Abstract:
A permanent magnet generator having the unique feature of a speed proportionally adjusted air gap for self-regulation of coil output voltage over a wide range of operating rotational speed of a steam turbine to which the invention is coupled. The Permanent Magnet Generator rotor is supported by the turbine end shaft and the stator is supported by a bracket bolted to the turbine pedestal base or other rigid structure. The speed proportional air gap is accomplished through the use of a plurality of centrifugal flyweights in mechanical coupling to a spool piece under spring load and to corresponding rare earth magnets via linkage such that increasing rotor speed extends the flyweights outward from the rotor center of rotation and draws the rare earth magnets closer to the rotor center of rotation and thus increases the air gap.

Description:
RELATED APPLICATIONS 
       [0001]    This is a continuation-in-part of co-pending U.S. Ser. No. 13/233,805, filed 15 Sep. 2011. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention is a permanent magnet generator designed to be coupled with a power source such as a steam turbine. It is ideally suited for application in nuclear power plants. Natural disasters, for example the May 2011 disaster following the tsunami at the Tokyo Electric Fukushima Nuclear Power Plant, evidence a flaw in the design of emergency reactor cooling water systems with potentially devastating consequences. Lessons learned from past disasters include the irrefutable conclusion that the electrical power primary and backup systems feeding the reactor core cooling injection pump drive steam turbines are subject to failure by natural disaster leading to a potential reactor core melt down, danger to and loss of human life, and long term irreversible environmental damages. 
         [0003]    In order to lessen the probability of disaster and its associated consequences, the steam turbine controls for the Safety Related (as defined by the United States Congressional Federal Register 10 CFR 50 Part B) steam turbines need to have no reliance upon the plant power feeds. The Safety Related steam turbine control components of the speed governor and electric valve actuator for operating the turbine governor valve need to be fully self-powered by a source of the mechanical energy of the controlled turbine, thereby independent of the external power sources or plant-run power feeds that are commonly subject to failure in a natural disaster. 
         [0004]    A complication of Safety Related turbine speed control is the “open governor valve” start position to place the turbine in operation. To be prepared for emergency pumping tasks, the Safety Related turbine governor valve actuator has the governor valves initially open in a fail-safe position under spring load. When the steam pressure is applied to the turbine (by an external valve), the turbine immediately begins acceleration from rest. In common applications, there is no acceleration control. Some nuclear plants have lessened acceleration by implementing small bypass steam lines admitting less steam flow and resulting in more gradual turbine rotor acceleration, but all operate on a similar starting logic. Any proposed turbine speed control system has the task of becoming functional at a low turbine shaft speed, at or near 500 revolutions per minute (RPM), and responding to limit the initial speed surge. Failure to respond by closing the turbine governor valve to the speed throttling position quickly results in excess acceleration and turbine over-speed trip, or safety shut-down of the turbine. Original equipment turbine speed control systems from the previous century were plagued with poor responding hydraulic control systems which often could not retard the acceleration quick enough due to susceptibility to operating oil contamination, air in the hydraulic oil and friction from long term inactivity. 
         [0005]    Conventional permanent magnet generators can be coupled to turbine shafts to produce electrical power, but cannot provide electrical power over the required wide speed range, typically 500 RPM to 5,000 RPM due to basic electromagnetic properties. If a permanent magnetic generator coupled to a Safety Related turbine is designed for proper coil output voltage at 500 RPM for a control system power feed, the coil output voltage will increase proportional to further turbine speed increase. This results in a ten-fold over-voltage output at 5,000 RPM which exceeds potentials and which will likely destroy electrical components in the rectification and shunt voltage regulation or limitation circuits. 
         [0006]    Newer generation Safety Related turbine speed control designs have implemented the use of electric actuators utilizing electric motors and roller screw engagement devices to position governor valves. Although the electric actuator represents a vast improvement over the previous hydraulic systems in reliability and reduced maintenance requirements, until this invention there was no means to power the electric actuator and connecting servo drive other than with plant AC or DC busses which are typically the items of failure in a natural disaster, including the tsunami at Fukushima. 
