Abstract:
A system for generating supplemental electrical power from the low-pressure (LP) turbine spool of a turbofan engine includes a high-speed, high magnetic pole count, generator, a gearbox, a controller and a power converter. The LP turbine spool is mechanically coupled to the generator portion by the gearbox for driving the generator portion. The controller portion has a speed-sensing element for sensing the LP turbine speed. The controller portion disables the power converter when the generator exceeds a predetermined speed, and enables the power converter when the generator portion is less or equal to the predetermined speed. The effective load on the generator is reduced to approximately zero when the LP turbine spool exceeds the predetermined speed, permitting the generator to be electrically bound up to the predetermined speed and mechanically bound in excess of the predetermined speed.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to an electrical generator for a gas turbine engine, and more particularly to a high-speed generator with high pole count. 
   BACKGROUND OF THE INVENTION 
   A gas turbine engine generally includes one or more compressors followed in turn by a combustor and high and low pressure turbines. These engine components are arranged in serial flow communication and disposed about a longitudinal axis centerline of the engine within an annular outer casing. The compressors are driven by the respective turbines and compressor air during operation. The compressor air is mixed with fuel and ignited in the combustor for generating hot combustion gases. The combustion gases flow through the high and low pressure turbines, which extract the energy generated by the hot combustion gases for driving the compressors, and for producing auxiliary output power. 
   The engine power is transferred either as shaft power or thrust for powering an aircraft in flight. For example, in other rotatable loads, such as a fan rotor in a by-pass turbofan engine, or propellers in a gas turbine propeller engine, power is extracted from the high and low pressure turbines for driving the respective fan rotor and the propellers. 
   It is well understood that individual components of turbofan engines, in operation, require different power parameters. For example, the fan rotational speed is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be very low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separate high and low turbines with independent power transmitting devices are necessary to drive the fan and core compressor in aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power. 
   Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand up to twice the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly. 
   Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from additional sources within the engine, rather than to rely solely on the HP engine spool to drive the electrical generators. The LP engine spool provides an alternate source of power transfer, however, the relatively lower speed of the LP engine spool typically requires the use of a gearbox, as slow-speed electrical generators are often larger than similarly rated electrical generators operating at higher speeds. 
   However, extracting this additional mechanical power from an engine when it is operating at relatively low power levels (e.g., at or near idle descending from altitude, low power for taxi, etc.) may lead to reduced engine operability. Traditionally, this power is extracted from the high-pressure (HP) engine spool. Its relatively high operating speed makes it an ideal source for mechanical power to drive electrical generators that are attached to the engine. However, it is desirable at times to increase the amount of power that is available on this spool, by transferring torque and power to it via some other means. 
   Many solutions to this transformation are possible, including various types of conventional transmissions, mechanical gearing, and electromechanical configurations. One such solution is a turbine engine that utilizes a third, intermediate-pressure (IP) spool to drive a generator independently. However, this third spool is also required at times to couple to the HP spool. The means used to couple the IP and HP spools are mechanical clutch or viscous-type coupling mechanisms. 
   U.S. Pat. No. 6,895,741, issued May 24, 2005, and entitled “Differential Geared Turbine Engine with Torque Modulation Capacity”, discloses a mechanically geared engine having three shafts. The fan, compressor, and turbine shafts are mechanically coupled by applying additional epicyclic gear arrangements. The effective gear ratio is variable through the use of electromagnetic machines and power conversion equipment. 
