Abstract:
A device and method for controlling the output of a wide speed range high reactance permanent magnet machine based PGS is provided. The windings of a permanent magnet machine are coupled to a three-phase diode bridge. A transistor is used for temporarily short-circuiting said diode bridge. A capacitor smoothes the voltage at a voltage detection point. A control unit generates a signal that switches the transistor in response to a voltage detected at the voltage detection point. The control unit signal modifies the duty cycle of the switching of the transistor in response to variations in the speed of the power generator to maintain a desired voltage at the voltage detection point.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to high speed generators and, more specifically, to apparatus and methods for regulating voltage to a DC power distribution bus over a wide speed range in a high reactance permanent magnet machine based electrical power generation system. 
         [0002]    Electrical power generation systems (PGS) play a significant role in the modern aerospace/military industry. Recently, this is particularly true in the area of more electric architecture (MEA) for aircraft and spacecraft. The commercial aircraft business is moving toward no-bleed air environmental control systems (ECS), variable-frequency (VF) power distribution systems, and electrical actuation. 
         [0003]    Military ground vehicles have migrated toward hybrid electric technology, where the main propulsion is performed by electric drives. Therefore, substantial demand for increased power generation has emerged. Future space vehicles will require electric power generation systems for thrust vector and flight control actuation. These systems must be more robust and offer greatly reduced operating costs and safety compared to the existing Space Shuttle power systems. 
         [0004]    These new aerospace trends have significantly increased power generation needs. This has led to increased operating voltages to reduce system losses, weight, and volume. New power, quality and electromagnetic interference (EMI) requirements have been created to satisfy both quality and performance needs. The overall result has been a significant increase in the installed electric power, creating challenges in accommodating this equipment in the new platforms. Therefore, overall system performance improvement and power density increases are necessary for the new-generation hardware to satisfy MEA. Decreasing the cost of power generation systems will make the new platforms more affordable. 
         [0005]    Wide Speed Range (WSR) PGS applicable to MEA must satisfy a complex set of requirements. The main function of such a system is electrical power generation; hence the system must provide conversion of the mechanical power supplied by the prime mover to conditioned electrical power supplied to the distribution bus. Generation is typically defined as continuous power at 100 percent load. Increasing the load to 150 percent for a limited time may be required. The percentage of increase and time required for overloading varies from application to application. 
         [0006]    Another requirement for WSR PGS applicable to MEA is steady-state regulation, which requires that the system maintain the output voltage constant within certain limits when the loads and other conditions are changed gradually. Transient regulation is a requirement that the system maintains the output voltage constant within certain limits when the loads and other conditions are changed rapidly. Transient limits are typically wider than steady-state limits. Typical regulation requirements can be found in MIL-STD-704E. Electromagnetic interference (EMI), both conducted and radiated emissions, are important requirements for an EPGS to provide proper operation of the installed electronics. At the same time, the electronic equipment including PGS should not be susceptible to the specified radiated emissions. 
         [0007]    DC bus short-circuit protection is another requirement which must provide adequate protection when an external short-circuit fault occurs at the DC distribution bus. Feeder short-circuit protection function is also required to prevent excessive current flow in the electric machine and the interface electric machine power electronics to reduce damages that may lead to a hazardous condition. Power electronics short-circuit protection is required to prevent excessive current flow in the power electronics unit. Overvoltage protection is required to prevent excessive voltage across a power distribution bus. Overvoltage protection prevents damage of the electronics connected to the distribution bus. 
         [0008]    Electric machines used in auxiliary power unit (APU) applications typically operate at constant speed or with small variation. The main engines of an airplane normally operate with a speed range where the ratio of maximum to minimum operating speed is about 2 to 1. This speed variation creates additional difficulties for a power generation system in providing regulated power within the entire speed range. There are some applications where the speed of the prime mover, for instance a helicopter engine, changes by a factor of up to 20. This wide speed range creates even more challenges due to variation of the electromotive force (emf) voltage of the machine with the speed. 
         [0009]    The synchronous permanent magnet machine (PMM) presents a very competitive design that outperforms other electric machines in most applications when weight and size are critical. However, the rotor flux in a typical PMM is fixed and cannot be controlled or disengaged when a short-circuit is initiated. Unlike other machines where the excitation of the rotor flux can be controlled and even disabled quickly, a PMM continues to generate emf until the rotor stops rotating. Therefore, the PMM presents a hazard in some applications, leading to its limited use, particularly in the aerospace industry. 
         [0010]    The High Reactance Permanent Magnet Machine (HRPMM) is one type of PMM in which, should it become shorted, the phase current magnitude can be internally limited to levels capable of being sustained either indefinitely, within the thermal limits of the system, or until the rotor speed can be reduced to zero. In some prior HRPMM power topologies the functional and protection requirements may be resolved. However, the operating speed range may still be quite narrow. 
