Abstract:
A cooling system for an electric machine is provided. The cooling system includes an airflow diverter feature configured to provide even distribution of airflow to the electric machine. The airflow diverter feature includes a circular disk mounted to the housing of the electric machine. The disk defines a plurality of vents forming an axial air passage. A plurality of radial air passages are in fluid communication with the axial air passage. The airflow diverter feature defines at least one inlet and at least one outlet. The inlet comprises at least one interior choke and the outlet comprises at least one exhaust choke. The interior choke and the exhaust choke are sized to control a rate of air flow through the radial air passages.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/581,587, filed Dec. 29, 2011, entitled “ELECTRIC MACHINE COOLING SYSTEM” and U.S. Provisional Patent Application No. 61/581,597, filed Dec. 29, 2011, entitled ELECTRIC MACHINE COOLING SYSTEM, the content of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     This application is related to air cooled electric machinery with an airflow diverter feature configured to provide even distribution of airflow to the electric machine. 
     SUMMARY 
     Air cooled permanent magnet rotors and stator systems may be designed to evenly move air across the end windings into the interior of the rotor through axial channels. The system may use an airflow diverter to evenly distribute the airflow angularly about the electric machine. In addition, the diverter may use various baffles or openings to direct the airflow toward the rotor air passages so that at least a portion of the airflow is forced first forced around the end windings of the stator. 
     After the airflow cools the end windings, the air may be split into parallel paths, a portion enters the rotor, and a portion enters the exterior of the stator. Air that enters the rotor is ejected radially through spacer vents which function as a radial fan system to generate considerable cooling for the generator rotor and stator. At high rotational speeds, the rotor and spacer vents may develop considerable head pressure and can generate large mass flow rates through the interior of the rotor. At high flow rates, air may separate from the outer wall of the rotor axial flow channels. This leaves a large “vena contracta” or separated wake that spans a formidable length of the axial channel. If rotor spacer vents are located in these regions, the separated wake will prevent air from moving into the spacer vent and then moving radially outward—thus starving the cooling in these local spacer vents. 
     The system described herein may utilize a specially designed flow restriction located in the interior of the generator which serves to slow the air moving through the rest of the rotor. This reduced velocity, if properly engineered, can eliminate the separated flow entering the rotor axial channels and can ensure that the airflow through the rotor spacer vents is more evenly distributed (e.g. providing an even volumetric flow rate). This creates a generator with lower peak temperatures and more evenly distributed temperature distributions. More evenly distributed temperature distributions may be beneficial to lower the power used to control temperature as well as enhance reliability (evenness of temperature is better for longevity of winding insulation). 
     The carefully shaped flow restrictor may be achieved with very low cost geometry modification of the standard spacer vent construction. Thus the cooling improvements described herein may be achieved at very low cost. 
     Accordingly, in some implementations a cooling system for an electric machine is provided. The cooling system may include an airflow restriction feature configured to provide airflow to a radial air passage of the electric machine. 
     In some implementations, the system includes a rotor for an electric machine. The rotor may include an axial air passage configured to receive airflow through an inlet port. The axial air passage may be in fluid communication with a plurality of radial air passages. The axial air passage may have a first cross-sectional area, for example, where the axial air passage is connected to a first radial air passage. Further, the axial air passage may have an internal choke portion with a second cross-sectional area that is less than the first cross sectional area, such that a portion of the airflow is provided to the first radial air passage. 
     The second cross sectional air passage area may be less than 90% of the first cross sectional area, and preferably in some implementations about 75% of the first cross sectional area. The second cross sectional area air passage may also be greater than 60% of the first cross sectional area. 
     The rotor may be constructed from a plurality of disks, where each disk includes a plurality of vents that form the axial air passage. The internal choke portion may be formed by a disk with vent opening that is smaller than the vent opening other disks. In addition, the internal choke portion may have a generally trapezoidal shape. Further, the trapezoidal or other shapes forming the choke may be radially spaced about the disk to form a segmented annular shape. 
