Abstract:
An apparatus and method generate electric current within a specified frequency range from a rotor operating within a broad range of rotational speeds by reducing the number of poles of the generator at higher rotational speeds. At higher rotational speeds, the generator circuit is altered so that a flow of current through half of a plurality of windings is reversed and the polarity in the said half of the windings is reversed. Two adjacent windings with the same polarity create a single pseudo pole, which effectively reduces the number of poles in the generator by half, and reduces the frequency of the electric current produced by the generator. Thus, the generator is operable to produce current within a specified frequency range from a rotor operating within a broad range of rotational speeds.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to an electric generator, and more specifically to a pole shifting electric generator. 
         [0002]    A typical gas turbine engine has a high-pressure (HP) spool and a low-pressure (LP) spool. The LP spool typically operates across a broader range of rotational speeds, and the HP spool typically operates within a narrower range of rotational speeds. 
         [0003]    Vehicles incorporating gas turbine engines, such as aircraft, require significant amounts of electric power for operation. In some aircraft applications, a generator driven at these speeds would produce electric current within a frequency range of 360-800 Hz. This frequency range is acceptable (i.e., a frequency ratio of 2.22:1). The rotational speed of an HP spool extends across a speed ratio of about 2.22:1. Therefore, the high-pressure (HP) spool of a turbine engine is typically used to generate electricity for an aircraft system. However, due to modern aircraft efficiency requirements, the demand for electric power is increasing beyond the power extraction potential of the HP spool. 
         [0004]    The rotational speed of an LP spool varies over a much broader range, e.g. a speed ratio of 4.44:1. Although power extraction from the LP spool is possible, the broader range of rotational speeds of the LP spool would produce current whose frequency exceeds the 360-800 Hz range at higher speeds. When the LP spool is operating at lower speeds (e.g. across a 2.22:1 speed ratio), the LP spool is operable to produce current within the acceptable 360-800 Hz range. However when the LP spool operates in the higher range, which exceeds the 2.22:1 speed ratio, the frequency of the current produced by the generator would exceed the desired 360-800 Hz range. Of course, all of the mentioned ranges are examples only. 
         [0005]    If aircraft circuitry designed for 360-800 Hz electrical current receives current with a frequency that exceeds this range, the aircraft circuitry can be damaged. Alternatively, aircraft circuitry can be designed to accommodate a wider frequency range of current, but this would result in an unacceptable increase in weight and volume of the circuitry. 
       SUMMARY OF THE INVENTION 
       [0006]    In one disclosed embodiment, an electric generator is installed on a device that operates across a wide range of speeds. As disclosed, it may be on a low-pressure (LP) spool of a gas turbine engine, where the LP spool operates within a broad range of rotational speeds. The generator, comprising a plurality of windings, is excited by a first exciter field. During a first, lower speed range each pole of the generator has two adjacent poles with opposing polarity. During a second, higher speed range, a second exciter field is activated to alter the generator circuit so that a flow of current through half of the plurality of windings is reversed. For each winding in which the flow of current is reversed, the polarity is also changed. The result is that each winding of the generator has a first adjacent winding with the opposite polarity and a second adjacent winding with the same polarity. Two adjacent windings with the same polarity form a single pseudo pole, which effectively reduces the number of poles in the rotor in half, and reduces the frequency of the electric current produced by the generator, enabling the generator to continue producing electricity within a desired frequency range while operating at higher rotational speeds. 
         [0007]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows a turbine engine. 
           [0009]      FIG. 2  schematically illustrates a circuit of a generator according to one embodiment of the present invention, during a first, lower LP spool rotational speed range. 
           [0010]      FIG. 3  illustrates the embodiment of  FIG. 2  during a second, higher LP spool rotational speed range. 
           [0011]      FIG. 4  illustrates a four pole rotor according to one embodiment of the present invention during the first, lower LP spool rotational speed range. 
           [0012]      FIG. 5  illustrates an eight pole rotor according to one embodiment of the present invention during the first, lower LP spool rotational speed range. 
