Patent Publication Number: US-10778126-B2

Title: Synchronous electrical power distribution system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/366,954 filed Dec. 1, 2016, which is a non-provisional application claiming priority under 35 USC § 119(e) to U.S. provisional application 62/267,143, “SYNCHRONOUS ELECTRICAL POWER DISTRIBUTION SYSTEM STARTUP AND CONTROL” filed Dec. 14, 2015, and which also claims priority under 35 USC § 119(e) to U.S. provisional application 62/369,184, “SYNCHRONOUS ELECTRICAL POWER DISTRIBUTION SYSTEM” filed Jul. 31, 2016, all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to synchronous alternating current systems and, in particular, to synchronous generators. 
     BACKGROUND 
     Synchronous power systems are commonly used for efficiently powering electrical motors that drive fans, compressors, pumps, and other types of loads. Asynchronous electrical machines produce (e.g., motors) or consume (e.g., generators) torque only in conditions where the mechanical speed is different than the electrical speed. The magnitude of the difference of mechanical and electrical speeds is commonly referred to as “slip”. Asynchronous motors produce at least partial, and up to full rated, torque at all mechanical speeds less than the electrical voltage speeds, thus allowing acceleration rapidly to near matching electrical and mechanical speeds when connected to an electrical bus operating at constant speed, or “line start”. The ratio of electrical speed with respect to mechanical speed of an electrical machine is defined by the number of magnetic pole pairs of the specific design. Examples of asynchronous motors are induction motors which function based on Eddy current phenomena and hysteresis motors which rely on magnetic hysteresis phenomena. Induction motors are commonly used to drive mechanical loads from fixed speed national electric grids due to their “line start” capacity. 
     Synchronous electrical machines produce (e.g., motors) or consume (e.g., generators) torque only in conditions where the mechanical speed is equal to the electrical speed and the rotor and stator magnetic poles are misaligned. Synchronous machines commonly cannot “line start” due to the impractical requirement to connect the nonrotating motor to the rotating electrical grid at precisely aligned stator and rotor magnetic poles and develop sufficient torque to accelerate the rotor to electrical speed before misalignment exceeds ninety degrees electrical, where accelerating torque decreases and becomes negative at one hundred eighty degrees electrical. Synchronous machines are uncommon for driving mechanical loads from fixed speed national electrical grids due to the need to add “line start” functionality. A synchronous generator may provide such a system with the electrical power needed to spin the electrical motors that drive the loads. In some systems, the generator and load driving motors may be accelerated to operating speed using power electronics, a pony motor, and/or extra induction rotor devices; all of which may increase losses (thereby decreasing efficiency) and add mass to the system. 
     SUMMARY 
     In one example, the disclosure is directed to a system that includes a synchronous generator configured to supply at an output a voltage and a current to a plurality of synchronous loads, and an exciter configured to provide a variable field current to the synchronous generator to control a magnitude and phase of the voltage and the current of the output of the synchronous generator. The system also includes a controller configured to control a variable exciter voltage to control the field output by the exciter, and the corresponding magnitude of the voltage and the current output by the synchronous generator. The controller further configured to damp oscillations in a power angle between the voltage and the current by dynamic adjustment of the variable exciter voltage. 
     In another example, the disclosure is directed to a system that includes a synchronous generator configured to supply polyphase electrical power to a plurality of synchronous motor loads, a sensor configured to sense a voltage and a current of an output of the synchronous generator and a controller. The controller is configured to determine a desired power angle based on the voltage and the current received from the sensor to damp oscillations in a measured power angle between the voltage and the current. The system also includes an exciter configured to excite the synchronous generator to control at least one of the voltage and the current of the output of the synchronous generator. The controller is configured to control the exciter based on the desired power angle to dynamically adjust the excitation of the synchronous generator to damp the oscillations in the measured power angle between the voltage and the current. 
     In yet another example, the disclosure is directed to method that includes the steps of exciting a synchronous generator with an field current provided by an exciter, and controlling, with a controller, an exciter voltage to control the field current output by the exciter and a magnitude of at least one of a voltage and a current supplied at an output of the synchronous generator for a plurality of synchronous loads. The method also includes the steps of identifying, with the controller, power angle oscillations between the voltage and the current; and controlling the exciter with the controller by dynamic modulation of the exciter voltage to damp the identified power angle oscillations. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example synchronous power system for providing electrical power from an alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure. 
         FIG. 2  is a schematic diagram illustrating a portion of an example synchronous power system for providing electrical power from an alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure. 
         FIG. 3  is a conceptual diagram illustrating a portion of an example synchronous power system for providing electrical power from an alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure. 
         FIG. 4  is a flow chart illustrating example operations performed by a controller of an example synchronous power system for providing electrical power from an alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure. 
         FIG. 5  is a diagram illustrating a variable exciter voltage as compared to rotor speed of an example synchronous power system for providing electrical power from an alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure. 
         FIG. 6  is a conceptual diagram illustrating a portion of an example synchronous power system for providing electrical power from an alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure. 
         FIG. 7  illustrates an example graph of the effect of damping sub-harmonic currents. 
         FIG. 8  is an operational flow diagram example of actively damping power angle oscillations. 
         FIG. 9  is a block diagram illustrating operation of a controller in an example configuration for dynamically adjusting the voltage output signal. 
         FIG. 10  are example diagrams illustrating a transition of a voltage output signal between AC excitation and DC excitation in accordance with shaft speed. 
     
    
    
