Abstract:
A starter-generator for an aircraft engine comprises a variable dynamoelectric machine alternatively operable as a motor or as a generator, having a rotor. A support motor is coupled to the variable dynamoelectric machine to assist the machine. A torque converter selectively couples and decouples the rotor to the engine, coupling the rotor to the engine at some point when the dynamoelectric machine is operated as a motor. The engine may be started by the dynamoelectric machine when operated as a motor through a first power train including the torque converter and may drive the dynamoelectric machine as a generator through a second power train.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a starter-generator for an aircraft engine, such as a turbine engine. 
   A turbine engine employed in an aircraft may be started by supplying compressed air to an accessory air turbine motor having reduction gearing to drive the engine. Compressed air is provided by an auxiliary power unit. These pneumatic systems require numerous air ducts, seals and air valves that are not only bulky but heavy. Moreover, these systems add undesirable complexity to the aircraft, reducing reliability and increasing cost for aircraft operators. 
   Recently, aircraft manufacturers have commenced using electric starters for turbine engines. Such a starter adds little additional componentry and wiring to the aircraft because the starter takes advantage of the aircraft&#39;s existing electrical system. Thus, the starter does away with many of the components used to start the engine by compressed air. 
   One approach to starting a turbine engine electrically is to employ a single dynamoelectric machine that operates as both a starter and a generator. Typically, this machine comprises a rotor and stator that serve the dual function of cranking the engine to start and operating as a generator driven by the turbine engine after start. The machine supports this dual function to eliminate the need for separate machines, associated mounting pads, and gearing on the engine accessory gearbox. One such starter-generator system uses a synchronous generator to operate as an induction motor to start the turbine engine. However, the use of such a device as an induction motor to start the engine creates the risk of damaging the integrity of the device. Potential burning of rotating diodes, very high current through the damper bars and the effect of inrush currents on field windings all pose risks to the device. 
   One kind of dynamoelectric machine that may be employed as a starter-generator is a variable frequency generator. The generator is used with a variable frequency electrical system of an aircraft that has componentry receptive to electrical voltage at a frequency that may vary with engine speed. Such a system allows the starter-generator to be restarted by an auxiliary power unit or another variable frequency power source, such as from another generator driven by an engine in mid-flight. 
   The need to avoid overburdening of the variable frequency starter-generator during start-up is of particular importance because of the need to preserve the mid-flight restart capability of the aircraft. That is, a starter-generator burned out by overloading during start-up will not be useful in a restart condition. 
   A need therefore exists to safeguard the variable frequency dynamoelectric machine during engine start up. 
   SUMMARY OF THE INVENTION 
   The present invention employs a variable frequency starter-generator coupled to a support motor, which operates to drive the starter-generator up to an initial synchronous speed. 
   As known, the starter-generator may operate as both a motor and a generator. In contrast to existing devices, however, a support motor is coupled to the starter-generator accelerate it to a designated synchronous speed. The support motor may also operate as another generator driven by the engine in a power generation mode. 
   Mechanical linkages may exist between the engine and the starter-generator to reduce the load on the engine or the load on the starter-generator. A torque converter may selectively couple and decouple the starter-generator to the engine. When the starter-generator is operated as a motor, the torque converter permits the starter-generator to rotate the turbine engine. When the starter-generator is operated as a generator, the torque converter decouples the direct connection from the engine to the starter generator. In power generation mode, the starter-generator is driven by the engine and outputs a variable frequency electric current to the electrical system of the aircraft. The support motor may be a permanent magnet generator, which may also serve to generate electricity for the aircraft. 
   The torque converter may decouple the starter-generator from the engine until such time that the starter-generator has reached a specific speed. Upon reaching this speed, the torque converter may then couple the engine to the starter-generator. The support motor assists the starter-generator in reaching this predetermined speed. A control unit sensing the speed of the starter-generator and engine may serve to control the torque converter via a proportional flow control valve. 