         [0007]    Previous work has established some degree, but not total turbine self-powering. For example, U.S. Pat. No. 5,789,822 to Calistrat et al. utilizes the low power generation of magnetic speed probes to self-power the electronic governor, but does not address the much greater electrical power demand of operating the governor steam valve and therefore must use a non-electric, hydraulic-positioned governor valve with accompanying high failure potential and complexity. 
         [0008]    Other work has centered on designs of permanent magnet generators for general applications which either have no voltage regulating capability or use complex electrical apparatus to compensate for limited variable speed operation. Due to the required radiation survival criteria for Safety Related turbine applications, complex electrical apparatus is not feasible, nor reliable, and the extreme range of speed of operation of a Safety Related turbine, again typically 500 RPM to 5,000 RPM, at a 1:10 ratio, is beyond the compensating ranges of the prior art. Any suitable device must withstand an environment having radiation levels on the magnitude of 10 5  rads. 
         [0009]    Further art has uniformly centered on devices and configurations to improve the generation efficiency of permanent magnet generators, but none is like the subject invention which utilizes a decrease in generation efficiency to simplify regulation and make the power feed system more robust with fewer failure potential components. 
         [0010]    The physical components of permanent magnet generators in basic form consist of sets of permanent magnets and wire-wound coils in proximity under a relative velocity. A key property of permanent magnet generators is the magnet-to-coil proximity, also known as the “air gap”. The magnetic flux density of the magnets decreases proportionately with the magnitude of the air gap. The generated voltage across a coil is proportional to the flux density at a given relative velocity, and increases proportionately with relative velocity. 
         [0011]    The generated voltage can be expressed with the following formula: V=N dI′/dt where 
         [0012]    V=voltage generated at each stator coil 
         [0013]    I′=instantaneous value of the magnetic flux cutting the stator coil under magnetic rotation 
         [0014]    I=peak magnetic flux density at near-zero air gap 
         [0015]    s=air gap distance 
         [0016]    N=number of stator coil turns and 
         [0017]    I′=I/s 
         [0018]    Therefore a voltage compensation of increasing velocity can be accomplished by simultaneous increasing the air gap at the cost of decreasing generator efficiency. Since efficiency is of minor importance in light of Safety Related turbine operation, and the overall device mechanical load on the turbine is small, sacrificing efficiency for robust power generation is a good trade off. 
       Definitions 
       [0019]    The following definitions are to be given to terms used herein. These definitions are in addition to the customary definitions of the terms. If a conflict should arise, the subject term is to be given both definitions.
       10-CFR-50 Part B The United States Congressional Federal Register reference for rules concerning nuclear plant equipment.   Air Gap The distance between the stator coil and the rotor magnet of a brushless generator device.   Centrifugal Flyweight A weight in a rotor which pivots about a pin at a radius at increasing angle with rotor speed.   Electric Valve Actuator A brushless motor operated  mechanical screw assembly which when the motor turns in either direction causes an operating rod to extend and retract, thus converting the rotary motion to linear motion, for example a linear stroke.   Permanent Magnet Generator An electrical generator consisting of one or more permanent magnets in a rotating element and one or more wire coils in a stationary element.   Rare Earth Magnet High magnetic flux density permanent magnets constructed of Neodynmium Iron Boron, Samarium Cobalt, Ceramic, or Alnico materials.   Safety Related turbine Steam turbine which drives water pumps in nuclear power plants that are defined as emergency cooling devices in 10-CFR-50. The most common safety related turbines are:
           Auxiliary Feed Pump Turbines and Emergency Feed Pump Turbines in pressurized water reactor plants.   Reactor Core Coolant Injection, High Pressure Safety Injection, and Low Power Safety Injection turbines in boiling water reactor plants.   
           Servo Drive An electronic control device which operates a brushless motor by constructing three phase alternating current from a direct current feed bus, employing isolated gate bipolar transistors to rapidly switch power waveforms under timing control of a digital signal processor.   Steam Turbine A prime mover which converts steam enthalpy to rotational torque at various speeds.   Turbine Governor Valve The speed and load controlling steam valve on a steam turbine.   Voltage Regulation The process of maintaining stable voltage under varying load and generation conditions.       