         [0011]    As can be seen, there is a need for a PMM-based power generation systems that can supply power to a DC bus within a wide speed variation while satisfying the functional and safety requirements discussed above. 
       SUMMARY OF THE INVENTION 
       [0012]    In one aspect of the invention, a device for controlling a variable speed electrical power generator comprises: a permanent magnet machine generating an output voltage across output terminals, the permanent magnet machine having a plurality of stator windings; a diode bridge connected across the plurality of stator windings; a transistor for at least temporarily short-circuiting the diode bridge; a capacitor for smoothing the output voltage detected across the pair of output terminals; and a control unit for generating a signal that switches the transistor in response to a voltage detected across the pair of output terminals, the control unit signal modifying the duty cycle of the switching of the transistor in response to variations in the speed of the power generator to maintain a desired voltage across the pair of output terminals. 
         [0013]    In another aspect of the invention a variable speed permanent magnet machine connected to a load comprises: a permanent magnet rotor; a stator assembly mounted adjacent the rotor and including a plurality of electrical windings disposed in a plurality of slots between a plurality of stator teeth and having a stator winding resistance R S , the electrical windings being electrically connected to a permanent magnet machine output adapted to deliver generated output voltage from the permanent magnet machine; a voltage control circuit providing for a boost in the output voltage in a first rotational speed range, the voltage control circuit also providing limiting of output current to a pre-selected value in a second rotational speed range; wherein, in use, the movement of the rotor induces an alternating voltage and current in the electrical windings of a first polarity and the first alternating voltage and current induces a second alternating voltage and current of a second polarity in the electrical windings, and the voltage control circuit limiting of output current being provided by the second alternating voltage and current of a second polarity. 
         [0014]    In a further aspect of the present invention, a method for controlling a wide speed range high reactance permanent magnet machine in a plurality of speed ranges comprises: determining an output voltage across output terminals of a circuit including a wide speed range high reactance permanent magnet machine having stator windings, a diode bridge connected across the stator windings, a solid state switch connected across the diode bridge and a PWM controller circuit connected to the solid state switch; if the wide speed range high reactance permanent magnet machine is in a low speed range, and if the detected output voltage is lower than a desired output voltage, increasing the duty cycle of the PWM controller to increase the detected output voltage; and if the detected output voltage is higher than the desired voltage, decreasing the duty cycle of said PWM controller to decrease the detected output voltage. 
         [0015]    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 
         [0016]      FIG. 1  is a radial cross-sectional view of a high reactance permanent magnet machine in accordance with one embodiment of the invention; 
           [0017]      FIG. 2  is a graph of the voltage versus current characteristics of a high reactance permanent magnet machine at various speeds in accordance with one embodiment of the invention; 
           [0018]      FIG. 3  is a block diagram of the wide speed range electric power generation system using a high reactance permanent machine in accordance with one embodiment of the invention; 
           [0019]      FIG. 4  is a graph of a regulation curve showing the duty cycle a pulse modulated switch shown in  FIG. 3  over a range of speeds; 
           [0020]      FIG. 5  is a timing diagram of selected output voltages and currents for the wide speed range electric power generation system shown in  FIG. 3 ; and 
           [0021]      FIG. 6  is a flow chart of a process for regulating output voltage in a wide speed range high reactance permanent magnet machine in accordance with one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    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. 
         [0023]    The present invention generally provides a wide speed range, high reactance permanent magnet machine (HRPMM) that may provide regulated voltage over wide variations in the rotational speed of the prime mover, and hence the HRPMM rotor. For example, the ratio of the maximum to minimum rotor speed may be as high as 20 to 1. At low machine rotational speeds, in a boost mode, the output voltage may be increased by using a pulse width modulated switch across a diode bridge to store energy in the electric machine and transfer this energy to the electric machine output. As the machine speed increases, the duty cycle of the switch may be increased to increase the output voltage. At high machine speeds, in a current limiting mode, the synchronous reactance of the HRPMM may be used to limit the current to the desired level, while the pulse width modulated switch may maintain a relatively constant duty cycle. This may be done by designing the machine parameters such that the short-circuit current is close to the operating current. 
         [0024]    The present invention may be applicable to high speed generators where the voltage to a DC power distribution bus must be regulated over a wide speed range. One example is in helicopters where the speed of the prime mover of the generator may vary by a factor of 20 to 1. The present invention also provides an optimized solution for power generation in various applications such as MEA systems in aircraft and spacecraft, hybrid electric ground vehicles and other applications where weight and size are critical, including auxiliary power units. 
         [0025]    Prior art based PGSs have not generally been able to provide a regulated voltage over wide rotor speed ranges. In contrast, the present invention can provide regulated voltage over rotor speed ranges as wide as 20 to 1. Further, unlike the prior art, the present invention employs a varying duty cycle of a pulse width modulated switch at slow machine speeds and close to constant duty cycle at high machine speeds. Prior art permanent magnet machines also have generally used separate inductors to provide voltage boost. In contrast, the present invention may use the inductance of the PMM to provide a voltage boost instead of using a separate inductor for this purpose. 
