Patent Publication Number: US-2013232941-A1

Title: Apparatus for extracting input power from the low pressure spool of a turbine engine

Description:
BACKGROUND OF THE INVENTION 
     Turbine engines, and particularly gas turbine engines, also known as combustion turbine engines, are rotary engines that extract energy from a flow of combusted gases passing through the engine onto a multitude of turbine blades. Gas turbine engines have been used for land and nautical locomotion and power generation, but are most commonly used for aeronautical applications such as for airplanes, including helicopters. In airplanes, gas turbine engines are used for propulsion of the aircraft. 
     Gas turbine engines can have two or more spools, including a low pressure (LP) spool that provides a significant fraction of the overall propulsion system thrust, and a high pressure (HP) spool that drives one or more compressors and produces additional thrust by directing exhaust products in an aft direction. A triple spool gas turbine engine includes a third, intermediate pressure (IP) spool. 
     Gas turbine engines also usually power a number of different accessories such as generators, starter/generators, permanent magnet alternators (PMA), fuel pumps, and hydraulic pumps, e.g., equipment for functions needed on an aircraft other than propulsion. For example, contemporary aircraft need electrical power for avionics, motors, and other electric equipment. A generator coupled with a gas turbine engine will convert the mechanical power of the engine into electrical energy needed to power accessories. 
     It is known to use DC generators and variable frequency (VF) generators for extracting power from high pressure spools of gas turbine engines. But heretofore it has not been feasible to use such generators to extract power from low pressure spools because of the wild speed ranges of low pressure spools, which at the high end exceeds the acceptable speed tolerances of DC and VF generators. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A power generation system for extracting power from a low pressure (LP) spool of a turbine engine includes a DC or variable frequency generator, an LP drive assembly, and a control mechanism. The LP drive assembly has an input mechanically coupled to the LP spool and an output mechanically coupled to the generator. The control mechanism has a controller with a matrix of tabular commands that map the input to a desired output so that the desired output is a speed range that is lower than a speed range of the input. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic cross-sectional diagram of a gas turbine engine for an aircraft. 
         FIG. 2  is a schematic block diagram of a first embodiment of an electrical power generation system for the gas turbine engine of  FIG. 1  using a variable frequency generator. 
         FIG. 3  is a schematic block diagram of a second embodiment of an electrical power generation system for the gas turbine engine of  FIG. 1  using a DC generator. 
         FIG. 4  is a schematic diagram of a mechanism for speed range reduction in the embodiments of  FIGS. 2 and 3 . 
         FIG. 5  is an exemplary chart showing a relationship for mapping output speeds in the in the embodiments of  FIGS. 2 and 3 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The described embodiments of the present invention are directed to power extraction from an aircraft engine, and more particularly to an electrical power system architecture which enables production of electrical power from a turbine engine, preferably a gas turbine engine. It will be understood, however, that the invention is not so limited and has general application to electrical power system architectures in non-aircraft applications, such as other mobile applications and non-mobile industrial, commercial, and residential applications. 
       FIG. 1  is a schematic cross-sectional diagram of a gas turbine engine  10  for an aircraft. Engine  10  includes, in downstream serial flow relationship, a fan section  12  including a fan  14 , a booster or low pressure (LP) compressor  16 , a high pressure (HP) compressor  18 , a combustion section  20 , a HP turbine  22 , and a LP turbine  24 . A HP shaft or spool  26  drivingly connects HP turbine  22  to HP compressor  18  and a LP shaft or spool  28  drivingly connects LP turbine  24  to LP compressor  16  and fan  14 . HP turbine  22  includes an HP turbine rotor  30  having turbine blades  32  mounted at a periphery of rotor  30 . Blades  32  extend radially outwardly from blade platforms  34  to radially outer blade tips  36 . 
