Abstract:
An apparatus for regulating a transient response of an output signal of an electrical generator. The apparatus comprises a tapped output winding means for providing a first AC signal and a second AC signal. The first and second AC signals have respective RMS values. The RMS value of the first AC signal is greater than the RMS value of the second AC signal. An AC switching means for selecting between the first AC signal or the second AC signal, and thereby providing a switched AC signal which has a duty cycle. A rectifier means for rectifying the switched AC signal and providing a rectified DC signal. The rectified DC signal has a DC signal component, a square wave signal component and a ripple signal component. The square wave signal component has a duty cycle. The duty cycle of the square wave signal component is equal to the duty cycle of the switched AC signal. A filter means for filtering the rectified DC signal and for providing the output signal. The filter means is operable to filter the rectified DC signal by averaging the square wave component and filtering the ripple component. The AC switching means is responsive to the output signal to adjust the duty cycle of the switched AC signal to regulate the output signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to regulating transient responses of an output signal of electric generators.  
         [0003]     2. Description of Related Art  
         [0004]     When a load is suddenly decreased or removed from an output of an electric generator, the output voltage typically increases during a transient response. This increase in output voltage during the transient response is due primarily to two factors. The first factor is a delayed response of an electric generator controller that regulates an exciter current, which, by flowing through an exciter winding, provides an exciter magnetic field. An amount of energy coupled by transformer action is influenced by the exciter current alone during the transient response, given that a mechanical rotating force of the generator is constant during this period. A decrease in the exciter current decreases the amount of energy coupled by transformer action. Since the exciter current does not adjust instantly after the load is decreased, the amount of energy coupled by transformer action does not adjust instantly, and energy output of the generator remains constant initially. However, the energy going to the load is decreased, and consequently the energy output tends to increase the output voltage.  
         [0005]     The second factor is due to a quantity of stored magnetic energy that is in the exciter winding of the electric generator immediately prior to the decrease or the removal of the load. In the ideal case, if the electric generator controller responded immediately to the removal of the load by reducing an exciter current which reduces the energy coupled to the output by transformer action, there is still the quantity of stored magnetic energy in the exciter winding. The quantity of stored magnetic energy is transferred by transformer action to an armature winding, where it again increases the output voltage of the generator.  
         [0006]     Conventional electric generators, for example a brushed generator, have the exciter winding on a rotor. During the transient response, the conventional electric generators dissipate the quantity of stored magnetic energy in the exciter winding in a resistive impedance. The quantity of stored magnetic energy of the exciter winding is diverted to the resistive impedance when an output voltage is detected as having an over-voltage situation.  
         [0007]     However, there are situations when it is difficult to employ the resistive impedance to discharge the energy in the exciter winding. For example, when the electric generator is a brushless generator. In this situation, an exciter circuit consists of an exciter field winding on a stator, an exciter armature winding on the rotor and a generator field winding on the rotor. The generator field winding, in this case, has a quantity of stored magnetic energy that must be dissipated when the sudden change or removal of the load occurs. Since the generator field winding is on the rotor, which does not have any electrical connections to the stator, it is difficult to employ electrical circuitry for the purposes of dissipating the quantity of stored magnetic energy in the generator field winding.  
         [0008]     Conventional brushless generators as disclosed in U.S. Pat. No. 6,628,104 by Yao, for example, provide an impedance circuit that selectively and temporarily absorbs excitation field current in the free-wheeling path of the excitation field winding to reduce voltage overshoot of the generator upon occurrence of an operating transition, such as a transition from high load to low load. In one implementation, the impedance circuit is an RC circuit, a by-pass switch is provided across the RC circuit. When excitation current in the free-wheeling path is not to be absorbed by the RC circuit, the by-pass switch is ON, thereby providing a low-impedance path for the excitation current. A by-pass driver controls the by-pass switch to change the by-pass switch from ON to OFF based on one or more detection signals, e.g., indicating a load transition or power-up, thereby introducing the impedance circuit into the free-wheeling path to effect decay of the excitation current from the generator. This solution has the disadvantage that the energy stored in the generator field winding is not absorbed by the impedance circuit, and therefore the energy in the generator field winding can cause damaging over-voltage conditions on the output of the generator.  
         [0009]     In another situation, when the load is suddenly increased on the output of the electric generator, the output voltage typically decreases in the transient response. This decrease is primarily due to the delayed response of the electric generator controller. The increased output energy requirement is not initially provided for by the amount of energy coupled to the output by transformer action, which is primarily influenced by the exciter current. The increased load tends to sink charge from the output capacitance at a rate greater than the amount of charge sourced to the output capacitance by transformer action, and consequently the output voltage drops. Clearly, the electric generator by Yao does not offer a solution to this problem.  
         [0010]     To solve these problems a novel method and apparatus are required that prevents the energy stored in the generator field winding from causing an over-voltage condition on the output when the load is decreased, and prevents a decrease in output voltage when the load is increased.  