   Another mechanical linkage may operate to decouple the engine from the torque converter when the rotational speed of the turbine engine, once started, outpaces the speed of the torque converter as driven by the starter-generator. Another linkage may decouple the engine from the starter-generator when during start mode the rotational speed of the starter-generator outpaces the speed of the engine. In addition, the support motor may be powered by its own power supply. The starter-generator may be powered by a second power source which switches on when the starter-generator reaches a predetermined speed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
       FIG. 1  illustrates a schematic representation of the invention, showing the flow of mechanical power during engine start. 
       FIG. 2  illustrates the flow of mechanical power of the invention of  FIGS. 1 and 2 , showing the flow of mechanical power in power generation mode. 
       FIG. 3  illustrates a schematic representation of the invention during in-flight start mode. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a schematic representation of the inventive starter-generator  10 . Like existing starter-generators, inventive starter-generator  10  employs dynamoelectric machine  14 , which operates as both a motor and a generator. Dynamoelectric machine is a variable frequency generator having rotor  22 , which turns when the field windings of dynamoelectric machine  14  are charged by an alternating current from a power source. Support motor  26 , such as a permanent magnet motor and generator, is mechanically linked to rotor  22  to accelerate it to a designated synchronous speed. Thus, as support motor  26  turns so too does rotor  22  of dynamoelectric machine  14 . 
   Torque converter  18  may selectively couple and decouple the movement of rotor  22  to engine  16 , such as a turbine engine for an aircraft. Torque converter  18  may be a hydraulic torque converter, which when filled with hydraulic fluid from hydraulic source  20  provides a coupling between rotor  22  and engine  16 . As known, hydraulic source  20  includes a reservoir of hydraulic fluid that may pass to torque converter  18  through proportional flow control valve  21 . Then, rotor  22  drives engine  16  to turn. When hydraulic fluid is discharged from torque converter  18 , rotor  22  is decoupled from engine  16 . Accordingly, in this state, rotor  22  will not drive engine  16 . 
   When torque converter is decoupled, dynamoelectric machine  14  may commence operation as a generator. Engine  16  is mechanically linked to dynamoelectric machine  14  through coupling  46 . Couplings  42  and  46  permit the reduction of drag caused by components of starter-generator  10  during various modes of operation. Coupling  42  provides a mechanical link between torque converter  18  and engine  16  such that engine  16  is driven by torque converter  18  when filled with hydraulic fluid as long as the speed of torque converter  18  exceeds the speed of engine  16 . However, once the speed of engine  16  exceeds the speed of torque converter  18 , coupling  42  decouples engine from torque converter  18  by permitting engine  16  to overrun torque converter  18 . For example, when engine  16  has reached a self-sustaining speed, it may overrun torque converter  18 . Thus, coupling  42  limits torque converter  18  from creating a load on engine  16 . Coupling  42  may be an overrunning clutch. 
   Coupling  46  provides a mechanical link between engine  16  and rotor  22 . The linkage is such that rotor  22  is coupled to engine  16  as long as the output speed of engine  16  exceeds the speed of torque converter  18 . If the speed of rotor  22  exceeds the output speed of engine  16 , such as during start-up, then coupling  46  decouples engine  16  from rotor  22 , allowing rotor  22  to overrun engine  16 . In this way, engine  16  does not load dynamoelectric machine  14  during start up at coupling  46 . Coupling  46  may also be an overrunning clutch. 
     FIG. 1  illustrates the workings of these foregoing components at the initiation of start mode of engine  16 . Specifically, power is supplied to support motor  26  by first power source  58 , which may be an alternating current from a three phase inverter supplied to support motor  26 . Support motor  26  may be a permanent magnet motor and generator. Support motor  26  may be controlled by control unit  50 . Control unit  50  may oversee operation of support motor  26 . A DC source  27  may supply power to exciter  28  to operate dynamoelectric machine  14 . DC source  27  may be integrated into control unit  50  or it may just communicate with control unit  50  to oversee operation of exciter  28 . Support motor  26  turns rotor  22  of dynamoelectric machine  14 , assisting its start-up by accelerating dynamoelectric machine  14  to close to synchronous frequency of the second power source  62 , say 400 Hz. This acceleration overcomes accessory and gear train drag. At synchronous speed, such as 400 hertz, dynamoelectric machine  14  may then receive power from second power source  62 , such as a three phase 115 volt alternating current power supply from an auxiliary power unit or a ground cart, through electrical switch A and electrical switch B, which are closed to permit current to flow from second power source  62  to dynamoelectric machine  14 . Second power source  62  may operate at 400 hertz and then continue to drive dynamoelectric machine  14  as a synchronized motor at this speed. Control unit  50  may turnoff power to support motor  26 . 
