Abstract:
A self-excited, switched reluctance generator obtains excitation energy from a capacitor bank via an excitation bus during normal operation. During a short-circuit or load fault, one or more phases of the generator provide power to the excitation bus while the remaining phases send current to a faulted positive bus. The system does not require an external battery or power source for excitation energy. The system is capable of resuming normal operation after the load bus fault.

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to self-excitation of a switched reluctance generator during a short-circuit or load bus fault without the need for a battery or auxiliary DC power source. 
     Switched reluctance electric generators are often used in aerospace applications, where they typically provide electrical power for the vehicle in which they reside. In such aerospace applications, a generator must be able to provide current during short-circuit and fault conditions on a positive load bus. 
     A load fault can occur when the maximum load current for a particular generator system is exceeded. A common type of load fault is a short circuit, which is an accidental low-resistance connection between two nodes of a circuit that are meant to be at different voltages. A load fault would typically happen not in the generator itself but somewhere in the load that it is powering. If a fault occurs, the output voltage of the generator drops to zero. During such a load fault, current from the generator is still needed to operate protective devices such as circuit-breakers and fuses. 
     Switched reluctance electric generators also require excitation energy to operate. This excitation energy must come from a DC power source. Since a switched reluctance generator itself is a source of DC power, the generator typically relies on its own DC output as a source of excitation energy. This is known as self-excitation, because the device is producing its own excitation energy. 
     In the event of a short-circuit or load fault, the generator still requires excitation energy to provide current for protective devices, however because of the short-circuit or fault condition, the generator can no longer use its DC output as a source of this energy. In such situations, a switched reluctance generator needs an alternate source of excitation energy to keep operating. Without the necessary excitation energy to sustain the generator in the event of a fault, the entire system would simply discharge and stop producing power. However, if a source of excitation energy is present during a fault, the generator can continue producing current and can return to its normal voltage and resume normal operation. 
     A conventional solution to this need for excitation energy involves a capacitor bank connected to a positive and negative bus of the generator. In this setup, the generator takes a small amount of energy from the capacitor bank for excitation during each electrical cycle. In the event of a fault on a load bus, this capacitor bank will completely discharge. A drawback of this conventional setup is that once the capacitor bank discharges, it can no longer excite the switched reluctance generator to operate, because the capacitor bank has been discharged to zero volts. 
     Another solution to this problem involves maintaining a separate excitation bus connected to an external power source, such as a battery or other auxiliary generator in the electrical system. Although this overcomes the earlier problem, it introduces additional problems. One drawback to this solution is overall diminished reliability due to the increased complexity and the need for additional hardware. Another drawback is an increase in the overall weight of the generator, which is undesirable in aerospace applications, due to the additional power source. 
     It is therefore the objective of this invention to provide a novel scheme for self-excitation of a switched reluctance generator in the event of a load bus fault, which does not require a battery or auxiliary generator in the electrical system. 
     SUMMARY OF THE INVENTION 
     A typical switched reluctance generator has three or more phases. Each phase comprises at least one diode, at least one switch, and an inductive winding. Each inductive winding corresponds to a diametrically opposite pair of stator poles of the generator. The power transfer of the generator is controlled by timing the current pulses in each phase with respect to a corresponding rotor position. 
     Excitation energy is necessary to start the generator and to get the generator to begin producing electricity. Once the generator is running and is producing electricity, additional excitation energy is necessary to sustain the generator. A switched reluctance generator typically has three or more phases. Each phase of the generator takes turns requiring excitation energy as the rotor rotates. All of the phases do not require excitation energy simultaneously. Each phase in the generator contains its own inductive winding. Excitation energy is stored in the magnetic field created by this winding. Typically a shaft position sensor is used to determine which phase requires excitation energy at any given time. 
     In the event of a load bus fault, the present invention provides a method of utilizing a portion of the current from one phase of the switched reluctance generator to provide the requisite excitation energy while the remaining phases deliver energy to the faulted positive bus as required. This eliminates the need for a battery or other auxiliary generator. This invention thus provides both weight reduction and reliability improvement. In addition, even though this generator relies on a capacitor bank for excitation energy under normal operating conditions as in the prior art, the problem of the prior art where once the capacitor bank is discharged the generator can no longer operate, is avoided. Unlike the prior art the capacitor bank is never fully discharged. In addition, this generator may also be used a motor. Due to this dual functionality as either a generator or a motor, it is common to refer to a switched reluctance generator as a switched reluctance machine. 
     The present invention can be expanded to a wide variety of switched reluctance generator designs that have a plurality of phase windings or coils. This includes, but is not limited to, designs with varying numbers of phases (e.g. 3, 4, 5, or 6) and varying stator and rotor pole combinations (e.g. 6-4, 12-8, 8-6, etc.). In addition, this invention is not limited to aerospace systems, but could be employed in any application that requires a fault tolerant electric generator. 
     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 
         FIG. 1  schematically shows the present invention in an example environment of an aircraft. 