   High-speed electric machines are almost always manufactured with low pole counts, lest the magnetic materials experience excessive core losses at higher frequencies that results in an inefficient motor design. This is primarily related to the fact that the soft material used in the vast majority of present motors is a silicon-iron alloy. It is well known that losses resulting from changing a magnetic field at frequencies greater than about 400 Hz in conventional silicon-iron based materials causes the material to heat, frequently to a point where the device cannot be cooled by any suitable means. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a new system and apparatus for high-speed generators that can rotate mechanically up to a very high speed but generate electrical power based on a lower rotational speed. Usually one of the key limitations for the design of high-speed generators is the number of magnetic poles because it determines the fundamental electrical frequency and hence the power converter PWM frequency. The PWM frequency cannot exceed a certain limit in order to keep the converter switching losses within acceptable ranges. The number of magnetic poles has a significant effect on the size of the stator and rotor back iron and hence the size and weight of the generator. A generator with a high number of magnetic poles must be small and light-weight to stay within the frequency limit imposed by the converter operation. In addition, a high number of magnetic poles allows the use of tooth winding configuration that is fault-tolerant, which is a key issue in several applications especially the aerospace applications. If a machine operating at a wide speed range (in a direct-drive configuration) is only required to generate power over a narrower portion of its wide speed range, it is suggested to gear up the speed so that the number of magnetic poles are only limited by the frequency at the top speed for power generation. The machine can still mechanically rotate up to the maximum speed. Gearing up the speed will help reduce the size of the machine. Limiting the frequency at a much lower speed will allow the use of a higher number of magnetic poles, which will have a significant effect on reducing the size and weight of the machine. This machine can be of any type, e.g. switched reluctance, permanent magnet etc. Also it can be combined with other machines in the form of double-sided dual-rotor generators or single-stator dual-rotor generators for further reduction of overall system size and weight. This machine can either be a radial-flux or an axial-flux machine. 
   Another source of power within the engine is the low-pressure (LP) spool, which typically operates at speeds much slower than the high-pressure (HP) spool, and over a relatively wider speed range. Tapping this low-speed mechanical power source without transformation typically results in impractically large generators. The LP spool has a wider operating speed range, typically 1100-4500 rpm, however, the LP generator may require electrical power generation corresponding to about 2200 rpm during idle-descent, even though it will still be spinning up to 4500 rpm since it cannot be disengaged from the LP spool. One means of reducing the size of the generator is by stepping up the speed range, e.g., using a gear box, so that the machine is sized for a smaller torque for the same power. Since an active power converter controls the generator, practical limitations are imposed on the converter PWM frequency in order to reduce the converter switching losses and hence achieve good overall system efficiency. This limitation on the PWM frequency defines a limitation on the maximum machine fundamental frequency that in turn defines the number of magnetic poles. If this limitation is imposed at a lower speed, the machine can have a higher number of magnetic poles, resulting in a thickness reduction in the back iron of the stator and rotor portions. Hence, the overall machine size and weight is reduced, a critical parameter for aerospace applications. 
   The present invention is directed to a system for generating supplemental electrical power from the low-pressure (LP) turbine spool of a turbofan engine. The system includes a high-speed, high magnetic pole count, generator, a gearbox, a controller and a power converter. The LP turbine spool is mechanically coupled to the generator portion by the gearbox for driving the generator portion. The controller portion has a speed-sensing element for sensing the LP turbine speed. The controller portion disables the power converter when the generator exceeds a predetermined speed, and enables the power converter when the generator portion is less or equal to the predetermined speed. The effective load on the generator is reduced to approximately zero when the LP turbine spool exceeds the predetermined speed, permitting the generator to be electrically bound up to the predetermined speed and mechanically bound in excess of the predetermined speed. 
   The present invention is also directed to system for generating supplemental electrical power from the low-pressure (LP) turbine spool of a turbofan engine, the system having a generator portion for generating electrical power having a stator back iron portion and a rotor back iron portion, the stator back iron portion and the rotor back iron portion having significantly reduced thickness relative to low magnetic pole count generators. The system also includes a gearbox for driving the generator portion, a controller portion for controlling an output of the generator, a power converter for converting generator power to power a load and the LP turbine spool being mechanically coupled to the generator portion by the gearbox for driving the generator portion. Further the system includes the controller portion in electrical communication with a speed-sensing element for sensing a speed of the LP turbine spool. During operation of the engine, the controller portion is configured to disable the power converter in response to the speed of the generator portion exceeding a predetermined speed, and to enable the power converter when the speed of the generator portion does not exceed the predetermined speed, such that the effective load on the generator portion is reduced to approximately zero when the LP turbine spool exceeds the predetermined speed. 
   An advantage of the present invention is the use of a number of magnetic poles that is greater than conventional related devices that make feasible concentrated and isolated armature windings that exhibit increased fault tolerance. For even further reduction of overall generator weight, the high-speed generator can be combined with other HP spool generators in the form of double-sided dual-rotor or single-stator dual-rotor configurations. 
