Abstract:
A starting system for aircraft engines employs power from multiple power sources. Each engine is started with a starter motor that is driven by the same multiple power sources which collectively provide starting power. As engine speed increases during each starting cycle a voltage boost is progressively provided by a boost converter. The starting system allows use of voltages higher than output voltage of the power sources while allowing the power sources to remain connected to a main aircraft power distribution bus.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is in the field of electrical power systems and, more particularly, power conversion and distribution systems which operate in vehicles such as aircraft for purposes of performing electrical main engine starts. 
         [0002]    In a typical aircraft, electrical power may be distributed with a 28 volt direct current (Vdc) system. In prior-art aircraft, main engine starting may be performed electrically or pneumatically. In the case of prior-art electrical starting, a DC starter-generator may be connected to a 24 Vdc battery. In the case of prior-art pneumatic starting, an air-turbine starter motor may be driven from a high pressure air source. As aircraft designs improve, these prior-art starting techniques require improvement. 
         [0003]    One major field of design improvement is known in the aircraft industry as More Electric Aircraft (MEA). In the context of MEA design, pneumatic starting systems are considered undesirable. Consequently, electrical starting systems have become the systems of choice in MEA designs. It is also anticipated that in the near future brushless starter-generators will replace the brush type starter-generators which have been routinely used on aircraft with 28 Vdc distribution systems. 
         [0004]    In addition to a MEA design evolution, there is a design evolution in a direction of more efficient, higher power engines. These newer engines require increased torque and speed for starting. Prior-art electrical starting systems may not provide the requisite torque and speed to efficiently start newer engines. 
         [0005]    Another consideration is that main engines started pneumatically depend upon the availability of the aircraft auxiliary power unit (APU) as a source of high pressure air. When the APU is inoperative, a backup source of high pressure air must be located, usually in the form of a ground cart. These pneumatic ground carts are not as readily available as electric ground carts, and hence aircraft with electric start systems are considered easier to dispatch. 
         [0006]    Various efforts have been made in the prior-art to improve electrical starting systems. In one prior-art example, two 24 Vdc batteries are connected in series to drive an electrical starter with 48 Vdc. This arrangement requires use of multiple contactors. Also electrical isolation is needed to assure that 48 Vdc is not applied to any equipment on the aircraft which may not be tolerant of 48 Vdc. This isolation is typically provided through use of a dedicated starter bus which is electrically separable from main power distribution buses of the aircraft. 
         [0007]    Such a prior art system may be understood by referring to  FIG. 1 . In  FIG. 1 , a prior art power distribution system is designated generally by the numeral  10 . The system  10  may be a starter and system for engines  12  and  14  of an aircraft (not shown). The system  10  may also be a generator system for the aircraft. The system  10  may comprise starter-generators  16  and  18 , control units  20  and  22 , aircraft power buses  24  and  26  and power sources or batteries  28  and  30 . 
         [0008]    The prior-art system  10  may be configured so that, in a power generation mode, direct current (DC) power may be provided to the aircraft power buses  24  and  26  at a voltage that is approximately equal to an output voltage of the batteries  28  and  30  and the starter-generators  16  and  18 . In a typical aircraft, the voltage on the buses  24  and  26  may be about 28 Vdc. Various electrical devices may be connected to the buses  24  and  26 . These devices are symbolically represented as blocks numbered  32  and  34 . In a typical aircraft the devices  32  and  34  may be rated to operate at a 28 Vdc bus voltage. 
         [0009]    The prior-art system  10  may also be configured to provide engine starting at a voltage higher than the 28 Vdc bus voltages. This may be accomplished by connecting both of the batteries  28  and  30  in series and employing the series connected batteries  28  and  30  to start the engines sequentially. For example the engine  12  may be started by driving the starter-generator  16  using a 48 Vdc power source. In this starter mode of operation, provision must be made to avoid energizing the aircraft power buses  24  and  26  with the 48 Vdc starting voltage. 
         [0010]    In order to avoid applying 48 Vdc to the aircraft power buses  24  and  26 , a start bus  36  may be provided in the system  10 . It may be seen that various contactors and connection paths may be employed to perform engine starting through the start bus  36 . The following Table 1 explains how various combinations of contactors shown in  FIG. 1  and their respective switching states may provide requisite current routing of the prior-art system  10 . 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 (Prior Art) 
               
