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The Applicability of Electrically Driven Accessories for Turboshaft Engines

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The Applicability of Electrically Driven Accessories for Turboshaft Engines E AMEICA SOCIEY O MECAICA EGIEES G 4 E. 4 S., Yr, .Y. 00 e Sociey sa o e esosie o saemes o oiios aace i aes o iscussio a meeigs o e Sociey o o is iisios o Secios, m ® o ie i is uicaios. iscussio is ie oy i e ae is u ise i a ASME oua. aes ae aaiae om ASME o mos ae e meeig. ie i U.S.A. Copyright © 1993 by ASME h Applblt f Eltrll rvn Ar fr rbhft Enn Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78910/V03BT16A069/2403672/v03bt16a069-93-gt-313.pdf by guest on 26 September 2021 M. Simon Jarvis Warren J. Ostergren General Electric Aircraft Engines Lynn, Massachusetts Bert Smith Aviation Applied Technology Directorate U. S. Army Aviation and Troop Command Fort Eustis, Virginia Abstract more detailed description of the electric accessories and the engine Improved electrical power generation and actuation systems mounting, and electronics cooling considerations. Given the pre- offer new design approaches for performing the engine control and liminary design of an electric system, a trade study was completed accessory functions in helicopter propulsion systems. Present and the results are presented with a summary of the hardware helicopter technology utilizes turboshaft engines with mechanically experience. driven accessories. These accessories perform the functions of starting, fuel and lube pumping, variable stator actuation and inlet particle separation. This paperdiscusses the applicability of replacing Current Accessory Systems the mechanically driven accessories with their electrically driven Present gas turbine aircraft engines require control and accessory counterparts. An electric accessory system is defined which includes; systems to provide stable control of the engine turbo machinery, a switched reluctance starter/generator and its associated control lubrication, and starting. In order to ensure stable, efficient control unit; an electric pumping and actuation system; and the engine of the engine, the control system provides metered fuel to the engine mounting for the starter/generator. A comparison between the combustor, actuation for the compressor variable stators and lubri- mechanically and electrically driven accessory systems is performed cation to the engine bearings. A typical turboshaft engine control on the basis of cost, weight and reliability. Experience to date with system accomplishes these functions with accessories such as the switched reluctance machines and electrically driven turboshaft fuel boost and high pressure pumps, lube pump, alternator for accessory systems is summarized. The benefits of electrically electrical power, hydromechanical control unit, and electrical con- driven accessories are shown andrecommendations for future activity trol unit. These accessories are driven by the engine gearbox. The for this important technology are discussed. gearbox is also utilized for starting the engine. Introduction Electrical Accessory Systems Advances in electronic semiconductors and electric motors In addition to shaft power, typical turboshaft airframe appli- have now enabledtraditional turboshaftengine controls and accessory cations also require large amounts of electrical power. Traditionally designs to incorporate these technologies. The key developments this electrical power is provided through generators that are mounted that permit this are in the area of switched reluctance machines and on the main rotor gearbox or transmission. By combining the the associated electronics. Given an efficient power source, more starting and generating functions into one machine a turboshaft reliable, lighter and less expensive motors are also in a position to engine becomes capable of providing to the airframe bothmechanical compete with the traditional engine gearbox drives. This provides and electrical power. In addition the engine can benefit from the for simpler, more responsive fuel and lube delivery systems. It also availability of this electric power by replacing the engine gearbox offers new challenges for the engine and cooling system design. drive to the fuel and lube pumps and the hydraulic power for the This paper will give a brief description of the engine accessories that variable stator actuator with electric motor drives. A conceptual are considered for electric technologies, an overview of the starter/ electric accessory control system block diagram is shown in figure generator system including a description of a switched reluctance 1. machine and the associated electronics. This will be followed by a Presented at the international Gas Turbine and Aeroengine Congress and Exposition Cincinnati, Ohio May 242, This paper has been accepted