Abstract:
One embodiment of the present invention is a unique method of controlling the output of a synchronous electrical machine. Another embodiment is a unique method of controlling the output of a synchronous electrical machine for powering a load. Still another embodiment is a unique aircraft power generation system. Other embodiments include apparatuses, systems, devices, hardware, methods, and combinations for fluid driven actuation systems. Further embodiments, forms, features, aspects, benefits, and advantages of the present application will become apparent from the description and figures provided herewith.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to power generation, and more particularly, to synchronous power generation. 
       BACKGROUND 
       [0002]    Synchronous power generation systems that effectively operate under varying speed and load conditions, remain an area of interest. Some existing systems have various shortcomings, drawbacks, and disadvantages relative to certain applications. Accordingly, there remains a need for further contributions in this area of technology. 
       SUMMARY 
       [0003]    One embodiment of the present invention is a unique method of controlling the output of a synchronous electrical machine. Another embodiment is a unique method of controlling the output of a synchronous electrical machine for powering a load. Still another embodiment is a unique aircraft power generation system. Other embodiments include apparatuses, systems, devices, hardware, methods, and combinations for synchronous power generation systems and the control thereof. Further embodiments, forms, features, aspects, benefits, and advantages of the present application will become apparent from the description and figures provided herewith. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: 
           [0005]      FIG. 1  illustrates some aspects of a non-limiting example of an aircraft in accordance with an embodiment of the present invention. 
           [0006]      FIG. 2  schematically illustrates some aspects of a non-limiting example of a power generation system in accordance with an embodiment of the present invention. 
           [0007]      FIG. 3  schematically illustrates some aspects of non-limiting example of a power generation system with a power conversion system in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    For purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nonetheless be understood that no limitation of the scope of the invention is intended by the illustration and description of certain embodiments of the invention. In addition, any alterations and/or modifications of the illustrated and/or described embodiment(s) are contemplated as being within the scope of the present invention. Further, any other applications of the principles of the invention, as illustrated and/or described herein, as would normally occur to one skilled in the art to which the invention pertains, are contemplated as being within the scope of the present invention. 
         [0009]    Referring to  FIG. 1 , there are illustrated some aspects of a non-limiting example of a system  10  in accordance with an embodiment of the present invention. In one form, system  10  is an aircraft, referred to herein as aircraft  10 . In other embodiments, system  10  may be any type of engine powered system or vehicle, including one or more types of air-vehicles; land vehicles, including and without limitation, tracked and/or wheeled vehicles; marine vehicles, including and without limitation, surface vessels, submarines and/or semi-submersibles; amphibious vehicles, or any combination of one or more types of air, marine and land vehicles. In various forms, system  10  may be manned and/or autonomous. 
         [0010]    In one form, aircraft  10  includes a fuselage  12 , wings  14 , an empennage  16  and one or more propulsion systems  18 . In one form, aircraft  10  is a twin engine turbofan aircraft. In other embodiments, aircraft  10  may be any fixed-wing aircraft, including turbofan aircraft and turboprop aircraft, or may be any rotary wing or hybrid rotary wing and fixed wing aircraft. In various embodiments, aircraft  10  may have a single engine or a plurality of engines. In various embodiments, aircraft  10  may employ any number of wings  14 . Empennage  16  may employ a single or multiple flight control surfaces. 
         [0011]    Referring now to  FIG. 2 , some aspects of a non-limiting example of a power generation system  20  in accordance with an embodiment of the present invention is schematically depicted. In one form, aircraft  10  employs power generation system  20  for supplying electrical power to a load  22 . In one form, load  22  is a dc load. In other embodiments, load  22  may be any electrical load. Power generation system  20  includes an engine  24 , a synchronous generator  26  and a power conversion system  28 . In one form, engine  24  is a propulsion engine for aircraft  10 , and is considered a part of propulsion system  18 . In other embodiments, engine  24  may be any engine associated with aircraft  10 , e.g., a propulsion engine or an auxiliary power unit (APU). In still other embodiments, engine  24  may be any engine associated with any system. In one form, engine  24  is a gas turbine engine. In a particular form, engine  24  is a turbofan engine. In other embodiments, engine  24  may be any type of gas turbine engine, or may be a hybrid engine, a piston engine or any other type of engine. 