     DETAILED DESCRIPTION 
     The techniques and circuits described in this disclosure may enable a controller of an example synchronous power system to synchronize a generator to one or more load driving motors by carefully controlling the field current of an exciter to the generator and rotational acceleration or speed of the prime mover shaft. As such, the example synchronous power system may perform generator to load-motor synchronization without suffering from an increase in mass or decrease in efficiency that is commonly caused by power electronics, pony motors, and induction rotor devices which are typically used to synchronize other power systems. 
       FIG. 1  is a conceptual diagram illustrating system  100  as an example synchronous power system for providing polyphase electrical power from at least one alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure. The polyphase electrical power may be balanced polyphase electrical power, such as three phase or six phase balanced electrical power. System  100  includes prime mover  102 , exciter  103 , generator  104 , motors  106 A- 106 N (collectively referred to as “motors  106 ), and loads  108 A- 108 N (collectively referred to as “loads  108 ”). System  100  also includes controller  112  for controlling each of components  102 ,  103 ,  104 ,  106 , and  108 . 
     Prime mover  102  is configured to provide mechanical energy to system  100  by rotating or spinning shaft  110 . Prime mover  102  is any type of machine, whether an engine or a motor, that is configured to produce mechanical energy for use in a synchronous power system. Examples of prime mover  102  include heat engines (e.g., internal or external combustion engines), electrical motors, pneumatic motors, hydraulic motors, jet engines, or any other type of machine that can be controlled so as to vary the rotational speed or acceleration of shaft  110 . In some examples, the acceleration or speed of prime mover  102  can be finely controlled during start-up. For example, prime mover  102  may be controllable so that the speed of prime mover  102  increases during a first phase of a start-up period (e.g., one to two seconds, up to about thirty seconds), from substantially zero to one percent of its operational speed. Once prime mover  102  reaches one percent of its operational speed, prime mover  102  may be controllable so that the speed of prime mover  102  increases much more quickly during a second phase of the start-up period (e.g., thirty to fifty seconds), from one percent to eighty or one hundred percent of its operational speed. 
     Exciter  103  and generator  104 , in combination, convert the mechanical energy provided by prime mover  102  into a suitable form of electrical energy for powering and spinning motors  106  to drive loads  108 . Alternatively, exciter  103  and generator  104  may be on separate shafts, or exciter  103  may not be a shaft driven device. Exciter  103  is configured to provide or otherwise output a field current I FIELD  (also referred to as a “magnetizing current”) to generator  104 . Generator  104  uses the field current I FIELD  to magnetize the electromagnets in its rotor such that when the rotor spins with shaft  110 , generator  104  produces an alternating current at electrical bus  114 . Exciter  103  may produce the field current I FIELD  by producing an electromotive force (EMF) which induces an alternating (AC) current, and then by rectifying the AC current, exciter  103  outputs the field current I FIELD  in a direct (DC) current form. 
     In the example of  FIG. 1 , generator  104  is an AC generator. In some examples, generator  104  is configured to output variable frequency, three-phase AC power onto bus  114 . In other examples, generator  104  may output any poly-phase (e.g., two or more phase) AC power onto a single bus such as bus  114  or multiple buses. In the example of  FIG. 1 , exciter  103  is a brushless field exciter (e.g., a rotating-rectifier exciter). Exciter  103  may be any type of exciter that can produce a controllable field current I FIELD . 
     Motors  106  represent any type of synchronous, asynchronous, or hybrid combination thereof, motor for receiving AC electrical power provided by a synchronous power system, such as polyphase electrical power provided by system  100 . In the example of  FIG. 1 , motors  106 , such as synchronous motors, are electrically coupled to generator  104  via bus  114 . For example, motors  106  may be propulsion motors for an aircraft or marine craft, for example, for driving propellers. Motors  106  may include additional sensors and/or feedback circuitry for providing information (e.g., voltage, current, speed, frequency, phased, etc.) back to the components of system  100  that are used to control motors  106 , such as controller  112 . 
     Loads  108  represent any type of motor-driven load. In the example of  FIG. 1 , loads  108  are mechanically coupled to motors  106 , such as synchronous motors. Examples of loads  108  include propellers, fans, compressors, pumps, screws, or any other type of load that is driven by an electrical motor, such as one of motors  106 , and do not exhibit zero speed or static torque. Thus, the loads  108  may exhibit a linear increase in counter torque as the rotational speed of individual loads  108  increases with a corresponding increase in the speed of a motor  106 . The loads may be non-linear loads having torque that is monotonic to speed so that as speed increases, torque increases. In other words, torque may be continuous through a range of speed such that the motors may have a uniformly smooth torque curve. 
     System  100  includes controller  112 , which is configured to synchronize generator  104  to the motors  106  by controlling a level of the field current I FIELD  being output from exciter  103  based on a speed of shaft  110 . For the sake of brevity and clarity, controller  112  is shown as, generally, being operatively coupled to any or all of components  102 ,  103 ,  104 ,  106 , and  108 ,  110 , and  114 . In other words, controller  112  is configured to provide signals and information to, and receive information from (e.g., as feedback), each of the different components of system  100 . For example, controller  112  may send information to prime mover  102  to vary the acceleration or speed of shaft  110 . As another example, controller  112  may send information to exciter  103  to vary or otherwise control the field current I FIELD  provided to generator  104 . 
     Controller  112  may comprise any suitable arrangement of hardware that may include software or firmware configured to perform the techniques attributed to controller  112  that are described herein. Examples of controller  112  include any one or more computing systems, computing devices, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. Thus, there may be any number of independently operating controllers  112  in the system  100  that may or may not be in direct communication with each other. Controller  112  that includes software or firmware also includes hardware, such as one or more processors, processing units, processing components, or processing devices, for storing and executing the software or firmware contained therein. 
     In general, a processor, processing unit, processing component, or processing device is a hardware device that may include one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. Although not shown in  FIG. 1 , controller  112  may include a memory configured to store data. The memory may be any form of storage medium that is other than transitory, and may include any volatile or non-volatile media, such as a random access memory (RAM), read only memory (ROM), non-volatile RAM (NVRAM), electrically erasable programmable ROM (EEPROM), flash memory, and the like. In some examples, the memory may be external to controller  112  (e.g., may be external to a package in which controller  112  is housed) and may include or comprise any suitable storage medium, such as a non-transitory storage medium, for storing instructions that can be retrieved and executed by a processor of controller  112 . 
     In some examples, controller  112 , or any portion thereof, may be an internal component or feature of any of components  102 ,  103 ,  104 ,  106 , or  108 . In other words, any one or more of components  102 ,  103 ,  104 ,  106 , or  108  may include controller  112 , or any feature or characteristic associated with controller  112  that is described herein, as an internal component. 
     In operation, controller  112  may provide a signal or command (directly or indirectly) to prime mover  102  that causes shaft  110  to begin spinning with a particular rotational speed or acceleration in accordance with the signal or command provided by controller  112 . Controller  112  may provide an additional signal or command to exciter  103  that causes exciter  103  to produce a particular field current I FIELD  based at least partially on the signal or command from controller  112 . The field current and speed with which shaft  110  spins may cause generator  104  to output a two or more phase AC electrical signal across electrical bus  114 . Motors  106  may be energized by the AC electrical signal received via bus  114  to drive loads  108 . 
     By providing signals and/or commands to prime mover  102 , exciter  103 , and generator  104 , controller  112  may synchronize generator  104  to motors  106  by carefully controlling the field current I FIELD  exciter  103  provides to generator  104  and by also carefully controlling the acceleration or speed of shaft  110 . As such, controller  112  may alone perform generator to load-motor synchronization. Accordingly, system  100  may not suffer from an increase in mass or decrease in efficiency that is commonly caused by power electronics, pony motors, and induction rotor devices which are typically used to synchronize generators to motors of other power systems. 
       FIG. 2  is a schematic diagram illustrating system  200  as a portion of an example generator of an example synchronous power system, such as system  100  of  FIG. 1 , for providing polyphase electrical power from an alternating current generator, such as a synchronous generator, to one or more motors, such as synchronous motors, in accordance with one or more aspects of the present disclosure. For the sake of brevity and ease of description, system  200  is described within the context of  FIG. 1 . For example, exciter  203  and generator  204  of system  200  represent examples of, respectively, exciter  103  and generator  104  of system  100 . Exciter  203  and generator  204  are controllable by controller  212  of system  200  which represents an example of controller  112  of system  100 . 
     Exciter  203  represents an example of a brushless exciter and is configured to output field current I FIELD  to generator  204 . Exciter  203  is controllable by controller  212  such that signals or commands from controller  212  in the form of a voltage output signal (exciter voltage) cause exciter  203  to output a variable level field current I FIELD . Exciter  203  includes exciter rotor  232 , exciter stator  234 , and rectifier  236 . Exciter  203  may include other components required to produce field current I FIELD . 
     Rectifier  236  rectifies an AC current output from exciter rotor  232  to a DC field current I FIELD  output that is used by generator  204  to magnetize generator rotor  222 . In some examples, rectifier  236  is a full-bridge rectifier. 
     Exciter stator  234  may include an exciter field coil, which is a set of stationary coils. In other words, the exciter field coil does not move or spin with movement of a prime mover shaft. Exciter stator  234  may be energized, by controller  112  using a controlled voltage source  238 , to induce a current in the exciter stator  234 . The voltage source  238  may supply the exciter voltage. The voltage source  238  may transition the exciter voltage between an AC voltage signal and a DC voltage signal such that an AC current, a DC current or some combination of an AC current waveform and a DC current waveform may be induced with the exciter stator  234 . Accordingly, the exciter voltage may selectively include an AC component and a DC component. The level of the AC component and the DC component in the exciter voltage may be selectively and/or independently varied by the controller based on a rotational speed of the exciter rotor  232 . Thus, a waveform of the exciter voltage may selectively include at least one of an AC component or a DC component. In addition, the controller may transition a level of the AC component lower and transition of a level of the DC component higher based on an increase in rotational speed of the exciter while electric power output of the synchronous generator is occurring. 