       FIG. 2  is a schematic block diagram of an electrical power system architecture  40  according to a first embodiment of the invention. The system architecture  40  includes multiple engine systems, shown herein as including at least a left engine system  42  and a right engine system  44 . The left and right engine systems  42 ,  44  may be substantially identical; therefore, only the left engine system  42  will be described in detail for the sake of brevity. The left engine system  42  can include the HP and LP spools  26 ,  28  of the gas turbine engine  10  shown in  FIG. 1 , although the system architecture  40  has application to other engines as well. The left engine system  42  shown herein uses mechanical power provided by two spools, the HP spool  26  and the LP spool  28 . However, the system architecture  40  could also be implemented on an engine having more than two spools, such as a 3-spool engine having an intermediate pressure spool in addition to the HP and LP spools. The system architecture  40  can further include an auxiliary power unit (APU)  46  of the aircraft and an external power source (EPS)  48 . As shown herein, the APU  46  and EPS  48  each have a DC output  50 ,  52 , respectively. 
     In the embodiment illustrated, the left engine system  42  includes a first variable frequency starter generator  56 , configured to produce variable frequency (VF) AC power from mechanical power supplied by the HP spool  26 , and a second variable frequency generator  58  configured to produce variable frequency (VF) AC power from mechanical power supplied by the LP spool  28 . 
     The HP spool  26  can be operably coupled with the first variable frequency starter generator  56  by an HP drive assembly having an input mechanically coupled to the HP spool  26  and an output mechanically coupled to the first variable frequency starter generator  56 . One embodiment of the HP drive assembly is an accessory gearbox  64 , where the first variable frequency starter generator  56  can be mounted and coupled to the accessory gearbox  64 . Within the accessory gearbox  64 , power may also be transferred to other engine accessories. The first variable frequency starter generator  56  converts mechanical power supplied by the HP spool  26  into electrical power. 
     The first variable frequency starter generator  56  can also provide a starting function to the aircraft wherein it functions a motor to start the engine  10 . Alternatively, the first variable frequency starter generator  56  on the HP side of the left engine system  42  may not necessarily provide a starting function to the aircraft. In such case, a separate starter motor connected to the accessory gearbox  64  can be provided to perform the starting function for the aircraft. Furthermore, the left engine system  42  may include multiple generators drawing mechanical power from the HP spool  26  to produce power in order to provide a measure of redundancy. 
     The second variable frequency generator  58  may be identical to the first variable frequency starter generator  56 , but for the starting function. In this situation, however, because of the fluctuating speed ranges of the LP spool  28 , the LP spool  28  is operably coupled with the first variable frequency starter generator  56  by speed range reduction assembly  74  having an input mechanically coupled to the LP spool  28  and an output mechanically coupled to the second variable frequency generator  58 . One embodiment of the speed range reduction assembly includes a controller  76  (see  FIG. 4 ) that reduces the range of the variable speed input from the LP spool  28  to a range within the tolerances of the second variable frequency generator  58 . The first variable frequency starter generator  56  converts mechanical power supplied by the HP spool  26  into VF electrical power output. 
     Although the embodiment shown herein is described as using one second variable frequency generator  58  on the LP side of the left engine system  42 , another embodiment of the invention may use multiple second variable frequency generators  58  drawing mechanical power from the LP spool  28  to produce AC power in order to provide a measure of redundancy. Furthermore, while a separate second variable frequency generator  58  and speed range reduction assembly  74  are discussed herein, an integrated drive generator which combines the speed range reduction assembly  74  and the second variable frequency generator  58  into a common unit can alternatively be used. 
     Power output  68  from the first variable frequency starter generator  56  is supplied to a first electrical AC bus  86 . Similarly, power output  78  from the second variable frequency generator  58  is supplied to a second electrical AC bus  94 . Some AC power  90  is drawn from the first electrical AC bus  86  to an AC/DC converter  84  for converting the AC power output  90  to a DC power output  92  which is fed to an electrical DC bus  98 . 
