Abstract:
An apparatus performs mechanical phase synchronization of multiple AC generators in an electrical power distribution system. The apparatus according to one embodiment comprises a gearbox mechanically coupled to a prime mover, multiple AC generators, each having a rotor driven by mechanical energy from the prime mover supplied via the gearbox to generate AC electrical power, and a mechanical linkage keying orientation of each rotor to the gearbox, thereby mechanically synchronizing phase of the multiple AC generators.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention claims priority under 35 U.S.C. §119(e) of U.S. Provisional Application No. 60/629,423 filed Nov. 22, 2004, the entire contents of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical systems for vehicles, and more particularly to a method and apparatus for achieving mechanical phase synchronization of multiple AC generators in an electrical power distribution system. 
     2. Description of the Related Art 
     Large vehicles such as aircrafts and ships require large amounts of energy to drive all onboard equipment. Increasing size and weight of energy producing systems, such as generators and motors, puts a limit on feasible size and complexity of large vehicles. “More electric” vehicles, which use electrical energy generators while eliminating bulky and vulnerable hydraulic systems, are being designed and produced. To satisfy the demand of electrical energy for “more electric” vehicles, large electrical generators are required. AC Variable Frequency generators provide an attractive solution, as they keep weight and cost down while increasing reliability. However, single AC Variable Frequency generators can not typically meet the power requirements of a vehicle with high electrical power demand, because an AC Variable Frequency generator capable of providing the required large electrical output would easily exceed technological, spatial, design, construction and installation limitations for both the generator and its prime mover. 
     In order to stay within spatial and technological limitations of generators while providing the large required electrical energy output, multiple smaller AC Variable Frequency generators can be combined to meet the required electrical energy output. However, the outputs of AC Variable Frequency generators that are not synchronized in phase and voltage cannot be combined, as current and voltage would be transferred between AC Variable Frequency generators without reaching the electrical distribution system of a vehicle. If the mismatch in frequency and voltage between AC Variable Frequency generators is large, their combined output can become unstable, leading to a shutdown or damage to the vehicle&#39;s electric system. 
     As a result, the use of multiple AC generators that are not synchronized requires one isolated bus for each AC generator to perform the necessary distribution of electrical generated power. As the number of prime movers and AC generators per prime mover increases for a vehicle application, the complexity of the electrical distribution system and of the electrical control system becomes extensive. 
     A few publications have studied systems that combine AC generators. One such technique is described in “Voltage Regulator Load Division Using Real and Reactive Generator Output Power Components to Control the Exciter”, Abdul Rashid, U.S. Pat. No. 5,077,485. With the method described in this work, a constant speed drive control and feedback system is used to combine generators to produce 400 Hz AC output. The constant speed drive control and feedback system adds significant complexity to the system. Moreover, this technique is not applicable to AC variable frequency generators installations. 
     Another technique is described in “Methods and Apparatus for Synchronizing Multiple Motor Driven Generators”, Robert Lee and Suresh Gupta, U.S. Pat. No. 4,575,671. In this publication, however, electric motors located between a prime mover and generators are de-energized for manual synchronization of generators using a slipping pole technique. 
     A disclosed embodiment of the application synchronizes multiple AC generators by utilizing a method and an apparatus for mechanical phase synchronization of multiple AC generators. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus for achieving mechanical phase synchronization of multiple AC generators in an electrical power distribution system, and to an electrical power distribution system with mechanically phase-synchronized AC generators. According to a first aspect of the present invention, an apparatus for mechanical phase synchronization of multiple AC generators comprises: a gearbox mechanically coupled to a prime mover; multiple AC generators, each having a rotor driven by mechanical energy from the prime mover supplied via the gearbox to generate AC electrical power; and a mechanical linkage keying orientation of each rotor to the gearbox, thereby mechanically synchronizing phase of the multiple AC generators. 
