals as shown in FIG. 2.  
         [0045]    Alternative Mechanical Structural Embodiments of the Integrated Turbogenerator  
         [0046]    The integrated turbogenerator disclosed above is exemplary. Several alternative structural embodiments are known.  
         [0047]    In one alternative embodiment, air  22  may be replaced by a gaseous fuel mixture. In this embodiment, fuel injectors may not be necessary. This embodiment may include an air and fuel mixer upstream of compressor  40 .  
         [0048]    In another alternative embodiment, fuel may be conducted directly to compressor  40 , for example by a fuel conduit connecting to compressor impeller housing  44 . Fuel and air may be mixed by action of the compressor impeller  42 . In this embodiment, fuel injectors may not be necessary.  
         [0049]    In another alternative embodiment, combustor  50  may be a catalytic combustor.  
         [0050]    In another alternative embodiment, geometric relationships and structures of components may differ from those shown in FIG. 1A. Permanent magnet motor/generator section  10  and compressor/combustor section  30  may have low pressure catalytic reactor  80 A outside of annular recuperator  90 , and may have recuperator  90  outside of low pressure catalytic reactor  80 A. Low pressure catalytic reactor  80 A may be disposed at least partially in cylindrical passage  59 , or in a passage of any shape confined by an inner wall of combustor  50 . Combustor  50  and low pressure catalytic reactor  80 A may be substantially or completely enclosed with an interior space formed by a generally annularly shaped recuperator  90 , or a recuperator  90  shaped to substantially enclose both combustor  50  and low pressure catalytic reactor  80 A on all but one face.  
         [0051]    Alternative Use of the Invention Other than in Integrated Turbogenerators  
         [0052]    An integrated turbogenerator is a turbogenerator in which the turbine, compressor, and generator are all constrained to rotate based upon rotation of the shaft to which the turbine is connected. The invention disclosed herein is preferably but not necessarily used in connection with a turbogenerator, and preferably but not necessarily used in connection with an integrated turbogenerator.  
         [0053]    Turbogenerator System Including Controls  
         [0054]    Referring now to FIG. 2, a preferred embodiment is shown in which a turbogenerator system  200  includes power controller  201  which has three substantially decoupled control loops for controlling (1) rotary speed, (2) temperature, and (3) DC bus voltage. A more detailed description of an appropriate power controller is disclosed in U.S. patent application Ser. No. 09/207,817, filed Dec. 8, 1998 in the names of Gilbreth, Wacknov and Wall, and assigned to the assignee of the present application which is incorporated herein in its entirety by this reference.  
         [0055]    Referring still to FIG. 2, turbogenerator system  200  includes integrated turbogenerator  1  and power controller  201 . Power controller  201  includes three decoupled or independent control loops.  
         [0056]    A first control loop, temperature control loop  228 , regulates a temperature related to the desired operating temperature of primary combustor  50  to a set point, by varying fuel flow from fuel control element  50 P to primary combustor  50 . Temperature controller  228 C receives a temperature set point, T*, from temperature set point source  232 , and receives a measured temperature from temperature sensor  226 S connected to measured temperature line  226 . Temperature controller  228 C generates and transmits over fuel control signal line  230  to fuel pump  50 P a fuel control signal for controlling the amount of fuel supplied by fuel pump  50 P to primary combustor  50  to an amount intended to result in a desired operating temperature in primary combustor  50 . Temperature sensor  226 S may directly measure the temperature in primary combustor  50  or may measure a temperature of an element or area from which the temperature in the primary combustor  50  may be inferred.  
         [0057]    A second control loop, speed control loop  216 , controls speed of the shaft common to the turbine  70 , compressor  40 , and motor/generator  10 , hereafter referred to as the common shaft, by varying torque applied by the motor generator to the common shaft. Torque applied by the motor generator to the common shaft depends upon power or current drawn from or pumped into windings of motor/generator  10 . Bi-directional generator power converter  202  is controlled by rotor speed controller  216 C to transmit power or current in or out of motor/generator  10 , as indicated by bi-directional arrow  242 . A sensor in turbogenerator  1  senses the rotary speed on the common shaft and transmits that rotary speed signal over measured speed line  220 . Rotor speed controller  216  receives the rotary speed signal from measured speed line  220  and a rotary speed set point signal from a rotary speed set point source  218 . Rotary speed controller  216 C generates and transmits to generator power/converter  202  a power conversion control signal on line  222  controlling generator power converter  202 &#39;s transfer of power or current between AC lines  203  (i.e., from motor/generator  10 ) and DC bus  204 . Rotary speed set point source  218  may convert to the rotary speed set point a power set point P* received from power set point source  224 .  