     A motor-starter controller  96  can selectively provide power from the electrical DC bus  98  to the first variable frequency starter generator  56  to initiate a starting procedure for the aircraft. The motor-starter controller  96  can be integrated with the first variable frequency starter generator  56  for engine starting by connecting the motor-starter controller  96  to first variable frequency starter generator  56  as shown  FIG. 2 . 
     The first and second electrical buses  86 ,  94  are configured to supply AC power to one or more loads (not shown) that require a AC power supply. The first and second electrical buses  86 ,  94  can be selectively connected to enable loads to be shared by the HP spool  26  and the LP spool  28 . 
     In operation, with the gas turbine engine  10  started, HP turbine  22  rotates the HP spool  26  and the LP turbine  24  rotates the LP spool. The accessory gearbox  64  is driven by the rotating HP spool  26 , and transmits mechanical power from the HP spool  26  to the first variable frequency starter generator  56 . The first variable frequency starter generator  56  converts mechanical power supplied by the HP spool  26  into electrical power and produces the AC power output  68 . The speed range reduction assembly  74  is driven by the rotating LP spool  28 , and transmits mechanical power from the LP spool  28  to the second variable frequency generator  58 . The second variable frequency generator  58  converts the mechanical power supplied by the LP spool  28  into electrical power and produces the AC power output  78 . The power outputs  68 ,  78  can be respectively provided to the electrical AC buses  86 ,  94  configured to supply AC power to one or more loads (not shown) that require a AC power supply. Depending on the type of load drawing power, the AC power extracted by the system architecture  40  may undergo further processing before being used by the loads. The DC power outputs  50 ,  52  of the APU  44  and the EPS  48 , if converted, can also be provided to the electrical AC buses  86 ,  94 . 
     The left and right engine systems  42 ,  44 , APU  46  and EPS  48  can provide DC power to various loads of the aircraft as needed. The various DC outputs of the left engine system  42 , the right engine system  44 , the APU  46 , and the EPS  48  are preferably integrated with appropriate switches to provide no break power transfer (NBPT) to the aircraft. 
       FIG. 3  is a schematic block diagram of an electrical power system architecture  400  according to a second embodiment of the invention. The system architecture  400  includes multiple engine systems, shown herein as including at least a left engine system  420  and a right engine system  440 . The left and right engine systems  420 ,  440  may be substantially identical; therefore, only the left engine system  420  will be described in detail for the sake of brevity. The left engine system  420  can include the HP and LP spools  26 ,  28  of the gas turbine engine  10  shown in  FIG. 1 , although the system architecture  400  has application to other engines as well. The left engine system  420  shown herein uses mechanical power provided by two spools, the HP spool  26  and the LP spool  28 . However, the system architecture  400  could also be implemented on an engine having more than two spools, such as a 3-spool engine having an intermediate pressure spool in addition to the HP and LP spools. The system architecture  40  can further include an auxiliary power unit (APU)  46  of the aircraft and an external power source (EPS)  48 . As shown herein, the APU  46  and EPS  48  each have a DC output  50 ,  52 , respectively. 
     In the embodiment illustrated, the left engine system  420  includes a first autotransformer unit (ATU) integrated generator  560 , shown herein as a first variable frequency starter generator  560 , configured to produce variable frequency (VF) AC power from mechanical power supplied by the HP spool  26 , and a second ATU integrated generator  580  configured to produce variable frequency (VF) AC power from mechanical power supplied by the LP spool  28 . 
     The first variable frequency starter generator  560  includes a power generation section  600  and an ATU section  620 . The ATU section  620  may be integrated with the power generation section  600  by integrating some of the electrical windings necessary for power transformation on the electrical winding of the power generation section  600  which can effectively eliminate winding duplication in the power generation section  600  and the ATU section  620 , and can translate into weight and cost savings for the aircraft. 
