Abstract:
A method of starting an aircraft engine is provided. The method includes providing motive power from a generator for starting an engine during an engine start mode and deriving electrical power by way of the generator from rotation of the engine during a generate mode, transmitting the motive power from the generator to the engine during the engine start mode by way of a constant speed drive (CSD) and regulating a frequency of the electrical power output from the generator during the generate mode by way of the CSD and coupling a generator and CSD controller (GCC) to the generator and the CSD and operating the generator and the CSD by the GCC to execute at least the engine start mode and the generate mode.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to a method used for starting an aircraft engine using a synchronous generator and a constant speed drive. 
     Many aircraft in service have a synchronous generator coupled to each engine through a constant speed drive (CSD). As each of the engines turn, the corresponding CSD controls an associated synchronous generator to produce a constant frequency output current that can be used in various aircraft systems. For example, a synchronous generator may be employed to provide power to the air conditioning system or other electrical systems on a commercial aircraft. 
     Given the power rating required to drive these electrical loads, most synchronous generators should be capable of starting the aircraft engine. The problem with this configuration is that starting an aircraft engine requires that the engine spins at varying speed ranges outside the narrow frequency band (i.e., about 400 Hz) of commonly used CSDs. As a result, in most aircraft designs, an air turbine starter is used to start the engine as there does not exist a method to vary the speed output from the CSD. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a method of starting an aircraft engine is provided. The method includes providing motive power from a generator for starting an engine during an engine start mode and deriving electrical power by way of the generator from rotation of the engine during a generate mode, transmitting the motive power from the generator to the engine during the engine start mode by way of a constant speed drive (CSD) and regulating a frequency of the electrical power output from the generator during the generate mode by way of the CSD and coupling a generator and CSD controller (GCC) to the generator and the CSD and operating the generator and the CSD by the GCC to execute at least the engine start mode and the generate mode. 
     According to another aspect of the invention, a starter-drive generator (SDG) system for starting an aircraft engine is provided and includes a generator and a constant speed drive (CSD) configured to perform a main engine start through the use of an external power supply and to perform a frequency control function. 
     According to yet another aspect of the invention, a starter-drive generator (SDG) system for starting an aircraft engine is provided and includes a generator to provide motive power for starting an engine during engine start mode and to derive electrical power from rotation of the engine during generate mode, a constant speed drive (CSD) to transmit the motive power from the generator to the engine during the engine start mode and to transmit the motive power from the engine to the generator during generate mode and a generator and CSD controller (GCC) coupled to the generator and the CSD, which is configured to regulate the power between the generator and the engine to execute at least the engine start mode and the generate mode. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is defined and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a starter-drive generator (SDG) system in accordance with embodiments; 
         FIG. 2  is a top level schematic of a phase synchronization algorithm; 
         FIG. 3  is a schematic diagram of a phase lock loop (PLL) operation algorithm; 
         FIG. 4  is a schematic diagram of a constant speed drive (CSD) with two wobbler plates in accordance with embodiments; and 
         FIG. 5  is a schematic diagram for illustrating a control law of the CSD of  FIG. 4 . 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The description provided below relates to a starter-drive generator (SDG) system used for starting an aircraft engine using a synchronous generator and a constant speed drive (CSD). One or more embodiments disclosed herein may allow a CSD to perform main engine start through the use of an external constant frequency power supply such as a ground cart, an auxiliary power unit (APU), etc., in addition to performing its typical frequency control function. 
     With reference to  FIG. 1 , an SDG system  10  is provided for use with an aircraft or another suitable rotor-machine. The SDG system  10  includes a main bus  11 , a motor drive  12 , an AC bus  13 , a generator  14 , a CSD  15 , a generator and CSD controller (GCC)  16  and an engine  17 . As shown in  FIG. 1 , one side of the AC bus  13  is connected to outputs of the main bus  11  and the motor drive  12 . The other side of the AC bus  13  is connected to the generator  14  with the CSD  15  and the engine  17  connected to the generator  14  is series. The GCC  16  is disposed in parallel with the generator  14  and the CSD  15 . 