     According to a second aspect of the present invention, an electrical power distribution system with mechanically phase-synchronized AC generators comprises: a gearbox mechanically coupled to a prime mover; an electrical power distribution bus; multiple AC generators, each having a rotor driven by mechanical energy from the prime mover supplied via the gearbox to generate AC electrical power; and a mechanical linkage keying orientation of each rotor to the gearbox, thereby mechanically synchronizing phase of the multiple AC generators. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further aspects and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a general block diagram of an electrical system containing an electrical power distribution system according to an embodiment of the present invention; 
         FIG. 2A  is a block diagram of an electrical power distribution system with non-synchronized AC generator primary bus layout included in an electrical system; 
         FIG. 2B  is a block diagram of an electrical power distribution system with a synchronized AC generator primary bus layout included in an electrical system according to an embodiment of the present invention illustrated in  FIG. 1 ; 
         FIG. 3  illustrates an exemplary electrical power distribution system with non-synchronized AC generator primary bus layout; 
         FIG. 4  illustrates an exemplary electrical power distribution system with a synchronized AC generator primary bus layout according to an embodiment of the present invention illustrated in  FIG. 2B ; 
         FIG. 5  is a block diagram of an exemplary AC variable frequency generator included in an AC power generator system according to an embodiment of the present invention illustrated in  FIG. 2B ; 
         FIG. 6  illustrates a shaft engagement of an AC variable frequency generator included in an AC power generator system according to an embodiment of the present invention illustrated in  FIG. 2B ; 
         FIG. 7A  illustrates an exemplary uniformly indexed splined shaft that engages an AC variable frequency generator included in an AC power generator system; 
         FIG. 7B  illustrates an exemplary non-uniformly indexed splined shaft that engages an AC variable frequency generator included in an AC power generator system according to an embodiment of the present invention illustrated in  FIG. 2B ; 
         FIG. 8  illustrates an interface drive shaft with a keyed spline pattern for rotationally aligning an AC variable frequency generator to a similarly indexed gearbox shaft according to an embodiment of the present invention illustrated in  FIG. 6 ; 
         FIGS. 9A ,  9 B and  9 C illustrate an arrangement and technique for fine rotational alignment of an AC variable frequency generator output shaft to rotor phase position in accordance with an embodiment of the present invention illustrated in  FIG. 8 ; and 
         FIG. 10  illustrates a synchronized chain of AC variable frequency generators associated with an AC power generator system according to an embodiment of the present invention illustrated in  FIG. 2B . 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the invention are more specifically set forth in the accompanying description with reference to the appended figures.  FIG. 1  is a general block diagram of an electrical system containing an electrical power distribution system according to an embodiment of the present invention. The electrical system  100  illustrated in  FIG. 1  includes the following components: an AC power generator system  11 ; an energy output system  15 ; and individual systems and equipment  17 . The AC power generator system  11  and the energy output system  15  form an electrical power distribution system  110 . Operation of the electrical system  100  in  FIG. 1  will become apparent from the following discussion. 
     AC power generator system  11  provides electrical power to electrical system  100  that may be a vehicle, a laboratory facility, a large appliance, or another electrically driven system. AC power generator system  11  may be associated with an automobile engine; an aircraft engine; a ship engine; a generator; or a collection of generators. Airborne vehicle use is the most likely application since generator sizes are limited for airborne vehicle applications. Portable power stations (up to around 1 MW) driven by diesel or turbine engines, ground based vehicles, and ships may also have generator size limitations. Energy output system  15 , which distributes the electrical power produced by AC power generator system  11  to individual systems and equipment  17 , may contain distribution busses, switches, contactors, and motors. Individual systems and equipment  17  are elements that enable functioning of services in electrical system  100 . Such services may be an electric motor, an automatic braking system, an electrical light that can be turned on inside a vehicle, a remote control system onboard an aircraft, etc. 