     The HP spool  26  can be operably coupled with the first variable frequency starter generator  560  by an HP drive assembly having an input mechanically coupled to the HP spool  26  and an output mechanically coupled to the power generation section  620 . One embodiment of the HP drive assembly is an accessory gearbox  640 , where the first variable frequency starter generator  560  can be mounted and coupled to the accessory gearbox  640 . Within the accessory gearbox  640 , power may also be transferred to other engine accessories. The power generation section  600  of the first variable frequency starter generator  560  converts mechanical power supplied by the HP spool  26  into electrical power and produces a power supply  660  having three phase outputs. The ATU section  620  of the first variable frequency starter generator  560  functions to both transform the three phase outputs of the power supply  660  into a nine phase power output  680  and to step up the voltage of the power supply. 
     The first variable frequency starter generator  560  also provides a starting function to the aircraft. Alternatively, the first variable frequency starter generator  560  on the HP side of the left engine system  420  may comprise a generator that does not provide a starting function to the aircraft. In this case, a separate starter motor connected to the accessory gearbox  600  can be provided to perform the starting function for the aircraft. Furthermore, the left engine system  420  can include multiple generators drawing mechanical power from the HP spool  26  to produce power in order to provide a measure of redundancy. 
     The second variable frequency generator  580  includes a power generation section  700  and an ATU section  720 . The LP spool  28  can be operably coupled with the second variable frequency generator  580  by an LP drive assembly having an input mechanically coupled to the LP spool  28  and an output mechanically coupled to the power generation section  700 . One embodiment of the speed range reduction assembly  740  includes a controller that reduces the range of the variable speed input from the LP spool  28  to a range within the tolerances of the second variable frequency generator  58 . As shown herein, the speed range reduction assembly  740  can be mechanically coupled to the second variable frequency generator  58  and drives the power generation section  700  at a variable speed different than the input speed. The power generation section  700  of the second variable frequency generator  58  converts mechanical power supplied by the LP spool  28  into electrical power and produces a power supply  760  having three phase outputs. The ATU section  720  of the second variable frequency generator  58  functions to both transform the three phase outputs of the power supply  760  into a nine phase power output  780  and to step up the voltage of the power supply. 
     Although the embodiment shown herein is described as using one second variable frequency generator  580  on the LP side of the left engine system  42 , another embodiment of the invention may use multiple second variable frequency generators  58  drawing mechanical power from the LP spool  28  to produce AC power in order to provide a measure of redundancy. Furthermore, while a separate second variable frequency generator  58  and speed range reduction assembly  740  are discussed herein, an integrated drive generator which combines the speed range reduction assembly  740  and second variable frequency generator  58  into a common unit can alternatively be used. 
     The power output  680  from the integrated first variable frequency starter generator  560  is supplied to a first AC/DC converter for converting the AC power output  680  to a DC power output  800 . As illustrated, the first AC/DC converter can include a first rectifier device  820  and a first filter  840  for converting the AC voltage to DC voltage and for evening out the current flow before being supplied to a first electrical DC bus  860 . Similarly, the power output  780  from the second variable frequency generator  580  is supplied to a second AC/DC converter for converting the AC power output  780  to a DC power output  880 . As illustrated, the second AC/DC converter can include a second rectifier device  900  and a second filter  920  for converting the AC voltage to DC voltage and for evening out the current flow before being supplied to a second electrical DC bus  940 . 
     A motor-starter controller  960  can selectively provide power from the first electrical bus  860  to the first variable frequency starter generator  560  to initiate a starting procedure for the aircraft. The motor-starter controller  960  can be integrated with the first variable frequency starter generator  560  for engine starting by connecting the motor-starter controller  960  to the specific location of the first variable frequency starter generator  560  as shown  FIG. 3 . The three phase motor-starter controller  960  is connected to the three phase power supply  660  to drive the first variable frequency starter generator  560  as a three phase starter for engine starting. 
     The first and second electrical buses  860 ,  940  are configured to supply DC power to one or more loads (not shown) that require a DC power supply. The first and second electrical buses  860 ,  940  can be selectively connected to enable loads to be shared by the HP spool  26  and the LP spool  28 . 
