Abstract:
A lightning mitigation system for use on an aircraft employs parasitic capacitance associated with a motor/generator to dissipate voltage provided as a result of a lightning strike. The motor/generator includes a set of windings defined by an outer periphery and a case that surrounds the set of windings. A parasitic capacitance is defined by the airgap separating the windings of the motor/generator from the case. A motor controller is electrically connected to the set of windings and includes a filter circuit. The filter circuit includes an equivalent capacitance that is selected based on the parasitic capacitance associated with the motor/generator such that a lightning strike results in a large portion of the voltage being dissipated by the parasitic capacitance of the motor/generator.

Description:
BACKGROUND 
     The present invention is related to electrical systems employed by aircraft, and in particular to electrical systems that provide lightning strike mitigation. 
     By some estimates, each aircraft in the U.S. commercial fleet is struck by lightning at least once a year. The effects of lightning strikes are typically mitigated by the use of electrically conductive materials, such as aluminum, as an exterior component or skin of the aircraft. The electrically conductive material provides a low-resistance path for the lightning to follow, preventing the lightning strike from damaging other components of the aircraft. 
     The next generation of aircraft employs composite materials to form the frame of the aircraft, such as the fuselage and wings. In addition, the next generation of aircraft has been deemed a ‘more electric aircraft’, which means the aircraft will rely more heavily on electric systems, as opposed to traditional mechanical and pneumatic systems. The electrical systems, disposed around the airplane, including on the wings and fuselage, provide a low-resistance path that can conduct lightning strikes, potentially damaging the electrical systems on the aircraft not capable of handling high voltages. This risk can be mitigated by the addition of high-voltage filters, but these filters are heavy and expensive, thereby increasing the cost of the aircraft as well as decreasing the fuel-efficiency of the aircraft. 
     SUMMARY 
     A lightning mitigation system for electrical systems on an aircraft makes use of parasitic capacitance associated with a motor/generator to mitigate the voltage associated with a lightning strike. The system includes a motor/generator comprised of at least one set of windings and a motor/generator case located a distance d around the outer periphery of the at least one set of windings. The at least one set of windings and the motor case located around the periphery of the case results in a parasitic capacitance being defined between the case and the windings. A motor controller is electrically connected to the at least one set of windings and includes a filter circuit a capacitance that is sized based on the parasitic capacitance associated with the motor/generator case such that the voltage associated with a lightning strike is dissipated, in large part, by the parasitic capacitance associated with the motor case and windings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of an electrical system according to an embodiment of the present invention that provides lightning strike mitigation. 
         FIG. 2  is a cross-sectional view of a motor/generator according to an embodiment of the present invention illustrating the airgap between the motor/generator case and set of windings. 
         FIG. 3  is a simplified circuit diagram representation of the electrical system according to an embodiment of the present invention that illustrates the lightning strike mitigation provided by the motor/generator. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a novel, inexpensive solution to the problem of mitigating lightning strikes on modern aircraft having exterior structures comprised largely of composite materials that do not provide electrical paths for mitigating lightning. In response to the use of composite materials on the exterior of aircraft, as well as the advent of more electric aircraft that rely on semiconductor components that are sensitive to large voltages caused by lightning strikes, mitigation of lightning strikes has relied on large, heavy, and therefore expensive filters to prevent lightning strikes from damaging electrical components. The present invention obviates the need for large filters by taking advantage of parasitic capacitance formed as a result of the airgap between the motor/generator case and the motor/generator windings. In this way, the present invention provides an inexpensive solution to lightning strike mitigation. 
       FIG. 1  is a circuit diagram of electrical system  10  according to an embodiment of the present invention that provides lightning strike mitigation. System  10  includes direct current (dc) input feeder  12 , motor controller  14 , electromagnetic interference (EMI) filter  15 , alternating current (ac) feeder  16 , motor/generator system  18 , motor/generator case  20 , motor/generator windings  22 , and representation of lightning strike voltage  24 . 
     Motor/generator system  18  may operate in both a motor mode and a generator mode. In the motor mode, dc voltage provided by dc input feeder  12  is converted to an ac voltage by motor controller  14 . The ac voltage is provided by ac feeder  16  to windings  22  for generating motive force in motor/generator system  18 . 
     Motor controller  14  includes a plurality of semiconductor devices (e.g., transistors) that allow motor controller  14  to selectively convert dc power to ac power (e.g., during the motor mode) or ac power to dc power (e.g., during the generating mode). Voltage surges, such as those caused by lightning strikes, can result in damage to the semiconductor devices. To protect the semiconductor devices from damage, large filters are typically required to absorb or mitigate voltage caused by a lightning strike. In addition, motor controller  14  includes EMI filter  15 , which is shown separately in  FIG. 1  for ease of illustration, but is typically included as part of motor controller  14 . EMI filter  15  is typically employed to remove electromagnetic interference associated with ac power generated by motor/generator  18 . In the embodiment shown in  FIG. 1 , EMI filter  15  includes an individual capacitive element C 1 , C 2 , and C 3  connected between each phase of power and a ground connection (e.g., typically the chassis of the motor controller). 
     As described in more detail with respect to  FIG. 3 , the capacitance associated with EMI filter  15  can be designed in conjunction with the parasitic capacitance associated with motor/generator  18  to provide a capacitive network that results in a majority of the voltage generated as a result of a lightning strike being dissipated by the parasitic capacitance associated with motor/generator  18 . In this way, the present invention takes advantage of parasitic capacitance provided as a result of the air gap located between motor/generator case  20  and motor/generator windings  22  to prevent voltage associated with a lightning strike from damaging motor controller  14 . 