       FIG. 2A  is a block diagram of an electrical power distribution system  110 A with non-synchronized AC generator primary bus layout included in an electrical system  100 . The electrical power distribution system  110 A includes the following components: N number of engine/generator systems  30   1 ,  30   2 , . . . ,  30   N ; R1 number of General Control Units (GCUs)  40   1     —     1 ,  40   1     —     2 , . . .  40   1     —     R1  and R1 number of busses  50   1     —     1 ,  50   1     —     2 , . . . ,  50   1     —     R1  connected to engine/generator system  30   1 ; R2 number of GCUs  40   2     —     1 ,  40   2     —     2 , . . . ,  40   2     —     R2  and R2 number of busses  50   2     —     1 ,  50   2     —     2 , . . .  50   2     —     R2  connected to engine/generator system  30   2 ; and so on, up to RN number of GCUs  40   N     —     1 ,  40   N     —     2 , . . . ,  40   N     —     RN  and RN number of busses  50   N     —     1 ,  50   N     —     2 , . . . ,  50   N     —     RN  connected to engine/generator system  30   N . Energy produced by engine/generator system  30   1  is distributed to individual systems and equipment  17  through busses  50   1     —     1 ,  50   1     —     2 , . . . ,  50   1     —     R1 . GCUs  40   1     —     1 ,  40   1     —     2 , . . . ,  40   1     —     R1  control energy production of engine/generator system  30   1 . Operation of engine/generator systems  30   2 , . . . ,  30   N  is similar to operation of engine/generator system  30   1 . GCUs  40   i     —     k , where subscript “i” takes values between 1 and N, and subscript “k” takes values between 1 and Ri, may be microprocessors, digital circuits, analog circuits or any combination thereof. Bus  50   i     —     k  connects to engine/generator system  30   i  through contactor  37   i     —     k , and to individual systems and equipment  17  through contactor  38   i     —     k , where subscript “i” takes values between 1 and N, and subscript “k” takes values between 1 and Ri. More than one contactor may connect a bus to individual systems and equipment  17 . Contactors may also connect any two busses. Engine/generator systems  30   1 ,  30   2  . . .  30   N  and GCUs  40   1     —     1 ,  40   1     —     2 , . . .  40   1     —     R1 ,  40   2     —     l ,  40   2     —     2  . . .  40   2     —     R2 , . . .  40   N     —     1 ,  40   N     —     2  . . .  40   N     —     RN  belong to AC power generator system  11 . Busses  50   1     —     1 ,  50   1     —     2  . . .  50   1     —     R1 ,  50   2     —     1 ,  50   2     —     2  . . .  50   2     —     R2 , . . .  50   N     —     1 ,  50   N     —     2  . . .  50   N     —     RN  and contactors  37   1     —     1 ,  37   1     —     2  . . .  37   1     —     R1 ,  37   2     —     1 ,  37   2     —     2  . . .  37   2     —     R2 , . . .  37   N     —     1 ,  37   N     —     2  . . .  37   N     —     RN ,  38   1     —     1 ,  38   1     —     2  . . .  38   1     —     R1 ,  38   2     —     l ,  38   2     —     2  . . .  38   2     —     R2 , . . .  38   N     —     1 ,  38   N     —     2  . . .  38   N     —     RN  belong to energy output system  15 . Any additional contactors connecting busses to individual systems and equipment  17 , and to other busses, are part of energy output system  15 . 