     Controller  112  may control the voltage level of the voltage source  238  (exciter voltage) via the voltage output signal to vary the level of the current that is induced by exciter stator  234 . The voltage source  238  is illustrated with dotted lines since the voltage source  238  may be included in the controller  212 , and may be controlled using a voltage regulation circuit or through other voltage regulation techniques. Alternatively, the voltage source  238  may be a separate device or system that receives the voltage output signal from the controller  212  and produces the exciter voltage, or may be included in the exciter  203  and receives the voltage output signal. For purposes of brevity, the exciter voltage will be described as being controlled by the controller  212  using an output voltage signal, although it should be recognized that the controller  212  may provide the exciter voltage or control output of the exciter voltage. 
     Exciter rotor  232  may include an exciter armature, which is a set of balanced coils, coupled to shaft  110  (not shown) of system  100 , which is driven by prime mover  102  of system  100 , and controlled by controller  212  to spin at a variable speed or acceleration. In other words, unlike the exciter field coil which may remain stationary, the exciter armature may move or spin with movement of a prime mover shaft. The balanced coils of exciter rotor  232  are connected through rectifier  236  to generator rotor  222 . When the exciter armature of exciter rotor  232  is rotating or spinning, the magnetic flux produced by the exciter field coil of exciter stator  234  is provided by the exciter armature coils of exciter rotor  232  to rectifier  236 . This change in magnetic flux in the exciter armature coils of exciter rotor  232  generates an electromotive force (EMF). This EMF induces current in the field winding of generator rotor  222  during a first portion of the EMF AC cycle. The flux produced by the exciter armature coil of exciter rotor  232  then decreases as it leaves the magnetic flux region of exciter field coil of exciter stator  234 , and an opposite EMF is generated. Rectifier  236  naturally applies the EMF in a consistent manner to induce current flow in one direction, as field current I FIELD , through the field coil of generator rotor  222 . 
     Generator  204  is configured to output an AC power to electrical bus  214 . Generator  204  is controllable by controller  212  such that a signal and/or command (voltage output signal) from controller  212  controls the exciter voltage, which may cause generator  204  to output AC power at a variable power level or variable frequency at bus  214 . Generator  204  includes generator rotor  222  and generator stator  224 . 
     Generator rotor  222  may include a rotating field coil that spins or rotates with shaft  110  of system  100  congruently with the spinning or rotation of exciter rotor  232 . The field coil of generator rotor  222  is typically much more inductive than the rotor coils of exciter rotor  232 , and as such, the field coil of generator rotor  222  may filter the fundamental frequency of field current I FIELD  (i.e., the rectified exciter current). Field current I FIELD  from exciter  203  magnetizes generator rotor  222 . 
     Generator stator  224  includes a set of stationary coils which may not move or spin with movement of shaft  110 . As generator rotor  222  spins with the spinning of shaft  110 , the resultant magnetic field produced by field current I FIELD  running through the rotating field coil of generator rotor  222  induces an AC current out of generator stator  224  at bus  214 . 
       FIG. 3  is a conceptual diagram illustrating system  300  as a portion of an example synchronous power system, such as system  100  of  FIG. 1 , for providing polyphase electrical power from an alternating current generator, such as a synchronous generator, to one or more motors, such as synchronous motors, in accordance with one or more aspects of the present disclosure. For the sake of brevity and ease of description, system  300  is described within the context of system  100  of  FIG. 1  and system  200  of  FIG. 2 . For example, exciter  303  and generator  304  of system  300  represent examples similar to, respectively, exciter  103  and generator  104  of system  100  or exciter  203  and generator  204  of system  200 . Exciter  303  and generator  304  are controllable by controller  312  of system  300  which represents an example similar to the controllers  112  and  212  of systems  100  and  200 . 
     System  300  includes prime mover  302  as an example of prime mover  102  of system  100 . Prime mover  302  produces mechanical energy that spins shaft  310  which causes rotor  332  of exciter  303  and rotor  322  of generator  304  to also spin or rotate as exciter  303  and generator  304  may be both mechanically coupled to shaft  310 . In other words, rotor  332  and rotor  322  may be mechanically coupled to prime mover  302  via shaft  310 . In other examples, exciter  303  and generator  304  may be on separate shafts, or exciter  303  may not be a shaft driven device. 
     System  300  further includes motor  306  and load  308 . Motor  306  is driven by a three-phase AC electrical signal output from generator  304  onto link  314 . In the example of system  300 , motor  306  is a synchronous propulsor motor which is mechanically coupled to load  308 . In the example of  FIG. 3 , load  308  is a fan or a propeller, or another load having a linear torque curve. In other examples, system  300  may include more than one motor  306  and more than one load  308 , including any and all other examples of motors  106  and loads  108  described above with respect to system  100 . 
     Controller  312  of system  300  may send and receive information for controlling the speed at which shaft  310  spins, the current or voltage level at bus  314 , and/or the speed at which motor  306  spins load  308 . For example, controller  112  may provide a signal or command to prime mover  302  that causes prime mover  302  to spin shaft  310  with a particular speed or acceleration defined by the signal from controller  312 . Controller  312  may provide a signal or command (voltage output signal) to exciter  303  that causes exciter  303  to produce a particular field current I FIELD  in accordance with the signal or command from controller  312  that provides the exciter voltage. The field current I FIELD  produced by exciter  303  and the speed with which shaft  310  spins may cause generator  304  to output a three-phase AC electrical signal across electrical bus  314 . Motor  306  may use the AC electrical signal received via bus  314  to drive load  308 . 
       FIG. 4  is a flow chart illustrating example operations performed by a controller of an example synchronous power system, such as system  100  of  FIG. 1 , for providing polyphase electrical power from an alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure.  FIG. 4  is described below within the context of  FIGS. 1-3 . For the sake of brevity, operations  400 - 420  are described as being performed by controller  112  of  FIG. 1  although controllers  212  and  312  may also perform operations  400 - 420 . 
     Controller  112  may synchronize generator  104  to motors  106  by providing signals and commands, to prime mover  102 , exciter  103 , and generator  104 , that carefully control the field current I FIELD  exciter  103  provides to generator  104  and by also carefully controlling the rotational acceleration or speed of shaft  110 . In other words, since controller  112  has control with respect to the start of prime mover  102  and its acceleration of shaft  110 , and since controller  112  has control with respect to the field current I FIELD  provided to generator  104 , and since system  100  powers loads  108  that have a torque that increases linearly in dependence on rotational speed, then controller  112  may control system  100  through means already designed into the downstream system. As such, controller  112  may alone perform generator to load-motor synchronization without the need for additional power electronics, pony motors, and induction rotor devices that are typically used to synchronize generators to motors of other power systems. 
     To implement the control concept provided by controller  112 , motors  106  and generator  104  may be well matched and controller  102  may control the field current I FIELD  provided by exciter  103  to enable rotor magnetic flux at generator  104  even when shaft  110  is at substantially zero speed. In other words, with motors  106  and generator  104  being matched, the combined electrical ratings of motors  106  are within the operating capacity of generator  104 , and generator  104  has sufficient capacity to produce excess (e.g., 125%) of the combined voltages required by motors  106  for short periods (e.g., 5 seconds). Substantially zero speed of the shaft  110  or generator  104  refers to the lowest speed in which the generator  104  can source sufficient terminal current to supply the very small static friction torque, negligible load torque, and torque to accelerate the moment of inertia of the motors  106  to the same electrical speed before the generator  104  rotates more than about ¼ of an electrical revolution. By controlling the speed of shaft  110  and the energizing voltage provided to exciter  103 , controller  112  may be able to control the “synchronization” of motors  106  and loads  108  without additional power electronics, pony motors, and induction rotor devices. Synchronization may be controlled since when prime mover  102  and generator  104  start (e.g., as shaft  110  first begins to rotate and starts to increase from a substantially zero speed to an operational speed over a period of time ranging from seconds to minutes) the load torques associated with corresponding motors  106  and loads  108 , which are of negligible magnitude at low speeds, correspondingly increase as speed increases. Controller  112  may control system  100  based on the following theory of operations. 
     For controller  112  to induce a terminal voltage (V GEN ) of generator  104  (e.g., a voltage sufficient to initiate rotation of motor  106  and loads  108 ), during system start-up and at extremely low rotational rotor speeds (ω) of generator  104 , controller  112  may energize the field coil of the stator of exciter  103  to induce a voltage with a significantly high magnitude and frequency. That is, the voltage used to energize the field coil of the stator of exciter  103  may have a combination of magnitude and frequency that causes the magnetic flux produced by the stator of exciter  103  to couple across the exciter air-gap between the stator and rotor, so as to produce a field current I FIELD  from the rotor of exciter  103 , that is sufficient for initiating and maintaining rotation of the motors  106 , without exceeding the voltage rating of the exciter stator insulation. Thus, the field current I FIELD  is capable of producing the maximum generator phase voltage at whatever rated speed the generator is rotating. For example, consider Table 1, which shows variation in field current I FIELD  and terminal voltage V GEN  given variations in rotor speeds (ω) and the magnitude of the exciter voltage at exciter  103 . Note: The values shown in Table 1 are examples only for the purposes of illustration; actual values vary depending on system parameters and load requirements. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                 Terminal 
               
