Abstract:
A closed loop cooling system and method for an oil-free electric motor of an aircraft cabin air compressor. A closed-loop fan, which may be coupled to a motor rotor opposite a compressor impeller, circulates the required cooling air in a closed loop through a motor stator and bearings, a motor cooler, and a ducting used to complete the flow circuit via a ram air cooled heat exchanger. The cooling air may be pressurized to cabin pressure. The system and method of the present invention eliminates the possibility of contaminating trace amounts of oil in cabin supply air.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/482,502, filed on Jun. 24, 2003. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to a method and apparatus for cooling a motor, and in particular, to a novel closed-loop system for cooling a compressor having air bearings and an air-cooled motor.  
           [0003]    Environmental control systems for the more electric aircraft (MEA) use a dedicated compressor to pressurize ambient air for use as fresh air in the cabin. The compressor may be, for example, a single stage centrifugal configuration driven by a variable speed permanent magnet motor. In traditional environmental control systems, the pressurized air for the cabin is bled from the main engine or the auxiliary power unit. The air can sometimes become contaminated with an odor due to the leakage of trace amounts of oil, for example, through shaft seals.  
           [0004]    An essential attribute of the MEA environmental control system (ECS) is that the method of compressing ambient air be oil-free. To meet this requirement, the compressor rotating assembly is supported by air bearings and the motor is air-cooled. In order for the air bearings to operate properly, the motor/bearing cavity should be maintained at near cabin pressure. The cooling airflow rate required to maintain satisfactory stator winding temperatures is significant, about 0.3-0.4 lb/min per kW of motor power. At the aircraft maximum cruising altitude, the motor driven compressor can deliver 0.65 lb/min per kW of motor power. Furthermore, the low air density at high altitudes increases the need for large motors with the consequent payload increase. For this reason, the cabin air compressors are by far the largest users of electric power on the more electric aircraft. Given these parameters, it is not feasible to use all fresh air with an open-loop system for motor cooling due to the substantial equipment weight and fuel burn penalties.  
           [0005]    An apparatus that attempts to use fresh air within an open-loop system to cool a motor is disclosed in U.S. Patent Application Publication No. 2004/0005228 A1 of Agrawal, et al. The publication describes an oil-free motor-driven compressor/blower for use in fuel cell systems. Fresh, ambient air may be used to cool the motor. The device disclosed in the publication is not suitable for use at high altitudes or aboard flight vehicles, as fresh, ambient air at these conditions is of low density. Using low-density air requires a larger compressor and excess power to achieve the same function as using higher density air for motor cooling. Furthermore, continuous compression of low-density fresh ambient air would be required to use the device of Agrawal et al. The device in the Agrawal publication is not suitable for use aboard an aircraft due to the constraints on equipment size, weight, and power usage of aircraft components.  
           [0006]    As can be seen, there is a need for an improved apparatus and method for cooling an electric motor for driving an aircraft cabin air compressor without contaminating the cabin air with lubricants. Furthermore, there is a need for an apparatus and method for cooling an air-cooled motor that functions efficiently during flight.  
         SUMMARY OF THE INVENTION  
         [0007]    In one aspect of the present invention, an apparatus for cooling a motor comprises a closed-loop fan for supplying forced air to the motor; and a motor cooler, wherein a recycle air exits the motor and enters the motor cooler; and wherein the motor cooler provides cooled air to the closed-loop fan.  
           [0008]    In a further aspect of the present invention, a compressor cooling system for a vehicle comprises a motor including a motor housing; a compressor driven by the motor such that the compressor compresses a ram air; a closed-loop fan for supplying forced air into the motor housing; and a motor cooler for receiving a recycle air from the motor housing, wherein the motor cooler cools the recycle air to provide cooled air to the closed-loop fan.  
           [0009]    In still a further aspect of the present invention, a compressor cooling system for a vehicle comprises a motor including a motor housing; a compressor driven by the motor such that the compressor compresses a ram air; a closed-loop fan for withdrawing forced air from the motor housing; and a motor cooler for receiving a recycle air from the motor housing, wherein the motor cooler cools the recycle air to provide cooled air to the closed-loop fan.  
           [0010]    In another aspect of the present invention, a cabin air compressor cooling system for an aircraft comprises a motor; a fan for supplying forced air into the motor; a motor cooler such that a recycle air exits the motor and enters the motor cooler; and a compressor driven by the motor and disposed between the motor and the motor cooler such that the compressor compresses a ram air after the ram air flows across the motor cooler; wherein the motor cooler provides cooled air to the fan.  
           [0011]    In yet another aspect of the present invention, a compressor cooling system for an aircraft comprises a motor; a compressor driven by the motor, wherein the compressor compresses a ram air; a backface cavity situated between the motor and the compressor; a closed-loop fan for supplying forced air into the motor; a motor cooler such that a recycle air exits the motor and enters the motor cooler; wherein the motor cooler provides cooled air to the closed-loop fan; and an air conditioning pack for receiving a compressed air from the compressor.  
           [0012]    In still a further aspect of the present invention, a method for cooling a motor comprises forcing air into the motor; drawing recycle air from the motor into a motor cooler; cooling the recycle air within the motor cooler to produce a cooled air; and feeding the cooled air into the motor.  