         FIG. 2  is a schematic representation of a switched reluctance generator employing the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  schematically shows the present invention in an example environment of an aircraft  10 . The aircraft  10  comprises a switched reluctance generator  12 , a load  14 , and a connection  16 . Depending on the size and type of the aircraft, numeral  12  might represent a plurality of switched reluctance generators. Load  14  represents any device in the aircraft that requires electricity. The generator  12  is connected to the load  14  by the connection  16 . The connection  16  represents both a positive and a negative bus of the generator  12 . Although  FIG. 1  shows an aircraft, this invention is not limited to use in an aircraft.  FIG. 1  is merely presented to show one use of the present invention. 
       FIG. 2  shows circuit  20  for a switched reluctance generator, comprising three phases  22 ,  24 , and  26 . An alternative embodiment could include additional phases. Each phase includes an inductive winding, two switches, and two diodes. While it is possible that each phase might comprise just one switch and just one diode, there will typically be two of each. Phase  22  comprises switches  48  and  50 , diodes  52  and  54 , and winding  42 . Phase  24  comprises switches  56  and  58 , diodes  60  and  62 , and winding  44 . Phase  26  comprises switches  64  and  66 , diodes  68  and  70 , and winding  46 . Diode  34  and switch  36  are placed in parallel on the positive bus  30  between phases  22  and  24 . Depending on the voltage and current ratings required, the switches may be semiconductor devices such as MOSFETs or IGBTs. Capacitor bank  38  acts as a filter for the generator system. 
     Capacitor bank  40  provides excitation energy for the system under normal load conditions. Excitation energy is sent from capacitor bank  40  into excitation bus  28 , and then through either switch  48 , switch  56 , or switch  64 . The generator circuit provides power to a load (not shown). The load would be located between points P 1  and N 1 . A load fault or short circuit would typically occur between points P 1  and N 1  and not in one of the three phases  22 ,  24 , or  26 . 
     This circuit topology allows independent control of current in each phase  22 ,  24 , and  26  of the generator. In addition, this circuit permits the system to operate as either an electric generator or as a motor. Each inductive winding  42 ,  44 , and  46  corresponds to a pair of diametrically opposite stator poles (not shown) in the generator. The power transfer is controlled by timing the current pulses in each phase with respect to the corresponding position of a rotor in the generator, so that when the rotor poles are moving into alignment with the excited stator poles, current flows into the winding. 
     Under normal load conditions, when the switched reluctance generator  12  is operating as a self-excited generator, switch  36  is closed or ‘ON.’ The controller  80  acts as a sensor by monitoring the voltage across the positive bus  30  and negative bus  32 , and by monitoring the voltage across the excitation bus  28  and negative bus  32  to detect when a fault occurs. 
     A switching operation in the circuit is performed by the controller  80 . The controller may consist of analog and digital circuits. If the control circuit is digital, then the controller  80  can perform the switching electronically with a microprocessor. The semiconductor devices  34  and  36  may be replaced by a suitable mechanical switch, such as a relay or contactor. A worker skilled in this art would recognize how to provide an appropriate method to switch. The three inputs of the controller  80  are the input  82  from the negative bus  32 , the input  83  from the excitation bus  28 , and the input  84  from the positive bus  30 . The controller is connected to each switch in the generator via output connections  86 ,  88 ,  90 ,  92 ,  94 ,  96 , and  98 . 
     Capacitor bank  40  provides excitation energy for the system under normal load conditions, with each phase taking turns drawing current from the capacitor bank  40 . However the capacitor bank  40  never fully discharges under normal load conditions. When a fault is detected on the positive bus  30 , switch  36  must be opened or forced ‘OFF.’ 
     When the controller  80  detects a fault, the controller forces switch  36  open into an ‘OFF’ position. Diode  34  prevents the excitation bus  28  from discharging into the faulted positive bus  30 . At this point current from phase  22  would flow into the excitation bus  28 . Excitation bus  28  would then deliver the requisite excitation energy current to the phases  24  and  26  to sustain the generator. Phases  24  and  26  would not simultaneously require excitation energy. The excitation energy would first be provided to one of the phases, and then to the next phase. Meanwhile, phases  24  and  26  would provide current to the faulted positive bus  30  until the fault is cleared. The need for an external power source is eliminated because phase  22  provides the requisite excitation energy and capacitor bank  40  does not fully discharge. When the main bus voltage P 1 -N 1  rises back to its normal value, the switch  36  can be re-closed and the generator system can resume normal operation. 
       FIG. 2  shows a particular embodiment of the invention where diode  34  and switch  36  are connected to the output of phase  22 . In this embodiment, phase  22  is able to be the phase that supplies excitation energy in the event of a fault, with phases  24  and  26  providing current to the faulted positive bus  30 . Depending on the placement of diode  34  and switch  36 , any of the phases can provide excitation energy to the other phases.  FIG. 1  is not intended to limit phase  22  as being the only phase that can excite the other phases in the event of a fault. 
     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.