           [0013]      FIG. 6  illustrates the four pole rotor of  FIG. 4  during the second, higher LP spool rotational speed range. 
           [0014]      FIG. 7  illustrates the eight pole rotor of  FIG. 5  during the second, higher LP spool rotational speed range. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]      FIG. 1  illustrates an example gas turbine engine  10 . The engine  10  comprises a low pressure (LP) spool  12 , and a high pressure (HP) spool  14 . The LP spool  12  comprises a fan  16 , a low pressure compressor  18 , and a low pressure turbine  20 . The HP spool  14  comprises a high pressure compressor  22  and a high pressure turbine  24 . A combustor  26  is adjacent to the HP spool  14 . The fan  16  sends air through the low pressure compressor  18 , which sends air through the high pressure compressor  22 . The combustor  26  injects fuel into the air, which is burned, producing heat and driving both the high pressure turbine  24  and the low pressure turbine  20 . A generator  30  is shown coupled to the LP spool  12 , however the location of generator  30  is an example location, and the generator  30  could be coupled to the LP spool  12  in other locations. 
         [0016]      FIG. 2  schematically illustrates a generator  30 , according to one embodiment of the present invention, that could be coupled to an LP spool of a gas turbine engine. A generator control unit  32  provides current to a first exciter stator  34 , which creates a magnetic field. The magnetic field, or first exciter field, induces an alternating current in a first exciter armature  36 . The exciter armature  36  sends three phases of AC current to a first set of rectifiers  38 , which convert the three phases of AC current to DC current. This DC current flows from the set of rectifiers  38  through a plurality of windings  50 ,  52 ,  54 , and  56 . The windings are driven to rotate with the LP spool, so that the current flowing through the windings induces a current in a stationary main stator  64 . The windings and an associated rotor may be connected through appropriate gearing to be driven with the LP spool. From the main stator  64 , current flows to a load  68  through phases  66   a ,  66   b , and  66   c  and through neutral connection  66   d.    
         [0017]    Transistors  40 ,  42 ,  44 , and  46  perform a switching operation in the generator  30 . In one example, the transistors could be MOSFETs. When the LP spool is operating within a first, lower range of rotational speeds, transistor switches  40  and  44  are closed, and transistor switches  42  and  46  are open. In this switching configuration, current enters a rotor  48  through the winding  56  with a south polarity and exits the rotor  48  through the winding  50  with a north polarity. The current then reenters the rotor  48  through the winding  54  with a north polarity and exits the rotor  48  through the winding  52  with a south polarity. The first range of rotational speeds may extend across a speed ratio of 2.22:1. When operating within this first range of rotational speeds, the generator produces a current within a desired frequency range. An example desired frequency range is 360-800 Hz. During this first range of rotational speeds, a second exciter stator  58 , a second exciter armature  60 , and a second set of rectifiers  62  are all inactive. 
         [0018]    As shown in  FIGS. 2 and 3 , the stationary components are the generator control unit  4 , the exciter stators  34  and  58 , the main stator  64 , and the aircraft load  68 . The exciter armatures  36  and  60 , the rectifiers  38  and  62 , the transistors  40 ,  42 ,  44 , and  46 , and the windings  50 ,  52 ,  54 , and  56  are all driven by the LP spool to rotate while the generator is operating. Current is able to flow into the rotating windings because the stationary exciter stators  34  and  58  induce current in the rotating exciter armatures  36  and  60 . Furthermore, current is able to flow to the aircraft load  68  because the rotating windings induce a current in the stationary main stator  64 . 
         [0019]      FIG. 3  shows the generator of  FIG. 2  during a second, higher range of rotational speeds. The second range of rotational speeds may extend across a rotational speed ratio of 4.44:1. As the rotational speed of the LP spool increases, the frequency of the current produced by the generator  30  also increases, and can exceed the desired frequency range. When generator control unit  32  detects that the rotational speed of the LP spool has entered the second range of rotational speeds, the second exciter stator  58  is activated. When the generator control unit  32  provides current to the second exciter stator  58 , a magnetic field, or second exciter field, is created. The second exciter field only performs a switching operation in the generator  30  and does not actually provide current to the windings  50 ,  52 ,  54 , or  56 . 