         [0026]      FIG. 1  shows a radial cross-section of a synchronous high reactance PMM  10  having a laminated tooth stator in accordance with one embodiment of the present invention. It will be appreciated by those skilled in the art that a HRPMM may be similar in construction to a conventional reactance PMM, with the basic difference being in the machine synchronous-reactance value, as determined by various design parameters described in more detail below. The HRPMM  10  may include a stator  12  having a plurality of stator windings  14  disposed in slots  16  between a plurality of stator teeth  18 . It may be noted that the present invention could also be practiced using a stator ring toothless design. The stator  12  may be liquid or gas cooled in a conventional manner by placing a housing and cooling passages around the back iron. A number of alternative cooling and housing approaches could be implemented. 
         [0027]    Also shown in  FIG. 1  is a rotor  20  which may be a permanent magnet two-pole rotor suitable for high-speed implementations. In lower speed applications the present invention could also be implemented with a rotor having more than two poles. The rotor  20  may include a magnet  22  enclosed in an inconel sleeve  24  for structural integrity. An air-gap  28  may exist between the stator  12  and the rotor  20 , which may provide a cooling air passage. A bore seal  26  may be placed in the air-gap to create a separation between stator  12  and rotor  20 , if required. Additional cooling flow, typically air, can be provided in the air-gap  28  for a better thermal result if required. The losses of this machine may be primarily concentrated in the stator. The losses in the rotor may be negligible. 
         [0028]    A conventional aluminum housing  30  may surround the stator  12 . An aluminum spacer  32  with air cooling slots  34  may be provided between the housing  30  and the spacer  32  to provide additional means for cooling the HRPMM  10 . 
         [0029]      FIG. 2  shows the V-I characteristics of the HRPMM  10  of the present invention for a specific application at different speeds. In particular, 
         [0030]      FIG. 2  shows the amplitude of the machine terminal voltage at  38 ,  40  and  42  as a function of the current through the load. Curves  110 ,  112 ,  114 ,  116  and  118  show the V-I characteristics of the HRPMM  10  at 1,250 Hz, 640 Hz, 320 Hz, 160 Hz and 80 Hz respectively. Curve  110  represents the V-I curve at the highest operating speed, where the frequency is 1250 Hz. The back emf voltage on curve  110  is 118 Vrms L-N (line-to-neutral). For each of the curves  110 ,  112 ,  114 ,  116  and  118 , as the current through the load  64  (shown on the horizontal axis) increases, the machine terminal voltage at terminals  38 ,  40 , and  42  decreases. The emf voltage across terminals  38 ,  40 , and  42  may reduce linearly with the speed reduction. The curve  118  at the lowest speed of 80 Hz appears below all the other curves. At that speed of 80 Hz shown in curve  118 , the frequency is 80 Hz and the back emf voltage is 7.55 Vrms. 
         [0031]    The short-circuit current point  120  may be approximately the same for all V-I curves. This phenomenon may be due to the relation expressed in equation 2, as shown below, where R S  can be ignored with a good approximation for practical purposes. While not explicitly shown in  FIG. 2 , it can be appreciated that for different speeds, E EMF  and X S  may be changing linearly. Therefore, the ratio E EMF  over X S  may be constant for different speeds, which represents the short-circuit value. 
         [0032]      FIG. 3  is a block diagram showing additional details of the HRPMM based PGS  20  in accordance with one embodiment of the invention. In particular,  FIG. 3  shows a voltage control circuit  36 . Three machine terminals  38 ,  40  and  42  attached to the HPRMM may supply a three phase AC voltage to the voltage control circuit  36 . Six diodes  44 ,  46 ,  48 ,  50 ,  52  and  54  may be arranged to form a three phase bridge rectifier circuit  56  connected to input terminals  57 ,  59 . A solid-state switch  58  may be connected in parallel with the rectifier diodes  44 - 54  to short the input terminals  57 ,  59  through the rectifier. Solid state switch  58  may be a conventional MOSFET or IGBT transistor. A diode  60  may be connected between the solid state switch  58  and a capacitor  62  to prevent reverse discharge of the capacitor  62  during the shorting period. Capacitor  62  and load  64  may both be connected across output terminals  69 ,  71 . The capacitor  62  may be connected in series with the diode  60  such that it may be charged and supply load  64  with 34 Vdc. Capacitor  62  also may filter out voltage ripple due to the rectification and switching. Pulse width modulation (PWM) control circuit  66  may use PWM to drive the solid state switch  58  to maintain the desired 34 Vdc at the capacitor  62  terminals. The PWM frequency may be selected constant at 20 KHz. A protection device  67  may comprise a variety of devices such as resistors, capacitors and thyristors, and is provided to protect the HRPMM  10  in various failure modes, such as the failure of solid state switch  58  in an open state. 
         [0033]    In accordance with one embodiment of the invention, the HRPMM  10  may be designed with particular dimensions and materials to meet certain requirements. As a high reactance PMM it should have a synchronous reactance in the range of 1 m to 10 m. Also, as described below, the HRPMM may be configured such that the operating current across terminals  69 ,  71  is equal to the short-circuit current, that is, the current across terminals  69 ,  71  when load  64  is shorted, as described below. 
         [0034]    In accordance with the invention, an HRPMM  10  that meets the above-discussed objectives is configured with various PMM parameters determined as described below. Key parameters of a HRPMM may be the phase-generated voltage E EMF , and the synchronous impedance of the machine Z S . If these two values are known explicitly, the mathematical analysis of the HRPMM may be relatively straightforward. The generated current, 1M, can be calculated, utilizing circuit analysis theory, as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     M 
                   