     In some implementations, the rotor may include an exhaust choke. As such, an axial air passage may be formed in the rotor and configured to receive an airflow through an inlet port. The axial air passage may be in fluid communication with a plurality of radial air passages. The axial air passage may have a first cross-sectional air passage area allowing a certain airflow. Further, the axial air passage may have a choke portion, such as an exhaust choke, with a second cross-sectional area that is less than the first cross sectional area. The exhaust choke may be located downstream from the plurality of radial air passages such that a substantial airflow is provided to the plurality radial air passages. 
     For the exhaust choke, the second cross sectional area of the axial air channel may be less than 40% of the first cross sectional area, and preferably in some implementations may be about 25% of the first cross sectional area. Further, the second cross sectional area of the axial air channel may be greater than 10% of the first cross sectional area. 
     In addition, the rotor may be formed from a plurality of disks, each disk having a plurality of vents that form the axial air passage. The exhaust choke may be formed by a disk with vent opening that is smaller than other disks of the plurality of disks. Further, the opening at the exhaust choke may have a generally semi-circular shape. 
     In addition, it should be understood that the inner choke and the exhaust choke may be used together. Further, any number of chokes may be used with varying cross section areas. For example, the vent forming the axial passage at each choke may have a subsequently reduced cross sectional area from the inlet port to the exhaust port of the axial air passage. Each choke section may be formed by reducing the cross sectional area primarily from the inner edge of the axial air passage. As such, the choke portion may extend into the axial air passage radially from the center of the rotor. 
     As such, in some implementations, the rotor may include an axial air passage formed in the rotor and configured to receive an airflow through an inlet port. The axial air passage may be in fluid communication with a plurality of radial air passages. The axial air passage may have a first cross-sectional area where the axial air passage is connected to a first radial air passage of the plurality of air passages and the axial air passage may have a first choke portion with a second cross-sectional area that is less than the first cross sectional area such that a portion of the airflow is provided to the first radial air passage. In addition, the axial air passage may also have a second choke portion (e.g. an exhaust choke) with a third cross-sectional area that is less than the first cross sectional area downstream from the plurality of radial air passages such that a substantial airflow is provided to the plurality radial air passages. 
     Further, it is understood that the first and second chokes may have any combination of the characteristics described above or elsewhere in this application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  is a cut away side view of an electric machine with a heat exchanger; 
         FIG. 2  is a cut away side view of an electric machine; 
         FIG. 3  is a flow chart illustrating a method for cooling an electric machine; 
         FIG. 4  is a color illustration of airflow through an electric machine; 
         FIG. 5  is a color illustration of airflow through an electric machine where no interior choke is provided in the airflow; 
         FIG. 6  is a color illustration of airflow through an electric machine with choking provided in the airflow; 
         FIG. 7  is a graph illustrating the temperature of the magnet, stator lamination, and winding with respect to the distance from the driving end of an electric machine with no interior choke; 
         FIG. 8  is the temperature of the magnet, stator lamination, and winding that a given distance from the driving end of the electrical machine with a 25% interior choke and 75% exhaust choke; 
         FIG. 9  is a graph illustrating a temperature of the magnet, stator lamination, and winding with respect to the distance of the driving end of the electrical machine with 50% interior choke and a 75% exhaust choke; 
         FIG. 10  is an end view of a rotor assembly illustrating the axial air channels with an interior and exhaust choke; 
         FIG. 11  is a front view of a rotor plate that forms part of the axial air channel; 
         FIG. 12  is a plate that forms part of the radial air channels of the rotor; 
         FIG. 13  is a plate that forms the interior choke of the rotor; 
         FIG. 14  is a plate that forms the exhaust choke of the rotor; 
         FIG. 15  is a plate that illustrates the stator radial vents; 
         FIG. 16  is a plate that illustrates the air diverter; 
         FIG. 17  is a wire frame drawing that illustrates one implementaion of the air diverter located within an electrical machine; 
         FIG. 18  is a wire frame drawing that illustrates another implementation of the air diverter located within an electrical machine; and 
         FIG. 19  is a wire frame drawing that illustrates yet another implementaion of the air diverter located within an electrical machine. 