 
       SUMMARY OF THE INVENTION 
       [0033]    The present invention comprises a permanent magnet generator which is designed to couple to a power source, such as a turbine output shaft. The permanent magnet generator utilizes an internal mechanism consisting of a plurality of centrifugal fly weights positioning a coil spring-opposed spool piece which is in turn linked mechanically to magnets in respective alignment to provide magnet travel towards the center of rotation with increased speed due to the greater centripetal force of the flyweight assembly than the rare earth magnet assembly. A plurality of stater coils are positioned along the interior of an annular ring. The resulting magnet motions increase the air gap between the magnets and the stator coils lending fewer flux lines and less flux density to generate less voltage. This increasing air gap action with increased rotational speed is used to counter the inherent increase in coil voltage output in a self-regulating manner. 
         [0034]    The permanent magnet generator of the present invention includes a rotor with a plurality of radially positioned linearly movable magnets, the rotor mounted to a steam turbine output shaft, a stationary annular stator with a plurality of radially positioned coils positioned about the rotor and the plurality of magnets being movable to vary an air gap between the magnets and the coils. The centrifugal flyweights may be mechanically coupled with linkage bars and a spring-opposed spool piece to each magnet thereby providing an increasing magnet-to-coil air gap with increasing steam turbine output shaft speed. The magnets are movable responsive to a rotational speed of said rotor. The magnets have sufficient radial position travel to reduce a flux density for purposes of regulating a voltage output from the coils. The centripetal force of the flyweights applied over a pivot moment is greater than the centripetal force of the magnets as applied to the linkage bars and spool piece with the difference in centripetal forces resisted by a coil spring adjacent to the spool piece thereby metering net magnet motion with speed. In my preferred embodiment, the magnets are rare earth magnets. One or more coils may provide a governor speed feedback, the governor speed feedback may in turn be coupled to a steam turbine speed control system. A shunt circuit coupled to at least one coil provides a feed bus for controlling turbine speed. 
         [0035]    In another embodiment, and similar to the previously described embodiment, the permanent magnet generator includes a rotor with a plurality of radially positioned linearly moveable magnets, the rotor mounted to a steam turbine output shaft, a stationary annular stator with a plurality of radially positioned coils positioned about the rotor and the plurality of magnets being moveable to vary an air gap between the magnets and the coils. In this embodiment, the centrifugal flyweight assemblies and linkage bars are replaced by multiple pivot arm assemblies, as will be described. 
         [0036]    The invention may also be described as a steam turbine speed control system including the permanent magnet generator described above coupled to a steam turbine governor valve. Alternatively, the permanent magnet generator may include a rotor coupled to a turbine output shaft, the rotor housing a plurality of linearly movable magnets radially arranged about said rotor, each of said magnets being coupled to a centrifugal flyweight, an annular stator having a plurality of coils being positioned about said rotor and a variable air gap being formed between each magnet of said plurality of magnets and each coil of said plurality of coils depending upon a rotational speed of the rotor. Again, each centrifugal flyweight may be coupled to a spring-opposed spool piece by one or more linkage bars. The magnets have sufficient radial position travel relative to the coils to reduce a flux density for regulating voltage output from the coils. Four magnets and four coils are preferred; however, other equivalent counts of magnets and coils could be used in the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is a side view of the permanent magnet generator; 