         [0012]    Synchronous generator  26  is coupled to and powered by engine  24 . In one form, synchronous generator  26  is coupled to engine  24  via a direct drive, e.g., whereby engine  24  and synchronous generator  26  have the same rotational speed. In other embodiments, synchronous generator  26  may be coupled to engine  24  via one or more transmissions, or may be mounted on a shaft or spool of engine  24 . Synchronous generator  26  includes a field winding  30  and a armature (stator) winding  32 . In one form, synchronous generator  26  is a three phase synchronous generator having output phase legs  32 A,  32 B and  32 C. In other embodiments, synchronous generator  26  may output any number of phases, and may be, for example, a 5-phase machine. Power conversion system  28  is coupled to synchronous generator  26  and to load  22 . Power conversion system  28  is configured to control the output of synchronous generator  26  and supply electrical power from synchronous generator  26  to dc bus  34  for powering load  22 . In some embodiments, power generation system  20  may include or be coupled to an energy storage system  36  in parallel with load  22 , e.g., a battery and/or one or more other types of energy storage systems coupled to dc bus  36 . In other embodiments, power generation system  20  may not include an energy storage system such as energy storage system  36 . 
         [0013]    Referring to  FIG. 3  some aspects of a non-limiting example of power generation system  20  in accordance with an embodiment of the present invention are schematically depicted. Power conversion system  28  includes a plurality of electronic elements or devices, each of which may be formed as a discrete element or formed from as one or more sub-elements. In various embodiments, the elements of power conversion system  28  may be formed on a single chip, e.g., an application specific integrated circuit (ASIC) or as a plurality of chips and/or other discrete components. In one form, power conversion system  28  includes electronic elements (devices) in the form of a power converter  40 ; a gate drive  42 ; a pulse width modulation (PWM) signal driver  44 ; controller blocks  46 ,  48  and  50 ; error blocks  52 ,  54  and  56 , a direct quadrature (d-q) to a-b-c reference frame transform block  58 ; a summer block  60 ; transfer functions  62  and  64 ; a flux value output unit  66 ; a speed sensor  68 ; an integrator block  70 ; an error block  72 ; a controller block  74 ; a field converter  76  and phase leg current sensors  78  and  80 . 
         [0014]    Power converter  40  is communicatively coupled to phase legs  32 A,  32 B and  32 C of synchronous generator  26 . Power converter  40  is communicatively coupled to energy storage system  36  and load  22  via dc bus  34 . Gate drive  42  is communicatively coupled to with power converter  40  and with PWM signal driver  44 . Controller blocks  46 ,  48  and  50  are communicatively coupled to error blocks  52 ,  54  and  56 , respectively. In one form, controller blocks  46 ,  48  and  50  are proportional-integral (PI) controllers. In other embodiments, other controller types may be employed in addition to or in place of PI controller blocks. 
         [0015]    PWM signal driver  44  is configured to generate PWM logic level control signals based on the output of transform block  58  and the phase current output of synchronous generator  26 . Gate drive  42  is communicatively coupled to PWM signal driver  44  and with power converter  40 . Gate drive  42  is configured to receive the PWM logic signals from PWM signal driver  44  and to output driver signals to power converter  40  for controlling the operation of power converter  40 . Power converter  40  includes a plurality of power semiconductor switching devices (not shown), for example and without limitation, insulated gate bipolar transistors (IGBTs), that perform power switching to convert the output of synchronous generator  26  to direct current for use on dc bus  34 . In other embodiments, other switching devices may be employed, e.g., metal-oxide semiconductor field effect transistors (MOSFETs). 
         [0016]    Transform block  58  includes a torque component current (I qs *) value input; an angular position e value input; a flux component current (I ds *) value input; and reference phase current i a *, i b * and i c * value outputs. Transform block  58  is configured to transform direct-quadrature reference frame data into 3-phase a-b-c reference frame data. In other embodiments, other transforms may be employed. Error blocks  52 ,  54  and  56  are communicatively coupled to i a *, i b * and i c * outputs, respectively, of transform block  58 . Error blocks  52  and  54  are also communicatively coupled to phase current sensors  80  and  78 , respectively. Phase current sensors  80  and  78  are configured to sense the current in phase legs  32 A and  32 B, respectively, convert the current values into voltage values, and to output the voltage values as values indicative of phase leg  32 A current and phase leg  32 B current, respectively. The inputs of summer block  60  are communicatively coupled to phase current sensors  80  and  78 , and the output of summer block  60  is communicatively coupled to error block  56 . Summer block  60  is configured to determine, e.g., calculate, a current value indicative of phase leg  32 C current, based on the output of phase current sensors  80  and  78 , e.g., based on the equation, i a +i b +i c =0. Summer block  60  is configured to pass the current value indicative of phase leg  32 C current to error block  56 . 