               
                 Exciter 
                 Exciter 
                 Field Current - 
                   
                 Voltage-V GEN   
               
               
                 Voltage 
                 Frequency 
                 I FIELD   
                 Rotor Speed - ω 
                 @ rated speed 
               
               
                 (V) 
                 (Hz) 
                 (A) 
                 (RPM) 
                 (V) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                  6 V 
                 0 
                 ~0 
                 1 
                 ~0 
               
               
                 260 V 
                 200 
                 13 
                 1 
                 ~0.19 
               
               
                 200 V 
                 200 
                 10 
                 1000 
                 145.5 
               
               
                  6 V 
                 0 
                 10 
                 1000 
                 145.5 
               
               
                 200 V 
                 200 
                 10 
                 3300 (rated) 
                 480 
               
               
                  2 V 
                 0 
                 3.3 
                 10000 
                 480 
               
               
                   
               
            
           
         
       
     
     According to Table 1, with an exciter voltage of 6V, 0 Hz, when a rotor speed (ω) of generator  104  is substantially zero (or low), the field current I FIELD  out of exciter  103  may be negligible or zero, which may only be sufficient to produce a terminal voltage V GEN  at generator  104  (e.g., which is not of sufficient magnitude to drive motors  106 ). On the other hand, at an exciter voltage of 260V, 200 Hz, when the rotor speed (ω) of generator  104  is substantially zero (or low), the field current I FIELD  out of exciter  103  may be 10 A and may be sufficient to produce a terminal voltage V GEN  of generator  104  that is approximately 0.19V (e.g., which may produce sufficient current magnitude to drive motors  106  up to the low synchronous speed). 
     As the rotor speed (ω) increases, the required magnitude of the exciter voltage becomes less and less, to produce a sufficient field current I FIELD  out of exciter  103  that is sufficient to produce a terminal voltage V GEN  of generator  104  sufficient to drive motors  106 . For example, at an exciter voltage of only 200V, 200 Hz, when the rotor speed (ω) of generator  104  is approximately ⅓ the operational speed (e.g., 1000 RPM), the field current I FIELD  out of exciter  103  may still be 10 A and may be sufficient to produce a terminal voltage V GEN  of generator  104  that is approximately 145.5V (e.g., which may be of sufficient magnitude to drive motors  106 ). 
     Lastly, Table 1 shows that when the rotor speed (ω) of generator  104  is relatively high (e.g., at 1000 RPM or some other operational speed), an exciter voltage of only 6V, 0 Hz may produce a field current I FIELD  out of exciter  103  of 10 A which may be sufficient to produce a terminal voltage V GEN  of generator  104  that is also approximately 145.5V. In other words, Table 1 shows that, when the exciter voltage of exciter  103  is increased to a relatively high frequency (e.g. 200 Hz), by increasing the magnitude of the exciter voltage to sufficiently high levels (e.g., 200V), exciter  103  may produce a field current I FIELD  that is sufficiently high (e.g., 10 A), even at substantially zero or low speed (ω), to produce a terminal voltage V GEN  that is sufficient for driving motors  106  to begin and maintain rotation. As the rotor speed (ω) of the generator  104  increases beyond a threshold speed (e.g., ⅓ operational or ⅓ max speed), the magnitude of the exciter voltage can be reduced and still cause exciter  103  to produce a sufficiently high field current I FIELD  to drive the motors  106 . When the rotor speed (ω) reaches a predetermined rotation speed, such as an operational speed or a maximum speed, the AC component of the exciter voltage can be removed entirely and the exciter voltage can be a nominal DC voltage (e.g., 6V, 0 Hz). See  FIG. 5  for a graphical view of the relationship between exciter voltage and rotor speed (ω). 
     The above theory of operations may enable controller  112  to re-configure exciter  103  from operating as an “inside out” field wound motor, to operating as a transformer, such as an air gap transformer. In other words, when synchronizing between the generator  104  and motors  106  as the shaft  110  is increasing from substantially zero speed up to its operational speed, controller  112  may provide an exciter voltage to exciter  103  that is of sufficiently “high magnitude and frequency” for inducing the terminal voltage V GEN  at bus  114  that is needed to initiate and maintain rotation of motors  106  and load  108  synchronous with the increasing rotational speed of the generator  104 . 
     In operation, referring to  FIG. 4 , controller  112  may determine speed of the shaft  110  that mechanically couples the prime mover  102 , such as a jet engine, of the system to the AC generator  104  of the system ( 400 ). For example, during a period of time that is associated with the start-up of prime mover  102 , controller  112  may provide a signal and/or command to prime mover  102  that causes shaft  110  to begin increasing from a zero speed to an operational speed. At the start of system  100 , controller  112  may receive sensor information indicating a speed of shaft  110  as prime mover  102  begins mechanically spinning or rotating shaft  110 . In other examples, controller  112  may infer the speed of shaft  110  based on voltage and/or current measurements taken within the system  100  (e.g., from AC generator  104 , for example). In any case, this start-up phase (e.g., lasting anywhere from between zero and five seconds) also causes the rotors of exciter  103  and generator  104  to begin spinning congruently with shaft  110 . While the speed of shaft  110  is at substantially zero, or at any time before shaft  110  is at a predetermined full operational or rated speed at which AC generator  104  drives each of motors  106  and loads  108 , controller  112  may induce excitation in system  100  so as to cause motors  106  and loads  108  to “spin-up” or be induced (energized) to rotate in-synch with AC generator  104 . 
     Controller  112  may determine, based on the speed of the shaft, a level of a field current I FIELD  needed to excite the AC generator  104  and synchronize the AC generator  104  to one or more electrical motors  106  that are electrically coupled to the AC generator and are configured to drive one or more mechanical loads  108  ( 410 ). For example, controller  112  may utilize a function or a look-up table of values to determine the level of field current I FIELD  needed by AC generator  104  to produce a terminal voltage V GEN  at bus  114  that is of sufficient strength (e.g., magnitude and frequency) to begin turning motors  106  and loads  108  as shaft  103  spins with substantially zero or less than operational speed. In some examples, controller  112  may input the speed into a function or look-up table and determine, based on the function or look-up table, that the level of the field current I FIELD  is at a maximum level of current when the speed of the shaft is at substantially zero speed or that the level of the field current is at a minimum level when the speed of the shaft is at an a operational speed (e.g., 3300 RPM or some other speed needed to drive AC generator  104  to produce the required V GEN  at bus  114 ). 
     In some examples, in addition to determining the speed of the shaft  110 , controller  112  may determine a power factor of the AC generator  104  and changes, over time, in the speed of the shaft  110  and the power factor of the AC generator  104 . In this case, controller  112  may determine the level of the field current I FIELD  (needed to excite the AC generator sufficiently to maintain synchronized rotation of the AC generator with rotation of one or more electrical motors that are electrically coupled to the AC generator and configured to drive one or more mechanical loads) based on the speed of the shaft  110 , the power factor of the AC generator, and changes, over time, in the speed of the shaft  110  and the power factor of the AC generator. In other words, the function, look-up table, and/or algorithm that controller  112  may use to determine the field current I FIELD  needed for a particular load condition may be dependent on more than just the rotational speed of the shaft  110 . Controller  112  may input at least one of the rotational speed, the power factor, or changes in the speed and/or the power factor, into a function and/or look-up table and determine, based on an output from the function and/or look-up table, the level of the field current I FIELD . 
     Controller  112  may adjust the field current I FIELD  to maintain the power factor of the AC generator in a predetermined range, such as greater than −0.9, less than +1.1, or otherwise near 1.0, as the power factor fluctuates in the predetermined range. As the speed of the shaft  110  changes and the power factor changes, controller  112  may update its determination at any given time regarding the level of field current I FIELD  needed to excite the AC generator and maintain the AC generator  104  synchronized to one or more electrical motors  106  that are electrically coupled to the AC generator  104  and configured to drive one or more mechanical loads  108  at the given time. In other words, the function and/or look-up table used by controller  112  may factor in changes in speed and/or power factor to cause controller  112  to adjust the field current I FIELD  accordingly. 
     Controller  112  may control the exciter  103  of the system  100  to cause the exciter  103  to output the level of the field current I FIELD  to excite the AC generator  104  and synchronize the AC generator  104  to the one or more electrical motors ( 420 ). For example, controller  112  may synchronize AC generator  104  with motors  106  by varying the level of the field current I FIELD  being output from exciter  103  during start-up of system  100 , or at any other time, in response to controller  112  determining that the speed of the shaft  110 , the power factor of the AC generator  104 , and/or changes, over time, in the speed of the shaft  110  and the power factor of the AC generator  104 . 
     In any case, if controller  112  determines that due to the speed or acceleration of shaft  100 , that system  100  is a candidate for synchronization via exciter field current I FIELD  control, controller  112  may control the field current I FIELD  using a voltage output signal or command to control the exciter voltage. Although referred to herein as a “voltage output signal,” control of exciter  103  by the controller  112  to output the field current I FIELD  may be a command, a variable excitation voltage output by the controller  112 , or a control signal provided directly to the exciter  103  to create the exciter voltage, or to a power supply or other device that may directly or indirectly create the exciter voltage to induce the exciter  103  to output the field current I FIELD . The voltage output signal may cause application of an exciter voltage to the exciter  103  that has a sufficient magnitude or frequency to induce (even when the shaft  110  is at substantially zero speed) a field current I FIELD , and therefore a terminal voltage V GEN , at the AC generator  104  that causes the one or more electrical motors  106  to drive the one or more mechanical loads  108 . For instance, in some examples, the terminal voltage V GEN  is a minimum voltage needed by motors  106  to accelerate loads  108  from a substantially zero speed. By utilizing the principles of Table 1, controller  112  may apply a relatively high level of exciter voltage at a relatively high frequency, to the armature of exciter  103  such that a field current I FIELD  is induced out of exciter  103 , even if shaft  110  is not spinning or spinning slowly. As the speed of shaft  110  increases to operational speed, controller  112  may reduce the magnitude of the exciter voltage back down to predetermined operating levels associated with the operational speed(s). 
     In some examples, controller  112  may apply the exciter voltage directly (e.g., via an internal voltage source  238 ) using the voltage output signal. In other examples, exciter  103  may include a variable voltage source  238  and controller  112  may control the variable voltage source of exciter  103  to output the exciter voltage based on the voltage output signal to produce a sufficiently high voltage or frequency at the field coil of exciter  103  to induce a terminal voltage V GEN  at AC generator  104  that causes motors  106  to drive loads  108 . 
     In some examples, controller  112  may continue to monitor the speed of shaft  110 , the power factor of AC generator  104 , the magnitude of the terminal voltage V GEN , the level of field current I FIELD  out of exciter  103 , and the rotational speed or acceleration of loads  108  and dynamically adjust the amount of excitation that controller  112  applies to exciter  103  accordingly. For example, controller  112  may dynamically adjust the exciter voltage to exciter  103  by decreasing a magnitude of the exciter voltage in response to determining an increase in the rotational speed of the shaft  110  or an increase in a speed of the one or more mechanical loads. For example, controller  112  may dynamically decrease the magnitude of the exciter voltage proportionally to the level of increase in the speed of the shaft  110 , or an increase in the speed of the one or more mechanical loads. In other words, at speeds where the DC excitation becomes effective, as the speed of shaft  110  increases or as the speed of the one or more mechanical loads  108  increases, controller  112  may decrease the level of exciter voltage or in some examples, may transition to modulated low voltage DC excitation, since the increasing speed of shaft  110  or the increasing of the speed of the one or more loads  108  may naturally lead to an increase in the level of field current I FIELD  out of exciter  103 , and thereby lead to an increase or maintaining of the level of the terminal voltage at bus  114 . 
     In some examples, as also described elsewhere, controller  112  may monitor the power factor of AC generator  104  and dynamically adjust the field current by adjusting the magnitude or frequency of the exciter voltage so as to substantially maintain unity power factor. As used herein, substantially maintaining unity power factor refers to maintaining the power factor within a predetermined range of unity such as +/−0.1, such that the power factor ranges from 0.90 lagging to 0.90 leading. For example, controller  112  may dynamically vary the excitation voltage magnitude and frequency to increase the field current I FIELD  to move the power factor to the lagging region (e.g., in response to determining the power factor is greater than one or “leading”). Conversely, controller  112  may dynamically vary the excitation voltage magnitude and frequency to decrease the field current I FIELD  to move the power factor to the leading region (e.g., in response to determining the power factor is less than one or “lagging”). 
     In some examples, controller  112  may apply the excitation voltage to the exciter, so as to induce a field current I FIELD  and terminal voltage, at low speeds by setting the magnitude of the exciter voltage to a maximum voltage when the speed of the shaft is at substantially zero speed and setting the magnitude of the exciter voltage to a minimum voltage when the speed of the shaft is at an operational speed. In other words, controller  112  may utilize the principles of Table 1 and as described above to use a relatively high magnitude and frequency exciter voltage when the speed of shaft  110  is low (e.g., less than operational speed) and use a lower magnitude and frequency exciter voltage when the speed of shaft  110  is high (e.g., at operational speed). 
     By energizing exciter  103  with a particular high level and high frequency voltage in this way, controller  112  may control the field current I FIELD  output from exciter  103  even at low rotational speeds. Controller  112  may control exciter  103  using a speed independent exciter armature or exciter voltage, and therefore, dynamically control the field current I FIELD  providing the magnetic flux of the rotor of generator  104  so as to permit a significant terminal voltage V GEN , even at very low shaft speeds. The significant terminal voltage V GEN  may induce current flow in the attached load motors  106  and thus torque, thereby accelerating load motors  106  to match the electrical speed of generator  104 . 
