Abstract:
A system that selectively alters a phase angle of an electrical generator. The system has a powered input shaft for transmitting power and an output shaft for receiving power for delivery to the electrical generator. A coupler portion interconnects the input shaft with the output shaft and transmits power therebetween. The coupler portion is selectively moveable in an axial direction between a first axial position wherein the input shaft and output shaft are disposed at a first relative angular position, and a second axial position wherein the input shaft and output shaft are disposed at a second relative angular position different than the first position.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to a method of synchronizing generators mechanically, and more specifically, to a method and system for controlling an angular phase shift between two electrically paralleled generators mounted to a common drive source, and includes a means to detect a phase error between two paralleled generators, and to control an actuator to reduce the error to within predetermined limits. 
   BACKGROUND OF THE INVENTION 
   As airplane architecture is increasingly dependent upon electrical systems to operate the aircraft, the size and criticality of the primary electrical generation systems also increases. As an example, the Boeing 787 employs two engines to drive four variable frequency starter/generators (VFSGs). In this arrangement, two starter/generators are mounted to each engine gearbox. Both of the generators do operate at the same speed and frequency, since they are directly geared from the same engine shaft, but the AC power output of the two generators is not synchronized in phase. Therefore, the power generated by the two generators cannot be tied directly together. In order to protect each generator, the distribution buses and related equipment connected to each generator must be physically separated. Additional bus tie breakers must also be installed to power each bus while the normal generator is not available. Such physical limitations increase the complexity, size, weight, and cost of the power distribution system. 
   Connecting two or more generators in parallel to a common bus could reduce this complexity and enable improved load balancing, but requires precise synchronization of both generators. If the generators are not precisely synchronized, large currents known as “circular currents” will be present, causing a loss in efficiency, excessive heat dissipation, and the potential to damage the generators. Typical six-pole generators should be mechanically aligned within approximately one degree. Known methods, such as indexing gears and keying shafts, could be applied, but these methods alone would not provide sufficient tolerances to allow the generators to be paralleled. 
   Additionally, many former airplane designs employed a Constant Speed Drive (CSD) mechanism to produce fixed 400 Hz power from a variable engine shaft speed. CSD mechanisms have been used, notably on the Boeing 747-400 airliner, to synchronize the main generators so that paralleled buses are feasible. However, CSD systems are heavy, expensive, and unreliable, and have been dropped in favor of directly geared variable frequency generators. 
   In designing new generations of aircraft, reducing the volume and weight of the power systems is critical. The four-bus architecture currently used in aircraft power distribution systems consumes an excessive amount of the equipment bay. 
   Therefore, what is needed is the ability to reduce the physical barriers and number of bus breakers required to operate the electrical systems of an aircraft, by configure the four generators that are required to power the aircraft in a two-bus architecture. Such an arrangement simplifies the electrical panels, reduces the amount of electrical equipment that is required, and relaxes the separation requirements internal to the panels as well as for wiring routed throughout the aircraft. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a method and apparatus for detecting and controlling the phase angle between a pair of generators synchronized on a common bus. The present invention is further directed to a system for controlling the phasing of multiple generators driven from a common drive source. 
   In one aspect, the present invention is directed to an aircraft electrical power system. The system includes one or more pairs of starter/generators having parallel outputs configured to power a common electrical bus. A power transmission device is provided, having a pair of rotary output elements and an input element. A rotary drive source drives the input of the power transmission device. At least one adjustable coupler mechanism is controllable to adjust a phase angle of a rotor shaft of the at least one starter/generator. The starter/generators include one coupler mechanism each, to couple the rotor shaft of the starter/generator to one of the rotary output elements. A controller is configured to monitor an output electrical phase angle of each starter/generator and control the coupler mechanism in response to a detected phase error between the respective starter/generator output electrical phase angles by adjusting a relative angular position between one of the rotor shafts and the coupled rotary output element such that the phase error is substantially eliminated. 