       SUMMARY OF THE INVENTION  
       [0011]     A first aspect of the present invention includes an apparatus for regulating a transient response of an output signal of an electrical generator. The apparatus comprises an output winding means which provides a first AC signal and a second AC signal. A switching means for combines the first AC signal and the second AC signal to provide a switched signal. A filter unit has an input and an output. The filter unit is between the switched signal and the output signal. The output of the filter unit provides the output signal. The switching means is responsive to the output signal to control the combining of the first and second AC signals to regulate the transient response of the output signal.  
         [0012]     In a second aspect of the present invention there is a generator which provides an output signal. The generator comprises a stator and a rotor disposed about the stator. An exciter regulator is responsive to the output signal of the generator and provides an exciter field signal. The exciter regulator is connected to the stator. An exciter field coil is responsive to the exciter field signal and provides an exciter magnetic field. The exciter field coil is on the stator. An exciter armature coil is responsive to the exciter magnetic field and provides an exciter armature signal. The exciter armature coil is on the rotor. A generator field coil is responsive to the exciter armature signal and provides a generator field magnetic field. The generator field coil is on the rotor. There is also a generator armature coil means which provides a first AC signal and a second AC signal. A switching means combines the first AC signal and the second AC signal to provide a switched signal. A filter unit has an input and an output. The filter unit is between the switched signal and the output signal. The output of the filter unit provides the output signal.  
         [0013]     In another aspect of the present invention a method is provided for regulating a transient response of an output signal of a generator. A first AC signal and a second AC signal from a generator armature winding means are provided. The first AC signal and the second AC signal are combined by a switching means in order to provide a switched signal. The switched signal is filtered in order to provide the output signal. The output signal is monitored in order to adjust the switching of the the first and second AC signals to maintain the output signal at a set-point value.  
         [0014]     An advantage of the present invention is the avoidance of using expensive and bulky conventional filtering components such as inductors and capacitors. This is increasingly true at higher power levels when the generator producing the power is usually slower in it&#39;s transient response time. The burden to filter the DC voltage in the transient response in conventional generators, until the generator can compensate for the load change, is one which requires massive inductors and capacitors.  
         [0015]     Another advantage of the present invention is that the voltage ratings of the AC switch and DC switch, which typically comprise MOSFET devices, need only be the difference between the first and second AC signals and first and second DC signals respectively, which are substantially less than the respective peak AC and DC values. This permits a significant cost savings, size reduction, and boosts efficiency because power MOSFET development and cost reductions have focused more on lower voltage type devices, which inherently are able to carry higher values of current with minimal power loss. These lower voltage type MOSFET devices make use of low cost high volume assembly and packaging techniques, as opposed to devices rated for both high voltage and high current which are typically packaged as more specialized power modules that are substantially more costly to purchase and often require large amounts of manual labour.  
         [0016]     Additionally, the use of conventional switch-mode AC-DC power convertors are still relatively expensive and highly specialized to design compared to the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The present invention will be more readily understood from the following description of preferred embodiments thereof given, by way of example, with reference to the accompanying drawings, in which:  
         [0018]      FIG. 1  is a schematic view of a brushless generator;  
         [0019]      FIG. 2  is a schematic view of an output stage in  FIG. 1  for a first embodiment of the invention;  
         [0020]      FIG. 3  are waveform diagrams of signals from  FIG. 2  during normal operation;  
         [0021]      FIG. 4  are waveform diagrams of the signals in  FIG. 3  during a decrease in a load;  
         [0022]      FIG. 5  are waveform diagrams of the signals in  FIG. 3  during an increase in a load;  
         [0023]      FIGS. 6A, 6B  and  6 C are schematic views of circuit elements of  FIG. 2 ;  
         [0024]      FIG. 7  is a schematic view of the output stage in  FIG. 1  for another embodiment of the invention;  
         [0025]      FIG. 8  is a schematic view of a rectifier circuit of  FIG. 7 ;  
         [0026]      FIG. 9  is a schematic view of the output stage in  FIG. 1  for a second embodiment of the invention;  
         [0027]      FIG. 10  is a schematic view of a filter circuit of  FIG. 9 ;  
         [0028]      FIG. 11  is a schematic view of the output stage in  FIG. 1  for a third embodiment of the invention;  
         [0029]      FIG. 12  is a schematic view of an H-bridge circuit connected to the output of  FIG. 9 ;  
         [0030]      FIG. 13  is a schematic view of an H-bridge circuit connected to the output of  FIG. 11 ;  
         [0031]      FIG. 14  is a schematic view of an output stage in  FIG. 1  according to a fourth embodiment of the invention;  
         [0032]      FIG. 15  is a schematic view of an output stage in  FIG. 1  according to a sixth embodiment of the invention; and  
         [0033]      FIG. 16  is a schematic view of an output stage in  FIG. 1  according to a seventh embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     The operation and structure of a brushless generator can be understood by referring first to  FIG. 1 . A brushless generator indicated generally by reference numeral  10  includes a stator  12 , a rotor  14  and a regulator  16 . The stator  12  is stationary and includes an exciter field coil  18  and an output stage  20 . The output stage conventionally contains a generator armature coil assembly, a rectifier unit and a filter unit. The rotor  14  is rotated by an external mechanical force, such as an engine or some accessory thereof, and includes an exciter armature coil assembly indicated generally by reference numeral  22  and a generator field coil  24 .  
