Abstract:
An assembly for a direct current (DC) synchronous machine, according to an exemplary aspect of the present disclosure includes, among other things, a stationary portion including a direct current (DC) armature winding and a rotating portion including alternating current (AC) field winding configured to supply DC output to the DC armature winding. A rotating inverter is configured to selectively communicate current to the AC field winding such that a frequency of the current is adjusted to approach synchronization with a position of the rotating portion. A method for generating DC output from a DC synchronous machine is also disclosed.

Description:
BACKGROUND 
       [0001]    This disclosure relates generally to power generation, and more specifically to a direct current (DC) synchronous machine. 
         [0002]    Synchronous machines are known. Synchronous machines include a stationary portion and a rotating portion, where the rotating portion and the stationary portion each have at least one winding. 
         [0003]    One application of synchronous machines is a starter/generator application for a gas turbine engine. Synchronous starter/generators are configured to function as a motor to first start the gas turbine engine. Once the engine is running, the synchronous starter/generator system can operate as a generator. 
         [0004]    Some synchronous machines utilize a rotating DC field winding and a three-phase stator armature winding. When operating as a motor, the synchronous machine is coupled to an alternative current (AC) power source in order to supply motive power to a device with moving parts, such as the starter function or a pump or compressor. When operating as a generator, the synchronous machine is configured to supply AC current to one or more loads such as avionics equipment or motor driven loads on an aircraft. When DC output is required, the output voltage from the synchronous machine is rectified at the three-phase stator armature winding to produce direct current. There are challenges associated with current harmonics that adversely affect the generator efficiency, large volumetric and power density, and thermal losses on such rectifiers. 
       SUMMARY 
       [0005]    A synchronous machine, according to an exemplary aspect of the present disclosure, includes, among other things, a direct current (DC) armature winding and a rotating portion including a rotating inverter coupled to an alternating current (AC) field winding. The AC field winding is separated from the DC armature winding to define an air gap. The rotating inverter is configured to communicate current to the AC field winding such that a frequency of the current is adjusted to approach synchronization with a position of the rotating portion. A method for generating DC output from a synchronous machine is also disclosed. 
         [0006]    The various features and advantages of disclosed embodiments will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a DC synchronous machine. 
           [0008]      FIG. 2  schematically illustrates selected portions of the DC synchronous machine of  FIG. 1 . 
           [0009]      FIG. 3  schematically illustrates a second embodiment of a DC synchronous machine. 
           [0010]      FIG. 4  schematically illustrates a third embodiment of a DC synchronous machine. 
           [0011]      FIG. 5  schematically illustrates a fourth embodiment of a DC synchronous machine. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The disclosed embodiments of a synchronous machine include a rotating inverter to provide stable DC output. The DC synchronous machine can be configured using various energy sources and various field winding configurations as discussed below. While the disclosed embodiments are primarily discussed as DC generators, it should be noted that the general configuration can function in both motor and generator modes. 
         [0013]      FIG. 1  illustrates a DC synchronous machine  100  configured to provide DC supply to one or more loads  102 . In this disclosure, like reference numerals designate like elements where appropriate and reference numerals with the addition of one-hundred or multiples thereof designate modified elements that are understood to incorporate the same features and benefits of the corresponding original elements. 
         [0014]    The DC synchronous machine  100  includes a stationary portion  104 , or stator, arranged about a rotating portion  106 , or rotor, to define an air gap  108 . The rotating portion  106  can be coupled to a prime mover  110  via a shaft  112 . Example prime movers  110  can include gas turbine engines for aircrafts and ground-based systems using diesel engines. The rotating portion  106  and the stationary portion  104  include wire coils. During operation in a generator mode, a rotating magnetic field is generated by supplying current to the wire coils at the rotating portion  106  while the rotating portion rotates relative to the stationary portion  104 . 