   In another aspect, the present invention is directed to a synchronization system for synchronizing a plurality of starter/generators on an engine, and connected in parallel on a common bus. The system includes a primary rotary drive shaft drivingly connected to the engine. A power transmission device has an input and a plurality of output drive shafts. The power transmission device is driven by the primary rotary drive shaft and is configured to power the plurality of starter/generators at a predetermined rotational frequency. At least one coupler mechanism is connected between an input shaft of one of the starter/generators and one of the output drive shafts of the power transmission device. A controller monitors an output electrical phase angle of the starter/generators and controls the coupler mechanism in response to a predetermined deviation between the respective starter/generator electrical output phase angles. The coupler mechanism is selectively moveable to adjust a relative angular position of the input shaft relative to the associated output shaft. Also, the output electrical phase of the starter/generator with which the coupler portion is coupled is adjustable in response to movement of the coupler mechanism, to substantially synchronize the output electrical phase of one of the starter/generators with the output electrical phase of the other starter/generators. 
   In another aspect, the present invention is directed to a method of synchronizing a pair of electrical generators for connection to a common electrical bus. The method includes the steps of driving a rotor shaft of each generator from a common drive source; providing a transmission device between each generator and the common drive source; coupling the rotor shaft of at least one generator to an input shaft of the transmission device with a coupler device, the coupler device being selectively moveable to adjust a relative angular position between the rotor shaft and the input shaft; sensing a phase difference between each electrical generator output current; determining whether circulating currents are present based on a sensed phase difference between the generator output currents; and, in response to a sensed phase difference, moving the coupler axially to a second position between the rotor shaft and the input shaft until the sensed phase difference is substantially eliminated. 
   An advantage of the present invention is the means to control the angular phase shift of the electrical outputs between two generators that are driven from a common drive source. 
   Another advantage of the present invention is that by controlling the angular phase shift of the generator output, the two generators may be electrically connected on a common bus, saving space and weight in the power distribution system, and enabling load sharing, with a minimal impact to the generators. 
   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 schematic diagram of an exemplary synchronization arrangement for a pair of generators driven by a common shaft. 
       FIG. 2  is an exploded perspective view of the synchronization mechanism of the present invention. 
       FIG. 3  is an exploded perspective view of the synchronization mechanism showing the actuator assembly, coupler and shaft arrangement. 
       FIG. 4  shows a line diagram of the axial coupling and de-coupling distances. 
       FIG. 5  is a method for synchronization of generators according to the present invention. 
   

   Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , an exemplary system of synchronously controlling a pair of variable frequency (VF) starter/generators  100  on a bus  102 , is implemented using the synchronization mechanism  10  described below (See, e.g.,  FIGS. 2 &amp; 3 ). VF starter/generators  100  have substantially identical electrical and mechanical ratings, e.g., input and output power, and are preferably identical or matching generator sets. A primary rotary shaft  104 —e.g., a high-pressure (HP) spool or a low-pressure (LP) spool of a turbine engine—is drivingly connected to a gearbox  106 . The shaft has an input or drive gear  112  meshing concurrently with and driving a pair of output gears  108  and  110 . The output gears  108 ,  110  have the same gear ratio with the drive gear  112 , causing the frequency of the VF starter generators  100  to be matched. The secondary gears  108 ,  110  have output shafts  12  inserted into the synchronization mechanism  10 . It should be noted that the arrangement of the gearbox  106  is shown schematically, and that the gearbox may include multiple stages of gearing to achieve the desired rotational frequency of the VF starter/generators, as will be readily appreciated by persons having ordinary skill in the art. Other power transmission means may be substituted for the gearbox  106 , such as a belt-driven pulley arrangement, a hydraulic transmission, multiple power take-offs, etc., provided that the drive means or prime mover is a single shaft and the power transmission means are configured to match the rotational speed and power of the starter/generators. Also, the system is operable as a generator or as a starter. When the system is operated as a motor for starting the turbine engine, the power train description is reversed, i.e., the primary rotary shaft  104  of the HP or LP spools is the output that is driven by VF starter/generators  100  through the secondary gears  108 ,  110  of the gearbox  106 . 