           [0013]    In yet a further aspect of the present invention, a method for cooling a motor comprises forcing air into an electric motor by using a closed-loop fan; flowing air across a stator and rotor of the electric motor for cooling; drawing recycle air from the electric motor into an air-to-air heat exchanger; driving a compressor with the electric motor, wherein the compressor compresses a ram air; cooling the recycle air within the air-to-air heat exchanger to produce a cooled air; cooling the air-to-air heat exchanger with another ram air; and feeding the cooled air into the electric motor; wherein the electric motor comprises an oil-free electric motor and includes an air bearing. The cooled air into the electric motor also may provide cooling air for the air bearing.  
           [0014]    These and other aspects, objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a block diagram schematically representing a cooling system for an air-cooled motor, according to the present invention;  
         [0016]    [0016]FIG. 2 is a schematic of a cooling system, according to an embodiment of the present invention;  
         [0017]    [0017]FIG. 3 is a schematic of a cooling system, according to another embodiment of the present invention;  
         [0018]    [0018]FIG. 4 is a cross-sectional view of a motor and compressor, according to an embodiment of the present invention;  
         [0019]    [0019]FIG. 5 is a flow chart of a method for cooling a motor, according to an embodiment of the present invention; and  
         [0020]    [0020]FIG. 6 is a flow chart of a method for cooling a motor, according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    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.  
         [0022]    Broadly, the present invention provides a closed-loop, oil-free cooling system, such as for an air-cooled, cabin air compressor of a more electric aircraft (MEA). This system can include a motor cooler that uses ram air to cool a stream of air that removes heat from an electric motor that is used to drive the compressor for, example, an environmental control system of an aircraft. Although the following description will describe the present invention as being used in aircraft, the following description should be understood to be applicable to other suitable uses, such as ground vehicles and stationary installations to provide cooling air. The present invention may be used in systems that drive electric generators, hydraulic pumps, or other equipment requiring constant speed operation.  
         [0023]    Because the present invention recycles cooling air in a closed-loop and does not rely on oil-lubricated bearings, it has several advantages over prior art motor cooling systems. For example, the cooling system of the present invention features: decreased motor size, because continuous compression of low-density fresh air at high altitudes is not required; operation without the risk of contaminating cabin air with traces of lubricant oil; the ability to sustain extreme temperatures; and either no scheduled maintenance, or a reduced maintenance schedule because lubricant levels do not have to be checked. Thus, the present invention differs from the prior art in that a motor cooler uses a closed-loop air system to remove heat from an oil-free electric motor having air bearings.  
         [0024]    In more specifically describing the present invention, and as can be appreciated from FIG. 1, the present invention provides a compressor cooling system  10  which may be located aboard an aircraft. Ram air  30  may enter a compressor  20  which may compress the ram air  30  to provide compressed air  60 . A first portion  62  of compressed air  60  may be ducted to an air conditioning pack  134 . The compressor  20  may be coupled to a motor  40 . In some cases, an electric motor may be used to drive the compressor  20 . As an example, motor  40  may comprise a variable speed permanent magnet motor. As heat is produced by the motor  40 , a cooling air  70  may be provided to the motor  40  by a closed-loop fan  50 . As the cooling air  70  removes heat from the motor  40 , the hotter recycle air  80  may leave the motor  40  and enter a motor cooler  100 . A ram air  90  (not necessarily the same as ram air  30 ) may flow across the motor cooler  100  to remove heat from the recycle air  80  to produce a cooled air  110 . The cooled air  110  may leave the motor cooler  100  and may be supplied to the closed-loop fan  50  for repeated cooling of the motor  40 . The closed-loop fan  50  may be driven by the motor  40  or by a dedicated motor (not shown).  
         [0025]    Another embodiment of a cooling system of the present invention is shown schematically in FIG. 2. A portion of the compressed air  60  may be fed to an air conditioning pack  134  for cooling cabin air. Compressor cooling system  10  may comprise motor  40  that includes a motor housing  46 . Inside the motor housing  46 , a shaft  48  may rotate to drive a rotor  44  adjacent to a stator  42 . A closed-loop fan  50  may move a forced air  112 , which may flow within the motor housing  46  and across the shaft  48 , the stator  42 , and the rotor  44  The closed-loop fan  50  may be driven by the shaft  48 . The closed-loop fan  50  may be affixed to the motor housing  46  or to another part of the motor  40 . However, the closed-loop fan  50  may also adequately move the forced air  112  without being affixed or attached to the motor housing  46  or any other part of the motor  40 . For example, in an alternative embodiment (not shown), closed-loop fan  50  may be a stand-alone unit disposed at a location remote from motor  40 .  
         [0026]    While the elements in the compressor cooling system  10  may be shown in FIG. 2 in axial alignment (for example, compressor  20  and motor  40 ), it should be understood that the present invention may or may not have the elements in axial alignment.  