     As the components of system  100  spin-up to a predetermined operational speed, the exciter field energizing voltage may increase in frequency, decrease in AC magnitude, and an additional DC component may increase. Near operational speed, the AC component of the exciter field voltage may be eliminated and controller  112  may use techniques, such as power factor control, to control the DC component to ensure continued synchronization of load motors  106  under varied load conditions. 
       FIG. 5  is a diagram illustrating a variable exciter voltage  500  as compared to rotor speed (ω)  502  of an example synchronous power system, such as system for providing polyphase electrical power from an alternating current generator to one or more motors, in accordance with one or more aspects of the present disclosure. For example, with reference to Table 1,  FIG. 5  shows the variable exciter voltage  500  delivered to an exciter, such as exciter  103 , being at 260V, 200 Hz when the rotor speed (ω)  502  of a generator, such as generator  104 , is low or (substantially zero RPM). As the rotor speed (ω)  502  of generator increases, a controller, such as controller  112 , may decrease the magnitude of the exciter voltage. For instance, when the rotor speed (ω)  502  of generator reaches approximately 1000 RPM or ⅓ its operational speed, controller  112  may apply a 100V, 200 Hz exciter voltage to the exciter. And eventually, once the rotor speed (ω)  502  of generator reaches approximately 3300 RPM and higher, up to is maximum operational speed, the controller may decrease the magnitude of exciter voltage further, eventually only applying only a minimal 5V, 0 Hz exciter voltage to the exciter. 
     In examples where the generator includes a relatively high impedance when compared to a relatively low impedance of motors, such as motors  106 , the voltage at the output of the generator may be largely dictated by the motors. For example, the impedance of the generator may be three or four per unit (p.u.) and the impedance of the motors, as viewed from the generator may be one or two p.u. In such examples, changes in the exciter voltage supplied to the generator may result in changes in a magnitude of current output of the generator with relatively little change in voltage output of the generator. In addition, a relatively high per unit generator with relatively low per unit load motors and fixed exciter current may have a significantly reduced increase in power with motor electrical displacement angle. 
     During a startup condition, such as when the speed of the generator is substantially zero and first begins to rotate, or at rotational speeds of less than full speed, such as less than 50% of rated speed of the generator, the motors and the generator may be synchronously rotating. Under these conditions, the synchronous coupling, or magnetic coupling, between the generator and the motors may be a relatively “loose” coupling or a relatively low “stiffness” in the magnetic coupling of the generator rotor and the motor rotors due to the low rotational speed conditions. (e.g. low change in electrical torque transfer with electrical angle of displacement of motor(s) with respect to generator) For example, a high per unit generator with low per unit load motors and fixed exciter current may have a significantly reduced increase in power with motor electrical displacement angle. 
     As described herein, a “loose coupling” or “stiffness” refers to the capability of the rotors of the motors and the generator to maintain electrical phase synchronization during changing operating conditions, such as perturbations within the system  100 . Such perturbations or disturbances may be, for example, a result of changes in the load, such as load  108 , on one or more motors, changes in rotational speed of the generator, and/or changes in the field current supplied to the generator. Examples of other changing operating conditions may include changes in the rotational speed of both the generator  104  and corresponding synchronized motors  106 . The robustness of the magnetic coupling due to synchronization of the generator rotor and the motor rotors may be affected by system operating conditions such as the rotational speed, the magnitude of current flow to the motors, and the power factor angle. As the rotational speed of the generator  104  increases, and/or the current flow to the motors  106  increases, the magnetic coupling between the generator  104  and the motors  106  may increase in stiffness making a loss of synchronism between the generator  104  and the motor  106  less likely to occur. In addition, a power factor angle between the voltage and current that is lagging may result in a stiffer coupling when compared to, for example, a unity power factor. 
     After synchronization of the generator with the motors, such that rotational speeds are substantially equal, changes in system operating conditions may result in mechanical modes occurring at one or more resonant frequencies of rotating inertia of the load that coincides with a phase delay of the inductance of the generator. A mode may be self-sustaining and reinforce decoupling action between the generator and the motors. Modes may occur at any frequency where the system is underdamped. Under underdamped system conditions, torque oscillations (or torque ripple) may develop between the generator and the motors, and be reinforced at the motors  106  such that the power angle between voltage and the current being supplied at the output of the generator  104  begins to oscillate creating a resonant mode. The torque oscillations and corresponding changes in the power angle may occur at a subharmonic frequency to the frequency of the voltage and current. As the rotational speed of the generator and synchronized motors changes, such as during a ramped speed system startup, different power angle oscillations (and corresponding torque oscillations) may occur at different sub-harmonic frequencies. 
       FIG. 6  is a block diagram of an example system  600  that includes a generator  604  having a rotor  605  and providing a voltage and current (polyphase electrical power) at an output  614  of the generator  604  in accordance with a field current I FIELD    609  supplied by an exciter  603  as controlled by a controller  612  to supply motors  606 , such as synchronous motors that are driving loads to form synchronous loads. For the sake of brevity and ease of description, system  600  is described within the context of system  100  of  FIG. 1 , system  200  of  FIG. 2  and system  300  of  FIG. 3 . For example, exciter  603  and generator  604  of system  600  represent examples similar to, respectively, exciter  103  and generator  104  of system  100  or exciter  203  and generator  204  of system  200  or exciter  303  and generator  304  of system  300 . Exciter  603  and generator  604  are controllable by controller  612  of system  600 , which represents an example similar to the controllers  112  and  212  and  312  of systems  100  and  200  and  300 . 
     The controller  612  may monitor the voltage and/or current of two or more phases at the output  614  of the generator  604  using a sensor  616 . The output  614  may also be considered the system bus, or system voltage and current. The sensor  616  may be a current transformer (CT), a potential transformer (PT) or any other form of voltage and/or current measurement device capable of outputting measurement signal(s) to the controller  612 . Based on the sensed voltage and/or current, the controller  612  may identify torque oscillations between the generator  604  and the motors  606  within a mode at a sub harmonic frequency. In other examples, torque oscillations within a mode may be identified by the controller  612  from other sensed inputs provided by other forms of sensors, such as a position sensor for the shaft  110  of the generator  604 . The subharmonic frequency of a mode may be below the rotational speed of the shaft  110  and corresponding AC frequency of the sensed voltage and current. For example, the subharmonic frequency can be low, such as 1 to 4 Hz, when the frequency of rotation of the shaft (the voltage and current frequency) is much higher, such as 300 or 400 Hz. 
     An example in  FIG. 6  includes the voltage or current signal  620  illustrated at a relatively high frequency and the sub harmonic frequency being within a subharmonic envelope  622  at a relatively low frequency. Any number of resonant modes may occur at different resonant sub harmonic frequencies during the ramped speed startup of the generator  104  and motors  106 . Occurrence of such sub harmonic frequencies may be dependent on, for example, machine parameters such as moments of inertia, internal impedance, and distribution impedance. In addition, sub harmonic frequencies may be any frequency less than the frequency of the voltage and current. 
     During one of these resonant modes, current demand by the motors  606  may correspondingly oscillate due to the effect of the oscillation of the power angle and the corresponding complex power requirements of the motor  606  (e.g. oscillatory changes in reactive power (VAR) requirements at the motor). Due to the oscillations in the corresponding power angle and complex power at the resonant frequency, if the generator  604  and the motors  606  are loosely magnetically coupled, the magnitude of the oscillations may increase until one or more of the motors  606  lose synchronization with the generator  604  such that the generator  604  and one or more of the motors  606  are no longer magnetically coupled. The loss of synchronization may also be referred to as “slipping a pole” since the poles of the generator rotor and the poles of the motor rotor are no longer electrically magnetically coupled between corresponding poles. As an example analogy for understanding by the reader, the synchronous operation of the generator  604  and motors  606  can be viewed as an “electronic mass spring damper” that is either underdamped, overdamped, or critically damped at a given frequency. As such, variations in synchronization between the generator  604  and motors  606  during an underdamped condition can be thought of as being analogous to changes in the loading of a mechanical spring. Such variations in the synchronization (spring loading) may be reduced or damped by damping the oscillations of the power angle (and corresponding oscillations in torque loading) so as to avoid loss of synchronization conditions when the system is otherwise underdamped. 
     Damping of the power angle oscillations may be performed using the controller  612  and the exciter  603  by selective changes of the field current I FIELD  at the exciter  603  to counteract the power angle oscillations and resulting torque oscillations. The controller  612  may dynamically modulate a level of a voltage output signal  624  supplied to the exciter  603  (directly or indirectly) to correspondingly increase and decrease the stiffness of the coupling between generator  604  and the motors  606 . The timing by the controller  612  for increasing and decreasing the stiffness of the coupling may be at substantially the same frequency as the resonant frequency of a mode such that the torque oscillations and corresponding oscillations in the power angle during the mode are damped, or reduced. Modulation of the voltage output signal may be timed by the controller  612  with respect to the phasing of the current output by the generator  604  such that increases in the level of the voltage output signal are 180 degrees out of phase with the phasing of the current/voltage inducing the undesirable power angle oscillations (torsional oscillations) in order to provide a canceling or counteracting effect and thereby actively damp out the oscillations. Thus, modulation of the voltage output signal may have the effect of changing the system from be critically damped or underdamped to being over damped. 
     Damping of the power factor, power angle, or reactive power oscillations may effectively damp out sub-harmonic oscillations in the terminal currents of the generator  104 .  FIG. 7  illustrates an example graph of the effect of damping sub-harmonic currents. In  FIG. 7 , generator phase currents on the y-axis  702  are damped over a period of time illustrated along the x-axis  704 . In the illustrated example, at time t=0 until time t=0.1, there are relatively large sub-harmonic oscillations in power factor (power angle). In this example, the power factor is initially oscillating between approximately 100% and 42% (i.e. power angle between 0 degrees and 65 degrees lagging). When active damping is enabled at about time t=0.1 to t=0.25, the power angle oscillations are reduced and the power angle is stabilize at a desired 0 degrees (for this example). This example is for purposes of explanation only, and may not represent the power angle oscillation levels, frequencies, power factors, etc. of a particular system, and therefore does not limit the scope of the present disclosure. 
     Determination by the controller  612  of the level of the field excitation to be applied to the exciter  603  may be based on a determination of a desired reactive power, power angle or power factor. The desired reactive power, power angle or power factor may be determined based on system operating conditions and/or objectives. For example, it may be desirable for the system to substantially maintain zero reactive power (unity power factor), or some non-zero amount of reactive power at full rated speed. The desired reactive power, power angle or power factor may be a fixed value independent of system operating conditions, such as rotational speed of the generator  604 . Alternative, the controller  612  may dynamically determine the desired reactive power, power angle or power factor based on system operating conditions, such as rotation speed of the generator  604 . 
     In some examples of system operation, the controller  612  may use a lookup table to dynamically determine a desired reactive power, power angle or power factor. In other examples, modeling, adaptive control, fuzzy logic or any other control scheme may be used to dynamically determine a desired reactive power, power angle or power factor for the system. 
     Table 2 is an example of a table for dynamic determination by the controller  612  of a desired power angle using the voltage and current measured, for example, by the sensor  616  at the output of the generator  604 . In other examples, Table 2 could be used to determine a desired power factor or reactive power or other desired parameter. In Table 2, a magnitude of the voltage and current output by the generator  604  are inputs to the controller  612 , which are multiplied to determine a total apparent power (kVA) along a vertical axis in Table 2. In addition, a horizontal axis in Table 2 provides the voltage magnitude of the output voltage of the generator  604  at the output  614 . In example systems where system voltage is dominated by low-impedance synchronous loads, such as permanent magnet synchronous motors, the voltage may be proportional to speed of rotation of the shaft of the generator  604 . In other examples, the rotational speed of the generator  604 , or some other parameter indicative of speed may be used. 
     In this example, as indicated in Table 2, the desired power angle becomes progressively closer to zero (power factor becomes closer to unity) as power increases, since efficiency is improved at higher generator power output resulting in tighter magnetic coupling between the generator  604  and the motors  606 . At lower power levels, a progressively more lagging power factor is provided in this example to leave greater margin in the stiffness of the magnetic coupling between the generator  604  and the motor  606  to allow for potential load imbalances. In this example, the desired power factor also becomes closer to one as speed increases. This is because in this example system, the magnetic coupling between generator rotor and load rotors is determined to be stiffer at higher speed, so that greater torque margin is provided. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Reference Lagging 
                 |V| (Volts) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Power Angle (deg) 
                 10 
                 50 
                 100 
                 200 
                 350 
                 600 
                 1000 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 |V|*|I| 
                 1 
                 48 
                 45 
                 42 
                 39 
                 36 
                 33 
                 30 
               