         [0035]     In operation, the exciter field coil  18  is excited by an exciter field current I EF  from the regulator  16  producing an exciter magnetic field. The exciter armature coil assembly  22  rotates through the exciter magnetic field and consequently a 3-phase exciter armature signal V EA1 , V EA2  and V EA3  is induced in the assembly. The induced 3-phase exciter armature signal V EA1 , V EA2  and V EA3  is rectified by a bridge rectifier assembly indicated generally by reference numeral  26  which provides a DC exciter armature voltage V EA  and a DC exciter armature current I EA .  
         [0036]     The generator field coil  24  is excited by the DC exciter armature current I EA  producing a generator field magnetic field. The generator field magnetic field modulates in time and space since the generator field coil  24  is on the rotor  14  which rotates. The output stage is responsive to the generator field magnetic field and provides an output voltage V o . For a conventional brushless generator, the structure of the output stage includes the generator armature coil assembly, the rectifier unit and the filter unit.  
         [0037]     The regulator  16  has an output measurement unit  28 , for example an operational amplifier and pulse width modulated optocoupler, and a control unit  29 , for example a PID controller. The output measurement unit  28  provides an output sample signal  27 , representative of the output voltage V O , or an output current in other embodiments, to the control unit  29 . The control unit  29  is responsive to the output sample signal  27  and serves to adjust the exciter field current I EF  so as to maintain the output voltage V O  at a set-point value.  
         [0038]     In a preferred embodiment of the present invention, the output stage  20  has a structure as illustrated in  FIG. 2 . A generator armature coil  30  has a first tap terminal  32 , a second tap terminal  34  and an end terminal  36 . The first tap terminal  32  provides a first induced AC signal V IAC1  with respect to the end terminal  36 , and the second tap terminal  34  provides a second induced AC signal V IAC2 , also with respect to the end terminal. The end terminal  36  provides a first reference voltage V R1 .  
         [0039]     The first induced AC signal V IAC1  has a first RMS value and the second induced AC signal V IAC2  has a second RMS value. In the present embodiment, the first RMS value is greater than the second RMS value.  
         [0040]     A first AC switch  40  has a first control terminal  42 , a first switch terminal  44  and a second switch terminal  46 . The first switch terminal  44  receives the first induced AC signal V IAC1 . The second switch terminal  46  can provide a switched AC signal V SAC .  
         [0041]     A second AC switch  50  has a second control terminal  52 , a third switch terminal  54  and a fourth switch terminal  56 . The third switch terminal  54  receives the second induced AC signal V IAC2 . The fourth switch terminal  56  can also provide the switched AC signal V SAC .  
         [0042]     A rectifier unit  60  has a first input terminal  61  and a second input terminal  62 . The first input terminal  61  receives the switched AC signal V SAC . The second input terminal  62  receives the first reference voltage V R1 . The rectifier unit  60  further includes a first output terminal  64 , which provides a first DC voltage V DC1 , and a second output terminal  65 , which provides a second reference voltage V R2 .  
         [0043]     A filter unit  70  has a first input terminal  72 , which receives the first DC voltage V DC1 , a second input terminal  74 , which receives the second reference voltage V R2 , a first output terminal  76 , which provides the output voltage V O , and a second output terminal  78 , which provides a ground reference GND for the output voltage V O .  
         [0044]     A switch controller  80  has a first DC input terminal  81 , which receives the output signal V O , a second DC input terminal  82 , which receives the ground reference GND, a first AC input terminal  83 , which receives the first induced AC signal V IAC1 , a second AC input terminal  84 , which receives the second induced AC signal V IAC2 , and an AC reference input terminal  85  which receives the first reference signal V R1 . The switch controller  80  further includes a first output terminal  86 , which provides a first switch control signal V SWC1 , and a second output terminal  87 , which provides a second switch control signal V SWC2 .  
         [0045]     The first control terminal  42  of the first AC switch  40  receives the first switch control signal V SWC1 , provided by the switch controller  80 . Similarly, the second control terminal  52  of the second AC switch  50  receives the second switch control signal V SWC2 , also provided by the switch controller  80 .  
         [0046]     In operation, the generator armature coil  30  couples energy from the generator field winding  24 , as shown in  FIG. 1 , by linking a modulating flux of the generator field magnetic field and thereby inducing the first and second induced AC signals, V IAC1  and V IAC2  respectively.  
         [0047]     The switched AC signal V SAC  signal alternates between the first induced AC signal V IAC1  and the second induced AC signal V IAC2 . This alternation is provided by the switch controller  80  consecutively enabling and disabling the first AC switch  40  and then the second AC switch  50 . The switch controller  80  does not allow both the first AC switch  40  and the second AC switch  50  to be enabled simultaneously.  
         [0048]     The first AC switch  40  is enabled when the switch controller  80  asserts the first switch control signal V SWC1 . When the first AC switch  40  is enabled the first switch terminal  44  is shorted to the second switch terminal  46 , and consequently the switched AC signal V SAC  equals the first induced AC signal V IAC1 .  
