Abstract:
A brushless exciter for a synchronous generator or motor generally includes a stator and a rotor rotatably disposed within the stator. The rotor has a field winding and a voltage rectifying bridge circuit connected in parallel to the field winding. A plurality of firing circuits are connected the voltage rectifying bridge circuit. The firing circuit is configured to fire a signal at an angle of less than 90° or at an angle greater than 90°. The voltage rectifying bridge circuit rectifies the AC voltage to excite or de-excite the field winding.

Description:
REFERENCE TO GOVERNMENT CONTRACTS  
       [0001]     The U.S. Government may have certain rights in this technology under Contract Number DE-FC36-02GO11101 awarded by the Department of Energy. 
     
    
     BACKGROUND  
       [0002]     Large generators are driven by a prime mover to produce a supply of electric energy. A synchronous generator rotor is energized by an exciter providing to the generator&#39;s field winding a supply of DC power effective to produce a magnetic field. An annular stator surrounding the rotor contains a plurality of windings in which electricity is induced by the rotating magnetic field.  
         [0003]     Providing the supply of DC power to the rotor involves transferring the DC power from a stationary element to the rotating element. One method for transferring the DC power includes the use of slip-rings rotating with the rotor in combination with stationary brushes that contact the slip-rings. The use of slip-rings in this manner are subject to reliability and arcing problems. The arcing problems can present a hazard when the generator has to operate in volatile gas environments such as near oil and gas plants or in military applications  
         [0004]     An improved technique for transferring power from the stationary element to the rotating element uses a brushless exciter in which a DC field is applied to a stationary exciter winding. One or more windings rotating with the rotor pass through the magnetic field produced by the stationary exciter winding thereby producing AC power. The exciter AC power is rectified in a rectifier located on the rotor to produce the required DC excitation. Also, wound rotor induction machines can be used with a rotating rectifier connected to the rotor windings.  
         [0005]     When these synchronous generators make use of a superconductor in the rotor field, the electrical time constant of super cooled or high temperature superconducting (HTS) field windings in can be greater than one hour due to the fact that HTS field winding internal resistance approaches zero at HTS temperatures. In the generator output voltage regulation, a fast response of the current in the field windings is required to compensate any changes in load at the generator terminals. This requires that the exciter have a negative forcing function capability to de-excite the field windings when required by an output voltage regulator. For the case of static exciters, this is accomplished by using a stator mounted thyristor or silicon controlled rectifier (SCR) bridge connected to the rotor-mounted field winding via slip rings since a thyristor bridge can produce a negative DC voltage.  
         [0006]     Unfortunately, in the case of brushless exciters, the typical rotating diode bridge used does not allow for the application of negative forcing voltage to the field coil. All proposed topologies require the use of force-commutated devices like SCR&#39;s, FET&#39;s, etc. mounted in the rotor to produce negative DC voltage across the field windings. Mounting and controlling the devices is challenging. Redundancy requirements add to the system complexity. Moreover, the number of control signals to be transmitted to the rotor increases with the addition of each semiconductor.  
         [0007]     Accordingly, there is a need for an improved brushless exciter with less redundancy so as to reduce the complexity as well as the number of control signals transmitted to the rotor.  
       BRIEF SUMMARY  
       [0008]     Disclosed herein are brushless exciters and methods of use. In one embodiment, a brushless exciter comprises a stator; and a rotor rotatably disposed within the stator, the rotor having a field winding, a voltage rectifying bridge circuit connected in parallel to the field winding, the voltage rectifying bridge circuit comprising a silicon controlled rectifier and a firing circuit configured to fire a signal at an angle less than 90° and at an angle greater than 90° to the silicon controlled rectifier.  
         [0009]     In another embodiment, a brushless exciter comprises a stator; and a rotor rotatably disposed within the stator, the rotor having a field winding, a diode rectifying bridge circuit connected in series to a voltage rectifying bridge circuit, wherein the diode rectifying bridge circuit and the voltage rectifying bridge circuits are connected in parallel to the field winding, wherein the diode rectifying bridge comprises a diode configured to fire at an angle less than 90°, and wherein the voltage rectifying bridge circuit comprises a silicon controlled rectifier and a firing circuit configured to fire a signal at an angle greater than 90° to the silicon controlled rectifier.  
         [0010]     A method of exciting and de-exciting a field winding of a brushless synchronous generator comprises providing a gated firing circuit connectively disposed to a voltage rectifying circuit connected to the field winding; receiving a first gate signal at an angle less than 90° from the gated firing circuit; exciting the field winding with a rectified AC voltage via the voltage rectifying circuit; receiving a second gate signal at an angle greater than 90° from the firing circuit; and de-exciting the field winding with a rectified AC voltage via said voltage rectifying circuit.  
