Abstract:
A permanent magnet comprises a primary winding, a secondary winding, a permanent magnet, an output terminal for connection to an external load, and a switching mechanism with two modes. In a first mode of the switching mechanism, the primary winding is connected between neutral and the output terminal, and the varying magnetic flux from the permanent magnet induces a nonzero voltage at the output terminal. In the second mode, the secondary winding provides a return path to neutral for the primary winding, thereby providing negligible voltage and current at the output terminal and substantially canceling change in magnetic flux from the permanent magnet.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to permanent magnet generators, and more particularly to methods and apparatus for de-energizing permanent magnet generators. 
         [0002]    Electromagnetic generators convert mechanical energy into electricity by producing changing magnetic flux from mechanical movement. Changes in magnetic flux through windings of wire induce voltage in these windings according to Faraday&#39;s law, producing deliverable electric energy. Permanent magnet generators (PMGs) produce varying flux through generator windings either with permanent magnet rotors, or with permanent magnet stators. Generators with permanent magnet rotors drive rotating permanent magnets to produce magnetic fields that vary at fixed stator winding locations. Generators with permanent magnet stators use stationary magnets to produce magnetic fields through which rotor windings travel, for the same ultimate effect. For simplicity, PMGs will hereafter be assumed to use permanent magnet rotors, although those skilled in the art will understand that generators can analogously be constructed with stationary magnets and rotating windings. 
         [0003]    In some situations it becomes necessary to rapidly de-energize a generator, either for operator safety or to prevent damage to generator components. Operating an energized generator while shorts are present between generator windings, for instance, can cause rapid resistive burnout of generator wiring. When such harmful faults are detected, it is often desirable for generators to quickly and automatically de-energize to avoid further damage. It may also be necessary to de-energize a generator to avoid dangerous high voltages. Generators which produce flux with field windings rather than permanent magnets can rapidly de-energize by cutting off currents to field windings. PMGs, although desirable for many reasons, are more difficult to rapidly de-energize. So long as permanent magnet rotors continue to rotate, they ordinarily induce currents in stator windings. Rotating magnets produce varying magnetic flux governed by the right-hand-rule. Whenever net flux passing through stator windings varies, this induces currents tending to oppose the change in flux. As a result, a PMG will ordinarily continue to produce voltage as long as the permanent magnet rotor turns. 
         [0004]    A variety of techniques have been developed to de-energize PMGs. A PMG can be de-energized by mechanically halting rotor movement, but this method is slow, and usually necessitates decoupling the generator drive shaft from its mechanical power source. As a faster alternative, some systems include specialized stator windings attached to an external power supply. When rapid flux cancellation is needed, these systems force current through the specialized stator windings to produce a countervailing magnetic flux, canceling changes in flux generated by the rotating permanent magnets. Another system (U.S. Pat. No. 7,777,384) provides conductive shunts which route flux away from stator windings, thereby preventing currents from being induced on stator windings. During normal operation, these shunts are saturated, and flux passes through the stator windings as usual. In a fault condition, the shunts can be “opened” by halting saturation, to de-energize the stator windings. A third system (U.S. Pat. No. 7,443,070) similarly uses shunts to route flux away from stator windings, but disconnects these shunts mechanically during normal generator operation. 
         [0005]    PMGs are preferably capable of rapidly de-energizing without sacrificing normal operational power. As described above, existing mechanisms for de-energizing PMGs add considerable bulk or complexity. Simpler, smaller mechanisms are highly desirable. 
       SUMMARY 
       [0006]    The present invention is directed toward permanent magnet generators having a primary winding, a secondary winding, a permanent magnet, an output terminal for connection to an external load, and a switching mechanism with two modes. In a first mode of the switching mechanism, the primary winding is connected between neutral and the output terminal, and the varying magnetic flux from the permanent magnet induces a nonzero current and voltage at the output terminal. In the second mode, the secondary winding provides a return path to neutral for the primary winding, thereby providing negligible voltage or current at the output terminal and substantially canceling change in magnetic flux from the permanent magnet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective view of a section of a permanent magnet generator. 
           [0008]      FIG. 2  is a cross-sectional view of the permanent magnet generator of  FIG. 1 , illustrating a permanent magnet rotor and a stator with stator windings. 
           [0009]      FIG. 3A  is a schematic view of one phase of the rotor and stator of  FIG. 1 . 
           [0010]      FIG. 3B  is a simplified schematic view of the stator configuration of  FIG. 3A . 
           [0011]      FIG. 4A  is a simplified schematic view of an alternative stator configuration. 
           [0012]      FIG. 4B  is a simplified schematic view of the normal operation mode of the stator configuration of  FIG. 4A . 
