Abstract:
A method for generating electrical power may include the steps of rotating a rotor of a generator at a speed in excess of about 12,000 revolutions per minute (rpm) to about 25,000 rpm and producing power with the generator at a rate in excess of about 800 kilowatts (kW). The generator has a power/weight ratio no smaller than about 3 kW/lbs. A rotor is cooled with cooling oil internally circulated through the rotor of the generator so that contact of cooling oil with external surfaces of the rotor may be precluded. The stator is also cooled with oil that is prevented from contacting the external surfaces of the rotor. Pressurized airflow may be produced in a gap between the rotor and a stator of the generator to preclude entry of cooling oil into the gap.

Description:
GOVERNMENT RIGHTS 
       [0001]    This invention was made with Government support under Tactically Exploited Reconnaissance Node (TERN)-Phase II contract HR0011-13-C-0099 subcontract 25172 awarded by DARPA. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to producing high power output from compact electrical generators. More particularly, the invention relates to cooling such generators and mitigation of windage losses that may result from such cooling. 
         [0003]    There is an increasing need for high power generators in the 500 kilowatts (kW) to 1 megawatt (MW) range for hybrid-electric propulsion and directed energy weapons. At the same time, these applications require low weight and volume. Small and light-weight generators must be operated at high rotational speeds in order to produce high power outputs. High amounts of heat are produced during operation of such generators. Oil cooling is typically employed to maintain operating temperatures of such generators at a tolerable level. 
         [0004]    Oil-based cooling systems are advantageously compact and efficient, but there has heretofore been a practical limit to their effective application. At rotational speeds of about 12,000 revolutions per minute (rpm) or more, a generator may experience high windage friction forces that develop when oil enters a gap between a rotor and a stator of the generator. As the rotor rotates at a high speed, windage friction may heat the oil in the gap beyond its temperature limit if not replenished with an adequate amount of lower temperature oil. This constant oil bath may have the counterproductive effect of producing higher rotating friction or windage reducing the overall efficiency of the generator. In that context, heat-reducing benefits of oil-based cooling may be offset by heat production resulting from windage friction arising from a presence of oil in the air gap. Undesirable windage friction may also arise if cooling oil is sprayed in the generator and the oil spray is allowed to contact exterior surfaces of the rotor. 
         [0005]    As can be seen, there is a need for generator cooling system that will retain its effectiveness at high rotational speeds. More particularly, there is a need for such a system that may eliminate windage friction resulting from cooling oil contact with exterior surfaces of a rotor and/or presence of oil in an air gap between a rotor and a stator of the generator. 
       SUMMARY OF THE INVENTION 
       [0006]    In one aspect of the present invention, an oil-cooled electrical generator comprises: a stator; a rotor; a gap between the stator and the rotor; an air passageway passing through the stator to the gap; a source of pressurized air in fluid communication with the air passageway; and passageways for cooling oil, wherein the passageways for cooling oil are fluidly isolated from the gap. 
         [0007]    In another aspect of the present invention, apparatus for cooling a generator comprises: a cooling oil spray nozzle directed at end turns of windings of a stator of the generator; a spray shield having an internal chamber fluidly isolated from the spray nozzle; wherein the internal chamber is in fluid communication with a gap between a rotor of the generator and the stator. 
         [0008]    In still another aspect of the present invention, a method for generating electrical power comprises the steps of: rotating a rotor of a generator at a speed in excess of about 12,000 rpm to about 25,000 rpm; producing power with the generator at a rate in excess of about 800 kilowatts (kW); cooling the rotor with cooling oil; and producing pressurized airflow in an gap between the rotor and a stator of the generator to preclude entry of cooling oil into the gap, wherein the generator has a power/weight ratio no smaller than about 3 kW/lbs. 