     
    
    
     DETAILED DESCRIPTION 
     An electrical machine with a heat exchanger is provided in  FIG. 1 . The electrical machine  100  may be a generator, a motor, or other electrical machine. The heat exchanger  110  may be in fluid communication with the electrical machine  100  to cool the airflow through the electrical machine  100 . The electrical machine  100  may include a drive end  112  that may be connected to a mechanical load in the case of a motor or may be driven by a turbine, such as an air turbine, hydraulic turbine, or other power input source in the case of a generator. The drive end  112  is connected to the rotor shaft  114  and rotates therewith. The rotation of the rotor shaft  114  also rotates the rotor assembly  122  and the fan blades  116 . The rotation of the fan blades  116  pulls air from the inlet port  118  into the electrical machine  100 . 
     The airflow from the air inlet  118  is directed by the diverter  120 . The air diverter  120  forces air across the front end of the windings in the stator assembly  124 . The air is then allowed to flow through air channels in the rotor assembly  122 . Airflow may be apportioned through the air channels in the rotor assembly  122  by devices such as chokes located through the rotor assembly  122 . The airflow may then be collected through the fan blade assembly  116  and distributed to the output port  126 . The output port  126  provides the airflow from the electrical machine  100  to the heat exchanger  110 . 
     The airflow from the electrical machine  100  circulates through the heat exchanger  110 , as denoted by arrow  128 . Heat is removed from the airflow  128  by an airflow  132  which is circulated from the inlet port  130  of the heat exchanger  110  to the outlet port  134  of the heat exchanger  110 . Accordingly, the heat from the airflow  128  of the electric machine  100  transfers the heat generated by the electric machine  110  to the airflow  132  which is then transported away from the electric machine  100  as it leaves the heat exchanger  110 . 
     It is contemplated within this disclosure that the rotor  122  and the stator  124  may be switched such that the rotor  122  is located outside of the stator  124 . In the configurations shown, the rotation of the rotor  122  pushes the air radially through the stator. However, in an implementation where the rotor is outside the stator, rotation of the rotor would pull the air through the stator and, in the same manner, air would be directed to the fan assembly  116  and distributed to the heat exchanger  110 . 
       FIG. 2  is an illustration of an electrical generator which may be an implementation of the electrical machine  100  in  FIG. 1 . Airflow is received from the inlet port  118  and distributed circumferentially around the electric machine. The airflow is shown as being distributed to a top portion of the electric machine by arrow  212  and to a bottom portion of the electric machine by arrow  210 . However, it should be noted that the stator and rotor would typically be cylindrical in nature and, therefore, the airflow would be distributed circumferentially around the entire electrical machine by the air diverter  120 . The air diverter  120  would provide the airflow  212  to the windings  215  of the electric machine. The air diverter  120  may provide the air through openings within a plate while other solid portions of the plate may block airflow, thereby diverting the airflow to the openings which are distributed circumferentially around the plate and aligned radially with the end turns of the windings. 
     The airflow will be drawn through the electrical machine by the fan assembly  116  and also by rotation of the rotor assembly  122 . Accordingly, the airflow  212  may be split into an airflow  214  that is drawn around the outside of the stator assembly  124  by the fan assembly  116  and also airflow  216  which is drawn through axial air channels  218  in the rotor assembly  122 . The airflow  216  is drawn into the axial air channels  218  by rotation of the rotor assembly  122 . 
     The rotation of the rotor assembly  122  pushes air through radial air channels  220  in the stator assembly  122 . Each of the radial air channels  220  in the rotor assembly  122  are aligned with corresponding radial air channels  222  in the stator assembly  224 . For example, air channels  220  in the rotor may be aligned in the axial dimension with the air channels  222  at the same axial location in the stator. Multiple air channels  220  and  222  have the same axial location but are radially spaced around the rotor and stator. 
