Abstract:
A controlled frequency generating system (CFG) may be constructed with a main generator and an exciter driven by a common shaft. Excitation power may be provided from the common shaft; as distinct from prior-art systems which may require independent excitation power sources. While controlling the output voltage and frequency of the main generator, the bi-directional controller extracts power from a main generator output and may supply the extracted power to supplement excitation power when needed at certain rotational speeds. The controller may extract power from the exciter when, at other rotational speeds, the exciter produces excess power. The extracted excess power may be delivered to the output of the main generator to maintain a desired level of output power at a desired frequency, irrespective of speed of rotation of the CFG.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is in the field of control of electrical machines and, more particularly, control of electrical machines employed for generating controlled-frequency electrical power with a variable-speed engine. 
         [0002]    Current aircraft electrical generators are required to produce power at a constant frequency of 400 Hz, even though an aircraft engine to which the generator system is attached has variable speed. The traditional solution is to use a hydro-mechanical transmission to convert the variable engine speed to a constant speed at the generator input shaft. A typical hydro-mechanical transmission may be large, heavy and expensive. Aircraft owners would prefer to have a smaller, lighter, and less costly alternative. 
         [0003]    This has led to development of controlled frequency generators (CFG&#39;s) that can be set to have a constant output frequency irrespective of their shaft speed. Prior art CFG&#39;s may require use of a separate source of frequency-controlled excitation power. Because CFG&#39;s are required to operate over a wide speed range, the prior-art excitation power source must have capability for delivering a widely varying amount of excitation power to an exciter power controller (EXPC). Such prior-art separate excitation power sources may consume space and add weight to an aircraft. 
         [0004]    Additionally, in some modes of operation of the prior-art CFG, power needs may change direction so that power may flow out from an excitation winding of the CFG into the EXPC and from there to the excitation power source in the power system. When load-off events occur, transients may arise in prior-art CFG&#39;s. Accurate control may be difficult to accomplish when such transients develop. 
         [0005]    As can be seen, there is a need to eliminate a requirement for a separate source of excitation power in a CFG. Additionally, there is a need to provide ease of transient control for a CFG or eliminate a need for such control. 
       SUMMARY OF THE INVENTION 
       [0006]    In one aspect of the present invention a controlled frequency generating system (CFG) comprises a main generator and an exciter driven with a common shaft. A bidirectional controller extracts excitation power from an output of a main generator when the CFG operates at a rotational speed at which supplemental power input to the exciter stator is required. The bi-directional controller extracts power from the exciter when the main generator operates at a rotational speed at which the exciter produces power in excess of excitation requirements. 
         [0007]    In another aspect of the present invention a controller for maintaining a desired level of excitation in a controlled frequency generator system (CFG) comprises a first inverter interconnected with stator windings of a main generator of the CFG, and a second inverter interconnected with stator windings of an exciter of the CFG. The inverters are interconnected to bi-directionally transfer energy between the exciter stator windings and the main generator stator windings. 
         [0008]    In still another aspect of the present invention a method for generating electrical power at a controlled frequency comprises the steps of driving an exciter and a main generator with a common shaft, determining, on the basis of rotational speed of the shaft, a proper level of excitation required to maintain a desired frequency and voltage magnitude; and directing some main-generator power to provide supplementary excitation power when a rotational speed produces excitation power that is less than a desired level of excitation. 
         [0009]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram of a controlled frequency generating system (CFG) in accordance with the invention; 
           [0011]      FIG. 2  is a block diagram of a control system in accordance with the invention; 
           [0012]      FIG. 3  is a block diagram of an interconnection arrangement for the control system of  FIG. 2  in accordance with the invention; 
           [0013]      FIG. 4  is a block diagram showing an energy flow pattern for a first rotational speed of the CFG in accordance with the invention; 
           [0014]      FIG. 5  is a block diagram showing an energy flow pattern for a second rotational speed of the CFG in accordance with the invention; 
           [0015]      FIG. 6  is a block diagram showing an energy flow pattern for a third rotational speed of the CFG in accordance with the invention; and 
           [0016]      FIG. 7  is a flow chart of a method in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0018]    Broadly, the present invention may be useful in operating CFG&#39;s. More particularly, the present invention may provide control for a CFG operating at a variable speed. The present invention may be particularly useful in aircraft and aerospace vehicles which employ such CFG&#39;s as sources of electrical power. 