                   = 
                   
                     
                       
                         E 
                         EMF 
                       
                       
                         
                           Z 
                           S 
                         
                         + 
                         
                           Z 
                           L 
                         
                       
                     
                     = 
                     
                       
                         E 
                         EM 
                       
                       
                         
                           ( 
                           
                             
                               R 
                               S 
                             
                             + 
                             
                               j 
                                
                               
                                   
                               
                                
                               
                                 X 
                                 S 
                               
                             
                           
                           ) 
                         
                         + 
                         
                           [ 
                           
                             
                               R 
                               L 
                             
                             + 
                             
                               j 
                                
                               
                                 ( 
                                 
                                   
                                     X 
                                     L 
                                   
                                   - 
                                   
                                     X 
                                     C 
                                   
                                 
                                 ) 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0035]    In Equation (1), R S  is the stator winding resistance and X S  is the synchronous reactance. The load  64  is represented by R L  (load resistance), X L  (reactance) and X C  (load admittance). The load resistance absorbs the real power delivered by the generator. The reactance represents the reactive load with inductive nature and the admittance represents the reactive load with capacitive behavior. 
         [0036]    The short-circuit current of the HRPMM  10 , for example the current at terminals  38 ,  40 , and  42  when the load  64  is shorted, can be obtained from Equation (1) by postulating the load parameters to equal zero. The result is Equation (2). 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     SC 
                   