   In the generator mode, the output shafts  12  of the gearbox  106  simultaneously drive a pair of rotor shafts  14  of VF starter generators  100 . Each output shaft  12  is connected to the rotor shaft  14  through a synchronization mechanism  10 . Each VF starter/generator  100  produces an electrical power output on lines  114 ,  116 , respectively. Controller  118  senses the output phase currents for each generator  100 , and compares the two as described above. The controller  118  adjusts the phase angle for each VF starter generator  100  until the phase angles match within predetermined tolerances. When the respective phase angles are matched within the predetermined tolerance, the VF starter generators  100  are connected to the common bus  102  via contactors  120  and  122 , respectively. The contactors  120 ,  122  are actuated by controller  118 . The controller  118  may optionally be configured to open either or both of the contactors  120 ,  122 , if the VF starter generators  100  drift out of phase by a predetermined unacceptable tolerance during flight. 
   There is also a controller  124 ,  126  associated with each VF starter generator  100 . The controllers  124 ,  126  are configured to connect either or both the VF starter generators  100  to an external electrical power source (not shown) for electrically starting the turbine engines. The external power source is connected through contactors  126 ,  128  to power lines  114 ,  116  respectively, to energize the VF starter generators  100 , which in turn drive the HP spool or LP spool  104  through gearbox  106 . Phase control may optionally be implemented during electrical starting of the engine. After a predetermined startup interval, the contactors  128  and  130  open to isolate the external power source or sources from the VF starter generator  100  outputs. 
   The above-described system is implemented in an aircraft wherein a pair of generators  100  are connected to each engine for powering electrical loads associated with the aircraft. In this way, segregation of power apparatus is not required, and the associated electrical distribution equipment is correspondingly in size. The synchronization system provides inherent load balancing between the commonly driven generator pairs. 
   Referring next to  FIGS. 2-4 , the present invention is directed to a synchronization mechanism generally designated as  10 . The synchronization mechanism  10  can automatically advance or retard the phasing of one or two generators  100  by a few degrees, in order to synchronize two generators  100  operating in parallel. The range of adjustment must be adequate to cover the tolerances within the gearbox, but is expected to be approximately five degrees. Typically, the generators  100  are drivingly connected to a gear reducer through the synchronization mechanism. The synchronization mechanism  10  receives input shaft  12 , which is typically the output shaft of a gear reducer, and is attached to a rotor shaft  14 . A shaft coupler  16  joins the rotor shaft  14  and the input shaft  12 , and an actuator  18  controls the axial position of the coupler  16 . Current transformers or any other suitable current measuring sensor  117  are used to detect circular currents in the generator outputs, and a controller for controlling the relative phase of the two generators in response to a sensed circulating current. 
   In a preferred embodiment, the input shaft  12  from the gear reducer has helical splines  20  at one end. The helical splines  20  have a slight axial twist, and may be machined onto the generator shaft or threadably attached to a gearbox output shaft. The rotor shaft  14  has an annular open end  22  with internal splines  24 . The internal splines  24  of the rotor shaft  16 , and the external splines  28  of the coupler are preferably straight splines, however, the splines may also have a slightly helical configuration, e.g., to account for shaft twist. The coupler  16  has internal splines  26  and external splines  28  mating with the splines of the input shaft  12  and rotor shaft  14 , to engage both of the input shaft  12  and the rotor shaft  14 , to link them together. Other splined arrangements may be used as well, e.g., the rotor shaft  14  having internal helical splines  24 , and the input shaft  12  having straight splines  20 ; or the rotor shaft  14  and the input shaft  12  having helical splines with opposite twists. In principle the splined arrangements between the input shaft  12  and the rotor shaft  14  need merely be variably aligned relative to one another, and complementary with the corresponding or mating splined arrangement on the coupler  16 . Both the rotor shaft  14  and the input shaft  12  are keyed with the coupler  16  so that they engage only when the generator poles are nearly aligned. 