         [0019]    In contrast to prior-art CFG&#39;s, which employ separate sources of excitation power, the present invention may, among other things, provide excitation power from mechanical energy extracted from a main shaft of the CFG. The present invention may employ a bidirectional excitation controller to, depending on speed of the CFG, direct power from a main generator to an exciter or alternatively direct power from the exciter to an output of the main generator. The bidirectional controller may facilitate smooth transitions between various speed ranges of the CFG. Undesirable abrupt transient conditions may be reduced. Consequently, transient response of the inventive CFG may be readily controlled. 
         [0020]    Referring now to  FIG. 1 , a CFG is designated generally by the numeral  10 . The CFG  10  may comprise a main generator  12 , an exciter  14  and EXPC  16 . An output  12 - 1  of the main generator  12  may be connected to a main power bus  18  through a current path  20 . The EXPC  16  may be connected to the exciter  14  through a current path  22 , to the main generator output  12 - 1  through a current path  24  and to the main power bus  18  through a current path  26 . The main generator  12  and the exciter  14  may be driven with a common shaft  30 . 
         [0021]    In an exemplary embodiment, the CFG  10  may comprise an electrical power source for an aircraft (not shown). The CFG system  10  may use a rotor winding scheme as described in pending U.S. patent application Ser. No. 11/962,672, filed Dec. 21, 2007, which application is incorporated by reference herein. In such a rotor winding scheme there may be a plurality of windings with two phases, which are 90 degrees apart in space and 90 degrees shifted electrically. Rotor flux may be controlled to rotate at a speed that creates a desired frequency of output power (e.g. 400 Hertz [Hz]). 
         [0022]    Referring now to  FIG. 2 , the CFG  10  is shown in more detail. It may be seen that the EXPC  16  may comprise an inverter  40  connected to the exciter  14  and an inverter  42  connected to the main generator  12 . The EXPC  16  may also comprise a control block  44  for providing control signals to the inverter  40  and a control block  46  for providing control signals to the inverter  42 . 
         [0023]    The control block  44  may be provided with input data relating to various parameters. For example, the control block  44  may be provided with input signals  48  through  58 . Signal  48  (Vmain_ref) may comprise reference voltage of the main generator  12 . Signal  50  (Freq-main_ref) may comprise reference frequency of the main generator  12 . Signal  52  (Vdc-fdbk) may comprise feedback of DC voltage. Signal  54  (Idc_fdbk) may comprise feedback of DC current. Signal  56  (pos/speed) may comprise position and/or speed of rotation of the main generator  12 . Signal  58  (Vmain_fdbk) may comprise feedback of voltage of main generator  12 . 
         [0024]    The control block  46  may also be provided with input data relating to various parameters. For example, the control block  46  may be provided with signals  50 ,  56 ,  58  and  60  through  66 . Signal  60  (Vdc_ref) may comprise a dc reference voltage. Signal  62  (Vdc_fdbk) may comprise feedback of DC voltage. Signal  64  (lex-main_fdbk) may comprise feedback of current passing from the exciter  14  to the main generator  12 . Signal  58  (Vmain_fdbk) may comprise the feedback of voltage of main generator  12 . Signal  56  (pos/speed) may comprise position and/or speed of rotation of the main generator  12 . Signal  50  (Freq-main_ref) may comprise reference frequency of the main generator  12 . Signal  66  (Qref) may comprise reference reactive power. 
         [0025]    In operation, the control block  44  may utilize set points from the signals  48 , (Vmain_ref) and  50 , (Freq-main_ref) along with feedback signal  52 ,  54 ,  56  and  58  to provide a control signal  74  to the inverter  40 . In this regard the control block  44  may control output frequency and magnitude of voltage supplied by the inverter  40 . 
         [0026]    The control block  46  may utilize set points from the signal  60  (Vdc_ref),  50  (Freq-main_ref) and  66  (Qref) along with feedback signals  62 ,  64 ,  56  and  58  to provide a control signal  76  to the inverter  42 . The control block  46  may control output magnitude and phase of voltage supplied by the inverter  42 . This control may be achieved by sensing magnitude and phase of output voltage of the main generator  12 . The inverter  42  may be controlled to provide matching magnitude and phase. At the same time reactive power (Q) may be minimized. 
         [0027]    Referring now to  FIG. 3 , an interconnection diagram for the CFG  10  is shown. It may be seen that stator windings  80  of the exciter  14  may be interconnected with the inverter  40  via input/output filters  88 . Stator windings  82  of the main generator  12  may be interconnected with the inverter  42  via input/output filters  90 . Interconnection between inverters  40  and  42  may include a bulk DC link capacitor  86  and a discharge resistor and switch combined as a dynamic brake  84 . 