           [0038]      FIG. 2  is a front view of the permanent magnet generator; 
           [0039]      FIG. 3A  is a section view of the permanent magnet generator taken at line  3 A- 3 A in  FIG. 2 ; 
           [0040]      FIG. 3B  is a similar section view taken at line  3 B- 3 B in  FIG. 2 ; 
           [0041]      FIG. 4  is a section view of the permanent magnet generator taken at line  4 - 4  in  FIG. 1 ; 
           [0042]      FIG. 5  is a side view showing the permanent magnet generator coupled to a turbine output shaft; 
           [0043]      FIG. 6  illustrates the connection of the stator coils to a rectifier and shunt circuit to complete the control system power supply and the speed signal output; 
           [0044]      FIG. 7  illustrates the permanent magnet generator in a block diagram connected to the shunt circuit, turbine governor and turbine governor valve electric actuator; 
           [0045]      FIG. 8  is a side view of the end plug assembly and linkage bar connections; 
           [0046]      FIG. 9  is a section view of the end plug assembly and linkage bars taken at line  9 - 9  in  FIG. 8 ; 
           [0047]      FIG. 10  is a side view of the spool piece assembly and linkage bars; 
           [0048]      FIG. 11  is a section view of the spool piece assembly and linkage bars taken at line  11 - 11  in  FIG. 10 ; 
           [0049]      FIG. 12  is a perspective view of a flyweight pivot bracket; 
           [0050]      FIG. 13  is a perspective view of a flyweight; 
           [0051]      FIG. 14  is a perspective view of a magnet assembly and linkage bars; 
           [0052]      FIG. 15  is a side view of another permanent magnet generator; 
           [0053]      FIG. 16  is a front view of the permanent magnet generator shown in  FIG. 15 ; 
           [0054]      FIG. 17A  is a section view of the permanent magnet generator illustrated in  FIGS. 15 and 16  and taken at line  17 A- 17 A of  FIG. 16 ; 
           [0055]      FIG. 17B  is a section view of the permanent magnet generator illustrated in  FIGS. 15 and 16 , taken at line  17 B- 17 B of  FIG. 16 , and showing the rotational motion of the pivot arms under increasing turbine speed; 
           [0056]      FIG. 18  is a section view of the permanent magnet generator illustrated in  FIG. 15  and taken along line  18 - 18  thereof; 
           [0057]      FIG. 19A  is an exploded view of a pivot arm and magnet assembly for use with the generator shown in  FIGS. 15-18 ; and 
           [0058]      FIG. 19B  is a fragmentary section view of the pivot arm assembly illustrated in  FIG. 17A  and taken along line  19 B- 19 B thereof. 
           [0059]      FIGS. 20A and 20B  are views illustrating movement of pivot arm and attached magnet assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0060]    The permanent magnet generator  10  of the present invention couples to a source of rotational motion, such as a turbine. The permanent magnet generator consists primarily of two components including a rotor assembly  52  and a stator assembly  110 . As shown in  FIGS. 1 ,  2  and  5 , the turbine mating flange  20  bolts to the turbine output shaft through flange bolt holes (not shown) or may thread on as per the turbine end shaft design, and also bolts to the inboard cylinder  22 . Inboard cylinder  22  in turn bolts by flange  24  to the outboard cylinder  26  and forms the rotor assembly  52 . The turbine bearings (not shown) support the permanent magnet generator rotor  52 . The permanent magnet generator  10  stator assembly  110  supporting structure consists of a base  130 , inboard stator bracket  112  and outboard stator bracket  114  forming a welded assembly with the stator wheel  122  affixed between the stator brackets  112  and  114 . In operating position, the stator wheel  122  is centered about the rotor outboard cylinder  26 . Support brackets  116  and  118  also secure the inboard stator bracket  112  and outboard stator bracket  114  to the base  130 . 
         [0061]    As further shown in  FIG. 1 , rotor assembly  52  components of the permanent magnet generator include flyweight assemblies  30 , flyweight brackets  36 , spool piece assembly  40 , a coil spring  50  and end plug  90 . 