         [0017]    Speed sensor  68  is configured to sense the rotational velocity (frequency) ω of synchronous generator  26 . In one form, speed sensor  68  is mounted on synchronous generator  26 . In other embodiments, speed sensor  68  may be mounted in other locations or its function may be derived using the sensed currents from phase current sensors  78  and  80 . In still other embodiments, speed sensor  68  may be configured to sense the rotational speed of the output shaft of engine  24  that is coupled to synchronous generator  26 . Integrator block  70  is communicatively coupled to speed sensor  68 . Integrator block  70  is configured to integrate the output of speed sensor  68  (operating frequency ω of synchronous generator  26 ) to generate angular positional data θ indicative of the angular (rotational) position of synchronous generator  26 . Integrator block  70  is communicatively coupled to the θ input of transform block  58 , and is configured to pass the positional data θ to transform block  58 . 
         [0018]    Error block  72  is configured to determine the difference between the desired dc bus  34  voltage (V REF ) and the actual dc bus  34  voltage (V B ) (e.g., V B  as measured by a voltage sensor, not shown). Controller block  74  is communicatively coupled to error block  72 . In one form, controller block  74  is a proportional-integral (PI) controller. In other embodiments, other control types may be employed in addition to or in place of a PI controller. Controller block  74  is configured to integrate and amplify the difference error obtained via the comparison of desired voltage V REF  and the actual voltage V B  to obtain a reference current I REF  in the form of a numerical value. 
         [0019]    Controller block  74  is communicatively coupled to flux value output unit  66 . Controller block  74  is configured to pass I REF  to both flux value output unit  66  and transfer function  62 . Flux value output unit  66  is also communicatively coupled to speed sensor  68  and to transfer function  62 . Flux value output unit  66  is configured to receive the operating frequency ω (rotational speed) of synchronous generator  26  from speed sensor  68 , and to receive and I REF  from controller block  74 . Flux value output unit  66  is configured to output data, e.g., a flux value, based on the current I REF  and the current operating frequency ω. In one form, the output of flux value output unit  66  is a field component current I fld  numerical value. In one form, the output of flux value output unit  66  varies with I REF  and operating frequency ω. Since I REF  is based on the difference error obtained via the comparison of desired voltage V REF  and the actual voltage V B , the output of flux value output unit  66  varies with the difference error. In one form, flux value output unit  66  includes a lookup table (LUT)  66 A, e.g., stored in a memory, in software, firmware and/or hardware (not shown). In one form, LUT  66 A stores data, e.g., data in the form of a plurality of flux levels for controlling the operation of synchronous generator  26  under a plurality of different synchronous generator  26  speed and load conditions anticipated in field-service and/or testing of synchronous generator  26  and/or other components of power generation system  20 . In other embodiments, other forms of data may be employed, e.g., continuous and/or discontinuous equations that provide a flux level output that varies based on speed and load conditions. Flux value output unit  66  is configured to pass the field component current I fld  value to transfer function  62  and to transfer function  64 . 
         [0020]    Transfer function  62  is configured to combine I fld  and I REF , and output a torque component current I qs * value based thereon. Transfer block  62  modifies I REF  based on the output of output of flux value unit  66  to obtain I qs . Transfer function  62  is communicatively coupled to transform block  58 , and supplies torque component current I qs * value as input to transform block  58 . It is noted that during the operation of power generation system  20 , the flux component current (I ds *) value input to transform block  58  is set to zero. Hence, the operation of transform block  58  is based on positional data θ and torque component current I qs *. 
         [0021]    Transfer function  64  is communicatively coupled to flux value output unit  66 . Flux value output unit  66  passes field component current I fld  value as input to transfer function  64 , which converts field component current I fld  value into a field reference voltage V fld(ref)  value. Transfer function  64  is communicatively coupled to field converter  76 , and passes the field reference voltage V fld(ref)  value to field converter  76 . Field converter  76  is configured to amplify and convert the field reference voltage V fld(ref)  value into a field winding voltage, which is supplied from field converter  76  to field windings  30 . Field converter  76  controls the excitation to the generator field winding, and hence controls the generator output voltage. 