         [0049]     The second AC switch  50  is enabled when the switch controller  80  asserts the second switch control signal V SWC2 . When the second AC switch  50  is enabled, the third switch terminal  54  is shorted to the fourth switch terminal  56 , and consequently the switched AC signal V SAC  equals the second induced AC signal V IAC2 .  
         [0050]      FIG. 3  illustrates waveforms during normal operation of the first switch control signal V SWC1 , the second switch control signal V SWC2  and a duty cycle of the switched AC signal V SAC . The duty cycle of the switched AC signal V SAC  is defined by the percentage of time the switched AC signal V SAC  equals the first induced AC signal V IAC1 . Note that the waveforms of the first and second switch control signals V SWC1  and V SWC2  have a break-before-make dead space  49 . This prevents the shorting of the first tap terminal  32  with the second tap terminal  34 .  
         [0051]     Referring back to  FIG. 2 , the rectifier unit  60  rectifies the switched AC signal V SAC  and provides the first DC voltage V DC1 . The first DC voltage V DC1  is a composite voltage comprising a composite DC voltage, a composite square wave voltage and a composite ripple voltage. The composite square wave voltage has a composite duty cycle which is identical to the duty cycle of the switched AC voltage V SAC . The composite DC voltage and the composite ripple voltage are inherent in the rectification of the switched AC voltage V SAC . The composite square wave voltage is a result of the alternating nature of the switched AC voltage V SAC . The filter unit  70  serves to low pass filter the composite ripple voltage and the composite square wave voltage of the first DC voltage V DC1 , and provides the output voltage V O . In this example, the filter unit  60  provides the output voltage V O  that is essentially equivalent to the sum of the composite DC voltage, an average value of the composite square wave voltage and a reduced amount of the composite ripple voltage. The output voltage V O  is consequently applied to the load.  
         [0052]     The transient condition is next considered when the load is decreased. As discussed above, in the case for the conventional brushless generator illustrated in  FIG. 1 , the output voltage would tend to increase because of the delayed response of the controller  29  and the quantity of stored magnetic energy  40  in the generator field coil  24 . In the present embodiment of the output stage  20 , illustrated in  FIG. 2 , the switch controller  80  detects that the output voltage V O  increases, and accordingly decreases the duty cycle of the switched AC voltage V SAC . By decreasing the duty cycle, the percentage of time the switched AC voltage V SAC  equals the first induced AC signal V IAC1  decreases, and consequently the percentage of time the switched AC voltage equals the second induced AC signal V IAC2  increases. This is illustrated by the waveforms of the first and second switch control signals V SWC1  and V SWC2  respectively and the duty cycle of the switched AC voltage V SAC  in  FIG. 4 .  
         [0053]     The decrease in duty cycle of the switched AC voltage V SAC  affects the first DC voltage V DC1  by correspondingly decreasing the composite duty cycle of the composite square wave to match the duty cycle of the switched AC voltage V SAC . By decreasing the composite duty cycle the average value of the composite square wave also decreases. Therefore, the output voltage V O  decreases accordingly since it is the sum of the composite DC voltage, the average value of the composite square wave, and a reduced amount of composite ripple voltage. In summary, a decreased load causing an increase in output voltage V O  leads to a decrease in the output voltage V O  by operation of the circuit of the present embodiment.  
         [0054]     In a case where an amount of load decreased is sufficient to cause the output voltage to increase regardless of how much the duty cycle of the switched AC voltage V SAC  is decreased, the switch controller  80  can temporarily disable both the first AC switch  40  and the second AC switch  50 . This will sink charge from the output capacitance of the generator, and prevent charge from being sourced to the output capacitance, which consequently decreases the output voltage. The switch controller can also, periodically, enable either one of the first or second AC switches,  40  or  50  respectively, to source charge to the output capacitance in order to regulate the output voltage at the set-point value.  
         [0055]     The transient condition is now considered when the load is increased. In this case for the brushless generator illustrated in  FIG. 1 , the output voltage would tend to decrease because of the delayed response of the controller  29 . In the present embodiment, the switch controller  80  illustrated in  FIG. 2  detects that the output voltage V O  decreases, and accordingly increases the duty cycle of the switched AC voltage V SAC . By increasing the duty cycle, the percentage of time the switched AC voltage V SAC  equals the first induced AC signal V IAC1  increases, and consequently the percentage of time the switched AC voltage equals the second induced AC signal V IAC2  decreases. This is illustrated by the waveforms of the first and second switch control signals V SWC1  and V SWC2  respectively and the duty cycle of the switched AC voltage V SAC  in  FIG. 5 .  
         [0056]     The increase in duty cycle of the switched AC voltage V SAC  affects the first DC voltage V DC1  by correspondingly increasing the composite duty cycle of the composite square wave to match the duty cycle of the switched AC voltage V SAC . By increasing the composite duty cycle the average value of the composite square wave also increases. Therefore, the output voltage V O  will tend to increase since it is the sum of the composite DC voltage, the average value of the composite square wave, and a reduced amount of composite ripple voltage. In summary, an increased load causing a decrease in output voltage V O  leads to an increase in the output voltage V O  by operation of the circuit of the present embodiment.  