     The main bus  11  may be formed as a 3-phase AC, constant frequency bus. During a start mode of the engine  17 , the main bus  11  may be connected to an external power supply (e.g., ground cart, second generator, APU, etc.). During a generate mode, the main bus may be connected to normal aircraft loads (e.g., air conditioning and electrical systems). The motor drive  12  is used to spin the generator  14  to a synchronous speed, which is close to the frequency of the external power supply. The motor drive  12  may include an active rectifier at a front end thereof and a 6-switch inverter on the back end. When the motor drive  12  is not employed for engine  17  starting operations, the motor drive  12  may be used for a secondary function such as an environmental control system (ECS) of the aircraft. The AC bus  13  is formed as a “two-sided” panel filled with contactors and is used to re-configure power flow within the aircraft. 
     The generator  14  may be a wound field synchronous generator and is configured to provide motive power for starting the engine  17  during engine start mode and to derive electrical power from the engine  17  during generate mode. During engine start, the generator  14  is used as a motor for the engine  17  through an ignition sequence thereof. During the generate mode, the generator  14  is used create electrical power based on rotational energy received from the engine  17 . The CSD  15  includes a hydraulic gearbox used to regulate shaft torque between the engine  17  and the generator  14 . The CSD  15  can be configured in an output or an input summed configuration and uses two servo valves to control torque transfer. The GCC  16  is configured to perform synchronous generator excitation control and to perform servo valve control for the CSD  15 . During the engine  17  start mode, the GCC  16  regulates an exciter field in the generator  14  in order to help regulate torque produced by the generator  14  and the CSD  15 . During the engine  17  generate mode, the GCC  16  regulates the exciter field to control generator  14  terminal voltage. 
     As noted above, the SDG system  10  may be configured to operate in two modes of operation: the start mode and the generate mode. In the start mode, external power supplied from the external constant frequency power supply via the main bus  11  is used to spin the engine  17  up to light-off speed. In the generate mode, the SDG system  10  functions in a similar fashion as existing CSD driven generators. 
     For the start mode, system initialization proceeds as follows. In a system initialization operation, the AC bus  13  configures its contactors such that the motor drive  12  is connected to the generator  14  and the main bus  11  is disconnected from the generator  14 . The GCC  16  supplies start mode excitation to the generator  14  and controls the engine shaft speed to zero RPM in accordance with an algorithm that will be described below. The motor drive  12  then performs a precharge sequence to charge a DC link and enables a phase-lock-loop (PLL) in order to calculate a frequency and phase of a power source. 
     In a synchronization operation, the motor drive  12  starts an active rectifier to generate a DC link bus. The active rectifier may include standard voltage and current regulators. The motor drive  12  also starts a phase regulator, which controls the speed of the generator  14  in order to achieve the desired phase command. In a transition to main bus  11  operation, once the generator  14  has achieved the desired phase command with respect to the main bus  11 , the motor drive  12  is disabled, the AC bus  13  opens the motor drive  12  contactor and the AC bus  13  closes the main bus  11  contactor. In an engine  17  stating operation, with the main bus  11  contactor closed, the GCC  16  begins to accelerate the engine  17  by commanding the CSD  15  to begin applying torque to the engine  17  shaft. The GCC  16  will control the engine speed as required to start the engine  17 . Finally, in a transition to generate operation, when the engine  17  achieves sufficient speed that torque assist is no longer required, the GCC  16  de-excites the generator  14  and re-configures the CSD  15  for generate mode. In the generate mode, the SDG system  10  function in a similar manner as conventional CSD driven generators. 
     With reference to  FIGS. 2 and 3 , control algorithms for executing the operations described above will now be described.  FIG. 2  is a top level schematic of a phase synchronization algorithm. The phase synchronization algorithm includes a phase lock loop (PLL)  20 , a frequency regulator  21  (i.e., a first or frequency PI controller) and a phase regulator  22  (i.e., a second or phase PI controller). The PLL  20  measures the AC voltage source and calculates the corresponding AC source frequency and phase. These calculated values are used as commands for the frequency regulator  21  and the phase regulator  22 . The frequency regulator  21  is thus enabled when a frequency error is greater than 5 Hz. Once the generator frequency is within 5 Hz of the command, the phase regulator  22  takes over and adjusts frequency in order to maintain the desired phase command. The desired phase command is adjusted by a phase advance term  23 . The phase advance term  23  is based on the amount of phase change (due to the generator  14  slowing down) that is expected during the power source transition from the motor drive  12  to the main bus  11 . 