         [0020]    The second exciter field induces an alternating current in the second exciter armature  60 . The second exciter armature  60  sends three phases of AC current to a second set of rectifiers  62 , which convert the three phases of AC current to DC current. This DC current alters the switching configuration of the generator  30  so that transistors  40  and  44  are opened, and transistors  42  and  46  are closed. In this switching configuration, current still enters the rotor  48  through the winding  56  with a south polarity and exits the rotor  48  through the winding  50  with a north polarity, however due to the change in switching configuration, current now reenters the rotor  48  through the winding  52  which now has a north polarity, and exits the rotor  48  through the winding  54 , which now has a south polarity. 
         [0021]    The polarity of each winding  50 ,  52 ,  54 , and  56  is determined by the orientation of the winding. As is well-known in the art, one can orient a winding so that when an electric current flows through the winding a desired polarity is created in the winding. When the flow of current through a winding is reversed, the polarity of the winding is also reversed. When adjacent windings have the same polarity, the generator  30  treats them as a single, pseudo pole. This effectively reduces the number of poles in the generator by half. The reduction in the quantity of poles simulates a lower rotational speed, and therefore reduces the frequency of current produced by the generator. Even though the LP spool is rotating within a range of rotational speeds that extends across a 4.44:1 speed ratio, with the pseudo poles it is effectively operating within a range of rotational speeds that extends across a 2.22:1 speed ratio, and thus still producing current within the desired frequency range. 
         [0022]      FIG. 4  illustrates a rotor  80  having four windings  82 ,  84 ,  86 , and  88 .  FIG. 4  illustrates the rotor  80  during the first, lower range of rotational speeds of the LP spool. As shown in  FIG. 4 , the windings have alternating polarity, with each winding having an adjacent winding with opposite polarity. For example, winding  84  has a south polarity, and adjacent windings  82  and  86  have a north polarity. This results in the rotor  80  having four distinct poles. 
         [0023]    Similarly, as shown in  FIG. 5 , the rotor  90  has eight windings  92 ,  94 ,  96 ,  98 ,  100 ,  102 ,  104 , and  106 . During the lower rotational speeds of the LP spool, these windings also have alternating polarity, with each winding having an adjacent winding with opposite polarity. For example, winding  94  has a south polarity, and adjacent windings  92  and  96  have a north polarity. This results in the rotor  90  having eight distinct poles. 
         [0024]      FIG. 6  illustrates the rotor  80  and windings  82 ,  84 ,  86 , and  88  of  FIG. 4  during the second, higher rotational speed range of the LP spool. A second exciter field (not shown) has reversed the direction of the flow of current through windings  84  and  86 , which has also reversed the polarity of windings  84  and  86 . Now each winding no longer has two adjacent windings of opposite polarity. Each winding has a first adjacent winding with the opposite polarity, and a second adjacent winding with the same polarity. When adjacent poles have the same polarity, they become a single pseudo pole. The four pole rotor  80  becomes a pseudo two pole rotor, as adjacent windings  82  and  84  both have a north polarity, and adjacent windings  86  and  88  both have a south polarity. 
         [0025]      FIG. 7  illustrates the rotor  90  and windings  92 ,  94 ,  96 ,  98 ,  100 ,  102 ,  104 , and  106  of  FIG. 5  during the higher rotational speed range of the LP spool. A second exciter field (not shown) has reversed the polarity of windings  92 ,  98 ,  100 , and  106 . The eight pole rotor  90  becomes a pseudo four pole rotor, as the number of poles is effectively reduced in half due to each winding having a first adjacent winding with the opposite polarity and a second adjacent winding with the same polarity. 
         [0026]    This application extends to any generator comprising 4*N windings, where N is a positive, even integer. Also, while disclosed as associated with an LP spool, it may have application in other generator applications that operate over a broad speed range. 
         [0027]    In addition, although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.