         [0057]     In a case where an amount of load increased is sufficient to cause the output voltage to decrease regardless of how much the duty cycle of the switched AC voltage V SAC  is increased, the switch controller  80  can temporarily continuously enable the first switch  40 . This will source the maximum amount of charge to the output capacitance of the generator.  
         [0058]     The first and second AC switches,  40  and  50  respectively, have a structure illustrated in  FIG. 6A  for this particular embodiment. Two n-channel MOSFETs, indicated generally by reference numerals  100  and  101  respectively, have respective sources S connected together and respective gates G connected together. Alternative embodiments could use other types of AC switches, such as a pair of p-channel MOSFETs.  
         [0059]     The rectifier unit  60  in this example is a bridge rectifier, which is commonly known in the art, and is illustrated in  FIG. 6B . The filter unit  70 , in its simplest form, can be a capacitor as illustrated in  FIG. 6C .  
         [0060]     The present embodiment can be adapted to provide an AC output voltage by not including the rectifier unit  60 . In this case, the first input terminal  72  of the filter unit  70  receives the switched AC voltage V SAC  directly, and the second input terminal  74  receives the first reference voltage V R1 . The operation is similar in principal to the previously described embodiment in that the duty cycle of the switched AC voltage V SAC  is varied so that the averaging effect of the filter unit  70  on the switched AC voltage varies the output voltage V O  accordingly. Note that the first DC input terminal  81  of the controller  80  is also adapted to receive and monitor the AC output voltage.  
         [0061]     A second embodiment of the invention is illustrated in  FIG. 7  wherein like elements to the previous embodiment have like reference numerals with an additional suffix “0.7”. This embodiment is a 3-phase version of the previously described single-phase embodiment. Like elements of each phase of the embodiment illustrated in  FIG. 7  have like reference numerals with an additional suffix “.x”, wherein x denotes the phase and is either 1, 2 or 3.  
         [0062]     The operation of this embodiment is similar to the single-phase embodiment. A notable difference is a rectifier unit  60 . 7  that is responsive to three switched AC voltages V SAC7.1 , V SAC7.2  and V SAC7.3  and provides a first DC voltage V DC1.7  and a reference voltage V R2.7 . The rectifier unit can have a structure as illustrated in  FIG. 8 . The first and second AC switches of each phase can have the same structure as illustrated in  FIG. 6A . The filter unit can, again, simply be a capacitor.  
         [0063]     As with the single-phase embodiment, the 3-phase present embodiment can be adapted to provide an AC output voltage, in this case a 3-phase AC output voltage, by not including the rectifier unit  60 . 7 . In this case, the filter unit  70 . 7  receives and filters the first switched AC voltage V SAC7.1 , the second switched AC voltage V SAC7.2  and the third switched AC voltage V SAC7.3  to provide the 3-phase output voltage. The switch controller  80 . 7  is adapted to receive and monitor the 3-phase output voltage.  
         [0064]     Another embodiment of the invention is illustrated in  FIG. 9  wherein like parts have like reference numerals with an additional suffix “0.9”. A generator armature coil  130  has a first tap terminal  132  and an end terminal  136 . The first tap terminal  132  provides an induced AC signal V IAC1.9  with respect to the end terminal  136 , which provides a reference voltage V R100 . Another generator armature coil  230  has a first tap terminal  232  and an end terminal  236 . The first tap terminal  232  provides an induced AC signal V IAC2.9  with respect to the end terminal  236 , which provides a reference voltage V R200 .  
         [0065]     The first induced AC signal V IAC1  has a first RMS value and the second induced AC signal V IAC2.9  has a second RMS value. In the present embodiment, the first RMS value is less than the second RMS value.  
         [0066]     A rectifier unit  160  has a first input terminal  161  and a second input terminal  162 . The first input terminal  161  receives the induced AC signal V IAC1.9 . The second input terminal  162  receives the reference voltage V R100 . The rectifier unit  160  further includes a first output terminal  164 , which provides a DC voltage V DC101 , and a second output terminal  165 , which provides a second reference voltage V R101 .  
         [0067]     Another rectifier unit  260  has a first input terminal  261  and a second input terminal  262 . The first input terminal  261  receives the induced AC signal V IAC2.9.  The second input terminal  262  receives the reference voltage V R200 . The rectifier unit  260  further includes a first output terminal  264 , which provides a DC voltage V DC201 , and a second output terminal  265 , which provides a second reference voltage V R201 .  
         [0068]     A first filter unit  170  has a first input terminal  172 , which receives the DC voltage V DC101 , a second input terminal  174 , which receives the reference voltage V R101 , a first output terminal  176 , which provides a DC voltage V DC102 , and a reference terminal  178 .  
         [0069]     A second filter unit  270  has a first input terminal  272 , which receives the DC voltage V DC201 , a second input terminal  274 , which receives the reference voltage V R201  a first output terminal  276 , which provides a DC voltage V DC202 , and a second output terminal  278 , which provides a reference voltage V R.9 . The reference terminal  178  of the filter unit  170  receives the DC voltage V DC202 .  