       FIG. 2B  is a block diagram of an electrical power distribution system  110 B with a synchronized AC generator primary bus layout included in an electrical system  100  according to an embodiment of the present invention illustrated in  FIG. 1 . The amount of energy output to individual systems and equipment  17  by electrical power distribution system  110 B is the same as the amount of energy output to individual systems and equipment  17  by electrical power distribution system  110 A in  FIG. 2A . The electrical power distribution system  110 B includes the following components: N number of engine/generator systems  30   1 ,  30   2  . . .  30   N ; N number of General Control Units (GCUs)  40   1 ,  40   2  . . .  40   N ; and N number of busses  50   1 ,  50   2  . . .  50   N . Engine/generator system  30   1  is controlled by GCU  40   1  and sends generated electrical power to bus  50   1  which connects to contactors  37   1  and  38   1 ; engine/generator system  30   2  is controlled by GCU  40   2  and sends generated electrical power to bus  50   2  which connects to contactors  37   2  and  38   2 ; and so on, to engine/generator system  30   N  which is controlled by GCU  40   N  and sends generated electrical power to bus  50   N  which connects to contactors  37   N  and  38   N . Energy produced by electrical power distribution system  110 B is sent to individual systems and equipment  17 . Engine/generator systems  30   1 ,  30   2  . . .  30   N  and GCUs  40   1 ,  40   2  . . .  40   N  belong to AC power generator system  11 . Busses  50   1 ,  50   2  . . .  50   N  and contactors  37   1 ,  37   2  . . .  37   N  and  38   1 ,  38   2  . . .  38   N  belong to energy output system  15 . 
       FIG. 3  illustrates an exemplary electrical power distribution system  110 C with non-synchronized AC generator primary bus layout included in an electrical system  100 . Electrical power distribution system  110 C includes the following components: engine/generator systems  31 ,  32  and  33 ; GCUs  41 ,  42 ,  43 ,  44 ,  45 ,  46 ; AC generators  61 ,  62 ,  63 ,  64 ,  65 ,  66 ; busses  51 ,  52 ,  53 ,  54 ,  55 , and  56 ; and contactors  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77 ,  78 ,  79 ,  80 ,  81 ,  82 ,  83 ,  84 ,  85 ,  86 ,  87 ,  88 . AC generator  61  is associated with engine/generator system  31 , is controlled by GCU  41 , and sends generated electrical power to bus  51  which connects to contactors  71 ,  77 , and  78 ; AC generator  62  is associated with engine/generator system  31 , is controlled by GCU  42 , and sends generated electrical power to bus  52  which connects to contactors  72 ,  79 , and  80 ; AC generator  63  is associated with engine/generator system  32 , is controlled by GCU  43 , and sends generated electrical power to bus  53  which connects to contactors  73 ,  81 , and  82 ; AC generator  64  is associated with engine/generator system  32 , is controlled by GCU  44 , and sends generated electrical power to bus  54  which connects to contactors  74 ,  83 , and  84 ; AC generator  65  is associated with engine/generator system  33 , is controlled by GCU  45 , and sends generated electrical power to bus  55  which connects to contactors  75 ,  85 , and  86 ; and AC generator  66  is associated with engine/generator system  33 , is controlled by GCU  46 , and sends generated electrical power to bus  56  which connects to contactors  76 ,  87 , and  88 . Engine/generator systems  31 ,  32  and  33  may contain additional AC generators. AC generators  61 ,  62 ,  63 ,  64 ,  65 ,  66  may be AC fixed frequency or variable frequency generators, providing high electrical power output. AC generators  61  and  62 ,  63  and  64 , and  65  and  66  associated with each engine/generator system are not synchronized in phase and voltage. The outputs of non-synchronized AC generators cannot be combined. If, for example, the outputs of non-synchronized AC generators  61  and  62  were combined, current and voltage would be transferred between generators  61  and  62  without reaching energy output system  15 . If the frequency and voltage mismatch between generators  61  and  62  outputs is large, the combined output of generators  61  and  62  can become unstable, leading to shutdown of electrical system  100  by its electrical protection systems or, in case of failure of electrical protection systems, leading to damage of electrical system  100 . Since the outputs of non-synchronized AC generators cannot be combined, each AC generator requires one individual GCU and one individual bus in order to generate and distribute electrical power. Hence, the more AC generators are present in electrical power distribution system  110 C, the more busses and GCUs are needed for electrical power production and distribution. As the number of engines and AC generators per engine/generator system increases in electrical system  100 , the complexity of electrical power distribution system  110 C and of GCUs control interface becomes extensive. 