           [0013]      FIG. 4C  is a simplified schematic view of the cancellation mode of the stator configuration of  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a perspective view of permanent magnet generator  10 , including drive shaft  12 , rotor  14  (with permanent magnets  20 ), stator  16  (with stator coil  22 ), and air gap  18 . Drive shaft  12  is rotatable by an energy source (not shown) such as rotational motion of a vehicle drive system, wind, water, or another prime mover. Drive shaft  12  turns rotor  14 , which contains cylindrically arranged permanent magnets  20 . Stator  16 , which surrounds rotor  14 , contains multiple stator coils  22 , each with multiple windings. Stator coils  22  are dense coils of looping conductive wire. Rotor  14  is separated from stator  16  by air gap  18 , a narrow open space. It will be understood by one skilled in the art that, although this description focuses on generators having permanent magnet rotors, the invention could analogously be constructed with a permanent magnet stator and multiple rotor coils. 
         [0015]      FIG. 2  is a cross-section of permanent magnet generator  10  along cross-section line  2 - 2  from  FIG. 1 .  FIG. 2  depicts generator  10 , including drive shaft  12 , rotor  14  (with permanent magnets  20 ), stator  16  (with stator coils  22 ), and air gap  18 . Permanent magnets  20  are anchored to rotor  14 , and stator coils  22  are anchored to stator  16 . Magnetic fields from permanent magnet  20  cross air gap  18  and pass through stator coils  22  with magnetic flux Φ pm  (not shown in  FIG. 2 ; see  FIG. 3 ). As drive shaft  12  rotates rotor  14 , the movement of permanent magnets  20  causes Φ pm  to vary, causing a voltage to develop across stator coils  22 . The change in Φ pm  produces an electromotive force ε in volts described by Faraday&#39;s law: 
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         [0000]    Electromotive force ε induces a current I stator  in stator coils  22 , which can be coupled to an external load to supply power. 
         [0016]      FIG. 3A  is a schematic view of permanent magnet generator  10  illustrating a stator coil configuration of the present invention.  FIG. 3A  illustrates rotor  14  (with permanent magnet  20 ), stator  16  (with stator coils  22 ), magnetic flux Φ pm , magnetic flux Φ c , switching contact S, switch control  24 , neutral N, and output terminal T out  with output voltage V. Stator coils  22  comprise primary windings  22   a  (with current I pri ) and secondary windings  22   b  (with current I sec ). 
         [0017]    Every set of primary windings  22   a  in stator coil  22  is uniquely paired with a set of secondary windings  22   b . Primary windings  22   a  and secondary windings  22   b  have an identical number of turns. Primary windings  22   a  extend from neutral post N to output terminal T out , while secondary windings  22   b  and switching contactor S form a cancellation circuit between neutral post N and output terminal T out . As shown in  FIGS. 3A and 3B , secondary windings  22   b  extend from switching contactor S to neutral post N. Alternatively, switching contactor S could be located between secondary windings  22   b  and neutral post N, and secondary windings  22   b  could extend between output terminal T out  and switching contactor S. 
         [0018]    As rotor  14  turns, the movement of permanent magnet  20  produces magnetic flux Φ pm  through primary windings  22   a  and secondary windings  22   b , as described above. The induced current through primary windings  22   a  is I pri , and the induced current through secondary windings  22   b  is I sec . I pri  and I sec  together make up I stator  (I stator =I pri +I sec ). During normal operation of the circuit configuration of  FIGS. 3A and 3B , I sec =0, and I stator =I pri . 
         [0019]    The opening and closing of switching contactor S is controlled by switch control  24 . Switching contactor S is open during normal generator operation, and disconnects secondary windings  22   b  so that I sec  is zero. In this state, current I pri  is supplied to output terminal T out . V, the voltage at output terminal T out , is nonzero value so long as rotor  14  continues to turn. Current I pri  tends to oppose change in flux Φ pm , according to Lenz&#39;s law (as does I sec , when present), producing a countervailing flux Φ c . Because of load current limiting, change in Φ c  will always be less than change in Φ pm , and will never fully cancel change in Φ pm . Higher load resistances produce lower currents (V=IR), and therefore lower induced flux. Lower resistances produce higher currents which induce greater flux. In the limit of a perfect short circuit condition (i.e. zero load resistance), change in Φ c  will cancel change in Φ pm . The relationship between output voltage V, magnetic flux Φ pm  (produced by permanent magnet generator  20 ) and magnetic flux Φ c  (produced by the currents I pri  and I sec ) is described by: 
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         [0020]    When switching contactor S is closed, PMG  10  enters a cancellation mode in which it rapidly de-energizes. Closing switching contactor S shorts primary windings  22   a  through secondary windings  22   b , effectively bypassing output terminal T out . Because internal resistances in secondary windings  22   b  are much lower than the generator load at output terminal T out , substantially all of primary current I pri  will flow in series from primary windings  22   a  to secondary windings  22   b , such that I pri =I sec . Because the internal resistance of secondary winding  22   b  is very small, voltage V at output terminal Tout will be approximately zero, and the rate of change of Φ c  will approach the rate of change of Φ pm , as indicated by equation [2]. Thus, closing switching contactor S allows change in Φ c  to approximately cancel change in Φ pm , thereby substantially de-energizing PMG  10  (see equation [1], above). 