         [0028]    The inverters  40  and  42  may function as voltage source inverters (VSI&#39;s) with current control. The inverters  40  and  42  may each perform fast inner-loop current controlling which may be implemented in a typical direct-quadrature (D-Q) vector control frame. For the inverter  42 , gate driving may be performed to control D-Q currents so that output voltage of the inverter  42  may be identical in magnitude and phase with the output  12 - 1  of the main generator  12 . At the same time, reference D-axis current may be set to zero to force reactive power to zero. 
         [0029]    For the inverter  40 , D-axis current and Q-axis current may be controlled to maintain a desired magnitude and frequency at the output  12 - 1  of the main generator  12 . In this context, the exciter  12  may be provided with power input or power extraction (PWe_c) in accordance with the following expression: 
         [0000]        PW e   —   c={[Ns×Pm]/[N ×( Pm+Pe )]−1}× PW shaft   (eqn.1) 
         [0030]    where:
       Ns is the synchronous speed of the main generator;   N is the mechanical speed of a shaft of the main generator;   Pm is the pole number of the main generator   Pe is the pole number of the exciter; and   PWshaft is mechanical power available from a main shaft       
 
         [0036]    Referring now to  FIGS. 4 through 6 , the effects of implementing the control system described above may be understood.  FIGS. 4 through 6  may symbolically illustrate energy flow between and among various components of an illustrative embodiment of the inventive CFG  10 . In the particular embodiment illustrated in  FIGS. 4 through 6 , the CFG  10  may have a synchronous speed of 12,000 revolutions per minute (RPM), the exciter  14  may have two poles and the main generator  12  may have four poles. 
         [0037]      FIG. 4  may illustrate a particular operating speed of 8,000 RPM. At 8,000 RPM, the CFG  10  may be capable of providing power to the bus  18  directly from the main generator  12 . In other words, power to the bus (PWb) may be equal to power from the main generator (PWm). PWm may be comprised of two sources of energy. Some of the power (PWm) may be extracted directly from the shaft  30  (PWm_s) and some may be provided by the exciter, (PWe_m). The exciter  14  may extract 33.3% of the total bus power PWb from the shaft  30 . This exciter-extracted power may be referred to as PWe_s. It may be seen that at 8,000 RPM, PWe_s and PWe_m may be equal. As a consequence, the bidirectional EXPC  16  may be in a state of neither supplying power to the exciter  14  nor extracting power from the exciter  14 . 
         [0038]      FIG. 5  may illustrate energy flow of the illustrative CFG  10  at rotational speed 6,000 RPM. At 6,000 RPM, the main generator  12  may provide power PWm that may be 133.3% of the desired bus power PWb. This is because, as compared to 8,000 RPM operation, the exciter power PWe_m may deliver to the main generators up to 66.6% of the desired bus power PWb. In this 6,000 RPM case, the EXPC  16  may extract some of the main generator output power PWm. This extracted power may be referred to as PWm_c and may have a magnitude of about 33.3% of the desired bus power PWb. Thus, even though PWm may exceed a desired PWb, excess energy does not find its way to the bus  18 . 
         [0039]    The EXPC  16  may direct the extracted power PWm_c and deliver it to the exciter  14  as power PWc_e. It may be seen the sum of PWc_e and PWe_s may be equal to PWe_m, i.e. the power provided to the main generator  12  by the exciter  14 . It may also be seen that power extracted from the shaft  30  by the exciter  14  and the main generator  12  may remains the same as the that extracted in the 8,000 RPM case (i.e. PWe_s @6,000 RPM=PWe_s @8,000 RPM; and PWm_s @6,000 RPM=PWm_s @8,000 RPM). 
         [0040]      FIG. 6  may illustrate energy flow of the illustrative CFG  10  at rotational speed 11,900 RPM. At 11,900 RPM, the main generator  12  may provide power PWm that may be only 67.2% of the desired bus power PWb. This is because, as compared to 8,000 RPM operation, the exciter power PWe_m may deliver to the main generators about 0.5% of the desired bus power PWb. In this 11,900 RPM case, the EXPC  16  may extract some power from the exciter  14  (i.e., PWe_c). This extracted power may be provided directly to the bus  18  or the output  12 - 1  of the main generator  12  to supplement PWm. The extracted power which is delivered directly to the bus  18  may be referred to as PWc_m. The extracted power PWc_e and PWc_b may have a magnitude of about 32.8% of the desired bus power PWb. Thus, even though PWm may be less than a desired PWb, a full value of PWb (i.e. 100%), may reach the bus  18 . 