       FIG. 4  illustrates an exemplary electrical power distribution system  110 D with a synchronized AC generator primary bus layout according to an embodiment of the present invention illustrated in  FIG. 2B . Electrical power distribution system  110 D includes the following components: engine/generator systems  31 ,  32  and  33 ; GCUs  47 ,  48 ,  49 ; AC generators  61 ,  62 ,  63 ,  64 ,  65 ,  66 ; busses  57 ,  58 ,  59 ; and contactors  131 ,  132 ,  133 ,  134 ,  135 ,  136 ,  137 ,  138 ,  139 ,  140 ,  141 ,  142 ,  143 . AC generators  61  and  62  associated with engine/generator system  31  are synchronized by synchronizing geartrain  121 , are controlled by GCU  47 , and send generated electrical power to bus  57  which connects to contactors  131 ,  132 ,  137  and  138 ; AC generators  63  and  64  associated with engine/generator system  32  are synchronized by synchronizing geartrain  122 , are controlled by GCU  48 , and send generated electrical power to bus  58  which connects to contactors  133 ,  134 ,  139 ,  140  and  141 ; and AC generators  65  and  66  associated with engine/generator system  33  are synchronized by synchronizing geartrain  123 , are controlled by GCU  49 , and send generated electrical power to bus  59  which connects to contactors  135 ,  136 ,  142  and  143 . Engine/generator systems  31 ,  32  and  33  may contain additional synchronized AC generators. AC generators  61 ,  62 ,  63 ,  64 ,  65 ,  66  may be AC variable frequency (VF) generators providing high electrical power output. Synchronizing geartrains  121 ,  122  and  123  synchronize generator pairs  61  and  62 ,  63  and  64 , and  65  and  66  respectively, in phase rotation and in voltage regulation. 
     Generators that are phase synchronized are also voltage synchronized, since the phase position is defined by the voltage waveform. Voltage outputs from the two parallel generators in each pair need to be similar in value, since an imbalance in voltage outputs would produce a potential difference between the two parallel generators. Such a potential difference would cause circulating (reverse) currents on the primary power feeder cables. 
     Before closing the line contactors from each generator to the common bus, the GCUs adjust the voltages of paired generators to the same level. When line contactors are closed, current will not be transferred between the two generators in each pair, since both generator outputs are at the same voltage. Hence, the generators&#39; power will be delivered to the bus. The GCUs may measure current to ensure that the generators share the load equally. In addition, GCUs control excitation to both generators by a suitable designed transfer function, to maintain the collective voltage output within accepted limits of voltage for the system. 
     When two or more AC VF generators are synchronized in phase and voltage, they can be driven from one gearbox and function as one unit. The outputs of synchronized AC VF generators can thus be safely combined, so that that only one GCU and only one bus are necessary to control synchronized AC VF generators. When all VF AC generators associated with every engine/generator system are synchronized, the total number of buses and GCUs in electrical power distribution system  110 D can be reduced to the number of engine/generator systems in AC power generator system  11 . In synchronized electrical power distribution system  110 D for example, the total numbers of busses and GCUs have been reduced to 50% of total numbers of buses and GCUs present in non-synchronized electrical power distribution system  110 C from  FIG. 3 . Smaller numbers of busses and GCUs greatly reduce the complexity of electrical power distribution system  110 D, reducing cost and weight while increasing reliability. In synchronized electrical power distribution system  110 D the total number of contactors connected to busses is also reduced compared to the total number of contactors in non-synchronized electrical power distribution system  110 C. 