             
          
           
               
                   
                   
                 Start 
                 Start 
                 Power 
               
               
                   
                 Contactor 
                 Engine 
                 Engine 
                 Generation 
               
               
                   
                 Number 
                 12 
                 14 
                 Mode 
               
               
                   
                   
               
               
                   
                 LC1 
                 Open 
                   
                 closed 
               
               
                   
                 LC2 
                   
                 open 
                 closed 
               
               
                   
                 SC1 
                 closed 
                 open 
                 open 
               
               
                   
                 SC2 
                 Open 
                 closed 
                 open 
               
               
                   
                 SC3 
                 closed 
                 closed 
                 open 
               
               
                   
                 BC1 
                 Open 
                 open 
                 closed 
               
               
                   
                 BC2 
                   
                   
                 closed 
               
               
                   
                 BC3 
                 Open 
                 open 
                 closed 
               
               
                   
                   
               
             
          
         
       
     
         [0011]    In another prior-art example an auxiliary power unit (APU) generator may be connected in series with a battery to produce a 48 Vdc starting voltage. In this arrangement the APU generator must be designed to produce the high currents needed for starting. Such a robust APU generator may not be desirable on an aircraft because of its size, weight and cost. Multiple contactors and buses are also required to temporarily interconnect the APU generator and the battery and preclude application of 48 Vdc to 28 Vdc rated equipment. 
         [0012]    As can be seen, there is a need to provide a system of electrical power distribution and control that enhances operation of an electrical engine starter. Additionally, there is a need to reduce the number, size and weight of components used to control electrical power distribution for such enhanced operation of the engine starter. 
       SUMMARY OF THE INVENTION 
       [0013]    In one aspect of the present invention an apparatus for distribution of electrical power comprises at least two electrical power sources each having an output voltage, an electric motor, at least two boost converters, a first interconnection path between a first one of the at least two electrical power sources, a first one of the at least two boost converters, a second interconnection path between a second one of the at least two electrical power sources, a second one of the at least two boost converters, a third interconnection path between at least two of the and the motor, and a controller for selectively operating the boost converters to apply power to the motor from at least two of the power sources at a voltage higher than the output voltage of either of the at least two power sources. 
         [0014]    In another aspect of the present invention an apparatus for starting a plurality of aircraft engines comprises at least two starter-generators, at least two power sources each having a limited output voltage, a first one of the starter-generators being interconnected with a first set of at least two boost converters and at least two multi-phase inverters, a second one of the starter-generators being interconnected with a second set of at least two boost converters and at least two multi-phase inverters, a first set of selectively operable interconnection paths between the at least two power sources and the first set of boost converters and inverters, a second set of selectively operable interconnection paths between the at least two power sources and the second set of boost converters and inverters and one or more controllers for selectively operating the boost converters and inverters so that the starter generators are operated with a voltage greater than the output voltage of any one of the power sources. 
         [0015]    In still another aspect of the present invention a method for operating an aircraft electrical system comprises the steps of supplying electrical power to a starter motor from at least two power sources connected in parallel, wherein each of the at least two power sources has an output voltage and providing a voltage boost to current flowing from both of the at least two power sources so that the motor is provided with voltage higher than the output voltage of either of the at least two power sources. 
         [0016]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a block diagram of a power distribution control system in accordance with the prior art; 
           [0018]      FIG. 2  is a block diagram of a power distribution control system in accordance with the present invention; 
           [0019]      FIG. 3  is a block diagram of a control unit portion of the control system of  FIG. 2 ; and 
           [0020]      FIG. 4  is a flow chart of a method of starting an engine in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0022]    Broadly, the present invention may be useful in controlling vehicle power distribution. More particularly, the present invention may provide for enhanced operation of an electrical engine starter. The present invention may be particularly useful in vehicles such as aircraft. 
         [0023]    In contrast to prior-art power distribution and engine starting systems, which employ a dedicated starter bus, among other things, the present invention may provide for enhanced starter operation by controlling power distribution on a main power bus of an aircraft. The present invention, instead of utilizing series connected power sources, may employ a novel interconnection system, and boost converters and inverters to produce controlled power to a starter-generator in order to enhance starter system operation. 
         [0024]    Referring now to  FIG. 2 , a power distribution system in accordance with the present invention is designated generally by the numeral  100 . The system  100  may be a starter system for engines  112  and  114  of an aircraft (not shown). The system  100  may also be a generator system for the aircraft. The system  100  may comprise starter-generators  116  and  118 , control units  120  and  122 , aircraft power buses  124  and  126  and power sources such as batteries  128  and  130 . 
         [0025]    It may be noted that the system  100  does not embody a starter bus. This is because there is no need to electrically isolate the aircraft power buses  124  and  126  during startup of either of the engines  112  or  114 . The batteries  128  and  130  may not be connected in series during startup. Consequently the buses  124  and  126  may remain connected to the respective batteries  128  and  130  without risk of exposing the buses  124  and  126  to a voltage higher than a battery voltage of about 24-28 Vdc. Consequently, 28 Vdc-rated electrical devices, represented symbolically as blocks  132  and  134 , may be interconnected with their respective buses  124  and  126  during both engine-starting and power-generation operation of the system  100 . 
         [0026]    During engine starting, the system  100  may utilize the novel control units  120  and  122  to provide a boosted voltage to the respective starter-generators  116  and  118 . The boosted voltage provided to the starter-generators  116  and  118  may not be applied to the buses  124  and  126 . 
         [0027]    The following Table 2 may explain how various combinations of contactors shown in  FIG. 2  and their respective switching states may provide requisite current routing of the inventive system  100 . 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Start 
                 Start 
                 Power 
               