     A PLL operation algorithm is presented in  FIG. 3 . As shown in  FIG. 3 , the PLL  20  reads the input AC voltages and calculates both the frequency of the AC source and the phase (or electrical angle) of the AC source. 
     With reference to  FIGS. 4 and 5 , control algorithms and operations of the CSD  15  are shown. As shown in  FIG. 4 , the CSD  15  includes a rotatable shaft  30  that is coupled to the generator  14  and the engine  17 , a V-block wobbler  31  and an FV-block wobbler  32 . The V-block wobbler  31  and the FV-block wobbler  32  cooperatively operate to control a rotation speed of the rotatable shaft  30 . The V-Block wobbler  31  is operably coupled to the F-unit servo  40  and the FV-block wobbler  32  is operably coupled to the FV-unit servo  41 . Both the F-unit servo  40  and the FV-unit servo are operably coupled to the GCC  16 . During operation, the GCC  16  outputs electrical signals to the F-unit servo  40  and the FV-unit servo  41 . Those electrical signals are converted into hydraulic control pressures that are transmitted to the V-block wobbler  31  and that FV-block wobbler  32 , respectively. 
     That is, the GCC  16  is responsible for controlling the torque transferred through the CSD  15 . The GCC  16  performs this by controlling the F-unit servo  40  and the FV-unit servo  41 , which regulate the flow of hydraulic fluid within the CSD  15 . For the purposes of the present description, the GCC  16  controls the CSD  15  through the synchronization mode, the engine start mode, the transition to generate mode and the generate mode. 
     During the synchronization mode, the GCC  16  attempts to regulate the engine shaft speed (i.e., the rotatable shaft  30 ) to zero RPM. This is done by controlling the FV-block wobbler  32  (which is coupled to the rotatable shaft  30  via a differential trim ring gear) by way of the FV-unit servo  41 . This is necessary as the engine shaft may be windmilling and the motor controller may not be sized sufficiently to overcome this windmilling torque. 
     During the engine start mode for an input summed CSD configuration, the GCC  16  uses the FV-unit servo  41  and the F-unit servo  40  to place the FV-block wobbler  32  on its start mode hard stop and the V-block wobbler  31  on its fully de-stroked wobbler stop, respectively. The V-block wobbler  31  is then controlled according to the control law shown in  FIG. 5  while the FV-block wobbler  32  is maintained on its start mode wobbler hard stop. 
     During the engine start mode for an output summed CSD configuration, the FV-block wobbler  32  is placed at a neutral (0°) wobbler angle initially and the V-block wobbler  31  is placed on its fully de-stroked wobbler stop. During the first portion of the engine start mode, the FV-block wobbler is controlled according to the control law shown in  FIG. 5  while the V-block wobbler  31  is maintained on its de-stroke hard stop. Once the FV-block wobbler  32  hits its start mode wobbler hard stop, the V-block wobbler is controlled according to the control law shown in  FIG. 5  while the FV-block wobbler  32  is maintained on its start mode hard stop. 
     As shown in  FIG. 5 , the servo position of each of the F-unit servo  31  and the FV-unit servo  32  is regulated using a respective PI controller  50 . The input to the PI controller  50  is the calculated error between the power command  51  and the power measured in the generator  14 . The power command  51  is based on the engine shaft speed and is both rate limited by rate limiter  52  and saturated to a maximum limit by max power command limit  53 . The feedback power provided by feedback loop  54  is calculated using the dq voltage and current measured by the GCC  16 . 
     During the transition to generate mode, the GCC  16  will stroke the F-block wobbler  31  to the generate mode hard stop with the V-block wobbler  32  positioned on the fully de-stroked hard stop in preparation for the generate mode. In the generate mode, the GCC  16  will regulate the V-block wobbler  31  to control the terminal frequency as is done in conventional IDG/CSD systems. The FV-block wobbler  32  will remain on its generate mode hard stop as it is not used during the generate mode. 
     As a result of the use of the invention as described above, an air turbine start and engine pad can be removed to thus reduce aircraft weight, an improved start capability is provided as the engine start is electrically controlled as opposed to being controlled by a pneumatic system, the use of a constant frequency bus, as used on many aircraft and ground power carts, is permitted and a need for additional controllers on the aircraft is eliminated as functions are implemented into existing controllers. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.