     The classification of AC generators, such as VF or CF type generators, refers to the type of prime mover system that the AC generators are permitted to operate and interface with. CF generators are defined as generators with a design frequency bandwidth between 380 Hz and 420 Hz. By contrast, VF generators have frequency bandwidths outside the 380 Hz to 420 Hz range. The frequency bandwidth of VF generators is usually centered around or about 400 Hz, but is free to vary over the governing prime mover speed range. Variable frequency may also have design implications for the AC generators, by impacting their frequency range. AC generators  61 ,  62 ,  63 ,  64 ,  65 ,  66  can be other kinds of generators as well besides AC VF generators. As an example, AC generators  61 ,  62 ,  63 ,  64 ,  65 ,  66  can be engine generators used in propulsion engine applications. In propulsion engine applications, the generators spin at the speed of the turbine that corresponds to engine thrust or horsepower requirements. The speed on a propulsion engine normally varies over a wider range that the range of VF generators. Hence, the output frequency of engine generators can typically fall outside what is considered by convention to be a fixed or constant frequency between 380 Hz and 420 Hz. The present embodiment indexes the alignment of the phases between generators that are driven from the same prime mover, regardless of generators&#39; type, speed, frequency, or classification. The present embodiment provides a synchronized phase relationship between multiple AC generators driven at the same variable or constant speed, and by the same prime mover. 
     The current embodiment synchronizes generators regardless of the frequency at which the generators run. Generators used in airborne equipment, e.g. designed to run on a supply frequency nominally around 400 Hz, can be synchronized. AC VF generators with high frequency tolerance band, as well as constant frequency (CF) generators with lower frequency tolerance band, can be synchronized. Generators for other system applications may have frequency values in various ranges. For example, ground-based systems running with commercial equipment are designed for use with 60 Hz power supplies. Prime movers can vary in their optimally efficient speed ranges. Those ranges are typically much different from ranges that could directly drive a generator. Therefore, a speed reducing or multiplying gearbox can be used to obtain a suitable drive speed for generators from a prime mover. Typical choices of generators (and associated drive speeds) for vehicle applications are, by design, in multiples of 2. Hence, 2 pole (24,000 rpm), 4 pole (12,000 rpm) and 6 pole (8,000 rpm) generators may be used. 
     Some trimming of the excitation for the AC generators may be necessary in order to compensate for machine construction variability. The need for excitation trimming arises from variations that may occur in the electrical transfer function properties of generators. An important control parameter for excitation trimming is the load sharing of two or more generators when connected in parallel. To control the load sharing of two or more generators connected in parallel, the GCU can control the excitation of the generators. The control ensures that the voltage output of generators is the same prior to connecting the generators in parallel. After the line contactors are closed and the generators are connected in parallel, the GCU can regulate the sum of generator outputs by comparing the outputs to the required voltage of the system. GCU can also ensure that the output currents from parallel generators are close enough in magnitude. Hence, circulating currents between generators are prevented. Measurement of output current per channel may also be performed, to ensure equal load sharing between AC generators. Current transformers may be included in the generator system for this purpose. Alternately, the current transformers may be substituted with Hall effect sensors or similar devices to provide control feedback for load sharing purposes. The current transformers or other sensors can be part of a differential protection loop used to detect feeder cable faults. A current transformer in a differential protection loop may also be used to provide load-sharing feedback and additional functions without additional system hardware. The GCUs can then use the feedback to modify generator field excitation. 
       FIG. 5  is a block diagram of an exemplary AC variable frequency generator included in an AC power generator system  11  according to an embodiment of the present invention illustrated in  FIG. 2B . Exemplary AC VF generator  230  is associated with an engine/generator system  31 A belonging to AC power generator system  11 . Engine/generator system  31 A also includes a prime mover  250  and a gearbox  238 . Prime mover  250  provides mechanical energy and may be a turbine or another system driven by water, wind, or fuel. Gearbox  238  is a mechanical engagement system that picks up on shaft  252  mechanical energy generated by prime mover  250 . Gearbox  238  then sends the mechanical energy to AC VF generator  230  through gearbox interface spline section  244 , which is connected to generator shaft  246 . Shafts  244  and  246  may have different rotational speeds. AC VF generator  230  includes the following components: a main generator  236 ; an exciter  234 ; a permanent magnet generator  232 ; and a GCU  254 . Main generator  236 , exciter  234  and/or permanent magnet generator  232  have rotors that rotate as driven by generator shaft  246 . GCU  254  controls main generator  236 , exciter  234  and permanent magnet generator  232 . Main generator  236 , exciter  234  and permanent magnet generator  232  are electromechanical systems that may include armature windings, rectifiers, and permanent magnets for energy generation. Other types of AC VF generators may also be used in AC power generator system  11 . 