               
                   
                 Contactor 
                 Engine 
                 Engine 
                 Generation 
               
               
                   
                 Number 
                 112 
                 114 
                 Mode 
               
               
                   
                   
               
             
             
               
                   
                 LC10 
                 closed 
                   
                 closed 
               
               
                   
                 LC20 
                   
                 closed 
                 closed 
               
               
                   
                 SC10 
                 closed 
                 open 
                 open 
               
               
                   
                 SC20 
                 open 
                 closed 
                 open 
               
               
                   
                 BC10 
                   
                   
                 closed 
               
               
                   
                 BC20 
                   
                   
                 closed 
               
               
                   
                 BTB0 
                 open 
                 open 
                 open 
               
               
                   
                   
               
             
          
         
       
     
         [0028]    It may be seen that the contactor LC 10  along with its respective interconnecting conductors may provide an interconnection path between the battery  128  and the control unit  120 . The contactor SC 10  along with its interconnecting conductors may provide an interconnection path between the battery  130  and the control unit  120 . Similarly, the contactor LC 20  along with its respective interconnecting conductors may provide an interconnection path between the battery  130  and the control unit  122 . The contactor SC 20  along with its respective interconnecting conductors may provide an interconnection path between the battery  128  and the control unit  122 . For purposes of illustration, these interconnection paths may be referred as interconnection paths PLC 10 , PSC 10 , PLC 20  and PSC 20  respectively. 
         [0029]    Referring now to  FIG. 3 , one of the control units, for example the control unit  120  is illustrated. The control unit  120  may comprise boost converters  300  and  302 , a controller  304 , multi-phase inverters  306  and an exciter power supply  308 . The control unit  120  may be configured to provide six-phase alternating current (AC) to the starter-generator  116  of  FIG. 2 . The control unit  120  may also convert six-phase AC to 28 Vdc when the starter-generator  116  is operated in a power generation mode. 
         [0030]    The boost converter  300  may be connected with the battery  128  through the bus  124  and the interconnection path LC 10  of  FIG. 2 . The boost converter  302  may be connected with the battery  130  through the bus  126  and the interconnection path PSC 10  of  FIG. 2 . It can be seen that use of the two boost converters  300  and  302  connected to the two batteries  128  and  130  provides an effective parallel connection between the batteries  128  and  130 . In other words, electrical energy from both of the batteries  128  and  130  may be employed to drive the starter-generator  116  through an AC interconnection path  303 . 
         [0031]    The boost converters  300  and  302  may operate in a conventional manner to provide a voltage boost to current which flows through the converters  300  and  302 . As voltage is boosted, current is reduced. If a single one of the batteries, e.g. the battery  128  were to be used as a sole source of starting power for the engine  112  of  FIG. 2 , then a voltage boost performed by the boost converter  300  may produce an intolerable diminishment of current delivered to the starter-generator  116 . Conversely, if the boost converter  300  were not used, current from the single battery  128  may be delivered to the starter-generator  116  at an intolerably low voltage. 
         [0032]    When the batteries  128  and  130  are both used to power the control unit  120 , their combined current output may be sufficient to withstand diminishment of current that may result from voltage boosting by the boost converters  300  and  302 . 