for publication in the Transactions of the ASME Discussion of it will be accepted at ASME Headquarters until September 0, Sae Geeao Sa oque brrd o n (bI Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78910/V03BT16A069/2403672/v03bt16a069-93-gt-313.pdf by guest on 26 September 2021 Aica Ieace owe 20 c IeeCoee Ui a" KW AEC Gnrt r Moo Coo Ui lvrd t C (kW l , Mlr b d t n Q Alx Mnr Ml Figure 2. Start and Generating Requirement Figure 1. Engine Electric Interfaces Show A Simple Architecture Engine Starter/Generator System The engine starter is required for less than one minute for each start attempt and is not used after completion of engine starting. It is generally sized to produce adequate torque to start the engine. Figure 2 shows a typical starting and generating requirement curve. Electric power generation is initiated after the starter has been turned off. This situation lends itself to a natural integration of the starting and generating functions into one electricalmachine; the starter/generator. Since this machine acts as a starter for the engine, it is implied that it should be mounted to the engine compressor shaft. This enables extracting power, in the generation mode, directly from the com- pressor/gas generator shaft. Several different machine technologies have been examined as candidates for implementing the starter/generator functions. The main issues for starter/generator machine selection are high rotational speed capability (in excess of 50,000 RPM for some directly coupled machines on small turboshaft engines), power density, reliability, and cost. Overall, it was concluded that the switched reluctance machine(SRM) offered the best compromise of these criteria for aircraft engine applications. The switched reluctance machine is a brushless synchronous machine with salient rotor and stator poles, concentrated phase windings, and no permanent magnets or rotor windings. A diagram of the machine is shown in figure 3. The switched reluctance Figure 3 Switched Reluctance Machine Cross Section Shows machine is inherently low in cost. Its rotor is constructed from a A Simple Rotor and Stator Configuration simple stack of laminations shrunk fit onto a shaft, making it a rugged and low cost structure. This simple construction also gives the SRM construction cost. rotor the capability of operating at very high engine speeds without Switched reluctance machines also have the potential to run in the containmentproblems associated with rotor windings or permanent very high temperature environments. Because it has no permanent magnets. magnets, the maximum operating temperature of the machine is Likewise the stator assembly process is simple and inexpensive. limited primarily by the insulation system. Machine designs have Each phase of the machine is made up of two coils wound around been explored for operating temperatures up to 400 degrees Celsius. diametrically opposed stator poles and electrically connected in Simplicity, ruggedness, and high temperature capability lead to series. The concentrated stator winding can be externally form- high reliability in switched reluctance machines. Further discussion wound and slipped over the salient stator poles resulting in lower on the operation of the SRM can be found in the literature. [4],[5] Hendanr SRM ie teaGaa ues ue Ws Moo um Mo ^^g AcWa , p Mic° ocesso uaass 11 Seo I _ ,_s _ K.I _ I " Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78910/V03BT16A069/2403672/v03bt16a069-93-gt-313.pdf by guest on 26 September 2021 Moo Coo Ui 20 c om s n AEC Sys Figure 4. Switched Reluctance Controller Block Diogram Figure 6. Directly Metered Fuel Flow Simplifies Fuel System Design Inverter/Converter Unit power devices have made possible compactbrushless DC motors and The main function of the inverter/converter unit(ICU) is to generators, additional developments in high temperature power supply unidirectional current to the machine during starting and electronic devices and packaging techniques are needed to reduce the accept unidirectional current from the machine during generation. size and cost further and thereby enable the electric accessory drive TheICU is made up of acontroller and apower bridge. The controller technology to realize its full potential. is a microprocessor-based digital control that is capable of high There is considerable flexibility in the topology of the power bandwidth control of the power switches in the power bridge. The inverter/converter module used to drive the switched reluctance controller block diagram is shown in figure 4 and the power bridge machine. The topology shown in figure 5 was chosen for the starter/ in figure 5. The power bridge contains the high power capacitors, generator because it offers the maximum control flexibility and has diodes and solid state switches necessary to
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