         [0070]     A first DC switch  140  has a first control terminal  142 , a first switch terminal  144  and a second switch terminal  146 . The first switch terminal  144  receives the DC voltage V DC102 . The second switch terminal  146  can provide a switched DC voltage V SDC .  
         [0071]     A second DC switch  240  has a first control terminal  242 , a first switch terminal  244  and a second switch terminal  246 . The first switch terminal  244  receives the DC voltage V DC202 . The second switch terminal  246  can also provide the switched DC voltage V SDC .  
         [0072]     A filter unit  70 . 9  has a first input terminal  72 . 9 , which receives the switched DC voltage V SDC , a second input terminal  74 . 9 , which receives the reference voltage V R.9 , a first output terminal  76 . 9 , which provides the output voltage V O.9 , and a second output terminal  78 . 9 , which provides a ground reference GND. 9  for the output voltage V O.9 .  
         [0073]     A switch controller  80 . 9  has a first DC input terminal  81 . 9 , which receives the output signal V O.9 , a second DC input terminal  82 . 9 , which receives the ground reference GND. 9 , a first output terminal  86 . 9 , which provides a first switch control signal V SWC1.9 , and a second output terminal  87 . 9 , which provides a second switch control signal V SWC2.9 .  
         [0074]     In operation, the generator armature coils  130  and  230  couple energy from the generator field winding  24 , as shown in  FIG. 1 , by linking a modulating flux of the generator field magnetic field and thereby inducing the first and second induced AC signals, V IAC1.9  and V IAC2.9  respectively.  
         [0075]     The armature coils  130  and  230  provide the induced AC signals V IAC1.9  and V IAC2.9  respectively, to the rectifier units  160  and  260  respectively. The rectifier units  160  and  260  rectify the induced AC signals V IAC1.9  and V IAC2.9  respectively, and provide the DC voltages V DC101  and V DC201  respectively.  
         [0076]     The switched DC voltage V SDC  is generated by alternating between the DC voltages V DC102  and V DC202 . This alternation is controlled by the switch controller  80 . 9  consecutively enabling and disabling the first DC switch  140  and then the second DC switch  240 . The switch controller  80 . 9  does not allow both the first DC switch  140  and the second DC switch  240  to be enabled simultaneously.  
         [0077]     The first DC switch  140  is enabled when the switch controller  80 . 9  asserts the first switch control signal V SWC1.9 . When the first DC switch  140  is enabled the first switch terminal  144  is shorted to the second switch terminal  146 , and consequently the switched DC voltage V SDC  equals the DC voltage V DC102 .  
         [0078]     Similarly, the second DC switch  240  is enabled when the switch controller  80 . 9  asserts the second switch control signal V SWC2.9 . When the second DC switch  240  is enabled the first switch terminal  244  is shorted to the second switch terminal  246 , and consequently the switched DC voltage V SDC  equals the DC voltage V DC202 .  
         [0079]     The waveforms of the first and second switch control voltage V SWC1.9  and V SWC2.9  are similar to the corresponding waveforms in  FIG. 3 . A duty cycle is defined by the percentage of time the switched DC voltage V SDC  equals the DC voltage V DC102 . Again, the waveforms of the first and second switch control voltages V SWC1.9  and V SWC2.9  have a break-before-make dead space. This prevents the shorting of the DC voltage V DC102  with the DC voltage V DC202 .  
         [0080]     Referring back to  FIG. 9 , the switched DC voltage V SDC  is a composite voltage comprising a composite DC voltage, a composite square wave voltage and a composite ripple voltage. The composite square wave voltage has a composite duty cycle, which is identical to the duty cycle of the switched DC voltage V SDC . The composite DC voltage and the composite ripple voltage are inherent in the rectification function provided by rectifier units  160  and  260  respectively. The composite square wave voltage is a result of the alternating nature of the switched DC voltage V SDC . The filter unit  70 . 9  serves to low pass filter the composite ripple voltage and the composite square wave voltage of the switched DC voltage V SDC , and provides the output voltage V O.9 . In this example, the filter unit  70 . 9  provides the output voltage V O.9  that is essentially equivalent to the sum of the composite DC voltage, an average value of the composite square wave voltage and a reduced amount of the composite ripple voltage. The output voltage V O.9  is consequently applied to the load.  
         [0081]     In response to an increase or decrease in load, the circuit of the present embodiment operates similarly to previous embodiments by varying the duty cycle of the switched DC voltage V SDC .  
         [0082]     The first and second rectifier units  160  and  260  respectively can have the structure illustrated in  FIG. 6B . The first and second filter units  170  and  270  respectively can have the structure illustrated in  FIG. 6C . The first and second DC switches  140  and  240  respectively are typically MOSFET transistors. The filter unit  70 . 9  can have the structure illustrated in  FIG. 10 , wherein an inductor L 1  and a capacitor C 1  form an LC circuit, which is commonly known the art.  