         [0033]    It may be realized that the booster converters  300  and  302  may provide voltage boosting only in a forward direction. In other words an output voltage of one of the booster converters  300  or  302  to their respective multi-phase inverters  306  may be higher than an input voltage from the aircraft power buses  124  and  126 . But, the boost converters may not feed back a boosted voltage to the aircraft power buses  124  and  126  of  FIG. 2 . Consequently, aircraft electrical equipment represented by the blocks  132  and  134  of  FIG. 2  may remain connected to their respective aircraft power buses  124  and  126  during startup of the engines  112  and  114  without risk of the equipment being exposed to overvoltage. 
         [0034]    The controller  304  may be embodied using a conventional digital signal processor (DSP). The controller may be programmed to selectively activate the boost converters  300  and  302 . The selective activation of the boost converters  300  and  302  may be performed responsively to sensed rotational speed of the starter-generator  116 . This may be desirable in conditions that require voltage boosting to overcome back electromotive force (BEMF) from the starter-generator  116 . BEMF may vary in magnitude as a function of rotational speed of the starter-generator  116 . At low speeds BEMF may be low enough to preclude a need for voltage boosting. In such a case, the boost converters  300  and  302  may be de-activated and undiminished current may be delivered to the starter-generator  116 . At higher rotational speeds, BEMF may increase and the boost converters  300  and  302  may be commanded by the controller  304  to boost voltage to overcome the increased BEMF of the starter-generator  116 . In this way, an optimized current-voltage relationship may be dynamically produced for electrical power provided to the starter-generator  116 . At a beginning of a startup, low speed rotation of the engine  112  may require high torque. High torque may be optimally produced with undiminished current. As rotational speed increases, torque requirements may decrease while BEMF may increase. The controller  304  may command the boost converters  300  and  302  to provide progressively increasing voltage boosting to overcome the progressively increasing BEMF. 
         [0035]    It may be noted that the control unit  120  may comprise by-pass contactors  310 ,  312  and  314 . These by-pass contactors may be employed to produce a circuit path through which current may be transferred directly between the starter-generator  116  and the aircraft power buses  124  and  126  of  FIG. 1  after being rectified by the inverters  306 . These by-pass contactors  310 ,  312  and  314  may be particularly useful to improve efficiency of the system  100  by offsetting adverse effects of series power transistors and inductor parasitic resistances that may be associated with the boost converters  300  and  302 . 
         [0036]    In one embodiment of the present invention, a method is provided for controlling power distribution on a vehicle such as, for example, an aircraft. In that regard the method may be understood by referring to  FIG. 4 . In  FIG. 4 , a flow chart portrays various aspects of an inventive method  400 . 
         [0037]    In a step  402 , electrical power may be supplied to a first boost converter (e.g. power from the battery  128  to the boost converter  300 ). In a step  404 , electrical power may be supplied to a second boost converter (e.g., power from the battery  130  to the boost converter  302 ). In a step  406 , power supplied to the first boost converter may be provided with a voltage boost (e.g. within the boost converter  300 ). In a step  408 , power supplied to the second boost converter may be provided with a voltage boost (e.g. within the boost converter  302 ). In a step  410 , power from the first boost converter may be supplied to a first multi-phase inverter. In a step  412  the power from the second boost converter may be supplied to a second multi-phase inverter. In a step  414 , combined output of the first and second multi-phase inverters may be employed to drive a starter-generator to start an engine (e.g. the starter-generator  116  may start the engine  112 ). 
         [0038]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.