         [0022]    The operation of power generation system  20  includes steady state operation and dynamic operation. Dynamic operation includes variations in speed, e.g., of synchronous generator  26 , and in load, e.g., dc bus  34  load. During the operation of power generation system  20 , it is desirable that synchronous generator  26  be operated at a desired efficiency, e.g., an optimal efficiency. In order to do so, embodiments of the present invention employ the torque component current for controlling the stator winding (armature) currents, and the flux component current to control the field current based on the flux data stored in flux value output unit  66 . This may enable the optimization of the field control loop to operate the synchronous generator  26  at high, e.g., maximum efficiency. In one form, the flux component current I ds  input of (d-q) to a-b-c reference frame transform block  58  is set to zero, and hence, armature (stator) winding  32  currents are determined primarily by the torque component (torque component current I qs * value). In addition, in one form, the excitation of synchronous generator  26  is controlled by the field current reference (field component current I fld  value) output by flux value output unit  66 . In one form, the optimum reference value for field voltage for a given speed (operating frequency ω) and load current (e.g., dc bus  34  load current) is obtained from flux value output unit  66 . In another aspect, embodiments of the present invention employ instantaneous current controllers for the armature based on the field oriented components as inputs, which may enable a faster response to electrical load changes, e.g., on dc bus  34 , e.g., relative to conventional systems. In yet another aspect, embodiments of the present invention eliminate the need for closing the loop for the field control based on stator ac voltage. 
         [0023]    Thus, in one form, a control strategy for regulating the dc voltage of the power converter fed from a three phase synchronous generator in accordance with an embodiment of the present invention includes the following: The desired dc voltage V REF  is compared with the actual do voltage V B  and the error is integrated and amplified to obtain the current reference I REF . In one form, based on the speed (operating frequency ω) and load current, flux levels, e.g., optimum flux levels that correspond to high efficiency operation of synchronous generator  26 , are pre-calculated and stored in LUT  66 A. These stored values are used for deriving the torque and flux reference currents for the control of synchronous generator  26 . 
         [0024]    In one form, the torque component current is used for controlling the stator winding (armature) currents, and the flux component current is used to control the excitation current based on the flux profile stored in the LUT  66 A. In one form, the flux component current I ds * for the d-q/abc transformation is set to zero, so that the armature currents are determined mainly by the I qs * component. In one form, the speed (operating frequency ω) of the synchronous generator  26  is integrated in integrator block  70  to obtain the angle θ required for the transformation. In one form, the excitation of the synchronous generator  26  is controlled by the field current reference value obtained from the flux profile look-up table LUT  66 A. In one form, the optimum reference value for field voltage for a given speed (operating frequency ω) and load current is obtained from the LUT  66 A. In some embodiments, the need for closing the loop for the field control based on stator ac voltage is eliminated. Since the final required voltage is dc (e.g., 270V dc or any suitable value), there is no need to regulate the stator output voltage. In one form, power converter  40  is used for regulating the dc voltage. In one form, the abc current references are compared with the measured currents of the armature and processed through the PI controllers  46 ,  48  and  50 . The signals are used to derive the pulse width modulation signals to control the switching times of the power devices in power converter  40 . In one form, the current loops also control the instantaneous currents of synchronous generator  26 . 
         [0025]    During the operation of power generation system  20 , error block  72  compares the desired dc bus  34  voltage (V REF ) and the actual dc bus  34  voltage (V B ) to generate a difference error. In one form, the actual dc bus  34  voltage is an instantaneous value obtained during the operation of synchronous generator  26 . In various embodiments, the determination may be made continuously, at one or more frequency intervals, or according to some other sampling scheme. The difference error resulting from the comparison is passed to controller block  74 , which integrates and amplifies the signal, forming the reference current I REF  value. The rotational speed (operating frequency ω) of synchronous generator  26  is obtained via speed sensor  68 . In various embodiments, the rotational speed may be obtained continuously, or, for example, sampled, e.g., at one or more frequency intervals, or according to some other sampling scheme. 
         [0026]    A flux component current and a torque component current are then determined based on the comparison, the operating frequency ω, and a predetermined value. In one form, the predetermined value is a reference flux value determined based on the comparison and the operating frequency ω of synchronous generator  26 . In a particular form, the flux value is obtained from flux value output unit  66  via LUT  66 A during the operation of synchronous generator  26 , e.g., for each change in speed and load conditions, based on reference current I REF  value and operating frequency ω. Each predetermined flux level value corresponds to a potential result of the comparison and a potential operating frequency ω of synchronous generator  26 . In one form, the operating frequency ω is an instantaneous value obtained during operation of synchronous generator  26 . In other embodiments, other values may be employed, e.g., time-averaged frequency values. 