         [0062]    Rotational motion from the turbine output shaft  12  is imparted to the inboard  22  and outboard  26  cylinders that make up the rotor assembly  52  through flanges  20  and  24 . Now referring to  FIGS. 3A and 3B , outward motion of the flyweight assembly  30  under increasing rotational speed of the rotor assembly  52  causes the flyweights  32  to pivot about pins  34  fixed by the flyweight bracket  36  and transmit force to a spool piece assembly  40  which freely moves within a bushing  42 . Flyweight assemblies  30  are in positions at an initial functioning rotational speed of 500 RPM as shown in  FIG. 3A  and in positions at a rotational speed equivalent to the over speed trip set point of the turbine as shown in  FIG. 3B . They change position as shown by arrows A in  FIG. 3B . The spool piece assembly  40  motion and resulting force is opposed by a coil spring  50  located within the inboard cylinder  22 . As the spool piece assembly  40  is displaced in the inboard direction as shown by arrow B in  FIG. 3B , connected inboard linkage bars  60  pull at the lower pivot points of the magnet assembly  70  and rotate outboard linkage bars  64  about the pivot pins  82  of the end plug assembly  90  with the effect of the rare earth magnets  72  moving within their magnet bushings  74  and being drawn inward and away from the stator coils  120  as shown by arrows C. This action increases the permanent magnetic generator  10  air gap  100  with increasing turbine speed and provides the basis for voltage regulation. 
         [0063]    The sectional view of  FIG. 4  shows the linkage connections at the magnet assembly  70  and shows one of many configurations of four magnets  72  and four or an equivalent number of stator coils  120 . It is to be understood that other counts and configurations of magnets  72  and coils  120  could be used without departing from the present invention. 
         [0064]    Preferred but not essentially specified materials: inboard cylinder  22 , flange  24 , outboard cylinder  26  are each aluminum. The turbine mating flange  20 , spool piece assembly  40 , end plug assembly  90 , stator brackets  112 ,  114 ,  116  and  118  are each nickel-plated steel. The spool piece bushing  42 , the magnet bushing  74 , and all linkage bar pivot bushings are oil-impregnated bronze. The stator wheel  122  is a phenolic material. The stator coils  120  are magnet wire coils potted to the stator wheel  122  with an appropriate compound. The flyweights  32  are dense-alloy. The magnets  72  are neodymium iron boron cemented to a nickel-plated steel cup  76  with pivot bosses  68 . The coil spring  50  is spring steel or stainless steel. 
         [0065]    The size and number of magnets  72 , stator coils  120  and number of wire turns, and gauge of magnet wire are determined by the power requirements of the control system of the target steam turbine unit at low speed, 500 RPM typically. This power is small by conventional generator comparison, falling between 500 Watts and 2,000 Watts. While not required, rare earth magnets are preferable in the magnetic assemblies  70 . The flyweight assembly  30  mass is then adjusted to produce a force on the spool piece assembly  40  at the maximum target turbine operating speed (shown in  FIG. 3B ) sufficient to displace the magnet assemblies  70  inward a distance to reduce the magnetic flux density and thus the coil voltage proportional to the inverse ratio of the maximum target turbine operating speed divided by 500 RPM. 
         [0066]      FIG. 5  shows the mechanical installation of the permanent magnet generator  10  on the target turbine output shaft  12  including two part coupling  14  having a flange  16  connected to the turbine end shaft. As described above, flange  20  is connected to the inboard cylinder  22 . Flanges  16  and  20  are bolted together as shown. The stator base  130  is also shown in  FIG. 5  bolted to the turbine pedestal base  18 . 
         [0067]      FIG. 6  schematically depicts the typical permanent magnet generator  10  electrical connections. The plurality of stator coils  120  are wired to a full wave rectifier diode bank  140  which has its output smoothed by an alternating current capacitor  142  and then fed through resistor  144  to the voltage shunt circuit  146  comprised of a Zener diode  148  and transistor  150 , with direct current capacitor  152  attaching to the final direct current supply output  154 . In addition, a rectified coil leg  156  is tapped for use as a speed reference output  158 . The permanent magnet generator  10  of the invention provides a steady shunt current to pass the transistor  150  when the turbine is operating at speeds greater than 500 RPM. This shunt current is available immediately to supply at all times if the attached, load increases. This shunt current reduces the size required of the direct current capacitor  152 . 