         [0021]    While switching contactor S is closed, primary windings  22   a  runs in series with secondary windings  22   b . Thus, shorting primary winding  22   a  through secondary winding  22   b  effectively provides twice as many winding turns for the production of Φ c  as if primary windings  22   a  were shorted directly. According to Ampere&#39;s law, Φ∝N I. By doubling the effective number of turns in stator coil  22 , the current needed to produce Φ c  is halved. This reduction in current protects stator coils  22  against resistive burnout. 
         [0022]      FIG. 3B  is a simplified schematic view of stator coil  22  of  FIG. 3A , showing only one phase of primary windings  22   a , secondary windings  22   b , neutral post N, output terminal T out , switching contact S, and switch control  24 . Switching contact S is an electrical switch capable of connecting or isolating secondary windings  22   b , and is open during normal operation of PMG  10 . Switch control  24  actuates switching contact S. In one embodiment, switch control  24  opens and closes switching contact S in response to operator input. In another embodiment, switch control  24  may open and close switching contact S automatically in response to sensed PMG conditions. Switch control may automatically close switching contact S in response, for instance, to detection of a harmful generator fault, thereby immediately de-energizing PMG  10  to avoid damage. During normal operation of PMG  10 , switching contactor S is open, primary windings  22   a  generate power, and secondary windings  22   b  are electrically isolated. In a cancellation mode of PMG  10 , switching contactor S is closed, and primary windings  22   a  are shorted through secondary windings  22   b , as previously discussed with respect to  FIG. 3A . 
         [0023]      FIG. 4A  is a simplified schematic view of an alternative circuit configuration for stator coil  22 .  FIG. 4A  illustrates primary windings  22   a , secondary windings  22   b , neutral post N, output terminal T out , voltage V, and switching contactors S 1A , S 1B , S 2A , and S 2B .  FIGS. 4B and 4C  are simplified schematic views of states of the circuit configuration of  FIG. 4A , illustrating primary windings  22   a , secondary windings  22   b , neutral post N, output terminal T out , and voltage V.  FIG. 4B  illustrates a normal operation mode, while  FIG. 4C  illustrates a cancellation mode. Switching contactors S 1A , S 1B , S 2A , and S 2B  are contactors like switching contactor S. 
         [0024]    During normal operation, the circuit configuration of  FIGS. 3A and 3B  only utilizes primary windings  22   a  to generate power, leaving secondary winding  22   b  disconnected. The alternative configuration illustrated in  FIG. 4A  enables the entirety of stator coil  22 —both primary windings  22   a  and secondary windings  22   b —to be utilized for power generation, but requires slightly more sophisticated wiring. 
         [0025]    During ordinary operation, switching contactors S 1A  and S 1B  are closed, and switching contactors S 2A  and S 2B  are open, such that primary windings  22   a  and secondary windings  22   b  operate in parallel, as indicated by  FIG. 4B . This configuration doubles the utilized copper area of the winding, and therefore reduces copper power loss relative to the configuration of  FIGS. 3A and 3B . In the cancellation mode illustrated by  FIG. 4C , switches are reversed: switching contactors S 2A  and S 2B  are closed, and switching contactors S 1A  and S 1B  are open. In this mode, primary windings  22   a  are shorted through secondary windings  22   b , as discussed previously. This configuration produces zero voltage V at output terminal T out , and cancels change in flux through stator coil  22  by approximately matching change in Φ c  to change in Φ pm , As discussed with respect to  FIG. 3A , neither flux cancellation nor voltage cancellation will be absolute. Because secondary windings  22   b  (like primary windings  22   a ) possess some slight internal resistance, a very small fraction of current I pri  will exit output terminal T out , rather than flowing through secondary windings  22   b  as I sec . As a result, a small voltage V (negligible, for most purposes) will be present at T out  even while stator coils  22  are in the cancellation mode ( FIG. 4B ). Similarly, change in Φ c  will never entirely cancel change in Φ pm , but will sufficiently cancel change in Φ pm  to substantially de-energize stator coils  22 . 
         [0026]    The present invention offers a mechanism for substantially canceling flux through stator coil  22  without halting permanent magnet rotor  14 , and with little additional manufacturing complexity. No external power source is required for flux cancellation, and the additional wiring needed for cancellation is minimal. The only moving parts used by this cancellation system are switching contactors, which are simple, small, and well-known in the art. As disclosed in the embodiment of  FIGS. 4A through 4C , all windings of stator coils  22  are used during normal operation. 
         [0027]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.