         [0083]     An advantage of this embodiment over previous embodiments is a reduced amount of ripple voltage on the output voltage V O.9 . When the phase of the first induced AC signal V IAC1.9  is  180  degrees out of phase with the second induced AC signal V IAC2.9 , a ripple voltage component of the DC voltage V DC101  is cancelled by a ripple voltage component of V DC201 .  
         [0084]     Referring to  FIGS. 9 and 12 , the present embodiment can be adapted to include a conventional H-bridge circuit after the filter circuit  70 . 9  in order to synthesize an AC output voltage VH O.9  of any desired frequency and any peak amplitude up to the maximum voltage available from the filter circuit.  
         [0085]     Another embodiment of the invention is illustrated in  FIG. 11  wherein like elements to the previous embodiments have like reference numerals with an additional suffix “0.11”. This embodiment is a 3-phase version of the previous single-phase embodiment illustrated in  FIG. 9 . Like elements of each phase of the embodiment illustrated in  FIG. 11  have like reference numerals with an additional suffix “.x”, wherein x denotes the phase and can be either 1, 2 or 3.  
         [0086]     The operation of this embodiment is essentially identical to the single-phase embodiment of  FIG. 9 . A notable difference is rectifier units  160 . 11  and  260 . 11  that are responsive to three induced AC signals V IAC1.11.1 , V IAC1.1.2  and V IAC1.11.3 , and V IAC2.11.1 , V IAC2.11.2  and V IAC2.11.3  respectively. The rectifier units  160 . 11  and  260 . 11  can have the structure illustrated in  FIG. 8 .  
         [0087]     Referring to  FIGS. 11 and 13 , the 3-phase embodiment can be adapted to include a conventional H-bridge circuit after the filter circuit  70 . 11  in order to synthesize an AC output voltage V HO.11  of any desired frequency and any peak amplitude up to the maximum voltage available from the filter circuit.  
         [0088]     An advantage of the previously described embodiments, and of the invention in general, is the avoidance of using expensive and bulky conventional filtering components such as inductors and capacitors. This is increasingly true at higher power levels when the generator producing the power is usually slower in its transient response time. The burden to filter the DC voltage in the transient response in conventional generators, until the generator can compensate for the load change, is one which requires massive inductors and capacitors.  
         [0089]     Additionally, the use of conventional switch-mode AC-DC power convertors is still relatively expensive and highly specialized to design compared to the embodiments of the present invention.  
         [0090]     The embodiment of  FIG. 9  has distinct advantages over the other embodiments. Not only does it enhance DC output transient response, but it also provides a smaller, simpler generator than the embodiment of  FIG. 11 , and it greatly filters away AC ripple of rectified single-phase AC compared to all the other embodiments.  
         [0091]     In another embodiment of the present invention, the output stage has a structure as illustrated in  FIG. 14 , wherein like parts have like reference numerals with an additional suffix “0.14”. This embodiment is similar to the first embodiment of  FIG. 2 , the differences being described below. This embodiment has a filter unit  70 . 14 , a switch controller  80 . 14  and a rectifier unit  60 . 14  as in the embodiment of  FIG. 2 , in other examples the rectifier unit is not required. When the rectifier unit  60 . 14  is included then an output voltage V O.14  is a DC voltage. If it is not included then the output voltage V O.14  is an AC voltage.  
         [0092]     A first generator armature coil  300  has a first terminal  301  and a second terminal  302 . The first terminal  301  is a reference and the second terminal provides a first induced AC signal V IAC1.14  with respect to the first terminal. A second generator armature coil  304  has a first terminal  305  and a second terminal  306 . The first terminal  305  is a reference and the second terminal provides a second induced AC signal V IAC2.14  with respect to the first terminal  305 .  
         [0093]     An H-bridge circuit  308  has first, second, third and fourth terminals  312 ,  314 ,  316  and  318  respectively, and an I/O port  320 . The H-bridge circuit  308  has four AC switches  310 , which can comprise MOSFET devices. The second terminal  302  of the first generator armature coil  300  is connected to the first terminal  312  of the circuit  308 . The first terminal  305  of the second generator armature coil  304  is connected to the third terminal  316  of the circuit  308 , and the second terminal  306  of the armature coil  304  is connected to the fourth terminal  318  of the circuit  308 . The fourth terminal  314  of the circuit  308  is connected to a terminal  61 . 14  of the rectifier unit  60 . 14 , similar to the rectifier unit in  FIG. 2 , and provides a switched AC signal V SAC.14  to the terminal  61 . 14 . The first terminal  301  of the first generator armature coil  300  is connected to a terminal  62 . 14  of the rectifier unit  60 . 14 . Note that in other examples without the rectifier unit  60 . 14 , the second terminal  314  of the H-bridge circuit  308  and the first terminal  301  of the first generator armature coil  300  are connected to terminals  72 . 14  and  74 . 14  respectively of the filter unit  70 . 14 .  