         [0015]    A controller  114  is coupled to the DC synchronous machine  100  to cause various characteristics of the rotating magnetic field to be adjusted, as discussed in detail below. The DC synchronous machine  100  can be coupled to a DC-DC power converter  156  powered by a power source  152  in order to generate mechanical output in a motor mode to drive the prime mover  110 . The DC synchronous machine  100  can also be configured to function in a generator mode to supply direct current constant voltage to one or more loads  102 . Controller  114  is configured to receive a load voltage feedback signal and communicate a current magnitude command to the current regulator  138  in response to the load voltage feedback signal to maintain load voltage at a pre-determined level or range. As an example, the DC synchronous machine  100  can be a starter/generator which operates to start rotation of a gas turbine engine, and then is driven by the gas turbine engine to generate current. 
         [0016]    Referring to  FIG. 2 , the DC synchronous machine  100  can be configured to generate a rotating magnetic field based on a supply of three-phase alternating current (AC). The stationary portion  104  includes a stator armature winding  116 , and the rotating portion  106  includes AC field winding  118 . Energy is supplied to the rotating portion  106  by a rotor energy source  120 . In the illustrative embodiment, the rotor energy source  120  is a synchronous exciter including exciter field winding  122  located at the stationary portion  104  and an exciter armature winding  124  located at the rotating portion  106 . A rotor energy supply  126  is coupled to the exciter field winding  122  and is configured to selectively energize the exciter field winding  122 . Energy is transferred across the air gap  108  from the exciter field winding  122  to induce alternating current in the exciter armature winding  124 . In alternative embodiments, the rotor energy source  120  is a permanent magnet exciter configured to transfer energy across the air gap  108 . 
         [0017]    A supply of alternating current from the exciter armature winding  124  is communicated to a rotating inverter  132  through a rotating rectifier  128 . The rotating rectifier  128  can include one or more diodes  130  configured to rectify the AC supply to a DC supply. 
         [0018]    The rotating inverter  132  can include a three-phrase bridge including transistor(s)  134  and diode(s)  136  configured to invert the direct current to alternating current, and to selectively provide alternating current to the AC field winding  118 . AC supply to the AC field winding  118  generates a rotating magnetic field between the AC field winding  118  and stator armature winding  116 . The rotating magnetic field induces a current in the stator armature winding  116  to generate DC output at terminals  103   a ,  103   b.    
         [0019]    The rotating portion  106  includes a current regulator  138  configured to selectively adjust a frequency, a phase and/or a current magnitude of current communicated from the rotating inverter  132  to the AC field winding  118 . The current regulator  138  can be coupled to the gate of one or more transistor(s)  134  to selectively adjust the current output supplied from the rotating inverter  132  to the AC field winding  118 . 
         [0020]    The controller  114  is coupled to the current regulator  138  and is configured to cause the current regulator  138  to selectively adjust current supplied from the rotating inverter  132  to the AC field winding  118  such that the frequency of the current supplied is synchronized with a position of the rotating portion  106 , shaft  112 , or the rotating magnetic field relative to the stationary portion  104  by use of a position sensor  154 , for example. Synchronization refers to adjusting current supplied to each phase of a field winding to cause the wave cycles per second (frequency, Hz) of the current at each phase to be equivalent to the revolutions per second (angular frequency, ω) of a rotor. 
         [0021]    Various techniques for determining the relative position of the rotating portion  106  can be utilized. In some embodiments, the current regulator  138  is configured to utilize one or more position sensors  154 , such as resolver and Hall effect devices, to selectively adjust current based on rotor position. Further ways for determining a rotor position and synchronizing a frequency of a supply current with a rotor position are disclosed in co-pending U.S. patent application Ser. No. ______ (Client Reference No. ID-0035805; Attorney Docket No. 67036-814PUS1), entitled “Sensorless Control of a DC Synchronous Machine” filed on even date herewith. Aspects of this function from the co-pending application are incorporated herein by reference. 