   Another advantage is the ability to extract electrical power from the LP spool and combination of HP and LP generators, to provide significant weight and size reduction in the generator. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal sectional view schematic illustration of an exemplary aircraft turbofan gas turbine engine. 
       FIG. 2  is a diagrammatic representation of the generator drive train of the present invention. 
       FIG. 3  is a diagram of the active and passive speed ranges of the generator. 
       FIG. 4  is a schematic representation of a low-pole count prior art permanent magnet machine. 
       FIG. 5  is a schematic representation of a high-pole count permanent magnet machine of the present invention. 
       FIG. 6  is a schematic representation of a low-pole count prior art permanent magnet machine with distributed overlapping windings. 
       FIG. 7  is a schematic illustration of a high-pole count permanent magnet machine of the present invention having concentrated non-overlapping windings. 
       FIG. 8  is an alternate embodiment of the high speed, high pole count generator configured in combination with another generator driven by the HP turbine. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Shown in  FIG. 1  is an exemplary turbofan engine  10  having a generally axially extending axis or centerline  12  generally extending in a forward direction  14  and an aft direction  16 . The bypass turbofan engine  10  includes a core engine  18  (also called a gas generator) which includes a high pressure compressor  20 , a combustor  22 , and a high pressure turbine (HPT)  23  having a row of high pressure turbine blades  24 , all arranged in a serial, axial flow relationship. High-pressure compressor blades  64  of the high-pressure compressor  20  are fixedly connected in driving engagement to the high pressure turbine blades  24  by a larger-diameter annular core engine shaft  26  which is disposed coaxially about the centerline  12  of the engine  10  forming a high pressure spool  21 . 
   A combustor  22  in the core engine  18  mixes pressurized air from the high-pressure compressor  20  with fuel and ignites the resulting fuel and air mixture to produce combustion gases. Some work is extracted from these gases by the high-pressure turbine blades  24  causing the blades  24  to rotate, and by this rotation driving the high-pressure compressor  20 . The combustion gases are discharged from the core engine  18  into a power turbine or low-pressure turbine (LPT)  27  having a row of low-pressure turbine blades  28 . The low-pressure turbine blades  28  are fixedly attached to a smaller diameter annular low-pressure shaft  30  that is disposed coaxially about the centerline  12  of the engine  10  within the core engine shaft  26  forming a low-pressure spool  29 . The low-pressure shaft  30  rotates axially spaced-apart first and second stage fans  31  and  33  of an engine fan section  35 . The first and second stage fans  31  and  33  include first and second stage rows of generally radially outwardly extending and circumferentially spaced-apart first and second stage fan blades  32  and  36 , respectively. 
   A fan bypass duct  40  circumscribes the second stage fan  33  and the core engine  18 . Core discharge airflow  170  is discharged from the low pressure turbine  27  to mix with a bypass airflow  178  discharged from the fan bypass duct  40  through a rear variable area bypass injector (VABI)  53 . Mixing takes place in a tail pipe  69  in which exhaust flow is formed, which is discharged through a variable area exhaust nozzle  122 . An optional afterburner  130  may be used to increase the thrust potential of the engine  10 . 
   Referring next to  FIG. 2 , an electromagnetic generator  50  of the present invention is coupled with a gearbox  44  through a connecting shaft  46 . The gear box  44  is driven by LP spool  30  through a power take-off (PTO)  42 . The ratio of the gear box  44  is designated as (x). The ratio x is normally a multiplier in the range of five to ten, although the range may be higher or lower for specific applications if so required. The electromagnetic generator is designed to rotate mechanically at speeds up to 4500×revolutions per minute (rpm). During idle descent of an aircraft, when the engine  10  is operating in a range of 1100 rpm to 2200 rpm, the generator  50  is required to generate electrical power up to 2200×rpm. During normal flight operation the engine  10  operates in a higher speed range of 2200 rpm to 4500 rpm. 