     To aid in the distribution of air through the radial air channels along the axial length of the rotor assembly  122 , one or more devices, for example chokes, may be located within the axial air channel  218  at one or more axial locations along the length of the rotor assembly  122 . For example, an internal choke  228  may be located within the first few radial channels. The internal choke  228  may block a portion of the airflow  216 , thereby aiding distribution of the airflow through the first few radial channels. The internal choke  228  may block 25% of the cross-sectional area of the axial air channel  218  although various other percentages may be used depending on the location of the choke within the air channel and the axial location of the choke within the air channel. In addition, a plurality of chokes may be used and located at variously axial locations along the length of the rotor assembly  122 . Further, each choke may block an increased amount of cross-sectional area of the channel  218  as the axial location increases from the intake of the axial channel  218  to the exit of the axial channel  218 . For example, while the interior choke  228  may block 50% of the cross-sectional area of the channel  218 , a choke that is closer to the exit of the axial channel  218  may block a larger percentage of the cross-sectional area, for example choke  230  at the exhaust of the axial channel  218  may block 75% of the cross-sectional area of the channel  218 . 
     Accordingly, the airflow that is diverted through the radial channels, denoted by arrows  232 , may join up with the airflow  214  diverted along the outside of the stator. Airflows  232  and  214  may then be drawn back across the exhaust side of the windings into the intake opening  234  of the fan assembly  116 . Further, a portion of the airflow  216  through the axial air channel  218  may exit the exhaust end past the exhaust choke  230 , as denoted by arrow  233 . The airflow  233  may join up with the airflows  214  and  232  entering the intake port  234  of the fan assembly  116 . Accordingly, the airflows are then communicated to the exhaust port  126 , as denoted by arrow  236 . 
     A method for cooling an electric machine, such as a generator is provided by the flow chart in  FIG. 3 . The method  300  starts in block  310 . In block  310 , cool air enters electric machine. For example, the cool air may enter from a heat exchanger at a drive end along the top of the electric machine. In block  312 , the airflow is routed by an air diverter. The air diverter balances the air distribution around the perimeter of the housing. As noted in block  314 , the air diverter routes the airflow across the winding end. For example, the airflow pulled through the air diverter flows across the end turns of the windings and into the center of the rotor. Most of the airflow may be pulled into the center of the rotor through air channels oriented axially through the rotor assembly. Some airflow may immediately move outside of the stator core traversing around the outside of the stator. Other portions of the airflow may then be pulled into the center of the rotor core, as noted by block  316 . Rotation of the rotor core causes a natural pumping action that is combined with the shaft fan pumping action to draw the air to the non-drive end of the electric motor. In block  318 , the rotation of the rotor causes the air in the rotor to be expelled through radial vents towards the outer portion of the rotor. The radial vents may be located along the axial length of the rotor thereby drawing air through the entire rotor. For example, I-beams in the radial vents may naturally act as a centrifugal fan to pump the air radially to the outer edge of the rotor. 
     An interior choke inside the rotor may slow down the incoming air, as denoted by block  320 . The interior choke minimizes the separation of the air from the outer surface of the axial air channels, as discussed elsewhere in this application. Without the choke, the separation of air from the outer radial surface of the air channels which may starve the airflow through the first few radial vents and, thereby, create a hot spot in the magnets, windings, or laminates. In block  322 , air continues through the rotor and is distributed through the radial vents pulling heat from the rotor laminations and magnets. 
     An additional exhaust choke may be located at the non-drive end of the rotor to ensure that most of the air space is forced through radial vents. The exhaust choke helps to ensure that most of the air is forced through the radial vents rather than traveling axially through the entire rotor and collecting at the fan entrance. Air from the rotors blow into stator radial vents as denoted by block  324 . The stator radial vents are axially aligned with the rotor radial vents. The air from the rotor flows radially through the stator pulling heat from the stator core and windings, as denoted by block  326 . 
     The air that flows radially through the stator collects in the outer axial stator air passageway. Air then moves to the non-drive end through the stator axial air passageway, as denoted in block  328 . Air is then routed across the non-drive end windings, as denoted in block  330 . The air passing over the end turns cools the windings by pulling heat from the end turns. The airflows from the end turns to the center of the shaft mounted fan. Air then enters the shaft mounted fan and is discharged radially, as denoted in block  332 . 