         [0011]     The above described and other features are exemplified by the following figures and detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Referring now to the figures, wherein the like elements are numbered alike:  
         [0013]      FIG. 1  illustrates a top level schematic view diagram of an exemplary embodiment of a single brushless exciter;  
         [0014]      FIG. 2  illustrates a top level schematic view diagram of a firing circuit of  FIG. 1 ;  
         [0015]      FIG. 3  illustrates a top level schematic view diagram of an exemplary embodiment of a dual brushless exciter using two synchronous machines;  
         [0016]      FIG. 4  illustrates a top level schematic view diagram of an exemplary embodiment of a single brushless exciter for a synchronous motor using a wound rotor induction machine; and  
         [0017]      FIG. 5  illustrates a top-level schematic view diagram of an exemplary embodiment of a dual brushless exciter for a synchronous motor using two wound rotor induction machines. 
     
    
     DETAILED DESCRIPTION  
       [0018]     Disclosed herein is a brushless exciter  10  for a synchronous generator or motor  20  as shown in  FIG. 1 . The brushless exciter  10  has a plurality of silicon controlled rectifiers (also referred to as SCRs or thyristors) configured as a single three-phase bridge network  11  mounted on the rotor of the synchronous generator  20 . The bridge network  11  is configured with the anode of the SCR  13  connected to the cathode of SCR  13 ′ and to one of the winding of a three-phase synchronous generator  19  used as an exciter machine. Another anode of the SCR  13  is connected to the cathode of SCR  13 ′ and to another windings of the three-phase exciter synchronous generator  19 . The anodes of SCRs  13 ′ are all connected together and to one end of the field winding  12  mounted on the rotor of the synchronous generator  20 . The cathodes of SCRs  13  are all connected together and to the other of the end of the field winding  12  of the synchronous generator  20 . All of the gates of SCRs  13  and  13 ′ are connected to their respective firing circuits  26  and the firing circuits are connected to the output of a voltage regulator  22  via a light activated or optical transformer  23 . This configuration enables the bridge network  11  to provide positive and negative voltages to the field winding  12 .  
         [0019]     The current, through the field winding  12 , can be monitored via a connection to one side of a rotating transformer  21 ,  FIG. 1 . This measurement can also be made by measuring directly the AC current output of the exciter windings. The other side of the transformer  21  is connected to the input of the voltage regulator  22 . As the load connected to the output of synchronous generator  20  increases and decreases, the voltage measured across the generator terminals will increase and decrease, respectively.  
         [0020]     When the voltage decreases at the terminals of the synchronous generator  20 , the output voltage regulator will increase the field current command. The actual field current is sensed and telemetrically transmitted to the voltage regulator  22 . If the value of the actual field current is below its commanded value, the regulator will generate a positive output, increasing the field current of the field exciter machine. The higher field current will generate a large AC voltage at the input of the thyristor bridge. The positive output of the voltage regulator  22 , will also force a signal across the optical transformer  23 . This sends a current through the diode of the opto-coupler  27 , short circuiting the resistor  39  of the SCR&#39;s firing circuit  26 , forcing a faster charging of the capacitor  40  which reaches the firing voltage with a fast time constant. The firing circuit  26  then fires all the SCR&#39;s of the bridge with the small firing angle  28 , generating at the output of the thyristor bridge a positive voltage amplitude proportional to the field voltage of the exciter machine that will tend to increase the current in the generator field winding. The increase in field current will then cause an increase in the generator output voltage.  
         [0021]     If the generator output terminal is above the desired level, the command for the field current will decrease, eventually becoming smaller than the measured field current. The field current regulator changes the DC field of the exciter machine, and if the error is too large, the polarity of its output will change to a negative value. If the output of the generator becomes negative, the command for the field current of the exciter machine will change polarity as well. A negative field in the exciter machine will only change the phase relation of its AC winding current and will not change the SCR bridge voltage. To reverse its polarity, the voltage regulator  22  will send a signal through the optical transformer  23  that will interrupt the current flowing through the opto-coupler  27 . This will cause the insertion of the resistor  39  into the circuitry, delaying the charging of the capacitor  40 . The slower charging of this capacitor delays the angle of firing to the position indicated as  29  in  FIG. 2 , causing a firing angle for the SCR bridge of about 150 electrical degrees. This large firing angle generates a negative voltage across the terminals of the thyristor bridge, forcing a decrease in the field current thus reducing the voltage at the generator terminal at the same time.  
         [0022]     It should be understood that the field voltage regulator  22  will have to operate as a comparator with hysteresis in order to avoid too high a rate of switching of the signal through optical transformer  23 . The level of hysteresis should take into account the field time constant and the maximum voltage from the exciter machine.  
         [0023]     In another form of control, the field current feedback could be eliminated and the command for the field of the exciter machine could be directly derived from the generator voltage regulator. The command for the angle of the SCR-bridge could then be derived from the sign i.e., plus or minus, of the output of the voltage regulator, increasing the field current by applying a positive voltage to the field coil and decreasing it by not sending the current through opto-coupler  27 .  
         [0024]     The described circuit provides an additional level of safety i.e. a fail-safe, by always forcing the decrease of the field current when there is a loss of power or a break in the optical signal going to the opto-coupler  27 .  