         [0009]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a cross-sectional view of a generator in accordance with an exemplary embodiment of the invention; 
           [0011]      FIG. 2  is cross-sectional view of the generator of  FIG. 1  taken along line  2 - 2  in accordance with an exemplary embodiment of the invention; 
           [0012]      FIG. 3  is detailed view of a portion of the cross-sectional view of 
           [0013]      FIG. 2  in accordance with an exemplary embodiment of the invention; 
           [0014]      FIG. 4  is a perspective view of a stator lamination of the generator of  FIG. 1  in accordance with an exemplary embodiment of the invention; 
           [0015]      FIG. 5  is detailed view of a portion of the lamination of  FIG. 4  in accordance with an exemplary embodiment of the invention; 
           [0016]      FIG. 6  is a cross-sectional view of the generator of  FIG. 1  taken along line  6 - 6  in accordance with an exemplary embodiment of the invention; 
           [0017]      FIG. 7  is a cross-sectional view of a portion of the view of  FIG. 2  taken along the line  7 - 7  in accordance with an exemplary embodiment of the invention; and 
           [0018]      FIG. 8  is a flow chart of a method for generating electrical power in accordance with an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0020]    Various inventive features are described below that can each be used independently of one another or in combination with other features. 
         [0021]    The present invention generally may provide a system for producing high power output from a compact electrical generator. More particularly, the invention may provide a capability for reducing windage friction in the generator by precluding entry of cooling oil into an air gap between a rotor and a stator, and contact of cooling oil with other exterior surfaces of a rotor of the generator. 
         [0022]    Referring now to  FIG. 1 , a cross sectional view illustrates an oil-cooled generator  100  constructed in accordance with an exemplary embodiment of the invention. The generator  100  may include a housing  102 , a stator  104  and a rotor  106 . A back iron  108  may surround the stator  104 . The back iron  108  may be provided with a spiral grove  110  along its length. Cooling oil  112  may be circulated through the spiral groove  110 . Thus, the spiral grove  110  may be considered to be a cooling-oil passageway  110 . 
         [0023]    The stator  104  may include windings  114 , laminations of a first type  116  (hereinafter laminations  116 ) and one or more laminations of a second type  158  (hereinafter center lamination  158 ), as will be explained hereinbelow. Spray nozzles  122  may deliver cooling-oil spray  124  onto the end turns  120 . Cylindrical spray shield  126  with internal chambers  127  may be positioned to deflect the cooling-oil spray  124 . First portions  128  of outer surfaces  130  of the spray shields  126  may be adhesively bonded to the stator  104 . Second portions  132  of the outer surfaces  130  of the spray shields  126  may be sealed against the housing  102  with seals such as O-rings  134 . In other words, the oil spray  124  may be constrained within spray chambers  129  externally of the spray shields  126 . Thus, cooling oil spray  124  may be precluded from contacting outer surfaces of the rotor  106 . Consequently, the rotor  106  may be rotated at a high rotational speed without encountering windage friction forces arising from presence of the cooling oil spray  124  in the generator  100 . 
         [0024]    Cooling oil  112  may be circulated through cooling oil passageways  137  internally through the rotor  106  and through bearings  138  which support a shaft  140  of the rotor  106 . The bearings  138  may be fluidly isolated from interiors of the spray shields  126  with seals  142 . Consequently, cooling oil  112  circulating through the rotor  106  may be precluded from contacting external surfaces of the rotor  106  even though the rotor  106  is advantageously cooled by the cooling oil  112 . 
         [0025]    Pressurized airflow  144  may be introduced into a gap  146  between the rotor  106  and the stator  104 . The airflow  144  may preclude entry of cooling oil  112  into the gap, thus further assuring that external surfaces of the rotor  106  are not contacted by the cooling oil  112 . 
         [0026]    As a consequence of precluding contact of cooling oil  112  with external surfaces of the rotor  106 , the rotor  106  may be rotated at high rotational speeds without experiencing windage friction forces from the cooling oil  112 . 