                   = 
                   
                     
                       
                         E 
                         EMF 
                       
                       
                         Z 
                         S 
                       
                     
                     = 
                     
                       
                         E 
                         EM 
                       
                       
                         ( 
                         
                           
                             R 
                             S 
                           
                           + 
                           
                             j 
                              
                             
                                 
                             
                              
                             
                               X 
                               S 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0037]    The short-circuit current depends primarily on two basic machine parameters, E EMF  and Z S . For a conventional PMM, E EMF  and Z S  may be selected such that the short-circuit current is several times larger than the operating or nominal current. A reactance-per-unit quantity can be introduced to define the relative reactance (reactance per unit) X PU =I RATED /I SC . For a conventional reactance machine, X PU  may be from 0.2 to 0.3. In contrast, for the HRPMM  10  in accordance with one embodiment of the invention, E EMF  and Z S  are selected in such a way that the short-circuit current between terminals  69 ,  71  is similar to the operating current and X PU  is from 0.8 to 1.0. One skilled in the art will appreciate the particular physical and electrical features of the HRPMM  10  that may be configured using known design techniques to achieve this X PU . 
         [0038]      FIG. 4  shows a curve  122  of the duty cycle of the solid state switch  58  as a function of the machine frequency, which is linearly proportional to the speed. The curve  122  representing this relationship is called a regulation curve. The PWM control circuit  66  may use a closed loop control system to measure the output voltage across terminals  69 ,  71  and across the load  64 . Connections  68  and  70  are connected to output terminals  69  and  71  respectively. PWM control circuit  66  generates a PWM signal in connection  72  based on the measured output voltage across terminals  69 ,  71  such that it may maintain the voltage of the load at 34 Vdc. 
         [0039]    There are two distinct regions in the regulation curve  122 : a boost region  124  and a current limiting region  126 . The boost region  124  occurs when the rectified machine voltage across input terminals  57  and  59  is below the output regulated voltage across terminals  69 ,  71 . In the boost region  124 , approximately in the range of 80 Hz to 180 Hz, the solid state switch  58  may short the machine terminals  57 ,  59  in order to increase the current across terminals  57  and  59  and store more energy in the machine winding  14 . Upon the opening of solid state switch  58 , the energy may be released from the winding  14  to the capacitor  62 . In this way, a voltage boosting operation, which increases the output voltage across terminals  69 ,  71 , may be achieved. Boosting operation is described in equation 3 where V out  is the output regulated voltage across terminals  69 ,  71 , V L-L P  is the input boosted voltage across terminals  57 ,  59 , which is in fact the machine line-to-line peak voltage, and D is the duty cycle. The duty cycle is defined as D=t on /T where T is the period of the PWM signal on line  72  for solid state switch  58  and t on  is the on-time of the PWM signal. Equation 3 does not account for the non-ideal characteristics of the diodes, switches and electric machine. 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     out 
                   
                   = 
                   
                     
                       V 
                       
                         L 
                         - 
                         LP 
                       
                     
                     