         [0058]    A third control loop, voltage control loop  234 , controls bus voltage on DC bus  204  to a set point by transferring power or voltage between DC bus  204  and any of (1) Load/Grid  208  and/or (2) energy storage device  210 , and/or (3) by transferring power or voltage from DC bus  204  to dynamic brake resistor  214 . A sensor measures voltage DC bus  204  and transmits a measured voltage signal over measured voltage line  236 . Bus voltage controller  234 C receives the measured voltage signal from voltage line  236  and a voltage set point signal V* from voltage set point source  238 . Bus voltage controller  234 C generates and transmits signals to bi-directional load power converter  206  and bi-directional battery power converter  212  controlling their transmission of power or voltage between DC bus  204 , load/grid  208 , and energy storage device  210 , respectively. In addition, bus voltage controller  234  transmits a control signal to control connection of dynamic brake resistor  214  to DC bus  204 .  
         [0059]    Power controller  201  regulates temperature to a set point by varying fuel flow, adds or removes power or current to motor/generator  10  under control of generator power converter  202  to control rotor speed to a set point as indicated by bi-directional arrow  242 , and controls bus voltage to a set point by (1) applying or removing power from DC bus  204  under the control of load power converter  206  as indicated by bi-directional arrow  244 , (2) applying or removing power from energy storage device  210  under the control of battery power converter  212 , and (3) by removing power from DC bus  204  by modulating the connection of dynamic brake resistor  214  to DC bus  204 .  
         [0060]    Referring to FIG. 3 a , it illustrates permanent magnet turbogenerator  1  including a permanent magnet motor or generator section  10 . Permanent magnet generator  10  includes stator  14  and rotatable permanent magnet shaft or rotor  28 . Stator  14  includes stator teeth  400 . Rotor  28  includes permanent magnet rotor sleeve  12 , shield  420 , permanent magnet  430  and journal bearings  15 A and  15 B. Permanent magnet  430  may be a single piece or multiple pieces held together inside sleeve  12 . The journal bearings rotatably support sleeve  12 . Journal bearings  15 A and  15 B are preferably a compliant foil hydrodynamic fluid film-type of bearing, such as that described in U.S. Pat. No. 5,427,455, which is hereby incorporated by reference in its entirety.  
         [0061]    Shield  420  may be disposed within rotor  28 . Shield  420  may be located between the exterior surface  435  of magnet  430  and the inner surface  440  of sleeve  12 . Accordingly, shield  420  would be spaced apart from stator teeth  430  by sleeve  12 . Shield  420  may be sized and shaped in a configuration that cooperates with sleeve  12  and magnet  430 . For example, sleeve  12  and magnet  430  may be substantially cylindrical, thus one preferred shape for shield  420  is a cylinder. In one embodiment, shield  420  would be plated to the inner surface of sleeve  12 . In another embodiment, shield  420  would be a foil wrapped around magnet  430 .  
         [0062]    Permanent magnet  430  may be inserted into permanent magnet sleeve  12  with a radial interference fit by any number of conventional techniques, such as heating permanent magnet sleeve  12  and supercooling permanent magnet  430 , hydraulic pressing, using pressurized lubricating fluids, tapering the inside diameter of the permanent magnet sleeve  12  and/or the outer diameter of the permanent magnet  430 , and other similar methods or combinations thereof.  
         [0063]    Referring now to FIG. 3 b , it illustrates rotor  28  including sleeve  12 , shield  420  and magnet  430 . Shield  420  is disposed between sleeve  12  and magnet  430 . Sleeve  12  and shield  430  have portions  12   a ,  12   b , and  420   a ,  420   b , respectively, that may extend beyond axial ends  430   a ,  430   b  of permanent magnet.  
         [0064]    Referring now to FIG. 3 c , it illustrates shield  420  and magnet  430 . Shield  420  is assembled over magnet  430 . Shield  420  may be interference fit to sleeve  12  by thermal fitting or hydraulic expansion of sleeve  12 . Excess material  420   a  on the ends of sleeve  12  is swaged to bring shield  420  down over the axial end faces of the magnet  430 . It should be appreciated, however, that any appropriate manufacturing method may be used. Excess material  420   a  may partially or completely cover the axial end faces of magnet  430 . Shield  420  may be formed as a lining material within an inner surface of sleeve  12 . Shield  420  may extend beyond ends of permanent magnet  430 .  
         [0065]    Shield  420  may include material that is highly conductive and non-magnetic compared to the sleeve and magnet such that eddy current losses are reduced. Examples of such material include copper, aluminum, silver, gold or any other suitably conductive and non-magnetic material may be used. Shield  420  comprises a material that has a conductivity that is at least about a magnitude of five times higher than the larger of the conductivities of materials comprising sleeve  12  and magnet  430 . Conductivity of shield material may typically be in the range of 6×10 6  to 7×10 6  Siemens/meter.  
         [0066]    Because shield  420  is highly conductive compared to sleeve  12  and magnet  430 , it provides a flow path for the eddy currents. Eddy currents are generated substantially in shield  420 , rather than in sleeve  12  and magnet  430 . Because shield  420  has low electrical resistance relative to the electrical resistance of sleeve  12  or the magnet  430 , a minimal amount of heat is generated by the eddy currents that flow in shield  420 .  