         [0068]      FIG. 7  include all components of the turbine speed control system  180  powered by the permanent magnet generator  10 . The permanent magnet generator  10  feeds the rectifier  140  which in turn feeds the voltage shunt  146  of which output becomes the positive DC bus. This positive DC bus powers both the electronic governor  160  and the servo drive  162 . The electronic governor  160  produces a bipolar velocity demand output proportional to speed error which is input to the servo drive  162 . The servo drive  162  establishes the governor-requested velocity of the electric valve actuator  164  operating shaft  166  which is coupled to the turbine governor valve  168 . The turbine governor valve  168  is situated between the turbine steam supply conduit  170  and the conduit  172  that leads to the turbine nozzles (not shown). Thus positive speed error results in proportional turbine governor valve  168  opening velocity and conversely negative speed error results in proportional turbine governor valve  168  closing velocity. 
         [0069]      FIGS. 8 and 9  detail the end plug assembly  90 . Each outboard linkage bar  64  connection is made using a shoulder screw/pivot pin  82  recessed into the end plug body  98  and mounting through oil-impregnated bronze bushings  96  within the outboard linkage bars  64 . 
         [0070]      FIGS. 10 and 11  show the spool piece assembly  40 , consisting of spring seat  44 , hardened washer  46  and body  48  attaching to linkage bars  60  using shoulder screws  66  in the same manner as the end plug assembly  90  of  FIGS. 8 and 9 . 
         [0071]      FIG. 12  depicts the flyweight bracket  36 . The bracket  36  includes an opening  38  through which each bracket  36  is coupled by pivot pins  34  to each flyweight  32 . 
         [0072]      FIG. 13  depicts the flyweight assembly  30  including flyweight  32 , pressed bushing  54 , and roller  56  with axle  58 . 
         [0073]      FIG. 14  details the magnet assembly  70 . Each rare earth magnet  72  is bonded to the nickel-plated magnet cup  76 . A yoke  78  is attached by pins  80  and optional bushings (not shown) to both the steel cup mounting boss  68  and the inboard linkage bars  60  and outboard linkage bars  64 . 
         [0074]    With reference now to  FIGS. 15-19 , another preferred embodiment of a permanent magnet generator  200  may be seen. As in the previous embodiment, the permanent magnet generator  200  of this embodiment couples to a source of rotational movement, such as a turbine output shaft  12 . Similar to the previously described permanent magnet generator  10 , the permanent magnet generator  200  in these views includes a rotor assembly  52 A and a stator assembly  110  (including stator wheel  122 ), the stator assembly  110  being substantially identical to that described with reference to permanent magnet generator  10 . The permanent magnet generator  200  further includes a cylinder  28  having an end plug  90 . The permanent magnet generator  200  stator assembly  110  supporting structure consists of a base  130 , and brackets  112  and  114  forming a welded assembly with the stator wheel  122  affixed between the stator brackets  112  and  114 . In operating position, the stator wheel  122  is centered about the cylinder  28 . Support brackets  116  and  118  also secure the stator brackets  112  and  114  to the base  130 . 
         [0075]    With particular attention to  FIGS. 17A ,  19 A and  19 B, the rotor assembly  52 A components of the alternative permanent magnet generator  200  and pivot arm assemblies  210  may be seen. As shown particularly in  FIGS. 19A and 19B , a pivot arm assembly  210  includes a pivot arm  212 , pivot rod  214 , pivot block  216 , torsion spring  218 , and bushings  220 . The pivot arm  212  includes a distal end  224  and a proximal end  226 . The proximal end  226  has a surface area designed to mount the flyweight  32 , and the distal end  224  is pivotally coupled to the magnet assembly  270 . As shown, the flyweight  32  is preferably bolted directly to the proximal end  226 . It is to be understood that the flyweight  32  may be comprised of single or multiple components, depending on desired use. The pivot arm  212  also includes a pivot rod aperture  230  to receive the pivot rod  214  therein. A torsion spring  218  is attached in holes  232 A,  232 B in the pivot rod  214  and pivot block  216 , respectively. The pivot arms  212  are pinned by the pivot rods  214  to pivot blocks  216  with mechanically limited freedom of motion set by arm-to-block internal clearance. The torsion springs  218  provide a preload force in such direction to yield the outward magnet assembly  270  travel limit. This is accomplished by the torsion spring  218  fixed in holes  232 A,  232 B. With continued attention to  FIG. 19A  and also to  FIG. 19B , the pivot arm assembly  210  may be seen to include pins  238 . Pins  238  secure the pivot rod  214  to the pivot arm  212 . The pins  238  are preferably inserted into apertures  240  in the pivot arm  212  and further into aligned apertures  242  in the pivot rod  214 . 