                       1 
                       - 
                       D 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0040]    When the HRPMM  10  operates at the lowest speed, the frequency may be 80 Hz and the back emf voltage may be 7.55 Vrms L-N, as shown in curve  118  in  FIG. 2 . At this point the solid state switch  58  may operate at 70 percent duty cycle, as seen in the boost region  124  of curve  122  in  FIG. 4 , to keep the voltage across the load  64  at 34 Vdc. 
         [0041]    As the speed of the HRPMM  10  is increased in the boost region  124 , the frequency and back emf voltage increase and lower duty cycle may be required. Another data point  128  shown in  FIG. 4  is at 100 Hz and 9.44 Vrms respectively, where the duty cycle may be reduced to 51.8 percent. The PWM duty cycle may continue to drop gradually until it is almost equal to zero at frequency=180 Hz and the back emf voltage is 16.99 Vrms L-N, which is shown at data point  130 . When the frequency increases slightly above 180 Hz, the PWM duty cycle may increase again rapidly from zero to close to 80 percent at data point  132 . 
         [0042]    Above about 210 Hz, for example, at data point  134 , the voltage control circuit  36  may transition from the boosting mode  124  to the current limiting mode  126 . The duty cycle of the regulation curve may remain relatively constant for the high frequencies of the current limiting region  126 . Thus, in the lower speed region  124 , a higher duty cycle may boost the output voltage, but in the high speed region, from about 180 Hz to about 210 Hz, increases in the duty cycle will lower the output voltage. Above 210 Hz the duty cycle has a small effect on output voltage, because of current limiting as discussed below. A characteristic point  136  may be at the highest speed of operation where the frequency may be 1,250 Hz, the back emf voltage may be 118 Vrms L-N, and the PWM may operate at 79.38 percent duty cycle. Different duty cycles values can be expected at different load values and during transients. These transients can be expected when fast speed or load changes occur. 
         [0043]    In the current limiting mode of operation, the voltage control circuit  36  may use the synchronous reactance of the HRPMM to limit the output current. In particular, the elements of the WSR HRPMM  10  shown in  FIG. 1  may be selected to increase the total impedance of the machine so as to create a desired synchronous inductance and thus, a leakage impedance. The synchronous inductance can be accurately defined and controlled by defining an appropriate shape or configuration for the stator  12 , and by selecting appropriate materials for construction of the stator  12 . 
         [0044]    A simulation of the WSR HRPMM based EPGS shown in  FIG. 3  may be used to confirm the expected results. In one simulation at frequency=120 Hz with input voltage 11.325 Vrms, the duty cycle of the PWM was 38.7 percent. Also, in the simulation the output voltage at the load was very close to 34 Vdc with current value very close to 11.76 Amps. Therefore, the delivered power to the load is close to 400 W, which is the desired power at the load.  FIG. 5  shows the curves of the output voltages and currents at the solid state switch  58  and at the load  64  for one such simulation. In particular, the current at the load  64  is shown at line  74  is constant at 11.76 A. The current at switch  58  is shown at line  76  is 18.27 A. The current out of the rectifier circuit  56  is shown at line  78  and the voltage output at the load  64  is constant at 34V as shown at line  80 . Line  82  shows the voltage at solid state switch  58  with is 34.32V. 
         [0045]      FIG. 6  shows a flowchart of a process  84  of regulating voltage in a WSR HRPMM based PGS in accordance with one embodiment of the invention. Step  86  may comprise starting voltage control circuit to WSR HRPMM based PGS. The voltage control circuit  36  may comprise the diode bridge  56 , the solid state switch  58  and the PWM control circuit  66  shown in  FIG. 3 . In step  88  the output voltage across terminals  69  and  71  may be detected. 
         [0046]    Step  92  may determine if the speed of the WSR HRPMM is in the low range. This low range may correspond to the frequency range in  FIG. 4  of about 80 Hz to about 180 Hz. If step  92  determined that the speed was in the low range, then the process  84  may move to step  94  to determine if the detected output voltage is lower than the desired output voltage. If so, the duty cycle of the PWM controller circuit  66  may be increased in step  98 , which may have the effect of boosting the voltage. The process  84  may next return to step  88 . 
         [0047]    If step  94  determines that the detected output voltage is higher than the desired output voltage, step  96  will decrease the PWM duty cycle to lower output voltage. The process  84  may next return to step  88 . 
         [0048]    Returning now to step  92 , if is determined that the WSR HRPMM is not in the low speed range, then the process may move from step  92  to step  100  which may determine if the detected output voltage is lower than the desired output voltage. If it is, the process moves to step  102  where the duty cycle is decreased to increase the output voltage. If step  100  determined that the measured output voltage was not lower than the desired output voltage, then step  104  will increase the duty cycle to decrease the output voltage, after which the process  84  may return to step  88 . 
         [0049]    As can be appreciated by those skilled in the art, the present invention provides an WSR HRPMM  10  that can deliver regulated voltage to a DC power distribution bus with a number of advantages. It can operate over an extended speed range of up to a factor of 20. It provides a simple power topology, using only one switch and seven diodes. In boost operation at low speed no dedicated inductance is required because the inductance of the electric machine is used as an energy storage element. In current limiting operation at high speed the synchronous reactance of the HRPMM is used for current limiting. 
         [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.