               
                 (kVA) 
                 5 
                 44 
                 40.8 
                 37.7 
                 34.5 
                 31.3 
                 28.2 
                 25 
               
               
                   
                 10 
                 40 
                 36.7 
                 33.3 
                 30 
                 26.7 
                 23.3 
                 20 
               
               
                   
                 20 
                 36 
                 32.5 
                 29 
                 25.5 
                 22 
                 18.5 
                 15 
               
               
                   
                 35 
                 32 
                 28.3 
                 24.7 
                 21 
                 17.3 
                 13.7 
                 10 
               
               
                   
                 60 
                 28 
                 24.2 
                 20.3 
                 16.5 
                 12.7 
                 8.8 
                 5 
               
               
                   
                 100 
                 24 
                 20 
                 16 
                 12 
                 8 
                 4 
                 1 
               
               
                   
                 200 
                 6 
                 5 
                 4 
                 3 
                 2 
                 1 
                 0 
               
               
                   
               
            
           
         
       
     
       FIG. 8  is an operational flow diagram example of actively damping power angle oscillations (torque oscillations) that is applicable to any of the system described, however, for ease of understanding, reference will be made to  FIG. 6  unless otherwise noted. During operation, voltage and current may be sensed by at the generator terminals  604  by a sensor  616 . ( 802 ) The voltage and current signals from the sensor  616  may then be transformed such as by Clark&#39;s transform (α, β) for a fixed frame of the stator to obtain two-dimensional Cartesian coordinates. ( 804 ) For example, in a Clark&#39;s transformation, current differences between phase sets may be transformed into their own two dimensional (2D) stationary frame. Using the two-dimensional Cartesian coordinates, the magnitude and vector angles for the voltage and current may be determined, using, for example, arctan functions with appropriate filtering, or with a phase-locked loop to track angles. ( 806 ) The controller  612  may use oscillations between the voltage and current vector angles, to identify corresponding torque oscillations. The viability of different approaches to obtain the magnitude and vector angles for the voltage and current may be dependent on other factors, such as voltage/current sensor quality, or processing power. 
     If, for example, a position sensor providing the shaft position of the generator  604  was provided to the controller  612 , such that the position of the rotor was known by the controller  612 , a Park&#39;s transform could be used in which the quadrature axis (q) provides the torque producing component and the direct axis (d) provides the reactive component. Oscillations in the reactive component could then be used by the controller  612  to identify corresponding torque oscillations. In  FIG. 8 , the use of sensed voltage and current is described, however, it should be understood that other sensed parameters, such as the shaft position may also be used by the controller  612  to detect and damp power angle oscillations and corresponding torque oscillations. 
     Using, for example, the vector angles, and magnitudes of the voltage and current, an actual parameter, such as the actual power angle, reactive power and/or power factor at the present shaft speed may be determined. ( 808 ) Filtering of the actual parameter, such as the reactive power, power angle, or power factor term may optionally be applied by the controller  612 . ( 810 ) In example configurations, the controller  612  may provide one or more filters. Alternatively, or in addition, one or more separate and independent filter devices may be controlled by the controller  612 . 
     Filtering may be applied to the actual parameter depending, for example, on the bandwidth of controller  612  in controlling the exciter  603  and/or the desired feedback characteristics. For example, frequency filtering may be applied so the controller  612  is focused on oscillations in sub harmonic frequencies where modes are likely to occur, such as in a predetermined range of frequencies. Thus, the frequency filtering may be a notch filter, low pass filter, high pass filter, or a filter may be omitted if the entire frequency spectrum is used, or the controller  612  is capable of focusing on the desired sub-harmonic frequencies. 
     If implemented, the goal of any such filtering is to ensure any un-desirable oscillation frequencies of the actual parameter are passed through the filter so they can be cancelled with regulation using the exciter  603 . Frequencies, or ranges of frequencies, which should not be considered by the controller  612  to determine the voltage output signal  624  can be blocked by the filtering, which may improve stability of the control loop. In some examples, the controller  612  may dynamically apply filtering in accordance with an operational parameter, such as the speed of the shaft, the magnitude of output current from the generator  604 , or the magnitude of the voltage at the generator output  614 . Alternatively, or in addition, the filter may include one or more fixed filters that are dynamically switched in and out by the controller  612  based on operational parameters. Alternatively, or in addition, the filter may be one or more filters that are fixed and consistently applied to the actual parameter. 
     The determined voltage and current vector angles and magnitudes may be used as variables by the controller  612  to determine a desired parameter, such as a reactive power, power angle or power factor value that results in an over damped condition in a given system at the present shaft speed. ( 812 ) For example, the variables may be used to track zero reactive power (unity power factor), or some non-zero amount of reactive power based on system objectives and conditions. A “desired parameter” may be system specific and may vary from system to system depending on the particular system configuration and characteristics. 
     In example operation, the controller  612  may determine a magnitude of KVA as a variable based on the absolute value of the measured voltage and current. The controller  612  may use the determined KVA and measured voltage in conjunction with Table 2 to dynamically determine the desired parameter as a desired power angle, as the speed of the shaft dynamically changes. The dynamically determined desired power angle may be used as a first setpoint for the system. In other examples, models, adaptive control, fuzzy logic, or any other decision based process or mechanism could be used to determine the desired parameter used as the first setpoint for the system. 
     Using the desired value as the first setpoint, and the dynamically changing calculated actual parameter (filtered or unfiltered), the controller  612  may determine an error signal using an actual parameter regulator included in the controller  612 . ( 814 ) The error signal may represent power angle oscillations due to differences between the desired parameter and the actual parameter varying at one or more sub-frequencies. The error signal may be used by the controller  612  to determine a field excitation level command. ( 816 ) The field excitation level command may be used as a second setpoint to regulate the reactive power, power angle or power factor to damp power angle oscillations and corresponding torsional oscillation. In an example, the field excitation level command may be expressed as an RMS exciter current. The actual parameter regulator may include a dynamic feedback controller such as proportional integral derivative (PID) controller, a lead-lag controller, an adaptive controller or model-based controller. Alternatively, or in addition, the actual parameter regulator may regulate the actual parameter by operation as a neural network, fuzzy logic, or any other form of controller scheme. The actual parameter regulator may be tuned so that any un-desirable oscillations in power angle, reactive power, or power factor are damped out with application of field excitation in such a way that oscillation energy is removed. 
     The determined field excitation level command is used by the controller  612  to dynamically adjust the voltage output signal  624  to counteract undesirable power angle oscillations and corresponding torque oscillations. ( 818 ) The dynamically adjusted voltage output signal  624  results in the exciter  603  applying a desired field excitation level at the generator. ( 820 ) As described in detail elsewhere, voltage output signal  624  is dynamically adjusted in accordance with the sub-frequency at which power angle oscillations have been identified by the controller  612  to correspondingly adjust the exciter voltage and corresponding output current of the generator  604 . As such, the dynamic adjustment of the output current of the generator  604  at the sub-frequency has the effect of damping oscillations of the power angle and corresponding torque oscillations at the sub-frequency. 
       FIG. 9  is a block diagram illustrating operation of the controller  612  in an example configuration for dynamically adjusting the voltage output signal  624 . In  FIG. 9 , the configuration of the controller  612  is illustrated as having an outer control loop  902  and an inner control loop  904 . In other examples, the controller  612  may be configured as a single multi-input control loop, or additional control loops. In addition, implementation of the controller  612  to dynamically adjust the voltage output signal  624  may be accomplished in any number of ways to meet the described functionality, configuring the controller  612  as a state-space based controller, a nonlinear controller, a model-predictive controller, fuzzy-logic or neural network controller, or any other form of controller that can accomplish the described functionality. 
     The configuration in  FIG. 9  illustrates an example of regulation of the power angle and generator field excitation in which the bandwidth of the inner control loop  904  may be made sufficiently faster than the outer control loop bandwidth  902  in order to maintain suitable phase margin for stability and robustness purposes. The outer control loop  902  includes a regulator  906 , such as a PID controller or lead-lag based controller. The regulator  906  may provide the error signal  908 , which may be the difference between the desired parameter  909 , such as a desired power angle, and the measured parameter  910 , such as a measured power angle. The error signal  908  output by the regulator  906  may be the field excitation level, which may be expressed as an RMS exciter current command. 
     The regulator  906  may operate in a determined bandwidth of frequencies. If the bandwidth of the regulator  906  (combined with the bandwidth of any filtering done on the measurements) is sufficient to act on the oscillatory frequencies identified as a mode, the regulator may be tuned to damp power angle oscillations in a stable manner. Alternatively, or in addition, system models may be used instead of or in addition to PID or lead-lag control to increase performance using model-based control methods. Other example control architectures include model-reference adaptive control, L1 adaptive control, H-∞ adaptive control, fuzzy logic, and neural network. 
     The inner loop control  904  may receive the error signal  908  at a multiplier  912 . The multiplier  912  may also receive an AC/DC component  914  of the excitation signal. The AC/DC component  914  of the excitation signal is developed based on an AC compensation frequency setpoint  916 , such as in rad/s, and a shaft speed  918  of the generator  604  to transition between an AC signal and a DC signal. 
     An AC component generator  920  may generate an AC component of the excitation signal. The AC component generator  920  receives the AC compensation frequency setpoint  916 , which may be multiplied at a multiplier  922  by a clock signal provided by a system clock  924  and converted from radians to a time varying unitary magnitude sinusoid by a converter  926  to generate a per unit AC sinusoidal component contribution to the exciter voltage. 
     A DC component generator  928  may generate a DC component of the excitation signal. The DC component receives the shaft speed  918  which may be a measured shaft speed provided by a shaft speed sensor, or an estimated shaft speed determined by the controller  612  based on other system parameter(s) such as the stator voltage frequency of the generator  604 . An actual shaft speed measurement may be provided by a sensor such as speed sensor on the generator  604 . An estimated shaft speed may be determined, for example, based on phase-locked loop tracking of the current or voltage angle waveforms at the output  614  of the generator  604 , a “sensor-less” speed estimation algorithm using the voltage and current measurements at the output  614  of the generator  604 , or a magnitude of generator terminal voltage, which may be approximately proportional to speed in some systems. In other example systems, the shaft speed may the exciter  603  shaft speed instead of the generator shaft speed. The shaft speed  918  may be used to determine a DC component contribution to the exciter voltage in connection with a DC component contributor  930 . 
     In this example, the DC component contribution to the exciter voltage may be determined from a table using the shaft speed, such as a table of DC component contribution v. shaft speed. As further discussed elsewhere, the DC component contribution may be dynamically changed in accordance with changes in the speed of the exciter to transition between a DC exciter voltage and an AC exciter voltage. The DC component may be represented with a gamma value  932  in a predetermined range, such as between zero and one, where one indicates a fully DC exciter voltage signal, and zero represents a fully AC exciter voltage signal. In other examples, other measurement/calculation technique may be used to determine the DC component contribution in the exciter voltage. 