   When the generator  50  is driven by gearbox  44  at speeds in excess of 2200×rpm, a generator controller  52  disables a power converter  54  connected to the generator output, for example, by using contactors or by disabling gate signals to semiconductor devices within the power converter, effectively reducing the load  56  on the generator  50  to zero. This allows the machine to operate at high speeds while at the same time having a high number of magnetic poles, and thereby preventing the generator  50  from exceeding the fundamental frequency limit at 2200×rpm. The fundamental frequency limit is imposed by the maximum practical pulse width modulated (PWM) frequency that can be achieved in the electrical active power converter. Depending on the power level, the limit on the PWM frequency is set based on the switching capabilities of the semiconductor devices used as well as the available thermal management of the power converter. The switching losses increase in proportion to the PWM, thus affecting the power converter and system efficiencies. Also, higher PWM frequency generates more heat, and thus requires greater cooling capacity. The high number of magnetic poles allows the use of concentrated isolated armature windings that are fault-tolerant in nature. 
     FIG. 3  is a diagram showing the speed range in which there is active generation versus the speed range in which the generator  50  is in a passive mechanical rotation mode. The active speed range  200  preferably occurs between 100× and 2200×, which as indicated above, is the normal operating range of the engine  50  during idle descent. The passive speed range  202  is above 2200×, up to about 4500×, which is the engine speed during normal flight. 
   In the embodiment shown in the figures the generator  50  is a permanent magnet, but this generator  50  may be any type of suitable generator such as, but not limited to, e.g., switched reluctance, permanent magnet, wound-field, and other configurations, as well as a radial-flux or axial-flux machine. Also the generator  50  can have any number of phases. 
   Referring next to  FIGS. 4 and 5 , which respectively show a prior art stator and rotor, and the stator and rotor of the present invention, the reduced size of the stator and rotor back iron is illustrated. In  FIG. 4 , a low-pole count machine  60  is shown having twelve slots  62  and four poles  64 . The slots are defined by adjacent tooth portions  67 . The poles  64  are affixed to the rotor back iron  68   a , and the slots  62  receive pole windings (not shown), which are coiled around stator tooth portions  67 . The tooth portions  67  extend radially inward from stator back iron  68   a . By contrast, in  FIG. 5 , a high-pole count generator  50  of the present invention has twelve slots  62  and ten poles  64 . The stator back iron  66   b  and the rotor back iron  68   b  are significantly reduced in thickness in the high-pole count generator  50 , relative to low-pole count generator  50 . The reduced size is achievable due to the inverse relationship between the number of poles and the magnetic flux per pole, i.e., as the number of poles gets higher, the magnetic flux/pole gets lower. The lower flux/pole requires less back iron to accommodate the same magnetic flux density. 
   Referring next to  FIGS. 6 and 7 , an example of a prior art machine and a high pole count generator are shown for comparison purposes. In the low-pole count machine of  FIG. 6 , distributed overlapping windings of prior are shown and the distributed overlapping windings of the present invention are shown. Phase windings designated A, B and C are shown as overlapping one another. For example, phase winding A is connected between slots (s 1 ) and (s 4 ); phase winding B connected between slots (s 2 ) and (s 11 ); and phase winding C connected between slots (s 3 ) and(s 6 ). Since each phase is connected between non-adjacent slots, there is overlap in the flux paths circulating in the respective stator back iron  66  and tooth portions  67 . By contrast,  FIG. 7  illustrates the high-pole count machine  50 . The phase windings in generator  50  are concentrated with phase windings A, B and C connected between adjacent slots, and thus providing non-overlapping, fault-tolerant stator windings. The non-overlapping, concentrated phase windings A, B and C improve the machine fault-tolerance because there is minimum coupling between the various phases. For example, in case of a fault in phase A, phases B and C will not be significantly affected, and the machine can still continue to produce a useful level of power, and the machine could produce the rated power if the machine phases were rated above the actual rated power. Referring to  FIG. 8 , the high-speed LP generator  50  can be part of a combination machine  80 , having one or more HP spool generators  82  in the form of double-sided dual-rotor or single-stator dual-rotor configurations. The double-sided dual rotor configuration permits significant reduction in the overall frame size and cooling equipment sizes, as well as reduced weight. Further reduction in the size and weight of the shared stator yoke may be achievable, depending on the vector summation of the magnetic fluxes present in the dual machine configurations. 
   While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.