       FIG. 6  illustrates a shaft engagement of an AC VF generator  237  included in an AC power generator system  11  according to an embodiment of the present invention illustrated in  FIG. 2B . AC VF generator  237  is associated with an engine/generator system  33 B that belongs to AC power generator system  11 . AC VF generator  237  includes a stator  260  and a rotor  261 . Rotor  261  rotates as driven by generator interface section  262 , which receives rotation energy from gearbox interface spline section  244 . Rotor  261  may be an electrical winding with DC current. As rotor  261  rotates, the DC current in rotor  261  creates a magnetic field that in turn creates an electric current in stator  260 . The output of stator  260  is an AC voltage. AC VF generator  237  may be an AC VF generator such as the one illustrated in  FIG. 5 , with rotor  261  and stator  260  belonging to main generator  236 ; or it may be any other type of AC VF generator. Generator interface section  262  and gearbox interface spline section  244  may be splined shafts that engage splined recesses in rotor  261  and in gearbox  238  respectively. 
     If no phase synchronization of AC VF generator  237  is present, as is the case in existing designs, the spline pattern of generator interface section  262 , of gearbox interface spline section  244 , and of corresponding recesses in rotor  261  and gearbox  238  are uniformly indexed. In that case, engagement of generator interface section  262  to rotor  261 , and of gearbox interface spline section  244  to gearbox  238  would be done randomly around a uniformly indexed spline surface. 
       FIG. 7A  illustrates an exemplary uniformly indexed splined shaft  262 A that engages an AC VF generator  237  included in an AC power generator system  11 . Surface  275  of shaft  262 A presents uniformly indexed splines that engage a similarly splined recess in rotor  261  of an AC VF generator  237 . Uniformly indexed surface  275  does not perform any synchronization function. 
       FIG. 7B  illustrates an exemplary non-uniformly indexed splined shaft  262 B that engages an AC VF generator  237  included in an AC power generator system  11  according to an embodiment of the present invention illustrated in  FIG. 2B . Indexed splined shaft  262 B has non-uniform laminations  277  on surface  276 . The indexed splined shaft  262 B drives the rotor of AC VF generator  237 . The rotor slides over the non-uniform laminations  277  while positioning the generator poles (phases). Similar non-uniform laminations  278  for the female shaft that accepts indexed splined shaft  262 B are present. 