     In addition to being supplied to a summer  934 , the gamma value  932  may also be provided to a u{circumflex over ( )}2  938  for use in generation of the AC component contribution. The output of the u{circumflex over ( )}2  938  may be subtracted from a first constant (1)  940  at a difference  942 , and an output of the difference  942  may be provided to a multiplier  944  where it is multiplied by a second constant (2)  946  and output to a square root  948 . The output of the square root  948  may provide the peak in per unit of desired AC exciter current to be multiplied by the time varying per unit sinusoid at a multiplier  950  to provide the resulting time varying sinusoidal per unit current command component  952 . The time varying sinusoidal per unit current command component  952  may be summed at the summer  934  with the gamma value  932  to form the AC+DC per unit current command component  914  of the excitation signal. The AC+DC per unit current command component  914  is calculated such that the effective root of the mean of the square is equivalent, independent of the DC component magnitude. The output of the multiplier  912  may be an exciter instantaneous current command  956 , provided in units such as amps, which may be provided to a summer  958  and an inverse exciter model  960  as a current command. In this example, a regulator  962 , such as a PID controller, is used in conjunction with the inverse exciter model  960  to generate the voltage output signal. An output signal  964  of the regulator  962  may be summed with an output signal  966 , such as a required voltage signal, of the inverse exciter model  960  at a summer  968  to provide a terminal voltage command  970 . 
     The output signal  966  of the inverse exciter model  960  may serve as a feed-forward term in order to increase response rate of the controller  612  in instances where the controller  612  may not have the bandwidth to otherwise operate at a sub frequency where a mode is identified. In this capacity, the inverse exciter model  960  may provide a voltage output that is summed with the output signal  964  of the regulator  962 . The voltage command may be limited based on the voltage available from a supply of DC voltage, such as a DC power supply  972 . In addition, anti-windup protection may be used in the regulator  962 . 
     The voltage command  970  and the supply of DC voltage may be provided to an amplifier  976 , such as an H-bridge. The amplifier  976  may be controlled by the controller  612  to amplify the voltage command  970  using the DC voltage to produce the desired voltage output signal  624 , which is provided to the exciter  603 . In an example, an H-bridge may be controlled by the controller  612  to perform PWM modulation to produce the desired output voltage  624  at the terminals of the brushless exciter  603 . A measurement of the exciter current from the exciter  603 , which may also be a terminal voltage input, may be used as a feedback term to calculate an exciter current error signal  978  provided to the regulator  962 . In another example, the current measurement may also be used to dynamically update parameters of the inverse exciter model  960  such that the inverse exciter model  960  would be capable of more quickly and accurately converging to voltage level required for a particular sub-frequency. 
     In some examples, based on voltages available and bandwidth requirements, it may also be advantageous to account for phase delay between the voltage output signal and the current response of the exciter  603 . Accounting for phase delay may be accomplished by estimation of what this phase lag may be using the inverse exciter model  960 . The estimate may be dynamically determined during changing operating conditions, or may be a predetermined time constant, such as an off-line calculated time constant, depending on system sensitivities. Since an AC portion of the exciter current command  956  is periodic at a known fixed frequency, a lagging time constant corresponding to an integer multiple of this period minus the modelled exciter lag time constant (using the smallest integer multiple required to still get a positive lagging time constant) could then be applied to the error signal input into the regulator  962 . Effectively, this may allow the regulator  962  to track a ‘future’ current exciter instantaneous current command by assuming there is little change in the exciter instantaneous current command value from cycle to cycle. In some example systems, the AC proportion of the excitation current frequency may typically be 200 Hz or greater, and the fundamental power angle oscillation frequencies (generator oscillation frequencies) to damp out may be in the 10&#39;s of Hz or lower, so there may be sufficient phase margin for the assumption of little change in the exciter instantaneous current. 
     The output signal  966  of the inverse exciter model  960  may be used to account for lag in a similar manner, so that the reference voltage outputs from both the regulator  962  and the inverse exciter model  960  are each tracking the periodic reference current at the same phasing in time. Effectively, this lag approach can greatly improve the gain accuracy of the current tracking loop at higher frequencies, however, such an approach may sacrifice phase accuracy. Since the phase accuracy of a relatively high frequency excitation current does not affect its overall RMS value or the generator primary field excitation level, but the gain accuracy does directly affect excitation level, this tradeoff may be desirable. 
       FIG. 10  are example diagrams illustrating a transition of the exciter voltage between AC excitation and DC excitation in accordance with shaft speed. In the example of  FIG. 10 , the exciter  603  and the generator  604  may rotate on a common shaft  166 , as illustrated in  FIG. 1 . In other examples, the exciter  603  may be rotated by a separate shaft driven by the same prime mover driving the generator  604 , or by a different source of mechanical rotational energy. Although described hereafter in a common shaft configuration, it should be understood that the exciter  603  may be separately driven. 
     Transition of the exciter voltage between AC excitation and DC excitation may occur anywhere along the range of shaft speed from zero speed to full rated speed. In an example system, at a shaft speed from 0% to about 25% of rated speed, the controller  612  may control the exciter voltage to include only an AC component. The transition of the exciter voltage between AC excitation and DC excitation may occur anywhere between about 25% and 35% of rated speed, and above about 35% of rated speed the controller  612  may control the exciter voltage to include only a DC component. In other examples, the speed range where the exciter voltage includes only the AC component, the speed range where the exciter voltage includes both the AC component and the DC component (during the transition), and the speed range where the exciter voltage includes only the DC component may be different. In some examples, the rotational speed of the exciter may match the rotational speed of the generator, regardless of whether the exciter and the generator are on a common shaft. 
     In  FIG. 10 , a first diagram  1002  illustrates the transition from AC to DC excitation, and a second diagram  1004  illustrates the DC contribution to total RMS excitation. In the first diagram  1002 , an exciter input current in amps (A)  1006  is illustrated as transitioning from an AC waveform to a DC waveform over a period of time (t)  1008 . In the second diagram  1002 , in alignment with the time (t)  1008 , the % contribution of the DC component (γ) to the RMS excitation is illustrated to correspond with the transition from of the exciter voltage from an AC waveform to a DC waveform. As illustrated in the example of  FIG. 10 , the excitation controller  612  may maintain a true RMS value of an exciter current during the transition period as the level of the AC component and the level of the DC component included in the waveform of the exciter voltage are varied based on the rotational speed of the exciter. 
     In order to provide excitation at low rotational speed of the generator  604 , an AC current waveform may be used as the exciter voltage at the exciter terminals to avoid decoupling of the exciter rotor and the exciter stator. At substantially zero or low speed of the generator shaft, there may not be enough change in flux per time across the air gap of the exciter  203  for the flux to couple between the exciter stator  234  and the exciter rotor  232 . ( FIG. 2 ) Once the shaft speed of the exciter  203  is above a determined system specific threshold (such as about 30% rated shaft speed), then a transition may occur to the use of DC current as an input to the exciter stator terminals without decoupling the exciter rotor and the exciter stator. Thus, the controller  612  may initiate a decrease in the AC component and a corresponding increase in the DC component during a transition period in response to the rotational speed of the exciter  203  increasing above a determined threshold. In other words, the controller  112  may control the exciter voltage to produce flux in the air gap with the AC component within a first range of rotational speed  1014  starting from zero speed and ramping the speed through a part of the transition. In addition, the controller  112  may control the exciter voltage to produce flux in the air gap within a second range of rotational speed  1016  with the DC component starting within the transition and ramping up to full rated speed. As illustrated in  FIG. 10 , the first range of rotational speed  1014  and the second range of rotational speed  1016  may overlap during the transition, and the second range of rotational speed  1016  may include rotational speeds that are greater than any rotational speeds included in the first range of rotational speed  1014 . In the example transition period shown in  FIG. 10 , the AC component is included in the exciter voltage from 0.0 to 0.095 seconds, and the DC component is included from 0.02 to 0.1 seconds. In other examples, the transition period may be faster, or slower than that illustrated in  FIG. 10 . 
     In examples of the present system only one excitation circuit may be used to supply both AC and DC components. As described herein, the single excitation circuit may include the excitation controller  112  and the source of the AC and DC components of the exciter voltage, which are provided based on the voltage output signal  624 . The single excitation circuit may smoothly transition the excitation waveform of the exciter  603  between AC and DC. The transition between AC and DC excitation waveform may be accomplished in a manner which preserves the true RMS value of the exciter current, but varies the contribution of AC and DC components towards that total true RMS value. Thus, during the transition the controller  612  may maintain a true RMS value of the exciter current substantially constant by offsetting variations in the AC component and the DC component. The excitation controller  612  may control the waveform of the exciter voltage to include at least one of the AC component and the DC component throughout the time the field current is varied and the generator is outputting variable electric power. 
     In  FIG. 10 , an example transition in the time domain is illustrated. In this example, a constant excitation level of three Amps RMS is maintained, while dynamically transitioning between a fully AC waveform (at 400 Hz) to fully a DC waveform of the exciter voltage, based on the voltage output signal  624 . This transition of the exciter voltage from AC to DC may occur while shaft speed is increasing through a determined speed range. In addition, the speed of the generator  604  may be increasing through a determined speed range that corresponds to the increasing speed of the exciter  603 . Thus, the controller  612  may control the level of the AC component of the exciter voltage so that the generator  604  outputs electric power at the zero speed condition or at a time rotation of the exciter  603  and the generator  604  begins. In addition, during the transition, the controller  612  may decrease the level of the AC component of the exciter voltage while increasing the DC component of the exciter voltage as a rotational speed of the exciter  603  and the generator  604  increases. Further, following the transition, the controller  612  may control the level of the DC component of the exciter voltage so that the generator  604  outputs electric power at a ramped speed up to full rated speed. 
     Dynamically transitioning while decreasing speed through the determined range of shaft speed may be reversed (from a fully DC waveform to a fully AC waveform). Thus, during the transition period, the controller  612  may decrease the contribution of the AC component and increase the contribution of the DC component to maintain a linear transition of the exciter voltage as the rotational speed of the exciter  603  increases. Alternatively, the controller  612  may decrease the contribution of the DC component and increase the contribution of the AC component to maintain a linear transition of the exciter voltage as the rotational speed of the exciter decreases. 
     While transitioning from the AC component to the DC component, or from the DC component to the AC component, the controller  612  may maintain a liner transition of the exciter voltage as the rotational speed of the exciter increases and decreases. During the transition period, when both the AC component and the DC component are present in the exciter voltage, the AC waveform may be modulated on the DC waveform as illustrated in  FIG. 10 . The exciter voltage may be controlled to include only the AC component at zero speed, or low rotational speed, of the exciter  603 , and may be transitioned to include only the DC component above a predetermined exciter speed threshold. During the time when the exciter voltage includes only the AC component, a combination of the AC component and the DC component, or only the DC component, the generator  604  may continuously generate electric power at the output  614  using the field current produced with the exciter voltage. 
     In one or more examples, the operations described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the operations may be stored on or transmitted over, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media, which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium. 
     By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transient media, but are instead directed to non-transient, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     Instructions may be executed by one or more processors, such as one or more DSPs, general purpose microprocessors, ASICs, FPGAs, or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware. 
     The subject-matter of the disclosure relates, among others, to the following aspects: 
     1. A system comprising:
         at least one synchronous generator configured to supply at an output a voltage and a current to a plurality of synchronous loads;   at least one exciter configured to provide a field current to the synchronous generator to control a magnitude and phase of the voltage and the current of the output of the synchronous generator; and   at least one controller configured to control a variable exciter voltage to control the field current output by the exciter, and the corresponding magnitude of the voltage and the current output by the synchronous generator;   the controller further configured to damp oscillations in a power angle between the voltage and the current by dynamic adjustment of the variable exciter voltage.       