         [0076]    With specific reference to  FIGS. 17A and 17B , motion of the pivot arm assemblies  210  under increasing rotational speed of the rotor assembly  52 A may be seen. As shown, the pivot arms  212 , along with attached flyweights  32  pivot about pivot rods  214 . The pivot arms  212  are fixed by the pivot rods  214  to pivot blocks  216 . As seen in  FIG. 17B , increased rotational speed pivots the pivot arm proximal end  226  in the direction of arrows A. The pivot arm assemblies  210  are in positions at an initial functioning rotational speed of 500 RPM as shown in  FIGS. 17A and 20A , and in positions at a rotational speed equivalent to the over speed trip set point of the turbine as shown in  FIGS. 17B and 20B . The initial position of each pivot arm  212  as shown in  FIGS. 17A and 20A , is set by the torsion spring  218  rotating the pivot arm  212  on the pivot rod  214  until the limit of arm to block  216  clearance is reached. As shown in  FIG. 17B , as the rotor increases rotational speed, a greater moment on the proximal end  226  containing the flyweight  32  as compared to the distal, magnet supporting end  224  results in a rotational force opposing the torsion spring  218 . At the desired minimum speed, (typically 500 RPM), the forces are balanced. As the rotational speed increases beyond the desired minimum speed, the greater flyweight  32  moment causes the pivot arm  212  to rotate about the pivot rod  214  until the full magnet travel distance is reached. As may be viewed particularly in  FIG. 20B , surface  234 A of the pivot arm  212  contacts surface  236 A of the pivot block  216 , to limit movement. Typically the travel distance is 0.5 inch at rated turbine speed, for example 5000 RPM. As may be further seen in  FIG. 17B , the distal end  224  and attached magnet assembly  270  move in the direction of arrow C until surfaces  234 A,  236 A contact and limit further movement. In contrast, and as seen in  FIG. 20A , when the pivot arm  212  is in its initial position, surfaces  234 B and  236 B are in contact with one another while surfaces  234 A and  236 A are spaced from one another. The pivot arm assembly  210  motion and resulting force is opposed by a torsion spring  218  located within the pivot arm  212  (see particularly  FIG. 19A ). As the proximal end  226  of the pivot arm  212  is displaced in the direction of arrow A, the pivot arm rotates about the pivot rod  214  to move the distal end  224  in with attached magnet  72  in the direction of arrow C, with the effect of the rare earth magnets  72  moving within their magnet bushings  74  and being drawn inward and away from the stator coils  120 , as shown. This action increases the permanent magnetic generator  200  air gap  100  with increasing turbine speed and provides the basis for voltage regulation. 
         [0077]    The sectional view of  FIG. 18  shows the connections at the magnet assembly  270  and shows one of many configurations of four magnets  72  and four or an equivalent number of stator coils  120 . It is to be understood that, other counts of magnets  72  and coils  120  could be used without departing from the present invention. 
         [0078]      FIG. 19A  details the magnet assembly  270  for use with the alternative permanent magnet generator  200 . As in the previous embodiment, each rare earth magnet  72  is bonded to a nickel-plated magnet cup  76 . A link  178  is attached by pins  80  and optional bushings (not shown) to both the steel cup mounting boss  68  and the distal end  224  of pivot arm  212 . 
         [0079]    The permanent magnet generator  200  depicted in  FIGS. 15-19  presents a simplified assembly procedure and simplified adjustment procedure as compared to the embodiment described earlier. Further, since the generator  200  requires fewer, less complex components, there is a manufacturing cost reduction. 
         [0080]    The foregoing is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.