   Sensor  54  senses speed of dynamoelectric machine  14 . Once dynamoelectric machine reaches a predetermined speed, say 400 hertz, control unit  50 , which is in communication with sensor  54 , then instructs proportional flow control valve  21  to commence filling torque converter  18  with hydraulic fluid from hydraulic source  20 . As hydraulic fluid begins to fill torque converter  18 , torque converter  18  begins to rotate engine  16  through first coupling  42 , creating power train  34 . Once engine  16  reaches a self-sustaining speed, it will outpace the speed of torque converter  18 . Coupling  42  permits this event to happen without significant drag on engine  16 , as explained above, by allowing engine  16  to overrun torque converter  18 . 
   Engine sensor  64  senses when engine  16  has reached a self-sustaining speed. Engine sensor  64  may be part of engine  16  or in communication with the output of engine  16 , or it may be integrated into the input shaft of the starter drive generator. When such a condition is sensed, control unit  50  then discharges hydraulic fluid from torque converter  18  decoupling rotor  22  from engine  16 . Hydraulic fluid is returned to hydraulic source  20 . Power from second power source  62  is disconnect at electrical switch A. 
   As shown in  FIG. 2 , engine  16  then serves to drive dynamoelectric machine  14  through second coupling  46 , which drives rotor  22 , creating power train  38 . The engine  16  forms a power train with second coupling  46  and rotor  22  for a power generation mode of operation. Dynamoelectric machine  14  is thus driven to generate electric power at a frequency related to speed of engine  16 . This variable frequency power is then directed to aircraft electrical bus  66  by closing switch C and opening switch A, which supplies alternating current electrical power to electrical components of the aircraft at variable frequency of engine  16 . Moreover, support motor  26 , a permanent magnet generator, may also supply power to control circuitry due to its mechanical link with dynamoelectric machine  14 . Such power may be passed through a voltage regulator of a general control unit of the aircraft. Power supplied to exciter  28  may be varied by the voltage regulator to control the output voltage. 
   As shown in  FIG. 3 , the inventive starter-generator is also capable of in-flight start. In the event of engine  16  stall, alternative power source  67  is in communication with aircraft electrical bus  66  and may supply electrical power through bus  66  and closed electrical switches C and B. Alternative power source  67  may be an on-board auxiliary power unit or a variable frequency power source, such as the electrical output of another variable frequency generator driven by another engine connected to the same bus. In the event the alternative power source  67  is a 400 Hz auxiliary power unit, then the start sequence is the same as described above. 
   However, in the event alternative power source  67  is a variable frequency source, say 400 to 800 Hz, such as from aircraft bus, then support motor  26  accelerates dynamoelectric machine  14  to frequency of the variable frequency source. If this frequency, as sensed by speed sensor  54 , exceeds a certain frequency desired for turning engine  16 , say 400 Hz, then control unit  50  operates proportional flow control valve  21  to partially fill torque converter  18  so that torque converter  18  turns engine  16  to self-sustaining speed. Partial filling of torque converter  18  is controlled in a manner so if the frequency from alternative power source  67  exceeds the desired frequency, say 400 Hz, torque converter  18  supplies the sane mechanical power to engine  16  achieved with a speed equivalent to the desired frequency, say 400 Hz. This is important to limit high driving power requirements and as such avoid high loads, intense heating up of the oil and detrimental effects upon actuated engine. Known techniques exist for determining the exact amount of the filling of torque converter  18  required to ensure maintenance of the desired frequency given the speed sensed by sensor  54 . 
   The aforementioned description is exemplary rather that limiting. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed. However, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. Hence, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For this reason the following claims should be studied to determine the true scope and content of this invention.