     Air leaves the non-drive end and may, for example, be provided to a heat exchanger at a top of the electric machine at the non-drive end. If the heat is provided to a heat exchanger, the hot air enters the counter-flow air to exchange the heat thereby cooling the airflow, as denoted by block  336 . Cooled air may then be provided from the heat exchanger back to the electric machine, as provided in block  310 , where the cycle may continue. 
       FIG. 4  is a color illustration of the airflow through an electric machine. The airflow is denoted at the intake by reference numeral  410 . Cooler air is shown by the blue color and hotter air at the top of the display temperature range is shown as a red color. Accordingly, it can be seen that the airflow  410  starts at the intake as blue and traverses to the non-drive end absorbing heat from the electric machine. Therefore, the increase in temperature is denoted by the green and yellow colors at the non-drive end of the airflow. Further, it is noted that a vacuum illustrated at circle  412  may form at the drive end of the axial rotor shaft. As such, the airflow would separate from the outer surface of the axial air channel that travels through the rotor. 
     The separation may starve the first few radial air channels. However, the airflow then is directed to the outer surface, as denoted by reference numeral  414 , as the airflow progresses longitudinally along the axial air channel. The separation at reference numeral  412  may starve the first few radial channels of airflow thereby causing overheating in portions of the drive end of the electric machine. Devices, such as chokes, may be located within the axial air channel to redistribute the airflow in a balanced manner through all the radial channels. 
       FIG. 5  is a color depiction of airflow illustrating an electric machine where no inner choke is utilized and a 75% exhaust choke is utilized. The airflow is denoted at the entrance port by reference numeral  510 . The airflow through the first channel is denoted by reference numeral  512  and the airflow through the second radial channel is denoted by reference numeral  514 . The flow through the first channel  512  and the second channel  514  is much smaller in volume than the airflow through the latter radial channels. Therefore, the airflow is less able to cool the drive end of the rotor and stator. This can be visualized by the yellowish color of the airflow near the drive end which denotes a higher temperature in the air circulating around the stator and rotor. 
       FIG. 6  is a color depiction of airflow illustrating an electric machine where a 25% inner choke and a 75% exhaust choke is utilized. The airflow is denoted at the entrance port by reference numeral  610 . Compared with  FIG. 5 , the airflow through the first channel  612  and the airflow through the second radial channel  614  are greatly increased. Accordingly, the rotor and stator towards the drive end are much cooler, as visualized by the darker green color. Further, good airflow is maintained in the radial passages in the middle of the rotor and stator due to the exhaust choke at the non-drive end of the axial channel. 
       FIG. 7  is a graph illustrating the temperature of the rotor interior (e.g. magnets), stator laminations, and windings with respect to the axial distance from the drive end of the electric machine. The temperature of the windings is illustrated by line  710 . The temperature of the stator laminations is denoted by reference numeral  712  and the temperature of the magnets is denoted by reference numeral  714 . Generally, the temperature of the rotor interior is less than the temperature of the stator laminations. Further, the temperature of the stator laminations is generally less than the temperature of the windings. The graph provided in  FIG. 7  relates the temperature of the magnet, stator laminations, and windings with no interior choke and a 75% exhaust choke. This also corresponds to the airflow in  FIG. 5 . These temperatures are with a 63° C. air from a heat exchanger and a 40° C. ambient temperature. 
       FIG. 8  is a graph illustrating the temperature of the magnet, stator laminations, and windings with respect to the axial distance from the drive end of the electric machine. The temperature of the windings is illustrated by line  810 . The temperature of the stator laminations is denoted by reference numeral  812  and the temperature of the magnets is denoted by reference numeral  814 . The graph provided in  FIG. 8  relates the temperature of the magnet, stator laminations, and windings with a 25% interior choke and a 75% exhaust choke. In this context, a 25% choke relates to, for example, a 25% reduction in the cross sectional area of the air channel. This also corresponds to the airflow in  FIG. 4 . These temperatures are with a 63° C. air from a heat exchanger and a 40° C. ambient temperature. 