         [0027]    Referring now to  FIGS. 1-5 , it may be seen how the airflow  144  may be produced within the gap  146 . A source  148  of pressurized air may be in fluid communication with a port  150  in the housing  102 . The port  150  may be aligned with air passageway  152  that may pass through a land  154  of the back iron  108 . The air passageway  152  may be in fluid communication with an annular groove  156  of a center lamination  158 . The center lamination  158  may be provided with air-passageway holes  160  passing from the annular groove  156  to the gap  146 . Consequently, the gap  146  may be in fluid communication with the source  148  of pressurized air. 
         [0028]    The source  148  may provide pressurized air to the gap  146  so that the airflow  144  in the gap  146  may take place at a pressure of about 3 psi to about 5 psi, as an example. The airflow  144  may pass through the gap  146  and into the internal chambers  127  of the spray shields  126 . The airflow  144  may emerge from the spray shields  126  through drain holes  136 . The holes  136  may be positioned so that they are oriented within the generator  100  to facilitate gravity drain at different generator attitude orientations. With such positioning of the holes  136 , the oil spray  124  from the spray nozzles  122  may be precluded from entering the spray shields  126  through the holes  136 . 
         [0029]    By referring now to  FIGS. 6 and 7 , it may be noted that the stator windings  114  may be held in position with wedges segments  164  that are interlocked with notches  166  formed in the stator laminations  116 . However, the center lamination  158  may be constructed without wedge-supporting notches. Thus, the wedge segments  164  may terminate at the center lamination  158 . In other words, unlike prior art generator configuration in which a single wedge may extend along an entire length of a stator, the windings  114  of the generator  100  may be supported with partial-length wedge segments  164 . A first wedge segment  164  may extend from a first end of the stator  104  to the center lamination  158 . A second wedge segment  164  may extend from an opposite end of the stator  104  to the center lamination  158 . 
         [0030]    It may be seen that rotation of the rotor  106  may be unencumbered by windage friction from the cooling oil  122 . Consequently, the generator  100  may be particularly useful in applications that may require a compact and lightweight configuration for a generator while, at the same time, producing a high power output. For example, the generator  100  may be constructed with an overall weight of about 250 pounds to about 300 pounds and it may have a capability of generating about 800 kW to about 1,000 kW. In order to produce such high power output, the generator  100  may need to operate with rotor rotational speeds of about 12,000 to about 25,000 rpm. 
         [0031]    Referring now to  FIG. 8 , a flow chart illustrates an exemplary embodiment of a method  800  for generating electrical power with a compact generator that weighs no more than about 300 pounds and can output about 1,000 kW of useful electrical power. In other words, the generator may have a power/weight ratio of about 3 kW/lbs. Cooling of the generator may require extraction of no more than about 21 kW of heat loss energy from the generator when the generator is producing full power. 
         [0032]    In a step  802 , a rotor of the compact generator may be rotated at a high rotational speed (e.g., the rotor  106  of the generator  100  may be rotated at a speed of at least about 12,000 rpm). In a step  804 , stator windings of the generator may be sprayed with cooling oil (e.g., Cooling oil spray  124  may be sprayed from nozzles  122  onto end turns  120  of stator windings  114  of the generator  100 ). In a step  806 , cooling oil may be internally circulated through the stator spiral groove  110 . In a step  808 , a rotor of the generator may be cooled with internally circulating cooling oil (e.g., cooling oil  112  may be circulated internally through the rotor  106 ). In a step  810 , airflow may be produced through a gap between the rotor and the stator of the compact generator to preclude entry of cooling oil into the gap (e.g., pressurized air may be provided from the source  148  through a center lamination  158  of the stator  104  into the gap  146  to produce the airflow  144 , which airflow continues through the spray shields  126  and exits from the holes  136  in the spray shields  126 ). In a step  812 , sprayed cooling oil may be deflected away from the rotor (e.g., the spray shields  126  to deflect the cooling oil spray  124  from the rotor  106  so that the rotor  106  is not encumbered with windage friction forces that may have been produced by contact with the cooling oil spray  124 ). In a step  814 , electrical power may be produced by the compact generator at a rate of at least about 800 kW while heat loss energy of less than about 21 kW may be extracted by the circulated cooling oil. 
         [0033]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.