         [0067]    Eddy currents, which would otherwise penetrate through the sleeve and cause eddy current losses in the rotor, are greatly reduced by shield  420 . Shield  420 , however, may cause an increase in eddy current losses caused by tooth ripple harmonics. To minimize the effects of tooth ripple harmonics, shield  420  may be spaced by the annular width of the sleeve  12  from the stator teeth  400 . Consequently, the existence of shield  420  would reduce winding harmonics losses much more than it would increase tooth ripple losses. Therefore, the total eddy current loss would be reduced.  
         [0068]    Tables 1 and 2 below illustrate eddy current losses for various shield thicknesses. Table 1 includes harmonic losses data based on an INCONEL™ sleeve and a copper shield. INCONEL™ is a registered trademark of Inco Alloys International, Inc. Table 2 includes harmonic losses data based on a carbon fiber sleeve and a copper shield.  
                                                                                 TABLE 1                           Eddy Current Losses With An INCONEL ™ Sleeve*                Switching and Other Winding               Harmonics Caused Losses            Shield       Sleeve   Shield   Tooth   Total Eddy       Thickness   Magnet   Section   Section   Ripple   Current Loss       (mil)   Section (W)   (W)   (W)   Loss (W)   (W)                    20   1.1   27   19   39   86.1       15   1.9   26   23   29   79.9       10   4.1   27   30   21   82.1       5   13   34   39   15   101.0       0   65   94    0   10   169.0                          
 
         [0069]    [0069]                                                                                 TABLE 2                           Eddy Current Losses With A Carbon Fiber Sleeve*                Switching and Other Winding               Harmonics Caused Losses            Shield       Sleeve   Shield   Tooth   Total Eddy       Thickness   Magnet   Section   Section   Ripple   Current Loss       (mil)   Section (W)   (W)   (W)   Loss (W)   (W)                    20   1.3   0   22   2   25.3       15   2.3   0   27   1.6   30.9       10   5.2   0   37   1.2   43.4       5   19   0   57   0.7   76.7       0   140   0    0   0   140.0                            
         [0070]    Shield  420  may be formed in a thickness that provides acceptable electrical conductive performance. In a currently preferred embodiment, shield  420  is approximately 0.020 inches thick.  
         [0071]    Positioning shield  420  inside sleeve  12 , minimizes the total eddy current losses for various shield thicknesses. In examples shown in Tables 1 and 2, increasing the shield thickness from 0 to 20 mils per inch of diameter may slightly increase tooth ripple loss from 0 W to 2 W. However, a considerable reduction in total eddy current losses from 140.0 W to 25.3 W may be realized because a total of eddy current losses associated with the rotor is the based on “switching losses,” “winding harmonics losses” and “tooth ripple losses,” and the reduction in magnitude of “winding harmonics losses” considerably overcomes the increase in “tooth ripple losses.” 
         [0072]    Alternatively, referring to FIG. 3 d , shield  420  may be positioned outside sleeve  12 . To minimize potential tooth ripple loss and eddy current losses, shield  420  may be spaced at a sufficient distance from stator teeth  400 . Shield  420  may be spaced from the stator teeth  400  at a distance approximately equal to the annular width of the sleeve  12 . The spacing could be provided using a higher diameter stator or a lower diameter rotor. Shield  420  may have portions  420   a ,  420   b  that extend beyond axial ends  14   a ,  14   b  of permanent magnet  430 .  
         [0073]    Referring to FIG. 3 e , in accordance with another embodiment, shield  420  may be positioned as a central core and permanent magnet  430  may surround shield  420 . Permanent magnet  430  may be ring shaped, and may be a single piece or multiple pieces held together inside sleeve  12 . Shield  420  may be formed in a diameter that provides acceptable electrical conductive performance.  
         [0074]    In another embodiment illustrated in FIG. 3 f , sleeve  12  may be made of a highly conductive nonmagnetic material, and a separate shield may not be provided. Because sleeve  12  would be highly conductive compared to magnet  430 , sleeve  12  would provide a flow path for the eddy currents. Eddy currents would be generated substantially in sleeve  12 , rather than in magnet  430 . Because sleeve  12  would have low resistance relative to resistance of magnet  430 , a minimum amount of heat would be generated by the eddy current that would flow in sleeve  12 .  
         [0075]    Referring to FIG. 4 a , in accordance with another embodiment, rotor  500  may have a shaft  510  and journal bearings  15 A and  15 B rotatably supporting the shaft  510 . Rotor  500  would be disposed in motor/generator  10  and would be surrounded by stator  14 . Mechanical link  76  may mechanically constrain compressor impeller  42  to shaft  510  causing rotor  500  and the magnet disposed therein to rotate when compressor impeller  42  rotates. Shield  420  may be located between permanent magnet  430  and shaft  510 . Permanent magnet  430  may be ring shaped, and may be a single piece or multiple pieces held together inside sleeve  12 . Alternatively, referring to FIG. 4 b , shield  420  may be positioned between sleeve  12  and magnet  430 . In another alternate embodiment illustrated in FIG. 4 c , rotor  500  would include shaft  510  surrounded by permanent magnet  430 . Shield  420  would be located between permanent magnet  430  and shaft  510 . Journal bearings  15 A and  15 B would rotatably support shaft  510 .  
         [0076]    While the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.