       FIG. 9  is a graph illustrating the temperature of the magnet, stator laminations, and windings with respect to the axial distance from the drive end of the electric machine. The temperature of the windings is illustrated by line  910 . The temperature of the stator laminations is denoted by reference numeral  912  and the temperature of the magnets is denoted by reference numeral  914 . The graph provided in  FIG. 9  relates the temperature of the magnet, stator laminations, and windings with a 50% interior choke and a 75% exhaust choke. This also corresponds to the airflow in  FIG. 6 . These temperatures are with a 63° C. air from a heat exchanger and a 40° C. ambient temperature. 
     In one example, design goal may be to keep the variation in the stator lamination to less than 10 degrees C. variation while minimizing the average magnet temperature and minimizing the peak winding temperature. In this case the pressure drop may also be observed to minimize fan power. The preferred configuration for one implementation was achieved with the 25% inlet choke and 75% exhaust choke which achieved a 7 degree C. stator temperature variation (max to min) and had an average magnet temperature of 31.9 deg C. The improved performance of this implementation is illustrated by the limited variation in line  812  and  814  in  FIG. 8 . 
     This improved performance is clarified when comparing, with the system having no interior choke and 75% exhaust choke in  FIG. 7 . The system of  FIG. 7 , has a 9 degree C. stator temperature variation and had an average rotor interior temperature of 30.1 deg C. The improved performance can also be compared with the system having 50% interior choke and 75% exhaust choke in  FIG. 9 . The system of  FIG. 9  had a 13 degree C. stator temperature variation (max to min) and had an average magnet temperature of 28.3 deg C. (fan power was also increasing). Thus it is possible to “over choke” the interior choke. The exhaust choke was less sensitive although it did seem to function better at about 75% (or greater) or the shaft mounted fan aft of the rotor could pull too much air and the rear rotor vents could starve. 
       FIG. 10  is an end view of a rotor assembly  122 . The rotor may be made up of a number of plates in the form of round disks. The disks may then be stacked and fastened to form the rotor assembly  122 . The axial channels in the rotor assembly  122  are denoted by reference numeral  218 . The interior choke  228  can be seen blocking approximately 25% of the axial channel  218 . Further, the exhaust choke  230  may be seen blocking a larger portion of the axial channel  218 . For example, the exhaust choke  230  may block approximately 75% of the cross-sectional area of the axial channel  218 . Further, a hole may be provided in each of the plates allowing the rotor shaft  114  to extend therethrough. The rotor shaft  114  may be keyed to each of the plates thereby causing rotation of the rotor assembly  122  based on rotation of the rotor shaft  114 . 
       FIG. 11  is one of a plurality of plates that may be used to build the rotor assembly  122 . The plate  1110  may be formed of a laminated steel or other magnetically conductive materials. The rotor plate  1110  may be in the form of a disk such as a circular disk. The plate  1110  may include a plurality of holes  1114  arranged circumferentially around a center of the plate. The holes  1114  may form a portion of the axial air channel through the rotor assembly  122 . The plate  1110  may also include a hole  1118  through the center of the plate allowing the rotor shaft to extend therethrough. In addition, the hole  1118  may include one or more keyways  1112  allowing the rotor shaft to engage the plate  1110  and rotate it along with the rotation of the rotor shaft. In addition, the plate may include a plurality of holes  1116  allowing fasteners to extend therethrough, thereby fastening the plurality of plates together to form the rotor assembly  122 . 
       FIG. 12  is rotor vent plate that may be used to build the rotor assembly  122 . The rotor vent plate  1210  may be formed of a laminated steel or other magnetically conductive materials. The rotor plate  1210  may be in the form of a disk such as a circular disk. The plate  1210  may include a plurality of holes  1214  arranged circumferentially around a center of the plate. The plate  1210  may also include a hole  1218  through the center of the plate allowing the rotor shaft to extend therethrough. In addition, the hole  1218  may include one or more keyways  1212  allowing the rotor shaft to engage the plate  1210  and rotate the plate  1210  with the rotation of the rotor shaft. 