       FIG. 8  illustrates an interface drive shaft with a keyed spline pattern for rotationally aligning an AC VF generator  237  to a similarly indexed gearbox shaft according to an embodiment of the present invention illustrated in  FIG. 6 . Shaft  314  performs the function of synchronizing geartrain  121 ,  122  or  123  in  FIG. 4  by way of allowing only one rotational alignment during the generator to gearbox assembly process. Shaft  314  has two parts: generator interface spline section  262  and gearbox interface spline section  244 . The rotor assembly  261  of AC VF generator  237  is driven by generator interface spline section  262  which is engaged through a suitable female spline coupling. Gearbox  238  drives gearbox interface spline section  244  through a similar coupling on the gearbox. Generator interface spline section  262  and gearbox interface spline section  244  are separated by a shear section cut-away  304 . The surface  310  of gearbox interface spline section  244  contains a spline pattern with a keying arrangement such as a keyway slot  301  having several splines blanked over. The surface  312  of generator interface spline section  262  contains a spline pattern with a similar keyway slot  316 . Rotor laminations in rotor assembly  261  of AC VF generator  237  contain a similarly keyed female mating spline that engages generator interface spline section  262 , and gearbox  238  contains a similarly keyed female mating spline that engages gearbox interface spline section  244 . The keying arrangement on shaft  314  allows AC VF generator  237  to engage to gearbox  238  in only one orientation. Other non-uniform spline patterns for shaft surfaces  310  and  312  allowing AC VF generator  237  to engage gearbox  238  in only one orientation may also be used. When gearbox  238  drives multiple AC VF generators placed in parallel on the non-uniform spline pattern shaft  314 , all AC VF generators will have the same rotational orientation with respect to gearbox interface spline section  244  and with respect to each other; hence all AC VF generators will be synchronized between each other. The shear cut-away section  304  is machined to the tolerances necessary to provide proper shear torque. The manufacturing processes of gearbox  238  and of all parallel AC VF generators  237  engaged by shaft  314  ensure that keying arrangements  301  and  316  are in alignment for proper positioning of rotors  261  of all AC VF generators  237 . Gearbox  238  may also contain multiple gearbox interface spline sections  244  property synchronized to each other by design and construction. Likewise, all parallel AC VF generators  237  are designed and constructed so as to ensure that the rotor and stator laminations in each AC VF generator are in the same location, and the keyed splines on all generator interface spline sections  262  are oriented the same way for each AC VF generator. 
       FIGS. 9A ,  9 B and  9 C illustrate an arrangement and technique for fine rotational alignment of an AC VF generator  237  output shaft to rotor phase position in accordance with an embodiment of the present invention illustrated in  FIG. 8 . Views from different angles of the arrangement for fine rotational alignment of AC VF generator  237  output shaft to rotor phase position are present in  FIGS. 9A ,  9 B and  9 C. The arrangement in  FIGS. 9A ,  9 B and  9 C may be used when design and quality assurance techniques require that an AC VF generator phase synchronization be done with high precision. The arrangement in  FIG. 9A  includes two flanges  410  and  418  that enable angular adjustment. Flange  418  provides rotor shaft input for AC VF generator  237 , while flange  410  provides gearbox drive shaft input  408  for a gearbox  238 , using female mating spline  404  with keyway slot  406  with several splines blanked over. Locking nuts  412 ,  414  and  416  connect flanges  410  and  418 . Slotted holes  422 ,  424  and  426  in flange  410  in  FIG. 9B  receive bolts  412 ,  414  and  416  from flange  418 , on screws  427 ,  428  and  429 . Slotted holes  422 ,  424  and  426  have larger diameter than screws  428 ,  430  and  432 , allowing for several degrees of angular adjustment between flanges  410  and  418 . Generator rotor shaft input  420  to flange assembly connects a generator rotor to flange  418 . Spline keying  406  is used to initially position flanges  410  and  418  with respect to each other, then fine adjustment is accomplished using slotted bolt connections holes  422 ,  424  and  426 . Locking nuts  412 ,  414  and  416  are first loosened for adjustment and then tightened to clamp flanges  410  and  418  together. The angular adjustment between flanges  410  and  418  can be implemented as part of the final test and calibration of AC VF generator  237 . 
       FIG. 10  illustrates a synchronized chain of AC variable frequency generators associated with an AC power generator system  11  according to an embodiment of the present invention illustrated in  FIG. 2B . Gearbox interface spline section  244  of gearbox  238  synchronizes AC generators  237   1 ,  237   2 , . . . ,  237   Q  through rotor shafts  262   1 ,  262   2 , . . . ,  262   Q  that connect to rotors  261   1 ,  261   2 , . . . ,  261   Q . Rotor shafts  262   1 ,  262   2 , . . . ,  262   Q  perform the function of synchronizing geartrains  121 ,  122  and  123  in  FIG. 4 . One GCU  500  controls all synchronized generators  237   1 ,  237   2 , . . . ,  237   Q , and one bus  502  distributes the combined electrical output of all generators.