   As indicated above, the coupler splines  26 ,  28  mate with and link the rotor shaft  14  with the input shaft  12 . Any axial movement of the coupler changes the angular position of the rotor shaft  14  relative to the input shaft  12 . Preferably, the input shaft  12  extends slightly beyond the end of rotor shaft  14 , so that by moving the coupler  16  axially beyond the end of the rotor shaft  16 , the coupler  16  disengages from the rotor shaft  14 , while still maintaining mechanical engagement with the input shaft  12 , which mechanically disconnects the generator. This relationship is illustrated linearly in  FIG. 4 . At the left end limit of travel of the coupler  16 , the coupler  16  and rotor shaft  14  are disconnected, while the coupler  16  is still engaged with the input shaft  12 , due to the extension 
   The actuator  18  is operable to drive the axial position of the coupler  16 . The actuator may be any type of actuator for effecting linear movement of an object on a rotating shaft, e.g., hydraulic piston, magnetically actuated positioners, or other means. In the embodiment shown in  FIGS. 2 and 3 , axial movement of the coupler  16  is controlled by an actuator motor  30 . A carrier plate  32  has an axial opening  34  with a coupler bearing  36  for receiving the coupler flange  38 . The coupler flange is rotatable in the coupler bearing  36 , and restrained from linear movement relative to the carrier  32 . The carrier plate  32  has a plurality of threaded holes for receiving threaded posts  42 . The threaded posts have toothed sprockets  44  attached at one end. An endless toothed belt  46  engages the toothed sprockets for rotating the toothed sprockets  44 . Actuator motor  48  drives the toothed belt  46  in response to control signals generated by the controller in response to the sensed circulating currents which indicate a phase deviation between two generators connected in parallel. The actuator motor  48  positions the carrier plate  32  by advancing or retracting the threaded posts  44 , which are rotated by the toothed belt  46 , clockwise or counterclockwise as appropriate, to move the carrier plate in either direction linearly and coaxially with the coupler  16 , and shafts  12 ,  14 . As the axial position of the coupler  16  coincides with the position of the carrier plate  32 , movement of the carrier plate  32  moves the coupler  16  linearly along the axis  50  of the shafts  12 ,  14 , and mechanically adjusts the relative rotational angle between the shafts  12 ,  14 , or disconnects the shafts  12 ,  14  if driven beyond the end of the rotor shaft  14 . 
   In another embodiment, the present invention may implement a synchronization system in an aircraft maintenance method as follows: 
   Referring to  FIG. 5 , in step  200 , a pair of generators  100  is initialized by a first operation of the engine; the generators are configured to be driven by a common drive shaft  104  through a gearbox  106 . In step  202 , a phase difference is sensed by a phase monitoring device, to determine whether circulating currents are present between the respective generators; in step  204 , data is then transmitted by the phase monitoring device to a digital computer (personal computer, programmable controller, laptop, etc.) through an appropriate interface; in step  206 , the computer generates a maintenance report indicating whether the generator outputs are synchronized, and setting forth the deviation, if any, in the respective phase angles; then, in step  208 , a technician adjusts the phase angles by operating the phase control actuator until both of the generators are synchronized; in step  210 , the technician fixes the phase actuator in the synchronized positions, so that the generators are locked in phase and do not self-adjust; in step  212 , the phase monitoring device detects and compares the generator outputs at predetermined intervals, e.g., after each flight or at every scheduled preventive maintenance shutdown; and in step  212 , the method is repeated as necessary. The method of  FIG. 5  may be implemented in instances where it is impractical or unnecessary to provide continuous feedback control of the synchronization mechanism  10 . 
   Since the drive shaft that powers the gearbox  106  is common to both generators  100 , the gear ratios of the gearbox  106  are the same, and the generators  100  are substantially identical, the frequency of the two generators is inherently the same. Therefore, the singular control parameter of the present invention is the phase angles of the outputs of the generators  100 , to avoid circular currents and to achieve load balancing, between generators. 
   In another embodiment, the generator synchronization system may be controlled by a single synchronization mechanism  10 , by sensing the phase angle of the first generator  100  as a reference signal, and modulating the output phase of the second generator  100  to synchronize the second generator phase angle with the first generator phase angle, substantially eliminating circulating currents. 
   It should also be understood that variations might be made to the synchronizing mechanism within the spirit and scope of the present invention. For example, the synchronization mechanism  10  may be disposed internally in the engine gearbox, by incorporating a sliding helical gear similar to the synchronization mechanism helical splines  20  to accomplish the adjustment. Furthermore, an alternate synchronization mechanism (not shown) may include gears with a herringbone arrangement, rather than with helical splines, to neutralize the effects of axial thrust. 
   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.