     The plate  1210  may include I-beams  1220  welded to a surface of the plate and extending radially. The I-beams  1220  may be located around the circumference of the plate  1210 . The I-beams  1220  may be periodically spaced about the circumference, for example with equal angular spacing. Further, one or more I-beams may also be attached to portions  1222  of the plate extending between the holes  1214 , as denoted by reference numeral  1224 . The rotation of the plate  1210  causes the I-beams  1220  to force the air radially to the outer edge of the plate  1210 . Accordingly, the rotor may act like a centrifugal pump. In addition, the plate  1210  may include a plurality of holes  1216  allowing fasteners to extend therethrough, thereby fastening multiple plates together to form the rotor assembly  122 . 
       FIG. 13  is a plate for restricting air flow that may be used to build the rotor assembly  122 . The plate  1310  may be formed of a laminated steel or other magnetically conductive materials. The rotor plate  1310  may be in the form of a disk such as a circular disk. The plate  1310  may include a plurality of holes  1314  arranged circumferentially around a center of the plate. The holes  1314  may form a portion of the axial air channel through the rotor assembly  122 . Specifically, the holes  1314  may have a smaller cross sectional area than the holes  1114  through plate  1110 . As such, the portion of the plate  1310  extending into the axial air channel defined by holes  1114  forms a choke to redistribute airflow through the axial air channel. 
     The plate  1310  may be located after the first few radial air channels or vents. Alternatively, multiple plates  1310  may be located along the rotor assembly, for example having holes  1314  with different shapes and/or cross sectional areas. As such, plate  1310  may form an interior choke for the rotor assembly  122 . The holes  1314  may have less than 90% of the cross sectional area of the holes  1114 . The holes  1314  may have a cross section area more than 60% of the cross section area of the holes  1114 . In some applications, the cross section area of the holes  1314  may be about 75% of the cross sectional area of holes  1114 . 
     The plate  1310  may also include a hole  1318  through the center of the plate allowing the rotor shaft to extend therethrough. In addition, the hole  1318  may include one or more keyways  1312  allowing the rotor shaft to engage the plate  1310  and rotate it along with the rotation of the rotor shaft. In addition, the plate may include a plurality of holes  1316  allowing fasteners to extend therethrough, thereby fastening the plurality of plates together to form the rotor assembly  122 . 
       FIG. 14  is a plate for restricting air flow that may be used to build the rotor assembly  122 . The plate  1410  may be formed of a laminated steel or other magnetically conductive materials. The rotor plate  1410  may be in the form of a disk such as a circular disk. The plate  1410  may include a plurality of holes  1414  arranged circumferentially around a center of the plate. The holes  1414  may form a portion of the axial air channel through the rotor assembly  122 . Specifically, the holes  1414  may have a smaller cross sectional area than the holes  1114  through plate  1110 . As such, the portion of the plate  1410  extending into the axial air channel defined by holes  1114  forms a choke to redistribute airflow through the axial air channel. 
     The plate  1410  may be located at the exhaust end of the axial air channel. As such, plate  1410  may form an exhaust choke for the rotor assembly  122 . The holes  1414  may have less than 40% of the cross sectional area of the holes  1114 . The holes  1414  may have a cross section area more than 10% of the cross section area of the holes  1114 . In some applications, the cross section area of the holes  1414  may be about 25% of the cross sectional area of holes  1114 . 
     The plate  1410  may also include a hole  1418  through the center of the plate allowing the rotor shaft to extend therethrough. In addition, the hole  1418  may include one or more keyways  1412  allowing the rotor shaft to engage the plate  1410  and rotate it along with the rotation of the rotor shaft. In addition, the plate may include a plurality of holes  1416  allowing fasteners to extend therethrough thereby fastening the plurality of plates together to form the rotor assembly  122 . 
       FIG. 15  is a plate that forms the radial stator vents. The plate  1510  may be made of a laminated steel or other magnetically conducting material. The plate  1510  may have an opening  1512  allowing the rotor to extend therethrough. The plate  1510  may be in the shape of a disk and may include projections  1514  extending inwardly toward the center of the plate  1510  and as such, towards the rotor. The plate may include I-beams  1516  welded to a surface of the plate and extending radially. The I-beams may extend from the edge of the plate along the center of the projections inwardly towards the rotor. The projections  1514  may form channels  1518  that may be oriented axially with respect to the stator. 