     2. The system of claim  1 , wherein the controller is configured to identify oscillations in the power angle between the voltage and the current as a frequency of the voltage and current is ramped between substantially zero and a rated speed of the synchronous generator. 
     3. The system as in either claim  1  or  2 , wherein the controller is configured to identify oscillations in the power angle between the voltage and the current in sub harmonic frequencies that are less than a frequency of the voltage and current. 
     4. The system as in any of claims  1 - 3 , wherein the controller is configured to damp oscillations in the power angle between the voltage and the current by dynamic adjustment of the excitation to provide energy to counteract oscillation energy provided to the synchronous loads. 
     5. The system as in any of claims  1 - 4 , wherein the controller is configured to damp oscillations at a plurality of resonant modes occurring at different resonant sub harmonic frequencies during a ramped speed startup of the synchronous generator and the plurality of synchronous loads comprising motors synchronized with the synchronous generator. 
     6. The system as in any of claims  1 - 5 , wherein the controller comprises a first control loop and a second control loop, wherein an output of the first control loop representative of an exciter current command is provided as an input set point to the second control loop, an output of the second control loop being a voltage output signal to dynamically adjust the variable exciter voltage. 
     7. The system as in any of claims  1 - 6 , further comprising a sensor configured to measure at least one parameter of the synchronous generator and provide a sensor input signal to the controller representative of the parameter, the controller configured to identify oscillations in the power angle based on the sensor input signal being at least one of the voltage or the current. 
     8. The system as in any of claims  1 - 7 , wherein the controller is configured to dynamically adjust the variable exciter voltage to counteract torque oscillations between the synchronous generator and the synchronous loads. 
     9. The system as in any of claims  1 - 8 , wherein the controller is configured to determine an error between a desired power angle and the power angle, the error indicative of the oscillations in the power angle and used by the controller to damp the oscillations in the power angle. 
     10. A system comprising:
         at least one synchronous generator configured to supply polyphase electrical power to a plurality of synchronous motor loads;   at least one sensor configured to sense a voltage and a current of an output of the synchronous generator;   at least one controller configured to determine a desired power angle based on the voltage and the current received from the sensor to damp oscillations in a measured power angle between the voltage and the current; and   at least one exciter configured to excite the synchronous generator to control at least one of the voltage and the current of the output of the synchronous generator, the controller configured to control the exciter based on the desired power angle to dynamically adjust the excitation of the synchronous generator to damp the oscillations in the measured power angle between the voltage and the current.       

     11. The system of claim  10 , wherein the oscillations in the measured power angle are at a first frequency, and the voltage and current are at a second frequency, the second frequency being greater than the first frequency. 
     12. The system of claim  10  or  11 , wherein the controller is configured to control the exciter based on the desired power angle to dynamically adjust the excitation of the synchronous generator to adjust a stiffness in a magnetic coupling between a rotor of the synchronous generator and a rotor of each of the synchronous motor loads. 
     13. The system as in any of claims  10 - 12 , wherein the controller is configured to control the exciter based on the desired power angle to dynamically modulate the excitation of the synchronous generator at a harmonic frequency of the oscillations of the measured power angle. 
     14. The system as in any of claims  10 - 13 , wherein the controller is further configured to filter the voltage and the current to a predetermined range of frequencies less than a frequency of the voltage and current, and to identify the oscillations of the measured power angle as being within the predetermined range of frequencies. 
     15. The system as in any of claims  10 - 14 , wherein the controller is configured to determine a KVA output of the synchronous generator, and use the KVA output and the voltage to determine the desired power angle. 
     16. A method comprising:
         exciting at least one synchronous generator with an field current provided by at least one exciter;   controlling, with at least one controller, an exciter voltage to control the field current output by the exciter and a magnitude of at least one of a voltage and a current supplied at an output of the synchronous generator for a plurality of synchronous loads;   identifying, with the controller, power angle oscillations between the voltage and the current; and   controlling the exciter with the controller by dynamic modulation of the exciter voltage to damp the identified power angle oscillations.       

     17. The method of claim  16 , wherein identifying, with the controller, power angle oscillations between the voltage and the current comprises dynamically determining, with the controller, an error between a desired power angle and a measured power angle and controlling an exciter voltage of the exciter with the controller to counteract the identified power angle oscillations. 
     18. The method of claim  16  or  17 , further comprising synchronizing the synchronous generator and the synchronous loads at substantially zero speed of the synchronous generator and zero speed of the synchronous loads. 
     19. The method of claim  18 , further comprising damping power angle oscillations, with the controller, at a plurality of resonant modes occurring at different resonant sub harmonic frequencies during a ramped speed increase from the substantially zero speed, the synchronous generator and the synchronous loads synchronized with the synchronous generator being included in the ramped speed increase. 
     20. The method as in any of claims  16 - 19 , wherein controlling the exciter with the controller to dynamically modulate the exciter voltage comprises adjusting a stiffness of a magnetic coupling between the synchronous generator and the plurality of synchronous loads comprising motors to damp the identified power angle oscillations. 
     Various examples have been described. These and other examples are within the scope of the following claims.