         [0022]    The current regulator  138  can be coupled to one or more current sensors  140  to determine the current output at each phase of the rotating inverter  132 . The current regulator  138  is configured to selectively adjust the current supply to the rotating inverter  132  based on the current measurements at each sensor  140   a ,  140   b ,  140   c  of the rotating inverter  132 , and position information from the position sensor  154 , for example. 
         [0023]    By arranging the rotating inverter  132  in the rotating portion  106 , the overall system can take advantage of the amplification effect that occurs when power is transferred across the air gap  108  from the AC field winding  118  to the stator armature winding  116 , providing similar output voltage as arrangements having a high power rectifier at the stationary portion. Thus, generator efficiency can be improved by reducing high power electronic and thermal management requirements. 
         [0024]      FIG. 3  schematically illustrates a second embodiment of a DC synchronous machine  200 . In this embodiment, the rotor energy source  220  is a high frequency transformer including a primary winding  242  located at a stationary part  204  and a secondary winding  244  located at a rotating portion  206 . A rotor energy supply  226  coupled to the primary winding  242  is configured to supply current to and energize the primary winding  242 . Energy is transferred across the air gap  208  from the primary winding  242  to induce alternating current in the secondary winding  244 . AC supply from the secondary winding  244  is communicated to the rotating inverter  232  through a rotating rectifier  228 . 
         [0025]      FIG. 4  schematically illustrates a third embodiment of a DC synchronous machine  300 . In this embodiment, the rotor energy source  320  is a rechargeable energy source, such as a supercapacitor (shown) or lithium ion battery, located in the rotating portion  306 . An output contactor  329  is coupled to the stator armature winding  316  and is configured to cause the DC synchronous machine  300  to operate in charge and supply modes. The mode selection is initiated by the controller  314  based on a state of charge of the rechargeable energy source. During supply mode, power will flow from the rechargeable rotor energy source  320  to a rotating inverter  332 . During charge mode, power is supplied to the stator armature winding  316  from a high frequency charger  327 . The high frequency voltage induced in the rotating AC field winding  318  is rectified by the rotating inverter  332  which operates as a six-pulse rectifier by turning off transistors, or as an active rectifier and reversing the power flow to the rotating re-chargeable energy source  320 . The rate of charge current to the rotor energy source  320  can be controlled by the high frequency charger  327  or by the rotating inverter  332  operating as an active rectifier. 
         [0026]      FIG. 5  schematically illustrates a fourth embodiment of a DC synchronous machine  400  configured to generate a rotating magnetic field based on two-phase current. In this embodiment, AC field winding  418  is a two-phase winding which can include a direct winding  418   a  and a quadrature winding  418   b . A rotating inverter  432  can include two H-bridge circuits  432   a ,  432   b  coupled to direct winding  418   a  and quadrature winding  418   b . The H-bridges  432   a ,  432   b  can include transistor(s)  434  and diode(s)  436  configured to selectively provide a supply of alternating current to the AC field windings  418  which is phase shifted by 90 electrical degrees. The field current causes a rotating magnetic field between the AC field winding  418  and stator armature winding  416  in order to induce direct current in the stator armature winding  416 . 
         [0027]    The embodiments  200 ,  300 ,  400  of a DC synchronous machine in utilize synchronization techniques described above with reference to  FIG. 2 . These include synchronization of the frequency of supply current with the rotor position utilizing one or more positions sensors to determine rotor position, or synchronization of the frequency of supply current with the rotor position utilizing various electrical parameters, such as voltage, detected at various locations within the DC synchronous machine to determine rotor position as described above. While exact synchronization is preferable, this disclosure extends to attempting to approach synchronization in the disclosed manner. 
         [0028]    Although the different examples have a specific component shown in the illustrations, embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. It should also be understood that any particular quantities disclosed in the examples herein are provided for illustrative purposes only. 
         [0029]    Furthermore, the foregoing description shall be interpreted as illustrative and not in any limiting sense. A worker of ordinary skill in the art would understand that certain modifications could come within the scope of this disclosure. For these reasons, the following claims should be studied to determine the true scope and content of this disclosure.