       FIG. 2  is a cross-sectional view of motor/generator  18  according to an embodiment of the present invention. The cross-sectional view illustrates the presence of air gap  26  between motor/generator case  22  and motor/generator windings  24 . In this embodiment, the motor/generator windings represent the stator portion of motor/generator  18 , wherein a rotor portion (not shown) would be included within the interior portion of the stator. 
     The parasitic capacitance provided by the combination of motor/generator case  22 , motor/generator windings  24  and air gap  26  is dependent on the distance d of air gap  26  and the dielectric or breakdown voltage of air gap  26 . In an exemplary embodiment, the distance d of air gap  26  is substantially uniform around the circumference of windings  24 . Without a uniform air gap, points along the interior of motor/generator case  22  that extend close to motor/generator windings  24  provide breakdown paths for large voltages. By providing a substantially uniform distance, a uniform parasitic capacitance is provided that is capable of withstanding the high voltages caused by a lightning strike without breakdown of the dielectric (e.g., air) within air gap  26 . 
     In addition, the breakdown voltage associated with air gap  26  can be further increased by circulating the air within motor/generator case  22  and motor/generator windings  24 . By circulating air within air gap  26 , particles ionized as a result of a large breakdown voltage are removed (as a result of the circulation) from air gap  26 . In one embodiment, air is circulated within air gap  26  as a result of the rotation of the rotor (not shown). That is, as the rotor rotates, the air flow developed as a result of the rotation is provided to airgap  26 . In this way, ionized particles are removed from airgap  26  prior to formation of an ionized path forming between motor/generator case  22  and motor/generator windings  24 . 
       FIG. 2  illustrates the formation of a parasitic capacitance associated with motor/generator  18 . The parasitic capacitance represents half of the capacitive network employed to mitigate the effect of a lightning strike. The remainder of the capacitive network is realized by selectively designing the capacitance associated with EMI filter  15  (as shown in  FIG. 1 ). Typically (although not always), the parasitic capacitance associated with motor/generator  18  cannot be easily modified or varied. Thus, design of the capacitive network often depends on analyzing or experimentally detecting the parasitic capacitance associated with motor/generator  18  and then designing EMI filter  15  to provide a capacitive network that results in the majority of the voltage developed by a lightning strike being mitigated by the parasitic capacitance associated with motor/generator  20 . 
       FIG. 3  is a simplified circuit diagram representation of the electrical system according to an embodiment of the present invention that illustrates selection of the filter circuit associated with motor controller  14  to mitigate the effects of a lighting strike. In particular, the simplified diagram provides a framework by which the desired amount of capacitance can be analyzed using a single point system (i.e., without the multiple phases of ac power) that allows capacitances associated with motor/generator  18  and the filter circuit to be analyzed as a series circuit. To this end, previously introduced components are illustrated in this circuit diagram as a single point and are denoted by the suffix prime. For instance, EMI filter  15  (as shown in  FIG. 1 ) is represented as an equivalent capacitance labeled  15 ′, three-phase ac feeder lines  16  (as shown in  FIG. 1 ) are represented as single-phase line  16 ′, motor/generator case  20  (as shown in  FIG. 1 ) is represented as point  20 ′, motor/generator windings  22  (as shown in  FIG. 1 ) are represented as point  22 ′, and airgap  26  between motor/generator windings  22  and motor/generator case  20  is represented as capacitor  26 ′. 
     Analyzed as a single point system, EMI filter  15  can be represented as the sum of each individual capacitive element C 1 , C 2 , and C 3 , allowing EMI filter  15  to be represented as equivalent capacitance  15 ′. Likewise, the parasitic capacitance associated with motor/generator  18  can be represented as a single capacitive element  26 ′. The resulting capacitive network is expressed as a series connection of parasitic capacitance  26 ′ and equivalent filter capacitance  15 ′. Assuming the parasitic capacitance  26 ′ associated with motor/generator  18  is known and fixed, the equivalent filter capacitance  15 ′ can be designed such that the voltage generated as a result of a lightning strike (approximately 6000 V) is dissipated largely by the parasitic capacitance  26 ′. As a result of the lightning mitigation provided by the capacitive network that includes the parasitic capacitance associated with motor/generator  18 , motor controller  14  can be designed without expensive and large filters otherwise required to protect components from a lightning strike. 
     For example, in an exemplary embodiment the parasitic capacitance provided by the motor case, windings, and airgap is equal to approximately six nanoFarads (nF). In order to mitigate the approximately six-thousand volts provided by a strike of lightning without damage to semiconductor employed by motor controller  14 , the equivalent capacitance  15 ′ of the EMI filter is selected to equal approximately three-hundred nF. In this way, approximately 5,800 volts (i.e., 98% of the voltage surge provided by the lightning strike) are dissipated by the parasitic capacitance provided by the case and windings, leaving only approximately 120 volts to be dissipated by the equivalent capacitance  15 ′ associated with the EMI filter. 
     In this way, the present invention takes advantage of parasitic capacitance associated with motor/generator case and windings to mitigate the effect of lightning strikes. In particular, recognition of the parasitic capacitance provided by the motor/generator case and windings allows filter components employed by motor controller  14  to be appropriately sized such that the majority of the voltage is dissipated by parasitic capacitance of the motor/generator case. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.