       FIG. 16  is a plate that forms the air diverter  120 . The plate  1610  may be formed of a metal, plastic, or other material sufficiently sturdy for diverting air flow through the electric machine. The plate  1610  may be in the form of a disk such as a circular disk. The plate  1610  may be mounted to the housing of the electrical machine and, therefore, may be stationary. The plate  1610  may include a plurality of holes  1616  arranged circumferentially around a center of the plate. As such, the holes  1616  may from an annual passage through the plate  1610 . The holes  1616  may force the airflow to the outer portion of the electric machine. In particular, the holes  1616  may be located radially outside the windings such that a portion of the airflow is forced to travel from the air inlet across the end turns of the windings before entering the rotor axial channels. 
     The annular opening  1618  formed by the holes  1616  may have an inner diameter that is larger than the diameter of a circular pattern formed by the axial air channels of the rotor. Further, the annular opening  1618  may have an inner diameter that is larger than the rotor diameter. In addition, the annular opening  1618  may have an outer diameter that larger than the diameter of the stator windings, such that the airflow is forced past the stator windings prior to entering the axial air channels. 
     The air diverter may be located axially adjacent to the windings such that the air flow is not allowed to travel directly to the axial air channels of the rotor without interacting with the end turns of the windings. The plate  1610  may also include a hole  1614  through the center of the plate allowing the rotor shaft to extend therethrough. 
       FIG. 17  is an illustration identifying one implementation of an air diverter. The housing of the electric machine  1710  may include an air inlet port  1714  and an air exhaust port  1716 . Air may flow through the inlet port  1714  and be diverted by the air diverter  1712  peripherally around the circumference of the housing  1710 . In  FIG. 17 , the air diverter  1712  may be formed by a disk shaped plate like the air diverter shown in  FIG. 16 . As such, projections from the air diverter  1712  may form openings that direct the air over the end turns of the winding in the stator. As such, the air would flow over the end turns of the windings and then towards the rotor and through axial air channels in the rotor to cool internally both the rotor and stator. The air will then proceed from the rotor and stator to the exit port  1716  such that the heat may be removed from the electric machine. 
       FIG. 18  is an illustration identifying one implementation of an air diverter. The housing of the electric machine  1810  may include an air inlet port  1814  and an air exhaust port  1816 . Air may flow through the inlet port  1814  and be diverted by the air diverter  1812  peripherally around the circumference of the housing  1810 . In  FIG. 18 , the air diverter  1812  may be a baffle formed by a partially conical or cylindrical surface. The baffle may be extending radially inward from the housing of the electrical machine. As such, the partially conical or cylindrical surface of the air diverter  1812  may direct a portion of the airflow from the inlet port  1814  over the end turns of the winding in the stator. As such, the air would flow over the end turns and the windings and then towards the rotor and through axial air channels in the rotor to cool internally both the rotor and stator. The air will then proceed from the rotor and stator to the exit port  1816  such that the heat may be removed from the electric machine. 
       FIG. 19  is an illustration identifying one implementation of an air diverter. The housing of the electric machine  1910  may include an air inlet port  1914  and an air exhaust port  1916 . Air may flow through the inlet port  1914  and be diverted by the air diverter  1912  peripherally around the circumference of the housing  1910 . In  FIG. 19 , the air diverter  1912  is formed by a cylinder located around the rotor shaft and extending radially outward. The cylinder may be have a radius greater than the distance from the center of the rotor shaft to the axial air channels. As such, the cylinder may direct the air over the end turns of the winding in the stator. As such, the air would flow over the end turns and the windings and then towards the rotor and through axial air channels in the rotor to cool internally both the rotor and stator. The air will then proceed from the rotor and stator to the exit port  1816  such that the heat may be removed from the electric machine. 
     While a particular implementation of the above described concepts may be a permanent magnet machine, the concepts are equally applicable to electrical machines in general. Other types of electrical machines incorporating the above described elements may include, but are not limited to, wound-field synchronous, induction, switched reluctance, or variable reluctance machines. Further, any of the elements described above may be implemented alone or in combination regardless of the particularly described exemplary embodiments. 
     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of the principles of this application. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this application, as defined in the following claims.