     In operation, with the gas turbine engine  10  stared, HP turbine  22  rotates the HP spool  26  and the LP turbine  24  rotates the LP spool. The accessory gearbox  640  is driven by the rotating HP spool  26 , and transmits mechanical power from the HP spool  26  to the first variable frequency starter generator  560 . The first variable frequency starter generator  560  converts mechanical power supplied by the HP spool  26  into electrical power and produces the DC power output  800 . The speed range reduction assembly  740  is driven by the rotating LP spool  28 , and transmits mechanical power from the LP spool  28  to the second variable frequency generator  580 . The second variable frequency generator  580  converts the mechanical power supplied by the LP spool  28  into electrical power and produces the DC power output  880 . The power outputs  800 ,  880  can be respectively provided to the electrical buses  860 ,  940  configured to supply DC power to one or more loads (not shown) that require a DC power supply. Depending on the type of load drawing power, the DC power extracted by the system architecture  400  may undergo further processing before being used by the loads. The DC power outputs  50 ,  52  of the APU  44  and the EPS  48  can also be provided to the electrical buses  860 ,  940 . 
     The left and right engine systems  42 ,  44 , APU  46  and EPS  48  can provide DC power to various loads of the aircraft as needed. The various DC outputs of the left engine system  42 , the right engine system  44 , the APU  46 , and the EPS  48  are integrated with appropriate switches to provide no break power transfer (NBPT) to the aircraft. 
       FIG. 4  is a schematic diagram of the speed range reduction assembly  74 ,  740 . The speed range reduction assembly  74 ,  740  comprises a conventional constant speed drive (CSD)  300  which may be based on a continuously variable transmission or a hydraulic system. As mentioned above, the CSD  300  may be coupled to the output of the LP spool  28 , and integrated with or otherwise coupled to a variable frequency generator  56 ,  58 ,  560 ,  580 , and to a controller  32 . The controller  32  is configured to receive feedback signals  34  from the variable frequency generator  56 ,  58 ,  560 ,  580 , and process them with an algorithm of tabular commands  36  to alter the speed of the CSD  300  and the consequent input to the variable frequency generator  56 ,  58 ,  560 ,  580 . 
       FIG. 5  illustrates graphically how the output speeds of the CSD  300  are determined by the controller  76 . Plot A is an empirically determined curve showing an exemplary relationship between input speeds from the LP spool  28  and output speeds from the CSD  300  for a variable frequency generator having high efficiencies at high speeds. Plot B is an empirically determined curve showing an exemplary relationship between input speeds from the LP spool  28  and output speeds from the CSD  300  for a variable frequency generator having high efficiencies at low speeds. And Plot C is an empirically determined curve showing an exemplary purely proportional relationship between input speeds from the LP spool  28  and output speeds from the CSD  300  for a variable frequency generator. Actual values for the curves depend on many factors, including the specifications of particular generators, and they can be determined empirically and/or by testing or virtual modeling. Exemplary speed ranges of LP spool may be 4:1 or 5:1, and they can be reduced to 2:1, which is an exemplary range of the same proximity for a standard VF generator. 
     In operation, the controller  32  applies the algorithm of the curve correlating to a given generator to reduce the speed range of the output of the CSD  300  from the higher speed range of the input from the LP spool  28 . As the LP spool  28  rotates, the controller  32  continuously receives signals from the input to the CSD  300  and maps the output speed of the CSD  300  to the input speed based on the algorithm. The algorithm may be implemented by the controller  76  using tabular commands extracted from the selected curve. Ideally, the mapping can be optimized for the most efficient operation of the generator. 
     One advantage that may be realized in the practice of some embodiments of the system architecture disclosed herein is that DC and VF generators that are readily available for extracting power from HP spools can now be operated with LP spools, thereby saving significant cost in separate development and sourcing for generators that are readily available for extracting power from LP spools. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.