         [0027]    A backface cavity  130  may be disposed between the motor  40  and the compressor  20 . The backface cavity  130  may be open to the external environment. As the forced air  112  travels the length of the motor  40 , a portion of forced air  112  may exit, or leak, through the backface cavity  130  as seal leakage air  128 . To replace any seal leakage air  128 , a make-up air  124  may be added, for example, to the recycle air  80 . As an example, make-up air  124  may be obtained by ducting a portion of the compressed air  60  for combination with recycle air  80 . Optionally, the make-up air  124  may be filtered with a scavenge flow filter  120 , which may remove waste  122  from the make-up air  124 . The make-up air  124  may be added, optionally controlled by a valve (not shown), to the recycle air  80  as needed.  
         [0028]    The motor cooler  100  may comprise an air-to-air heat exchanger. A heat exchanger  132 , which may be incorporated into the air conditioning pack  134 , may be placed adjacent to the motor cooler  100 . The motor cooler  110  may be cooled by a ram air  90 .  
         [0029]    A schematic representation of a further embodiment of a compressor cooling system  10  is shown in FIG. 3. The embodiment of compressor cooling system  10  shown in FIG. 3 may have the same or similar elements, features, and characteristics as described above with reference to FIG. 2. However, in the embodiment shown in FIG. 3, the compressor cooling system  10  may be configured such that the ram air  90  compressed by the compressor  20  is the same ram air  90  that flows across the motor cooler  100 . A first portion of a single source of ram air  30  may be compressed by the compressor  20 , and a second portion  92  of the single source of ram air  30  may flow through the motor cooler  100 .  
         [0030]    As shown in FIG. 3, the compressor  20  also may be located between the motor  40  and the motor cooler  100 . While the elements in the compressor cooling system  10  may be shown in FIG. 3 in axial alignment (for example, motor cooler  100 , compressor  20 , and motor  40 ), it should be understood that the present invention may or may not have the elements in axial alignment. While placing the compressor  20  between the motor  40  and the motor cooler  100  is one way to configure the compressor cooling system  10  such that the compressor  20  compresses a ram air  90  after the ram air  90  flows across the motor cooler  100 , it should be understood that the present invention may be configured in other ways such that the same ram air  90  that is compressed by the compressor  20  is also the same ram air  90  that flows across and cools the motor cooler  100 .  
         [0031]    In FIG. 4, a cross-sectional view of a motor  40  and a compressor  20 , according to a further embodiment of the present invention is shown. The embodiment of motor  40  shown in FIG. 4 may have the same or similar elements, features, and characteristics as described above with reference to FIGS. 2 and 3. However, differences from FIGS. 2 and 3, or more details, may be shown in FIG. 4. For example, the compressor  20  may be attached to a compressor discharge  22 , as compared with the motor  40  in FIGS. 2 and 3, which do not show a compressor discharge  22 . Shaft seal  24  may be the site where seal leakage air  128  (shown in FIGS. 2 and 3) may exit, or leak, through the backface cavity  130 . The motor  40  may comprise at least one air bearing  26 , typically a plurality of air bearings  26 , and more typically exclusively air bearings  26  (i.e., motor  40  may typically lack oil-lubricated bearings), as compared to the prior art oil-lubricated systems.  
         [0032]    With reference to FIG. 5, a method  200  for cooling a motor  40 , according to yet another embodiment of the present invention, is described. Method  200  may comprise a step  210  of forcing forced air  112  into a motor  40 . Thereafter, step  220  may involve drawing recycle air  80  from the motor  40  into a motor cooler  100 . Thereafter, step  230  may comprise combining make-up air  124  from the compressed air  60  with the recycle air  80 . Thereafter, step  240  may comprise cooling the recycle air  80  within the motor cooler  100  to produce a cooled air  110 ; and step  250  may comprise feeding the cooled air  110  into the motor  40 . Optionally, the forcing step  210  may include using a closed-loop fan  50  for forcing the air  112  into the motor  40 . The motor  40  may be an oil-free motor, such as an electric motor having air bearings. The feeding step  250  may include using a closed-loop fan  50  for feeding the cooled air  110  into the motor  40 .  
         [0033]    With reference to FIG. 6, a method  300  for cooling a motor  40 , according to still another embodiment of the present invention, is described. Method  300  may comprise a step  310  of forcing air  112  into an electric motor  40  by using a closed-loop fan  50 . Thereafter, a step  320  may comprise flowing air  112  across a stator  42  and a rotor  44  of the electric motor  40  for cooling. Thereafter, a step  330  may involve drawing recycle air  80  from the electric motor  40  into an air-to-air heat exchanger. Thereafter, a step  340  may comprise driving a compressor  20  with the electric motor  40 , wherein the compressor  20  compresses a ram air  30 . Thereafter, a step  350  may involve cooling the recycle air  80  within the air-to-air heat exchanger to produce a cooled air  110 . Thereafter, a step  360  may comprise cooling the air-to-air heat exchanger with another ram air  90 , and a step  370  may involve feeding the cooled air  110  into the electric motor  40 , wherein the electric motor  40  comprises at least one air bearing  26 . Optionally, a step  380  may comprise combining make-up air  124  from the compressed air  60  with the recycle air  80 .  
         [0034]    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.