         [0025]     The field winding  12 ,  FIG. 1  may, if desired, be constructed from any conducting material. Examples of conducting materials are copper, aluminum and high temperature superconducting material (HTSC). The field winding  12  carries a large amount of current and its contribution to over all system power loss and weight is significant. If desired, the field winding  12  may be cryogenically cooled below 100° Kelvin (K) with any coolant, which is hereinafter defined as the cryogenic temperature. Examples of coolants are helium liquefied at 55° K and nitrogen liquefied at 77° K. Cryogenic cooling or super-cooling of the field winding  12  reduces the resistance of the field winding thereby enabling the field winding to conduct higher current with less conductor material. Using less conductor material reduces the size and weight of the field winding  12 . Making the wiring from high temperature superconducting material reduces the losses and size/weight of the wiring. However, if the use of high temperature superconducting material was impractical, even cooling copper or aluminum wiring will improve its conductivity and performance.  
         [0026]     Another embodiment of the disclosed technology is two independent-series connected sources of different polarities  30 ,  FIG. 3  providing excitation to the field winding  61 . The sources are two rotating synchronous generators  31  and  32  with three-phase armature windings on the rotors and field windings  33  and  34  on the stator and driven by the same shaft  35 . The SCR bridge  36  is fired at a constant angle of greater than 90° and preferably at an angle of about 150° with a firing circuit  37  which is similar to the firing circuit  26  with the resistor  39  in the circuit or replacing the series connection of resistors  38  and  39  by one resistor of the value of their sum.  
         [0027]     The two synchronous generators are combined into one machine with two independent sets of windings. Their independence may be achieved, for instance, by using two sets of windings of different pole numbers, e.g., four and six poles. Because the pole numbers differ, there is no electromagnetic coupling between the winding sets. Both the armature and field windings would share a common core, rotor and stator. Only one source field winding  33  or  34  is excited at a time thus enabling three modes of operation. Mode one, source field winding  33  is on and source field winding  34  is off. This mode provides excitation to field winding  61  via diode bridge  50 . Mode two, source field winding  34  is on and source field winding  33  is off. This mode provides de-excitation to field winding  61  via SCR bridge  36  which will provide the field coil with a negative voltage reducing the field current amplitude. Mode three, source field windings  33  and  34  are both off. In this mode no excitation is provided to the field winding  61 .  
         [0028]     The rotor and stator yokes need not be oversized to carry any more flux than that required by a single machine designed for the lowest pole number. The slots for the field and armature windings would be larger to contain both sets of windings. However, the combined machine would be significantly smaller than two separated machines for the same efficiency and controllability.  
         [0029]     An exciter cannot be used for a variable speed motor. The synchronous generators that provide the energy to the exciter would stop working at very low speed, making it impossible to control the field current. In another embodiment  51 ,  FIG. 4  of the disclosed technology, a three-phase wound rotor asynchronous generator  52  is used to control the excitation of the field winding  53 . When the asynchronous generator  52  is excited with a balanced AC voltage applied to the stator the generator produces a balanced AC voltage of a magnitude depending on: the primary excitation voltage, the rotating frequency and the turns ratio between the stator and the rotor. The secondary voltage can be controlled in order to achieve controllable voltage amplitude at the output of the SCR bridge  54  rotating with the field winding. If the speed is bi-directional, the output voltage would still be zero when the asynchronous generator  52  is rotating at a synchronous speed. However, the amplitude could still be controlled if the phase sequence of the primary voltage is changed as a function of the direction of rotation. The same method of controlling the field winding  53  excitation as discussed above may, if desired, be used with this method of excitation.  
         [0030]     In another embodiment  55 ,  FIG. 5  of the disclosed technology, dual three-phase wound rotor asynchronous generators  56  and  57  are used to control the excitation of the field winding  58 . When the asynchronous generator  56  is excited with a balanced AC voltage applied to the stator, the generator produces a balanced AC voltage of a magnitude that depends on: the primary excitation voltage, the rotating frequency and turns ratio between the stator and the rotor. The secondary voltage can be controlled in order to achieve controllable voltage amplitude at the output of the diode bridge  59  rotating with the field winding  58 . The SCR bridge  60  is fired at a constant angle greater than 90° and preferably at an angle of about 150° with a firing circuit  37  which is similar to the firing circuit  26 . The same method of controlling the field winding  58  excitation as discussed above may, if desired, be used with this method of excitation.  
         [0031]     The terms “first”, “second”, and the like do not denote any order or importance, but rather are used to distinguish one element from another, and the terms “the”, “a”, and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Furthermore, all ranges reciting the same quantity or physical property are inclusive of the recited endpoints and independently combinable.  
         [0032]     While the disclosed technology has been described with reference to an exemplary embodiment, 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 disclosed technology. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosed technology without departing from the essential scope thereof. Therefore, it is intended that the disclosed technology not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosed technology, but that the disclosed technology will include all embodiments falling with the scope of the appended claims.