         [0027]    In one form, the excitation current of field winding  30  of the synchronous generator  26  is controlled using the flux component current to achieve the desired DC voltage. In one form, the flux component current is a field winding flux component current (e.g., field component current I fld  value). Field component current I fld  value is passed to transfer function  64 , which converts field component current I fld  value into a field reference voltage V fld(ref)  value. The field reference voltage V fld(ref)  value is passed to field converter  76 , which converts the field reference voltage V fld(ref)  value into a field winding voltage, which is then supplied from field converter  76  to field windings  30  to control field windings  30 . 
         [0028]    In one form, armature (stator) winding  32  current of synchronous generator  26  is controlled based on the stator winding torque component current. In one form, the torque component is obtained by passing the output of flux value output unit  66 , field component current I fld  value, along with I REF , to transfer function  62 , which outputs torque component current I qs * value based on field component current I fld  value and I REF . Torque component current I ds * value is provided from transfer function  62  as an input to transform block  58 . In addition, the angular position θ value is determined by integrating operating frequency ω at integrator block  70 , which supplies the angular position θ value as an input to transform block  58 . As mentioned above, in one form, the flux component current I ds * input of (d-q) to a-b-c reference frame transform block  58  is set to zero. 
         [0029]    Transform block  58  determines stator winding reference current i a *, i b * and i c * values based on the torque component current I ds * value, the angular position θ value, and the flux component current I ds  input being set to zero. Phase current sensors  80  and  78  sense the current in phase legs  32 A and  32 B of synchronous generator  26 , respectively, and generate current values indicative of phase leg  32 A current and phase leg  32 B current, respectively, which are supplied to summer block  60 . Summer block  60  calculates a current value indicative of phase leg  32 C current. The current values representing the current in phase legs  32 A,  32 B and  32 C are supplied to error blocks  52 ,  54  and  56 , respectively, which determine the differences (errors) between the stator winding reference current i a *, i b * and i c * and corresponding measured phase leg  32 A,  32 B and  32 C current values. The errors are supplied to controller blocks  46 ,  48  and  50 , which process and amplify the errors for input to PWM signal driver  44 . PWM signal driver  44  generates PWM signals from the errors, and supplies the PWM signals to gate driver  42 . Gate driver  42  controls the switching devices of power converter  40  to yield the desired voltage and current output on dc bus  34 . 
         [0030]    Embodiments of the present invention include a method of controlling an output of a synchronous electrical machine, comprising: comparing a desired DC voltage to an actual DC voltage; determining an operating frequency of the synchronous electrical machine; determining a flux component current and a torque component current based on: the comparison; the operating frequency; and a predetermined value; controlling a stator winding current in a stator winding of the electrical machine based on the torque component current; and controlling an excitation current of a field winding of the electrical machine using the flux component current to achieve the desired DC voltage. 
         [0031]    In a refinement, the flux component current is a field winding flux component current; and wherein torque component current is a stator winding torque component current. 
         [0032]    In another refinement, the predetermined value is a flux value determined based on the comparison and the operating frequency. 
         [0033]    In yet another refinement, the flux value is obtained from a lookup table during operation of the electrical machine. 
         [0034]    In still another refinement, the predetermined value is obtained from a lookup table during operation of the electrical machine. 
         [0035]    In yet still another refinement, the lookup table includes a plurality of predetermined values; and wherein each predetermined value corresponds to a potential result of the comparison and a potential operating frequency. 
         [0036]    In a further refinement, the operating frequency is an instantaneous operating frequency value obtained during operation of the electrical machine; and wherein the actual DC voltage is an instantaneous actual voltage value obtained during operation of the electrical machine. 
         [0037]    In a yet further refinement, the method further comprises determining an angular position of the synchronous electrical machine; and determining a reference stator winding current value based on the torque component current and the angular position. 
         [0038]    In a still further refinement, the method further comprises comparing the reference stator winding current value with a measured stator winding current value from a stator winding of the synchronous electrical machine to determine a stator winding current error. 
         [0039]    In a yet still further refinement, the method further comprises generating a pulse width modulation signal based on the stator winding current error. 