         [0094]     The switch controller  80 . 14  has an I/O port  86 . 14  which is connected to the I/O port  320  of the H-bridge circuit  308  by control bus  88 . The switch controller outputs a switch control signal V SWC1.14  on the control bus which operates to enable and disable the AC switches  310  such that the second induced AC signal V IAC2.14  is either added or subtracted from the first induced AC signal V IAC1.14 . The resulting combination is the switched AC signal V SAC.14 . The switch controller  80 . 14  controls the adding and subtracting of the second induced AC signal V IAC2.14  to the first induced AC signal V IAC1.14  in order to regulate the output voltage V O.14 .  
         [0095]     In another embodiment of the present invention, the output stage has a structure as illustrated in  FIG. 15 , wherein like parts have like reference numerals with an additional suffix “0.15”. This embodiment is similar to the previous embodiment of  FIG. 14  and to the embodiment of  FIG. 9 , the differences being described below. This embodiment has a filter unit  70 . 15  and a switch controller  80 . 15  as in the embodiment of  FIG. 9 . The filter unit  70 . 15  provides an output voltage V O.15 .  
         [0096]     A first generator armature coil  130 . 15  has a first terminal  132 . 15  and a second terminal  136 . 15 . The first terminal  132 . 15  provides an induced AC signal V IAC1.15  with respect to the second terminal  136 . 15 , which provides a reference voltage V R100.15 . A second generator armature coil  230 . 15  has a first terminal  232 . 15  and a second terminal  236 . 15 . The first terminal  232 . 15  provides an induced AC signal V IAC2.15  with respect to the second terminal  236 . 15 , which provides a reference voltage V R200.15 .  
         [0097]     A rectifier unit  260 . 15  has a first input terminal  261 . 15  and a second input terminal  262 . 15 . The first input terminal  261 . 15  receives the induced AC signal V IAC2.15 . The second input terminal  262 . 15  receives the reference voltage V R200.15 . The rectifier unit  260 . 15  further includes a first output terminal  264 . 15 , which provides a DC voltage V DC201.15 , and a second output terminal  265 . 15 , which provides a second reference voltage V R201.15 .  
         [0098]     An H-bridge circuit  308 . 15  has first, second, third and fourth terminals  312 . 15 ,  314 . 15 ,  316 . 15  and  318 . 15  respectively, and an I/O port  320 . 15 . The H-bridge circuit  308 . 15  has four DC switches  310 . 15 , which can comprise MOSFET devices. The first output terminal  264 . 15  of the rectifier unit  260 . 15  is connected to the first terminal  312 . 15  of the circuit  308 . 15 . The first terminal  132 . 15  of the first generator armature coil  130 . 15  is connected to the fourth terminal  318 . 15  of the circuit  308 . 15 , and the second terminal  136 . 15  of the armature coil  130 . 15  is connected to the third terminal  316 . 15  of the circuit  308 . 15 . The second terminal  314 . 15  of the circuit  308 . 15  is connected to a first terminal  72 . 15  of the filter unit  70 . 14  and provides a switched DC signal V SDC.15 . The second output terminal  265 . 15  of the second rectifier unit  260 . 15  is connected to a second terminal  74 . 5  of the filter unit  70 . 15 .  
         [0099]     The switch controller  80 . 15  has an I/O port  86 . 15  which is connected to the I/O port  320 . 15  of the H-bridge circuit  308 . 15  by control bus  88 . 15 . The switch controller receives an indication of the phase of the induced AC signal V IAC1.15  on the control bus  88 . 15  and outputs a switch control signal V SWC1.15  on the control bus which operates to enable and disable the DC switches  310 . 15  such that the induced AC voltage V IAC1.15  is either added or subtracted from the DC voltage V DC201.15 . The resulting combination is the switched DC signal V SDC.15 . The switch controller  80 . 15  controls the adding and subtracting of the induced AC voltage V IAC1.15  to the DC voltage V DC201.15  in order to regulate the output voltage V O.15 . It is understood that the switch controller  80 . 15  toggles its control of the H-bridge circuit  308 . 15  on alternate AC phases such that the induced AC signal V IAC1.15 is essentially rectified.    
         [0100]     In another embodiment of the present invention, the output stage has a structure as illustrated in  FIG. 16 , wherein like parts have like reference numerals with an additional suffix “0.16”. This embodiment is similar to the previous embodiments of  FIG. 14  and  15 , comprising a first generator armature coil  304 . 16  with an induced AC signal V IAC2.16 , a second generator armature coil  300 . 16 , an H-bridge circuit  308 . 16  and a switch controller  80 . 16 . A control bus  88 . 16  connects I/O port  320 . 16  of the H-bridge circuit  308 . 16  with I/O port  86 . 16  of the switch controller  80 . 16 . In this embodiment a series rectifier  330  allows the H-bridge circuit  308 . 16  to comprise DC switches  310 . 16 . The switch controller  80 . 15  is aware of the phase of the induced AC signal V IAC2.16  over the control bus  88 . 16  in order to toggle the control polarity of switch control signal V SWC1.16  output by the switch controller on the control bus. A rectifier unit  60 . 16  is optional depending on whether output voltage V O.16  is to be DC or AC.  
         [0101]     As will be apparent to those skilled in the art, various modifications may be made within the scope of the appended claims.