         [0041]    It may be seen the sum of PWm_c and PWm may be equal to 100% of PWb. It may also be seen that power extracted from the shaft  30  by the exciter  14  and the main generator  12  are the same as that extracted in the 8,000 RPM and 6,000 RPM cases. This is because 11,900 RPM is a speed that is close to the 12,000 RPM synchronous speed of the illustrative CFG  10 . But, at synchronous speed, PWe_m may become zero and thus may represent a limiting operating condition for the illustrative CFG  10 . 
         [0042]    It may also be seen that for all of the illustrated speeds of  FIGS. 4 through 6 , the bus power PWb may be derived exclusively from the shaft  30 . In other words, there may be no need to provide power to the exciter  14  or the EXPC  16  from any power source other than the shaft  30 . 
         [0043]    Referring back now to equation 1 and to  FIG. 3 , it may be seen that whenever PWe_c is positive, power may flow from the EXPC  16  into the exciter  14 . Conversely, when the value of PWe_c is negative, power may flow from the exciter  14  into the controller  16  and ultimately into the bus  18  to supplement power from the main generator  12 . In  FIG. 3 , it may be seen that a damping resistor  84  may reduce undesirable transient effects when current flow switches direction as rotational speed of the illustrative CFG  10  may change from being less than 8,000 RPM to being greater than 8,000 Rpm, or vice versa, or when load-off transients occur on the bus  18 . Additionally, it may be seen that filter sets  88  and  90  may be provided to reduce adverse effects of harmonics that they be generated by the inverters  40  and  42 . It may also be noted that the bulk DC link capacitor  86  may function as a starting capacitor and may provide excitation current for a brief period to facilitate initial power production by the CFG  10  at start-up. 
         [0044]    It may be seen that when the EXPC  16  is configured as described above, the CFG  10  may be considered to be self-excited. Thus the CFG  10  may have a capability to produce controlled output frequency and voltage, to synchronize the EXPC  16  to the output of the main generator  12  and to minimize (target is zero) reactive power. 
         [0045]    Referring now to  FIG. 7 , an exemplary method  700  for practicing the present invention is illustrated in a flow chart. In a step  702 , mechanical power may be transmitted to an exciter of the CFG (e.g. the shaft  30  may supply mechanical energy to the exciter  14 ). In a step  704 , mechanical power may be transmitted to a main generator of a controlled frequency generator (CFG) from a shaft (e.g., the shaft  30  may supply mechanical energy to the main generator  12 ). 
         [0046]    In step  706  a controller may determine whether the exciter requires additional power or is producing excess power (e.g., based on speed of rotation of the CFG and equation 1, the EXPC  16 , in the context of controlling frequency and output voltage of the CFG  10 , may provide power to the exciter  14  to supplement power from the shaft  30 . Or alternatively, the EXPC  16  may extract excess power from the exciter  14 ). In the event that a determination is positive, steps  708  and  710  may be initiated. In the event that the determination is negative, steps  712  and  714  may be initiated. In the event of a zero determination all of the steps  708  through  714  may remain uninitiated. 
         [0047]    In the case of a positive determination in step  706 , the step  708  may be performed to extract power from a main generator of the CFG. Simultaneously, in a step  710 , the extracted power may be supplied to the exciter by the controller. For example, the EXPC  16  may extract power from the main generator  12  and deliver the extracted power to the exciter  14  so that the exciter  14  may provide proper excitation of the main generator. In a step  716 , mechanical power from step  702  may be combined with electrical power from step  710  (e.g., PWe_s may be combined with PWc_e to produce PWe_m). In a step  718 , power from steps  704  and  716  may be combined to provide a desired power level at a bus to which the CFG may be connected (e.g., PWe_c may be added to PWm_s. But since PWm_c has been subtracted from PWm in step  708 , PWm may be at a desired power level at the bus  18 ). 
         [0048]    In the case of a negative determination in step  706 , the step  712  may be performed to extract power from the exciter and provide the extracted power to the controller. Simultaneously, in the step  714 , the extracted power of step  712  may be supplied directly to the bus. For example, the exciter  14  may extract an amount of power (PWe_s) from the shaft  30  which is in excess of an amount needed for proper excitation (PWe_m). This excess power (PWe_c) may be supplied directly to the bus  18 . In a step  720 , power from step  704  may be combined with power from step  714  and from a step  722  (wherein main generator electrical power may be extracted) to provide a desired level of power at the bus (e.g., PWm_c may be added to PWm because PWm by itself is not large enough to provided a desired power level at the bus  18 ). 
         [0049]    In the case of a zero determination in step  706 , there may be no requirement for energy transfer into or out from the controller. In other words, mechanical power to the exciter may be equal to required excitation power. Output of the main generator may then be equal to a desired power level at the bus. 
         [0050]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.