         [0040]    Embodiments of the present invention include a method of controlling an output of a synchronous electrical machine for powering a load, comprising: providing an electronic element configured to output a flux value based on a first input value and a second input value, wherein the first input value is based on a difference value between a desired output value and a corresponding actual output value; wherein the second input value is an operating frequency of the synchronous electrical machine; and wherein the flux value varies with changes in the difference value and the operating frequency; operating the synchronous electrical machine; determining the first input value while operating the synchronous electrical machine; detecting the second input value while operating the synchronous electrical machine; inputting the first input value and the second input value into the electronic element; outputting the flux value based on the first input value and the second input value; determining a flux component current and a torque component current based on: flux value; controlling a stator winding current in a stator winding of the electrical machine based on the torque component current; and controlling an excitation current of a field winding of the electrical machine using the flux component current to achieve the desired output value. 
         [0041]    In a refinement, the method further comprises storing a relationship between a plurality of flux values and a plurality of combinations of first input values and second input values in the electronic element, wherein the flux value is determined based on the relationship, the first input value and the second input value. 
         [0042]    In another refinement, the electronic element includes a lookup table stored therein and configured to output the flux value based on the first input value and the second input value. 
         [0043]    In yet another refinement, the method further comprises determining an angular position of the synchronous electrical machine based on the operating frequency of the synchronous electrical machine. 
         [0044]    In still another refinement, the method further comprises determining a reference stator winding current value based on the torque component current and the angular position. 
         [0045]    In yet still another refinement, the method further comprises comparing the reference stator winding current value with a measured stator winding current value from a stator winding of the synchronous electrical machine to determine a stator winding current error value; and generating a pulse width modulation signal based on the stator winding current error value. 
         [0046]    In a further refinement, the operating frequency is a rotational speed of the synchronous electrical machine. 
         [0047]    In a yet further refinement, the first input value is a load reference current value. 
         [0048]    In a still further refinement, the output of the synchronous electrical machine is performed without using field winding current control for the synchronous electrical machine based on stator winding ac voltage of the synchronous electrical machine. 
         [0049]    Embodiments of the present invention include an aircraft power generation system, comprising: an engine; a synchronous generator coupled to and powered by the engine, wherein the synchronous generator has a field winding and a stator winding; a first electronic element configured to output a flux value based on a first input value and a second input value, wherein the first input value is based on a difference value between a desired output value of the power generation system and a corresponding actual output value; wherein the second input value is an operating frequency of the synchronous generator; and wherein the flux value output by the first electronic element varies with changes in the difference value and the operating frequency during the operation of the synchronous generator; a second electronic element configured to output the first input value; a third electronic element coupled to the first electronic element and the second electronic element, and configured as a transfer function operable to convert the flux value and the first input value into a torque component current value configured for controlling a stator winding current in the stator winding; and a fourth electronic element coupled to the first electronic element and configured to convert the flux value into a field winding reference value configured for controlling an excitation current in the field winding. 
         [0050]    In a refinement, the aircraft power generation system further comprises: an integrator coupled to the synchronous generator and configured to integrate the operating frequency to provide an angular position of the synchronous generator; and means for controlling a stator winding current in the stator winding based on the angular position and the torque component current value. 
         [0051]    In another refinement, the means for controlling is configured to generate a reference stator winding current value, and is configured to compare the reference stator winding current value with a measured stator winding current value to yield a stator winding current error value; and wherein the means for controlling is configured to control the stator winding current based on the stator winding current error value. 
         [0052]    In yet another refinement, the means for controlling is configured to generate a pulse width modulation signal based on the stator winding current error value. 
         [0053]    In still another refinement, the aircraft power generation system further comprises a DC bus powered by the synchronous generator, wherein the desired output value is a reference voltage, and wherein the actual output value is a DC bus voltage. 
         [0054]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment(s), but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as permitted under the law. Furthermore it should be understood that while the use of the word preferable, preferably, or preferred in the description above indicates that feature so described may be more desirable, it nonetheless may not be necessary and any embodiment lacking the same may be contemplated as within the scope of the invention, that scope being defined by the claims that follow. In reading the claims it is intended that when words such as “a,” “an,” “at least one” and “at least a portion” are used, there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. Further, when the language “at least a portion” and/or “a portion” is used the item may include a portion and/or the entire item unless specifically stated to the contrary.